194th AAS Meeting - Chicago

Contributions of the Center for Solar Research (NJIT) to the 194th Meeting of the American Astronomical Society, 30 May - 3 June 1999, Chicago, IL

Extreme-Ultraviolet Jets and H-alpha Surges in Solar Microflares

Jongchul Chae, Jiong Qiu, Haimin Wang, and Philip R. Goode

Big Bear Solar Observatory, New Jersey Institue of Technology, Big Bear City, CA 92314, U.S.A.

Abstract. We analyzed simultaneous EUV data from the Transition Region And Coronal Explorer (TRACE) and H-alpha data from Big Bear Solar Observatory (BBSO). In the active region studied, we found several EUV jets that repeatedly occurred where pre-existing magnetic flux was canceled by newly emerging flux of opposite polarity. The jets look like, but are usually smaller and shorter lived than Yohkoh soft X-ray jets. The EUV jets have a typical size of 4000-10,000 km, a transverse velocity of 50-100 km s-1, and a lifetime of 2-4 minutes. Each of the jets was ejected from a loop-like bright EUV emission patch at the moment when the patch reached its peak emission. We also found dark H-alpha surges that are correlated with these jets. A careful comparison, however, revealed that the H-alpha jets are not cospatial with the EUV jets. Instead, the EUV jets are are identified with bright jetlike features in the H-alpha line center, which are distinct from dark surges. Our results support a picture in which H-alpha surges and EUV jets represent different kinds of plasma ejection - cool and hot plasma ejections along different field lines - which must be dynamically connected to each other. We emphasize the importance of observed flux cancellation and a small erupting filament in understanding the acceleration mechanisms of EUV jets and H-alpha surges.

Large-Scale Structures of Solar Flares

Carsten Denker, William Marquette, Haimin Wang, Philip R. Goode, and Anders Johannesson

Big Bear Solar Observatory, New Jersey Institue of Technology, Big Bear City, CA 92314, U.S.A.

Abstract. Since December 1997, the Big Bear Solar Observatory (BBSO) has provided daily, contrast enhanced, H-alpha full disk images of unsurpassed quality, temporal resolution of about 30 s, and spatial resolution of about 2 arcsec which allow us to study the evolution of small-scale structures and low-contrast features. This data set has the right qualities to allow us to study large-scale phenomena associated with major solar flares such as Moreton waves, transient brightening of the H-alpha network, filament eruptions and disappearances. In 1998, 31 flares of magnitude M3.0 or larger were observed by the Geosynchronous Operational Environmental Satellites (GOES). Eight of them occurred during the typical observing hours at BBSO and seven were actually covered by H-alpha full disk observations presented here. We provide a detailed description of various chromospheric disturbances initiated by the flares, the influence of magnetic fields on their appearance, and their association with coronal mass ejections.

The OVRO Solar Array During Max Millenium

Dale Gary1 and Gordon J. Hurford2

(1) Physics Department, New Jersey Institute of Technolgy, University Heights, Newark, NJ 07102-1982
(2) Space Science Laboratory, University of California at Berkeley, Berkeley, CA 94720-7450

Abstract. The Owens Valley Radio Observatory (OVRO) Solar Array is a solar-dedicated radio imaging instrument that obtains flare and active region data in the range 1-18 GHz. The array is currently undergoing extensive software and hardware upgrades, to be completed by mid-2000 - in time for the launch of the HESSI spacecraft. The hardware improvements include adding two antennas to the 5 existing ones, bringing the number of antennas to 7 and the number of baselines to 21. This addition will improve the image quality and double the east-west spatial resolution, to 5 arcsec at 10 GHz.

Along with the hardware improvements, the computer control and data systems are being completely revamped to modernize the data handling and make the data available and useful to outside users on a daily basis. Certain standard data products (e.g. calibrated total power spectra of flares), will be available by this summer, via web server, for open use by the solar community. Complete interferometry data will be available by the time of the HESSI launch, in self-contained data files that can be analyzed using freely-available, user-friendly IDL widget-driven software. It is planned to model the analysis software after the HESSI analysis software, so that those who have mastered the use of the HESSI software will be able to use the OVRO software without difficulty. This is possible since the HESSI and OVRO data are parallel in many ways. Both obtain spatial data in the form of time profiles of spatial fourier components, at many spectral points. Both pertain to emission from both high-energy and thermal electrons in the corona. The two data-sets are highly complementary, and it is expected that the OVRO data will be of great interest to many researchers working on HESSI data. Ultimately, the full OVRO data will be archived at the HESSI European Data Center (HEDC).

