Magnetic local time dependency of radiation belt electron precipitation: impact on polar ozone

The radiation belts are regions in the near-Earth space where solar wind electrons are captured by the Earth’s magnetic field. A portion of these electrons is continuously lost into the atmosphere where they cause ionisation and chemical changes. Driven by solar activity, electron forcing leads to ozone variability in the polar regions. Understanding possible dynamical connections to regional climate is an on-going research activity which supports the assessment of greenhouse gas driven climate change by better definition of the solar-driven variability. In the context of the Coupled Model Intercomparison 5 Project Phase 6 (CMIP6), energetic electron and proton precipitation is included in the solar forcing recommendation for the first time. For radiation belt electrons, CMIP6 forcing is from a daily, zonal mean proxy model. This zonal mean model ignores the well-known dependency of precipitation on magnetic local time (MLT), i.e. its diurnal variability. Here we use the Whole Atmosphere Community Climate Model with lower ionospheric chemistry extension (WACCM-D) to study the effect of MLT dependency of electron forcing on the polar ozone response. We analyse simulations applying MLT-dependent and 10 MLT-independent forcings, and contrast ozone responses in monthly mean data as well as in monthly means of individual local time sectors. We consider two cases: 1) year 2003 and 2) extreme, long-duration forcing. Our results indicate that the ozone responses to MLT-dependent and MLT-independent forcings are very similar, and the differences found are small compared to those related to overall uncertainties in electron forcing. We conclude that electron forcing that ignores the MLT dependency will still provide an accurate ozone response in long-term climate simulations. 15

positive and negative ions and is designed for particle precipitation studies in the mesosphere and upper stratosphere (Verronen et al., 2016). This allows for detailed simulations of the ion-neutral chemistry interaction leading to HO x production, in contrast 60 to simple parameterizations that are typically used. We analyse monthly mean results as well as monthly averages at different local times, and discuss the differences in the ozone impact in the context of overall uncertainties in the MEE forcing.

Model and Simulations
Here we use CESM version 1.0.5 with WACCM-D, similar to the setup used by Andersson et al. (2016). Our WACCM version is 4, for more details on the model see Marsh et al. (2013). The model was run at 1.95 • ×2.5 • latitude×longitude resolution with 88 pressure levels between the ground and the top altitude of 6 × 10 −6 hPa (≈140 km). The specified dynamics configuration was used, i.e. surface pressure and horizontal winds and temperatures up to 50 km were taken from NASA's Modern-Era Retrospective Analysis for Research and Applications (MERRA) (Rienecker et al., 2011). Standard EPP input includes precipitation in the auroral regions by electrons with a characteristic energy of 2 keV and a Maxwellian energy distribution as well as ionization due to solar protons at energies between 1 and 300 MeV. In addition, we apply ionization due 70 to galactic cosmic rays from the NAIRAS model as described by Jackman et al. (2016).
For the radiation belt electron precipitation, we used the APEEP proxy model version 2 by van de Kamp et al. (2018). Fitted to satellite-based electron observations and using the geomagnetic Ap index as the sole driver, APEEP provides integrated electron fluxes above 30 keV energy and energy-flux gradients at McIlwain L shells between 2 and 10, i.e. between 44 • and 72 • of magnetic latitude. This latitude region is primarily influenced by electrons from the outer Van Allen radiation belt (e.g.

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Baker et al., 2018). APEEPv2 can output daily zonal averages or daily averages for eight MLT sectors. Compared to the earlier version 1 (van de Kamp et al., 2016), APEEPv2 applies a more conservative noise floor screening for satellite data and provides, in addition to daily zonal means, daily MLT-dependent output over eight three-hour sectors. The purpose of the APEEP models is to allow for multi-decadal climate simulations with electron forcing, e.g. APEEPv1 atmospheric ionization rates are included in the solar forcing recommendation of the Coupled Model Intercomparison Project Phase 6 (CMIP6), as described in Matthes 80 et al. (2017). For the purpose of this study, the new provision is MLT-dependent MEE ionization production rates. Figure 1 shows ionization rates at 88 km altitude from APEEPv2 for the period between 1998 and 2012. The variation with solar activity is clear, with the lowest ionization seen in 2009 during the solar minimum. The strongest ionization is in the declining phase of solar cycle, peaking in 2003. The ionization typically maximises at magnetic latitudes 60 • -70 • .
We performed WACCM-D simulations using three different APEEP ionization inputs: 1) no input (Zero), 2) APEEPv2 85 zonal mean (ZM), and 3) APEEPv2 MLT-dependent (MLT). Note that we calculated APEEPv2 zonal mean ionization from APEEPv2 MLT ionization to make sure that the daily total energy input is the same for simulations with input 2 and 3. Two

Results
Obviously, the MLT-dependent forcing produces results that should have differences to those from the ZM forcing if we looked at hourly output from WACCM-D. This particularly applies to species which have short chemical lifetimes, such as the ions.

