Formation of polar ionospheric tongue of ionization during minor geomagnetic disturbed conditions

Abstract

Previous investigations of ionospheric storm‐enhanced density (SED) and tongue of ionization (TOI) focused mostly on the behavior of TOI during intense geomagnetic storms. Little attention has been paid to the spatial and temporal variations of TOI during weak to moderate geomagnetic disturbed conditions. In this paper we investigate the source and development of TOI during a moderate geomagnetic storm on 14 October 2012. Multi‐instrumental observations including GPS total electron content (TEC), Defense Meteorological Satellite Program (DMSP) in situ measured total ion concentration and ion drift velocity, SuperDARN measured polar ion convection patterns, and electron density profiles from the Poker Flat Incoherent Scatter Radar (PFISR) have been utilized in the current analysis. GPS TEC maps show salient TOI structures persisting for about 5 h over high latitudes of North America on 14 October 2012 in the later recovery phase of the storm when the magnitudes of IMF By and Bz were less than 5 nT. The PFISR electron density profiles indicate that the extra ionization for TEC enhancements mainly occurred in the topside ionosphere with no obvious changes in the bottomside ionosphere and vertical plasma drifts. Additionally, there were no signatures of penetration electric fields in the equatorial electrojet data and upward ion drifts at high latitudes. At the same time, strong subauroral polarization streams with ion drift speeds exceeding 2.5 km/s carried sunward fluxes and migrated toward lower latitudes for about 5° based on the DMSP cross‐track drift measurements. Based on those measurements, we postulate that the combined effects of initial build‐up of ionization at midlatitudes through daytime production of ionization and equatorward (or less poleward than normal daytime) neutral wind reducing downward diffusion along the inclined filed lines, and an expanded polar ion convection pattern and its associated horizontal plasma transport are important in the formation of the TOI.

Simulation and Observations of the Polar Tongue of Ionization at Different Heights During the 2015 St. Patrick's Day Storms

“…Near the geomagnetic poles, the vertical plasma transport due to the horizontal thermospheric wind is zeroed. Our results are in agreement with previous findings by Liu et al (), Liu, Wang, Burns, Solomon, et al (). These authors have demonstrated that the TOI is enhanced due to equatorward upwelling of plasma at the dayside subauroral latitudes in the region of plasma accumulation before its removal to the polar cap.…”

“…Sojka et al () reported that under conditions of enhanced magnetospheric convection, TOI is more pronounced. The main characteristics of the TOI occurrence during strong geomagnetic storms have been studied using multi‐instrumental observations and theoretical models (e.g., Foster et al, ; David et al, ; Liu et al, , Liu, Wang, Burns, Solomon, et al, ; Liu, Wang, Burns, Yue, et al, ; Thomas et al, ). Often, occurrence of TOI is linked to another storm time plasma feature—SED, a localized plume‐like increase of ionospheric electron density (particularly the total electron content [TEC]), which is transported poleward from a source region at middle latitudes (Coster & Scone, ; Foster, ).…”

Morphology of high‐latitude plasma density perturbations as deduced from the total electron content measurements onboard the Swarm constellation

“…Resultant antisunward convection in the middle of the polar cap can lead to (segmented or unsegmented) TOI [e.g., Zhang et al , ; Walsh et al , ], which is elongated across different L shells by definition. Note that TOIs can occur during strong as well as moderate to weak geomagnetic storms [e.g., Liu et al , , and references therein]. As our study imposes no constraints on geomagnetic activity, TOIs will also contribute to the statistics in the polar cap, as shown in Figures and .…”

Relative importance of horizontal and vertical transports to the formation of ionospheric storm‐enhanced density and polar tongue of ionization

“…Storm time, single‐site observations from the Millstone Hill Incoherent Radar (ISR) frequently show dramatic F region electron density enhancements around the duskside, which is termed the “dusk effect” [e.g., Papagiannis et al ., ; Mendillo et al ., ; Evans , ; Anderson , ; Buonsanto , , , ]. Based on Millstone Hill ISR 2‐D electron density measurements, Foster [] renamed this phenomenon SED, which is typically characterized by a latitudinally distinct region of sunward convection F region plasma, high electron densities, an elevated F region peak, a significantly enhanced topside ionosphere, and low electron temperatures near sunset at middle latitudes [ Foster , ; Liu et al ., , ]. Cherniak and Zakharenkova [] reported that high‐latitude ionospheric irregularities tend to occur in the edge of SED/TOI having steep ionospheric density gradients during the St. Patrick's Day event in 2015.…”

“…In some literatures, people do not distinguish between SED and TOI since these two are quite similar but occur at different latitudes. However, Liu et al [2015] reported that TOI can also occur during geomagnetic quiet or weakly disturbed conditions at favorable universal local times independent of SED occurrence. TOI tends to form during the universal times when high-latitude two-cell convection patterns are closer to solar-produced middle-latitude plasma source region, facilitating the poleward plasma transportations.…”