Plasma perturbations created by transverse ion heating events in the magnetosphere

Abstract

Transversely heated ions provide an important mechanism for the transport of ionospheric ions to the distant parts of the magnetosphere. The heating occurs via wave‐particle interactions. The transport of such ions has been generally treated by a test particle approach, according to which ions move upward adiabatically under the action of the mirror force. The perturbations in the plasma created by the heating are studied here for the first time by solving a self‐consistent set of plasma transport equations, in which heating is included phenomenologically. The nature of the perturbation is found to depend on the field‐aligned extent of the heating region and on the heating time. When the heating occurs in a relatively localized region, the perturbation consists of a density cavity topped by a density enhancement (bump). The plasma in the bump is that expelled from the cavity. For weak heating the bump propagates upward without any steepening near the density fronts. For relatively strong heating the bump steepens into a forward‐reverse shock pair. The forward shock (F) forms near the leading edge while the reverse shock (R) is at the top of the density cavity, near the trailing edge of the bump. When the R‐F shock pair has formed, the velocity profile shows double “sawtooth” structure. Such shock structures are similar to the features of interplanetary shocks associated with solar wind transients. Parallel temperatures of the ions are reduced inside the cavity and increased in the density bump adiabatically. The transversely heated ions occur inside the upper part of the cavity and in the density enhancement. The dynamics of the transversely heated ions is not determined by the mirror force alone. The electric fields developing in the perturbation significantly affect their upward motion; the electric fields near the lower edge (R) of the density bump are downward, and they retard the upward motion. The electric and mirror forces on the transversely heated ions are compared. The electric fields near the leading edge (F) of the density bump are upward, and when they are sufficiently strong they compress the ions in the ambient polar wind. This creates an ion population heated only in T∥ just near the leading edge of the perturbation. During the course of its evolution the electric field distribution shows pulselike features. For impulsive heating the distribution evolves into a predominantly unipolar upward pointing electric field near the leading edge. Such an electric field pulse moves upward and its field strength depends on the electron temperature and the density gradient across F. The perturbation produced by transverse heating extending over long distances is characterized by an upward expanding density cavity with its lower end anchored at the lower edge of the heating region. Inside the cavity the spatial distributions of density, flow velocity, and parallel and perpendicular temperatures reach a quasi‐steady state while above it they continue to evolve. The dynamics of the ions in the cavity, including their pitch angle distribution, is described by a test particle approach under the influence of the mirror force. When the heating ceases, the cavity fills from the bottom through the expanding polar wind, and the transversely heated ions inside the cavity ride on top of this polar wind expansion.