Substorm onset process: Ignition of auroral acceleration and related substorm phases

Abstract

The substorm onset process was studied on the basis of the vertical evolution of auroral acceleration regions derived from auroral kilometric radiation (AKR) spectra and Pi pulsations on the ground. The field-aligned auroral acceleration at substorm onset demonstrated two distinct phases. Low-altitude acceleration (h~3000-5000 km), which accompanied auroral initial brightening, prebreakup Pi2, and direct current of ultralow frequency (DC-ULF) pulsation were first activated and played an important role (precondition) in the subsequent substorm expansion phase onset. Prebreakup Pi2 is suggestive of the ballooning mode wave generation, and negative decrease in DC-ULF suggests increasing field-aligned current (FAC). We called this stage the substorm initial phase. A few minutes after this initial phase onset, high-altitude acceleration, which accompanied auroral breakup and poleward expansion with breakup Pi1 and Pi2 pulsations, suddenly broke out in an altitude range from 8000 to 16,000 km. Thus, substorm expansion onset originated in the magnetosphere-ionosphere (M-I) coupling region, i.e., substorm ignition in the M-I coupling region. Statistical investigations revealed that about 65% of earthward flow bursts observed in the plasma sheet were accompanied by enhanced low-altitude AKR, suggesting that flow braking of bursts causes FAC and resulting low-altitude field-aligned acceleration in the M-I coupling region. On the basis of these observations, we propose a substorm onset scenario in which FAC that originated from the braking of plasma flow bursts first enhances low-altitude acceleration (substorm initial phase onset) and then the increasing FAC induces current-driven instability in the M-I coupling region, which leads to high-altitude acceleration and resulting substorm expansion phase onset.

ULF Waves above the Nightside Auroral Oval during Substorm Onset

“…This means that low-amplitude highfrequency ULF waves cannot be visually identified without analysis at least on a logarithmic scale. One interesting aspect, however, is the assertion by Morioka et al [2014] that 40-50 s period ULF waves are the signature of a magnetotail instability, which is entirely consistent with the results presented in this chapter. Moreover, we do not limit ourselves to a narrow frequency band; we use instead a broader frequency range that overlaps both the long-period Pi1 and short-period Pi2 bands, since there is no physical distinction between ULF waves of 30 s and 50 s periods, other than the artificial boundary at 40 s initially identified by Jacobs et al [1964] as a matter of practicality.…”

“…Morioka et al hypothesized that the increase in ULF wave amplitudes at auroral latitudes during expansion phase onset may instead be related to new acceleration regions developing at high altitudes in the magnetosphere-ionosphere coupling region, not the inner plasma sheet. From this, Morioka et al [2014] conclude that the magnetic Pi2 waves (40-50 s period) observed are likely a signature of a magnetospheric instability, whereas the Pi1 waves (10-30 s period) are a signature of intensified FACs in conjugate M-I coupling regions. Although Morioka et al [2014] provide compelling evidence that the AKR generation during substorms is a two-step process, it is necessary to question the ULF data more deeply.…”

“…From this, Morioka et al [2014] conclude that the magnetic Pi2 waves (40-50 s period) observed are likely a signature of a magnetospheric instability, whereas the Pi1 waves (10-30 s period) are a signature of intensified FACs in conjugate M-I coupling regions. Although Morioka et al [2014] provide compelling evidence that the AKR generation during substorms is a two-step process, it is necessary to question the ULF data more deeply. Briefly, Figure 8 of Morioka et al [2014] shows three limited frequency ranges derived from fluxgate measurements, but analyzed as a variometer using dH/dt as opposed to H(t), effectively forming a poor high-pass filtering of the data.…”

“…Studying the two-step evolution of Auroral Kilometric Radiation (AKR), Morioka et al [2014] present analysis of conjugate ground magnetometer data to their AKR signatures. Morioka et al hypothesized that the increase in ULF wave amplitudes at auroral latitudes during expansion phase onset may instead be related to new acceleration regions developing at high altitudes in the magnetosphere-ionosphere coupling region, not the inner plasma sheet.…”

“…Although Morioka et al [2014] provide compelling evidence that the AKR generation during substorms is a two-step process, it is necessary to question the ULF data more deeply. Briefly, Figure 8 of Morioka et al [2014] shows three limited frequency ranges derived from fluxgate measurements, but analyzed as a variometer using dH/dt as opposed to H(t), effectively forming a poor high-pass filtering of the data. As seen in Figure 7.2 and Figure 7.5, ULF wave onset across all frequencies occur within 10 s, and the waves can only be seen to grow exponentially out of the magnetic noise on a logarithmic scale or by quantitative analysis through tools such as AWESOME or HilbertHuang transform.…”