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Title: Studies of solar white-light flares and small-scale magnetic structures observed in the near infrared
Author: Xu, Yan
View Online: njit-etd2005-140
(xvii, 111 pages ~ 11.6 MB pdf)
Department: Federated Physics Department of NJIT and Rutgers-Newark
Degree: Doctor of Philosophy
Program: Applied Physics
Document Type: Dissertation
Advisory Committee: Wang, Haimin (Committee co-chair)
Denker, Carsten J. (Committee co-chair)
Goode, Philip R. (Committee member)
Gary, Dale E. (Committee member)
Wu, Zhen (Committee member)
Date: 2005-08
Keywords: Sun
High angular resolution
Atmospheric motions
Availability: Unrestricted

Using the most advanced infrared imaging technology as developed by NJIT, detailed study of solar white-light flares and small magnetic structures, such as faculae and pores, are presented in this dissertation. The investigations focus on near-infrared observations at 1.56 μm, which are good proxy of the deepest layer of the solar photosphere.

I made fundamental contributions in two areas of near infrared (NIR) solar physics: (1) the first detection and understanding of white-light flares in the NIR and (2) clearly demonstrated non-existence of "dark faculae". Several high-resolution observations have been carried out at BBSO and National Solar Observatory/Sacramento Peak. The data benefited from a newly developed state-of-the-art near-infrared photometric system and the high-order adaptive optics system. In addition to the near-infrared observations, visible continuum and G-band images were obtained simultaneously, as well as the data from satellites, such as Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SoHO) and the Ramaty High Energy Solar Spectroscopic Imager (RHESSI), were acquired for comparison.

For flare study, the very first near-infrared observations of two white-light flares are presented. The flare morphology and dynamics are briefly summarized as follows: (1) Significant intensity enhancements appeared in the near-infrared and visible continua and G-band. The maximum intensity enhancements are much higher than the prediction of any existing models: 25% to 66% of the NIR continuum and 45% to 76% for the visible white-light; (2) The flares were typical two ribbon flares. During the impulsive phase, two major flare ribbons moved apart. The flare ribbons in the near-infrared and other wavelengths were both temporally and spatially well correlated with RHESSI hard X-ray; (3) All ribbons showed a brighter core surrounded by a faint halo structure. The ribbon separation speeds were about 28 km/s in the first and 24 km/s in the second event based on MR observations. The derived electric fields in the reconnection current sheet Ec are about 23 V cm-1 and 22 V cm-1, respectively; (4) The MR emission and the impulsive HXR emission up to 800 keV were well correlated with a small delay of less than two minutes; (5) The high resolution and high cadence images gave us the first chance to measure the cooling time of flares at photospheric height. We found that the cooling process could be characterized by two steps: A quick temperature drop, which is related to the cooling process of the bright cores, and a relatively slow decay related to the halo structures. The time scale is in the order of less than 30 seconds and a few minutes for these two steps, respectively. The findings and results can be explained by combination of several existing models.

The high-resolution data are also used for the study of small magnetic structures. Images observed in different wavelengths represent the properties of faculae or pores in different layers of the solar atmosphere from the bottom of the photosphere using near-infrared at 1.56 μm to the upper photosphere using G-band. Both statistical studies and individual examinations show that the contrasts of faculae and pores have the same sign in both the near-infrared and visible continua. In other words, no so-called "dark faculae" that are bright in the visible but dark in the near-infrared was found. The previously observed "dark faculae" are most likely unresolved pores or due to non-simultaneous observations. In addition, a threshold for the magnetic flux density that separates pores from faculae was determined.

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