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  • 11
    Electronic Resource
    Electronic Resource
    Clark lake microbursts: On a lower limit to type III burst brightness temperatures (1986)
    Springer
    Solar physics 107 (1986), S. 135-157 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Further observations of solar microbursts by the Clark Lake radioheliograph are reported. The microbursts have properties consistent with weak type III bursts, with the implication that type III's can have brightness temperatures as low as 106 K. We explore the importance of this result. A single model to explain the stronger type III bursts and the weaker microbursts is sought. We show that none of the models for stabilizing the strongest type III electron streams can explain the observed microbursts: these models have threshold levels of Langmuir waves which imply emission (due to spontaneous scattering off ions) with brightness temperatures in excess of those observed. It appears that either some vital physics is still missing from models for type III bursts, or that microbursts should have properties significantly different from those of type III bursts. In the latter case further observations should allow important tests of type III models.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00155348
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  • 12
    Electronic Resource
    Electronic Resource
    Coronal bright points at 6 cm wavelength (1987)
    Springer
    Solar physics 108 (1987), S. 99-111 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We report the results of the first observations of solar coronal bright points at 6 cm wavelength using the Very Large Array (VLA), with a spatial resolution of ∼ 1″.2. The maximum brightness temperature of the sources observed is ≈ 3 × 104 K with a mean value of ≈ 1 × 104 K (above the quiet Sun value). The lifetime of most sources is between 5 and 20 min. The average diameter of the sources is about 5–15″ arc. The sources are gaussian-like near the footpoint of miniature loops and they appear in groups. The observations indicate that significant fluctuations in the brightness temperature (sometimes quasi-periodic) and in the spatial extents of these sources can occur over periods of a few minutes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00152080
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  • 13
    Electronic Resource
    Electronic Resource
    Large-scale structure of the sun's corona from radio observations using the Clark Lake Radioheliograph (1987)
    Springer
    Solar physics 108 (1987), S. 113-129 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present meterwave maps of the solar corona made with the Clark Lake Radioheliograph at 30.9, 50, and 73.8 MHz for one solar rotation. We compare and contrast them with optical data: 10830 Å maps, white-light coronagraph images (SOLWIND and Mauna Loa K coronameter) and forbidden line scans. Most of the sources in the radio maps persist for two days or more, and appear to rotate approximately with the solar rate. A coronal hole seen against the disk at all three frequencies shows interesting similarities and significant differences with the optical signatures of the hole. Elongated features of the 50 MHz corona correspond rather well to the azimuthal position of white light streamers seen in SOLWIND images. Synoptic charts made from the radio maps show overall similarities to synoptic charts constructed from (limb) coronagraph data. Some of the differences may result from the different weightings given by the radio and optical data to density and temperature, or by the different sensitivities to non-radial geometries. We show that the combined use of meter wave and optical images provide considerable new insights into the three-dimensional structure of the low to middle corona.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00152081
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  • 14
    Electronic Resource
    Electronic Resource
    Microbursts at meter-decameter wavelengths (1990)
    Springer
    Solar physics 127 (1990), S. 165-183 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We study the characteristics of microbursts using a large data base obtained with the multifrequency radioheliograph of the Clark Lake Radio Observatory. Most of the new observations were made during July 29, 1985 to August 2, 1985; we also include for statistical studies the microburst data used in our earlier studies. We perform a statistical analysis of many characteristics such as frequency drift, source size and brightness temperature and compare them with the properties of normal type III bursts. We investigate the coronal structures and surface activities associated with some of the events. We find that (i) the brightness temperature is in the range 6 × 105 K to 6 × 107 K; (ii) the drift rate of the microbursts is slightly smaller than that of normal type III bursts, implying electron beams with speeds ∼0.2c. We explore various theoretical interpretations of the observed low brightness temperatures. We show that the microbursts can be explained as due to spontaneously emitted Langmuir waves by electron beams whereas normal type III bursts are due to coherently emitted plasma waves in a two-stream instability. We estimate the range of number densities for electron beams responsible for microbursts.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00158521
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  • 15
    Electronic Resource
    Electronic Resource
    Polarization features of solar radio emission and possible existence of current sheets in active regions (1994)
    Springer
