1998 AGU Meeting - Boston

Photospheric Magnetic Field Changes Associated With Transition Region Explosive Events

J. Chae1, H. Wang1, C. Lee1, P. Goode1, and U. Schühle2

(1) Big Bear Solar Observatory, New Jersey Institute of Technology, Big Bear City, CA 92314, U.S.A.
(2) Max-Planck-Institut für Aeronomie, D-37191 Katlenburg-Lindau, Germany

Abstract. From a comparison of SOHO SUMER spectral data and a time series of BBSO magnetograms, we present observational clues to the physical origin of transition region explosive events: First, explosive events rarely occur in the interior of strong magnetic flux concentrations, but rather are preferentially found in regions with weak and mixed polarity fluxes which display magnetic neutral lines. Second, the majority of explosive events happen during the `cancellation' of photospheric magnetic flux. Third, there is a strong tendency for explosive events to occur repeatedly, as bursts, while local photospheric magnetic flux continuously decreases due to cancellation. These results strongly support the idea that transition region explosive events are a manifestation of magnetic reconnection occurring in the quiet Sun. Further, one may infer from the third result that the explosive events represent repetitive fast magnetic reconnections in the transition region, which are initiated by slow magnetic reconnections occurring beneath.

Speckle Interferometric Observations of Solar Fine Structures

C. Denker, P.R. Goode, and H. Wang

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

Abstract. In the autumn of 1997, we started a program of high spatial resolution imaging at the Big Bear Solar Observatory (BBSO). We used a fast (90 frames/s) 12-bit CCD camera to observe various solar features at different wavelength settings (continuum at 520 nm, G-band at 430.6 nm, and H-alpha center line). We present almost diffraction limited observations of solar granulation, H-alpha bright points, pores, and a small delta-spot configuration. The speckle masking method (Weigelt 1977, Opt. Comm. 21, 55) was also used to reconstruct high spatial resolution magnetograms. The high quality of the present observations and measurements of the Fried parameter confirm recent seeing tests at BBSO by the National Solar Observatory (Beckers 1997, Sol. Phys. 176, 23) demonstrating the excellent seeing conditions at lake site observatories.

VLA Observations of the Corona at 74 MHz: A Search for Radio CMEs

D.E. Gary1, T.S. Bastian2, A. Vourlidas3, and N. Kassim4

(1) Physics Department, New Jersey Institute of Technology, Newark, NJ 07102-1982, U.S.A.
(2) P.O. Box O, Socorro, NM 87801, U.S.A.
(3) Naval Research Lab, Mail Code 7660, Washington, DC 20375-5320, U.S.A.
(4) Naval Research Lab, Mail Code 7213, Washington, DC 20375-5351, U.S.A.

Abstract. The VLA antennas were recently equipped with 74 MHz receivers (lambda = 4 m). We present the first solar observations with the new system. The resulting images show the highest dynamic range (ranging from 500:1 in 40 min of integration to 1800:1 for a 6 hr observing run) ever obtained at this frequency. With such good sensitivity it is of interest to try to detect CMEs. During three days of observing, our observations overlapped at least partially with the occurrence of two CMEs, one on 1998 Jan 31, and another on 1998 Feb 1, both well observed with the LASCO coronagraph experiment on the SOHO spacecraft. We find no unambiguous enhancement associated with either event, with a conservative upper limit of Delta Tb < 3000 K for the radio brightness temperature. We compare this with the expected brightness of the CMEs as measured with LASCO, and give prospects for future detections.

We also use the 74 MHz images, along with simultaneous images at 1.45 GHz and 327 MHz to investigate the large-scale spatial structure of the corona. In particular, on 1 Feb, the circular polarization of the 74 MHz emission was measured, and is compared with expectations from raytracing calculations in a quiet magnetized corona. We investigate the possibility of using such polarization measurements at 74 MHz to directly measure the magnetic field strength at a height of more than 1.5 solar radii from Sun center.

Excitation of Solar Oscillations: Observation and Theory

P.R. Goode1, T.R. Rimmele2, and L. Strous1

(1) Big Bear Solar Observatory, New Jersey Institute of Technology, Big Bear City, CA 92314, U.S.A.
(2) National Solar Observatory, Sunspot, New Mexico, NM 88349, U.S.A.

Abstract. Rimmele, et al. (1995, ApJ 444, L119) used their simultaneous observations of the Sun's seismic events and overlaid granular field to show that the events occur in dark lanes which become abruptly darker immediately preceeding the events. They also demonstrated that power in the seismic events is comparable to that required to drive the Sun's p-modes. Goode, et al. (1998, ApJ 495, L27) used the same data to demonstrate that the power from the seismic events is converted into normal mode power as the ripples travel away from the events. An important remaining question is the degree of association among these seismic events and those calculated by Nordlund and co-workers. Here we report the preliminary results of direct comparisons, beginning with line profile data, of observation and theory. Individual classes of events will be compared and discused.

