Spectral characteristics of steady quiet-time EMIC waves observed at geosynchronous orbit

Abstract

We have studied the spectral properties of quiet-time electromagnetic ion cyclotron (EMIC)waves following a steady quiet condition, which is defined with Kp values ≤1 during 12 h, using GOES 10,11, and 12 magnetometer data for solar minimum years 2007-2008. We identified 6584 steady quiet-timeEMIC wave samples using a semiautomated procedure. Approximately 82% of the samples were observedin the morning-to-early afternoon sector (0700-1500 magnetic local time) with a maximum occurrencenear noon, and their peak frequencies were mostly in the He band. We found that the occurrence rate ofsteady quiet-time EMIC waves is higher than that of EMIC waves for all or quiet geomagnetic conditions(Dst>0 nT or AE < 100 nT) reported in previous studies by a factor of 2 or more. The frequency ratio fpeak(sample's peak frequenc y)/fH+(the local proton gyrofrequency) of the He-band waves (∼0.11-0.16) understeady quiet conditions is lower than that (∼0.14-0.24) in previous studies. These results may be due tothe fact that the plasmasphere expanded more frequently to the geosynchronous region under extremelyquiet geomagnetic conditions in 2007-2008 than the periods selected in previous studies. The amplitudeand frequency of He-band EMIC waves for nonlinear wave growth are examined as changing cold plasmadensity at geosynchronous orbit. We confirm that the spectral properties of observed EMIC waves are ingood agreement with the nonlinear theory.