Magnetic mapping of auroral signatures of comet SL9 in the Jovian magnetosphere

Pergamon Planet. Space Sci., Vol. 45, No. 10, pp. 13151331. 1997

cj. 1997 Elsevier Science Ltd

PII: S0032-0633(97)00109-8

All rights reserved. Printed in Great Britain 0032-0633197 $17.00+0.00

Magnetic mapping of aurora1 signatures of comet SL9 in the Jovian magnetosphere

R . PrangC,‘” I. Engle,’ M. Dunlop,3 S. Maurice4f and D. Rego’$

‘Institut d’Astrophysique Spatiale, Bat 121. CNRS-UniversitC Paris XI, 91405 Orsay Cedex. France ‘Physics Dept., U.S. Naval Academy, Annapolis, MD 21402-5026, U.S.A. Space and Atmospheric Physics, Imperia1 College, London. SW7 2BZ, U.K. 4ESTEC/SSD/S0. ESA, 2200 AG Noordwijk, The Netherlands ‘Space Physics Laboratory. University of Michigan, 2455 Hayward, Ann Arbor, MI 48109, U.S.A.

Received 15 November 1996; accepted 24 April 1997

Correspondence to: R. Prangk *Also at IAP. Paris, France. _FPresent address : Space and Atmospheric Science Group, Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A. $Present address : Department of Physics and Astronomy, UCL, London WClE 6BT, U.K.

1. Introduction

Comet Shoemaker-Levy 9 (SL9) was split into more than 20 major fragments during its last close fly-by of Jupiter in July 1992. The various subnuclei sequentially impacted Jupiter during a series of collisions distributed between July 16 and 21, 1994. The impact sites were all located at mid south latitudes ( -40’ to -44 ‘) and on the far side of Jupiter, close to the morning limb (within a few degrees), thus not in direct view from Earth. Before colliding with Jupiter’s atmosphere, the fragments traveled several days within the magnetosphere. Based on earlier observations and modeling (such as that of Saturn’s rings environment or of comet-solar wind interactions), the dust in the comae was expected to be negatively charged, and any hypothetical volatized species partially ion- ized. under the combined effects of photo-ionization, of interaction with the ambient plasma, and of bombard- ment by the magnetospheric energetic particles. thus dis- turbing the electrodynamical equilibrium of the magnetosphere/ionosphere system. Consequentially, a variety of plasma processes, depending upon the geometry of the field lines and the characteristics of the comae, were

1316 R. PrangC et al: Magnetic mapping of aurora1 signatures of comet SL9 in the Jovian magnetosphere

expected to trigger energetic particle precipitation along field lines into the Jovian ionosphere (e.g. Geophysical Research Letters special issue, Vol. 21, 1994, pp. 1047- 1067). In particular, some studies suggested that significant field-aligned currents might be generated by enhanced conductivity along the coma (IO-like interaction) or by the motion of charged dust relative to the corotating magnetospheric plasma. These currents in turn are expected to trigger field-aligned driven instabilities and particle precipitation along field-lines (Farrell et al., 1994 ; Ip and Prange, 1994). The observable signature of such precipitation should be of the aurora1 type, in the form of collisionally excited FUV emissions at the magnetic footprints of the nuclei, and of radio emissions along each magnetic field line passing through a fragment nucleus. The overall process was anticipated to depend upon the relative velocity of the coma with the corotating magnetosphere and on the local magnetospheric plasma characteristics (temperature, density, etc.) and could differ as a function of distance and from a region in the magnetosphere to another (on dayside or nightside closed field-lines, in the cusp, or in a neighborhood of non- corotating nightside open field lines connected to the southern polar cap). Generally, an interaction actually taking place and leading to aurora1 and radio signatures would be expected to appear at the closest magnetic footprint of the nucleus, i.e. in the southern high latitude atmosphere. However, particular configurations could allow conjugate “auroral” signatures to appear in the northern hemisphere. This could occur if the comet fragment passed through a closed field-line.

The study of the aurora1 signatures presented two interesting characteristics. First, as a consequence of the trajectories of the fragments, which approached Jupiter from the sunward-dusk sector, the magnetic footprints remained on the dayside until the very last hour when the fragment nuclei traveled over the south pole to the

Magnetic Axis Away from the sun ,

predawn mid-latitude impact site. Even when the nuclei reached the nightside, aurora1 signatures, if any, could be seen from the Earth’s vicinity above the planet limb, since aurora1 FUV emission occur more than 500-1000 km above the cloud tops, and radio emission yet higher. Secondly, aurora1 signatures were observable for several days, since each fragment spent between 5 and lOdays within the Jovian magnetosphere.

The study of such interactions was also challenging in various respects.

The nature of magnetospheric effects was controlled by the relative geometry of SL9 comae and the local Jovian magnetic field, modulat.ed by the 10 h rotation period of Jupiter (magnetic axis tilted by 10” from the rotation axis), and we had to consider the comet path, not in a fixed planetocentric frame of reference, but in the corotating tilted magnetic frame. By chance, the comet path sampled a variety of magnetospheric regimes, from day to night sides, from dusk to dawn, and because of the high latitude of the trajectory from closed to open magnetic field lines near the polar cap boundary (Fig. 1). Therefore, we could expect periods during which plasma interactions, if any, would give rise either to a single aurora1 signature in the south, or to a conjugate pair of signatures in the north and south. In addition, conjugate signatures may, or may not, be simultaneously observable from Earth orbit depending on the distortion of the field lines.

