Search

Noticeable F‐region electron density (NmF2) depletions were observed in the winter‐nighttime polar cap ionosphere during solar minimum from the Vertical Incidence Pulsed Ionospheric Radar (VIPIR) with Dynasonde analysis at Jang Bogo Station (JBS) in Antarctica. We focus on the F‐region density depletion events (known as polar holes) following a steady quiet condition that is defined with Kp values ≤ 1+ during 6 hr. Forty‐five polar holes were identified by JBS VIPIR/Dynasonde (JVD) in 2019. All of the events started over a wide range of nightside magnetic local time (22‐05 MLT) with a peak occurrence at 01–03 MLT. JVD measured exponential NmF2 decrease in the nightside MLT (∼19–2.5 hr) zone with e‐fold decay times distributed in the range of ∼0.5–∼3.5 hr before the onset of a polar hole. The e‐folding times decrease along the longitude from dusk toward midnight. The horizontal ion drift velocity (Vhor) estimated from JVD monotonically goes down from ∼190 m/s at 18 MLT to ∼100 m/s near magnetic midnight, and the NmF2 is depleted as Vhor decreases prior to the polar hole formation. The observations of the exponential NmF2 decrease and the positive correlation between NmF2 and Vhor prior to the polar holes are discussed in light of possible formation mechanisms of polar holes, including temporal variations and spatial structure of the polar ionosphere. Key Points: Polar holes were mostly observed at ∼22−06 MLT under extremely quiet geomagnetic conditionsExponential electron density decrease occurred ∼1.0−4.5 hr prior to the onset of the polar holesThe exponential declination of NmF2 due to recombination loss is the reason for the formation of the polar holes [ABSTRACT FROM AUTHOR]

The IAGA‐endorsed Polar Cap Indices for the northern and southern hemispheres, PCN and PCS, are compared for 1998–2018, inclusive. Potential effects of the slightly different, and changing, magnetic coordinates of the two magnetic stations employed, Thule (Qaanaaq) in Greenland and Vostok in Antarctica, are investigated. It is shown that the agreement in overall behavior of the two indices is very close indeed but that PCS consistently correlates slightly better with solar wind parameters than PCN. Optimum lags for these correlations are 19 min for 1‐min data and 37 min for hourly averages. The correlations are significantly highest for the predicted magnetopause reconnection voltage, which is a linear predictor of PCN and PCS for all 1‐hr data and for all but the largest 0.1% of 1‐min values. The indices show lower correlation and marked non‐linearity (tending to saturation) at all levels with the estimated magnetopause reconnection electric field or the estimated power input into the magnetosphere. The PCN index is shown to correlate closely with the transpolar voltage measured by the northern‐hemisphere SuperDARN radar network and both PCN and PCS clearly show the Russell‐McPherron effect of dipole tilt and the Y‐component of the interplanetary magnetic field. However the patterns in time‐of‐year and Universal Time (UT) are complicated by lobe reconnection during northward‐IMF, the effect of which on the indices is shown to be predominantly a summer hemisphere phenomenon and gives a UT dependence on the IMF Y‐component that is predicted theoretically. Plain Language Summary: The Polar Cap Indices are generated from geomagnetic recordings made at Thule (Qaanaaq) in Greenland and Vostok in Antarctica and are used as monitors of the coupling of solar wind energy and momentum into Earth's magnetosphere. These stations are at similar locations relative to the nearby magnetic poles of the Earth but there are small differences, the effect of which is investigated. There has been debate about the processing of the data to generate the indices and the extent to which the results from the two hemisphere do ‐ and should ‐ agree with each other. It is shown that the overall agreement of the northern and southern hemisphere indices is very good indeed. It has been proposed that these indices reflect a number of aspects of coupling of the solar wind and Earth's magnetosphere, but it is shown here that they are optimum indicators of the voltage generated by magnetic reconnection between the geomagnetic field and the interplanetary magnetic field. This is shown to be consistent with the variations of the indices with time‐of‐year and time‐of‐day. Key Points: Simultaneous 1‐min values can differ, but the distributions of the north and south polar cap indices over 1998–2018 are very similarBoth indices give significantly higher correlations with the predicted voltage of open flux generation and with observed transpolar voltageBoth indices show a Russell‐McPherron effect plus a northward‐IMF lobe reconnection effect that is predominantly in the summer hemisphere [ABSTRACT FROM AUTHOR]

