[Show abstract][Hide abstract]ABSTRACT: The goal of the Miniature X-ray Solar Spectrometer (MinXSS) CubeSat is to explore the energy distribution of soft X-ray (SXR) emissions from the quiescent Sun, active regions, and during solar flares, and to model the impact on Earth's ionosphere and thermosphere. The energy emitted in the SXR range (0.1 to 10 keV) can vary by more than a factor of 100, yet we have limited spectral measurements in the SXRs to accurately quantify the spectral dependence of this variability. The MinXSS primary science instrument is an Amptek, Inc. X123 X-ray spectrometer that has an energy range of 0.5-30 keV with a nominal 0.15 keV energy resolution. Two flight models have been built. The first, MinXSS-1, has been making science observations since 2016 June 9, and has observed numerous flares, including 40 C-class and 7 M-class flares. These SXR spectral measurements have advantages over broadband SXR observations, such as providing the capability to derive multiple-temperature components and elemental abundances of coronal plasma, improved irradiance accuracy, and higher resolution spectral irradiance as input to planetary ionosphere simulations. MinXSS spectra obtained during the M5.0 flare on 2016 July 23 highlight these advantages, and indicate how the elemental abundance changes from primarily coronal to more photospheric during the flare. MinXSS-1 observations are compared to the Geostationary Operational Environmental Satellite (GOES) X-Ray Sensor (XRS) measurements of SXR irradiance and estimated corona temperature. Additionally, a suggested improvement to the calibration of the GOES XRS data is presented.
[Show abstract][Hide abstract]ABSTRACT: There are still uncertainties regarding the formation mechanisms for storm-enhanced density (SED) in the high and subauroral latitude ionosphere. In this work, we deploy the Thermosphere Ionosphere Electrodynamic General Circulation Model (TIEGCM) and GPS total electron content (TEC) observations to identify the principle mechanisms for SED and the tongue of ionization (TOI) through term-by-term analysis of the ion continuity equation, and also identify the advantages and deficiencies of the TIEGCM in capturing high- and subauroral latitude ionospheric fine structures for the two geomagnetic storm events occurring on 17 March 2013 and 2015. Our results show that, in the topside ionosphere, upward E × B ion drifts are most important in SED formation and are offset by antisunward neutral winds and downward ambipolar diffusion effects. In the bottomside F-region ionosphere, neutral winds play a major role in generating SEDs. SED signature in TEC is mainly caused by upward E × B ion drifts that lift the ionosphere to higher altitudes where chemical recombination is slower. Horizontal E × B ion drifts play an essential role in transporting plasma from the dayside convection throat region to the polar cap to form TOIs. Inconsistencies between model results and GPS TEC data were found: (1) GPS relative TEC difference between storm-time and quiet-time has “holes” in the dayside ion convection entrance region, which do not appear in the model results; (2) The model tends to overestimate electron density enhancements in the polar region. Possible causes for these inconsistencies are discussed in this article.
Full-text available · Article · Aug 2016 · Journal of Geophysical Research: Space Physics
[Show abstract][Hide abstract]ABSTRACT: O/N2, measured by the Global Ultraviolet Imager on board the Thermosphere Ionosphere Mesosphere Energetics Dynamics satellite, has large longitudinal variations at the solstices, which are simulated well in upper atmosphere general circulation models. These longitudinal variations are caused by the displacement of the Earth's magnetic poles from the geographic ones. The location of a magnetic pole affects the latitude at which the winds, driven by heating in summer, converge in the subauroral region of the winter hemisphere. In the magnetic pole's longitude sector, this convergence occurs at relatively low latitudes, which results in the maximum values of O/N2 also occurring at relatively low latitudes. These latitudes have a relatively small solar zenith angle, contributing to a strong winter anomaly. In the zonally opposite longitude sector, maximum values of O/N2 occur at relatively high latitudes because the summer-to-winter wind convergence also occurs at relatively high latitudes. These high latitudes have a relatively large solar zenith angle, so ionization is weak, contributing to a weak winter anomaly. Therefore, the displacement between the magnetic and geographic poles not only results in a strong longitudinal variation of O/N2 but also results in a strong longitudinal variation of the ionosphere winter anomaly.