Months before the collisions, we studied the magnetic field line configurations and fragment footprints as a function of time in order to select optimal conditions for the detection of aurora1 signatures. We focused on conditions for which field aligned currents along a coma flux tube had been predicted. As reported in Clarke et al. (1995) and in Prange et al. (1995) we detected an unusual south aurora1 emission. Its specific characteristics are estab-

Magnetic Axis

Toward the sun

Fig. 1. Projection of the comet train in the Jovian magnetosphere in the noon-midnight meridian as on July 20, 1994. Two tracks have been plotted against the magnetic field lines for two opposite orientations of the Jovian magnetic axis. The field lines are computed with the solar wind controlled model (see text)

R. Prangt et al: Magnetic mapping of aurora1 signatures of comet SL9 in the Jovian magnetosphere 1317

lished in Section 2. In a preliminary study, Prange et al. suggested that this emission could lie at the magnetic footprint of fragment Q, on an open magnetic field line. In Section 3, we show that a consistent picture of the comet magnetic footprint(s) locations and of the comet- magnetosphere configuration as a function of time requests the use of two different complementary magnetic field models. A classical model including the full planetary field component and the plasma disc contribution is used to study the location of the footprints of the various fragments still in the magnetosphere, their association with the FUV spot. and to discuss our ability to dis- tinguish between them (Section 3.1). A second model, associating the solar wind interaction with the internal field sources, approximated by a precessing planetary dipole with the symmetric current sheet, is used to a general study of the high latitude instantaneous field-line passing through the comet (subsequently referred to as CIMFL) as a function of time along the comet path, and especially to identify the changes between open and closed field lines and the effects of distortion on the local time along field lines. The results of this model, less classical. are developed in Section 3.2, in the form of the temporal evolution of the CIMFL of a “typical” fragment for selected orientations of the Jovian magnetic axis. The evolution of fragment Q is then studied in more details until collision. Finally we discuss the implications of the emission characteristics and of the field modeling on the nature of the aurora1 process and on any relationship of this south FUV event with FUV emissions seen in a subsequent set of images and with X-ray emission detected later in the north (Waite et al., 1995) (Section 4).

2. July 20, 1994 HST observations

When trying to define the optimal period for our observations, we focused on conditions for which field aligned currents along a coma flux tube had been predicted: within the last ll2R, (Ip and Prange, 1994) and farther than about 10RJ (Farrell et al., 1994) or for large comae. This was performed with the assistance of the complementary magnetic field models described below. Within the operational constraints of the Hubble Space Telescope and the Science Team priorities, we selected an orbit on July 20, 1994, from 14 :09 to 14:47 UT. At that time, within half an hour of its predicted impact time, P2 was expected to be within 2R, of Jupiter’s center (lR, from the surface), on a closed field line whose north conjugate footprint was visible from Earth, while the extended complex Q2-Ql was at about 7RJ, and the rest of the train farther than IOR, on the dayside. Unfortunately, due to unavoidable uncertainties in the prediction of impact times, the impact P2 occurred finally more than 1 h after the beginning of the observations.

Four images were taken between 14 :lO UT and 14 :47 UT with the Wide Field Planetary Camera 2. The first two exposures (400s) were taken with filters F160WB + F 13OLP which isolate the Lyman bands of molecular hydrogen. The last two (300 s) were taken with

the Wood’s filter Fl60WB only, providing the H Lyman ~1, Hz Werner and Lyman band emission. We have isolated in Fig. 2 the southern hemisphere of Jupiter from these images, and overplotted a grid of coordinates. The four images exhibit a high latitude bright oval encircling the south pole. In Fig. 2(a) and (c), an additional bright spot appears on the polar cap. This spot appears at the same location in both cases within 1 S-2 pixels (pixel size is O.l”, i.e. ~380 km on the disc on Jupiter at that date). As reported by Clarke et al. (1995), the brightness of these spots is 200 kR and 240 kR, comparing with the brightness of the aurora1 oval, and well above the average level of the polar cap (20 kR). The spots are elongated in the east-west direction, and cover about lo’--20” in longitude, significantly more than the blurring due to the planetary rotation (4’ and 3-). The north polar region did not reveal any comparable feature in any of the four images.

More than five years of observations with HST (FOC and WFPC2) have established that the global morphology of the Jovian aurorae is fairly stable, with the brightest emissions localized along narrow ovals around the poles, and at times, with a broad transpolar emission in the north (Gerard et al., 1993 ; Turgeon and Caldwell, 1993 ; Prange, 1994; Clarke et al., 1995; Prange et al., 1996, 1997). Temporal variability has been recorded, consisting essentially of global intensity changes of one or the other of these features, of localized brightenings extending along the permanent bright oval (Gerard et al., 1994) or of an equatorward extension of the oval emission at specific longitudes. The oval present in the images under study looks very similar, in shape and size, to the aurora1 ovals seen in these previous observations, and identified as a region equatorward of the boundary of the polar cap (Prange, 1994). By contrast, we have never observed any- thing like this bright spot inside the south aurora1 oval. Although the very high latitude of the spot makes precise localisation more difficult, we believe that it is located inside the aurora1 oval, on or close to the polar cap because it appears unambiguously on the planet side of the limb as defined by the aurora1 oval, and because the farside of the latter, which lies behind the limb, is not visible in the contiguous Fig. 2(b) and (d) (it is not visible either in other images taken with FOC at about the same CML). Even a brightening of the far side of the aurora seen from behind the limb would not appear at this location. nor with such well defined edges. Similarly. we have never observed so far any periodical modulation of the aurora1 emission. Therefore, on the basis of our present knowledge of the aurora1 morphology. we have ruled out any natural mechanism (such as solar wind effects), and we have assumed that it was related to some interaction between the comet train and the magnetosphere, trans- mitted to the ionosphere by means of a field aligned current system.

We have mentioned above that the two spots appear practically at the same location on the Jovian polar cap. Neglecting any possible small effect of the wavelength dependent distortion (the first two image do not include the Lyman M contribution whereas the last two do), we find that the observed 1.5-2 pixel shift (550-750 km on


the surface) is less than the 4 pixel shift which would result from the corotation of the spot between 14 : 10 and 14 :29. In addition this shift is in the direction opposite to corotation. Since all the “normal” amoral features observed so far were corotating, we believe this is an additional argument in favor of an excitation process at the magnetic footprint of a comet fragment, practically fixed in local time during the course of the observations (in fact, slowly moving opposite to the corotation direction).

3. Magnetic field models

In this section, we wish to study the magnetic footprint location and the shape of the CIMFL as a function of time along each comet fragment path, and especially along that of the large complex Q2/Ql, in order to establish which one best matches the location and characteristics of the FUV spots observed.

The trajectories of fragments P2 and followings have been provided in jovicentric coordinates by Chodas (personal communication, 1994) within a distance of lOOR, from Jupiter. The paths of the various nuclei did not differ noticeably, except during the last couple of hours before impact, when a general trend was observed along the train toward lower latitudes of the south pole fly-by (by a few tenths of a degree) and of corresponding shorter distances of the impact sites from the morning limb (by a few degrees): fragment Q thus represents an average case among the fragment train, and its ephemeris was used to calculate the following sample representative results except very close to impact.