The ionospheric density displays hemispheric asymmetries in the polar region due to various hemispheric differences, for example, in the offset between geographic and geomagnetic poles and in the geomagnetic field strength. Using ground‐based ionospheric measurements from Vertical Incidence Pulsed Ionospheric Radar with Dynasonde analysis at Jang Bogo Station (JBS), Antarctica and from EISCAT Svalbard Radar (ESR) where both sites are located mostly in the polar cap, we investigate the hemispheric differences in the ionospheric density between the northern and southern hemispheres for geomagnetically quiet and solar minimum condition. The results are also compared with Thermosphere Ionosphere Electrodynamic Global Circulation Model (TIEGCM) simulations. The observations show larger density and stronger diurnal and seasonal variations at JBS in the southern hemisphere than at Svalbard in the northern hemisphere. The diurnal variations of the density peak height are also observed to be much larger at JBS. In both hemispheres, the ionospheric density is significantly reduced in winter due to the limited solar production at high geographic latitudes, but TIEGCM considerably overestimates winter density, which is even larger than summer density, especially in the northern hemisphere. Also existed are the differences in the equinoctial asymmetry between the observations and the simulations: the daytime F‐region density is observed to be larger in fall than in spring in both hemispheres, but TIEGCM shows the opposite. In general, most of the observed asymmetrical density are much weaker in the model simulation, which may result from lack of proper magnetospheric forcings and neutral dynamics in the model. Plain Language Summary: The global ionospheric density is not symmetric around the equator due to various reasons. The main reason for the asymmetry would be the asymmetric characteristics of the geomagnetic field. The ionospheric plasma motions are strongly controlled by the magnetic field lines and the magnetospheric energy inputs dominate the polar ionosphere. Therefore, the asymmetric geometry of the magnetic field lines and the offset between the geomagnetic and geographic poles can produce asymmetric density distributions of the ionosphere. However, the investigation of the hemispheric asymmetry has been mostly restricted to the low and mid‐latitude ionosphere and little study has been conducted in the polar ionosphere, mainly due to the lack of observations in the southern hemisphere. Since the establishment of the Jang Bogo Station (JBS), Antarctica in 2014, Korea Polar Research Institute (KOPRI) has been operating various instruments to observe the ionosphere in association with the thermosphere and the magnetosphere. These observations allow us to investigate the hemispheric asymmetry of the ionospheric density by comparing with the observations in the northern polar region. It is found that the diurnal and seasonal variations of the ionospheric density are much stronger at JBS in the southern hemisphere than in Svalbard in the northern hemisphere. Key Points: The polar ionospheric density shows larger magnitude and stronger diurnal and seasonal variations at Jang Bogo Station than in SvalbardThe daytime density shows an equinoctial asymmetry which is larger in fall equinox than in spring equinoxThermosphere Ionosphere Electrodynamic Global Circulation Model significantly overestimates the winter density at high latitudes and shows the opposite equinoctial asymmetry to the observations [ABSTRACT FROM AUTHOR]

Nearly all studies of impulsive geomagnetic disturbances (GMDs, also known as magnetic perturbation events MPEs) that can produce dangerous geomagnetically induced currents (GICs) have used data from the northern hemisphere. In this study, we investigated GMD occurrences during the first 6 months of 2016 at four magnetically conjugate high latitude station pairs using data from the Greenland West Coast magnetometer chain and from Antarctic stations in the conjugate AAL-PIP magnetometer chain. Events for statistical analysis and four case studies were selected from Greenland/AAL-PIP data by detecting the presence of >6 nT/s derivatives of any component of the magnetic field at any of the station pairs. For case studies, these chains were supplemented by data from the BAS-LPM chain in Antarctica as well as Pangnirtung and South Pole in order to extend longitudinal coverage to the west. Amplitude comparisons between hemispheres showed (a) a seasonal dependence (larger in the winter hemisphere), and (b) a dependence on the sign of the By component of the interplanetary magnetic field (IMF): GMDs were larger in the north (south) when IMF By was >0 (<0). A majority of events occurred nearly simultaneously (to within ±3 min) independent of the sign of By as long as |By| ≤ 2 |Bz|. As has been found in earlier studies, IMF Bz was <0 prior to most events. When IMF data from Geotail, Themis B, and/or Themis C in the near-Earth solar wind were used to supplement the time-shifted OMNI IMF data, the consistency of these IMF orientations was improved. [ABSTRACT FROM AUTHOR]