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With low solar activity and unusual progression, Solar Cycle 24 lasted from December 2008 to December 2019 and is considered to be the weakest cycle in the last 100 years. During such quiet solar background conditions, the wave forcing from lower atmosphere will have a perceivable effect on the ionosphere. This study examines the ionospheric response to meteorological phenomenon of Sudden Stratospheric Warming (SSW) events during Solar Cycle 24 (Arctic winter 2008/09 to 2018/19). Ionospheric response to each of these identified warming periods is quantified by studying ground – based Global Positioning System (GPS) derived vertical Total Electron Content (VTEC) and its deviation from monthly median (ΔVTEC) for four longitudinal chains, selected from worldwide International GNSS service (IGS) stations. Each chain comprises of eight stations, chosen in such a way as to cover varied latitudes both in Northern and Southern Hemispheres. A strong latitude – dependent response of VTEC perturbations is observed after the peak stratospheric temperature anomaly (ΔTmax). The semidiurnal behaviour of VTEC, with morning increase and afternoon decrease, is mostly observed at near-equatorial stations. This vertical coupling between lower and upper atmosphere during SSW is influenced by prominent 13-14 days periodicities in VTEC observations, along with other periodicities of 7, 5, and 3 days. It is seen that the ionospheric response increases with increase in solar activity. Further, under similar ionizing conditions, quite similar ionospheric response is observed, irrespective of ΔTmax and type of SSW event being major or minor. However, under similar SSW strength (ΔTmax), no prominent pattern in ionospheric response is observed. The causative mechanism for the coupling processes in the atmosphere during these SSW events is discussed in detail.

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.

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.

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.

We investigate the regularities of a change in Spread-F F probability during day-to-day, under varying solar variability, latitudinal behavior and their response to geomagnetic storm in equatorial and low-mid latitude stations. The occurrence characteristics of Spread-F irregularities, is obtained from daily hourly ionosonde data from a low-mid latitude station, Delhi (28.6°N, 77.2°E), for more than half a solar cycle (2001 to 2007). The latitudinal behavior of Spread-F is studied using ionosonde data from anomaly crest station, Ahmedabad (23.01°N, 72.36°E) and equatorial station, Kodaikanal (10.2°N, 77.5°E) for low, moderate and high solar activity periods. The maximum percentage occurrences of Spread-F were observed during the low solar activity year 2007, we believe, the low plasma and neutral density during 23/24 solar cycle minimum could be an important factor leading to the generation and propagation of TIDs and gravity waves. An anti-solar activity correlation to Spread-F occurrence is reported during all the seasons at different stations which are because of instability generated by the trans-equatorial meridional winds. There is a substantial variation during pre and post midnight hours in F region height from equatorial to low latitudes in response to magnetic disturbances. Concurrence was observed in the occurrence time of Spread-F to different storm events during different storm phases. The established irregularities and their behavior in Indian region are qualitatively interpreted and discussed.

[1] The present paper deals with the morphological (volume emission rate as a function of altitude and latitude) study of greenline dayglow emission under equinox conditions. The morphology is obtained from the emission profiles of greenline dayglow emission using the/glow model. The/glow model is updated by incorporating newly discovered temperature dependent rate coefficient of the reaction N2 (A3Σu+) + O. The recently proposed correction for the atomic oxygen density in MSIS 90 neutral atmosphere model is also included at mesospheric altitudes to achieve the consistency with the WINDII measurements in this altitude region. The modeled morphology of greenline dayglow emission is presented between 50°S and 50°N latitudes for the months of March and April. It has been found that this emission shows asymmetry between the northern and the southern hemispheres. The uneven transport of atomic oxygen due to dynamical processes in both the hemispheres plays an important role in producing the asymmetry in the mesospheric region. The asymmetry in the thermospheric region is likely due to the changing contributions with altitude of the various sources which are responsible for the production of O(1S) during the daytime.