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  • 1990-1994  (18)
  • 1985-1989  (34)
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  • 1
    Electronic Resource
    Electronic Resource
    Simultaneous multi-frequency imaging observations of solar microwave bursts (1989)
    Springer
    Solar physics 121 (1989), S. 153-161 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We review the results of simultaneous two-frequency imaging observations of solar microwave bursts with the Very Large Array. Simultaneous 2 and 6 cm observations have been made of bursts which are optically thin at both frequencies, or optically thick at the lower frequency. In the latter case the source structure may differ at the two frequencies, but the two sources usually seem to be related. However, this is not always true of simultaneous 6 and 20 cm observations. The results have implications for the analysis of non-imaging radio data of solar and stellar flares.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00161693
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    On the reconciliation of simultaneous microwave imaging and hard X-ray observations of a solar flare (1991)
    Springer
    Solar physics 132 (1991), S. 125-136 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have compared microwave imaging data for a small flare with simultaneous hard X-ray spectral observations. The X-ray data suggest that the power-law index δ of the energy distribution of the radiating electrons is 5.3 (thick-target) which differs significantly from the estimate (δ = 1.4) from a homogeneous optically-thin gyrosynchrotron model which fits the radio observations well. In order to reconcile these results, we explore a number of options. We investigate a double power-law energy spectrum for the energetic electrons in the flare, as assumed by other authors: the power law is steep at low energies and much flatter at the higher energies which produce the bulk of the microwaves. The deduced break energy is about 230 keV if we tentatively ignore the X-ray emission from the radio-emitting electrons: however, the emission of soft photons by the flat tail strongly contributes to the observed hard X-ray range and would flatten the spectrum there. A thin-target model for the X-ray emission is also inconsistent with radio data. An inhomogeneous gyrosynchrotron model with a number of free parameters and containing an electron distribution given by the thick-target X-ray model could be made to fit the radio data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00159134
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    UVSP and VLA observations of the 24 June 1980 flare: Asymmetric or isotropic beaming? (1990)
    Springer
    Solar physics 129 (1990), S. 343-349 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Observations of the 15:22 UT flare of 24 June 1980 were made using the Very Large Array (VLA) at 6 cm wavelength simultaneously with the Hard X-ray Imaging Spectrometer (HXIS) aboard the Solar Maximum Mission. It was found that at the peak of the impulsive phase, the brightest microwave point appeared to lie between the soft (3.6–8.0 keV) and hard (22–30 keV) X-ray maxima, which were themselves separated by ∼ 20″ (Kundu et al., 1984). Since the publication of these results, we have analyzed the imaging data from the Ultraviolet Spectrometer Polarimeter (UVSP) with the goal of narrowing the possible interpretations of the event. Like the VLA and HXIS, the UVSP observations provide information about the location of the primary electrons; the observations taken together suggest that the fast electrons were symmetrically distributed within the flare loop.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00159045
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  • 4
    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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    Paper (German National Licenses)
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  • 5
    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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    Paper (German National Licenses)
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  • 6
    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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  • 7
    Electronic Resource
    Electronic Resource
    Solar cycle variation of the microwave spectrum and total irradiance (1994)
    Springer
    Solar physics 152 (1994), S. 167-173 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have extended the proxy relationship between irradiance and microwaves by using the daily solar fluxes from Toyokawa Observatory at 1000, 2000, 3750 and 9400 MHz in addition to the Ottawa 2800 MHz flux for the years 1980–1989. It turns out that the flux at 1000 MHz is better correlated with irradiance than the flux at higher frequencies-an unexpected result. We have also found that the spectrum of the flux shows shape changes that are related to the number and type of active regions. Because of this the five-frequency spectral measurements of microwave flux allow one to separate the sunspot and coronal features, providing an improved proxy of solar variability.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01473200
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  • 8
    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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  • 9
    Electronic Resource
    Electronic Resource
    Meterwave observations of a coronal hole (1987)
    Springer
    Solar physics 111 (1987), S. 419-428 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present meterwave maps showing a coronal hole at 30.9, 50.0, and 73.8 MHz using the Clark Lake Radioheliograph in October 1984. The coronal hole seen against the disk at all three frequencies shows interesting similarities to, and significant differences from its optical signatures in He i λl10830 spectroheliograms. Using the model of coronal holes by Dulk et al. (1977) we derive the electron density from the radio observations of the brightness temperature. The discrepancy between the density value derived from the Skylab EUV data and that computed from our radio data is even larger than in Dulk et al. 's comparison at similar and higher frequencies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00148529
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  • 10
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    Three-dimensional coronal structures using clark lake observations (1994)
    Springer
    Details
    Publication Date: 1994-03-01
    Print ISSN: 0038-0938
    Electronic ISSN: 1573-093X
    Topics: Physics
    Published by Springer
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