First ever VLF monitoring of the lunar occultation of a solar flare during the 2010 annular solar eclipse and its effects on the D-region electron density profile Sujay Pal a,b,n , Surya K. Maji b , Sandip K. Chakrabarti a,b a S. N. Bose National Centre for Basic Sciences, JD Block, Sector-III, Salt Lake, Kolkata 700098, India b Indian Centre for Space Physics, 43 Chalantika, Garia St. Road, Kolkata 700084, India article info Article history: Received 4 April 2012 Received in revised form 12 August 2012 Accepted 14 August 2012 Available online 24 August 2012 Keywords: Solar eclipse Solar flare D-region Ionosphere VLF Electron density Occultation abstract A ground based Very Low Frequency (VLF) radio receiver of Indian Centre for Space Physics located at Khukurdaha (22127 0 N, 87145 0 EÞ monitored the VLF signal at 19.8 kHz from the NWC station during a partial solar eclipse (maximum obscuration 75%) which took place on January 15, 2010. The receiver and the transmitter were on two opposite sides of the annular eclipse belt. During the same period, a solar flare also occurred and it was partly blocked by the lunar disk. Thus the resultant signal was perturbed both by the eclipse and by the flare. The deviation of the signal from the normal value was obtained by subtracting from the average diurnal signal on days bracketing the eclipse. The deviation was analysed. We compare the data from GOES-14, HINODE and RHESSI satellites during the event. We got a clear depression in the data during the period of the partial eclipse. Most interestingly, there was also a flaring activity in the sun which reached its peak (C-type) just after the time when the eclipse was near maximum. By superposing the lunar disk on the image obtained by HINODE mission, we pin- point the time frame of blocking hard and soft X-rays. We extract the time variation of the electron density profile in the D-region of the ionosphere due to occulted solar flare from the combined effect of the eclipse and the flare. We also compare the results with a normal solar flare. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Very Low Frequency (VLF) radio waves are used for long distance communication for their very low attenuation while propagating through the Earth-ionosphere wave guide. Indian Centre for Space Physics has been monitoring VLF radio wave transmitting stations from several of its monitoring stations. On January 15, 2010, there was an annular solar eclipse as seen from the southern regions of India. At Khukurdaha ð22127 0 N, 87145 0 EÞ station, the eclipse was partial with a maximum obscuration of about 75%. From this station, we have been monitoring NWC transmitter operating at 19.8 kHz. We report the results of our observations and interpretations in this paper. Periodic variation of the solar radiation on the upper atmo- sphere each day due to sunrise and sunset gives rise to a period- ical variation in its charge density. After the sunrise, the extreme ultraviolet and the soft X-ray radiations create the D-region lowering the ionospheric height to  60270 km. Solar eclipse has always been a very important calculable event which changes the ionosphere in a predictable time frame. During a partial eclipse, the ionosphere experiences a reduction in intensity of the UV and soft X-ray photons from the solar disk. This gives us an opportunity to study the global variation of electron concentra- tion in the ionosphere which could be useful to understand the ion chemistry. Long wavelength propagation models inside the Earth-ionosphere wave guide may be tested as well. The effects of the solar eclipse on the amplitude and phase of VLF signals over the both short ð o3000 kmÞ and long ð 43000 kmÞ propagation paths have been reported by many authors (Bracewell, 1952; Sen Gupta et al., 1980; Lynn, 1981; Buckmaster and Hansen, 1986; Mendes da Costa et al., 1995). Clilverd et al. (2001) reported the results of monitoring the VLF signals from multiple transmitters and receivers paths during the total solar eclipse of August 11, 1999 in Europe. They calculated the variation of electron density at 77 km altitude throughout the period of solar eclipse, which showed a linear variation in electron production rate with solar ionizing radiation. Pal et al. (2012) reported the effects of total solar eclipse of 2009 in India for several propagation paths and also modelled the VLF ampli- tude deviations due to the eclipse using the Long Wave Propaga- tion Capability (LWPC) (Ferguson, 1998) code. Chakrabarti et al. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/pss Planetary and Space Science 0032-0633/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pss.2012.08.016 n Corresponding author at: Indian Centre for Space Physics, 43 Chalantika, Garia St. Road, Kolkata 700084, India. Tel.: þ91 33 2335 5706/7/8x442; fax: þ91 33 2335 3477. E-mail address: sujay@bose.res.in (S. Pal). Planetary and Space Science 73 (2012) 310–317