Challenging Minisatellite Payload Observations Research Articles

Third‐Order Structure Functions of Zonal Winds in the Thermosphere Using CHAMP and GOCE Observations

AbstractWe use multi‐year observations of cross‐track winds (u) from the CHAllenging Minisatellite Payload (CHAMP) and the Gravity Field and Steady State Ocean Circulation Explorer (GOCE) to calculate third‐order structure functions in the thermosphere as a function of horizontal separation (s). They are computed using the mean (〈δu3〉) and the median and implemented over non‐polar satellite paths in both hemispheres. On height averages, 〈δu3〉 is shown to scale with s2 for s ≃ 80–1,000 km, in agreement with equivalent estimates in the lower atmosphere from aircraft observations. Conversely, follows an s3 power law for almost the whole s range, consistent with the two‐dimensional turbulence scaling law for a direct enstrophy cascade. These scaling laws appear independent of winds in distinct atmospheric regions. Furthermore, the functions are predominantly positive, indicating a preferential cyclonic motion for the wind.

Solar Flux Dependence of Upper Thermosphere Diurnal Variations: Observed and Modeled

AbstractUpper thermosphere mass density over the declining phase of solar cycle 23 is investigated using a day‐to‐night ratio (DNR) of thermosphere properties to evaluate how much relative change occurs climatologically between day and night. Challenging Minisatellite Payload (CHAMP) observations from 2002 to 2009, MSIS 2.0 output, and TIEGCM V2.0 simulations are analyzed to assess their relative response in DNR. The CHAMP observations demonstrate nightside densities decrease more significantly than dayside densities as solar flux decreases. This causes a steadily increasing CHAMP mass density DNR from around two to greater than four with decreasing solar flux. The MSIS 2.0 nightside densities decrease, with decreasing solar flux, more significantly than the dayside, resulting in the same trend as CHAMP. TIEGCM V2.0 displays an opposite trend in density DNR with decreasing solar flux due to dayside densities decreasing more significantly than nightside densities. A sensitivity analysis of the two models reveals the TIEGCM V2.0 to have greater sensitivity in temperature to levels of solar flux, while MSIS 2.0 displayed a greater sensitivity in mean molecular weight. The pressure DNR from both models contributed the most to the density DNR value at 400 km. As solar flux decreases, the two models' estimate of pressure DNR deviate appreciably and trend in opposite directions. The TIEGCM V2.0 dayside temperatures during middle‐to‐low solar flux are too cold relative to MSIS 2.0. Increasing the dayside temperature values by about 50–100 K and decreasing the nightside temperature slightly would bring the TIEGCM V2.0 into better agreement with MSIS 2.0 and CHAMP observations.

Satellite In Situ Electron Density Observations of the Midlatitude Storm Enhanced Density on the Noon Meridional Plane in the F Region During the 20 November 2003 Magnetic Storm

AbstractIonospheric storm enhanced density (SED) has been extensively investigated using total electron content deduced from GPS ground and satellite‐borne receivers. However, dayside in situ electron density measurements have not been analyzed in detail for SEDs yet. We report in situ electron density measurements of a SED event in the Northern Hemisphere (NH) at the noon meridian plane measured by the Challenging Minisatellite Payload (CHAMP) polar‐orbiting satellite at about 390 km altitude during the 20 November 2003 magnetic storm. The CHAMP satellite measurements render rare documentation about the dayside SED's life cycle at a fixed magnetic local time (MLT) through multiple passes. Solar wind drivers triggered the SED onset and controlled its lifecycle through its growth and retreat phases. The SED electron density enhancement extended from the equatorial ionization anomaly to the noon cusp. The midlatitude electron density increased to a maximum at the end of the growth phase. Afterward, the dayside SED region retreated gradually to lower magnetic latitudes. The observations showed a hemisphere asymmetry, with the NH electron density exhibiting a more significant enhancement. The simulations using the Thermosphere Ionosphere Electrodynamic General Circulation model show a good agreement with the CHAMP observations. The simulations indicate that the dayside midlatitude electron density enhancement has a complicated dependence on vertical ion drift, neutral wind, magnetic latitude, MLT, and the height of the F2 layer. Finally, we discuss the notion of using the mean cross‐polar cap electric field as a proxy for assessing the effects of solar wind drivers on producing midlatitude electron density enhancement.

