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  • Springer  (25)
  • 1985-1989  (10)
  • 1980-1984  (15)
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  • 1
    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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  • 2
    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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  • 3
    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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  • 4
    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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  • 5
    Electronic Resource
    Electronic Resource
    Observations of solar filaments at 8, 15, 22 and 43 GHz (1981)
    Springer
    Solar physics 71 (1981), S. 311-328 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract On April 3, 4, 6, and 8, 1978, solar observations were made using the Haystack 120 ft telescope at 8, 15, 22, and 43 GHz. Hα filtergrams obtained at the Sacramento Peak Observatory on the same days showed an average of more than 30 filaments or filament fragments (per day) on the disk. Most of these appeared as depressions in brightness temperature at 15 and 22 GHz. Because of the relatively low spatial resolution at 8 GHz, only a few appeared at that frequency, and presumably because of lower opacity in filaments at higher frequencies, few depressions were visible at 43 GHz. At 15 and 22 GHz, more depressions appeared than Hα filaments, but virtually all the radio depressions overlay magnetic neutral lines. Taking the data sets for each day as independent samples, we found that at 22 GHz, 46 of the 77 radio depressions were associated with Hα filaments; at 15 GHz the correlation was smaller; only 27 out of 48 being associated with the Hα filaments. The data imply that the microwave depression features are the result of absorption by filaments and perhaps also the result of other effects of the associated filament channel, but not necessarily coronal depletion. The effects of filament absorption are, statistically, about twice as effective as other phenomena (such as absorption by material invisible in Hα, for example) in creating the radio depression. A center-to-limb study of a single large filament clearly showed that at 15 and 22 GHz the absorption by cool hydrogen supported above the neutral line was the predominant factor in producing the observed depression at radio frequencies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00167554
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  • 6
    Electronic Resource
    Electronic Resource
    Thermal and nonthermal phenomena in solar flare loops at 20 cm wavelength and in X-rays (1983)
    Springer
    Solar physics 83 (1983), S. 3-14 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present X-ray images from the P78-1 satellite for a long-lasting burst at 20 cm wavelength mapped with the Very Large Array on 19 May, 1979 by Velusamy and Kundu (1981). The decimeter wave observations were originally interpreted in terms of two models, one invoking thermal electrons radiating at low harmonics of the gyrofrequency, and the other invoking mildly relativistic electrons emitting gyrosynchrotron radiation. If indeed the 20 cm source is thermal, it should also be visible in soft X-rays, while if it is nonthermal, the soft X-ray emission should be weak or spatially or temporally distinct from the 20 cm burst. We find that only one of the three 20 cm sources was approximately co-spatial with the soft X-ray source, and that it was only partially thermal. The 20 cm burst is therefore primarily decimeter type IV emission from mildly relativistic electrons of the post-flare phase. The long lifetime (≳ 2h) and smooth temporal variation of the burst belie its nonthermal nature and suggest continuous acceleration as well as long term storage of energetic electrons.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00148239
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  • 7
    Electronic Resource
    Electronic Resource
    Sharp edges in solar microwave spectra: Neutral current sheets or cyclotron lines? (1984)
    Springer
    Solar physics 93 (1984), S. 305-315 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have mapped two solar active regions using the VLA at three closely spaced frequencies (4496, 4716, and 4996 MHz) in an attempt to determine the origin of the steep spectra (indexγ ≈ −5 to −8) sometimes observed with large single telescopes. One of the regions observed indeed shows an anomalously large slope (γ ≈ −6) compared to the usual (γ ≈ −2 to −2.5). The other region shows a similar slope (γ ≈ −5) but with a larger range of statistical error. Two possible explanations for such steep edges in solar spectra are (1) transmission effects of neutral current sheets, and (2) the appearance of cyclotron lines. The internal evidence of the microwave maps and simultaneous optical observations favor an explanation in terms of cyclotron lines.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02270842
