Statistics of Higher Harmonics SRS at Low Latitude Station, Shillong

Abstract

This is the first statistical ground investigation of unique low latitude Spectral Resonance Structures (SRS) that increase beyond $5Hz$, crossing the fundamental Schumann resonance at few occurrences. The study is carried out using high resolution magnetic field variation data obtained from induction coil magnetometer (ICM) installed at very low latitude Indian station, Shillong ($25.56°N$ geographic latitude, $91.86°E$ geographic longitude, $dipoleL=1.08$) for the duration 2013–2018. This study focuses on the characteristics of SRS whose harmonic frequencies reach $5Hz$ and above. The study reveals that the occurrence of SRS events above $5Hz$ is higher in winter as compared to summer, also the occurrence is higher during low solar activity period as compared to period of solar maximum. It is seen that both the cavities, viz. Ionospheric Alfven Resonator (IAR) and Magnetospheric Alfven Resonator (MAR) support such SRS and have their respective quality factor (Q-factor). The seasonal variability of the Q-factor of these cavities also shows a trend similar to the occurrence, with higher values of Q-factor during winter as compared to summer. Dependence of SRS events on parameters such as height of the IAR cavity, ion mass density, conductivities, which affects the Q-factor reveals that IAR above $5Hz$ is supported by a larger height of the cavity with very low ion densities. On the other hand, the MAR cavity efficiency is higher when the Alfven wave conductance is much larger than the Pedersen conductivity.