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.
While high-latitude and equatorial density irregularities and scintillations have been explored in detail, studies on mid-latitude (30°-60° magnetic latitude (MLAT)) density irregularities and scintillation are scarce, and their properties are far less understood. Statistically, radio signal scintillations are indeed much more common in the equatorial and high-latitude ionosphere, whereas those are rare in the mid-latitude ionosphere (Basu et al., 2002;Prikryl et al., 2015). Global surveys of satellite-based GPS amplitude scintillations and losses of lock suggest their occurrences are only a fraction of a percent depending on longitudes (Tsai et al., 2017;Xiong et al., 2018). Despite the low occurrence of disturbances in the mid-latitude ionosphere, a limited number of studies have shown that substantial strengths of density irregularities and scintillation occur during geomagnetically disturbed times. Ledvina et al. (2002) gave the first report of mid-latitude amplitude scintillation during a storm, and they showed that the scintillation is associated with steep density gradients. In-situ observations of the ionosphere density found evidence of density irregularities in a mid-latitude trough during the same storm (Mishin & Blaunstein, 2008). The density irregularities in their event followed a power law with a power law index between −5/3 and −2. They suggested that instability processes such as the gradient drift instability (GDI), temperature gradient instability (TGI), and ionosphere feedback instability at the trough density gradient in the subauroral polarization stream (SAPS) region, play a role in forming the turbulence and energy cascading to smaller scales.
We demonstrate scintillation analysis from a network of geodetic Global Positioning System (GPS) receivers which provide data at 1-second resolution. We introduce proxy phase (σ TEC) and amplitude (SNR 4) scintillation indices and validate them against the rate of change of TEC index (ROTI) and S 4. Additionally, we validate scintillation observations against a Connected Autonomous Space Environment Sensor scintillation receiver. We develop receiver-dependent scintillation event thresholding using hardware-dependent noise variance. We analyze 6 days adjacent to the 7-8 September 2017 geomagnetic storm, using 169 receivers covering magnetic latitudes between 15 • and 65 • in the American longitude sector. We leverage the available spatial sampling coverage to construct 2-D maps of scintillation and present episodic evolution of scintillation intensifications during the storm. We show that low-latitude and high-latitude scintillation morphology match well-established scintillation climatology patterns. At midlatitudes, spatiotemporal evolution of scintillation partially agrees with known scintillation patterns. Additionally, the results reveal previously undocumented midlatitude scintillation-producing structures. The results provide an unprecedented view into the spatiotemporal development of scintillation-producing plasma irregularities and provide a resource to further exploit scintillation evolution at large spatial scales.
Measurements from a dense network of GPS receivers have been used to clarify the relationship between substorm auroras and GPS signal corruption as manifested by loss of lock on the received signal. A network of nine receivers was deployed along roadways near the Poker Flat Research Range in central Alaska, with receiver spacing between 15 and 30 km. Instances of large‐amplitude phase fluctuations and signal loss of lock were registered in space and time with auroral forms associated with a sequence of westward traveling surges associated with a substorm onset over central Canada. The following conclusions were obtained: (1) The signal corruption originated in the ionospheric E region, between 100 and 150 km altitude, and (2) the GPS links suffering loss of lock were confined to a narrow band (<20 km wide) along the trailing edge of the moving auroral forms. The results are discussed in the context of mechanisms typically cited to account for GPS phase scintillation by auroral processes.
The Mahali Global Positioning System (GPS) array (9 receivers, 15–30 km baseline distance) in central Alaska has probed auroral structures in a field‐aligned direction during a geomagnetic substorm on 7 October 2015. We present results from a collaborative study of GPS phase scintillation, optical emission brightness, and ionospheric density perturbations, by virtue of data fusion procedure from the Mahali GPS array, all‐sky imager (ASI), and the Poker Flat Incoherent Scatter Radar (PFISR). First, we present observations in a traditional way using colocated GPS‐ASI sensors, giving us a principal pattern of the phase scintillation with respect to auroral brightness, free of any mapping ambiguities. Next, we use an assumption that the plasma irregularities are located at an altitude of 120 km, we map the optical data to this altitude, and we extend the GPS‐ASI study over the entire field of view of the GPS receiver array. We obtain a repeatable and persuasive pattern, revealing that GPS phase scintillation is clustered at the auroral edges. Moreover, investigation of the colinear ISR observations supports the altitude assumption of scintillation producing irregularities, and PFISR‐derived electric field estimates suggest that the source for irregularities is gradient drift instability. The phase scintillation was observed on all GPS receivers, phase scintillation exceeded once cycle during several electrojet intensifications, and several events lasted for more than a minute. Finally, phase scintillation was observed during all surge events, independent of the particular auroral morphology.
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