ULF Wave Associated Density Irregularities and Scintillation at the Equator

This paper presents independent multi-instrument observations that addresses the physical mechanisms of how ULF wave associated electric fields initiate ionospheric density fluctuations and scintillation at the equator. Since the magnetic field at the equator is entirely embedded in a relatively high collision and high conductivity medium, the condition may not be possible for the geomagnetic field to fluctuate due to the damped/penetrated ULF wave. This implies the fluctuating electric field at the equator may not be produced through equatorial dynamo action due to fluctuating magnetic fields. Instead the oscillating field penetrates from high-latitudes through the TM0 (zero order transverse magnetic) mode and produce fluctuating induced magnetic field, and thus modulate the vertical drift to oscillate. We estimated the ULF associated electric field at high-latitudes and equatorial region, and demonstrated that only 15% of the fluctuating electric field from the auroral region can make to the equatorial region. We also calculated the corresponding vertical drift that oscillate with nearly identical periodicity (6-9 min) as the ULF waves in the Pc5 band. Because of its large amplitude and long periods compared to other ULF wave frequency bands, the Pc5 wave associated electric field can easily penetrate to the lower latitude region. The oscillating vertical drift at the equator has an amplitude of 2.5 - 7.0 m/s, which is 25 -50% of the typical quiet time dayside maximum value of the vertical drift at the equator, which is strong enough fluctuation to easily produce significant ionospheric density fluctuations and trigger scintillation at the equatorial region. In this paper, we present multi-instrument observations that clearly confirm the role of ULF wave penetration for the formation of density irregularities and scintillation at the equator.