[Show abstract][Hide abstract] ABSTRACT: Rapid specification of ionization rates and ion densities in the upper atmosphere is essential when many evaluations of the atmospheric state must be performed, as in global studies or analyses of on-orbit satellite data. Though many models of the upper atmosphere perform the necessary specification, none provide the flexibility of computational efficiency, high accuracy, and complete specification. We introduce a parameterized, updated, and extended version of the GLobal AirglOW (GLOW) model, called GLOWfast, that significantly reduces computation time and provides comparable accuracy in upper atmospheric ionization, densities, and conductivity. We extend GLOW capabilities by (1) implementing the nitric oxide empirical model, (2) providing a new model component to calculate height-dependent conductivity profiles from first principles for the 80–200 km region, and (3) reducing computation time. The computational improvement is achieved by replacing the full, two-stream electron transport algorithm with two parameterizations: (1) photoionization (QRJ from Solomon and Qian (2005)) and (2) electron impact ionization (F0810 from Fang et al. (2008, 2010)). We find that GLOWfast accurately reproduces ionization rates, ion and electron densities, and Pedersen and Hall conductivities independent of the background atmospheric state and input solar and auroral activity. Our results suggest that GLOWfast may be even more appropriate for low characteristic energy auroral conditions. We demonstrate in a suite of 3028 case studies that GLOWfast can be used to rapidly calculate the ionization of the upper atmosphere with few limitations on background and input conditions. We support these results through comparisons with electron density profiles from COSMIC.
Journal of Geophysical Research: Space Physics 05/2015; 120(6):n/a-n/a. DOI:10.1002/2015JA021146 · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Model simulations of temperature and density trends in the upper thermosphere are generally consistent with satellite drag data, but some discrepancies remain. The most important of these is that satellite drag analyses under solar minimum conditions have measured density change of about −5% per decade near 400 km altitude, while model simulations of upper atmosphere cooling due to anthropogenic increases in carbon dioxide and other trace gases have predicted about half that rate. For solar moderate and maximum conditions, agreement is better. The rate of change is less during higher solar activity, because higher levels of nitric oxide cooling compete with the anthropogenic cooling. However, some past modeling studies used global mean models, and others attempted to scale to decadal rates from scenarios where carbon dioxide was doubled. Both of these approaches have shortcomings. Therefore, we have performed new, fully 3D simulations, using the NCAR Thermosphere-Ionosphere-Mesosphere Electrodynamics General Circulation Model (TIME-GCM), to better quantify secular change rates at various levels of solar activity. These simulations use a twelve-year baseline (approximately one solar cycle) in order to more directly compare with measured rates. Our new findings are in better agreement with observations for solar minimum conditions, approximately −5% per decade at 400 km, and are also still in reasonable agreement at solar maximum, approximately −2% per decade. This confluence of observation and simulation strengthen the case that some of the best evidence of the impact of anthropogenic global change on the upper atmosphere is the continued systematic decrease of thermospheric density.
Journal of Geophysical Research: Space Physics 02/2015; 120(3). DOI:10.1002/2014JA020886 · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A newly implemented helium module in The National Center for Atmospheric Research-Thermosphere Ionosphere Electrodynamics General Circulation Model (NCAR-TIEGCM) offers the first opportunity in three decades to describe helium behavior in the context of a first principles, self-consistent model, and to test early theories of wintertime helium bulge formation. This study shows general agreement with the findings of Reber and Hays  but articulates the definitive role of vertical advection in the bulge formation. Our findings indicate vertical advection and molecular diffusion are the dominate processes responsible for the solstice helium distribution. Horizontal winds indirectly contribute to the helium bulge formation by their divergent wind field that leads to vertical winds in order to maintain thermosphere mass continuity. As a minor gas, thermospheric helium does not contribute to mass continuity and its distribution is dictated by more local interactions and constraints.
