The rotation of sunspots in the solar active region NOAA 10930

The rotation of sunspots in the solar active region NOAA 10930 was investigated on the basis of the data on the longitudinal magnetic field and the Doppler velocities using magnetograms and dopplergrams taken with the Solar Optical Telescope installed aboard the HINODE mission. Under the assumption of axial symmetry, areally-mean vertical, radial, and azimuthal components of the magnetic field and velocity vectors were calculated in both sunspots. The plasma in the sunspots rotated in opposite directions: in the leading sunspot, clockwise, and in the following sunspot, counterclockwise. The magnetic flux tubes that formed sunspots of the active region on the solar surface were twisted in one direction, clockwise. Electric currents generated as a result of the rotation and twisting of magnetic flux tubes were also flowing in one direction. Azimuthal components of magnetic and velocity fields of both sunspot umbrae reached their maximum on December 11, 2006. By the start of the X3.4 flare (December 13, 2006), their values became practically equal to zero.     

The coronal disturbance of 1972, August 12

Using the observed data for metric and hectometric type III radio bursts, the dependence of burst characteristics on the solar longitude has been examined over a wide frequency range. It is found that there exists an east-west asymmetry for the extension of metric type III bursts into hectometric wavelength range. In particular, hectometric bursts are rarely observed for solar flares associated with metric bursts eastward solar longitude 60°E. Furthermore, for eastern longitudes, the low frequency radio observations show a large dispersion in drift time interval.     

The limb flare of August 11, 1972

A limb flare is described that occurred above a complex and very active sunspot. Four stages can be distinguished: the flash-phase, the spray-phase, the surge-phase and the loop-phase. Each of them had a duration that was longer than that of the preceding one. In the spray ascending speeds up to 745 km s^–1 and accelerations up to 1.3 km s^-2 were recorded. The loop-phase has been observed in the coronal lines 5303 and 5694 Å. The yellow line, being very weak before the flare, became extremely strong in the loop and surpassed five times the intensity of the green line. X-ray bursts and ionospheric disturbances of long duration demonstrate that not only the flare itself but also the loop was a source of X-rays. Most of the radio-bursts can be ascribed to specific features in the H-records of the event.Astronomishe Mitteilungen der Eidgenössischen Sternwarte Zürich Nr. 318.     

The flares of August 1972

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.     

A morphological study of a solar active region in August 1972: II. A correlation analysis of morphologies of flares and spots

A correlation analysis of the morphology of nine solar flares of a large solar active region in August 1972 and the morphology of the fine structure of sunspot group in this region has led to the following conclusions:

• 1.1. There is a certain correlation between the outburst of the first large flare, occurring at 0355 UT. of August 2nd and the morphological variations of photospheric spots both in time and in spatial positions.
• 2.2. All the preliminary bright points of the nine flares on the both sides of the filament and their main morphological development are also closely correlated with the spiral structure of the spots “O” and “B”.
• 3.3. All the directions of the bright ribbons of flares and filaments (consisting of paralled fibrils) in the chromosphere, the serpent-like long fibres of penumbra on the east of the spot “O” and the line H_∥ = 0 in the photosphere are consistent with one another. This can be regarded as a morphological evidence in favour the opinion that the outburst of flares is propagated along the horizontal magnetic field on the surface of the sun.

     

Magnetic fields,loop prominences and the great flares of August, 1972

Lines of magnetic force, computed under the assumption that the solar corona is free of electric currents, have been compared with loop prominence systems associated with three flares in August, 1972. The computed fields closely match the observations of loops at a height of 40000 km at times 3–4 h after onset of the associated flares. Inferred magnetic field intensities in the loops range from 1300 G where the loops converge into a sunspot to 50–80 G at 40 000 km above the photosphere. The first-seen and lowest-lying loops are sheared with respect to the calculated fields. Higher loops conform more closely to the current-free fieldlines. A model of Barnes and Sturrock is used to relate the degree of shear to the excess magnetic energy available during the flare of August 7. On various lines of evidence, it is suggested that magnetic energy was available to accelerate particles not only during the impulsive phase of the flare, but also during the following 2–3 h. The particle acceleration region seems to be in the magnetic fields just above the visible loops. The bright outer edges of the flare ribbons are identified as particle impact regions. The dense knots of loop prominence material fall to the ribbons' inner edges.On leave from Tel Aviv University, Tel Aviv, Israel.     

