Statistical Study of Mercury's Energetic Electron Events as Observed by the Gamma‐Ray and Neutron Spectrometer Instrument Onboard MESSENGER

Abstract

We present results from a statistical analysis of Mercury's energetic electron (EE) events as observed by the gamma‐ray and neutron spectrometer instrument onboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. The main objective of this study is to investigate possible anisotropic behavior of EE events using multiple data sets from MESSENGER instruments. We study the data from the neutron spectrometer (NS) and the gamma‐ray spectrometer anticoincidence shield (ACS) because they use the same type of borated plastic scintillator and, hence, they have very similar response functions, and their large surface areas make them more sensitive to low‐intensity EE events than MESSENGER's particle instrumentation. The combined analysis of NS and ACS data reveals two different classes of energetic electrons: “Standard” events and “ACS‐enhanced” events. Standard events, which comprise over 90% of all events, have signal sizes that are the same in both the ACS and NS. They are likely gyrating particles about Mercury's magnetic field following a 90° pitch angle distribution and are located in well‐defined latitude and altitude regions within Mercury's magnetosphere. ACS‐enhanced events, which comprise less than 10% of all events, have signal sizes in the ACS that are 10 to 100 times larger than those observed by the NS. They follow a beam‐like distribution and are observed both inside and outside Mercury's magnetosphere with a wider range of latitudes and altitudes than Standard events. The difference between the Standard and ACS‐enhanced event characteristics suggests distinct underyling acceleration mechanisms.

Review of Mercury’s dynamic magnetosphere: Post-MESSENGER era and comparative magnetospheres

“…Furthermore, as dipolarizations and flux ropes interact with the surrounding plasma environment, they generate additional asymmetries in the magnetotail, including en- ergetic electron injections, fast sunward flows, magnetic pileup, and substorm current wedge formation. Dipolarizations, for example, are a dominant source of energetic electron acceleration within Mercury's magnetosphere (Dewey et al, 2017), resulting in energetic electron injections to be more frequent at dawn than at dusk (Baker et al, 2016), independent of electrons' eastward drift about the planet (Walsh et al, 2013;Nikoukar et al, 2018;Dong et al, 2019). A fraction of these energetic electrons precipitate to the planet, and in the absence of a thick atmosphere, collide with the planet's surface to produce X-ray fluorescence and space weathering (Starr et al, 2012).…”

MESSENGER Gamma Ray Spectrometer and Epithermal Neutron Hydrogen Data Reveal Compositional Differences Between Mercury's Hot and Cold Poles

“…Following earlier studies (Lawrence et al, 2013(Lawrence et al, , 2017, the duration of these events was determined manually by looking through the spectra for increased background (see the list of SEP events given in Lawrence et al, 2016). Similarly, those data acquired during energetic electron events (EEEs; Lawrence, Anderson, et al, 2015;Nikoukar et al, 2018) were also removed as they complicate the derivation of neutron count rates. This removal of EEEs resulted in the loss of just a few spectra per orbit.…”

Possible Global Generation Region of Nonlinear Whistler‐Mode Chorus Emission Waves at Mercury

“…In the tentative energetic electron enhancement events at Mercury, energetic electrons have a pancake PAD mostly distributed at 90 deg. based on the MESSENGER observations (Ho et al., 2011; Nikoukar et al., 2018). Our simulations also reconstruct the pancake PAD and show that the tentative energetic electron enhancements can evolve to a flattop PAD with repetitive chorus generation at Mercury, because the resonant electrons are effectively accelerated in the same manner as in the Earth’s radiation belt (Hikishima et al., 2009).…”

“…To validate the chorus wave generation in the day and night sectors and the contribution of evolution of PAD of energetic electrons via wave–particle interactions for the observation results showing the mainly 90‐deg. PAD (Ho et al., 2011; Nikoukar et al., 2018), we conducted a one‐dimensional full particle‐in‐cell (PIC) simulation along a magnetic field line in the Mercury magnetospheric environment. The PIC simulation is based on previous works on chorus emission waves in the Earth’s inner magnetosphere (Hikishima et al., 2009), but the parameters are widely different from the typical values used for the Earth (e.g., higher f p / f c , lower initial hot electron velocity, highly stretched field line, larger loss cone, etc.).…”

“…The most burst events show dominantly a 90‐deg. pitch angle distribution (PAD) (Nikoukar et al., 2018), and the mechanisms for forming such a PAD are not fully understood. The origin of these electron burst events and understanding the similarity and the difference in the behavior of electron dynamics between Mercury’s and Earth’s magnetospheres may elucidate particle acceleration mechanisms underlying in our Solar Systems.…”