A. K. Upadhayaya

F2 region response to geomagnetic disturbances across Indian latitudes: O (1S) dayglow emission

The morphology of ionospheric storms has been investigated across equatorial and low latitudes of Indian region. The deviation in F-2 region characteristic parameters (f(o)F(2) and hF) along with modeled green line dayglow emission intensities is examined at equatorial station Thiruvananthapuram (8.5 degrees N, 76.8 degrees E, 0.63 degrees S geomagnetic latitude) and low-latitude station Delhi (28.6 degrees N, 77.2 degrees E,19.2 degrees N geomagnetic latitude) during five geomagnetic storm events. Both positive and negative phases have been noticed in this study. The positive storm phase over equatorial station is found to be more frequent, while the drop in ionization in most of the cases was observed at low-latitude station. It is concluded that the reaction as seen at different ionospheric stations may be quite different during the same storm depending on both the geographic and geomagnetic coordinates of the station, storm intensity, and the storm onset time. Modulation in the F-2 layer critical frequency at low and equatorial stations during geomagnetic disturbance of 20-23 November 2003 was caused by the storm-induced changes in O/N-2. It is also found that International Reference Ionosphere 2012 model predicts the F-2 layer characteristic (f(o)F(2) and hF) parameters at both the low and equatorial stations during disturbed days quite reasonably. A simulative approach in GLOW model developed by Solomon is further used to estimate the changes in the volume emission rate of green line dayglow emission under quiet and strong geomagnetic conditions. It is found that the O(S-1) dayglow thermospheric emission peak responds to varying geomagnetic conditions.

Morphology of Ionospheric F2 region variability associated with Sudden Stratospheric Warmings

The effect of Sudden Stratospheric Warming (SSW) on the F2 region ionosphere has been extensively analyzed for the major event of year 2009, apart from a few reports on other major and minor events. Morphology of ionospheric responses during SSW can be better comprehended by analyzing such warming events under different solar, geomagnetic and meteorological conditions. We investigate the features of F2 region variability following the SSW events of 2010, 2011, 2012, 2013, 2014, 2015 and 2016, using ionosonde data from the Asian region covering a broad latitudinal range from 26.6°N to 45.1°N. We find perceptible ionospheric variations in electron densities during these warming events which is accompanied by a large variation of ~117% within enhancements, as compared to a meagre variation of ~11% within depressions, during these events. We also examine six months’ data at these latitudes and longitudes and find that the maximum and minimum variation in F2 layer critical frequency is observed during each SSW period. The influence of quasi-stationary-16-day planetary waves is seen during these SSW events. Further, a recently proposed parameter “SSW integrated strength” by Vieira et al. [2017] to characterize SSW event with respect to ionosphere is also examined. It is seen that it does not fit well for these seven SSW events at these latitudes and longitudes.

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.