摘要:

The energy spectrum of galactic cosmic ray Helium was measured in two different balloon experiments launched four days apart from Canada: SMILI‐I on 1‐Sept‐1989 and MASS on 5‐Sept‐1989. A slow Forbush decrease began on 4‐Sept‐1989 and had not reached its maximum at the time of the MASS flight. Comparison of the balloon measurements shows a fractional decrease of 0.37 to 0.15 in the Helium flux between 200 and 450 MeV/nucleon (1.2–2.0 GV). The rigidity dependence is analyzed in two models and found to be steeper than previous observations. Interplanetary particle data and ground‐based Neutron Monitor results are consistent with the balloon observations. Probable sources for this Forbush decre

ISSN:0094-8276    

DOI:10.1029/93GL01972

年代:1993

数据来源: WILEY

摘要:

Escape rates of oxygen atoms from dissociative recombination of O2+above the Martian exobase are computed in light of new information from ab initio calculations of the dissociative recombination process, and our recently revised understanding of the Martian dayside ionosphere. Only about 60% of the dissociative recombinations occur in channels in which the O atoms are released with energies in excess of the escape velocity. Futhermore, we find that the computed escape fluxes for O depend greatly on the nature of the ion loss process that has been found necessary to reproduce the topside ion density profiles measured by Viking. If it is assumed that the ions are not lost from the gravitational field of the planet, as required by an analysis of nitrogen escape, the computed average O escape rate is 3 × 106cm−2s−1, much less than half the H escape rates inferred from measurements of the Lyman alpha dayglow, which are in the range (1 – 2) × 108cm−2s−1. Suggestions for restoring the relative escape rates of H and O to the stoichiometric ratio of water are explored. The final resolution of this problem may have to await a future aeronomy miss

ISSN:0094-8276    

DOI:10.1029/93GL01118

年代:1993

数据来源: WILEY

摘要:

The ion acoustic beam instability is suggested as a mechanism to produce wave turbulence observed in the Venus mantle at frequencies 100 Hz and 730 Hz. The plasma is assumed to consist of a stationary, cold O+ion plasma and a flowing, shocked solar wind plasma. The O+ions appear as a beam relative to the flowing ionosheath plasma which provides the free energy to drive the instability. The plasma is driven unstable by inverse electron Landau damping of an ion acoustic wave associated with the cold, ionospheric O+ions. The instability can directly generate the observed 100 Hz waves in the Venus mantle, as well as the observed 730 Hz waves through the Doppler shift of the frequency caused by the satellite motion.

ISSN:0094-8276    

DOI:10.1029/93GL01984

年代:1993

数据来源: WILEY

摘要:

Waves are generated in front of planetary bow shocks by a variety of plasma instabilities and they occur over a wide range of frequencies and wavelengths. In the strongly Doppler shifting environment of solar wind, multiple spacecraft observations are needed to determine unambiguously the wave properties such as frequency, phase velocity, and wavelength. The separations of the spacecraft required for these studies are restricted because the waves have finite coherence lengths. The multiple spacecraft must be located within a coherence length of each other in order that they are studying the same wave. In this study, we have used high time resolution simultaneous magnetic field data from the dual ISEE spacecraft to study the coherence lengths of upstream ULF waves. We examine the cross‐correlation between ISEE 1 and 2 observations for different spacecraft separations and determine the coherence lengths for upstream 30‐second waves, 3‐second waves and one‐Hz waves. We find that the observed coherence lengths are consistent with those estimated from the bandwidth of the spectral peak, and that these lengths vary markedly from less than 100 km to over 1 RE. In order to study all these wave phenomena, a multiple spacecraft mission such as the upcoming ESA Cluster mission would need to be capable of assuming a wide variety of possible sepa

ISSN:0094-8276    

DOI:10.1029/93GL01985

年代:1993

数据来源: WILEY

摘要:

