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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Regular paper 26 Jun 2018

Regular paper | 26 Jun 2018

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Annales Geophysicae (ANGEO).

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

Nikolai Østgaard1, Jone P. Reistad1, Paul Tenfjord1, Karl M. Laundal1, Theresa Rexer2, Stein E. Haaland3,1, Kristian Snekvik1, Michael Hesse1, Steve E. Milan4,1, and Aanders Ohma1 Nikolai Østgaard et al.
  • 1Birkeland Centre for Space Science, Department of Physics and Technology, University of Bergen, Norway
  • 2University of Tromsø, Norway
  • 3Max-Planck Institute, Göttingen, Germany
  • 4Department of Physics and Astronomy, University of Leicester, UK

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 deg, 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 most of the time asymmetric. The magnetic storm on August 17, 2001 offers a unique opportunity to study the dynamics of the asymmetric geospace. IMF BY was 20–30nT and tilt angle was 23 deg. Auroral imaging revealed conjugate features displaced by 3–4 hours magnetic local time. The latitudinal width of the dawnside aurora was quite different (up to 6 deg) in the two hemispheres. The auroral observations together with convection patterns derived entirely from data indicate both dayside, lobe and tail reconnection in the north, but most likely only dayside and tail reconnection in the southern hemisphere. Increased tail reconnection during substorm expansion phase reduces the asymmetry.

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Nikolai Østgaard et al.
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Publications Copernicus
Short summary
In this paper we take the advantage of having two auroral imaging missions giving simultaneous data of both the southern and northern aurora. Combined with all available in-situ measurements from space and global ground-based networks, we explore the asymmetric behavior of geospace. We find large auroral asymmetries and different reconnection geometry in the two hemispheres. During substorm expansion phase asymmetries are reduced.
In this paper we take the advantage of having two auroral imaging missions giving simultaneous...