摘要:

We test Lee's (1983) quasi‐linear theory of ion acceleration using ISEE‐3 measurements of the November 12, 1978 quasi‐parallel interplanetary shock. His theory accounts with varying degrees of precision for the energetic proton spatial profiles; the dependence of the spectral index of the power law proton velocity distribution upon the shock compression ratio; the power law dependence of the upstream proton scalelength upon energy; the absolute magnitude of the upstream proton scale length; the behavior of the energetic proton anisotropy upstream and downstream of the shock; the behavior of the alpha‐particle proton ratio upstream; the equality of the spatial scale lengths at the shock of the upstream waves and of the protons that resonate with them; and the dependence of the integrated wave energy density upon the proton energy density at the shock. However, the trace magnetic field frequency spectra disagree with his theory in two ways. The part of the spectrum that can resonate with the observed protons via first‐order cyclotron resonance is flat, whereas Lee's theory predicts anf−7/4frequency dependence for the November 12 shock. Higher frequency waves, which could not resonate with the observed upstream protons, increased in amplitude as the shock approached, suggesting that they too were generated b

ISSN:0148-0227    

DOI:10.1029/JA091iA11p11917

年代:1986

数据来源: WILEY

摘要:

We have studied 34 forward interplanetary shocks observed by ISEE 3 during 1978 and 1979. Magnetic field and high‐energy particle data have been used, and for each shock the first critical Mach number has been determined. The first surprising result is that the majority of the observed shocks appear to be supercritical, and consistent with their supercritical character, many shocks have a foot and/or an overshoot in the magnetic field structure. Large‐amplitude low‐frequency waves (period ∼20 s in the spacecraft frame) are commonly observed upstream of all supercritical shocks (except for a few quasi‐perpendicular shocks) and also upstream of the few subcritical shocks. Intense particle events are frequently observed at many shocks: spikes at quasi‐perpendicular shocks and energetic storm particle events associated with quasi‐parallel shocks can be comparably intense. The correlation of the high‐energy particle peak flux with various shock parameters is in agreement with the acceleration mechanisms proposed by p

ISSN:0148-0227    

DOI:10.1029/JA091iA11p11929

年代:1986

数据来源: WILEY

摘要:

Altitude profiles of median values of total ion density Niand constituent ion densities [O+], [H+], and the molecular ion group density [O2++ M29+] (=[O2+] + [NO+]+[N2+]+[CO+]) for the central nightside Venus ionosphere are constructed from measurements made by the Pioneer Venus (PV) orbiter retarding potential analyzer (ORPA) during the first five seasonal passages of the PV spacecraft through the nightside hemisphere. The profiles cover the altitude range 145–900 km altitude. The altitude profiles for Ni, [O+], and [O2++ M29+] are compared with previously predicted profiles derived from numerical models in which the ionization source was O+transported from the dayside ionosphere and electron impact produced ionization, respectively. The numerically derived profile for O+transport reproduces the median profiles of Ni, [O+], and [O2++ M29+] quite satisfactorily. The extent to which electron impact ionization is required remains uncertain. The temporal variability of a quantity Q from its median value Qmis characterized by a variance parameter such that two thirds of the measurements Q fall within the limits Qm/σ ≤ Q ≤ σ Qm. The value of σ for Ni, [O+] and [H+] above an altitude of 150 km is 2.5. Below 150 km the variance for Niand [O2++ M29+] is approximately 1.5, but the statistics from which it is derived are less reliable. ORPA‐derived median plasma density Niand [O2++ M29+] agree closely with the average (and median) value of the peak nightside electron density Ne,maxmeasured by the PV radio occultation experiment when Niand [O2++ M29+] are extrapolated a short altitude interval to that of Ne,max. The standard deviation of Ne,maxalso agrees well with the variances of Niand [O2++ M29+] calculated for altitudes below 150 km. The ORPA median Niprofile at low altitudes is a factor of two smaller than the median Neprofile derived from published PV Langmuir probe (OLP) medians. At higher altitudes the Nimedian profile is 80–85% that of Ne. The plasma densities measured by PV in situ instruments in the altitude range 200–250 km exceed those derived from Venera 9 and 10 radio occultation data by a factor of the order of 10. The Venera measurements were made at sunspot minimum and, if representative of the median density, imply an order of magnitude variation over a solar cycle in the median O+flux transported from the days

ISSN:0148-0227    

DOI:10.1029/JA091iA11p11936

年代:1986

数据来源: WILEY

摘要:

The origin of large‐amplitude hydromagnetic waves in the Venus magnetosheath down‐stream of the quasi‐parallel bow shock is investigated by means of numerical simulation. It is shown that the most likely source of these waves is the bow shock itself, rather than an instability involving the solar wind and oxygen ions of planetary origin. Pickup of O+ions by these waves is also examined and shown to be in agreement with previous test particle calculations. The effect of mass loading on the structure of the shock is also disc

ISSN:0148-0227    

DOI:10.1029/JA091iA11p11951

年代:1986

数据来源: WILEY

摘要:

Among several different types of radio emissions discovered at Uranus during the Voyager 2 encounter in January 1986 is a very sporadic, bursty signal which consists of very narrow bands lying in the frequency range from about 3 to 10 kHz. The bursty emission was virtually undetectable from the dayside portion of the Voyager 2 trajectory but was observed out to beyond 300RUduring the outbound trajectory through the predawn sector. While the narrowband tones making up this emission are reminiscent of escaping continuum radiation observed near earth, Jupiter, and Saturn, the Uranian signals show large amplitude variations on time scales of 1 s, suggesting a very different type of generation mechanism.

