Sangwoo Lee

Small Magnetic Bipoles Emerging in a Filament Channel

Observations have shown that quiescent prominences or filaments have a hemispheric magnetic pattern of chirality. Motivated by the question of whether the filament chirality is of subsurface origin or not, we have studied small magnetic bipoles that emerged in a quiescent filament channel at latitude N45°. During our 5 day observing run, performed in 1999 October, a huge filament erupted and reformed shortly in the same filament channel. Using high-cadence, long-integration line-of-sight magnetograms taken at Big Bear Solar Observatory, we identified a total of 102 bipoles that showed an average total flux of 2.8 × 1019 Mx, an average separation of 7400 km at the time of full development, and an emergence rate of 430 hr-1 per the entire solar surface area. These properties indicate that most of the bipoles are ephemeral regions. The most important finding in the present study is that the magnetic axes of the bipoles emerging in the filament channel are systematically oriented; a negative (trailing) pole is observed to be located preferentially to the south-east of its companion positive (leading) pole. This preferred orientation does not match either the Hale law of active region orientation or a theory that attributes the axial field of a filament to emerging bipoles. We propose two possible subsurface field configurations of bipoles consistent with the observed preferential orientation and discuss physical implications of our results for understanding filament magnetic fields.

A Rapid Change in Magnetic Connectivity Observed Before Filament Eruption and Its Associated Flare

To gain insight to the cause of filament eruptions and flares on the Sun, we observed a filament that erupted in active region NOAA 8597. The observations consisted of Hα filtergrams at three wavelengths (line center and ±0.5 A) and line-of-sight magnetograms. All were taken on 1999 June 24 at Big Bear Solar Observatory. We found from the time sequence of Hα images that the filament eruption was preceded by a rapid change in connectivity in a bundle of filament threads. The thread bundle was initially sharply curved near its one end of the filament and suddenly flipped and then became straight in the new orientation. The flipped segment of the thread bundle swept over a 100 × 50 area on the solar surface in about half an hour. At the latter stage of the connectivity change, we observed a downward draining of material along the thread bundle that had a transverse component of 50 km s-1. After that, the filament body split into two parallel parts, one part erupted while the other part remained, and the two-ribbon flare occurred. We also found canceling magnetic features in the vicinity of the initial location of the thread end, which displayed a flux decrease during the Hα connectivity change. Our results show clear and direct evidence that magnetic reconnection takes place in the low atmosphere prior to eruption. This preeruption reconnection seems to be very different from a posteruption coronal reconnection, which is believed to lead to a two-ribbon flare.

Time-dependent Flare Signature in Hydrogen Balmer Lines

In recent studies of Hα flare imaging spectroscopy performed by Leka et al. and de la Beaujardiere et al. in 1993, Hα line profiles have been investigated as a function of space to locate the physical process underlying observed spectral profiles. In this Letter, we discuss time evolution of hydrogen Balmer line profiles at a fixed spatial point recorded by the multichannel spectrograph at Nanjing University in China during the 1991 October 27 white-light flare which occurred in NOAA 6891. It is found that the Balmer emission lines with a central reversal show up at the beginning of the Hα flare but change to single-peaked profiles in about 30 s. This result may, according to Canfield et al.s model in 1993, indicate that the Balmer line enhancement even at one flare site could be caused by two physical processes, namely, initially by the high-energy particle precipitation and subsequently by the high coronal pressure. We also present evidence for downward motion and the Stark broadening effect on the observed Balmer line wings to examine further the physical consequences of the particle precipitation. Time correlation of the Balmer line emission with hard X-ray and continuum brightening is briefly discussed based on the spectral information.

Spectral analysis of H?, H?, and H? lines of a white-light flare (3B/X6.1) on October 27, 1991

We have analyzed time series of Hα, Hβ, and Hγ line profiles taken from a 3B/X6.1 flare which occurred on October 27, 1991 in active region NOAA 6891. Each set of the spectra was taken simultaneously for the first 10 min of the flare event with a low and non-uniform time resolution of 10–40 s. A total of 22 sets of Hα, Hβ, and Hγ were scanned by a PDS with absolute intensity calibration to derive the dynamics and energetics of material in the flare region. Our results are as follows: (1) The Balmer line emitting region is accelerated downward to about 50 km s-1 for the first 50 s and then is decelerated to about 10 km s-1 for the next 150 s. (2) The radial velocity peak precedes the Balmer line intensity peak by about 40 s. (3) The total energy radiated from these Balmer lines is estimated to be 4.9 × 1029 erg.