Mechanisms responsible for asymmetric aurora between the conjugate hemispheres
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The work presented in this thesis is part of ongoing research within one of the research groups at the Birkeland Centre for Space Science (BCSS) at the Department of Physics and Technology, University of Bergen. Members of this group have earlier found that the two hemispheres can exhibit large asymmetries in the auroral display, contrary to what have usually been assumed. The group at BCSS is focusing on aspects of the solar wind - magnetosphere - ionosphere interactions that can lead to a different outcome in the two polar hemispheres in terms of e.g. convection speeds, magnetic ﬁeld perturbations, electric currents, and auroral emissions.
As a part of this ongoing effort, this thesis investigates two mechanisms proposed to be important in creating north-south asymmetries in the auroral display. To do this, simultaneous images from space of both the aurora borealis and the aurora australis are used to identify periods and regions when asymmetric aurora occurs. During the work on this thesis, a systematic search was performed resulting in the hitherto largest dataset of such simultaneous global imaging of the Earth’s dual auroras.
In Paper I [Reistad et al., 2013] we ﬁnd that non-conjugate aurora, being features only visible, or signiﬁcantly brighter in one hemisphere, is rather common during active auroral displays. Furthermore, we found that 10 out of 15 identiﬁed non-conjugate features were consistent with at least one of the two mechanisms investigated in detail in this thesis.
Two mechanisms has been investigated in detail to learn more about their importance. In Paper II [Reistad et al., 2014] we ﬁnd that the poleward part of the dusk side auroral oval in the northern hemisphere is on average brighter when the Interplanetary Magnetic Field (IMF) is pointing away from the Sun, compared to when IMF is pointing toward the Sun, during southward directed IMF. The opposite result is found when looking at the dusk side auroral oval in the southern hemisphere. We interpret the results as a possible effect of the asymmetric Solar Wind (SW) dynamo, suggested to be responsible for asymmetric energy transfer from the SW into the conjugate hemispheres. This mechanism has been suggested to explain event studies of asymmetric aurora, but this is the ﬁrst time it is investigated statistically. Therefore, these results suggest that this mechanism can be of general importance and its accumulative effect of asymmetric energy transfer from the SW into the conjugate hemispheres might be important.
The second mechanism investigated is related to the dynamic effects from IMF By on how ﬁeld-lines convect from the nightside toward the dayside. It has earlier been suggested that IMF By could affect the two hemispheres differently but a detailed description has been lacking. Our group at BCSS has developed an updated understanding that can explain, with a higher level of detail than earlier, how the SW forcing during IMF By can lead to hemispheric asymmetries in convection speeds, Birkeland currents, and aurora along a closed ﬁeld-line. This is presented in Paper III [Tenfjord et al., 2015]. In Paper IV [Reistad et al., 2016] we show observations that support this understanding, where an event with highly displaced aurora between the hemispheres are presented, together with observations of asymmetric ionospheric convection and Birkeland currents, consistent with our understanding of the IMF By inﬂuence.
The main conclusions in this thesis are:
→ Non-conjugate features are common in active auroral displays
→ IMF Bx inﬂuences the duskside auroral intensity during southward IMF oppositely in the two hemispheres
→ The opposite auroral response to IMF Bx in the conjugate hemipsheres is consistent with a more efﬁcient SW dynamo on average in one hemisphere compared to the other
→ Field-lines with asymmetric footpoints (induced by IMF By) will release magnetic stress into one hemisphere as they convect toward and around the Earth from the nightside. We name this the process of restoring symmetry
→ Signatures consistent with the process of restoring symmetry are observed in both ionospheric convection data and in average maps of BCs, suggesting that this mechanism can be responsible for substantial asymmetric forcing of the two hemispheres
Paper I: J. P. Reistad, N. Østgaard, K. M. Laundal, and K. Oksavik, On the non-conjugacy of nightside aurora and their generator mechanisms, Journal of Geophysical Research, Vol. 118, doi:10.1002/jgra.50300, 2013. This article is not available in BORA. The published version is available at: 10.1002/jgra.50300
Paper II: J. P. Reistad, N. Østgaard, K. M. Laundal, S. Haaland, P. Tenfjord, K. Snekvik, K. Oksavik, and S. E. Milan, Intensity asymmetries in the dusk sector of the poleward auroral oval due to IMF Bx, Journal of Geophysical Research, Vol. 119, doi:10.1002/2014JA020216, 2014. The article is available in BORA at: http://hdl.handle.net/1956/10386
Paper III: P. Tenfjord, N. Østgaard, K. Snekvik, K. M. Laundal, J. P. Reistad, S. Haaland, and S. E. Milan, How the IMF By induces a By component in the closed magnetosphere and how it leads to asymmetric currents and convection patterns in the two hemispheres, Journal of Geophysical Research, Vol. 120, doi:10.1002/2015JA021579, 2015. The article is available in BORA at: http://hdl.handle.net/1956/11843
Paper IV: J. P. Reistad, N. Østgaard, P. Tenfjord, K. M. Laundal, K. Snekvik, S. Haaland, S. E. Milan, K. Oksavik, H. U. Frey, and A. Grocott, Dynamic effects of restoring footpoint symmetry on closed magnetic ﬁeld-lines, Journal of Geophysical Research, Vol 121, doi:10.1002/2015JA022058, 2016. The article is available in BORA at: http://hdl.handle.net/1956/12120