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At least 379 records · Page 21
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NASA NTRS1972

Thermal radio emission from Jupiter and Saturn

Microwave brightness temperature calculations have been carried out for a number of model atmospheres for Jupiter and Saturn. The models considered are characterized by helium to hydrogen number mixing ratios which range from 0 to 0.2. Gaseous ammonia is assumed to be a trace constituent in all the models. The ammonia abundance below the (NH3) cloud level is a free parameter which is determined by comparing the observed thermal spectrum of Jupiter and the total spectrum of Saturn with the model calculations. The theoretical microwave spectra corresponding to those models in which the ammonia abundance is that expected from an atmosphere containing a solar abundance of elements are found to be in generally good agreement with the observations. This result is shown to be nearly independent of the helium to hydrogen ratio for the models considered. There is no evidence at this time that a non-thermal component contributes to Saturn's microwave spectrum.

Gulkis, S.↗
NASA NTRS1973

Modulation of the Jovian decametric radio emission by IO

It is argued that the sweeping of trapped energetic MeV protons by Io can result in induced emissions. The instability is caused by the density gradient occurring at the edge of the cavity. It is shown that the azimuthal Larmor drift of the energetic protons which is transverse to both the local magnetic field and the density gradient can excite extraordinary waves with frequencies slightly above the local electron cyclotron frequency. It is found that the emission coefficient is proportional to the square root of (n sub p/n sub t), where n sub p and n sub t denote the density of the energetic protons and the density of the thermal electrons, respectively. The theory also predicts a lower frequency cutoff. This cutoff frequency depends upon the density of thermal electrons in the source region.

Wu, C. S.↗
NASA NTRS1974

Modulation lanes in the dynamic spectra of Jupiter's decametric radio emission

High-resolution dynamic spectra of Jupiter's decametric emission exhibit lane patterns, referred to as modulation lanes. These lanes appear tilted in the orthogonal time-frequency plane, and the sign and magnitude of their drift rate is determined by the central meridian longitude of System III at Jupiter. The lanes are recorded both on L- and S-bursts, giving rise to only a relatively few basic spectral types. Their simultaneous appearance, however, produces rather complicated spectral events. The origin of the modulation lanes is unknown. A few possible mechanisms are suggested.

Riihimaa, J. J.↗
NASA NTRS1975

A search for periodic variations of solar radio emission at 3.7- and 11.1-cm wavelengths

Periodic time variations in the emission of quiet solar regions were sought with the NRAO three-element interferometer at 3.7 and 11.1 cm. The visibility function of a source as obtained with a tracking interferometer is an implicit function of time, because of projected baseline changes, and therefore only observations taken when the baseline changes vary slowly with time were used. The visibility function shows significant time variations, but a power-spectrum analysis of the data revealed no unique periodicity.

Kundu, M. R.↗
NASA NTRS1976

The variability of the total flux density and polarization of Jupiter's decimetric radio emission

Results are presented for a continuing program to monitor the Jovian 13-cm flux at weekly intervals and for an observational program to measure the linear polarization on an annual basis. The initial results of these programs are compared with existing data. Some implications about the radiation belts are discussed. When the new data are combined with previously published 11- and 21-cm measurements, it is found that the nature of the time variations during the past 13 years is similar but not identical at these wavelengths and that there is no convincing evidence that the variations are correlated with either solar activity or Jupiter's distance from the sun. The peak degree of polarization of the synchrotron has not changed more than 1% from the mean value of 24.5% during the 12-year interval considered.

Klein, M. J.↗
NASA NTRS1976

A search for radio emission from ionized hydrogen in M15

A sensitive search at 3.6 cm for thermal emission from ionized gas in the globular cluster M15 (NGC 7078) reveals no emission greater than the experimental error, which is 0.003 Jy. Based on this flux-density limit, the mass of ionized gas within 3 pc of the cluster center is less than 26 solar masses. This result suggests that the ionized-gas content of M15, which contains an intense X-ray source, is not greatly enhanced compared with similar globular clusters without known X-ray sources. This mass limit is an order of magnitude greater than the amount of gas required to produce the X-ray flux by accretion onto massive black holes.

