Ionospheric model-observation comparisons:E layer at Arecibo Incorporation of SDO-EVEsolar irradiancesJan J. Sojka1, Joseph B. Jensen1, Michael David1, Robert W. Schunk1, Tom Woods2, Frank Eparvier2,Michael P. Sulzer3, Sixto A. Gonzalez3, and J. Vincent Eccles4

1Center for Atmospheric and Space Sciences, Utah State University, Logan, Utah, USA, 2LASP, University of ColoradoBoulder, Boulder, Colorado, USA, 3Arecibo Observatory, Center for Geospace Studies, SRI International, Arecibo, Puerto Rico,4Space Environment Corporation, Providence, Utah, USA

Abstract This study evaluates how the new irradiance observations from the NASA Solar DynamicsObservatory (SDO) Extreme Ultraviolet Variability Experiment (EVE) can, with its high spectral resolutionand 10 s cadence, improve the modeling of the E region. To demonstrate this a campaign combining EVEobservations with that of the NSF Arecibo incoherent scatter radar (ISR) was conducted. The ISR providesE region electron density observations with high-altitude resolution, 300m, and absolute densities using theplasma line technique. Two independent ionospheric models were used, the Utah State UniversityTime-Dependent Ionospheric Model (TDIM) and Space Environment Corporation’s Data-Driven D Region(DDDR) model. Each used the same EVE irradiance spectrum binned at 1 nm resolution from 0.1 to 106 nm. Atthe E region peak the modeled TDIM density is 20% lower and that of the DDDR is 6% higher than observed.These differences could correspond to a 36% lower (TDIM) and 12% higher (DDDR) production rate if thedifferences were entirely attributed to the solar irradiance source. The detailed profile shapes that includedthe E region altitude and that of the valley region were only qualitatively similar to observations. Differenceson the order of a neutral-scale height were present. Neither model captured a distinct dawn to dusk tilt in theE region peak altitude. A model sensitivity study demonstrated how future improved spectral resolution ofthe 0.1 to 7 nm irradiance could account for some of these model shortcomings although other relevantprocesses are also poorly modeled.

1. Introduction

The E region of the terrestrial ionosphere is still only qualitatively understood even though quantitativemeasurements of this region have been made for over 80 years (seven solar cycles). Within this region theroles and relative importance of the known physical and chemical processes is not completely understood.To first order the dayside E region plasma is produced from the ionization of the neutral gas by the solarirradiance. A variety of processes lead to further ionization (cascade ionization due to energeticphotoelectrons, for example). Chemical reactions further redistribute the ion composition, and there is apotential altitude redistribution driven by neutral winds and atmospheric waves (i.e., planetary, tidal, andgravity wave) [Schunk and Nagy, 2009]. Difficulties in carrying out quantitative analysis of observations orphysical modeling have historically revolved about the difficulty of observing, hence specifying, the solarirradiance driver and the neutral atmosphere’s altitude profiles of composition and density. Althoughadvanced photoelectron transport models exist, without a knowledge of the altitude distribution of thephotoelectrons, i.e., the photoelectrons generated from the solar irradiance, only qualitative modeling ispossible. Similar concerns exist about the knowledge of the neutral atmosphere in this altitude regime inwhich minor species such as nitric oxide (NO) play a significant ionospheric role, especially in the lower E andD region. Dynamics, winds and waves, are also difficult to model and routinely measure in this region. Hence,E region modeling efforts to apply theoretical understanding have been hampered by the difficulty ofknowing the above drivers (solar irradiances and atmospheric dynamics) or environment (compositionaldensity) specification with good accuracy. We are glad to report this situation has changed for the solarirradiance driver, first by the observations of the Solar EUV Experiment (SEE) instrument on ThermosphereIonosphere Mesosphere Energetics and Dynamics and most recently by the Extreme ultraviolet VariabilityExperiment (EVE) instrument on Solar Dynamics Observatory (SDO). Both instruments have very good

SOJKA ET AL. ©2014. American Geophysical Union. All Rights Reserved. 3844

PUBLICATIONSJournal of Geophysical Research: Space Physics

RESEARCH ARTICLE10.1002/2013JA019528

Key Points:• New solar irradiances used to improvemodeling of the ionospheric E-region

• NASA SDO satellite and NSF AreciboIncoherent Scatter Radar Campaign

• Identify shortcomings in ionosphericmodeling and solar observationmeasurements

Correspondence to:J. J. Sojka,jan.sojka@usu.edu

Citation:Sojka, J. J., J. B. Jensen, M. David, R. W.Schunk, T. Woods, F. Eparvier, M. P. Sulzer,S. A. Gonzalez, and J. V. Eccles (2014),Ionospheric model-observation compari-sons: E layer at Arecibo Incorporation ofSDO-EVE solar irradiances, J. Geophys. Res.Space Physics, 119, 3844–3856,doi:10.1002/2013JA019528.

