摘要:

We have studied the diffusion in velocity space of cometary ions using the distributions of ions measured by the implanted ion spectrometer on Giotto during the inbound pass. The measurements were transformed into a frame comoving with the solar wind and oriented with the magnetic field. The observations show the evolution of the pitch angle distribution in the solar wind turbulence to form a shell from the initial ring. Diffusion in energy takes place simultaneously but on a longer time scale. Comparison with theory is inhibited by the lack of a suitable spatial model, but the simple arguments we can make indicate that pitch angle diffusion, and the process of parallel pickup, take place more slowly than theory suggests.

ISSN:0148-0227    

DOI:10.1029/JA094iA08p09983

年代:1989

数据来源: WILEY

摘要:

For several years it has been known that traveling interplanetary shocks can accelerate ions to energies of ≥70 MeV/nucleon. Recently, a subset of these shocks has been found to accelerate electrons as well, sometimes to energies of 7 MeV. Between 1978 and 1985 the Pioneer 10/11 spacecraft have observed 11 traveling interplanetary shocks in the outer heliosphere, 7 to 28 AU, which accelerate protons and helium to energies>11–20 MeV/nucleon. Six of these shock events also accelerate electrons to the energy range 2–7 MeV. These are the only traveling interplanetary shocks seen by the Pioneer spacecraft where electrons are accelerated to relativistic energies. In this paper we give the results of analyses of the solar wind plasma, interplanetary magnetic field, and energetic particle data for the 11 shocks. Our analysis has determined that there appear to be three necessary conditions in order for a traveling interplanetary shock to simultaneously accelerate ions to at least 11–20 MeV/nucleon and electrons to relativistic energies: (1) that the upstream magnetic field have a precursor wave in the frequency range 0.003‐0.04 s−1; (2) that the shock has a compression ratior(=ρ2/ρ1) of between 3 and 4, and has the scattering centers converge at |ΔV|≥50 km/s; and (3) that the relativistic electrons be accelerated via the Fermi mechanism. We propose a model to explain the simultaneous appearance of relativistic electrons and high‐energy ions at an outer heliospheric traveling shock. In this model the Fermi mechanism accelerates the electrons in two stages to relativistic energies, and the shock‐drift mechanism accelerates the ions to energies ≥11–20 MeV/nucleon. An analysis of the momentum spectra of various charged particle species leaves indeterminate the origin of the seed populations of both shocks which accelerate only ions and those which accelerate both ions an

ISSN:0148-0227    

DOI:10.1029/JA094iA08p09995

年代:1989

数据来源: WILEY

摘要:

A hot, suprathermal population of electrons is often present near the Earth's bow shock. The overall morphology of this hot population as observed with the Los Alamos/Garching fast plasma experiments on ISEE 1 and 2 in the energy range from 1 to 20 keV is much as described previously by others working primarily with more energetic particle measurements. In particular, at energies below 20 keV the flux of suprathermal electrons is most intense immediately downstream from the shock and decreases in intensity with increasing penetration into the magnetosheath. Upstream fluxes of electrons in this energy range are generally considerably weaker than are the downstream fluxes. The major new results from our ISEE measurements are as follows: (1) The suprathermal electrons are commonly found downstream from perpendicular and quasi‐perpendicular portions of the shock but not downstream from quasi‐parallel portions. (2) The suprathermal electron spectrum extends smoothly out of the shocked solar wind spectrum generally as a power law in energy with an exponent in the range from −3 to −4. (3) Angular distributions for the suprathermal electrons are generally isotropic immediately downstream of the shock ramp, but an anisotropy perpendicular to the magnetic field usually develops with increasing penetration into the magnetosheath. (4) Suprathermal electrons are often first observed within the shock layer itself as a field‐aligned beam escaping upstream. We interpret these observations in terms of a qualitative model wherein the suprathermal electrons are accelerated out of the solar wind thermal and halo populations as the solar wind convects across perpendicular portions of the shock. Subsequently, the suprathermal electrons leak back upstream along the magnetic field to form the backstreaming fluxes of energetic electrons commonly observed in the Earth's foreshock region. Although the mechanism of acceleration is presently uncertain, we believe it is unlikely that these electrons are accelerated by magnetic mirroring of upstream electrons at the shock or that they result from a simple adiabatic mapping of electron distributions from upstream to d

