Searching for Solar Storms in Greenland Ice Cores: Resolving the Controversy over the Use of Nitrate Spikes as Proxy Evidence for Solar Particle Events

Sun-to-Ice traces the behavior of solar energetic particles from their acceleration near the Sun's surface to their penetration of Earth's atmosphere. For this final part of the journey, simulations from the Whole Atmosphere Community Climate Model (WACCM) allow us to study the impact of solar particles on the physics and chemistry of Earth's atmosphere.

High energy particles from the Sun initiate a cascade of ionization and dissociation reactions in the atmosphere that produce nitric oxide (NO) and the hydroxyl radical (OH), enhancing the families of odd nitrogen (NOy = NO+NO2+NO3+2N2O5+HNO3+HO2NO2+ClONO2+BrONO2) and odd hydrogen (HOx = H+OH+HO2). Both of these chemical families participate in the catalytic destruction of stratospheric ozone and can affect atmospheric photochemistry, temperature, and winds.

Measurements of the full spectra of solar energetic particles have only been available since the launch of cosmic ray detectors aboard satellites in the 1960s. In previous decades, the intensity and energies of particles from solar storms were inferred through ground-based measurements of secondary particles from ion chamber, neutron monitors, and muon detectors. Prior to the 1930s, however, little is known about the size and frequency of solar particle events, aside from eyewitness accounts of sunspots and solar flares.

One controversial area of research links spikes in nitrate ions measured in polar ice cores to historical solar particle events. It is theorized that the production of NOy in the upper atmosphere ultimately leads to enhanced deposition of nitrate ions (NO3-) at the surface (Figure 1). Our goal through Sun-to-Ice has been to use the WACCM model to quantify the increase in nitrate expected during recent solar events and to identify transport processes that might lead to the rapid deposition of nitrate to the surface. In addition, we explore alternative explanations for the nitrate spikes in ice cores that do not involve solar particle events.

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The National Center for Atmospheric Research (NCAR) Whole Atmosphere Community Climate Model (WACCM) is an extension of the global, three dimensional Community Earth System Model (CESM), uniquely reaching upper atmospheric altitudes capable of capturing the impact of solar energetic particles on the atmospheric. Below are figures from a WACCM simulation of the 9 November 2000 Solar Proton Event (SPE). Figure 2 shows the enhancement of NOx (NOx = NO + NO2) and the reduction of O3 throughout the stratosphere and mesosphere following the event. NOx enhancements slowly descend within the polar vortex and persists at levels of 3-5 ppbv at ~30 km through March. Corresponding reductions of O3 are 5-10% background levels. Figure 3 demonstrates favorable comparison between WACCM calculated NO2 and POAM III satellite measurements during this timeframe.

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Because energetic protons streaming toward Earth travel along magnetic field lines, their atmospheric impact remains focused on the poles. Conditions during polar winter allow enhanced NOy to persist and descend within a stable polar vortex. Figure 4 provides an animation of a meridional cross section of zonally averaged NOx during this simulation. The animation clearly shows the production of NOx at the poles following the SPE event along with the descent of enhanced NOx within the Arctic polar vortex to lower altitudes (10 hPa is ~ 30 km; 1 hPa is ~ 50 km).

As NOx descends it is oxidized into other species in the odd nitrogen (NOy) family, primarily soluble HNO3 that can deposit to the surface through wet deposition (snowfall). Figure 5 shows the descent of total NOy following the 9 Nov 2000 SPE, with the center plot indicating SPE enhancement. The left plot shows background NOy levels in mole ratios and the plot to the right shows background NOy in number density. The 9 Nov 2000 SPE results in a thin ~5 km layer of 5-10 ppbv enhanced NOy around 25-30 km in comparison to the thick background pool of 10-15 ppbv NOy in the lower stratosphere, with the majority of background NOy in the stratosphere below the altitudes of SPE enhancement.

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Figure 6 presents an animation of the 9 Nov 2000 solar proton event enhancement of total column NOy within the polar vortex throughout the simulation. Total column NOy remains below 20%, suggesting that even if the total enhancement of NOy were deposited to the surface, the maximum nitrate spike seen in surface snow would not exceed 20%, much smaller than the 200-500% peaks in ice cores currently attributed to SPEs. The 9 November 2000 event did not produce enough atmospheric NOy to account for nitrate spikes observe in surface snow and ice.

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The WACCM simulations also allow us to quantify the effects of SPE events on stratospheric ozone through catalytic destruction involving the HOx and NOx families. Reductions in O3 can impact the radiative properties of the atmosphere, leading to changes in temperature that influence winds and global dynamics. Variations in stratospheric O3 can also influence the photochemistry of the lower atmosphere in sunlit regions. Figure 7 shows an animation of the reduction of total column O3 following the 9 Nov 2000 event, continuing through March 2001. The reductions in total column ozone remain below 5%, constrained primarily within the polar vortex throughout winter and mixing with lower latitudes in spring.

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Although the WACCM simulations fail to calculate enough NOy enhancements to account for nitrate spikes at the surface, results from the simulation of the 9 November 2000 suggests that polluted plumes might be able to explain sporadic enhancements of nitrate measured in snow and ice. Figure 8 shows an animation of NOy in the lower troposphere (850 mb, noting that WACCM uses sigma coordinates to adjust to higher elevations). Although the polar vortex tends to isolate air in upper atmosphere from lower latitudes, there is enough mixing in the troposphere to allow polluted plumes from North American and Europe to travel to ice core locations such as Summit, Greenland. Daily observations of a suite of ions in surface snow at Summit, Greenland (Figure 9) show that the majority of nitrate ion variation can be attributed to tropospheric sources such as pollution, biomass burning, sea salt, and dust.

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The WACCM modeling studies of recent solar particle events have failed to produce enough odd nitrogen in the atmosphere to associate nitrate spikes in snow and ice with high energy solar protons. Instead, these simulations suggest that observed fluctuations in nitrate in snow and ice can be explained by tropospheric sources and transport from lower latitudes. The next step for Sun-to-Ice will be to consider much larger solar particle events with higher energies, exploring the potential for events that might be capable of enhancing NOy directly in the troposphere, allowing more impulsive deposition of nitrate ions to the surface.

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