摘要:

This paper addresses the collisionless damping of whistlers observed as precursors standing upstream of oblique, low‐Mach number terrestrial bow shocks. The linear theory of electromagnetic waves in a homogeneous Vlasov plasma with Maxwellian distribution functions and a magnetic field is considered. Numerical solutions of the full dispersion equation are presented for whistlers propagating at an arbitrary angle with respect to the magnetic field. It is demonstrated that electron Landau damping attenuates oblique whistlers and that the parameter which determines this damping is βe. In a well‐defined range of parameters, this theory provides damping lengths which are the same order of magnitude as those observed. Thus electron Landau damping is a plausible process in the dissipation of upstream whistlers. Nonlinear plasma processes which may contribute to precursor damping are also discussed, and criteria for distinguishing among these are descr

ISSN:0148-0227    

DOI:10.1029/JA090iA01p00099

年代:1985

数据来源: WILEY

摘要:

In this study the propagation and growth of whistler mode waves generated by electron beams within the earth's bow shock are investigated using a planar model for the bow shock and a model electron distribution function. Within the shock, the model electron distribution function possesses a field‐alignedT⊥>T∥beam that is directed toward the magnetosheath. Waves with frequencies between about 1 and 100 Hz with a wide range of wave normal angles are generated by the beam via Landau and anomalous cyclotron resonances. However, because the growth rate is small and because the wave packets traverse the shock quickly, these waves do not attain large amplitudes. Waves with frequencies between about 30 and 150 Hz with a wide range of wave normal angles are generated by the beam via the normal cyclotron resonance. The ray paths for most of these waves are directed toward the solar wind, although some wave packets, because of plasma convection, travel transverse to the shock normal. These wave packets grow to large amplitudes, because they spend a long time in the growth region. The results suggest that whistler mode noise within the shock should increase in amplitude with increasing upstream θBn. The study provides an explanation for the origin of much of the whistler mode turbulence observed at the bow

ISSN:0148-0227    

DOI:10.1029/JA090iA01p00105

年代:1985

数据来源: WILEY

摘要:

The effects of a finite electron pressure on quasi‐parallel collisionless shocks are examined by means of numerical simulations. The simulation is performed using a nonperiodic hybrid code in which the ion dynamics are followed exactly and the electrons are treated as a massless fluid of finite pressure. The results of the simulations show that the electron pressure significantly improves the stationarity of the shock structure. The finite electron pressure also leads to an electrostatic potential across the shock as expected. The potential structure is localized and stable at a low‐Alfvén Mach number (MA= 2), but for a higher Mach number (MA= 4) it is nonlocal and unstable. In either case the change in potential energy is on the order of the downstream electron parallel thermal energy. Power spectral analysis of the upstream waves shows that they are right‐hand polarized whistlers phase standing in the shock

ISSN:0148-0227    

DOI:10.1029/JA090iA01p00115

年代:1985

数据来源: WILEY

摘要:

Heating at collisionless shocks due to the kinetic cross‐field streaming instability, which is the finite beta (ratio of plasma to magnetic pressure) extension of the modified two stream instability, is studied. Heating rates are derived from quasi‐linear theory and compared with results from particle simulations to show that electron heating relative to ion heating and heating parallel to the magnetic field relative to perpendicular heating for both the electrons and ions increase with beta. The simulations suggest that electron dynamics determine the saturation level of the instability, which is manifested by the formation of a flattop electron distribution parallel to the magnetic field. As a result, both the saturation levels of the fluctuations and the heating rates decrease sharply with beta. Applications of these results to plasma heating in simulations of shocks and the earth's bow shock are descri

ISSN:0148-0227    

DOI:10.1029/JA090iA01p00123

年代:1985

数据来源: WILEY

摘要:

The evolution of the ion and electron distribution functions across a set of 10 low‐Mach number, nominally subcritical, quasi‐perpendicular shocks is examined in high time resolution (full distribution every 3 s) using data from the ISEE 1 and 2 spacecraft. Both ions and electrons sometimes show slight preheating upstream of the shock, but otherwise the ion and electron temperatures rise together in the magnetic ramp and show no further increase downstream. Contrary to the usual assumption based on early laboratory and theoretical work that at subcritical shocks the bulk of the energy dissipation occurs as resistive heating of the electrons, it is found that the ion temperature increase exceeds that of the electrons. This difference is attributed to the distinction between dispersive shocks, such as those studied here, and resistive shocks, such as those observed in most laboratory studies. The increase in ion temperature is predominantly in the perpendicular direction and is due to heating of the entire distribution rather than to the formation of a high‐energy tail. The perpendicular temperature increase is typically a factor of 10–20, much greater than the usual assumption of adiabatic heating. The downstream to upstream ratio of perpendicular electron temperature is equal to the magnetic field ratio (∼2–2.5). The electrons also show significant heating in the parallel direction, with the downstreamT∥/T⊥∼1–1.2. The downstream electron distribution exhibits the characteristic flattop seen downstream of supercritical shocks, and there is evidence for the field‐aligned electron beam identified previously within those shocks. As previously reported, the downstream ion and electron total temperatures are nearly equal. These observations are interpreted as evidence for the simultaneous operation of several plasma instabilities, including the modified two‐stream instability, driven by the cross‐field current within the shock, and the ion acoustic instability, driven by the fi

ISSN:0148-0227    

DOI:10.1029/JA090iA01p00137

年代:1985

数据来源: WILEY

摘要:

The distribution of the shock normal angles, θBn, can be calculated for all the heliocentric distances when the distributions of andare available, where θBnis the angle between the upstream magnetic field and the shock normal. The average shock normal is assumed to propagate approximately in the radial direction. However, the distribution of the angles, θnr′between the shock normals and radial direction is assumed to follow a Rayleigh distribution. The distribution of is obtained by observations by Helios 1 and 2, Voyager 1, and Pioneer 10 spacecraft. The distribution of each component ofis assumed to follow a Gaussian. Our results show that even very close to the sun, the probability of observation of parallel and quasi‐parallel shocks is still smaller compared to the observation of quasi‐perpendicular and perpendicular shocks. Therefore it is concluded that the parallel and quasi‐parallel shocks do not easily form in the solar wind because of the fluctuating character of andand not because of any other physical mechanism. The observed distributions of θBnusing Helios 1 and 2 at heliocentric distances between 0.3 to 0.5 AU, 0.5 to 0.75 AU, and 0.75 to 1.0 AU are compared with our calculated distributions. The agreement is good, with the agreement at 0.75 to 1.0 AU being the best. The comparison is also made with the results from ISEE 3 observations at 1 AU. Our calculated distribution agrees very well with these o

ISSN:0148-0227    

DOI:10.1029/JA090iA01p00149

年代:1985

数据来源: WILEY

摘要:

Near‐sun spacecraft Doppler scintillation observations have been combined with Solwind coronagraph and Helios 1 plasma measurements to provide more definitive measurements of the evolution and propagation of interplanetary shock waves between the sun and earth orbit than have been available from previous observations. This study shows that substantial deceleration of fast shocks (shock speeds exceeding 1000 km s−1) takes place near the sun and that the amount of deceleration increases with shock speed. This is consistent with the significantly lower and rather narrow range of shock velocities observed by direct spacecraft near earth orbit. When coronal mass ejection (CME) speeds are available for the fast shocks, they are considerably lower than the speeds measured farther out but near the sun. This implies that either the fast shocks first accelerate before decelerating on their way out from the sun (assuming the CME front is identified with the shock) or the CME speeds do not represent and substantially underestimate the shock speeds in the outer corona. If the CME speeds underestimate the shock speeds of the fast shocks, they do not appear to do so for the slow shocks. If the shocks are being driven over distances indicated by the acceleration region or to the point where deceleration begins, then their velocity profiles imply that the slower shocks are being driven farther out than the faster shocks. The analysis of one piston‐driven shock shows that the velocity of the contact surface is about 0.58 that of the shock front vel

ISSN:0148-0227    

DOI:10.1029/JA090iA01p00154

年代:1985

数据来源: WILEY

摘要:

A comparison between Solwind observations of coronal mass ejections (CME's) and Helios 1 observations of interplanetary shocks during 1979–1982 indicates that 72% of the shocks were associated with large, low‐latitude mass ejections on the nearby limb. Most of the associated CME's had speeds in excess of 500 km/s, but some of them had speeds in the range 200–400 km/s. An additional 26% of the shocks may have been associated with CME's, but we were less confident of these associations because the sizes and locations of the CME's did not seem appreciably different from those of the numerous CME's without Helios shocks. Only 2% of the shocks clearly lacked CME's. As the average level of sunspot activity declined during 1982, the shock frequency also declined, but the observed shocks and some of their associated CME's had unusually high speeds well in excess of 1000

ISSN:0148-0227    

DOI:10.1029/JA090iA01p00163

年代:1985

数据来源: WILEY

摘要:

We compare fast (υ ≥ 500 km s−1) coronal mass ejections (CME's) with reported metric type II bursts to study the properties of CME's associated with coronal shocks. We confirm an earlier report of fast frontside CME's with no associated metric type II bursts and calculate that 33±15% of all fast frontside CME's are not associated with such bursts. Faster CME's are more likely to be associated with type II bursts, as expected from the hypothesis of piston‐driven shocks. However, CME brightness and associated peak 3‐cm burst intensity are also important factors, as might be inferred from the Wagner and MacQueen (1983) view of type II shocks decoupled from associated CME's. We use the equal visibility of solar frontside and backside CME's to deduce the observability of backside type II bursts. We calculate that 23±7% of all backside type II bursts associated with fast CME's can be observed at the earth and that 13±4% of all type II bursts originate in backside flares. CME speed again is the most important factor in the observability of backside typ

ISSN:0148-0227    

DOI:10.1029/JA090iA01p00177

年代:1985

数据来源: WILEY

摘要:

The solar flare‐initiated shock can be considered to be an initially driven shock that converts into a blast wave as it propagates through the interplanetary medium. In our simplified shock modeling, to estimate the time of the shock arrival at a specified position in space we assume that the shock is initially driven from the flare position at the velocity indicated by the type II solar radio burst. We assume the shock is driven until the initiating solar flare energy is somewhat expended. After this initially driven phase, the shock front then propagates with the characteristic speed expected of the shock front of a blast wave. In the interplanetary medium with its 1/r² density dependence, the blast wave shock front speed should be proportional tor−0.5. We find that thisr−0.5speed dependence is a general characteristic of the shock front speed when the speeds are determined in the solar wind reference frame. The blast wave “rides over” the preexisting solar wind so that the disturbance speed of the shock front at any instant of time is the speed of the shock front added vectorially to the solar wind speed. Application of these principles enables a consistent “timing” of solar flare‐initiated shock waves that is event independent. We show that this concept is consistent with the time position profile derived from the analysis of kilometric type II events that are associated with inter

ISSN:0148-0227    

DOI:10.1029/JA090iA01p00183

年代:1985

数据来源: WILEY