Flare Irradiance Spectral Model Research Articles

Energetic Electron Flux Predictions in the Near‐Earth Plasma Sheet From Solar Wind Driving

AbstractSuprathermal electrons in the near‐Earth plasma sheet are important for inner magnetosphere considerations. They are the source population for outer radiation belt electrons and they pose risks to geosynchronous satellites through their contribution to surface charging. We use empirical modeling to address relationships between solar driving parameters and plasma sheet electron flux. Using Time History of Events and Macroscale Interactions during Substorms, OMNI, and Flare Irradiance Spectral Model Version 2 data, we develop a neural network model to predict differential electron flux from 0.08 to 93 keV in the plasma sheet, at distances from 6 to 12 RE. Driving parameters include solar wind (SW) density and speed, interplanetary magnetic field (IMF) BZ and BY, solar extreme ultraviolet flux, IMF BZ ultra‐low frequency (ULF) wave power, SW‐magnetosphere coupling functions Pα1 and NXCF, and the 4‐hr time history of these parameters. Our model predicts overall plasma sheet electron flux variations with correlation coefficients of between 0.59 and 0.77, and median symmetric accuracy in the 41%–140% range (depending on energy). We find that short time‐scale electron flux variations are not reproduced using short time‐scale inputs. Using a recently published technique to extract information from neural networks, we determine the most important drivers impacting model prediction are VSW, VBS, and IMF BZ. SW‐magnetosphere coupling functions that include IMF clock angle, IMF BZ ULF wave power, and IMF BY have little impact in our model of electron flux in the near‐Earth plasma sheet. The new model, built directly on differential flux, outperforms an existing model that derives fluxes from plasma moments, with the performance improvement increasing with increasing energy.

Energy dependent response of the dayside Martian ionospheric electrons to solar forcing

We study the response of Auger electrons, He-II photoelectrons, and thermal electrons in the dayside Martian ionosphere to the solar soft X-rays (SXR; 0.1–7 nm), 30.5 nm, and the extreme ultraviolet (EUV; 20–90 nm) irradiances, respectively. For this purpose, the suprathermal electron fluxes measured by the Solar Wind Electron Analyzer (SWEA) and densities of the thermal electrons measured by Langmuir Probe and Waves (LPW) instruments, both on the Mars Atmosphere and Volatile Evolution (MAVEN) mission, have been used. The data used in the present study span from January 2015 (Martian Year (MY) 32, solar longitude, Ls = 263°) to December 2019 (MY 35, Ls = 110°), which falls in the declining phase of solar cycle 24. The solar irradiances are taken from the extreme ultraviolet monitor instrument on MAVEN and also from the Flare Irradiance Spectral Model for Mars. The results of the present study show that the fluxes of the suprathermal electrons and the densities of the thermal electrons are drastically reduced during solar minimum. The Auger electrons show a significant correlation and have an almost linear relationship with the SXR irradiance; both of which are independent of altitude. The response of the He-II photoelectrons to 30.5 nm solar irradiance deviates slightly from the linear relationship, particularly in the altitude range of 225–350 km and the solar zenith angle (SZA) range of 45°-65°. At these altitude and SZA ranges, the thermal electrons show a power law dependence on the EUV irradiance. The responses of the He-II photoelectrons and thermal electrons to their respective solar irradiances decrease on either side of the altitude and SZA range of maximum response (225–350 km and 45°-65°, respectively). The energy dependent response of the electrons to the solar irradiances and their altitude and SZA variation are explained by considering their dependence on electron temperature, and ionization and the neutral heating efficiencies. The responses of the He-II photoelectrons and thermal electrons to their respective solar irradiances decrease near the terminator.