Received 9 OCT 2013Accepted 15 APR 2014Accepted article online 16 APR 2014Published online 8 MAY 2014

spectral coverage of the solar irradiance in question with SEE making one observation every 90min and EVEone every 10 s [Woods et al., 2005, 2012]. Spectra from both instruments are calibrated on a regular basis, andhence, E region researchers have the much needed quantitative and accurate knowledge of one of thedrivers. For nonflare studies both the SEE 90min and EVE 10 s irradiance are useful; however, during flareevents it is the EVE 10 s spectrum that is the key constraint on modeling.

The solar irradiances from 2 to 20 nm contain photons whose energies range from 629 eV to 62 eV,respectively. The average energy needed for subsequent ionization by photoelectrons is 35 eV [Banks andKockarts, 1973], given that the photoelectrons generated by this band of the irradiance have many multiplesof 35 eV; this results in a significant cascade of ionization. This has been well appreciated but difficult tomodel if the irradiance is not well constrained [Lilensten et al., 1989; Buonsanto et al., 1992; Titheridge, 1996].This difficulty has led E region researchers to develop proxy photoelectron scaling effects on the primaryphoto ionization. Lilensten et al. [1989] used a single altitude-dependent parameterization of aphotoionization multiplication factor to account for the cascade of ionization. Rasmussen et al. [1988] used adifferent approach in which a corresponding multiplier was obtained by dividing the photoelectron energyby 35 eV for each photoelectron energy bin. A major campaign to compare ionospheric models andobservations was held over a 4 year interval in the 1990s. The campaign was an NSF coupling energetics anddynamics of atmospheric regions workshop called “Problems Related to Ionospheric Models andObservations” (PRIMO). A summary of the PRIMO findings was published [Anderson et al., 1998] in which oneof the findings called for all the modelers to scale their photoelectron cascade production by a factor of 2 inthe E region in order to be in better agreement with ionosonde and incoherent scatter radar (ISR)measurements made at Millstone Hill, MA. In contrast to this result, the Student Nitric Oxide Explorer missionprovided new insights of the E region [NO] and the irradiances [Bailey et al., 2000]. These results enabledSolomon et al. [2001] to reevaluate earlier model comparisons with theMillstone Hill ISR E region observationsof Buonsanto et al. [1992] and adjust their photoelectron scaling factor of the cascade of ionization by a factorof 4. Solomon [2006] describes a variety of E region models, both empirical and physics based. Theconsequences of uncertainties in the physical processes are described, and comparisons are made withempirical climate representations of the E region. The observations of the escape of photoelectron from theionosphere provide a method by which a model of the ionospheric photoionization process can beevaluated. Richards and Peterson [2008] provide an extensive study of this comparison with details of modeldifficulties that are not too dissimilar to those of Solomon [2006]. In addition to the physical processes ofionization and secondary ionization, the question of the spectral variability of the solar irradiances over thesoft X-ray range has led to continuously improving semiempirical models needed in the absence of suitablemeasurements. Richards et al. [2006] describe such a development and its comparison with observations.Today with continuous solar irradiance observations and complete spectral coverage the generation ofphotoelectrons now depends on the knowledge of the major neutral species of [N2], [O2], and [O], and thesespecies are arguably reasonably well understood for most conditions; specifically nonstorm conditions.

This study is the first campaign in which the SDO-EVE instrument provides the solar irradiances for E regionmodeling that two independent ionospheric models use to generate the E layer over Arecibo, Puerto Rico. Thesignificant new step is that during this campaign the ISR at Arecibo collected very high resolution (300m inaltitude), calibrated electron density profiles through the E region and above. In this study we investigate to whatextent the new EVE solar irradiance provides a resolution to specifying one of the drivers for E region modeling.Section 2 describes the instrumentation and models used in this study. The initial campaign is described insection 3 including the solar irradiances and E region observations. Section 4 shows the model results andcomparisons with the ISR E region observations. A discussion of what appears to bemissing as well as comparisonwith International Reference Ionosphere (IRI) is given in section 5. Section 6 presents a summary of our findings.

2. Instrumentation and Models

This study brings together three independent sources of information. The first is the SDO-EVE instrumentwhich provides the observation of solar irradiance, the source of the midlatitude ionosphere. Theseirradiances then drive two independent ionospheric models that simulate the E region. The third and finalstep is to compare the modeled ionospheric densities with those observed by the Arecibo ISR. Theseinstruments and models are described in the following subsections.

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2.1. SDO-EVE Instrument

This study uses the EUV solar irradiance made by the EVE instrument aboard NASA’s Solar DynamicsObservatory. This platform was launched in February 2010 as the current solar cycle 24 was beginning toramp up. The EVE instrument and its calibration, are well understood [seeWoods et al., 2012; Hock et al., 2012].A key aspect of the SDO satellite geosynchronous orbit is that it provides continuous solar observations withvery few Earth eclipses. Irradiance products from EVE are available in a variety of wavelength resolutions andtime cadences from the EVE website, http://lasp.colorado.edu/home/eve/data/data-access/. Three sensors ofthe EVE instrument are relevant in the present study. The sensor Multiple EUV Grating Spectrograph A(MEGS-A) is a spectrograph that provides 0.1 nm resolution in the wavelength range from 6 to 37 nm; MEGS-Bis a spectrograph likewise providing 0.1 nm resolution in the range 37–106 nm. The MEGS data productsprovide the irradiance in 0.02 nm bins. The EUV SpectroPhotometer (ESP) Quadrant Diode (QD) provides theirradiance over the 0.1 to 7 nm band.

