Substorm activity on January 11, 1994: Geotail observations in the distant tail during the leading phase of a corotating interaction region
Authors: Wilken B. , Zong Q.‐G. , Doke T. , Mukai T. , Yamamoto T. , Reeves G. D. , Maezawa K. , Kokubun S. , Ullaland S. 
Published: February 2004

On January 11, 1994 an interplanetary corotating interaction region (CIR) passed the Earth's magnetosphere (final phase of solar cycle 22). Ground‐based magnetometers, geosynchronous satellites, and Geotail (GSE −91, 15.5 and −3.5 RE), in or near the plasma sheet at the dusk flank of the magnetotail, detected a series of disturbances throughout this day, which culminated in the development of an isolated substorm between 1400 and 1710 UT (event 2). Small substorm activity early in the day (event 1) produced an energetic particle population with rather normal composition (relative helium abundance 10%). The CIR‐related high‐speed stream started to interact with the magnetosphere at about midday, which appears to have contributed to the initiation of the major substorm at 1400 UT (event 2). A geosynchronous Los Alamos National Laboratory (LANL) satellite at 2100 LT detected a rapid flux dropout for electrons at 1400 UT followed by a transient recovery around 1500 UT. The main injection phase, which started at 1551 UT, resulted in a massive electron flux increase. Throughout the 120‐min long growth phase of this substorm the energetic particle spectrometer (HEP‐LD) on board Geotail measured a rather slow flux buildup for protons and helium ions in the distant plasma sheet (PS). The azimuthal angular distributions showed only very small anisotropies in this phase. An unusual feature is the high helium abundance of 26% compared to about 10% normally found in the magnetosphere. After the onset of the electron injection at 6.6 RE, HEP‐LD observed the appearance of tailward beams of energetic protons and helium ions (relative helium abundance 26%) in the PS/plasma sheet boundary layer (PSBL) and oxygen ions in the central plasma sheet (CPS). The oxygen ions were delayed by 27 min relative to the P/He beam. Plasmoid‐like structures in the magnetic field accompanied the streaming ions. The synoptic observations in the geostationary orbit and in the distant tail suggest a source location at X=−50 RE (near‐Earth neutral line (NENL)) for this substorm. Substorm‐related extraction of oxygen ions from the polar ionosphere and subsequent drift to the acceleration region (NENL) can explain the observed delay for these ions, event 3, the last disturbance of the day, started at 2000 UT and lasted until 2300 UT. The proton/helium population was again broad in angular distribution and helium‐rich in composition (20.7%) but the geostationary/distant tail association is less dynamic. The appearance of accelerated helium‐rich energetic plasmas in event 2 and 3, confined to the plasma sheet and absent in the lobes, is most likely a result of the arrival of solar wind plasma with an enhanced helium content, which gained access to the magnetosphere shortly before or during the growth phase of the event 2 substorm. Helium‐rich solar wind compositions are frequently observed in CIR events.