Solar Asphericities from BBSO Synoptic Data and MDI Splittings

Philip R. Goode1, Wojtek A. Dziembowski1, 2, and William Marquette1

(1) Big Bear Solar Observatory, New Jersey Institute of Technology, Big Bear City, CA 92314, U.S.A.
(2) Warsaw University Astronomical Observatory, Al. Ujazdowskie 4, Pl-00-478 Warszawa, Poland, and Copernicus Astronomical Center, Poland

Abstract. Beneath the photosphere, on average the Sun is almost a perfect sphere. Historically, the main photospheric asymmetry studied has been the magnetic activity cycle as reflected in the Maunder butterfly diagram. In recent years, more subtle signatures of asymmetries varying with the solar cycle have been found. We have used temporal averages of BBSO synoptic maps from the activity minimum which has just ended to extract the low degree Legendre dependence of the data. We present preliminary comparisons of the degree of correlation between these Legendre coefficients varying through the solar cycle with the temporally corresponding low degree Legendre coefficents from MDI splitting data to enable us to garner another perspective, the role of the magnetic field in the Sun's cycle dependent asphericity.

Dynamical Characteristics of H-alpha Fine Features on the Quiet Sun

Chik-Yin Lee, Haimin Wang, and Jongchul Chae

Big Bear Solar Observatory, New Jersey Institue of Technology, Big Bear City, CA 92314, U.S.A.

Abstract. The Littrow spectrograph in the Big Bear Solar Observatory (BBSO) is an unique grating system which can produce a 4-dimensional (x, y, lambda, t) data array. We are mainly interested in studying H-alpha fine features like upflow events, spicules/fibrils, and dark grains by means of constructed spectroheligrams. We have found from upflow events: (1) The typical profile indicates a spectral character in which absorption is only observed in the blue wing. (2) The line-center optical thickness is lower than that of spicules. (3) The motion has no receding phase as seen in H-alpha- as shown by Doppler signals. The lifetime, size, and velocity of upflow events are found to be 1.4 minutes, 2.5 arcsec (FWHM), and 15 km s-1 respectively. Dark grains may be considered to be the least energetic upflow events in the sense that its lifetime and size are about half that of upflow events. The total birthrate is estimated to be 200 s-1, which is about one third of that of explosive events. The association of the magnetic origin of explosive events has been reported by a previous study on the use of BBSO/magnetograms and SOHO/SUMER UV images. The electron temperature (104 K) and density (1010 cm-3) of upflow events can easily be found. The global particle and kinetic energy flux are determined to be 1015 cm-2 s-1 and 105 ergs cm-2 s-1 respectively. Judged by the favorable amount of particle flux, we propose a scenario in that the predominant downflow observed in UV is due to hot and diffusive returning material of H-alpha upflow events. Comparisons with TRACE/EUV images are in process.

Magnetic Trapping of Flare Electrons and Microwave Emission

Jeongwoo Lee and Dale E. Gary

Physics Department, New Jersey Institute of Technology, Newark, NJ 07102-1982, U.S.A.

Abstract. The topic of trapping of flare-produced electrons in magnetic loops and their evolution under Coulomb collision has received considerable attention in relation to interpreting hard X ray observations, since the first work by Melrose and Brown in 1976. However, application of the idea to the study of microwave radiation has been more limited. Petrosian in 1982 illustrated how the magnetic field affects the trapping and beaming of electrons to predict spatial morphology of microwave emission given magnetic structure and location of a flaring loop. Mel'nikov in 1994 used a model for trap and precipitation to study relative intensities and time delays between microwaves and hard X rays. We present a detailed modeling of microwave emission from electrons undergoing Coulomb interaction in magnetic traps, designed for quantitative analysis of spatially-resolved, multiwavelength microwave observations such as those of the Solar Arrays at Owens Valley Radio Observatory (OVRO). Our main concern is to properly relate the precipitation rate and pitch angle diffusion to magnetic quantities of the flaring loop and injection parameters. In this approach, we use coronal field extrapolation and overlays of soft X ray loops to provide the magnetic quantities so that the microwave spectrum can be used mainly as the electron diagnostic. We discuss the model capabilities and apply the results to a flare that occurred in AR 7515 on 1993 June 3. This flare showed spectral flattening in the decay phase along with morphological variation suggestive of a magnetic trap around the loop top, and the spectral flattening is interpreted as driven by Coulomb collision in the magnetic trap.

Ultraviolet Flaring Loop Systems in an Eruptive Event

Jiong Qiu, Haimin Wang, Jongchul Chae, Chikyin Lee, and Philip R. Goode

Big Bear Solar Observatory, New Jersey Institute of Technology, Big Bear City, CA 92314, U.S.A.