Tests of a New Solar Flare Model Against D and E Region Ionosphere Data

AbstractWe present results from a suite of models designed to simulate solar flare effects on the D and E region of the ionosphere. This suite includes models of the solar spectrum, the ionosphere and of HF radiowave propagation. A central component of this system is the development of photoelectron ionization enhancement factors with higher energy resolution in the soft X‐ray spectral region that can be used to supplement existing ionization schemes currently implemented in upper atmospheric general circulation models. We tested this photoelectron model in the NCAR Thermosphere‐Ionosphere‐Mesosphere‐ Electrodynamics General Circulation Model (TIME‐GCM) and in a photochemical model of the D region. In both cases, we compared predicted flare response using two different input solar flare spectra. One is the Flare Irradiance Spectral Model (FISM) and the other is a physics based model called NRLFLARE. Our predictions for the E region were compared with incoherent scatter radar data and suggest that enhanced flux in the 1–2 nm spectral region, as indicated by NRLFLARE, is important for reproducing the observations. For the D region, we combined our theoretical results for the X1.3 flare of 7 September 2017 with ray tracing calculations that suggest 20–40 db of 6.4 MHz absorption. This agrees with previously published observations and model estimates, all of which suggest greater HF absorption than the operational D region absorption prediction model (swpc.noaa.gov/products/d‐region‐absorption‐predictions‐d‐rap). Finally, our theoretical comparison with previously published empirical models derived from very low frequency data was less clear due, in part, to large differences between the different empirical models.

Modeling the Impact of a Strong X‐Class Solar Flare on the Planetary Ion Composition in Mercury's Magnetosphere

AbstractWe model the impact of an extreme solar flare on the Mg+, Na+, O+ and He+ ion density distribution in Mercury's magnetosphere. The Flare Irradiance Spectral Model of the solar irradiance during the X9.3‐class flare on 6 September 2017 is used as input to the time‐dependent Latmos Ionized Exosphere ion density model. We find that the time‐evolution of the planetary ion distribution differs with respect to energy, location and species. There exist two ion energy populations on the dayside that experience different dynamical evolution. The peak ion density in the nightside plasma sheet is delayed by ∼7–8 min compared to the dayside. The maximum Mg+ density occurs ∼4 min before He+ and O+ in the whole magnetosphere. The time delay between different species does not necessarily occur for solar flares that erupt near the apparent solar limb, where the optical depth is large.

Lunar Photoemission Yields Inferred From ARTEMIS Measurements

AbstractPhotoemission yield (the number of emitted electrons per incoming photon) is one of the fundamental properties of solid materials but is not yet well constrained for the lunar surface for photon energies >∼20 eV. In this study, we constrain this yield for incident photons with energies of ∼10–500 eV with data from the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) mission along with solar irradiance spectra from Version 2 of the Flare Irradiance Spectral Model. We also report the first oxygen Auger electron observations at the Moon by the ARTEMIS spacecraft, which provides a unique feature to identify photoelectrons emitted from the lunar surface. With lunar photoelectron observations identified in both Earth's magnetotail lobes and the solar wind for four selected days, we infer a lower bound of 10−3 in yield for photon energies >∼20 eV. However, our investigation also reveals an uncertainty over ∼4 orders of magnitude in derived yields with a sensitivity study, owing to a poorly constrained photoelectron energy probability function. This uncertainty motivates future experiments on lunar samples to better characterize the lunar surface charging environment.

Model-based reproduction and validation of the total spectra of a\xa0solar flare and their impact on the global environment at the X9.3 event of September 6, 2017

