Challenging Minisatellite Payload Research Articles

Ionospheric response to CIR‐induced recurrent geomagnetic activity during the declining phase of solar cycle 23

AbstractThis paper presents an epoch analysis of global ionosphere responses to recurrent geomagnetic activity during 79 corotating interaction region (CIR) events from 2004 to 2009. The data used were GPS total electron content (TEC) data from the Madrigal Database at the Massachusetts Institute of Technology Haystack Observatory and the electron density (Ne) data obtained from CHAllenging Minisatellite Payload (CHAMP) observations. The results show that global ionosphere responses to CIR events have some common features. In high and middle latitudes, the total electron content (TEC) showed a significant positive response (increased electron densities) in the first epoch day. A negative TEC response occurred at high latitudes of the American sector following the positive response. The CHAMP Ne showed a daytime positive response in all latitudes and a nighttime negative response in the subauroral region. These negative TEC and Ne responses were found to be related to thermospheric composition (O/N2) changes during the storms. At all latitudes, the maximum of the TEC positive effect always occurred at 2–6 h after the CIR starting during local daytime and 10–18 h later for the CIR onset during local nighttime. Case studies indicate that the TEC and Ne positive response had a strong dependence on the southward component (Bz) of the interplanetary magnetic field and solar wind speed. This suggests that penetration electric fields that were associated with changes in solar winds might play a significant role in the positive ionospheric response to storms. During the recovery time of the CIR‐produced geomagnetic activity, the TEC positive disturbance at low latitudes sometimes could last for 2–4 days, whereas at middle to high latitudes the disturbance lasted only for 1 day in most cases. A comparison of the ionospheric responses between the American, European and Asian sectors shows that the ionosphere response in the North American sector was stronger than that in the other two regions. The response of foF2 to the CIR events in middle to high latitudes showed a negative response for 2–3 days after the first epoch day. This is different from the response of TEC, which was mostly positive during the same period of time.

Three-dimensional morphology of equatorial plasma bubbles deduced from measurements onboard CHAMP

Abstract. Total electron content (TEC) between Low-Earth-Orbit (LEO) satellites and the Global Navigation Satellite System (GNSS) satellites can be used to constrain the three-dimensional morphology of equatorial plasma bubbles (EPBs). In this study we investigate TEC measured onboard the Challenging Minisatellite Payload (CHAMP) from 2001 to 2005. We only use TEC data obtained when CHAMP passed through EPBs: that is, when in situ plasma density measurements at CHAMP altitude also show EPB signatures. The observed TEC gradient along the CHAMP track is strongest when the corresponding GNSS satellite is located equatorward and westward of CHAMP with elevation angles of about 40–60°. These elevation and azimuth angles are in agreement with the angles expected from the morphology of the plasma depletion shell proposed by Kil et al.(2009).

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.

North‐south asymmetry of the high‐latitude thermospheric density: IMF BY effect

AbstractPrevious studies have established that the y component of the interplanetary magnetic field (IMF By) plays a role in the north‐south asymmetry of the high‐latitude plasma convection and wind. The effect of the positive/negative IMF By in the Northern Hemisphere resembles the effect that the negative/positive IMF By would have in the Southern Hemisphere. In this study, we demonstrate that the IMF By effect can also contribute to the hemispheric asymmetry of the thermospheric density. We use high‐accuracy air drag measurements from the CHAllenging Minisatellite Payload (CHAMP) satellite and SuperMAG AE index during the period 2001–2006 to examine the response of the high‐latitude thermospheric density to geomagnetic activity. Our statistical analysis reveals that the density response at 400 km is greater in the Southern Hemisphere under positive IMF By conditions, and greater in the Northern Hemisphere under negative IMF By conditions. The results suggest that the IMF By effect needs to be taken into account in upper atmospheric modeling for an accurate description of high‐latitude densities during periods of enhanced geomagnetic activity.

