Van Allen Probes Observations of Electromagnetic Ion Cyclotron (EMIC) Rising Tones: Occurrence Statistics

Electromagnetic ion cyclotron (EMIC) waves usually occur in three frequency bands defined by the O+, He+, and H+ ion gyrofrequencies. In Fourier time-frequency power spectrograms of magnetic field data, EMIC waves sometimes feature discrete, rising tone structures that rapidly increase in frequency. We will present the occurrence rates of EMIC wave rising tones observed by the Van Allen Probes Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) fluxgate magnetometer from the start of the mission in 2012 through June 2016. Data from both Van Allen Probe A and Probe B will be presented. Most EMIC wave rising tones occurred in the H+ band and were triggered emission type events, where the discrete wave packet structures rapidly rise in frequency out of the main band of EMIC waves. A small percentage of EMIC wave rising tone events occurred in the He+ band, where the discrete wave packet structures were embedded within the main band of EMIC waves and appeared similar to whistler-mode chorus elements. For selected cases, we will present wave normal analysis and consider ion distributions associated with rising tones. These observations provide context for theoretical studies indicating that EMIC rising tones may be very efficient at scattering radiation belt electrons.