Dual structure of auroral acceleration regions at substorm onsets as derived from auroral kilometric radiation spectra

Abstract

[1] Two sources of auroral kilometric radiation (AKR) and their development prior to and during substorms were identified from high-time-resolution spectrograms provided by Polar/PWI ac electric field observations and were investigated in connection with the auroral acceleration process. 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 this 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 s), 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. The highaltitude source and related transient acceleration at substorm onset are apparently due to (1) intrinsically local instabilities such as current-driven instabilities or (2) transient short wavelength Alfvén waves coming from the magnetosphere. The low-altitude source, which is fairly stable and insensitive to substorm onset, may belong to the global quasistatic potential distribution over the auroral oval, which involves a large-scale inverted-V structure and a quasi-steady field-aligned current.

AKR modulation and global Pi2 oscillation

“…Keiling et al [2008] presented that fluctuation of the photon flux observed by the Polar satellite had same periodicity as ground Pi2s. Morioka et al [2007] presented clear one-to-one correspondences between AKR breakup and Pi2 occurrence. They demonstrated that each appearance of the high-altitude AKR with sudden increase in power associated with the Pi2 pulsation.…”

“…[37] AKR is believed to emanate from the beams of auroral electrons, which are accelerated by the parallel electric potential [e.g., Morioka et al, 2007]. The temporal relation between AKR modulation and ground Pi2 oscillations suggests a coherent fluctuation between the parallel electric field (d k ) and upward FAC of the SCW (dj k ).…”

“…A gradual growth of the integrated AKR power started at latest 10:18:40 UT (and might be 10:18:10 UT) slightly before the onset of Pi2 at GAM. The AKR was emitted from lowaltitude range below ∼4500 km, and the altitude range was a typical one for the substorm growth phase (4000-5000 km [Morioka et al, 2007]). The ZYK D component also started to increase at 10:18:40 UT (possibly Pi2 precursor [Kepko and Kivelson, 1999]), then the increase became more steep at the time of GAM onset.…”

“…Then the AKR source expanded toward higher-altitude range (∼8000 km) after the first pulse of the AKR enhancement. The altitude range was a typical one for the substorm expansion phase (6000-12000 km [Morioka et al, 2007]). The two gray lines ("line a" and "line b"), which are superposed on the GAM H plot, roughly trace the trend of the nonoscillated magnetic variations.…”

Universal time control of AKR: Earth is a spin‐modulated variable radio source

“…[6] AKR has been studied in relation to auroral breakup and auroral particle acceleration [e.g., Gurnett, 1974;Kaiser and Alexander, 1977;Fairfield et al, 1999] and is widely accepted to be generated by rapidly accelerated electrons accompanying sudden auroral activation in the polar magnetosphere [e.g., Anderson et al, 1998;Strangeway et al, 2001;Morioka et al, 2007]. Figure 2 shows the frequency-time spectrogram (f-t diagrams) of AKR observed by GEOTAIL and AL/AU index on 5 December 1995.…”