Low-Energy Ions as a Major Source of Magnetospheric Plasma: Statistics and Consequences

Abstract:

Magnetospheric ions with energies less than tens of eV originate from the ionosphere. The low energy indicates the origin of the plasma but also severely complicates detection of the positive ions onboard sunlit spacecraft at higher altitudes, which often become positively charged to several tens of volts. We discuss some methods to observe low-energy ions, including a technique based on the detection of the wake behind a charged spacecraft in a supersonic flow.

Low-energy ions typically dominate the density and flux in large regions of the Terrestrial magnetosphere, also at high altitudes. This is true both on the nightside and the dayside, during all parts of the solar cycle. The loss of this initially low-energy plasma to the solar wind is one of the primary pathways for atmospheric escape. The global outflow is of the order of 10²⁶ ions/s and often dominates over the outflow at higher energies. The outward flux increases by a factor of 2 with increasing solar EUV flux during a solar cycle. The total outward flux may increase another factor of 2 due to an increased polar cap area at high geomagnetic activity. On the dayside, the increased density due to low-energy plasma will lower the Alfvén velocity and the magnetopause reconnection rate. In addition, the low-energy ions with a gyro radius between the gyro radii of electrons and hot ions introduce a new lengths scale important for magnetic reconnection and Hall currents.