Multipoint observations of magnetospheric compression‐related EMIC Pc1 waves by THEMIS and CARISMA

Following a long interval (many days) of sustained very quiet geomagnetic conditions, electromagnetic ion cyclotron (EMIC) wave activity was seen by the CARISMA array (www.carisma.ca) on the ground for several hours simultaneously with enhanced solar wind density and related magnetic compression seen at GOES 12 on 29th June 2007. The THEMIS C, D, and E satellites were outbound in a “string‐of‐pearls” configuration and each observed EMIC waves on L‐shells ranging from 5 to 6.5. THEMIS resolved some of the spatial‐temporal ambiguity and defined the radial extent of EMIC activity to be ∼1.3 Re. The band‐limited EMIC waves were seen slightly further out in radial distance by each subsequent THEMIS satellite, but in each case were bounded at high‐L by a decrease in density as monitored by spacecraft potential. The EMIC wave activity appears to be confined to a region of higher plasma density in the vicinity of the plasmapause, as verified by ground‐based cross‐phase analysis. The structured EMIC waves seen at THEMIS E and on the ground have the same repetition period, in contradiction to expectations from the bouncing wave packet hypothesis. Compression‐related EMIC waves are usually thought to be preferentially confined to higher L's than observed here. Our observations suggest solar wind density enhancements may also play a role in the excitation of radially localised EMIC waves near the plasmapause.