Showing papers by "Jiuhou Lei published in 2015"

Response of the topside and bottomside ionosphere at low and middle latitudes to the October 2003 superstorms

Abstract: Ionospheric observations from the ground-based GPS receiver network, CHAMP and GRACE satellites and ionosondes were used to examine topside and bottomside ionospheric variations at low and middle latitudes over the Pacific and American sectors during the October 2003 superstorms. The latitudinal variation and the storm time response of the ground-based GPS total electron content (TEC) were generally consistent with those of the CHAMP and GRACE up-looking TEC. The TECs at heights below the satellite altitudes during the main phases were comparable to, or even less than, the quiet time values. However, the storm time CHAMP and GRACE up-looking TECs showed profound increases at low and middle latitudes. The ground-based TEC and ionosonde data were also combined to study the TEC variations below and above the F2 peak height (hmF2). The topside TECs above hmF2 at low and middle latitudes showed significant increases during storm time; however, the bottomside TEC below hmF2 did not show so obvious changes. Consequently, the bottomside ionosphere made only a minor contribution to the ionospheric positive phase seen in the total TEC at low and middle latitudes. Moreover, at middle latitudes F2 peak electron densities during storm time did not have the obvious enhancements that were seen in both the ground-based and topside TECs, although they were accompanied by increases of hmF2. Therefore, storm time TEC changes are not necessarily related to changes in ionospheric peak densities. Our results suggest that TEC increases at low and middle latitudes are also associated with effective plasma scale height variations during storms.

Pathways of F region thermospheric mass density enhancement via soft electron precipitation

Abstract: The efficiencies of pathways of thermospheric heating via soft electron precipitation in the dayside cusp region are investigated using the coupled magnetosphere-ionosphere-thermosphere model (CMIT). Event-based data-model comparisons show that the CMIT model is capable of reproducing the thermospheric mass density variations measured by the CHAMP satellite during both quite and active periods. During the 24 August 2005 storm event (Kp = 6−) while intense Joule heating rate occurs in the polar cusp region, including soft electron precipitation is important for accurately modeling the F region thermospheric mass density distribution near the cusp region. During the 27 July 2007 event (Kp = 2−) while little Joule heating rate occurs in the polar cusp region, the controlled CMIT simulations suggest that the direct pathway through the energy exchange between soft electrons and thermospheric neutrals is the dominant process during this event, which only has a small effect on the neutral temperature and mass density at 400 km altitude. Comparisons between the two case studies show that the indirect pathway via increasing the F region Joule heating rate is a dominant process during the 24 August 2005 storm event, which is much more efficient than the direct heating process.

A simulation study on the impact of altitudinal dependent vertical plasma drift on the equatorial ionosphere in the evening

Abstract: We carry out a simulation study on the impact of altitudinal dependent plasma drift on the equatorial ionosphere in the evening, under geomagnetically quiet conditions. Our study used the vertical plasma drift velocity data measured by an incoherent scatter radar at Jicamarca (11.95 ◦ S, 76.87 ◦ W). The data covered the local sunset period on 15 and 16 November 2004. The plasma drift had significant altitudinal variations in the vertical component, which is perpendicular to the magnetic field. We employed SAMI2 (SAMI2 is another model of the ionosphere) to evaluate the effect of the altitude-dependent ion drift on the equatorial ionosphere. Three types of plasma drift velocity inputs were used in our simulations. The first input is calculated from an empirical model, the second is a height-averaged drift obtained from the observed drift velocity, and the third one corresponds to the observed altitudinal dependent drift data. A strong equatorial ionization anomaly occurred in the results of all numerical experiments. Additional layers (F3 layers) in electron densities over the equatorial F region and "arch" latitudinal structures extending to lower middle latitudes were seen in the simulations driven by the observed altitudinal dependent drift. We further show that neutral winds do not have a significant effect on the simulated F3 layers. The results of our numerical experiments suggest that the simulated additional ionospheric layers and arch structures are associated with the altitudinal gradients in the vertical plasma drift velocity.

A numerical study of the effects of migrating tides on thermosphere midnight density maximum

Abstract: In this study, we employed the National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) and the extended Canadian Middle Atmosphere Model (eCMAM) to investigate the role of the migrating terdiurnal tide on the formation and variation of the thermosphere midnight temperature maximum (MTM) and midnight mass density maximum (MDM). The migrating terdiurnal tide from the eCMAM was applied at the TIEGCM's lower boundary, along with the migrating diurnal and semidiurnal tides from the Global-Scale Wave Model. Several numerical experiments with different combinations of tidal forcing at the TIEGCM's lower boundary were carried out to determine the contribution of each tide to MTM/MDM. We found that the interplay between diurnal, semidiurnal, and terdiurnal tides determines the formation of MTM/MDM and their structure in the upper thermosphere. The decrease of thermospheric mass density after MDM reaches its maximum at ~02:00 local time is mainly controlled by the terdiurnal tide. Furthermore, we examined the generation mechanisms of the migrating terdiurnal tide in the upper thermosphere and found that they come from three sources: upward propagation from the lower thermosphere, in situ generation via nonlinear interaction, and thermal excitation.

Feasibility study on the derivation of the O+‐O collision frequency from ionospheric field‐aligned observations

Abstract: Recently, Vickers et al. (2013) used a simplified ion momentum equation to derive the ion-neutral (O+-O) collision frequency and thermospheric atomic oxygen density from the European Incoherent Scatter Svalbard Radar data. It was assumed that the vertical neutral wind at high latitudes is generally small under geomagnetically quiet conditions so that it was neglected in their derivation. Although the vertical neutral wind may indeed be small, the field-aligned neutral wind component, which needs to be taken into consideration in the ion momentum equation, can be large. Simulations with the Thermosphere-Ionosphere Electrodynamics Global Circulation Model have been carried out to evaluate the effect of the neutral wind on the estimation of the ion-neutral collision frequency from the ion momentum equation. Our results reveal that, even at high latitudes, the effect of the neutral wind can be significant in the derivation of the ion-neutral collision frequency from ionospheric field-aligned observations, especially at night.

Statistical analysis of thermospheric density response to solar wind sector structure

Abstract: The global averaged thermospheric mass density data during 1967–2007 are analyzed to investigate thermospheric response to solar wind sector structure. Well-defined solar wind sectors have polarities of interplanetary magnetic field (IMF) toward (+Bx, −By) or away (−Bx, +By) from the Sun for multiple days. In March, thermospheric mass densities increase from the away to toward polarities, with a maximum density perturbation of ~23% at 400 km with respect to the 11 day averages; they decline from the toward to away polarities, and the maximum reduction at 400 km can reach 12%, which is associated with a weakened heating effect of the geomagnetic activity. In September, thermospheric densities respond in an opposite way to the same sector structure as compared with the March results. In solstice seasons, thermospheric density variations in response to solar wind sector structure are typically smaller than 10% relative to the 11 day averages. Besides the seasonal dependence, relative density changes tend to become greater at higher altitude and lower solar activity. However, during solar minimum the density variations at 550 km are not substantially larger than those at 400 km due to the possible descending of the transition altitude between helium and atomic oxygen. Moreover, the corotating interaction region (CIR) has a high probability to occur around the sector boundary. Consequently, the CIR effects account for one third of the density enhancement at 400 km during ineffective-effective sectors, whereas the density reduction associated with effective-ineffective sectors can be impaired by about 50%, which is independent of altitude.