Low-energy He+ and H+ distributions and proton cyclotron waves in the afternoon equatorial magnetosphere

Details Low-energy He+ and H+ distributions and proton cyclotron waves in the afternoon equatorial magnetosphere Low-energy He+ and H+ distributions and proton cyclotron waves in the afternoon equatorial magnetosphere

Jun 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996jgr...10113255f&link_type=abstract

Journal of Geophysical Research, Volume 101, Issue A6, p. 13255-13266

Physics

Plasma Physics

12

Magnetospheric Physics: Magnetospheric Configuration And Dynamics, Magnetospheric Physics: Plasma Waves And Instabilities, Space Plasma Physics: Charged Particle Motion And Acceleration, Space Plasma Physics: Wave/Particle Interactions

Scientific paper

Characteristics of low energy (~10 to 50 eV) H+ and He+ distributions in the afternoon magnetosphere from L=4 to 9 are studied using Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) data. He+ distributions in this study routinely display features indicating resonant heating by proton cyclotron waves in the outer magnetosphere (L>7) whereas protons exhibit minimal evidence of heating. Low energy H+ distributions have T⊥>>T∥ for L<5 but T⊥<7. The change is due to a decrease in T⊥ with increasing L and is consistent with adiabatic cooling in the perpendicular direction as the ions convect sunward. By contrast, low-energy He+ distributions have T⊥>>T∥ independent of L. The failure of the He+ to cool adiabatically indicates that He+ experiences transverse heating as it convects sunward through the outer magnetosphere. The occurrence of proton cyclotron waves in the afternoon magnetosphere is evaluated using AMPTE/CCE magnetometer data and shows that low energy ions convecting sunward from the afternoon plasmasphere have a 20% chance of being exposed to cyclotron waves by the time they reach L=5,50% by L=6 and >95% by L=7. The majority of cyclotron waves in the early afternoon outer magnetosphere occur above the local He+ gyrofrequency and will interact resonantly with He+ but nonresonantly with H+. The combined effects of adiabatic sunward convection and interactions between the low-energy ions and proton cyclotron waves therefore account for the observed characteristics of both the H+ and He+ distributions in the afternoon magnetosphere.

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