For the first time fengyun3 C satellite-global navigation satellite system occultation sounder achieved spaceborne Bei Dou system radio occultation

The radio occultation (RO) technique using signals from the global navigation satellite system, is widely used to observe the atmosphere for applications such as numerical weather prediction (NWP) and global climate monitoring. Since 1995, there have been turborogue sounder on board global positioning system/meteorology, black jack sounder on board challenging minisatellite payload and gravity recovery and climate experiment, IGOR sounder on board constellation observing system for meteorology, ionosphere and climate, GRAS on board meteorological operational, which have been recieving a large number of RO data, but their observed signals come only from global positioning system (GPS). These RO data have been wildly used in NWP and climate monitoring, however they cannot meet the requirements for high accuracy and real time atmosphere observation, in this case compatible RO sounder to obtain more RO observations is significant. Global navigation satellite system occultation sounder (GNOS) on board the fengyun3 C (FY3 C) satellite, which is the first Bei Dou system (BDS)/GPS compatible RO sounder in the world, was launched on 23 September 2013. Up to now, lots of RO observations have been obtained. In this study, the components of GNOS are introduced; one-day GNOS RO events and their global distribution are analyzed; compared with the GPS RO observations, the accuracy and consistency of BDS real-time positioning results and BDS RO products are analyzed. The preliminary results show that the BDS can enhance the number of RO events by 33.3%; the average deviation and standard deviation of BDS real time positioning results are 6 m and 7 m, respectively; the BDS/GPS difference standard deviation of refrectivity, temperature, humidity, pressure and ionospheric electron density are lower than 2%, 2 K, 1.5 g/kg, 2%, and 15.6%, respectively. The BDS observations/products are consistent with those of GPS, therefore BDS RO products can bring benefit to numerical wheather prediction and global chlimate change analysis.

Intraannual variability of tides in the thermosphere from model simulations and in situ satellite observations

AbstractIn this paper, we provide insights into limitations imposed by current satellite‐based strategies to delineate tidal variability in the thermosphere, as well as the ability of a state‐of‐the‐art model to replicate thermospheric tidal determinations. Toward this end, we conducted a year‐long thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model (TIME‐GCM) simulation for 2009, which is characterized by low solar and geomagnetic activity. In order to account for tropospheric waves and tides propagating upward into the ∼30–400 km model domain, we used 3‐hourly MERRA (Modern‐Era Retrospective Analysis for Research and Application) reanalysis data. We focus on exospheric tidal temperatures, which are also compared with 72 day mean determinations from combined Challenging Minisatellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) satellite observations to assess the model's capability to capture the observed tidal signatures and to quantify the uncertainties associated with the satellite exospheric temperature determination technique. We found strong day‐to‐day tidal variability in TIME‐GCM that is smoothed out when averaged over as few as ten days. TIME‐GCM notably overestimates the 72 day mean eastward propagating tides observed by CHAMP/GRACE, while capturing many of the salient features of other tidal components. However, the CHAMP/GRACE tidal determination technique only provides a gross climatological representation, underestimates the majority of the tidal components in the climatological spectrum, and moreover fails to characterize the extreme variability that drives the dynamics and electrodynamics of the ionosphere‐thermosphere system. A multisatellite mission that samples at least six local times simultaneously is needed to provide this quantification.

Noise features of the CHAMP vector magnetometer in the 1–25 Hz frequency range

CHAMP (CHAllenging Minisatellite Payload), a German small satellite mission to study the Earth's gravity field, magnetic field and upper atmosphere, ended in space on 19 September 2010. Thanks to the good quality of the satellite and to several altitude maneuvers, the satellite provided continuous and reliable observations including house-keeping data at different processing levels for more than 10 years. Among them the high-resolution vector magnetic field data, provided by the FGM (FluxGate vector Magnetometer) at 50Hz sample rate, are one of the most important data products. The 50Hz data allows us to investigate all signals from DC to 25Hz. Here we report on noise features found in the frequency range above 1Hz. There are predominantly three types of disturbances found regularly in the vector magnetic field data. One is at ∼2.1Hz and harmonic signals, an other is an 8Hz single frequency signal. The features of these two signals are characterized and analyzed, which helps us to trace the signals to the source of disturbance. The 2.1Hz harmonic signals mainly depend on the incident angle of sunlight, which points at the star tracker instrument ASC (Advanced Stellar Compass), as the signal depends on the level of blinding. The 8Hz sinusoidal signal is continuously present and its amplitude is controlled by the temperature inside of the satellite. For higher temperatures the signal gets smaller, and it practically disappears above 20°C. Only the FGM x-axis is affected by the 8Hz signal. The third artificial signal affects the FGM y-axis. A frequency sweep from the Nyquist frequency down to zero and up again occurs when the readings of the FGM y-axis go through zero. This is probably related to a ringing of the analog-to-digital converter at the sign change. The amplitudes of all these noise features are generally below 50pT.

