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.
Highlights
Over the last 2 decades it has been well established that the transverse component, BY, of the interplanetary magnetic field (IMF) leads to longitudinal displacement of the aurora in the conjugate hemispheres (Liou et al, 2001b; Østgaard et al, 2004, 2005, 2011b; Wang et al, 2007; Liou and Newell, 2010)
As similar auroral features in the two hemispheres can be considered as an illuminated footprint of conjugate magnetic field lines, these findings provide evidence of an “added” BY component in the closed magnetosphere with the same polarity as the IMF BY
As two substorms occurred during this 2 h period, we find that increased tail reconnection does not transport BY components into the closed magnetosphere, but rather reduces the asymmetry by partly removing the magnetic pressure in the lobes
Summary
Over the last 2 decades it has been well established that the transverse component, BY , of the interplanetary magnetic field (IMF) leads to longitudinal displacement of the aurora in the conjugate hemispheres (Liou et al, 2001b; Østgaard et al, 2004, 2005, 2011b; Wang et al, 2007; Liou and Newell, 2010). The magnetic storm on 17 August 2001 offers a unique opportunity to study how all these effects are dynamically interrelated when we have a large IMF BY component (> 20 nT), large tilt angle (23◦) and two substorms with increased tail reconnection in their expansion phase This magnetic storm was studied by Longley et al (2017b) who used conjugate auroral imaging to determine the dawn–dusk offset of the polar cap between the hemispheres and compared those with four different MHD model predictions. To establish a convection pattern we use data from SuperDARN (Greenwald et al, 1995), SuperMAG
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