Appeared in Annales Geophysicae, 13, 505-516, 1995

Potential Merits for Substorm Research from Imaging

of Charge-Exchange Neutral Atoms

Ioannis A. Daglis and Stefano Livi

Max-Planck-Institut f�ur Aeronomie, Katlenburg-Lindau, Germany

Abstract. The in situ observations of the Earth magnetosphere performedover the past decades of space research have provided a rather good under-standing of many partial localized processes of the magnetospheric substorm.The continuing lack of global observations inhibits the construction of acoherent picture of the substorm as a whole, which is actually determinedby the coupling of the partial processes. In this context the importance ofglobal observations for the advancement of magnetospheric substorm studiesis critical. This paper presents brie y a promising technique of global obser-vations, namely the Imaging of Charge Exhange Neutral Atoms, or shortlyNeutral Atom Imaging (NAI) of the magnetosphere. Model and theoreticalestimates of charge-exchange neutral atoms, as well as appropriate spacecraftorbit and intrumentation requirements are presented and discussed for speci�cregions of interest and vantage points. The potential merits of NAI for thesubstorm research are presented along with possible combinations with othertypes of observational methods for maximum science gain. Substorm issuesthat would bene�t from NAI should include among others the assessment ofthe ionospheric contribution to the hot magnetospheric plasma, the relativeimportance of various ionospheric ion source regions, the resolution of spatialand temporal characteristics of substorm ion injections. NAI observations canbe precious complements to local observations and lead to the understandingof how local processes, many of which are resolved quite well today, combineto form the global process of the magnetospheric substorm.

1. Introduction

Imaging of charge-exchange neutral atoms, or shortlyneutral atom imaging (NAI), is a relatively new tech-nique that enables global imaging of magnetosphericplasmas, as well as plasma ion populations in gen-eral [Roelof , 1987; Hsieh et al., 1992; Witte et al.,1993; Hsieh and Gruntman, 1993, e.g.,]. In the case ofthe Earth magnetosphere, neutral atoms are producedthrough charge exchange [Smith and Bewtra, 1978, e.g.]between hot (E > 1 keV) magnetospheric or ionosphericplasma and the geocorona. The geocorona is an exo-spheric extension of relatively cold (�1000 K) neutralatoms, which resonantly scatter solar Lyman � radia-tion [Chamberlain, 1963; Rairden et al., 1986]. Sinceoxygen atoms must have an energy of about 10 eVto overcome the Earth's gravitational �eld, while lightatoms and molecules need much less energy to escape,the geocorona is actually a hydrogen gas. The neutralhydrogen density falls o� quickly with radial distance

(Figure 1), however, even at r=10Re it has substan-tial values (�10 cm�3). Magnetospheric/ionosphericions charge exchange with the geocorona producing neu-tral atoms; the produced neutrals leave the interac-tion region with essentially the same energy and ve-locity direction as the incident ion (Figure 2). Thusthe geocorona acts like an imaging screen for magneto-spheric/ionospheric ions. Remote sensing of the gener-ated neutrals can provide line-of-sight integrated obser-vations of the source populations.

Charge exchange had already been suggested in 1959by Dessler and Parker [1959] as the mechanism for dis-sipating ring current energy. Some low-latitude auroraground-based observations [Tinsley , 1981] lent supportto the role of energetic neutral atoms, and the interestin neutral atom imaging grew after the detection of en-ergetic neutral particles emanating from Jupiter [Kirschet al., 1981b] and Saturn [Kirsch et al., 1981a].

The importance of global imaging and particularly

1

2 Daglis and Livi

Figure 1. (not available in the pdf version of the pa-per) The Chamberlain models of the exospheric hydro-gen density radial pro�le for three temperatures. Themodel at T = 1050 K provided the best �t to 4 yearsof Dynamics Explorer 1 Lyman � observations of thegeocorona [Rairden et al., 1986].

neutral atom imaging has been increasingly recognizedduring the last few years [Williams , 1990; Akasofu,1991; Williams et al., 1992], and some proposals havebeen made for NAI experiments [Orsini et al., 1992;Wurz et al., 1993] and NAI missions [Smith et al., 1993;Sauvaud et al., CESR, Toulouse, France, 1994; Franket al., 1994], the experiment described by Orsini et al.

[1992] actually being scheduled to y on the SAC-Bsatellite [Gulich and White, 1993] in 1995.

The very �rst indication of energetic neutral atomsat auroral altitudes was presented by Meinel [1951]who detected a blue-shifted hydrogen H� line corre-sponding to 76 keV H in the magnetic zenith duringan aurora. The �rst ENA spectrometer was introducedby Bernstein et al. [1969]. The basic working princi-ple of the spectrometer was the conversion of neutralsinto ions in thin foils and their subsequent detection.This technique is used in many of the modern proposedENA spectrometers. The spectrometer of Bernsteinet al. [1969] was own aboard a Javelin sounding rocketlaunched from Wallops Island, Virginia, on March 7,1970; it detected high uxes of keV neutral hydrogenatoms at 800 km altitude [Wax et al., 1970]. The au-thors suggested a solar or interstellar origin of the de-tected ENA. A few years later, Hovestadt and Scholer

[1976] suggested that energetic (300 - 500 keV) parti-

cles observed by IMP 7 at 33 RE geocentric distance andcoming from the direction of the Earth were energeticneutral atoms.

Pioneering work in neutral atom imaging has beendone by E. C. Roelof, who has concentrated on energeticneutral atom imaging. Roelof et al. [1985] presentedthe �rst energetic neutral atom (ENA) observations ofthe global decay of the magnetospheric ring current,obtained from the ISEE 1 MEPI instrument. Roelof

[1987] extended then these observations to obtain the�rst-ever global image of the ring current, by iteratingmodel ring current distributions in a dipole magnetic�eld until their convolution through charge exchangewith the model geocorona reproduced the actually ob-served ENA patterns. Furthermore, Roelof [1989] usedthis image to derive a quantitative picture of the three-dimensional current system driven by global pressureasymmetries inferred from the ring current image.

The concept of the magnetospheric substorm was in-troduced by Coroniti et al. [1968] in order to includethe global disturbance characteristics and to emphasizethe importance of the magnetosphere in auroral zoneobservations. In the meantime it has become clear thatthe magnetospheric substorm is the fundamental dy-namic process of the magnetosphere. Increased energyinput from the solar wind (during times of southwardIMF) disturbs the magnetosphere equilibrium and theenergy imparted to the system is dissipated explosivelythrough the substorm process. The e�ects include auro-ral displays, magnetospheric particle energization, mag-netic �eld recon�guration and plasmoid formation in themagnetotail. Neutral atom imaging can provide simul-taneous global images of the Earth's magnetosphere,including the critical for substorms regions of the innermagnetotail and the auroral ionosphere. The cross-scalecoupling of various magnetospheric components duringsubstorms can be visualized and identi�ed through NAIobservations. Furthermore, global images can providethe framework within which local observations can beplaced and combined to form the whole.

The increasing recognition of the importance of iono-spheric ions in the magnetospheric substorm process haslead to some recent e�orts to assess the dynamic charac-teristics of ionospheric ion source regions [Hesse et al.,1993], the energization and transport of ionosphericions to the near-Earth magnetosphere [Banaszkiewiczand Wodnicka, 1993; Daglis et al., 1994a] and the in- uence of ionospheric ions on substorm development[Daglis et al., 1990, 1991; Cladis and Francis , 1992;Daglis and Axford , Max-Planck-Institut f�ur Aeronomie,Katlenburg-Lindau, Germany, 1994].

Detailed knowledge of the spatial and temporal char-acteristics of the global ion out ow on characteristicsubstorm timescales would improve vastly the under-standing of the importance of ionospheric ion out owto magnetospheric substorms. In situ measurements

Neutral Atom Imaging of Substorms 3

can provide a detailful insight into the processes of in-dividual out owing (a good example was the swedishViking mission), but since the processes of ion out oware rather transient and localized, in situ measurementscannot provide a comprehensive, global view of the in-stantaneous con�guration of ion out ow. Neutral atomimaging of good time resolution (with timescales of �minutes, giving a signi�cant number of images withineach substorm phase) would enable the determination oflocations of ion out ow as well as their temporal varia-tions. Such information is invaluable for the assessmentof the geospace cross-scale coupling during substorms.

