Showing papers by "Jiuhou Lei published in 2012"

A comparison of the effects of CIR‐ and CME‐induced geomagnetic activity on thermospheric densities and spacecraft orbits: Case studies

Abstract: Enhanced energy input from the magnetosphere to the upper atmosphere during geomagnetic storms has a profound effect on thermospheric density and consequently near-Earth satellite orbit decay. These geomagnetic storms are caused by two different processes. The first is coronal mass ejections (CMEs) and the second is corotating interaction regions (CIRs). CME-driven storms are characterized by large maximum energy input but relatively short duration, whereas CIR-driven storms have relatively small maximum energy input but are of a considerably longer duration. In this paper we carried out a statistical study to assess the relative importance of each kind of storm to satellite orbital decay. The results demonstrate that CIR storms have a slightly larger effect on total orbital decay than CME storms do in a statistical sense. During the declining phase and the minimum years of a solar cycle, CIR storms occur frequently and quasiperiodically. These storms have a large effect on thermospheric densities and satellite orbits because of their relatively long duration. Thus, it is important to fully understand their behavior and impact.

Annual and semiannual variations of thermospheric density: EOF analysis of CHAMP and GRACE data

Abstract: [1] In this paper, observations from CHAMP and GRACE during 2002–2010 are used to study the seasonal variations of thermospheric density by characterizing the dominant modes of thermospheric density variability as empirical orthogonal functions (EOFs). Our results showed that the first three EOFs captured most of the density variability, which can be as large as 98% of total density variability. Subsequently, the obtained mean field, first three EOFs and the corresponding amplitudes of three EOFs are applied to construct a thermospheric density model at 400 km to study seasonal variations of thermospheric density under geomagnetically quiet conditions. Thermospheric density shows strong latitudinal dependence in seasonal variation, although it usually has maxima near the equinoxes and minimum in the local winter at middle and high latitudes. Semiannual variations imbedded in the annual variations are seen at all latitudes; annual variations however become dominant in the southern hemisphere. Specifically, the observations show that the annual amplitude can reach as large as 40–50% of the annual mean at high latitudes in the southern hemisphere and it decreases gradually from the southern to northern hemisphere. The semiannual component to the annual mean is about 15–20% without significant latitudinal dependence. Additionally, the relative amplitudes of annual and semiannual variations in the MSISE00 density agree fairly well with the observations, albeit the MSISE00 gives an opposite solar activity dependence for the annual and semiannual variations compared with the positive F107 dependence seen in the observations.

Thermosphere and ionosphere response to subauroral polarization streams (SAPS): Model simulations

Abstract: An empirical model of subauroral polarization streams (SAPS) has been incorporated into the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) This SAPS driven TIEGCM is used to simulate the effect of SAPS on the global thermosphere and ionosphere during a moderate geomagnetically active period between day of year (DOY) 329 and 333 in 2008 Model results show: (1) SAPS caused an increase in global thermospheric temperature which became stronger with time This neutral temperature increase was more significant in subauroral and auroral regions Joule heating by the SAPS and the redistribution of this heat by dynamic processes were the primary mechanisms for the simulated global neutral temperature changes (2) In the SAPS driven TIEGCM, the strong ion drag effect in the subauroral SAPS channel drove large changes in thermospheric winds Zonal neutral winds had either an extra, separate channel of westward flow in the subauroral region in the afternoon-midnight sector or a broad westward wind jet that merged with the regular duskside auroral westward zonal neutral wind driven by the high latitude convection pattern The exact latitudinal profile of the zonal winds depended on local time (3) The response of neutral temperature and wind to SAPS was more significant at higher altitudes and exhibited seasonal/hemispheric asymmetry (4) The heating to the thermosphere by SAPS also resulted in changes in thermospheric composition with upwelling of molecular rich air in subauroral and auroral regions and downwelling of atomic oxygen rich air at other latitudes These changes in thermospheric composition contributed to the deeper and more extended ionospheric electron density depletions in subauroral middle latitude regions, as well as electron density increases along the equatorward edge of the SAPS channel in the afternoon sector

Overcooling in the upper thermosphere during the recovery phase of the 2003 October storms

