Multi‐Instrument Investigation of the Polar Holes

Abstract

During the solar minimum the F-region plasma density can become extremely low, and in some exceptional cases the F-layer can completely disappear. These F-region density depletions are particularly deep during geomagnetically quiet nights, when slow plasma convection creates polar holes, extending 100-1000 km in size, where the electron density is several orders of magnitude lower than the background values. Polar holes are believed to be formed during the periods of the very slow anti-sunward convection, when the plasma is trapped just poleward of the auroral oval in the absence of any ionization sources. At the same time, fast convection with rapid vertical plasma transport has also been associated with the polar hole formation. Combining the electron density measurements from the Resolute Bay Incoherent Scatter Radar (RISR) and Poker Flat ISR (PFISR), the state of art variational data assimilation tool Ionospheric Data Assimilation Four-Dimensional (IDA4D), and the convection data from the Super Dual Auroral Radar Network (SuperDARN) array, the polar hole formation and evolution are experimentally investigated. The F-region plasma is traced back in time prior to the polar hole formation to analyze the favorable conditions for the formation of the polar holes. The role of plasma convection in the formation of the polar holes is experimentally investigated. The results show that the polar holes can be formed in the twilight condition and without being trapped in the closed convection loops. The formation of the polar holes were equally possible during slow and strong convection flows, predominantly with a standard two-cell convection configuration. FORSYTHE ET AL.

Dynasonde Observations of Ionospheric Polar Holes Under Quiet Geomagnetic Conditions at Jang Bogo Station, Antarctica

“…Similar characteristics of polar holes, including occurrence location, horizontal scale size, duration time, and geomagnetic conditions for the polar hole formation, have been reported using observations in space and on the ground (Benson & Grebowsky, 2001;Bjoland et al, 2021;Brinton et al, 1978;Crowley et al, 1993;Forsythe et al, 2021;Hoegy & Grebowsky, 1991;Jenner et al, 2020). However, we believe that our analyses are the first to report the relationship between antisunward plasma convection speed and exponential electron density depletion prior to the polar hole formation.…”

“…Such a NmF2 decrease is accompanied by a decrease in V hor . This positive correlation between NmF2 and V hor implies that the occurrence of the electron density depletion is related to slow antisunward plasma convection in darkness and in the absence of an ionization source (Benson & Grebowsky, 2001; Brinton et al., 1978; Forsythe et al., 2021; Sojka et al., 1981a, 1981b). Figure 7e shows that the median HmF2 does not depend on the JBS MLT, staying at HmF2 ∼ 330 km from MLT = 20 hr to MLT = 01 hr.…”

“…It has been known that the polar hole forms as a result of slow antisunward convection flow across the dark polar cap and attendant ion recombination (e.g., Benson & Grebowsky, 2001; Brinton et al., 1978; Forsythe et al., 2021). Sojka et al.…”

Hemispheric Asymmetry of the Polar Ionospheric Density Investigated by ESR and JVD Radar Observations and TIEGCM Simulations for the Solar Minimum Period

“…Although the ionospheric density is mainly determined by solar EUV produced ionization even at high latitudes, the energetic particle precipitation enhances mostly the nighttime E‐region density in the auroral region, and the dayside densities in the E‐ and F‐regions are also enhanced by energetic proton and soft electron precipitation, respectively, in the cusp region. Furthermore, the plasma convection induced by magnetospheric electric fields redistributes the ionospheric plasma by transporting the plasma from the dayside to the nightside in the F‐region and often produces the characteristic polar ionospheric features such as the tongue of ionization (TOI), plasma patches, ionospheric density trough, plasma hole, etc (e.g., Forsythe et al., 2021; Foster et al., 2005; Zhang et al., 2017).…”

Resolving the High‐Latitude Ionospheric Irregularity Spectra Using Multi‐Point Incoherent Scatter Radar Measurements

“…Enhancements in plasma density, such as polar cap patches (Crowley, 1996), can form in a number of ways, such as from the transportation and mixing of dense plasma (like extreme ultraviolet photoionized plasma) with relatively depleted plasma (Zhang et al., 2013), or from particle impact ionization (e.g., Goodwin et al., 2015; MacDougall & Jayachandran, 2007; Oksavik et al., 2006; Perry & Maurice, 2018; Walker et al., 1999; Weber et al., 1984). Plasma density depletions, such as polar holes, can also result through transportation (Forsythe et al., 2021; Sojka et al., 1981a, 1981b), but also result through enhanced plasma recombination (Perry et al., 2015; Zettergren & Semeter, 2012). The drivers of these processes, such as magnetospheric coupling (Goodwin et al., 2019), determines the size and occurrence of ionospheric irregularities.…”