On the problem of active longitudes of sunspots and flares

The active longitudes of indices for sunspot activity and solar flares were detected and investigated by the method of isoline for the period July 1, 1957 to December 31, 1962. In the most active hemisphere of the sun the active longitudes of sunspot and flares appear to coincide. It is shown that the rate of concentration in the active longitudes is the highest for more important formations. Arguments for the reality of the active longitudes of sunspot areas are advanced. In conclusion the question of the influence of the uncertainty of the solar rotation period on the detection of active longitudes of flares is considered.     

The role of spiral sunspots in the solar activity of October 1972

From a collection of photographs of 192 knots relating to 68 flares of the activity region McMath 12094 of October 1972, and of spots and prominences, a correlation analysis was made which led to the following conclusions:

• 1.1. Flare activity began in the region above the leader spot A(N) which had the highest twisted magnetic energy of all the spots in the group. It rapidly developed to the region above the follower S-spots, and then to regions outside the spot area. This evolutionary process was in step with the morphological changes and the rotational movement of the spots in the group.
• 2.2. The flare activity around Spot A began in the interior of the twisted magnetic tube and was in step with the untwisting of magnetic features, and the flare knots spread from the vicinity of the umbra to the outside of the penumbra. When the knotted structure completely untwisted itself, the overall flare activity decreased in the region.
• 3.3. A spiral spot was found to be a strong centre of attraction below a giant coronal prominence. This shows that coronal prominences are formed through the contraction of strong electrical currents in the corona.

We also discussed the question whether the twisted magnetic structure of spot A could have provided most of the energy in the flare activity and lead to an instability.     

Particle trapping and acceleration during the August 1972 event

Several features of the August 1972 events are studied using neutron monitor data together with solar wind streamlines calculated on the basis of an approximate kinematic approach. Examination of the evolution of these streamlines shows that the streamline which passes from the Earth undergoes dramatic changes during the main phase of these events. In a few hours this streamline, which was estimated with HEOS-2 solar wind velocity data, was decreased (i.e., compressed) to a total radial extend of 0.2 AU (at the beginning of 5 August), although its initial length was 1 AU. An exact MHD time-dependent solution by Dryer et al. (1978a) gives similar results.The relative cosmic ray increase (3–7 UT, 5 August), immediately after the deep F.d., is attributed to trapping and acceleration of particles between two shock waves. Similar acceleration was found by Pomerantz and Duggal (1974) and Levy et al. (1976) for another cosmic ray increase during this event.The extremely large solar wind velocities during the main phase of the event are not only due to the large energy of the flare but also to the fact that the ambient solar wind was already almost empty because of the sweeping action of previous shock waves.Work performed partly at the University of Athens and partly at Imperial College of Science and Technology.     

Force-free magnetic fields and flares of August 1972

On the basis of observations (Zirin and Tanaka, 1973) inferring the presence of shear in magnetic fields, the amount of extractable energy stored in a class of force-free magnetic fields is evaluated for the flares of August 1972, using the formulations developed by Nakagawa and Raadu (1972). It is shown that the evaluated energy storage could be built up by the proper motions of sunspots in the active region McMath 11976 during July 31 and August 7. Then for the flare of August 7, a detailed analysis is made of the manner of energy release in the post maximum phases deduced from the configuration of flare loops. It is shown that the observed flare loops could be represented closely by the force-free magnetic fields and that the evaluated rate of energy release is consistent with observed rate given by the soft X-ray emission. The results of analysis suggest that the flare of August 1972 could be identified with the relaxation of an energetic force-free magnetic field towards lower energy states. The limitations and possible future extension of this type of analysis are discussed.Visiting scientist from the Tokyo Astronomical Observatory, Mitaka, Tokyo, Japan.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Multiple flows and the fine structure of the transition region around sunspots

The fine structure in the flow field in the transition region above and surrounding a sunspot is determined fromCIV 1548 line profiles, observed with the High Resolution Telescope and Spectrograph (HRTS) during the Spacelab 2 mission. The observed line profiles show one, two, or three distinct velocity components within the resolution element of 1 × 1. Supersonic flows occur in small regions where the line profile has two or three components. The line component that shows supersonic speed often is weaker than the subsonic line component, which may explain why some observers have been unable to detect the supersonic flow component. The broadening of individual line components shows non-thermal velocities close to 20 km s^–1. This suggests that turbulence is less important than usually considered.The presence of multiple flows, which also occurs in quiet solar regions, suggests that the transition region above sunspots has a sub-arc-second fine structure, perhaps consisting of thin fibrils. The Evershed flow in the transition region appears to have a correspondingly complex character, possibly consisting of sub- and supersonic siphon flows along the individual fibrils. Time changes in the flow field over 5 min may correspond to characteristic times of individual fine structures. Possible explanations of the net downward directed mass flux are presented.     

