Engineering topics

Zirin, Harold

Publications and source records attributed to Zirin, Harold.

At least 19 records

NASA NTRS1999

Variability of Solar UV Irradiance Related to Bright Magnetic Features Observed in Call K-Line: Relationship between Lyman alpha and K-line Report for UARS funding agency

In this report we comment on the relationship between the Lyman alpha and Calcium K-line emission from the Sun. We firstly examine resolved Lyman alpha images (from TRACE) and resolved K-line images. We find that the Lyman alpha emission is consistent with a linear dependence on the K-line emission. As this is in conflict with the analysis of Johannesson et al.(1995, 1998) we proceed by comparing the disk integrated Lyman alpha flux as a function of ratio between the disk integrated Mg II core and wing fluxes (Johannesson et al (1998) having previously found a linear dependence between this index and the BBSO K-line index). We find that a reasonably good fit can be obtained, however note the discrepancies which lead Johannesson et al to consider the square root relationship. We suggest an alternative interpretation of the discrepancy.

Zirin, Harold↗

NASA NTRS1996

Correlative Studies of Solar Flares

In October 1992, post-doc Haimin Wang was awarded a Compton GRO Fellowship by NASA to study the flares observed simultaneously by BATSE (Burst and Transient Experiment), OVRO (Owens Valley Radio Observatory), Yohkoh, and BBSO (Big Bear Solar Observatory). During the past four years, a number of flares have been analyzed in detail. Substantial information on the physics of flares came out of these studies.

Zirin, Harold↗

NASA NTRS1994

Observations of vector magnetic fields in flaring active regions

We present vector magnetograph data of 6 active regions, all of which produced major flares. Of the 20 M-class (or above) flares, 7 satisfy the flare conditions prescribed by Hagyard (high shear and strong transverse fields). Strong photospheric shear, however, is not necessarily a condition for a flare. We find an increase in the shear for two flares, a 6-deg shear increase along the neutral line after a X-2 flare and a 13-deg increase after a M-1.9 flare. For other flares, we did not detect substantial shear changes.

Chen, Jimin↗

NASA NTRS1994

The roots of coronal structure in the Sun's surface

We have compared the structures seen on X-ray images obtained by a flight of the NIXT sounding rocket payload on July 11, 1991 with near-simultaneous photospheric and chromospheric structures and magnetic fields observed at Big Bear. The X-ray images reflect emission of both Mg X and Fe XVI, formed at 1 x 10(exp 6) K and 3 x 10(exp 6) K, respectively. The brightest H(alpha) sources correspond to a dying sub-flare and other active region components, all of which reveal coronal enhancements situated spatially well above the H(alpha) emission. The arches appear to lie in a small range of angle in the meridian plane connecting their footpoints. Sunspots are dark on the surface and in the corona. For the first time we see an emerging flux region in X-rays and find the emission extends twice as high as the H(alpha) arches. Many features which we believe to correspond to `X-ray bright points' (XBPs) were observed. Whether by resolution or spectral band, the number detected greatly exceeds that from previous work. All of the brighter XBPs correspond to bipolar H(alpha) features, while unipolar H(alpha) bright points are the base of more diffuse comet-like coronal arches, generally vertical. These diverge from individual features by less than 30 deg, and give a good measure of what the `canopies' must do. The H(alpha) data shows that all the H(alpha) features were present present the entire day, so they are not clearly disappearing or reappearing. We find a new class of XBPs which we call `satellite points', elements of opposite polarity linked to nearby umbrae by invisible field lines. The satellite points change rapidly in X-ray brightness during the flight. An M1.9 flare occurred four hours after the flight; examination of the pre-flare structures reveals nothing unusual.

Golub, Leon↗

NASA NTRS1993

Strong transverse fields in delta-spots

Spectroscopic measurements of the strength and direction of transverse magnetic fields in six delta-spots are presented. The field direction is determined by the relative strength of the pi- and sigma-components at different polarizer orientations, and is, with one exception, parallel to the neutral line and as strong as the umbral field. Field strengths determined by line splitting are as high as 3980 G.

