The asymmetric geospace as displayed during the geomagnetic storm on 17 August 2001

Abstract

Abstract. Previous studies have shown that conjugate auroral features are displaced in the two hemispheres when the interplanetary magnetic field (IMF) has a transverse (Y) component. It has also been shown that a BY component is induced in the closed magnetosphere due to the asymmetric loading of magnetic flux in the lobes following asymmetric dayside reconnection when the IMF has a Y component. The magnetic field lines with azimuthally displaced footpoints map into a “banana”-shaped convection cell in one hemisphere and an “orange”-shaped cell in the other. Due to the Parker spiral our system is most often exposed to a BY-dominated IMF. The dipole tilt angle, varying between ±34∘, leads to warping of the plasma sheet and oppositely directed BY components in dawn and dusk in the closed magnetosphere. As a result of the Parker spiral and dipole tilt, geospace is asymmetric most of the time. The magnetic storm on 17 August 2001 offers a unique opportunity to study the dynamics of the asymmetric geospace. IMF BY was 20–30 nT and tilt angle was 23∘. Auroral imaging revealed conjugate features displaced by 3–4 h magnetic local time. The latitudinal width of the dawnside aurora was quite different (up to 6∘) in the two hemispheres. The auroral observations together with convection patterns derived entirely from measurements indicate dayside, lobe and tail reconnection in the north, but most likely only dayside and tail reconnection in the Southern Hemisphere. Increased tail reconnection during the substorm expansion phase reduces the asymmetry.

DMSP Observations of High‐Latitude Dayside Aurora (HiLDA)

“…Previous studies mainly focus on the large-scale HiLDA spots, as their high emission intensity and large size can be recognized from UV imagers onboard high-altitude satellites (Frey et al, 2003;Korth, 2004;Østgaard et al, 2018). There is one earlier study based on Viking images (on which more details can be recognized than on higher altitude global imagers such as IMAGE, however with a much reduced field-of-view) where Murphree et al (1990) shows several cases of arc-like forms separated from the dayside oval, probably detecting a HiLDA event without naming it that way.…”

“…HiLDA events were mostly observed in the northern hemisphere (Murphree et al, 1990;Frey et al, 2003Frey et al, , 2004Østgaard et al, 2018;Han et al, 2020). A statistical study based on the IMAGE FUV data showed that the occurrence of HiLDA in the northern hemisphere is typically connected with strong positive IMF B y , positive B z , negative B x , and low solar wind density (Frey et al, 2004).…”

“…The formation of HiLDA in the upward FAC region was considered to be driven by the lobe reconnection (Carter et al, 2018;Østgaard et al, 2018;Frey et al, 2019).…”

“…The formation of HiLDA in the upward FAC region was considered to be driven by the lobe reconnection (Carter et al, 2018;Frey et al, 2019;Østgaard et al, 2018). In the northern hemisphere, the lobe reconnection during northward IMF with strong positive B y drives a large clockwise lobe convection cell and an upward FAC dawnside of the reconnection site.…”

Separation and Quantification of Ionospheric Convection Sources: 2. The Dipole Tilt Angle Influence on Reverse Convection Cells During Northward IMF

“…Earlier studies have found strong evidence that lobe reconnection is more efficient in the hemisphere inclined toward the Sun, that is, the local summer hemisphere (Crooker & Rich, 1993;Frey et al, 2004;Koustov et al, 2017;Østgaard et al, 2018;Wilder et al, 2010;Yakymenko et al, 2018), from observing the sunward ionospheric convection velocity in the dayside polar cap during northward IMF. However, most studies have focused on the dependence on the solar wind electric field, E sw = v sw B T , where B T is the trans- (Koustov et al, 2017;Sundberg et al, 2009;Wilder et al, 2009;Yakymenko et al, 2018), likely to be the most important controlling factor on the lobe reconnection rate during purely northward IMF.…”