摘要:

The present observational knowledge of the size distributions of solar surface magnetic structures—sunspots, sunspot groups and active regions—and their temporal evolution, is reviewed in the context of how such information may provide important clues to the nature of the solar dynamo and the underlying causes of solar variability. The ability of such information to distinguish between the competing theoretical mechanisms of flux tube fragmentation and coalescence is briefly discussed.

ISSN:0094-243X    

DOI:10.1063/1.42863

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We discuss large scale numerical simulations as a tool for obtaining qualitative understanding of the processes directly and indirectly responsible for coronal heating. The actual heating process in the low beta coronal plasma is most likely driven by transfer of magnetic energy from the subsurface high beta region, where magnetic energy is created as an energetically insignificant byproduct of solar convection and rotation. Based on the results of recent numerical experiments, we discuss some of the processes involved.

ISSN:0094-243X    

DOI:10.1063/1.42872

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The role of magnetic flux tubes as communication channels connecting the photosphere with the chromosphere and above is reviewed.

ISSN:0094-243X    

DOI:10.1063/1.42864

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

According to a recent study comparing magnetic fields extrapolated from photospheric measurements with chromospheric and coronal observations, the electric current helicity in active regions possesses a predominant sign in each of the two solar hemispheres. This indicates that the generation of atmospheric electric currents, which are needed as the energy source for flares and coronal heating, is a global phenomenon connected with the rotation of the Sun rather than a consequence of plasma motions within individual active regions uncorrelated between different active regions. Theoretically, the evolution of the atmospheric magnetic field may be understood as the continual distortion of an existing equilibrium by disturbances propagating upward from the photosphere and subsequent fast relaxation to a new, neighboring equilibrium. Here current helicity proves to be an important quantity decisive for whether the (mean) magnetic field can evolve along a stable path with growing free energy. Disruptive disturbances with an explosive release of magnetic energy, in particular flares, are generally thought to be due to current sheet formation and magnetic reconnection. Reconnection may be defined as a discontinuous change of the field line connectivity to the photosphere. Then field lines running into a magnetic null point or such that are tangential to the photosphere should play a particular role. By comparing extrapolated magnetic fields with flare observations examples have been found suggestive of a connection between tangential field lines and flare activity.

ISSN:0094-243X    

DOI:10.1063/1.42865

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Intrinsically weak magnetic fields are difficult to identify, since flux measurements (magnetograms) cannot by themselves distinguish between filling‐factor and field‐strength effects. The first real determinations of intrinsically weak (less than kG) fields have in fact been made only this year, using the StokesVprofiles of an infrared line pair near 1.56 &mgr;m. Many cases of discrete magnetic elements with field strengths as low as 0.4 kG have been found, immediately adjacent (within a couple of arcsec) to the normal strong‐field fluxtubes that have strengths in the range 1.4–1.6 kG and a magnetic polarity that can be both the same or opposite to that of the adjacent magnetic component.There appears to be a continuous sequence of bipolar magnetic regions of various scales, down to the spatial resolution limit, from active regions to ephemeral regions and inner‐network fields. It seems likely that this sequence continues in the form of a subarcsec mixed‐polarity or ‘‘turbulent’’ field that permeates the 99% of the photospheric volume not occupied by the kG flux tubes in the network. A one‐sigma upper limit of 100 G to the strength of this hitherto ‘‘invisible’’ field has been set from line‐broadening constraints, which indicates that this small‐scale field is intrinsically weak. Arguments are given why the spatial spectrum of flux emergence should saturate when scales approaching the photon mean free path in the photosphere (about 100 km) are approached, which is the range of scales that may be opened up to exploration by LEST and OSL.It is shown how correlations (‘‘active longitudes’’) in the pattern of small‐scale flux emergence lead to a replenishment of the global or ‘‘background’’ magnetic‐field pattern at high heliographic latitudes in a time as short as weeks, more than two orders of magnitude faster than predicted by numerical models of the Babcock‐Leighton type. There is thus a close link between the small‐scale dynamics and the global solar‐cycle evolution.

ISSN:0094-243X    

DOI:10.1063/1.42873

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The High Resolution Telescope and Spectrograph (HRTS) instrument contains a broadband spectroheliograph which has been tuned on sounding rocket flights to cover a passband centered on 1600 A˚, where the predominant flux contributor is continuum emission from the temperature minimum region (approximately 70% of the integrated intensity in quiet regions). I discuss the HRTS observations of the temperature minimum region in quiet areas and their relation with magnetic field, 5 minute oscillations, and heating. The brightness temperature of solar fine structure elements composing the supergranular network is found to be linearly proportional to the local absolute value of magnetic field strength. In cell centers, there is evidence for a 250 s period oscillation occurring in 10 arc sec scale patches, which, however, is energetically unimportant to the local heating budget. I discuss an interpretation in which a basal heating and 5 minute type oscillations occur globally, while the network bright points occur in magnetic regions, heated perhaps from partial dissipation of Alfven waves (whose energy flux is linearly proportional to B) in individual elemental 1500 G (at the photosphere) flux tubes which expand to form the temperature minimum fine structure bright points.

ISSN:0094-243X    

DOI:10.1063/1.42874

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Explosive events are highly‐dynamic, small‐scale phenomena commonly observed in spectra of transition zone lines. Their velocities are near 100 km s−1, sizes near 1500 km, and time scales near 60 s. They occur at a height of 1000–2000 km, below typical transition zone structures. It has been demonstrated that some explosive events are caused by emerging magnetic flux and there is good evidence to indicate that the great majority are related to magnetic flux cancellation. The most probable mechanism for their generation is magnetic reconnection. The characteristics of explosive events and flux cancellation taken together show that magnetic reconnection in the quiet sun apparently proceeds readily in rapid bursts along the neutral line separating opposite flux elements that are convected together by photospheric flows.

ISSN:0094-243X    

DOI:10.1063/1.42875

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The global nonlinear dynamics of magnetic field lines in plasmoids with a pair of nulls, whereB=0, is studied. The aim of this analysis is to describe the separatrix surfaces on which singularities can occur in ideal magnetohydrodynamics because of topological changes in the field. These separatrix surfaces should locate the boundary layers associated with three‐dimensional reconnection in the presence of resistivity or inertia. It is found that the field lines exhibit chaotic scattering with several properties in common with plasmoid models without nulls (in which one component of the magnetic field never changes sign). In particular, the singular surfaces can be fractal, implying complex current density structures down to the dissipation scale. These generic features are expected to exist in typical coronal magnetic geometries exhibiting three‐dimensional reconnection and the formation of current sheets.

ISSN:0094-243X    

DOI:10.1063/1.42878

出版商:AIP    

年代:1992

数据来源: AIP

ISSN:0094-243X    

DOI:10.1063/1.42876

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Helioseismic studies have revealed that the only sharp change in the Sun’s internal rotation occurs near the interface between the convective zone and the radiative interior. This region is generally regarded as the source of the solar activity cycle. Other helioseismic clues to the properties of the interface concern the magnetic field and the temporal stability of rotation there.

ISSN:0094-243X    

DOI:10.1063/1.42877

出版商:AIP    

年代:1992

数据来源: AIP