摘要:

We describe and analyze in detail the coronal mass ejection of 18 August 1980, using images from the coronagraph on the Solar Maximum Mission (SMM) satellite. The event occurred at the site of a large coronal helmet streamer and evolved into the three‐part structure of a bright frontal shell, followed by a relatively dark space surrounding a bright filamentary core as seen in many mass ejections of the SMM epoch. The bright core can be identified as material from a prominence whose eruption was observed from the ground; this identification is based on (1) the looplike and filamented appearance of the core, (2) its motion along a trajectory that is a good extrapolation of the motion deduced from ground‐based observations of the prominence eruption, and (3) direct observations of Hα emission when the core is in the coronagraph field of view. The mass of the frontal shell is equal to that of the coronal helmet streamer (to the ∼30% accuracy with which mass estimates can be made), indicating that the shell is the coronal material previously in the helmet streamer, displaced and set into motion by the erupting prominence and surrounding cavity. The mass ejected in the bright core (or prominence) is estimated to be ∼50% larger than the “coronal” material in the frontal loop. The total mass of 2.5×1015g and energy of 5×1031ergs estimated for this mass ejection are both greater than in typical ejections of the Skylab era but are comparable to the average mass and energy in an interplanet

ISSN:0148-0227    

DOI:10.1029/JA091iA10p10951

年代:1986

数据来源: WILEY

摘要:

Coronal mass ejections detected with the Solar Maximum Mission coronagraph/polarimeter are often accompanied by erupting prominence material observed both in Hα and in the electron scattering continuum. Hα emission is concentrated in bright filaments moving radially outward. The same filaments are seen in the electron scattering continuum as regions of enhanced brightness. In this paper we develop a diagnostic method based on the observed Hα and continuum brightness to derive the electron density, line of sight thickness, and degree of ionization of hydrogen as functions of the temperature of the prominence filaments. Our method differs from that of Poland and Munro (1976) in the treatment of Ly α excitation. Analysis of data from the event of August 18, 1980, illustrates that the rising prominence material has decreased density, increased temperature, and increased ionization of hydrogen relative to quiescent prominences in the lower corona. Hydrogen is found to be 90–99% ionized, electron densities are near 108cm−3, and the temperature is near 20,000 K. The increased ionization is due mainly to the decreased density. Use of the results is made here and in an accompanying paper by Illing and Hundhausen to determine the total mass ejected in th

ISSN:0148-0227    

DOI:10.1029/JA091iA10p10961

年代:1986

数据来源: WILEY

摘要:

Beyond about 10RJfrom Jupiter the configuration of magnetic field lines in Jupiter's magnetosphere is strongly affected by the presence of an equatorial current sheet. Using a Voyager 1 magnetic field model that explicitly includes the effects of this current sheet to describe the magnetic field line geometry, we have reexamined charged particle observations from Pioneer 11's high‐latitude flyby of Jupiter in 1974. We find that if the trajectory of Pioneer 11 is mapped to the equatorial plane along the model magnetic field lines, significant features in the time‐intensity profiles of trapped protons and electrons, including one microsignaturelike feature, are found to correspond to shells of closed field lines crossed by the orbits of the satellites Ganymede and Callisto. Although many of the characteristics of the features, including their magnitude, persistence, and similarity of appearance for protons and electrons, are difficult to explain, we suggest that these features are signatures of interaction of the trapped particles with Ganymede and Callisto. If this interpretation is correct, it offers explanations for several large features in the time‐intensity profile of trapped particles observed during Pioneer 11's flyby that could not be explained using earlier magnetic field models which did not include effects of the current sheet, and it leads to reinterpretation of some features previously associated with Ganymede as signatures of Callisto. It also implies that the configuration of the current sheet during the Pioneer 11 flyby in 1974 was very similar to that observed by Voyager 1 in 1979 and provides confirmation of the essential accuracy of current sheet magnetic field models for describing the magnetic field configuration out to radii of about 30RJfrom Jupiter. Finally, the characteristics of the signatures described suggest interesting problems for future research on the dynamics and interactions of trapped particles with satellites in the outer regions of the Jovian trapped radiation

ISSN:0148-0227    

DOI:10.1029/JA091iA10p10975

年代:1986

数据来源: WILEY

摘要:

