摘要:

Explosive events are highly energetic, small‐scale phenomena which are frequently detected throughout the quiet and active Sun. They are seen in profiles of spectral lines formed at transition zone temperatures as exceptionally Doppler‐shifted features, typically at 100 km s−1to the red and/or blue of the rest wavelength. Sufficient observational evidence has now been developed to demonstrate that some explosive events are associated with the emergence of new magnetic flux. In these cases it is likely that the acceleration of plasma is caused by the magnetic reconnection resulting from flux emergence. We take as a working hypothesis the proposal that all explosive events are the result of magnetic reconnection. Since explosive events tend to occur on the edges of high photospheric magnetic field regions, we identify them with reconnection that occurs during the cancellation of photospheric magnetic flux (Martin, 1984; Livi et al., 1985). The combined observational characteristics of photospheric flux cancellation and transition zone explosive events provide powerful diagnostic information concerning the nature of magnetic reconnection. Reconnection in the quiet solar atmosphere apparently proceeds in bursts at sites much smaller than the boundary between opposite polarity flux elements that are observed to cancel in magnetograph sequences. Equating the velocity of the expelled transition zone plasma with the Alfvén speed yields magnetic field strengths of 20 G at the site of reconnection. The speed at which the reconnection proceeds is commensurate with the rapid rates predicted by Petschek

ISSN:0148-0227    

DOI:10.1029/90JA02572

年代:1991

数据来源: WILEY

摘要:

ISEE 3 field and plasma data are used to investigate the frequency of occurrence of isolated, large‐amplitude rotational (RD) and tangential (TD) discontinuities in different types of solar wind flow. It is found that there are relatively more TDs in solar wind that originates in closed field regions and is ejected into interplanetary space by coronal transients than in the solar wind that originates in open field regions. An exception is the plasma in bidirectional electron streaming events which has very few discontinuities of any type. The speed of the wind from open field regions is approximately linearly related to the number of RDs per hour; such a relation does not exist for the wind associated with coronal mass ejections. These results are consistent with the hypothesis that the convection‐driven shuffling of magnetic foot points at the solar surface leads to TDs, magnetic reconnection, and heating of the corona on closed field lines as suggested by Parker, while in the open field regions the disturbances created by the shuffling are carried off by waves which contribute to the acceleration of the solar wind. It is also demonstrated that when the proton β is low, the plasma cannot support either type of discontin

ISSN:0148-0227    

DOI:10.1029/91JA00566

年代:1991

数据来源: WILEY

摘要:

Magnetic clouds are large (<0.25 AU) interplanetary regions with topologies consistent with those of magnetic loops. They are of interest because they may be an interplanetary signature of coronal mass ejections. Clouds have been identified in solar wind data by their magnetic properties and by the presence of bidirectional particle fluxes. Two possible closed magnetic topologies have been considered for clouds: (1) an elongated bottle with field lines rooted at both ends in the Sun and (2) a detached magnetic bubble or plasmoid consisting of closed field lines. The inferred topologies are also consistent with open field lines that converge beyond 1 AU. We have used solar energetic particles (SEPs) as probes of the cloud topologies. The rapid access of SEPs to the interiors of many clouds indicates that the cloud field lines extend back to the Sun and hence are not plasmoids. The small modulation of galactic cosmic rays associated with clouds also suggests that the magnetic fields of clouds are not closed.

ISSN:0148-0227    

DOI:10.1029/91JA00659

年代:1991

数据来源: WILEY

摘要:

A large interplanetary magnetic cloud has been observed in the mid‐December 1982 data from ISEE 3. It is estimated to have a heliocentric radial extent of ≳0.4 AU, making it one of the largest magnetic clouds yet observed at 1 AU. The magnetic field measured throughout the main portion of the cloud was fairly tightly confined to a plane as it changed direction by 174° while varying only moderately in magnitude. Throughout nearly the entire duration of the cloud's passage, IMP 8 was located in the Earth's dawn magnetosheath providing observations of this cloud's interaction with the bow shock and magnetopause; the cloud is shown to maintain its solar wind characteristics during the interaction. Near the end of the cloud passage, at 0806 UT on December 17, ISEE 3 (and IMP 8 at nearly the same time) observed an oblique fast forward interplanetary shock closely coincident in time with a geomagnetic storm sudden commencement. The shock, moving much faster than the cloud (radial speeds of 700 and 390 km/s, respectively, on the average), was in the process of overtaking the cloud. The indexDstdecreased monotonically by ≈130 nT during the 2‐day cloud passage by the Earth and was well correlated with theBzcomponent of the interplanetary magnetic field. There was no significant decrease in the cosmic ray intensity recorded by ground‐based neutron monitors at this time of rather strong, smoothly changing fields. However, a Forbush decrease did occur immediately after the interplanetary shock, during a period of significant field turbulence. Thus a large, smooth, interplanetary helical magnetic field configuration engulfing the Earth does not necessarily deflect cosmic rays sufficiently to cause a Forbush decrease, but there is a suggestion that such a decrease may be caused by particle scattering by turbulent magnetic fields. Finally, the field observations within the cloud are reasonably well described by a model developed by Burl

ISSN:0148-0227    

DOI:10.1029/91JA00670

年代:1991

数据来源: WILEY

摘要:

The one‐dimensional hydrodynamics of flows subjected to mass loading are considered anew, with particular emphasis placed on determining the properties of mass‐loading shocks. This work has been motivated by recent observations of the outbound Halley bow shock (Neubauer et al., 1990), which cannot be understood in terms of simple hydrodynamical or magnetohydrodynamical descriptions. By including mass injection at the shock, we have investigated the properties of the Rankine‐Hugoniot conditions on the basis of a geometric formulation of the entropy condition. Such a condition, which is more powerful than the usual thermodynamical formulation, serves to determine those solutions to the Rankine‐Hugoniot conditions which correspond to a physically realizable downstream state. On this basis a concise theoretical description of hydrodynamic mass‐loading shocks is obtained. We show that mass‐loading shocks have more in common with combustion shocks than with ordinary nonreacting gas dynamical shocks. It is shown that for decelerated solutions to the Rankine‐Hugoniot conditions to exist, the upstream flow speedu0must satisfyu0>ucrit>cs, wherecsis the sound speed. Besides the usual supersonic‐subsonic transition, mass‐loading fronts can also admit a decelerating supersonic‐supersonic transition, the structure of which consists of a sharp decrease in the flow velocity preceding a recovery and an increase in the final downstream flow speed. We suggest the possibility that such structures may describe the inbound Halley bow shock (Coates et al., 1987a). Both parallel and oblique shocks are considered, the primary difference being that oblique shocks are subjected to a shearing stress due to mass loading. It is conjectured that such a shearing may de

ISSN:0148-0227    

DOI:10.1029/91JA00616

年代:1991

数据来源: WILEY

摘要:

Linear and nonlinear electron damping of the whistler precursor wave train to low Mach number quasi‐perpendicular oblique shocks is studied using a one‐dimensional electromagnetic plasma simulation code with particle electrons and ions. In some parameter regimes, electrons are observed to trap along the magnetic field lines in the potential of the whistler precursor wave train. This trapping can lead to significant electron heating in front of the shock for low βe(∼10% or less). Use of the 64‐processor Caltech/JPL Mark IIIfp hypercube concurrent computer has enabled us to make long runs using realistic mass ratios (mi/me= 1600) in the full particle in‐cell code and thus simulate shock parameter regimes and phenomena not previously studied n

ISSN:0148-0227    

DOI:10.1029/91JA00655

年代:1991

数据来源: WILEY

摘要:

This paper compares calculated and measured energy spectra of implanted H+and O+ions on the assumption that the pickup geometry is quasi‐parallel and about 1% of the waves generated by the cometary pickup process propagate backward (toward the comet). The model provides a good description of the implanted O+and H+energy distribution near the pickup energies. The thickness of the implanted ion velocity distribution shells was nearly constant between 2.50×106km and 1.20×106km (just outside the shock) along the inbound Giotto trajectory. The explanation is that the velocity diffusion coefficient and characteristic diffusion time vary approximately as 1/randr, respectively, and therefore their product (which determines the velocity shell thickness) remains nearly const