CHAMP climate data based on the inversion of monthly average bending angles

Abstract. Global Navigation Satellite System Radio Occultation (GNSS-RO) refractivity climatologies for the stratosphere can be obtained from the Abel inversion of monthly average bending-angle profiles. The averaging of large numbers of profiles suppresses random noise and this, in combination with simple exponential extrapolation above an altitude of 80 km, circumvents the need for a "statistical optimization" step in the processing. Using data from the US–Taiwanese COSMIC mission, which provides ~1500–2000 occultations per day, it has been shown that this average-profile inversion (API) technique provides a robust method for generating stratospheric refractivity climatologies. Prior to the launch of COSMIC in mid-2006, the data records rely on data from the CHAMP (CHAllenging Mini-satellite Payload) mission. In order to exploit the full range of available RO data, the usage of CHAMP data is also required. CHAMP only provided ~200 profiles per day, and the measurements were noisier than COSMIC. As a consequence, the main research question in this study was to see if the average bending-angle approach is also applicable to CHAMP data. Different methods for the suppression of random noise – statistical and through data quality prescreening – were tested. The API retrievals were compared with the more conventional approach of averaging individual refractivity profiles, produced with the implementation of statistical optimization used in the EUMETSAT (European Organisation for the Exploitation of Meteorological Satellites) Radio Occultation Meteorology Satellite Application Facility (ROM SAF) operational processing. In this study it is demonstrated that the API retrieval technique works well for CHAMP data, enabling the generation of long-term stratospheric RO climate data records from August 2001 and onward. The resulting CHAMP refractivity climatologies are found to be practically identical to the standard retrieval at the DMI (Danish Meteorological Institute) below altitudes of 35 km. Between 35 and 50 km, the differences between the two retrieval methods started to increase, showing largest differences at high latitudes and high altitudes. Furthermore, in the winter hemisphere high-latitude region, the biases relative to ECMWF (European Centre for Medium-range Weather Forecasts) were generally smaller for the new approach than for the standard retrieval.

Global ionospheric and thermospheric response to the 5 April 2010 geomagnetic storm: An integrated data‐model investigation

AbstractWe present a case study of the 5 April 2010 geomagnetic storm using observations and numerical simulations. The event was driven by a fast‐moving coronal mass ejection and despite being a moderate storm with a minimum Dst near −50 nT, the event exhibited elevated thermospheric density and surges of traveling atmospheric disturbances (TADs) more typically seen during major storms. The Thermosphere‐Ionosphere‐Mesosphere‐Electrodynamics General Circulation Model (TIMEGCM) was used to assess how these features were generated and developed during the storm. The model simulations gave rise to TADs that were highly nonuniform with strong latitude and longitude/local time dependence. The TAD phase speeds ranged from 640 m/s to 780 m/s at 400 km and were ~5% lower at 300 km and approximately 10–15% lower at 200 km. In the lower thermosphere around 100 km, the TAD signatures were nearly unrecognizable due to much stronger influence of upward propagating atmospheric tides. The thermosphere simulation results were compared to observations available from the Gravity Field and Steady‐State Ocean Circulation Explorer (GOCE), CHAllenging Minisatellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) satellites. Comparison with GOCE data shows that the TIMEGCM reproduced the cross‐track winds over the polar region very well. The model‐data comparison also revealed some differences, specifically, the simulations underestimated neutral mass density in the upper thermosphere above ~300 km and overestimated the storm recovery tome by 6 h. These discrepancies indicate that some heating or circulation dynamics and potentially cooling processes are not fully represented in the simulations, and also that updates to some parameterization schemes in the TIMEGCM are warranted.

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.

