摘要:

In 1978, W. Van Tend and M. Kuperus proposed a simple catastrophe model for magnetically driving coronal mass ejections, prominence eruptions, and two‐ribbon flares. Their model, which is based on simple circuit concepts, suggests that a stable configuration containing a current filament will lose equilibrium when the filament current exceeds a critical value. Here we use a two‐dimensional numerical simulation to test how the Van Tend‐Kuperus model works in an ideal MHD fluid. The simulation exhibits the expected loss of mechanical equilibrium near the predicted critical value, but the current filament moves only a short distance upward before coming to rest at a new equilibrium. However, this new equilibrium contains a current sheet which is resistively unstable to magnetic reconnection, and if magnetic reconnection occurs rapidly, the filament can continue to move upward at Alfvénic

ISSN:0148-0227    

DOI:10.1029/JA095iA08p11919

年代:1990

数据来源: WILEY

摘要:

Empirical models of solar EUV irradiance variations developed by Hinteregger et al. (1981) and Tobiska and Barth (1990) are compared over time scales of the 27‐day solar rotation and the 11‐year solar cycle. Relative to the Hinteregger et al. model, the Tobiska and Barth model predicts the variations of the longest‐wavelength EUV emissions (λ>80 nm), which are entirely chromospheric, to be somewhat less during solar rotation and considerably less over the solar cycle. In contrast, emissions at the shortest EUV wavelengths (λ<20 nm), which are predominantly of coronal origin, are predicted by the Tobiska and Barth model to vary considerably more during solar rotation and somewhat more during the solar cycle than by the Hinteregger et al. model. Within the wavelength region 20–80 nm, chromospheric emission variations are generally underpredicted on all time scales by the Tobiska and Barth model compared with that of Hinteregger et al., with differences between the predicted variations in the strongest chromospheric emission lines (He II 30.378 nm, He I 58.433 nm, and H I Lβ 102.572 nm) exceeding a factor of 2. Neither models nor measurements yet provide a consistent picture of long‐term variability in the EUV portion of the sun's spectrum. This is related to discrepancies of apparent instrumental origin which are identified between the rocket measurements of the EUV spectrum in August 1979 and in November 1988, to which the models are each independently tied. It is demonstrated that simple calculations of active region emission can provide useful constraints on empirical variability models, at least for chromospher

ISSN:0148-0227    

DOI:10.1029/JA095iA08p11933

年代:1990

数据来源: WILEY

摘要:

Magnetic field and plasma data collected by the Helios spacecraft between 0.3 and 1 AU near the activity minimum of solar cycle 21 have been analyzed to establish spectral characteristics of compressive fluctuations in the inner solar wind and their relation to the morphology of the plasma flow and magnetic field. The compressive turbulence level is found to be closely related to the stream structure or latitudinal location with respect to the heliospheric current sheet. Compressive turbulence in low‐speed flows is more fully developed and intense. The spectra are radially invariant and come close to a −5/3 spectral law. In contrast, fast stream turbulence becomes increasingly compressive, in terms of radially growing amplitudes of δn/nand δB/B, with increasing heliocentric distance. The spectra reveal a flatter high‐frequency part, which gradually seems to get straightened out at large solar distances. Single case studies, as well as averaged spectra and their spectral features, are presented and discussed in the context of the theoretical turbulence lit

ISSN:0148-0227    

DOI:10.1029/JA095iA08p11945

年代:1990

数据来源: WILEY

摘要:

Interplanetary magnetic clouds, although not dominant, are a relatively common feature of the solar wind at 1 AU. Their diameters at 1 AU fall in the range of 0.2–0.4 AU, and they have enhanced field strength (B ≃ 15–30 nT at 1 AU), and lower plasma temperature and density than the surrounding plasma. The internal field is a magnetic force‐free configuration, and therefore the current density (J) is proportional toBeverywhere:J= αB, giving ▽×B= αB. If α is constant throughout the cloud (Burlaga, 1988), then ▽²B= −α²B, which has a cylindrically symmetric field solution that is consistent with observations: the axial field is proportional to the zeroth‐order Bessel function ofr, whereris the perpendicular distance from the cloud's axis, the tangential component is proportional to the first‐order Bessel function, and the radial component is zero. We have developed a least squares program that fits magnetic field data within a cloud to these functions and which estimates various properties of the cloud, such as its size, maximumB, and inclination of its axis, as well as closest approach distance of the spacecraft. Results of a study of 18 clouds observed at 1 AU indicate that the most probable direction of the cloud's axis is within 15° of the ecliptic plane and ≃100° from the Sun's direction when it is projected into the ecliptic plane. A broad range of orientations is observed with some extending to 80° from the ecliptic. Other statistical properties are presented, and three