ISSN:0148-0227    

DOI:10.1029/JA094iA08p10011

年代:1989

数据来源: WILEY

摘要:

It is well known that for sufficiently high Mach numbers (typicallyMf>2) ion reflection is the primary process by which ion energy dissipation is initiated at quasi‐perpendicular, collisionless shocks in space. In this paper we present evidence from ISEE 2 measurements that coherent ion reflection is also often an important element of the ion energy dissipation process at high Mach number, quasi‐parallel shocks. Such coherent reflection is not evident in a number of published hybrid simulations of quasi‐parallel shocks wherein the dissipation process appears to be associated primarily with incoherent scattering of solar wind ions in the upstream and downstream wave fields. However, such reflection is present in other simulation results. The main evidence for coherent ion reflection as an important element in achieving dissipation at quasi‐parallel shocks is the following: (1) A cold secondary (to the solar wind) beam of ions is often observed within the ramp of the shock where the magnetic field intensity rises precipitously and where the major electron heating occurs. Typically this secondary beam of ions has a velocity space position roughly consistent with specular reflection of a portion of the solar wind ions incident on the shock. (2) Downstream from the shock, a relatively dense and cool core ion population is typically observed, accompanied by either multiple ion bunches or, more commonly, a suprathermal ion shell. (These populations are in addition to the extremely hot, tenuous diffuse ions which appear nearly ubiquitously both upstream and downstream from the shock.) The downstream core and shell (or bunches) are believed to represent directly transmitted solar wind ions and reflected and later scattered ions, respectively, with the motions of the latter population accounting for a large fraction of the ion energy dissipated at th

ISSN:0148-0227    

DOI:10.1029/JA094iA08p10027

年代:1989

数据来源: WILEY

摘要:

We present a method of computing the dayside global Earth magnetic field which is in equilibrium with the plasma pressure based on satellite observations at a local region of the magnetosphere. The method, which utilizes a perturbation around a dipole magnetic field, involves computation of the global plasma pressure profile,P⊥(r, θ) andP∥(r, θ) based on the equatorial (anisotropic) pressure dataP⊥(r, π/2) andP∥(r, π/2), derivation of the current profile which satisfies the equilibrium condition,J × B − ▽· P = 0, and computation of the magnetic field using the current profile and the boundary current produced by the solar wind. The method is applied for the Active Magnetospheric Particle Tracer Explorers data, and the result of the computation is found to compare reasonably well with the observed magnetic field profile near the ge

ISSN:0148-0227    

DOI:10.1029/JA094iA08p10039

年代:1989

数据来源: WILEY

摘要:

The kinetic Alfvén wave, an Alfvén wave with a perpendicular wavelength comparable to the ion gyroradius, can diffuse ions both in velocity and coordinate spaces with comparable transport rates. This may lead to the generation of ion beams in the plasma sheet boundary layer (PSBL). To investigate the ion beam generation process numerically, a two‐dimensional quasi‐linear code was constructed. Assuming that the plasma β (the ratio of plasma pressure to the magnetic pressure) varies from β = 1 to β ≪ 1 across the magnetic field, the dynamics of the ion beam generation in the PSBL was studied. It was found that if we start with an ion distribution function which monotonically decreases with velocity along the magnetic field and a density gradient across the magnetic field, ions diffuse in velocity‐coordinate space until nearly a plateau is established along the diffusion path. Depending on the topology of the magnetic field at the lobe side of our simulation system, i.e., open or closed field lines, the ion distribution function may or may not reach a steady state. If the field lines are open there, i.e., if the diffusion extends into the lobe, the double diffusion process may provide a mechanism for continuously transferring the ions from the central plasma sheet to the lobe. We comment on the effect of the particle loss on the establishment of the pressure balance in the

