Oblique Interplanetary Shocks Research Articles

The Extended Field-aligned Suprathermal Proton Beam and Long-lasting Trapped Energetic Particle Population Observed Upstream of a Transient Interplanetary Shock

Abstract The properties of the suprathermal particle distributions observed upstream of interplanetary shocks depend not only on the properties of the shocks but also on the transport conditions encountered by the particles as they propagate away from the shocks. The confinement of particles in close proximity to the shocks, as well as particle scattering processes during propagation to the spacecraft, lead to the common observation of upstream diffuse particle distributions. We present observations of a rare extended anisotropic low-energy (≲30 keV) proton beam together with a trapped ≳500 keV proton population observed in association with the arrival of an oblique interplanetary shock at the Advanced Composition Explorer, the Interplanetary Monitoring Platform-8, and the Wind spacecraft on 2001 January 31. Continuous injection of particles by the traveling shock into a smooth radial magnetic field region formed in the tail of a modest high-speed solar wind stream produced an extended foreshock region of energetic particles. The absence of enhanced magnetic field fluctuations upstream of the shock results in the observation of a prolonged anisotropic field-aligned beam of ≲30 keV protons as well as a population of higher-energy (≳500 keV) protons with small pitch-angle cosine (μ ∼ 0) extending far from the shock.

Unusual Location of the Geotail Magnetopause Near Lunar Orbit: A Case Study

AbstractThe Earth's magnetopause is highly variable in location and shape and is modulated by solar wind conditions. On 8 March 2012, the ARTEMIS probes were located near the tail current sheet when an interplanetary shock arrived under northward interplanetary magnetic field conditions and recorded an abrupt tail compression at ∼(‐60, 0, ‐5) RE in Geocentric Solar Ecliptic coordinate in the deep magnetotail. Approximately 10 minutes later, the probes crossed the magnetopause many times within an hour after the oblique interplanetary shock passed by. The solar wind velocity vector downstream from the shock was not directed along the Sun‐Earth line but had a significant Y component. We propose that the compressed tail was pushed aside by the appreciable solar wind flow in the Y direction. Using a virtual spacecraft in a global magnetohydrodynamic (MHD) simulation, we reproduce the sequence of magnetopause crossings in the X‐Y plane observed by ARTEMIS under oblique shock conditions, demonstrating that the compressed magnetopause is sharply deflected at lunar distances in response to the shock and solar wind VY effects. The results from two different global MHD simulations show that the shocked magnetotail at lunar distances is mainly controlled by the solar wind direction with a timescale of about a quarter hour, which appears to be consistent with the windsock effect. The results also provide some references for investigating interactions between the solar wind/magnetosheath and lunar nearside surface during full moon time intervals, which should not happen in general.

Structure of the Front of a Collisionless Oblique Interplanetary Shock Wave from High Time Resolution Measurements of Solar-Wind Plasma Parameters

Data from the Fast Solar Wind Monitor (BMSW) of the science payload onboard the SPEKTR-R satellite and data from instruments on board the WIND spacecraft are used to study statistically the structure of the front of oblique interplanetary shock waves with respect to the θBn angle and the fulfillment of the Rankine–Hugoniot conditions at the fronts of collisionless shock waves. The experimental wavelength of oscillations upstream of the ramp is compared with the estimated theoretical wavelength to determine that the dispersion of oblique magnetosonic waves plays the decisive role in the formation of the fronts of quasiperpendicular (45° ≤ θBn < 90°) collisionless interplanetary shock waves with small Mach numbers МA < 3 and a parameter of β1 < 1. Comparison of the Rankine–Hugoniot relations MA(ρ2/ρ1), which were measured at the fronts of 47 interplanetary shock waves with β1 < 5 and Alfven Mach numbers of 1 < МА < 10 by calculations performed within the ideal magnetic hydrodynamics (MHD), has revealed that the effective adiabatic index γ, which characterizes the processes inside the shock front, lies mainly within the range from 2 to 5/3.