[Show abstract][Hide abstract] ABSTRACT: The modulation of geomagnetic activity on the equatorial thermosphere anomaly (ETA) in thermospheric temperature under the high solar activity condition is investigated using the Thermosphere Ionosphere Electrodynamics General Circulation (TIEGCM) simulations. The model simulations during the geomagnetically disturbed interval, when the north-south component of the interplanetary magnetic field (Bz) oscillates between southward and northward directions, are analyzed and also compared with those under the quiet time condition. Our results show that ionospheric electron densities increase greatly in the equatorial ionization anomaly (EIA) crest region and decrease around the magnetic equator during the storm time, resulting from the enhanced eastward electric fields. The impact of both the direct heat deposition at high latitudes and the modulation of the storm-time enhanced EIA crests on the ETA are subsequently studied. The increased plasma densities over the EIA crest region enhance the field-aligned ion drag that accelerates the poleward meridional winds and consequently their associated adiabatic cooling effect. This process alone produces a deeper temperature trough over the magnetic equator as a result of the enhanced divergence of meridional winds. Moreover, the enhanced plasma-neutral collisional heating at higher latitudes associated with the ionospheric positive storm effect causes a weak increase of the ETA crests. On the other hand, strong changes of the neutral temperature are mainly confined to higher latitudes. Nevertheless, the changes of the ETA purely due to the increased plasma density are overwhelmed by those associated with the storm-time heat deposition, which is the major cause of an overall elevated temperature in both the ETA crests and trough during the geomagnetically active period. Associated with the enhanced neutral temperature at high latitudes due to the heat deposition, the ETA crest-trough differences become larger under the minor geomagnetic activity condition than under the quiet time condition. However, when geomagnetic activity is further elevated, the ETA crests tend to be masked by high temperatures at middle and high latitudes.
Journal of Geophysical Research: Space Physics 08/2014; 119(8). DOI:10.1002/2014JA020152 · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This study considers whether spikes in nitrate in snow sampled at Summit, Greenland from August 2000 to August 2002 are related to solar proton events. After identifying tropospheric sources of nitrate on the basis of correlations with sulfate, ammonium, sodium, and calcium, we use the three-dimensional global Whole Atmosphere Community Climate Model (WACCM) to examine unaccounted for nitrate spikes. Model calculations confirm that solar proton events significantly impact HOx, NOx, and O3 levels in the mesosphere and stratosphere during the weeks and months following the major 9 November 2000 solar proton event. However, SPE-enhanced NOy calculated within the atmospheric column is too small to account for the observed nitrate peaks in surface snow. Instead, our WACCM results suggest that nitrate spikes not readily accounted for by measurement correlations are likely of anthropogenic origin. These results, consistent with other recent studies, imply that nitrate spikes in ice cores are not suitable proxies for individual SPEs and motivate the need to identify alternative proxies.
[Show abstract][Hide abstract] ABSTRACT: The CHAMP satellite has revealed new observations of the upper F region ionosphere and the thermosphere. This chapter reviews these new findings at low and equatorial latitude, from the point of view of coupling between the atmosphere and the magnetic field and between different atmospheric regions. The chapter deals with the gross features in the electron density and temperature and in the neutral density and wind. These features include the equatorial anomaly, the electron temperature morning overshoot, equatorial plasma irregularities, the zonal wind jet, terminator waves, or findings of the wave-4 structure in plasma and neutrals. Large-scale thermospheric features revealed by CHAMP are described from three perspectives. They are the thermosphere response to solar forcing, to ionospheric forcing, and to lower atmosphere forcing.
[Show abstract][Hide abstract] ABSTRACT:  The total electron content (TEC) data measured by the Jason, CHAMP, GRACE and SAC-C satellites, the in-situ electron densities from CHAMP and GRACE, and the vertical E × B drifts from the ROCSAT satellite have been utilized to examine the ionospheric response to the October 2003 superstorms. The combination of observations from multiple satellites provides a unique global view of ionospheric storm effects, especially over the Pacific Ocean and American regions, which were under sunlit conditions during the main phases of the October 2003 superstorms. The main results of this study are: 1) There were substantial increases in TEC in the daytime at low and middle latitudes during both superstorms; 2) The enhancements were greater during the October 30 superstorm and occurred over a wider range of local times; 3) They also tended to peak at earlier local times during this second event; 4) These TEC enhancement events occurred at the local times when there were enhancements in the upward vertical drift; 5) The strong upward vertical drifts are attributed to penetration electric fields, suggesting that these penetration electric fields played a significant role in the electron density enhancements during these superstorms. Overall, the main contribution of this study is the simultaneous view of the storm-time ionospheric response from multiple satellites, and the association of local time differences in ionospheric plasma response with measured vertical drift variations.
