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Ann. Geophys., 24, 1905-1917, 2006
www.ann-geophys.net/24/1905/2006/
© European Geosciences Union 2006
Method to locate the polar cap boundary in the nightside ionosphere and application to a substorm event
A. T. Aikio1, T. Pitkänen1, A. Kozlovsky1,2, and O. Amm3
1Department of Physical Sciences, University of Oulu, P.O. Box 3000, FIN-90014, Finland
2Sodankylä Geophysical Observatory, Sodankylä, Finland
3Finnish Meteorological Institute, Helsinki, Finland
Abstract. In this paper we describe a new method to be used for the polar cap boundary
(PCB) determination in the nightside ionosphere by using the EISCAT Svalbard
radar (ESR) field-aligned measurements by the 42-m antenna and southward
directed low-elevation measurements by the ESR 32 m antenna or northward
directed low-elevation measurements by the EISCAT VHF radar at Tromsø. The
method is based on increased electron temperature (Te) caused by
precipitating particles on closed field lines. Since the Svalbard
field-aligned measurement provides the reference polar cap Te height
profile, the method can be utilised only when the PCB is located between
Svalbard and the mainland. Comparison with the Polar UVI images shows that
the radar-based method is generally in agreement with the PAE (poleward
auroral emission) boundary from Polar UVI.
The new technique to map the polar cap boundary was applied to a substorm
event on 6 November 2002. Simultaneous measurements by the MIRACLE
magnetometers enabled us to put the PCB location in the framework of
ionospheric electrojets. During the substorm growth phase, the polar cap
expands and the region of the westward electrojet shifts gradually more apart
from the PCB. The substorm onset takes place deep within the region of closed
magnetic field region, separated by about 6–7° in latitude from the
PCB in the ionosphere. We interpret the observations in the framework of the
near-Earth neutral line (NENL) model of substorms. After the substorm onset,
the reconnection at the NENL reaches within 3 min the open-closed field line
boundary and then the PCB moves poleward together with the poleward boundary
of the substorm current wedge. The poleward expansion occurs in the form of
individual bursts, which are separated by 2–10 min, indicating that the
reconnection in the magnetotail neutral line is impulsive. The poleward
expansions of the PCB are followed by latitude dispersed intensifications in
the westward electrojet with high latitudes affected first and lower
latitudes later. We suggest that reconnection bursts energize plasma and
produce enhanced flows toward the Earth. While drifting earthward, part of
the plasma population precipitates to the ionosphere producing
latitude-dispersed enhancements in the WEJ.
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