Substorm onsets obtained from satellite and ground-based observations

Morioka, A. (1), T. Tokunaga (2), and K. Yumoto (3)

(1) Planetary Plasma and Atmospheric Research Center, Tohoku University, Sendai, Japan
(2) Graduate School of Sci., Kyushu University, Fukuoka, Japan
(3) Space Environment Research Center, Kyushu University, Fukuoka, Japan

The initiation of an explosive auroral substorm expansion (energy dissipation) is usually called "substorm onset" (Akasofu, 1964), although conceptually a substorm may begin with the growth phase (energy accumulation) [McPherron, 1970]. Substorms are clearly driven by solar wind energy input and there must be storage of energy in the magnetotail for their excitation. There are both directly driven and unloading aspects, with the unloading taking place in many forms (cf. Baker et al., 1984).

A main object of substorm research has been the understanding of its expansion onset, and many mechanisms have been proposed. However, there is no unequivocal agreement on one mechanism of the expansion onset. Also there is intense debate on the region of its initiation within the magnetotail where the expansion onset originates. A number of plasma instabilities were proposed to be candidates for the substorm initiation, and most of consider the magnetotail current sheet to be the region of instability. There are different scenarios of substorm expansion phase based on current disruption and reconnection models in the magnetotail.

In the present paper, we will demonstrate that auroral acceleration region is in the altitude range of 1-2 Re at substorm onsets. Morioka et al. (2007) recently investigated dual structure of auroral acceleration regions at substorm onsets by analyzing auroral kilometric radiation (AKR) spectra. The substorm onsets were also identified by low-latitude Pi 2 pulsations observed at Kakioka. Two sources of the AKR and their development prior to and during substorms were derived from high-time-resolution spectrograms provided by Polar/PWI ac electric field observations. One source is a low-altitude source region corresponding to middle-frequency AKR (MF-AKR), and the other is a high-altitude source region corresponding to low-frequency AKR (LF-AKR).

The former appears during the substorm growth phase in the altitude range of 4000-5000 km and is active both before and after substorm onset. A few minutes before the onset, the intensity of low-altitude source gradually increases, showing precursor-like behavior. It does not change drastically at the onset and is mostly insensitive to it. At Pi 2 onset, in contrast, high-altitude AKR appears abruptly with intense power in a higher and wider altitude range of 6000 to 12,000 km. The increase in its power is explosive (increasing 1000 times within 20 seconds), suggesting the abrupt growth of the parallel electric fields that cause bursty auroral electron beams. The statistically derived probability of both sources existing at substorm onset is ~70%, indicating that this duality of AKR sources is a common feature of substorms.

It is concluded that the onset region is as close as 1-2 Re where the high-altitude source with related transient acceleration at substorm onset is apparently due to 1) intrinsically local instabilities such as current-driven instabilities, or 2) transient Alfven waves with a shorter wavelength. The low-altitude source, which is fairly stable and insensitive to substorm onset, may belong to the global quasi-static potential distribution over the auroral oval, which involves a large-scale inverted-V structure and a quasi-steady field-aligned current.