摘要:

A unified treatment of the dynamics of the random twisting and untwisting of a solar magnetic loop by photospheric motion is presented. For motions fast compared to Alfve´n transit time, the shear Alfve´n waves damp rapidly on the stochastic field lines of the loop. For slow motions, the loop passes through a sequence of quasi‐static equilibria but the response is also punctuated by impulsive events identified as nanoflares. The statistics of these events indicate a Poisson process; the frequency of these events scales as &Dgr;E−1Mwhere &Dgr;EMis the energy in each event. Their contribution to heating is estimated.

ISSN:0094-243X    

DOI:10.1063/1.42866

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We present an analytical example of a series of magnetostatic equilibria with an endpoint. Numerical simulation demonstrates that oscillatory behavior sets in at the endpoint, with a typical amplitude of 50 km/sec. We suggest thisin situwave generation is an energy source for coronal heating.

ISSN:0094-243X    

DOI:10.1063/1.42867

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We investigate the onset conditions for direct resistive heating instabilities coupled with radiative processes within a twisted magnetic flux rope of axisymmetry and relate the helical patterns wrapping along the rope initiated by such instabilities to the collective appearance of compact X‐ray loops in a certain phase of active region development in the solar corona. Since the emergence and the subsequent evolution of a gigantic magnetic flux rope in the solar atmosphere involve complicated physical processes, it is expected that such instabilities occurring in an ensemble of many current sheaths embedded in a stressed, twisted, and bulged magnetic flux rope will manifest as collective X‐ray loop structures on various spatial scales and with varieties of large‐scale morphologies.

ISSN:0094-243X    

DOI:10.1063/1.42868

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The Alfve´n modes in inhomogeneous cylindrical current‐carrying plasmas is studied solving numerically the Hain‐Lu¨st equation. We have obtained the eigenfrequencies for different current density and it is shown that the distance of the frequencies from the lower edge of the Alfve´n continuum depends on its profile. We have shown, using the WKB approximation, that there exist a MHD discrete Alafe´n mode. The numerical results of the discrete Alfve´n frequencies are compared with the one obtained using the WKB approach. This study is suitable to understand oscillations and heating in solar prominences and also chromospheric‐coronal heating and oscillations by Alfve´n waves.

ISSN:0094-243X    

DOI:10.1063/1.42869

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A normal incidence multimirror telescope, the X‐ray Ultraviolet Imager, for high resolution imaging of the solar atmosphere in the soft X‐ray/XUV region, is being developed as part of the scientific payload of the NASA Orbiting Solar Laboratory. The X‐ray Ultraviolet Imager is formed by two units: a high resolution telescope (0.25 arcsec pixel size and 8×8 arcmin2field of view) and a wide field one (2.3 arcsec pixel size and 5×5 solar radii2field of view). The two systems complement each other and allow a full coverage of solar features from the small scale (200 km on the sun) to the global phenomena. Each system consists of 8 channels with multilayer mirrors, imaging at different wavelengths. In each channel the mirror coating is optimized to select a narrow spectroscopic window corresponding to an intense line in the region 40–400 A˚. In order to provide imaging and temperature diagnostics from the chromosphere to the upper corona, 8 wavelengths are chosen to cover the broad temperature range from 105to 107K. Four images, two high resolution and two full disk ones, are simultaneously obtained by the X‐ray Ultraviolet Imager, at a cadence which in flares can be of 0.4–1 s.

ISSN:0094-243X    

DOI:10.1063/1.42870

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Observations made with the Normal Incidence X‐ray Telescope (NIXT) have shown that some X‐ray emitting structures observed with NIXT resemble very closely the corresponding H&agr; plages. We have used hydrostatic models of coronal loops to explain such observations as strong emission from the lower section of high‐presssure coronal arches.

ISSN:0094-243X    

DOI:10.1063/1.42871

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We investigate the effect of plasma viscosity in the hydrodynamics of coronal flares. To this end we compute two otherwise identical models of a typical coronal compact flare, one including and the other neglecting viscosity terms from the relevant hydrodynamic equations. We find significant differences which may affect the diagnosis of observed high resolution X‐ray spectra.

ISSN:0094-243X    

DOI:10.1063/1.42859

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We review two mechanisms which can lend a non‐local character to energy transport in the solar atmosphere, heat flux propagating in the form of collisionless electrons, and non‐equilibrium ionization of hydrogen driven by ambipolar diffusion. Application of these processes to modeling of the lower transition region and upper chromosphere is considered.

ISSN:0094-243X    

DOI:10.1063/1.42860

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The evolution of MHD fluctuations advected by the solar wind is profoundly affected by the spherical wind expansion: the latter in an important source of anisotropy, the solar plasma becoming increasingly stretched in the transverse direction as it recedes from the sun. As a consequence, the nonlinear evolution of compressive velocity fluctuations in the transverse direction is ‘‘frozen’’ at a finite time in the low frequency limit; at higher frequencies the evolution, e.g., of Alfve´n waves, is easily predicted only in the linear limit. In the fully three‐dimensional and nonlinear case, numerical simulations are necessary: here we present preliminary numerical results from simulations of MHD turbulence in a plasma volume expanding with the wind.

ISSN:0094-243X    

DOI:10.1063/1.42861

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

On October 16, 1990, a remarkable active prominence was observed, in H&agr; line, during 113 minutes. The time gap between successive profile recordings was 20 seconds and 339 profiles have been recorded together. We focused our attention on Doppler shift measurements as well as on significant changes in the prominence intensity. We also give a possible explanation of a sudden increment in the intensity of H&agr; line in dependence on the fact if the whole profile originates in one particular place in the prominence or if it is a superposition of the radiation incident from two different, physically unrelated parts of the prominence.

ISSN:0094-243X    

DOI:10.1063/1.42862

出版商:AIP    

年代:1992

数据来源: AIP