Nitric oxide (NO) is produced by energetic particle precipitation (EPP) in the mesosphere‐lower thermosphere (MLT) region, and during the polar winter, NO can descend to stratospheric altitudes where it destroys ozone. In this paper, we study the general scenario, as opposed to a case study, of NO production in the thermosphere due to energetic particles in the auroral region. We first investigate the relationship between NO production and two geomagnetic indices. The analysis indicates that the auroral electrojet index is a more suitable proxy for EPP‐produced NO than the typically used midlatitude Ap index. In order to study the production and downward transport of NO from the lower thermosphere to the mesosphere, we perform superposed epoch analyses on NO observations made by the Solar Occultation For Ice Experiment instrument on board the Aeronomy of Ice in the Mesosphere satellite. The epoch analysis clearly shows the impact of the 27 day solar cycle on NO production. The effect is observed down to an altitude range of about 50 km to 65 km, depending on the hemisphere and the occurrence of stratospheric warmings. Initially, a rapid downward transport is noted during the first 10 days after EPP onset to an altitude of about 80–85 km, which is then followed by a slower downward transport of approximately 1–1.2 km/d to lower mesospheric altitudes in the order of 30 days.

Energetic electrons from the magnetosphere deposit their energy in the atmosphere and lead to production of nitric oxide (NO) in the mesosphere and lower thermosphere. We study the atmospheric NO response to a geomagnetic storm in April 2010 with WACCM (Whole Atmosphere Community Climate Model). Modeled NO is compared to observations by Solar Occultation For Ice Experiment/ Aeronomy of Ice in the Mesosphere at 72-82 ∘ S latitudes. We investigate the modeled NOs sensitivity to changes in energy and chemistry. The electron energy model input is either a parameterization of auroral electrons or a full range energy spectrum (1-750 keV) from National Oceanic and Atmospheric Administration/Polar Orbiting Environmental Satellites and European Organisation for the Exploitation of Meteorological Satellites/Meteorological Operational satellites. To study the importance of ion chemistry for the production of NO, WACCM-D, which has more complex ion chemistry, is used. Both standard WACCM and WACCM-D underestimate the storm time NO increase in the main production region (90-110 km), using both electron energy inputs. At and below 80 km, including medium-energy electrons (>30 keV) is important both for NO directly produced at this altitude region and for NO transported from other regions (indirect effect). By using WACCM-D the direct NO production is improved, while the indirect effects on NO suffer from the downward propagating deficiency above. In conclusion, both a full range energy spectrum and ion chemistry is needed throughout the mesosphere and lower thermosphere region to increase the direct and indirect contribution from electrons on NO.

Abstract. 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 on-board 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 8 years of SOFIE observations, covering the period 2007–2015, and focus on the Southern Hemisphere (SH), rather than on dynamical variability in the Northern Hemisphere (NH) 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 in the thermosphere. Number densities are more similar during winter, though the altitude of peak NO density, which reaches between 102 and 106 km in WACCM and between 98 and 104 km in SOFIE, is most separated during winter. Using multiple linear regression (MLR) and superposed epoch analysis (SEA) methods, 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−1 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 (MEE) and D-region ion chemistry that are not represented in the model.

In April 2010, a coronal mass ejection and a corotating interaction region on the Sun resulted in an energetic electron precipitation event in the Earth's atmosphere. We investigate direct and indirect nitric oxide (NO) response to the electron precipitation. By combining electron fluxes from the Total Energy Detector and the Medium Energy Proton and Electron Detector on the National Oceanic and Atmospheric Administration's Polar‐orbiting Operational Environmental Satellites, we obtain a continuous energy spectrum covering 1–750 keV. This corresponds to electrons depositing their energy at atmospheric altitudes 60–120 km. Based on the electron energy deposition, taking into account loss due to photolysis, the accumulated NO number density is estimated. When compared to NO measured at these altitudes by the Solar Occultation for Ice Experiment instrument on board the Aeronomy of Ice in the Mesosphere satellite, the NO direct effect was detected down to 55 km. The main variability at these altitudes is, however, dominated by the indirect effect, which is downward transported NO. We estimate the source of this descending NO to be in the upper mesosphere at ∼75–90 km.

Nitric oxide (NO) produced in the polar middle and upper atmosphere by energetic particle precipitation depletes ozone in the mesosphere and, following vertical transport in the winter polar vortex, in the stratosphere. Medium‐energy electron (MEE) ionization by 30–1,000 keV electrons during geomagnetic storms may have a significant role in mesospheric NO production. However, questions remain about the relative importance of direct NO production by MEE at altitudes ~60–90 km versus indirect NO originating from auroral ionization above 90 km. We investigate potential drivers of NO variability in the southern‐hemisphere mesosphere and lower thermosphere during 2013–2014. Contrasting geomagnetic activity occurred during the two austral winters, with more numerous moderate storms in the 2013 winter. Ground‐based millimeter‐wave observations of NO from Halley, Antarctica, are compared with measurements by the Solar Occultation For Ice Experiment (SOFIE) spaceborne spectrometer. NO partial columns over the altitude range 65–140 km from the two observational data sets show large day‐to‐day variability and significant disagreement, with Halley values on average 49% higher than the corresponding SOFIE data. SOFIE NO number densities, zonally averaged over geomagnetic latitudes −59° to −65°, are up to 3 × 108/cm3 higher in the winter of 2013 compared to 2014. Comparisons with a new version of the Whole Atmosphere Community Climate Model, which includes detailed D‐region ion chemistry (WACCM‐SIC) and MEE ionization rates, show that the model underestimates NO in the winter lower mesosphere whereas thermospheric abundances are too high. This indicates the need to further improve and verify WACCM‐SIC with respect to MEE ionization, thermospheric NO chemistry, and vertical transport.