QUASI-PERIODIC SIMILAR-TO-5-60 S FLUCTUATIONS OF VLF SIGNALS PROPAGATING IN THE EARTH-IONOSPHERE WAVE-GUIDE - A RESULT OF PULSATING AURORALPARTICLE-PRECIPITATION

Subionospheric very low frequency and low-frequency (VLF/LF) transmitt er signals received at middle-latitude ground stations at nighttime we re found to exhibit pulsating behavior with periods that were typicall y in the similar to 5-60 s range but sometimes reached similar to 100 s. The amplitude versus time shape of the pulsations was often triangu lar or zigzag-like, hence the term ''zigzag effect.'' Variations in th e envelope shape were usually in the direction of faster development t han recovery. Episodes of zigzag activity at Siple, Antarctica (L simi lar to 4.3), and Saskatoon, Canada (L similar to 4.2), were found to o ccur widely during the predawn hours and were not observed during geom agnetically quiet periods. The fluctuations appeared to be caused by i onospheric perturbations at the similar to 85 km nighttime VLF reflect ion height in regions poleward of the plasmapause. We infer that in th e case of the Saskatoon and Siple data, the perturbations were centere d within similar to 500 km of the stations and within similar to 100-2 00 km of the affected signal paths. Their horizontal extent is inferre d to have been in the range similar to 50-200 lan. The assembled evide nce, supported by Corcuff's [1996] recent research at Kerguelen (L sim ilar to 3.7), suggests that the underlying cause of the effect was pul sating amoral precipitation. The means by which that precipitation pro duces ionospheric perturbations at 85 lan is not yet clear. Candidate mechanisms include (1) acoustic waves that propagate downward from pre cipitation regions above the similar to 85 km VLF reflection level; (2 ) quasi-static perturbation electric fields that give rise to ExB drif ts of the bottomside ionosphere; (3) secondary ionization production a nd subsequent decay at or below 85 km. Those zigzag fluctuations exhib iting notably faster development than recovery probably originated in secondary ionization produced near 85 km by the more energetic (E>40 k eV) electrons in the incident electron spectrum.