Influence of the Magnetic Field Strength and Solar Activity on the Thermospheric Zonal Wind

AbstractBased on Thermosphere‐Ionosphere Electrodynamics General Circulation Model simulation and CHAllenging Minisatellite Payload observations, the effects of the geomagnetic field intensity and solar activity on the thermospheric zonal wind and related physical mechanisms are investigated. The weakening of the magnetic field results in an increase in the westward wind during the daytime and a decrease in the eastward wind at night, and leading to a decreasing superrotation. The weakening solar activity causes a reduction in the zonal wind and superrotation. The theoretical term analysis shows that when the magnetic field is weakened, the vertical upward drift velocity of the plasma increases, resulting in a decrease in the electron density in the F layer. The weakening of eastward acceleration of viscous force and ion drag results in an enhanced westward wind. The downward drift velocity of ions increases at night, resulting in an increase in the electron density, while the ion‐neutral velocity difference decreases. The weakening of eastward acceleration of pressure gradient and viscous force at night are main reasons for the decreased eastward wind. The reduced solar activity leads to a decrease in the pressure gradient and ion drag. Combined with the change of viscous force, these processes cause the decrease in the superrotation. The geomagnetic field configuration is responsible for the UT variation (potential longitudinal change) in the superrotation. When the magnetic field is weakened, although the average neutral wind decreases, the Pedersen conductivity of the F‐layer is quadrupled. Therefore, the meridional current system driven by the F‐layer dynamo is enhanced accordingly.

The Role of Strong Meridional Neutral Winds in the Formation of Deep Equatorial Ionization Trough in CHAMP Observations

AbstractIonospheric plasma density data from the Planar Langmuir Probe onboard the CHAllenging Minisatellite Payload (CHAMP) are used to investigate the latitudinal profile of the ionospheric plasma density. Along with the well‐known profiles with features of the equatorial ionization anomaly and equatorial plasma bubbles, a third type is observed as smooth profiles with large‐scale deep equatorial trough (DET) where the plasma density is extremely low. A global survey focusing on magnetically quiet conditions reveals that the DET profiles are generally a post‐sunset phenomenon, and their appearance depends on longitude and season. During equinoxes in active solar years, the strong pre‐reversal enhancement electric field is believed to uplift the ionosphere and produce the DET profiles. While around June solstice, the DET profiles are clustered at W– E longitude, where the zonal electric field is weak but the magnetic meridional component of the neutral wind is most significant. The SAMI2 model is then employed to simulate the ionospheric plasma distribution in the magnetic meridional plane with enhanced neutral wind, which successfully reproduced the main features of the observed DET profile. The results indicate that the neutral wind can obviously affect the equatorial ionosphere by transporting plasma in the magnetic flux tube through neutral‐ion interaction. The neutral wind is previously known to work with the inclined magnetic field at mid‐latitude, while this study emphasizes its effect around the magnetic equator especially when the electric field forcing () is weak or absent.

Equatorial Nighttime Thermospheric Zonal Wind Jet Response to the Temporal Oscillation of Solar Wind

AbstractThis work investigates the response of the equatorial thermospheric wind jet to the temporal oscillation of the Bz component of the interplanetary magnetic field using Thermosphere Ionosphere Electrodynamics General Circulation Model simulations and CHAllenging Minisatellite Payload observations. The thermospheric wind jet is formed in the magnetic equatorial region with the strongest zonal winds. Both the morning westward jet and the nighttime eastward wind jet at the dip equator decrease during the oscillation periods of IMF Bz. Ion drag dominates the deceleration of the equatorial eastward wind jet in the nighttime, with a minor contribution from the viscous force. The relative motion between the ions and neutrals has a dominant influence on the ion drag at the 60–180°W and 107–180°E geographic longitude; at other longitudes, both relative velocity (∆velocity) and electron density changes (∆Ne) are dominant. Moreover, the decrease in the equatorial eastward wind jet at 20 magnetic local time, which is also driven by ion drag, is significant above 270 km. The generation of the equatorial westward wind jet disturbance is significantly influenced by ∆velocity at all altitudes, whereas ∆Ne becomes important below 450 km.

