Carsten J. Denker

Rapid Change of δ Spot Structure Associated with Seven Major Flares

A large fraction of major flares occur in active regions that exhibit a δ configuration. The formation and disintegration of δ configurations is very important in understanding the evolution of photospheric magnetic fields. In this paper we study the relationship between the change in δ spot structures and associated major flares. We present a new observational result that part of penumbral segments in the outer δ spot structure decay rapidly after major flares; meanwhile, the neighboring umbral cores and/or inner penumbral regions become darker. Using white-light (WL) observations from the Transition Region and Coronal Explorer (TRACE), we study the short-term evolution of δ spots associated with seven major flares, including six X-class flares and one M-class flare. The rapid changes, which can be identified in the time profiles of WL mean intensity are permanent, not transient, and thus are not due to flare emission. The co-aligned magnetic field observations obtained with the Michelson Doppler Imager (MDI) show substantial changes in the longitudinal magnetic field associated with the decaying penumbrae and darkened central areas. For two events for which vector magnetograms were available, we find that the transverse field associated with the penumbral decay areas decreased while it increased in the central darkened regions. Both events also show an increase in the magnetic shear after the flares. For all the events, we find that the locations of penumbral decay are related to flare emission and are connected by prominent TRACE postflare loops. To explain these observations, we propose a reconnection picture in which the two components of a δ spot become strongly connected after the flare. The penumbral fields change from a highly inclined to a more vertical configuration, which leads to penumbral decay. The umbral core and inner penumbral region become darker as a result of increasing longitudinal and transverse magnetic field components.

RAPID CHANGES OF MAGNETIC FIELDS ASSOCIATED WITH SIX X-CLASS FLARES

In this paper, we present the results of the study of six X-class flares. We found significant changes in the photospheric magnetic fields associated with all of the events. For the five events in 2001, when coronagraph data were available, all were associated with halo coronal mass ejections. Based on the analyses of the line-of-sight magnetograms, all six events had an increase in the magnetic flux of the leading polarity of order of a few times 1020 Mx while each event had some degree of decrease in the magnetic flux of the following polarity. The flux changes are considered impulsive because the changeover time, which we defined as the time to change from preflare to postflare state, ranged from 10 to 100 minutes. The observed changes are permanent. Therefore, the changes are not due to changes in the line profile caused by flare emissions. For the three most recent events, when vector magnetograms were available, two showed an impulsive increase of the transverse field strength and magnetic shear after the flares, as well as new sunspot area in the form of penumbral structure. One of the events in this study was from the previous solar cycle. This event showed a similar increase in all components of the magnetic field, magnetic shear, and sunspot area. We present three possible explanations to explain the observed changes: (1) the emergence of very inclined flux loops, (2) a change in the magnetic field direction, and (3) the expansion of the sunspot, which moved some flux out of Zeeman saturation. However, we have no explanation for the polarity preference; i.e., the flux of leading polarity tends to increase while the flux of following polarity tends to decrease slightly.

High-Resolution Observations of Multiwavelength Emissions during Two X-Class White-Light Flares

We observed two X-class white-light flares (WLFs) on 2003 October 29 (~20:40 UT) and November 2 (~17:16 UT) using the Dunn Solar Telescope (DST) and its High-Order Adaptive Optics (HOAO) system in several wavelengths. The spatial resolution was close to the diffraction limit of DSTs 76 cm aperture, and the cadence was as high as 2 s. This is the first time that WLFs have been observed in the near-infrared (NIR) wavelength region. We present a detailed study in this paper comparing photospheric continuum observations during the two events with corresponding line-of-sight magnetograms from the Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager (MDI) and hard X-ray (HXR) data from the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). We also discuss several models that provide possible mechanisms to explain these continuum enhancements, especially in the NIR.