We consider the foundations of 1‐ and 2‐dimensional shock simulations in which the physical quantities are independent of a coordinate which is not parallel to the magnetic field. We show analytically that in such simulations the ions are effectively tied to the convected magnetic lines of force because of the presence of an ignorable spatial coordinate. This conclusion has important consequences. In particular we conclude that the acceleration of energetic charged particles at quasi‐perpendicular shocks cannot be properly studied in such simulations because the role of perpendicular diffusion cannot be properly eval

ISSN:0094-8276    

DOI:10.1029/93GL01973

年代:1993

数据来源: WILEY

摘要:

The properties of small‐amplitude waves propagating in a homogeneous anisotropic plasma are investigated using an MHD double‐polytropic model that incorporates the CGL double‐adiabatic model in one extreme and the isothermal model in the other. It is found that the properties of fast and intermediate mode waves remain qualitatively the same as in ordinary MHD but that, in certain parameter regimes, three inversions occur for slow‐mode waves: (1) their phase speed exceeds that of intermediate waves; (2) they behave like fast‐mode waves in that, across them, the plasma density and magnetic field increase or decrease together; (3) rarefaction waves rather than compression wave

ISSN:0094-8276    

DOI:10.1029/93GL01706

年代:1993

数据来源: WILEY

摘要:

One‐ and two‐dimensional initial value hybrid computer simulations are used with a magnetosheath parameter model to study the consequences of the growth of the mirror and ion cyclotron anisotropy instabilities. Magnetosheath observations have demonstrated inverse correlations between the proton and helium ion temperature anisotropies and the proton parallelβ. Using the maximum growth rate as a fitting parameter, linear Vlasov instability theory for the proton cyclotron and helium cyclotron anisotropy instabilities reproduces both observed correlations. Furthermore, results from the asymptotic states of simulations of the ion cyclotron instabilities qualitatively reproduce both observat

ISSN:0094-8276    

DOI:10.1029/93GL01700

年代:1993

数据来源: WILEY

摘要:

We report the first fully three‐dimensional determinations of the bulk motions of ion and electron plasmas in Earth's magnetotail at geocentric radial distances near the lunar orbit. The two regions that are examined are the plasma sheet and the plasma mantle. The magnitudes and directions of the bulk velocities are the same for the ion and electron plasmas in the plasma mantle. However, in the plasma sheet the electron bulk speeds are often substantially greater than the ion bulk speeds. This situation arises because the component of electron flow along the magnetic field is greater than the ion parallel flow. Most of the bulk flow of the electrons is often attributable to the presence of field‐aligned currents, notE×Bconvection of plasmas. Hence previously reported two‐dimensional electron velocity distributions do not give a reliable determination of mass transport and convection in the plasma sheet at these distances in the magne

ISSN:0094-8276    

DOI:10.1029/93GL01450

年代:1993

数据来源: WILEY

摘要:

The structure of the quiet‐time magnetotail is studied using a test particle simulation. Vlasov equilibria are obtained in the regime whereυD=Eyc/Bzis much less than the ion thermal velocity and are self‐consistent in that the current and magnetic field satisfy Ampere’s law. Force balance between the plasma and magnetic field is satisfied everywhere. The global structure of the current sheet is found to be critically dependent on the source distribution function. The pressure tensor is nondiagonal in the current sheet with anisotropic temperature. A kinetic mechanism is proposed whereby changes in the source distribution results in a thinning of the current

ISSN:0094-8276    

DOI:10.1029/93GL01976

年代:1993

数据来源: WILEY

摘要:

In this report we will discuss recent determinations of wavelength and convection of Electron Cyclotron Harmonic (ECH) emissions as observed by the CRRES Plasma Wave Experiment (PWE). One motivation for this study is that these emissions may power the keV electron precipitation generating the diffuse aurora and so it is important to establish typical wave intensities. Problems encountered in doing this include wave attenuation because ECH wavelengths are typically close to the antenna length and the fact that it is difficult to study the effect of these waves on local electrons because of wave convection near the geomagnetic equator. We will also discuss a technique for approximating the emission wavelength with only one two‐element electric dipole antenn

ISSN:0094-8276    

DOI:10.1029/93GL00482

年代:1993

数据来源: WILEY