Abstract: [1] Infrared radiative emissions by carbon dioxide (CO2) and nitric oxide (NO) are the major cooling mechanisms of the lower thermosphere. During geomagnetically active periods, the NO density and cooling rate in the auroral regions increase significantly as a result of particle precipitation and Joule heating. Previous studies have shown that the time for NO density to recover to quiet time levels is longer than that of the thermosphere temperature or density recovery. This study explores the implications of these different recovery rates for the post-storm thermosphere. Thermosphere densities retrieved from the CHAMP and GRACE accelerometer measurements and NO cooling rates measured by TIMED/SABER are used to examine their variations during the post-storm period of the October 2003 geomagnetic storms. It was found that thermosphere densities at both CHAMP and GRACE altitudes recovered rapidly and continuously decreased below the quiet time densities during the post-storm period, especially at middle latitudes. Compared with the quiet time values, the maximum depletion in the CHAMP and GRACE densities after the storm is about 23–36%, and the estimated decrease of thermospheric temperature is as large as 70–110 K. Our analysis suggests that the elevated NO cooling rate, resulting from the slower recovery of NO densities in the post-storm period, is a plausible cause for this apparent post-storm overcooling of the thermosphere.

The impact of helium on thermosphere mass density response to geomagnetic activity during the recent solar minimum

Abstract: [1] High-resolution mass density observations inferred from accelerometer measurements on the CHAMP and GRACE satellites are employed to investigate the thermosphere mass density response with latitude and altitude to geomagnetic activity during the recent solar minimum Coplanar orbital periods in February 2007 and December 2008 revealed the altitude and latitude response in thermosphere mass density for their respective winter hemispheres was influenced by the relative amount of helium and oxygen present The CHAMP-to-GRACE (C/G) mass density ratio depends on two terms; the first proportional to the ratio of the mean molecular weight to temperature and the second proportional to the vertical gradient of the logarithmic mean molecular weight For the relative levels of helium and oxygen in February 2007, the winter hemisphere C/G mass density response to geomagnetic activity, although similar to the summer hemisphere, was caused predominantly by changes in the vertical gradient of the logarithmic mean molecular weight In December 2008, the significant presence of helium caused the mean molecular weight changes to exceed temperature changes in the winter hemisphere leading to an increase in the C/G ratio with increasing geomagnetic activity, in opposition to the decrease observed in the summer hemisphere that was caused primarily by temperature changes The observed behavior is indicative of composition effects influencing the mass density response and the dynamic action of the oxygen to helium transition region in both latitude and altitude will lead to complex behaviors in the mass density at GRACE altitudes throughout the extended solar minimum from 2007 to 2010

The effect of ∼27 day solar rotation on ionospheric F2 region peak densities (NmF2)

Abstract: [1] Ionospheric F2 region peak electron densities (NmF2) observed from 11 ionosonde stations in the East Asian-Australian sector from 1969 to 1986 have been used to investigate the effect of ∼27 day solar rotation on the ionosphere. These stations were located from the magnetically equatorial regions to the middle latitudes in both hemispheres. We found that, averaged over all stations and for 18 years, the normalized standard deviation of the midday ∼27 day variations of NmF2 was 8% and that of the midnight variations was 10%. We applied different data analysis methods, including Fourier transform, band-pass filter, and multiple linear regression analysis, to determine quantitatively the sources of the observed ∼27 day variations of NmF2 and their relative contributions to these variations. Our results show that the ∼27 day variations in solar radiation and geomagnetic activity, caused by solar rotation, are the main drivers of the ionospheric ∼27 day variations. They accounted for more than 85% of the variations seen in the NmF2 ∼27 day variation, and their contributions became about 95% at higher latitudes. At geomagnetically low latitudes, the contribution of the ∼27 day variation in solar EUV radiation was greater than that of the ∼27 day variation in geomagnetic activity. However, the contribution from geomagnetic activity became more significant and was even larger than the contribution of solar radiation at higher latitudes, especially at midnight. At all latitudes the correlation between the ∼27 day variations of NmF2 and solar radiation was evidently positive, whereas that between NmF2 and geomagnetic activity was positive at geomagnetically low latitudes and became negative at higher middle latitudes. We did not found large seasonal or solar cycle changes in the ∼27 day variations of NmF2. These variations, however, did show significant differences between the two hemispheres.

Positive ionospheric storm effects at Latin America longitude during the superstorm of 20–22 November 2003: revisit

Abstract: . Positive ionospheric storm effects that occurred during the superstorm on 20 November 2003 are investigated using a combination of ground-based Global Positioning System (GPS) total electron content (TEC), and the meridian chain of ionosondes distributed along the Latin America longitude of ~280° E. Both the ground-based GPS TEC and ionosonde electron density profile data reveal significant enhancements at mid-low latitudes over the 280° E region during the main phase of the November 2003 superstorm. The maximum enhancement of the topside ionospheric electron content is 3.2–7.7 times of the bottomside ionosphere at the locations of the ionosondes distributed around the mid- and low latitudes. Moreover, the height of maximum electron density exceeds 400 km and increases by 100 km compared with the quiet day over the South American area from middle to low latitudes, which might have resulted from a continuous eastward penetration electric field and storm-generated equatorward winds. Our results do not support the conclusions of Yizengaw et al. (2006), who suggested that the observed positive storm over the South American sector was mainly the consequence of the changes of the bottomside ionosphere. The so-called "unusual" responses of the topside ionosphere for the November 2003 storm in Yizengaw et al. (2006) are likely associated with the erroneous usage of magnetometer and incomplete data.

