University of Oulu

Häkkilä, T., Verronen, P. T., Millán, L., Szeląg, M. E., Kalakoski, N., and Kero, A.: Odd hydrogen response thresholds for indication of solar proton and electron impact in the mesosphere and stratosphere, Ann. Geophys., 38, 1299–1312,, 2020.

Odd hydrogen response thresholds for indication of solar proton and electron impact in the mesosphere and stratosphere

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Author: Häkkilä, Tuomas1; Verronen, Pekka T.1,2; Millán, Luis3;
Organizations: 1Space and Earth Observation Centre, Finnish Meteorological Institute, Helsinki, Finland
2Sodankylä Geophysical Observatory, University of Oulu, Sodankylä, Finland
3Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 9.7 MB)
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Language: English
Published: Copernicus Publications, 2020
Publish Date: 2021-03-01


Understanding the atmospheric forcing from energetic particle precipitation (EPP) is important for climate simulations on decadal time scales. However, presently there are large uncertainties in energy flux measurements of electron precipitation. One approach to narrowing these uncertainties is by analyses of EPP direct atmospheric impacts and their relation to measured EPP fluxes. Here we use observations from the microwave limb sounder (MLS) and Whole Atmosphere Community Climate Model (WACCM) simulations, together with EPP fluxes from the Geostationary Operational Environmental Satellite (GOES) and Polar-orbiting Operational Environmental Satellite (POES) to determine the OH and HO₂ response thresholds to solar proton events (SPEs) and radiation belt electron (RBE) precipitation. Because of their better signal-to-noise ratio and extended altitude range, we utilize MLS HO₂ data from an improved offline processing instead of the standard operational product. We consider a range of altitudes in the middle atmosphere and all magnetic latitudes from pole to pole. We find that the nighttime flux limits for day-to-day EPP impact detection using OH and HO₂ are 50–130 protons cm⁻² s⁻¹ sr⁻¹ (E>10 MeV) and 1.0–2.5×104 electrons cm⁻² s⁻¹ sr⁻¹ (E = 100–300 keV). Based on the WACCM simulations, nighttime OH and HO₂ are good EPP indicators in the polar regions and provide best coverage in altitude and latitude. Due to larger background concentrations, daytime detection requires larger EPP fluxes and is possible in the mesosphere only. SPE detection is easier than RBE detection because a wider range of polar latitudes is affected, i.e., the SPE impact is rather uniform poleward of 60∘, while the RBE impact is focused at 60∘. Altitude-wise, the SPE and RBE detection are possible at ≈ 35–80 and ≈ 65–75 km, respectively. We also find that the MLS OH observations indicate a clear nighttime response to SPE and RBE in the mesosphere, similar to the simulations. However, the MLS OH data are too noisy for response detection in the stratosphere below 50 km, and the HO₂ measurements are overall too noisy for confident EPP detection on a day-to-day basis.

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Series: Annales geophysicae
ISSN: 0992-7689
ISSN-E: 1432-0576
ISSN-L: 0992-7689
Volume: 38
Issue: 6
Pages: 1299 - 1312
DOI: 10.5194/angeo-38-1299-2020
Type of Publication: A1 Journal article – refereed
Field of Science: 115 Astronomy and space science
Dataset Reference: MLS data are available from the NASA Goddard Space Flight Center Earth Sciences (GES) Data and Information Services Center (DISC;, last access: 17 December 2020, Livesey et al., 2018). All model data used are available on request from the corresponding author. CESM source code is distributed through a public subversion code repository (, last access: 17 December 2020, UCAR, 2020).
Copyright information: © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License.