Role of Diffusion in Transport of Cold Dense Plasma Near the Magnetopause Deep Into the Magnetosphere: THEMIS Observations

We investigated a boundary region between hot-tenuous and cold-dense plasma sheets (HT-CDPSs) in order to understand transport of cold dense plasma near the magnetopause deep into the magnetosphere. In this presentation, we will show a case study of THEMIS observations in which three of the THEMIS spacecraft observed an HT-CDPS boundary in the dawn flank. The HT-CDPS boundary was detected sequentially at THEMIS-E (farthest from the Earth), at THEMIS-A, and at THEMIS-D (nearest from the Earth), which suggests earthward transport of the cold dense plasma. Decrease in ion density toward the Earth indicates that the earthward transport of the cold-dense plasma is not simply due to earthward convective motion of the flux tube filled with cold-dense plasma. 3D particle data show that ions (cold electrons) have a perpendicular (parallel) anisotropy during the farthest boundary crossing, while the strength of these anisotropies weakens during the following boundary crossing. Electromagnetic fluctuations below proton cyclotron frequency were observed at least during the farthest boundary crossing. We found that a diffusion coefficient estimated from the observed ion density is an order of magnitude larger than the typical coefficients estimated from simulations of diffusive processes, e.g., kinetic Alfven waves and Kelvin-Helmholtz instability at the magnetopause. We will discuss whether the observed electromagnetic fluctuations can contribute to the transport of cold plasma in the magnetosphere.