    Solar physics 155 (1994), S. 339-350 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We show that it is possible to account for the polarization features of solar radio emission provided the linear mode coupling theory is properly applied and the presence of current sheets in the corona is taken into account. We present a schematic model, including a current sheet that can explain the polarization features of both the low frequency slowly varying component and the bipolar noise storm radiation; the two radiations face similar propagation conditions through a current sheet and hence display similar polarization behavior. We discuss the applications of the linear mode coupling theory to the following types of solar radio emission: the slowly varying component, the microwave radio bursts, metric type U bursts, and bipolar noise storms.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00680599
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  • 16
    Electronic Resource
    Electronic Resource
    Non-radial magnetic field structures in the solar corona (1994)
    Springer
    Solar physics 150 (1994), S. 317-323 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We report on the structure and geometry of coronal magnetic fields inferred from the observations of meter-decimeter type III and moving type IV radio bursts, associated with a Hα flare. This is the first report of type III radio bursts from the Nançay radioheliograph after it acquired the two-dimensional multifrequency capability. Dispersion of the radio source positions with frequency suggests that open and closed field lines are considerably inclined to the radial direction which is consistent with the connectivity observed in the magnetogram. We suggest that multiple arch systems are involved in the type IV emission. From the polarization and dispersion characteristics of the type IV source, we infer that the emission is due to fundamental plasma emission.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00712893
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  • 17
    Electronic Resource
    Electronic Resource
    Three-dimensional coronal structures using clark lake observations (1994)
    Springer
    Solar physics 150 (1994), S. 325-337 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have undertaken a study of coronal features observed at meter-decameter wavelengths using the Clark Lake radioheliograph. Among the coronal structures we have studied are the radio manifestations of coronal streamers on the solar disk and above the solar limb. We have analyzed the radio data quantitatively, using ray-tracing models for comparison with the maps. Our study provides information about the streamers' three-dimensional shapes, scales, and density profiles, for comparison with related observations using white-light coronagraphs.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00712894
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  • 18
    Electronic Resource
    Electronic Resource
    A new microwave/X-ray diagnostic for the thermal phase of solar flares (1986)
    Springer
    Solar physics 105 (1986), S. 87-99 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have observed 10 solar bursts during the thermal phase using the Haystack radio telescope at 22 GHz. We show that these high frequency flux observations, when compared with soft X-ray band fluxes, give useful information about the temperature profile in the flare loops. The microwave and X-ray band fluxes provide determinations of the maximum loop temperature, the total emission measure, and the index δ of the differential emission measure (q(T)/T = cTδ−1). The special case of an isothermal loop (δ = ∞) has been considered previously by Thomas et al. (1985), and we confirm their diagnostic calculations for the GOES X-ray bands, but find that the flare loops we observed departed significantly from the isothermal regime. Our results (δ = 1–3.5) imply that, during the late phases of flares, condensation cooling (δ ≈ 3.5) competes with radiative cooling (δ ≈ 1.5). Further, our results appear to be in good agreement with previous deductions from XUV rocket spectra (δ ≈ 2–3).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00156379
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  • 19
    Electronic Resource
    Electronic Resource
    SIGNATURES OF CORONAL CURRENTS IN MICROWAVE IMAGES (1997)
    Springer
    Solar physics 174 (1997), S. 175-190 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Microwave emission from solar active regions at frequencies above 4 GHz is dominated by gyroresonance opacity in strong coronal magnetic fields, which allows us to use radio observations to measure coronal magnetic field strengths. In this paper we demonstrate one powerful consequence of this fact: the ability to identify coronal currents from their signatures in microwave images. Specifically, we compare potential-field (i.e., current-free) extrapolations of photospheric magnetic fields with microwave images and are able to identify regions where the potential extrapolation fails to predict the magnetic field strength required to explain the microwave images. Comparison with photospheric vector magnetic field observations indicates that the location inferred for coronal currents agrees with that implied by the presence of vertical currents in the photosphere. The location, over a neutral line exhibiting strong shear, is also apparently associated with strong heating.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1004956422669
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  • 20
    Electronic Resource
    Electronic Resource
    Two-Sided-Loop-Type X-ray Jets and Metric Radio Bursts (1998)
    Springer
    Solar physics 178 (1998), S. 173-178 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have searched for nonthermal radio signatures in the form of metric type III bursts in conjunction with two-sided-loop-type X-ray jets observed by the Yohkoh/SXT experiment. We have found no evidence of type III bursts in association with this particular type of X-ray jets in contrast to the positive evidence of type III's in association with anemone-type X-ray jets. This result is consistent with the simulation results of Yokoyama and Shibata (1995), which show that anemone-type jets are produced by vertical/oblique plasma flow whereas the two-sided-loop-type jets are produced by horizontal plasma flow.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1005018109815
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