We attempted to reproduce the total electron content (TEC) variation in the Earth's atmosphere from the temporal variation of the solar flare spectrum of the X9.3 flare on September 6, 2017. The flare spectrum from the Flare Irradiance Spectral Model (FISM), and the flare spectrum from the 1D hydrodynamic model, which considers the physics of plasma in the flare loop, are used in the GAIA model, which is a simulation model of the Earth's whole atmosphere and ionosphere, to calculate the TEC difference. We then compared these results with the observed TEC. When we used the FISM flare spectrum, the difference in TEC from the background was in a good agreement with the observation. However, when the flare spectrum of the 1D-hydrodynamic model was used, the result varied depending on the presence or absence of the background. This difference depending on the models is considered to represent which extreme ultraviolet (EUV) radiation is primarily responsible for increasing TEC. From the flare spectrum obtained from these models and the calculation result of TEC fluctuation using GAIA, it is considered that the enhancement in EUV emission by approximately 15–35 nm mainly contributes in increasing TEC rather than that of X-ray emission, which is thought to be mainly responsible for sudden ionospheric disturbance. In addition, from the altitude/wavelength distribution of the ionization rate of Earth's atmosphere by GAIA (Ground-to-topside Atmosphere and Ionosphere model for Aeronomy), it was found that EUV radiation of approximately 15–35 nm affects a wide altitude range of 120–300 km, and TEC enhancement is mainly caused by the ionization of nitrogen molecules.

A New Model for Ionospheric Total Electron Content: The Impact of Solar Flux Proxies and Indices

AbstractWe present a new high‐resolution empirical model for the ionospheric total electron content (TEC). TEC data are obtained from the global navigation satellite system (GNSS) receivers with a 1° × 1° spatial resolution and 5‐min temporal resolution. The linear regression model is developed at 45°N, 0°E for the years 2000–2019 with 30‐min temporal resolution, unprecedented for typical empirical ionospheric models. The model describes dependency of TEC on solar flux, season, geomagnetic activity, and local time. Parameters describing solar and geomagnetic activity are evaluated. In particular, several options for solar flux input to the model are compared, including the 10.7 cm solar radio flux (F10.7), the Mg II core‐to‐wing ratio, and formulations of the solar extreme ultraviolet flux (EUV). Ultimately, the extreme ultraviolet flux presented by the Flare Irradiance Spectral Model, integrated from 0.05 to 105.05 nm, best represents the solar flux input to the model. TEC time delays to this solar parameter on the order of several days as well as seasonal modulation of the solar flux terms are included. The Ap3 index and its history are used to reflect the influence of geomagnetic activity. The root mean squared error of the model (relative to the mean TEC observed in the 30‐min window) is 1.9539 TECu. A validation of this model for the first 3 months of 2020 shows excellent agreement with data. The new model shows significant improvement over the International Reference Ionosphere 2016 (IRI‐2016) when the two are compared during 2008 and 2012.

The Flare Irradiance Spectral Model‐Version 2 (FISM2)

AbstractThe Flare Irradiance Spectral Model (FISM) is an important tool for estimating solar variability for a myriad of space weather research studies and applications, and FISM Version 2 (FISM2) recently was released. FISM2 is an empirical model of the solar ultraviolet irradiance created to fill spectral and temporal gaps in the satellite observations. FISM2 estimates solar ultraviolet irradiance variations due to the solar cycle, solar rotations, and solar flares. The major improvement provided by FISM2 is that it is based on multiple new, more accurate instruments that have now captured almost a full solar cycle and thousands of flares, drastically improving the accuracy of the modeled FISM2 solar irradiance spectra. Specifically, these new instruments are the Solar Dynamics Observatory (SDO)/Extreme Ultraviolet Variability Experiment (EVE), Solar Radiation and Climate Experiment (SORCE)/X‐ray Photometer System (XPS), and SORCE/Solar Stellar Irradiance Comparison Experiment (SOLSTICE). FISM2 is also improved to 0.1‐nm spectral bins across the same 0‐ to 190‐nm spectral range and is already being used in research to estimate space weather changes due to solar irradiance variability in planetary thermospheres and ionospheres.