Estimation of Winds from GPS Radio Occultations

AbstractGPS radio occultations (RO) offer the possibility to map winds in the upper troposphere and lower stratosphere (UTLS) region because geopotential height is the independent coordinate of retrieval. Most other sounders do not offer this possibility because their independent coordinate of retrieval is pressure. To estimate the precision with which GPS radio occultation data can map winds, dry pressure profiles are simulated from the Interim European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-Interim) at the actual locations of the Challenging Minisatellite Payload (CHAMP) and the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) soundings for the year 2007. Monthly wind maps were created by using Bayesian interpolation on subsampled ERA-Interim data in 3–5-day bins and subsequent averaging over a month. Mapping winds in this approach requires that 1) geostrophy approximates winds; 2) dry pressure approximates pressure in the UTLS; and 3) geopotential height can be mapped accurately given sparse, nonuniform distributions of data. This study found that, under these conditions, it is possible to map monthly winds near the tropopause with an accuracy of 5.6 m s−1 with CHAMP alone and 4.5 m s−1 with COSMIC alone. The dominant contributors to uncertainty are undersampling of the atmosphere and ageostrophy, particularly at the leading and trailing edges of the subtropical jet. The former is reduced with increased density of GPS RO soundings. The latter cannot be reduced even after iteration for balanced winds. Nevertheless, it is still possible to capture the general wind pattern and to determine the position of the subtropical jet despite the uncertainty in its magnitude. COSMIC radio occultation measurements from 2006 through 2011 were used to estimate monthly geostrophic winds maps in UTLS. The resultant wind dataset is posted online.

Typical comparisons between plasma bubble and blob distributions at different altitudes over the Indian sector using a unique combination of satellite-based observations – a case study

We report in this article, for the first time, on the altitudinal variations of ionospheric irregularities during post-sunset hours between 14 and 17 October 2001 at Indian longitudes by using a unique combination of Challenging Mini Satellite Payload (CHAMP), Republic of China Satellite (ROCSAT-1), and Defense Meteorological Satellite Programme (DMSP) satellites located at about 400, 650, and 850 km, respectively. It became possible for us to study the width of plasma bubbles and plasma blobs in the longitudinal (zonal) direction at different altitudes along with temporal variations of ion velocity components in horizontal (Vy) and vertical (Vz) directions as probed by ROCSAT-1 and DMSP satellites. The dominant features noticed in Vy and Vz components are their anti-correlation relationship at the ROCSAT-1 satellite’s altitude (mirroring effect). The most important observation from this study is that plasma bubbles are found to have occurred immediately after post-sunset hours, whereas plasma blobs were observed after three hours following the bubbles’ appearance time and continued until pre-sunrise hours at higher altitudes between around ±5° and ±20° from the magnetic equator, which indicate that the plasma blobs emanate from plasma bubbles that generated at the bottomside of the ionospheric F-layer shortly after sunset on similar lines to a few recent research studies. It is therefore believed that the polarized electric fields generated inside plasma bubbles might have played a role in the generation of higher-altitude plasma blobs. Importantly, the calculated power spectra of bubbles and blobs show a nearly equal trend, indicating that they are intricately connected with each other.

An empirical model of the auroral oval derived from CHAMP field-aligned current signatures – Part 2

Abstract. In this paper we introduce a new model for the location of the auroral oval. The auroral boundaries are derived from small- and medium-scale field-aligned current (FAC) based on the high-resolution CHAMP (CHAllenging Minisatellite Payload) magnetic field observations during the years 2000–2010. The basic shape of the auroral oval is controlled by the dayside merging electric field, Em, and can be fitted well by ellipses at all levels of activity. All five ellipse parameters show a dependence on Em which can be described by quadratic functions. Optimal delay times for the merging electric field at the bow shock are 30 and 15 min for the equatorward and poleward boundaries, respectively. A comparison between our model and the British Antarctic Survey (BAS) auroral model derived from IMAGE (Imager for Magnetopause-to-Aurora Global Exploration) optical observations has been performed. There is good agreement between the two models regarding both boundaries, and the differences show a Gaussian distribution with a width of ±2° in latitude. The difference of the equatorward boundary shows a local-time dependence, which is 1° in latitude poleward in the morning sector and 1° equatorward in the afternoon sector of the BAS model. We think the difference between the two models is caused by the appearance of auroral forms in connection with upward FACs. All information required for applying our auroral oval model (CH-Aurora-2014) is provided.

