Magnetic flux transport of decaying active regions and enhanced magnetic network     

Haimin Wang  Harold Zirin  Guoxiang Ai 《Solar physics》1991,131(1):53-68

Several series of coordinated observations on decaying active regions and enhanced magnetic network regions have been carried out jointly at Big Bear Solar Observatory (BBSO) and Huairou Solar Observing Station of the Bejing Astronomical Observatory in China. The evolution of magnetic fields in several regions was followed closely for 3 to 7 days. The transport of magnetic flux from the remnants of decayed active regions was studied. Three related topics are included in this paper. (1) We studied the evolution and lifetime of the magnetic network which defines the boundaries of supergranules. The results are consistent with our earlier studies: network cells have an average lifetime of about 70 hours; 68% of new cells appeared by growing from a single network magnetic element; 50% of decaying cells disappeared by contracting to a network element. (2) We studied the magnetic flux transport in an enhanced network region in detail, and found the diffusion rate to be negative, i.e., there was more flux moving towards the decayed active region than away from it. We found several other cases where the magnetic diffusion rate does not agree with Leighton's model. The slow diffusion rate is likely due to the fact that the average velocity of larger magnetic elements, which carry most of the magnetic flux, is less than 0.1 km s^–1; their average lifetime is longer than 100 hours. (3) We briefly described some properties of Moving Magnetic Features (MMFs) around a sunspot (detailed discussion on MMFs will be presented in a separate paper). In this particular case, the MMFs did not carry net flux away from the central spot. Instead, the polarities of MMFs were essentially mixed so that outflowing positive and negative fluxes were roughly balanced. During the 3-day period, there was almost no net flux accumulation to form a moat. The cancellation of MMFs of opposite polarities at the boundary of the super-penumbra caused quite a few surges and H brightenings.  相似文献   

    

A. Johannesson  W. Marquette  H. Zirin 《Solar physics》1995,161(1):201-204

We have compared three years of daily CaII K-line images from the Big Bear Solar Observatory (BBSO) with HI Lyman irradiance data from the Upper Atmosphere Research Satellite (UARS). The daily full-disk CaII K-line images are reduced to a new index of integrated excess emission, which reproduces both the 27 day rotational modulation and the solar cycle decrease in Ly irradiance. Our analysis shows that while plages reproduce the 27-day variation quite well, the total K-line emission excess above the quiet background is needed to reproduce the secular solar cycle trend in the Ly irradiance. The resulting K-line index exhibits a high degree of correlation (0.9) with the time series of measured Ly flux.  相似文献   

On the comparison of filter magnetographs and the advanced stokes polarimeter     

Harold Zirin 《Solar physics》1995,159(1):203-206

Liteset al. (1994) (hereafter LMS) have recently published a comparison of the response of their magnetograph (the Advanced Stokes Polarimeter (ASP)) to that they expect from filter-based magnetographs (FM). Not surprisingly, they conclude the ASP is better. They claim that only their instrument is quantitative, and others are not, and that the transverse field strength and azimuth measured by filter magnetographs may be in error by up to 50%. While the calculation is formally correct it ignores the high sensitivity attained by real FM's accumulating thousands of difference frames. Further, FM's have been cross-compared and tested empirically, without any such errors appearing. We point out that the two instruments have different roles, but the functional use of the FM is far superior to the ASP for solar research. The ASP may give accurate results for unresolved unipolar fields; it gives totally erroneous results when the field rapidly changes direction within its resolution element, as occurs in delta spots.  相似文献   

The Hα/Hβ ratio in solar flares     

Harold Zirin  Margaret Liggett  Alan Patterson 《Solar physics》1982,76(2):387-392

We have measured the ratio of H to H central intensities in the peak kernels of 14 flares, using simultaneous filtergrams. The ratio is typically one with some scatter. By contrast, in stellar flares the ratio is about 0.8.  相似文献   

Coronal loops and active region structure     

D. F. Webb  H. Zirin 《Solar physics》1981,69(1):99-118

We intercompared synoptic H, Ca K, magnetograph and Skylab soft X-ray and EUV data for the purpose of identifying the basic coronal magnetic structure of loops in a typical active region and studying its evolution. We focussed on a complex of activity in July 1973, especially McMath 12417. Our principal results are: (1) Most of the brightest loops connected the bright f plage to either the sunspot penumbra or to p satellite spots; no non-flaring X-ray loops end in umbrae; (2) short, bright loops had one or both ends in regions of emergent flux, strong fields or high field gradients; (3) stable, strongly sheared loop arcades formed over filaments; (4) EFRs were always associated with compact X-ray arcades; and (5) loops connecting to other active regions had their bases in outlying plage of weak field strength in McM 417 where H fibrils marked the direction of the loops. We conclude that a typical loop brightens in response to magnetic field activity at its feet, which heats the plasma. This suggests that the loop acts as a trap for gas convected from its base.  相似文献   

Effects of ionospheric horizontal gradients on differential GPS   总被引：2，自引：0，他引：2  

Mardina Abdullah  Hal J. Strangeways  David M. A. Walsh 《Acta Geophysica》2007,55(4):509-523

This paper outlines the effect of horizontal ionospheric gradients on transionospheric path propagation such as for the case of GPS signals. The total electron content (TEC) is a function of time of day, and is much influenced by solar activity and also the receiving station location. To make the model applicable for long baselines, for which the ionosphere is not generally well correlated between receiving stations, the ionospheric gradients should be taken into account. In this work the signal path is determined using a modified ray-tracing technique together with a homing-in method. Results show that horizontal gradients can have a significant effect on GPS positioning for both single station positioning and differential GPS. For differential GPS, the ionospheric delay can, however, be either increased or decreased compared with the case of no gradient, depending on the gradient direction.  相似文献   

