Characteristics of energetic (³ 30 keV/nucleon) ions observed by the Wind/STEP instrument upstream of the Earth's bow shock

M. I. Desai, G. M. Mason, J. R. Dwyer, J. E. Mazur, T. T. von Rosenvinge and R. P. Lepping

Department of Physics, University of Maryland, College Park


We investigate here the characteristics of energetic ions (0.03-2.0 MeV/nucleon) during 1225 upstream events observed by the Energetic Particles: Anisotropy, Composition, and Transport/Suprathermal Energetic Particle (EPACT/STEP) instrument on board the Wind spacecraft from 1994 day 325 to 1999 day 92. We find that (1) the event occurrence rate showed significant variations with changes in the solar cycle, (2) the occurrence rate increased when both the solar wind speed and the geomagnetic activity index were enhanced, (3) most events were observed within ± 80 RE in YGSE and inside ~ 100 RE in XGSE, although the events occurred at all locations of the Wind orbit, (4) ~ 73 % of the events were observed when the interplanetary magnetic field was radial and in the ecliptic plane, and when the spacecraft was most likely magnetically connected to the bow shock, (5) the events lasted typically between 10 min to 3 hours and exhibited strong sunward field-aligned flow, (6) ~ 25 % of the ion events were accompanied by 20-48 keV electrons as measured by the Wind/3DP instrument, (7) the energy spectra 100-300 keV protons and 30-300 keV/nucleon He-Fe during ~ 70 % of the events obeyed power laws with g between 3 and 5, while the energy spectra of He and CNO for ~ 30 % of the events softened above ~ 80 keV/nucleon obeyed power laws with g ~ 3-5, (8) the total energy ion spectrum above ~ 0.5 MeV energy was dominated by heavier ions during the events, (9) a substantial fraction ( ³ 40 %) of the spectra for all species extended above ~ 150 keV/e, and (10) the heavy ion composition of the events was similar to typical solar wind values. We compare the above findings with the main predictions of the magnetospheric leakage and the Fermi acceleration models and find that neither model can satisfactorily account for our results. We highlight the new challenges and requirements for both models.

J. Geophy. Res., Vol. 105, No. A1, 61-78, 2000