EMIC waves observed at geosynchronous orbit under quiet geomagnetic conditions (Kp ≤ 1)

Abstract

We statistically study the local time distribution of the helium band electromagnetic ion cyclotron (EMIC) waves observed at geosyn- chronous orbit when geomagnetic activity was low (Kp ≤ 1). In order to iden- tify the geosynchronous EMIC waves, we use high-time resolution magnetic field data acquired from GOES 10, 11, and 12 over a two-year period from 2007 and 2008 and examine the local time distribution of EMIC events. Un- like previous studies, which reported high EMIC occurrence in the postnoon sector with a peak around 1500-1600 magnetic local time (MLT) during mag- netically disturbed times (i.e., storm and/or substorm), we observed that quiet- time EMIC waves mostly occur in a region from morning (∼0600 MLT) to afternoon (∼1600 MLT) with a peak around 1100-1200 MLT. To investigate whether the quiet-time EMIC wave occurrence has a causal relationship with magnetospheric convection enhancement or solar wind dynamic pressure vari- ations, we performed a superposed epoch analysis of solar wind parameters (solar wind speed, density, dynamic pressure, and IMF Bz) and geomagnetic indices (AE and SY M ？H). From the epoch analysis we found that solar wind dynamic pressure variation is more important parameter than AE and SY M ？ H for quiet-time EMIC wave occurrence.