Evolution of Mid‐latitude Density Irregularities and Scintillation in North America During the 7–8 September 2017 Storm

Abstract

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.

The Origin of Midlatitude Plasma Depletions Detected During the 12 February 2000 and 29 October 2003 Geomagnetic Storms

“…Because the growth rate of the gradient drift instability is positive under these conditions, the northeast side boundary of the SED provides a favorable condition for the growth of perturbations. The development of irregularities and scintillations at the poleward boundary of the SED was also identified during the 8 September 2017 storm, and the formation of the turbulences at the boundary was explained in terms of the gradient drift instability (Nishimura et al., 2021). These studies along with our observations indicate that plasma depletions can develop in midlatitudes by local plasma instabilities.…”

“…This interpretation is largely based on the morphology of midlatitude depletions and the simultaneous detection of EPBs at the same magnetic meridian. However, midlatitude depletions are also explained in association with other sources such as traveling ionospheric disturbances (TIDs) (Kil et al., 2016; Nishioka et al., 2009), poleward streaming of plasma depletions (Zakharenkova & Cherniak, 2020), or plasma density gradient formed by storm enhanced density (SED) (Nishimura et al., 2021; Sun et al., 2013). From the total electron content (TEC) maps over Japan, Nishioka et al.…”

Multi‐Scale Density Structures in the Plasmaspheric Plume During a Geomagnetic Storm

“…GPS receivers have been used to show that density irregularities are present in the plume (Heine et al., 2017; Mrak et al., 2020). The plume can evolve to multiple channels and density irregularities are localized to their steep TEC gradients (Nishimura et al., 2021). Super Dual Auroral Radar Network (SuperDARN) echoes are also enhanced at the TEC gradients, indicating that density irregularities extend down to 10s of meter wavelengths (Nishimura et al., 2021).…”

“…The plume can evolve to multiple channels and density irregularities are localized to their steep TEC gradients (Nishimura et al., 2021). Super Dual Auroral Radar Network (SuperDARN) echoes are also enhanced at the TEC gradients, indicating that density irregularities extend down to 10s of meter wavelengths (Nishimura et al., 2021). Velocity observations in the ionosphere have shown that SAPS also contain fine‐scale (∼tens of km) structures that propagate quasi‐periodically (Erickson et al., 2002; Foster et al., 2004; Makarevich & Bristow, 2014; Oksavik et al., 2006).…”

The nighttime ionospheric response and occurrence of equatorial plasma irregularities during geomagnetic storms: a case study

“…In the eastern Asian sector, unique severe EPIs events occurred as depleted plasma density structures that extend northeastward from low latitudes to mid-latitudes on 8 September 2017 were confirmed in the Swarm in-situ Ne measurements as reported by Aa et al (2018). In addition, in the American sector, the midlatitude irregularities that resided near the Storm Enhanced Density (SED) base and ionosphere main trough were also reported to be generated via the gradient drift instability (Nishimura et al, 2021); however, this kind of plasma irregularities are mainly located close to the subaurora region, and they are not the same as the low-latitude EPIs as addressed in our study. Even though, the mechanism of such subauroral plasma irregularities during a storm and the question whether these irregularities are related to the expansion of EPIs from low latitudes are still of interest, which require further study.…”