Kenneth R. Lang

Structure of a solar active region from Ratan 600 and very large array observations

Observations de la region active solaire AR 3804 les memes jours de juillet 1982 avec le RATAN 600 et le VLA

A Source book in astronomy and astrophysics, 1900-1975

132 selections containing the writings of important specialists in astrophysics, such as Eddington, Einstein, Gamow, Dyson and Bok, are presented. Attention is given to the solar system, variable and dying stars, and to stellar atmospheres, spectra, evolution and nucleosynthesis. Other subjects include the distribution of stars and the space between them, normal galaxies, radio galaxies, quasars, relativity, and cosmology. Some topics presented are the photoelectric photometry of the stars, radar determinations of the rotations of Venus and Mercury, the abundances of chemical elements in stellar atmospheres, and atomic synthesis and stellar energy. Also considered are T Tauri variable stars, the radio frequency detection of interstellar hydrogen, galactic magnetic fields and the origin of cosmic radiation, fluctuations in cosmic radiation at radio frequencies, and the relation between the expansion and the mean density of the universe.

V.L.A. Observations of Solar-Active Regions. I. The Slowly Varying Component,

Abstract : Very Large Array (V.L.A.) synthesis maps of the total intensity and the circular polarization of the active region AR 2032 at 6 cm wavelength are presented and compared with H alpha photographs and Zeeman effect magnetograms of the same region with second-of-arc accuracy. The maps indicate that the radio emission is dominated by a small (approx. 30 inches), bright (10 to the 6th power K), circularly polarized (40% to 100%) source whose detailed morphological features are correlated with the chromospheric plage seen as bright regions on the H alpha photographs. The large brightness temperatures of the radio features indicate that they are the coronal counterparts of the chromospheric plage. The regions of enhanced emission at 6 cm wavelength exhibited substantial structural changes on a time scale of one day, and these changes were correlated with similar changes in the chromospheric plage. Weak or undetectable radio emission was found in the regions directly overlying sunspots; but two small (approx. 10 inches), bright (approx. 10 to the 6th power K) radio sources were found at the outer edges of one sunspot where the magnetic field gradient is large. Although the degree of circular polarization varied from 40% to 100% in different regions of the dominant, plage-associated source, the entire source had one magnetic polarity and the general magnetic structure was correlated with the longitudinal magnetic field seen on magnetograms of the lower lying photosphere.

Solar active region physical parameters inferred from a thermal cyclotron line and soft X-ray spectral lines

Simultaneous high-resolution observations of coronal loops were made at the 20-cm wavelength with the VLA and at soft X-ray wavelengths with the SMM FCS. The images obtained at both wavelengths have nearly identical sizes and ellipsoidal shapes, with the emission stretching between and across regions of opposite magnetic polarity in the underlying photosphere. The results indicate that the radiation at 20 cm and soft X-ray wavelengths originates from the same region, and that 20 cm maps can image X-ray coronal loops. The X-ray spectral lines were used to obtain values of electron temperature, T(e), of about 2.6 x 10 to the 6th K and electron density, N(e), of about 3.1 x 10 to the 9th/cu cm. These parameters were used to show that the layers emitting 20-cm radiation can be optically thick to either thermal bremsstrahlung or thermal gyroresonance radiation, depending upon unknown but plausible values of loop thickness, magnetic scale height, and magnetic field strength. 13 references.

Millisecond Radio Spikes from the Dwarf M Flare Star AD Leonis

Arecibo radio observations of millisec bursts of radio signals at 1415 MHz from AD Leonis are reported. The observed burst had an ellipticity of 0.95, 50-100 percent circular polarization, and a flux density maximum of 30 mJy. The 50 sec burst featured five quasi-periodic oscillations with a mean periodicity of about 3.2 sec. A second, less intense burst that occurred 20 sec later was 100 percent circularly polarized. The area emitting the bursts covered an estimated 0.005 of the radius of AD Leonis and had an electron density of 6 billion/cu cm and a longitudinal magnetic field strength of 250 gauss, if the source was an electron-cyclotron maser. A coherent plasma source would require, for the first harmonic, an electron density of 20 billion/cu cm and a magnetic field much less than 500 gauss. A second harmonic of the plasma frequency would require an electron density of 6 billion/cu cm and a field strength much less than 250 gauss. The possibility that the source was periodic oscillations in coronal loops is discussed. 45 references.

Very Large Array Observations of Solar Active Regions. IV. Structure and Evolution of Radio Bursts from 20 cm Loops.

