Hong Sik Yun

Temperature Dependence of Ultraviolet Line Average Doppler Shifts in the Quiet Sun

The existence of prevailing redshifts in the UV lines formed in the solar transition region raises an important question concerning its physical origin and its role in the mass and energy balance of the outer solar atmosphere. A series of UV spectral lines observed by SUMER has been analyzed to obtain the spatial average of Doppler shifts in the quiet Sun as a function of temperature. The UV lines used for the analysis cover temperatures ranging from 104 to 106 K. The wavelength calibration has been done in reference to the coolest chromospheric lines such as neutral lines of silicon and sulfur. The positioning of the line center in blended lines has been made by employing a constrained multi-Gaussian fitting technique. The error in the measured average of the Doppler shifts is estimated to be smaller than 1 km s-1. Our results show that the average Doppler shift at the base of the transition region is about 1-2 km s-1, increasing with temperature with a peak value of 11 km s-1 near T = 2.3 × 105 K. Then it decreases but remains still above zero (5 km s-1 in Ne VIII lines and 4 km s-1 in Mg X lines). We find that this behavior can be explained by the dominance of emission from plasma flowing downward from the upper hot region to the lower cool region along flux tubes with varying cross section. Assuming that pressure and mass flux are constant along a flux tube, the cross section of a typical flux tube has been estimated as a function of temperature. It turns out that the cross section is nearly constant below T = 105 K and then expands by a factor of about 30 at T = 106 K. This behavior is fairly well represented by an analytical functional form, A(T)/A(Th) = [1 + (Γ2 - 1)(T/Th)ν]1/2/Γ with parameters of Th = 106 K, Γ = 30, and ν = 3.6.

Spatio-spectral Maximum Entropy Method. I. Formulation and Test

The spatio-spectral maximum entropy method (SSMEM) has been developed by Komm and coworkers in 1997 for use with solar multifrequency interferometric observation. In this paper we further improve the formulation of the SSMEM to establish it as a tool for astronomical imaging spectroscopy. We maintain their original idea that spectral smoothness at neighboring frequencies can be used as an additional a priori assumption in astrophysical problems and that this can be implemented by adding a spectral entropy term to the usual maximum entropy method (MEM) formulation. We, however, address major technical difficulties in introducing the spectral entropy into the imaging problem that are not encountered in the conventional MEM. These include calculation of the spectral entropy in a generally frequency-dependent map grid, simultaneous adjustment of the temperature variables and Lagrangian multipliers in the spatial and spectral domain, and matching the solutions to the observational constraints at a large number of frequencies. We test the performance of the SSMEM in comparison with the conventional MEM.

Hα, Extreme-Ultraviolet, and Microwave Observations of the 2000 March 22 Solar Flare and Spontaneous Magnetic Reconnection

The evolution of a GOES class X1.1 solar flare, which occurred in NOAA Active Region 8910 on 2000 March 22, is discussed using observations from the Owens Valley Solar Array (OVSA), Big Bear Solar observatory (BBSO), Transition Region and Coronal Explorer (TRACE), and the Michelson Doppler Imager (MDI) on board Solar and Heliospheric Observatory (SOHO). During the impulsive phase, a set of coronal loops are visible in the TRACE 171 A ˚ (� 1 � 10 6 K) wavelength band, which is confined to a small volume in the center of the large ��� -type active region. This is rapidly followed by the emergence of bright Hribbons that coincide with the EUV emission. Radio images show a single source encompassing the Hribbons at 5 GHz, but at higher frequencies a double source is seen within the area bounded by the compact Hand EUV emissions. We interpret the observation under the idea of the confined flare in contrast with the more com- monly cited, eruptive flare. We use a schematic magnetic reconnection geometry based on the MDI magneto- gram to suggest that the EUV loops show some parts of a separatrix, and that the radio and Hsources coincide with the whole part of the separatrix and its footpoints, respectively. First of all, it explains why this flare lacks the separating motion of Hribbons, a signature for eruptive flares. Second, the very short dura- tion of microwave bursts in spite of the large amount of soft X-ray flux is explicable under this scenario, since energy release via spontaneous reconnection in a confined magnetic structure can be very rapid. Third, the confined magnetic geometry is also considered favorable for preserving chromospheric evaporation and plasma turbulence as inferred from the OVSA microwave spectrum. In addition, a coronal mass ejection as detected in the LASCO coronagraph after this flare is briefly discussed in relation to the above flare model. Subject headings: MHD — radiation mechanisms: nonthermal — Sun: flares — Sun: magnetic fields — Sun: radio radiation — Sun: UV radiation

