UCLA

The Gamma Ray and Neutron Detector (GRaND) onboard the Dawn spacecraft is sensitive to energetic protons, energetic electrons, and strong magnetic fields. This allows us to probe the local space environment while Dawn was in orbit about Vesta and Ceres. Understanding how these planets interact with the solar wind provides information about properties of the planetary surface and its interior. At Vesta, we searched for evidence of a past dynamo by attempting to indirectly detect remanent crustal fields. We scanned the GRaND data for solar wind electrons reflected by local crustal fields and by sensing effects of the magnetic field on the BGO spectrum. We did not find any evidence for reflected electrons or shifts in the BGO spectrum caused by magnetism associated with Vesta. Based on the sensitivity of the instrument, we conclude the upper bound for the magnetic dipole moment to be 6*10^8 Tm^3.

At Ceres, we have interpreted sudden bursts of energetic electrons detected by GRaND to be an electron foreshock produced by reflection and acceleration of solar wind electrons encountering a bow shock surface. This phenomenon occurred on a timescale of ~10 days, suggesting the bow shock to be a transient feature, most likely produced by a temporary exosphere. We show that a temporary exosphere at Ceres can be produced by solar energetic particle events which are capable of sputtering water ice from the surface.