Manish Mithaiwala

Linear and nonlinear Landau resonance of kinetic Alfvén waves: Consequences for electron distribution and wave spectrum in the solar wind

Kinetic Alfven wave turbulence in solar wind is considered and it is shown that non-Maxwellian electron distribution function has a significant effect on the dynamics of solar wind plasmas. Linear Landau damping leads to the formation of a plateau in the parallel electron distribution function which diminishes the Landau damping rate significantly. Nonlinear scattering of waves by plasma particles is generalized to short wavelengths and it is found that for the solar wind parameters this scattering is the dominant process as compared to three-wave decay and coalescence in the wave vector range 1/ρi<k<ωpe/c. Incorporation of these effects leads to the steepening of the wave spectrum between the inertial and the dissipation ranges with a spectral index between 2 and 3. This region can be labeled as the scattering range. Such steepening has been observed in the solar wind plasmas.

Weak turbulence in the magnetosphere: Formation of whistler wave cavity by nonlinear scattering

We consider the weak turbulence of whistler waves in the in low-β inner magnetosphere of the earth. Whistler waves, originating in the ionosphere, propagate radially outward and can trigger nonlinear induced scattering by thermal electrons provided the wave energy density is large enough. Nonlinear scattering can substantially change the direction of the wave vector of whistler waves and hence the direction of energy flux with only a small change in the frequency. A portion of whistler waves return to the ionosphere with a smaller perpendicular wave vector resulting in diminished linear damping and enhanced ability to pitch-angle scatter trapped electrons. In addition, a portion of the scattered wave packets can be reflected near the ionosphere back into the magnetosphere. Through multiple nonlinear scatterings and ionospheric reflections a long-lived wave cavity containing turbulent whistler waves can be formed with the appropriate properties to efficiently pitch-angle scatter trapped electrons. The prim...

Quasilinear evolution of plasma distribution functions and consequences on wave spectrum and perpendicular ion heating in the turbulent solar wind

The measured spectrum of kinetic Alfven wave fluctuations in the turbulent solar wind plasma is used to calculate the quasi-linear evolution of the initially stable electron and ion distribution functions. The resulting ion distribution function is found to be unstable to electromagnetic left hand polarized ion cyclotron-Alfven waves as well as right hand polarized magnetosonic-whistler waves. These waves can pitch angle scatter the ion super-thermal velocity component to provide perpendicular ion heating. Additionally, right hand polarized waves transfer some part of kinetic Alfven wave flux to whistler waves.The measured spectrum of kinetic Alfven wave fluctuations in the turbulent solar wind plasma is used to calculate the electron and ion distribution functions resulting from quasi-linear diffusion. The modified ion distribution function is found to be unstable to long wavelength electromagnetic ion cyclotron waves. These waves pitch angle scatter the parallel ion velocity into perpendicular velocity which effectively increases the perpendicular ion temperature.

Weak turbulence theory of the nonlinear evolution of the ion ring distributiona)

The nonlinear evolution of an ion ring instability in a low β magnetospheric plasma is considered. The evolution of the two-dimensional ring distribution is essentially quasilinear. Ignoring nonlinear processes the timescale for the quasilinear evolution is the same as for the linear instability 1/1τQL~γL τQL~γL. However, when nonlinear processes become important, a new timescale becomes relevant to the wave saturation mechanism. Induced nonlinear scattering of the lower-hybrid waves by plasma electrons is the dominant nonlinearity relevant for plasmas in the inner magnetosphere and typically occurs on the timescale 1/1τNL~ω(M/m)W/WnT nT τNL~ω(M/m)W/WnT nT, where W is the wave energy density, nT is the thermal energy density of the background plasma, and M/Mm m is the ion to electron mass ratio, which has the consequence that the wave amplitude saturates at a low level, and the timescale for quasilinear relaxation is extended by orders of magnitude.

Stability of an ion-ring distribution in a multi-ion component plasma

The stability of a cold ion-ring velocity distribution in a thermal plasma is analyzed. In particular, the effect of plasma temperature and density on the instability is considered. A high ring density (compared to the background plasma) neutralizes the stabilizing effect of the warm background plasma and the ring is unstable to the generation of waves below the lower-hybrid frequency even for a very high temperature plasma. For ring densities lower than the background plasma density, there is a slow instability where the growth rate is less than the background-ion cyclotron frequency and, consequently, the background-ion response is magnetized. This is in addition to the widely discussed fast instability where the wave growth rate exceeds the background-ion cyclotron frequency and hence the background ions are effectively unmagnetized. Thus, even a low density ring is unstable to waves around the lower-hybrid frequency range for any ring speed. This implies that effectively there is no velocity threshold...

