Electron calibration of instrumentation for low energy, high intensity particle measurements at Mercury

Abstract

Unique identification of the high intensity, impulsively accelerated charged particle fluxes discovered during Mariner 10's first encounter with Mercury (March 1974) requires a detailed knowledge of the responses of the two University of Chicago charged particle telescopes on the Mariner 10 spacecraft to low energy fluxes over a wide dynamic range of flux levels. The results of detailed analyses show that these telescopes can separate and identify unambiguously the presence of electron and proton fluxes for a wide range of electron spectra and intensities in the relevant overall energy range of ∼30 keV to ∼2 MeV. We reaffirm the detection of electrons with energies in the range 200–600 keV in portions of the burst events. Armstrong et al. (1975) argued that our reported low energy telescope (LET) proton fluxes which were observed during the most intense portions of the B and C events could be due to two‐ and three‐fold electron pile‐up resulting from electrons in the energy range ∼100–200 keV. Using proton measurements made with nearly identical LET's on Pioneer spacecraft near Jupiter and, in addition, using detailed laboratory calibrations, we find that their estimates are in error by orders of magnitude. Then, using our calibration results, we searched for those electron intensities and energy spectra which conceivably could simulate the charged particle telescope responses at Mercury I. From laboratory electron calibrations and calculations we find that assumed fluxes of electrons having power‐law differential energy spectra with spectral index ∼7–9 and intensities of ≈4 × 108 electrons/(cm² s sr) for energies greater than 35 keV, give rise to a calculated counting rate in our LET which is completely dominated by at least seven‐fold pulse pile‐up. These fluxes can simulate the observed Mercury I counting rates, but this imposes a rate of energy input of up to ≈30 ergs/(cm² s sr). All other electron spectra defined for energies >35 keV having a maximum differential intensity at an energy ≳50 keV fail to simulate simultaneously the Mercury 1 LET and main telescope (MT) responses. Thus the electron fluxes required to simulate our measurements of the Mercury I B and C events must be a factor ∼10²–10³ greater than the electron fluxes required to interpret those measurements as resulting from the simultaneous presence of electron and proton fluxes. We therefore favor an interpretation in terms of simultaneous electron and proton fluxes.