Each fragment approached Jupiter from the southern dusk side sector. At the beginning of our simulation (lOOR,), their common latitude was - 52” and their longitude with respect to the noon meridian, or local time, was +62” (positive in the rotation direction). The fragment local time remained stable for a long period, before it traveled toward the south polar region, crossing the noon meridian and the dawn sector during the few hours pre- ceding impact. The latitude progressively increased toward very high values, driving the fragment close to or within the polar cap.

In modeling the comet-magnetosphere interaction, it is not specifically the latitude of the fragment which is crucial but rather its location in the corotating magnetic frame of reference. The excursion from the average motion due to the periodic precession of the magnetic axis is significant, in particular in terms of magnetic latitude (Z + lo’), and is important in determining the nature of the instantaneous field lines as a function of time, throughout multiple rotation periods. Figure 1 shows the remaining train of fragments in the heliospheric frame of reference, at two particular given times corresponding to extreme configurations of the magnetosphere (the magnetic field lines are the actual model ones) and illustrates how a magnetic field line passing through an incoming fragment varies with time. As a result its length and equatorial distance increase and decrease, depending on the

orientation of the magnetic axis of Jupiter and on the fragment distance to the planet. The fragment may even reach open field lines for a while and later return to another magnetic neighborhood where the field lines are closed. A different, complementary view of this effect was provided by Figure 1 in (Prange et al., 1995) drawn in the corotating magnetic frame of reference.

In addition, the magnetic field in the outer magnetosphere is also partly controlled by the solar wind, which introduces local time dependent components. As the fragments cross the magnetosphere from dusk to dawn, this results in additional variations of the magnetic field lines, mainly noticeable during the last few hours before impact. Because of the combination of these two effects, the local magnetic field line attached to any given fragment changes constantly. Its variation is almost periodic when the fragment is far from Jupiter and the effect of its proper motion is almost negligible, and it becomes progressively aperiodic as the fragment azimuthal motion fastens. It must therefore be modeled carefully.

The magnetic field lines are also highly distorted near the top of the atmosphere due to the strong asymmetry of the internal planetary component. The distortion, modulated by the magnetic longitude of field lines at the rotation period of Jupiter must also be taken into account in order to derive precise “surface” field line footprints.

To fulfil all the above requirements, we need an accurate knowledge of the field line geometry inside the Jovian magnetosphere, for different orientations of the magnetic axis (MA) and a wide range of distances to the planet center. Unfortunately, there does not exist yet any such comprehensive model of the Jovian magnetosphere valid for a large range of local times, from the magnetopause to the surface of the planet. Therefore, we had to combine the capabilities of two different models. Preliminary results of this combined modeling were already been used to assist in the preparation of observations (IUE and HST-FUV programs), and some were also made available to the observing community via the electronic “Bulletin Board” hosted by the University of Maryland Astronomy Program.

3. I . Near-surface model

The first model is a version of the O6 model which allows the surface magnetic footprint of any inner-to-middle magnetosphere location to be calculated with a reasonably good accuracy. It has been obtained by inversion of the Voyager magnetometer measurements. It includes high order multipolar planetary components, and an equatorial current sheet centered on the magnetic axis which represents the magnetic effects of the trapped magnetospheric plasma (Connerney et al., 1981 ; Connerney, 1992), so that it is suitable for uses close to the planet. But it assumes cylindrical symmetry (bi dimensional model) and cannot account for any local time, solar wind related effect. This version has been modified to account for

R. Prange 41 LZI: Magnetic mapping of aurora1 signatures of comet SL9 in the Jovian magnetosphere 1319

Fig. 2. Four consecutive views of Jupiter’s south pole from the FUV images taken on July 20. 1994 around 14: 30 UT. using the HST WFPC?. with filters F160WB + F 13OLP (Lyman bands of molecular hydrogen), (a), (b), and F160WB only (H Lyman r, H, Werner and Lyman bands, (c). (d)). We have adjusted an oblate spheroid to visualize the planet limb at the wavelength of the aurora1 emissions. with a grid of coordinates every 30’.. and we have overplotted the theoretical footprint of the magnetic shell crossing the equator at 30R, in the 0, model (gray dotted curve). The corotating dark spot is the impact site of fragment H. The FUV aurora1 oval is mainly visible near its dawn and bright dusk ansae. It is much less visible around the central meridian, where it seems to he a few degrees poleward of the model oval (longitude dependent uncertainty in the magnetic field model discussed below). Note the unusual bright spot on images (a) and (c)

R. Prange et al: Magnetic mapping of aurora1 signatures of comet SL9 in the Jovian magnetosphere 1321

Ulysses observations of Jupiter’s magnetic field (courtesy of M. K. Dougherty).

Based on the post-collision admitted impact times (Chodas and Yeomans, 1994) of each fragment of the train from P2 to W (July 20,15 : 23 UT to July 22,OS : 6 UT) we have computed their path in the corotating magnetic frame of reference. The resulting paths are pseudo-helices around the “guiding” mean fragment path which result from the combination of the 10” wobbling of the magnetosphere (due to the tilt of the Jovian magnetic axis) and of the decreasing fragment distance to the planet (see also Fig. 1 of Prange et al. (1995) and explanation therein). Figure 3 shows the surface projection of the pseudo-helices and illustrates how it varies from fragment to fragment. The path of a selection of fragments, Q, S, U and V, is mapped down the magnetic field and projected onto

Fragment Q

Fragment U

\ Impact ‘(. 270 ” ,.

the southern polar atmosphere in the system III corotating frame of reference. Only distances within z 20R, are considered (corresponding roughly to the last day before collision) as the surface trace of the path outside this distance lies very close to the first ring in each plot. There are no major differences among the various fragments. In each case, the south footprint spirals around near the magnetic pole, then it escapes toward lower latitude to the impact site, nearly along a meridian. Each trace remains at high latitude, always inside the 30R, model magnetic footprint, which is generally associated with the “normal” main aurora1 oval, until about l-l .5 h before impact. However, minor differences are visible, essentially in the shape, size and orientation of the last loop and in the escape direction (in particular, the footprint of some fragments still sur- rounds the pole whereas others remain on the same side).

