Preearthquake anomalous ionospheric signatures observed at low-mid latitude Indian station, Delhi, during the year 2015 to early 2016: Preliminary results

We have analyzed five major earthquake events measuring greater than 6 on Richter scale (M > 6) that occurred during the year 2015 to early 2016, affecting Indian region ionosphere, using F-2 layer critical parameters (f(o)F(2), h(m)F(2)) obtained using Digisonde from a low-mid latitude Indian station, Delhi (28.6 degrees N, 77.2 degrees E, 19.2 degrees N geomagnetic latitude, 42.4 degrees N dip). Normal day-to-day variability occurring in ionosphere is segregated by calculating F-2 layer critical frequency and peak height variations (Delta f(o)F(2), h(m)F(2)) from the normal quiet time behavior apart from computing interquartile range. We find that the ionospheric F-2 region across Delhi by and large shows some significant perturbations 3-4 days prior to these earthquake events, resulting in a large peak electron density variation of similar to 200%. These observed perturbations indicate towards a possibility of seismo-ionospheric coupling as the solar and geomagnetic indices were normally quiet and stable during the period of these events. It was also observed that the precursory effect of earthquake was predominantly seen even outside the earthquake preparation zone, as given by Dobrovolsky et al. (1979). The thermosphere neutral composition (O/N-2) as observed by Global Ultraviolet Imager, across Delhi, during these earthquake events does not show any marked variation. Further, the effect of earthquake events on ionospheric peak electron density is compared to the lower atmosphere meteorological phenomenon of 2015 sudden stratospheric warming event