Abstract. The BBSO/TRACE joint campaign on September 27, 1998 observed an eruptive flare event which lasted for half an hour. The observation covered several ultraviolet lines from transition region and H-alpha line center and off-band emissions from chromosphere with high spatial resolution which allows detailed study on the flare plasma at wide temperature range. Various flare loop systems are found in the active region depicting different scenarios of the event. The flare was started by low atmosphere explosive reconnection which triggered vehement loop eruption, heating of both large scale and low-lying pre-existent loops, and mass ejection of both hot (up to 106K) and cool (104K) plasmas. Due to reconnection of open field lines, new flaring loop archade was formed after the eruption and was clearly seen in EUV emissions. From high resolution UV, EUV and H-alpha filtergrams, fine structure of both low-lying and coronal loops can be identified. The multi-wavelength observations on this event enable us to distinguish different mechanisms of energy and mass transfer in different loop systems displaying different radiative and dynamic behaviours, and to investigate the early stage of the flare when lower atmosphere instability is essential in triggering the major flare and the relation between lower and higher atmospheres is complex.

New Vectormagnetographs at the Big Bear Solar Observatory

Thomas Spirock, Carsten Denker, Philip R. Goode, and Haimin Wang

Big Bear Solar Observatory, New Jersey Institute of Technology, Big Bear City, CA 92314, U.S.A.

Abstract. During the past two years the Big Bear Solar Observatory has begun an aggressive program to upgrade the observatory's instrumentation. In the forefront of this effort are improvements to the current vectormagnetograph and the development of two new vector magnetograph systems - two in visible light and one in the near infrared. In the first case, the current filter-based video-magnetograph, on the 10 inch vacuum-refractor, is being replaced by a 1k by 1k pixel, 12-bit, 15 frames per second CCD camera, and higher quality polarization optics to improve our measurements using the CaI line at 6103 Å. Secondly, a Fabry-Pérot based imaging magnetograph with a spatial resolution of 0.15 arcsec per pixel having a temporal resolution of approximately 1 min for the Stokes-V and approximately 4 min for the full Stokes vector with a band-pass of 80 mÅ is being developed for the 26 inch vacuum-reflector to scan the line of the FeI line at 6302.5 Å. In the near infrared, an automated spectrograph based vectromagnetograph, using a 12-bit, 320 by 240 pixel, 30 frame per second InGaAs CCD camera, is being developed to study the FeI lines at 1.56485 µm (g=3) and 1.56529 µm (g=1.5). Current plans and the status of each instrument will be discussed and test results will be presented.

The Rotation Rate Near the Solar Poles

John R. Varsik

Big Bear Solar Observatory, New Jersey Institute of Technology, Big Bear City, CA 92314, U.S.A.

Abstract. Magnetic field based measurements of the solar rotation rate at the solar poles will be presented. These results have been obtained using the BBSO video magnetograph and analyzed using local correlation tracking of the magnetic flux knots which make up the overall polar fields. The measurements are from the period 1995 to 1999 and cover the early rising phase of Cycle 23 and latitudes from 45 to nearly 90 degrees. Measurements based on time series mosaics (time resolution ~25 minutes, duration one day) as well as twice-daily images will be presented. Comparison will be made to the results of Snodgrass (1983).

Studies of Microflares and C5.2 Flare of September 27, 1998

Haimin Wang, Jongchul Chae, Jiong Qiu, Chik-Yin Lee and Philip R. Goode

Big Bear Solar Observatory, New Jersey Institute of Technology, Big Bear City, CA 92314, U.S.A.

Abstract. On September 27, 1998, Big Bear Solar Observatory (BBSO) and Transition Region and Coronal Explorer (TRACE) coordinated observations from 16:00 to 19:00 UT to study properties of microflares on AR NOAA 8340. Fortuitously, a C5.2 flare occurred at 16:30 UT in this active region. H-alpha and magnetograph movies were obtained at BBSO; CIV 1550 Å, FeIX 171 Å and FeXII 195 Å movies were obtained by TRACE; both with a cadence about 1 minute. In this paper, we concentrate on the study of magnetic properties of 70 CIV microflares, as well as their relationship to the C5.2 flare. We obtained the following results: (1) We found two kinds of microflares: microflares of transient brightenings with a time scale of 1 to 5 minutes (impulsive events) and microflares lasting half an hour or longer (persistent events). 90% of the microflares are impulsive events. Most of the events in this category are associated with well defined magnetic neutral lines, but some of them are found in non-neutral line areas. All of seven persistent events are found at parasitic magnetic configurations with inclusions of small magnetic flux within dominant magnetic flux of opposite polarity. (2) More than a third of the impulsive microflares occurred near the C5.2 flare site indicating that a local instability is responsible for both the C5.2 flare and microflares. This indirectly support the avalanche theory of flare energy release, which predicts that a big flare may be associated with many small flares. Based on the morphologies of those events, we postulate that the persistent events may be due to loop interaction; while impulsive events may be associated with sheared loop structure.