Fragment S

Fragment V

Fig. 3. South polar projection of the magnetic footprint of fragments Q, S, V and Q on the surface of Jupiter, during the last day before impact, using a version of the 0, model. The theoretical footprint of the 30R, magnetic shell is superimposed. As discussed in the text, the high order terms of the internal field are not accurately known. Connerney et al. (1993) have shown that this results in a maximum latitudinal uncertainty of less than 10” for the magnetic shell passing through IO (6R,) around SIII longitude lO(r120”, which becomes much smaller for the outermost magnetic shells of interest here (this accounts for the small discrepancy between the model oval and the aurora1 emission locus near CML in Fig. 2). For -90” < ii,, < 60”, the latitudinal uncertainty is essentially negligible. The longitudinal uncertainty is the same order of magnitude (Connerney, private communication)


Impact: 20.832“..*.. II.

“,,, 41142 at time -‘......____ ,,,/ ‘:~: of observations ‘,,f : ‘. ‘I

.20 7.3 ’ . _/ ‘,., 2O:jS

‘~,, Mapped equatorial ’ . . . . shell: 30 R, --X._, :., I ’ “\ l. ,’ , ’ I,’ ,’

. . (b) 1

‘. _~ ,f’.

‘. . . ‘. -.

,/’ ‘...._. ,/’

~ I ;

Fig. 4. Time tagged polar projections of the magnetic footprint of fragment Q on the surface of Jupiter in both hemispheres

They are related to the particular SIII longitude of each fragment at a given time before impact (i.e. at a given distance from the planet).

Figure 4(a) is a detailed reproduction of Fig. 3 for the complex Ql/Q2, from July 20.08 to July 20.832 (Q impact time). The fraction of this path which corresponds to the time period of our observations is highlighted as a thick segment, and we see that it is reasonably well apart from the rotation axis. Figure 4(b) presents the conjugate footprint spiral in the northern hemisphere. It shows that the north track vanishes several times when the southern location of the fragment Q cannot be mapped along a field line to the planet surface in the northern hemisphere. This corresponds to periods when the wobbling of the magnetosphere, combined with the fragment motion, places the fragment on a polar cap (open) field line in this

P2 Q RtoW

I

Fig. 5. The magnetic footprint of the comet train as seen from Earth at the time of the observations. In bold, are the locations for fragments P2, Q and for the cluster R to W. The grid of coordinates and the theoretical aurora1 oval already used in Fig. 3 are superimposed to assist comparison with the observed spots. It appears clearly that the footprint closest to the FUV spots is Q’s (practically at the center of the spot). As discussed in text and Fig. 3, the model footprint uncertainties are negligible in latitude for Q to W and can reach a few degrees for P. To estimate the longitude accuracy, we have compared with another, newer, magnetic field version fitted on the 10 flux tube footprint (Con- nerney, private communication), and found that the latter shifts the footprints of all fragments by 7-9” toward smaller SIII longitudes, i.e. the footprint of Q nearly coincides with the right end of the spots, and the footprints of R to W lie even farther from the spot than with 0,

model. This is particularly the case at the time of our observations.

In order to compare our model predictions more easily with our HST images, we have plotted in Fig. 5 the magnetic projection of the train of comet fragments still in the magnetosphere (extending from P2 to W) as it would be seen from Earth (geocentric frame of reference). The footprint of each individual fragment is highlighted when it can be spatially resolved. To help comparison with the data, we have also plotted the same grid of coordinates as in Fig. 3 and the same aurora1 oval (30R,, O6 model). The train trace crosses the very high latitude polar region. On this trace, the footprint of P2 is on the morning side (E 60 east of CML), close to the planet limb (latitude z 87”). It is obviously not located (on the planet, and with respect to the aurora1 oval) as the bright FUV spot. The footprints of R to W, grouped at latitude z 80”, and ~65” from CML, seem too close to the evening limb to be identified with the bright spot. The footprint of the extended complex Q2-Ql, near the center of the oval, at latitude z 81.5’ and longitude ~50-55” from CML in the afternoon sector, by contrast, matches fairly well the observed FUV spot.

Our ability to identify the fragment footprint depends upon the uncertainties in the data and in the model. The images have been corrected for geometrical distortion of the disc emission (at /1.x 1900A), but the wavelength dependence of the distortion, nteded to locate accurately the aurora1 emissions (2 I 1650 A) has not yet been incor- porated. This correction could shift the spot emissions by z 2-3 pixels (J. T. Clarke, personal communication), but it is expected that the aurora1 oval would be shifted by a

R. Prangi: rt al: Magnetic mapping of aurora1 signatures of comet SL9 in the Jovian magnetosphere 1323

very similar amount, so that the relative geometry of the FUV aurora1 pattern would not be noticeably affected, in any case much less than the ~7-8 pixels between the

footprints of Q and R (Q and P2 are even significantly farther from one another). The accuracy of the magnetic field model is estimated from the comparison of the actual footprint of IO with the theoretical 6R, oval in Connerney et al. (1993). Although the real footprint is shifted poleward of the theoretical one by as much as 10’ near II,,, = 110’ for these low latitude field lines, this shift must decrease significantly for ovals of higher latitude. Furthermore, the model locus is nearly correct for longitudes less than 60” or more than 270”, such as those of the fragment Q to W footprints at the time of the observations. In longitude, the model accuracy is usually estimated to z + 5-10” (Connerney, private communication). We have used another possible model, and found that the R-W train would then be an even less likely candidate (see discussion in the caption of Fig. 5). Therefore, we feel rather confident in our identification.

Finally, it is interesting to note that our observations occurred practically near the transition from closed to open field lines, i.e. crossing of the polar cap boundary (Fig. 4(b)). However, this model does not properly describe high latitude field lines which are controlled by the relative geometry of the Jovian magnetic axis and of the solar wind direction ; it is therefore unable to account for the shape of boundary field lines that far from the planet, and we have to make use of a second model to infer correctly their nature.