ISSN:0148-0227    

DOI:10.1029/JA094iA08p10047

年代:1989

数据来源: WILEY

摘要:

A study of the relationship between diffuse auroral and plasma sheet electron distributions in the energy range from 50 eV to 20 keV in the midnight region was conducted using data from the P78‐1 and SCATHA satellites. From 1½ years of data, 14 events were found where the polar‐orbiting P78‐1 satellite and the near‐geosynchronous SCATHA satellite were approximately on the same magnetic field line simultaneously, with SCATHA in the plasma sheet and P78‐1 in the diffuse auroral region. For all cases the spectra from the two satellites are in good quantitative agreement. For 13 of the 14 events the pitch angle distribution measured at P78‐1 was isotropic for angles mapping into the loss cone at the SCATHA orbit. For one event the P78‐1 electron flux decreased with pitch angle toward the field line direction. At SCATHA the distributions outside the loss cone were most commonly butterfly or pancake, although distributions peaked toward the field line were sometimes observed at energies below 1 keV. Electron distributions, as measured where there is isotropy within the loss cone but anisotropy outside the loss cone, are inconsistent with current theories for the scattering of electrons by electrostatic waves. Using P78‐1 data to specify the pitch angle distribution in the loss cone for the distribution measured at SCATHA, the electron precipitation lifetimes were calculated for the 14 events. Because the distributions are anisotropic at pitch angles away from the loss cone, the calculated lifetimes significantly exceed the lifetimes in the limit when the flux is isotropic at all pitch angles. The computed precipitation lifetimes are found to be weakly dependent on magnetic activity. The average lifetimes exceed those for the case of isotropy at all pitch angles by a factor between 2 and 3 forKp≤2 and approxi

ISSN:0148-0227    

DOI:10.1029/JA094iA08p10061

年代:1989

数据来源: WILEY

摘要:

Energetic (>6 keV) electron data from the SEEP payload on the low altitude (∼200 km) polar orbiting S81‐1 satellite indicate a high rate of occurrence of short duration (<0.6 s) electron precipitation bursts. Characteristics of events observed at night (2230 MLT) versus daytime (1030 MLT) and at midlatitudes (2

ISSN:0148-0227    

DOI:10.1029/JA094iA08p10079

年代:1989

数据来源: WILEY

摘要:

Electron distribution functions with a peak oblique to the magnetic field, adjacent to but distinct from loss cone features, have been observed by the DE 1, Viking and S3‐3 spacecraft in passes through the nightside auroral zone, polar cap, dayside cusp and extended dayside auroral oval. Using particle simulations, we investigate two types of wave excitation and particle acceleration mechanisms which may contribute to producing these electron conic distributions. The first involves excitation of upper hybrid waves by the electron loss cone, and the subsequent perpendicular heating of the background and thermal electrons. The second involves excitation of downward propagating parallel modes by an auroral electron beam which frequently accompanies the upflowing electron conics. These modes provide parallel acceleration, which modifies the electron distribution function. Those electrons which are not lost to the atmosphere and mirror back up the magnetic field line give rise to enhancements in the distribution function at the edge of the loss con

ISSN:0148-0227    

DOI:10.1029/JA094iA08p10095

年代:1989

数据来源: WILEY

摘要:

One‐dimensional particle simulations have been carried out to study the low‐frequency broadband electrostatic noise that propagates almost perpendicularly from the magnetic field line when a nonrelativistic electron beam is injected into space from a spacecraft. ForTe=Tithe electrostatic ion cyclotron waves appear as well as the waves near the lower hybrid frequency. When the magnetic field is reduced so that Ωe≪ωpein a nonisothermal plasma,Te>Ti, oblique ion acoustic instabilities appear to propagate almost perpendicular to the magnetic field. In addition, a very low frequency mode at ω≪Ωiis found to be generated by the electrons flowing into the conductor. Both the injected beam electrons as well as the ambient electrons flowing into the spacecraft are responsible for generating those instabilities, which accelerate ions perpendicular to the mag

ISSN:0148-0227    

DOI:10.1029/JA094iA08p10103

年代:1989

数据来源: WILEY