Seasonal and Hemispheric Asymmetries of F Region Polar Cap Plasma Density: Swarm and CHAMP Observations

AbstractOne of the primary mechanisms of loss of Earth's atmosphere is the persistent “cold” ( 20 eV) ion outflow that has been observed in the magnetospheric lobes over large volumes with dimensions of order several Earth radii. As the main source of this cold ion outflow, the polar cap F region ionosphere and conditions within it have a disproportionate influence on these magnetospheric regions. Using 15 years of measurements of plasma density Ne made by the Swarm spacecraft constellation and the Challenging Mini Satellite Payload (CHAMP) spacecraft within the F region of the polar cap above 80° Apex magnetic latitude, we report evidence of several types of seasonal asymmetries in polar cap Ne. Among these, the transition between “winter‐like” and “summer‐like” median polar cap Ne occurs 1 week prior to local spring equinox in the Northern Hemisphere (NH) and 1 week after local spring equinox in the Southern Hemisphere (SH). Thus, the median SH polar cap Ne lags the median NH polar cap Ne by approximately 2 weeks with respect to hemispherically local spring and fall equinox. From interhemispheric comparison of statistical distributions of polar cap plasma density around each equinox and solstice, we find that distributions in the SH are often flatter (i.e., less skewed and kurtotic) than those in the NH. Perhaps of most significance to cold ion outflow, we find no evidence of an F region plasma density counterpart to a previously reported hemispheric asymmetry whereby cold plasma density is higher in the NH magnetospheric lobe than in the SH lobe.

A New Method for Deriving the Nightside Thermospheric Density Based on GUVI Dayside Limb Observations

AbstractWe propose a new method to derive the nightside thermospheric density by extending Global Ultraviolet Imager (GUVI) dayside limb observations using empirical orthogonal function (EOF) analysis. First, we acquire the GUVI dayside total mass density during 2002–2005 to construct a preliminary empirical model (EM). Simultaneously, we decompose the background thermospheric density from U.S. Naval Research Laboratory Mass Spectrometer and Incoherent Scatter Radar Extended model into different EOFs. The decomposed EOFs are then used to fit the continuous density from EM, to develop a new nightside extended model (NEM). The preliminary EM and developed NEM are further evaluated with Challenging Minisatellite Payload (CHAMP) satellite observations. Higher correlation coefficients and smaller relative standard errors between CHAMP observations and the NEM results are obtained than those between CHAMP observations and the EM results, and the NEM results are in good agreement with the CHAMP observations in time series during both daytime and nighttime, which all prove the NEM method is effective to the reproduction and extension of GUVI original dayside observations. Furthermore, the NEM reveals two typical seasonal variation features, the semiannual variation and equinoctial asymmetry of thermospheric density. The model provides an effective tool to derive the nightside thermospheric density and explore the thermospheric intrinsic structure, and needs the further development to achieve more widespread application of the thermosphere.

Comparison of Thermospheric Density Between GUVI Dayside Limb Data and CHAMP Satellite Observations: Based on Empirical Model

