Iver H. Cairns

Interstellar pickup ions and quasi-perpendicular shocks: Implications for the termination shock and interplanetary shocks

A new mechanism for the acceleration of pickup ions by repeated reflections from the electrostatic cross shock potential of a quasi-perpendicular shock is presented. The acceleration mechanism, multiply reflected ion (MRI) acceleration, offers a resolution to the issue of injecting pickup ions into an efficient particle energization scheme, and the injection efficiency for pickup ions is found to be inversely proportional to ion mass and proportional to charge. By studying the particle energy gain in the motional electric field (where a steady shock frame is assumed) the energized pickup ion spectrum can be computed. Extremely hard power law spectra (E −1.5 , for example) emerge from the upstream pickup ion distribution. The maximum energy that a reflected pickup ion can gain is found to be proportional to the square of the product of the Alfven speed and (r−1), where r is the shock compression ratio. For solar wind conditions at either interplanetary shocks or the termination shock the upper energy limit is typically in excess of 0.5 MeV. It is suggested here that MRI acceleration provides an efficient mechanism for injecting low-energy pickup ions into a subsequent acceleration process such as diffusive Fermi acceleration. Such a two-step acceleration scheme alleviates many of the difficulties which plague ion energization models at perpendicular shocks. The structure of a quasi-perpendicular shock modified by shock reflection of pickup ions is discussed in general terms. By way of application we present a detailed study of the MRI acceleration mechanism at the termination shock for a wide range of parameters and discuss the implications for the anomalous cosmic ray component. The acceleration of pickup ions by an interplanetary traveling shock is also discussed, and the observations made by Ulysses [Gloeckler et al., 1994] are addressed. The puzzling aspects of the Gloeckler et al. [1994] observations appear to be explained quite naturally by shock energization based on repeated pickup ion reflections. Observational tests of MRI acceleration may be possible by using pickup He + at either the terrestrial or Jovian bow shock or by using cometary ions at a cometary bow shock.

Fundamental and Harmonic Emission in Type III Solar Radio Bursts – I. Emission at a Single Location or Frequency

A model of type III solar radio bursts is developed that incorporates large-angle scattering and reabsorption of fundamental emission amid ambient density fluctuations in the corona and solar wind. Comparison with observations shows that this model accounts semiquantitatively for anomalous harmonic ratios, the exponential decay constant of bursts, burst rise times, and the directivity of fundamental emission. It is concluded that the long emission tail on interplanetary type III bursts is mostly fundamental emission, while much of the anomalous time delay of fundamental relative to harmonic emission from a given location must be ascribed to other causes.

First plasma wave observations at Neptune

The Voyager 2 plasma wave instrument detected many familiar plasma waves during the encounter with Neptune, including electron plasma oscillations in the solar wind upstream of the bow shock, electrostatic turbulence at the bow shock, and chorus, hiss, electron cyclotron waves, and upper hybrid resonance waves in the inner magnetosphere. Low-frequency radio emissions, believed to be generated by mode conversion from the upper hybrid resonance emissions, were also observed propagating outward in a disklike beam along the magnetic equatorial plane. At the two ring plane crossings many small micrometer-sized dust particles were detected striking the spacecraft. The maximum impact rates were about 280 impacts per second at the inbound ring plane crossing, and about 110 impacts per second at the outbound ring plane crossing. Most of the particles are concentrated in a dense disk, about 1000 kilometers thick, centered on the equatorial plane. However, a broader, more tenuous distribution also extends many tens of thousands of kilometers from the equatorial plane, including over the northern polar region.

Fundamental and Harmonic Emission in Type III Solar Radio Bursts – II. Dominant Modes and Dynamic Spectra

Recent theoretical estimates of the emissivity of fundamental and harmonic radiation in type III solar radio bursts are combined with calculations of electron beam evolution, radiation scattering and propagation delays to estimate dynamic spectra at a remote observer. The burst intensity, brightness temperature, temporal evolution, and dominant mode of emission are then calculated. A simple explanation of the recently observed low-frequency cutoff to type III emission is found and it is noted that some type III beams may propagate without significant radio emission. Criteria for observation of harmonic structure in dynamic spectra are also obtained. The results are shown to be consistent with a wide range of observations.

MHD simulations of Earth's bow shock at low Mach numbers: Standoff distances

Global, three-dimensional, ideal MHD simulations of Earth`s bow shock are reported for low Alfven Mach numbers M{sub A} and quasi-perpendicular magnetic field orientations. The simulations use a hard, infinitely conducting magnetopause obstacle, with axisymmetric three-dimensional location given be a scaled standard model, to directly address previous gasdynamic (GD) and field-aligned MHD (FA-MHD) work. Tests of the simulated shocks` density jumps X for 1.4 {approx_lt} M{sub A} {approx_lt} 10 and the high M{sub A} shock location, and reproduction of the GD relation between magnetosheath thickness and X for quasi-gasdynamic MHD runs with M{sub A} {much_gt} M{sub S}, confirm that the MHD code is working correctly. The MHD simulations show the standoff distance {alpha}{sub s} increasing monotonically with decreasing M{sub A}. Significantly larger {alpha}{sub s} are found at low M{sub A} than predicted by GD and phenomenological MHD models and FA-MHD simulations, as required qualitatively by observations. The GD and FA-MHD predictions err qualitatively, predicting either constant or decreasing {alpha}{sub s} with decreasing M{sub A}. This qualitative difference between quasi-perpendicular MHD and FA-MHD simulations is direct evidence for {alpha}{sub s} depending on the magnetic field orientation {theta}. The enhancement factor over the phenomenological MHD predictions at M{sub A} {approximately} 2.4 agrees quantitativelymorexa0» with one observational estimate. A linear relationship is found between the magnetosheath thickness and X, modified both quantitatively and intrinsically by MHD effects from the GD result. The MHD and GD results agree in the high M{sub A} limit. An MHD theory is developed for {alpha}{sub s}, restricted to sufficiently perpendicular {theta} and high sonic Mach numbers M{sub S}. Observational and further simulation testing of this MHD theory, and of its predicted M{sub A}, {theta}, and M{sub S} effects, is desirable. 10 refs., 6 figs.«xa0less

