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

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‐time EMIC wave samples using a semiautomated procedure. Approximately 82% of the samples were observed in the morning‐to‐early afternoon sector (0700–1500 magnetic local time) with a maximum occurrence near noon, and their peak frequencies were mostly in the He band. We found that the occurrence rate of steady 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 frequency)/ fH+ (the local proton gyrofrequency) of the He‐band waves (∼0.11–0.16) under steady quiet conditions is lower than that (∼0.14–0.24) in previous studies. These results may be due to the fact that the plasmasphere expanded more frequently to the geosynchronous region under extremely quiet geomagnetic conditions in 2007–2008 than the periods selected in previous studies. The amplitude and frequency of He‐band EMIC waves for nonlinear wave growth are examined as changing cold plasma density at geosynchronous orbit. We confirm that the spectral properties of observed EMIC waves are in good agreement with the nonlinear theory. Key Points Steady quiet‐time He‐band EMIC waves are mostly observed in the morning‐to‐early afternoon sector The fpeak/ fH+ values of the He‐band waves are closer to fO+ than fHe+ The spectral properties of observed EMIC waves are in good agreement with the nonlinear wave growth theory