Engineering topics

Lang, Kenneth R.

Publications and source records attributed to Lang, Kenneth R..

27 records · Page 2

NASA NTRS1987

Coordinated ultraviolet and radio observations of selected nearby stars

All of the US2 shifts assigned were successfully completed with simultaneous International Ultraviolet Explorer (IUE) and the Very Large Array (VLA) observations of the proposed target stars. The target stars included dwarf M flare stars and RS CVn stars. The combined ultraviolet (IUE) and microwave (VLA) observations have provided important new insights to the radiation mechanisms at these two widely-separated regions of the electromagnetic spectrum. The VLA results included the discovery of narrow-band microwave radiation and rapid time variations in the microwave radiation of dwarf M flare stars. The results indicate that conventional radiation mechanisms cannot explain the microwave emission from these stars. In general, ultraviolet variations and bursts occur when no similar variations are detected at microwave wavelengths and vice versa. Although these is some overlap, the variations in these two spectral regions are usually uncorrelated, suggesting that there is little interaction between the activity centers at the two associated atmospheric levels.

Lang, Kenneth R.↗

NASA NTRS1987

VLA observations of a solar noise storm

The first Very Large Array (VLA) observations of the sun at 92 cm wavelength (328 MHz) are presented. A 3-hr solar noise storm was detected; it consisted of burst-like spikes superimposed on a slowly varying background, and both storm components were 95 + or - 5 percent right-hand circularly polarized. A long duration soft X ray event preceeded the radio radiation by 30 m, suggesting a disturbance moving outwards at a velocity of v = 78 km/s. The 92 cm noise storm was resolved with an angular resolution of 9 sec for time intervals as short as 13 s. During the onset and early phases, the storm consisted of four compact sources, each with an angular diameter of 40 sec, oriented within an elongated source with angular dimensions of 40 sec by 200 sec. During the subsequent hour the most intense emission was located in two 40 sec sources separated by 100 sec. Snapshot maps revealed a persistent elongated source ta successive Peaks, with a scatter in the source position. A systematic position shift of Delta Theta sub I greater than or = 15 sec can be produced by the earth's ionoshere, but these effects can be removed by frequent observations of a nearby calibrator source. The observations confirm previously reported trends for a decrease in source size at higher frequencies, but they suggest a hitherto unresolved complexity in source structure.

Lang, Kenneth R.↗

NASA NTRS1987

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.

Willson, Robert F.↗

NASA NTRS1986

Coronal diagnostics.

This is an introduction to this part of the proceedings and an overview of coronal diagnostics. The current understanding of coronal loops is summarized. Included are observations from ground-based radio telescopes and from X ray telescopes lofted above the atmosphere, as well as theoretical interpretations of these observations. Also included in these introductory remarks is a discussion of the three dimensional structure of coronal loops. Alternative radiation mechanisms are then described within the context of both the radio and X ray emission. Various methods of determining the strength and structure of the coronal magnetic field are then described, followed by the coronae of nearby stars and future prospects for radio diagnostic of coronal loops.

Lang, Kenneth R.↗

NASA NTRS1986

Coronal plasmas on the sun and nearby stars

The current understanding of the quiescent, or non-flaring, microwave emission from solar active regions is summarized. The thermal radiation mechanisms that account for most of the quiescent emission is reviewed, while it is also pointed out that current-amplified magnetic fields or non-thermal radiation may be required in some instances. The 20 cm radiation of coronal loops and the thermal cyclotron lines that accurately specify their magnetic field strength are discussed. The 20 cm and X ray emission of the coronal plasma are then compared. The coronae of nearby stars is next discussed, where coherent radiation processes seem to prevail. Some thoughts toward directions for future exploration are given.

Lang, Kenneth R.↗

NASA NTRS1986

Compact, variable, moving sources observed on the sun at 2 centimeters wavelength

The Very Large Array (VLA) was used to observe the active region AT 4508 in the C-configuration between 1530 and 2330 UT on June 4, 1984. The position of this region was N06 E57 at 1300 UT on this day. Followup observations were made between 1500 and 2300 UT on January 17, 1986 in the D-configuration. Observations were compared with Mt. Wilson magnetograms. Results are discussed.

Lang, Kenneth R.↗

NASA NTRS1986

Simultaneous IUE and VLA observations of YZ Canis Minoris, Ad Leonis and Lambda Andromedae

The IUE and VLA simultaneously observed the dwarf M flare stars YZ Canis Minoris and AD Leonis, and the RS CVn star Lambda Andromedae. Narrow-band, slowly varying radiation near 20cm wavelength was detected from YZ Canis Minoris. The radiation cannot be attributed to conventional emission processes, but may be explained by a coherent emission mechanism. The Mg II h and k line intensities also underwent changes in intensity, although the connection between the microwave and ultraviolet variations is unclear. Impulsive microwave bursts were detected from AD Leonis, but there were no significant variations in the Mg II line intensities. Enhancements in several ultraviolet emission lines were detected on timescales of 1 hr from Lambda Andromedae without accompanying variations at 6cm or 20cm wavelength.

Willson, Robert F.↗

NASA NTRS1986

Solar burst precursors and energy build-up at microwave wavelengths

We summarize high-resolution microwave observations (VLA) of heating and magnetic triggering in coronal loops. Magnetic changes that precede solar eruptions on time scales of tens of minutes involve primarily emerging coronal loops and the interaction of two or more loops. Thermal cyclotron lines have been detected in coronal loops, suggesting the presence of hot current sheets that enhance emission from relatively thin layers of enhanced temperature and constant magnetic field. These current sheets may play a role in the excitation of solar bursts. A filament-associated source with a high brightness temperature and steep radiation spectrum occurs above a region of apparently weak photospheric field. This source might be attributed to currents that enhance coronal magnetic fields. Compact (phi=5 sec) transient sources with lifetimes of 30 to 60 minutes have also been detected in regions of apparently weak photospheric field. We conclude by comparing VLA observations of coronal loops with simultaneous SMM-XRP observations.

Lang, Kenneth R.↗

NASA NTRS1986

Radio wavelength observations of magnetic fields on active dwarf M, RS CVn and magnetic stars

The dwarf M stars, YZ Canis Minoris and AD Leonis, exhibit narrow-band, slowly varying (hours) microwave emission that cannot be explained by conventional thermal radiation mechanisms. The dwarf M stars, AD Leonis and Wolf 424, emit rapid spikes whose high brightness temperatures similarly require a nonthermal radiation process. They are attributed to coherent mechanisms such as an electron-cyclotron maser or coherent plasma radiation. If the electron-cyclotron maser emits at the second or third harmonic gyrofrequency, the coronal magnetic field strength equals 250 G or 167 G, and constraints on the plasma frequency imply an electron density of 6 x 10 to the 9th/cu cm. Radio spikes from AD Leonis and Wolf 424 have rise times less than or equal to 5 ms, indicating a linear size of less than or equal to 1.5 x 10 to the 8th cm, or less than 0.005 of the stellar radius. Although Ap magnetic stars have strong dipole magnetic fields, they exhibit no detectable gyroresonant radiation, suggesting that these stars do not have hot, dense coronae. The binary RS CVn star UX Arietis exhibits variable emission at 6 cm wavelength on time scales ranging from 30 s to more than one hour.

Lang, Kenneth R.↗