摘要:

We present initial results from a survey of the effects of interplanetary shocks on energetic ≥2 keV electrons and ≥47 keV ions, as observed by the field, plasma, and energetic particle experiments on the ISEE 3 spacecraft. Shock normals, velocities, Mach numbers, and upstream and downstream plasma parameters were determined for 37 forward shocks out of a total of 55 shocks observed between August 1978 and November 1979. We find that a minimum shock velocity along the upstream magnetic field of ∼250 km/s is required for an interplanetary shock to have a significant effect on acceleration of ≥2 keV electrons or ≥47 keV ions. Shocks with no effect on the energetic particle populations also had relatively small ratios of downstream to upstream magnetic field magnitudes. These results suggest that magnetostatic reflection off the shock itself is a significant mechanism in the acceleration process. Both energetic electron and ion flux variations associated with shocks can be classified into four general types: (1) no significant variation at all, (2) a spike of a few minutes duration at or near the shock, (3) a steplike postshock increase, and (4) a slow rise beginning several hours before the shock (energetic storm particle event). Essentially, every quasi‐perpendicular shock with θBn≳ 70° produced a shock spike in the proton fluxes, while every quasi‐parallel shock (θBn≲ 50°) produced a proton energetic storm particles event, provided the shock velocity was greater than the above stated threshold. Electron spikes were also observed for most, but not all, shocks with θBn≳ 70°. The most common effect observed in the electron fluxes was a steplike postshock increase of a factor of ∼2. These had no obvious dependence on θBn, but were found for every shock with speed greater than ∼175 km/s. Shock effects in the electron fluxes were about as common as for protons, but were limited to the 2–10 keV energy range except for 3 events which extended up to ∼50 keV. We find that significant ambient populations of both ≥2 keV electrons and ≳47 keV ions are present in the interplanetary medium prior to every shock. These particles could be the “se

ISSN:0148-0227    

DOI:10.1029/JA090iA01p00001

年代:1985

数据来源: WILEY

摘要:

We present observations of the 35‐ to 1600‐keV proton intensity‐time profiles and the three‐dimensional 35‐ to 56‐keV anisotropy distributions recorded during two interplanetary shock events on ISEE 3, and discuss these in the light of current particle acceleration models. The large April 5, 1979, event associated with a quasi‐parallel shock shows an extended foreshock region with a strong increase of the upstream proton flux, a downstream plateaulike profile, upstream flow from the shock, and downstream isotropy in the solar wind frame of reference. The small March 9, 1979, event has a structured intensity‐time profile, a narrow shock spike, and anisotropic angular distributions both upstream and downstream, the anisotropies immediately behind the shock exhibiting an intensity peak at pitch angles around 90°. The April event, representative for a class of large energetic storm particle events, shows many observational features which are in agreement with predictions made by diffusive shock acceleration models. The March event, representative for a class of events with irregular profiles and mainly associated with quasi‐perpendicular shocks, exhibits features which are characteristic of shock drift acceleration. We conclude that both acceleration models are operative in association with int

ISSN:0148-0227    

DOI:10.1029/JA090iA01p00012

年代:1985

数据来源: WILEY

摘要:

We present results of a detailed analysis of three‐dimensional anisotropies of protons in the energy range 35–1000 keV observed in association with interplanetary shocks on ISEE 3. We compare observations of high time resolution anisotropies made close to the shock in seven energy channels with theoretical predictions for Fermi acceleration and shock drift acceleration, and we find good evidence for both types of acceleration. We find a small number (six) of events exhibiting the signature of Fermi acceleration, and a somewhat larger number (20) exhibiting the signature of shock drift acceleration, the “Fermi” events being associated with strong, fast quasi‐parallel shocks and the “shock drift” events being associated with weaker, slower quasi‐perpendicular events. In the solar wind frame of reference the Fermi events have moderate upstream anisotropies, with flow away from the shock persisting for periods of one to two hours, the anisotropy decreasing with increasing energy, whereas downstream these events are isotropic. These events exhibit slow quasi‐exponential intensity increases of 1–2 orders of magnitude, peaking at the shock, and slowly decaying after the shock, often rising to a secondary peak some hours later. The shock drift events have large upstream first‐order anisotropies close to the shock, with flow away from the shock, and moderate downstream first‐order anisotropies, with flow toward the shock. The most notable feature of the shock drift events is a large negative second harmonic immediately downstream of the shock, signifying protons gyrating around the magnetic field at pitch angles of around 90°. These events have shock spike intensity increases lasting for a few minutes or tens of minutes. At all energies the largest intensity increases are observed with the Fermi events. Since the Fermi events are associated with the largest fluxes of solar flare protons, this may be due to a combination of solar particle background and protons accelerated in t

