The 4 June 2011 neutron event at Mercury: A defense of the solar origin hypothesis.

Abstract

We address the claim that an increase in the flux of neutrons detected by the Neutron Spectrometer (NS) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft in orbit about Mercury at 15:45 UTC on 4 June 2011 was generated by the impact of energetic ions onto spacecraft. We find this claim to be unwarranted. The claim is grounded on the erroneous assumption that the NS singles count rate is triggered only by energetic ions. Rather, because any mix of energetic ions, electrons, photons, and neutrons can trigger NS singles, these data do not provide a reliable constraint on the presence of energetic ions. The absence of an enhancement in the count rate of 1635‐keV gamma rays, as monitored by the MESSENGER Gamma‐Ray Spectrometer, provides independent evidence that a fluence of energetic protons sufficiently high to generate the neutron enhancement was not present during the neutron event. The interpretation that currently best matches the available data is that the neutron enhancement on 4 June 2011 was the result of solar neutrons.