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.     

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.     

Shock waves generated by the intense solar flare of 1972, August 7, 15:00 UT

The dynamic radio spectrum of the class 3B solar flare of 1972, August 7, 15: 00 UT, over the band 10 to 2000 MHz is examined. Type II and type IV bursts in the spectrum are interpreted in terms of a piston-driven shock, which appeared to be travelling at a velocity of about 1500 km s^?1 and which generated pulsations in the band 100 to 200 MHz as it passed through the corona. The progress of the shock through the interplanetary plasma was subsequently monitored by Malitson et al. with radio equipment covering the band 0.03 to 2.6 MHz on the IMP-6 satellite.

     

The helium-enriched interplanetary plasma from the proton flares of August/September, 1966

This paper describes the solar wind plasma ejected by the proton flares of August/September, 1966, in McMath Region 8461. The discussion will serve a dual purpose. First it will help complete the record on the events of August/September 1966. Secondly we will discuss the helium enrichment of the interplanetary plasma associated with the flares. This is the fifth case reported in which major flares produce helium enriched interplanetary plasma. Relative helium abundances of greater than 15% are typical. These findings are interpreted in terms of a solar atmosphere that contains helium enriched regions.     

Analysis of the August 7, 1972 white light flare: Light curves and correlation with hard X-rays

Cinematic, photometric observations of the 3B flare of August 7, 1972 are described in detail. The time resolution was 2 s; the spatial resolution was 1–2″. Flare continuum emissivity at 4950 Å and at 5900 Å correlated closely in time with the 60–100 keV non-thermal X-ray burst intensity. The observed peak emissivity was 1.5 × 10¹⁰ erg cm^?2 s^?1 and the total flare energy in the 3900–6900 Å range was ～10³⁰ erg. From the close temporal correspondence and from the small distance (3″) separating the layers where the visible emission and the X-rays arose, it is argued that the hard X-ray source must have had the same silhouette as the white light flare and that the emission patches had cross-sections of 3–5″. There was also a correlation between the location of the most intense visible emissions near sunspots and the intensity and polarization of the 9.4 GHz radio emission. The flare appeared to show at least three distinct particle acceleration phases: one, occurring at a stationary source and associated with proton acceleration gave a very bluish continuum and reached peak intensity at ～ 1522 UT. At 1523 UT, a faint wave spread out at 40 km s^?1 from flare center. The spectrum of the wave was nearly flat in the range 4950–5900 Å. Association of the wave with a slow drift of the microwave emission peak to lower frequencies and with a softening of the X-ray spectrum is interpreted to mean that the particle acceleration process weakened while the region of acceleration expanded. The observations are interpreted with the aid of the flare models of Brown to mean that the same beam of non-thermal electrons that was responsible for the hard X-ray bremsstrahlung also caused the heating of the lower chromosphere that produced the white light flare.     

The GLE-associated flare of 21 August, 1979

We use a variety of ground-based and satellite measurements to identify the source of the ground level event (GLE) beginning near 06∶30 UT on 21 August, 1979 as the 2B flare with maximum at ～06∶15 UT in McMath region 16218. This flare differed from previous GLE-associated flares in that it lacked a prominent impulsive phase, having a peak ～9 GHz burst flux density of only 27 sfu and a ?20 keV peak hard X-ray flux of ?3 × 10^-6 ergs cm^-2s^-1. Also, McMath 16218 was magnetically less complex than the active regions in which previous cosmic-ray flares have occurred, containing essentially only a single sunspot with a rudimentary penumbra. The flare was associated with a high speed (?700 km s^-1) mass ejection observed by the NRL white light coronagraph aboard P78-1 and a shock accelerated (SA) event observed by the low frequency radio astronomy experiment on ISEE-3.     

The proton flare of August 28, 1966

The proton flare of August 28, 1966 began on H records at 15^h21^m35^s UT. It presented an unusually complex development with flare emission occurring in two distinct plages. The brightest part of the flare attained maximum intensity, 152 % of the continuum, between 15^h30^m and 15^h32^m UT. Photometric measurements show that a long-enduring part of the flare continued to decline in intensity until at least 21^h20^m UT.The flare developed first in parts of the plages that were near the extremities of a filament and a complex system of curvilinear absorption structures, possibly an eruptive prominence in projection. During the rise to maximum intensity a large expanding feature moved southward from the site of the flare with a velocity 700 km/sec. Its appearance on monochromatic records of the chromosphere made in the center of H and 0.5 Å on either side was consistent with the effect of an advancing phenomenon that induces a small shift of the H absorption line, first to longer, and then to shorter wavelengths.Two bright flare-filaments were obvious aspects of the event by 15^h28^m and dominated the main phase of the flare. Loop-type prominences were observed in absorption as early as 15^h40^m.This investigation was supported in part by the Office of Naval Research through funds from Nonr-1224(19), and in part by the National Science Foundation through funds from GA-632.     

Energetic protons from the solar flare of March 24, 1966

Ten to 100 meV protons from the solar flare of March 24, 1966 were observed on the University of California scintillation counter on OGO-I. The short rise and decay times observed in the count rates of the 32 channels of pulse-height analysis show that scattering of the protons by the interplanetary field was much less important in this event than in previously observed proton flares. A diffusion theory in which D = M _r is found to be inadequate to account for the time behavior of the count rates of this event. Small fluctuations of the otherwise smooth decay phase may be due to flare protons reflected from the back of a shock front, which passed the earth on March 23.     