Journal of Geophysical Research: Space Physics 03/2014; DOI:10.1002/2013JA019575 · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT:  The last solar minimum period was anomalously extended and low in EUV irradiance compared with previous solar minima. It can readily be expected that the thermosphere and ionosphere must be correspondingly affected by this low solar activity. While there have been unanimous reports on the thermospheric changes, being cooler and lower in its density as expected, the ionospheric responses to low solar activity in previous studies were not consistent with each other, probably due to the limited ionospheric observations used for them. In this study, we utilized the measurements of total electron content (TEC) from TOPEX and JASON-1 satellites during the periods of 1992 to 2010, which includes both the last two solar minimum periods, in order to investigate how the ionosphere responded to the extremely low solar activity during the last solar minimum compared with previous solar minimum. Although the global daily mean TECs show negligible differences between the two solar minimum periods, the global TEC maps reveal that there are significant systematic differences ranging from about -30% to +50% depending on local time, latitude and season. The systematic variations of the ionospheric responses seem to mainly result from the relative effects of reduced solar EUV production and reduced recombination rate due to thermospheric changes during the last solar minimum period.
Journal of Geophysical Research: Space Physics 03/2014; 119(3). DOI:10.1002/2013JA019407 · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The solar minimum period between solar cycles 23 and 24 was the longest since the beginning of space-based measurements, and many manifestations of solar activity were unusually low. Thermospheric neutral density was about 30% lower than during the previous solar minimum, but changes in the ionosphere between the two solar minima are more controversial. Solar radiation, geomagnetic activity, and anthropogenic increases in greenhouse gases, can all play a role in these changes. In this paper, we address the latter of these potential contributions, the degree to which secular change driven by greenhouse gases, primarily CO2, could be responsible for the observed changes. New 3D model simulations find a global mean density decrease at 400 km of 5.8% between the two recent solar minima, which is larger than earlier 1D model results, and in better agreement with observations. From these model simulations and from other observational work, we estimate that the contribution of secular change to global mean neutral density decrease between the two recent solar minima is less than ~6%. The contribution of secular change to the global average decrease of F-region ionosphere peak density (NmF2) and altitude (hmF2), near mid-day, is estimated to be 1.5% and 1.5 km, respectively. However, secular changes in the ionosphere exhibit large variations with local time, geographic location, and season. The mid-day change of NmF2 seen in the model simulations ranged between +6% and -9%, and the change of hmF2 ranged between +11 km and -11 km, depending on geographic location.
Journal of Geophysical Research: Space Physics 03/2014; 119(3). DOI:10.1002/2013JA019438 · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT:  The solar minimum period during 2008–2009 was characterized by lower thermospheric density than the previous solar minimum and lower than any previously measured. Recent work used the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model to show that the primary cause of density changes from 1996 to 2008 was a small reduction in solar extreme ultraviolet (EUV) irradiance, causing a decrease in thermospheric temperature and hence a contracted thermosphere. There are similar effects in the ionosphere, with most measurements showing an F region ionosphere that is unusually low in density, and in peak altitude. This paper addresses the question of whether model simulations previously conducted, and their solar, geomagnetic, and anthropogenic inputs, produce ionospheric changes commensurate with observations. We conducted a 15 year model run and obtained good agreement with observations of the global mean thermospheric density at 400 km throughout the solar cycle, with a reduction of ~30% from the 1996 solar minimum to 2008–2009. We then compared ionosonde measurements of the midday peak density of the ionospheric F region (NmF2) to the model simulations at various locations. Reasonable agreement was obtained between measurements and the model, supporting the validity of the neutral density comparisons. The global average NmF2 was estimated to have declined between the two solar minima by ~15%. In these simulations, a 10% reduction of solar EUV plays the largest role in causing the ionospheric change, with a minor contribution from lower geomagnetic activity and a very small additional effect from anthropogenic increase in CO2.
