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  • 1980-1984  (102)
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  • 1
    Electronic Resource
    Electronic Resource
    Positional characteristics of meter-decameter wavelength bursts associated with hard X-Ray bursts (1982)
    Springer
    Solar physics 79 (1982), S. 107-119 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Several type III, type II, and type IV bursts were observed on April 25 and 26, 1979 with the Clark Lake Radio Observatory's E-W and N-S swept frequency interferometers in the range 20–110 MHz. The radio bursts were associated with hard X-ray bursts in the energy range 26–154 keV, as observed by ISEE-3. The type III bursts, which were associated with impulsive hard X-rays, were observed to great heights (∼ 3.1R ⊙ from disk center at 28 MHz) and their location indicates that the electron streams responsible for them were injected at the footpoints of magnetic field lines which diverge in the corona. With one exception, all the type III bursts occurred in dense coronal regions. Two gradual hard X-ray bursts were observed to occur in association with a type IV without type II, and a type IV-type II burst. For the gradual burst (observed on April 25) associated with a type IV only, it is believed that part of the energetic electrons responsible for meter-decameter type IV are trapped in a plasmoid behind a weak shock, as evidenced by the absence of a type II, while another part is located in low lying magnetic loops producing centimeter and hard X-radiation. The type II burst associated with the other gradual hard X-ray burst (observed on April 26) started approximately 9 min after the impulsive hard X-ray burst peak. This rather long delay between the type II onset and the impulsive maximum is believed to represent the time interval over which the shock becomes strong enough to produce a detectable type II higher in the corona.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00146976
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  • 2
    Electronic Resource
    Electronic Resource
    Decameter studies of the 5 September 1973 flare (1981)
    Springer
    Solar physics 71 (1981), S. 65-75 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We discuss the spectra and positions of the meter-decameter wavelength radio sources associated with the 5 September 1973 flare. We discuss the evolution of the size of the type II burst source and show that it fluctuates by a factor of 10, or larger. Consequently, the potential and kinetic energies associated with the shock are uncertain by the same factor. By comparing the positions of the type II and type III sources we conclude that while the shock wave associated with the type II was guided along high loops, the type III electrons were injected along open field lines which diverged within a short height in the corona. The characteristics of a particularly interesting type III burst with a low-frequency cut-off are discussed. We argue that nearby loops were not disrupted by the shock and that the energetic electrons produced during the event must have been injected at several sites and guided along open field lines at large distances from the flare to produce type III bursts.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00153607
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  • 3
    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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  • 4
    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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  • 5
    Electronic Resource
    Electronic Resource
    The spatial distribution of 6 centimeter gyroresonance emission from a flaring X-ray loop (1984)
    Springer
    Solar physics 92 (1984), S. 271-281 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We compare simultaneous high resolution soft X-ray and 6 cm images of the decay phase of an M3 X-ray flare in Hale Region 16413. The photographic X-ray images were obtained on an AS & E sounding rocket flown 7 November, 1979, and the 6 cm observations were made with the VLA. The X-ray images were converted to arrays of line-of-sight emission integrals and average temperature throughout the region. The X-ray flare structure consisted of a large loop system of length ∼ 1.3 arc min and average temperature ∼8 × 106 K. The peak 6 cm emission appeared to come from a region below the X-ray loop. The predicted 6 cm flux due to thermal bremsstrahlung calculated on the basis of the X-ray parameters along the loop was about an order of magnitude less than observed. We model the loop geometry to examine the expected gyroresonance absorption along the loop. We find that thermal gyroresonance emission requiring rather large azimuthal or radial field components, or nonthermal gyrosynchrotron emission involving continual acceleration of electrons can explain the observations. However, we cannot choose between these possibilities because of our poor knowledge of the loop magnetic field.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00157251
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  • 6
    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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  • 7
    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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  • 8
    Electronic Resource
    Electronic Resource
    X-Ray and microwave observations of active regions (1983)
    Springer
    Solar physics 85 (1983), S. 267-283 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We compare coordinated, high spatial resolution (2–3 arc sec) observations at 6 cm and in soft X-rays with photospheric magnetograms and optical filtergrams of two active regions. The correspondence of the brightest centimetric components in these regions with coronal loops, sunspots and pores, chromospheric structures and the photospheric magnetic field was determined. Our principal results are: The association between the microwave components and coronal X-ray and photospheric magnetic field structures is complex; in general X-ray emission was not associated with the microwave components. A majority of the components were not associated with sunspots, although the brightest (T b ≥ 4 × 106 K) components overlay regions of strong photospheric field or high field gradients. Several of the components coincided with the apparent bases of shorter coronal loops and 4 with the tops of X-ray loops. The X-ray and magnetic field observations are used to constrain possible centimetric emission mechanisms. Thermal bremsstrahlung can not be a significant contributor to this bright microwave emission. Thermal gyro-resonance absorption is consistent with some of the observations, but untenable for those components which are bright in microwaves, lack X-ray emission, and overlie regions of weak magnetic field. As an explanation for the brightest (T b ≥ 4 × 106 K) components, the g-r theory requires coronal loops with significant currents but very low densities. Alternatively, a nonthermal mechanism implies that the emission arises from the transition region and suggests that discrete regions of continuous particle acceleration may be common in active regions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00148654
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  • 9
    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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  • 10
    Electronic Resource
    Electronic Resource
    Time variability and structure of quiet Sun sources at 6 cm wavelength (1982)
    Springer
    Solar physics 76 (1982), S. 221-237 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Using the Westerbork Synthesis Radio Telescope (WSRT) we produced a synthesized map of a quiet Sun region on June 15, 1976, and studied the structure and time variability of the quiet emitting regions at 6 cm wavelength with a spatial resolution of 6 arc sec. Comparison of the 12hr synthesis map with Ca+ K filtergram shows that bright and dark features on the 6 cm quiet Sun synthesized map correspond to the chromospheric networks and cells observed in Ca+ K. All 6 cm bright features lie over bright Ca+ K network elements. The reverse correlation is not true, that is, not all bright Ca+ K network features have their 6 cm counterparts. Comparison with the photospheric magnetogram shows that about 72% of the photospheric magnetic field enhancements (¦B¦ ≥ 5 G) are coincident with 6 cm emissive regions. Only one 6 cm feature could be positively identified with a bipolar magnetic structure. This implies that no more than 20–25% of the 6 cm emitting features could be associated with X-ray bright points. Intercomparison of our 12hr two-dimensional synthesis map, a 4hr two-dimensional synthesis map (around meridian) and the one-dimensional fan beam scans of the quiet Sun region at 6 cm, along with the Ca+ K filtergram and photospheric magnetogram shows that: (1) All of the 15 time-varying elements at 6 cm were located on Ca+ K networks; (2) about 40% of the 15 time varying elements at 6 cm are coincident with enhancements of the photospheric magnetogram; (3) individual time-varying sources have minimum source size (FWHM) of ∼15 arc sec and maximum brightness temperature of ∼105 K; (4) the life time of the time varying sources varies from a few minutes to several tens of minutes; (5) the intensity of the sources varies by factors of 2 to 7 over time periods of ∼1 min to tens of minutes; and (6) the sources tend to disappear for periods of up to tens of minutes and to reappear at the same locations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00170985
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