Roles of thermospheric neutral wind and equatorial electrojet in pre-reversal enhancement,deduced from observations in Southeast Asia

Previous studies have proposed that both the thermospheric neutral wind and the equatorial electrojet(EEJ)near sunset play important roles in the pre-reversal enhancement(PRE)mechanism.In this study,we have used observations made in the equatorial region of Southeast Asia during March–April and September–October in 2010–2013 to investigate influences of the eastward neutral wind and the EEJ on the PRE’s strength.Our analysis employs data collected by the Gravity Field and Steady-State Ocean Circulation Explorer(GOCE)satellite to determine the zonal(east-west direction)neutral wind at an altitude of~250 km(bottomside F region)at longitudes of 90°–130°E in the dusk sector.Three ionosondes,at Chumphon(dip lat.:3.0°N)in Thailand,at Bac Lieu(dip lat.:1.7°N)in Vietnam,and at Cebu(dip lat.:3.0°N)in Philippines,provided the data we have used to derive the PRE strength.Data from two magnetometers—at Phuket(dip lat.:0.1°S)in Thailand and at Kototabang(dip lat.:10.3°S)in Indonesia—were used to estimate the EEJ strength.Our study is focused particularly on days with magnetically quiet conditions.We have found that the eastward neutral wind and the EEJ are both closely correlated with the PRE;their cross-correlation coefficients with it are,respectively,0.42 and 0.47.Their relationship with each other is weaker:the cross-correlation coefficient between the eastward neutral wind and the EEJ is just 0.26.Our findings suggest that both the eastward neutral wind and the EEJ near sunset are involved in the PRE mechanism.Based on the weak relationship between these two parameters,however,they appear to be significantly independent of each other.Thus,the wind and the EEJ are likely to be influencing the PRE magnitude independently,their effects balancing each other.

Global distributions of storm-time ionospheric currents as seen in geomagnetic field variations

To investigate temporal and spatial evolution of global geomagnetic field variations from high-latitude to the equator during geomagnetic storms, we analyzed ground geomagnetic field disturbances from high latitudes to the magnetic equator. The daytime ionospheric equivalent current during the storm main phase showed that twin-vortex ionospheric currents driven by the Region 1 field-aligned currents （R1 FACs） are intensified significantly and expand to the low-latitude region of-30~ magnetic latitude. Centers of the currents were located around 70~ and 65~ in the morning and afternoon, respectively. Corresponding to intensification of the R1 FACs, an enhancement of the eastward/westward equatorial electrojet occurred at the daytime/nighttime dip equator. This signature suggests that the enhanced convection electric field penetrates to both the daytime and nighttime equa- tor. During the recovery phase, the daytime equivalent current showed that two new pairs of twin vortices, which are different from two-cell ionospheric currents driven by the R1 FACs, appear in the polar cap and mid latitude. The former led to enhanced north- ward Bz （NBZ） FACs driven by lobe reconnection tailward of the cusps, owing to the northward interplanetary magnetic field （IMF）. The latter was generated by enhanced Region 2 field-aligned currents （R2 FACs）. Associated with these magnetic field variations in the mid-latitudes and polar cap, the equatorial magnetic field variation showed a strongly negative signature, produced by the westward equatorial electrojet current caused by the dusk-to-dawn electric field.

Longitudinal Variability of the Vertical Drift Velocity Inferred from Ground-Based Magnetometers and C/NOFS Observations in Africa

This study aims at discussing longitudinal effects on the variability of the vertical E × B drift velocity at low latitudes, specifically over African sector. To this effect, observations from ground-based magnetometers and the Ion Velocity Meter experiment onboard C/NOFS satellite are analyzed in conjunction with equatorial electric field and neutral wind model estimates under geomagnetically quiet conditions in the years 2012-2013. Notwithstanding the limitation in data over Africa, the combination of ground-based and in-situ observations confirmed the existence of longitudinal differences in the E × B between the Atlantic, Western and Eastern African sectors. This was well reproduced by the equatorial electric field model (EEFM) which showed that during noon, the peak of the equatorial electric field (EEF) was the lowest in the Atlantic sector, with an increasing trend towards the Eastern longitude. The Horizontal Wind Model 14 (HWM14) showed that the eastward zonal (poleward meridional) wind velocity was the lowest (highest) in the Eastern sector. Furthermore, the zonal (meridional) wind increased (decreased) from the Eastern to the Atlantic sector. These results highlight the contribution of the neutral wind velocity in driving the longitudinal difference in the vertical drift velocity over Africa.