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Based on a comprehensive catalogue with more than 4000 substorm entries from the years 2000-2005 (Frey and Mende, 2006), the spatial distribution of the substorm-related magnetic signature at mid and low latitudes around local midnight is investigated. For the first time, the magnetic signature of the substorm current wedge formation is studied also in near-Earth satellite data from CHAMP. The average maximal deflection measured on board the satellite is smaller by a factor of 2 than that determined from ground observations. The near-Earth and ground-based magnetic field observations cannot be described adequately by a simple current wedge model. In order to explain the measured magnetic field deflections, an improved near-Earth current system associated with magnetospheric substorms has to be considered (Ritter & Lühr, 2008). It comprises four elements:

1. a gradual decrease of the tail lobe field;
2. a re-routing of a part of the cross-tail current through the ionosphere to Region-1;
3. eastward ionospheric currents at low and mid latitudes driven by Region-2 field-aligned currents;
4. a partial ring current connected to these Region-2 field-aligned currents (FACs).

To estimate the latitudinal variation of the average magnetic signature, magnetic field measurements during substorm events at 4 ground observatories located along a meridian from the equator to mid-latitudes (BNG, TAM, AQU, NGK) were stacked (magnetic activity level: Kp>2). The magnetic deflections are practically identical during the substorm expansion phase. During the recovery phase, however, the signal decays faster at higher latitudes.

The average substorm signature from CHAMP data was obtained by stacking night time orbital arcs before and after a substorm onset were stacked in a superposed epoch analysis. The local time interval chosen for stacking the data aimed at catching orbits crossing the current wedge loop between the upward and the downward FAC paths connecting the tail current with the polar ionosphere. The differences between the B-field before and after the substorm onsets yield the substorm signature at CHAMP (magnetic activity level: Kp>2). At the equator this signature is half the amplitude than on the ground.

In order to interpret the average magnetic signature that results from the superposed epoch analyses of the satellite and observatory measurements, we computed the response to a simple substorm current wedge model (Clauer & McPherron, 1974). However, this current wedge model can predict the substorm signatures only qualitatively but fails to account for the details, eg. at higher latitudes and the z-component. If the model includes additionally a decay of the tail lobe field and shielding Region-2 FACs at 60° of invariant latitude feeding an eastward ionospheric current, the amplitudes fit much better to the observed magnetic field.

Full story in Ritter & Lühr (2008), 10.5194/angeo-26-2781-2008 |PDF |

References

• Clauer, C. R. and McPherron, R. L. (1974): Mapping of local time, universal time development of magnetosphere substorms using mid-latitude magnetic observations, J. Geophys. Res., 79.
• Frey, H. U. and Mende, S. B. (2006): Substorm onsets as observed by IMAGE-FUV, Int. Conf. Substorms, 8.
• Ritter, P. and Lühr, H. (2008): Near-Earth magnetic signature of magnetospheric substorms and an improved substorm current model. Annales Geophysicae, 26, 9, 2781-2793.