摘要:

Helioseismology began in earnest in the mid-1970s. By 1977 helioseismology had demonstrated that the solar convection zone was about twice as deep as was previously thought to be the case. Since then helioseismology has also provided a measurement of the radial profile of the sound speed in the solar interior and it has also ruled out solar models which would attempt to solve the solar neutrino problem through a lowering of the temperature of the core, while at the same time showing that the effects of the diffusion of helium and other elements must be incorporated in the computation of the most accurate models of the solar interior. The field has also provided measurements of the radial and latitudinal profiles of the sun’s angular velocity over the outer half of the solar interior (as measured by radius). It has also provided evidence for helical flow patterns in the motions of the very shallow, sub-photospheric layers. Finally, the frequencies of the solar p-mode oscillations have also been demonstrated to vary with changing levels of solar activity. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51420

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The intensity of chromospheric structures observed in He I&lgr;1083 nm is strongly modulated by overlying coronal radiation. For this reason, observations in He I&lgr;1083 nm and their detailed comparison with photospheric magnetic fields and the X-ray corona allows us to deduce some aspects of coronal structures and of the topology of the coronal magnetic fields, particularly important at times when there are no direct coronal disk observations available. This paper discusses what He I&lgr;1083 nm spectroheliograms can tell us about the short- and long-term evolution of the coronal magnetic fields, focusing on coronal holes, the large-scale, long-lived two ‘ribbon’ flare that follow the filament eruptions, and small-scale, short-lived dark points. The National Solar Observatory/Kitt Peak He I&lgr;1083 nm and magnetic field data are compared with direct observations of the coronal structure from theYohkohSoft X-ray Telescope. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51459

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The possibility to produce small scales and then to efficiently dissipate energy has been studied byMalara&etal; [1992b] in the case of MHD disturbances propagating in an weakly dissipative incompressible and inhomogeneous medium, for a strictly 2D geometry. We extend this work to include both compressibility and the third component for vector quantities. Numerical simulations show that, when an Alfve´n wave propagates in a compressible nonuniform medium, the two dynamical effects responsible for the small scales formation in the incompressible case are still at work: energy pinching and phase-mixing. These effects give rise to the formation of compressible perturbations (fast and slow waves or a static entropy wave). Some of these compressive fluctuations are subject to the steepening of the wave front and become shock waves, which are extremely efficient in dissipating their energy, their dissipation being independent of the Reynolds number. Rough estimates of the typical times the various dynamical processes take to produce small scales show that these times are consistent with those required to dissipate inside the solar corona the energy of Alfve´n waves of photospheric origin. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51376

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The soft X-ray telescope (SXT) aboardYohkohhas discoveredcoronal X-ray jetsassociated with small flares (microflares–subflares) in X-ray bright points (XBPs), emerging flux regions (EFRs), or active regions (ARs). These newly discovered jets may provide clues to solving the coronal heating mechanism and acceleration of high speed solar wind. The recent development of observations and theoretical modeling of X-ray jets are reviewed with emphasis upon the role of magnetic reconnection in generating these jets. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51355

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Coronal mass ejections and flares may generate shock waves travelling through the solar corona from which some are able to enter into the interplanetary space. These shocks manifest themselves in solar and interplanetary type II radio bursts. Their nonthermal radio radiation indicates that electrons are accelerated to suprathermal and/or relativistic velocities at these shocks. As well known by extraterrestrial in-situ measurements supercritical, quasi-parallel, collisionless shocks are accompanied by large amplitude magneto-hydrodynamic waves in the upstream region. These waves act as strong magnetic mirrors, at which charged particles can be reflected and accelerated. Thus, thermal electrons gain energy due to multiple reflections between two large amplitude MHD waves and reach suprathermal and relativistic velocities. This mechanism of accelerating electrons is discussed for circumstances in the solar corona and the interplanetary space. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51354

