Footpoints Of Field Lines Research Articles

Magnetic Topology and Current Sheet Formation

AbstractWe describe a mechanism for coronal heating. The basic idea is that since the photospheric flux is observed to consist of a complex pattern of positive and negative polarity regions, the topology of the coronal magnetic field (in particular the connectivity) must be discontinuous over a complex network of surfaces and magnetic null points in the corona. Consequently, photospheric motions of the field line footpoints, even if arbitrarily smooth, result in discontinuous stressing of the field. This produces coronal current sheets, reconnection at the null points, and rapid heating.

Positional characteristics of meter-decameter wavelength bursts associated with hard X-Ray bursts

Several type III, type II, and type IV bursts were observed on April 25 and 26, 1979 with the Clark Lake Radio Observatory's E-W and N-S swept frequency interferometers in the range 20–110 MHz. The radio bursts were associated with hard X-ray bursts in the energy range 26–154 keV, as observed by ISEE-3. The type III bursts, which were associated with impulsive hard X-rays, were observed to great heights (∼ 3.1R ⊙ from disk center at 28 MHz) and their location indicates that the electron streams responsible for them were injected at the footpoints of magnetic field lines which diverge in the corona. With one exception, all the type III bursts occurred in dense coronal regions. Two gradual hard X-ray bursts were observed to occur in association with a type IV without type II, and a type IV-type II burst. For the gradual burst (observed on April 25) associated with a type IV only, it is believed that part of the energetic electrons responsible for meter-decameter type IV are trapped in a plasmoid behind a weak shock, as evidenced by the absence of a type II, while another part is located in low lying magnetic loops producing centimeter and hard X-radiation. The type II burst associated with the other gradual hard X-ray burst (observed on April 26) started approximately 9 min after the impulsive hard X-ray burst peak. This rather long delay between the type II onset and the impulsive maximum is believed to represent the time interval over which the shock becomes strong enough to produce a detectable type II higher in the corona.

Evolution of open magnetic structures on the sun - The SKYLAB period

High-resolution harmonic analysis of the measured photospheric magnetic field of the sun is used to construct models of open magnetic structures over a period of 11 solar rotations. The models successfully reproduce the surface location and topology of all coronal holes during the Skylab period. In addition, there is persistent evidence in the models that open field lines are associated with active regions in a systematic way. These associations are listed for the period studied; they suggest that open field lines are a basic feature of solar magnetism. Specific examples of the evolution of coronal holes and of calculated open structures are presented. Quantitative study of the measured field strength within and neighboring a hole confirms the fact that coronal hole regions are indistinguishable by local magnetic properties. However, the calculated field strengths at the footpoints of open field lines within coronal holes show distinct evolutionary patterns and may indicate that, at least in young coronal holes, a significant amount of magnetic flux is closed. Problems of studying magnetic field divergence by using these models are discussed.

Differential rotation and the structure and energy content of coronal magnetic fields

It is argued that differential rotation of the photospheric magnetic fields will induce currents in the corona. The work done against surface magnetic stresses will increase the energy content of the coronal magnetic field. The electrical conductivities are high and the foot points of field lines move with the differential rotation. The force-free field equations are solved with this constraint to obtain a minimum estimate of the energy increase for a quadrupole field. During a solar rotation the magnetic energy increases by 25%. Local release of this energy in the corona would have a significant effect. The expansion of field lines as a result of the differential rotation should increase the amount of flux and the field strength in the solar wind region.