Nowcast of an EUV dynamic spectrum during solar flares

In addition to X-rays, extreme ultraviolet (EUV) rays radiated from solar flares can cause serious problems, such as communication failures and satellite drag. Therefore, methods for forecasting EUV dynamic spectra during flares are urgently required. Recently, however, owing to the lack of instruments, EUV dynamic spectra have rarely been observed. Hence, we develop a new method that converts the soft X-ray light curve observed during large flare events into an EUV dynamic spectrum by using the Solar Dynamics Observatory/Atmospheric Imaging Assembly images, a numerical simulation, and atomic database. The simulation provides the solution for a coronal loop that is heated by a strong flare, and the atomic database calculates its dynamic spectrum, including X-ray and EUV irradiances. The coefficients needed for the conversion can be calculated by comparing the observed soft X-ray light curve with that of the simulation. We apply our new method to three flares that occurred in the active region 12673 on September 06, 2017. The results show similarities to those of the Flare Irradiance Spectral Model, and reconstruct some of the EUV peaks observed by the EUV Variability Experiment onboard the Solar Dynamics Observatory.

Solar flares observed by Rosetta at comet 67P/Churyumov-Gerasimenko

Context. The Rosetta spacecraft made continuous measurements of the coma of comet 67P/Churyumov-Gerasimenko (67P) for more than two years. The plasma in the coma appeared very dynamic, and many factors control its variability. Aims. We wish to identify the effects of solar flares on the comet plasma and also their effect on the measurements by the Langmuir Probe Instrument (LAP). Methods. To identify the effects of flares, we proceeded from an existing flare catalog of Earth-directed solar flares, from which a new list was created that only included Rosetta-directed flares. We also used measurements of flares at Mars when at similar longitudes as Rosetta. The flare irradiance spectral model (FISM v.1) and its Mars equivalent (FISM-M) produce an extreme-ultraviolet (EUV) irradiance (10–120 nm) of the flares at 1 min resolution. LAP data and density measurements obtained with the Mutual Impedence Probe (MIP) from the time of arrival of the flares at Rosetta were examined to determine the flare effects. Results. From the vantage point of Earth, 1504 flares directed toward Rosetta occurred during the mission. In only 24 of these, that is, 1.6%, was the increase in EUV irradiance large enough to cause an observable effect in LAP data. Twenty-four Mars-directed flares were also observed in Rosetta data. The effect of the flares was to increase the photoelectron current by typically 1–5 nA. We find little evidence that the solar flares increase the plasma density, at least not above the background variability. Conclusions. Solar flares have a small effect on the photoelectron current of the LAP instrument, and they are not significant in comparison to other factors that control the plasma density in the coma. The photoelectron current can only be used for flare detection during periods of calm plasma conditions.

Effects of the September 2005 Solar Flares and Solar Proton Events on the Middle Atmosphere in WACCM

AbstractThis work investigates middle atmosphere effects of the September 2005 solar flares and solar proton events (SPEs). X‐17 and X‐6.2 flares occurred on 7 and 9 September, respectively, while two moderate SPEs occurred on 10 and 15 September. Flare ionization and dissociation were calculated in the Whole Atmosphere Community Climate Model (WACCM) using the Flare Irradiance Spectral Model. Proton measurements from the Geostationary Operational Environmental Satellite system were used to compute solar proton ionization. SPEs are shown to have a larger impact than solar flares on the polar stratosphere and mesosphere; however, flares have a larger influence on the sunlit and equatorial lower thermosphere. The two flares differed significantly with respect to photon spectrum. The larger, X‐17 flare was stronger during the impulsive phase, while the X‐6.2 flare was stronger during the gradual phase. This resulted in the X‐17 flare causing more initial ionization but for a shorter duration. The simulated flare impacts also differed because specific wavelengths of the flares influenced the atmosphere above the model top. Model‐measurement comparisons show that WACCM captures the overall timing and spatial distribution of the observed electron enhancements, indicating a reasonable simulation of flare and SPE‐induced ionization. Both the SPEs and flares caused odd nitrogen increases in the mesosphere. Odd hydrogen produced in the lower mesosphere by the SPEs led to short‐lived ozone decreases of nearly 100%. The flares caused small temperature increases in the lower thermosphere but had no effect on the stratosphere.

Photoemission and electrostatic potentials on the dayside lunar surface in the terrestrial magnetotail lobes.

