Solar cycle, seasonal, and diurnal variations of subauroral ion drifts: Statistical results

Abstract

The solar cycle, seasonal, and diurnal variations of the subauroral ion drifts (SAIDs) are investigated for the first time to use such a large database of 18,226 SAID events observed by the DMSP satellites during 1987-2012. Statistical results show that SAIDs occur mostly at 60.1°invariant latitude and 2230 magnetic local time with a typical half width of 0.57°, move equatorward during high solar activities with large widths, and have two occurrence peaks in spring and fall equinoxes and two valleys in summer and winter solstices. The seasonal variation of SAID latitude has two valleys in spring and fall, and SAID width has a valley distribution with a minimum in summer. SAIDs exhibit a clear day-to-night difference in latitude. The diurnal variation of SAID width has a morning valley and an afternoon peak. The generation mechanism of SAID associated with the electron precipitation and the downward field-aligned current is also supported in this study.

Hemispheric asymmetry of subauroral ion drifts: Statistical results

“…Figure shows that the maximum probabilities of the normalized SAID occurrence occur at ~62° MLAT and ~22:15 MLT for Kp < 4, and at ~58° MLAT and ~22:15 MLT for Kp ≥ 4 in the NH, whereas in the SH the maximum probabilities of the SAID occurrence are at ~−60° MLAT and ~22:45 MLT for Kp < 4 and at ~−56° MLAT and ~22:45 MLT for Kp ≥ 4. The maximum probability of the SAID occurrence can better describe the occurrence locations of SAIDs in the two hemispheres for different geomagnetic activity conditions [ Figueiredo et al ., ; He et al ., ]. Figure also shows that SAIDs in the SH occur generally at a location that is ~2° MLAT lower in MLAT and 0.5 h later in MLT than that in the NH.…”

“…The variations of the peak velocities with geomagnetic activity or hemisphere are not so evident as those of SAPS in Wang et al . [], possibly resulting from the fact that there are many events with WID peaks exceeding the IDM's threshold of −3000 m/s, and the exact peak velocity is thus difficult to be determined [ He et al ., ]. Furthermore, the SAID events in our database may be observed at different phases of their lifetimes which can be 0.5–3 h [ Anderson et al ., ] and during which the peak velocity of the SAID may be highly dynamic and can change from −1000 m/s to −3000 m/s or more.…”

“…The occurrence and characteristics of SAIDs [ Anderson et al ., , ; Karlsson et al ., ; He et al ., ] are closely related to auroral electron/ion precipitation, field‐aligned currents (FACs), and ionospheric conductance, all of which have hemispheric asymmetries [ Christiansen et al ., ; Hurtaud et al ., ; Luan et al ., ]. Anderson et al .…”

“…He et al . [] recently described solar cycle, seasonal, and diurnal variations of SAIDs using a large database of SAID events observed by the DMSP satellites.…”

“…For this purpose, the large SAID database developed by He et al . [] using DMSP observations from 1987 to 2012 has been used to systematically investigate the north‐south asymmetries of SAIDs for the first time in this paper. We will examine the north‐south asymmetry in the occurrence distribution, shape, drift velocity, and geomagnetic activity variations of SAIDs.…”