Experimental characterization of broadband electrostatic noise due to plasma compression

Abstract

For a wide variety of laboratory and space plasma environments, theoretical predictions state that plasmas are unstable to inhomogeneous flows over a very broad frequency range. Such sheared flows are generated in the Earth's magnetosphere and intensify during active periods. Specifically, for a velocity shear oriented perpendicular to a uniform background magnetic field, the shear scale length (L E ) compared to the ion gyroradius ( ρ i ) determines the character of the shear-driven instability that may prevail. An interpenetrating plasma configuration is used to create a transverse velocity shear profile in a magnetized plasma column, a condition similar to that found in the natural boundary layers. The continuous variation of ρ i /L E and the associated transition of the instability regimes driven by the shear flow mechanism are demonstrated in a single laboratory experiment. Broadband wave emission correlated to increasing/decreasing stress (i.e., ρ i /L E ), a characteristic signature of a boundary layer crossing, is found under controlled and repeatable conditions. This result holds out the promise for understanding the cause and effect of the in situ observation of broadband electrostatic noise.

A Census of Plasma Waves and Structures Associated With an Injection Front in the Inner Magnetosphere

“…Another possible path for wave growth is that sharp electric field gradients perpendicular to a guiding magnetic field, imposed by gradients in plasma velocity and/or density, can drive a broad spectrum of wave instabilities, including some that generate broadband electrostatic structures. This path is described in laboratory studies (Amatucci et al, 2003;DuBois et al, 2014;Tejero et al, 2011) as well as analytic and numerical results (Ganguli et al, 1994(Ganguli et al, , 2002Romero & Ganguli, 1993). While the cadence of the Van Allen Probes particle data is not sufficient to rigorously test this idea with this event, magnetotail dipolarization fronts measured by the MMS mission (Burch et al, 2015) may be rapid enough to conduct quantitative tests.…”

Inhomogeneities of plasma density and electric field as sources of electrostatic turbulence in the auroral region

“…The influence of plasma density gradients on ion-electron-hybrid instability in laboratory experiments has been considered recently in the works. 22,23 The study of this effect is particularly important in various areas of space physics, for instance, in the study of auroral zone phenomena. Accounting of plasma density inhomogeneities may lead to changes in the excitation threshold of the instability and to a change in the growth rate.…”

A New Perspective for Dipolarization Front Dynamics: Electromagnetic Effects of Velocity Inhomogeneity

“…Specifically, the electron-ion hybrid (EIH) instability is driven by the transverse velocity shear with intermediate scale length ( e < L E < i ), when ions are effectively unmagnetized and electrons have shear corrected velocity distributions (e.g., Ganguli et al, 1988aGanguli et al, , 1988bRomero et al, 1992). The kinetic EIH modes have been verified in a number of space and laboratory experiments and numerical simulations (e.g., Amatucci et al, 1996;DuBois et al, 2014;Liu et al, 2014Liu et al, , 2017Romero & Ganguli, 1993;Scales, Bernhardt, Ganguli, Siefring, & Rodriguez, 1994) and are suggested to be important mechanisms for the generation of broadband electrostatic fluctuations. However, previous EIH studies have been mostly focused on the electrostatic emissions and assumed uniform magnetic field for simplicity (e.g., Romero & Ganguli, 1993;Romero et al, 1992).…”