We present here the first in situ statistical survey of structured Pc1 pearl pulsations compared with unstructured electromagnetic ion cyclotron (EMIC) waves observed by the Van Allen Probes spacecraft. This data set was compiled from observations spanning 8 September 2012 through 31 August 2015 and comprises over 1630 h of total EMIC wave activity, of which 291 h exhibited pearl structure. Additionally, we have identified 29 wave events demonstrating periodically oscillating wave packets, mostly about the magnetic equator, indicated by the reversal of Poynting flux along the background magnetic field. We have found several stark differences between Pc1 pearl pulsations and unstructured EMIC waves. While unstructured EMIC waves demonstrate the predicted behavior of a higher occurrence across the dayside with enhanced wave power at dusk, pearl pulsations occur uniformly across magnetic local time, with a small enhancement in the late morning sector. Pearl pulsations were more often observed during magnetospherically quiet periods, particularly in the late recovery period of geomagnetic storms. The mean excitation frequency of pearl pulsations was observed to be independent of the local ion cyclotron frequency, and individual wave investigations indicate that the modulation period also remained constant for the duration of the event over a finite range in L. We examine three possible generation mechanisms—the bouncing wave packet model, modulation by ultralow-frequency Pc4 and Pc5 waves, and the formation of an ion cyclotron resonator—but are unable to definitively confirm the validity of any one model.
Bibliographical noteFunding Information:
This work was supported by Iowa subcontract 1000556126 to UNH in support of the VAP spacecraft and EMFISIS/MAG instruments and by NSF at UNH under grants PLR-1141987 and AGS-1202827. Additional support was provided by NASA through NASA Space Grant Fellowship NNX15AH79H. All data used for this analysis are freely available through the various instrument SOCs found at http://rbspgway.jhuapl.edu/data_instrumentationSOC.
- EMIC waves
- Pc1 pearl pulsations
- Van Allen Probes