AbstractThe Global Ultraviolet Imager (GUVI) aboard the Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics (TIMED) satellite senses far ultraviolet airglow emissions in the thermosphere. The retrieved altitude profiles of thermospheric neutral density from GUVI daytime limb scans are significant for ionosphere‐thermosphere study. Here, we use the profiles of the main neutral density to derive the total mass density during the period 2002–2007 under geomagnetic quiet conditions (ap < =12). We attempt to compare the obtained total mass density with the Challenging Minisatellite Payload (CHAMP) observations, making use of an empirical model (GUVI model hereafter). This GUVI model is aimed to solve the difficulty of the direct comparison of GUVI and CHAMP observations due to their different local times at a given location in a given day. The GUVI model is in good agreement with CHAMP observations with the small standard deviations of their ratios (less than 10%) except at low solar flux levels. The correlation coefficients are greater than 0.9, and the relative standard errors are less than 20%. Comparison between the GUVI model and CHAMP observations during solar minimum shows a large bias (~30%). The large bias at low solar flux levels might be due to the limitation of F10.7 as an extreme ultraviolet radiation flux proxy and the fitting method. Our results demonstrate the validity and accuracy of our model based on GUVI data against the density data from the CHAMP satellite.

An empirical model of the thermospheric mass density derived from CHAMP satellite

Abstract. In this study, we present an empirical model, named CH-Therm-2018, of the thermospheric mass density derived from 9-year (from August 2000 to July 2009) accelerometer measurements from the CHAllenging Mini-satellite Payload (CHAMP) satellite at altitudes from 460 to 310 km. The CHAMP dataset is divided into two 5-year periods with 1-year overlap (from August 2000 to July 2005 and from August 2004 to July 2009) to represent the high-to-moderate and moderate-to-low solar activity conditions, respectively. The CH-Therm-2018 model describes the thermospheric density as a function of seven key parameters, namely the height, solar flux index, season (day of year), magnetic local time, geographic latitude and longitude, as well as magnetic activity represented by the solar wind merging electric field. Predictions of the CH-Therm-2018 model agree well with CHAMP observations (within 20 %) and show different features of thermospheric mass density during the two solar activity levels, e.g., the March–September equinox asymmetry and the longitudinal wave pattern. From the analysis of satellite laser ranging (SLR) observations of the ANDE-Pollux satellite during August–September 2009, we estimate 6 h scaling factors of the thermospheric mass density provided by our model and obtain the median value equal to 1.267±0.60. Subsequently, we scale up our CH-Therm-2018 mass density predictions by a scale factor of 1.267. We further compare the CH-Therm-2018 predictions with the Naval Research Laboratory Mass Spectrometer Incoherent Scatter Radar Extended (NRLMSISE-00) model. The result shows that our model better predicts the density evolution during the last solar minimum (2008–2009) than the NRLMSISE-00 model.

Longitudinal variation in zonal winds at subauroral regions: Possible mechanisms

AbstractLongitudinal differences in thermospheric zonal winds (ΔUy) are investigated in the subauroral region for different seasons and under solar maximum and medium conditions by using Challenging Minisatellite Payload observations. Prominent wave‐1 longitudinal and diurnal variations of ΔUy are observed, along with an antiphase relationship between the Northern and Southern Hemispheres. These structures persist over the whole year and are independent of solar activity. ΔUy values are greater at nighttime than at daytime, and values in the south are greater than those in the north in local summer and winter. Model simulations confirm observed results in large‐scale structures, and the nonzero dipole tilt is found to be vital for the longitudinal variation of the zonal wind. The neutral air pressure gradient caused by the day‐night difference in solar heating is a major contributor to the observed ΔUy. The pressure effects are larger at nighttime than at daytime and larger in the Southern Hemisphere than in the Northern Hemisphere. Ion drag reduces the compatibility between the modeled and observed ΔUy as expected, with larger effects at nighttime than at daytime. Viscous force also reduces the compatibility between the modeled and observed ΔUy with greater effects at daytime, except at nighttime in the Southern Hemisphere. Similarly, the Coriolis force makes the difference between the modeled and observed ΔUy larger. The sum of these factors can explain, in general, the observed local time and hemispheric asymmetry features in longitudinal variation of the zonal wind.