Klein, M. J.↗
NASA NTRS1977

Maps of Jovian radio emission

Observations were made of Jupiter with the Westerbork telescope at all three frequencies available: 610 MHz, 1415 MHz, and 4995 MHz. The raw measurements were corrected for position errors, atmospheric extinction, Faraday rotation, clock, frequency, and baseline errors, and errors due to a shadowing effect. The data was then converted into brightness distribution of the sky by Fourier transformation. Maps of both thermal and nonthermal radiation were developed. Results indicate that the thermal disk of Jupiter measured at a wavelength of 6 cm has a temperature of 236 + or - 15 K. The radiation belts have an overall structure governed by the trapping of electrons in the dipolar field of the planet with significant beaming of the synchrotron radiation into the plane of the magnetic equator.

Depater, I.↗
NASA NTRS1981

The kilometric radio emission spectrum - Relationship to auroral acceleration processes

Using current theories for the generation of auroral kilometric radiation, the possibilities for obtaining information on the auroral acceleration processes from the spectrum of this radiation are discussed. It is shown, for example, that under certain conditions the low-frequency cutoff provides a direct indication of the upper altitude limit of the acceleration region. What is more, certain discrete features in the kilometric radiation spectrum show a very close similarity to whistler-mode emissions, thereby suggesting that the basic instability mechanisms are quite similar. Certain of these discrete features have frequency drifts which can be interpreted in terms of the propagation of shock-like disturbances along the auroral field lines at velocities near the ion-acoustic speed.

Gurnett, D. A.↗
NASA NTRS1983

Identification of radio emission from the Io flux tube

The identification of a specific Jovian decametric emission with the particular flux tube near the orbit of Io giving rise to it would substantially improve theoretical understandings of the relation of the Io flux tubes with the arc-like structures seen on time vs. frequency plots of decametric emissions observed by the Voyager spacecraft. The present paper shows how the instantaneous Io flux tube may be identified with particular emission features. It is demonstrated that the emissions designated Io caused emission (ICE) may be interpreted as representing sections of arc structures emitted by the interaction of Io with successive flux tubes. The model is capable of accounting for the change of displacement angle of the ICE structures as the sub-observed Jovian longitude changes. The identification permits the mapping of emissions onto the causative flux tubes for Jovian longitudes from 240 to 360 deg.

Riddle, A. C.↗
NASA NTRS1983

Dependence of radio emission in large H alpha flares 1967-1970 upon the orientation of the local solar magnetic field

Published data on ionizing radiation (as reflected in sudden ionospheric disturbances), H-alpha importance, 10-cm flux, dynamic spectrum, and 200-MHz flux are analyzed for a group of 84 solar flares (from 1967 to 1970) classified by Pudovkin and Chertkov (1976). The group consists of 29 flares with southern orientation of the 100,000-km-scale overlying magnetic field, 32 with northern orientation, and 23 with indeterminate orientation; all have H-alpha importance greater than about 2. Slightly greater X-ray and optical emissions, but one-order-of-magnitude greater prompt 200-MHz and 10-cm fluxes are found in southern as compared to northern-oriented flares. It is inferred that the amount of electromagnetic flare energy radiated promptly from the corona, as compared with that radiated from the chromosphere, was significantly affectly by the orientation of the overlying large-scale magnetic field during the period of observation. This trend is shown to be consistent despite some periodic variation.

Roelof, E. C.↗
NASA NTRS1985

Corotating Interaction Regions in Stellar Winds: Particle Acceleration and Non-thermal Radio Emission in Hot Stars

A co-rotating interaction region (CIR) forms in a stellar wind when a fast stream from a rotating star overtakes a slow stream. The CIR's were studied in detail in the solar wind over the past decade primarily because they are efficient sources of particle acceleration. Here, CIR's in OB star winds are used to explain two properties of such winds: deposition of non-radiative energy in the wind far from the stellar surfaces and acceleration of non-thermal particles.

Mullan, D. J.↗
NASA NTRS1985

Analysis of Jovian low frequency radio emissions

The density of ions in the Io plasma torus and the scattering of these ions by low frequency electromagnetic emissions detected by Voyager 1 were studied. The ion density profile was investigated using whistler dispersion measurements provided by the Voyager plasma instrument. The scale height and absolute density of H+ ions in the vicinity of the plasma torus were determined by combining the measured plasma densities with the whistler dispersion measurements. A theoretical analysis of the modes of propagation of low frequency electromagnetic emissions in the torus was undertaken. Polarization reversal effects and rough estimates of the ion diffusion coefficient were utilized. Numerical evaluation of the ion diffusion coefficients in the torus were made using the observed Voyager 1 wave intensities. Results show that the observed wave intensities produce significant ion diffusion effects in the ion torus.

Gurnett, D. A.↗