Polar Scientists Find Unexpected Energetic Ions in the Earth's Magnetic Cusp
Fritz, T. A., J. Chen, R. B. Sheldon, H. E. Spence, J. F. Fennell, S. Livi, C. T. Russell, and J. S. Pickett, Cusp energetic particle events measured by POLAR spacecraft, Phys. and Chem. of the Earth, 24, 135, 1999.POLAR carries the full range of particle instrumentation to resolve details of the complete spectrum of cusp energetic ions:
HYDRA: total ion distributions from
10 eV to 20 keV
TIMAS: plasma ion composition from
15eV/Q to 32 keV/Q
CAMMICE: energetic ion composition from
1 keV/Q to 60 MeV/Q.
CEPPAD IPS: total ion distribution from
30 keV to 1 MeV
The full energy range from thermal solar wind to MeV energies is present in the cusp and the detailed spectrum is used to unravel possible sources for these ions. 
The cusp is traditionally associated
with particles of solar wind origin:
ions 80 eV - 5 keV
electrons 20 eV - 1 keV
Polar often observes highly
energetic ions in the cusp:
10 keV up to several MeV
These are more typical of the trapped ring current and radiation belt populations.
A significant portion of the particle spectrum is consistent with solar wind accelerated through the quasi-parallel bow shock.
Karlheinz Trattner of Lockheed finds that the spectrum up to 10keV is consistent with direct access of shocked solar wind. The range from 10 - 150 keV fits the bow shock accelerated ions. Connectivity from the bow shock to cusp has never before been so directly implicated.
This may establish a new connection process between the solar wind and the magnetosphere. The origin of the high energy tail is still a topic of considerable scientific debate.
The Earth's cusps are singular regions in the magnetosphere at which all field lines threading the cavity boundary (magnetopause) come together. Particles on field lines in the cusp have access to basically all of geospace. The cusp has two important features:
1. It is the only place the solar wind has direct access to low altitudes.
2. Through convection, it is the only place in the magnetosphere that is virtually connected to everywhere.
Ted Fritz and Jiasheng Chen of Boston University have suggested that acceleration mechanisms local to the cusp act to create the energetic particle spectrum within the cusp and have identified intense magnetic fluctuations as the power source.
Discovery of new energetic ion population in the cusp; a significant portion of which originates through a direct connection to the bow shock. Origin of highest energy portion of spectrum remains under dispute; the evolving Polar orbit offers the best opportunity for resolution.
Given the cusp's connectivity, the presence of these Cusp Energetic Particles (CEP) has major implications for theories regarding energetic particle populations within the magnetosphere (the radiation belts and ring current) dramatically altering current perceptions of the sources and dynamics of these regions.
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