## Wednesday, May 27, 2020

### Looking At Some Plasma Instabilities Applicable to the Sun There are many variants and types of plasma instabilities on the Sun which play both major and minor  roles in events ranging from X-class solar flares to coronal mass ejections and even filament ejection-displacement.  In this post I look at a few of these.

Conveniently, many of these instabilities are defined in terms of specific velocities. One of the more important of these is known as the electron drift velocity  v  d  :

v  d  = I/ n e

where I is the current, n the number density (per cubic meter, for example) and e the electron charge. Often   v   is defined instead based on current density (J) where: J = I/A (current in amps flowing through a magnetic flux tube or channel of some cross-section area A)

So: v  d  = J / n e

In many other cases, where J, I are not so easily obtained, v  d    will be the electron drift velocity in a combined magnetic and gravitational field, i.e.:

v  d     = m e  g s  c / e B

where B is the magnetic induction, e the electronic charge (1.6 x 10-19 C) ,  s = 273  ms -2  the gravitational acceleration on the Sun, c is the velocity of light (2.98 x 10 8   m/s) and m e the electron mass (9.1 x  10 -31 kg) .

In the case of the ion-acoustic instability, the threshold for its onset is when:

v  d    > 43  v  s

that is, the drift velocity is at least 43 times the ion sound speed, where:

v  s   = Ö  (kT/ m i)

where k is the Boltzmann constant (1.38 x 10 -23  J/K), T the (Kelvin) temperature and m i  the ion mass (e.g. m  ~ 1.7 x 10 -27 kg).

Sen and White in a 1972 paper dealing with the role of the Hall effect in flares, showed that the two-stream instability is incepted when the drift velocity:

v  d    >   v  ith

where  v  ith   is the ion-thermal velocity:

v  ith   =   Ö  (2kT/ m i)

(rms value = Ö  3(kT/ m i)

In two-stream instability, when an electron flow is suddenly injected into a plasma – say for a coronal loop – the particles’ (Maxwellian) velocity distribution acquires a “bump” on its "tail" (higher velocity end of the distribution), consistent with two streams- an unperturbed one ( f  ov) and perturbed one ( f  eb ) applicable to the electron beam (See diagram ). In the region where the slope is positive (df   /d v > 0) there is a greater number of faster i.e.  than slower particles so a greater amount of energy is transferred from particles to associated (e.g. Alfven) waves. Since  f  eb contains more fast than slow particles a wave is excited, and there is inverse Landau damping such that plasma oscillations with vph (phase velocity) in the positive gradient region are unstable.

Resonant electrons (at v  ph   >   w e  / k) where w is the electron plasma frequency, i.e.

w e     =  [ne e2/ me  εo] ½

are the first to be affected by the local wave-particle interactions and have distributions altered by the wave electric field, E1, such that the total energy balance:

E1 (TOT) = ½ E1 w + ½  E1 k

referencing the wave and kinetic (particle) contributions respectively.

Thus, for E1(TOT) = const. then as the electron velocity decreases, the particle kinetic energy decreases and the wave energy density increases.

In Landau damping the exact opposite occurs, so the gradient df(v)/d v decreases, and with it the wave amplitude, while the particle kinetic energy increases- i.e. wave energy lost is fed to the particles (electrons) which gain energy.

Note also that the "plasma beam instability" is just the finite temperature analog for the two stream instability. In other words, if one has a "two stream instability" one de facto also has the beam instability.

Lastly, for the Buneman instability, the relevant condition is:

v  d   >  v  eth

where  v  eth    is the electron thermal velocity:

v  eth    =   Ö  (2k T e/ m e)

where T e   is the electron temperature and k the Boltzmann constant.

The rms (root mean square) value for   v  eth    is   Ö 3 (k T e/ m e)

The objective for much better solar flare predictions is to be able to interpret which of these instabilities is near onset for a given solar plasma condition. This is not easy by any means, and probably requires much better optical resolution than available at present. However, solar physicists will "truck on" using all the resources they have to effect the best forecasts of which they are capable.