Multi‐pass whistler gain in a magnetospheric cavity due to induced nonlinear scattering

[1]xa0Multi-pass gain in whistler energy is possible in the radiation belts if the wave amplitude is sufficiently large. Large amplitude magnetospherically reflected whistlers can induce nonlinear scattering and form a magnetospheric cavity filled with nearly isotropic spectrum of wave vectors perpendicular to the magnetic field with low obliqueness. This increases the wave-particle resonance time and maintains a large pitch angle scattering rate. Enhanced pitch angle scattering and precipitation of trapped electrons into the radiation belt loss cone provides a proportionate gain in the whistler energy.

Convective amplification of electromagnetic ion cyclotron waves from ring‐distribution protons in the inner magnetosphere

[1]xa0The growth of electromagnetic ion cyclotron (EMIC) waves due to a ring distribution of hydrogen ions is examined. Though these distributions are more commonly implicated in the generation of equatorial noise, their potential for exciting EMIC waves is considered here. It is shown that since the ring distribution is nonmonotonic in perpendicular velocity, the amplification achieved by this instability is greater than bi-Maxwellian distributions for typical observed anisotropies, because the waves can maintain resonance over a much longer part of its trajectory. For ring speeds (Vr) close to the Alfven speed (VA), the growth rate is largest at parallel propagation but decreases less rapidly toward oblique angles compared with a bi-Maxwellian and can have a second peak for shorter wavelengths. The ring distribution parallel thermal speed is deduced from the observation of equatorial noise because a statistical survey of this parameter is not available. A ring density to background plasma density of a few percent is sufficient to achieve moderate gain. The gain is also shown to increase with L shell since the perpendicular wave number does not rise as rapidly along the raypath as compared with smaller L. The analysis suggests that EMIC wave activity should be closely associated with equatorial noise.

Generation of a ULF wave resonator in the magnetosphere by neutral gas release

[1]xa0ULF waves generated by the release of a neutral gas in the equatorial plane of the Earths magnetosphere will be trapped between two turning points forming a resonator. This effectively prolongs the lifetime of the ensuing turbulence making it more useful for pitch angle scattering trapped energetic electrons. In the multispecies plasma environment of the magnetosphere, the presence of a small amount of helium causes the waves to reflect when their frequency matches the Buchsbaum frequency as they travel along field lines away from the equator. For a proton-electron-helium plasma, the Buchsbaum frequency is near the helium cyclotron frequency. With typical abundances of helium in the inner radiation belts, there is virtually no loss of wave energy by cyclotron damping at the turning points as the waves reflect between conjugate Buchsbaum points. Wave energy is eventually dissipated by collisional effects and radial convection.

Laboratory studies of nonlinear whistler wave processes in the Van Allen radiation belts

Important nonlinear wave-wave and wave-particle interactions that occur in the Earths Van Allen radiation belts are investigated in a laboratory experiment. Predominantly electrostatic waves in the whistler branch are launched that propagate near the resonance cone with measured wave normal angle greater than 85°. When the pump amplitude exceeds a threshold ∼5×10−6 times the background magnetic field, wave power at frequencies below the pump frequency is observed at wave normal angles (∼55°). The scattered wave has a perpendicular wavelength that is nearly an order of magnitude larger than that of the pump wave. Occasionally, the parametric decay of a lower hybrid wave into a magnetosonic wave and a whistler wave is simultaneously observed with a threshold of δB/B0∼7×10−7.

Collisionless and collisional dissipation of magnetospherically reflecting whistler waves

[1]xa0In this letter we consider the dissipation of magnetospherically reflecting whistler waves from both ionospheric sources and from sources outside the plasmasphere such as chorus. We use a simple spatially dependent model of the suprathermal electron population, a standard cold plasma density model based on a diffusive equilibrium, and a range of plasmaspheric temperatures to demonstrate that the often-ignored (electron-ion) collisional damping is usually at least the same order of magnitude and often an order of magnitude larger than the dissipation due to collisionless damping (Landau and transit-time). Furthermore the collisional damping is sensitive to the cold plasmaspheric temperature, which depends on location (night/day-side) and solar conditions. These results indicate that accurate spatially dependent models of plasmaspheric temperatures as well as suprathermal electron fluxes are necessary for modeling the dissipation of magnetospherically reflecting whistler waves.