The present paper is an extension of an internal re-port [Daglis and Livi , Max-Planck-Institut f�ur Aeronomie,Katlenburg-Lindau, Germany, 1994] presenting the mer-its from global neutral atom imaging for substorm re-search. Other global imaging techniques such as dopplershift measurements of emissions at various wavelengths[Chakrabarti et al., 1986; Minow and Smith, 1993, e.g.],UV imaging [Anger et al., 1987], or X-ray imaging[Calvert et al., 1985] will not be discussed here.

2. Neutral Atom Imaging for Substorm

Research

First we would like to present brie y the line-of-sightintegral which relates the geospace ion population to theneutral particle ux at a given observation point. Thegeneral formula giving the di�erential ux of charge-exchange neutral hydrogen of energy E originating atany point ~R and arriving at the observation point ~R0

along the direction s is:

jH(~R0; s; E)=1R

0

[jp(~R;E)�pH (E)nH(~R)�jH(~R; s; E)DH(s; E)]ds

(1)

where

jH(~R0; s; E) ux of H arriving at ~R0 along s withenergy E, (cm2 s sr keV)�1;

jH(~R; s; E) ux of H arriving at ~R along s with en-ergy E, (cm2 s sr keV)�1;

jp(~R;E) ux of protons at ~R and of energy E,(cm2 s sr keV)�1;

�pH(E) charge exchange cross section for pro-tons of energy E in target gas H;

nH(~R) number density of geocorona H, cm�3;DH(s; E) destruction of energetic H or the por-

tion of H generated at ~R but not arriv-ing at ~R0; complete formula given byOrsini et al. [1994].

The line integral runs from the observation point toin�nity along the look direction. The �rst term on theright side of the equation represents the production andthe second term the loss of charge-exchange neutral hy-drogen. The same formula holds for other neutral parti-cles (like oxygen and helium). Processes contributing to

Figure 2. Principle of energetic neutral atom produc-tion.

4 Daglis and Livi

the destruction of charge-exchange neutrals are inversecharge exchange, reionization e�ects due to photoion-ization and due to reinteraction of the ENA with theion population generating them. The destruction termis rather small and is important only for observationpoints at low altitudes, where the interaction of the neu-tral particles with the ionospheric thermal plasma hasto be taken into account.

One should however note that though global neutralatom images contain a lot of morphological information,the extraction of relevant quantitative information fromNAI observations is not trivial. The remote images ofgeospace structures are composed of integrals along thelines-of-sight from the observation point (given by equa-tion 1). Therefore, an unfolding of the product jion �nHfrom the line-of-sight integrals is required in order toextract physical parameters from the neutral atom im-ages. The unfolding can be done on the basis of aniterative comparison of the actual images with modelimages until the di�erences are minimized. The modelimages are simulated by convolving the NAI instrumentresponse with the neutral atom emission as computedfrom parametric models of the ion distributions. Thiskind of iterative \forward modelling" was introduced byRoelof [1987]. While the modelling of Roelof [1987] wasinteractive, automated forward modelling has been pre-sented by Roelof et al. [1992]. Furthermore, Roelof et al.[1993] proposed the use of non-parametric models of iondistributions to serve as the basis of the comparison im-age simulations. Roelof et al. [1993] illustrated their ap-proach using the Rice University time-dependent Mag-netospheric Speci�cation Model to generate ENA im-ages. For the purpose of constructing comparison im-ages, large databases of in situ measured ion distribu-tions are very valuable. Such databases can be veryuseful as an a priori information of gross local charac-teristics to anticipate. Such an information would be,for example, the strong O+ dawn-dusk asymmetry dur-ing highly active times reported by Daglis et al. [1994c].

In summary, the main di�culty in the process of ex-traction of the source ion distributions from NAI ob-servations stems from the fact that the neutral atomimages represent integral information (as evident fromequation 1). Besides, the neutral atom images involvethe spatially varying geocoronal H density along withthe source ion ux. Thus, the orbit of the NAI space-craft is of crucial importance and must satisfy the objec-tive of the mission. For example, in order to image theplasma sheet / ring current, the orbit has to be inclined.An equatorially orbiting spacecraft would hardly succedin resolving dynamic structures in the plasma sheet /ring current through NAI.

We proceed to the part concerning substorm researchnow. Several issues of substorm research would bene-�t from neutral atom imaging. Some of them wouldbe the global monitoring of compositional changes, the

assessment of the ionospheric contribution to the hotmagnetospheric plasma, the investigation of the rela-tionship between ionospheric ion out ow and energydissipation at the ionosphere, the relative importance ofvarious ionospheric ion source regions, and the resolu-tion of spatial and temporal characteristics of substormion injections.

After three decades of space research, it has becomea generally accepted viewpoint that not only is ion com-position in uenced by the plasma processes in the mag-netosphere, but in fact can itself have a major in uenceon the processes. NAI observations can determine thecomposition of the charge-exchange neutrals and there-fore also the composition of the original hot plasma.

The role of the ionosphere both as a particle sourceand as an active element in dynamic magnetosphericprocesses was long time neglected or questioned; the ex-ceptions were rare during the �rst two decades of spacereasearch [Dessler and Hanson, 1961; Axford , 1970].The discovery of O+ ions in the magnetosphere [Shelleyet al., 1972] and the repeated con�rmation of signi�cantionospheric contribution to the magnetospheric ion pop-ulation in the 1980's [Peterson et al., 1981; Lennartssonand Sharp, 1982; Gloeckler and Hamilton, 1987, e.g.,]lead to the acceptance that the ionosphere is a ma-jor plasma source for the magnetosphere and that attimes it can even become the dominant source [Hamil-ton et al., 1988]. Furthermore, the in uence of iono-spheric contribution on the substorm process has beenassessed by several studies [Baker et al., 1985; Dagliset al., 1990, 1991; Cladis and Francis , 1992; Rothwellet al., 1994; Daglis and Axford , Max-Planck-Institut f�urAeronomie, Katlenburg-Lindau, Germany, 1994].

Although the signi�cance of the ionosphere as amagnetospheric particle source has been established,and even an ionospheric dominance has been proposed[Chappell , 1988], the exact magnitude of the iono-spheric contribution to the magnetospheric plasma re-mains uncertain. Moreover, there have been surpris-ingly few investigations of the dependence of the iono-spheric ion source on substorm conditions [Daglis et al.,1992, 1994b], as well as the relation of ionospheric ionout ow with the di�erent substorm phases [Opgenoorthet al., 1994]. Another important factor is variations in(both solar wind and magnetospheric) magnetic �eldmagnitude and/or orientation, that certainly in uencethe processes leading to ionospheric out ow; the in-duced electric �elds (during magnetic �eld recon�gura-tions in the near-Earth magnetosphere) might also playa central role in the acceleration and transport of out- owing ionospheric ions [Banaszkiewicz and Wodnicka,1993; Daglis et al., 1994a]. Our knowledge of these in-terrelationships is still very limited. Hence there is aclear need for global ion out ow observations able todistinguish temporal and spatial e�ects, in order to clar-ify the relation of ionospheric ion out ow with the sub-

Neutral Atom Imaging of Substorms 5

storm process. Most interesting is the relation of ionout ow with the dissipation of energy originating fromthe solar wind - magnetosphere dynamo, as well as withthe triggering of the expansion phase of the substorm.

Obtaining global images of the ionospheric ion out- ow from the cleft fountain and the auroral region wouldresult in a major advance in understanding the geospacecross-scale coupling during substorms. Neutral atomimaging would be a precious tool in this e�ort. Sinceboth major ionospheric ion species O+ and H+ chargeexchange with the geocoronal neutral H, there is an in-tense ux of neutral O and H in the high-latitude mag-netosphere, the predominant region of ionospheric out- ow to the magnetosphere [Moore, 1984].

Recently Hesse et al. [1993] demonstrated that neu-tral atom imaging of the cusp/cleft ion fountain is fea-sible and would lead to images of both the spatial andtemporal evolution (Figure 3). In their study theyused a cusp ion fountain model to provide the sourceion uxes for the charge-exchange process and subse-quent line-of-sight integrations to estimate the observ-able uxes of neutrals. Within the energy range un-der consideration (30-300 eV) the O+ - H cross sectionvaries between 1.05 and 1.35 � 10�15 cm2 [Fite et al.,1962]. The radial dependence of geocorona neutral hy-drogen density was determined by Rairden et al. [1986]on the basis of the DE 1 geocorona imaging and theChamberlain model of the exospheric hydrogen density[Chamberlain, 1963]. Typical uxes of charge-exchangeneutrals derived for various spacecraft locations with aconstant geocentric distance of 2 RE were in the rangeof 2�103 to 104/cm2 s sr. The source strengths of thetwo main high-latitude ionospheric ion sources (day-side cleft and nightside auroral region) have comparablemean values, that are of the order of 109 cm�2s�1 [Pol-lock et al., 1990; Lu et al., 1992]. Hence, the results ofHesse et al. [1993] are valid for auroral NAI too. Weshall discuss the orbit/instrumentation requirements forimaging the ionospheric out ow in the Instrumentationsection.

