On the Role of Lightning in Coupling Geospace and the Neutral Atmosphere Through the Excitation of the Ionospheric Alfvén Resonator

Abstract:

A potentially important but poorly understood example of the interaction between the Earth’s climate system and the Near-Earth space environment concerns the excitation of the ionospheric Alfvén resonator (IAR) by lightning. The motivation for this research was to examine the characteristics of the relationship and coupling between the lightning produced electromagnetic pulse and the transmission of these pulses into the IAR cavity in the form of Alfvén waves. Induction coil magnetometer data from the Canadian Array for Real-time Investigations of Magnetic Activity (CARISMA; www.carisma.ca) and lightning data from the World-Wide Lightning Location Network (WWLLN; www.wwlln.net) was used to research the magnetic ground response due to lightning and the implied reflection coefficient of the IAR. Significantly, the results demonstrate clear evidence in support of local and non-local lightning as a driver of Alfvén waves in the IAR. Furthermore, evidence of ionospheric coupling is provided by the presence of a radial component in the outwardly traveling B pulse from the lightning strike which is not expected for a mode propagating in the Earth-ionosphere cavity in the absence of mode coupling into the IAR. Setting thresholds on the amplitude of the primary and reflected pulse shows a strong correlation between the number of reflective events and active IAR hours. The median ratio amplitude of these pulses was found each hour to infer an effective overall reflection coefficient of the IAR upper boundary. During active IAR, typically from around local dusk to local midnight, the inferred reflection coefficient is 0.3-0.4 reaching a maximum of 0.6. An explanation for the implied high effective reflection coefficient at Ministik station, Alberta, Canada, as compared to other stations is also discussed in the context of variations in local ground conductance. Together these findings provide important insights into lightning related electromagnetic processes which couple atmospheric electrodynamics to those in near-Earth geospace. The overall impact and consequences of such coupling remains to be fully determined.