Zirin, Harold↗

NASA NTRS1992

Joint vector magnetograph observations at BBSO, Huairou Station and Mees Solar Observatory

Joint vector magnetograph observations were carried out at Big Bear Solar Observatory (BBSO), Huairou Solar Observing Station (Huairou), and Mees Solar Observatory (MSO) in late September 1989. Comparisons of vector magnetograms obtained at the three stations show a high degree of consistency in the morphology of both longitudinal and transverse fields. Quantitative comparisons show the presence of noise, cross-talk between longitudinal field and transverse field, Faraday rotation and signal saturation effects in the magnetograms. We have tried to establish how the scatter in measurements from different instruments is apportioned between these sources of error.

Wang, Haimin↗

NASA NTRS1992

Flows around sunspots and pores

Observations are reported yielding high-resolution white-light data regarding the flow pattern of penumbral material and granules around sunspots and pores. The data are culled from optical observations near the limit of the telescope modulation transfer function, and correlation tracking is employed to track the motions of penumbral features and granules. Dark fibrils and bright grains in the sunspots' inner penumbra regions move toward the umbra, and the opposite occurs in the outer penumbra. Before pore formation and during the pores existence granular flows are found to converge on the features. The paper also details the evolution of an emerging flux region, and two types of photospheric dark alignments are described in the region of new flux emergence.

Wang, Haimin↗

NASA NTRS1991

Motions, fields, and flares in the 1989 March active region

The results of observations of NOAA AR 5395 are presented. The region was observed every day from limb to limb for significant periods, and nine of the ten class-X flares were recorded. The region was found to be a great Delta group, dominated by spots of following (f) polarity, which moved rapidly westward, producing large changes in magnetic structure which increased the shear and led to great flares. Aside from its great size, the region was unusual in that normally p spots dominate and move westward. In this case there was a 4:1 flux imbalance; 80 percent of the flux measured was of following polarity. The major following spot in the region was found to move with a near-constant acceleration, eventually reaching 0.25 km/s. Rapid spot motion was discovered in all other superactive regions. Small p and f spots move out from either side of the large f spot, and curl around it in curved trajectories. The moving penumbral material coalesces into new umbrae.

Wang, Haimin↗

NASA NTRS1991

Magnetic flux transport of decaying active regions and enhanced magnetic network

Several series of coordinated observations on decaying active regions and enhanced magnetic network regions on the sun were carried out jointly at Big Bear Solar Observatory and at the Huairou Solar Observing Station of the Bejing Astronomical Observatory in China. The magnetic field evolution in several regions was followed closely for three to seven days. The magnetic flux transport from the remnants of decayed active regions was studied, along with the evolution and lifetime of the magnetic network which defines the boundaries of supergranules. The magnetic flux transport in an enhanced network region was studied in detail and found to be negative. Also briefly described are some properties of moving magnetic features around a sunspot. Results of all of the above studies are presented.

Wang, Haimin↗

NASA NTRS1991

BEARALERTS - A successful flare prediction system

The BEARALERT program of predicting solar flares or rapid development of activity in certain sunspot groups is described. The program was instigated in December 1987 by the Big Bear Solar Observatory (BBSO) in order to test the current understanding of the flare process by making public predictions via electronic mail. Neither the exact timing of the flare nor the possibility of emergence of new active regions can be predicted. However, high-resolution observations of the magnetic configuration, H-alpha brightness and structure, and other properties of a region have enabled the BBSO astronomers to announce the onset of 15 of 23 major active regions over a two-year period. The criteria for prediction are given and discussed, along with those for filament eruption. The BEARALERT success evaluation is discussed, and the progress made in prediction success is evaluated.

Zirin, Harold↗

NASA NTRS1990

Optical properties of impulsive flares

The optical and morphological properties of impulsive flares are examined, showing that all impulsive flares occur close to sunspots and on magnetic inversion lines with steep gradients. Impulsive flares are defined and flare brightness variations and the magnetic structure of flares are discussed. The basic characteristics of impulsive flares are described and the evolution of a typical impulsive flare is outlined.

Zirin, Harold↗

NASA NTRS1990

Microwave structure of the quiet sun at 8.5 GHz

Multifrequency VLA observations of the quiet sun near 8.5 GHz are presented. Two regions of the sun were observed, one dominated by an enhanced network corresponding to a decayed active region, and the other corresponding to an enhanced network with no active features. The full-day synthesis maps for both show nearly perfect correspondence to H-alpha images, and to longitudinal magnetograms. The coronal loops were observed to appear as regions of radio emission with no underlying longitudinal magnetic fields, being aligned with H-alpha fibrils in the photosphere, and connecting regions of opposite magnetic polarity. The emission can be modeled as optically thin free-free emission from a coronal loop with a peak axial density of approximately 2.4-2.8 x 10 to the 9th/cu cm, for an assumed coronal temperature of 1-2 x 10 to the 6th K. The quiet chromosphere sources are measured, and the significance of these measurements for existing chromospheric models is discussed.

