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Production and transport mechanisms of NO in observations and models
Stockholm University, Faculty of Science, Department of Meteorology .ORCID iD: 0000-0003-3679-6744
Stockholm University, Faculty of Science, Department of Meteorology .
2018 (English)In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 18, no 12, p. 9075-9089Article in journal (Refereed) Published
Abstract [en]

A reservoir of Nitric Oxide (NO) in the lower thermosphere efficiently cools the atmosphere after periods of enhanced geomagnetic activity. Transport from this reservoir to the stratosphere within the winter polar vortex allows NO to deplete ozone levels and thereby affect the middle atmospheric heat budget. As more climate models resolve the mesosphere and lower thermosphere (MLT) region, the need for an improved representation of NO related processes increases. This work presents a detailed comparison of NO in the Antarctic MLT region between observations made by the Solar Occultation for Ice Experiment (SOFIE) instrument onboard the Aeronomy of Ice in the Mesosphere (AIM) satellite and simulations performed by the Whole Atmosphere Community Climate Model with Specified Dynamics (SD-WACCM). We investigate 7 years of SOFIE observations and focus on the Southern hemisphere, rather than on dynamical variability in the Northern hemisphere or a specific geomagnetic perturbed event. The morphology of the simulated NO is in agreement with observations though the long term mean is too high and the short term variability is too low. Number densities are more similar during winter, though the altitude of peak densities, which reaches between 102–106 km in WACCM and between 98–104 km in SOFIE, is most separated during winter. Using multiple linear regressions and superposed epoch analyses we investigate how well the NO production and transport are represented in the model. The impact of geomagnetic activity is shown to drive NO variations in the lower thermosphere similarly across both datasets. The dynamical transport from the lower thermosphere into the mesosphere during polar winter is found to agree very well, with a descent rate of about 2.2 km/day in the 80–110 km region in both datasets. The downward transported NO fluxes are however too low in WACCM, which is likely due to medium energy electrons and D-region chemistry that are not represented in the model.

Place, publisher, year, edition, pages
2018. Vol. 18, no 12, p. 9075-9089
National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-152063DOI: 10.5194/acp-2017-1188ISI: 000436579500009OAI: oai:DiVA.org:su-152063DiVA, id: diva2:1176961
Available from: 2018-01-23 Created: 2018-01-23 Last updated: 2018-07-23Bibliographically approved
In thesis
1. Solar Forcing of Nitric Oxide in the Upper Atmosphere
Open this publication in new window or tab >>Solar Forcing of Nitric Oxide in the Upper Atmosphere
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The forcing of the Sun on Earth's atmosphere manifests itself via solar radiation and energetic particle precipitation (EPP), which variations are most noticeable in the upper regions of the atmosphere. A key species in the lower thermosphere, which is influenced by solar forcing, is nitric oxide (NO). An NO reservoir is present in the lower thermosphere, from which NO-rich air can be transported downward into the mesosphere and stratosphere, where it takes part in catalytic ozone destruction cycles. For climate models to correctly simulate the solar forcing on our climate, the processes of NO production and destruction, as well as the descent into the lower atmosphere, must be understood and accurately represented.

In this thesis, observations from the Solar Occultation For Ice Experiment (SOFIE) instrument onboard the Aeronomy of Ice in the Mesosphere (AIM) satellite are used to investigate temporal characteristics of NO in the mesosphere and lower thermosphere. We have developed a diagnostic method to determine the relative importance of the NO physical drivers throughout the lower thermosphere. The method shows that, at high latitudes, precipitating auroral electrons dominantly drive NO variations. Comparisons with NO measurements by the Student Nitric Oxide Experiment (SNOE), made almost a decade earlier, reveal that the impact of this forcing on NO appears to be invariant throughout the 11 year solar cycle.

On shorter timescales, we have shown a clear signature of the reoccurring 27 day geomagnetic impact on NO concentrations during summer and winter, with subsequent descent into the lower mesosphere during winter. The occurrence of medium energy electrons, which precipitate to mesospheric altitudes, results in a further increase of the descending NO flux. This complicates the determination of the relative contribution of the EPP direct and indirect effect on NO, i.e. separating direct NO production from downwards transported NO, respectively, in NO enhancements at a certain altitude. Using a full-range energy spectrum from the Polar-orbiting Operational Environmental Satellites (POES), we have been able to disentangle the direct and indirect EPP effect on Southern hemispheric NO during a geomagnetic storm in 2010.

Simulations of NO by the Whole Atmosphere Community Climate Model with Specified Dynamics (SD-WACCM) model reveal that the model predicts a too high climatological mean, while the short term variability is too low, as compared to SOFIE. However, even though the dynamical transport in both model and observations agrees very well, the descending NO fluxes are too low in the model.

In conclusion, the results of this thesis provide a better understanding of NO variability from an observational standpoint and will enable better model representations in the future.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University, 2018. p. 38
Keywords
nitric oxide, NO, mesosphere, lower thermosphere, MLT, energetic particle precipitation, EPP, medium energy electrons, MEE, SOFIE, SNOE, WACCM, POES
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-152065 (URN)978-91-7797-139-9 (ISBN)978-91-7797-140-5 (ISBN)
Public defence
2018-03-09, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2018-02-14 Created: 2018-01-23 Last updated: 2018-02-12Bibliographically approved

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