Despite the need to accurately predict and assess the lunar electrostatic environment in all ambient conditions that the Moon encounters, photoemission and electrostatic potentials on the dayside lunar surface in the terrestrial magnetotail lobes remain poorly characterized. We study characteristics and variabilities of lunar photoelectron energy spectra by utilizing Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) and Apollo measurements in combination with the Flare Irradiance Spectral Model (FISM). We confirm that the photoelectron spectral shapes are consistent between ARTEMIS and Apollo and that the photoelectron flux is linearly correlated with the FISM solar photon flux. We develop an observation-based model of lunar photoelectron energy distributions, thereby deriving the current balance surface potential. The model predicts that dayside lunar surface potentials in the tail lobes (typically tens of volts) could increase by a factor of 2 - 3 during strong solar flares.

The MAVEN EUVM model of solar spectral irradiance variability at Mars: Algorithms and results

AbstractSolar extreme ultraviolet (EUV) radiation is a primary energy input to the Mars atmosphere, causing ionization and driving photochemical processes above approximately 100 km. Because solar EUV radiation varies with wavelength and time, measurements must be spectrally resolved to accurately quantify its impact on the Mars atmosphere. The Mars Atmosphere and Volatile EvolutioN (MAVEN) EUV Monitor (EUVM) measures solar EUV irradiance incident on the Mars atmosphere in three bands. These three bands drive a spectral irradiance variability model called the Flare Irradiance Spectral Model (FISM)‐Mars (FISM‐M) which is an iteration of the FISM model by Chamberlin et al. (2007, 2008) for spectral irradiance at Earth. In this paper, we report the algorithms used to derive FISM‐M and its associated uncertainties, focusing on differences from the original FISM. FISM‐M spectrally resolves the solar EUV irradiance at Mars from 0.5 to 189.5 nm at 1min cadence, and 0.1 nm resolution in the 6–106 nm range or 1 nm resolution otherwise. FISM‐M is suitable for both daily average and flaring spectral irradiance estimates and is based on the linear association of the broadband EUVM measurements with spectral irradiance measurements, including recent high time cadence 0.1 nm resolution measurements from the EUV Variability Experiment (EVE) on the Space Dynamics Observatory (SDO) between 6 and 106 nm. In addition, we present examples of model outputs for EUV irradiance variability due to solar flares, solar rotations, Mars orbit eccentricity, and the solar cycle, between October 2015 and November 2016.

Development of a data-verified ionospheric model with an ionosonde network

As a base model for space weather application, we develop an ionospheric model (SAMI2-CNU) by revising the open source SAMI2 model. The revision includes the photoionization of X-rays in the range below 50 A and photoelectron impact ionization. As a solar flux input to the model, we utilize the real-time solar extreme ultraviolet (EUV) and X-ray (1 ~ 105.0 nm) fluxes of the FISM (flare irradiance spectral model). In order to verify the SAMI2-CNU model, we compare the peak densities of the E and the F2 layers with those from the GIRO (Global Ionospheric Radio Observatory) network for 110 days with the F10.7 range of 70 - 280 in the last solar cycle. The model provides larger values of NmF2 by 30 - 40%, but the NmE values are reasonably close to the ionosonde values. By carrying out a linear correlation analysis between the model and the ionosonde values, we derived scale factors of 0.67 and 0.41 for the photoionization rates in the F2 and the E regions, respectively. With these scale factors, the SAMI2-CNU model allows us to match the ionosonde NmF2 and NmE values within 7% on the average. We also simulated ionospheric changes during eight solar flare events and found averaged 20% and 46% enhancements of NmF2 and NmE, respectively, over the pre-flare values. Although the SAMI2-CNU model has the same problem with matching the NmF2 enhancement as the original SAMI2, it provides reasonable estimates of the NmE increase, as well as reasonable matches of the NmF2 and the NmE values with the ionosonde values. Thus, the SAMI2-CNU model can be used as a physics model in the data-assimilated model that is being developed for the regional ionosphere around the Korea peninsula.

