Abstract
From the UCSD OSO-7 X-ray experiment data, we have identified 54 X-ray bursts with 5.1–6.6 keV flux greater than 103 photon cm−2 keV−1 which were not accompanied by visible Hα flare on the solar disk. By studying OSO-5 X-ray spectroheliograms, Hα activity at the limb and the emergence and disappearance of sunspot groups at the limb, we found 17 active centers as likely seats of the X-ray bursts beyond the limb. We present the analysis of 37 X-ray bursts and their physical parameters. We compare our results with those published by Datlowe et al. (1974a, b) for disk events.
The distributions of maximum temperature, maximum emission measure, and characteristic cooling time of the over-the-limb events do not significantly differ from those of disk events. We show that of conduction and radiation, the former is the dominant cooling mechanism for the hot flare plasma. Since the disk and over-the-limb bursts are similar, we conclude that the scale height for X-ray emission in the 5–10 keV range is large and is consistent with that of Catalano and Van Allen (1973), 11000 km, for primarily 1–3 keV emission.
Twenty-five or about 2/3 of the over-the-limb events had a non-thermal component. The distribution of peak 20 keV flux is not significantly different from that of disk events. However, the spectral index at the time of maximum flux is significantly different for events over the limb and for events near the center of the disk; the spectral index for over-the-limb events is larger by about δγ = 3/4. If hard X-ray emission came only from localized sources low in the chromosphere we would expect that hard X-ray emission, would be occulted over the limb; on the contrary, the observation show that the fraction of soft X-ray bursts which have a nonthermal component is the same on and off of the disk. Thus hard X-ray emission over extended regions is indicated.
Similar content being viewed by others
References
Brown, J. C.: 1971, Solar Phys. 18, 489.
Brown, J. C.: 1974, in G. Newkirk (ed.) ‘Coronal Disturbances’, IAU Symp. 57, 395.
Brunk, H. D.: 1960, An Introduction to Mathematical Statistics, Ginn and Co., Boston, p. 351.
Catalano, C. P. and Van Allen, J. A.: 173, Astrophys. J. 185, 335.
Chubb, T. A.: 1972, in E. R. Dyer (ed.), Solar Terrestrial Physics / 1970, Part I, D. Reidel Publ. Co., p. 99.
Culhane, J. L. and Philips, K. J. H.: 1969, Solar Phys. 11, 117.
Culhane, J. L., Vesecky, J. F. and Philips, K. J. H.: 1970, Solar Phys. 15, 394.
Datlowe, D. W. and Lin, R. P.: 1973, Solar Phys. 32, 459.
Datlowe, D. W., Hudson, H. S., and Peterson, L. E.: 1974a, Solar Phys. 35, 193.
Datlowe, D. W., Elcan, M., and Hudson, H. S.: 1974b, 39, 155. Solar Phys. 39, 155.
Frost, K. J. and Dennis, B. R.: 1971, Astrophys. J. 165, 655.
Harrington, T. M., Malloy, J. O., McKenzie, D. L., and Peterson, L. E.: 1972, IEEE Trans. Nuc. Sci. NS-19, 596.
Hudson, H. S.: 1974, in R. Ramaty and R. G. Stone (eds.), High Energy Phenomena on the Sun, Symposium Sept. 28–30, 1972, p. 207.
Hudson, H. S. and Okhi, K.: 1972: Solar Phys. 23, 125.
Kane, S. R.: 1974, in G. Newkirk (ed.), ‘Coronal Disturbances’, IAU Symp. 57, 105.
Kane, S. R. and Anderson, K. A.: 1970, Astrophys. J. 162, 1003.
Kane, S. R. and Donnelly, R. F.: 1971, Astrophys. J. 164, 151.
Kreplin, R. W. and Taylor, R. G.: 1971, Solar Phys. 21, 452.
Lin, R. P.: 1974, in R. Ramaty and R. G. Stone (eds.), High Energy Phenomena on the Sun, Symposium Sept. 28–30, 1972, p. 439.
McKenzie, D. L.: 1972, Astrophys. J. 175, 481.
McKenzie, D. L., Datlowe, D. W., and Peterson, L. E.: 1973, Solar Phys. 28, 175.
Neupert, W. M., Thomas, R. J., and Chapman, R. D.: 1974, Solar Phys. 34, 349.
Newton, H. W. and Nunn, M. L.: 1951, Monthly Notices Roy. Astron. Soc. 111, 413.
Roy, J.-R.: 1972, Solar Phys. 26, 418.
Rust, D. M. and Roy, J.-R.: 1971, in R. Howard (ed.) ‘Solar Magnetic Fields’, IAU Symp. 43, 569.
Rust, D. M. and Roy, J.-R.: 1974, The Late June 1972 ‘CINOF’ Flares, AFCRL Research Report. TR-74-0201.
Takakura, T., Ohki, K., Shibuya, N., Fujii, M., Matsuoka, M., Miyamoto, S., Nishimura, J., Oda, M., Ogawara, Y., and Ota, S.: 1971, Solar Phys. 16, 454.
Vaiana, G. S., Krieger, A. S., and Timothy, A. F.: 1973a, Solar Phys. 32, 81.
Vaiana, G. S., Davis, J. M., Giacconi, R., Krieger, A. S., Silk, J. K., Timothy, A. F., and Zombeck, M.: 1973b, Astrophys. J. (Letters) 185, L47.
Vorpahl, J. A.: 1974, in R. Ramaty and R. G. Stone (eds.), High Energy Phenomena on the Sun, Symposium Sept. 28–30, 1972, p. 222.
Wood, A. T. and Noyes, R. W.: 1972, Solar Phys. 24, 180.
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Roy, JR., Datlowe, D.W. X-Ray bursts from solar flares behind the limb. Sol Phys 40, 165–182 (1975). https://doi.org/10.1007/BF00183160
Received:
Issue Date:
DOI: https://doi.org/10.1007/BF00183160