摘要:

The main features of 2‐ to 3‐kHz heliospheric emissions are analyzed in the context of a general heliospheric “breathing” as inferred from the Voyager 2 solar wind average ram pressure data. Following McNutt (1988) we suggest triggers for the three 3‐kHz emission events seen to date, and we obtain good agreement in timing and expected 2fnof;p(postshock) frequency for termination shock distances ∼90 AU. This determination seems to correspond to epochs when the heliosphere is larger than on the average. We suggest that the visibility of the individual 3‐kHz events and their observed upward frequency drift are enhanced when the postulated global heliospheric expansion results in formation of a transient, compressed external plasma barrier around the heliopause that prevents for several months the escape of the radiation. We suggest that the 2‐kHz emission band is due to typical solar wind density maxima interacting with the termination shock and that the disappearance of that emission after mid‐1987 is caused by the 2fnof;p(postshock) frequency moving into the 2.4‐kHz spacecraft interference band during a period of closer shock distance corresponding to years of lower solar wind ram pressure. From all the events considered, we estimate the average termination shock distance to be in

ISSN:0148-0227    

DOI:10.1029/92JA02395

年代:1993

数据来源: WILEY

摘要:

Magnetometer data obtained over the course of the previous solar cycle by the Pioneer Venus orbiter (PVO) at ∼ 0.7 AU and IMP 8 at 1.0 AU are used to compare the long‐term behavior of the interplanetary magnetic field (IMF) at these two heliocentric distances. Similarities include an enhancement in the typical or median field magnitude during the declining phase of the solar cycle as compared to solar maximum or minimum, slight decreases in the Parker spiral angle from the declining phase through solar minimum, similar trends in the Alfvénic and magnetosonic Mach numbers, and the remarkably consistent sector structure noted previously. Differences include the temporal behavior of the high‐field tail of the field distribution, showing that high fields are most frequently observed during solar maximum at the Earth but during the declining phase of activity at Venus. This latter feature suggests that the perceived occurrence history of large fields from transient disturbances such as coronal mass ejections is a sensitive function of position within the helio

ISSN:0148-0227    

DOI:10.1029/92JA02235

年代:1993

数据来源: WILEY

摘要:

We discuss properties of solar energetic particle (SEP) events observed by both Helios spacecraft in the time period March 1976 to March 1980, in particular variations of the intensity time profiles with angular distance between the flare and the observer's magnetic footpoint. Special attention is paid to the neutral lines of the large‐scale coronal magnetic field. For individual events we will show that sector boundaries can have a strong influence on intensities and time‐scales of SEP events. This can be seen best in events in which both spacecraft are connected more or less symmetrically around the flare site with a sector boundary between them. In a statistical approach we discuss variations of onset and maximum times as well as maximum intensities with angular distance between the flare and the observer's magnetic footpoint for 39 SEP events. The use of two spaceprobes has the advantage that unknown flare parameters, such as the number of injected particles and the time of acceleration, can be eliminated by studying only relative variations of time and intensity with angular distance. These variations were analyzed under consideration of the sector boundaries of the large‐scale coronal magnetic field. We find (1) particles can be observed on both sides of sector boundaries during both, impulsive and gradual, events, (2) the onset times of ∼0.5‐MeV electrons can be ordered by the occurrence of sector boundaries, inside the flare sector we can define some kind of “azimuthal propagation velocity” Δϕ/Δtonswith ≥4°/min, outside the flare sector this velocity is ≤1°/min, (3) the azimuthal propagation of ∼20‐MeV protons seems to be systematically slower, (4) there seems to be no difference in the variations of onset times or maximum intensities with angular distance between impulsive and gradual events, and (5) within the broad scatter the variation of the maximum intensity with angular distance is not systematically influenced by the crossing of sector boundaries, despite the obvious influence of sector boundary crossings in individual events. These observations support the association of the fast propagation region (FPR) with the large‐scale polarity cells of the coronal magnetic field. The “transport” inside the FPR can be understood in terms of an open cone of propagation. The “transport” outside the FPR can be interpreted in terms of a propagation

ISSN:0148-0227    

DOI:10.1029/92JA02778

年代:1993

数据来源: WILEY

摘要:

Shapes of proton and alpha particle velocity distribution functions in the high‐speed solar wind are examined for the time period from 1973 to 1987. Whereas enhanced high‐energy extensions are a near universal feature of the proton distributions, no such enhancements are observed in alpha particle distributions. Such distributions are examined and documented in detail. An interpretation of this observation is presented in terms of a two‐component model of coronal heating and acceleration. We suggest that a modest amount of heating results from the damping of waves, whereas additional heating and the bulk of the acceleration is associated with the direct conversion of magnetic energy flux to plasma convection and enthalpy flux through the process of magnetic reconnection. This model predicts that the ambient flow of plasma from coronal holes is characterized by a low‐speed and a low‐mass flux. In particular, the proton flux is insufficient to drag alpha particles out of the solar gravitational potential well. This should lead to a considerable enhancement of the helium abundance close to the Sun. Plasma jets rich in helium and accelerated by reconnection close to the Sun buffet the ambient plasma from below. The slow‐moving ambient plasma is coupled to the faster‐moving jets by Coulomb friction close to the Sun, and then through microinstabilities far from the Sun. The net effect of this interaction is a high‐speed, double‐streaming proton population and a single component, but faster alpha particle flow at large helio

