Comparative Analysis of the Measured and Modeled Equatorial Thermospheric Wind Climatology

<p>The thermospheric winds play an important role in the vertical and horizontal couplings of the upper atmosphere by modulating neutral and plasma dynamics. A large variety of observation techniques and numerical as well as empirical models have been developed to understand the behavior of thermospheric winds. The Fabry-Perot interferometer (FPI) is a widely used ground- and satellite-based optical instrument for the thermospheric winds observations in the upper atmosphere. Due to solar contamination of the fainter airglow emission during the daytime, most of the ground-based interferometric wind measurements are limited to the nighttime period only. Despite these constraints, the Second‐generation, Optimized, Fabry‐Perot Doppler Imager (SOFDI) is designed for both daytime and nighttime measurements of thermospheric winds from OI 630‐nm emission and is currently operating at the Huancayo, Peru, near the geomagnetic equator. In this study, we present a comparative analysis of the observed SOFDI wind climatological data and several other modeled results including, but not limited to, Horizontal Wind Model 2014 (HWM-14), Coupled Thermosphere Ionosphere Plasmasphere Electrodynamics (CTIPe) model with and without implementing Prompt Penetration Electric Field (PPEF), Whole Atmosphere Model (WAM), SAMI3 model, and Magnetic mEridional NeuTrAl Thermospheric (MENTAT) model. We examine the relative performances of these models in the context of the direct-measured thermospheric winds. The day and nighttime modeled winds show an excellent agreement with the SOFDI wind data at the equatorial latitude, except for the daytime zonal winds. Further, this analysis gives a comprehensive picture of how well the measured winds provided by the SOFDI instrument and various models represent the features of the equatorial thermosphere. We also investigate and give an overview of the sources, drivers, effects, and possible mechanisms of the wind variability in the low-latitude thermosphere.</p>

Climatology of intermediate descending layers (or 150 km echoes) over the equatorial and low-latitude regions of Brazil during the deep solar minimum of 2009

In this work, we have performed a study for the first time on the climatology of the intermediate descending layers (ILs) over Brazilian equatorial and low-latitude regions during the extreme solar minimum period of 2009. The result of this study shows that the occurrence frequency of the ILs is very high, being > 60 % over São Luís (2∘ S, 44∘ W; inclination: −3.8∘) and > 90 % in Cachoeira Paulista (22.42∘ S, 45∘ W; inclination: −33.5∘). In most cases the ILs occur during the day at altitudes varying from 130 to 180 km and they may descend to lower altitudes (∼100 km) in a time interval of a few minutes to hours. The main driving force for the ILs at the low-latitude region, may be considered to be the diurnal tide (24 h) followed in smaller dominance by the semidiurnal (12 h), terdiurnal (8 h) and quarter-diurnal (6 h) components. In the magnetic equatorial sector, similar behavior was seen, with the exception of the semidiurnal tide, which in general does not appear to have influenced the IL's dynamics (except in summer). Additionally, the IL mean descent velocity over São Luís and Cachoeira Paulista shows a day-to-day variability that may be associated with a wave-like perturbation with a periodicity of some days. Some peculiarities in the IL dynamics were noted, such as the presence of the ILs during the night hours. Ascending and descending ILs appeared to have been formed from some connection with the ionospheric F layer. Quite often, these characteristics are observed in the presence of strong signatures of the gravity wave propagation as suggested by the F layer traces in the ionogram. The descending intermediate layer over Brazil appears to have been formed through a process of F1 layer base detachment. An interesting case study showed that an ascending ILs, initially detected at ∼130 km, reached the base of the F2 layer, due probably to the gravity wave propagation and/or the effect of a prompt penetration electric field.

Climatology of intermediate descending layers (150 km) over the equatorial and low latitude regions of Brazil during the deep solar minimum of 2009

