摘要:

On May 13 1998, active region NOAA 8218 was observed in the context of the SOHO/JOP80 campaign by an array of 8 different ground-based and space-born instruments. The emphasis was set on imaging of small-scale dynamics in this relatively small but rapidly evolving AR. In particular, SOHO/EIT (195 Å), TRACE (171 Å) and YOHKOH/SXT produced subfield image sequences at their respective highest possible rates. We searched for wave and instability phenomena by using an automated recognition scheme. This result in a wide inventory of propagating disturbances and localized transient brightenings. By comparing the soft X-ray signature as recorded by SXT with the EUV-signature as collected by EIT and TRACE, we are able to distinguish between various types of active region transients. As such we find that the strongest brightenings observed by EIT are indeed the EUV counterparts of the previously reported ARTBs seen by SXT. Weaker brightenings seen by EIT do often not have an X-ray counterpart. Moreover, in an extended system of faint quasi-open loops, we find propagating disturbances, with speeds of the order of 100 km/s, both in EIT and TRACE images. These are interpreted as sonic perturbations. The brightenings will be discussed in this paper while the propagating disturbances are described in the presentation by Eva Robbrecht at this conference. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324937

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

The magnetic field in the photospheric layers of the Sun outside sunspots is found to be in the form of concentrated magnetic flux tubes of 1–2 kG field strength. The model of convective collapse in a thin flux tube, where an instability causes “draining” of the tube and concentration of the magnetic field, has proved useful in attempting to explain the inferred field concentrations. Here we explore the mechanism of convective instability allowing for the inflow of matter in the upper reaches of the tube. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324938

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

The present paper considers Alfve´n waves in an inhomogeneous medium, under a non-uniform external magnetic field and in the presence of a non-uniform flow, to obtain the wave equations for the velocity and magnetic field perturbations in the case of propagation along a vertical uniform external magnetic field in an atmosphere with a vertical mean flow velocity and satisfying mass conservation. The wave equation has a critical layer where the mean flow velocity equals the Alfve´n speed, and in the case of an isothermal atmosphere, the wave equation for the magnetic field perturbation can be reduced to a Bessel type with complex order and imaginary variable. Thus it is possible to obtain exact solutions for all frequencies and distances, discussing the cases in which there are propagating or evanescent waves far below or far above the critical layer, i.e., the latter acts as a reflecting layer. The solution in the vicinity of the critical layer allows the amplitude and phase of the wave field to be plotted, as a function of distance from the critical layer (measured in scale heights), for different values of the dimensionless frequency and initial Alfve´n number (ratio of flow velocity to Alfve´n speed). One application is the solar atmosphere, for which the MHD equations are appropriate, and the wavelengths are large compared with the density scale height, thus excluding the ray approximation. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324939

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

We consider dissipative Alfve´n waves in a viscous atmosphere with a magnetic field, so that the electrical conductivity is considered as a non-diagonal tensor. Under a vertical uniform magnetic field we consider transverse magnetic and velocity perturbations. We obtain two decoupled second-order systems to right and left polarized Alfve´n waves. The first electrical diffusivity&khgr;1and the viscous diffusivity &eegr; are associated with “real” damping; the second electrical diffusivity&khgr;2is associated with “imaginary damping”. Boundary conditions are imposed in order to obtain the velocity perturbation spectrum of an Alfve´n wave. The velocity perturbation is plotted versus altitude over a distance of five scale heights, for different values of the parameters. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324940

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

In this paper we report on the nonresonant conversion of convectively unstable linear gravity modes into acoustic oscillation modes in shear flows. The convectively unstable linear gravity modes can excite acoustic modes with similar wave-numbers. The frequencies of the excited oscillations may be qualitatively higher than the temporal variation scales of the source flow, while the frequency spectra of the generated oscillations should be intrinsically correlated to the velocity field of the source flow. We anticipate that this nonresonant phenomenon can significantly contribute to the production of sound waves in the solar convection zone. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324941

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

De Groof &etal; ’98 [1] and ’00 [2] studied the time evolution of fast magnetosonic and Alfve´n waves in a coronal loop driven by radially polarized footpoint motions in linear ideal MHD. Footpoint driving seems to be an efficient way of generating resonant absorption since the input energy is mainly stored in body modes which keep the energy in the loop. The most important feature in this study is the stochastic driving of the loop. While in earlier models with a periodic driver or a single pulse, the loop is only heated at one single layer, we now find multiple resonance layers which results in a more globally heated loop. Moreover, these resonances (created on a realistic time scale) have length scales which are small enough to explain energy dissipation. An important aspect to take into account is the mass transfer between corona and chromosphere since the density becomes time dependent and consequently, the resonant surfaces shift throughout the loop [3]. Combined with the multiple resonances we found in the previous study, this result can lead to the globally heated coronal loops we observe. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324942

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

High cadence TRACE data (JOP 83) in the 171 Å bandpass are used to report on several examples of outward propagating oscillations in the footpoints of large diffuse coronal loop structures close to active regions. The disturbances travel outward with a propagation speed between 70 and 160 km s−1. The variations in intensity are of the order of 2&percent;–4&percent;, compared to the background brightness and these get weaker as the disturbance propagates along the structure. From a wavelet analysis at different positions along the structures, periods in the 200–400 seconds range are found. It is suggested that these oscillations are slow magneto-acoustic waves propagating along the loop, carrying an estimated energy flux of4×102&hthinsp;ergs&hthinsp;cm−2&hthinsp;s−1.©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324943

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Phase mixing was introduced by Heyvaerts and Priest [1] as a mechanism for heating plasma in open magnetic field regions. Here we include a stratified density and a diverging background magnetic field. We present numerical and WKB solutions to describe the effect of stratification and divergence on phase mixing of Alfve´n waves. It is shown that the decrease in density lengthens the oscillation wavelengths and thereby reduces the generation of transverse gradients. However, the divergence of the field lines shortens the wavelengths and thus enhances the generation of gradients. Furthermore we found that in a stratified atmosphere, ohmic heating is spread out over a greater height range whereas viscous heating is not strongly influenced by the stratification. A wavelet analysis indicated that the wavelet transform could provide us with information about the medium the waves are traveling through. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324944

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Recently it has been shown that for strong upstream magnetic field stationary three-dimensional (3D) magnetohydrodynamic (MHD) bow shock flows exhibit a complex double-front shock topology with particular segments of the shock fronts being of the intermediate MHD shock type. The large-scale stability of this new bow shock topology is investigated. It is found that large-amplitude perturbations may cause the disintegration of the intermediate shocks—which are indeed known to be unstable against perturbations with integrated amplitudes above critical values— but that in the driven bow shock problem there are always shock front segments where intermediate shocks are reformed dynamically, resulting in the reappearance of the new double-front topology. This shows that the new bow shock topology, and shock segments of intermediate type in general, may be found in MHD plasma flows even when there are large-amplitude perturbations. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324945

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

The magnetic field considered to reside at the base of the convection zone is presumed to vary over the solar activity cycle. We examine the effect of such a buried magnetic layer on the properties of solarp-modes. Analytical and numerical solutions to the dispersion relation for these modes are presented. Frequency changes due to the stored magnetic field are found to be negligible in comparison with the low- and intermediate-degree frequency shifts reported over the solar activity cycle. Nonetheless, there are grounds for inferring the signature of such a buried field through examining shifts of various degree. Thep-mode frequencies are increased proportionally to the square of the field strength at the base of the convection zone and depend upon the thickness of the magnetic layer. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324946

出版商:AIP    

年代:1900

数据来源: AIP