Full-text available · Article · Jun 2016 · Journal of Geophysical Research: Space Physics
[Show abstract][Hide abstract]ABSTRACT: We examine the solar cycle variability of thermospheric composition (O/N2) at the solstices. Our observational and modeling studies show that the summer-to-winter latitudinal gradient of O/N2 is small at solar minimum but large at solar maximum; O/N2 is larger at solar maximum than at solar minimum on a global-mean basis; there is a seasonal asymmetry in the solar cycle variability of O/N2, with large solar cycle variations in the winter hemisphere and small solar cycle variations in the summer hemisphere. Model analysis reveals that vertical winds decrease the temperature-driven solar cycle variability in the vertical gradient of O/N2 in the summer hemisphere but increase it in the winter hemisphere; consequently, the vertical gradient of O/N2 does not change much in the summer hemisphere over a solar cycle, but it increases greatly from solar minimum to solar maximum in the winter hemisphere; this seasonal asymmetry in the solar cycle variability in the vertical gradient of O/N2 causes a seasonal asymmetry in the vertical advection of O/N2, with small solar cycle variability in the summer hemisphere and large variability in the winter hemisphere, which in turn drives the observed seasonal asymmetry in the solar cycle variability of O/N2. Since the equatorial ionization anomaly suppresses upwelling in the summer hemisphere and strengthens downwelling in the winter hemisphere through plasma-neutral collisional heating and ion drag, locations and relative magnitudes of the equatorial ionization anomaly crests and their solar cycle variabilities can significantly impact the summer-to-winter gradients of O/N2 and their solar cycle variability.
Full-text available · Article · Apr 2016 · Journal of Geophysical Research: Space Physics
[Show abstract][Hide abstract]ABSTRACT: The Miniature X-ray Solar Spectrometer (MinXSS) is a 3-Unit (3U) CubeSat developed at the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado, Boulder (CU). Over 40 students contributed to the project with professional mentorship and technical contributions from professors in the Aerospace Engineering Sciences Department at CU and from LASP scientists and engineers. The scientific objective of MinXSS is to study processes in the dynamic Sun, from quiet-Sun to solar flares, and to further understand how these changes in the Sun influence the Earth's atmosphere by providing unique spectral measurements of solar soft x-rays (SXRs). The enabling technology providing the advanced solar SXR spectral measurements is the Amptek X123, a commercial-off-the-shelf (COTS) silicon drift detector (SDD). The Amptek X123 has a low mass (~324 g after modification), modest power consumption (~2.5 W), and small volume (2.7" x 3.9" x 1.0"), making it ideal for a CubeSat. This paper provides an overview of the MinXSS mission: the science objectives, project history, subsystems, and lessons learned that can be useful for the small-satellite community.
Full-text available · Article · Mar 2016 · Journal of Spacecraft and Rockets
[Show abstract][Hide abstract]ABSTRACT: We investigate the inter-hemisphere circulation at the solstices, in order to understand why O/N2 is larger in the northern hemisphere winter than in the southern hemisphere winter. Our studies reveal that the equatorial ionosphere anomaly (EIA) significantly impacts the summer-to-winter wind through plasma-neutral collisional heating, which changes the summer-to-winter pressure gradient, and ion drag. Consequently, the wind is suppressed in the summer hemisphere as it encounters the EIA but accelerates after it passes the EIA in the winter hemisphere. The wind then converges due to an opposing pressure gradient driven by Joule heating in auroral regions, produces large O/N2 at subauroral latitudes. This EIA effect is stronger near the December solstice than near the June solstice because the ionospheric annual asymmetry creates greater meridional wind convergence near the December solstice, which in turn produces larger O/N2 in the northern hemisphere winter than in the southern hemisphere winter.