2.2. Utah State University Time-Dependent Ionospheric Model

The Time-Dependent Ionospheric Model (TDIM) was initially developed as a midlatitude multi-ion

NOþ;Oþ2 ;N

þ2 ; and Oþ� �

model by Schunk and Walker [1973]. The time-dependent ion continuity and

momentum equations were solved as a function of altitude for a corotating plasma flux tube includingdiurnal variations and all relevant E and F region processes. This model was extended to include high-latitudeeffects due to convection electric fields and auroral particle precipitation by Schunk et al. [1975, 1976]. Asimplified ion energy equation was also added, which was based on the assumption that local heating andcooling processes dominate (valid below 500 km). The addition of plasma convection and particleprecipitation models is described by Sojka et al. [1981a, 1981b]. Schunk and Sojka [1982] extended theionospheric model to include ion thermal conduction and diffusion thermal heat flow. Also, the electronenergy equation was included by Schunk et al. [1986], and consequently, the electron temperature is nowrigorously calculated at all altitudes. The theoretical development of the TDIM is described by Schunk [1988],while comparisons with observations are discussed by Sojka [1989].

In using the EVE irradiance spectrum the interface to the model is via a 1 nm binned spectrum. Theabsorption and ionization cross sections are similarly binned at 1 nm see section 2.3 for details. Results ofdeveloping and testing this 1 nm binned spectrum have been presented by Sojka et al. [2013].

2.3. Data-Driven D Region Model

The Data-Driven D Region model (DDDR) [Eccles et al., 2005] solves for a time history of an altitude profile ofelectron density, total negative ion density, and total positive ion density in the D and E region (40–130 km)for any geographic position and geophysical condition. The DDDR is a first-principle model based onion-neutral chemistry resulting from the ionization driven by solar spectrum, night sky spectrum [Strobelet al., 1980], energetic solar protons, and auroral electrons. The solar spectrum from 0.05 nm to 104 nm plusthe Lyman Alpha line is typically generated by the EUVAC model [Richards et al., 1994] and an internalEUV-X-ray model. EUVAC is driven by F10.7 indices, and the internal EUV-X-ray model spectrum depends onGOES X-ray measurements as well as F10.7. For this study the DDDR solar spectrum is provided by the EVEinstrument observations. The atmosphere is obtained from NRLMSISE-00 Picone et al. [2002] with [NO]augmentation from the MODTRAN model [Anderson et al., 1986]. Absorption cross sections and ionizationcross sections are based on Richards et al. [1994] and Banks and Kockarts [1973] for X-ray energies and Bossyand Nicolet [1981] for Lyman Alpha. The auroral region ionization is generated from the characteristic energyand energy flux of the Hardy electron precipitation model [Hardy et al., 1987]. The DDDR Solar Proton Eventsmodule generates ionization profiles using geomagnetic rigidity cutoff formulae and ionization altitudeparameterization driven by the GOES energetic proton spectrum. Secondary electron ionization isapproximated by the 35 eV per ion-electron pair rule of thumb [Rasmussen et al., 1988]. The resultingtime-dependent system of ordinary differential equations is solved using a recent version of ODEPACK[Hindmarsh, 1983]. The DDDR model is the D and E region model within the HF absorption modelcurrently deployed at the Community Coordinated Modeling Center called Absorption by the D and Eregion of HF signals with normal incidence, which calculates nonderivative absorption of HF signalsoccurring during propagation through the D region [Webb et al., 2009]. The unknown parameters in theDDDR model EUV and X-ray spectrum have been adjusted to provide good comparisons with HF absorption

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measurements and VLF propagationcharacteristics, observed in the HFInvestigation of D-Region IonosphericVariation Experiment HIDIVE and frequencyAGILE experiments [Eccles et al., 2005; Riceet al., 2009a, 2009b].

2.4. Arecibo ISR

The Arecibo Observatory is located in PuertoRico at a geographic latitude of 18.3°N andlongitude of 66.7°W. This ISR observatorywas commissioned in 1963 and has anoperating frequency of 430MHz. Its peakpower is 2.5MW and has a 6% duty cyclecapability. Power can be fed into a twoseparate antenna systems, a linefeed or aGregorian feed. In this mode the ISRbecomes a dual-beam system [Gong et al.,2012]. A detailed description of the ISRmeasurement technique is found in Zhouet al. [1997] and Zhou and Sulzer [1997].

3. Arecibo E Layer Campaign

Both the EVE and Arecibo ISR instrumentshave restrictions on their duty cycle and/orscheduling constraints; hence, coordinatedcampaigns are required. For the firstcoordinated campaign 2 days were selected,8 and 9 February 2012, both instrumentswould carry out predetermined optimummodes that would enable E region modelingto reach a new stage of simulation andgeophysical insight. These special modes andoperational outcomes are presented anddiscussed in the next two subsections.