ISSN:0148-0227    

DOI:10.1029/JA090iA01p00019

年代:1985

数据来源: WILEY

摘要:

Observations of particle spectra, intensity, and enhancement of alpha particles over protons at diffuse ion events at the quasi‐parallel earth bow shock are compared to a Monte Carlo simulation of diffusive shock acceleration. The simulation includes the back reaction of accelerated particles on the shock structure, particle escape at an upstream free escape boundary, and a low energy per nucleon threshold for thermal leakage of downstream, shock‐heated particles into the upstream region. The simulation assumes that the same scattering operator that gives rise to shock acceleration can also describe a viscous shock governed by hydrodynamic turbulence. This implies that accelerated ions can be drawn directly from the thermal solar wind with no separate superthermal seed population. Good agreement between the simulation and observations made during nearly radial magnetic field configurations lends support to thermal leakage of downstream, shock‐heated ions as the mode of injection for diffuse ion e

ISSN:0148-0227    

DOI:10.1029/JA090iA01p00029

年代:1985

数据来源: WILEY

摘要:

We present a simple model unifying the distinct energetic ion populations and their associated low‐frequency hydromagnetic waves within earth's ion foreshock. Ions initially injected onto magnetic field lines at the shock excite hydromagnetic waves which pitch angle scatter the ions back toward the shock. The ions are represented by inhomogeneous inward (toward the shock) and outward traveling beams, and the transition rate between beams is determined by an effective quasi‐linear pitch angle diffusion coefficient for the transition. The intensities of waves resonant with the beams are calculated from wave kinetic equations utilizing linear wave growth rates which in turn are determined by the instantaneous, local beam densities. The coupled equations for the spatial and temporal evolution of the ion densities and wave intensities along a given magnetic field line are solved numerically assuming steady injection of ions at the shock following the initial magnetic connection of the field line to the bow shock. The initial interplanetary waves are assumed to be unpolarized on average and to propagate predominantly away from the sun relative to the solar wind. We find that (1) the ion anisotropy near the shock decreases slowly during the initial minutes of magnetic contact, but then makes a rapid transition to a steady, “diffuse” distribution; (2) the diffuse ion density declines steeply away from the shock with a scale length of ∼5RE, forms a broad minimum, and then increases with increasing anisotropy to form an “intermediate” ion distribution; (3) upstream of the intermediate ions the density forms a broad “reflected” ion distribution with high anisotropy extending to the foreshock boundary; (4) the reflected and intermediate ion distributions are confined to spatial bands which are aligned with the foreshock boundary and have ∼5‐REwidths; and (5) the waves associated with intermediate ions are strongly right‐hand polarized in the solar wind frame whereas the waves associated with diffuse ions exhibit a net polarization wi

ISSN:0148-0227    

DOI:10.1029/JA090iA01p00039

年代:1985

数据来源: WILEY

摘要:

