Trimpi occurrence and geomagnetic activity: Analysis of events detected at Comandante Ferraz Brazilian Antarctic Station (L = 2.25)

We present an analysis of the occurrence of Trimpi events observed at Comandante Ferraz Brazilian Antarctic Station (EACF), at L = 2.25, as observed by the amplitude of very low frequency (VLF) signals transmitted from Hawaii (NPM 21.4 kHz) from April 1996 to August 1999. The event parameters (total duration, amplitude variation, time incidence, and type (negative or positive)) were analyzed for 4394 events detected in the first year (solar minimum and relatively low Trimpi activity). The Trimpi occurrence was compared with geomagnetic activity for the whole period. A higher incidence of Trimpi events was observed near the equinoxes, with minima at the solstices following the same behavior as the Ap indices annual variation. During the day, their incidence peaked at ∼0700 and ∼1045 UT in accordance with the occurrence of lightning flashes in the geomagnetic conjugate regions but apparently not in agreement to the daily mean geomagnetic variation (Kp index) found to be peaked (in entire years sampling average) at 0300 UT and weaker at 0900 UT for the whole 4‐year period. The relationship between Trimpi occurrence and rise in solar activity for cycle 23 is discussed. A sudden strong incidence of Trimpi events around 5 May 1998 and 27 August 1998 was attributed to pronounced magnetic storms (Dst index).