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Multipoint Observations of Magnetospheric Compression related EMIC Pc1 Waves by THEMIS and CARISMA M E Usanova1 I R Mann1 I J Rae1 Z C Kale 1 V Angelopoulos2 J W Bonnell3 K H Glassmeier4 H U Auster4 and H J Singer5 1 Department of Physics University of Alberta Edmonton Alberta Canada Department of Earth and Space Sciences University of California at Los Angeles USA 3 Space Sciences Laboratory University of California Berkeley California USA 4 Institut f r Geophysik und Extraterrestrische Physik Technische Universit t Braunschweig Braunschweig Germany 5 NOAA Space Weather Prediction Center Boulder Colorado USA formerly Z C Dent 2 Abstract 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 Compressionrelated 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 Introduction Pc1 0 2 to 5 Hz pulsations are continuous geomagnetic field fluctuations believed to be generated by the electromagnetic ion cyclotron EMIC instability free energy often being provided by equatorial hot ions with temperature anisotropy Tperp Tpar e g Cornwall 1965 EMIC wave occurrence has been linked to variations in the solar wind flow and interplanetary magnetic field IMF In particular there is a class of dayside Pc1 events that show a strong correlation between EMIC power and increases in solar wind pressure e g Anderson and Hamilton 1993 Arnoldy et al 2005 Olson and Lee 1983 suggested that magnetospheric compressions cause an increase in hot proton temperature anisotropy Anderson and Hamilton 1993 confirmed that the probability of observing EMIC waves in space increases during magnetospheric compressions concluding that plasma in the outer dayside magnetosphere is often close to marginal stability such that EMIC waves can be stimulated by even modest compressions Anderson et al 1992 suggested that the EMIC growth rate which is inversely proportional to the Alfven velocity VA B0 0 0 peaks at two locations at high L shells where the geomagnetic field is relatively weak and just inside the plasmapause where the ambient plasma density is high Steep plasmapause density gradients may also provide special propagation conditions that cause local enhancements in growth rates so that waves can grow significantly even outside the plasmapause despite the density drop Horne and Thorne 1993 Engebretson et al 2002 observed spatially localized EMIC waves where continuous wave emissions were seen on the ground for extended periods of time In space the waves were observed on the Polar satellite for only a few minutes occurring only in radially narrow regions outside the plasmapause from L 5 11 Engebretson et al 2002 suggested that plasma sheet protons convecting sunward from the nightside magnetosphere were responsible for the EMIC wave generation Inside the plasmapause Pc1 activity associated with high solar wind density is more unusual Zolotukhina et al 2007 reported observations of Pc1 2 waves at low and high latitude ground stations during storm recovery phase suggesting that EMIC activity can be modulated by fast magnetosonic waves launched by the solar wind impacting the magnetopause We present coordinated ground satellite observations of compression related dayside structured e g Mursula et al 1997 EMIC Pc1 waves from 29th June 2007 The EMIC waves occur coincidentally with a strong solar wind density enhancement following several days of sustained quiet geomagnetic conditions Kp 3 Dst 10 nT On the ground structured EMIC wave activity with a wavepacket repetition period of 3 minutes was observed for several hours simultaneously with the enhancement in solar wind density In space the EMIC waves were observed coherently by three THEMIS spacecraft D then C and then E for a period of 35 minutes as they consecutively crossed the same region of space in a string of pearls configuration Multipoint spaceground observations enabled us to determine the location of the waves in the magnetosphere and conclude that the EMIC wave activity was spatially localised and confined to a region of low spacecraft potential interpreted here as just inside the plasmapause Observations and Data The orbits of the five Time History of Events and Macroscale Interactions during Substorms THEMIS Sibeck and Angelopoulos 2008 spacecraft lined up in a stringof pearls with apogee at 15 4 Re and an orbital period of 36 hours during the initial phase of the mission from launch on February 17 until September 2007 We use data from the THEMIS fluxgate magnetometer FGM Auster et al 2008 and electric field investigation EFI Bonnell et al 2008 instruments The Canadian Array for Real time Investigations of Magnetic Activity CARISMA www carisma ca is the continuation and expansion of the CANOPUS magnetometer array CARISMA has an upgraded cadence 8 samples s is expanded through the deployment of new stations and uses new GPS timed data loggers and a new data transmission system The sites used for this study and details of the orbits of THEMIS C D and E from 14 00 UT to 16 00 UT are shown in Figure 1 At around 14 00 UT the onset of a significant solar wind density enhancement n 35 cm 3 embedded in a slow solar wind V 350 km s arrived at the subsolar magnetopause This triggered a strong magnetospheric compression

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