Plasma Environment Around Comet 67P / Churyumov-Gerasimenko at its Perihelion

Near the perihelion of comet 67P/Churyumov-Gerasimenko (67P/CG) the instrument of the Rosetta Plasma Consortium (RPC) detected signatures of a diamagnetic cavity at a distance of about 150 km from the nucleus. The MAGnetometer (MAG) instrument observed crossing events into a magnetic field-free region where the magnitude of the magnetic field dropped by more than 20 nT to near zero. The Ion and Electron Sensor (IES) also observed decrease in flux of electrons with energies between 40-250 eV by more than a factor of two. This drop in the electron spectrum is perhaps due to the absence of the solar wind inside the diamagnetic cavity. The colder electrons (E<40 eV) are abundant inside and outside of the cavity and show no change in flux and the super hot electrons with energies greater than 250 eV have gyroradii large enough to not be affected at these crossing events. In this study, we show the IES electron differential flux spectrum inside and outside the cavity and compare the observations with model results. The model cases are for (1) pure photoelectron spectrum inside the cavity, (2) photoelectrons with a reflecting (trapping) boundary condition at 150 km from the nucleus, (3) photoelectrons with weak solar wind electron spectrum as a boundary condition. Model case 2 best describes the IES electron spectrum inside the cavity while model case 3 corresponds to the electron spectrum just outside of the cavity. We also studied the correlation between count numbers of electrons of different energies with the magnitude of the magnetic field. No apparent linear correlation was observed, however, for all electron energies, the highest electron counts are recorded at the highest magnetic field magnitudes.