Simulations of the equatorial thermosphere anomaly: Field-aligned ion drag effect

Abstract: [1] In this paper the impact of the field-aligned ion drag on equatorial thermosphere temperature and density is quantitatively investigated on the basis of the National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model (NCAR TIEGCM) simulations under high solar activity (F107 = 180). The increase of upward vertical winds over the magnetic equator associated with the additional divergence of meridional winds, caused by the inclusion of field-aligned ion drag, leads to a reduction in thermosphere temperature and density at the magnetic equator through enhanced adiabatic cooling. We found that the field-aligned ion drag has an obvious impact on the thermosphere only over the magnetic equatorial region in the daytime and evening sectors, whereas it has less effect on the equatorial thermosphere anomaly (ETA) crests. The daytime neutral temperature over the magnetic equator is reduced by about 30 K, for altitudes above 250 km without significant altitudinal variations, when field-aligned ion drag is included in the simulation. The thermosphere density in the magnetic equatorial region starts to change slightly at 300 km and depletes by about 5% at 400 km, while experiencing a greater decrease with altitude. Furthermore, the trough produced in the neutral temperature and density corresponds well with the magnetic dip equator. The ETA features during 12:00–18:00 LT become obvious as a result of the inclusion of the field-aligned ion drag. Specifically, our results show that at 400 km the crest-trough differences in neutral temperature are about 30–60 K, and the crest-trough ratios in thermosphere density are 1.03–1.06, comparable with observations.

Superposed epoch analyses of thermospheric response to CIRs: Solar cycle and seasonal dependencies

Abstract: Thermospheric response to Corotating Interaction Regions (CIRs) has been studied previously; however, its solar cycle and seasonal effects have not been fully investigated. Thermospheric mass density at 400 km measured by the CHAMP satellite during 2001-2008 and Sigma O/N2 from the TIMED/GUVI instrument covering a period from 2002 to 2008 are used to investigate the solar cycle and seasonal dependencies of the thermospheric response to CIRs. Our results reveal: (1) solar minimum CIRs compared to solar maximum counterparts have larger solar wind speeds before and after the stream interface. However, solar wind dynamic pressure and merging electric field are slightly larger at solar maximum than solar minimum. (2) CIR-induced variations of Sigma O/N2 are characterized by high latitude depression and low latitude enhancement, a distinction from global enhancement of neutral density at a fixed altitude. These relative thermospheric changes are dependent on solar cycle, with a more pronounced increase in neutral density at all latitudes and a stronger decrease in Sigma O/N2 at high latitude at solar minimum than at solar maximum. (3) A seasonal asymmetry is presented in the relative deviations of thermospheric mass density and composition. On the dayside, the peak increases of neutral density at high latitudes on average are similar to 40% in the summer hemisphere and similar to 26% in the winter hemisphere. Nighttime neutral density changes are more remarkable than that in the same latitudinal bands of daytime and have the same seasonal preference of enhancement as the dayside. At the daytime, Sigma O/N2 at high latitudes suffers more reduction in the summer hemisphere than in the winter hemisphere. At middle latitudes, Sigma O/N2 reduces in the winter hemisphere; nevertheless, it increases slightly in the summer hemisphere.

Simulations of the equatorial thermosphere anomaly: Physical mechanisms for crest formation

Abstract: [1] Several mechanisms including heat transport due to zonal winds, chemical heating and field-aligned ion drag have been proposed to explain the formation of the Equatorial Thermosphere Anomaly (ETA), but the cause of the ETA crests in thermosphere temperature is still a mystery. Our companion study (Lei et al., 2012) has revealed that the field-aligned ion drag mainly contributes to the ETA trough, but has little effect on the ETA crests. In this study, the mechanisms of heat transport associated with zonal winds and chemical heating due to recombination are examined to assess their contributions to the production of the ETA crests on the basis of National Center for Atmospheric Research Thermosphere-Ionosphere-Electrodynamics General Circulation Model (NCAR-TIEGCM) simulations. Our sensitivity simulations demonstrate that neither heat transport due to zonal winds nor chemical heating is able to explain the formation of the ETA crests. Instead, we found that the formation of the ETA crests is attributed to plasma-neutral heating which has two peaks in the topside ionosphere aside the magnetic equator. These two peaks, which are largely controlled by the magnetic field, are the results of energy transfer from thermal electrons and ions to the neutrals through collisions due to their temperature differences, albeit the ultimate source of this heating is solar radiation which produces photoelectrons that mainly depend on solar zenith angle. The TIEGCM simulations show that the crests of the ETA always locate poleward by 10°–15° with respect to those of the Equatorial Ionosphere Anomaly (EIA), although the trough location of the ETA resembles that of the EIA. The location of the ETA crests is associated with the two-hump structure in plasma-neutral collision heating which is small inside the EIA region and larger at the poleward edge of this region.

