Differences in the characteristics and triggering mechanisms of two successive AL index onsets on 21st January 2005

Abstract

Southward component of Interplanetary Magnetic Field (IMF) leads to the entry of energetic particles of solar origin into the magnetosphere. A part of this energy is subsequently released from the night-side as a geomagnetic substorm via the loading and unloading process in the magnetotail. However, other than this classical understanding of substorm, there exists an alternate particle entry mechanism, which mainly triggers weak to moderate substorms. So far, there is no clear understanding of how an intense substorm can be triggered without appreciable southward turning of IMF. In this paper, we have carried out a detailed multi-instrument analysis of a very strong substorm associated with the impingement of solar wind pressure impulse that took place on 21st January 2005 when IMF was not southward. It is observed that during the initial *10 minutes of the substorm onset, the AL index reached *–2000 nT, with particle precipitation being centred near dawn and not midnight. The usual substorm expansion phase with typical signatures of Near-Earth-Neutral-Line (NENL) magnetic reconnection, dipolarization and plasmoid formation started when IMF turned southward after *10 minutes of the Brst onset on this day. Using the present case study, we have tried to explain what are the characteristics of shock-triggered substorms and where the energy comes from to power such strong events. The dominance of dawn centered precipitation during the initial phase can be attributed to increased particle interaction with the ULF waves enhanced due to prevalent K–H instability caused by strong velocity shear in the dawn sector.