ISSN:0148-0227    

DOI:10.1029/92JA02260

年代:1993

数据来源: WILEY

摘要:

We have surveyed the risetimes of allE>15 MeV solar energetic proton (SEP) events observed on the NOAA GOES spacecraft between 1984 and 1989 and associated with well‐connected (W20‐W70) solar flares. Twelve events occurred during the times of observations by the coronagraph on the SMM spacecraft, and each event was associated with a coronal mass ejection (CME) observed above the west limb. In each of the three SEP events with the longest flux risetimes a second CME was observed before the time of peak flux. While all the initial CMEs were associated with prominent solar flares, the subsequent CMEs were not accompanied by obvious X ray or radio bursts. We interpret the long risetimes of two of the three SEP events in terms of a second SEP injection due to the second

ISSN:0148-0227    

DOI:10.1029/92JA02605

年代:1993

数据来源: WILEY

摘要:

The measurements carried out by the plasma spectrometer ASPERA, on‐board the Phobos 2 spacecraft show that the Martian bow shock is characterized by a sudden increase of ionization of the neutral corona. It acts as a source of new ions that can strongly modify the process of ion heating behind the shock front. The loss of momentum of solar wind protons due to their interaction with exospheric ions may lead to an increase in the effective scale of the obstacl

ISSN:0148-0227    

DOI:10.1029/92JA02374

年代:1993

数据来源: WILEY

摘要:

We examine the evolution with heliocentric distance of the amplitude of fluctuations in the interplanetary magnetic and velocity fields assuming a model of homogeneous, steady state turbulence. Based on observations and recent results, both the Alfvén ratio and the normalized cross helicity are taken to be constant compared to other variations, and the turbulence is assumed to be nearly incompressive. The fluctuation amplitudes are found by taking the heating of the plasma by the fluctuations to be negligible; quasi‐steady state turbulence with damping balanced by large‐scale energy input will lead to the same conclusions for inertial range fluctuations. While the assumptions of this model contrast sharply with those for purely Alfvénic fluctuations, we find that the radial dependence of the amplitude of the fluctuations for reasonable parameters is very nearly that found from both WKB analysis and recent turbulence modeling. The robustness of this result suggests why some predictions of WKB theory are apparently correct in solar wind conditions where the theory is not expected to be

ISSN:0148-0227    

DOI:10.1029/92JA02398

年代:1993

数据来源: WILEY

摘要:

Highly structured bursts of Langmuir waves produced by energetic electrons ejected from a solar flare have been observed using wideband plasma wave measurements on the Galileo spacecraft. The wideband sampling system on Galileo provides digital electric field waveforms at sampling rates up to 201,600 samples s−1, much higher than any previous instrument of this type. The solar flare of interest occurred on December 10, 1990, while the spacecraft was at a radial distance of 0.98 AU from the Sun. This flare emitted a stream of energetic electrons and an associated type III radio event, both of which were detected by Galileo. Starting about 1 hour after the onset of the flare, a large number of intense Langmuir wave bursts were detected near the local electron plasma frequency, which was about 25 kHz. The Langmuir wave bursts, which lasted about 1.5 hours, coincided with the arrival of the solar electrons. The bursts are highly structured and consist mainly of isolated wave packets with durations as short as 1 ms and beat‐type waveforms with beat frequencies ranging from 200 to 500 Hz. The peak electric field strengths are about 1.7 mV m−1. The highly structured envelopes of these waves are strongly suggestive of nonlinear parametric decay processes such as those predicted by various theories dealing with the saturation of beam‐driven electrostatic instabilities. However, the intensities are too low for strong turbulence effects to be im

ISSN:0148-0227    

DOI:10.1029/92JA02838

年代:1993

数据来源: WILEY

摘要:

It is demonstrated that the outer corona extends radially in the form of a sheet from the neutral line on the source surface (a spherical surface of 2.5 solar radii) or from an even lower level during the period of sunspot maximum, as well as during the period of sunspot minimum. During the period of sunspot maximum the neutral line lies nearly at right angles to the heliographic equator. Assuming that the long coronal streamers extend from the neutral line (as is known to be the case during the period of sunspot minimum), the expected geometry is constructed and is compared with coronal photographs taken during the eclipse. It resembles the observed coronal photographs seen from the Earth on July 11, 1991. In particular, the expected geometry can explain details of the observed features, including highly inclined (with respect to the heliographic equator) long coronal streamers and polar plumelike features. It is also explained why in the past the corona was believed to be nearly spherical during the period of sunspot maximum.

ISSN:0148-0227    

DOI:10.1029/92JA02402

年代:1993

数据来源: WILEY

摘要:

A new two‐fluid model which describes mass loading in the solar wind (e.g., the interaction of the solar wind with a cometary coma or the local interstellar medium) is presented. The self‐consistent back‐reaction of the mass‐loaded ions is included through their effective scattering in low‐frequency MHD turbulence and the invocation of a diffusive approximation. Such an approximation has the advantage of introducing self‐consistent dissipation coefficients into the governing equations, thereby facilitating the investigation of the internal structure of shocks in mass‐loading environments. To illustrate the utility of the new model, we consider the structure of cometary shocks in the hypersonic one‐dimensional limit, finding that the incoming solar wind is slowed by both mass loading and the development of a large cometary ion pressure gradient. The shock is broadened and smoothed by the cometary ions with a thickness of the order of the cometary ion

ISSN:0148-0227    

DOI:10.1029/92JA02896

年代:1993

数据来源: WILEY