Second harmonic plasma emission involving ion sound waves

The theory for second harmonic plasma emission by the weak turbulence (or random phase) processes L + L + or - S to T, proceeding in two three-wave steps, L + or - S to L prime and L + L prime to T, where L, S and T denote Langmuir, ion sound and electromagnetic waves, respectively, is developed. Kinematic constraints on the characteristics and growth lengths of waves participating in the wave processes, and constraints on the characteristics of the source plasma, are derived. Limits on the brightness temperature of the radiation and the levels of the L prime and S waves are determined. Expressions for the growth rates and path-integrated wave temperatures are derived for simple models of the wave spectra and source plasma.

Theory for low‐frequency modulated Langmuir wave packets

Langmuir wave packets with low frequency modulations (or beats) observed in the Jovian foreshock are argued to be direct evidence for the Langmuir wave decay L yields L-prime + S. In this decay, pump Langmuir waves L, driven by an electron beam, produce backscattered product Langmuir waves L-prime and ion sound waves S. The L and L-prime waves beat at the frequency and wavevector of the S waves, thereby modulating the wave packets. Beam speeds calculated using the modulated Jovian wave packets (1) are reasonable, at 4-10 times the electron thermal speed, (2) are consistent with theoretical limits on the decay process, and (3) decrease with increasing foreshock depth, as expected theoretically. These results strongly support the theory. The modulation depth of some wave packets suggests saturation by the decay L yields L-prime + S. Applications to modulated Langmuir packets in the Venusian and terrestrial foreshocks and in a type III radio source are proposed.

First test of stochastic growth theory for Langmuir waves in Earth's foreshock

This paper presents the first test of whether stochastic growth theory (SGT) can explain the detailed characteristics of Langmuir-like waves in Earths foreshock. A period with unusually constant solar wind magnetic field is analyzed. The observed distributions P(log E) of wave fields E for two intervals with relatively constant spacecraft location (DIFF) are shown to agree well with the fundamental prediction of SGT, that P(log E) is Gaussian in log E. This stochastic growth can be accounted for semi-quantitatively in terms of standard foreshock beam parameters and a model developed for interplanetary type III bursts. Averaged over the entire period with large variations in DIFF, the P(log E) distribution is a power-law with index ∼ −1; this is interpreted in terms of convolution of intrinsic, spatially varying P(log E) distributions with a probability function describing ISEEs residence time at a given DIFF. Wave data from this interval thus provide good observational evidence that SGT can sometimes explain the clumping, burstiness, persistence, and highly variable fields of the foreshock Langmuir-like waves.

Towards an MHD theory for the standoff distance of Earth's bow shock

An MHD theory is developed for the stand- off distance as of the bow shock and the thickness A,_s of the magnetosheath, using the empirical Spreiter et al. relation Ares = kX and the MHD density ratio X across the shock. The theory includes as special cases the well-known gasdynamic theory and associated phe- nomenological MHD-like models for A,n, and as. In general, however, MHD effects produce major differ- ences from previous models, especially at low Alfven (MA) and sonic (Ms) Mach numbers. The magnetic field orientation, Ma, Ms, and tlle ratio of specific heats 7 are all important variables of the theory. In contrast, the fast mode Mach number need play no direct role. Three principal conclusions are reached. First, the gas- dynamic and phenomenological models miss important dependances on field orientation and Ms and generally provide poor approximations to the MHD results. Sec- ond, changes in field orientation and Ms are predicted to cause factor of-_ 4 changes in Ares at low MA. These effects should be important when predicting the shocks location or calculating 3 from observations. Third, us- ing Spreiter et al.s value for k in the MHD theory leads to maximum as values at low AIA and nominal Ms that are much smaller than observations and MHD simula- tions require. Resolving this problem requires either the modified Spreiter-like relation and larger k found ira recent MHD simulations and/or a breakdown in the Spreiter-like relation at very low MA.

Fundamental and Harmonic Emission in Type III Solar Radio Bursts – III. Heliocentric Variation of Interplanetary Beam and Source Parameters

The parameters of type III solar radio sources have been observed to vary approximately as powers of the heliocentric distance. Recent theoretical studies of fundamental and harmonic emission are used to express the power-law exponents in terms of five basic ones. The results are then used to obtain a best fit to these five exponents, consistent with observed values of a set of 13 exponents.