We show that the source region for Saturnian kilometric radiation (SKR) which originates in the high latitude near‐noon dayside ionospheric can be mapped via the Saturn magnetic field to the outer edge of the dayside equatorial plasmasheet. The plasmasheet is known to be unstable at this boundary due to the outward centrifugal forces generated by the heavily mass‐loaded equatorial Saturnian magnetosphere. Also previous work has shown that Voyager 1 observations of MHD waves near the plasmasheet are consistent with their being driven by the centrifugal flute instability. The resulting field‐aligned MHD waves at higher latitudes can trap and accelerate electrons. For the plasma environment measured by Voyager, field‐aligned accelerated electrons up to energies of several keV are expected to be deposited into the SKR ionospheric source region. A necessary condition for the acceleration of the electrons to several keV is a population of hot electrons inside of the plasmasheet, near the unstable boundary. The hot electrons are produced as a byproduct of the pick‐up of exospheric atoms from Saturn's moons. The solar wind dependence of the SKR is thus a consequence of the elongation of the Saturn nightside magnetosphere with greater solar wind pressure leading to greater centrifugal pressures on the nightside plasmasheet. Although the theory presented correctly gives the local time dependence of the emissions, we suggest that the strong emission peak at a fixed Saturnian longitude is a consequence of locally reduced electron plasma frequency to electron gyrofrequency ratio. The lack of a nightside source of SKR is consistent with the disruption of the plasmasheet due to loss of plasma down the Saturn ma

ISSN:0148-0227    

DOI:10.1029/JA091iA10p10989

年代:1986

数据来源: WILEY

摘要:

Field‐aligned Birkeland currents and the twist of magnetic tail field lines are calculated for an axially symmetric pole‐on magnetosphere. The magnetic field components in the magnetospheric tail result from linear solutions to the MHD equilibrium problem. The field line twist angle, ε = arc tan (Bϕ/Bz), is shown to be dependent on the radial distance measured from the tail axis, on the amount of thermal plasma confined in closed magnetotail flux tubes, and on the ionospheric Pedersen conductivity. The field lines are maximally twisted in the force‐free tail configurations; they are minimally twisted if the magnetotail reaches the state of maximum plasma pressure, the so‐called Harris sheet limit. The field line twist results from the planetary rotation, which leads to the development of a toroidal magneticBϕcomponent and to differentially rotating magnetic field lines. If centrifugal forces become important, the thermal plasma pressureP(A) in the Grad‐Shafranov equation assumes the formP(A, r) which allows centrifugal forces to be included in the MHD equilibrium formalism. The equilibrium model reveals that magnetopause field lines do not rotate because the magnetopause is held in place by the solar wind stream. For a given ionospheric conductivity and a given thermal plasma content within the tail, the field line helix is twisted most near the center of the circular tail plasma sheet. The field‐aligned Birkeland currents reach their maximum strength on tail field lines that confine the plasma sheet. Thus polar aurorae are expected to appear around both the dayside and nightside magnetic poles at latitudes where the plasma sheet field lines map into the planet's ionosphere. This theoretical model was originally developed for the anticipated pole‐on magnetosphere of Uranus. Thus references to the Uranian magnetosphere that appear in this study and in the quoted literature apply to hypotheses that had been developed before the closest approach of Voyager 2 at Uranus on January 24, 1986. However, the results of this study are sufficiently general to allow their application to other rotating axially symmetric plasma‐magnetic field confi

ISSN:0148-0227    

DOI:10.1029/JA091iA10p10995

年代:1986

数据来源: WILEY

摘要:

In order to obtain an improved estimate of Neptune's magnetic field and hence the size and structure of the magnetosphere, a new scaling law for planetary magnetic fields has been developed. Starting from magnetostrophic balance between the coriolis force and thej × Bponderomotive force, we have derived a scaling relation which can be used to calculate magnetic field strengths using only the observable properties of a planet. Specifically, using the planet's mean density, radius, mass, rotation rate and internal heat source luminosity, we can obtain an estimate of the magnitude of the planet's magnetic field from the same parameters for the earth and earth's magnetic field. The estimated magnetic field is, however, an upper bound and good agreement with observations is expected only if the planet's dynamo is fully developed. This is apparently true for earth, Jupiter, Saturn, and Mercury for which good agreement is obtained between predicted and observed magnetic fields. In contrast, the moon, Venus, and Mars apparently lack currently active internal dynamos and force balance may not hold. From a comparison of theory and observations, we conclude that planetary dynamos are two state systems with either zero intrinsic magnetic field or a field near the upperbound determined from magnetostrophic balance. The calculated upper limit for the Uranus magnetic field is 0.3 G at the equator 1 bar level. The value for Uranus' magnetic field reported by Ness et al. based on Voyager observations is 0.23 G. Given this excellent agreement between calculation and observation and also noting that Neptune possesses a large internal heat source, we expect Neptune's magnetic field at the 1 bar level to be between 0.5–0.4 G. The expected bow shock stand‐off distances range from 20–40RN(Neptune radii). Hence, Neptune's satellite Triton at 14.6RNlies within the magnetosphere. Nonthermal radio emission generation may be possible in regions of sufficiently low polar ionospheric plasma densities. Finally, we show that agreement between observed and predicted magnetic fields derived from Blackett's “Bode's law” scaling relation are fortuitous and are owing to a weak sensitivity of predictions to some combinations of parameters appearing in our s

