In this chapter we examine, as before, electromagnetic fluctuations in a homogeneous, magnetized, collisionless plasma. In contrast to the previous chapter, however, we admit anisotropies in the distribution functions. In particular we consider a two-temperature bi-Maxwellian zeroth-order distribution function; this permits the growth of temperature anisotropy instabilities. Section 7.1 outlines the derivation of the dispersion equation; Section 7.2 discusses the properties of modes driven unstable by a proton temperature anisotropy, whereas Section 7.3 discusses the properties of electron temperature anisotropy instabilities. Section 7.4 is a brief summary.

Our emphasis in this chapter is on instabilities driven by T⊥j > T‖ a condition that is observed more often in space plasmas than the converse T‖ > T⊥. The reason for this discrepancy is simple: although space plasmas do not necessarily exhibit a bias toward perpendicular heating processes, perpendicular heating does not much change the mobility of the heated particles, whereas parallel heating enables the particles to move more rapidly along B0. Thus parallel-heated particles may leave the region of energization more quickly, implying that T‖ > T⊥ should be a less frequently observed condition. Of course, parallel-heated particles may appear elsewhere as a magnetic-field aligned beam streaming against a cooler background plasma; the electromagnetic instabilities driven by such configurations have quite different properties from temperature anisotropy instabilities, and are studied in detail in the next chapter.