The Very Large Array (VLA) has been used to study the structuree and evolution of six solar bursts near 20 cm wavelength. In most ccases the burst emission has been resolved into looplike structures with total lengths, Lapprox.3 x 10/sup 9/ cm, brightness temperatures T/sub B/approx.10/sup 7/-10/sup 8/ K, and degrees of circular polarization p/sub c/< or approx. =90%. changes in the total intensity and circular polarization of the bursts occur on time scales as short as 10 s. The individual peaks of one multiple-component burst originated in different locations within a magnetically complicated region. Preburst heating occurred minutes before the onset of the impulsive phase of one burst, and circular polarization changes occurred minutes before the onset of the impulsive phase of another burst. In one case, a loop system emerged in the vicinity of the impulsive source, and two adjacent loop systems may have emerged and triggered the burst.

Shock Waves in Xenon Flashtubes and Tube Deterioration

The xenon‐filled arc discharge tube has found wide applications as an optical pumping source for ruby lasers. An experimental study of the shock waves in the flashtube at laser pumping energies and their effect on tube deterioration has been made. The plasma discharge was investigated by taking high‐speed image converter and streak photographs of the discharge and by measuring the relative shock amplitude at the tube walls with a piezoelectric transducer. The velocity of the luminous boundary of the plasma discharge was proportional to the input energy in the range from 60 to 500 J and varied from 90 to 900 m/sec. The luminous boundary velocity remained constant for pressures from 100 to 1500 Torr. The relative shock amplitude measured by the electromechanical transducer was independent of the tube diameter up to 12 mm, increased linearly with the input energy, and increased with gas pressure. Shock data were correlated with efficiency measurements to determine optimum tube and operational parameters. The...

Simultaneous SMM flat crystal spectrometer and Very Large Array observations of solar active regions

Abstract : We compare high-resolution images of the quiescent emission from two solar active regions at 20 cm (VLA) and soft X-ray (SMM-FCS) wavelengths. There are regions where the X-ray coronal loops have been completely imaged at 20 cm wavelength. In other regions, the X-ray radiation was detected without detectable 20 cm radiation, and vice versa. The X-ray data were used to infer average electron temperatures Te of about 3,000000 K and average electron densities of about 2.5 x ten to the ninth power cc for the X-ray emitting plasma in the two active regions. The thermal bremsstrahlung of the X-ray emitting plasma is probably optically thin at 20 cm wavelength. Although the 20-cm brightness temperatures TB were always less than Te, suggesting optically thin layers, the thermal gyroresonance radiation from the X-ray emitting plasma ought to be optically thick at 20 cm wavelength. The low TB can be explained if a higher, cooler plasma covers the hotter X-ray emitting plasma. Thermal gyroresonance radiation must account for the intense 20-cm radiation near and above sunspots where no X-ray radiation is detected. The potential of 20-cm (VLA) and soft X-ray (SMM-FCS) comparisons is discussed.

Impulsive microwave burst and solar noise storm emission resolved with the VLA

The evolution of a microwave burst at 20.7 cm wavelength and a type I noise storm at 91.6 cm wavelength are examined using VLA. The magnetic loops in the two spectral regions are studied. The sizes and brightness temperatures of the 20.7 cm burst sources are compared with those predicted by multithermal and nonthermal models of microwave burst emission of Dulk and Dennis (1982). The data reveal that: (1) the precursor, impulsive, and post-impulsive phases of the 20.7 cm burst are located in spatially separated sources; (2) the gradual enhancement of a 91.6 cm noise storm continuum source suggests a feed-back mechanism exists between activity in higher and lower lying loops; and (3) the derived magnetic field strengths at 20.7 cm for the nonthermal models are 35-160 G with core field strengths of 125-220 G. 50 refs.

Multiple wavelength microwave observations of the RS Canum Venaticorum stars UX Arietis, HR 1099, HR 5110, and II Pegasi

The variabilities, core size and magnetic field of the RS CVn star UX Arietis was measured with the VLA at pairs of frequencies near 1415 MHz and 4835 MHz on June 10, 1985. Data were also gathered on HR 1099, HR 5110 and II Peg. UX Arietis exhibited variability on time scales ranging from 30 sec to 1 hr at 4835 MHz, but no detectable variations at 1415 MHz. An upper limit of 900 billion cm was placed on the size of the core emitting region, which is estimated to have a magnetic field strength of 15 G. The 30 sec variations are attributed to absorption by thermal plasma between the G5 and K1 companions of the UX Arietis system. 37 references.