Suprathermal Tails Research Articles

Parametric instabilities generated by an energetic electron beam

The large amplitude electron plasma oscillations produced by a low density energetic electron beam can parametrically decay into growing non-resonant electron and ion waves. A similar parametric instability occurs in a plasma driven by a sufficiently strong applied electric field whose frequency is near the electron plasma frequency. A two-specie one-dimensional particle simulation model was used to study the beam-generated parametric instability and the results are compared with corresponding one-specie beam and external field simulations. For low beam densities the individual non-resonant modes grow approximately at the rate predicted by parametric instability theory. The growth of these modes eventually causes the resonant mode energy to decay from its saturation value at a rate proportional to the original parametric growth rate. The electron distribution function fe(v) develops suprathermal tails which cause enhanced non-thermal electric field energy fluctuations whose spectrum W(k<or=0.1ke) approximately k-2.

Nonlinear production of suprathermal tails in auroral electrons

Differential energy spectra of electrons observed in an auroral breakup show evidence that an incident electron beam of energy E = 6–13 keV, width of 2–5 keV, and variable intensity was stabilized over a period of at least several minutes by the oscillating two-stream instability in the manner previously described by Papadopoulos and Coffey. The critical observed feature predicted by these authors' calculation is a tail with differential flux dj/dE ∝ E−0.5. This tail is much too intense to be attributed to the effects of collisions with atmospheric constituents. It is possible that the tail is part of the beam itself, i.e., produced higher up, but there are at least two arguments against this: first, the observations indicate that the tail approaches isotropy, whereas the beam is mostly downcoming, and second, the tail is observed as low as 130 km; the portion of the tail below 1.5 keV would have been absorbed by the atmosphere at greater heights if it had been incident from above. The observations support the hypothesis that the tail electrons are produced locally and by the proposed stabilization process. Implications are briefly discussed.

Heated electron distributions from resonant absorption

A simplified model of resonant absorption of obliquely incident laser light has been developed. Using a 1.5 dimensional electrostatic simulation computer code, it is shown that the inclusion of ion motion is critically important in determining the heated electron distributions from resonant absorption. The electromagnetic wave drives up an electron plasma wave. For long density scale lengths (L≃103λDe), the phase velocity of this wave is very large (ω/k≳10Vth) so that if heating does occur, a suprathermal tail of very energetic electrons is produced. However, the pressure due to this wave steepens the density profile until the density gradient scale length near the critical density (where the local plasma frequency equals the laser frequency) is of order 20λDe. The electrostatic wave is thus forced to have a much lower phase velocity (ω/k≃2.5Vth). In this case, more electrons are heated to much lower velocities. The heated electron distributions are exponential in velocity space. Using a simple theory it is shown that this property of profile steepening applies to most of a typical laser fusion pulse. This steepening raises the threshold for parametric instabilities near the critical surface. Thus, the extensive suprathermal electron distributions typically produced by these parametric instabilities can be drastically reduced.

Theory and simulation of resonant absorption in a hot plasma

A detailed theoretical and simulation study of resonant absorption in a hot plasma is presented which isolates the behavior of the plasma for times short compared to an ion response time. The extent to which an electron fluid model can describe the absorption process in the kinetic regime is discussed. At high intensities the absorbed energy is observed to be deposited in a suprathermal tail of electrons whose energy varies approximately as the square root of the incident power. The density profile modification due to the ion response to the pondermotive force is also discussed.

Electron heating due to parametric instability turbulence

The turbulent electron heating due to laser-plasma instabilities at the critical density (laser frequency equals plasma frequency) is considered. In the regime where the laser energy is much less than the plasma thermal energy, simulations show that the wave energy spectrum takes on approximate k−2 shape concomitant with the formation of a suprathermal electron tail. Test particle calculations demonstrate that these tails are produced by velocity space diffusion due to the plasma waves. Quasilinear theory predicts a linear heating rate and an exponential shape for the electron tail, in agreement with the simulation result. The fluid equations, including mode coupling terms, are solved, and it is found that the instability saturation level and k−2 spectrum are due to mode coupling. Using the resulting fields, the electron distribution function is evolved, giving reasonable heating rates and suprathermal tail formation. Calculations in the underdense region (laser frequency greater than plasma frequency) show that the heated distribution function has fewer high energy electrons. Finally, an approximate analysis of heating in a finite interaction region is given.

Polar ionospheric disturbances and solar corpuscular emissions

It has been found that the study of polar radio blackouts due to abnormal ionization in the lower ionosphere yields considerable evidence indicating the existence of energetic solar particles associated with solar flares. Polar radio blackouts are classified into two characteristic types, one is the polar cap blackout and the other is the auroral zone blackout. It is shown that the polar cap blackout appears with some hours delay after a major solar radio outburst of type IV, and the blackout is confined within the geomagnetic latitude of 60°–65°. The estimated energies of particles causing this are of about 10–100 MeV. The auroral zone blackout then follows, being accompanied with geomagnetic storms and aurorae, and it may be caused by the so-called auroral particles of 1 MeV or less. The energy spectrum of solar particles associated with solar flares is revealed from the present result together with all information from various observations related to solar and terrestrial disturbances. It is concluded that solar particles have a conspicuous suprathermal non-Maxwellian tail extending from a few keV up to relativistic energy range, though the bulk of corpuscular clouds consists of rather low energy particles and hence likely to be in the Maxwellian distribution. Some discussions on the nature of solar corpuscular clouds and their effect upon the terrestrial ionosphere are also given.

Solar corpuscular radiation and polar ionospheric disturbances

Study of disturbances in the polar inosphere yields considerable evidence of the existence of energetic solar particles associated with solar flares. Radio blackouts due to enhanced ionizations are classified into two characteristic types: the polar-cap blackout, and the auroral-zone blackout. It is shown that the polar-cap blackout appears with some hours' delay after a major solar radio out-burst of type IV, and is confined within the geomagnetic latitude of about 60° to 65°. The estimated energies of particles causing it are of about 10 to 100 Mev. The auroral-zone blackout then follows, accompanied by geomagnetic storms and auroras; it may be caused by the so-called auroral particles of 1 Mev or less. The energy spectrum of solar particles associated with solar flares is discussed in view of the present results and of information from various observations of solar and terrestrial disturbances. It is concluded that solar particles have a conspicuous suprathermal non-Maxwellian tail extending from a few kev up to relativistic energy ranges, though the main part of corpuscular clouds consists of particles of rather low energy, which therefore are likely to be in the Maxwellian distribution. The nature of solar corpuscular clouds and their effect upon the terrestrial ionosphere are also discussed.