The super substorm incidence in response to interplanetary shock front impacts

Ma, Xiaohan

Abstract:

The interaction between interplanetary (IP) shocks and the Earth’s magnetosphere would generate/excite various types of geomagnetic phenomena, thus affect severely near Earth space environment. In this work, we analyzed 184 events out of 246 IP shocks from 2001 to 2013 to investigate the super substorms (SME/AE >1000nT) triggered by IP shock passage. All 246 shock events have been categorized into 3 groups according to the direction of the interplanetary magnetic field (IMF) Bz in the Geocentric Solar Magnetospheric (GSM) coordinates at the time of shock arrival: either associated with southward IMF or with northward IMF and with no significant north/south direction IMF. There are 106 IP shocks with southward IMF pre-condition events, and 78 IP shocks with northward IMF pre-condition events.

The statistical analysis shows that the maximum auroral electrojet (SME) indices are significantly increased when the IMF is southward. After the shock arrival, they keep increasing from 100nT to a peak value, the median value of which is 700 nT and the 3rd quartile is over 1100 nT. Meanwhile, the increments of SME indices are much higher on the dawn sectors (02:00LT∼06:00LT). However, when the IMF is northward, the changes of SME indices are less distinct.

Besides, the geomagnetic activities are much more intense around autumn equinox when the direction of IMF is away the Sun, and much more intense around spring equinox when the direction of IMF is toward the Sun, which can be attributed to the Russell-McPherron (R-M) effect. It is found that a strong substorm will be definitely occurred if the IP shock events associated with positive R-M effect together with southward IMF pre-condition in the autumn (the SME indices of all that kind of events are at least larger than 700nT).

Thus, a southward pre-condition IMF Bs in the ambient interplanetary space ahead of an IP shock , especially with positive R-M effect can be treated as a precursor of strong or even super substorm.