3.1. EVE 8 February 2012 Campaign

The restriction on full spectral usage of EVE every 10 s arises from a conservation of the lifetime of the EVEMEGS-B spectrometer. Normal operations provides a full spectrum (5–103 nm) for 5min every hour, for theother 55min only MEGS-A coverage is available (5–36 nm). However, for the campaign it was requested thatduring the sunlit hours at Arecibo, Puerto Rico, that MEGS-B be run continuously. This is the special EVEMEGS-B campaignmode used when flare activity is high and for this Arecibo campaign. Hence, on 8 February2012 this mode was run until midnight universal time which captures the daylight hours at Arecibo.

The solar irradiance, both spectrally and temporally, did not vary significantly during this 24 h period. This wasalso the case 24 h earlier and later. The maximum solar irradiance disturbance was a C7 class flare monitoredby GOES on the prior day. The EVE operations on both the preceding day (seventh) and following day (ninth)returned to normal operations. The quiescence in the solar irradiance was also found in the daily radio fluxindex F10.7. In the week centered on 9 February the F10.7 ranged between 100 and 101.

Unfortunately, the campaign’s first day was marred by the Arecibo ISR suffering a power system failure.Hence, on 8 February 2012 no ISR observations were obtained; however, on the ninth a full daytime set of ISRobservations was obtained. Therefore, for the purposes of this study attention was placed on the EVE 9February observations. These were compared with the earlier full-day EVE operations. Figure 1 (bottom),shows the EVE irradiance 1 nm resolution spectrum in photon flux units (photons/cm2 s) as the day’s average

Figure 1. The EVEmean solar photon flux (photons/cm2 s) was obtainedduring daytime hours at Arecibo on 9 February 2012. The flux is binnedinto 1 nm wide bins from 0.01 to 103 nm. Values of the flux at 31, 98,and 103 nm are 76, 35, and 33 × 108 photons/cm2 s, respectively.(top) The standard deviation for each bin over the sunlit hours atArecibo on 9 February 2012. (bottom) The EVE irradiance 1 nm resolu-tion spectrum in photon flux units (photons/cm2 s) as the day’s averagefor 9 February 2012.

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for 9 February 2012. The MEGS-B operationbeyond 36 nmwas for 5min every hour whilebelow this wavelength the irradiance wasavailable every 10 s. For completeness, threewavelength bins exceeded the 25 × 108

photons/cm2 s; 31, 98, and 103 nm and theirrespective photon fluxes are 76 × 108,35 × 108, and 33× 108 photons/cm2 s.Figure 1 (top) shows the correspondingstandard deviation in the same units as themean values of Figure 1 (bottom). From acomparison of these two figures it is evidentthat the temporal variability was less than0.1% of the mean across the entire EUVspectrum. This lack of temporal variabilityalso applies between the full EVE spectra on8 February and that shown on 9 February.The difference between the means on these2 days is in fact less than the standarddeviations shown. During these 2 days aweak B9.9 flare occurred on 9 February at12:16 UT. This magnitude of flare has almost

no signature in the E region given the level of the background daytime irradiance during this period. Thephoton fluxes shown in Figure 1 (bottom) are used to drive the two ionospheric models used in this study.

3.2. Arecibo ISR 9 February 2012 Campaign

On 8 February 2012, the start of the campaign, the ISR power system was inoperable. After successfulmaintenance/repair the ISR operated in the full EVE campaign mode during daylight hours on 9 February 2012.A total of 22 high-resolution electron density profiles (EDPs) were obtained. Each EDP was generated from asequence of pulse codes as well as plasma line measurements that had a cadence of approximately 30min. Inthe E region to achieve the 300m altitude resolution a 90 s integration time was used. Each profile extendedfrom 60 km to 2500 km altitude, and two key requirements were met in generating these profiles. The first wasthat in the F1 layer, valley, E region and below, the altitude resolution is significantly less than the 1 km target,typically 300m. A second objective was to have these ISR densities calibrated using the Arecibo ISR plasma linecapability. This techniquewas used from 2500km down through the F2 layer to the F1 layer almost reaching thevalley region. Figure 2 provides a close up of the observed E region, and three profiles are shown, the noonprofile (thick line), an early morning 08:00 SLT (thin line), and afternoon 16:00 Solar Local Time (dashed line). InFigure 2 it is evident that the ionospheric structure (namely the E peak and valley) change altitude in time, arrowspointing to the right identify the E region peak in each EDP and arrows pointing left identify the valley region.Above the valley region is the F1 layer while below 95 km is the D region. The electron densities approximatelytrack the expected local timemorphology with a noonmaximum [Chu et al., 2009;Moore et al., 2006;Nicolls et al.,2012; Pavlov and Pavlova, 2013; Titheridge, 1997]. Two different techniques were used to obtain the electrondensity profiles shown in Figure 2 and for all 22 profiles of this study. The coded long pulse (clp) data havethe smallest error bars and are reliable at altitudes above ~110 km and densities> 1.2–1.3 ×1011m�3.Altitude resolution is 300m in altitude×10 s in time. Below that region (i.e., where clp does not work) adifferent pulse is used. The 13baud Barker code is reliable to 90 km or below, even though resolution is muchlower, 600m×90 s. The crossover between the two pulse codes can easily be seen as a large increase innoise (near 130 km for the 16:00 profile and 110 km for the noon (12:00)). Each profile is calibrated againstthe plasma line measurement at altitudes above 140 km. This calibration is then used to extend calibrationsto lower altitudes. One profile structure of note does show up at noon in the E region peak, see featurelabeled A in Figure 2. The measurement uncertainty depends on the electron density and range (altitude)since fixed integration times are used. In Figure 2 the dawn and dusk profiles have the lowest densities andtheir measurement uncertainties extend up to 10%. However, at noon at the higher altitudes this uncertaintyis only a few percent. This spike in density does not fall within the expected measurement uncertainty but

