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Data from ionosonde and GPS total electron content (TEC) observations reveal a number of ionospheric phenomena that occurred in response to the total solar eclipse on 21 August 2017 over North America. The eclipse started over the US west coast at ∼16:00 UTC (08:00 LT) and ended over the US east coast at ∼20:00 UTC (15:00 LT). We identify the growth of plasma density irregularities and turbulence on the bottomside ionosphere during the eclipse totality (at ∼10:34 LT over the Idaho station), signified by the distinct appearance of spread-F echoes in the ionosonde data. These spread-F echoes appeared in both O-mode and X-mode traces, and they lasted for approximately 10 minutes. Preceding the appearance of the spread echoes, an uplift of the ionospheric F-layer and steepening of the bottomside ionospheric density gradient were seen in the ionosonde data. In addition, data from the ionosonde observations also show some characteristic signatures of traveling ionospheric disturbances (TIDs) at ∼300 km altitude during the eclipse. The TIDs in the ionosonde observations were observed roughly 30 minutes after the time of maximum eclipse, and they have a wave periodicity of approximately 10 minutes. In other words, these TIDs were in the wake of the eclipse totality (lagging behind the umbra), and the wave period was shorter than the medium-scale or short-scale TIDs typically seen in the GPS TEC data. Finally, large reductions in TEC and ionospheric plasma densities (by 33%–45%) as a response to the eclipse were observed in both the GPS TEC and ionosonde data. The ionospheric density in the ionospheric E-region reached minimum roughly at the same time as the maximum eclipse, while the ionospheric F-region density and GPS TEC reached minimum 20–30 minutes after the maximum eclipse.

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.

The response of equatorial ionosphere to the magnetospheric origin DP 2 current system fluctuations is examined using ground-based multiinstrument observations. The interaction between the solar wind and fluctuations of the interplanetary magnetic field (IMF) Bz, penetrates nearly instantaneously to the dayside equatorial region at all longitudes and modulates the electrodynamics that governs the equatorial density distributions. In this paper, using magnetometers at high and equatorial latitudes, we demonstrate that the quasiperiodic DP 2 current system penetrates to the equator and causes the dayside equatorial electrojet (EEJ) and the independently measured ionospheric drift velocity to fluctuate coherently with the high-latitude DP 2 current as well as with the IMF Bz component. At the same time, radar observations show that the ionospheric density layers move up and down, causing the density to fluctuate up and down coherently with the EEJ and IMF Bz.

Geomagnetically induced currents (GICs) caused by interplanetary shocks represent a serious space weather threat to modern technological infrastructure. The arrival of interplanetary shocks drives magnetosphere and ionosphere current systems, which then induce electric currents at ground level. The impact of these currents at high latitudes has been extensively researched, but the magnetic equator has been largely overlooked. In this paper, we investigate the potential effects of interplanetary shocks on the equatorial region and demonstrate that their magnetic signature is amplified by the equatorial electrojet. This local amplification substantially increases the region's susceptibility to GICs. Importantly, this result applies to both geomagnetic storms and quiet periods and thus represents a paradigm shift in our understanding of adverse space weather impacts on technological infrastructure.

The feasibility of predicting the daily occurrence of Global Positioning System scintillation events using forecasts of common geophysical indices to drive a physics-based model of the system is demonstrated over a 5 month period for the African and Asian longitude sectors. The output from the Wing Kp model, which uses solar wind data to predict the geomagnetic activity level up to 4 h in advance, was used to drive the National Center for Atmospheric Research thermosphere/ionosphere model, from which the strength of the Rayleigh-Taylor instability growth rate was calculated to determine the likelihood of scintillation. It is found that the physics-based model demonstrates superior skill to an empirical scintillation model (Wideband Model (WBMOD)) in forecasting scintillation suppression events during seasons when scintillation is common. However, neither of the models driven in this way possess the ability to forecast isolated scintillation events during transitional and off-peak seasons.

Describing the day-to-day variability of Equatorial Plasma Bubble (EPB) occurrence remains a significant challenge. In this study we use the Thermosphere-Ionosphere Electrodynamics General Circulation Model (TIEGCM), driven by solar (F10.7) and geomagnetic (Kp) activity indices, to study daily variations of the linear Rayleigh-Taylor (R-T) instability growth rate in relation to the measured scintillation strength at five longitudinally distributed stations. For locations characterized by generally favorable conditions for EPB growth (i.e., within the scintillation season for that location), we find that the TIEGCM is capable of identifying days when EPB development, determined from the calculated R-T growth rate, is suppressed as a result of geomagnetic activity. Both observed and modeled upward plasma drifts indicate that the prereversal enhancement scales linearly with Kp from several hours prior, from which it is concluded that even small Kp changes cause significant variations in daily EPB growth.

[1] While the mechanism for producing plasma irregularities in the dusk sector is believed to be fairly well understood, the cause of the formation of irregularities and bubbles during the postmidnight sector is still unknown, especially for magnetically quiet periods. This paper presents a case study of the strong postmidnight bubbles that often occur during magnetically quiet periods primarily in June solstice, along with a 4 year (2009–2012) statistical study that shows strong occurrence peak during June solstice predominantly in the African sector. We also confirm, for the first time, the presence of Rayleigh-Taylor (RT) instability during postmidnight hours by using the physics-based model for plasma densities and RT growth rates. Finally, we consider several possible sources of the eastward electric fields that permit the RT instability to develop and form bubbles in the postmidnight local time sector.

[1] Previously, ionospheric signatures of plasmaspheric plumes were reported only at the North American longitude sector. This led to the hypothesis that the geomagnetic field configuration at those longitudes played a vital role in the observation of plasmaspheric plume signatures only over the American continent. Combining ground-based GPS total electron content (TEC), EISCAT incoherent scattering radar (ISR), and DMSP F15 ion drift meter observations we have observed greatly elevated density over the European continent during storm recovery phase on 12 September 2005. The TEC seen over Europe has a tongue of enhanced ionization extending to higher latitudes, which is identical to the plasmaspheric plume signatures that have been often observed over North America. Therefore, our observations clearly demonstrate that ionospheric signatures of plasmaspheric plumes are not limited to the North American sector and suggest that they may be observed at any longitude as long as a dense array of instruments are available to identify these signatures. Citation: Yizengaw, E., M. B. Moldwin, and D. A. Galvan (2006), Ionospheric signatures of a plasmaspheric plume over Europe, Geophys. Res. Lett., 33, L17103, doi:10.1029/2006GL026597.

[1] Tomographic imaging provides a powerful technique for obtaining images of the spatial distribution of ionospheric electron density in an altitude versus geographic latitude grid at a specified longitudinal sector. The method, which involves monitoring the total electron content (TEC) of the ionosphere using signals transmitted from the global positioning system (GPS), has the ability to reveal the detailed structure of the ionosphere including the mid-latitude trough. The tomographic reconstruction of ionospheric electron density structures are performed at different specific longitudinal sectors both in the northern and southern hemisphere. At the same time the plasmapause position is estimated from IMAGE EUV images and mapped onto tomographic images. The simultaneous location of the altitude extension of the mid-latitude trough and the plasmapause position are essentially co-located.