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.

Galactic cosmic ray variations in the inner heliosphere from solar distances less than 0.5 AU: Measurements from the MESSENGER Neutron Spectrometer

“…Based on instrument modeling, this triple-coincidence count rate measures the integral flux of protons and alpha particles greater than 125 MeV/nucleon and thus serves as a proxy for GCR flux for the vicinity around the MESSENGER spacecraft. To date, energetic particle data from the NS have been used to make corrections to neutron data measured during the first year of orbital operations [Lawrence et al, 2013] and to understand the interaction of charged particles with the MESSENGER spacecraft for studies of solar neutrons Lawrence et al, 2014Lawrence et al, , 2015. In addition, Rodgers et al [2015] reported GCR variations in the inner heliosphere for distances ranging from 0.3 to 0.85 AU based on NS measurements from the MESSENGER cruise phase that lasted from 2004 to 2011.…”

Long‐duration neutron production by nonflaring transients in the solar corona

“…[] and later defended in Lawrence et al . []. However, we also note that the count rate of fast neutrons between 0.5 and 8 MeV registered by the MESSENGER NS in response to these normalized H and He spectra incident on MESSENGER spacecraft material should be proportional to the LG2dbl count rate if the neutrons are generated locally by the energetic ion environment at MESSENGER.…”

“…This fact drove our conclusion that the neutrons observed in this event could not have been produced on the spacecraft by local energetic particles. This conclusion is strengthened by the absence of detection of an enhancement of a Mg(p,γ) line at 1635 keV by the MESSENGER GRS [see also Lawrence et al ., ].…”

“…Their main criticisms are answered by Lawrence et al . [] and our present manuscript, which surveys all such neutron events at MESSENGER between December 2007 and March 2014 that occurred when MESSENGER was magnetically coupled to spacecraft at 1 AU that carried fully functioning energetic ion spectrometers. Six such events were found that contained a sufficient flux of neutrons that they could not have all been generated by the interaction of solar energetic particle (SEP) ions from the local energetic particle environment of MESSENGER with spacecraft and NS detector material.…”

“…One event that occurred on 4 June 2011 was not accompanied by ions having energies above the 45 ± 10 MeV double‐coincidence detection threshold of the NS [ Lawrence et al ., ]. In addition, the MESSENGER Gamma‐Ray Spectrometer (GRS) did not detect an enhancement of a proton‐generated Mg gamma‐ray line, which would have had to be present if energetic ions having a sufficient enough flux during the 4 June 2011 event [ Lawrence et al ., , ] were interacting with spacecraft material to generate the observed neutrons. As a consequence, the 4 June 2011 event provides very strong evidence that the MESSENGER NS has detected solar neutrons.…”