Hydromagnetic wave excitation by ionized interstellar hydrogen and helium in the solar wind

Abstract

Interstellar atoms penetrate the heliosphere, are ionized by solar UV radiation or charge exchange with solar wind ions, and are "picked up" by the solar wind onto a ring distribution in velocity space. The ring distribution is unstable to the generation of hydromagnetic waves with growth time scales which are small compared with those of the continual pickup process and convection with the solar wind. First, the growth rates for parallel propagation are derived, presented, and compared with previous work. Then the spatially homogeneous quasi-linear equations describing the subsequent hydromagnetic wave excitation and pickup ion velocity diffusion in pitch angle and energy are presented under the assamption of wave propagation parallel to the ambient magnetic field. Neglecting ion energy changes, analytical expressions for the time-asymptotic wave spectra accompanying the time-asymptotic isotropic ion distribution are derived. The results indicate that pickup helium has a very small (unobservable) effect on the solar wind wave spectrum, but that pickup hydrogen results in substan_.tial modifications at cyclotron resonant frequencies (•, 10 -2 Hz at •,7 AU). Finally, based on the previous expressions, the radial evolution of the pickup-hydrogen-modified wave spectra for both polarizations and propagation directions is computed analytically including the degradation of the wave power in the divergent solar wind. The predicted modifications beyond •, 5 AU are substantial and could be observable at spacecraft frequencies greater than •-5 x 10 -3 Hz if not degraded by turbulent wave-wave interactions or stochastic ion acceleration. The predicted enhancement by a factor of order 10 at •, 7 AU involves •, 10% of the power in the ambient field, is left-polarized (Alfv•nic as opposed to magnetosonic) in the solar wind frame, is unpolarized in the spacecraft frame, and includes an equal admixture of propagation directions. -3 mines the local interstellar densities' n n = 0.065 ___ 0.01 cm and n.e = 0.01 _+ 0.0045 cm -3 [Chassefiere et al., 1986]. As soon as an interstellar neutral is ionized it is accelerated by the solar wind electric field, E = -c-•Vsw x B, where Vsw is the solar wind velocity and B is the ambient solar wind magnetic field, and "picked-up" onto a ring distribution about B in velocity v space. In the frame of the solar wind the ring is characterized by a speed Ivl-Vsw and a pitch angle equal to the angle between B and Vsw, such that the ions propagate sunward. The ring is initially broad due to variations in Vsw and, especially, B during the pickup process and is broadened further by ion scattering in the solar wind electric and magnetic field fluctuations. Since the phase speed of the irregularities which most effectively scatterer the ions is approximately the Alfv6n speed V A and VA << Vsw, the ions are initially scattered in pitch angle to form a thin spherical shell in velocity space. Adiabatic deceleration of the ions in the divergent solar wind combined with continual pickup of new ions yields a th...

Turbulent Heating of the Distant Solar Wind by Interstellar Pickup Protons

“…Immediately after ionization, the new pickup ion begins to gyrate about the solar wind magnetic field, so that a distribution of new pickup ions may be represented by a ring beam in velocity space, f 0 $ ðv À V sw Þ ðl À l 0 Þ, where v and l are the ion speed and cosine of the pitch angle, respectively, measured in the plasma frame, the initial value of l in the magnetic field B is l 0 ¼ B r =B, and ðxÞ is the Dirac delta function. Such a distribution is unstable to the resonant generation of MHD waves (Wu & Davidson 1972;Lee & Ip 1987;Bogdan, Lee, & Schneider 1991) and will interact with these waves, as well as with any ambient resonant waves, to scatter to an approximately isotropic shell in velocity space. This scattering occurs on a timescale l , which is typically less than the convective timescale c ¼ r=V sw , where r is the heliocentric radial position.…”

Contributions to the cross shock electric field at supercritical perpendicular shocks: impact of the pickup ions

“…The production process and feature of the pickup ions in the solar wind have been studied quite well (Sagdeev et al 1986;Lee and Ip 1987). Neutral atoms and molecules, penetrating from the local interstellar medium or escaping from comets, are ionized by photoionization, electron impact or charge exchange with the solar wind.…”

Numerical Solution of the Time‐dependent Kinetic Equation for Anisotropic Pitch‐Angle Scattering

“…Since a neutral particle has a very slow inÑow speed with respect to the Sun (D 20 km s~1 depending on the species), the newborn ion has a supersonic initial speed comparable to the solar wind speed U in the reference frame of the solar wind. Thus, since the energy di †usion timescale (Lee & Ip 1987 ;Zank 1999), where is the pitch-angle q v D U2/V A 2 q k q k timescale, pitch-angle di †usion dominates on the timescales on which we solve our transport equations. It is therefore a reasonable approximation to neglect the e †ects of energy di †usion in the transport equation.…”

“…The power in the waves responsible for scattering pickup ions may be highly variable and depends on heliocentric distance and latitude, thus leading to a variable scattering rate (Lee & Ip 1987 ;Zank, Matthaeus, & Smith 1996). In addition, as discussed by Zank & Cairns (2000), the wave growth rate can be stochastic when driven by pickup ions, making the scattering rate yet more variable.…”