Detailed information on the morphology of iono-spheric ion out ow, with respect to solar wind condi-tions and substorm phase is invaluable for the assess-ment of the importance of ionospheric ions in substormevolution. A number of proposals have been made inthe past, concerning the role of increased ionosphericion concentration in the near-Earth magnetosphere.

Baker et al. [1982] for example, argued that a re-duction of the plasma sheet half-width combined withenhanced O+ densities shall lead to a an increase ofthe ion tearing mode instability growth rate (due to de-magnetization of O+ ions). Baker et al. [1985] showedthat during two substorms, one with low O+/H+ den-sity ratio and a later one with O+/H+ � 1, the relativelocation of the current wedge was di�erent. This was in-terpreted as the �rst observational evidence of the local

Figure 3. (not available in the pdf version of the paper)Computed neutral oxygen uxes from charge exchangeof the cusp/cleft out owing O+ [Hesse et al., 1993]. The uxes are of the order of 103.

time dependence of substorm onset instability growthdue to the O+/H+ ratio. Daglis et al. [1991] showedthat during the substorm growth phase, energized iono-spheric ions could contribute signi�cantly to an excessof parallel ion pressure, leading to the well known near-Earth magnetic �eld tail-like recon�guration [McPher-

ron, 1972; Kaufmann, 1987]. A tail-like recon�gurationof the magnetic �eld ultimately leads to the substormexpansion onset. Cladis and Francis [1992] estimatedthat in response to increased O+ parallel pressure inthe near-Earth magnetotail, shears in the convection ve-locity shall develop near the anks of the O+ injectionregion; the authors suggested that substorms may betriggered by velocity shear instabilities that e�ectivelycreate anomalous resistivity in the near-midnight regionof the neutral sheet. Rothwell et al. [1994] suggestedthat O+ density striations in the near-Earth nightsidemagnetosphere might drive auroral arc formation.

Obviously, the NAI determination of the abundanceof O+ in the plasma sheet and ring current during sub-storms is highly desirable. Fortunately, the charge-exchange cross section �(E) at energies �100 keV ismuch larger (by a factor of �50) for O+ than for H+

(Figure 4). Furthermore, it has been demonstrated thatthe energy range above 20 keV is most important duringsubstorms, since during substorm expansion the bulk ofthe ion energy density is contained in this energy range[Daglis et al., 1994b]. Finally, it has been shown thatO+ can compete with the major magnetospheric ion

6 Daglis and Livi

Figure 4. (not available in the pdf version of the pa-per) Plots of the charge-exchange cross sections �(E)as a function of energy for energetic H+ and O+ ongeocoronal H. It is obvious that the energy dependenceof �(E) is critical: while its value is approximately thesame for the two ion species at E=10 keV, it is higherby a factor of �50 for O+ at E=100 keV. This fact mayprove decisive for NAI observations of the geospace dur-ing very active times, when high energies (�20-300 keV)contain the bulk of the ion energy density and O+ ionscompete, or even dominate, H+ ions.

species H+, or even become the dominant ion species inthe ring current during highly disturbed time periods[Hamilton et al., 1988; Roeder et al., 1994]. However, itis not known how the O+ ions are transported to the in-ner plasma sheet / ring current, and if their dominanceis localized or rather spatially extended. One can antic-ipate that an e�ective NAI monitoring of substorms cangive a more precise picture of the O+ role in substormsand storms dynamics.

Coordinated neutral atom imaging of the near-Earthmagnetotail and UV/visible/X-ray imaging of the au-roral ionosphere [Chakrabarti et al., 1986, e.g.] couldresolve the issue of relative timing of the substorm ex-pansion onset at the magnetosphere (injection of ener-getic ions) and the auroral ionosphere (brightening ofan auroral arc). This could be done most e�ectivelythrough coordinated observations of imaging satellites.The UV imager on board the swedish Viking satellite[Anger et al., 1987] has demonstrated how powerful au-rora imaging can be, while simulated ENA images of ioninjections in the near-Earth plasma sheet [Frank et al.,1994] have shown the feasibility of substorm injectionimaging (see Plate 2 following).

Daglis et al. [1994b] examined the correlation be-tween the ion energy density in the near magnetotailand the auroral electrojet intensity during substorms.The results show very strong correlation in the caseof (ionospheric origin) O+, and no correlation for (so-lar wind origin) He++. This indicates direct feeding ofthe near-Earth magnetotail with ionospheric origin ionsduring substorms, when energy dissipation at the iono-sphere increases. The results of Daglis et al. [1994b]are consistent with the observations of Grande et al.

[1992], showing a pronounced increase of O+ contentin the near-Earth magnetosphere at the second injec-tion during a twin substorm injection, which indicatesan ionospheric response to the increased energy dissipa-tion associated with the substorm onset. CoordinatedNAI observations of ionospheric ion out ow and variousionospheric ground-based observations can investigatethe relationship between the ion out ow and the energydissipation at the ionosphere. Some examples would beEISCAT, STARE and rio-imager systems.

EISCAT (European Incoherent Scatter) is an inco-herent scatter radar system, which provides informa-tion about the density, temperature, mass and veloc-ity of ionospheric ions [R�ottger , 1991]. Very interest-ing EISCAT observations of large �eld-aligned bulk ionup ows from the topside ionosphere during auroral ac-tivity have been published by Wahlund et al. [1992].Opgenoorth et al. [1994] recently presented EISCAT ob-servations of large �eld-aligned ion up ows concurrentwith growth and expansion phase auroral arcs, also con-sistent with the above mentioned results ofGrande et al.[1992] and Daglis et al. [1994b]. It is our feeling thatsuch ground-based observations could complement per-

Neutral Atom Imaging of Substorms 7

101

102E

ne

rgy

(ke

V)

H+ ions Activity: low

101

102

102

104

106

Flu

x inte

nsity

H+ ions Activity: very high

2 4 6 8L-shell

101

102

En

erg

y (k

eV

)

O+ ions Activity: low

2 4 6 8L-shell

101

102

102

104

Flu

x inte

nsity

O+ ions Activity: very high

Plate 1. An arti�cial \global" picture of the energetic H+ (top panels) and O+ (bottom panels) near-equatorial(L=1.5{9.5, MLT=2300{0100 hrs) ux intensity pro�le, during quiet (left) and very active (right) time periods.The L-shell resolution is 0.25 RE . The �gures are a synthesis of 20 months of AMPTE/CCE-CHEMmeasurements.Note the ion injection structure in the high activity panels.

Plate 2. Simulated ENA image of a substorm ion injection (courtesy of E. C. Roelof). Note the similarity betweenthe ion injection structures in this Plate and in Plate 1 (high activity panels). The radial scale of the injectionstructures is �0.5 to 1 RE . In section 3 we discuss the feasibility of NAI of such structures.

8 Daglis and Livi

fectly with NAI satellite-borne observations.

STARE (Scandinavian Twin Auroral Radar Experi-ment) consists of a pair of coherent pulse doppler radarsand allows the study of temporal and spatial variationsof the plasma ow patterns in the ionosphere [Nielsen,1982]. Furthermore, the Joule heat production rate canbe estimated by using STARE electric �eld measure-ments and empirical model conductivities of the iono-sphere [Kosch and Nielsen, 1993]. Rio-imagers measurethe spatial variations of particle precipitation at a giventime and determine how this spatial variation changeswith time [Nielsen and Axford , 1977]. Unfortunatelyground-based in situ measurements alone provide ei-ther snapshots of ion out ow in a very limited regionor statistical samples of large regions lacking temporalinformation.

A primary issue for the resolution of ionosphere/magnetosphere coupling is the determination of therelative importance of the auroral acceleration regionand cleft ion fountain as ion sources for the magne-tosphere. Observational studies have indicated theimportance of both regions [Shelley , 1986; Lockwoodet al., 1985] and modeling studies have assessed thetransport and energization of ionospheric ions, fromthe cleft region [Cladis and Francis , 1992] as well asfrom the auroral region [Banaszkiewicz and Wodnicka,1993; Daglis et al., 1994a]. Since it is most proba-ble that di�erent ionospheric regions may be dominantion sources for speci�c substorm phases and/or dif-ferent levels of magnetospheric activity and/or di�er-ent levels of the solar wind - magnetosphere dynamoactivity [Daglis and Axford , Max-Planck-Institut f�urAeronomie, Katlenburg-Lindau, Germany, 1994], theassessment of this issue will be especially valuable forthe study of magnetosphere-ionosphere coupling.

