Journal cover Journal topic
Annales Geophysicae An interactive open-access journal of the European Geosciences Union
Journal topic

Journal metrics

IF value: 1.490
IF 5-year value: 1.445
IF 5-year
CiteScore value: 2.9
SNIP value: 0.789
IPP value: 1.48
SJR value: 0.74
Scimago H <br class='widget-line-break'>index value: 88
Scimago H
h5-index value: 21
Volume 27, issue 3
Ann. Geophys., 27, 1035–1046, 2009
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 27, 1035–1046, 2009
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.

  02 Mar 2009

02 Mar 2009

Statistical visualization of the Earth's magnetotail based on Geotail data and the implied substorm model

S. Machida1, Y. Miyashita2, A. Ieda3, M. Nosé4, D. Nagata1, K. Liou5, T. Obara6, A. Nishida2, Y. Saito2, and T. Mukai7 S. Machida et al.
  • 1Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto, Japan
  • 2Institute of Space and Astronautical Science/JAXA, Kanagawa, Japan
  • 3Solar Terrestrial Environment Laboratory, Nagoya University, Aichi, Japan
  • 4World Data Center for Geomagnetism, Kyoto University, Kyoto, Japan
  • 5The Johns Hopkins University, Applied Physics Laboratory, MD, USA
  • 6Aerospace Research and Development Directorate/JAXA, Ibaraki, Japan
  • 7Japan Aerospace Exploration Agency (JAXA), Tokyo, Japan

Abstract. We investigated the temporal and spatial development of the near-Earth magnetotail during substorms based on multi-dimensional superposed-epoch analysis of Geotail data. The start time of the auroral break-up (t=0) of each substorm was determined from auroral data obtained by the Polar and IMAGE spacecraft. The key parameters derived from the plasma, magnetic-field, and electric-field data from Geotail were sorted by their meridional X(GSM)–Z(proxy) coordinates.

The results show that the Poynting flux toward the plasma-sheet center starts at least 10 min before the substorm onset, and is further enhanced at X~−12 RE (Earth radii) around 4 min before the onset. Simultaneously, large-amplitude fluctuations occurred, and earthward flows in the central plasma sheet between X~−11 RE and X~−19 RE and a duskward flow around X=−10 RE were enhanced. The total pressure starts to decrease around X=−16 RE about 4 min before the onset of the substorm. After the substorm onset, a notable dipolarization is observed and tailward flows commence, characterised by southward magnetic fields in the form of a plasmoid.

We confirm various observable-parameter variations based on or predicted by the relevant substorm models; however, none of these can explain our results perfectly. Therefore, we propose a catapult (slingshot) current-sheet relaxation model, in which an earthward convective flow produced by catapult current-sheet relaxation and a converted duskward flow near the Earth are enhanced through flow braking around 4 min before the substorm onset. These flows induce a ballooning instability or other instabilities, causing the observed current disruption. The formation of the magnetic neutral line is a natural consequence of the present model, because the relaxation of a highly stretched catapult current-sheet produces a very thin current at its tailward edge being surrounded by intense earthward and tailward magnetic fields which were formerly the off-equatorial lobe magnetic fields. This location is the boundary between a highly stressed catapult current sheet and a Harris-type current sheet characterized by little stress. In addition, the flows induced around the boundary toward the current-sheet center may enhance the formation of the magnetic neutral line and the efficiency of magnetic reconnection. After magnetic reconnection is induced, it plays a significant role in driving the substorm.

Publications Copernicus