[1] A solar terminator wave has been revealed in thermospheric wind and density simultaneously observed by CHAMP. The wind terminator wave is out of phase with the density terminator wave. But both have wavefronts about 30° inclined to the terminator line at low latitudes, and wavelengths ranging between 3000–5000 km. They show a clear dawn-dusk asymmetry, with more pronounced wave signatures forming at dusk. Terminator wave is indiscernible in the dawnside wind. Most wave structures are observed at night, with some extension to the sunlit region around solstices. The midnight density maximum is seen to be closely connected to terminator wave structures, hence indicating a possible role of terminator waves in its formation.

/pdf/a-solar-terminator-wave-in-thermospheric-wind-and-density-1cpe0f56oo.pdf

A solar terminator wave in thermospheric wind and density simultaneously observed by CHAMP

[1] A one-year dataset spanning March 2011 to March 2012 of coincident observations of nighttime thermospheric zonal neutral winds, equatorial plasma bubble (EPB) velocities, and zonal plasma drifts is used to examine the relationship between the thermosphere and the ionosphere near the geomagnetic equator over Peru. Thermospheric neutral winds are determined by using a bistatic Fabry-Perot interferometer (FPI) experiment located at Merihill and Nazca in Peru. The ambient plasma drift velocities were obtained using the incoherent scatter radar at the Jicamarca Radio Observatory in Peru. The EPB zonal velocities were estimated utilizing images of the OI 630.0 nm emission recorded by a narrow-field optical imaging system at the Cerro Tololo Inter-American Observatory in Chile. The joint analysis of these datasets illustrates that the nighttime and night-to-night variations in the zonal neutral winds, EPB velocities, and plasma drifts are well correlated. This consistent result of the local time variations of the neutral winds with that of EPB and plasma drifts illustrates that the F-region dynamo is, in general, fully activated. However, at times, the magnitude of the EPB velocities and the plasma drifts are different from the neutral winds. It is plausible that such a difference is due either to the effect of polarization electric fields developed inside the EPB or due to the latitudinal gradient of the neutral winds and EPB velocity measurements since the EPB velocities are estimated at a higher latitude, corresponding to an apex altitude of ~400 km, than the wind estimates, which derive from an apex altitude of ~250 km.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/jgra.50238

Comparison of zonal neutral winds with equatorial plasma bubble and plasma drift velocities

. Field-aligned currents (FACs) generate magnetic deflections perpendicular to the ambient Earth magnetic field. We investigate the characteristics of FACs associated with equatorial plasma bubbles (EPBs) as deduced from magnetic field measurements by the CHAMP satellite. Meridional magnetic deflections inside EPBs show a clear hemispheric anti-symmetry for events observed before 21:00 LT: inward in the Northern Hemisphere and outward in the Southern Hemisphere. When an eastward electric field is assumed the magnetic signature signifies a Poynting flux directed downward along the magnetic field lines. This means that FACs are driven by a high-altitude equatorial source. Such a scheme cannot be drawn as strictly from our observations after 22:00 LT, possibly because of a westward turning of the electric field inside EPBs and/or a decay of EPBs later at night. The perpendicular magnetic deflection is tilted by 40° from the magnetic meridional plane in westward direction, which implies that the depleted volume of EPBs, as well as the FAC structure, is tilted westward by 40° above the magnetic equator. The peak-to-peak amplitude of the FAC density is found to range typically between 0.1–0.5 μA/m2. The field-aligned sheet current density and the diamagnetic current strength show no correlation.

/pdf/the-characteristics-of-field-aligned-currents-associated-51k6zbdvra.pdf

The characteristics of field-aligned currents associated with equatorial plasma bubbles as observed by the CHAMP satellite

[1] The decadal observations from CHAMP satellite have provided ample information on the Earth's upper thermosphere, reshaping our understandings of the vertical coupling in the atmosphere and near-Earth space. An empirical model of the thermospheric mass density is constructed from these high-resolution observations using the multivariable least-squares fitting method. It describes the density variation with latitude, longitude, height, local time, season, and solar and geomagnetic activity levels within the altitude range of 350–420 km. It represents well prominent thermosphere structures like the equatorial mass density anomaly (EMA) and the wave-4 longitudinal pattern. Furthermore, the empirical model reveals two distinct features. First, the EMA is found to have a clear altitude dependence, with its crests moving equatorward with increasing altitude. Second, the equinoctial asymmetry is found to strongly depend on solar cycle, with its magnitude and phase being strongly regulated by solar activity levels. The equinoctial density maxima occur significantly after the actual equinox dates toward solar minimum, which may signal growing influence from the lower atmosphere forcing. This empirical model provides an instructive tool in exploring thermospheric density structures and dynamics. It can also be easily incorporated into other models to have a more accurate description of the background thermosphere, for both scientific and practical purposes.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/jgra.50144