ISSN:0148-0227    

DOI:10.1029/90JA02750

年代:1991

数据来源: WILEY

摘要:

The properties of low frequency magnetic fluctuations generated by cometary ion pickup are examined by means of one‐dimensional hybrid simulations, in which newborn ions are created at a constant rate. The helicity and direction of propagation of magnetic fluctuations are investigated for various cometary ion injection angles, α, relative to the solar wind magnetic field. The parameter η represents the relative contribution of wave energy density propagating in the direction away from the comet, parallel to the beam. For small (quasi‐parallel) injection angles, α ∼ 0° and η is of order unity, while for larger (quasi‐perpendicular) angles, α ∼ 90° and η is about 0.5. At intermediate angles, α ∼ 60°, η can vary between 0 and 1, depending on the wave number. The wave properties are consistent with the instabilities (right‐hand cyclotron resonant modes at small α, left‐hand Alfven/ion cyclotron modes at large a) expected from linear theory. Consequences of these results for particle acce

ISSN:0148-0227    

DOI:10.1029/91JA00158

年代:1991

数据来源: WILEY

摘要:

The inductive interaction between a conducting body and a magnetized plasma in relative motion is formulated in terms of a source driving stationary magnetohydrodynamic waves in the frame of the body. Of particular interest is the existence of analytic solutions which describe the stationary wave pattern associated with a point source current. Such solutions are significant because not only do they, in principle, facilitate the construction of more general solutions by serving as Green's functions but also they provide an asymptotic description of the far field of the stationary wave pattern generated from each point of a source of finite size. The total pressure perturbation exhibits a Laplacian type behavior in the case of “subsonic” flow and a Mach cone like disturbance for “supersonic” flow. On the other hand, the associated electric potential response is mixed in nature in that it consists of compressive magnetoacoustic perturbations as well as the well‐known Alfvén wings corresponding to the shear Alfvén mode. In the subsonic case it is shown that although there is a strong disturbance in the potential on the Alfvén wings it is not narrowly confined to these wings by virtue of the two‐dimensional dipolelike contributions from the fast magnetoacoustic mode. In supersonic flow the potential disturbance is swept downstream and confined to the fast Mach cone, on which it is singular and within which lies the other singularity on th

ISSN:0148-0227    

DOI:10.1029/91JA00046

年代:1991

数据来源: WILEY

摘要:

On April 30 (day 120), 1985, the magnetosphere was compressed at 0923 UT and the subsolar magnetopause remained near 7REgeocentric for ∼2 hours, during which the four spacecraft Spacecraft Charging At High Altitude (SCATHA), GOES 5, GOES 6, and Active Magnetospheric Particle Tracer Explorers (AMPTE) CCE were all in the magnetosphere on the morning side. SCATHA was in the low‐latitude boundary layer (LLBL) in the second half of this period. The interplanetary magnetic field was inferred to be northward from the characteristics of precipitating particle fluxes as observed by the low‐altitude satellite Defense Meteorological Satellite Program (DMSP) F7 and also from absence of substorms. We used magnetic field and particle data from this unique interval to study ULF waves in the LLBL and their relationship to magnetic pulsations in the magnetosphere. The LLBL was identified from the properties of particles, including bidirectional field‐aligned electron beams at ∼200 eV. In the boundary layer the magnetic field exhibited both a 5–10 min irregular compressional oscillation and a broadband (Δƒ/ƒ ∼ 1) primarily transverse oscillations with a mean period of ∼50 s and a left‐hand sense of polarization about the mean field. The former can be observed by other satellites and is likely due to pressure variations in the solar wind, while the latter is likely due to a Kelvin‐Helmholtz (K.‐H.) instability occurring in the LLBL or on the magnetopause. Also, a strongly transverse ∼3‐s oscillation was observed in the LLBL. The magnetospheric pulsations, which exhibited position dependent frequencies, may be explained in terms of field line resonance with a broadband source wave, that is, either the pressure‐induced compressional wave or the K.‐H. wave generat

ISSN:0148-0227    

DOI:10.1029/91JA00612

年代:1991

数据来源: WILEY