Date of Award

8-31-2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Applied Physics - (Ph.D.)

Department

Physics

First Advisor

Haimin Wang

Second Advisor

Yan Xu

Third Advisor

Dale E. Gary

Fourth Advisor

Wenda Cao

Fifth Advisor

Lucia Kleint

Sixth Advisor

Neepa T. Maitra

Abstract

As one of the most intense activities on the solar surface, flares have been extensively observed and studied ever since the first report. The standard model of solar flares has been established and commonly accepted. However, many limitations from the researching tools have left some of the problems unsolved or controversial. For example, the density of electrons in the corona is lower than it is required to activate the observed emission in HXR, and the mechanism that these electron beams can penetrate down to lower chromosphere is unclear. Many theoretical scenarios were suggested, and more observations had been in need.

Multi-wavelength observations are powerful tools in revealing the details of solar flares. Following the improvement of research instruments, such as spacecraft, telescopes, charged-coupled devices (CCDs) and computing devices, we are able to make better use of the emissions for understanding the flare. For instance, Goode Solar Telescope in Big Bear Solar Observatory (BBSO/GST), equipped with a 1.6-meter major mirror, has been dedicated to solar observation. With a resolution up to about 0.03 arc-second per pixel, it is capable of providing detailed information of fine structures in solar flares. Interface Region Imaging Spectrograph (IRIS) offers images in the ultraviolet (UV) together with spectrograms over several wavelength windows, including tens of spectral lines that are powerful in diagnosing the flaring atmosphere. Solar Dynamic Observatory (SDO) records solar full-disk images in multiple wavelengths, from the extreme ultraviolet (EUV) to the visible continuum, covering a wide range of temperatures. Moreover, thanks to the improvement of computing power, more plausible codes are developed to calculate the flaring atmosphere.

Taking advantages of the high-resolution instruments and novel numerical modeling packages, the dissertation work cover several topics, from the energetics of white-light emission in macro-scope to the sub-arcsecond features on flare ribbons in multiple wavelengths and the corresponding modeling. As summarized below, the major results provide additional and important constraints in understanding the flare emission and instructive for future observations and developing of new modeling:

Using the SDO/HMI images and RHESSI hard X-ray (HXR) spectra, the relationship between white-light (WL) and HXR emissions id found. The correlation between HXR power-law indices and WL emissions indicates the importance of non-thermal electrons' energy distribution in stimulating the WL flares. This suggests the direct heating mechanism accounts for the core of the compact WL flares.

The WL flares, which are considered to be in the most violent class, and solar energetic particle (SEP) events are under survey, and no clear correlation is found between them. Straightforward speculation is that the acceleration process could be different for SEPs and the energetic electrons powering WLFs in the events analyzed.

Emissions from chromospheric spectral lines, Mg II k line and H? are observed using IRIS and BBSO/GST, respectively for the flare on 2015-06-22. Unique features of the line profiles are observed in narrow edge of the ribbon. Numerical study using combination code of RADYN and RH suggests the formation height and corresponding thermodynamic conditions of the distinct line feature.

Inspired by a study of solar flares in He I 10830 Å line that observed enhancement absorption in the frontiers of flare ribbons, we analyze the evolution of the line emission in numerical models and compare it with observations. The result suggests that the temperatures and free electron densities at heights of 1.3-1.5 Mm should be larger than ~104K and 6*1011cm3 are thresholds for the line to start being in emission.

With the high-resolution vector magnetogram, in wavelength of 1.56 ?m that is from the lower layer of the atmosphere, a transient rotation of the local magnetic field is observed in the leading edge of 2015-06-22 flare ribbon. The azimuth angle rotates closer to the extrapolated potential field. This newly observed magnetic field activity may be related to the energetic electron beams, but cannot be well explained using existing models.

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