Following the pioneering work of E. C. Roelof on theobservation of the terrestrial ring current through ener-getic neutral atom imaging [Roelof et al., 1985; Roelof ,1987], there has been a growing attention of the scien-ti�c community to the potential importance of neutralatom imaging for magnetospheric research, and severalsimulation studies were conducted during the last fewyears. Livi et al. [1992] andOrsini et al. [1994] modelledthe precipitation at low altitudes of energetic neutral Hand O atoms generated via charge exchange in the innermagnetosphere, for geocentric distances r=2{9 RE andfor MLT 2300{0100 hrs. They used a line-of-sight inte-gration code, including a modi�ed dipole magnetic �eldmodel, the typical geocoronal neutral H altitude pro�leand measured ion source data distributions from theCHEM spectrometer [Gloeckler et al., 1985] on boardAMPTE/CCE. The energetic neutral atom (ENA) pre-cipitation as detectable by a satellite orbiting at lowaltitudes (�1500 km) was calculated, and the relativecontribution of the ring current and the radiation belts,as well as the relative contribution of H+ and O+ to

the ENA generation were discussed. The calculationsof Orsini et al. [1994] gave ux levels of �1 to 104

cm�2s�1sr�1keV�1 at energies 100 keV down to 1 keVrespectively. The analysis of the relative contributionsby the ring current and the radiation belts indicatedthat the radiation belt contribution dominates, thus ob-scuring the ring current contribution. Since howeverthe ring current generated ENA distributions exhibit amore dynamical behaviour, more elaborate modellingmay help in discrimanating the ring current contribu-tion from the radiation belt contribution. Anyhow, thestudy of Orsini et al. [1994] demonstrated the feasibil-ity of using NAI to obtain global images of the innermagnetosphere from beneath.

At this point we would like to introduce an \arti-�cial" global imaging perspective of substorm e�ectson the energetic ion population of the magnetosphere.Plate 1 shows the color-coded ux intensities of H+ andO+ in the L-range 1.5 to 9.5 around midnight local time,for low and for high geomagnetic activity respectively.These data have been used as the input source distribu-tion for the ENAmodelling study byOrsini et al. [1994].The plate demonstrates in an indirect manner that neu-tral atom imaging of the magnetosphere will providepowerful diagnostics of the dynamic processes. In thevery active case (AE>1000 nT) a clearly new structureemerges, centered around L=7, which most probably isthe substorm injection front. Such structures, emergingduring di�erent substorm phases, would most e�ectivelybe monitored by NAI instrumentation. It is a most im-portant issue to resolve spatial and temporal charac-teristics of the energetic ion injections which mark thesubstorm expansion onset. Imaging of the ion injectionmorphology in the near-Earth magnetotail at and aftersubstorm onset would be crucial for the identi�cation ofthe acceleration processes. It would be also crucial forvisualizing and determining the participation of the var-ious magnetospheric regions to the plasma energization.The images obtained through NAI would yield a movieof the evolution of the injection front. The anticipatedresults of neutral atom imaging of the substorm injec-tions can help to assess the various existing substormmodels. Plate 2 by E. C. Roelof demonstrates the aboveremarks very e�ciently. It shows a simulated ENA im-age of the inner magnetosphere during an ion injectionevent. The vantage point is at r/RE = 5, MLat = 60o,MLT = 2100 hrs. The logarithmic color bar covers afactor of 103, normalized to the maximum ENA ux inthe image. In the end of the Instrumentation section weshall discuss the feasibility of neutral atom imaging ofsuch substorm-associated structures in the inner plasmasheet.

The limitations of in situ measurements in the vastmedium of the magnetosphere are not yet fully real-ized or accepted by the community: full local time orradial distance coverage of a region under exploration

Neutral Atom Imaging of Substorms 9

requires months of observations. The disadvantage isdouble: �rst one needs time-consuming measurementsbefore having the spatial information, and second, suchinformation does not have the time resolution appro-priate for the dynamics of substorms or even magneticstorms. While the panels of plate 1 required 20 monthsof measurements made by the CHEM ion spectrometeron board the AMPTE/CCE satellite, the NAI techniquecould give similar information in one orbital pass of thespacecraft, and with a time resolution (�5{10 min) al-lowing to follow individual substorm phases.

Undoubtedly the time resolution will depend onmany factors like the spacecraft orbit, the instrumentcharacteristics (geometrical factor, e�ciency, energythreshold, etc), and the target population (energy, uxlevels) to image. A desired high time resolution of sev-eral minutes is therefore not universally feasible. Wewould like to discuss the problem for two most interest-ing target regions, the auroral fountain region and theinner plasma sheet.

To optimize the imaging conditions of the target re-gion and population, the most favourable spacecraft or-bit has to be chosen. Imaging of the inner plasma sheetand the outer ring current would pro�t from a mediuminclination (around 60o) medium altitude (�5 to 7 RE)elliptical orbit. High spatial resolution images of thelow-altitude ENA emissions, imaging of the ring cur-rent from beneath and imaging of the ionospheric out- ow would be best served with a low altitude (�500km to 2 RE) orbit. Imaging of the auroral ion out- ow region would additionally require medium to highinclination (60o-75o). The angular coverage from lowaltitudes can be maximized with a spinning spacecraft.A disadvantage of low altitude orbits is their relativelyshort period, e.g. �100 min for a �1000 km altitude cir-cular orbit, or �200 min for a �10,000 km (apogee) al-titude elliptical inclined orbit. Short orbital period willinevitably hinder continuous monitoring of the evolu-tion of substorms, since substorms evolve on timescalesof �1 to 3 hrs. An intermediate orbit (� 3 to 5 RE

apogee, medium to high inclination) might serve imag-ing of both target regions (plasma sheet / ring currentand ionospheric out ow), provided multiple instrumen-tation suitable for the di�erent populations to be im-aged would be available on the same satellite. How-ever, with increasing altitude the spatial resolution ofNAI will decrease. For low resolution global imaging ofthe whole magnetosphere, high altitude (>10 RE) or-bits would be best. A three-axis oriented Earth pointingspacecraft would give global manifestations of substormand storm development and the gross characteristics ofionospheric response. The global viewing of the tempo-ral evolution of the geospace system and the cross-scalecoupling would compensate for the lack of high spatialresolution.

3. Instrumentation

Because neutral atom uxes are several orders ofmagnitude lower than charged particle uxes, highlye�cient instrumentation is required for neutral atomimaging. Several modelling studies have estimated the uxes of neutral atoms for various magnetospheric re-gions. We shall list a few numbers before going on withthe intrumentational requirements.

Fluxes of �30 keV H atoms and �70 keV O atomsoriginating from the ring current during a magneticstorm and imaged from a position at 60 deg magneticlatitude and 5 RE geocentric distance are expected to be>104 cm�2s�1sr�1 [Frank et al., 1994]. Fluxes of lowerenergy (�10 keV) H atoms in the near-Earth plasmasheet have been calculated to be in the range of 1 to100 cm�2s�1sr�1keV�1, depending on substorm phaseand position [Moore et al., 1992]. Assuming a spacecraftlocation at 5 RE geocentric distance and 65o magneticlatitude, Hesse et al. [1993] computed peak ux levelsof low energy (<1 keV) neutral hydrogen in the plasmasheet to range from 200 to 700 cm�2s�1sr�1 during sub-storm expansion. Modelling of the energetic neutral Hand O atoms precipitation at low altitudes (1400 km)resulted in ux levels of �103 cm�2s�1sr�1keV�1 at10 keV and �1 cm�2s�1sr�1keV�1 at 100 keV [Orsiniet al., 1994]. In the case of the cleft ion fountain theexpected peak uxes of 30-100 eV neutral atoms arebetween 103 and 105 cm�2s�1sr�1 [Hesse et al., 1993].

The neutral atom populations to be imaged can beroughly divided into three categories. The energeticneutral atoms with energies of �20 to 200 keV, themedium energy neutral atoms with energies of �1 to 30keV, and the low energy neutral atoms of less than �500eV energy. Spectrographs appropriate for all three pop-ulations have already been proposed in several studies[Orsini et al., 1992; Frank et al., 1994; Ghielmetti et al.,1994, e.g.,]. Conventional techniques can be used formeasuring neutral particles of the �rst two categories:these are techniques relying on direct detection via theenergy deposition on solid state detectors (�rst cate-gory), or on ionization and analysis in standard chargedparticle instrumentation (second category). General re-quirements for all NAI instruments are large geome-try factors and large detectors, because charge-exchangeneutral atom uxes are relatively low. As a consequenceof these requirements, three potential sources of back-ground have to be faced: photons (mainly EUV), ener-getic charged particles and cosmic radiation.

