As illustrated in Figure 13.1 the magnetosheath is the plasma region between the bow shock and the magnetopause in which the shocked solar wind and particles escaping from the magnetosphere are found. Near the nose of the bow shock the magnetosheath plasma is primarily a hot, dense, slow plasma with . However, this flow accelerates up towards the solar wind speed and becomes increasingly wind-like toward the flanks of the bow shock, as the shock becomes increasingly weak. MHD and gasdynamic simulations show the large scale structure of the magnetosheath with very good accuracy [e.g., Spreiter et al., 1966]. For instance, Figure 14.1 shows the spatially-varying characteristics of the magnetosheath plasma just described above. Note that the magnetosheath remains a collisionless plasma with collision frequencies much smaller than other relevant frequencies.
Figure 14.1: Contours of the plasma flow speed and temperature in the magnetosheath [Spreiter et al., 1966; Cravens, 1997].
The particles entering the magnetosheath through the shock have abundant sources of free energy for wave growth due to the ions and electrons both having temperature anisotropies (i.e., ), due to gyrating ions lying on a ring in velocity space, and due to the cross-shock potential creating a low energy hole in the electron distribution. Additionally, however, the properties of the bow shock vary with position, leading to changes in the temperature, flow speed, gyrospeed etc. of the particles injected into the magnetosheath. As discussed qualitatively in Lectures 1 and 10, growth of waves removes energy from the particles, decreases unstable gradients in the particle distribution, and ideally diffuses particles in velocity space and thermalizes the particle distributions by wave-particle scattering.