Asymmetric Reconnection at the Magnetopause Under the Presence of Low-Energy Ions of Magnetospheric Origin

Abstract:

Magnetic reconnection at the magnetopause is characterized by asymmetry of the plasmas involved. On one side, there is the shocked plasma of the sheath coming from the solar wind, and, on the other, the energetic plasma drifting from the magnetosphere towards the magnetopause. Recent studies have pointed that, in addition to the high-energy (~10⁴ eV) ions from the latter plasma, relevant amounts of positive low-energy ions (with energies below the equivalent spacecraft potential, i.e. below ~5-10 eV) are frequently present on the magnetospheric side of reconnection layer. These ions have small enough gyroradius to ExB drift inside the thin separatrix layer, and can modify processes in the layer. We use Cluster observations near the subsolar point to study the role of these low-energy ions at the magnetopause when reconnection is occurring. The presence of low-energy ions can be detected by observing a local wake electric field caused by a supersonic flow of low-energy ions passed a charged spacecraft. The density of these ions can be estimated by comparing the total density obtained from the plasma frequency or the spacecraft potential with observations by particle detectors of ions with high enough energy to reach the spacecraft. The dataset employed (2008) permits to resolve the current at scales below the ion inertial length (c/ω_pi) in the direction normal to the magnetopause. The terms jxB/en, -vxB and  ∇ p_e/en are compared in the thin layers with strong E_n fields observed during the magnetopause crossings. It is clear that the presence of low-energy ions modifies the reconnection process, since in some cases they are able to balance the whole E_n term via the -vxB drift without the need of jxB/en.