The energetic neutral atom instrumentation gener-ally can be assembled from following components. Acollimator with plates serrated and alternately biasedat certain potentials, a time-of- ight section with twofoils, and a solid-state detector. The serrated and biasedcollimator plates will reject charged particles [Chenget al., 1993, e.g.], while the valid time-of- ight mea-

10 Daglis and Livi

surements will determine the velocity of the incomingneutrals and the solid-state detector will determine theenergy of the incoming particles. The TOF measure-ments will further ensure the rejection of backgroundresponses (FUV, EUV, cosmic radiation). FUV andEUV uxes produce secondary electrons, but not corre-lated start and sto pulses. High-energy (above the rejec-tion cut-o� energy of the collimator) charged particlescan be identi�ed by their fast TOF signatures. The useof position-sensitive anodes behind the start/stop mi-crochannel plates in the TOF section will provide theparticle velocity direction. The pulse height of the mi-crochannel plate signal can be used to determine theneutral species, since the number of emitted secondaryelectrons depends on the atomic number of the neutral.

The front foils in such instruments set the lower limitin energy coverage through their thickness; thin foilspermit measurements to include lower energies, but onthe other hand the foils must be thick enough to at-tenuate the UV light at 121.6 nm (10.2 eV) from theresonantly scattered solar Lyman � emission from thegeocoronal hydrogen. The intensities range from �500Rayleighs to �10 kRayleighs, which is �5�107 to 109

photons cm�2s�1sr�1, depending on look direction andspacecraft orbit. The attenuation of UV is necessary sothat the sensor circuitry is not swamped by false counts.Since the photon transmittance of various foils is known[Hsieh et al., 1980, 1991; Cheng et al., 1993], the pho-ton background can be reduced as far as desired, butfor a time-of- ight instrument the increased scatteringdue to increased foil thickness will degrade the energyresolution and threshold, as well as the angular reso-lution. Thus, the use of thick foils involves a trade-o� between background rejection and energy/angularresolution. An other technique for the suppression ofEUV, the use of nuclear track �lters, has been pro-posed by Gruntman and Leonas [Space Research Insti-tute, Academy of Sciences, Moscow, 1986]. These �ltersconsist of thin foils, which are etched to create straightcylindrical channels of diameter smaller than the Ly-man � wavelength (e.g. � 100 nm). In this way onecan remove completely Lyman � photons, while neutralparticles pass through the channels una�ected. How-ever, the acceptance angles of such �lters are rathersmall (�10o), and this may result in severe scatteringof particles with the channel walls.

The principle of medium energy neutral atoms in-strumentation is conversion of the atoms to ions andelectrostatic analysis of these ions [McComas et al.,1992, e.g.]. This technique combines four standardelements: collimator, thin foil, electrostatic analyzer,time-of- ight detector. First a collimator de�nes theazimuthal �eld of view. As in the instrumentation ofthe �rst category, the plates of the collimator are al-ternately biased at a �xed potential to reject chargedparticles. After the collimator, an ultrathin carbon

foil ionizes a substantial fraction of the incident neu-tral atoms. The newly formed ions are postaccelerated,pass then through the electrostatic analyzer and tran-sit a second foil generating secondary electrons. Theseelectrons provide the start time for the time-of- ightsection. The stop time is delivered by a microchannelplate or a position-sensitive anode. The time-of- ightmeasurements can di�erentiate between the major mag-netospheric species hydrogen and oxygen, and they canprovide excellent rejection of background penetratingradiation.

This technique has good e�ciency for energies as lowas 1 keV, but it cannot be applied for imaging muchlower energy neutral atoms (third category), because atlower energies the yield of ions and secondary electronsdecreases rapidly. Recently new surface ionization tech-niques have been developed, that make use of low workfunction surfaces for converting neutral particles to ionsduring surface impact and re ection [B�urgi et al., 1990].Particularly low work functions have been obtained withmonocristalline tungsten W(110) substrates coated witha thin layer of Cs [Desplat and Papageorgopoulos , 1980].The Cs overlayer reduces the work function to �1.5 eVand, in combination with the high electron density ofthe W surface, facilitates the electron transfer to there ected particle. An instrument that relies on the con-cept of surface ionization is currently operating on theUlysses spacecraft [Witte et al., 1992] and has success-fully imaged low-energy (�40 eV) interstellar neutralhelium [Witte et al., 1993]. The other elements of aNAI instrument involving surface conversion are simi-lar to elements of the instruments of the �rst two cate-gories: an entrance collimation system, an electrostaticanalyzer and a time-of- ight mass analyzer with posi-tion sensing. Since the initial energies are very low,postacceleration is applied prior to the TOF section.

We shall apply the above numbers to the imaging oftwo important target regions now. The two regions arethe auroral ion fountain and the inner plasma sheet /ring current. As we have already mentioned, imagingof the auroral ion fountain would be best served by alow altitude, high inclination orbit. The apogee alti-tude should be su�ciently above the estimated heightof the bottom of the low-altitude auroral accelerationregion, that is at �1700 km [Lu et al., 1992], to guar-antee a noticeable amount of neutral atoms originat-ing from the charge exchange of accelerated up owingionospheric ions. The most appropriate instrumenta-tion should involve both the second and the third tech-nique described above. Considering a ux level of 103

to 104 cm�2s�1sr�1, an instrument with a geometricalfactor of 0.01 cm�2sr�1 would be needed to come upwith a count rate of 10 to 100 s�1. The geometricalfactor for detecting low-energy neutral atoms is energydependent, since it involves the energy-dependent ionconversion e�ciency of the neutral atoms at the con-

Neutral Atom Imaging of Substorms 11

version foil or surface. It has been shown [Frank et al.,1994] that for energies 1 to 20 keV, geometrical factorsof �0.05 cm�2sr�1 are feasible. Assuming however a ge-ometrical factor of 0.01 cm�2sr�1, an e�ciency of 10%,and a ux level of 103 to 104 cm�2s�1sr�1, we end upwith a count rate of 1 to 10 s�1. This will give a totalof 300 to 3000 counts in 5 min, or 900 to 9000 countsin 15 min, i.e. measurements with reasonable to verygood statistical value (1�=5.7%-1%), within timescalesthat are appropriate to dynamical substorm features.Of course, the actual sensitivity depends on the par-ticular instrument design. However, the above valuescan be considered realistic, and they are high enoughto make an investigation of the auroral ion out ow incharacteristic substorm timescales feasible.

Regarding NAI of the inner plasma sheet and ringcurrent during substorms, we have mentioned that theexpected charge-exhange neutral ux at energies of�30 to 70 keV at a spacecraft location at �60o mag-netic latitude and �5 RE geocentric distance is >104

cm�2s�1sr�1 [Frank et al., 1994]. Count rates of 10 s�1

(again assuming an e�ciency of 10%) would require ageometrical factor of �0.001 cm�2, which is rather easyto realize. Fluxes of the order of 102 cm�2s�1sr�1 andcount rates of 1 s�1 (e�ciency 10%) require a geometri-cal factor of 0.01 cm�2s�1sr�1. It is important to notethat the Energetic Neutral Particle Imager presentedby Frank et al. [1994] has a geometric factor per 4o� 4o

pixel of 0.01 cm�2s�1sr�1 and a total of 30 pixels. Ob-viously NAI of the inner plasma sheet is feasible and thescienti�c gain shall be considerable. The kind of struc-tures in the inner plasma sheet that can be resolvedthrough NAI depends both on the spacecraft orbit andon the design of the NAI instrument. For the space-craft location considered here, the distance normal tothe equatorial plane would be �4.3 RE . For 4o� 4o

pixels we would then have a spatial resolution (whenlooking normal to the equatorial plane) of �0.35 RE or�2200 km, which is very satisfying. Even when look-ing down to an equatorial geocentric distance of �6.8RE , where substorm-associated structures appear, thespatial resolution will be �0.7 RE , which is still satisfy-ing. Structures with scales close to those seen in Plate1 can be observed with such a resolution. Note that theL-shell resolution of Plate 1 is 0.25 RE , while the newstructures emerging during high activity periods havescales of �1 RE .

