Spontaneous and trigger-associated substorms compared: Electrodynamic parameters in the polar ionosphere

Abstract

[1] An attempt is made to study the difference, if any, between the response of the polar ionosphere to spontaneous substorms and that to trigger-associated substorms in terms of electrodynamic parameters including ionospheric current vectors, the electric potential, and the current function. The results show that, in the first approximation, the ionospheric parameters for the two types of substorms are quite similar. It is therefore conceived that spontaneous substorms are not very different from trigger-associated substorms in the development of substorm processes in the magnetosphere-ionosphere system. We demonstrate, however, that spontaneous substorms seem to have a more clearly identifiable growth phase, whereas trigger-associated substorms have a more powerful unloading process. Changes in the current intensity and the electric potential drop across the polar cap in the recovery phase are also quite different from each other. Both the current intensity and the cross-polar cap potential drop show a larger decrease in the recovery phase of trigger-associated substorms, but the potential drop decreases only slightly and the currents in the late morning sector are still strong for spontaneous substorms. We interpret these findings as an indication of the relative importance of the unloading process and the directly driven process in conjunction with the north-south polarity of the interplanetary magnetic field. There still exists a strong directly driven process in the recovery phase of spontaneous substorms. For trigger-associated substorms, however, both the directly driven process and the unloading process become weak after the peak time.

Substorm Occurrence and Intensity Associated With Three Types of Solar Wind Structure

“…This assessment is also consistent with the finding that triggered substorms tend to be associated with larger | AL | values (Hsu & McPherron, ). Liu et al () also reported that a larger response in the polar cap potential drops for triggered than for spontaneous substorms.…”

Variation and modeling of ultraviolet auroral oval boundaries associated with interplanetary and geomagnetic parameters

“…It should be noted that not all the response time of some processes is equal to this propagation time, especially the processes taking place in the magnetospheric tail. For instance, there are not only the trigger‐associated substorms responding to the variation of IMF in several minutes but also the spontaneous substorms which occur without obvious IMF triggering or when IMF changed several tens of minutes later [ Liu et al ., ]. Moreover, the nightside auroral brightness responds to the southward turning of the IMF with a peak delay time of ~60 min [ Liou et al ., ].…”

THEMIS and ground-based observations of successive substorm onsets following a super-long growth phase

“…Previous studies (e.g., Rostoker et al, 1983) have shown that the disturbances in the solar wind/IMF could trigger substorm. There exists a relatively stronger loading process for the substorm that occurred during a steady solar wind/IMF condition (Liu et al, 2011). Hsu and McPherron (2004) suggested the internal instability in the magnetosphere that causes the substorm is susceptible to external perturbations in the solar wind/IMF.…”