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Interchange motion as a transport mechanism for formation of cold‐dense plasma sheet

To evaluate whether interchange motion can provide the transport for the formation of the cold‐dense plasma sheet in the near‐Earth region, we present an event of cold‐dense plasma sheet observed by five THEMIS probes after the interplanetary magnetic field turned northward, as well as their comparisons with the simulation results from the Rice Convection Model (RCM) combined with a modified Dungey force‐balanced magnetic field solver. The observations of cold‐dense plasma at different locations show quite different characteristics: (1) closer to the flank, the appearance is more periodic and exhibits larger fluctuations in plasma moments and magnetic field; (2) further away from the flank, the cold plasma appears later; (3) in the mixture with the cold plasma, the decrease in high‐energy particle fluxes becomes less significant further away from the flank; (4) there is energy‐dispersion in the cold particles at some locations; and (5) near the magnetopause, the fluctuations have the characteristics of the Kelvin‐Helmholtz (K‐H) vortices and the colder‐denser plasma is likely to have lower entropy. In the RCM simulations, lower entropy plasma consisting of colder‐denser ions and electrons was periodically released locally at the outer boundary to represent the plasma created within a K‐H vortex. This lower entropy perturbation is interchange unstable and the resulting interchange motion through the magnetosphere‐ionosphere coupling pushes the colder‐denser plasma radially inward. The simulated particle energy spectrums at different locations qualitatively reproduce the observations, strongly suggesting that the seemingly different characteristics of cold‐dense plasma observed by different probes are all a result of the same interchange‐related transport mechanism.