Solar Spectral Proxy Irradiance from GOES (SSPRING): a model for solar EUV irradiance

Several currently operating instruments are able to measure the full EUV spectrum at sufficient wavelength resolution for use in upper-atmosphere modeling, the effects of space weather, and modeling satellite drag. However, no missions are planned at present to succeed the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) and Solar Dynamics Observatory (SDO) missions, which currently provide these data sources. To develop a suitable replacement for these measurements, we use two broadband EUV channels on the NOAA GOES satellites, the magnesium core-to-wing ratio (Mg II index) from the SOlar Radiation and Climate Experiment (SORCE) as well as EUV and Mg II time averages to model the EUV spectrum from 0.1 to 105 nm at 5-nm spectral resolution and daily time resolution. A Levenberg-Marquardt least squares fitting algorithm is used to determine a coefficient matrix that best reproduces a reference data set when multiplied by input data. The coefficient matrix is then applied to model data outside of the fitting interval. Three different fitting intervals are tested, with a variable fitting interval utilizing all days of data before the prediction date producing the best results. The correlation between the model results and the observed spectrum is found to be above 95% for the 0.1–50 nm range, and between 74% and 95% for the 50–105 nm range. We also find a favorable comparison between our results and the Flare Irradiance Spectral Model (FISM). These results provide a promising potential source for an empirical EUV spectral model after direct EUV measurements are no longer available, and utilize a similar EUV modeling technique as the upcoming GOES-R satellites.

Comparison of different solar irradiance models for the superthermal electron transport model for Mars

As the solar photon fluxes directly control the production of photoelectrons, it is important to examine the influence of different solar irradiance models on the photoelectron fluxes. In this study, we present the implementation of the two recent solar irradiance models, the Flare Irradiance Spectral Model (FISM) and the Heliospheric Environment Solar Spectral Radiation (HESSR) model, to our SuperThermal Electron Transport (STET) model. In addition, we have proposed a new modification to the Hinteregger-81 model. The resultant photoelectron fluxes from the three solar irradiance models, the Hinteregger-81 model, FISM and the HESSR model, are compared and mostly vary within a factor of 2.

MEASUREMENTS AND MODELING OF TOTAL SOLAR IRRADIANCE IN X-CLASS SOLAR FLARES

The Total Irradiance Monitor (TIM) from NASA's SOlar Radiation and Climate Experiment (SORCE) can detect changes in the Total Solar Irradiance (TSI) to a precision of 2 ppm, allowing observations of variations due to the largest X-Class solar ares for the first time. Presented here is a robust algorithm for determining the radiative output in the TIM TSI measurements, in both the impulsive and gradual phases, for the four solar ares presented in Woods et al. (2006), as well as an additional are measured on 2006 December 6. The radiative outputs for both phases of these five ares are then compared to the Vacuum Ultraviolet (VUV) irradiance output from the Flare Irradiance Spectral Model (FISM) in order to derive an empirical relationship between the FISM VUV model and the TIM TSI data output to estimate the TSI radiative output for eight other X-Class ares. This model provides the basis for the bolometric energy estimates for the solar ares analyzed in the Emslie et al. (2012) study.

Wavelength dependence of solar irradiance enhancement during X-class flares and its influence on the upper atmosphere

