Responses of the D region ionosphere to solar flares revealed by MF radar measurements

Electron density (N-e) and effective reflection height (H', where the electron density reaches its peak value) in the D region observed by medium frequency (ME) radar at Kunming (25.6 degrees N, 103.8 degrees E) were utilized to quantitatively study the ionospheric response to solar flares. It was found that the H' and N-e had profound responses to solar flares. The results showed a sharp decrease in the H' from 76 to 80 km during the non-flare period to nearly 58 km with flares, and a sudden increase in the N-e from below 4x 10(8) m(3) in non-flare period to 1.8x 10(9) m(3) during 13 M-level flares. The behavior of N-e was strongly and positively correlated with the variation of the X-ray flux, with the largest coefficient being 0.93, while the changes of H' exhibited a strongly negative correlation of -0.96 with that of the X-ray flux. Moreover, variations of H' and Delta N-e depended on not only the flare level, but also upon the onset time and duration of flare.