The inter-hemispheric difference in the impact of the geomagnetic storms of June 2015 and December 2015 is investigated with respect to quiet time seasonal asymmetry. A meridional chain of ground observatories along 95°E (GNSS receiver/Ionosonde), satellite in-situ measurements (SWARM/COSMIC/C-NOFS), Total Electron Content map and SAMI2/CTIPe model simulations are utilized. Symmetric negative (positive) effects prevailed during the main phase of June (December) storm but hemispheric asymmetry was manifested during the recovery phase. Differential VTEC and NmF2 response in addition to perturbations in VTEC by more than 30 TECU (~90-100%) were recorded. The SWARM observations confirmed that the topside density/TEC enhancement in the southern low latitude was much higher than 300%. The SWARM A/B pass of 23 June and ground TEC map showed a third latitudinal maxima around -45° dip angle in southern hemisphere low latitude in addition to the conventional EIA crests. Similarly an additional peak appeared at +45° dip in northern hemisphere in the SWARM A pass in the sunrise period of 21 December. The higher winter-side hmF2 and northward C/NOFS meridional flow velocity suggest that storm time Joule heating resulted in anomalous equator-ward winds surge in the winter hemispheres of 95°E which led to the formation of the additional storm time maxima at the pole-ward edge of the EIA region. Further modeling efforts are needed to capture this counter-intuitive feature for a better forecasting of the impact of space weather events over low latitude ionosphere.

The total energy transfer from the solar wind to the magnetosphere is governed by the reconnection rate at the magnetosphere edges as the IMF $B_z$ turns southward. The delayed response of the ring current to solar wind driving can account for the anomalous growth of the SYM-H under northward IMF $B_z$. The geomagnetic storm on 21-22 January 2005 is considered to be anomalous as the SYM-H index that signifies the strength of ring current, grows and has a sustained peak value lasting more than 6 hrs under northward IMF $B_z$ conditions. In this work, first the standard WINDMI model is utilized to estimate the growth and decay of various magnetospheric currents by using several solar wind-magnetopsehre coupling functions. However, it is found that the WINDMI model driven by any of these coupling functions is not fully able to explain the enhancement of SYM-H under northward IMF $B_z$. The SYM-H variations during the entire duration of the storm were only reproduced when the effects of the dense plasma sheet were included in the WINDMI model. The limitations of directly-driven models relying purely on the solar wind parameters and not accounting for the state of the magnetosphere are highlighted by this work.

We present, for the first time, a plasmaspheric hiss event observed by the Van Allen probes in response to two successive interplanetary shocks occurring within an interval of ~2 hours on December 19, 2015. The first shock arrived at 16:16 UT and caused disappearance of hiss for ~30 minutes. Significant Landau damping by suprathermal electrons followed by their gradual removal by magnetospheric compression opposed the generation of hiss causing the disappearance. Calculation of electron phase space density and linear wave growth rates showed that the shock did not change the growth rate of whistler mode waves within the core frequency range of plasmaspheric hiss (0.1 - 0.5 kHz) during this interval making conditions unfavorable for the generation of the waves. The recovery began at ~16:45 UT which is attributed to an enhancement in local plasma instability initiated by the first shock-induced substorm and additional possible contribution from chorus waves. This time, the wave growth rate peaked within the core frequency range (~350 Hz). The second shock arrived at 18:02 UT and generated patchy hiss persisting up to ~19:00 UT. It is shown that an enhanced growth rate and additional contribution from shock-induced poloidal Pc5 mode (periodicity ∼240 sec) ULF waves resulted in the excitation of hiss waves during this period. The hiss wave amplitudes were found to be additionally modulated by background plasma density and fluctuating plasmapause location. The investigation highlights the important roles of interplanetary shocks, substorms, ULF waves and background plasma density in the variability of plasmaspheric hiss.