Direct EUV/X‐Ray Modulation of the Ionosphere During the August 2017 Total Solar Eclipse

Abstract

The great American total solar eclipse of 21 August 2017 offered a fortuitous opportunity to study the response of the atmosphere and ionosphere using a myriad of ground instruments. We have used the network of U.S. Global Positioning System receivers to examine perturbations in maps of ionospheric total electron content (TEC). Coherent large-scale variations in TEC have been interpreted by others as gravity wave-induced traveling ionospheric disturbances. However, the solar disk had two active regions at that time, one near the center of the disk and one at the edge, which resulted in an irregular illumination pattern in the extreme ultraviolet (EUV)/X-ray bands. Using detailed EUV occultation maps calculated from the National Aeronautics and Space Administration Solar Dynamics Observatory Atmospheric Imaging Assembly images, we show excellent agreement between TEC perturbations and computed gradients in EUV illumination. The results strongly suggest that prominent large-scale TEC disturbances were consequences of direct EUV modulation, rather than gravity wave-induced traveling ionospheric disturbances.Plain Language Summary A total solar eclipse is a rare astronomical event, which offers an opportunity to study how the Sun interacts with our atmosphere in great detail. The solar irradiance at X-ray and extreme ultra violet wavelengths is so energetic that it ionizes the neutral gases in the upper atmosphere, whereas longer wavelengths penetrate deeper in the atmosphere and heat it up. The great American total solar eclipse on 21 August 2017 offered an extraordinary opportunity to study the atmospheric response to the occulted solar disk at a time of its largest energy deposition (around noon time). The ionosphere is affected by variations in the solar illumination and by temperature changes in the lower atmosphere. We identified four large-scale perturbations in ionospheric plasma using signals acquired from a large network of Global Positioning System receivers. For the first time, we show firm evidences that irregular solar flux deposition inside of the penumbra causes a continental-wide modulation of the ionospheric plasma. Specifically, the observed disturbances were caused by two sunspots which created four stark deviations from a uniform solar disk model. The discovery of a direct solar modulation is a revolutionary turnabout, since the previous reports attributed the disturbances to thermospheric or stratospheric sources.

Multispectral and Multi‐instrument Observation of TIDs Following the Total Solar Eclipse of 21 August 2017

“…A total solar eclipse had occurred 8 hr before the observed LSTIDs (at Carbondale, IL). While the LSTIDs were most likely generated by geomagnetic effects, the effect of a total solar eclipse on the IT system has also been well documented by recent and prior studies (e.g., Coster et al, ; Liu et al, ; Mrak et al, ). Furthermore, the MUF series at MH showed perturbations even prior to the start of the geomagnetically active time (Figure ).…”

“…We then subtracted the background vertical TEC to obtain DTEC residuals, using variable orders of polynomials (cf. Mrak et al, ). The carrier phase‐based differential approach provides an accuracy better than 0.03 TECu (Coster et al, ; 1 TECu = 10 16 e − /m 2 ), which enables one to resolve tiny but spatially coherent perturbations in TEC.…”

“…The carrier phase‐based differential approach provides an accuracy better than 0.03 TECu (Coster et al, ; 1 TECu = 10 16 e − /m 2 ), which enables one to resolve tiny but spatially coherent perturbations in TEC. The DTEC residuals were mapped to a geographical map at an altitude of 300 km and transformed from the naturally irregular spatial grid into a regular grid (e.g., Azeem et al, ; Mrak et al, ) with a resolution of 0.2° × 0.2° (geographical coordinates). Due to the size of the grid (spatial sampling) and slant‐to‐vertical mapping uncertainty, the minimum scale sizes that can be inferred from these maps are on the order of 100 km.…”

Topside Ionospheric Electron Temperature Observations of the 21 August 2017 Eclipse by DMSP Spacecraft

“…As each active region produced excess EUV and X‐ray emissions compared to the main solar disk, the illumination in these wavelengths at a given point on the Earth would undergo sharp changes when the lunar limb obscured these active regions rather than the smooth changes expected from a uniformly illuminated disk. Utilizing the Solar Dynamics Observatory observations at 19.3 nm in conjunction with Naval Observatory Vector Astrometry Software (Kaplan et al, ), Mrak et al () showed that a projection of the actual penumbra consists of regions of steep changes in gradients, manifesting themselves as overlapping circles moving eastward with the eclipse (see Figure 1b of Mrak et al, ). Further, they compared the time sequence of the locations of these EUV gradients with the measured total electron content (TEC) perturbations during the eclipse and found they matched, thus indicating that the source of the TEC perturbations was the uneven EUV illumination from the Sun.…”

“…Observations of the electron temperatures during two separate DMSP spacecraft passes through the penumbral shadow during this eclipse showed sporadic regions of cooled and noncooled electrons rather than the smooth decrease and increase in temperature in the shadow predicted by Huba and Drob (). Mrak et al (), who mapped the effects of two solar active regions producing excess EUV/X‐ray irradiation onto the ionosphere, showed that this unevenness in the solar illumination produced a complex and structured ionospheric response rather than the smooth variation that was predicted. Comparing these results with the observed electron temperatures indicates that this is likely the cause of the sporadic cooling seen by the DMSP spacecraft.…”