摘要:

Examination of the intensity changes in five outer coronal “looplike” transients observed by the Skylab coronagraph shows general tendencies for (1) greatest concentration of material at the flanks rather than at the tops of the bright loops that characterize these transients, (2) presence of a large region of depleted density within these loops, and (3) development of bright legs that contain most of the material in the transient and that display very little lateral motion as the top of the bright loop moves radially outward through the outer corona. These properties of looplike coronal transients provide useful constraints on theoretical models of this phenomenon. In particular, direct comparison of the observed density distributions with those predicted by models of compressional waves initiated by an impulsive energy release in the low corona are a necessary test of these models. These models predict a maximum enhancement at the top of the loop, rather than at the flanks and “legs” that move laterally with a significant fraction of the propagation speed of the loop top, in conflict with the observed tendencies. If the observed loops are taken to have the geometry assumed in the compressive wave models (“toroidal symmetry” about a rotation axis passing through the center of the sun), the predicted density enhancements are several times larger than those inferred from the observations. Agreement cannot be achieved without use of a geometry which conflicts with that used for the model c

ISSN:0148-0227    

DOI:10.1029/JA089iA04p02113

年代:1984

数据来源: WILEY

摘要:

We present the results of a statistical study on the proton energy spectra in the range of 35–1600 keV during the one‐hour interval centered on the time of arrival of the shock front at the spacecraft of 75 interplanetary shocks that cover the period from August 1978 until December 1980, using the low‐energy proton experiment on ISEE 3. The strength of the shocks was determined by calculating the ratio of the downstream to upstream plasma density by using the ion data obtained by the Los Alamos solar wind instrument. The shock events were sub‐divided into four different classes based on the behavior of their low‐energy (35–238 keV) spectral index. The signatures of the shock events and their spectral index‐time profiles in the different classes are (1) smooth profiles associated with oblique, strong, and fast shocks, which roughly corresponds with predictions following from diffusive shock acceleration theory; (2) irregular profiles mainly associated with quasi‐perpendicular shocks and (3) spikelike profiles associated with quasi‐perpendicular shock spike events, where the properties within both classes point at predominant shock drift acceleration; (4) flat profiles, mainly associated with weak shocks accompanied by little or no shock‐accelerated particles. Strong and fast oblique shocks are found to be the most effective particle accelerators. The spectrum at the shock can generally be described by two power laws with a breakpoint energy near 250 keV. For only 15% of the events the spectrum followed a power law over the full energy range. We found that the low‐energy spectral index, measured immediately downstream of shocks associated with clear flux enhancements, is related to the shock strength according to predictions from first‐order Fermi acceleration, irrespective of the assigned diffusive or drift

ISSN:0148-0227    

DOI:10.1029/JA089iA04p02122

年代:1984

数据来源: WILEY

摘要:

Recent models of preferential acceleration and heating of heavy ions in the solar wind have invoked a resonant interaction with parallel‐propagating ion cyclotron waves. However, these waves must have frequencies in the cold plasma stop band (Ωα<ω<ωc) to accelerate alpha particles to speeds faster than the bulk plasma. Such waves cannot exist in a plasma containing cold alpha particles. We investigate solutions to the WKB warm plasma dispersion relation in a proton‐alpha plasma to determine when parallel ion cyclotron waves can propagate at ω = Ωα. We present examples of the behavior of the dispersion curves in this resonant regime for several types of plasma states appropriate to the fast solar wind. We find that ion cyclotron waves can propagate at ω = Ωαfor a bounded set of plasma parameters. However, use of the correct dispersion relation is likely to make resonant cyclotron acceleration of solar wind alpha particles less feasible as a mechanism to produce the observed prefere

ISSN:0148-0227    

DOI:10.1029/JA089iA04p02133

年代:1984

数据来源: WILEY

摘要:

Microscopic dissipation processes in quasi‐perpendicular shocks are studied by two‐dimensional plasma simulations in which electrons and ions are treated as particles moving in self‐consistent electric and magnetic fields. Cross‐field currents induce substantial turbulence at the shock front reducing the reflected ion fraction, increasing the bulk ion temperature behind the shock, doubling the average magnetic ramp thickness, and enhancing the upstream field aligned electron heat flow. The short scale length magnetic fluctuations observed in the bow shock are probably associated with this tur

ISSN:0148-0227    

DOI:10.1029/JA089iA04p02142

年代:1984

数据来源: WILEY

摘要:

We have studied the structural elements, including shock ramps and precursor wave trains, of a series of oblique low‐Mach number terrestrial bow shocks. We used magnetic field data from the dual ISEE 1 and 2 spacecraft to determine the scale lengths of various elements of shock structure as well as wavelengths and wave polarizations. Bow shock structure under these conditions is essentially that of a large‐amplitude damped whistler mode wave which extends upstream in the form of a precursor wave train. Shock thicknesses, which are determined by the dispersive properties of the ambient plasma, are too broad to support current‐driven electrostatic waves, ruling out such turbulence as the source of dissipation in these shocks. Dissipative processes are reflected in the damping of the precursors, and dissipative scale lengths are ∼200–800 km (several times greater than shock thicknesses). Precursor damping is not related to shock normal angle or Mach number, but is correlated withTe/Ti. The source of the dissipation in the shocks does not appear to be wave‐wave decay of the whistlers, for which no evidence is found. We cannot rule out the possibility of contributions to the dissipation from ion acoustic and/or lower‐hybrid mode turbulence, but interaction of the whistler itself with upstream electrons offers a simpler and more self‐consistent explanation for the observed wav

ISSN:0148-0227    

DOI:10.1029/JA089iA04p02151

年代:1984

数据来源: WILEY

摘要:

A study of the short‐term (12‐hour) variation of the linear correlation between the solar wind‐magnetosphere energy coupling function ε and the electrojet indexAErevealed unexpectedly large variations of the correlation coefficientRand of the ratio between ε andAE. About half of allRvalues fell below the 95% confidence level, and the ratio between ε andAEvaried by more than 100% of the average value, even whenRwas large. This result reveals that the ε function is not a very good predictor of substorms and their intensities. Several possible causes for the variations are discussed. It is shown that some of the variations of the correlation coefficient occur because of a variable temporal relationship betwee

ISSN:0148-0227    

DOI:10.1029/JA089iA04p02162

年代:1984

数据来源: WILEY

摘要:

The configuration of the tail plasma sheet in earth's magnetotail has been calculated in connection with a three‐dimensional magnetosphericBfield model. This model is based on the idea that thermal plasma, tail currents, and magnetic field be in magnetohydrostatic equilibrium during time periods of magnetically quiet conditions. The tail configuration is generated by a separation method assuming a cylindrical magnetotail boundary with constant radius. The separation method restricts self‐consistency to planes perpendicular to the tail axis. The computed tail plasma sheet is flexible and reacts to changes of the earth's dipole tilt angle and changes of the solar wind pressure. Consequences for the plasma sheet configuration with respect to the assumed tail magnetopause shape and the separation method are the following: (1) the plasma sheet thickness increases in YGSMdirection toward the flanks of the tail; (2) the plasma sheet becomes thicker and more diffuse with increasing distance from the earth; (3) during the northern hemisphere summer, the neutral sheet is raised above the magnetospheric equatorial plane around local midnight but crosses this plane and is depressed below it near the flanks of the tail. The latter result agrees qualitatively with Fairfield's empirical neutral sheet model which he derived from spacecraft measurements of the tail field polarity. This agreement between theory and observational material provides a further piece of evidence that the magnetohydrostatic theory is an appropriate level for describing quantitatively the quiet state of the magnetosph

ISSN:0148-0227    

DOI:10.1029/JA089iA04p02169

年代:1984

数据来源: WILEY

摘要:

On day 143, 1978, 2230–2330 UT, the ISEE 2 plasma experiment detected the multiple population frequently found in the mantle and low‐latitude boundary layer regions of the magnetotail. This population consisted at this time of ionospheric O+ and H+ mixed with solar wind plasma (typically H+ and He++). A detailed analysis of the low‐energy population indicates that it can be further resolved into two distinguishable proton components. The ratio between the oxygen and the proton energies implies that one of the proton populations is of ionospheric origin and flows tailward together with the ionospheric oxygen. The angular distribution of the other proton population and of the He++ suggests a higher‐altitude origin, the presence of He++ ions suggesting that this is the solar wind proton co

ISSN:0148-0227    

DOI:10.1029/JA089iA04p02180

年代:1984

数据来源: WILEY

摘要:

Data from the Lockheed ion mass spectrometer on the polar‐orbiting satellite S3‐3 were used to determine the outflow from the high‐latitude ionosphere of terrestrial ions in the energy range of 0.5 to 16 keV during magnetically quiet times (Kp≤ 3) and magnetic storms (minimumDst<−80nT). Populations of ions with different histories were separated on a statistical basis to allow the use of all available data rather than just data in which outflowing ions were conspicuous. The major constituents of the outflow were H+and O+. A small component (<5%) of He+could be detected but was too weak to be measured reliably. Both the intensity of the outflow and its composition showed marked dependence on magnetic activity. During quiet times the total outflow in this energy range from both hemispheres was ∼1.3×1025s−1with the ratio O+/H+= 0.25. During storms the total outflow was higher, ∼7.2×1025s−1with a much larger O+co

ISSN:0148-0227    

DOI:10.1029/JA089iA04p02185

年代:1984

数据来源: WILEY

摘要:

Measurements made on the S3‐3 satellite within upgoing ion beams are used to determine what role the beam regions play in ionosphere‐magnetosphere coupling and auroral particle acceleration. Numerical integrations over the measured electron and ion distribution functions provide estimates of the contributions of these energetic particles to bulk properties of the auroral plasma. It is found that energetic ions carry most of the parallel momentum flux and that energetic electrons carry most of the energy flux and field‐aligned current within ion beam regions. We conclude that these energetic electrons carry upward field‐aligned current all the way from the dense ionosphere to the distant magnetotail. The net current density carried by energetic electrons is associated with non‐Maxwellian anisotropies (e.g., loss cones) in the electron distribution functions. These electrons are not well approximated by a drifting Maxwellian distribution. Microscopic stability analysis shows that the observed EHC waves can be driven by an ion beam. From the analysis, we find that there probably are very few cold electrons near the satellite when ion beams and EHC waves are detected. It also is suggested that the cold ion density may be maintained at a very low level within ion beams, although at larger density than that of the cold electrons. Cold electrons are likely to carry much of the downward field‐aligned current adjacent to ion beams, where ion conies and higher frequency waves are observed. It is noted that some field‐aligned current also could be transferred to cold electrons at the lower border of the acceleration region. Finally, momentum fluxes are used to investigate forces exerted by the acceleration process and the way these forces are transmitted from the ionosphere to the magnetosphere. The ion beam carries a substantial momentum flux away from the acceleration region. This momentum flux must be provided from below by another particle species. If hot (10 e V to 100 eV) secondary electrons with a density of 10 to 100 cm−3are trapped below the acceleration region, these electrons can supply the upward force needed to produce stable acceleration. Otherwise, either the whole acceleration region or structures within it will be accelerated. More complete measurements at the lower edge of the acceleration region are required to distinguish between thes

ISSN:0148-0227    

DOI:10.1029/JA089iA04p02195

年代:1984

数据来源: WILEY