ULF Wave-Associated Density Irregularities and Scintillation at the Equator

This paper presents independent multi-instrument observations that address the physical mechanisms of how ultralow-frequency (ULF) wave-associated electric fields initiate ionospheric density fluctuation 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 ULF wave activity. This implies that the fluctuating electric field at the equator may not be produced through equatorial dynamo action due to fluctuating magnetic fields. Instead, the electric field penetrates from high latitudes and produces fluctuating magnetic field as well as modulates the vertical drift and hence causes the density to fluctuate at the equatorial region. We demonstrate this by estimating the ULF associated fluctuating electric field at high latitudes and at the equatorial region by applying the appropriate attenuation factor as it penetrates to lower latitudes. The periodicity of both electric field and density fluctuations appears to be between 6 and 9 min, which is a typical period of ULF waves in the Pc5 range. Because of its large amplitude and long periods compared to other ULF wave frequency bands, the Pc5 wave-associated electric field, which can even be estimated using magnetograms with low sensitivity and low sampling rate (e.g., 1 min), can easily penetrate to the lower latitude region and produce significant ionospheric density fluctuations that can be strong enough to create scintillation at the equatorial region.