Misidentification of the source of a neutron transient detected by MESSENGER on 4 June 2011

Abstract

Low‐energy (1–10 MeV) neutrons emanating from the Sun provide unique information about accelerated ions with steep energy spectra that may be produced in weak solar flares. However, observation of these solar neutrons can only be made in the inner heliosphere where measurement is difficult due to high background rates from neutrons produced by energetic ions interacting in the spacecraft. These ions can be from solar energetic particle events or produced in passing shocks associated with fast coronal mass ejections. Therefore, it is of the utmost importance that investigators rule out these secondary neutrons before making claims about detecting neutrons from the Sun. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) neutron spectrometer recorded an hour‐long neutron transient beginning at 15:45 UTC on 4 June 2011 for which Lawrence et al. (2014) claim there is “strong evidence” that the neutrons were produced by the interaction of ions in the solar atmosphere. We studied this event in detail using data from the MESSENGER neutron spectrometer, gamma ray spectrometer, X‐ray Spectrometer, and Energetic Particle Spectrometer and from the particle spectrometers on STEREO A. We demonstrate that the transient neutrons were secondaries produced by energetic ions, probably accelerated by a passing shock, that interacted in the spacecraft. We also identify significant faults with the authors' arguments in favor of a solar neutron origin for the transient.

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

“…This task is necessary to answer the criticism of our analyses of the 31 December 2007 and 4 June 2011 neutron events given by Share et al . [, ]. Our methodology was first presented in Lawrence et al .…”

“…Before proceeding to a chronological analysis of the seven accepted neutron events, we present our methodology to fill the void in our knowledge of the energetic ion environment at MESSENGER by using the ion spectra measured at 1 AU by the energetic ion experiments on the STEREO A/B and the GOES/ACE spacecraft. This task is necessary to answer the criticism of our analyses of the 31 December 2007 and 4 June 2011 neutron events given by Share et al [ , 2015. Our methodology was first presented in Lawrence et al [2014] and later defended in Lawrence et al [2015b].…”

“…Both reports of the detection and identification of solar neutrons by the MESSENGER NS were criticized by Share et al . [, ] on the basis that we could not independently determine the energetic ion environment at MESSENGER using existing MESSENGER particle detectors for these events. Their main criticisms are answered by Lawrence et al .…”

“…Both reports of the detection and identification of solar neutrons by the MESSENGER NS were criticized by Share et al [ , 2015 on the basis that we could not independently determine the energetic ion environment at MESSENGER using existing MESSENGER particle detectors for these events. Their main criticisms are answered by Lawrence et al [2015b] 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.…”

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

“…On the basis, in part, of these results, Lawrence et al [2014] concluded that there was strong evidence that the detected neutrons were solar in origin. Share et al [2015] (hereafter shortened to Share et al) recently criticized Lawrence et al's [2014] study. In particular, Share et al concluded that the excess neutrons detected on 4 June 2011 were generated locally at the spacecraft.…”