High Resolution H-alpha Full Disk Observations of the Sun

A. Johannesson, C. Denker, W. Marquette, and H. Wang

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

Abstract. We present H-alpha full disk observations of the Sun obtained with the 20 cm Singer telescope at the Big Bear Solar Observatory. The full disk images were obtained with a 2032 x 2028 pixel, 8-bit Kodak MegaPlus 4.2 CCD camera. Here, we will report on recent improvements of our synoptic observing program. Since October 1997, we run a semi-automatic image processing routine from an IDL graphical user interface. The processed images are corrected by means of dark and flat field frames, and a limb darkening profile is substracted as well. The processed images show a variety of chromospheric fine structures - even prominences at the disk edge become easily discernible. The high contrast of the images, their high spatial resolution (approximately 1 arcsec/pixel), and the high temporal resolution (up to 3 frames/min) make these images an ideal choice to study the statistical properties of mini-filament eruptions, the chromospheric differential rotation and meridional flows within the chromosphere, as well as the evolution of active regions, filaments, flares, and prominences. In this presentation, we will show some of our preliminary results. To get an impression of the data quality achieved by the improved image processing routines, visit our Web page with the latest observations from the Big Bear Solar Observatory: http://www.bbso.njit.edu/cgi-bin/LatestImages.

Dynamic Characteristics of H-alpha Jets

C.Y. Lee, J. Chae, and H. Wang

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

Abstract. H-alpha jets are defined as dark features as seen from the far blue wing of spectroheliogram. We are interested in these features because they may result from small scale magnetic reconnection in the quiet sun region. In this paper, we examine the dynamic characteristics of H-alpha jets based on H-alpha spectrograph observations. The spectral data came from the BBSO high resolution 4-dimensional H-alpha (x,y,t,lambda) data array in the week long jointed BBSO/SOHO-JOP51 observation on August 26, 1997. An solar region approximately 120 arcsec x 95 arcsec centered at the coordinate 136E-400S was scanned repeatedly. A high temporal (0.5 minute) and spatial (1.0 arcsec) resolutions are achieved. We have totally 118 scans, each scan has 600 spectral images. All images are flat-fielded and corrected for contrast variations due to optical effects. Firstly, we determine line-of-sight velocity, Doppler width, and optical thickness by applying Cloud Model fitting to H-alpha propfiles. Secondly, spectroheliograms and movies at various wavelengths (center line at 656.3 nm and off band within +/- 0.1 nm) are constructed. By observing the evolution of selected features from movies complied from spectroheliograms, statistics such as life-time, size, and frequency of occurrence of selected features can be found. Combining these two approaches, we finally present the dynamic characteristics of H-alpha jets in comparison with other chromospheric features such as dark mottles, bright mottles, as well as filaments. A comparison with SOHO data will also be made.

OVRO and NRO Observations of a Solar Flare

J. Lee and D.E. Gary

Physics Department, New Jersey Institute of Technology, Newark, NJ 07102

Abstract. The solar flare occurred on June 3rd, 1993 was jointly observed by Owens Valley Radio Observatory (OVRO) and the Nobeyama Radio Observatory (NRO) Radioheliograph. The 17 GHz NRO data had a time profile similar to GOES soft X ray light curves, except for a pair of short-lived spikes, which as a whole might imply events of impulsive injection of nonthermal particles amid gradual evolution of thermal particles during the flare. We verify such a scenario by tracing their counterparts at successively low frequencies available in OVRO data, eventually to identify the nonthermal component and separate it from the thermal component in space.

Magnetic Cancellation, Magnetic Reconnection, Motions and Flares in an Active Region

W. Li1, J. Chae2, C.Y Lee2, P.R. Goode2, and H. Wang2

(1) Beijing Astronomical Observatory, Chinese Academy of Sciences, Beijing 100080, China
(2) Big Bear Solar Observatory, New Jersey Institute of Technology, Big Bear City, CA 92314, U.S.A.

Abstract. The discovery of canceling photospheric solar magnetic fields resulted from Big Bear line-of-sight magnetograms, by Martin, Livi, and Wang (1985, Austr. J. Phys. 38, 929). Recently, Wang and Shi (1993, Sol. Phys. 143, 119) further examined and clarified the picture of cancellation by means of a time sequence of vector magnetograms obtained from Huairou Solar Observing Station (HSOS). Wang and Shi also pointed out that the emergence of new flux and cancellation were closely related to each other. We have studied the relationship among magnetic cancellation, magnetic reconnection, and the proper motion of spots in an active region based on coordinated observations of the active region at HSOS and Big Bear Solar Observatory (BBSO) in October, 1991. We analyzed a complete time sequence of vector magnetograms, a continuous white light movie, and a line-of-sight magnetogram movie obtained from a 4-day series of observations at the time of the central meridian passage of active region NOAA 6891. From the analysis, we have found observational evidence that magnetic cancellation may be caused by moving magnetic features (MMF). Some MMFs, irrespective of whether they were newly emerged or old, collided with another flux of opposite polarity and resulted in a cancellation that occurred at the interface of two topologically separated magnetic loops. Therefore, our results strongly indicate that it is not only new flux emergence, but also the proper motion of magnetic features can play an important role in magnetic cancellation and flares.

