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High‐energy electron precipitation from the radiation belts can penetrate deep into the mesosphere and increase the production rate of NOxand HOx, which in turn will reduce ozone in catalytic processes. The mechanisms for acceleration and loss of electrons in the radiation belts are not fully understood, and most of the measurements of the precipitating flux into the atmosphere have been insufficient for estimating the loss cone flux. In the present study the electron flux measured by the NOAA POES Medium Energy Proton and Electron Detectors 0° and 90° detectors is combined together with theory of pitch angle diffusion by wave‐particle interaction to quantify the electron flux lost below 120 km altitude. Using this method, 41 weak and moderate geomagnetic storms caused by corotating interaction regions during 2006–2010 are studied. The dependence of the energetic electron precipitation fluxes upon solar wind parameters and geomagnetic indices is investigated. Nine storms give increased precipitation of >∼750 keV electrons. Nineteen storms increase the precipitation of >∼300 keV electrons, but not the >∼750 keV population. Thirteen storms either do not change or deplete the fluxes at those energies. Storms that have an increase in the flux of electrons with energy >∼300 keV are characterized by an elevated solar wind velocity for a longer period compared to the storms that do not. Storms with increased precipitation of >∼750 keV flux are distinguished by higher‐energy input from the solar wind quantified by the ϵparameter and corresponding higher geomagnetic activity. Use a new and better estimation of loss cone flux to study energetic electron precipitation driven by CIRsA linear relationship is found between the energy input into the magnetosphere and precipitation of relativistic electronsPrecipitation patterns vary in MLT and latitude, attributed to different wave‐particle interactions and energy dependent drift periods more...

Medium energy electron (MEE) (30–1,000 keV) precipitation enhances the production of nitric (NOx) and hydrogen oxides (HOx) throughout the mesosphere, which can destroy ozone (O3) in catalytic reactions. The dynamical effect of the direct mesospheric O3reduction has long been an outstanding question, partly due to the concurrent feedback from the stratospheric O3reduction. To overcome this challenge, the Whole Atmosphere Community Climate Model version 6 is applied in the specified dynamics mode for the year 2010, with and without MEE ionization rates. The results demonstrate that MEE ionization rates can modulate temperature, zonal wind and the residual circulation affecting NOxtransport. The required fluxes of MEE to impose dynamical changes depend on the dynamical preconditions. During the Northern Hemispheric winter, even weak ionization rates can modulate the mesospheric signal of a sudden stratospheric warming event. The result provides a first step in a paradigm shift for the understanding of the MEE direct effect. Medium energy electrons (MEE) precipitate into the mesosphere, where they produce nitric (NOx) and hydrogen oxides (HOx), which are both associated with ozone (O3) loss. During polar winter, NOxis long‐lived and can be transported all the way down to the stratosphere, where it can lead to a stratospheric O3loss, which has the potential to change the dynamics of the atmosphere. The dynamical impact of the mesospheric O3loss is, however, unclear. To study the impact of MEE on the mesospheric dynamics, two Whole Atmosphere Community Climate Model runs are compared, with and without MEE ionization. This study shows that MEE have the potential to modulate the dynamics of the mesosphere directly. Furthermore, it shows that when the atmosphere is unstable, even weak ionization rates are able to modulate the signal of a sudden stratospheric warming in the mesosphere. Medium energy electrons can directly impact the mesospheric temperature and windsDuring a sudden stratospheric warming event, even weak geomagnetic activity can alter the mesospheric wind fieldsMesospheric O3decrease modulates gravity wave filtering and leads to a dynamical response during unstable atmospheric conditions Medium energy electrons can directly impact the mesospheric temperature and winds During a sudden stratospheric warming event, even weak geomagnetic activity can alter the mesospheric wind fields Mesospheric O3decrease modulates gravity wave filtering and leads to a dynamical response during unstable atmospheric conditions more...

