Search

Solar proton events (SPEs) cause large‐scale ionization in the middle atmosphere leading to ozone loss and changes in the energy budget of the middle atmosphere. The accurate implementation of SPEs and other particle ionization sources in climate models is necessary to understand the role of energetic particle precipitation in climate variability. We use riometer observations from 16 riometer stations and the Whole Atmosphere Community Climate Model with added Dregion ion chemistry (WACCM‐D) to study the spatial and temporal extent of cosmic noise absorption (CNA) during 62 SPEs from 2000 to 2005. We also present a correction method for the nonlinear response of observed CNA during intense absorption events. We find that WACCM‐D can reproduce the observed CNA well with some need for future improvement and testing of the used energetic particle precipitation forcing. The average absolute difference between the model and the observations is found to be less than 0.5 dB poleward of about 66° geomagnetic latitude, and increasing with decreasing latitude to about 1 dB equatorward of about 66° geomagnetic latitude. The differences are largest during twilight conditions where the modeled changes in CNA are more abrupt compared to observations. An overestimation of about 1° to 3° geomagnetic latitude in the extent of the CNA is observed due to the fixed proton cutoff latitude in the model. An unexplained underestimation of CNA by the model during sunlit conditions is observed at stations within the polar cap during 18 of the studied events. CNA during 62 SPEs was studied with WACCM‐D and riometer observationsCNA is modeled well on average and in individual events with a nonlinearity correction for high levels of CNAThe fixed proton cutoff latitude in WACCM‐D at 60 degrees leads to overestimation of the extent of the CNA, especially in small to moderate SPEs

While observed mesospheric polar nitric acid enhancements have been attributed to energetic particle precipitation through ion cluster chemistry in the past, this phenomenon is not reproduced in current whole‐atmosphere chemistry‐climate models. We investigate such nitric acid enhancements resulting from energetic electron precipitation events using a recently developed variant of the Whole Atmosphere Community Climate Model (WACCM) that includes a sophisticated ion chemistry tailored for the D‐layer of the ionosphere (50–90 km), namely, WACCM‐D. Using the specified dynamics mode, that is, nudging dynamics in the troposphere and stratosphere to meteorological reanalyses, we perform a 1‐year‐long simulation (July 2009–June 2010) and contrast WACCM‐D with the standard WACCM. Both WACCM and WACCM‐D simulations are performed with and without forcing from medium‐to‐high energy electron precipitation, allowing a better representation of the energetic electrons penetrating into the mesosphere. We demonstrate the effects of the strong particle precipitation events which occurred during April and May 2010 on nitric acid and on key ion cluster species, as well as other relevant species of the nitrogen family. The 1‐year‐long simulation allows the event‐related changes in neutral and ionic species to be placed in the context of their annual cycle. We especially highlight the role played by medium‐to‐high energy electrons in triggering ion cluster chemistry and ion‐ion recombinations in the mesosphere and lower thermosphere during the precipitation event, leading to enhanced production of nitric acid and raising its abundance by 2 orders of magnitude from 10−4to a few 10−2ppb. D‐layer ion chemistry enhances production of mesospheric nitric acid through ion cluster recombination with a seasonal maximum in winterMedium‐to‐high energy electrons precipitating into the mesosphere in April–May 2010 further enhance the formation of nitric acidOccurring at a time of year with low background abundance, the enhancement brought nitric acid abundance close to its annual cycle maximum

Recently, a model for medium‐energy (30–1000 keV) radiation belt‐driven electron precipitation (ApEEP) has been put forward for use in decadal to century‐long climate model runs as part of the Climate Modelling Intercomparison Project, phase 6 (CMIP6). The ApEEP model is based on directly observed precipitation data spanning 2002–2012 from the constellation of low‐Earth‐orbiting Polar Operational Environmental Satellites (POES). Here, we test the ApEEP model's ability using its magnetic local time variant, ApEEP_MLT, to accurately represent electron precipitation fluxes from the radiation belts during a large geomagnetic storm that occurred outside of the span of the development data set. In a study of narrowband subionospheric very low frequency (VLF) transmitter data collected during March 2015, continuous phase observations have been analyzed throughout the entire St. Patrick's Day geomagnetic storm period for the first time. Using phase data from the U.K. transmitter, call‐sign GVT (22.1 kHz), received in Reykjavik, Iceland, electron precipitation fluxes from L= 2.8 to 5.4 are calculated around magnetic local noon (12 MLT) and magnetic midnight (00 MLT). VLF‐inferred >30‐keV fluxes are similar to the equivalent directly observed POES fluxes. The ApEEP_MLT >30‐keV fluxes for L< 5.5 describe the overall St. Patrick's Day geomagnetic storm‐driven flux enhancement well, although they are a factor of 1.7 (1.3) lower than POES (VLF‐inferred) fluxes during the recovery phase. Such close agreement in >30‐keV flux levels during a large geomagnetic storm, using three different techniques, indicates this flux forcing are appropriate for decadal climate simulations for which the ApEEP model was created. Remarkably reliable subionospheric VLF transmitter phase measurements provide >30‐keV electron precipitation fluxes for March 2015VLF‐inferred >30‐keV electron precipitation fluxes are similar to the equivalent POES >30‐keV loss‐cone fluxes in the same regionCMIP6 > 30‐keV electron precipitation fluxes are only 1.3 times lower than the VLF‐inferred fluxes during the 2015 St. Patrick's Day storm

Energetic particle precipitation into the upper atmosphere creates excess amounts of odd nitrogen and odd hydrogen. These destroy mesospheric and upper stratospheric ozone in catalytic reaction chains, either in situ at the altitude of the energy deposition or indirectly due to transport to other altitudes and latitudes. Recent statistical analysis of satellite data on mesospheric ozone reveals that the variations during energetic electron precipitation from Earth's radiation belts can be tens of percent. Here we report model calculations of ozone destruction due to a single event of pulsating aurora early in the morning on 17 November 2012. The presence of high‐energy component in the precipitating electron flux (>200 keV) was detected as ionization down to 68 km altitude, by the VHF incoherent scatter radar of European Incoherent Scatter (EISCAT) Scientific Association (EISCAT VHF) in Tromsø, Norway. Observations by the Van Allen Probes satellite B showed the occurrence of rising tone lower band chorus waves, which cause the precipitation. We model the effect of high‐energy electron precipitation on ozone concentration using a detailed coupled ion and neutral chemistry model. Due to a 30 min, recorded electron precipitation event we find 14% odd oxygen depletion at 75 km altitude. The uncertainty of the higher‐energy electron fluxes leads to different possible energy deposition estimates during the pulsating aurora event. We find depletion of odd oxygen by several tens of percent, depending on the precipitation characteristics used in modeling. The effect is notably maximized at the sunset time following the occurrence of the precipitation. Pulsating aurora may contribute to solar cycle variability of polar mesospheric odd oxygenModeled odd oxygen depletion is 14% locally at 75–80 km due to a 30 min eventLonger and harder electron precipitation could deplete odd oxygen by several tens of percent