Full-text available · Article · Feb 2016 · Journal of Geophysical Research: Space Physics
[Show abstract][Hide abstract]ABSTRACT: We provide the first ever characterization of the primary modes of ionospheric Hall and Pedersen conductance variability as empirical orthogonal functions (EOFs). These are derived from six satellite years of Defense Meteorological Satellite Program (DMSP) particle data acquired during the rise of solar cycles 22 and 24. The 60 million DMSP spectra were each processed through the Global Airlglow Model. Ours is the first large-scale analysis of ionospheric conductances completely free of assumption of the incident electron energy spectra. We show that the mean patterns and first four EOFs capture ∼50.1 and 52.9% of the total Pedersen and Hall conductance variabilities, respectively. The mean patterns and first EOFs are consistent with typical diffuse auroral oval structures and quiet time strengthening/weakening of the mean pattern. The second and third EOFs show major disturbance features of magnetosphere-ionosphere (MI) interactions: geomagnetically induced auroral zone expansion in EOF2 and the auroral substorm current wedge in EOF3. The fourth EOFs suggest diminished conductance associated with ionospheric substorm recovery mode. We identify the most important modes of ionospheric conductance variability. Our results will allow improved modeling of the background error covariance needed for ionospheric assimilative procedures and improved understanding of MI coupling processes.
Article · Dec 2015 · Journal of Geophysical Research: Space Physics
[Show abstract][Hide abstract]ABSTRACT: Nitrate ion spikes in polar ice cores are contentiously used to estimate the
intensity, frequency, and probability of historical solar proton events,
quantities that are needed to prepare for potentially society-crippling space
weather events. We use the Whole Atmosphere Community Climate Model to
calculate how large an event would have to be to produce enough odd nitrogen
throughout the atmosphere to be discernible as nitrate peaks at the Earth's
surface. These hypothetically large events are compared with probability of
occurrence estimates derived from measured events, sunspot records, and
cosmogenic radionuclides archives. We conclude that the fluence and spectrum of
solar proton events necessary to produce odd nitrogen enhancements equivalent
to the spikes of nitrate ions in Greenland ice cores are unlikely to have
occurred throughout the Holocene, confirming that nitrate ions in ice cores are
not suitable proxies for historical individual solar proton events.
Article · Nov 2015 · Journal of Geophysical Research Atmospheres
[Show abstract][Hide abstract]ABSTRACT: We have found that consideration of neutral helium as a major species leads to a more complete physics-based modeling description of the Earth's upper thermosphere. An augmented version of the composition equation employed by the Thermosphere-Ionosphere-Electrodynamic General Circulation Model (TIE-GCM) is presented, enabling the inclusion of helium as the fourth major neutral constituent. Exospheric transport acting above the upper boundary of the model is considered, further improving the local time and latitudinal distributions of helium. The new model successfully simulates a previously observed phenomenon known as the “winter helium bulge,” yielding behavior very similar to that of an empirical model based on mass spectrometer observations. This inclusion has direct consequence on the study of atmospheric drag for low-Earth orbiting satellites, as well as potential implications on exospheric and topside ionospheric research.
Full-text available · Article · Jul 2015 · Journal of Geophysical Research: Space Physics
[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.
Full-text available · Article · May 2015 · Journal of Geophysical Research: Space Physics
[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.
Full-text available · Article · Feb 2015 · Journal of Geophysical Research: Space Physics
[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.
Full-text available · Article · Aug 2014 · Journal of Geophysical Research: Space Physics
[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 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.
Full-text available · Article · Mar 2014 · Journal of Geophysical Research: Space Physics
[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.
Full-text available · Article · Mar 2014 · Journal of Geophysical Research: Space Physics
[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.
Article · Mar 2014 · Journal of Geophysical Research: Space Physics
[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.
Full-text available · Article · Oct 2013 · Journal of Geophysical Research: Space Physics