Near-Infrared Observations at 1.56 Microns of the 2003 October 29 X10 White-Light Flare

We present high-resolution observations of an X10 white-light flare in solar NOAA Active Region 10486 obtained with the Dunn Solar Telescope (DST) at the National Solar Observatory/Sacramento Peak on 2003 October 29. Our investigation focuses on flare dynamics observed in the near-infrared (NIR) continuum at 1.56 μm. This is the first report of a white-light flare observed at the opacity minimum. The spatial resolution was close to the diffraction limit of the 76 cm aperture DST. The data benefited from a newly developed high-order adaptive optics system and a state-of-the-art NIR complex metal oxide semiconductor focal plane array. In addition, we compared hard X-ray (HXR) data of RHESSI and magnetograms of the Michelson Doppler Imager on board SOHO with the NIR continuum images. Although it is still possible that some high-energy electrons penetrate deep to this layer, a more likely explanation of the observed emission is back-warming. During the impulsive phase of the flare, two major flare ribbons moved apart, which were both temporally and spatially correlated with RHESSI HXR ribbons. The maximum intensity enhancement of the two flare ribbons is 18% and 25%, respectively, compared to the quiet-Sun NIR continuum. The separation speed of the ribbons is about 38 km s-1 in regions with weak magnetic fields and decreases to about 19 km s-1, where stronger magnetic fields are encountered. The derived electric field in reconnection current sheet Ec is of the order of 45 V cm-1 during the flare maximum.

HIGH-CADENCE OBSERVATIONS OF AN IMPULSIVE FLARE

We analyzed high-cadence observations of a C5.7 flare on 1999 August 23 at Big Bear Solar Observatory (BBSO). The observing wavelength was 1.3 A in the blue wing of Hα, with a cadence of 0.033 s. In addition, the hard X-rays time profile obtained by the Burst And Transient Source Experiment (BATSE) and BBSO high-resolution magnetograms was compared with our Hα observations to understand in detail the particle precipitation in this event. The important results are as follows:

Synoptic Hα Full-Disk Observations of the Sun from Big Bear Solar Observatory – I. Instrumentation, Image Processing, Data Products, and First Results

The Big Bear Solar Observatory (BBSO) has a long tradition of synoptic full-disk observations. Synoptic observations of contrast enhanced full-disk images in the Ca ii K-line have been used with great success to reproduce the H i Lα irradiance variability observed with the Upper Atmosphere Research Satellite (UARS). Recent improvements in data calibration procedures and image- processing techniques enable us now to provide contrast enhanced Hα full-disk images with a spatial resolution of approximately 2′′ and a temporal resolution of up to 3 frames min−1.In this first paper in a series, we describe the instruments, the data calibration procedures, and the image-processing techniques used to obtain our daily Hα full-disk observations. We also present the final data products such as low- and high-contrast images, and Carrington rotation charts. A time series of an erupting mini- filament further illustrates the quality of our Hα full-disk observations and motivate one of the future research projects. This lays a solid foundation for our subsequent studies of solar activity and chromospheric fine structures. The high quality and the sunrise- to-sunset operation of the Hα full-disk observations presented in this paper make them an ideal choice to study statistical properties of mini-filament eruptions, chromospheric differential rotation, and meridional flows within the chromosphere, as well as the evolution of active regions, filaments, flares, and prominences.

Ultraviolet and Hα Emission in Ellerman Bombs

We present the first high-cadence time profiles of Ellerman bombs (EBs) at two wavelengths, 1.3 A in the blue wing of the Hα line and the UV continuum at 1600 A, and study their temporal correlation. Our results demonstrate that 46 out of 75 EBs exhibit a good correlation at the two wavelengths with a correlation coefficient greater than 50%, suggesting that a common energy release produces emission at the two wavelengths. We also find that the EBs with strong Hα emission tend to show a good Hα-UV correlation but that the weakly correlated or noncorrelated EBs are usually weak in Hα emission. More than half of the Hα-UV well-correlated EBs are located at the boundaries of unipolar magnetic areas; the others are located at, or close to, the magnetic inversion lines. However, the majority of the weakly or noncorrelated EBs are located at the magnetic inversion lines. Our results suggest that the physical mechanisms and the energy distributions are quite different in different types of EBs and that heating in the photosphere and temperature minimum region is very important for producing EBs. The high-cadence observations of EBs also confirm unambiguously that the light curves of EBs generally demonstrate a fast rise and a fast decay, with an average e-fold rising/decaying time of about 1 minute, which distinguishes EBs from the flare phenomenon.