3.2. Sob-wind-controlled model

The second model is based on a tri-dimensional rep- resentation of the Jovian magnetosphere, which super- imposes the solar wind contribution to sources of magnetic field which are interior to the magnetopause. The internal sources are approximated by a tilted dipole (that is why this model is not well suited for near-surface simulations) at the center of Jupiter, plus a current sheet similar to the one in the previous model. The shape of the boundary between the magnetosphere region and the surround- ing interplanetary medium is constructed by equating the internal magnetic pressure to the Newtonian pressure associated with the incident solar wind at all locations on the magnetopause. For details of the method, as applied to Jupiter for a zero-tilt of the planetary dipole, one can refer to Engle (1991) and references therein. The model has been later modified to include the predicted Jovian diurnal variation by tilting the magnetic axis by 10” from the normal of Jupiter’s orbital plane and allowing the natural precession with the period of the planet rotation, about 9 h 55 min (Engle, 1992 ; Bode, 1994). The resulting shape of the magnetopause and detailed structure of the solar-wind-driven currents on the magnetopause thus vary periodically with the precession and are represented accordingly by the model. The model simultaneously determines the deformation of the field associated with the solar-wind-driven currents.

To our knowledge, this model is the only one which allows to follow the complex deformation of the instantaneous magnetic field lines passing through any fragment

of the comet as Jupiter rotates and as the comet travels across the magnetosphere (inwards, eastwards, and ulti- mately towards the nightside). To estimate the effect of the varying orientation of the magnetosphere, we have used the two extrema represented by (i) the north magnetic axis (MA) directed toward the comet position, leading to the innermost location of a fragment in the magnetosphere, and (ii) away from the comet, which leads to a significantly higher magnetic latitude location. Early in the simulation, the temporal steps are such that during each particular interval, all the MA orientations are suc- cessively encountered (i.e. the other parameters do not vary significantly during a full rotation of Jupiter). Later, when the temporal steps decrease (local time and distance vary faster and faster), we should have used the actual MA orientation corresponding to each time period and each fragment. However, we have chosen to remain general and to present also the “MA toward” and “MA away” situations, because it is more explanatory. and provides a schematic guide for any or all of the fragments. The real case is studied, for several fragments at a particular time only (end of this section), and as a function of time for fragment Q only (Section 4).

We first need to briefly discuss our choice of (free) parameters used in the model. Whereas the planetary dipole intensity and orientation are fixed, the overall size of the magnetosphere and the current intensity in the equatorial sheet depend on the solar wind pressure. Based on Jupiter’s flybys, we know these parameters for two extreme opposite conditions : a “Voyager-type” model incorporating a compressed magnetosphere in a relatively dense and/or fast solar wind, and a “Ulysses-type” model representative of a much larger, and “flatter”, magnetosphere which generally results from a weaker solar wind pressure. At the time of the comet collision, as discussed in Prange et al. (1995) the magnetosphere appears to resemble a Voyager-type one. Therefore. we have used standard Voyager values for the various model parameters, and in particular, we have set the scale size of the magnetopause to 60R, (subsolar standoff distance). Nevertheless, we have also investigated the Ulysses-type model case, and we have found that the general behavior of the CIMFL is rather similar. The main differences are that the field lines seem to be less disturbed by the solar wind pressure and that the north-south asymmetry (see below) is less extreme than with the Voyager-type model, as a result of the weaker solar wind pressure associated with the larger subsolar standoff distance.

We find that the computation does not provide any field-line closing “below” the comet onto the south hemisphere surface until about 5 days before impact time IT (July 16 for the ephemeris of fragment Q used as a “test fragment”). We interpret this as the comet being on open dayside field lines, outside the magnetopause or in the outer dayside cleft, and then of course the shape of the computed field-line is without any physical significance. The first case showing a field line computed from the

1324 R. Prang6 et al: Magnetic mapping of am-oral signatures of comet SL9 in the Jovian magnetosphere

fragment location all the way to the planet is July 16.6 ( zIT-4.25 days), with the MA directed toward the comet. However, Fig. 6(a) shows that the connection does not occur to the south hemisphere as expected, but to the north. This suggests that the comet is still in a boundary region where the model cannot represent the field lines accurately enough. The first transit on a closed field line occurs on July 17.6 (%TI - 3.25 days) when the MA is toward the comet. Figure 6(b) shows this very long dayside closed field line in the dusk sector, extending to more than 60R, toward the dusk direction (i.e. almost to the magnetopause) and to 5 12R, in the sun direction. When the MA is directed away from the comet, the CIMFL is an open field line, connected to the planet inwards, and to the late evening sector outwards. The computation is stopped when the distance in Y (dawn dusk meridional projection) reaches 70R,, considered as the magnetopause. At this point, the field line is still very far below the equatorial plane (2 z -35RJ and hardly past Jupiter’s dawn dusk plane toward the nightside (X z -4RJ). We may probably consider such a field line as a cleft field line rather than a nightside field line. Therefore, on that date, the nucleus then oscillates between a closed dayside field line to an open field line toward the dusk. One can also notice the influence of the solar wind pressure on the XY projection (in the equatorial plane). It displaces the field- line away from the noon meridional plane and bends it toward the nightside more and more as the field line approaches the outermost regions of the magnetosphere. Also noticeable is the north-south asymmetry of the closed field lines, resulting from the tilt of the Jovian magnetic axis relative to the solar wind direction. This situation remains essentially the same until July 20 (Z IT - 1 day). However, as the comet penetrates deeper into the magnetosphere along a quasi linear path, first about 60” from the noon meridian, then at earlier local time, the length of the closed (“MA toward”) field lines tends to increase, both in the X and Y directions (toward the sun and toward dusk), as the comet is traveling toward higher magnetic latitudes.