On the origin of burst Pc1 pulsations produced in interaction with an oblique interplanetary shock

We examined the features of bursts of unstructured Pc1 geomagnetic pulsations recorded with period in the range T=2–5s on 19 November 2007 using simultaneous observations by the geosynchronous satellites GOES-10, 11, 12, a constellation of high-apogee satellites THEMIS and by the CARISMA ground-based network of magnetometers. The pulsation excitation resulted from contact of an oblique interplanetary shock wave (ISW) with the magnetosphere. At geosynchronous orbit, we found eastward drift of the source of Pc1 bursts observed first by GOES-11 (~09MLT), then by GOES-12 (~13MLT) and, finally, by GOES-10 (~14MLT). Ground-based observatories with ~40° longitudinal separation observed the excitation of oscillations with a delay to the west and east as compared with the median Fort Simpson observatory. An increase in frequency, seen at the sharp leading edge of oscillations, lasted for about 150s. We determined the propagation velocity of the pulsations׳ source from the difference between the first observations of the pulsations by the satellites and at the Earth. In order to interpret the observed patterns of pulsation we considered different mechanisms such as: (1) Eastward drifting clouds of energetic electrons accelerated due to compression of the magnetosphere; (2) Plasmaspheric bulges (or detached plasma); (3) Magnetopause surface waves generated in the region of contact with the ISW and resulting in undulation of the region of developing the cyclotron instability.

Direct Acceleration of Pickup Ions at the Solar Wind Termination Shock: The Production of Anomalous Cosmic Rays

We have modeled the injection and acceleration of pickup ions at the solar wind termination shock and investigated the parameters needed to produce the observed anomalous cosmic-ray (ACR) fluxes. A nonlinear Monte Carlo technique was employed that in effect solves the Boltzmann equation and is not restricted to near-isotropic particle distribution functions. This technique models the injection of thermal and pickup ions, the acceleration of these ions, and the determination of the shock structure under the influence of the accelerated ions. The essential effects of injection are treated in a mostly self-consistent manner, including effects from shock obliquity, cross-field diffusion, and pitch-angle scattering. Using recent determinations of pickup ion densities, we are able to match the absolute flux of hydrogen in the ACRs by assuming that pickup ion scattering mean free paths, at the termination shock, are much less than an AU and that modestly strong cross-field diffusion occurs. Simultaneously, we match the flux ratios He+/H+ or O+/H+ to within a factor ~5. If the conditions of strong scattering apply, no pre-termination-shock injection phase is required and the injection and acceleration of pickup ions at the termination shock are totally analogous to the injection and acceleration of ions at highly oblique interplanetary shocks recently observed by the Ulysses spacecraft. The fact that ACR fluxes can be modeled with standard shock assumptions suggests that the much discussed "injection problem" for highly oblique shocks stems from incomplete (either mathematical or computer) modeling of these shocks rather than from any actual difficulty shocks may have in injecting and accelerating thermal or quasi-thermal particles.

Ulysses observations of a discrete wavepacket upstream of an interplanetary shock

We present a single example of a previously unidentified type of upstream wave structure, found several minutes ahead of a forward propagating, oblique interplanetary shock observed on day 6, 1992, using Ulysses magnetometer data. The wave appears as an isolated high frequency wavepacket associated with a larger scale compressive structure. In the region between the wavepacket and the shock the magnetic field is moderately, but significantly, compressed and deflected suggesting the existence of a foreshock. The wavepacket is dispersive and wavelet analysis shows a near linear decrease in frequency from 0.3 to 0.1 Hz, continuing across the large scale structure which is assumed to be the source of these waves. We suggest that the feature is similar in nature to the shocklet and associated discrete wavepacket commonly found in planetary foreshocks, but further examples are required before a definite conclusion can be reached.

Acceleration of Solar Wind Ions by Nearby Interplanetary Shocks: Comparison of Monte Carlo Simulations withUlyssesObservations