Journal of Geophysical Research: Space Physics 10/2013; 118(10). DOI:10.1002/jgra.50561 · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT:  We conducted model simulations to examine how changes in concentration of radiatively active trace gases in the middle atmosphere affect long-term changes in the upper atmosphere. We focused our model study on the impact of increases in carbon dioxide (CO2), methane (CH4), and water vapor (H2O), and decreases in ozone (O3) between 1983 and 2003. We used both the National Center for Atmospheric Research Whole Atmosphere Community Climate Model and the Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model, global mean version, in this study. The model simulations indicate that CO2 is the main forcing mechanism of long-term changes in the thermsophere, with minor influences from O3, CH4, and H2O. At 400 km altitude, global mean thermospheric neutral density decreased by ~4.5% due to CO2 forcing alone, whereas it decreased by ~4.8% due to the combined forcing from all four gases. O3 depletion caused cooling in the stratosphere and mesosphere (maximum decrease of 0.5 K) due to reduced absorption of solar ultraviolet radiation, but had nearly no cooling effect in the thermosphere. However, due to thermal contraction in the stratosphere and mesosphere, O3 depletion caused a small decrease in thermospheric neutral density of ~0.25%. Increases in both CH4 and H2O may slightly warm the upper mesosphere and thermosphere due to increased chemical heating and absorption of solar ultraviolet radiation.
Journal of Geophysical Research: Space Physics 06/2013; 118(6). DOI:10.1002/jgra.50354 · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We investigated the relationship between the systematic
annual and semiannual variations in the ionosphere and
thermosphere using a combination of data analysis and
model simulation. A climatology of daytime peak density
and height of the ionospheric F2 layer was obtained from
GPS radio occultation measurements by the Constellation
Observing System for Meteorology, Ionosphere, and Climate
(COSMIC) during 2007–2010. These measurements were
compared to simulations by the NCAR Thermosphere-
Ionosphere-Electrodynamics General Circulation Model
(TIE-GCM). Model reproduction of the ionospheric annual
and semiannual variations was significantly improved
by imposing seasonal variation of eddy diffusion at the
lower boundary, which also improves agreement with
thermospheric density measurements. Since changes in
turbulent mixing affect both the thermosphere and
ionosphere by altering the proportion of atomic and
molecular gases, these results support the proposition that
composition change drives the annual/semiannual variation
in both the neutral and ionized components of the coupled
Geophysical Research Letters 05/2013; 40(1-6). DOI:10.1002/grl.50448 · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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 0–14 nm irradiance increases much more (~680% on average) than that in the 14–25 nm waveband (~65% on average), except at 24 nm (~220%). The average percentage increases for the 25–105 nm and 122–190 nm wavebands are ~120% and ~35%, respectively. The influence of 6 different wavebands (0–14 nm, 14–25 nm, 25–105 nm, 105–120 nm, 121.56 nm, and 122–175 nm) 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 0–14 nm waveband caused the largest enhancement of the globally integrated solar heating, the impact of solar irradiance enhancement on the thermosphere at 400 km is largest for the 25–105 nm waveband (EUV), which accounts for about 33 K of the total 45 K temperature enhancement, and ~7.4% of the total ~11.5% neutral density enhancement. The effect of 122–175 nm flare radiation on the thermosphere is rather small. The study also illustrates that the high-altitude thermospheric response to the flare radiation at 0–175 nm is almost a linear combination of the responses to the individual wavebands. The upper thermospheric temperature and density enhancements peaked 3–5 h after the maximum flare radiation.
Journal of Atmospheric and Solar-Terrestrial Physics 01/2013; 115. DOI:10.1016/j.jastp.2013.10.011 · 1.47 Impact Factor
[Show abstract][Hide abstract] ABSTRACT:  We report preliminary results of a global 3-D ionospheric electron density reanalysis demonstration study during 2002-2011 based on multisource data assimilation. The monthly global ionospheric electron density reanalysis has been done by assimilating the quiet days ionospheric data into a data assimilation model constructed using the International Reference Ionosphere (IRI) 2007 model and a Kalman filter technique. These data include global navigation satellite system (GNSS) observations of ionospheric total electron content (TEC) from ground-based stations, ionospheric radio occultations by CHAMP, GRACE, COSMIC, SAC-C, Metop-A, and the TerraSAR-X satellites, and Jason-1 and 2 altimeter TEC measurements. The output of the reanalysis are 3-D gridded ionospheric electron densities with temporal and spatial resolutions of 1 h in universal time, 5° in latitude, 10° in longitude, and ∼30 km in altitude. The climatological features of the reanalysis results, such as solar activity dependence, seasonal variations, and the global morphology of the ionosphere, agree well with those in the empirical models and observations. The global electron content derived from the international GNSS service global ionospheric maps, the observed electron density profiles from the Poker Flat Incoherent Scatter Radar during 2007-2010, and foF2 observed by the global ionosonde network during 2002-2011 are used to validate the reanalysis method. All comparisons show that the reanalysis have smaller deviations and biases than the IRI-2007 predictions. Especially after April 2006 when the six COSMIC satellites were launched, the reanalysis shows significant improvement over the IRI predictions. The obvious overestimation of the low-latitude ionospheric F region densities by the IRI model during the 23/24 solar minimum is corrected well by the reanalysis. The potential application and improvements of the reanalysis are also discussed.
