Studies of high-latitude ionospheric currents utilizing Swarm satellites
Workayehu, Abiyot Bires (2021-10-22)
https://urn.fi/URN:ISBN:9789526230481
Kuvaus
Tiivistelmä
Abstract
The work presented in this thesis studies the high-latitude ionospheric current systems in the North Hemisphere (NH) and South Hemisphere (SH). The main objective has been to statistically investigate the effect of geomagnetic activity, season and IMF directions on the currents, giving special emphasis on hemispheric asymmetry. The thesis makes use of magnetic field data measured by the European Space Agency (ESA) Swarm-A and -C satellites from both hemispheres. Based on the magnetic data, ionospheric currents have been estimated using the spherical elementary current system (SECS) method. The bootstrap statistical method was used to normalize the data from the two hemispheres as far as possible. The detailed results are as follows.
On average, the currents are larger in the NH than in the SH. Hemispheric asymmetry in the high-latitude ionospheric currents is larger during low geomagnetic activity (Kp < 2) than high geomagnetic activity (Kp ≥ 2), with NH/SH field-aligned current (FAC) ratio of 1.12 and 1.02. Asymmetry is also larger during local winter and autumn than local summer and spring, with stronger currents in the NH than in the SH.
The role of background ionospheric conductances on the hemispheric asymmetry in currents was studied. However, it does not show similar hemispheric asymmetry as the high-latitude ionospheric currents, indicating that solar illumination difference does not seem to be the reason for the asymmetry. It was also found that the ionospheric conductivity calculation using the IRI and MSISE models did not show the auroral oval, so the role of auroral precipitation could not be determined.
When making the statistical analysis for different IMF directions, it was found that the orientation of IMF has strong influence on the hemispheric asymmetry in the high-latitude currents, but this influence depends on local season. Hemispheric asymmetry in the high-latitude currents is larger for IMF By⁺ in NH (By⁻ in SH) than vice versa during both Bz⁺ (northward) and Bz⁻ (southward) IMF conditions. The strongest hemispheric asymmetry occurred in local winter and autumn for IMF By⁺ in NH (By⁻ in SH) afnd IMF Bz⁺ with NH/SH FAC ratio of about 1.18.
The role of electric field on the hemispheric asymmetry in high-latitude currents was studied using the cross polar cap potential (CPCP) difference values from the Super Dual Auroral Radar Network (SuperDARN) dynamic model. The results suggested that the convection electric field cannot explain the hemispheric asymmetry in the high-latitude ionospheric currents.
The statistical results also indicated that the sign of IMF By affects the latitudinal distribution and magnitude of auroral currents in a given hemisphere. This is known as the “explicit By effect ”. On average By⁺ in the NH and By⁻ in the SH causes larger currents than vice versa. The By effect on auroral currents in a given hemisphere during IMF Bz⁺ is in very good agreement with the By effect on the CPCP values, except during SH equinox and NH summer.
The factors and physical mechanisms causing the observed hemispheric asymmetries in the high-latitude ionospheric currents require still further investigations. In specific, the effect of auroral precipitation induced conductivities for the hemispheric asymmetry during different IMF conditions and different seasons should be studied by using measurements and modeling.
Notice
Printed version has incorrect ISBN:
ISBN (PDF) 978-952-62-3047-1 it should be 978-952-62-3048-1.
Huomautus
Painetussa virheellinen ISBN-tunnus:
ISBN (PDF) 978-952-62-3047-1 pitäisi olla 978-952-62-3048-1.
Original papers
Original papers are not included in the electronic version of the dissertation.
Workayehu, A. B., Vanhamäki, H., & Aikio, A. T. (2019). Field‐Aligned and Horizontal Currents in the Northern and Southern Hemispheres From the Swarm Satellite. Journal of Geophysical Research: Space Physics, 124(8), 7231–7246. https://doi.org/10.1029/2019ja026835
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(10), e2020JA028051. https://doi.org/10.1029/2020ja028051
Workayehu, A. B., Vanhamäki, H., Aikio, A. T., & Shepherd, S. G. (2021). Effect of Interplanetary Magnetic Field on Hemispheric Asymmetry in Ionospheric Horizontal and Field‐Aligned Currents During Different Seasons. Journal of Geophysical Research: Space Physics, 126(10), e2021JA029475. https://doi.org/10.1029/2021ja029475
Aikio, A. T., Vanhamäki, H., Workayehu, A. B., Virtanen, I. I., Kauristie, K., Juusola, L., Buchert, S., & Knudsen, D. (2018). Swarm Satellite and EISCAT Radar Observations of a Plasma Flow Channel in the Auroral Oval Near Magnetic Midnight. Journal of Geophysical Research: Space Physics, 123(6), 5140–5158. https://doi.org/10.1029/2018ja025409
Osajulkaisut
Osajulkaisut eivät sisälly väitöskirjan elektroniseen versioon.
Workayehu, A. B., Vanhamäki, H., & Aikio, A. T. (2019). Field‐Aligned and Horizontal Currents in the Northern and Southern Hemispheres From the Swarm Satellite. Journal of Geophysical Research: Space Physics, 124(8), 7231–7246. https://doi.org/10.1029/2019ja026835
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(10), e2020JA028051. https://doi.org/10.1029/2020ja028051
Workayehu, A. B., Vanhamäki, H., Aikio, A. T., & Shepherd, S. G. (2021). Effect of Interplanetary Magnetic Field on Hemispheric Asymmetry in Ionospheric Horizontal and Field‐Aligned Currents During Different Seasons. Journal of Geophysical Research: Space Physics, 126(10), e2021JA029475. https://doi.org/10.1029/2021ja029475
Aikio, A. T., Vanhamäki, H., Workayehu, A. B., Virtanen, I. I., Kauristie, K., Juusola, L., Buchert, S., & Knudsen, D. (2018). Swarm Satellite and EISCAT Radar Observations of a Plasma Flow Channel in the Auroral Oval Near Magnetic Midnight. Journal of Geophysical Research: Space Physics, 123(6), 5140–5158. https://doi.org/10.1029/2018ja025409
Kokoelmat
- Avoin saatavuus [37957]