In this work, we report for the first time the climatology of intermediate descending layers over Brazilian equatorial and low latitudes regions during the extreme solar minimum period of 2009. The occurrence frequency of this layer is very high, being > 60 % over São Luís (2° S; 44° W, I: −5.7°) and > 90 % in Cachoeira Paulista (22.42° S; 45° W, I: −34.4°). Our results reveal that in most of the cases the intermediate layers (IL’s) appear during the day at altitudes varying from 130 to 180 km and present a descent movement that reaches the lower altitudes (~ 100 km) in a time interval of a few minutes to hours. Differently from other longitudinal sectors, the diurnal tide (24 h) can be considered as the main cause of IL’s for the low latitude region, followed by a smaller dominance of semidiurnal (12 h), terdiurnal (8 h) and quarter-diurnal (6 h) tide components. In the equatorial sector, similar behavior was found, with the exception of the semidiurnal tide, which in general does not appear to influence the IL’s dynamics (except in summer). The IL’s mean descent velocities over São Luís and Cachoeira Paulista show a day-to-day variability that may be associated with gravity waves propagation. Some peculiarities in the IL’s dynamics have been noted, such as the presence of the IL’s during the night hours, ascending IL’s, simultaneous IL’s, and descending IL’s been formed from some connection with the ionospheric F layer. Quite often, these characteristics are observed in the presence of strong signatures in the ionogram F-layer trace similar to those caused by the gravity wave propagation. We will show further that the descending intermediate layer over Brazil can be formed through a process of F1 layer base detachment. Besides that, we will present an interesting case study in which an ascending IL’s, initially detected at ~ 140 km, reached the base of F2 layer, probably due to the gravity wave propagation and/or due to the effect of the prompt penetration electric field.

Signatures of Sudden Storm Commencement on the equatorial thermospheric dayglow

It has been observed that the OI 630.0 nm dayglow emission over a dip equatorial station, Trivandrum (8.5° N, 77° E, dip 0.5° N), India registered an abrupt increase of ~ 2000 R during the compression phase of the magnetosphere as dictated by a sudden increase in solar wind ram pressure. Furthermore, an unusual depletion of these emissions has been observed during the eastward interplanetary electric field (IEF), concomitant with southward excursion of IMF Bz. The ionosonde and magnetometer observations confirmed the effects of prompt penetration electric field (PPEF). Associated with the eastward PPEF, formation of F3 layers were also noticed. These unique results, which emphasize the effect of Sudden Storm Commencement/IEF on these equatorial daytime airglow emissions are discussed in context of changes in the equatorial zonal electric field and F region height variations associated with polar/auroral activities due to the magnetosphere-ionosphere coupling.

Longitudinal study of the ionospheric response to the geomagnetic storm of 15 May 2005 and manifestation of TADs

Response of low latitude ionosphere to the geomagnetic storm of 15 May 2005 has been studied using total electron content (TEC) data, obtained from three GPS stations namely, Yibal, Udaipur and Kunming situated near the northern crest of equatorial ionization anomaly at different longitudes. Solar wind parameters, north-south component of the interplanetary magnetic field (IMF Bz) and AE index data have been used to infer the strength of the geomagnetic storm. A large value of eastward interplanetary electric field at 06:15 UT, during the time of maximum southward IMF Bz has been used to infer the transmission of an eastward prompt penetration electric field (PPEF) which resulted in a peak in TEC at 07:45 UT due to the local uplift of plasma in the low latitudes near the anomaly crest over a wide range of longitudes. Wave-like modulations superposed over the second enhancement in TEC between 09:15 UT to 10:30 UT have been observed at all the three stations. The second enhancement in TEC along with the modulations of up to 5 TECU have been attributed to the combined effect of super plasma fountain and traveling atmospheric disturbances (TAD). Observed large enhancements in TEC are a cause of concern for satellite based navigation and ground positioning. Increased [O/N2] ratio between 09:15 UT to 10:15 UT when modulations in TEC have been also observed, confirms the presence of TADs over a wide range of longitudes.

Responses of equatorial anomaly to the October-November 2003 superstorms

The responses of Equatorial Ionization Anomaly (EIA) to the superstorms of October-November 2003 were investigated using the total electron content (TEC) measured with global positioning system (GPS) receivers in China, Southeast Asia, Australian (CSAA), and the American regions. Enhanced EIA was seen to be correlated with the southward turning of the interplanetary magnetic field Bz. In both the CSAA and American regions, EIA was intensified, corresponding to a large increase in the F-layer peak height (hmF2) measured by ionosonde and digisonde at middle and equatorial latitudes. However, the enhanced EIA was shown to be more significant during the daytime in the American region, which was associated with a series of large substorms when Bz was stable southward. The prompt penetration electric field and the wind disturbances dynamo electric field are suggested to be responsible for this observation according to current theory, although some features cannot be totally decipherable. Both the ionogram and magnetometer data show the existence of a weak shielding effect whose effect still needs further study. A clear asymmetric ionospheric response was shown in our TEC observations, even though it was only one month after autumnal equinox. The southern EIA crest was totally obliterated on 29 and 30 October in the CSAA region and on 31 October in the American region. Ion temperatures from the Defense Meteorological Satellite Program (DMSP) spacecraft revealed that the unequal energy injection at the polar region might be the reason for this effect. It is concluded that different physical processes have varying degrees of importance on the evolution of EIA in the CSAA and American regions.