摘要:

In the region upstream of comet Halley, pickup heavy ions of cometary origin were directly observed by the Implanted Ion Spectrometer on Giotto. Here we discuss diffusion of this population in pitch angle and in energy, during the approach to the comet and on the outbound leg. The two data sets are compared and qualitative ideas on scattering timescales are inferred. In addition the bulk parameters of these distributions have been computed and we present a comparison of the observed speed in the solar wind frame and the observed density with expectations. We find that pitch angle scattering occurs more slowly than expected with filled shells appearing at 2.5×106km, and that significant energy diffusion does not occur until the bow shock region. Also the shell distributions downstream of the shock flow at the bispherical bulk speed (related to the Alfvén speed) along the magnetic field with respect to the solar wind in accordance with conservation of energy between the pickup ions and the wave turbulenc

ISSN:0148-0227    

DOI:10.1029/JA095iA07p10249

年代:1990

数据来源: WILEY

摘要:

Alfvén waves propagating outward from the coronal base set the solar wind into oscillatory motion. The effect of this wave motion on the broadening and shape of the spectral lines of the resonantly scattered Ly α radiation emitted by a low‐density coronal hole is investigated at hights above 1.5Rsfrom Sun center. Usually it has been assumed that the broadening due to waves can be characterized by a kinetic temperature proportional to the wave velocity amplitude squared, δυ². It is shown that the accuracy of this approximation depends on the wave velocity amplitude and the assumed wave sp

ISSN:0148-0227    

DOI:10.1029/JA095iA07p10261

年代:1990

数据来源: WILEY

摘要:

A solar wind model with protons, electrons, O VII and O VI ions is studied. It is found that ionization and recombination processes lead to an approximately constant density ratio of the oxygen states (nOVI/nOVII≈ const ≪ 1) in the solar wind acceleration region. Although ionization and recombination have a significant effect on the flow speed of the O VI ions, these processes are not fast enough to bring the speed of O VI up to the flow speed of the O VII i

ISSN:0148-0227    

DOI:10.1029/JA095iA07p10269

年代:1990

数据来源: WILEY

摘要:

The stability of a spherical shell ion distribution function against low‐frequency hydromagnetic waves is discussed by relaxing the assumption of parallel propagation which was imposed in the preceding discussions published in the literature. It is found that Alfvén waves as well as magnetosonic waves with oblique angles of propagation can be excited by a shell distribution. In the present analysis, the emphasis is placed on the case in which the thickness of the spherical shell is sma

ISSN:0148-0227    

DOI:10.1029/JA095iA07p10273

年代:1990

数据来源: WILEY

摘要:

We have investigated the injection and interplanetary propagation of low‐energy protons that originated in a prompt solar particle event on July 20, 1981. The event was highly unusual in that solar flare protons could be seen from suprathermal energies (35 keV) to the energetic particle range (20 MeV). The observations were made in interplanetary space using the low‐energy ion instrument (DFH) and ULEWAT instrument on ISEE 3. At the higher energies, the appearance of the time‐intensity and time‐anisotropy profiles in this event were characteristic of diffusive particle propagation; however, owing to extremely quiet interplanetary conditions, it was possible to observe solar particles down to proton energies well below previous studies. We were able to fit the observed time‐intensity and time‐anisotropy profiles over the entire energy range using a model based on the spherically symmetric Fokker‐Planck equation including convection, diffusion, and adiabatic deceleration. From our model fits we find that (1) the radial interplanetary diffusion coefficientKincreases with heliocentric radiusrasK=K0rb, wherebis close to 1; (2) the scattering mean free path for protons, λ, increases with particle rigidityPas λ ∝P(0.22±0.03)over the range between 9 and 160 MV; and (3) at Earth orbit, λ values lie between 0.02 AU (for ∼0.5‐MeV electrons) and 0.17 AU (for protons, 10–20 MeV). Anisotropies in this event are long lasting (>48 hours) owing in part to an extended injection: the lower the energy, the longer the injection lasts. The extended period of injection appears to be evidence for diffusive coronal shock acceleration of protons during this event. Following the solar particle event maximum of intensity, a bidirectional streaming event was observed; during this period the particle anisotropies along with changes in the interplanetary magnetic field give evidence for the passage of a