Figure 2. Arecibo ISR EDPs at 08:00, 12:00, and 16:00 SLT. Left-point-ing horizontal arrows identify the valley between the E layer and F1layer, while right-pointing horizontal arrows identify the E layer peak.The noon structure labeled A is probably a narrow metallic ion layer.

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rather is a real feature probably associatedwith a thin metallic ion layer [Bishop et al.,2002; Zhou and Morton, 2005]. The threeexamples shown in Figure 2 represent thelowest density EDP (8:00 SLT) and thehighest density EDP (12:00 SLT) of the 22observed EDPs on 9 February 2012. This dataset will be compared with the modelsimulations in the next section.

4. Modeling and Comparison4.1. TDIM Simulations for 9February 2012

The EVE irradiance spectrum as presented insection 3.1 was the input for the TDIMmodelalong with the F10.7 and ap indices that dayfor the neutral atmosphere NRLMSISE-00[Picone et al., 2002]. It was assumed thatcorotation adequately described theionosphere convection for the 9 February2012 at 18° latitude. Simulations were then

carried out beginning 24 h prior to dawn on the study day, day 40 of 2012. The simulation was thencontinued for 2 days capturing the dayside ionosphere observed by the Arecibo ISR.

Figure 3 presents the TDIM-simulated Arecibo EDPs at 08:00, 12:00, and 16:00 SLT that correspond to the threeEDP times shown in Figure 2. The three EDPs are plotted using the same line styles and with the same arrowsto identify E layer peaks and valleys as in Figure 2. The noon profile, thick solid line, has distinctly high densityin the D and lower E region up to 100 km, while the E region peak density is about 20% lower than thatobserved at Arecibo. A 20% lower density could correspond to a 36% lower ionization rate if the entiredifference is associated with the solar irradiance source. The TDIM valley is similarly lower in density; however,the F1 layer is similar to the observations. Likewise good and poor EDP characteristics are evident in

comparing the 08:00 and 16:00 EDP. Thesignificance of the 20% type of differencecan be viewed as very good agreementwhen the observations and statisticalanalysis of other E region studies isconsidered, i.e., Chu et al. [2009], Nicolls et al.[2012], and the earlier work of Titheridge[1997, 2000]. However, as will be seen in thefollowing subsections there is significantwork still to be undertaken in order to modelthe E region processes accurately.

4.2. DDDR Simulations for 9February 2012

The DDDR model also simulated the Areciboionosphere using the same EVE irradiancespectrum that the TDIM simulation used.Figure 4 shows the DDDR EDPs at the sametimes as the observations and the TDIM. Atnoon, the thick solid line EDP, the E layer peakdensity is about 6% larger than the Areciboobservations and slightly lower in altitude,while the valley is significantly higher than

Figure 3. TDIM simulations of the ionosphere at Arecibo for thesame three local times shown in Figure 2. The arrows also identifythe valley (left-pointing arrows) and the E layer peak (right-pointingarrows). Thin dashed lines reproduce the Figure 2 Arecibo ISR EDPs.

Figure 4. DDDR simulations of the ionosphere at Arecibo for thesame three local times shown in Figure 2. The arrows also identifythe valley (left-pointing arrows) and the E layer peak (right-pointingarrows). Thin dashed lines reproduce the Figure 2 Arecibo ISR EDPs.

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observed. The 6% larger density couldcorrespond to a 12% higher ionizationrate if this difference is associated withthe solar irradiance source. The 08:00 and16:00 simulation also have similaritiesand differences with the observations. Inthe following sections specificintercomparisons between the twomodels and the observations are made.The EVE-SDO spectrum is used in bothmodels for photoionization process.Geosolar and magnetic indices from 9February 2012 are used as inputs to theMSIS model atmosphere (F10.7 = 97,F10.7a = 118, and ap daily = 6).

4.3. Comparison of Daytime E Layers

Although the models and theobservations have different EDP extents and LT cadences, they are presented in Figure 5 over the same localtime, 08:00 to 16:30 LT, and altitude, 60 to 190 km, with the same linear density color range. The lowest TDIMaltitude is 88 km while the DDDR only extends up to 130 km. Comparing the two model images associatedwith the ISR observations provides a clearer impression of the model strengths and limitations. The TDIMunderestimates the E layer density but captures the height of the peak at noon relatively well, while theDDDR has produced a thicker E layer density is slightly lower than observed. In both simulations thebottomside of the E layer reaches below 100 kmwhereas the Arecibo ISR observations indicate that this is notthe case. Both simulations generate a valley region between the E and F1 layers, but neither simulation of thevalley agrees well with the observations. The DDDR has better densities but is too high in altitude, while theTDIM is too low in density but captures the altitude better. Neither simulation captures the rather distinct tiltof the E layer from the dawn height of 113 km to its dusk height of 103 km. At this time our knowledge of howthe distinct dawn dusk tilt is generated is unknown. The argument can bemade that for the same solar zenithangle and the same irradiance the photoionization should be the same. Hence, assuming that theatmosphere is the same at dawn and dusk, the electron density profiles should be the same. Given that theobservations are not the same, it is probable that the atmospheres or atmospheric dynamics must bedifferent. Similar E region tilts have been reported by Titheridge [2000].

