Heppa III Intercomparison Experiment on Electron Precipitation Impacts: 2. Model‐Measurement Intercomparison of Nitric Oxide (NO) During a Geomagnetic Storm in April 2010

Abstract

Energetic particles from the Sun or the terrestrial magnetosphere precipitate into the atmosphere at high geomagnetic latitudes. Due to magnetospheric shielding, protons from solar coronal mass ejections can mainly precipitate into the atmosphere inside the polar caps polewards of ∼60° geomagnetic latitude. Electrons accelerated in the magnetotail to energies ranging from a few keV to hundreds of keV in auroral substorms precipitate in the auroral region. Electrons trapped in the outer radiation belt can be accelerated to energies from a few tens of keV to several MeV in geomagnetic storms initiated by solar coronal mass ejections or high-speed solar wind streams, and precipitate at mid-to-high latitudes, equatorwards of the auroral ovals (see review and references in Mironova et al. [2015]; Sinnhuber & Funke [2019]). The penetration depth of the precipitating particles depends on their energies, with auroral electrons mainly affecting the lower thermosphere above ∼90 km, radiation belt electrons precipitating further down into the middle atmosphere, even into the upper stratosphere (1 MeV electrons to ≈50 km, e.g. Fang et al. [2010]).

HEPPA III Intercomparison Experiment on Electron Precipitation Impacts: 1. Estimated Ionization Rates During a Geomagnetic Active Period in April 2010

“…A robust recommendation concluding which of these eight ionization rate estimates provides the most realistic representation of MEE ionization requires an independent validation in the form of direct electron flux observations and/or observations of the atmospheric impact such as electron density, bremsstrahlung, cosmic radio noise absorption, or chemical changes. The latter will be limited by the accuracy of the observations, as well as the models used to estimate the impacted variables from the ionization as demonstrated in the companion paper Sinnhuber et al (2021). Due to inadequate pitch angle coverage, most of the current particle detectors in space are unsuitable for accurately determining the flux of MEE precipitating into the atmosphere.…”

“…It will enable quantitative studies of the importance the atmospheric impact, as well as an evaluation of the relative importance of MEE compared to other ionization sources, such as aurora, SPEs, solar flares (EUV) and galactic cosmic rays using the upper and lower bound. In the companion paper, Sinnhuber et al (2021), the validity of three of these ionization rate data-sets, ApEEP, AIMOS, and OULU, is evaluated by comparing the output of four chemistry-climate models to observe NO densities. There we find that the differences in the amount of NO in the individual models are much larger than the differences between the multi-models mean using different ionization rates, however, multi-model mean results are consistent with the differences between ionization rate data-sets used.…”

“…It includes variations due to solar cycle, season, time of day, latitude, as well as activity indices such as daily solar flux (F10.7) and geomagnetic activity ( Ap ). Full chemical‐dynamical atmospheric models can also be applied, for example, WACCM, HAMMONIA, EMAC, KASIMA etc (see the companion paper Sinnhuber et al., 2021). These can be used to provide the atmosphere for ionization rate calculations, or the ionization rates can be calculated self‐consistently within the atmospheric model where the model itself will respond to for example, increased Joule heating due to the calculated ionization rates or enhanced radiative cooling due to NO formation.…”

“…Furthermore, three operating spacecraft observing nitric oxide in the mesosphere enable validation of the ionization rates by comparison with observations of nitric oxide in the mesosphere as presented in the companion paper Sinnhuber et al. (2021).…”

“…The main purpose of Part 1 is to give an overview of the available ionization rates and their different properties, in order to understand the uncertainty in the associated MEE impact on the atmosphere. An in‐depth intercomparison of the MEE response in different atmospheric models is provided by the companion paper (Sinnhuber et al., 2021).…”