Final Report Summary - EPPIC (Energetic Particle Precipitation Interconnection with Climate)
The aim of the work in this proposal was to follow the effects of energetic particle precipitation into the upper atmosphere, flowing down through the stratosphere, quantify the effect on tropospheric climate, and determine the signature of the forcing in observed climate variability. We did this as part of the ongoing study of non-anthropogenic forcing effects on the Earth's climate system. The influence of solar forcing on the atmosphere is still poorly understood. This is in part due to the multiple mechanisms that link solar output to the Earth's climate system. In the recent intergovernmental panel on climate change (IPCC) report most of the estimates of solar forcing were made using models driven by total solar irradiance. However, state-of-the-art research indicates that solar ultraviolet (UV) and energetic particle precipitation effects are significant in the stratosphere, and thus influencing the global circulation patterns of the Earth. In this project we described energetic particle forcing of the atmosphere as a result of the descent of nitric oxide into the stratosphere, to answer the questions about the influence of particle precipitation on global circulation patterns.
The project began by identifying robust signals in polar surface temperature maps which were associated with the variation in geomagnetic storm activity levels [Seppälä et al., 2009]. Temperature variations of ±4 K were detected in the polar regions in both the northern and southern hemispheres, and were strongest in the winter months. This initial work left two significant questions unanswered: 1) why is this climate forcing limited to the polar regions, and 2) why is it only seen during the winter? A strong candidate mechanism suggested itself, which was chemical modification of the upper atmosphere as a result of energetic particle precipitation generating odd nitrogen during geomagnetic storms. Because of the structure of the Earth's magnetic field and the fact that it guides energetic particles from the Sun and the Earth's radiation belts (the Van Allen belts) towards the northern and southern poles, chemical modification of the atmosphere by the generation of odd nitrogen is focused at the poles. Further, because odd nitrogen has a long lifetime (months) only during periods of darkness, it has a maximum effect during the polar night, and very limited effect during the polar summer. A final piece of the puzzle is that during the wintertime at the poles a strong polar vortex wind feature occurs at high altitudes, which generates large scale downward motion of the atmosphere, effectively transporting the long-lived odd nitrogen towards the lower atmosphere.
The work undertaken during the project focused on three primary areas of investigation. An initial area of investigation was to determine the levels of energetic particle precipitation that were entering the upper atmosphere. Several papers were published which used remote sensing instrumentation to either observe the changes in ionization levels produced by energetic particle precipitation [Clilverd et al., 2010; Rodger et al., 2010] or to directly observe changes in the amounts of high altitude odd nitrogen (NOx) produced during geomagnetic storms [Newnham et al., 2011].
A second area of investigation was in the effect that odd nitrogen could have on atmospheric ozone levels, which is a key factor in the temperature and dynamical balance of the whole atmosphere. The impact of energetic electron precipitation on ozone levels throughout a geomagnetic storm was determined in Rodger et al. [2010], while the impact of energetic proton precipitation on the production of odd nitrogen during two large solar proton events was determined in Verronen et al. [2011]. The transport of odd nitrogen from the mesosphere down to the stratosphere was investigated in Salmi et al. [2011]. The odd nitrogen in the event studied was generated by energetic electron precipitation at high altitude (~100 km) and the work determined the rate of descent of odd nitrogen, and the impact on stratospheric ozone levels that occurred as the descent happened.
The third area of investigation was the identification of the dynamical and atmospheric circulation changes that are induced by the chemical modification of the atmosphere by energetic particle precipitation. Baumgaertner et al. [2011] simulated the climate forcing observed by Seppälä et al. [2009] using a coupled climate model driven by large changes in the high altitude levels of odd nitrogen in order to simulate the effects of geomagnetic storms. Baumgaertner et al. [2011] was able to show that significant effects on the stratospheric circulation patterns occurred at the same time as the descent of odd nitrogen in the northern polar winter. As a culmination of the project, Seppälä et al. [2011] analysed the lower atmosphere responses during high geomagnetic activity levels, and found that winds, temperatures, and especially the propagation of atmospheric waves were altered, but primarily during the part of the 11-year solar cycle known as solar maximum.
The results of Seppälä et al. [2009, 2011] and the other papers generated by this project suggest that chemical modification of the atmosphere takes place almost continuously as a result of energetic particle precipitation. However, the region, the season, and the phase of the 11-year solar cycle are important factors in determining whether there is any significant influence of the energetic particle forcing on climate variability in the lower atmosphere.
