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Zirin, H.

Publications and source records attributed to Zirin, H..

At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

Studies of solar flares using optical, X-ray and radio data

X-ray, radio, H-alpha, and 3835-A data for a number of solar flares identified as sources of X-ray bursts are compared and analyzed. Eight of the more distinctive flares are described in detail, the exact value of H-alpha intensity is determined, and the production of the H-alpha emission is considered. It is shown that: (1) there is a good correspondence between the rise and peak of H-alpha emission and hard X-ray flux; (2) the H-alpha surface brightness of hard and soft X-ray bursts is the same for flares of the same soft X-ray flux; (3) all multiple spikes correspond to different steps in optical flares development; (4) smaller flares produce no observed brightening; (5) there is evidence that the low chromospheric network is not heated by conduction from above; and (6) the flare H-alpha emission can be explained by direct excitation of a preflare cloud or prominence by the flare electrons and their secondaries.

Zirin, H.↗
NASA NTRS1976

Production of a short-lived filament by a surge

An unusual solar event is investigated in which a short-lived cloud, very much like a filament, was formed by ejecta from a large surge. The temporal evolution of this surge is described, and evidence is presented which indicates that the short-lived cloud was a bona fide filament. The energetics of this event and the mass of the surge are estimated from radio and X-ray data obtained at the onset.

Zirin, H.↗
NASA NTRS1976

Further observations of the 10830-A helium line in stars and their significance as a measure of stellar activity

Measurements of the 10830-A He line in 198 stars are given along with data on other features in that spectral range. Nearly 80 per cent of all G and K stars show some 10830-A absorption or emission; of these, half are variable and one-quarter show emission. The results of Vaughan and Zirin (1968) are confirmed that the 10830-A line is not found in M stars, is weak in F stars, and is particularly strong in close binaries. The line is found in emission in extremely late M and S stars along with P-gamma, but the latter is not in emission in the G and K stars with 10830-A emissions. Variable He emission along with Ti I emission is found in the RV Tauri variables R Sct and U Mon. In R Aqr, the Fe XIII coronal line at 10747 A is found in emission along with a line at 11012 A, which may be singlet He or La II, as well as the 10830-A and P-gamma lines. There is possibly a five-year period in the appearance of 10830-A emission in theta Her. The nature of coronae or hot chromospheres in the various stars is extensively discussed without definite conclusion, except that the 10830-A intensity must be more or less proportional to the energy deposited in the chromosphere-corona by nonthermal processes.

Zirin, H.↗
NASA NTRS1976

Fe XIII line in R Aquarii

The emission line of the forbidden coronal transition of Fe XIII at 10,747 A has been detected in data from a spectral survey at R Aquarii. Strong emission lines of He I at 10,830 A and P gamma have been identified; a line at 11,013 A may be He I 3(1)S-6(1)P. It is suggested that the observed iron line is produced by a corona surrounding R Aquarii inter alia. The corona may be the source of the radio emission reported by Gregory and Seaquist (1974).

Zirin, H.↗
NASA NTRS1976

Production of a short-lived filament by a surge

A large surge was observed on September 17, 1971 part of which, after travelling 200,000 km across the surface, returned to the surface to form a filament. The filament lasted about 30 minutes, then rose up and returned to the source of the surge. This was interpreted as the filling of a semi-stable magnetic trap. Analysis of the microwave radio burst showed it to have been produced by a source optically thick at 8,800 MHz, with area 4 (arc min)squared and T approximately 275,000 deg, N squared sub eV approximately 7 x 10 to the 48th power. The soft x-ray burst showed a component at 12 x 1,00.000 deg with N squared sub eV approximately 3 x 10 to the 48th power.

