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--- un:plasma [2024/10/06 12:51] – asad
+++ un:plasma [2024/10/06 13:29] (current) – [4. Plasma Theory] asad
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 where $\tau_n$ is the average time between two collisions between an electron and a neutral. The bottom line is this: the plasma frequency must be much higher than the collision frequency.
+===== - Plasma Theory =====
+Plasma dynamics refers to the interaction of electric and magnetic fields with different types of charge carriers. Space plasmas have many external influences as well as many influences inside the plasma, making its dynamics very complex. To create its perfect mathematical model, the equations of motion for each particle must be known, and since each particle is affected by all the other particles, the equations for all particles must be  simultaneously. This is practically impossible and unnecessary, since we only want to know the average properties of the plasma. So four types of approximations or simplifying approaches are used to construct the plasma theory.
+//Single Particle Motion// is the simplest approach. Here only the effect of external electric and magnetic fields on an individual particle is considered. Here the overall behavior of the plasma is neglected. But this approach is sufficient to explain the type of low-density plasma found in ring currents.
+//Magnetohydrodynamics// is at the other extreme, where the entire plasma is considered as a whole, excluding individual particles. Plasma is here a single conductive fluid with some average properties, eg, density, temperature, velocity. Here the plasma is assumed to be in local equilibrium. This approach is good for studying low frequency waves in many conducting magnetic fluids.
+The //multi-fluid// approach is similar to magnetohydrodynamics, only here different species of particles are considered separately, eg, electrons, protons, heavier ions. A separate fluid is assigned to each species.
+//Kinetic theory// is the most advanced approach, as statistics are well used here. A variety of statistical distribution functions in phase space are used instead of equations of motion for individual particles.