Astrophys Space Sci (2014) 350:1–9 DOI 10.1007/s10509-013-1699-4 ORIGINAL ARTICLE Solar flare induced D-region ionospheric perturbations evaluated from VLF measurements Ashutosh K. Singh · A.K. Singh · Rajesh Singh · R.P. Singh Received: 30 June 2013 / Accepted: 10 November 2013 / Published online: 3 December 2013 © Springer Science+Business Media Dordrecht 2013 Abstract The results of very low frequency (VLF) wave amplitude measurements carried out at the low latitude sta- tion Varanasi (geom. lat. 14 ◦ 55 ′ N, long. 154 ◦ E), India dur- ing solar flares are presented for the first time. The VLF waves (19.8 kHz) transmitted from the NWC-transmitter, Australia propagated in the Earth-ionosphere waveguide to long distances and were recorded at Varanasi. Data are ana- lyzed and the reflection height H ′ and the sharpness factor β are evaluated. It is found that the reflection height decreases whereas sharpness factor increases with the increase of so- lar flare power. The H ′ is found to be higher and β smaller at low latitudes than the corresponding values at mid and high latitudes. The sunspot numbers were low during the considered period 2011–2012, being the rising phase of so- lar cycle 24 and as a result cosmic rays may impact the D- region ionosphere. The increased ionization from the flare lowers the effective reflecting height, H ′ , of the D-region roughly in proportion to the logarithm of the X-ray flare in- tensity from a typical mid-day unperturbed value of about 71–72 km down to about 65 km for an X class flare. The sharpness (β ) of the lower edge of the D-region is also sig- nificantly increased by the flare but reaches a clear saturation value of about 0.48 km −1 for flares of magnitude greater than about X1 class. Keywords Sun: flares, X-rays · Waves: VLF · Ionospheric perturbations A.K. Singh · A.K. Singh (B ) · R.P. Singh Atmospheric Research Lab., Department of Physics, Banaras Hindu University, 221005, Varanasi, India e-mail: abhay_s@rediffmail.com R. Singh Dr. KSK Geomagnetic Research Laboratory, Chamanganj, Jhunsi, Allahabad, India 1 Introduction The neutral atmosphere in the lower D-region (∼50–70 km) is ionized mainly by solar EUV radiation and galactic cos- mic rays. Although, the down going solar EUV radiation is increasingly absorbed by the increasing atmospheric den- sity, but the available free electron density become very small due to high electron attachment and recombination rates. The partially ionized lower D-region forms the up- per boundary of the Earth-ionosphere waveguide, while the oceans and ground form the lower boundary. Usually these boundaries are stable and allow VLF waves (∼3–30 kHz) propagation with little or no perturbation. Occasionally so- lar flares (Mitra 1974; Thomson et al. 2004), geomagnetic storms (Peter et al. 2006), solar proton events (Clilverd et al. 2006), γ -ray bursts (Tanaka et al. 2010), solar eclipse (Klobuchar and Whitney 1965; Singh et al. 2012), light- ning discharges (Rodger 1999), particle precipitations due to whistler mode wave particle interaction (Inan et al. 2010) and earthquakes (Hayakawa et al. 1996) could produce addi- tional ionization in the lower D-region and perturb the upper boundary of the Earth-ionosphere waveguide. As a result the amplitude and phase of the VLF waves propagating through the Earth-ionosphere waveguide also get perturbed (Pant 1993; Rodger 1999). Observations of these waves are used as one of the best probes available for characterizing the height and sharpness of the lower D-region (Thomson 2010; Thomson et al. 2011). Amongst the other techniques, rock- ets yields good results but they are expensive and transient and practically it is not possible to use the technique for diur- nal, seasonal and latitudinal studies. Other techniques such as incoherent scatter radar, balloons, satellites, etc have limi- tations due to low altitudes and large air densities (Inan et al. 2010).