Signature of electric field associated with localized electron precipitation in the polar cap region - Akebono (EXOS-D) results

Akebono (EXOS-D) observations demonstrated that the electric field in the polar cap region is very irregular and electron precipitation spikes are prevalent when the interplanetary magnetic field (IMF) is directed northward. Comparison of the electric field and the particle precipitation shows that most electron precipitations occur in the divE < 0 regions. The variation of the electric field was predominantly seen in the dawn-to-dusk (E_y) component rather than in the noon-midnight (E_x) component, when the observations were made in the nightside polar cap region. This means that the particle precipitation region is basically sun-aligned in the nightside polar cap region. However, in the dayside polar cap region, the contribution of the E_x component to divE is larger, which suggests that the orientation of the elongation is not exactly sun-aligned but also has a zonal component on the dayside. The average energy of the precipitating electrons was in a range from 100 eV to 200 eV. The magnetosheath/mantle is the likely source region of the precipitating electrons in the polar cap during northward IMF conditions.