4. Summary

The global perspective of magnetospheric substormsis still produced by interpretation of large sets of insitu, localized measurements lacking information on thetemporal and spatial relationship between the variousgeospace regions. In this paper we discussed why andhow neutral ion imaging can contribute to magneto-spheric substorm research. We presented estimates

of charge-exchange neutral uxes, and appropriate or-bit/instrumentation requirements. We have demon-strated that NAI of the auroral ion source and the innerplasma sheet during substorms is feasible. Summariz-ing, we would like to list some of the substorm researchissues, for which neutral atom imaging would be bene-�tial:

a. determination of the morphology of ionosphericion out ow, with respect to solar wind conditions andsubstorm phase.

b. determination of the morphology of ion injectionsin the near-Earth magnetotail.

c. comparative timing of the substorm expansiononset at the magnetosphere and the auroral ionosphere.

d. comparative timing of auroral brightening and ionout ow.

e. determination of the relationship between the en-ergy dissipation at the ionosphere and the correspond-ing ion out ow.

f. determination of the relative importance of theauroral acceleration region and cleft ion fountain as ionsources for the magnetosphere.

g. monitoring of the evolution of the substorm onsetinjection fronts.

h. visualization and identi�cation of the participa-tion of the various magnetospheric regions to the plasmaenergization during substorms.

NAI observations can be precious complements to lo-cal observations and lead to the understanding of howlocal processes, many of which are resolved quite welltoday, combine to form the global process of the mag-netospheric substorm.

Several versions of NAI instruments have been un-der development [Hsieh and Curtis , 1989; McEntire

and Mitchell , 1989], but unfortunately none of themhas been own in space; we hope that things willchange in future. The Magnetospheric Imaging Instru-ment (MIMI) is one of the selected experiments for theCassini mission to the Saturn system and includes aNAI sensor [Cheng et al., 1993]. Another ight opportu-nity for a NAI instrument is the SAC-B mission [Gulichand White, 1993], where the Imaging Spectrometerfor Energetic Neutral Atoms (ISENA) [Orsini et al.,1992] is scheduled to y with. A swedish microsatel-lite scheduled for launch in the near future shall carry aNAI instrument for ionospheric ion out ow observations(R. Lundin, personal communication). Finally, thespectrometer MEDINAS [Wilken et al., Max-Planck-Institut f�ur Aeronomie, Katlenburg-Lindau, Germany,1994] with ENA operation mode has been proposed forthe norwegian high-inclination NISSE mission.

Closing we would like to emphasize that an appropri-ately situated NAI instrument could e�ectively monitorglobal substorm activity continuously, while this is im-possible for single point in situ measuring instruments,

12 Daglis and Livi

no matter how many satellites are available, since themagnetosphere is a vast size system. Neutral atomimaging can provide a global framework for the local ob-servations, and promote our apprehension of the cross-scale coupling within dynamic geospace. In this con-text it is regretful that during the (NASA) ISTP and(ESA) STSP programs, with a unique constellation ofspacecraft in the near-Earth space, no NAI instrumentswill be operating in space. A combination of such aplethora of in situ measurements with global NAI ob-servations from an imaging satellite on a, preferably,medium to high-inclination orbit would have been mostfruitful. However, though no NAI-dedicated instru-mentation are / will be operating during ISTP/STSP,there are two charged-particle spectrometers operatingin space or under preparation, that have ENA detec-tion capabilities. These are the HEP-LD spectrometer[Doke et al., 1994] which is successfully operating onboard the japanese Geotail satellite, the RAPID spec-trometer [Wilken et al., 1993] which will be launchedwith the four Cluster spacecraft. Both instruments per-mit the identi�cation and analysis of energetic neutralatoms with energies above �20 keV.

Acknowledgments. We thank the ISENA PI StefanoOrsini for stimulating our interest in ENA imaging throughour cooperation in the ISENA experiment. We gratefullyacknowledge the Principal Investigator of the CHEM spec-trometer George Gloeckler (University of Maryland) and themany individuals at the University of Maryland, the Max-Planck-Institut f�ur Aeronomie, and the Technische Univer-sit�at Braunschweig, for making CHEM an excellent instru-ment. It is a pleasure to acknowledge informative discussionswith Don Mitchell and Ed Roelof. Plate 2 was kindly pro-vided by Ed Roelof. We thank both referees for their helpfulcomments.

References

Akasofu, S.-I., Auroral phenomena, in Auroral Physics,edited by C.-L. Meng, M. J. Rycroft, and L. A. Frank,pp. 3{12, Cambridge Univ. Press, New York, 1991.

Anger, C. D., et al., An ultraviolet auroral imager for theViking spacecraft, Geophys. Res. Lett., 14 , 387{390, 1987.

Axford, W. I., On the origin of radiation belt and auroral pri-mary ions, in Particles and Field in the Magnetosphere,edited by B. M. McCormac, pp. 46{59, D. Reidel, Nor-well, Mass., 1970.

Baker, D. N., E. W. Hones Jr., D. T. Young, and J. Birn,The possible role of ionospheric oxygen in the initiationand development of plasma sheet instabilities, Geophys.Res. Lett., 9 , 1337{1340, 1982.

Baker, D. N., T. A. Fritz, W. Lennartsson, B. Wilken, H. W.Kroehl, and J. Birn, The role of heavy ions in the localiza-tion of substorm disturbances on March 22, 1979: CDAW6, J. Geophys. Res., 90 , 1273{1281, 1985.

Banaszkiewicz, M., and E. B. Wodnicka, High-latitude oxy-gen transport to the equatorial plane in the recon�gu-rating magnetosphere, Ann. Geophys., 11, suppl. C, 390,1993.

Bernstein, W., R. L. Wax, N. L. Sanders, and G. T. Inouye,An energy spectrometer for energetic (1 - 25 keV) neu-tral atoms, in Small rocket instrumentation techniques,edited by K.-I. Maeda, pp. 224{231, North-Holland, Am-sterdam, Netherlands, 1969.

B�urgi, A., M. Oetliker, P. Bochsler, J. Geiss, and M. A.Coplan, Charge exchange of low-energy ions in thin car-bon foils, J. Appl. Phys., 68 , 2547{2554, 1990.

Calvert, W., H. D. Voss, and T. C. Sanders, A satellite im-ager for atmospheric X-rays, IEEE Trans. Nucl. Sci., NS-32 , 112{118, 1985.

Chakrabarti, S., J. C. Green, Y. T. Chiu, R. M. Robinson,G. R. Swenson, and D. S. Evans, Imaging of ionosphericplasma out ow in the magnetosphere: Veri�cation of anew concept, Adv. Space Res., 6 (3), 215{220, 1986.

Chamberlain, J. W., Planetary coronae and atmosphericevaporation, Planet. Space Sci., 11 , 901{960, 1963.

Chappell, C. R., The terrestrial plasma source: a new per-spective in solar-terrestrial processes from Dynamics Ex-plorer, Rev. Geophys., 26 , 229{248, 1988.

Cheng, A. F., E. P. Keath, S. M. Krimigis, B. H. Mauk,R. W. McEntire, D. G. Mitchell, E. C. Roelof, andD. J. Williams, Imaging neutral particle detector, RemoteSensing Reviews, 8 , 101{145, 1993.

Cladis, J. B., and W. E. Francis, Distribution in magneto-tail of O+ ions from cusp/cleft ionosphere: A possiblesubstorm trigger, J. Geophys. Res., 97 , 123{130, 1992.

Coroniti, F. V., R. L. McPherron, and G. K. Parks, Studiesof the magnetospheric substorm: 3. Concept of the mag-netospheric substorm and its relation to electron precip-itation and micropulsations, J. Geophys. Res., 73 , 1715{1722, 1968.

Daglis, I. A., and W. I. Axford, On the fast response of theionospheric ion source to increased energy input from thesolar wind and the magnetosphere, Report MPAE-W-100-94-16 , Max-Planck-Institut f�ur Aeronomie, Katlenburg-Lindau, Germany, 1994.

Daglis, I. A., and S. Livi, Neutral atom imaging of magne-tospheric substorms, Report MPAE-W-100-94-03 , Max-Planck-Institut f�ur Aeronomie, Katlenburg-Lindau, Ger-many, 1994.

Daglis, I. A., E. T. Sarris, and G. Kremser, Indications forionospheric participation in the substorm process fromAMPTE/CCE observations, Geophys. Res. Lett., 17 , 57{60, 1990.

Daglis, I. A., E. T. Sarris, and G. Kremser, Ionospheric con-tribution to the cross-tail current during the substormgrowth phase, J. Atmos. Terr. Phys., 53 , 1091{1098,1991.