The tilt exhibited by the Arecibo observations is not new. Empirical modeling based on Chapmanphotoionization profiles and comparisons with ionospheric E region climatology led Titheridge [2000] tosuggest that these E layer height differences between morning and evening at the same zenith angle weredue to diurnal changes in neutral composition and temperature. In our modeling the NRLMSISE-00 has notgenerated such a dawn dusk difference in E layer peak height. Another suggested mechanism to explain thistilt is to consider dynamics associated with neutral winds-tides [Chu et al., 2009]. Unfortunately neither theTDIM nor DDDR is able to adequately simulate E region dynamics.

Although no peak or valley characteristics are present in the EDP below the E layer peak, the models andobservations are quite different. At 95 km in Figure 5 the observed noon density is 3 × 1010m�3, while theTDIM and DDDR densities are 7 × 1010 and 8× 1010m�3, respectively. In both models this altitude regime ismost dependent on the 1 to 10 nm wavelength photons. This height is near the lower boundary of the TDIMas well as being an intermediate height between the E layer and D layer below.

In both the observations and simulations a distinct E layer peak and a valley above this peak are evident.Figures 6 and 7 respectively show the density (bottom) and altitude (top) of the E layer peak and valley. TheE region peak altitude was obtained by finding the altitude of themaximum density between 100 and 120 kmwhile the valley altitude was obtained by finding the altitude of the minimum density between 110 and140 km. The density of the E layer peak, Figure 6 (bottom), is well reproduced by the DDDR simulations (longdashed line); however, neither model captures the layer height or its downward trend (Figure 6, top). The twosimulations have noon (12:00 SLT) heights of 105–106 km whereas the observed noon height is 108–109 km.

Figure 5. Comparison between observations and simulations of the day-side E and F1 layers of the ionosphere at Arecibo on 9 February 2012. The(left) TDIM, (middle) ISR observations, and (right) DDDR electron densityare color coded on a linear scale. Note the TDIM has a lower boundary at88 km, and the DDDR has an upper boundary at 130km.

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At dawn the height discrepancy is larger,between 8 and 10 km. Toward dusk theArecibo observations indicate the presenceof a descending layer between 14:30 and15:30 SLT that merges with the simulatedheights. After 15:30 SLT the Areciboobservations are about 4 km higher thanthe simulations. Overall the majordiscrepancies between E region peaksimulations and observations are (i) that thesimulated peak heights are too low and (ii)the dawn to dusk lowering trend beingobserved is not captured in the modeling.

In Figure 7 the two panels compareobserved and simulated lowest density andaltitude of the valley region. Goodagreement is obtained for the lowestdensity in the valley in both models(Figure 7, bottom). While the height is wellreproduced by the TDIM (Figure 7, top), theDDDR valley height is significantly higher.However, the DDDR upper boundary liesclose to this altitude, and hence, thisdiscrepancy is not entirely unexpected. Theheight of the observed valley undergoes amarked jump between 14:30 and 15:30 SLTsolid line in Figure 7 (top). This abruptincrease in height is an artifact associatedwith the presence of a second density layerbetween 13:00 and 14:30 SLT that hascaused the valley (lowest density altitude)to move downward. However, at 14:30 SLT

this extra layer has faded allowing the lowest density altitude to move back up to the expected dusk levels.The additional structure is a descending layer associated with metallic ions.

The two model simulations are different from each other and straddle the observed peak density but not theentire observed E region profile. The TDIM is primarily an F region model with sufficient E region chemistry tosimulate the major chemistry reactions, but not all known chemistry. In contrast the DDDR is a D region andlower E region model in which local chemistry including negative ion chemistry processes are dominant.Hence, over the E region neither model is at its strongest in simulating all the processes. The differencesbetween the two models, even though the same irradiances, same atmospheres, and same cross sections areused, indicate the relevance of the other differences in chemistry and physics.

5. Discussion

The ionosphere is also empirically modeled. The internationally recognized empirical model is theInternational Reference Ionosphere (IRI) model [Bilitza and Reinish, 2008, and references therein]. This modelcontains various options one of which provides a different E region representation, the FIRI, a semiempiricalmodel based on rocket-borne radio wave sounding experiments [Friedrich and Torkar, 2001]. The IRI like otherempirical models is a set of functions where coefficients have been chosen to provide best agreement withmeasurements. Both the IRI and the specialized FIRI have been used to create noon profiles corresponding tothe time, date, and solar conditions on 9 February 2012 at Arecibo. These empirical profiles are compared tothe Arecibo ISR observations, and the TDIM and DDDR simulations. In Figure 8 the thick line is the observedionospheric density at noon. The plus symbol and the dashed thick lines respectively show the DDDR and

Figure 6. Quantitative comparison of the observed and simulated Elayer (bottom) peak density and (top) height. The ISR observations(solid line), TDIM simulation (short dash line), and DDDR simulation(long dashed line) are shown from 08:00 to 16:30 SLT.

