Determination of the polytropic index in the plasma sheet

Using eight months of magnetotail plasma data, we have done a statistical survey on the relation between ion density and pressure in the Earth's plasmasheet. More than 270,000 spin averaged (4.5s) samples of ion density and thermal pressure obtained in the central plasma sheet and the plasma sheet boundary layer were cross‐correlated in order to obtain typical values of the polytropic index γ for different tail regions and disturbance conditions. The plasma sheet ion population behaves, on average, adiabaticly both in the central plasma sheet and the plasma sheet boundary layer. However, a polytropic index of about 1.4 for the quiet plasma sheet indicates that the latter behaves like a poorly insulated vessel. Hence, there seems to be no quiet time magneto‐tail equilibrium (“ground state”), but rather continuous cooling until new energy is entering from the outside.