Daglis, I. A., E. T. Sarris, G. Kremser, and B. Wilken,On the solar wind-magnetosphere-ionosphere coupling:AMPTE/CCE particle data and the AE indices, in Studyof the Solar-Terrestrial System, ESA SP-346 , edited byR. Reinhard and J. J. Hunt, pp. 193{198, European SpaceAgency, Paris, 1992.

Daglis, I. A., M. Banaszkiewicz, and E. B. Wod-nicka, Coupling of the high-latitude and the equato-rial magnetosphere during substorms through the trans-port/acceleration of ionospheric ions, in Proc. Interna-tional Conference on Substorms 2, Fairbanks, U.S.A.,March 7-11 , edited by J. R. Kan, J. D. Craven, and S.-I.Akasofu, pp. 615{619, Univ. of Alaska, Fairbanks, 1994a.

Neutral Atom Imaging of Substorms 13

Daglis, I. A., S. Livi, E. T. Sarris, and B. Wilken, Energydensity of ionospheric and solar wind origin ions in thenear-Earth magnetotail during substorms, J. Geophys.Res., 99 , 5691{5703, 1994b.

Daglis, I. A., S. Livi, and B. Wilken, Equatorial magneto-spheric ion energy density between 2 and 9 Earth radiifor various activity levels, Ann. Geophys., 12, suppl. III,565, 1994c.

Desplat, J. L., and C. A. Papageorgopoulos, Interaction ofcesium and oxygen on W(110). I. Cesium absorption onoxygenated and oxidized W(110), Surf. Sci., 92 , 97{118,1980.

Dessler, A. J., and W. B. Hanson, Possible energy source forthe aurora, Astrophys. J., 134 , 1024{1025, 1961.

Dessler, A. J., and E. N. Parker, Hydromagnetic theory ofgeomagnetic storms, J. Geophys. Res., 64 , 2239{2252,1959.

Doke, T., et al., The energetic particle spectrometer HEPonboard the GEOTAIL spacecraft, J. Geomagn. Geo-electr., 46 , 713{733, 1994.

Fite, W. L., A. C. H. Smith, and R. F. Stebbings, Chargetransfer in collisions involving symmetric and asymmetricresonances, Proc. R. Soc. London, A268 , 527{536, 1962.

Frank, L. A., et al., Imagers for the magnetosphere, aurora,and plasmasphere, Optical Engineering , 33 (2), 391{408,1994.

Ghielmetti, A. G., E. G. Shelley, S. A. Fuselier, P. Wurz,P. Bochsler, F. Herrero, M. F. Smith, and T. S. Stephen,Mass spectrograph for imaging low-energy neutral atoms,Optical Engineering , 33 (2), 362{370, 1994.

Gloeckler, G., and D. C. Hamilton, AMPTE ion compositionresults, Phys. Scr., T18 , 73{84, 1987.

Gloeckler, G., et al., The charge-energy-mass (CHEM) spec-trometer for 0.3 to 300 keV/e ions on the AMPTE/CCE,IEEE Trans. Geosci. Remote Sens., GE-23 , 234{240,1985.

Grande, M., C. H. Perry, D. S. Hall, B. Wilken, S. Livi,F. S�raas, and J. F. Fennell, Composition signaturesof substorm injections, Substorms 1, Eur. Space AgencySpec. Publ., SP-335 , 485{490, 1992.

Gruntman, M. A., and V. B. Leonas, Experimental oppor-tunity of planetary magnetosphere imaging in energeticneutral atoms, Preprint 1109,, Space Research Institute,Academy of Sciences, Moscow, 1986.

Gulich, M., and C. White, Satelite de Aplicaciones Cienti�-cas SAC-B International Cooperative Mission, AIAA, 93(4257), 1{8, 1993.

Hamilton, D. C.,G. Gloeckler, F. M. Ipavich, W. St�udemann, B. Wilken,and G. Kremser, Ring current development during thegreat geomagnetic storm of February 1986, J. Geophys.Res., 93 , 14,343{14,355, 1988.

Hesse, M., et al., Imaging ion out ow in the high-latitudemagnetosphere using low-energy neutral atoms, Opt.Eng., 32 (12), 3153{3160, 1993.

Hovestadt, D., and M. Scholer, Radiation belt-producedenergetic hydrogen in interplanetary space, J. Geophys.Res., 81 , 5039{5042, 1976.

Hsieh, K. C., and C. C. Curtis, Remote sensing of planetarymagnetospheres: mass and energy analysis of energeticneutral atoms, in Solar system plasma physics, Geophysi-cal Monograph 54 , edited by J. H. Waite Jr., J. L. Burch,and R. L. Moore, pp. 159{164, AGU, Washington, D. C.,1989.

Hsieh, K. C., and M. A. Gruntman, Viewing the outer he-liosphere in energetic neutral atoms, Adv. Space Res., 13(6), 131{139, 1993.

Hsieh, K. C., E. Keppler, and G. Schmidtke, Extreme ultra-violet induced forward photoemission from thin carbonfoils, J. Appl. Phys., 51 (4), 2242{2246, 1980.

Hsieh, K. C., B. R. Sandel, V. A. Drake, and R. S. King, HLyman � transmittance of thin C and Si/C foils for keVparticle detectors, Nuc. Inst. Meth., B61 , 187{193, 1991.

Hsieh, K. C., C. C. Curtis, C. Y. Fan, and M. A. Gruntman,Techniques for the remote sensing of space plasma in theheliosphere via energetic neutral atoms: a review, in SolarWind Seven, edited by E. Marsch and R. Schwenn, pp.357{364, Pergamon Press, London, 1992.

Kaufmann, R. L., Substorm currents: Growth phase andonset, J. Geophys. Res., 92 , 7471{7486, 1987.

Kirsch, E., S. M. Krimigis, W.-H. Ip, and G. Gloeckler, X-ray and energetic neutral particle emission from saturn'smagnetosphere, Nature, 292 , 718{721, 1981a.

Kirsch, E., S. M. Krimigis, J. W. Kohl, and E. P. Keath, Up-per limits for x-ray and energetic neutral particle emissionfrom jupiter: Voyager 1 results, Geophys. Res. Lett., 8 ,169{172, 1981b.

Kosch, M. J., and E. Nielsen, Multi-radar estimates fromSTARE and SABRE of mid-latitude ionospheric Jouleheating, Eos Trans. AGU , 74 (43), 537, 1993.

Lennartsson, W., and R. D. Sharp, A comparison of the 0.1-17 keV/e ion composition in the near equatorial magne-tosphere between quiet and disturbed conditions, J. Geo-phys. Res., 87 , 6109{6120, 1982.

Livi, S., I. A. Daglis, B. Wilken, S. Orsini, M. Candidi,and K. C. Hsieh, Simulation of energetic neutral atomsprecipitation, Eos Trans. AGU , 73 (43), 460, 1992.

Lockwood, M., J. H. Waite Jr., T. E. Moore, J. F. E. John-son, and C. R. Chappell, A new source of suprathermalO+ near the dayside polar cap boundary, J. Geophys.Res., 90 , 4099{4116, 1985.

Lu, G., P. H. Rei�, T. E. Moore, and R. A. Heelis, Up owingionospheric ions in the auroral region, J. Geophys. Res.,97 , 16,855{16,863, 1992.

McComas, D. J., H. O. Funsten, J. T. Gosling, K. R.Moore, and M. F. Thomsen, Low energy neutral atomimaging, instrumentation for magnetospheric imagery, inInstrumentation for magnetospheric imagery, SPIE Vol.1744 , edited by S. Chakrabarti, pp. 40{50, Society ofPhoto-Optical Instrumentation Engineers, Bellingham,WA, 1992.

McEntire, R. W., and D. G. Mitchell, Instrumentationfor global magnetospheric imaging via energetic neu-tral atoms, in Solar system plasma physics, GeophysicalMonograph 54 , edited by J. H. Waite Jr., J. L. Burch,and R. L. Moore, pp. 69{80, AGU, Washington, D. C.,1989.

McPherron, R. L., Substorm related changes in the geomag-netic tail: the growth phase, Planet. Space Sci., 20 , 1521{1539, 1972.

Meinel, A. B., Doppler shifted auroral hydrogen emission,Astrophys. J., 113 , 50{54, 1951.

Minow, J. I., and R. W. Smith, Optical remote sensing ofthe ion convection pattern in the high-latitude ionospherefrom a polar orbiting satellite, Geophys. Res. Lett., 20 ,559{562, 1993.

