Modulation of Electromagnetic Ion Cyclotron Waves by Pc5 ULF Waves and Energetic Ring Current Ions

Abstract

We present a ground observation of modulations of strong electromagnetic ion cyclotron(EMIC) waves by short and long periodicities at Indian Antarctic station, Maitri. The signatures of thesewaves were evident in the magnetic field variations recorded by an induction coil magnetometer duringthe interval 4.7–7.2 UT on 17 September 2011, a moderately disturbed day. These waves were preceded bya gradual increase in the solar wind dynamic pressure, which started at 3.88 UT. The discrete rising toneEMIC waves were observed in the Pc1 frequency band (∼0.5–0.9 Hz). The investigation of the periodicitiesof the observed wave spectrogram shows the presence of short (≈2.4 min) and long (≈39–69 min)periodicities. Our analysis shows that the short periodicities are associated with the Pc5 Ultra LowFrequency (ULF) waves generated by magnetic field line oscillations, while long periodicities might beassociated with the ring current drifting ions. A new method, based on the cross-correlation technique,is adopted to determine sweep rates of the discrete rising tones. The average sweep rates estimated in therange of 0.44–1.9 mHz/s are relatively low as compared to the past reports of sweep rates derived from thesatellite observations of EMIC waves. We found that the higher sweep rates are associated with the strongerEMIC waves on the ground, which is in agreement with the theoretical studies. This suggests that thetheoretically proposed dependence of sweep rate on strength of EMIC wave in the generation region isretained even during the propagation of these waves to the ground.