Solar magnetic field and Solar activities

    Solar activities include various phenomena on different scales: granules, spikes, flow along fibrils on small scale, and sunspot, flare, supergranule on active region scale, and filament (prominence), corona hole, coronal mass ejection (CME) on heliospheric scale. Almost all of these activities are associated with magnetic field. The following texts describe the relationship of magnetic fields with solar flare, filament (prominence) eruption, CME.

1. Solar magnetic field and Solar flare
    The solar magnetic fields in photosphere of the Sun can be directly diagnosed by measuring the Zeeman Effect of spectral lines formed in the photosphere. The so-called Zeeman Effect is the effect of splitting a spectral line into several components in the presence of a static magnetic field. The measured polarization of the split components of the spectral lines depends on the magnitude and direction of the magnetic field vector. By measuring the circular and linear polarization of the spectral line, one can derive the magnetic field vector in principle with some assumption of the solar atmosphere. So far, the relative reliable measurement of magnetic field of the Sun is just for the photosphere.

Figure 1 The Carrington Flare observed in 1859

    It is found that the occurrence and process of solar flare are very closely related to the solar magnetic field, and now it is believed that the energy released during solar flares originates from the conversion of solar magnetic field energy. Figure 2 shows a line-of-sight (LOS) magnetogram of a large famous sunspot active region observed by Huairou Solar Observing Station (HSOS), National Astronomical Observatories of China (Small earth on the upper-right corner illustrates a relative scale). This active region with large area has very complex magnetic configuration, different from the common 'leading and following' dipole type, and therefore contains higher non-potential magnetic energy which leads to the eruption of a large flare. The magnetic field reconnection during flare changes the magnetic field topology, which represents the structure configuration of magnetic field evolving from the high-energy state to the low-energy state. After flare, the magnetic field inclines to the low energy state approaching the potential field, and the longitudinal field decreases along the neutral line region, accompanied by the transverse field increasing.

Figure 2 Line-of-sight magnetogram of a large active region

2. Solar magnetic field, filament eruption and CME
    In the solar chromosphere, there often appear some faint stripe structures, which absorb the background light on solar disk and are called filament. When these structures are observed at the solar limb, they become brighter than the dark cosmic background and resemble as a convex 'ear' of the Sun on large-scale case, and therefore are called prominence. Due to magnetic force, the filament composed of low temperature and high-density materials is suspended and supported in high temperature and low-density corona and chromosphere. Figure 3 shows a huge filament and corresponding LOS photospheric magnetic field. The filament normally appears on the boundary of opposite magnetic field (magnetic neutral line), and the magnetic field plays an important role in supporting the filament. Modern high-resolution imaging observation shows that the filament is very dynamic on the microscopic level. The traditional filament model can qualitatively illustrate how the filament suspends and what supports the filament, but it cannot explain filament oscillation, fiber structure, material falling, etc. The observation of filament magnetic field in high resolution will further clarify these problems.

Figure 3 Filaments on the solar disk and corresponding LOS magnetic fields

    Filaments can disappear or reappear due to the local heating, and it can also erupt completely. For the three processes, filament eruption, flare, and CME, it is difficult to thoroughly separate one from the others, and actually it is a variation of corona and chromosphere as a whole. Figure 4 shows the eruption process of an east-west extended filament on the north-west solar limb. In the initial stage, the filament is suspended along the neutral line of the large scale dipole magnetic field (left panel, SOHO/MDI); the filament loses stability and floats up (marked with 'F' in the middle panel, SOHO/EIT); the filament becomes a part of the CME after eruption (marked with 'P' in the right panel, SOHO/LASCO).

Figure 4 Eruption of a large filament and its relation with CME

    The numerical simulation of magnetohydrodynamics (MHD) demonstrates that a magnetic flux tube enters the corona from solar interior can form a stable configuration initially. Due to the various motion of magnetic field in the photosphere or the magnetic flux emergence, the corona accumulates excess free energy by shearing, winding or by new emerging process of the photospheric magnetic field. As soon as the instability condition is reached, the filament is driven to erupt and then subsequently form CME's and flares.





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