14 Daglis and Livi

Moore, K. R., D. J. McComas, H. O. Funsten, andM. F. Thomsen, Low energy neutral atoms in theEarth's magnetosphere: modeling, in Instrumentationfor magnetospheric imagery, SPIE Vol. 1744 , edited byS. Chakrabarti, pp. 51{61, Society of Photo-Optical In-strumentation Engineers, Bellingham, WA, 1992.

Moore, T. E., Superthermal ionospheric out ows, Rev. Geo-phys., 22 , 264{274, 1984.

Nielsen, E., The STARE system and some of its applications,in The IMS Source Book , edited by C. T. Russell and D. J.Southwood, pp. 213{224, AGU, Washington, D. C., 1982.

Nielsen, E., and W. I. Axford, Small scale auroral absorptionevents associated with substorms, Nature, 267 , 502{504,1977.

Opgenoorth, H. J., M. A. L. Persson, P. K. Toivanen, T. I.Pulkkinen, A. Aikio, I. A. Daglis, and M. Grande, NearEarth substorm onset: The possible role of ion out owsduring the substorm growth phase, paper presented atthe International Conference on Substorms 2, Univ. ofAlaska, Fairbanks, March 7-11, 1994.

Orsini, S., I. A. Daglis, M. Candidi, K. C. Hsieh, S. Livi,and B. Wilken, Model calculation of energetic neutralatoms precipitation at low altitudes, J. Geophys. Res.,99 , 13,489{13,498, 1994.

Orsini, S., et al., Proposal of an italian experiment forthe mission SAC-B. ISENA: Imaging particle Spectrom-eter for Energetic Neutral Atoms, in Instrumentationfor magnetospheric imagery, SPIE Vol. 1744 , edited byS. Chakrabarti, pp. 91{101, Society of Photo-Optical In-strumentation Engineers, Bellingham, WA, 1992.

Peterson, W. K., R. D. Sharp, E. G. Shelley, R. G. Johnson,and H. Balsiger, Energetic ion composition of the plasmasheet, J. Geophys. Res., 86 , 761{767, 1981.

Pollock, C. J., M. O. Chandler, T. E. Moore, J. H. Waite Jr.,C. R. Chappell, and D. A. Gurnett, A survey of upwellingion event characteristics, J. Geophys. Res., 95 , 18,969{18,980, 1990.

Rairden, R. L., L. A. Frank, and J. D. Craven, Geocoronalimaging with Dynamics Explorer, J. Geophys. Res., 91 ,13,613{13,630, 1986.

Roeder, J. L., J. F. Fennell, M. W. Chen, M. Schulz,M. Grande, and S. Livi, CRRES observations of the com-position of the ring current ion populations, Paper pre-sented at the 30th COSPAR Scienti�c Assembly, Ham-burg, Germany, 11-21 July 1994. Book of abstracts, p.181, 1994.

Roelof, E. C., Energetic neutral atom image of a storm-timering current, Geophys. Res. Lett., 14 , 652{655, 1987.

Roelof, E. C., Remote sensing of the ring current using en-ergetic neutral atoms, Adv. Space Res., 9 (12), 195{203,1989.

Roelof, E. C., D. G. Mitchell, and D. J. Williams, Energeticneutral atoms (E�50 keV) from the ring current: IMP7/8 and ISEE 1, J. Geophys. Res., 90 , 10,991{11,008,1985.

Roelof, E. C., B. H. Mauk, and R. R. Meier, Instru-ment requirements for imaging the magnetosphere inextreme-ultraviolet and energetic neutral atoms derivedfrom computer-simulated images, in Instrumentation formagnetospheric imagery, SPIE Vol. 1744 , edited byS. Chakrabarti, pp. 19{30, Society of Photo-Optical In-strumentation Engineers, Bellingham, WA, 1992.

Roelof, E. C., B. H. Mauk, R. R. Meier, K. R. Moore,and R. A. Wolf, Simulations of EUV and ENA magne-tospheric images based on the Rice Convection Model,in Instrumentation for magnetospheric imagery II, SPIEVol. 2008 , edited by S. Chakrabarti, pp. 202{213, Societyof Photo-Optical Instrumentation Engineers, Bellingham,WA, 1993.

Rothwell, P. L., M. B. Silevitch, L. P. Block, and C.-G.F�althammar, O+ phase bunching and auroral arc struc-ture, J. Geophys. Res., 99 , 2461{2470, 1994.

R�ottger, J., Incoherent scatter observations of the auroralionosphere with the EISCAT radar facility, in AuroralPhysics, edited by C.-L. Meng, M. J. Rycroft, and L. A.Frank, pp. 419{437, Cambridge University Press, NewYork, 1991.

Sauvaud, J.-A., et al., MAGICS: MAGnetospheric ImagingCircumterrestrial Satellite. A proposal to the EuropeanSpace Agency, Report 94-141 , CESR, Toulouse, France,1994.

Shelley, E. G., Magnetospheric energetic ions from theEarth's ionosphere, Adv. Space Res., 6 (3), 121{132, 1986.

Shelley, E. G., R. G. Johnson, and R. D. Sharp, Satelliteobservations of energetic heavy ions during a geomagneticstorm, J. Geophys. Res., 77 , 6104{6110, 1972.

Smith, M. F., et al., The High-Latitude Ion Transportand Energetics (HI-LITE) Explorer: A mission to in-vestigate ion out ow from the high-latitude ionosphere,in Instrumentation for magnetospheric imagery II, SPIEVol. 2008 , edited by S. Chakrabarti, pp. 40{56, Societyof Photo-Optical Instrumentation Engineers, Bellingham,WA, 1993.

Smith, P. H., and N. K. Bewtra, Charge exchange lifetimesfor ring current ions, Space Sci. Rev., 22 , 301{318, 1978.

Tinsley, B. A., Neutral atom precipitation, J. Atmos. Terr.Phys., 43 , 617{632, 1981.

Wahlund, J.-E., H. J. Opgenoorth, I. H�aggstr�om, K. J.Winser, and G. O. L. Jones, EISCAT observations oftopside ionospheric ion out ows during auroral activity:Revisited, J. Geophys. Res., 97 , 3019{3037, 1992.

Wax, R. L., W. R. Simpson, and W. Bernstein, Large uxesof 1-keV atomic hydrogen at 800 km, J. Geophys. Res.,75 , 6390{6393, 1970.

Wilken, B., et al., RAPID: The imaging energetic parti-cle spectrometer on Cluster, in Cluster: mission, payloadand supporting activities, ESA SP-1159 , edited by W. R.Burke, pp. 185{217, European Space Agency, Paris, 1993.

Wilken, B., I. A. Daglis, S. Livi, J. Woch, F. S�raas, S. Ulla-land, M. Grande, M. Lester, and S. Orsini, The Medium-Energy Ion and Neutral Atom Spectrometer (MEDINAS)for the NISSEmission, Experiment Description Document1.1 , Max-Planck-Institut f�ur Aeronomie, Katlenburg-Lindau, Germany, 1994.

Williams, D. J., Why we need global observations, in Mag-netospheric Physics, edited by B. Hultqvist and C. G.F�althammar, pp. 83{101, Plenum, New York, 1990.

Williams, D. J., E. C. Roelof, and D. G. Mitchell, Globalmagnetospheric imaging, Rev. Geophys., 30 , 183{208,1992.

Witte, M., H. Rosenbauer, E. Keppler, H. J. Fahr, P. Hem-merich, H. Lauche, A. Loidl, and R. Zwick, The inter-stellar neutral - gas experiment on ULYSSES, Astron.Astrophys. Suppl. Ser., 92 , 333{348, 1992.

Neutral Atom Imaging of Substorms 15

Witte, M., H. Rosenbauer, M. Banaszkiewicz, and H. J.Fahr, The Ulysses Neutral Gas Experiment: Determina-tion of the velocity and temperature of the interstellarneutral helium, Adv. Space Res., 13 (6), 121{130, 1993.

Wurz, P., P. Bochsler, A. G. Ghielmetti, E. G. Shelley,F. Herrero, and M. F. Smith, Concept for the HI-LITENeutral Atom Imaging Instrument, in Proceedings of theSymposium on Surface Science, edited by P. Varga andG. Betz, pp. 225{230, Kaprun, Austria, 1993.

I. A. Daglis and S. Livi, Max-Planck-Institut f�urAeronomie, Postfach 20, D-37189 Katlenburg-Lindau,F. R. Germany. (e-mail: SPAN.28673::linmpi::daglis;SPAN.28673::linmpi::livi)

This preprint was prepared with AGU's LATEXmacros v4, with

the extension package `AGU++' by P. W. Daly, version from .