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  • 8
    Electronic Resource
    Electronic Resource
    EUV arcades: Signatures of filament instability (1982)
    Springer
    Solar physics 81 (1982), S. 91-105 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have examined seven active regions of the Skylab period in the EUV (Harvard College Observatory), and in Hα and K3 (Observatoire de Meudon, spectroheliograms and patrols) in order to elucidate the magnetic geometry in the coronal environment of filaments. We have also looked for signatures of magnetic reconfigurations associated with instabilities (i.e. velocities or disappearances) of filaments. Out of sixteen Hα filaments observed, six were stable (lifetime ≥ 48h). All the filaments lay within coronal cavities as seen in lines formed above 1.5 × 106 K (Mgx λ625, Sixii λ521, Fexvi λ417, Fexv λ361). None of the stable filaments had arcades or arches spanning the cavities except (sometimes) at the ends of the filaments. On the other hand, most (8/10) of the unstable filaments (having concurrent Doppler shifts or a subsequent DB within 24h) had arcades or single arches spanning their cavities. The arches were observed in EUV lines with formation temperatures as low as 2–4 × 105 K (Oiv λ554, Ovi λ1032, Ne vii λ465), as well as in hotter lines. A statistical test shows that the arcade/instability vs non-arcade/stability association is significant at the 99% confidence level. We suggest 2 types of scenario relating arcades to instabilities. The more preferable scenario is closely related to the Kuperus/Van Tend model of filament disruptions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00151983
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  • 9
    Electronic Resource
    Electronic Resource
    Network to cell contrast at microwaves (1983)
    Springer
    Solar physics 85 (1983), S. 237-242 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Using different models deduced from EUV lines for the cell and for various network components, the corresponding radio brightness temperature in the millimetric and centimetric range of wavelengths are computed. The contrast C = [T b (network)]/[T b (cell)] and the difference ΔT = T b (network) - T b (cell) are compared with the few available observations of the quiet Sun inhomogeneities performed with sufficient angular resolution. The comparison shows a satisfactory agreement with most of the observations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00148652
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  • 10
    Electronic Resource
    Electronic Resource
    Active region magnetic fields inferred from simultaneous VLA microwave maps, X-ray spectroheliograms, and magnetograms (1982)
    Springer
    Solar physics 80 (1982), S. 233-249 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A series of VLA maps at 6 cm wavelength have been generated from observations of a solar active region (NOAA 2363) on 29 and 30 March, 1980. During the same period, X-ray spectroheliograms were acquired for this region in the lines of O viii, Ne ix, Mg xi, Si xiii, S xv, and Fe xxv, with X-rayn Polychromator (XRP) aboard the Solar Maximum Mission (SMM). Intervals of relative quiescence (i.e., when X-ray flares and centimeter wave bursts were not evident) were selected for microwave mapping. The resulting VLA maps have spatial resolution of 4″ × 4″, and generally show two or more sources whose slowly evolving substructures have spatial scales of 10″–30″. These maps were co-registered with Hα photographs (courtesy of AF/AWS SOON, Holloman and Ramey AFB) to an accuracy of ± 8″. Similarly, the X-ray spectroheliograms have been co-registered with white light photographs to about the same accuracy. Magnetograms from KPNO and MSFC have also been co-aligned, and the magnetic X-ray, and microwave features compared. In general we have found that (a) the peaks of X-ray and 6 cm emission do not coincide, although (b) the sources in the two wavelength domains tend to overlap. These facts in themselves are evidence for the existence of opacity mechanisms other than thermal bremsstrahlung. In order to quantify this assertion, we have computed differential emission measures to derive densities and temperatures. Using these and calculated force-free magnetic fields from Kitt Peak magnetograms, we present an assessment of the mechanism of gyroresonance absorption at low harmonics of the electron gyrofrequency as the source of opacity responsible for the microwave features. We conclude that large-scale currents must be present in the active region loops to account for the bright 6 cm sources far from sunspots.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00147971
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