Multiwavelength Study of Flow Fields in Flaring Super Active Region NOAA 10486

We present high-resolution observations of horizontal flow fields measured by local correlation tracking from intensity images in three wavelengths, i.e., G band (GB), white light (WL), and near-infrared (NIR). The observations were obtained on 2003 October 29 within the flaring super active region NOAA 10486, which was the source of several X-class flares, including an X10 flare that occurred near the end of the observing run. The data were obtained at National Solar Observatory/Sacramento Peak (NSO/SP) using the newly developed high-order adaptive optics (AO) system. We also use Dopplergrams and magnetograms from MDI on board SOHO to study the line-of-sight flow and magnetic field. We observe persistent and long-lived (at least 5 hr) strong horizontal and vertical shear flows (both in the order of 1 km s-1) along the magnetic neutral line (NL) until the X10 flare occurred. From lower photospheric level (NIR), the direction of the flows does not change up to the upper photosphere (GB), while the flow speeds in the shear motion regions decrease and, on the contrary, those in regions without shear motions increase with increasing altitude. Right after the X10 flare, the magnetic gradient decreased, while both horizontal and vertical shear flows dramatically enhanced near the flaring NL. Our results suggest that photospheric shear flows and local magnetic shear near the NL can increase after the flare, which may be the result of shear release in the overlying large-scale magnetic system or the reflection of a twisted or sheared flux emergence carrying enough energy from the subphotosphere.

Rapid Penumbral Decay following Three X-Class Solar Flares

We show strong evidence that penumbral segments decayed rapidly and permanently right after three X-class solar flares. Two of the three events occurred very recently in NOAA Active Region 10486, an X17 flare on 2003 October 28 and an X10 flare on 2003 October 29. The third X2.3 flare was observed in solar active region NOAA AR 9026 on 2000 June 6. The locus of penumbral decay is related to flare emission, albeit with distinct differences for each event. We present difference images highlighting the rapid changes between pre- and postflare states of the flaring active region, which show distinct decaying penumbral segments and neighboring umbral cores becoming darker. Because of the lack of spectroscopic data, we cannot exclude the possibility that the observed changes are due to changes in the temperature structure of the flaring atmosphere, or to a corresponding reduction in opacity for a section of both umbra and penumbra. However, we argue against this possibility because the observed intensity changes are permanent, not transient. We instead propose a possible explanation that magnetic fields change from a highly inclined to a more vertical configuration within approximately 1 hr after the flares; i.e., part of the penumbral magnetic field is converted into umbral fields.

Comparison of the 1998 April 29 M6.8 and 1998 November 5 M8.4 Flares

We combined, and analyzed in detail, the Hα and magnetograph data from Big Bear Solar Observatory (BBSO), full-disk magnetograms from the Michelson Doppler Imager (MDI) on board Solar and Heliospheric Observatory (SOHO), coronagraph data from the Large Angle Spectrometric Coronagraph (LASCO) of SOHO, Fe XII 195 A data from the Extreme ultraviolet Imaging Telescope (EIT) of SOHO, and Yohkoh soft X-ray telescope (SXT) data of the M6.8 flare of 1998 April 29 in National Oceanic and Atmospheric Administration (NOAA) region 8375 and the M8.4 flare of 1998 November 5 in NOAA region 8384. These two flares have remarkable similarities: 1. Partial halo coronal mass ejections (CMEs) were observed for both events. For the 1998 April 29 event, even though the flare occurred in the southeast of the disk center, the ejected material moved predominantly across the equator, and the central part of the CME occurred in the northeast limb. The direction in which the cusp points in the postflare SXT images determines the dominant direction of the CMEs. 2. Coronal dimming was clearly observed in EIT Fe XII 195 A for both but was not observed in Yohkoh SXT for either event. Dimming started 2 hr before the onset of the flares, indicating large-scale coronal restructuring before both flares. 3. No global or local photospheric magnetic field change was detected from either event; in particular, no magnetic field change was found in the dimming areas. 4. Both events lasted several hours and, thus, could be classified as long duration events (LDEs). However, they are different in the following important aspects. For the 1998 April 29 event, the flare and the CME are associated with an erupting filament in which the two initial ribbons were well connected and then gradually separated. SXT preflare images show the classical S-shape sheared configuration (sigmoid structure). For the 1998 November 5 event, two initial ribbons were well separated, and the SXT preflare image shows the interaction of at least two loops. In addition, no filament eruption was observed. We conclude that even though these two events resulted in similar coronal consequences, they are due to two distinct physical processes: eruption of sheared loops and interaction of two loops.