Pc2-3 geomagnetic pulsations on the ground, in the ionosphere, and in the magnetosphere: MM100, CHAMP, and THEMIS observations

Abstract. We analyze Pc2-3 pulsations recorded by the CHAMP (CHAllenging Minisatellite Payload) satellite in the F layer of the Earth's ionosphere, on the ground, and in the magnetosphere during quiet geomagnetic conditions. The spectra of Pc2-3 pulsations recorded in the F layer are enriched with frequencies above 50 mHz in comparison to the ground Pc2-3 spectra. These frequencies are higher than the fundamental harmonics of the field line resonances in the magnetosphere. High quality signals with dominant frequencies 70–200 mHz are a regular phenomenon in the F layer and in the magnetosphere. The mean latitude of the maximum Pc2-3 occurrence rate lies at L ≈ 3.5 in the F layer, i.e., inside the plasmasphere. Day-to-day variations of the L value of the CHAMP Pc2-3 occurrence rate maximum follow the plasmapause day-to-day variations. Polarization and amplitude of Pc2-3s in the magnetosphere, in the ionosphere, and on the ground allow us to suggest that they are generated as fast magnetosonic (FMS) waves in the outer magnetosphere and are partly converted into shear Alfven waves near the plasmapause. The observed ground-to-ionosphere amplitude ratio during the night is interpreted as a result of the Alfven wave transmission through the ionosphere. The problem of wave transmission through the ionosphere is solved theoretically by means of a numerical solution of the full-wave equation for the Alfven wave reflection from and transmission through a horizontally stratified ionosphere. The best agreement between the calculated and measured values of the ground-to-ionosphere amplitude ratio is found for k = 5 × 10−3 km−1, i.e., the observed ground-to-ionosphere amplitude ratio corresponds to a wave spatial scale which could provide a Doppler shift within a few percent of the apparent frequency of the Pc2-3 pulsations as recorded by a low-orbiting spacecraft.

Seasonal and latitudinal variations of the electron density nonmigrating tidal spectrum in the topside ionospheric F region as resolved from CHAMP observations

AbstractIn this study we present for the first time the nonmigrating tidal spectrum for the electron density in the topside ionosphere on global scale for different seasons at both solar maximum and minimum conditions. The electron density observations from the CHAMP (CHAllenging Minisatellite Payload) satellite provide evidence for prominent nonmigrating tides at different latitude regions. At middle and high latitudes the most prominent diurnal tides are DE1, D0, and DW2, as well as semidiurnal tides SW1 and SW3 preferably during equinox seasons. DE1 is only found in the northern middle and high latitudes, while D0 and DW2 are much stronger in the Southern Hemisphere. These tides are believed to be driven by the thermospheric winds through ion‐neutral interactions. At equatorial and low‐latitude regions the most prominent diurnal tides are DE2 and DE3. DE2 is found to be present at low‐latitude regions throughout the whole year with larger amplitudes in the Southern Hemisphere, while DE3 shows largest amplitudes (symmetric above the dip equator) at the equatorial ionization anomaly (EIA) crest region during September equinox. A general antiphase behavior between the EIA crest and trough is observed for the tides DE3, DE2, DW2, and SW3. We consider this as strong evidence for the modulation of the EIA electron density by the E layer zonal electric field via the ion fountain effect. An exception is the tidal component D0, which exhibits antiphase variations between the two hemispheres. The phase value at the equatorial trough is halfway between those of the two hemispheres. Presently, we cannot give an explanation for it, and study concerning special model effort is further needed.

The spatial distribution of region 2 field-aligned currents relative to subauroral polarization stream