We interpret this as a manifestation that the comet is traveling toward the cleft/cusp region. Simultaneously, the open field lines encountered with the MA away from the comet resemble more and more nightside field lines in late dusk/pre-midnight sector (Fig. 6(b) and (c)). By the middle of July 20, ( M IT - 8 h), the solar longitude of the comet starts to decrease, and the comet crosses the noon meridian on July 20.775 ( % IT - 1.5 h) and penetrates into the dawn sector. On July 20.78 ( zIT- 1.25 h), the CIMFL is, for the last time, a dayside closed field line in the dawn sector when the MA is toward the comet. When the MA is away, it is a nightside open field line, surprisingly in the dusk sector (Fig. 6(d)). This amazing situation results from the very high latitude reached by the comet at that time ( - 87”!) which permits it to circle around the magnetic pole during a rotation period. The comet then enters a region where it crosses open field lines for all orientations of the MA, but these high latitude field lines again lie in the dawn to predawn sector when the MA is directed toward the comet, and switch smoothly to

the dusk or post-dusk sector when it becomes away from the comet (Fig. 6(e)). During this period, the comet itself crosses the dawn terminator (still on the front side as viewed from Earth) about 20min later on July 20.79 (z IT- 1 h). Closed nightside field lines in the predawn sector are reached when the MA is toward the comet on July 20.81 ( zTI -0.55 h), whereas the comet is still on open field lines when the MA is away (Fig. 6(f) and (8)). Finally, the CIMFL are nightside closed field lines for all MA directions after July 20.82 ( zTI -0.3 h), and the length of the field line decreases until collision on July 20.832 (Fig. 6(h)).

The general result of this modeling is thus that each fragment of the comet spends the major part of its travel time in the magnetosphere on duskside magnetic field lines, oscillating from a dayside-closed field line configuration (when the north Jovian magnetic axis is directed toward the comet) to a nightside-open one (when it is away from the comet) as illustrated in Figure 3 of Prange et al. (1995). A few hours before impact, the comet stays for a short while on field lines which are open for all magnetic axis directions (but either dawn or dusk). In this phase, the details of the combined effects due to the magnetic axis direction and to the comet location are not trivial and depend strongly on the magnetic longitude of each fragment. Finally, the comet progressively enters a region of predawn nightside closed field lines. which become shorter and shorter as the impact time approaches.

The accuracy of this time history, and particularly of the succession of open and closed field lines, depends on the assumptions made in the solar-wind-controlled model. The major ones belong to two different classes: (i) the use of dipole plus rigidly corotating current sheet for the internal field, and (ii) the parameters describing the solar wind-magnetosphere relative configuration. The contribution of the higher order terms of the multipolar planetary field decreases quite rapidly with distance, and use of the dipole instead of the full multipolar model will not noticeably affect the shape of the magnetic field lines, hence the location and size of the cusp and polar cap, beyond 1.5 - 2R, from Jupiter’s center. Consequently, our conclusion regarding the alternate periods of closed and open field lines remains unaffected until the last 15-30 min before impact. During this final approach time interval, each comet fragment latitude has begun to decrease and it enters a magnetic neighborhood in which it remains continually on predawn closed field lines (Fig. 6(g) and (h)). The uncertainties upon the location and size of the cusp and polar cap introduced by the use of a dipolar planetary field therefore only affects the precise time(s), but not the nature, of this final transition. The model contains also the simplifying assumption of a rigid corotation of an axially symmetric current sheet, whose axis in turn precesses regularly about the planetary spin axis. That enabled the relative position of the current sheet elements to be described by a single parameter. However, this assumption is not valid at large equatorial distance, where most of the field lines of interest cross the equatorial plane. At such distances, the plasma rotates more slowly


(a) IT - 4 days C-60 R,)

@I IT - 1.5 day (-30 R,)

y --3 Y

(c) IT - 15 hours (-17R,)

-601/ -60Y

Cd) IT - 1.27 hour (-3.3RJ

North magnetic axis toward comet North magnetic axis away from comet

Fig. 6. Schematic time history of the field lines attached to a “test” fragment during its traversal of the Jovian magnetosphere, using the solar-wind-controlled magnetic model. Two orientations of the Jovian magnetic axis are considered, toward the fragment and away from the fragment. The star represents the instantaneous location of the fragment. The Sun direction (local noon) is toward positive X, 18 LT toward positive Y, and the rotation axis is along 2. Dotted lines, dashed lines and dotted-dashed lines are the field lines projections on the three othogonal planes of reference YZ, ZX. and XY, respectively. Distances are indicated in Jovian radii

326 R. Prangk et al: Magnetic mapping of aurora1 signatures of comet SL9 in the Jovian magnetosphere

(e> IT - 1.17 hour (-3.1 R,)

IT - 33 minutes (-2.2 R,)

(g) IT - 30 minutes (-1.95RJ

(h) IT - 15 minutes (-1.5R,)

North magnetic axis toward comet North magnetic axis away from comet

Fig. 6.-continued


than Jupiter, and this finally results in an additional bend- ing of the magnetic field lines which are delayed from their nominal position by an increasing function of radial distance (see Figure 1 l(b) of Khurana (1997)). This effect would tend to alter somewhat the specific location of the cusp footprint (representative of the boundary between open and closed field lines at large distances from the planet), but it should not affect the general nature of the diurnal variation described and studied above. The first order consequence of recognizing the existence of the lag of the outer current sheet will be to introduce a lag in the timetable of the transitions from open to closed field-lines and vice versa. The field lag would of course tend to be larger for larger distances (and earlier portions of a comet fragment’s track) than for the nearer-planet distances (during the last parts of a fragment’s path), implying that the transitions between open and closed field line configurations are increasingly delayed as the fragment approaches the planet. Khurana (1997) finds that this effect causes a delay of z 20-25’ on average at distances of 7&80R, near the equatorial plane. But compressed magnetospheres are less affected than expanded ones (less than 10” for Voyager 1, ~30” for Pioneer 10). This is confirmed by preliminary investigations which indicate that the extreme outer edge of the current sheet could lag the planet rotation by as much as 9G120” value for a fully inflated magnetosphere (Ulysses-type). As the SL9 fragments seem to have traveled in a compressed Voyager- type magnetosphere, we estimate that this would result in a 15-30min maximum delay in the transition times computed above. Concerning the approximations in the solar wind interaction, the affect of assuming that the solar wind impinges upon the magnetosphere perpendicular to the rotation axis (only a few degrees off the ecliptic plane) would result in only minor shifts in locations of the cusp footprint, corresponding to variations of the order of magnitude of a few minutes or less in the time of the transitions between open and closed field lines. The affect

of assuming that it impinges upon the magnetosphere along the sun--planet line introduces larger uncertainties in an absolute sense. However, the solar wind is generally flowing away radially, and in any case within 10” of the sun direction, so that this effect is also negligible in practice.