Journal of Geophysical Research Atmospheres 09/2012; 117(A9). DOI:10.1029/2012JA017968 · 3.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Changes in the thermosphere-ionosphere system caused by high-speed streams in the solar wind, and the co-rotating interaction regions they engender, are studied using a combination of model simulations and data analysis. The magnetospheric responses to these structures and consequent ionospheric drivers are simulated using the numerical Coupled Magnetosphere-Ionosphere-Thermosphere model and the empirical Weimer 2005 model, finding that the interplanetary magnetic field (IMF) is more important than solar wind speed and density per se in controlling magnetosphere-ionosphere coupling. The NCAR Thermosphere- Ionosphere-Electrodynamics General Circulation Model is then employed to calculate neutral density, nitric oxide cooling, and electron density, for comparison to space-based measurements from the STAR instrument on the CHAMP satellite, the SABER instrument on the TIMED satellite, and GPS occultations from the COSMIC mission, respectively. The recurrent, periodic changes observed under solar minimum conditions during 2008, and particularly during the Whole Heliospheric Interval (March-April of 2008), are simulated by the model and compared to these measurements. Numerical experiments were conducted to elucidate the mechanisms of solar wind and IMF forcing, setting the solar wind speed and density to nominal values, smoothing the IMF, and also setting it to zero. The results confirm the importance of IMF variations, particularly its north-south component (B z), but also show that when the average B z values are negative (southward), the interaction with increased solar wind speed amplifies the magnetosphere-ionosphere-thermosphere response. Conversely, during events when B z is on average positive (northward), even large increases in solar wind speed have small effects on the system
Journal of Geophysical Research Atmospheres 08/2012; 117. DOI:10.1029/2011JA017417 · 3.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: While it is widely known that coronal mass ejections and their related solar wind features are significant drivers of activity with geospace it is less known that corotating interaction regions (CIRs) and the high speed stream (HSS) periods that precede them are also drivers of activity within geospace. The most recent extended and weak solar minimum interval has brought renewed attention to the space weather impacts of CIR+HSS periods since the highly structured and relatively stable coronal hole features on the Sun resulted in numerous CIR+HSS periods. In this paper we examine two Carrington Rotations (CRs) using the Coupled Magnetosphere Ionosphere Thermosphere (CMIT) model. CR2060 lasted from August 14, 2007 to September 11, 2007 and contained three CIR+HSS periods. CR2068, also known as the Whole Heliosphere Interval (WHI), began on March 20, 2008 and lasted until April 16, 2008 and contained two CIR+HSS periods. For each CR simulations driven by both L1 solar wind observations from the OMNI data set and L1 conditions extracted from CORHEL heliospheric simulations were conducted. The heliospheric simulation results capture the velocity and density structures seen in the solar wind well for CR2060 and only get one of the CIR+HSS periods in CR2068. In each CR the heliospheric simulations produce a much weaker IMF and have less temporal variability in all parameters. We compare the results of the CMIT simulations for each CR to observations of the cross polar cap potential (CPCP), hemispheric power (HP), and SYM H index including the computation of RMS and cross correlation error metrics. We examine the response of the thermospheric density during these intervals by utilizing data from the CHAMP satellite. In the magnetosphere we use magnetic field data from the GOES spacecraft to asses the different simulations ability to describe the distribution and intensity the ULF wave power. Our results show that the L1 driven simulations under-estimates the SYM H index and HP and over-estimates the CPCP. We believe that over estimation of the CPCP is directly linked to the low HP highlighting the need for an improved precipitation model within CMIT. The ULF power in the L1 simulations compares well with the observations, especially for the compressional component important in radiation belt energization processes. In all cases, the CMIT simulations driven by the heliospheric simulation results produce dramatically inferior predictions highlighting the importance of having good IMF predictions in heliospheric model results and possibly indicating the importance of having fluctuations in the solar wind.