Artificial ionospheric wave number 4 structure below the F2 region due to the Abel retrieval of radio occultation measurements

Abstract: We analyzed the effect of the Abel inversion on the wave number 4 (WN4) structure from the GPS radio occultation (RO)---measured electron densities by using the FORMOSAT-3/COSMIC (F-3/C) observations under the equinox condition. The Abel-retrieved electron density from both the F-3/C observations and the simulated results by an empirical model with an imposed WN4 structure in the F layer are investigated. It is found that the Abel inversion can reproduce the real WN4 structure well in the F2 layer. However, it will result in pseudo and reversed-phase WN4 structure in the lower altitude (F1 and E layers). Quantitatively, relative ±15% WN4 signature in the F2 layer can produce ±40% artificial WN4 in the E and F1 layers. Analysis on the F-3/C data shows about ±15% WN4 signature in the F2 layer and ±50% WN4 with reversed-phase in the E and F1 layers. The F-3/C-observed WN4 structure in the E and F1 layers might be the combinations of the real WN4 signature and the artificial effects of Abel retrieval.

Terdiurnal migrating‐tide signature in ionospheric total electron content

Abstract: We report the latitudinal, seasonal and solar cycle variations of terdiurnal migrating tide (i.e., terdiurnal westward wave number 3, TW3) signature in ionospheric total electron content (TEC). Our investigations are based on the JPL global ionospheric maps (GIMs) during 1999-2011. The absolute amplitude of TW3 exhibits maximum values in the magnetic equatorial region, which reaches about 8 TECu under high solar activity and 1.5 TECu under low solar activity. The relative amplitude of zonal mean TEC in equatorial region, however, shows persistent patterns from 1999 to 2011 with little solar activity dependence. The relative amplitude of TW3 has a peak of similar to 10% in the magnetic equatorial region and a second one of similar to 7% at magnetic middle latitudes (30 degrees-50 degrees). At high latitudes in local winter seasons, the maximum relative amplitude occurs, which varies between 8 and 15% with slightly larger amplitudes in the Northern Hemisphere. The global structure of TW3 signature in TEC should shed some light on the coupling between the ionosphere-thermosphere and the lower atmosphere.

Auroral electrojets variations caused by recurrent high-speed solar wind streams during the extreme solar minimum of 2008

Abstract: The IMAGE network magnetic measurements are used to investigate the response of the auroral electrojets to the recurrent high-speed solar wind streams (HSSs) during the extreme solar minimum period of 2008. We first compare the global AU/AL indices with the corresponding IU/IL indices determined from the IMAGE magnetometer chain and find that the local IMAGE chain can better monitor the activity in MLT sectors 1230-2230 for IU and 2230-0630 for IL during 2008. In the optimal MLT sectors, the eastward and westward electrojets and their central latitude reveal clear 9-day periodic variations associated with the recurrent HSSs. For the 9-day perturbations, both the eastward and westward electrojet currents are better correlated with parallel electric field (EPAR) and electron hemispheric power (HPe) than with other forcing parameters. Interestingly, the eastward electrojet shows good correlations (r > 0.6) with EPAR and HPe only in part of its optimal MLT-sector, roughly 1200-1800, while the westward electrojet shows good correlations (r < -0.6) with EPAR and HPe in its whole optimal MLT sector. The poor correlations between the eastward electrojet and EPAR and HPe in the MLT sector 1800-2200 might be attributed to the impact of other magnetosphere-ionosphere coupling processes. The sensitivities of the eastward and westward electrojet currents to EPAR are close to 0.06 MA/(mV/m) and -0.12 MA/(mV/m), respectively, and the sensitivities of their central latitudes to EPAR are close to -2.83 Deg/(mV/m) and -2.14 Deg/(mV/m), respectively. The observed auroral electrojet response to the recurrent solar wind forcing provides new opportunities to study the physical processes governing the eastward and westward auroral electrojets.