The wavelength dependence of solar irradiance enhancement during flare events is one of the important factors in determining how the Thermosphere–Ionosphere (T–I) system responds to flares. To investigate the wavelength dependence of flare enhancement, the Flare Irradiance Spectral Model (FISM) was run for 61 X-class flares. The absolute and the percentage increases of solar irradiance at flare peaks, compared to pre-flare conditions, have clear wavelength dependences. The 0–14nm irradiance increases much more (~680% on average) than that in the 14–25nm waveband (~65% on average), except at 24nm (~220%). The average percentage increases for the 25–105nm and 122–190nm wavebands are ~120% and ~35%, respectively. The influence of 6 different wavebands (0–14nm, 14–25nm, 25–105nm, 105–120nm, 121.56nm, and 122–175nm) on the thermosphere was examined for the October 28th, 2003 flare (X17-class) event by coupling FISM with the National Center for Atmospheric Research (NCAR) Thermosphere–Ionosphere-Electrodynamics General Circulation Model (TIE-GCM) under geomagnetically quiet conditions (Kp=1). While the enhancement in the 0–14nm waveband caused the largest enhancement of the globally integrated solar heating, the impact of solar irradiance enhancement on the thermosphere at 400km is largest for the 25–105nm waveband (EUV), which accounts for about 33K of the total 45K temperature enhancement, and ~7.4% of the total ~11.5% neutral density enhancement. The effect of 122–175nm flare radiation on the thermosphere is rather small. The study also illustrates that the high-altitude thermospheric response to the flare radiation at 0–175nm is almost a linear combination of the responses to the individual wavebands. The upper thermospheric temperature and density enhancements peaked 3–5h after the maximum flare radiation.

Flare location on the solar disk: Modeling the thermosphere and ionosphere response

Solar flare enhancements to the soft X‐ray (XUV) and extreme ultraviolet (EUV) spectral irradiance depend on the location of the flare on the solar disk. Most emission lines in the XUV region (∼0.1 to ∼25 nm) are optically thin and are weakly dependent on the location of the flare, but in the EUV region (∼25 to ∼120 nm), many important lines and continua are optically thick, so enhancements are relatively smaller for flares located near the solar limb, due to absorption by the solar atmosphere. The flare irradiance spectral model (FISM) was used to illustrate these location effects, assuming two X17 flares that are identical except that one occurs near disk center and the other near the limb. FISM spectra of these two flares were used as solar input to the National Center for Atmospheric Research (NCAR) thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model (TIME‐GCM) to investigate the ionosphere/thermosphere (I/T) response. Model simulations showed that in the E region ionosphere, where XUV dominates ionization, flare location does not affect I/T response. However, flare‐driven changes in the F region ionosphere, total electron content (TEC), and neutral density in the upper thermosphere, are 2–3 times stronger for a disk‐center flare than for a limb flare, due to the importance of EUV enhancement. Flare location did not affect the timing of the ionospheric response, but the thermospheric response was ∼20 min faster for the disk‐center flare. Model simulations of I/T responses to an X17 flare on 28 October 2003 were consistent with measurements of TEC and neutral density changes.

Photoelectrons as a tool to evaluate spectral variations in solar EUV irradiance over solar cycle timescales

There is limited information about the relative magnitude of the spectral variations in the ionizing component of solar irradiance on solar cycle timescales. We found that the Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED)/Solar Extreme Ultraviolet Experiment (SEE) Version 9 irradiance values predict relatively more ionospheric heating at solar minimum than those from Version 8. These changes have direct impacts on solar cycle timescale variations in ionospheric and thermospheric energy inputs derived from them. Photoelectron observations from the Fast Auroral Snapshot (FAST) satellite obtained from 2002 to 2008 are used with solar irradiance data, photoelectron flux models, and models of solar irradiance to examine the solar cycle variations of irradiance in the 4–27 nm range derived from the XPS sensor in the TIMED/SEE instrument suite. Good (±50%) agreement is found between daily photoelectron observations and model predictions. The largest differences between observed and modeled fluxes are in the 4–10 nm range, where the Fast Auroral Snapshot data show that the SEE Version 9 irradiances are systematically low. Our analysis suggests that variation on solar cycle timescales in the TIMED/SEE Version 9 and Flare Irradiance Spectral Model irradiance derived from them are systematically low in the 18–27 nm region. Because of uncertainties in the absolute value of the observed photoelectron fluxes and solar irradiances, differences between observed and modeled photoelectron fluxes are not sufficient to determine more exactly the magnitude of variation on solar cycle timescales of solar irradiance in the 4–27 nm region. These suggestions can be confirmed by higher spectral resolution observations that will be made on the Solar Dynamics Observatory mission.