Study of Umbral Dots at 1.6 Microns

Jingshan Wang, Haimin Wang, Carsten Denker, Tom Spirock, and Philip R. Goode

Big Bear Solar Observatory, New Jersey Institute of Technology, Big Bear City, CA 92314, U.S.A.

Abstract. We used a 320 by 240 pixel InGaAs IR camera and a broadband filter centered at 1.565 µm to carry out a sequence of near IR observations at Big Bear Solar Observatory. The target is a delta sunspot and our objective is to study the properties of umbral dots in the opacity minimum. Because of the lower scatter light in IR, we can resolve the very center of the umbra. We discuss the contrasts, sizes, lifetimes and proper motions of umbral dots observed in IR and compared with visible observations.

Solar Differential Rotation Derived from H-alpha Full Disk Images by Means of Local Correlation Tracking

Martin F. Woodard1, Carsten Denker1, and Louis H. Strous2

(1) Big Bear Solar Observatory/New Jersey Institute of Technology, Big Bear City, CA 92314, U.S.A.
(2) Lockheed-Martin Solar and Astrophysics Lab, O/L9-41 B/252, 3251 Hanover Street, Palo Alto, CA 94304, U.S.A.

Abstract. We present the application of Local Correlation Tracking (LCT) techniques to time series of contrast-enhanced H-alpha full disk images taken as part of the synoptic observing program at Big Bear Solar Observatory (BBSO) during the summer of 1998. A typical set of daily H-alpha full-disk images consists of 600 to 800 individual frames, taken 30 to 60 s apart, with a 2k x 2k pixel Kodak 4.2 MegaPlus CCD camera at BBSO's Singer telescope. For each pair of successive images, we compute displacement vectors over a 64 x 64 cartesian grid covering the solar disk. The resulting daily-averaged flow maps show predominantly solar differential rotation and proper motions in active regions. We remap the flow maps to heliographic coordinates and determine a Legendre polynomial expansion of the daily differential rotation profile. We present preliminary findings regarding differential rotation based on different types of features seen in H-alpha, such as quiet-sun fibrils, plages, and dark filaments. We discuss the relation of our differential rotation profiles to profiles derived by other methods and address the question of time variability.

Study of Filament Disappearances and CMEs

Guo Yang and Haimin Wang

Big Bear Solar Observatory, New Jersey Institute of Technology, Big Bear City, CA 92314, U.S.A.

Abstract. Almost all H-alpha full-disk image data observed by BBSO from January 1997 to June 1998 were searched for filament disappearances. A list of the events was compiled for this period. Comparison with the data from EIT, LASCO and Yohkoh is being carried out to obtain the relationship between CME events and filament disappearances. The first result shows that many filament disappearance events seem to have no corresponding CME events.

The Changes of the Current Helicity Scaling Prior to a Strong Solar Flare

Vasyl B. Yurchyshyn1, 2, Valentina I. Abramenko2, and Vincenzo Carbone3

(1) Big Bear Solar Observatory, New Jersey Institute of Technology, Big Bear City, CA 92314, U.S.A.
(2) Crimean Astrophysical Observatory, 334413, Nauchny, Crimea, Ukraine
(3) Dipartimento di Fisica, Universita di Cosenza, Italy

Abstract. Recently it was shown (Abramenko et al., 1998, Sol. Phys. 178) the current helicity calculated by the photospheric magnetic field vector possesses a well pronounced scaling behavior. The sign-singularity of the 2D structures of current helicity can be studied by introducing the signed measure and by calculating some power-law exponent: cancellation exponent k. The time variations of the cancellation exponent seem to be related to flare activity of an active region: the periods of enhanced flaring are accompanied by a drop and subsequent rise of the cancellation exponent (Abramenko et al., 1998, Astron. Astrophys. 334). Here we continue to study the flare-related changes of cancellation exponent paying special attention to the preflare situation. We have found that the significant decrease of k begins 1-3 hours prior to strong flares. The reduce of the k means the changes in the transverse magnetic field structure. The preflare energy release produced by small-scale currents dissipation seems to supply observed preflare energetic phenomena such as preflare flocculi brightening and enhanced X-ray and microwave emission. So we are able to conclude that small-scale energy release in the transverse photospheric magnetic field take place prior to a strong flare. As soon as the evaluation of k needs only the calibrated vector magnetogram and some minutes for calculations the preflare drop of the cancellation exponent could be used for the short-time prediction of strong flares.

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