ISSN:0148-0227    

DOI:10.1029/JA095iA07p10279

年代:1990

数据来源: WILEY

摘要:

Magnetohydrodynamic (MHD) activity, including waves and turbulence, has been a focus of attention in solar wind research for several decades, owing to the wealth of available relevant spacecraft observations. Characterizations of the turbulence have generally been based on incompressible homogeneous turbulence theory. However, recent observations show that the fluctuations undergo systematic temporal evolution, suggesting couplings to large‐scale plasma and magnetic field inhomogeneities. Here we present in detail, and further develop, a theory of transport of small‐scale solar wind MHD turbulence, the preliminary results of which have been recently reported (Zhou and Matthaeus,Geophysical Research Letters, 16, 755, 1989). Large‐scale plasma velocity and magnetic fields are specified, and small‐scale incompressible turbulence evolves in response to the associated inhomogeneities, as well as in accordance with modeled local nonlinear couplings. Dynamical equations based on a two length scale expansion are derived, from which the evolution of various wave number spectra may be computed, including magnetic and kinetic energies, cross helicity, induced electric field, and the corresponding helicities. Several simple analytic solutions predict, with increasing heliocentric distance, a decrease of the preponderance of outward traveling type Alfvénic fluctuations and a lowering of the small‐scale kinetic to magnetic energy ratio of small scale. Both of these are consistent with Helios and Voyager observations. The theory provides a basis for further computations that may be compared with statistical features of solar wind ob

ISSN:0148-0227    

DOI:10.1029/JA095iA07p10291

年代:1990

数据来源: WILEY

摘要:

Observations of ionospheric composition made by DE 2 in the mid‐latitude ionosphere, show that during the solar maximum period of 1981, at altitudes near 900 km during the nighttime, the dominant light ion species is He+. Conditions may arise in which He+is the dominant ion species, exceeding the O+concentration by a factor of 2. This observation is contrasted by those at the same altitude near the dip equator. Here a deep minimum in the He+concentration accompanies observed maxima in the O+and H+concentrations and O+is always the major ion species. Calculations are used to illustrate that the distribution of neutral species at solar maximum, together with the appropriate ionization rates can easily account for the dominance of He+at mid‐latitudes. The relative abundance of the ionospheric species is a sensitive function of local time and the associated evolution of the topside O+concentration profile. Factors such as E×B drift may easily effect the precise nature of the observation. At the dip equator the existence of an E×B drift motion of the plasma is required to explain the He+minimum. Here the E×B drift is such that plasma observed near 900 km during the night, has resided at altitudes below 600 km during the day. Thus the He+concentration is only that which is produced in one day against the chemical loss process. This is contrasted with the situation at mid‐latitudes where the large flux tube volume provides a reservoir in which the He+can accumulate

ISSN:0148-0227    

DOI:10.1029/JA095iA07p10313

年代:1990

数据来源: WILEY

摘要:

