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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 18, issue 2
Ann. Geophys., 18, 182–190, 2000
https://doi.org/10.1007/s00585-000-0182-z
© European Geosciences Union 2000
Ann. Geophys., 18, 182–190, 2000
https://doi.org/10.1007/s00585-000-0182-z
© European Geosciences Union 2000

  29 Feb 2000

29 Feb 2000

Ionospheric measurements of relative coronal brightness during the total solar eclipses of 11 August, 1999 and 9 July, 1945

C. J. Davis1, M. Lockwood1, S. A. Bell2, J. A. Smith1, and E. M. Clarke3 C. J. Davis et al.
  • 1Space Science Department, Rutherford Appleton Laboratory, Chilton, OX11 0QX, OXON, UK
  • 2H M Nautical Almanac Office, Rutherford Appleton Laboratory, Chilton, OX11 0QX, OXON, UK
  • 3Dept. Science and Mathematics, Sheffeld Hallam University, City Campus, Pond Street, Sheffield, S11WB, UK
  • Correspondence to: C. J. Davis

Abstract. Swept-frequency (1-10 MHz) ionosonde measurements were made at Helston, Cornwall (50°06'N, 5°18'W) during the total solar eclipse on August 11, 1999. Soundings were made every three minutes. We present a method for estimating the percentage of the ionising solar radiation which remains unobscured at any time during the eclipse by comparing the variation of the ionospheric E-layer with the behaviour of the layer during a control day. Application to the ionosonde date for 11 August, 1999, shows that the flux of solar ionising radiation fell to a minimum of 25±2% of the value before and after the eclipse. For comparison, the same technique was also applied to measurements made during the total solar eclipse of 9 July, 1945, at Sörmjöle (63°68'N, 20°20'E) and yielded a corresponding minimum of 16±2%. Therefore the method can detect variations in the fraction of solar emissions that originate from the unobscured corona and chromosphere. We discuss the differences between these two eclipses in terms of the nature of the eclipse, short-term fluctuations, the sunspot cycle and the recently-discovered long-term change in the coronal magnetic field.

Key words: Ionosphere (solar radiation and cosmic ray effects) - Radio science (ionospheric physics) - Solar physics, astrophysics, and astronomy (corona and transition region)

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