An X-ray observation in the atmosphere during the August 7, 1972 solar flare

An observation carried out with a balloon-borne detector of an additional flux of secondary X-rays (E 30 keV) at large depths in the atmosphere is described. This excess is attributed to the emission of very hard X-rays during the solar flare of August 7, 1972. The propagation in the atmosphere of the secondary photons resulting from their electromagnetic interactions in the air is computed by utilizing the Monte Carlo method. The computations agree with the observed flux when a very hard solar X-ray spectrum is assumed.     

Magnetic flux changes associated with the solar flares of August 1972

The active region associated with Mt. Wilson sunspot group 18 935 (McMath, 11 976) which had a central meridian passage on August 4 and 5, 1972 produced a number of flares during transit. These included two importance 3B flares on August 4 and 7 as well as several of importance 1 and 2. Calculations of the total magnetic flux in this region were made during the period July 31 through August 9 using data from six observatories. For the 3B flare on August 4, the total flux changed from about 7.2 × 10²² Mx just before onset to about 5.6 × 10²² Mx two hours after onset. For the 3B flare on August 7, the flux was about 6.4 × 10²² Mx three hours before onset and about 5.2 × 10²² Mx three hours after onset. An importance 2B flare on August 2 had no measurable effect on the flux nor did any of several 1N or 1B flares which also occurred in this region during the period. The flux changes measured for the 3B flares occurred in the umbral and penumbral fields and no significant changes were observed in facular fields.The Aerospace Corporation, P.O. Box 92957, Los Angeles, Calif. 90009, U.S.A.     

Energetic properties of interplanetary plasma at the earth's orbit following the August 4, 1972 flare

Solar wind and interplanetary magnetic field data were obtained by the PROGNOZ 1 and PROGNOZ 2 satellites during the period following the August 4, 1972 (0621 UT) solar flare. A thermalized plasma was recorded one hour after the shock followed two hours later by the plasma piston with a bulk velocity higher than 1700 km s^-1. The comparison between the PROGNOZ and PIONEER 9 solar wind data shows an attenuation of the plasma properties with the deflection from the flare's meridian.     

The solar outburst on August 7, 1972 at 17 GHz and 35 GHz

The radio-burst on August 7, 1972, is discussed. On 17 GHz the peak flux was about 25000 SFU. Considering the decreasing phase of the burst, it was found that an exponential decrease as well as a power-law decrease can be used. The magnetic field in the origin of the burst should be of the order of 1000 G, while the exponent g due to a power-law of the energy distribution is estimated to be about g 3. The degree of circular polarisation shows an increase to about 25% during the ascending phase of the burst, while in the phase of maximal radiation and during the decrease the polarisation degree was small.     

Disturbances in the solar wind from IPS measurements in August 1972

From IPS and spacecraft measurements of the solar wind combined with geomagnetic observations, we identify the passage of three main disturbances through the solar wind from solar flares on August 2, 4 and 7. From a detailed study of the IPS data covering the third event, we conclude that the extent of the disturbance front at 1 AU covered about ±60° in longitude and more than 30° in latitude from the flare normal. If interpreted as a blast wave according to the model of De Young and Hundhausen (1971), the disturbance was ejected from the Sun into a cone of half-angle 45°±15°.     

Umbral boundaries,convection, and the depth of sunspots

The boundary between the umbra and penumbra of a sunspot is consistently observed to be very sharp, on the order of 500 km. Approximating the sunspot as a static region in a homogeneous medium with a radiative surface, temperature distributions resulting from a variety of convective motions exterior to the sunspot are calculated. The calculations suggest that, for the exterior convection to produce the observed boundary, the maximum depth of the region of inhibited convection below a sunspot umbra is on the order of 10³ km.