Abstract. To test the current-generation model of subauroral polarization stream (SAPS), we have investigated the relative positions of field-aligned currents (FACs) with respect to SAPS in a statistical way by using CHAMP (CHAllenging Minisatellite Payload) and DMSP (Defense Meteorological Satellite Program) satellite observations as well as model simulations. Comparative studies have been performed for consecutive CHAMP observations in different magnetic local time (MLT) sectors with respect to SAPS. The latitude of the peak westward zonal wind deduced from CHAMP measurements has been used to represent the location of the SAPS peak. Both the density and the sheet current strength of R2 (region 2) FACs are enhanced when SAPS occur. Subsequently R2 FACs decay in intensity and correspondingly the centers retreat poleward. The latitudes of the center of the R2 FAC, small- and medium-scale FACs, and SAPS shift equatorward with increasing MLT. The SAPS peaks are located between R2 and R1 (region 1) FAC peaks in all MLT bins under study. The SAPS peaks are closer to R2 centers in the later MLT sectors. The peaks of small- and medium-scale FACs are located poleward of SAPS, mainly in the upward R1 FACs region. The upward R1 FACs are partly closed by the downward R1 FACs in the dawn–morning sector. Based on model simulation, when R2 shifts equatorward to the subauroral region, the plasma flow also shifts equatorward with its peak located poleward of that of R2 FACs. Both the model and observations provide evidence that SAPS behave as caused by a magnetospheric current source.

Relation of zonal plasma drift and wind in the equatorial F region as derived from CHAMP observations

Abstract. In this paper we estimate zonal plasma drift in the equatorial ionospheric F region without counting on ion drift meters. From June 2001 to June 2004 zonal plasma drift velocity is estimated from electron, neutral, and magnetic field observations of Challenging Mini-satellite Payload (CHAMP) in the 09:00–20:00 LT sector. The estimated velocities are validated against ion drift measurements by the Republic of China Satellite-1/Ionospheric Plasma and Electrodynamics Instrument (ROCSAT-1/IPEI) during the same period. The correlation between the CHAMP (altitude ~ 400 km) estimates and ROCSAT-1 (altitude ~ 600 km) observations is reasonably high (R ≈ 0.8). The slope of the linear regression is close to unity. However, the maximum westward drift and the westward-to-eastward reversal occur earlier for CHAMP estimates than for ROCSAT-1 measurements. In the equatorial F region both zonal wind and plasma drift have the same direction. Both generate vertical currents but with opposite signs. The wind effect (F region wind dynamo) is generally larger in magnitude than the plasma drift effect (Pedersen current generated by vertical E field), thus determining the direction of the F region vertical current.

Theoretical study: Influence of different energy sources on the cusp neutral density enhancement

Simulations with the Global Ionosphere‐Thermosphere Model (GITM) show that both Poynting flux and soft electron precipitation are important in producing neutral density enhancements near 400 km altitude in the cusp that have been observed by the Challenging Minisatellite Payload (CHAMP) satellite. Imposing a Poynting flux of 75 mW/m2 in the cusp within the model increases the neutral density by 34%. The direct heating from 100 eV, 2 mW/m2 soft electron precipitation produces only a 5% neutral density enhancement at 400 km. However, the associated enhanced ionization in the F‐region from the electron precipitation leads to a neutral density enhancement of 24% through increased Joule heating. Thus, the net effect of the soft electron is close to 29%, and the combined influence of Poynting flux and soft particle precipitation causes a more than 50% increase in neutral density at 400 km, which is consistent with CHAMP observations in extreme cases. The effect of electron precipitation on the neutral density at 400 km decreases sharply with increasing characteristic energy such that 900 eV electrons have little effect on neutral density. Finally, the impact of 2 keV, 0.3 mW/m2 proton precipitation on the neutral density is negligible due to a lowering of the altitude of Joule heating.

Soft electron precipitation explains thermosphere mass enhancements

From 2000 to 2010, a German satellite carrying the Challenging Minisatellite Payload (CHAMP) orbited the Earth, tracking the properties of the thermosphere—the atmospheric layer that starts roughly 90 kilometers above the surface and coincides with the lower layers of the ionosphere. In analyzing CHAMP's observations, researchers came across an anomaly: 400 kilometers above the Earth in two different regions in the thermosphere, the density was persistently higher than scientists expected given model calculations. The density enhancements were found in two places: at the dayside cusp—the polar region where terrestrial magnetic field lines create a funnel for the solar wind—and in the auroral region on the planet's nightside. Though researchers had identified these anomalous regions, they have so far been unable to identify a mechanism to explain them.

