2008gl0344582008gl034458-p012008gl034458-p022008gl034458-p03Multipoint 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 1Department of Physics, University of Alberta, Edmonton, Alberta, Canada. 2Department of Earth and Space Sciences, University of California at Los Angeles, USA. 3Space Sciences Laboratory, University of California, Berkeley, California, USA. 4Institut für Geophysik und Extraterrestrische Physik, Technische Universität Braunschweig, Braunschweig, Germany. 5NOAA Space Weather Prediction Center, Boulder, Colorado, USA. *formerly Z. C. Dent. 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. Compression-related EMIC waves are usually thought to be preferentially confined to higher L’s thanobserved 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= B0μ0ρ0), 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 space-ground 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 “string-of-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