ISSN:0148-0227    

DOI:10.1029/JA095iA08p11957

年代:1990

数据来源: WILEY

摘要:

Hot flow anomalies (HFAs) are unique plasma structures observed in the vicinity of the Earth's bow shock. They are typically characterized by high temperature plasma which is strongly deflected relative to the solar wind and flanked by shock‐like density and magnetic field enhancements. One mechanism proposed to explain these structures involves the coupling of ions reflected off the bow shock with the solar wind. This coupling would convert the relative streaming energy between the reflected and solar wind ions into thermal energy. The hot plasma would then expand to form the observed HFA. This model predicts that the initial temperature of the HFA is roughly limited by the available energy in the relative streaming between the two ion beams, and that the observed temperature and density correspond to the coupled plasma after an expansion has occurred. We present a simple test of this mechanism by comparing the measured temperature and density with the values expected immediately following the assumed coupling and after expansion. We find that the relative streaming energy is in most cases sufficient to account for the measured temperature. However, the predicted HFA temperature after adiabatic expansion to the measured density is in many cases well below the measured temperature. This analysis indicates that if the reflected ion coupling mechanism is correct, the expansion is probably not adiabatic, and in some cases an additional process may be needed to account for the observed temperature. The observed anti‐correlation between the ion temperature and density both within individual HFAs and among the different HFAs provides further evidence that processes other than adiabatic expansion are occurr

ISSN:0148-0227    

DOI:10.1029/JA095iA08p11967

年代:1990

数据来源: WILEY

摘要:

The heating of solar wind alpha particles in hot diamagnetic cavities (HDCs) is examined both through data analysis and computer simulation. The Lockheed Plasma Composition Experiment on ISEE 1 shows more than an order of magnitude increase in the alpha temperature between the solar wind and the HDCs, analogous to the strong solar wind proton heating evidenced in the same transition. A one‐dimensional electromagnetic hybrid computer simulation is used to study alpha heating by ion/ion instabilities, where the initial condition is a cool field‐aligned proton beam streaming relative to the solar wind protons and alpha particles. Low beam densities excite the proton/proton right‐hand resonant instability which pitch angle scatters the beam without significantly heating the alphas. At larger beam densities, the proton/proton nonresonant instability saturates by strong trapping of all three ion components; after saturation the large amplitude magnetic fluctuations lead to stochastic scattering and heating of the ions such that the final temperatures of the alphas are typically greater than the final proton temperatures. These results provide further support to the hypothesis of Thomsenet al.(1988) that the nonresonant instability is the primary source of ion heating in hot diamagnetic cav

ISSN:0148-0227    

DOI:10.1029/JA095iA08p11975

年代:1990

数据来源: WILEY

摘要:

The linear dispersion relation of the beam‐plasma instability and the ISEE 1 wave observations in the electron foreshock are examined in order to study the existence of broadband emissions polarized in an oblique direction with respect to the direction of the electron beams reflected from the bow shock. The present work focuses on broadband emissions observed above the electron plasma frequency ωpe, in the frequency range 1 − 1.5 ωpeand above. The linear dispersion relation of waves is investigated numerically and the dependence of the unstable frequency bandwidth on the beam to plasma temperature ratio,Tb/Tp, and density ratio,Nb/Np, is studied. It is shown that electron plasma waves can initially grow in an oblique direction up to 2.5 ωpeand above, when the beam temperature is low,Tb/Tpbelow 10−2, and its density close to or aboveNb/Np= 0.01. The unstable frequency bandwidth over which emissions can grow is shown to be highly controlled by the density ratioNb/Np. The statistical study of the directivity of broadband emissions observed by ISEE 1 supports the existence of oblique emissions in the frequency range 1–1.5 ωpe, whereas the emissions with frequencies close to ωpepropagate mainly in the parall

ISSN:0148-0227    

DOI:10.1029/JA095iA08p11983

年代:1990

数据来源: WILEY

摘要:

We consider the acceleration of superthermal ions within ripples on the surface of a fast‐mode hydromagnetic shock. We confine attention to small‐amplitude surface ripples, characterized by widthLand amplitudeA, that are large compared to an energetic ion's gyroradiusrg, i.e.,rg≪A≪L. Furthermore, we focus upon shocks that are quasi‐perpendicular on average. To simplify the problem, we consider a perpendicular shock having a single surface ripple with a sinusoidal form. We investigate the effects of confinement, evolving geometry, and finite shock curvature that are associated with the ripple by integrating along the orbits of test particles (protons). We assume scatter‐free motion away from the shock and quasi‐static conditions. As an upstream magnetic field line convects through the surface ripple, it intersects the shock at two points, forming a temporary magnetic trap. A few particles injected into this trap undergo many reflections at the shock and are accelerated nonadiabatically (i.e., the first adiabatic invariant is not conserved) before being convected downstream. In some cases these particles form a high‐energy power law tail on the energy spectrum. Large‐amplitude, spike like flux enhancements of width ∼rgare coincident with the shock passage. These spikes are superposed upon broader, but less intense, flux increases with widths that are energy‐dependent but are typically ∼A. Flux‐time profiles and angular distributions in a given ripple vary markedly depending upon path through the ripple and distance from the shock. Angular distributions upstream range from unidirectional to bidirectional along the field, while those downstream are generally peaked nearly transverse to the field. The model‐predicted results reproduce many features of observed ion shock‐spike events, including their singly or multiply spiked impulsive structure, the variability of this structure with particle energy in a given event, and the large variety of structures

ISSN:0148-0227    

DOI:10.1029/JA095iA08p11993

年代:1990

数据来源: WILEY

摘要:

Plasma and magnetic field data from 223 orbits in the first nine tail seasons of Pioneer Venus are examined to provide a statistical description of the combined ion and magnetic field properties of the Venus magnetosheath and magnetotail. This description is based solely on instrumental responses, with no a priori assumptions about existing regions or their plasma and field characteristics. Over 22,000 plasma spectra taken from 7 to 12Rvdowntail are categorized as to whether they contain shocked solar wind, pickup ions, or no discernible plasma. The magnetic field characteristics associated with each plasma spectrum category are examined, and the average two‐dimensional cross‐tail ion flow and magnetic field structures of the Venus tail are determined at a resolution of 0.25×0.25Rv. Plasma flows everywhere tailward, slowing from 450 km/s in the sheath to less than 300 km/s at the tail axis. Weak outward deflections of 50 km/s or less are found within the tail. The magnetotail is found to be highly draped, with a field‐reversing current sheet not more than 0.25Rvthick. At these distances the tail is filled with shocked solar wind but also contains pickup ions and a plasma component with fluxes that are not detectable by the Pioneer Venus orbiter plasma analyzer. The bulk flow and field configurations imply a number density of 1.2 cm−3and a temperature of 9×106°K for a current sheet composed of 90% protons and 10% O+. These conditions are at or below the limit of instrumental detection. Weak magnetic field asymmetries are associated with the plasma dropouts. The undiscernible plasma component in the tail is consistent with a planetary ion population with fluxes below the instrumental detection threshold. A highE/qplasma population previously interpreted as planetary pickup ions (O+) is found asymmetrically both within the tail and in the adjacent sheath. The undiscernible and pickup ion plasmas (plasmas of Venusian origin) are observed from 5 to 15% of the time within the tail region. This rate depends on solar EUV flux, indicating a photoionization source. Thus the Venus tail is filled with plasma that is primarily shocked solar wind at sometimes undetectable fluxes, which coexists with a photoion population that produces asymmetries in the bulk plasma and magnetic field

ISSN:0148-0227    

DOI:10.1029/JA095iA08p12005

年代:1990

数据来源: WILEY

摘要:

The density, temperature, and velocity of plasma in the inner magnetosphere (L<12) of Saturn are well determined from observations by the Voyager plasma science experiment. We combine the ion and electron data sets from both Voyager flybys to produce an overview of the Saturnian plasma environment. A small spacecraft potential of −10 to −20 V is found to be a plausible mechanism for reconciling differences between the observed ion and electron densities. Electron and ion measurements from the Voyager 1 and 2 encounters are combined to produce a map of density contours insideL= 12. This contour map incorporates all the available Voyager thermal plasma data in this region under the assumption that the inner magnetosphere was stable during the nine months between encounters. In the model, heavy ions are confined to a narrow region near the equator because of their large temperature anisotropy and their low thermal velocity parallel to the magnetic field. The protons are spread much more widely in latitude. The model does not satisfactorily reproduce the densities of the suprathermal electrons observed insideL=7 , but these constitute only a few percent of the total electron density. The oxygen flux tube content decreases rapidly insideL= 5, indicating that losses occur in this region. The lifetime of neutral atoms in the inner magnetosphere is a strong function of latitude, with lifetimes varying from weeks to ye

ISSN:0148-0227    

DOI:10.1029/JA095iA08p12019

年代:1990

数据来源: WILEY