Determining the boundaries of the auroral oval from CHAMP field-aligned current signatures – Part 1

Abstract. In this paper we present the first statistical study on auroral oval boundaries derived from small- and medium-scale field-aligned currents (FACs, < 150 km). The dynamics of both the equatorward and poleward boundaries is deduced from 10 years of CHAMP (CHAllenging Minisatellite Payload) magnetic field data (August 2000–August 2010). The approach for detecting the boundaries from FACs works well under dark conditions. For a given activity level the boundaries form well-defined ellipses around the magnetic pole. The latitudes of the equatorward and poleward boundaries both depend, but in different ways, on magnetic activity. With increasing magnetic activity the equatorward boundary expands everywhere, while the poleward boundary shows on average no dependence on activity around midnight, which seems to be stationary at a value of about 72° Mlat. Functional relations between the latitudes of the boundaries and different magnetic activity indices have been tested. Best results for a linear dependence are derived for both boundaries with the dayside merging electric field. The other indices, like the auroral electrojet (AE) and disturbance storm time (Dst) index, also provide good linear relations but with some caveats. Toward high activity a saturation of equatorwards expansion seems to set in. The locations of the auroral boundaries are practically independent of the level of the solar EUV flux and show no dependence on season.

The Midlatitude Summer Night Anomaly as observed by CHAMP and GRACE: Interpreted as tidal features

AbstractThis paper presents a description of the Midlatitude Summer Night Anomaly (MSNA) in terms of solar tidal signatures, based on in situ observations from CHAMP (CHAllenging Minisatellite Payload) and GRACE (Gravity Recovery and Climate Experiment) during the solar minimum years 2008 and 2009. Our analysis is focusing on 40° to 60° magnetic latitude ranges in both hemispheres, where the reversed diurnal variations of the electron density are strongest. The results revealed that in the Southern Hemisphere the longitudinally symmetric tide D0 is particularly strong during December solstice. The well‐known Weddell Sea Anomaly is caused by a simultaneous constructive interference of three components D0, DW2, and SPW1. During June solstice the eastward propagating tide DE1 is the strongest in the Northern Hemisphere, which causes a wave‐2 longitudinal pattern. The two crests of the wave‐2 pattern at nighttime correspond well with the MSNA feature in the Northern Hemisphere. The MSNA feature over the USA continent is particularly strong, which can be explained by the combined contributions of the components DE1, D0, and DW2. The diurnally varying difference in electron density between the USA East and West Coast can also be explained by the phase propagation of the DE1. A similar effect has also been observed in the Asian region. The peak electron densities of the tidal component D0 appear around 0700 LT and 2000 LT in the Southern and Northern Hemispheres, respectively. The time shift suggests that the two hemispheres move in antiphase up and down. The planetary wave SPW1 exhibits an electron density crest near longitude sectors where the dip equator reaches far into the summer hemisphere.

Assimilation of GPS Radio Occultation Refractivity Data with WRF 3DVAR and Its Impact on the Prediction of a Heavy Rainfall Event

AbstractTo accommodate accurate analyses and forecasts of a heavy rainfall event over the Korean Peninsula, the authors assimilated the GPS radio occultation (RO) data by using the Weather Research and Forecasting Model (WRF) and its three-dimensional variational data assimilation system (3DVAR). The employed datasets are from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) and Challenging Minisatellite Payload (CHAMP) missions. The selected case was from late October 2006, which intensively hit the northeastern part of the Korean Peninsula with record-breaking rainfall. In this study, the local refractivity observation operator was used in assimilating GPS RO soundings. The results are more pronounced for the cycling assimilation of GPS RO data than for the one-time data assimilation. From all of the parameters investigated (temperature, geopotential height, specific humidity, and winds), the GPS RO soundings highly modified the moisture distribution in the lower troposphere and also changed the wind field via the model dynamics. For the heavy rainfall forecast, the quantitative accuracy of the precipitation forecast with the GPS RO data assimilation was in good agreement with observations in terms of the maximum rainfall amount and threat scores. The improved forecast in the experiment came from the exact positioning of the low pressure system and its consequent convergence near the rainfall area. When RO data and GPS precipitable water data were assimilated simultaneously, the moisture distribution changed horizontally and vertically such that it increased the amount of rainfall, and an accurate description of the convective system development was feasible.

