摘要:

We present the first global simulations of the modulation of galactic cosmic rays by a three‐dimensional solar wind with corotating interaction regions. The cosmic‐ray transport equation is solved in a computed wind and magnetic‐field model. The results show both the previously‐neglected small‐scale response to CIR's and global, drift‐dominated, effects which are similar to previous models. Our model predicts a correlation between the local magnetic field and the rate of decrease of the cosmic‐ray intensity which is similar to that observed. This is found to be due to inhibiteddiffusion.We suggest that such small‐scale variations are local and that they do not change significantly the global cosmic‐ray structure except, perhaps, nea

ISSN:0094-8276    

DOI:10.1029/91GL02307

年代:1991

数据来源: WILEY

摘要:

Recent Voyager 1 observations reveal reoccurrences of the low frequency interplanetary radio emissions. Three of the new events are weak transient events which rise in frequency from the range of 2–2.3 kHz to about 3 kHz with drift rates of ∼1.5 kHz/year. The first of the transient events begins in mid‐1989 and the more recent pair of events both were first detected in late 1990. In addition, there is an apparent onset of a 2‐kHz component of the emission beginning near day 70 of 1991. The new transient emissions are barely detectable on Voyager 1 and are below the threshold of detectability on Voyager 2, which is less sensitive than Voyager 1. The new activity provides new opportunities to test various theories of the triggering, generation, and propagation of the outer heliospheric radio emissions and may signal a response of the source of the radio emissions to the increased solar activity associated with the recent peak in the sola

ISSN:0094-8276    

DOI:10.1029/91GL02309

年代:1991

数据来源: WILEY

摘要:

The measurements carried out by the plasma spectrometer, ASPERA, onboard the PHOBOS‐2 spacecraft show that protons, originating in the extended hydrogen corona of Mars, were observed at altitudes ≤7500 km. The cyclotron instability of these pickup ions appears to generate Alfven waves observed by the MAGMA magnetometer. Analysis of the plasma data shows that weak pitch‐angle diffusion of a ring‐distribution of pickup protons occurs. The altitude profiles of pickup proton fluxes and number densities of the parent hydrogen atoms are

ISSN:0094-8276    

DOI:10.1029/91GL02082

年代:1991

数据来源: WILEY

摘要:

Recent observations at comets Giacobini‐Zinner and Halley suggest that simple non‐reacting gas dynamics or MHD is an inappropriate description for the bow shock. The thickness of the observed (sub)shock implies that mass‐loading is an important dynamical process within the shock itself, thereby requiring that the Rankine‐Hugoniot conditions possess source terms. This leads to shocks with properties similar to those of combustion shocks. We consider parallel magnetized shocks subjected to mass‐loading, describe some properties which distinguish them from classical MHD parallel shocks, and establish the existence of a new kind of MHD compound shock. These results will be of importance both to observations and numerical simulations of the comet‐solar wind

ISSN:0094-8276    

DOI:10.1029/91GL02242

年代:1991

数据来源: WILEY

摘要:

We use a newly developed 2‐D electrostatic code with particle ions and Boltzmann electrons to investigate the details of the ion/ion acoustic instability and the structure of electrostatic shocks. The ion/ion acoustic instability is thought to be operative in various regions of space plasmas, such as the plasma sheet boundary layer or the bow shock. Electrostatic shocks have also been hypothesized to exist in a variety of physical settings, such as the inner cometary coma or the auroral region. The simulation results show that for the parameters relevant to the plasma sheet boundary layer the saturation mechanism of the ion/ion acoustic instability is ion trapping. We also show that the 2‐D structure of electrostatic shocks is considerably different from that suggested by previous 1‐D simulations. The main reason for this difference is the presence of shock reflected ions, which through the ion/ion acoustic instability lead to the generation of large amplitude waves in the upstream region propagating obliquely to the shock normal. These waves play an important role in the shock dissipation pr

ISSN:0094-8276    

DOI:10.1029/91GL02241

年代:1991

数据来源: WILEY

摘要:

One‐dimensional hybrid simulations of quasi‐parallel collisonless shocks have been performed. Alpha particles are included in the simulations self‐consistently in such a way that sufficient simulation particles are present to give reasonable statistics. It is found that a fraction of the incoming solar wind protons and alpha particles is accelerated near the shock and escapes upstream. These particles constitute a diffuse upstream particle population. The density ratio bias factor, i.e., the density ratio of diffuse alpha particles to diffuse protons normalized to the solar wind He++to H+density ratio varies between 0.5 and 4.0, and depends on the solar wind He++to H+density ratio and on the shock Mach number. The density ratio bias factor for a low Mach number shock (MA= 4.6) isf∼ 0.5 when the alpha particles are treated as test particles and increases to ∼1 when the solar wind ratio of alpha to protons is 0.25. For a high Mach number shock ofMA= 8.9 with a very small alpha particle contribution to the solar wind we obtained a density ratio bias fact

ISSN:0094-8276    

DOI:10.1029/91GL02084

年代:1991

数据来源: WILEY

摘要:

A statistical analysis of 351 independent bow shock crossings and 233 independent magnetopause crossings by the ISEE‐I spacecraft from 1977 to 1980 was performed to determine the average positions and shapes of the bow shock and magnetopause. The standoff distance between the magnetopause and the bow shock depends on the compressibility of the plasma which in the ‘polytropic’ approximation is related to the ratio of specific heats, γ. Standoff distances for the bow shock and magnetopause were found to be 13.7RE(±0.2RE) and 10.3RE(±0.3RE) respectively. These distances are smaller than those observed during earlier epochs. The observed thickness of the magnetosheath is that expected for the compression of a gas whose polytropic index, γ, is 1.76±0.15. This value, representative of the entire magnetosheath, is consistent with the value of 1.67 deduced from the behavior of the plasma across individual shock transitions. A value of 1.67 is expected for an adiabatic process in a collisional, monatomic gas with three degrees of freedom with lower values for non‐adiabatic processes and higher values for anisotropic heating at the shock. The observed value of 1.76 indicates that heat flux does not much affect the position of the shock while the downstream anisotropy may have a

ISSN:0094-8276    

DOI:10.1029/91GL02090

年代:1991

数据来源: WILEY

摘要:

A theoretical analysis of the stability of a quasi‐neutral sheet with small normal magnetic componentBnis presented. It is demonstrated that contrary to the statement of Pellat et al. [1991], pitch angle diffusion (PAD) can remove the stabilizing effect ofBn. The PAD results in the escape of electrons to the region of negligibly small wave—particle interactions which decreases the electron contribution to the energy balance equation. In the limit of strong PAD the ion tearing mode growth rate gives the universal upper estimate of the rate of collisionless spontaneous reconnect

ISSN:0094-8276    

DOI:10.1029/91GL02245

年代:1991

数据来源: WILEY

摘要:

Controversy continues unabated concerning the detection of signatures of merging at low altitudes. One phenomenon required by nearly all models of merging is the acceleration of ions in the merging region by about the Alfvén velocity. We report the observation of spatially discrete patches of precipitating accelerated ions at the equatorward edge of the cusp. These regions of accelerated bulk flow velocity, which occur at or near the transition between boundary layer and cusp proper, are spatially distinct from the overall energy‐latitude dispersion in the main cusp and are best fitted by Maxwellian distributions with convection velocities of 450‐640 km/s; such velocities are 200–300 km/s faster than in the main cusp ion distribution. The acceleration cannot be attributed to electrostatic potentials, for the electron distributions in the patchy ion regions are also accelerated. Based on an initial survey of 13 such events, occurrence of distinctly separate accelerated ion patches appears to be primarily or exclusively a southwardBzphen

ISSN:0094-8276    

DOI:10.1029/91GL02088

年代:1991

数据来源: WILEY

摘要:

Particle and field data from a pass of the AUREOL‐3 satellite through the polar cusp, several minutes after the southward turning of the IMF, are analyzed in detail. Superposed on the classical cusp, characterized by the typical ion and electron precipitations, several very narrow arcs are detected where large fluxes of electrons and ions, accelerated to 2–4 keV, precipitate simultaneously. These localized arcs correspond to the upward current sheets of a succession in latitude of narrow, alternatively upward and downward field‐aligned current sheets. The data suggest that the satellite has crossed the ionospheric footprints of 2 adjacent flux transfer events separated by 100–150 km in latitude. Electric spikes and electromagnetic turbulence are typically associated with the region of downward c

ISSN:0094-8276    

DOI:10.1029/91GL02087

年代:1991

数据来源: WILEY