Observational characteristics of the white-light flare of August 9, 2011

We analyzed the monochromatic H_α and spectral (within a range of 6549–6579 Å) observational data for the 2B/X6.9 flare of August 9, 2011, that produced emission in the optical continuum. The morphology and evolution of the H_α flare and the position, time evolution, spectrum, and energetics of the white-light flare (WLF) kernels were studied. The following results were obtained: the flare erupted in the region of collision of a new and rapidly growing and propagating magnetic flux and a preexisting one. This collision led to a merger of two active bipolar regions. The white-light flare had a complex structure: no less than five kernels of continuum emission were detected prior to and in the course of the impulsive flare phase. Preimpulsive and impulsive white-light emission kernels belonged to different types (types II and I, respectively) of white-light flares. A close temporal agreement between the white-light emission maxima and the microwave emission peak was observed for the impulsive white-light emission kernels. The maximum flux, luminosity, and total energy emitted by the brightest impulsive WLF kernel equaled 1.4 × 10¹⁰ ergs cm^?2 s^?1, 1.5 × 10²⁷ ergs/s, and 5 × 10²⁹ ergs, respectively. The H_α profiles within the impulsive WLF kernels had broad wings (with a total extent of up to 26 Å and a half-width of up to 9 Å) and self-reversed cores. The profiles were symmetrical, but were shifted towards the red side of the spectrum. This is indicative of a downward motion of the entire emitting volume with a radial velocity of several tens of km/s. The intensity pattern in the wings did not correspond to the Stark one. The profiles were broadened by nonthermal turbulent motions with velocities of 150–300 km/s. The observed H_α profiles were analyzed and compared in their features to the profiles calculated for an intense heating of the chromosphere by nonthermal electrons accompanied by the development of a chromospheric condensation propagating downward. We came to the conclusion that the analyzed flare exhibited spectral features that may not be readily explained within the framework of chromosphere heating by a beam of nonthermal electrons.     

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.     

Studies of geomagnetic micropulsations observed at Ahmedabad during the magnetic storm of August 4 to 6, 1972 by power spectrum method

The magnetic field variations observed at Ahmedabad during the magnetic storm of August 4 to 6, 1972 are subjected to power Spectrum Analysis. The storm is divided in five parts depending upon its morphology and phase. Prominent peaks are observed in the range of 60–300 sec periods. The prominent periods and their respective power are studied in relation to the progress and phase of the storm. These are then related to the condition of the magnetosphere and the interplanetary medium observed at the time of the storm.     

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.     

Hydromagnetic emission of the interplanetary plasma

A new type of geomagnetic pulsation — the serpentine emission — has been discovered in the 0–2 Hz range. The particular feature which characterizes the serpentine emission is the wide modulation of the carrier frequency. A theory to account for this type of disturbance involves radiation from the solar wind. The frequency modulation is explained in terms of Doppler shifts and variation of the direction of the interplanetary magnetic field.     

Temporal variations of the interplanetary magnetic field at the Earth’s orbit and of the solar activity

This paper analyzes the time changes of the common oscillations of the spectra of the absolute value of the interplanetary magnetic field measured at the Earth’s orbit from 1964 to 1997 and of solar activity (the Wolf sunspot numbers). The frequency components of the spectra were determined using the method of nonlinear spectral analysis. Oscillations with common periods of T = 10.8, T = 8.8, and T = 3.73 years have been identified in the long-period part of the spectra, and their temporal variations are shown. We discuss the specific features of the spectral band in the short-period part of the spectra in the vicinity of the known periods of T ～ 1.3 years and T ～ 150 days that have been identified earlier in the solar data and in the solar wind parameters.     

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.     

Theory of quadrupolar sunspots and the active region of August, 1972

Energy is stored when the force-free magnetic field in an active region departs from a potential field, the departure showing up as a shear in the field. As soon as the field untwists, energy will be released to produce flares. Based on this idea, we derived an analytical solution of the equation of force-free field under the assumption of a constant force-free factor, and found expressions for seven important quantities for quadrupolar sunspots: the magnetic energy of the twisted field, that of potential field, the extractable free energy ΔM, the magnetic flux, the total current, the force-free factor and the field decay factor, in terms of three observables: the field intensity, the twist angle and the distance between two spots of the same polarity. The expression for ΔM can be useful in solar prediction work. For the active region of August, 1972, we found ΔM up to 6 × 10³² erg, sufficient to supply the energy of the observed flare activity. Observations of this active region are in good general agreement with our theoretical expectations: in the entire twisting of the quadrupolar sunspot group, each spot assumes the form of a complete spiral in the clockwise direction for each of the four spots.     

Observations of solar gamma ray continuum between 360 keV and 7 MeV on August 4, 1972

Measurements were made of the time-averaged gamma ray energy loss spectrum in the energy range 360 keV to 7 MeV by the gamma ray detector on the OSO-7 satellite during the 3B flare on August 4, 1972. The differential photon spectrum unfolded from this spectrum after subtracting the background spectrum and contributions from gamma ray lines is best described by a power law with spectral index of 3.4±0.3 between 360–700 keV and by an exponential law of the form exp (-E/E ₀) with E ₀ = 1.0±0.1 MeV above 700 keV. It is suggested that this spectrum is due to nonthermal electron bremsstrahlung from a population of electrons, with a strong break in the spectrum at 2 MeV. Since the observational data indicates that the matter number density must be n _H ? 5 × 10¹⁰ cm^-3 in the production region, the number of electrons above 100 keV required to explain the results is ?2 × 10³⁴.