Indian Journal of Radio & Space Physics Vol. 39, October 2010, pp. 296-301 Ionospheric modulations due to solar flares over Ahmedabad Som Sharma $ , H Chandra, Hari Om Vats, N Y Pandya & Rajmal Jain Physical Research Laboratory, Ahmedabad 380 009, India $ E-mail: somkumar@prl.res.in Received 28 July 2010; accepted 18 August 2010 Earth’s ionosphere responds quickly to the enhanced X-rays and ultraviolet (UV) radiation emitted from the Sun during solar flares. Ground and satellite based observations are important for unravelling the consequences of these essentially unpredictable events. Regular ionospheric soundings are made over Ahmedabad (23 o N, 72.4 o E) using KEL digital Ionosonde. Using this data, the influence of the excess X-ray emission from the flares / CMEs in different regions of the Earth’s ionosphere over Ahmedabad has been studied. Based on the X-ray spectral characteristics of the flares, the variation in the minimum frequency of reflection, f min , an indicator of the ionization in D-region of ionosphere, in contrast to daily quiet time variation, has been measured. The studies of the excess absorption, due to the solar flares, in the D-region of ionosphere over Ahmedabad are made for the events during 1996-2000. The deviation from Chapman’s layer behaviour of E-layer over Ahmedabad during few solar flares have been also found. Keywords: Solar flare, Ionospheric modulation, Magnetic field, Geomagnetic storm, Electron density PACS Nos: 94.20.de; 96.60.ge 1 Introduction Solar flares are transient phenomena taking place in the Sun and huge amount of energy (~10 27 ergs or even more), in the form of radiation and matter is thrown into the interplanetary medium in very short duration. This excess amount of energy triggers various geophysical processes in the ionosphere. Excess emission of X-rays from the Sun and other celestial sources are mainly responsible for enhanced ionization/absorption in the D-region of the ionosphere. Ananthakrishnan & Ramanathan 1 and Ananthakrishnan et al. 2 reported the terrestrial effects of the X-ray emission from celestial objects. Deshpande et al. 3 reported the effect of X-ray bursts from pulsar (PSR 0950+08). The minimum frequency of reflection in radio soundings by Ionosonde, f min , depends, apart from other factors, on the absorption from D-region of ionosphere and therefore, the changes in it are a fairly good indicator of the changes in the electron density in D-region. The value of f min is known to increase following the solar flares and for large solar flares there could be complete radio blackout in radio soundings. Effect of solar X-ray flares in D-region absorption has been studied earlier 4-5 . In this paper, the association between the solar flares and the enhanced absorption of radio waves due to the D-region of the ionosphere has been reported based on a large number of solar flares during 1997-2000. 2 Observations and Data Regular radio soundings are made from Ahmedabad since last five decades. In this study, the data has been used from ground based Ionosonde at Ahmedabad and X-ray intensities from Geostationary Operational Earth Satellite (GOES). Instrument on board GOES Satellite provides high quality X-ray flux data. GOES X-ray flux data contains 5 minute averages of solar X-ray output in the 1-8 Angstrom and 0.5-4.0 Angstrom pass bands. In this study, the hourly f min data has been used from Ionosonde and X-ray intensities from GOES for the year 1999. The response of the ionosphere over Ahmedabad during few solar flares events have also been studied using ionospheric data at Ahmedabad. 3 Results Ionospheric observations over Ahmedabad were made using KEL digital ionosonde. Variations in f min , recorded on ionogram, is an indicator of the absorption of radio signals passing through the D-region of the ionosphere. Figure 1(a) shows an example of the solar flare event of 12 May 1997. The total flare intensity is plotted as a function of time.