University of Oulu

Workayehu, A. B., Vanhamäki, H., & Aikio, A. T. (2020). Seasonal effect on hemispheric asymmetry in ionospheric horizontal and field‐aligned currents. Journal of Geophysical Research: Space Physics, 125, e2020JA028051.

Seasonal effect on hemispheric asymmetry in ionospheric horizontal and field‐aligned currents

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Author: Workayehu, A. B.1; Vanhamäki, H.1; Aikio, A. T.1
Organizations: 1Space Physics and Astronomy Research Unit, University of Oulu, Finland
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 11.5 MB)
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Language: English
Published: American Geophysical Union, 2020
Publish Date: 2021-03-24


We present a statistical investigation of the seasonal effect on hemispheric asymmetry in the auroral currents during low (Kp < 2) and high (Kp ≥ 2) geomagnetic activity. Five years of magnetic data from the Swarm satellites has been analyzed by applying the spherical elementary current system (SECS) method. Bootstrap resampling has been used to remove the seasonal differences between the hemispheres in the data set. In general, the currents are larger in the Northern Hemisphere (NH) than in the Southern Hemisphere (SH). Asymmetry is larger during low than high Kp and during local winter and local autumn than local summer and local spring. For all Kp conditions together, the NH/SH ratio for FACs in winter, autumn, spring, and summer are 1.17 ± 0.05, 1.14 ± 0.05, 1.07 ± 0.04, and 1.02 ± 0.04, respectively. The largest asymmetry is observed during low Kp in local winter, when the excess in the NH currents is 21 ± 5% in FAC, 14 ± 3% in curl‐free (CF) and 10 ± 3% in divergence‐free (DF) current. We also find that evening sector (13–24 MLT) contributes more to the high NH/SH ratio than the morning (01–12 MLT) sector. The physical mechanisms producing the hemispheric asymmetry are not presently understood. We calculated the solar‐induced ionospheric conductances during low Kp conditions from the IRI model. The model conductance NH/SH ratios are above 1 in autumn and spring, similar to the currents, but below 1 for winter, which is in contradiction with the currents. Therefore, we do not consider solar‐induced conductances as the main explanation for hemispheric asymmetry.

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Series: Journal of geophysical research. Space physics
ISSN: 2169-9380
ISSN-E: 2169-9402
ISSN-L: 2169-9380
Volume: 125
Issue: 10
Article number: e2020JA028051
DOI: 10.1029/2020JA028051
Type of Publication: A1 Journal article – refereed
Field of Science: 115 Astronomy and space science
Funding: This work was supported by the Academy of Finland (projects 314664 and 285474).
Academy of Finland Grant Number: 314664
Detailed Information: 314664 (Academy of Finland Funding decision)
285474 (Academy of Finland Funding decision)
Copyright information: © 2020. American Geophysical Union.