ISSN:0148-0227    

DOI:10.1029/JA091iA11p11958

年代:1986

数据来源: WILEY

摘要:

We studied the critical ionization velocity (CIV) process by first investigating a coupled system of equations describing the production of several ion species and electrons by impact ionization, their collisions with neutrals, and the heating of electrons. Analytic relations derived from this were tested with the help of a particle simulation including collisonal processes between neutrals and plasma particles. It was found that resistive heating of electrons plays an important role when the density of the neutrals is high and that electron heating due to lower hybrid waves is significant when the neutral density is low. In both cases we verified the control of the plasma production rate by the ratio of the beam velocity to the critical velocity.

ISSN:0148-0227    

DOI:10.1029/JA091iA11p11965

年代:1986

数据来源: WILEY

摘要:

We have studied the configuration of the polar cusps in a static MHD equilibrium model. By introducing two quasi‐linear equilibrium models, we show that one can obtain magnetospheric configurations with the magnetopause neutral points located tailward of the earth and at the same time with reasonable tail magnetic field geometry. We also conclude that by the pressure balance principle across the magnetopause, if the magnetopause is a smooth surface then the neutral points cannot be on the dayside boundar

ISSN:0148-0227    

DOI:10.1029/JA091iA11p11977

年代:1986

数据来源: WILEY

摘要:

The dynamical system composed of a charged particle moving in a two‐dimensional magnetic field containing a neutral point, with and without a perpendicular electric field, is studied. The system is found to be nonintegrable and chaotic. Lyapunov characteristic exponents are calculated for the various types of chaotic orbits. When parameters relevant to the magnetotail are used it is found that the stochastic nature of the motion is more important for a thick current sheet (20,000 km) than for a thin sheet (1000 km). Some consequences of this stochasticity are discussed, including the suggestion of a collisionless, “chaotic conductivity”; this represents the collisionlike randomizing effect of the chaotic dyn

ISSN:0148-0227    

DOI:10.1029/JA091iA11p11985

年代:1986

数据来源: WILEY

摘要:

The evolutionary dynamics of a magnetic neutral sheet are followed using a nonperiodic, two‐dimensional magnetoinductive particle‐in‐cell simulation. The purpose of this study is to observe the effects of particle and field boundary conditions. With open boundary conditions, particles can freely pass the simulation boundaries. Particles leaving the domain are lost forever, while other particles are continually reintroduced from the adjoining reservoirs. The statistical properties (density, flux, scalar pressure) of the reservoir particles, including gradients transverse to the boundaries, are specified. The field boundary conditions are similarly “open.” The effect of open boundary conditions on the ion tearing mode instability are studied by comparison with periodic simulations. The results show that there is little difference between the initial time evolution in the periodic and the open systems but that the boundary conditions can have a noticeable effect on the nonlinear phase of the tearing mode, especially on the properties of the magnetic islands. When there are no particles in the reservoir, particles at the ends of the simulation domain escape and an O‐type neutral line (magnetic island) is formed. When the reservoir particle distributions are maintained to give the same densities and pressures as existed in the initial distribution, the tearing instability leads to the ejection of all the particles from the simulation domain and the almost total collapse of the mag

ISSN:0148-0227    

DOI:10.1029/JA091iA11p11993

年代:1986

数据来源: WILEY

摘要:

Analytic properties of the ion beam acoustic instability are evaluated for reasonable magnetotail conditions and are found to be consistent with observations of broadband turbulence not only in the plasma sheet boundary layer but also in the lobes and central plasma sheet regions. Numerical calculations confirm many of the analytic results. Specifically, the dependence of the ion beam acoustic mode on particle distribution models is investigated. Without cold ions present the acoustic mode is stable. With cold streaming ions present the acoustic mode is unstable, but the growth rate is significantly enhanced when warm streaming boundary layer ions are simultaneously present. These results suggest that the interaction between the warm and cold ion streams is responsible for the peak in the electrostatic wave intensities observed within the plasma sheet boundary layer. In addition, the beam acoustic instability, for magnetotail models, is found to be due to resonant, not nonresonant, interactions. Relative streaming effects and dependence on ion species are considered and found to play significant roles in the behavior of beam acoustic growth as a function of wave normal angle and wave number.

ISSN:0148-0227    

DOI:10.1029/JA091iA11p12005

年代:1986

数据来源: WILEY