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TDIM-simulated profiles as shown in theearlier Figures 2–4. The diamond symbol linerepresents the IRI profile while the crosssymbol line represents the FIRI. Over the 100to 125km altitude range that is the E layerand the valley region the density spread isfrom 1.1× 1011m�3 to 1.75×1011m�3. Thiscorresponds to only a± 24% spread aboutthe mean density of 1.43×1011m�3. TheTDIM and DDDR physical models do nothave the same differences trends, i.e., TDIMpeak density is lower (20%) and DDDR peakdensity is higher (6%). This suggests thatthe± 24% range of density differences is notentirely associated with a modeledproduction rate error. Arguably, this is a veryreasonable density spread between twophysics-based and two empirical modelswith observations given that only 1 day ofobservations are available, and the modelsuse climatological representations for theatmosphere that lie within the model range.In this comparison with observations theE layer has been represented by IRI. However,many other data types have been used toprovide E region electron density profileswhich have been compared with a variety ofmodel techniques [Titheridge, 1997; Mooreet al., 2006; Chu et al., 2009;Nicolls et al., 2012;Pavlov and Pavlova, 2013]. However, in allsuch studies obtaining sufficient altituderesolution to enable this study’s model datacomparison is not possible other than asclimatology comparisons.

However, the density is not the mostcompelling aspect of this figure. The profileshapes represent how the solar irradianceinteracts with the neutral atmosphere in theE region on chemistry timescales of seconds.Therefore, the agreement in the profileshape and the peak and valley heights areindicative of how accurately the modelscapture the physics in the two physics-basedmodels. The IRI and FIRI are empirical andhence based on ionospheric densitymeasurements. In fact, both have a peakE region height that agrees well with thatobservation as compared to the two physicsmodels that have peak heights that arelower, DDDR is lower by 7 km, and TDIM islower by 4 km. These vertical distances aresignificant because the local thermospheric-scale height is on this order, 8 km.

Figure 7. Quantitative comparison of the observed and simulated(bottom) lowest density and (top) height of the valley between theE and F1 layers. The line styles are the same as used in Figure 6.

Figure 8. Comparison of noon E region profiles between ISR observa-tion (thick line), DDDR simulation (plus symbols), TDIM simulation(dashed thick line), IRI model (diamond symbols), and the FIRI model(cross symbols) for the Arecibo location.

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From the earlier Figure 6 the differencebetween the modeled and observedpeak heights persists all during daylighthours. This difference provides anopportunity to discuss what aspect ofthe physical models may contribute tothis difference. Additional observationsand model studies need to repeat thisanalysis to verify that indeed thisdifference is repeatable. For the purposeof this discussion a shortcoming in theEVE irradiance spectral resolution can beshown to potentially contribute to themodeled E region profile shape, hencethe altitude of the maximum density.The shortest EVE wavelengthmeasurements, 0.1 to 7 nm, are made byESP-QD as single-bin measurement overthe range. Hence, no spectral resolutionis available. In the earlier sections 2.1and 2.2 an overview of how the EVE

irradiance measurements from ESP-QD, MEGS-A, and MEGS-B are distributed between 0.0 and 105 nm in1 nm bins was described. The assumption being made in order to distribute the ESP single-irradiance valueover the 0 to 7 nm, seven bins, is that the irradiance spectrum is flat. This assumption, given that there is nosolar “flare” dynamics present is probably an acceptable one. However, the question still arises does the Elayer profile shape depend on the irradiance spectral shape over the 0 to 7 nm range?

This question is addressed by a further series of “what if” TDIM scenarios. These tests involve depositing all ofthe ESP irradiance into one of the seven 1 nm bins and rerunning the TDIM. Arguably, none of these newirradiance spectra would exist; however, the simulation will provide insight into how the E region depends onthese wavelengths.

In Figure 9 the noontime Arecibo TDIM simulations of the E region electron density are shown. The thick line isthe profile generated using the reference spectrum as shown earlier in the study (i.e., Figure 3). Then asimulation run in which the entire ESP irradiance is set to zero is shown by the thick dashed line. This provides ameasure of howmuch the longer wavelengths, ~90 to 105nm, contribute to the generation of the E region. Forthis case the E region peak and valley region above the peak has changed markedly with no peak or valley.