Journal of Atmospheric and Solar-Terrestrial Physics 07/2012; 83:-. DOI:10.1016/j.jastp.2012.01.005 · 1.47 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Ionospheric F2 peak electron densities (NmF2) measured at ten ionosonde stations have been analyzed to investigate ionospheric
day-to-day variability around the Whole Heliosphere Interval (WHI) in 2008 (Day of Year (DOY) 50 – 140). The ionosonde data
showed that there was significant global day-to-day variability in NmF2. This variability had 5-, 7-, 9-, 11-, 13.5-, and
16 – 21-day periodicities. At middle latitudes, the ionosphere appeared to respond directly to the solar-wind and interplanetary-magnetic-field
(IMF) induced geomagnetic-activity forcing, with the day-to-day variability having the same periods as those in the solar-wind/IMF
and geomagnetic activity. At the geomagnetic Equator, the ionosphere had a strong 7-day periodicity, corresponding to the
same periodicity in the IMF B
component. In the equatorial anomaly region, the ionosphere showed more complicated day-to-day variability, dominated by
the 9-day periodicity. In addition, there were also periodicities of 11 days and 16 – 21 days in the ionosonde data at some
stations. The ionosonde data were compared with the Coupled Magnetosphere Ionosphere Thermosphere (CMIT) simulations that
were driven by the observed solar-wind and IMF data during the WHI. The CMIT simulations showed similar ionospheric daily
variability seen in the data. They captured the positive and negative responses of the ionosphere at middle latitudes during
the first corotating interaction region (CIR) event in the WHI. The response of the model to the second CIR event, however,
was relatively weak.
Solar Physics 12/2011; 274(1):457-472. DOI:10.1007/s11207-011-9747-0 · 4.04 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We investigated how the rise rate and decay rate of solar flares affect the thermosphere and ionosphere responses to them. Model simulations and data analysis were conducted for two flares of similar magnitude (X6.2 and X5.4) that had the same location on the solar limb, but the X6.2 flare had longer rise and decay times. Simulated total electron content (TEC) enhancements from the X6.2 and X5.4 flares were ∼6 total electron content units (TECU) and ∼2 TECU, and the simulated neutral density enhancements were ∼15%-20% and ∼5%, respectively, in reasonable agreement with observations. Additional model simulations showed that for idealized flares with the same magnitude and location, the thermosphere and ionosphere responses changed significantly as a function of rise and decay rates. The "Neupert Effect," which predicts that a faster flare rise rate leads to a larger EUV enhancement during the impulsive phase, caused a larger maximum ion production enhancement. In addition, model simulations showed that increased E × B plasma transport due to conductivity increases during the flares caused a significant equatorial anomaly feature in the electron density enhancement in the F region but a relatively weaker equatorial anomaly feature in TEC enhancement, owing to dominant contributions by photochemical production and loss processes. The latitude dependence of the thermosphere response correlated well with the solar zenith angle effect, whereas the latitude dependence of the ionosphere response was more complex, owing to plasma transport and the winter anomaly.
Journal of Geophysical Research Atmospheres 10/2011; 116(A10). DOI:10.1029/2011JA016777 · 3.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Satellite drag data indicate that the thermosphere was lower in density, and therefore cooler, during the protracted solar minimum period of 2007-2009 than at any other time in the past 47 years. Measurements indicate that solar EUV irradiance was also lower than during the previous solar minimum. However, secular change due to increasing levels of CO2 and other greenhouse gases, which cool the upper atmosphere, also plays a role in thermospheric climate, and changes in geomagnetic activity could also contribute to the lower density. Recent work used solar EUV measurements from the Solar EUV Monitor (SEM) on the Solar and Heliospheric Observatory, and the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model, finding good agreement between the density changes from 1996 to 2008 and the changes in solar EUV. Since there is some uncertainty in the long-term calibration of SEM measurements, here we perform model calculations using the MgII core-to-wing ratio as a solar EUV proxy index. We also quantify the contributions of increased CO2 and decreased geomagnetic activity to the changes. In these simulations, CO2 and geomagnetic activity play small but significant roles, and the primary cause of the low temperatures and densities remains the unusually low levels of solar EUV irradiance.
Journal of Geophysical Research Atmospheres 07/2011; 116(7). DOI:10.1029/2011JA016508 · 3.43 Impact Factor