The Variable Solar Radio Spectrum
This paper reviews recent experimental and theoretical work concerning the slowly-varying component of solar radio emission, microwave bursts, and the four main spectral types of bursts observed in the meter and decimeter bands. Observational techniques are briefly summarized.
The slowly-varying component is believed to be thermal in origin and to come from overdense regions above plages and sunspots. The physical characteristics of this component are tabulated and recent theories of emission processes are discussed. Microwave bursts, as observed by both fixed-frequency and sweep-frequency receivers, are categorized and emission mechanisms for each category are considered. The characteristics and possible origins of Type-1 (noise storm) bursts, which originate high in the corona above active sunspots, are reviewed.
Type-II (slow-drift) bursts are believed to be generated by disturbances propagating outward through the corona, sometimes non-radially, at velocities of about 1000–1500 km/sec, in regions where the electron density is about 10 times that given by the Baumbach-Allen distribution. Measurements at meter wavelengths of the brightness distribution across the Type-II disturbances indicate that the radiation originates in a bright central emitting core, surrounded by an extended halo of scattered radiation. Observations with a swept-frequency interferometer show that the radiation generated in the second harmonic comes from directions corresponding to much lower heights in the solar atmosphere than the fundamental. It has been suggested that radiation in the second harmonic is preferentially back-scattered from the point of generation ; the second harmonic is then observed after reflection at much lower coronal heights.
Type-III (fast-drift) bursts, generated by an exciter moving at approximately 0.5 c, are also believed to be generated in density condensations and to have a core and halo brightness distribution similar to that of Type-II bursts. Examination of the burst profiles gives information on the dispersion of the exciter as it moves outward through the corona.
The complex properties of Type-IV (continuum) bursts, as observed throughout the complete radio spectrum (intensities, polarization, diameters, spectral characteristics, motions), are summarized and discussed in terms of existing models based on synchrotron and Cerenkov plasma wave emission processes.
Finally, the relation of the radio bursts to accompanying X-ray and high-energy nucléon emission is briefly examined in terms of existing theories of electromagnetic emission and particle acceleration in the solar atmosphere.
KeywordsBrightness Temperature Radio Emission Radio Burst Radio Spectrum Solar Radio
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- Allen, C. W.: 1963, Astrophysical Quantities (2nd Ed., Univ. of London, Athlone Press).Google Scholar
- Billings, D. E.: 1963, see page 280 of this volume.Google Scholar
- Boischot, A.: 1957, Compt. Rend. 244, 1326.Google Scholar
- Boischot, A. and Denisse, J. F.: 1957, Compt. Rend. 245, 2194.Google Scholar
- Boischot, A. and Simon, P.: 1959, Paris Symposium on Radio Astronomy (ed. by R. N. Bracewell), Stanford Univ. Press, p. 140.Google Scholar
- Christiansen, W. N. and Mathewson, D. S.: 1958, Proc. Inst. Radio Engr. 46, 127.Google Scholar
- Covington, A. E.: 1959, Paris Symposium on Radio Astronomy (ed. by R. N. Bracewell), Stanford Univ. Press, p. 159.Google Scholar
- Denisse, J. F.: 1960, URSI XIII General Assembly, London; andGoogle Scholar
- Denisse, J. F.: 1960, Information Bulletin Solar Radio Observatories, Europe 4, 3.Google Scholar
- Erickson, W. C.: 1963, General Dynamics-Astronautics Space Science Laboratory Report No. ERR-AN-233.Google Scholar
- Firor, J. W.: 1959, Paris Symposium on Radio Astronomy (ed. by R. N. Bracewell), Stanford Univ. Press, p. 136.Google Scholar
- Fokker, A. D.: 1960, Doctoral Thesis, Univ. of Leiden.Google Scholar
- Fokker, A. D.: 1961, Information Bulletin Solar Radio Observatories, Europe 8, p. 7.Google Scholar
- Groot, T. de: 1959, Paris Symposium on Radio Astronomy (ed. by R. N. Bracewell), Stanford Univ. Press p. 245.Google Scholar
- Ihsanova, V.: 1959, Paris Symposium on Radio Astronomy (ed. by R. N. Bracewell), Stanford Univ. Press, p. 171.Google Scholar
- Jager, C. de: 1962, Space Sci. Rev. 1, 487.Google Scholar
- Kakinuma, T.: 1956, Proc. Res. Inst. Aim. Nagoya Univ. 4, 78.Google Scholar
- Kakinuma, T.: 1958, Proc. Res. Inst. Atm. Nagoya Univ. 5, 71.Google Scholar
- Kundu, M. R.: 1959, Paris Symposium on Radio Astronomy (ed. by R. N. Bracewell), Stanford Univ. Press, p. 222.Google Scholar
- Kundu, M. R. and Haddock, F. T.: 1961, Trans. Inst. Rad. Eng. AP 9, 82.Google Scholar
- Kundu, M. R. and Smerd, S. F.: 1962, Information Bulletin Solar Radio Observatories, Europe 11, 4.Google Scholar
- Maxwell, A., Howard, W. E., and Garmire, G.: 1959, Scientific Report No. 14 under USAF Contract A F19(604)-1394 with Harvard University.Google Scholar
- Moreton, G. E.: 1962, J. Geophys. Res. 67, 1648.Google Scholar
- Schatzman, E. L.: 1963, see p. 313 of this volume.Google Scholar
- Schott, G. A.: 1912, Electromagnetic Radiation and the Mechanical Reactions Arising from It, Cambridge University Press.Google Scholar
- Seaton, M. J.: 1963, see p. 273 of this volume.Google Scholar
- Shklovsky, I. S.: 1946, Astron. J. U.R.S.S. 23, 333.Google Scholar
- Smerd, S. F.: 1963, see p. 398 of this volume.Google Scholar
- Suzuki, S.: 1961, Annals Tokyo Astron. Obs. 7, 75.Google Scholar
- Sweet, P. A.: 1958, IAU Symposium No. 6 (Stockholm, 1956) p. 123.Google Scholar
- Tanaka, H. and Kakinuma, T.: 1959, Paris Symposium on Radio Astronomy (ed. by R. N. Bracewell), Stanford Univ. Press, p. 215.Google Scholar
- Tanaka, H. and Kakinuma, T.: 1960, Proc. Res. Inst. Atm. Nagoya Univ. 7, 72.Google Scholar
- Tanaka, H. and Kakinuma, T.: 1962, J. Phys. Soc. Japan 17, A-2, 211.Google Scholar
- Uchida, Y.: 1962, J. Phys. Soc. Japan 17, A-2, 234.Google Scholar
- Vladimirsky, V. V.: 1948, Z. Eksperim. i Teor. Fiz. 18, 392.Google Scholar
- Weiss, A. A. and Sheridan, K. V.: 1962, J. Phys. Soc. Japan. 17, A-2, 223.Google Scholar
- Wild, J. P., Sheridan, K. V., and Trent, G. H.: 1959, Paris Symposium on Radio Astronomy (ed. by R. N. Bracewell), Stanford Univ. Press, p. 176.Google Scholar