Zirin, H.↗
NASA NTRS1975

Further observations of the lambda 10830 He line in stars and their significance as a measure of stellar activity

Measurements of the lambda 1030 He line in 198 stars are given along with data on other features in that spectral range. Nearly 80% of all G and K stars show some lambda 10830; of these, half are variable and 1/4 show emission. It was confirmed that lambda 10830 is not found in M stars, is weak in F stars, and is particularly strong in close binaries. The line is found in emission in extremely late M and S stars, along with P gamma, but P gamma is not in emission in G and K stars with lambda 10830 emissions. Variable He emission and Ti I emission are found in the RV Tauri variables R Scuti and U Mon. In R Aqr the Fe XIII coronal line lambda 10747 and a line at lambda 11012 which may be singlet He or La II are found, as well as lambda 10830 and P gamma. The nature of coronas or hot chromospheres in the various stars is discussed. It was concluded that the lambda 10830 intensity must be more or less proportional to the energy deposited in the chromosphere corona by non-thermal processes.

Zirin, H.↗
NASA NTRS1975

The helium chromosphere, coronal holes, and stellar X-rays

The solar chromosphere at the limb seen in D3 is an irregular bright band 1000 km thick with a dark band 1000 km thick beneath. The D3 chromosphere disappears in coronal holes. It is shown that the D3 emission, as well as the other He I and He II lines, can be explained quantitatively by photoionization by coronal back-radiation. A Chapman layer with N(He)H = 5 times 10 to the 17th power is formed near tau = 1 in the He I and He II continua. The chromospheric He emission or absorption is weak in coronal holes because there is no coronal back-radiation. Based on this model, the soft X-ray flux from stars with He 10830-A absorption lines is estimated as proportional to the 10830-A equivalent width and the apparent area.

Zirin, H.↗
NASA NTRS1974

Studies of K line filtergrams

The paper describes the material obtained on a Lyot filter for K at the Big Bear Solar Observatory. This material is compared with simultaneous H-alpha observations. K-line filtergrams are used to interpret K-line double reversal. The structure of the general chromosphere is discussed. It is found that the absorption of the cool clouds of the chromosphere, covering about one third of the area inside the cells, oscillating rapidly, and having a vertical extent around 3000 km, is not clearly seen in K3. In active regions K is close to H-alpha. Here all plagettes and plages are bounded by systems of strongly inclined bright and dark fibrils. Umbral flashes are found to be less visible in H-alpha, except that the running penumbral waves are bright in K and dark in H-alpha.

Zirin, H.↗
NASA NTRS1974

Solar flares

A review of the knowledge about solar flares which has been obtained through observations from the earth and from space by various methods. High-resolution cinematography is best carried out at H-alpha wavelengths to reveal the structure, time history, and location of flares. The classification flares in H alpha according to either physical or morphological criteria is discussed. The study of flare morphology, which shows where, when, and how flares occur, is important for evaluating theories of flares. Consideration is given to studies of flares by optical spectroscopy, radio emissions, and at X-ray and XUV wavelengths. Research has shown where and possibly why flares occur, but the physics of the instability involved, of the particle acceleration, and of the heating are still not understood.

Zirin, H.↗
NASA NTRS1973

The flares of August 1972

Observations of the August, 1972 flares at Big Bear and Tel Aviv, involving monochromatic movies, magnetograms, and spectra, are analyzed. The region (McMath 11976) showed inverted polarity from its inception on July 11; the great activity was due to extremely high shear and gradients in the magnetic field, as well as a constant invasion of one polarity into the opposite; observations in lambda 3835 show remarkable fast flashes in the impulsive flare of 18:38 UT on Aug. 2 with lifetimes of 5 sec, which may be due to dumping of particles in the lower chromosphere. Flare loops show evolutionary increases of their tilts to the neutral line in the flares of Aug. 4 and 7. Spectroscopic observations show red asymmetry and red shift of the H alpha emission in the flash phase of the Aug. 7 flare, as well as substantial velocity shear in the photosphere during the flare, somewhat like earthquake movement along a fault. Finally the total H alpha emission of the Aug. 7 flare could be measured accurately as about 2.5 x 10 to the 30th power erg, considerably less than coarser previous estimates for great flares.

Zirin, H.↗
NASA NTRS1973

19-20 May 1969, an example of type III emission during the impulsive phase of flares.

