The Convective Electric Field Influence on the Cold Plasma and Diamagnetic Cavity of Comet 67P

We studied the distribution of cold electrons (<1 eV) around comet 67P/Churyumov–Gerasimenko with respect to the solar wind convective electric field direction. The cold plasma was measured by the Langmuir Probe instrument and the direction of the convective electric field ${\boldsymbol{E}}$conv = −${\boldsymbol{v}}$ × ${\boldsymbol{B}}$ was determined from magnetic field (${\boldsymbol{B}}$) measurements inside the coma combined with an assumption of a purely radial solar wind velocity ${\boldsymbol{v}}$. We found that the cold plasma is twice as likely to be observed when the convective electric field at Rosetta's position is directed toward the nucleus (in the −${\boldsymbol{E}}$conv hemisphere) compared to when it is away from the nucleus (in the +${\boldsymbol{E}}$conv hemisphere). Similarly, the diamagnetic cavity, in which previous studies have shown that cold plasma is always present, was also found to be observed twice as often when in the −${\boldsymbol{E}}$conv hemisphere, linking its existence circumstantially to the presence of cold electrons. The results are consistent with hybrid and Hall magnetohydrodynamic simulations as well as measurements of the ion distribution around the diamagnetic cavity.