Thermospheric density perturbations in response to substorms

AbstractWe use 5 years (2001–2005) of CHAMP (Challenging Minisatellite Payload) satellite data to study average spatial and temporal mass density perturbations caused by magnetospheric substorms in the thermosphere. Using statistics from 2306 substorms to construct superposed epoch time series, we find that the largest average increase in mass density of about 6% occurs about 90 min after substorm expansion phase onset about 3 h of magnetic local time east of the onset region. Averaged over the entire polar auroral region, a mass density increase of about 4% is observed. Using a simple model to estimate the mass density increase at the satellite altitude, we find that an energy deposition rate of 30 GW applied for half an hour predominantly at an altitude of 110 km is able to produce mass density enhancements of the same magnitude. When taking into account previous work that has shown that 80% of the total energy input is due to Joule heating, i.e., enhanced electric fields, whereas 20% is due to precipitation of mainly electrons, our results suggest that the average substorm deposits about 6 GW in the polar thermosphere through particle precipitation. Our result is in good agreement with simultaneous measurements of the NOAA Polar‐orbiting Operational Environmental Satellite (POES) Hemispheric Power Index; however, it is about 1 order of magnitude less than reported previously.

Multisatellite observations of an intensified equatorial ionization anomaly in relation to the northern Sumatra earthquake of March 2005

AbstractHere we report multisatellite observations of ionospheric disturbances in relation to the occurrence of the M8.7 northern Sumatra earthquake of 28 March 2005. The DEMETER (Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions) and CHAMP (Challenging Minisatellite Payload) satellite data were investigated to find possible precursory and postevent phenomena. It was found that EIA (equatorial ionization anomaly) strength expressed in the apex height, derived from the CHAMP plasma density profile, was intensified along the orbits whose longitudes were close to the epicenter within about a week before and after occurrence of the earthquake. Increases in electron and O+ density along the orbits close to the epicenter were also observed in the DEMETER measurements. The normalized equatorial plasma density derived from the DEMETER measurements showed intensification about a week before and after the earthquake reaching maximum the day after the shock and afterward disappearing. In addition, similar behavior of the EIA enhancements related to the M8.0 Pisco earthquake of 15 August 2007 was observed. Surveys of space weather and geomagnetic activities excluded the possibility that these fluctuations were caused by changes in space weather or by a geomagnetic storm. Statistical analyses of the longitudinal variation revealed that the EIA was enhanced in the west of the epicenter and reduced in the east of the epicenter, and this fits the “increased conductivity” model. Based on these observations, we proposed a revised view of seismo‐ionospheric coupling in the region of the geomagnetic equator, to explain the EIA features observed in this study.

Global and frequent appearance of small spatial scale field-aligned currents possibly driven by the lower atmospheric phenomena as observed by the CHAMP satellite in middle and low latitudes

Using magnetic field data obtained by the Challenging Minisatellite Payload (CHAMP), we show global and frequent appearance of small-amplitude (1 to 5 nT on the dayside) magnetic fluctuations with period around a few tens of seconds along the satellite orbit in middle and low latitudes. They are different from known phenomena, such as the Pc3 pulsations. The following characteristics are presented and discussed in this paper: (1) The magnetic fluctuations are perpendicular to the geomagnetic main field, and the amplitude of the zonal (east–west) component is larger than that of the meridional component in general. (2) As latitude becomes lower around the dip equator, the period tends to become longer. (3) The amplitudes have clear local time dependence, which is highly correlated to the ionospheric conductivities in local time (LT) 06–18. (4) The amplitude of the fluctuations shows magnetic conjugacy to a certain extent. (5) The amplitude shows no dependence on solar wind parameters nor geomagnetic activity. (6) A seasonal dependence is seen clearly. The amplitudes in the northern summer and winter are larger than those in the equinoxes. In the northern summer, the amplitudes above the Eurasian and South American continents and their conjugate areas are larger. In the northern winter, those above the eastern Pacific Ocean are larger. We suggest that the above characteristics, (1) to (6), can be attributed to the small spatial scale field-aligned currents having a lower atmospheric origin through the ionospheric dynamo process.

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.