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Präzipitation von hochenergetischen Elektronen in die Atmosphäre: Eine Untersuchung anhand von Ballonmessungen und Modellstudien

Antragstellerin Dr. Miriam Sinnhuber
Fachliche Zuordnung Physik und Chemie der Atmosphäre
Förderung Förderung von 2020 bis 2023
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 429584791
 
Erstellungsjahr 2024

Zusammenfassung der Projektergebnisse

Starting point Electrons from the solar wind are accelerated in the radiation belts and magnetosheath during geomagnetic storms and auroral substorms. Depending on their pitch angle, some of these accelerated, energetic electrons can precipitate into the atmosphere at geomagnetic latitudes connecting to the radiation belts and auroral regions (broadly 55–80◦ of geomagnetic latitude). The altitude of precipitation depends on the particles energy, with auroral electrons, typically in the range of keV to tens of keV, mostly affecting the lower thermosphere and uppermost mesosphere above ≈80 km altitude, while radiation belt electrons can reach much higher energies of hundreds of keV to few MeV, and therefore affect lower altitudes down to the upper stratosphere. Dissociation and ionization of the most abundant species by collisions with the precipitating and secondary electrons lead to chains of ion and neutral reactions where radicals of the HOx (e.g., H, OH) and NOx (e.g., N, NO) chemical families are formed from the chemically inactive N2 and H2 O, and ozone is destroyed in catalytic cycles as a consequence. These processes are summarized in recent reviews [e.g., 15, 26, 27]. NOx is very long-lived in the dark middle atmosphere and can be transported down into the stratosphere during polar night. This so-called Energetic Electron Precipitaion EEP indirect effect [19] is observable at high latitudes in every polar winter [9] where observations reach into polar night. It leads to a significant ozone loss at the top of the stratospheric ozone layer [24], which in turn can start a chemical-radiative-dynamical forcing affecting atmospheric temperatures and dynamics down to tropospheric weather systems [e.g., 23, 21]. This geomagnetic forcing modulated by solar activity is included in chemistry-climate model experiments as part of the solar forcing of climate since CMIP 6 [13, 8]. However, it quickly became apparent that the recommended parameterized electron fluxes [16] and associated ionization rates [14, 17] are significantly lower than fluxes or ionization rates derived directly from observed fluxes, and consequently, the atmospheric impact is underestimated compared to observations as well [18, 25]. Due to the broad energy bins and large viewing angles of the telescopes the electron fluxes are commonly derived from1 , different approaches to derive ionization rates from these fluxes can lead to widely different ionization rates [17]. An additional source of uncertainty in deriving exact reproductions of the atmospheric impact with chemistry-climate models is the very complex D-region ion chemistry, which is neglected in most global models due to its large computational demands, but which can have a significant impact on, e.g., HNO3 [28] or active chlorine species [29, 7]. The aim of the HEPIC project was to address these issues by combining a long-term dataset of atmospheric ionization by precipitating electrons of relativistic (100 keV-few MeVs) energies observed since 1957 from a balloon-borne instrument at geomagnetic high (Apatity, Russia) and mid-latitudes (Moscow, Russia) with atmospheric models of different complexity and satellite observations of middle atmosphere trace gases. In particular, we wanted to address the following research questions: • What is the long-term impact of high-energy electron precipitation on the chemical composition of the middle atmosphere, and due to which processes? • What is the impact of the apparent underestimation of recommended ionization rates as suggested by the recent results on the composition, temperature and dynamics of the middle atmosphere? The balloon observations provide a measure of atmospheric ionization independent of the satellite observations commonly used in studies of the atmospheric impact. They were provided and analyzed by the Russian partners in St. Petersburg and Moscow. A one-dimensional ion chemistry model of the ion and neutral composition was used to assess the impact of the complex D-region ion chemistry with a focus on chlorine species and their additional impact on ozone, while full chemistry-climate models were used to assess the long-term impact of EEP on middle atmosphere composition, temperatures and dynamics. Dedicated model experiments were provided by KIT and the cooperation partners in Davos. Satellite data of trace gases were used assess the atmospheric impact of EEP and to evaluate model results. The selection and preparation of suitable satellite observations was the task of PhD students in St. Petersburg. 2.2 Progress and obstacles The project was granted in December 2019; the PhD position at KIT was filled in summer 2020, and Monali Borthakur started working on the project in November 2020. For the Russian partners, 1 POES/MEPED, ncei.noaa.gov/data/poes-metop-space-environment-monitor 3 the funding period started already with the beginning of the year 2020. In the first year of the project, a focus was therefore on the Workpackages (WPs) to be addressed by the Russian partners, on the analysis of the balloon data together with satellite observations to assess the atmospheric impact of EEP events (WPs 1 and 2, Section 2.3.1), and on the sensitivity of the derived ionization rates to varying input parameters (WP 1, see Sections 2.3.2). Model experiments were also carried out combining ionization rates derived from balloon observations, an assessment of the formation and loss of neutral species due to the resulting ion chemistry, and long-term impacts from a coupled chemistry-climate model, for a very unusual EEP event observed at geomagnetic midlatitudes in December 2009 (WPs 1, 2 and 3, Section 2.3.1). A project kick-off meeting was planned in March 2020, but had to be cancelled on very short notice when it became apparent that borders would be closed due to the Corona pandemic. After this, the cooperation was maintained by regular online meetings every ≈2–3 weeks. Three publications derived from this close cooperation: [4, 2, 3]. The cooperation was terminated in March 2022 after the Russian invasion of Ukraine. The main focus of the work at KIT was on model studies regarding the atmospheric impact of atmospheric ionization (WP 3), and as these could be carried out without input from the Russian partners, the project was continued at KIT until its regular end in November 2023. However, as the information about the EEP frequency and atmospheric ionization provided by the balloon observations was no longer available, the focus shifted to a more general assessment of the impact of atmospheric ionization on middle atmosphere composition and dynamics. Our ion-chemistry model ExoTIC was evaluated against satellite observations during a large solar proton event (Section 2.3.3). Preparation of the satellite observations was however originally the task of the Russian partners in St. Petersburg, but data of the MIPAS/ENVISAT instrument which were used for model evaluation are processed by another group at KIT2 , and this task could therefore be carried out by us. Model experiments were carried out with ExoTIC to assess the impact of chlorine ion chemistry on atmospheric composition and its impact on stratospheric and mesospheric ozone during large ionization events (Section 2.3.4). One publication derived from these studies [1]. To assess the impact of EEP on atmospheric temperatures and dynamics, a series of model experiments were designed and carried out with the high-top version of the EMAC interactive chemistry-radiationclimate model with different EUV and particle forcings during a sudden stratospheric warming (Section 2.3.5). About 280 ensemble members were calculated in total for the period January – February 2009. Due to the delays caused by redesigning the project, and as we had to take over some of the tasks of our former partners, the analysis of these model experiments continued after the official end of the project. First results are expected in the PhD thesis of Monali Borthakur likely in Autumn 2024. 2.

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