Various theoretical techniques have been devised to determine distribution functions of particles accelerated by the first-order Fermi mechanism at collisionless astrophysical shocks. The most stringent test of these models as descriptors of the phenomenon of diffusive acceleration is a comparison of the theoretical predictions with observational data on particle populations. Such comparisons have yielded good agreement between observations at the quasi-parallel portion of the Earth's bow shock and three theoretical approaches, namely, Monte Carlo kinetic simulations, hybrid plasma simulations, and numerical solution of the diffusion-convection equation. Testing of the Monte Carlo method is extended in this paper to the realm of oblique interplanetary shocks: here observations of proton and He2+ distributions made by the SWICS ion mass spectrometer on Ulysses at nearby interplanetary shocks (less than about 3 AU distant from the Sun) are compared with test-particle Monte Carlo simulation predictions of accelerated populations. The plasma parameters used in the simulation are obtained from measurements of solar wind particles and the magnetic field upstream of individual shocks; pickup ions are omitted from the simulations, since they appear, for the most part, at greater heliospheric distances. Good agreement between downstream spectral measurements and the simulation predictions are obtained for two shocks by allowing the parameter λ/rg, the ratio of the mean-free length to the ionic gyroradius, to vary in an optimization of the fit to the data; generally λ/rg 5, corresponding to the case of strong scattering. Simultaneous H+ and He2+ data, presented only for the 1991 April 7 shock event, indicate that the acceleration process is roughly independent of the mass or charge of the species. This naturally arises if all particles interact elastically with a massive background, as occurs in collisionless scattering off a background magnetic field, and is a patent property of the Monte Carlo technique, since it assumes elastic and quasi-isotropic of particles in the local plasma frame.

Diffusive Shock Acceleration in Oblique Magnetohydrodynamic Shocks: Comparison with Monte Carlo Methods and Observations

We report simulations of diffusive particle acceleration in oblique magnetohydrodynamical (MHD) shocks. These calculations are based on extension to oblique shocks of a numerical model for thermal leakage injection of particles at low energy into the cosmic-ray population. That technique, incorporated into a fully dynamical diffusion-convection formalism, was recently introduced for parallel shocks by Kang & Jones. Here, we have compared results of time-dependent numerical simulations using our technique with Monte Carlo simulations by Ellison, Baring, & Jones and with in situ observations from the Ulysses spacecraft of oblique interplanetary shocks discussed by Baring et al. Through the success of these comparisons, we have demonstrated that our diffusion-convection method and injection techniques provide a practical tool to capture essential physics of the injection process and particle acceleration at oblique MHD shocks. In addition to the diffusion-convection simulations, we have included time-dependent two-fluid simulations for a couple of the shocks to demonstrate the basic validity of that formalism in the oblique shock context. Using simple models for the two-fluid closure parameters based on test particle considerations, we find good agreement with the dynamical properties of the more detailed diffusion-convection results. We emphasize, however, that such two-fluid results can be sensitive to the properties of these closure parameters when the flows are not truly steady. Furthermore, we emphasize through example how the validity of the two-fluid formalism does not necessarily mean that steady state two-fluid models provide a reliable tool for predicting the efficiency of particle acceleration in real shocks.

Acceleration of solar wind ions by oblique interplanetary shocks

This paper compares observations of proton distributions made by the SWICS ion mass spectrometer on Ulysses at nearby interplanetary shocks with Monte Carlo simulations of particle acceleration at oblique collisionless shocks. The shock parameters are obtained from upstream measurements of the solar wind and magnetic field, and the input particles are drawn from convected Maxwellians, representing solar wind particles. Good agreement between downstream spectral measurements and the simulation predictions are obtained by allowing the parameter λ r g , the ratio of the mean-free scattering length to the ionic gyroradius, to vary in an optimization of the fit to the data. Generally λ r g is found to be less than about 20, which corresponds to the case of strong scattering.

Monte Carlo simulations of particle acceleration at oblique shocks: Including cross-field diffusion

The Monte Carlo technique of simulating diffusive particle acceleration at shocks has made spectral predictions that compare extremely well with particle distributions observed at the quasi-parallel region of the earth's bow shock. The current extension of this work to compare simulation predictions with particle spectra at oblique interplanetary shocks has required the inclusion of significant cross-field diffusion (strong scattering) in the simulation technique, since oblique shocks are intrinsically inefficient in the limit of weak scattering. In this paper, we present results from the method we have developed for the inclusion of cross-field diffusion in our simulations, namely model predictions of particle spectra downstream of oblique subluminal shocks. While the high-energy spectral index is independent of the shock obliquity and the strength of the scattering, the latter is observed to profoundly influence the efficiency of injection of cosmic rays into the acceleration process.