Multipoint Analysis of Source Regions of EMIC Waves and Rapid Growth of Subpackets

Electromagnetic Ion Cyclotron (EMIC) rising tone emissions are important in understanding the nonlinear wave evolution and interaction with the energetic particles. We present observations of rising tone emissions of EMIC waves by THEMIS A, D, and E spacecraft in the outer magnetosphere. These emissions with subpacket structures in the proton band were observed during the time interval 14:20 UT to 14:30 UT on September 9, 2010. The observed waves are left‐handed polarized with wave normal angles less than 30°. THEMIS A was closest to the equator and was in a higher L‐shell than THEMIS E and D. The smallest radial separation is ∼2,000 km between THEMIS E and D spacecraft. This configuration of THEMIS allows us to investigate the subpackets of rising tone EMIC waves observed simultaneously at 14:23 UT by three spacecraft. Hilbert Huang Transformation (HHT) is applied to show the variations of the instantaneous frequency and the observed wave amplitude. The direction of energy flow is determined from the analysis of the Poynting flux. There is a rapid nonlinear growth of the EMIC subpackets within one wavelength. Subpackets are dynamic in nature as their structure changes within one wave period, which is further supported by the nonlinear wave growth theory. Optimum and threshold amplitudes for the EMIC wave growth are calculated beside the nonlinear transition time (TN). Observed ion energies and pitch angle spectra of the ion fluxes are consistent with the energy associated with the Landau and cyclotron resonance conditions. Electromagnetic Ion Cyclotron (EMIC) waves are observed below the proton gyrofrequency and play an important role in the magnetospheric dynamics through the ion heating and precipitation of relativistic electrons. They often appear as a series of repetitive structures (known as subpackets) with increasing frequencies, known as rising tone emissions in their entirety. These rising tone emissions are believed to be generated near the geomagnetic equator by the anisotropic distribution of energetic ions (T⊥T∥>1). These emissions are self‐sustaining after the primary linear growth of the triggering wave at lower frequencies. Although previous simulations and theory showed that the source regions of these rising tone emissions move along the magnetic field line, direct observational evidence was missing. Our article provides a case study where these emissions are observed simultaneously by three THEMIS probes in the outer magnetosphere. Adopting a multipoint observation technique, we show there are scattered source regions with an extent greater than the EMIC wavelength, and the subpacket structure changes nonlinearly within one wave period. This analysis provides crucial information about the dynamics of the fine structures of rising emissions and gives an idea about the 3D extent of subpackets. Simultaneous observation of an Electromagnetic Ion Cyclotron (EMIC) wave event by THEMIS A, D, and E separated by a few 1,000 kmSource regions of EMIC waves are scattered around the equator close to the magnetopauseThe subpackets drastically change their amplitudes and frequencies within one wavelength of EMIC waves Simultaneous observation of an Electromagnetic Ion Cyclotron (EMIC) wave event by THEMIS A, D, and E separated by a few 1,000 km Source regions of EMIC waves are scattered around the equator close to the magnetopause The subpackets drastically change their amplitudes and frequencies within one wavelength of EMIC waves