Van Allen Probes Observations of Quasi-Periodic Whistler Mode Waves

Abstract:

Quasi-periodic whistler mode emissions (QP) are electromagnetic waves in the frequency range of a few hundred Hz to a few kHz observed in the inner magnetosphere that exhibit a periodic time modulation (tens of seconds to many minutes) of the wave intensity. These waves were first detected at high latitude ground based stations, but have also been detected by a number of spacecraft, including the twin Van Allen Probes. The Van Allen Probes include the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) Waves instrument. EMFISIS Waves simultaneously measures the vector wave magnetic field and the vector wave electric field in a continuous survey mode (typically with a 6 second cadence) along with a number of different burst modes to provide high time resolution waveforms (35,000 samples per second). These two modes allow a systematic survey of the occurrence of the QP waves and an investigation into any small scale structure of these emissions. By measuring all six wave components simultaneously, the wave propagation parameters, such as the wave normal angle and Poynting vector, of these plasma wave emissions are also obtained. A statistical survey of the occurrence and properties of the QP emissions detected by the Van Allen Probes has been performed. QP emissions have been detected at all Magnetic Local Times (MLT) and at all magnetic latitudes that Van Allen Probes sampled (-20^o to +20^o). The emissions are usually observed as either QP emissions which rise out of a band of hiss (QP hiss risers) or as QP emissions observed in a frequency band (QP bands), either separated from lower frequency hiss or detected when no hiss is present. The QP emissions have periods ranging from ~1 to 16 minutes, with most being around 2 to 4 minutes. Most QP events were detected for less than 1 hour, though some lasted for over 6 hours, and many were detected on successive orbits as the spacecraft returned to the same region of the magnetosphere. Furthermore, many events were detected by both spacecraft at the same time, even during large spacecraft separations, suggesting global events. These joint Van Allen Probes observations, along with simultaneous observations by other spacecraft such as Cluster and joint ground observations provide an opportunity to investigate the source and propagation properties of these waves.