Relationship between energetic particles and plasmas in the distant plasma sheet

Abstract

Measurements of ions from three different instruments on the IMP‐7 and 8 spacecraft are combined to yield the differential energy spectra of ions over the entire energy range of ∼ 100 eV to 4 MeV in the earth’s distant (∼ 30 to ∼ 40 Re) plasma sheet. These spectra, obtained during times of relatively small bulk flow velocities, span the intensity range from ∼ 10−5 to 105 (cm²sec sr keV)−1, varying smoothly over the entire energy range both when the plasma is cold (∼ 1 keV) and hot (∼ 9 keV). Overall, the shape of the spectrum resembles a Maxwellian but with a high energy (≳ 50 keV) tail described well by a power law (⧜E−γ). The high energy tail is displaced in a parallel fashion to higher or lower intensities when the plasma is hot or cold, respectively. The transition between the Maxwellian and the power law occurs at E ≃ (γ + 1)kT. It is found that the energetic particle populations in the plasma sheet appear to be directly related to the mean thermal energies of the corresponding plasmas.