Conjugate observations of electromagnetic ion cyclotron waves

We compare electromagnetic ion cyclotron (EMIC) waves observed by the elect ric field instrument on Polar to simultaneous Pc1 pulsations detected at th e subauroral Sodankyla station. We studied 44 passes where Polar was within +/-1.5 hours in magnetic local time (MLT) from Sodankyla during a Pc1 puls ation event. in 70% of these passes a matching EMIC wave was observed in sp ace. This shows that ground Pc1s indeed originate at high altitudes in the magnetosphere and have their source field line within a rather limited (+/- 1-1.5 hours) MLT range. It also limits horizontal ducting of waves in the l ongitude direction to less than similar to 1000-1500 km, and it gives a low er limit of similar to3 hours on the MLT extent of a typical EMIC wave band in space. Out of 69 EMIC events, 75% were observed at Sodankyla. This rest ricts the scale of ionospheric modification on the amplitude and frequency of waves. In several passes, two or three EMIC waves corresponding to diffe rent frequency branches were observed simultaneously. We discuss the locati on, frequency, amplitude, and ground visibility of the EMIC waves of these three branches separately. The results emphasize the high-latitude dayside as a source of EMIC waves observed on the ground.