ULF Wave‐Associated Density Irregularities and Scintillation at the Equator

Abstract

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.Plain Language SummaryThe ultralow‐frequency (ULF) wave, which is believed to be generated by strong solar wind dynamic pressure at the magnetopause, can penetrate to the ionosphere and modulate high‐latitude electric field that can penetrate to equatorial latitudes and cause density irregularities in the ionosphere. Especially in the dusk to midnight local time sector, when the background density is weaker and can easily be driven up and down by small magnitude of fluctuating electric field (vertical drift), the density fluctuation becomes stronger. Such density fluctuations create favorable conditions for the creation of rapid amplitude and phase fluctuations of radio signals, which affects several technological systems such as over the horizon high‐frequency radio communication outage and increased Global Navigation Satellite System navigation errors. Thus, ionospheric density fluctuations are as much an engineering concern as they are a scientific quest, and hence understanding the physics behind the contribution of ULF wave power for the formation of small‐scale ionospheric density fluctuations is very important to develop a model that can accurately capture the structure and dynamics of the global low‐latitude ionospheric irregularities.Key PointsULF modulates high-latitude electric fieldElectric field penetrates from high to low latitudesFluctuating electric field causes scintillation at the equator