The 2nd PANDA Symposium on Multimessager Astronomy - Jets and Shocks in the Universe

Two Types of Magnetic Reconnection in Coronal Bright Points and the Corresponding Magnetic Configuration

25 Apr 2013, 17:25

5m

Xi'an

Oral Talk

Speaker

Mr Qingmin Zhang (Purple Mountain Observatory, CAS)

Description

Coronal bright points (CBPs) are long-lived small-scale brightenings in the solar corona. They are generally explained by magnetic reconnection. However, the corresponding magnetic configurations are not well understood. We carry out a detailed multi-wavelength analysis of two neighboring CBPs on 2007 March 16, observed in soft X-ray (SXR) and EUV channels. It is seen that the SXR light curves present quasi-periodic flashes with an interval of ~1 hr superposed over the long-lived mild brightenings, suggesting that the SXR brightenings of this type of CBPs might consist of two components: one is the gentle brightenings and the other is the CBP flashes. It is found that the strong flashes of the bigger CBP are always accompanied by SXR jets. The potential field extrapolation indicates that both CBPs are covered by a dome-like separatrix surface, with a magnetic null point above. We propose that the repetitive CBP flashes, as well as the recurrent SXR jets, result from the impulsive null-point reconnection, while the long-lived brightenings are due to the interchange reconnection along the separatrix surface. Although the EUV images at high-temperature lines resemble the SXR appearance, the 171 Å and 195 Å channels reveal that the blurry CBP in SXR consists of a cusp-shaped loop and several separate bright patches, which are explained to be due to the null-point reconnection and the separatrix reconnection, respectively.

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In this paper, we carried out a detailed multi-wavelength
analysis of two neighboring CBPs captured by Hinode/XRT
and STEREO-B/EUVI on 2007 March 16. During the 5 hr
observations, the newly formed northern CBP was dynamic,
with strong flashes accompanied by SXR jets. The spot-like
feature in SXR can be resolved into a cusp-shaped loop under
the jet and several discrete kernels in 171Å and 195 Å. The preexisting
CBP, i.e., BP2, however, presented gentler variations,
probably due to the fact that it was in the late phase of
its lifetime, and no jets were found during the observations.
The potential field extrapolation revealed that the two bright
points are located above “embedded bipolar fields,” where
one polarity is surrounded by the opposite polarity. Both of
them are covered by a dome-like separatrix surface with a
magnetic null point above. The height of null points, which
characterizes the height of reconnection, is found to correlate
with the magnetic flux of the embedded polarities under the null
points.We suggest that the SXR light curves of the bright points
of this type present two components: quasi-periodic recurrent
flashes with a period of ∼1 hr and long-lived brightenings with
small-amplitude fluctuations. The CBP flashes result from the
impulsive null-point reconnection between the small-scale loops
and the unipolar field, which leads to the cusp-shaped structure
and jets guided by the spine field lines, whereas the long-lived
brightenings are due to the gentle interchange reconnection
along the separatrix surface between the anemone-like loops
and the unipolar field, which gives rise to bright kernels around
the cusp. The two types of reconnection in a single event might
be responsible for those CBPs associated with recurrent jets.
According to the 3D magnetic configuration of the CBPs, we
infer that the quasi-periodic CBP flashes might be caused by the
magnetic reconnection modulated by slow-mode wave along
the spine field lines or be due to the recycling of continuous
pumping of magnetic energy via the localized rotating motions
of the photosphere and the sporadic releasing of the magnetic
free energy. Further investigations are required in order to find
out how often the CBPs are associated with the “embedded
bipolar field.”