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"Plasmas are many-body systems, with enough mobile charged particles to cause some collective behavior ." [M.S. Murillo and J.C.Weisheit Physics Reports 302, 1-65 (1998)].

In the above definition what is meant by "collective behavior" ?

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Plasmas are controlled by long-range interactions (i.e., Coulomb potentials and magnetic fields) and so the particles respond accordingly. Since electric fields do work to get rid of themselves, a plasma will reach what is called a quasi-neutral state, i.e., equal number densities of oppositely charged particles or: $$ \sum_{s} \ Z_{s} \ n_{s} = 0 $$ where $Z_{s}$ is the charge state of species $s$ and $n_{s}$ is the number density of species $s$.

This can manifest in a fluid-like behavior and the amount which the fluid-like behavior dominates depends upon the plasma parameters like number density, magnetic field, or plasma $\beta$ given by: $$ \beta = \tfrac{ 2 \ \mu_{o} \ n_{o} \ k_{B} \ T }{ B_{o}^{2} } $$ where $n_{o}$ is the total number density, $T$ is the total temperature (see definition at https://physics.stackexchange.com/a/375611/59023), and $B_{o}$ is the magnetic field magnitude.

Highly collisional plasmas, e.g., solar photosphere, also behave like fluids as they are collisionally mediated. That is, the relevant frequencies like the cyclotron, $\Omega_{cs}$, and plasma, $\omega_{ps}$, frequencies are at or below particle-particle Coulomb collision frequencies (see definitions at https://physics.stackexchange.com/a/268594/59023).

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Collective behavior in Plasmas is the phenomenon where the way the plasma as a whole reacts/behaves is dependent on the behavior of each and every particle in the plasma. The overall behavior is the sum of the individual particles' behavior.

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