ISSN:0148-0227    

DOI:10.1029/JA091iA10p11003

年代:1986

数据来源: WILEY

摘要:

Observations from sensors on the IMP 8 and ISEE 3 spacecraft have been used to study the characteristics of interplanetary shock‐associated energetic particle events. In the energy range above 200 keV, we find that large ambient proton flux levels from the parent solar flare are associated with hard spectra downstream from the shock. At ∼40 keV/nucl, immediately behind the shock fronts the He/H flux ratio of shock‐accelerated particles is strongly correlated with the upstream He/H density ratio in the solar wind, but not with the flare He/H ratio. The correlation between the shock accelerated He/H and the solar wind He/H ratios is linear. Immediately downstream from the shock front, the energy spectral indices of protons and alpha particles are equal. Measured at the same energy/nucleon, the (46 keV/nucl) energetic particle He/H ratio is the same as the solar wind He/H ratio upstream from the shock, while at the same energy/charge, the energetic particle He/H ratio shows an enhancement by a factor of 4 to 5 compared with the solar wind. Many of these findings are consistent with a first‐order Fermi acceleration origin of the energetic particles. Suprathermal particles from the shock‐heated solar wind appear to be the primary seed population of the shock‐associated energetic particles near ∼40 keV/nucl. However, for shock accelerated energetic particles of energies in the range above 200 keV, a more energetic seed population appears to

ISSN:0148-0227    

DOI:10.1029/JA091iA10p11009

年代:1986

数据来源: WILEY

摘要:

A comprehensive set of experimental observations of a high β (2.4), supercritical (Mf= 3.8), quasi‐perpendicular (ΘBn1∼ 76°) bow shock layer is presented, and its local geometry, spatial scales, and stationarity are assessed in a self‐consistent, Rankine‐Hugoniot‐constrained frame of reference. Included are spatial profiles of the ac and dc magnetic and electric fields, electron and proton fluid velocities, current densities, electron and proton number densities, temperatures, pressures, and partial densities of the reflected protons. The transformation of the apparent time scales to the actual spatial scales is performed with unprecedented accuracy. The observed layer profile is shown to be nearly phase standing and one dimensional in a Rankine‐Hugoniot frame, empirically determined by the magnetofluid parameters outside the layer proper. One or both of these properties appear to collapse at the time resolution of 1.5 s in the specific geometry considered in this study. Several pieces of evidence are used to show this stationarity: (1) the similarity of the average magnetic structures seen on the two ISEE spacecraft; (2) the close agreement between the electric currents directly determined from the plasma data and those inferred from the magnetic data assuming the layer is one dimensional and time stationary; (3) the close agreement of the empirically determined scale lengths of the most prominent substructures with those determined by numerical simulations and previous laboratory studies; and (4) the close agreement between the theoretical Rankine‐Hugoniot‐determined normal plasma pressure jump and that of the observed electron and proton fluids. The resolved cross‐field electrical currents (with empirical error estimates) are observed to peak within the main magnetic ramp at a level well below the first stabilization threshold for ion acoustic turbulence suggested for low β shocks by Galeev (1976); clear evidence is also provided for smaller parallel currents throughout the main ramp and overshoot, with a predominant sense as if the shock electric field has caused the lighter electrons to lead the ions along the local magnetic field direction. The width of the shock depends on what structures are used to define it. The upstream pedestal or “foot” is nearly two upstream ion skin depths wide, but the main magnetic ramp is only 1/5 the upstream ion skin depth and thus considerably smaller than “conventional wisdom” and most simulations. The ramp scale length is directly corroborated by the current densities determine

ISSN:0148-0227    

DOI:10.1029/JA091iA10p11019

年代:1986

数据来源: WILEY

摘要:

At a high β (2.4), supercritical (Mf∼ 3.8), perpendicular (ΘBn1∼ 76°) shock, we have experimentally established for the first time a number of properties. The energy transformation within the resolved shock layer takes place in three stages: (1) the pedestal where ion gyromechanical energy is increased at the expense of the energy of the flow, (2) the ramp where flow energy is further diverted almost exclusively into magnetic and electron pressure, and (3) the downstream convected ion inertial length within which the ions start to make progress toward “thermalization” of the gyromechanical energy created in the pedestal. The scale of the magnetic ramp is clearly separated from that of the ion skin depth based on either of the Hugoniot asymptotic states. The cross‐shock electrical profile has been determined in both the normal incidence frame (NIF) and in the deHoffman‐Teller frame (HTF) by two different methods; the NIF potential drop was approximately 8 times that in the HTF. Potential overshoots were determined in both frames. The solar wind ion's convected inertial length in the downstream field is 10 times the scale of the magnetic ramp. The solar wind ions are shown to be significantly decelerated by the NIF electric force. Nearly all solar wind electrons have gyroradii less than the scale length of the magnetic field in the ramp; whenE≥ 500 eV this condition is no longer true. The thermal electron plasma remains well magnetized throughout the magnetic profile of the shock. The profile of the electron parallel temperature is closely correlated with that of the nonmonotonic deHoffman‐Teller potential profile, suggesting that the parallel energy change of the electrons is largely reversible. The perpendicular electron temperature is often, but not always, proportional to the magnetic intensity. The average cross‐field resistivity at the magnetic ramp was determined to be η⊥∼ 10−7cgs by three independent methods; the spatial variation of the resistivity through the shock layer is significant, including order of magnitude variations throughout the downstream overshoot structures. The maximum resistivity occurs near the end of the first low‐frequency magnetic overshoot. The parallel “resistivity” is usually negative and concentrated in regimes where the electron's HT potential energy has a local minimum. The cross‐field diffusion of the magnetic field within the ramp balances the steepening of the field profile, as demonstrated by the equality of the magnetic Reynold's length and the exponentia

ISSN:0148-0227    

DOI:10.1029/JA091iA10p11053

年代:1986

数据来源: WILEY

摘要:

Strong evidence is provided that the self‐consistently determined deHoffman‐Teller electric field controls the lowest‐order deformation and structure of the observed parallel electron distribution throughout a well‐resolved, supercritical, quasi‐perpendicular shock wave. Details of this control include velocity space boundary features predicted by reversible Vlasov theory in the dc force fields. Phase space signatures of collisionless dissipation are also explicitly illustrated for the first time. The observed contrast between the reversible Vlasov prediction using the deHoffman‐Teller potential clearly demonstrates the onset of irreversibility and the causes for the loss of electron energy implied by the negative parallel resistivity determined in paper 2. These measurements quantitatively illustrate the primary role of the deHoffman‐Teller electric field in increasing the electron parallel “temperature” across the shock and the secondary role of wave‐particle reactions which actually “cool” the reversibly energized distribution. Electric and magnetic field wave measurements have been used to determine wavelengths and indices of refraction of the turbulence within the shock layer in order to identify the collective effects responsible for this “collisionless” dissipation. The kinetic, modified two‐stream instability is identified as the principal wave mode (based on energy content) in the magnetic pedestal; at the main magnetic ramp these modes are joined by lower hybrid drift (LHD) modes, and an electron acoustic mode appears to be excitable within the magnetic overshoot. The reduced electron distribution has been empirically shown for the first time to have two peaks in this regime, but not elsewhere in the shock layer. The electron acoustic mode appears to be the primary agent for the anomalous resistivity across the shock ramp and overshoot, having the appropriate effective collision frequency and morphology; the bulk of the wave power and probable thermalizing (agent for the ions is the MTSSW (modified two‐stream instability due to transmitted solar wind ions) and LHD modes. By simulation the MTSSW mode in high β regime stabilizes by forming flat‐topped electron distributions; it is suggested that the MTSSW mode can remove the edges of the reversible Vlasov distributions which already have the basic ingredients (such as full width at half maximum) of the observed sheath distributions, leaving the flat‐topped distribution that is observed. The lower hybrid drifts are localized and strongest in the magnetic ramp, and the wave turbulence decays with distance behind the downstream convected ion inertial length (CIIL2), consistent with the removal of the driving free energy for this mode, which is the cross‐field drifts of the distinguishable ion subpopulations along the shock normal with respect to the electrons. The Vlasov and observed behaviors are contrasted with the extant particle shock simula

ISSN:0148-0227    

DOI:10.1029/JA091iA10p11075

年代:1986

数据来源: WILEY