At the precise time of our observations, all the fragments are still in the dusk sector except P2. They differ by their distance to the planet (~7 to 14R, from Q to W) and their system III longitude (solar longitude varies from z 5 1 to 60”). The parameters controlling the shape of the instantaneous field line passing through each of these last fragments are nearly similar. However, they vary slowly from one fragment to the next, with a continuous decrease of the latitude of the field line and a decrease of the XII longitude from Q to W, not considered in the previous study, so that one can follow the field line shape evolve according to the geometric configuration (Fig. 7) : from a clearly open nightside field line passing through fragment Q (higher latitude. closer to the “MA toward” configuration) to a marginally closed dusk field line through W (lower latitude, closer to the “MA away” configuration) for which the field lines reaches z7OR, in the

QllQ2 fragments

S fragment

W fragment

Fig. 7. Actual field line passing through various fragments of the train at the time of the observations as derived from the solar- wind-controlled magnetic model. The star indicates the location of the comet. Solid lines are field lines. Dotted lines, dashed lines and dotted-dashed lines are their YZ. ZX. and XY projections, respectively

XY direction, defined as the magnetopause distance in the model, with S displaying an intermediate shape (still just open on the dusk side). In this range of distances from the planet, the predictions of the model are fairly accurate. This finding is thus an important clue to interpret the lack of conjugate bright spot in the northern hemisphere as a consequence of the fragments being on open field lines at the times of the observations within the uncertainties of our assumptions.

1328 R. Prange et al: Magnetic mapping of aurora1 signatures of comet SL9 in the Jovian magnetosphere

4. Discussion

The above magnetic mapping suggests strongly that the FUV spots seen on July 20 are at the footprint of an open field line passing through fragment Q.

There are several related issues to discuss.

We could expect to find similar signatures in other images, related to the same or other fragments. Eight additional FUV sequences, two exposures each, were scheduled during the impact period. The south aurora1 oval was facing Earth (the best viewing geometry for high latitude emissions) in only one of them, on July 2 1. These images do not exhibit any particular feature on the polar cap. At that time, the fragments still in the magnetosphere were S (at NN 7R,) to W. This might indicate again that the event was specifically related to fragment Q, which was surrounded by the largest and more active dust coma, and that the later fragments did not interact as strongly with the magnetosphere. In the other images (before and after July 20), the CML was close to 180-200 and gave a good view to the north aurora, but the south aurora1 oval was at extremely high latitude, along the limb, and any extra emission at higher latitude would be very difficult to detect. For such CMLs, most of the fragments being on closed field lines, we could have also looked for unusual emission in the north. Unfortunately, the aurora1 pattern is much more complex than in the south. Except in the case detailed below, we did not identify any other FUV event, but this conclusion should be confirmed by a detailed analysis, beyond the scope of this study. Nevertheless, we have studied in more details the images obtained 2 h after the event, on July 20,16 : 10 to 16 :25 UT. In both images, a bright region lies slightly poleward of the average oval in the south, roughly at the location expected for the footprint of fragment Q (still at 5Rr) as derived from the near-surface model. But the viewing conditions (the oval is very close to the planet limb) make our conclusions be less firm than for

the images studied above. The expectation of a north conjugate spot depends on the nature of the field line. Therefore, we have modeled the complex temporal evolution of the instantaneous field line attached to fragment Q with the solar-wind-controlled model from the first event (14 : 10 UT) until impact (z 20 UT) taking into account the actual direction of the MA. The field line, first open and in the evening sector, closes at z 15 :40 UT and, after an excursion toward noon, it rotates back into the dusk sector and reopens at w 18 :20, then it sweeps across the night side before it finally closes again in the predawn sector 20 min before impact (Fig. 8). As discussed above these transition times may be delayed by up to z 15-30 min due to approximations in our model. Consequently, we expect that any aurora1 process would also have a signature in the north on the 16 : 12 to 16 :26 UT images as the field line is closed. Indeed, a couple of features could match this north footprint at first approximation. But the aurora1 morphology is naturally complex inside the north polar cap, and only evidence of an exact coincidence of the bright spot observed with the Q model footprint could be convincing. In the present state of our models, we are not able to compute the exact location of the north footprint of Q precisely enough. This will only be possible when the two models we used here independently at short and long distance from the fragment, will be integrated in a single self- consistent new model. Therefore, this second set of images provides a serious indication that the event, which would be related to Q, has lasted for several hours and may have given rise to conjugate north emissions, but we cannot give definite evidence of it.

3. X-ray observations were performed from about 13 UT to 18 UT with ROSAT, on the same day. The intensity in the north polar region increased above the back- ground level from z 15 :20 (just after P2 impact time) to past x 16 :00 (Waite et al., 1995, Fig. I), during a period which overlaps the first time interval when our

3

Y

-4oy

Fig. 8. Deformation of the magnetic field line of fragment Q from the UV event to impact time, computed above every 15 min with the solar-wind-controlled magnetic model (see discussion in the text)

R. Prang6 cr al: Magnetic mapping of aurora1 signatures of comet SL9 in the Jovian magnetosphere 1329

model predicts that fragment Q is on closed field lines. Considering the uncertainties in our timeline due to model assumptions, this can be considered as a good correlation, and it is tempting to think that both the FUV and north X-ray events are associated with the same process, triggered by the coma of fragment Q. In such a case, our model predicts that the X-ray source should be observed at high latitude and close to the central meridian, in contrast with the emissions observed at lower latitude on July 18 during impact K. This is precisely what is reported in Waite el al., 1995 (see also their Fig. 3). One more argument in favor of a remote effect of Q, rather than of the very small P2 fragment impact, is that this was the only X-ray burst which was not observed just at the time of an impact (by contrast with the bursts associated to K and W).

4. Finally, we have tried to infer indications on the nature of the plasma processes involved. The available obser- vational constraints that we can use are, in principle, the spectral signature of the incident particles, and the modulation of the precipitation.

The two wavelength ranges scheduled consecutively were designed to compare the relative Lyman M to H2 band brightnesses to model simulations by Rego et al. (1994, 1997). This model is aimed to provide the flux radiated at Lyman z and in the H2 bands by precipitating charged particles as a function of the incident energy and species. It includes the computation of the altitude distribution of the excitation rates. and of the wavelength dependent extinction due to absorption by the overlying atmosphere and radiative transfer effects. Assuming that the brightness increase between the first and the second UV spot was due to the contribution of Lyman 2, and taking into account the geometry of the line of sight, we derive an estimated energy range of % 100-400eV for precipitating electrons or z 1-2 keV for protons. However. one must take these estimates with particular care, because the process is not stationary and we have no evidence that the strength of the process has been the same in both cases.

