Occurrence frequencies of IMF triggered and nontriggered substorms

Abstract

[1] The occurrence of triggered and nontriggered substorm are examined in light of current interest in such issues as substorm identification, IMF B y variations, and potentially undetected small-scale solar wind perturbation. Global substorms are identified using a sudden, persistent decrease in the AL index. The onset of this global expansion is taken to be the time of the Pi 2 burst nearest in time to the beginning of the AL decrease. IMF triggers were identified both subjectively through visual scanning of the data and automatically with a computer algorithm. Both northward turnings of the IMF Bz and decreases in the amplitude of the By component were considered as possible triggers. Two different solar wind monitors were used in the investigation: IMP-8 in a circular orbit with a distance 12$35 Re to the Earth-Sun line and ISEE-2 in an elliptical orbit with a distance only 5$10 Re to the Earth-Sun line. The IMP-8 results show that the triggering probability does not depend on the distance of the monitor from the Earth-Sun line in the range 12-35 Re. The ISEE dataset shows that closer than 12 Re the triggering probability is the same as it is in the IMP-8 data set. Thus there appears to be no dependence of triggering on the location of the monitor provided it is within 35 Re of the Earth. We also demonstrate that including the By component does not significantly increase the probability of substorm triggering. Approximately 60% of all substorms appear to be triggered. Of the 40% for which we could not identify a trigger, 10% occurred while the IMF was northward. The data suggest that substorm onset is a consequence of an internal magnetospheric instability that is highly sensitive to changes in magnetospheric convection induced by a sudden change in the IMF, but that these changes are not always necessary.

Substorms during different storm phases

“…That approach gives a good global estimate for the substorm onset. This does not hold for all substorms since some substorms occur during northward IMF (Hsu and McPherron, 2003;Kullen and Karlsson, 2004). All three substorms of the present study are associated with more typical southward IMF conditions.…”

“…In the solar wind we observed a change in sign of the IMF B Y component, an increase in magnitude of IMF and abruptly decreasing solar wind dynamic pressure, which is a signature of a stream interface (Gosling et al, 1978). The studies by Hsu and McPherron (2003) and Kullen et al (2010) show that 50-60 % of the substorms are associated with a potential IMF B Z or B Y trigger. Furthermore, large changes in the dynamic pressure (>7 nPa) are only responsible for a fraction (1-3 %) the largest substorms.…”

Multiple magnetic dipolarizations observed by THEMIS during a substorm

“…The sudden increase of the AE index is usually used as the substorm onset signature by many researchers (e.g., Hsu and McPherron, 2003;Huang, 2005). From this study on substorm, we found that the time of sharp increase of AE index was within the substorm expansion phase, not at the substorm onset time.…”

PC index as a proxy of the solar wind energy that entered into the magnetosphere: 2. Relation to the interplanetary electric field E KL before substorm onset

“…The commonly accepted idea is that substorms are triggered by geoeffective changes in the solar wind parameters. The alternative concept suggests that substorms can be triggered by internal process (Hsu and McPherron 2003;Liu et al 2011;McPherron et al 1986). Indeed, occurrence of moderate and even intense substorms under conditions of relatively steady interplanetary (northward) magnetic field was noted in many studies (e.g., (Kullen and Karlsson 2004;Lee et al 2007;Wu et al 2002)).…”