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.
Much theoretical and observational work has been devoted to studying the occurrence of
- NSF-PAR ID:
- 10449290
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 46
- Issue:
- 19
- ISSN:
- 0094-8276
- Page Range / eLocation ID:
- p. 10726-10734
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The polar
F region ionosphere frequently exhibits sporadic variability (e.g., Meek, 1949,https://doi.org/10.1029/JZ054i004p00339 ; Hill, 1963,https://doi.org/10.1175/1520‐0469(1963)020<0492:SEOLII>2.0.CO;2 ). Recent satellite data analysis (Noja et al., 2013,https://doi.org/10.1002/rds.20033 ; Chartier et al., 2018,https://doi.org/10.1002/2017JA024811 ) showed that the high‐latitudeF region ionosphere exhibits sporadic enhancements more frequently in January than in July in both the northern and southern hemispheres. The same pattern has been seen in statistics of the degradation and total loss of GPS service onboard low‐Earth orbit satellites (Xiong et al. 2018,https://doi.org/10.5194/angeo‐36‐679‐2018 ). Here, we confirm the existence of this annual pattern using ground GPS‐based images of TEC from the MIDAS algorithm. Images covering January and July 2014 confirm that the high‐latitude (>70 MLAT)F region exhibits a substantially larger range of values in January than in July in both the northern and southern hemispheres. The range of TEC values observed in the polar caps is 38–57 TECU (north‐south) in January versus 25–37 TECU in July. First‐principle modeling using SAMI3 reproduces this pattern, and indicates that it is caused by an asymmetry in plasma levels (30% higher in January than in July across both polar caps), as well as 17% longer O+plasma lifetimes in northern hemisphere winter, compared to southern hemisphere winter. -
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Abstract Energetic particles of magnetospheric origin constantly strike the Earth’s upper atmosphere in the polar regions, producing optical emissions known as the aurora. The most spectacular auroral displays are associated with recurrent events called magnetospheric substorms (aka auroral substorms). Substorms are initiated in the nightside magnetosphere on closed magnetic field lines. As a consequence, it is generally thought that auroral substorms should occur in both hemispheres on the same field line (i.e., magnetically conjugated). However, such a hypothesis has not been verified statistically. Here, by analyzing 2659 auroral substorms acquired by the Ultraviolet Imager on board the NASA satellite “Polar”, we have discovered surprising evidence that the averaged location for substorm onsets is not conjugate but shows a geographic preference that cannot be easily explained by current substorm theories. In the Northern Hemisphere (NH) the auroral substorms occur most frequently in Churchill, Canada (~90°W) and Khatanga, Siberia (~100°E), up to three times as often as in Iceland (~22°W). In the Southern Hemisphere (SH), substorms occur more frequently over a location in the Antarctic ocean (~120°E), up to ~4 times more than over the Antarctic Continent. Such a large difference in the longitudinal distribution of north and south onset defies the common belief that substorms in the NH and SH should be magnetically conjugated. A further analysis indicates that these substorm events occurred more frequently when more of the ionosphere was dark. These geographic areas also coincide with regions where the Earth’s magnetic field is largest. These facts suggest that auroral substorms occur more frequently, and perhaps more intensely, when the ionospheric conductivity is lower. With much of the magnetotail energy coming from the solar wind through merging of the interplanetary and Earth’s magnetic field, it is generally thought that the occurrence of substorms is externally controlled by the solar wind and plasma instability in the magnetotail. The present study results provide a strong argument that the ionosphere plays a more active role in the occurrence of substorms.
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Abstract In this paper, we present a case study of the radial interplanetary magnetic field (IMF
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