Ground‐based auroral observations with a meridian‐scanning photometer and all‐sky camera were made at Rabbit Lake, Saskatchewan, Canada, simultaneously with nearby HILAT satellite passes during six winter periods near new moon from 1984 to 1986. Eight passes were selected in which HILAT overflew auroras visible from Rabbit Lake. Auroral intensity measurements made at Rabbit Lake were corrected for proton aurora and nightglow and compared with HILAT electron flux measurements at 800 km. The excitation efficiency for the 4278‐Å auroral emission by electron precipitation was found to be (0.29±0.08) kR/(erg/cm² s). Excitation efficiencies for 5577‐Å emission of (1.73±0.51) kR/(erg/cm² s) and (1.23±0.44) kR/(erg/cm² s) were measured on different days in auroras excited by 1.8‐keV and 3.1‐keV Maxwellian precipitation during moderate geomagnetic disturbance. After small night‐to‐night variations in atmospheric composition indicated by the MSIS‐86 model atmosphere were accounted for, the magnetic zenith column intensity ratio 6300/4278 was found to depend on the electron characteristic energy Eoapproximately as 6300/4278 ∼ 3.3 Eo−2.1for characteristic energies of a few keV. The 5577/4278 ratio was found to decrease by about 40% from 1 to 5 keV characteristic energy. These Results pertain mostly to weak auroras (I(5577 Å) = 2–6 kR). Most of these new measurements agree fairly well with previous results of other workers, but the two 5577‐Å excitation efficiencies and the decreasing 5577/4278 intensity ratio show that the green line excitation efficiency varies appreciably even for small changes in the precipitating electron spectrum and atmospheric composition. Published calculations of the auroral 6300/4278 and 5577/4278 ratios versus characteristic energy are compared with our measurements. The magnitude and variation of the 5577/4278 ratios are consistent with roughly 60% of green line emission resulting from O(¹

ISSN:0148-0227    

DOI:10.1029/JA095iA07p10321

年代:1990

数据来源: WILEY

摘要:

We have developed an efficient two‐stream auroral electron model to study the deposition of auroral energy and the dependence of auroral emission rates on characteristic energy. This model incorporates the concept of average energy loss to reduce the computation time. Our simple two‐stream model produces integrated emission rates that are in excellent agreement with the much more complex multistream model of Strickland et al. (1983) but disagrees with a recent study by Rees and Lummerzheim (1989) that indicates that the N2second positive emission rate is a strongly decreasing function of the characteristic energy of the precipitating flux. Our calculations reveal that a 10 keV electron will undergo approximately 160 ionizing collisions with an average energy loss per collision of 62 eV before thermalizing. The secondary electrons are created with an average energy of 42 eV. When all processes including the backscattered escape fluxes are taken into account, the average energy loss per electron‐ion pair is 35 eV in good agreement with laboratory re

ISSN:0148-0227    

DOI:10.1029/JA095iA07p10337

年代:1990

数据来源: WILEY

摘要:

Numerical simulations of critical ionization velocity (CIV) discharges have been performed, taking into account several collisional processes. The simulations are one‐dimensional with density variation in the direction of plasma‐neutral relative velocity. The heating of electrons by wave‐particle interactions results in an extended electron distribution with “hot tail” formation, which enables excitation and ionization of neutrals to occur. The tail formation is observed to persist with a maximum energy less than twice the beam energy. The collisional processes included in this simulation are ground state ionization, excitation and ionization of metastable states, ion‐neutral charge exchange, and electron‐neutral elastic collisions. It is found that near the critical velocity, metastable states participate strongly in ionization, while at high velocities the metastable effect becomes relatively unimportant. Electron‐neutral elastic collisions, within the approximations used in this model, appear to be unimportant. Charge exchange is found to be crucial in sustaining the discharge when the beam velocity is low but is unimportant when the beam velocity is high. Detailed results are shown for the case of neon, while threshold behavior is investigated for helium, xenon, and barium as well. The density dependence of ionization in neon and barium is investigated. Although we examine the effect of line excitation on thresholds for CIV in all species, xenon and barium are especially of interest because both species have been used or proposed to be used in space CIV experiments. We report that line excitation strongly drains energy for barium beams in space. As a result, despite a low ionization potential, the threshold velocity for critical ionization of barium is raised substantially. This suggests a reason why barium beam ejections, the most common method for investigating CIV in space, have so far achieved inconclusive results. We conclude that low ionization potential may not be the best criterion for choosing the optimal beams in space exp

ISSN:0148-0227    

DOI:10.1029/JA095iA07p10345

年代:1990

数据来源: WILEY