By analyzing observations from Van Allen Probes in its inbound and outbound orbits, we present evidence of coherent enhancement of cold plasmaspheric electrons and ions due to drift-bounce resonance with ultralow frequency (ULF) waves. From 18:00 UT on 28 May 2017 to 10:00 UT on 29 May 2017, newly formed poloidal mode standing ULF waves with significant electric field oscillations were observed in two consecutive orbits when Probe B was traveling inbound. In contrast to observations during outbound orbits, the cold (<150 eV) electrons measured by the HOPE instrument were characterized by flux enhancements several times larger and bidirectional pitch angle distributions during inbound orbits. The electron number density inferred from upper hybrid waves is twice as larger as during inbound orbits, which were also confirmed by an increase of spacecraft potential. The observed ULF waves are identified as second harmonic modes that satisfy the drift-bounce resonant condition of N = 1 with cold electrons. An enhancement of the plasmaspheric ion number density to restore charge neutrality of plasmas in inbound orbits is observed, which is associated with an increase of ULF wave periods. The observations suggest that the dynamics of plasmaspheric electrons is modified by ULF waves through drift-bounce resonance and that plasmaspheric ions are indirectly impacted.
Bibliographical noteFunding Information:
This work was supported by the National Natural Science Foundation of China (41421003 and 41627805). R. Rankin acknowledges the financial support from NSERC and the Canadian Space Agency. Processing and analysis of the HOPE data was supported by the Energetic Particle, Composition, and Thermal Plasma (RBSP-ECT) investigation funded under NASA’s Prime contract NAS5-01072. All RBSP-ECT data are publicly available at the Web site http://www.RBSP-ect.lanl.gov/. We acknowledge NASA CDAWeb (http://cdaweb.gsfc.nasa.gov/) for providing all data in this article. We thank Mirko Piersanti and another referee for their assistance in evaluating this paper.
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- ULF waves
- cold plasmaspheric electrons acceleration
- drift-bounce resonance
- modification of electron and ion density profile
- substorm activities