On the other hand, the spot, which is twice “on” and twice “off’ during the observations seems to be modulated with a 2Omin period. This rules out the effect of Alfven waves reflected between the coma and the Jovian ionosphere (similar to those suggested for the 10 flux tube), because their travel time would be less than 1 min along the high latitude, low density-low B fragment flux tube. This period would rather correspond to the travel time of = 3 keV protons, or 2 2 eV electrons, from the fragment to the surface and back. These values, although not per- fectly similar to the estimate derived from the spectral signature. are nevertheless also in the range of very low energy, non penetrating incident particles. If these particles were to be accelerated along field lines by parallel electric fields, they would not require large potential drops.

Processes like those predicted by Ip and Prange (1994) or Farrel ~,t al. (1994), are based on the onset of field aligned currents, whose intensity depends on the ambient plasma density. When the current density exceeds a given

critical value, it is known to trigger parallel acceleration and particle precipitation. However, the energy estimates above indicate that the acceleration process was not very efficient. We know also that, at the time of the observations, the fragments were on low density high latitude flux tubes. We suggest that the conditions on the relative velocity between the coma and the ambient plasma, and on the local plasma density, were just enough to sustain a critical current, and a marginal plasma instability. There- fore, any precipitation could have drained off enough particles to reduce significantly the plasma density in the instantaneous comet flux tube. The dynamo effect in the coma and the field aligned currents being similarly reduced, the particle precipitation process would have been switched off. The aurora1 precipitation would only be established again when a “new” flux tube would replace the partially empty one by corotation, and the critical conditions be fulfilled again. To account for the observed modulation period. we need a flux tube radius of the order of z 25 000 km at the distance of Q. This value is of the order of magnitude of a cometary coma. in particular if we consider the extended complex QZiQl. Translated into extension in the azimuthal direction, this corresponds to x 12 This compares also quite reasonably with the azimuthal east-west extent of the FUV bright spots deter- mined in Section 2 (lo-20 ). so that the spot could match not only the location of the magnetic projection of fragment Q, but also cover the entire projection of the disturbed flux tube passing through the dust coma. However, we can also note that the direction along which the FUV spot is elongated follows the surface projection of the comet train path (see Fig. 5). On the other hand, late images taken during the last day before impact show that gravitational effects have dramatically stretched the comae nearly along the fragment path. with the dust con- centrations extending almost half way between the various nuclei. Any current-driven electrodynamic coupling of the kind discussed above, between such a stretched coma and the Jovian ionosphere, would produce an aurora1 spot of the shape and orientation observed, even for a smaller radius of the coma, once degraded at the instrument spa- tial resolution. It is also presumable that such an elongated system would be less stable, and that the modulation of the blinking aurora could result from a relaxation process in the dust distribution itself.

Summary

We have used two different types of models of the magnetosphere. The first one, based on the Voyager 0, model, takes into account the full internal magnetic field and current sheet. but ignores the effect of the solar wind at the magnetopause. It is well suited for regions in the inner magnetosphere, and is also the one which can provide a best estimate of the longitude of the magnetic footprints taking into account the distortion of the magnetic field lines due to the multipole terms in the magnetic field. The second one is based on a tilted dipole plus equatorial current sheet. and takes into account the effect of the solar

1330 R. Prange et al: Magnetic mapping of aurora1 signatures of comet SL9 in the Jovian magnetosphere

wind on the magnetospheric cavity shape and field within (by means of magnetopause currents). It is well suited to represent the shape of magnetic field lines at large distances from Jupiter, to differentiate between open/closed and daysideinightside field lines depending on the sun/ magnetic dipole geometry, and to account for all non- axisymmetric “local time” effects. This model also allows to study various conditions for the solar wind/ magnetosphere interaction strength (“Voyager-type”, “Ulysses-type”). Combining these two models allows us to obtain a thorough description of the geometry of the interaction of comet SL9 with the Jovian magnetosphere and of the corresponding possible aurora1 signatures, all along the comet path. Using the parameters of the path of comet nucleus Q which represents an average case between the first and last fragments in the “pearl collar”, we have followed the trajectory of the comet throughout the magnetosphere and derived the nature of the instantaneous magnetic field line (inside/outside the magnetosphere, closed/open, daysidelnightside, dawn/dusk) as a function of distance to the planet and magnetic axis orientation (with respect to the sun and to the comet).

This allows us to interpret the transient bright UV spot, almost fixed in local time on the southern polar cap, as being due to some particle precipitation near the magnetic footprint of the pair Q2/Ql. Even if they did not give rise to the strongest impacts, these fragments were surrounded by the brightest, largest and most active comae of the train : the motion of such an extended and dense charged dust coma across the ambient magnetospheric plasma was a very favorable situation to develop an intense current system flowing along magnetic field lines and closing through the Jovian ionosphere. This current could have exceeded only slightly the critical density above which parallel acceleration takes place, as there are indication that the precipitating particles have only been accelerated to very low energies. The “blinking” nature of the spot could therefore be due to the fact that the field aligned current was only marginally critical, so that any decrease of the corotating local flux tube density due to pre- cipitations would lower them to subcritical densities, swit- ching off periodically the acceleration mechanism. An alternative explanation of the emission modulation could lie in an unstable distribution of the dust coma once it has been severely stretched along the fragment path by gravitational effects shortly before impact.

At the time of the bright spots, Q was on a dusk side open field line and did not have a northern footprint. However, the model predicts that the field line closed z 1 h after the end of the observations for % 2 h 40 min. A new series of images taken during this period might show a north and south signature at the footprint of Q, but the viewing conditions are not good and firm confirmation needs a more careful study. Interestingly, a significant X- ray emission burst in the north coincided also with the period of Q closed field lines. This might suggest an association between this X-ray emission and our UV spots.

Acknowledgement. We want to thank J. Clarke and W. Ip for fruitful discussions.

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Magnetic mapping of auroral signatures of comet SL9 in the Jovian magnetosphere

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