Abstract
Abstract. Comprehensive analysis of a moderate 600 nT substorm was performed using simultaneous optical observations inside the auroral oval and in the polar cap, combined with data from satellites, radars, and ground magnetometers. The onset took place near the poleward boundary of the auroral oval that is not typical for classical substorms. The substorm onset was preceded by two negative excursions of the interplanetary magnetic field (IMF) Bz component, with a 1 min interval between them, two enhancements of the antisunward convection in the polar cap with the same time interval, and 15 min oscillations in the geomagnetic H component in the auroral zone. The distribution of the pulsation intensity along meridian has two local maxima, namely at the equatorial and poleward boundaries of the auroral oval, where pulsations occurred in the out-of-phase mode resembling the field line resonance. At the initial stage, the auroral breakup developed as the auroral torch stretched and expanded poleward along the meridian. Later it took the form of the large-scale coiling structure that also distinguishes the considered substorm from the classical one. Magnetic, radar, and the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) satellite data show that, before the collapse, the coiling structure was located between two field-aligned currents, namely downward at the poleward boundary of structure and upward at the equatorial boundary. The set of GEOTAIL satellites and ground data fit to the near-tail current disruption scenario of the substorm onset. We suggest that the 15 min oscillations might play a role in the substorm initiation.
Highlights
Introduction1.1 Location of substorm onsets as inferred from satellite and ground observations
1.1 Location of substorm onsets as inferred from satellite and ground observations the substorm onset and development mechanisms were of high interest for many decades, there are still a number of issues under discussion
We describe the main features of the polar substorm inferred from ground observations to show that the most intense onset begins near the poleward boundary of the auroral oval so that the preceding onset-like features at lower latitudes look like pseudobreakup events (Sect. 3.1)
Summary
1.1 Location of substorm onsets as inferred from satellite and ground observations. The substorm onset and development mechanisms were of high interest for many decades, there are still a number of issues under discussion. The substorm studies use satellite plasma and field measurements in the magnetotail plasma sheet and simultaneous auroral and magnetic observations on the ground in the auroral zone where the plasma sheet is mapped onto the ionosphere. One of the longstanding problems is where and when key substorm processes initiate. The direct comparison of satellite measurements and ground data is hindered by the low accuracy of the mapping of magnetospheric processes to the ionosphere that are conditioned by the complex shape of geomagnetic field lines. Note that some kind of PBI is regarded as a substorm-onset trigger (Nishimura et al, 2015). To solve the above problem, one needs either an appropriate modification of the geomagnetic field model (Brito and Morley, 2017) or the involvement of some additional information (e.g., Shevchenko et al, 2010) to perform a more or less accurate conjugation of the satellite with ground instruments
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