The potential, including socio-economic impact, of the project is likely to be significant. The mechanism of energetic particle precipitation coupling to climate variability is one that is important for forecasts of polar wintertime temperature patterns, and will have particular emphasis for north western european wintertime climate. European winter conditions have been associated with geomagnetic activity levels in previous studies, and the work of this project has helped clarify a potential mechanism that explains the association, and allows more constrained forecasting to be undertaken.
The project began by identifying robust signals in polar surface temperature maps which were associated with the variation in geomagnetic storm activity levels [Seppälä et al., 2009]. Temperature variations of ±4 K were detected in the polar regions in both the northern and southern hemispheres, and were strongest in the winter months. This initial work left two significant questions unanswered: 1) why is this climate forcing limited to the polar regions, and 2) why is it only seen during the winter? A strong candidate mechanism suggested itself, which was chemical modification of the upper atmosphere as a result of energetic particle precipitation generating odd nitrogen during geomagnetic storms. Because of the structure of the Earth's magnetic field and the fact that it guides energetic particles from the Sun and the Earth's radiation belts (the Van Allen belts) towards the northern and southern poles, chemical modification of the atmosphere by the generation of odd nitrogen is focused at the poles. Further, because odd nitrogen has a long lifetime (months) only during periods of darkness, it has a maximum effect during the polar night, and very limited effect during the polar summer. A final piece of the puzzle is that during the wintertime at the poles a strong polar vortex wind feature occurs at high altitudes, which generates large scale downward motion of the atmosphere, effectively transporting the long-lived odd nitrogen towards the lower atmosphere.
The work undertaken during the project focused on three primary areas of investigation. An initial area of investigation was to determine the levels of energetic particle precipitation that were entering the upper atmosphere. Several papers were published which used remote sensing instrumentation to either observe the changes in ionization levels produced by energetic particle precipitation [Clilverd et al., 2010; Rodger et al., 2010] or to directly observe changes in the amounts of high altitude odd nitrogen (NOx) produced during geomagnetic storms [Newnham et al., 2011].
A second area of investigation was in the effect that odd nitrogen could have on atmospheric ozone levels, which is a key factor in the temperature and dynamical balance of the whole atmosphere. The impact of energetic electron precipitation on ozone levels throughout a geomagnetic storm was determined in Rodger et al. [2010], while the impact of energetic proton precipitation on the production of odd nitrogen during two large solar proton events was determined in Verronen et al. [2011]. The transport of odd nitrogen from the mesosphere down to the stratosphere was investigated in Salmi et al. [2011]. The odd nitrogen in the event studied was generated by energetic electron precipitation at high altitude (~100 km) and the work determined the rate of descent of odd nitrogen, and the impact on stratospheric ozone levels that occurred as the descent happened.
The third area of investigation was the identification of the dynamical and atmospheric circulation changes that are induced by the chemical modification of the atmosphere by energetic particle precipitation. Baumgaertner et al. [2011] simulated the climate forcing observed by Seppälä et al. [2009] using a coupled climate model driven by large changes in the high altitude levels of odd nitrogen in order to simulate the effects of geomagnetic storms. Baumgaertner et al. [2011] was able to show that significant effects on the stratospheric circulation patterns occurred at the same time as the descent of odd nitrogen in the northern polar winter. As a culmination of the project, Seppälä et al. [2011] analysed the lower atmosphere responses during high geomagnetic activity levels, and found that winds, temperatures, and especially the propagation of atmospheric waves were altered, but primarily during the part of the 11-year solar cycle known as solar maximum.
The results of Seppälä et al. [2009, 2011] and the other papers generated by this project suggest that chemical modification of the atmosphere takes place almost continuously as a result of energetic particle precipitation. However, the region, the season, and the phase of the 11-year solar cycle are important factors in determining whether there is any significant influence of the energetic particle forcing on climate variability in the lower atmosphere.
The potential, including socio-economic impact, of the project is likely to be significant. The mechanism of energetic particle precipitation coupling to climate variability is one that is important for forecasts of polar wintertime temperature patterns, and will have particular emphasis for north western european wintertime climate. European winter conditions have been associated with geomagnetic activity levels in previous studies, and the work of this project has helped clarify a potential mechanism that explains the association, and allows more constrained forecasting to be undertaken.