摘要:

Photospheric line-tying has a stabilizing effect that allows magnetic energy to build up in coronal loops until critical conditions are reached and the loop becomes unstable to them=1kink instability [1]. Recent research has concentrated on the non-linear evolution of instabilities in line-tied coronal loops. There are suggestions [2–5] that current sheets form during the non-linear evolution of the kink instability. We present 3D MHD simulations of the non-linear evolution of MHD instabilities in line-tied coronal loops. These simulations are carried out on a multi-processor cluster at St Andrews using a new 3D MHD Lagrangian remap code (Lare3d) which we shall discuss briefly. Results are presented for loops with different shear profiles to test the conditions for current sheet formation. We begin by presenting the test case of the Gold-Hoyle field, and compare our results with previous results [1,6]. New results for equilibria with no net axial current are presented and the formation of current sheets discussed. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324947

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

We discuss the effects of internal gravity waves on the mean radial differential rotation profile in the solar tachocline. Vertical transport of horizontal momentum arises from the radiative damping of inwardly traveling waves that are generated by low-frequency, convective fluid motions. For a viscosity typical of the radiative layers below the convection zone, the equilibrium profile of radial differential rotation is demonstrated to be unstable, possibly leading to turbulent mixing in the tachocline. The effect of a uniform, toroidal magnetic field is discussed. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324948

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Quasi-periodical oscillations of positions of coronal loops were observed in the extreme-ultraviolet band (171A, FeIX) with the imaging telescope onboard the TRACE spacecraft. Oscillating transversal displacements (swinging) of a long (∼130 Mm) thin (diameter ∼2 Mm) bright loop, were detected. The oscillations were excited by a flare (14th July 1998 at about 12:55 UT) at distance of about 60 Mm from the loop. The frequency of the oscillations was about 4 mHz (the period about 265 s) and was determined by the wavelet analysis. Neighboring perpendicular slits show synphase temporal behavior, suggesting that the oscillations are produced by akink global fast magnetoacoustic modeof the loop. With this interpretation, we determine the kink speed in the loop as 1040 km/s, which gives the Alfve´n speed of about770±40&hthinsp;km/s.The detection and analysis of post-flare oscillations of coronal loops provides us with an efficient tool for indirect determination of coronal parameters, e.g. the magnetic field found to be of about 20 G. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324949

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

On May 13, 1998 the EIT (Extreme-Ultraviolet Imaging Telescope) and TRACE (Transition Region And Coronal Explorer) instruments produced simultaneous high cadence image sequences of the same active region (AR 8218). TRACE achieved a 25 sec cadence in the FeIX/X(171 Å) bandpass while EIT achieved a 15 sec cadence (operating in ‘shutterless mode,’ SOHO JOP 80) in the FeXII(195 Å) bandpass. These high cadence observations in two complementary wavelengths have revealed the existence of weak transient disturbances in an extended coronal loop system. These propagating disturbances (PDs) seem to be a common phenomenon in this part of the active region. The disturbances originate from small scale brightenings at the footpoints of the loops and propagate along the loops. The apparent propagation speeds roughly vary between 65 and 150 km s−1which is close to the expected sound speed of the coronal loops. The measured propagation speeds seem to suggest that the transients are sound (or slow) wave disturbances. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324950

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

We consider slow resonant MHD waves in 1D planar equilibria with the unidirectional magnetic field. A nonlinear equation governing this waves in a slow resonant layer is derived. A periodic solution in the form of propagating wave with a permanent shape is found in the limiting case, where nonlinearity dominates dissipation. This solution is used to derive a connection formula that connects the values of the normal component of the velocity at two sides of the resonant layer. This connection formula is, in turn, used to study the interaction of an incoming sound wave with a slab containing an inhomogeneous magnetized plasma. The coefficient of the wave energy resonant absorption is calculated and compared with its counterpart obtained on the basis of linear theory. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324951

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

The nonlinear coupling of extraordinary and ordinary waves via kinetic Alfven waves (KAWs) is investigated on the basis of two fluid magnetohydrodynamics. The equation governing the time dependence of electric field of excited O-mode is found. We estimate the time of effective coupling between modes and corresponding interaction distance in solar corona. Our theoretical results show that the X- and O-mode couplings via Alfven waves can be efficient depolarization mechanism for the coronal radioemission. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324952

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

We study the sensitivity of frequency eigen-spectra of solar global modes on physical parameters describing the applied model. Starting from a simple configuration that includes only two distinct regions, the solar interior and the solar atmosphere in the Cartesian geometry, we gradually add new features that improve the initial model and cause changes in eigen-frequencies. These frequency variations are computed numerically and plotted as functions of free physical parameters of a particular model which then gives us a better insight into how various elements of the model influence the eigen-spectra of solarp-,f- andg-modes. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324953

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

The dispersive (kinetic) Alfve´n waves in the ion-cyclotron frequency regime (ICKAWs), with frequencies close to the ion cyclotron frequency and with wavelengths close to the ion gyroradius, are studied in view of their ability to interact strongly with the plasma of solar corona. The combined action of finite ion cyclotron frequency and finite ion gyroradius effects has a profound influence on the Alfve´n wave. The phase velocity of ICKAWs along the background magnetic field can be slower than the Alfve´n velocity (due to finite ion cyclotron frequency effects), but also faster than the Alfve´n velocity (due to finite ion gyroradius effects). The phase velocity depends on both the perpendicular and parallel wavenumbers, and spans a range of velocities from sub-Alfve´nic to super-Alfve´nic. Since these waves have a field-aligned component of electric field, they undergo Cherenkov resonant interaction and can be excited by the ion beam component set up in the low corona by the reconnection outflows from chromospheric reconnection events. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324954

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

The nonlinear parametric interaction of Alfven waves with magnetosonic and ion-acoustic waves is considered on the basis of two-fluid magnetohydrodynamics. A nonlinear dispersion relation describing three-wave interaction, instability growth rate have been calculated and estimated. The analyses of theoretical results shows that kinetic effects in the Alfven waves (the finite ion Larmour radius) are essential for the parametric interactions of waves. Nonlinear parametric processes studied in the paper could take place in the solar coronal loops, where plasma parameter is small. The products of the decay-magnetosonic and ion-acoustic waves, can effectively heat the coronal plasma in consequence of rapid dissipation. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324955

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Alternating current in flux tubes is one of the possible contributors to the heating of the solar corona. The mechanism of the generation of alternating currents in flux tubes by turbulent pressure fluctuations in convective zone is proposed. The thin-flux-tube approximation shows, that the linear and weakly nonlinear torsional waves in twisted flux tube, are accompanied by tube cross section fluctuations. Consequently, external pressure fluctuations generate not only slow MHD waves but torsional waves as well. The torsional waves are nothing else than alternating current. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324956

出版商:AIP    

年代:1900

数据来源: AIP