Empirical model of the thermospheric mass density based on CHAMP satellite observations

[1] We present results from the first extended period of coincident observations of thermospheric zonal neutral winds and equatorial plasma bubble (EPB) zonal drift velocities over northeastern Brazil during the October to December months of 2009 and 2010. The EPB zonal drift velocities are estimated utilizing images of the O I 630.0 nm emissions recorded by a wide-angle imaging system at Cajazeiras. Thermospheric neutral wind estimates are based upon common volume observations made by a bistatic Fabry-Perot interferometer (FPI) experiment using FPIs located at Cajazeiras and Cariri in Brazil observing the Doppler shift of the O I 630.0 nm emission. The results illustrate a similar pattern of nighttime and night-to-night variations in the zonal neutral winds and EPB zonal drift velocities. In general, the geomagnetic zonal neutral winds and the EPB velocities show an excellent agreement illustrating that the F region dynamo is fully developed. However, in the early evening hours the EPB zonal speed is slower than that of the background winds on several occasions. We conclude that this indicates that during the bubble evolution period in the early evening the F region dynamo is not fully activated.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2012JA017620

Comparison of nighttime zonal neutral winds and equatorial plasma bubble drift velocities over Brazil

[1] The thermospheric zonal wind forms a fast wind jet at the Earth's dip equator instead of the geographic equator. This remarkable feature is revealed in two sets of independent observations made two decades apart. One is from the CHAMP satellite during the year of 2002 and the other is from the DE-2 satellite during Aug. 1981–Feb. 1983. Both observations show that this wind jet is eastward at night with speed reaching 150 ms−1, and westward around noon with speed over 75 ms−1. These fast wind jets are observed during local times of fully developed equatorial ionization anomaly (EIA). On the other hand, a channel of slow wind is found on the dip equator during the period of 05–08 MLT, which corresponds to local times before the EIA develops. These features strongly suggest the ion drag being the principle cause for shifting the wind jet from the geographic equator to the dip equator.

/pdf/fast-thermospheric-wind-jet-at-the-earth-s-dip-equator-3jz025f6fe.pdf

Fast thermospheric wind jet at the Earth's dip equator

[1] Simulations with the NCAR Thermosphere - Ionosphere - Electrodynamics General Circulation Model (TIE-GCM) have been carried out to understand the cause of strong thermospheric zonal wind at the magnetic dip equator. The simulations show that the zonal winds blow strongly at the magnetic dip equator instead of at the geographic equator due to the latitude structure of ion drag. The fast winds at the dip equator are seen throughout the altitude between 280 km and 600 km, and the wind above 400 km is mainly accelerated via viscosity. A test simulation without viscosity verifies that the extension of the fast equatorial wind to heights above 400 km is maintained by viscous coupling with the winds at lower altitudes, in spite of there being an ion-drag maximum instead of relative minimum at the dip equator at high altitudes. Basically, viscosity is not so large compared to the pressure gradient and ion drag, but dynamics causes the pressure gradient and ion drag approximately to balance, and viscosity becomes important. The simulation results are consistent with the observations by the DE-2 and CHAMP satellites. Therefore we suggest that the zonal wind velocity in the low latitude region is controlled by ion drag and viscosity.

/pdf/on-the-formation-of-a-fast-thermospheric-zonal-wind-at-the-1f8oufo264.pdf

On the formation of a fast thermospheric zonal wind at the magnetic dip equator

The zonal neutral wind in the thermosphere flows strongly at the Earth’s magnetic dip equator instead of the geographic equator around 20 magnetic local time (MLT). On the other hand, the fast zonal plasma drift occurs in the low-latitude F region of evening ionosphere, but the velocity decreases at the magnetic dip equator. Therefore, the fast plasma drift velocity structure forms an arch in the frame of latitude and altitude in the evening. The fast zonal neutral wind occurs inside of the arch. Since the fast zonal plasma drift is strongly associated with equatorial ionization anomaly (EIA), we suggest that the ionosphere–thermosphere coupling associated with F-region dynamo is significantly important in the phenomenology of the low-latitude, F-region ionosphere and thermosphere. The fast neutral wind occurring in the evening F region over the magnetic dip equator may result in atmospheric super-rotation in the low-latitude thermosphere.

Tsutomu Kondo

Papers

Fast thermospheric wind jet at the Earth's dip equator

On the formation of a fast thermospheric zonal wind at the magnetic dip equator

Ionosphere-Thermosphere Coupling in the Low-Latitude Region