Effects of substorm dynamics on magnetic signatures of the ionospheric Alfven resonator

摘要

Spectral resonance structures (SRS) of the ionospheric Alfven resonator (IAR)measured by the induction magnetometer at the High Frequency Active Auroral ResearchProgram (HAARP) ionospheric observatory during a substorm on 28 February 2006 arepresented. The evolution of IAR SRS is compared to ionospheric parameters measuredby the colocated Digisonde, riometer and all-sky imager at HAARP. Initially, themagnetic IAR signatures (spectral resonance structures) exhibited an expected variationthat can be attributed to typical diurnal changes in ionospheric structure. At substormonset, the signatures disappeared because of either a suppression of resonance conditionsby substorm-related particle precipitation or enhanced power in the Pcl spectrum thatconcealed continuing IAR SRS. After the substorm, the SRS reappeared; however theharmonics had shifted to lower frequencies with tighter frequency spacing. For the firsttime, we show that this time-dependent behavior in IAR SRS is explained by increased Fregion densities resulting from electron precipitation. Similarities between observed IARharmonic frequencies and those calculated with a model suggest that variations in F regiondensity, especially f_oF_2,may often dominate the evolution of IAR eigenfrequencies. Thiscould potentially provide a mechanism for monitoring topside dynamics using IAR SRS.