The plus symbol profile indicates how the most energetic photons (X-rays) (0 to 1 nm) penetrate well belowthe standard E layer. In this simulation all the ESP irradiance has been deposited in the 0 to 1 nm spectral bin.In contrast, the simulation in which the irradiance is placed in the 1 to 2 nm bin generates a profile (crosssymbol line) that appears similar to the reference profile at the peak, and with low D region densities. Thisdramatic difference between the two adjacent irradiance wavelength bins arises from the marked differencein the ionization cross sections. In this part of the spectrum there are ionization cross-section structures [Sojkaet al., 2013]. The enhanced cross section for N2 in this wavelength band is associated with the Auger process[Richards and Peterson, 2008; Richards et al., 2006]. The Auger electrons have lower energy thanphotoelectrons created from the same photon wavelength. This difference is not accounted for in thepresent secondary ionization calculation. The limiting factor is that the ESP-QD measurement is an integralover 0.1 to 7 nm of which only a fraction produces Auger electrons. These lead to over 15 km difference in theatmospheric optical depth; 15 km in turn corresponds to at least two neutral-scale heights in this altituderegion. The dependence upon cross section continues to affect the simulated Ne profiles. The diamondsymbol line (2 to 3 nm bin) has a peak near 112 kmwhile the next wavelength bin, 3 to 4 nm, peak drop downto 100 km. Eventually, the profiles begin to approach the reference profile, bins 4 to 5 and 5 to 6 nm. However,their lower E region and D regions are less dense than the reference. This can be understood when theenergetic photoelectron generation is considered. A photon at 1 nm has an energy of 1241 eV while that at6 nm has an energy of 207 eV. In turn, the secondary ionization produced by these different energetic

Figure 9. E region electron density sensitivity on solar irradiance over the0.1 to 6 nm spectral range. The EVE-generated 1 nm wide spectral bin isshown as the reference profile (thick line). Removing the ESP contributionentirely is shown by the ESP zero profile (dashed thick line). The effects ofadding all the ESP irradiance to only one spectral bin are the other sixprofiles; each is identified in the key.

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photoelectrons is different. The 1 nm photon will produce about 35 further ionizations while that at 6 nm willonly produce 6. In these simulations of the E region ionosphere it is assumed that the mean free path for thephotoelectron is short enough that they lose their energy locally. Hence, between the ionization cross-section dependence on wavelength and the photoelectron wavelength dependence the significant E regionprofile structure differences can be understood. Thus, an outstanding observational need is to observe theirradiance spectrum over the ESP wavelength range.

In this study the emphasis has been on uncertainty in the solar irradiance as a contributing factor to modelfailure to replicate the observed ionospheric profile. Clearly this is overstated; at best the analysis providessensitivity of the profile shape to the solar irradiance. Several other factors can readily be identified thatplausibly apply but that cannot be constrained by observations. These include neutral composition, neutralwinds, photoelectron secondary ionization, shortcomings in the model to simulate processes accurately,chemistry schemes, and cross sections. Our objective is to evaluate howwell EVE has been to provide adequateinformation to fully constrain one of the model drives, perhaps one of the zeroth order drivers. The resultsindicate there is still a need to improve the spectral knowledge between 0.1 and 7nm for E region work.

A final consideration is what significance can be attached to these different profile shapes. These E regionprofiles are mainly responsible for the entire ionospheric Hall conductivity and to a large extent that of thePedersen conductivity. Hall and Pedersen conductivities were calculated for each of the profiles in Figure 9. TheHall conductivity varies by over 25% while the Pedersen varies somewhat less only 7%. Hence, the Pedersen toHall ratio is changing significantly by up to 30%. Significance of these changes lie in plasma electrodynamics inwhich ionospheric currents, electrojets, lie in the E region at low and middle latitudes while manymagnetospheric current systems close in the ionospheric E region [Heelis et al., 2012]. In these electrodynamicscenarios not only are the absolute conductivities important but their ratio is a key to how currents are oriented.

6. Summary

The study has provided insight of shortcomings as well as quantitative evaluation of how well models of theionospheric E region perform. This has been achieved by a campaign in which high spectral and timeresolution observation of the solar irradiance with simultaneous high-resolution altitude and absolutedensity measurements of the E region were made. These provided both highly constrained inputs andoutputs for two physics-based ionospheric models, TDIM and DDDR.

1. Overall the simulated ionospheric E region electron densities were within 25% of the observed.2. The detailed profile shapes of the E region peak and the valley region above were only qualitatively similar.3. Neither model reproduced a distinct dawn to dusk tilt in the E region peak altitude.4. At the ± 25% level the two physics-based models, DDDR, and TDIM, performed as well as the empirical

IRI/FIRI models when driven by the high spectral resolution EVE irradiance.5. A need exists to observe the 0.1 to 7nm irradiance with better than 1nm resolution in order for physics-

based models to capture the E region profile shape, or at least remove this dependence on the currentdifficulty of modeling the observed profile. The study finds that significant changes in this spectral distri-bution of the irradiance will change the E region height by several kilometers.

This research based on a campaign combining both solar irradiance measurements with high-resolution Eregion measurements is a first. The results of this study, especially the discrepancies in modeling, will initiatesubsequent campaigns. An extension of this campaignwill be to include ISRs at other latitudes such asMillstoneHill, Massachusetts; Poker Flat, Alaska; Resolute Bay, Canada; Sondrestrom, Greenland; and the ScandinavianEuropean Incoherent Scatter Scientific Association radars. An objective of this study to adequately specify oneof themain E region driver the solar irradiance has almost been achieved. Other drivers such as the atmosphere,or the atmospheric dynamics dependence of chemistry on temperature, are others that need to be addressed.

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