Ionospheric measurements of relative coronal brightness during the total solar eclipses

Swept-frequency (1±10 MHz) ionosonde measurements were made at Helston, Cornwall (50 060N, 5 180W) 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 ̄ux 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 SoÈ rmjoÈ le (63 680N, 20 200E) 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 di€erences between these two eclipses in terms of the nature of the eclipse, short-term ̄uctuations, the sunspot cycle and the recently-discovered long-term change in the coronal magnetic ®eld.