Examination of two multiple impulsive events in May 1969, and comparison of their impulsive Ha, hard X-ray, and microwave components with the observed type III emission. No good correlation was observed between meter intensity and the degree of simultaneous Ha outflow, although almost every type III burst had some associated Ha activity. Correspondingly, an inverse relationship existed between X-ray and type III emission - i.e., a strong meter burst accompanied weak X-radiation. This, along with the fact that sufficient energetic electrons were normally present to produce the X rays, implies that the trajectory, rather than electron spectrum or supply, determines whether type III noise is observed during an impulsive event.

Vorpahl, J.↗
NASA NTRS1973

Optical observations of the August 1972 flares

The optical appearance of the solar flares in McMath 11976 is described. The region showed complex, twisted magnetic fields from its birth on July 11, 1972. The flares occurred because of high field gradients and sheared field lines, which made transitions to lower energy field configurations possible. The H-alpha emission comes mainly from intense kernels about 10,000 kilometers across in each magnetic polarity. The measurement of the spectra and its relationship to the emission of impulsive flares is discussed. The peak emission of H-alpha from the kernels and the peak 3835 Angstrom flashes agree well with the hard X-ray peak.

Tanaka, K.↗
NASA NTRS1972

Observations of running penumbral waves.

Quiet sunspots with well-developed penumbrae show running intensity waves with period running around 300 sec. The waves appear connected with umbral flashes of exactly half the period. Waves are concentric, regular, with velocity constant around 10 km/sec. They are probably sound waves and show intensity fluctuation in H alpha centerline or wing of 10 to 20%. The energy is tiny compared to the heat deficit of the umbra.

Zirin, H.↗
NASA NTRS1972

The absence of flares in 3835 A and the heating of the chromosphere.

Simultaneous observations of flares in H-alpha and a band 15 A wide centered on 3835 A show no change whatever in 3835 A at the time of several flares, although the chromospheric network is easily visible. Flares are therefore transparent in this wavelength. Since the flare represents a test heat input from above, and no brightening of the network appears, we conclude it is impossible for the corona to produce the bright network by downward convection.

Zirin, H.↗
NASA NTRS1972

Fine structure of solar magnetic fields.

Extension of the deduction of magnetic fields from chromospheric structure to active regions and transverse fields. Fields independently predicted by these rules from a high-resolution H alpha filtergram are compared with a high-resolution magnetogram. The H alpha method has the advantage over conventional magnetograms that it shows transverse fields and relates the fields to the real sun. It has the disadvantage that higher spatial resolution is required and that it is difficult and time consuming in very complicated regions. The response of the chromosphere to magnetic fields is most consistent. Vertical field is invariably marked by bright plage, with brightness roughly proportional to the field strength (except for sunspots). All dark fibrils mark transverse fields and are parallel to field lines. All polarity changes are marked by dark fibrils, which may be transverse fibrils perpendicular to the field boundary, or filaments (prominences) which connect more distant points, and in which the field lines run nearly parallel to the boundary. The asymmetry between preceding and following polarity found by Veeder and Zirin (1970) does not exist; it was due to the low resolution of the Mount Wilson magnetograms.

Zirin, H.↗
NASA NTRS1972

The flares of August 1972

Analysis is made of observations of the August, 1972 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 impart and by heating by the energetic electrons and protons. The region showed twisted flux 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.

Zirin, H.↗
NASA NTRS1972

High resolution solar observations from space.

Present ground-based solar observations are limited by the transmission properties of the terrestrial atmosphere, the design of instruments, and the choice of observing locations. Recent improvements in the latter two have made possible observations with MTF around 7% at .5 arc sec and 15% at 1 arc sec. However, the instruments now appear to have been developed as far as the terrestrial atmosphere will permit, and new information will only be determined by space observations. The problems of construction, alignment and thermal control of a very high resolution solar telescope in space are discussed together with various optical systems. The problems of solar physics in which we would expect new information are also discussed, on the basis of recent high-resolution observations from the ground.

Zirin, H.↗