Gary, Dale E.↗

NASA NTRS1990

Flows, flares, and formation of umbrae and light bridges in BBSO region No. 1167

High-resolution observations of the large active region BBSO No. 1167 (Boulder No. 5060) are presented, based on data obtained with 65-cm telescope with spatial resolution of about 0.5 sec for much of two 11-hr periods. Rapid flux emergence in the large but simple active region observed for several days is analyzed. The magnetograms show complex fine structure, with some closely interwined regions of opposite polarity. It is found that, in a region of a new flux emergence, positive (leading polarity) flux flows along elongated channels immersed in the negative flux; and that moving magnetic features occur around all of the spots. Other aspects of this large region are also examined, such as characteristics of penumbra as related to umbra, and characteristics of combining spots of the same polarity. It is concluded that the emerging flux loops are discrete entities.

Zirin, Harold↗

NASA NTRS1990

Observations of vector magnetic fields with a magneto-optic filter

The use of the magnetooptic filter to observe solar magnetic fields in the potassium line at 7699 A is described. The filter has been used in the Big Bear videomagnetograph since October 23. It gives a high sensitivity and dynamic range for longitudnal magnetic fields and enables measurement of transverse magnetic fields using the sigma component. Examples of the observations are presented.

Cacciani, Alessandro↗

NASA NTRS1990

Solar flares

General characteristics of solar flares are discussed. They are a spectacular energy release arising from development of magnetic strain and shear, resulting from eruption of new magnetic flux near old. The release of magnetic energy is channeled into hard electrons (and possibly nucleons), which produce a series of other effects. The ultimate source of the magnetic complexity that produces flares is deep in the sun. A number of examples is given, and the difference between various types of flares is discussed.

Zirin, Harold↗

NASA NTRS1989

Seventy-five hours of coordinated videomagnetograph observations

Videomagnetograph observations obtained between September 24 and 29, 1987 are presented which illustrate the evolution of magnetic flux surrounding a stable sunspot. It is found that the dominant sunspot mainly ejects magnetic fields of opposite sign, and that the surrounding plage fields steadily contract and retreat inward toward the umbra, resulting in shrinking and weakening of the spot and plage. The extent of the moat is shown to be reduced by 50 percent in a 75-hour period, with the principal loss of flux probably due to concellation at the main neutral line. Five subflares were noted, three occurring prior to cancellation of the magnetic elements at the inversion line and two occurring during the development and disappearance of an ephemeral bipolar region.

Wang, Haimin↗

NASA NTRS1989

Observations of the 12 micron Mg I lines in various solar features

Measurements of the solar Mg I line at 812/cm are presented, confirming the ubiquitous narrow emission from the quiet sun with secant limb brightening observed by Brault and Noyes (1983). It is found that the central intensity of 12-micron emission lines is greater in plages than in the quiet sun, with an equivalent width up to 3 times greater than for the quiet sun. The magnetic fields measured in plages range from 300 to 800 Gauss. The secant-like limb brightening, and line ratios suggest that the lines are optically thin and are formed in collisional equilibrium with Mg II in a thin layer just above the temperature minimum.

Zirin, Harold↗

NASA NTRS1989

Video image selection studies of granules, pores, and penumbral flows near a large sunspot

The results of applying modern ground-based optical techniques using data obtained from high-resolution videotapes to the study of solar granulation are addressed. The average lifetime of granules is found to be 16 min, with larger granules having longer lifetimes. Two percent of granules explode, and the probability of a granule being hit by exploding granules at least once is 10 percent. A big exploding event covers about 10 granules. Many small pores change their size and darkness in less than a time scale of 60 min. The integrated darkness of one pore appears to be oscillating with a period of about 30 min. There are clear inflows of both bright and dark penumbra fibrils toward the umbra, from the outer edge of the penumbra. Outflow from the outer edge of penumbra is confirmed. This flow pattern may be related to the moving magnetic features and the moat structure surrounding the sunspots.

Zirin, Harold↗