Features of highly structured equatorial plasma irregularities deduced from CHAMP observations

Abstract. In this study five years of CHAMP (Challenging Mini-satellite Payload) fluxgate magnetometer (FGM) data is used to investigate the characteristics of Equatorial Plasma Bubbles (EPBs). We filtered the FGM data by using band-passes with four different cut-off periods to get the EPBs with different maximum spatial scale sizes in the meridional plane ranging from 76–608 km. Associated with the EPB observations at about 400 km, the typical altitude of CHAMP during the year 2000–2005, we also investigate the post-sunset equatorial vertical plasma drift data from ROCSAT-1 (Republic of China Satellite 1). Since the height of the F-layer is highly correlated with the vertical plasma drift and solar flux, we sorted the ROCSAT-1 data into different groups by F10.7. From the integrated vertical drift we have estimated the post-sunset uplift of the ionosphere. By comparing the properties of EPB occurrence for different scale sizes with the global distribution of plasma vertical uplift, we have found that EPBs reaching higher altitudes are more structured than those which are sampled by CHAMP near the top side of the depleted fluxtube. Such a result is in accord with 3-D model simulations (Aveiro and Hysell, 2010). Small-scale EPB structures are observed by CHAMP when the irregularities reach apex heights of 800 km and more. Such events are encountered primarily in the Brazilian sector during the months around November, when the post-sunset vertical plasma drift is high.

Data assimilation of thermospheric mass density

Accurate modeling of thermospheric mass density variation is of foremost importance to low Earth orbital prediction. The accuracy of empirical neutral density models based on global indices for solar and geomagnetic activity is inherently limited by the resolution of these indices. Assimilative modeling is appealing, as it provides a means to systematically identify and correct the inconsistencies between model specification and observations. In this paper we present a practical assimilative mass density specification methodology that optimally combines the mass density prediction by the Coupled‐Thermosphere‐Ionosphere‐Plasmasphere‐electrodynamics (CTIPe) model with in‐situ observations of neutral mass density by research satellites such as Challenging Minisatellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE). The methodology yields an analysis of the global density according to Bayes's rule under the assumption of Gaussian prior and observation error distribution (namely, by using the Optimal Interpolation or Kalman Filter update formula). To make best use of under‐sampled global neutral mass density observations, the background (prior) error covariance is built on the principal component analysis to represent the long‐distance correlation effectively, with an adaptive capability by using the maximum‐likelihood method. The neutral mass density specification at 400 km can be improved up to 50% beyond what has been attained by the CTIPe by assimilating CHAMP and GRACE density observations.

Seasonal and longitudinal variations of the solar quiet (Sq) current system during solar minimum determined by CHAMP satellite magnetic field observations

[1] Vector magnetometer observations from the Challenging Minisatellite Payload (CHAMP) satellite are used to determine the solar quiet (Sq) current system during the recent solar minimum. Observations from 2006 to 2008 are combined, and after removal of a main field model and accounting for field-aligned currents, the longitudinal and seasonal variation of the Sq currents are determined through the method of spherical harmonic analysis. Comparison with Sq currents derived from ground-based magnetometers in the African/European longitude sector reveals similar amplitudes and seasonal variations, indicating that the CHAMP observations can reliably determine the Sq current system. The seasonal variation is consistent with prior observations during solar minimum conditions and in the Northern Hemisphere exhibits a primarily annual variation with peak currents during local summer. The seasonal variation in the Southern Hemisphere is characterized by a semiannual variation with the maxima occurring around the equinoxes. Significant longitudinal variations are also observed, and they display a seasonal variability. During Northern Hemisphere summer, the predominant feature at local noon is a wave number 1 variation in longitude. During the remainder of the year, a wave 3 longitudinal structure is observed at this local time. The longitudinal variations are considered to be due to a combination of the orientation and strength of the geomagnetic field as well as the tidal winds in the lower thermosphere. Variations in tidal winds due to nonmigrating tides may influence the dynamo-generated electric fields and currents, resulting in the observed longitudinal variations of the Sq current function.