Energetic particle precipitation (EPP) causes ionization of the main constituents of the Earth's atmosphere which leads to the production of nitric oxide (NO) throughout the polar mesosphere and lower thermosphere (MLT). Due to the long lifetime of NO during winter, it can also be transported deeper into the atmosphere by the mesospheric residual circulation (the indirect EEP effect). This study investigates the mesospheric indirect NO response to EEP using Whole Atmosphere Community Climate Model (WACCM) version 6. In comparison to observations from the instrument Solar Occultation For Ice Experiment (SOFIE) on the AIM (Aeronomy of Ice in the Mesosphere) satellite, a wintertime underestimation is found in the modeled mesospheric NO amount. WACCM's temperature profile is found to be vertically shifted compared to observations by SOFIE and by The Sounding of the Atmosphere using Broadband Emission Radiometry instrument on the Thermosphere Ionosphere Mesosphere Energetics Dynamics satellite (SABER). The discrepancies in NO are therefore attributed to the model's ability to simulate the dynamics responsible for the indirect EEP effect. The drivers of this transport are investigated by sensitivity runs of WACCM's gravity wave forcing. Changing the amplitude of the non‐orographic gravity waves and the Prandtl number improves the modeled vertical distribution of NO and temperature in the MLT region. Nitric oxide (NO) is produced by energetic particle precipitation in the polar regions of the mesosphere and lower thermosphere, and its distribution rapidly increases with altitude. During winter the lifetime of NO is long enough for it to be transported by the downward atmospheric circulation in the mesosphere far from where it was produced. This makes the level of NO in the mesosphere dependent on both the local production and on transport processes. When comparing simulated NO in the model Whole Atmosphere Community Climate Model (WACCM) to observations by the satellite instrument Solar Occultation For Ice Experiment, too little NO is found in the mesosphere even when the mesospheric production is accounted for in the model. This may be related to model errors in the transport from the thermosphere. Comparing model temperatures to those observed by two different satellite instruments confirms that there are some dynamic deficiencies. The mesospheric circulation is driven by breaking gravity waves, so by changing where and how the wave energy and momentum are deposited in WACCM, the modeled circulation can be altered to improve the vertical NO distribution and the temperature profile in the polar mesosphere and lower thermosphere. The best correspondence between modeled and observed nitric oxide and temperature is found when the amplitude of the gravity waves is reduced or when the eddy diffusion is increased. Whole Atmosphere Community Climate Model underestimates the mesospheric nitric oxide amount and the temperature profile is vertically displaced compared to SOFIE and SABER observationsChanging the non‐orographic gravity wave amplitude or Prandtl number modify where and how the modeled gravity wave energy and momentum are depositedReducing the gravity wave amplitude improves both the vertical displacement of the summer mesopause and the wintertime mesospheric nitric oxide level Whole Atmosphere Community Climate Model underestimates the mesospheric nitric oxide amount and the temperature profile is vertically displaced compared to SOFIE and SABER observations Changing the non‐orographic gravity wave amplitude or Prandtl number modify where and how the modeled gravity wave energy and momentum are deposited Reducing the gravity wave amplitude improves both the vertical displacement of the summer mesopause and the wintertime mesospheric nitric oxide level more...

Accurate specification of ionization production by energetic electron precipitation is critical for atmospheric chemistry models to assess the resultant atmospheric effects. Recent model‐observation comparison studies have increasingly highlighted the importance of considering precipitation fluxes in the full range of electron energy and pitch angle. However, previous parameterization methods were mostly proposed for isotropically precipitation electrons with energies up to 1 MeV, and the pitch angle dependence has not yet been parameterized. In this paper, we first characterize and tabulate the atmospheric ionization response to monoenergetic electrons with different pitch angles and energies between ∼3 keV and ∼33 MeV. A generalized method that fully accounts for the dependence of ionization production on background atmospheric conditions, electron energy, and pitch angle has been developed based on the parameterization method of Fang et al. (2010, https://doi.org/10.1029/2010GL045406). Moreover, we validate this method using 100 random atmospheric profiles and precipitation fluxes with monoenergetic and exponential energy distributions, and isotropic and sine pitch angle distributions. In a suite of 6,100 validation tests, the error in peak ionization altitude is found to be within 1 km in 91% of all the tests with a mean error of 2.7% in peak ionization rate and 1.9% in total ionization. This method therefore provides a reliable means to convert space‐measured precipitation energy and pitch angle distributions into ionization inputs for atmospheric chemistry models. We tabulate the atmospheric ionization response to monoenergetic beams of precipitating electrons with different pitch anglesWe report a method to derive the ionization profile under any Earth atmosphere condition by any electron energy and pitch angle distributionThis method provides a reliable means to convert space measurements of precipitation into ionization input in atmospheric chemistry modeling more...

Energetic electron precipitation (EEP) is an important source of polar nitrogen oxides (NOx) in the upper atmosphere. During winter, mesospheric NOxhas a long chemical lifetime and is transported to the stratosphere by the mean meridional circulation. Climate change is expected to accelerate this circulation and therefore increase polar mesospheric descent rates. We investigate the Southern Hemispheric polar NOxdistribution during the 21st century under a variety of future scenarios using simulations of the Whole Atmosphere Community Climate Model (WACCM). We simulate stronger polar mesospheric descent in all future scenarios that increase the atmospheric radiative forcing. Polar NOxin the upper stratosphere is significantly enhanced in two future scenarios with the largest increase in radiative forcing. This indicates that the ozone depleting NOxcycle will become more important in the future, especially if stratospheric chlorine species decline. Thus, EEP-related atmospheric effects may become more prominent in the future. Variable space weather injects energetic particles into the upper and middle atmosphere, where it induces chemical changes. Importantly, it enhances the amount of “odd nitrogen” species (i.e., N, NO, NO2, and often denoted NOx), which can last a long time in the high latitudinal middle atmosphere during winter. NOxis transported downward over the pole with prevailing winter winds. In the stratosphere, this NOxcan destroy ozone, which is the main absorber of UV radiation at those altitudes. Our model simulations show that under climate change the mean atmospheric circulation will accelerate in the future. Thus, we predict higher polar NOxconcentrations in the stratosphere as more NOxis transported downward from higher altitudes, even if the level of energetic particle activity remains similar to historical levels. Thus, we expect the atmospheric response to energetic particle precipitation to become more prominent in the future if climate change continues. Winter polar NOxbelow 1 hPa is directly modulated by solar proton events and indirectly by electron precipitation via vertical transportVertical descent in the polar mesosphere during southern winter is stronger in future scenarios with larger radiative forcingPolar stratospheric NOxincreases over the latter part of the 21st century, even when particle precipitation is similar to historical levels more...