Effects of Non-LTE Radiative Loss and Partial Ionization on the Structure of the Transition Region

In this paper we address the question of how non-LTE radiative losses with partial ionization of hydrogen and helium affects the energetics and structure of the solar transition region. To accomplish this we have constructed theoretical models of a thin rigid magnetic flux tube with a steady material flow, which is embedded vertically in the solar atmosphere. These models include the effects of material flow, conduction, non-LTE radiative transfer in H and He, and partial ionization. We find from this study that the effect of non-LTE radiative transfer with partial ionization is significant near the base of the transition region at temperatures less than 2.5 × 104 K. This leads to a 1 order of magnitude increase in the differential emission measure in comparison with the optically thin approximation with complete ionization in the low (less than 2.5 × 104 K) temperature regime. Above this region the non-LTE and opacity effects are small. In the upflow case the conductive and convective energy processes dominate to such a large extent that non-LTE radiative process and partial ionization are not important. In this work we also confirm the previous work of other authors who provided the explanation for why downflowing transition region material is much more visible than upflowing material. We present the results in a manner that gives a good physical understanding as to why this occurs.

Stray-Light Effect on Magnetograph Observations

To examine the stray-light effect in magnetograph observations, we have determined the point spread functions of the vector magnetograph mounted on the Japanese Solar Flare Telescope based on two indirect methods, one analyzing the solar limb intensity profile, and the other using the Fourier power spectra of photospheric intensity distributions. Point spread functions consist of two parts: a blurring part which describes seeing and small-spread-angle stray light, and a scattering part which describes large-spread-angle stray light. The FWHM spatial resolution is typically 3.0′′, and the amount of scattered light is about 15% on clear days. We find that the blurring part is well described by a Moffat function whose Fourier transform is given by an exponential function. Our results indicate that polarization measurements of low-intensity magnetic elements like sunspots may be significantly underestimated due to the large-spread-angle stray light, and polarization measurements of magnetic elements which are smaller than 5–7′′ appear to be disturbed by small-spread-angle stray light.

Stray-light correction in magnetograph observations using the maximum entropy method

We have developed a method of stray-light correction which is applicable to filter-based magnetograph observations. Stray-light-corrected Stokes images are obtained by performing the deconvolution of observed Stokes images by the point spread function which is determined from the Stokes I image. For image deconvolution, the maximum entropy principle is used to guarantee that intensity should be positive and polarization degrees should be less than unity. We present an iterative algorithm for the maximum entropy method, which seeks the solution in Fourier space and thus accomplishes fast convergence. We find that our method is effective in correcting stray light which has a spread angle greater than the full width at half maximum of the point spread function. We also discuss the effect of stray light on magnetograph calibration.

Spatio-Spectral Maximum Entropy Method for Solar Microwave Imaging Spectroscopy

In a companion paper, we have presented so-called Spatio-Spectral Maximum Entropy Method (SSMEM) particularly designed for Fourier-Transform imaging over a wide spectral range. The SSMEM allows simultaneous acquisition of both spectral and spatial information and we consider it most suitable for imaging spectroscopy of solar microwave emission. In this paper, we run the SSMEM for a realistic model of solar microwave radiation and a model array resembling the Owens Valley Solar Array in order to identify and resolve possible issues in the application of the SSMEM to solar microwave imaging spectroscopy. We mainly concern ourselves with issues as to how the frequency dependent noise in the data and frequency-dependent variations of source size and background flux will affect the result of imaging spectroscopy under the SSMEM. We also test the capability of the SSMEM against other conventional techniques, CLEAN and MEM.