MESSENGER X-Ray Spectrometer and Energetic Particle Spectrometer Observations of Energetic Electrons during the Mercury Flybys

The X-Ray Spectrometer (XRS) onboard the MESSENGER spacecraft will determine elemental abundances on the surface of Mercury by measuring fluorescent X-ray emissions induced on the planet’s surface by the incident solar X-ray flux. The most prominent fluorescent lines are the Kα lines from the elements Mg, Al, Si, S, Ca, Ti, and Fe (1-10 keV). The Energetic Particle Spectrometer (EPS) observes both ions and electrons (>35 keV) accelerated in Mercury’s magnetosphere. EPS measurements will help characterize magnetospheric-particle contributions to Mercury’s magnetic field, as well as characterize energy conversion processes in the vicinity of, and within, the magnetosphere. Prior to entering orbit about Mercury in March 2011, MESSENGER has flown by the planet three times for spacecraft-trajectory gravity assists and scientific observations. Solar X-ray emissions during the flybys have been an order of magnitude lower than expected, and no X-ray signal has been detected from the planet. Nevertheless, the XRS has measured several count-rate spikes before (by ~5 minutes) and after (~7 minutes) closest approach whose signatures clearly identify their origin as electrons (~10-30 keV) interacting in the XRS detector material. Electron-induced fluorescence and bremsstrahlung are evident in the XRS gas proportional counters. Measured spectra are well modeled by kappa function electron distributions impinging on the XRS Mg and Al filters, Be windows, and Be-Cu collimator. However, no corresponding signal is observed by the EPS for these events. Modeling of at least one of these events suggests sufficient electron flux above 35 keV to be detectable by EPS. A possible scenario is that the XRS detected highly anisotropic and short-duration electron bursts that were not in the EPS field-of-view, implying an exceptionally narrow pitch angle distribution. The energetic electron observations made by the EPS and XRS provide a basis for assessing the electron distributions at Mercury during MESSENGER flybys.