Filament Disappearances During the Period of September 1991 through September 1994     

Haimin Wang  Arkadiusz E. Komenda  Frances Tang  Hal Zirin 《Solar physics》1998,178(1):109-117

Continuous full-disk H images recorded by the Big Bear Solar Observatory (BBSO) from 1 September 1991 to 19 September 1994 (the first three years of Yohkoh mission) were digitized and analyzed. The data set consists of nearly 10000 H images, one every half hour for the period when the BBSO was observing. Two statistical studies of the disappearing solar filaments based on this set of data are made: (1) The disk latitude distribution of all larger disappearing filaments with a minimum length of 70 arc sec, including the time of their disappearance. Of the 1095 such filaments, 439 disappeared during our continuous observations, 314 disappeared during the BBSO night gap, 162 disappeared during data gap (more than 94 hours) and 180 rotated beyond the west limb. If we plot latitudes as a function of time for all these disappeared filaments, it shows a uniform distribution in latitude. However, if we plot the distribution of larger disappeared filaments (200 arc sec or above), then the butterfly trend appears – position of filaments tends to drift to lower latitude as solar activity decreases. (2) The disk distribution of all detectable disappearing filaments, large and small, for the 9-months period, January 1994 to September 1994. We find that the size distribution of 351 collected disappeared filaments follows a power law with a power index of –1.40.  相似文献   

Study of supergranules     

Haimin Wang  Harold Zirin 《Solar physics》1989,120(1):1-17

Results of a detailed study on supergranule lifetime and velocity fields are presented. We show the correlation between the observed downdraft velocity and the network magnetic flux elements on the quiet sun. After excluding areas with magnetic flux density 25 G, we find that the upper limit of the supergranule vertical speed is 0.1 km s^–1 for both downdraft and updraft, and the r.m.s. speed is 0.03 km s^–1. By observing the evolution of individual supergranules, we find that the average lifetime of supergranules might be 50 hours. We describe different ways of formation and decay of supergranular cells. New cells usually form in an area containing no pre-existing supergranule velocity fields. Cells may disappear in two ways: fragmentation and fading away.  相似文献   

Evolution of magnetic fields and mass flow in a decaying active region     

Hongqi Zhang  Guoxiang AI  Haimin Wang  Harold Zirin  Alan Patterson 《Solar physics》1992,140(2):307-316

Five days of coordinated observation were carried out from 24–29 September, 1987 at Big Bear and Huairou Solar Observatories. Longitudinal magnetic fields of an p sunspot active region were observed almost continuously by the two observatories. In addition, vector magnetic fields, photospheric and chromospheric Doppler velocity fields of the active region were also observed at Huairou Solar Observatory. We studied the evolution of magnetic fields and mass motions of the active region and obtained the following results: (1) There are two kinds of Moving Magnetic Features (MMFs). (a) MMFs with the same magnetic polarity as the center sunspot. These MMFs carry net flux from the spot, move through the moat, and accumulate at the moat's outer boundary. (b) MMFs in pairs of mixed polarity. These MMFs are not responsible for the decay of the spot since they do not carry away the net flux. MMFs in category (b) move faster than those of (a). (2) The speed of the mixed polarity MMFs is larger than the outflow measured by photospheric Dopplergrams. The uni-polar MMFs are moving at about the same speed as the Doppler outflow. (3) The chromospheric velocity is in approximately the opposite direction from the photospheric velocity. The photospheric Doppler flow is outward; chromospheric flow is inward. We also found evidence that downward flow appears in the photospheric umbra; in the chromosphere there is an upflow.  相似文献   

The flares of August 1972     

Harold Zirin  Katsuo Tanaka 《Solar physics》1973,32(1):173-207

We present the analysis of observations of the August flares at Big Bear and Tel Aviv, involving monochromatic movies, magnetograms and spectra. In each flare the observations fit a model of particle acceleration in the chromosphere with emission produced by impact and by heating by the energetic electrons and protons. The region showed inverted polarity and high gradients from birth, and flares appear due to strong magnetic shears and gradients across the neutral line produced by sunspot motions. Post flare loops show a strong change from sheared, force-free fields parallel to potential-field-like loops, perpendicular to the neutral line above the surface.We detected fast (5 s duration) small (1') flashes in 3835 at the footpoints of flux loops in the August 2 impulsive flare at 1838 UT, which may be explained by dumping of > 50 keV electrons accelerated in individual flux loops. The flashes show excellent time and intensity agreement with > 45 keV X-rays. In the less impulsive 2000 UT flare a less impulsive wave of emission in 3835 moved with the separating footpoints. The thick target model of X-ray production gives a consistent model for X-ray, 3835 and microwave emission in the 18:38 UT event.Spectra of the August 7 flare show emission 12 Å FWHM in flare kernels, but only 1 to 2 Å wide in the rest of the flare. The kernels thus produce most of the H emission. The total emission in H in the August 4 and August 7 flares was about 2 × 10³⁰ erg. We belive this dependable value more accurate than previous larger estimates for great flares. The time dependence of total H emission agrees with radio and X-ray data much better than area measurements which depend on the weaker halo.Absorption line spectra show a large (6 km/s^-1) photospheric velocity discontinuity across the neutral line, corresponding to sheared flow across that line.This work has been supported by NASA under NGR 05 002 034, NSF Atmospheric Sciences program under GA 24015, and AFCRL under FI9628-73-C-0085.  相似文献