Statistical Observations of Pc1 Pearl Pulsations as Compared to Unstructured EMIC Waves Using the Van Allen Probes

Pc1 Pearl Pulsations are a time-modulated electromagnetic wave in the Pc1 band of the ULF spectrum. They are believed to be a subclass of electromagnetic ion cyclotron (EMIC) waves, and so are generated through the ion cyclotron instability. There have been rare isolated in situ observations of these pearl pulsations in the past, but the location and sensitivity of the Van Allen Probes spacecraft have provided us with an unprecedented number of events. Roughly a third of observed wave activity within the EMIC frequency range exhibits pearl structure, which is consistent with ground-based observations. Analysis of the central frequency dependence on the background field and their occurrence probability with respect to magnetic activity indices implies that there may exist a fundamental difference between the generation mechanisms for structured and unstructured EMIC waves. These waves often exhibit a rising frequency tone with a frequency sweep rate on the order of that observed within EMIC-triggered emissions. This frequency sweep in time has recently been proposed as exhibiting a more efficient method of wave-particle interactions with ultrarelativistic electrons. As such, we will examine conjunction events with the Polar-orbiting Operational Environmental Satellite spacecraft suggesting EMIC-driven electron precipitation. As of August of 2014, the Van Allen Probes spacecraft will have undergone one full precession around Earth, allowing for observations spanning the full range in MLT. We will therefore present here a comparative statistical analysis of EMIC waves, both pearl pulsations and unstructured, compiled over full coverage of the near-equatorial magnetosphere from L=~2-6.5.