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Coronal heating is at the origin of the X-ray emission and mass loss from the Sun and many other stars. While different scenarios have been proposed to explain the heating of magnetically confined and open regions of the corona, they all rely on the transfer, storage and dissipation of the abundant energy present in photospheric motions. Here we focus on theories which rely on magnetic fields and electric currents both for the energy transfer and storage in the corona. The dissipation of this energy, whether in the form of reconnection in current sheets (nanoflares ?) or the dissipation of MHD waves, depends crucially on the development of extremely small scales in the coronal magnetic field, where kinetic effects are likely to be fundamental. The question of whether coronal heating and flares may be viewed respectively as the macroscopic, low-energy average and the high-energy, temporally intermittent aspect of the same underlying driven, dissipative, turbulent system is also addressed, with emphasis placed on the main observational and theoretical stumbling blocks in the way of a confirmation or disproof of such a conjecture. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51400

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Faraday rotation (FR) measurements using linearly polarized radio signals from the two Helios spacecraft were carried out during the period from 1975 to 1984. This paper presents the results of a spectral analysis of the Helios S-band FR fluctuations observed at heliocentric distances from Fig. 3 to16 R⊙during the superior conjunctions 1979–1984. The mean intensity of the FR fluctuations usually does not exceed the noise level for solar offsets greater than≃15 R⊙.The rms FR fluctuation amplitude increases rapidly as the radio ray path approaches the Sun. Good representations of the radial variation were obtained with a two-term empirical formula. The equivalent two-dimensional FR fluctuation spectra are well modeled by a single power-law over the frequency range from 1 to 50 mHz. There is a tendency for an increase of the spectral index from 1.2 at solar offset distancesR>10 R⊙to 1.6 atR<6 R⊙,corresponding to a range for the three-dimensional spectral indexp=2.2–2.6. FR fluctuations thus display a somewhat lower spectral index compared with phase and amplitude fluctuations. Finally, a quasiperiodic component, believed to be associated with Alfve´n waves, was discovered in selected fluctuation spectra observed simultaneously at two ground stations. Characteristic periods and bulk velocities of this component were240±30 secand300±60 km/s,respectively. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51356

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

This paper summarizes (1) advances in our knowledge of coronal structures inferred from radio propagation measurements, and (2) gains in our understanding of the relationship between radio propagation and white-light coronagraph measurements. Radio propagation measurements confirm that streamers are ray-like structures as depicted in coronagraph pictures, but also reveal a hierarchy of filamentary structures throughout the corona, extending from the size of streamers down to scale sizes as small as about 1 km at the Sun(10−3 arcsec).Doppler scintillation measurements, therefore, open a new window on small-scale structure that has long eluded coronagraph measurements. In addition, high precision ranging measurements make it possible to investigate large-scale structures not yet observed in coronagraphs, such as plumes in equatorial coronal hole regions. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51417

出版商:AIP    

年代:1996

数据来源: AIP

ISSN:0094-243X    

DOI:10.1063/1.51357

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Observations of solar Lyman &agr; have been interpreted as indicating that the solar wind mass flux density is lower at polar latitudes than at equatorial latitudes. This ledLallement&etal; [1986] to make a parametric study of solar wind acceleration, along the lines of the earlier Munro-Jackson study [1977], in which they concluded that uncertainties in the polar mass flux were large enough to be consistent with two extreme opposites: (1) a substantial energy supply beyond classical thermal conduction is required, or (2) classical thermal conduction is adequate to drive the flow. This ambiguity has been clarified by Ulysses observations of the polar outflow [Phillips&etal;, 1995]. The polar mass flux density lies near or somewhat above the middle of the range studies byLallement&etal; [1986], which indicates that extended heating is likely to be going on out to at least∼5solar radii. Independent, purely energetic arguments can be made to estimate the required coronal source (electron) temperature that would be required to account for the observed energy flux density. An electron temperature of well above2×106 Kwould be required for the classical conduction flux density to be comparable to the total energy flux density; such a high temperature is unlikely in a coronal hole. These arguments strongly suggest that some extended heating or momentum transfer mechanism is required to drive the solar wind from the polar coronal hole. ©1996 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.51440

出版商:AIP    

年代:1996

数据来源: AIP