IR Magnetograph System at BBSO

T.J. Spirock, P.R. Goode, and H. Wang

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

Abstract. In this presentation, we report our first tests of an IR magnetograph system that is being setup on the spectrograph of the 65cm telescope at BBSO. We studied the two FeI lines at 1.56485 microns (g=3) and 1.56529 microns (g=1.5) which are near the opacity minimum in the solar atmosphere. A key ingredient in the system is a 320 x 240 pixel, 12-bit digital InGaAs CCD camera that can detect very weak signals. This camera has several characteristics specially suited to this project. Its quantum efficiency is greater that 70% from 1.0 microns to 1.6 microns which will allow short exposure times of few milliseconds and high frame rates up to 30 frames per second which will reduce the blurring effects of seeing and enable us to apply various post-processing techniques such as frame selection, image alignment, phase diversity or speckle interferometry. These techniques have been successfully applied to the new high frame rate BBSO visible light magnetograph. The IR magnetogram will be compared to their simultaneous visible light counterparts. The preliminary results of the first observations with this system will be presented.

Comparison of H-alpha and HeII 304 Å Macrospicules

H. Wang

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

Abstract. Simultaneous observations of limb macrospicules were carried out in H-alpha by Big Bear Solar Observatory (BBSO) and in HeII 304 Å by Extreme-Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO). For the first time, H-alpha and HeII macrospicules are compared with high spatial and temporal resolution, and with image enhancement. Data were obtained on October 6th, 7th and 8th, 1996. The target of the first and last day was the North pole; and on October, 7th, it was the quiet west limb. BBSO uses 12-bit digital camera to obtain high resolution H-alpha filtergrams at -0.6 Å, line center and +0.6 Å. The pixel resolution ranges between 0.17 arcsec and 0.34 arcsec, and temporal resolution, between 25 and 90 sec. EIT images have a fixed pixel resolution of 2.5 arcsec and temporal resolution between 1 and 6 minutes. We found the following: within the common field of view of BBSO and EIT, all 53 identified HeII 304 Å macrospicules have counterparts in H-alpha. However, morphologies of HeII 304 Å and H-alpha macrospicules are completely different. HeII 304 Å macrospicules are typically in the form of an elongated ejection; H-alpha macrospicules are either loop-like bright features, or much shorter jets. In the polar region, 55 (over 50%) H-alpha macrospicules do not show any corresponding HeII 304 Å macrospicules.

As expected, HeII 304 Å macrospicules occur much more frequently in the polar coronal hole area than in the equatorial region. However, H-alpha macrospicules occur at an equal rate in the pole and in the equatorial limb. Equatorial H-alpha macrospicules tend to be confined and lack of a jet-like structure.

We propose that macrospicules are a result of magnetic reconnection, likely due to network-ephemeral region or network-intranetwork interactions. If viewed from the disk, macrospicules might be H-alpha - 1.0 Å jets or a subset of `explosive events'. We propose that magnetic reconnection occurs at about the same rate in the polar coronal hole and in the quiet regions. H-alpha macrospicules are direct manifestation of magnetic reconnection. HeII 304 Å images detect substantially taller structure which are substantially hotter. Because of dominant vertical line configuration near the pole, reconnection tends to produce HeII 304 Å macrospicules; in the non-coronal hole regions, HeII macrospicules become undetectable because of the inclined magnetic field configuration.

Solar Near-Infrared Filter System

J. Wang1, H. Wang1, and N.M. Ravindra2

(1) Big Bear Solar Observatory, New Jersey Institute of Technology, Big Bear City, CA 92314, U.S.A.
(2) Department of Physics, New Jersey Institute of Technology, Newark, NJ 07102

Abstract. In this paper, we present a near-infrared (at 1564.8.5 Å) filter system, which is designed for the measurement of solar magnetic field at the Big Bear Solar Observatory (BBSO). It will consist of a prefilter, a liquid crystal analyzer, a wavelength-tunable birefringent filter and a Fabry-Perot Etalon. We will describe the birefringent filter and the associated optical components in detail. We expect to achieve a clean narrow bandpass (1/8 Å) with this system.

Evidence for Non-potential Magnetic Fields in the Quiet Sun

M.F. Woodard and J. Chae

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

Abstract. A comparison of BBSO H-alpha centerline filtergrams and video-magnetograms was made to investigate the existence of non-potential magnetic fields in the quiet solar chromosphere near magnetic network. We use the fibril structure in the H-alpha images as a proxy for the horizontal chromospheric magnetic field. The degree of non-potentiality of various subregions within an image was assessed by calculating the distribution of the angles between the proxy field and the horizontal component of the potential field extrapolated from the video-magnetograms. We found that for some subregions the potential field is a reasonable approximation to the proxy field, while for other subregions it is not. To explore the possible connection between field non-potentiality and heating of the chromosphere/corona we examined EUV images obtained by the Coronal Diagnostic Spectrometer on the SOHO satellite. Our analysis suggests that high EUV emission occurs in regions of strong, but not necessarily non-potential, fields. These results may signify that the current systems in the corona/transition region are not related in a simple way to those in the chromosphere.

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