Abstract
<p>We present an analysis of localized ionospheric perturbations detected at Comandante Ferraz Brazilian Antarctic Station (McIlwain parameter L~2.25) as fast-amplitude variations of very low frequency (VLF) signals transmitted from Hawaii (NPM, at 21.4 kHz), also known as Trimpi events. The study covers the first six months of 2007, during the period of minimum activity in the 23rd solar cycle. The occurrence of Trimpi events in the Antarctica peninsula region was studied in association with solar-wind parameters in the neighborhood of the Earth (geospace), along with the geomagnetic activity level (Ap, Dst indices). The analysis shows that the Trimpi events occurred predominantly during geomagnetically disturbed periods, but they have a more intricate association with the geospace regimes. The events achieve higher occurrence during the recovery phase of some geomagnetic storms, and also show a close association with electron flux enhancements in the belt region, especially those with higher energy. The higher event incidence occurred a few hours after what we call the 'angle bracket' phenomenon: when the solar wind velocity rises simultaneous with a decrease in its density.</p>
Highlights
The reception of very low frequency (VLF) radio waves (3-30 kHz) that propagate in the sub-ionospheric region has been used for many decades as a powerful tool for probing the physical characteristics of the lower ionosphere [e.g., Sechrist 1974, Bainbridge and Inan 2003]
A particular phenomenon detected in the VLF signal that is known as localized ionospheric disturbance is produced by the Van Allen belt electron precipitation induced by lightning-generated whistler waves
Concluding remarks The present study shows strong evidence that the geomagnetic activity and geospace weather regimes can be partners in electron precipitation events detected as localized ionospheric disturbances, the Trimpi events, through the use of VLF subionospheric propagating signals
Summary
The reception of very low frequency (VLF) radio waves (3-30 kHz) that propagate in the sub-ionospheric region has been used for many decades as a powerful tool for probing the physical characteristics of the lower ionosphere [e.g., Sechrist 1974, Bainbridge and Inan 2003]. Fast variations of the amplitude and phase of VLF signals (called VLF events here) are caused by the Van Allen belt electron precipitation that is induced by lightning-generated (ducted and nonducted) whistler waves, the well-known (classic and oblique) Trimpi events that can be observed only under nighttime conditions, when the ionosphere is not saturated by the solar radiation [Helliwell et al 1973, Johnson et al 1999]. The first results of our study indicate the possibility to use the Trimpi activity as a tool to empirically improve the geospace forecasting, while contributing to feed-back for the ionospheric models, providing lower ionospheric disturbance monitoring in that polar region
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