Abstract

This thesis studies the effect of energetic electron precipitation (EEP) on the Earth’s atmosphere, focusing on the northern wintertime stratosphere. EEP is driven by the interaction between the Earth’s magnetosphere and solar wind, a plasma stream from the Sun. Energetic electrons precipitate constantly from the near-Earth space to the high-latitude upper atmosphere. In the northern hemisphere, EEP has been found to affect in the lower atmosphere during the winter, most notably on the polar vortex, a wind system dominating winter stratosphere. Variations in polar vortex also extend to the surface, where EEP has been observed to affect large-scale weather modes.

The aim of this thesis is to clarify the EEP effect on wintertime middle atmosphere and its dependence on atmospheric conditions. The EEP effect is also studied together with the other solar-related driver, solar radiation, and internal drivers of the atmosphere. This work utilizes newly calibrated and corrected satellite measurements of precipitating electron fluxes and surface measurements of geomagnetic activity to quantify EEP activity. The atmosphere is studied by using reanalysis datasets which are based on comprehensive observations and cover the whole globe from the surface to about 50 kilometer altitude. In this work we use different statistical methods to reliably distinguish the atmospheric variability related to EEP.

Results of this thesis confirm the earlier findings that EEP significantly enhances the northern polar vortex. We also show that EEP is the main solar-related driver of the wintertime northern stratosphere. The EEP effect on the polar vortex is found to be particularly significant if the quasi-biennial oscillation (QBO), a wind mode in the equatorial stratosphere, is in the easterly phase. A remarkable interdependence is revealed between the EEP effect on the polar vortex and an extreme event of wintertime stratosphere, the sudden stratospheric warming (SSW). This thesis shows that an SSW occurs almost in every northern winter in which EEP activity is low and QBO is in the easterly phase. Moreover, the EEP effect was found to be enhanced during the few weeks preceding SSWs which are characterized by increased activity of planetary waves, dynamical disturbances propagating from the surface. This thesis also confirms that the EEP effect depends directly on planetary waves and, especially, on their latitudinal distribution. These findings help to better understand the mechanism of the EEP effect and, thus, improve the modelling and forecasting of the northern wintertime atmosphere.

Osajulkaisut / Original papers

Osajulkaisut eivät sisälly väitöskirjan elektroniseen versioon. / Original papers are not included in the electronic version of the dissertation.

1. Salminen, A., Asikainen, T., Maliniemi, V., & Mursula, K. (2019). Effect of energetic electron precipitation on the northern polar vortex: Explaining the QBO modulation via control of meridional circulation. Journal of Geophysical Research: Atmospheres, 124(11), 5807-5821. https://doi.org/10.1029/2018JD029296

   Rinnakkaistallennettu versio / Self-archived version

2. Salminen, A., Asikainen, T., Maliniemi, V., & Mursula, K. (2020). Comparing the effects of solar-related and terrestrial drivers on the northern polar vortex. Journal of Space Weather and Space Climate, 10, 56. https://doi.org/10.1051/swsc/2020058

   Rinnakkaistallennettu versio / Self-archived version

3. Asikainen, T., Salminen, A., Maliniemi, V., & Mursula, K. (2020). Influence of enhanced planetary wave activity on the polar vortex enhancement related to energetic electron precipitation. Journal of Geophysical Research: Atmospheres, 125(9), e2019JD032137. https://doi.org/10.1029/2019JD032137

   Rinnakkaistallennettu versio / Self-archived version

4. Salminen, A., Asikainen, T., Maliniemi, V., & Mursula, K. (2020). Dependence of sudden stratospheric warmings on internal and external drivers. Geophysical Research Letters, 47(5), e2019GL086444. https://doi.org/10.1029/2019GL086444

   Rinnakkaistallennettu versio / Self-archived version

5. Salminen, A., Asikainen, T., & Mursula, K. (2022). Planetary waves controlling the effect of energetic electron precipitation on the northern polar vortex. Geophysical Research Letters, 49(6), e2021GL097076. https://doi.org/10.1029/2021GL097076

   Rinnakkaistallennettu versio / Self-archived version