Charged particle acceleration via the shock drift mechanism at quasi‐perpendicular shocks has generally been analyzed by assuming uniform, time‐independent conditions at and near the shock. We present results from a model designed to study how the shock drift mechanism is modified when wave activity is included in the shock's upstream and downstream vicinities. The technique involves numerically following test particle trajectories in the wave‐shock system for predefined wave fields. In order to compare these results with those obtained in the scatter‐free (i.e., nonwave) case, we restricted particles to a single shock encounter, which is here defined as the period during which the particle remains within a gyrodiameter of the shock. As a particular example, we injected ensembles of ions into a system consisting of a quasi‐perpendicular shock moving through the interplanetary spectrum of ambient Alfvén waves. As compared with a single encounter in the scatter‐free limit, the inclusion of waves (1) increases particle transmission through the shock, (2) produces broader energy distributions for reflected and transmitted particles, with high‐energy tails at energies several times the maximum energy obtained in the scatter‐free case and (3) reduces anisotropies, particularly of reflected particles, but does not eliminate them. Also, for the range of energies studied, it was found that the approximate invariance of the magnetic moment for particle interactions with quasi‐perpendicular shocks is no longer valid when

ISSN:0148-0227    

DOI:10.1029/JA090iA01p00047

年代:1985

数据来源: WILEY

摘要:

By means of particle simulation, we study the excitation mechanism of low‐freqeuncy (0.01–0.05Hz) upstream hydromagnetic waves. Initially, we observe excitation of the right‐hand polarized waves propagating parallel to the field‐aligned ion beam, which is given as the free energy source. In the nonlinear stage we observed the phase space bunching of beam ions by the excited waves. We apply this bunching effect for the explanation of “gyrophased bunched” ions observed in the fores

ISSN:0148-0227    

DOI:10.1029/JA090iA01p00057

年代:1985

数据来源: WILEY

摘要:

The wave‐particle interactions of a hot ion beam streaming along a magnetic fieldBare studied. A second‐order theory of electromagnetic instabilities in a homogeneous, collisionless plasma at propagation parallel toBis used. The two instabilities most likely to be driven by a hot beam are the right‐hand and left‐hand resonant ion beam instabilities. If the conditions necessary for the validity of the theory are met, the two modes are found to reinforce one another. That is, each mode acts to produce a beam anisotropy which reduces its own growth rate but enhances the growth rate of the other. Thus this theory predicts that whenever sufficiently hot or “diffuse” ions are found at a collisionless shock and the plasma is sufficiently homogeneous that significant wave growth is possible, ion beam instabilities will act to produce a mixture of both right‐ and left‐hand polarized

ISSN:0148-0227    

DOI:10.1029/JA090iA01p00065

年代:1985

数据来源: WILEY

摘要:

Electrostatic waves are observed around the plasma frequency fpein the electron foreshock, together with electrons backstreaming from the bow shock. Using data from the sounder aboard ISEE 1, we show that this noise, previously understood as narrow band Langmuir waves more or less widened by Doppler shift or nonlinear effects, is in fact composed of two distinct parts: One is a narrow band noise, emitted just above fpe, and observed at the upstream boundary of the electron foreshock. This component has been interpreted as Langmuir waves emitted by a beam‐plasma instability. We suggest that it is of sufficiently large amplitude and monochromatic enough to trap resonant electrons. The other is a broad band noise, more impulsive than the narrow band noise, observed well above and/or well below fpe, deeper in the electron foreshock. The broad band noise has an average spectrum with a typical bi‐exponential shape; its peak frequency is not exactly equal to fpeand depends on the Debye length. This peak frequency also depends on the velocity for which the electron distribution has maximum skew. An experimental determination of the dispersion relation of the broad band noise shows that this noise, as well as the narrow band noise, may be due to the instability of a hot beam in a pla

ISSN:0148-0227    

DOI:10.1029/JA090iA01p00073

年代:1985

数据来源: WILEY

摘要:

The turbulent plasma development of Lee and Parks is applied to the solar wind approaching the earth's bow shock region. The ponderomotive force contribution is due to ion acoustic waves propagating in the direction of the ambient magnetic field. In this case, the envelope of the ion acoustic wave is shown to satisfy the cubic Schroedinger equation. Modulational instabilities exist for waves in the solar wind, thereby predicting the generation of solitary waves. This analysis further identifies that the ion acoustic waves which exhibit this instability have short wavelengths.

ISSN:0148-0227    

DOI:10.1029/JA090iA01p00095

年代:1985

数据来源: WILEY