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For some neutral species, like HO x , the APEEP-driven differences in production are partly masked by background diurnal variability of chemical production and loss, and are not seen as clearly as in the ionization rates shown in Figure 2.
However, since the APEEP models are designed to be used in multi-decadal climate simulations such as those conducted during CMIP6, it is more interesting to ask if the analysis of such simulations gives different answers if MLT-dependent APEEP forcing is applied. Typically, long climate simulations are analyzed using monthly mean data. Thus we concentrate on 105 WACCM-D monthly mean output first. Then, we also consider different local times (LSTs) separately from hourly output data saved separately. This would be similar to analysis of data from polar orbiting satellites, such measurements are typically made at limited local times for any given latitude.
Ozone changes caused by EPP have been suggested to drive the top-down dynamical coupling between middle atmosphere and troposphere. Thus we start our analysis directly with ozone, and then go on to ozone-affecting NO x and HO x . Polar cap 110 means shown in the following Sections were calculated as area-weighted (cosine-of-latitude scaling) averages at geographic latitudes 60 • −90 • . We concentrate more on the SH, because there geomagnetic latitudes span over a wider range of geographic latitudes than in the NH and thus cover a wider range of background conditions and diurnal variability, especially during winter. Thus we can expect that, overall, the MLT dependency of APEEP forcing should be more important factor in the SH atmosphere.  Figure 3a, the impact of MLT-dependent APEEP forcing on mesospheric ozone is strongest, with up to ≈10% depletion, between April and September at pressure levels between 0.1 and 0.01 hPa. The depletion is caused by increased HO x production. In the stratosphere, depletion up to ≈3% is seen from June to October, descending from 1 to 10 hPa. The stratospheric depletion and the descent are both caused by increased NO x 120 descending inside the polar vortex from production region in the mesosphere-lower thermosphere towards lower altitudes. The APEEP effects are moderate in October and November due to the major effect from the great Halloween solar proton event (e.g. Funke et al., 2011) which is included in all simulations. Above 0.01 hPa, the ozone effect becomes less consistent, i.e.
(e.g. Andersson et al., 2018). Figure 3b shows the impact of ZM APEEP forcing on ozone. The magnitude and extent of the response is clearly very similar to the response to MLT-dependent forcing shown in Figure 3a. For a more detailed view, Figure 3c shows the relative difference between simulations using the MLT and ZM APEEP forcing. APEEP Zero is used as a reference here, as it was in panels (a) and (b), so that the percentage numbers in the three panels are directly comparable. The main response patterns below 0.01 hPa 130 in panels (a) and (b) are not seen in panel (c), which indicates that applying MLT-dependency has little effect for the monthly mean ozone impact. Around the 0.01 hPa ozone minimum, there is a region with relative increases and decreases by a few percent.
In the following, we selected August 2003 for a closer study of effects at different LST. As seen in Figure (Figure 4d). From 0.1 to 0.01 hPa, the daytime depletion is at slightly lower altitude range than at night. However, this is mostly related to the diurnal variability of ozone concentration at these altitudes ( Figure 4d). Around 0.001 hPa, a few percent of increase is seen especially at nighttime. The increase comes from production of atomic oxygen, with a lower production from MLT-dependent forcing at noon-afternoon sectors. The APEEP contribution is also less important in the daytime when solar EUV dissociates oxygen molecules for ozone 145 production.
The response to ZM APEEP forcing is shown in Figure 4b, and it is again very similar to the response to MLT-dependent forcing. Figure 4c, shows the relative difference between simulations using the APEEP MLT and APEEP ZM forcing. Around 0.001 hPa, MLT forcing depletes 1-2% more ozone than ZM forcing at all LST. This is particularly seen in the early morning hours when MLT forcing produces more atomic oxygen than ZM. This effect reaches down to the 0.01 hPa ozone minimum.

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At 0.1-0.01 hPa, the MLT forcing adds to the ozone depletion by a few percent consistently at all LST compared to ZM, which is a minor difference when compared to the 7-15% impact seen in Panels (a) and (b) of Figure 4. Both MLT and ZM produce largest depletion from midnight to early morning. In the stratosphere, the differences are less than 1%. Figure 5 shows an example of our results from the Extreme Case, the data shown are monthly zonal means for February. Here we look at the MEE forcing region only, i.e. the mesosphere and above, because the three-month span of this simulation is not 155 long enough for NO x transport to cause full stratospheric effects. The ozone impacts below 0.1 hPa are thus small (not shown).
Although both hemispheres were equally forced with APEEP, except for the differences in the geographic extent of magnetic latitudes, the ozone effect is much clearer in the NH winter pole (Panels 5a and 5b) due to faster recovery in the summer pole through production driven by O 2 photodissociation. Depletion is seen at an altitude range between 0.01 and 0.5 hPa at latitudes 45 • −90 • degrees, with the strongest effect reaching 45% just below 0.01 hPa and North of 60 • . The depletion here is naturally 160 stronger than for the year 2003 because the extreme APEEP forcing was applied throughout the simulation. The response extends down to latitude ≈ 45 • , which is consistent with the extent of the APEEP forcing (see Figure 2). The simulations with ZM and MLT forcings give very similar results, and the differences are generally marginal in the range of a few percent, except in few small and isolated regions.

Odd nitrogen 165
Odd nitrogen (NO x ) chemical lifetime is days-to-months in the mesosphere-lower thermosphere, and NO x concentration can easily accumulate especially during polar winter conditions. Therefore, one would expect that a faithful representation of MLT dependency of APEEP forcing and NO x production is probably not crucial for the NO x distribution or NO x -driven ozone depletion in the upper stratosphere. Figure 6 shows the monthly zonal mean results for February from the Extreme Case. Largest increases, reaching up to and 170 beyond an order of magnitude, are seen between 0.1 and 0.001 hPa at polar latitudes, i.e. at the latitudes and altitudes where the APEEP forcing is applied. The increase extends from polar regions to all latitudes, the mid and low latitudes outside the forcing region showing a smaller but still > 100% impact in large regions. The SH effect is relatively stronger in magnitude than the NH effect due to the lower background concentration there. In the NH, the beginning effect of NO x descend inside the polar vortex extends the impact towards the stratopause. The MLT and ZM forcings produce again a very similar response, 175 in both magnitude and spatial extent, and the differences are small compared to the overall effect. However, the MLT forcing results in up to 1/10 less NO x in the peak response regions around 0.01 hPa than the ZM forcing, except at the very poles. At pressure levels below 0.05 hPa and above 0.003 hPa the relative differences are smaller.