Incoming energetic particles (EPs), primarily protons and electrons, mediate some chemical and physical processes in the Earth’s atmosphere. Variations in the incoming flux of EPs can thus result in perturbations to the chemical partitioning of the stratosphere.
Though EPs arise from multiple origins, each source process produces a characteristic energy spectrum. Galactic cosmic rays (GCRs) are a highly energetic source of protons originating from outside the solar system, with characteristic energies between J — J per nucleon. Solar cosmic rays (SCRs) originating from coronal mass ejections have energy spectra which typically peak around J per nucleon. Fluxes of GCRs and SCRs are inversely correlated with 11-year periodicity. When the 11-year sunspot cycle is at a maximum, so too are SCR fluxes, while GCR fluxes are at a minimum due to pressure from solar winds.
Regardless of their origin, sufficiently energetic EPs ( J) will collide with atmospheric gases and produce charged secondary products, classified by their soft (electrons, positrons, and photons) and hard (muons, pions, etc.) components. These secondary products will then interact with atmospheric species in a variety of ways. One such scheme resulting in the production of NO is presented below.
The reaction pathway is initiated following interaction with energetic secondary electrons, denoted .
Subsequent recombination/exchange chemistry further enhances atomic nitrogen.
Finally, reaction of atomic nitrogen with molecular oxygen produces nitric oxide.
Similarly, EPP events may produce HO. For more details, Mironova et al. (2015) provide a very thorough review on the interaction of EPPs with the atmosphere.
Once formed, HO and NO will engage mesospheric ozone loss processes. A schematic for the HO-mediated destruction of ozone in the middle atmosphere is presented in figure 1 — and NO-mediated processes will be quite similar. Ozone loss processes produced in the immediate environment of EPP interactions are frequently called direct effects. Direct effects from NO and HO produced by EPPs are a significant source of variability in mesospheric ozone (upwards of 25% of mesospheric ozone).
Figure 1: A graph theory representation of the HOx-mediated catalytic destruction of ozone in the middle stratosphere. Klobas thesis 2018.
Mesospheric HO from EPPs is not long-lived and does not have effects beyond the immediate environment in which it is formed; however, NO has a long lifetime and will transport to the middle stratosphere, where it will further engage ozone loss processing. Ozone loss following transport from the region in which the NO was formed is referred to as an indirect effect of EPPs. Ozone losses from indirect effects are estimated to be as large as 15% in the middle and upper stratosphere.
The action of these events is largely confined to the polar regions because (1) EPs are entrained to geomagnetic fields which direct particle fluxes to the magnetic poles, and (2) mesospheric and stratospheric subsidence of NO occurs at the poles.
One of the most fascinating papers I’ve ever read on the potentially disastrous effects of EPPs concerns a seemingly unlikely situation: a magnetic field reversal of the Earth concurrent with the transit of the Solar System through a region of high interstellar density. The story goes like this:
The density of the interstellar medium is heterogeneous. If the solar system were to transit a region with higher density, enhanced rates of EP precipitation would be expected as the heliosphere contracts. The coincidence of both situations seems highly improbable at first glance; however, a review of the expected frequencies and durations of these events provides the evidence to the contrary. The solar system has transited regions of enhanced density ( H atoms/cm vs 0.3 H atoms/cm currently) 135 times in the previous 4.5 Gy and the average duration of a transit was 1 My. The Earth’s magnetic field reverses stochastically, however on average once every 300 ky, the event itself lasting several thousand years. A statistical treatment thus indicates that there may have been as many as 7 instances in the prior 250 My of a magnetic field reversal coinciding with the transit of the solar system through a region of enhanced density — and that EP fluxes would remain elevated for up to a period of thousands of years.
Such 1000-year periods of enhanced GCR precipitation are projected to lead to widespread, long-term depletions of the stratospheric ozone layer. Pavlov et al. (2005) use a 2-D dynamical/chemical model to evaluate the new steady-state ozone solution following a 300-fold enhancement of GCR, finding that total column ozone decreases by 40\% globally, and by up to 80% at high latitudes after a 5 — 10 year adjustment period.
It’s a completely insane story with extraordinarily dire consequences. The fact that it’s not only possible, but expected to occur on a basis as frequent as the collision of a 5 km asteroid (one third of a Chicxulub impactor), with 1000-year implications is alarming; while Bruce Willis might be able to save us from an incoming asteroid, there’s nothing we can do to restart the geomagnetic field or redirect the velocity of the Solar System through less dense space.
To put this all in context, the Chicxulub event is estimated to have perturbed the ozone layer, the climate, and pretty much everything on Earth for about a decade or so, but I’ll write a post about that another time..
Two papers recently appeared on my google scholar alerts on the topic of EPPs.
The first, by Andersson et al. (2018), provides an accounting of total chemical forcing from EPP processes on ozone, while exploring the effect of medium-energy electrons (MEE, 300 — 1000 keV), whose effect has only recently been incorporated into chemical models [WACCM, this paper, SOCOL in Arsenovic et al. (2016)]. Andersson and workers find that the inclusion of MEE results in a strong enhancement of indirect ozone processing rates as low as 30 km — accounting for up to 8% of the ozone depending on the pressure level.
The second, by Meraner and Schmidt (2018), explores the climate impact of the radiative forcing effects from EPP-induced ozone loss. The authors find that the winter stratosphere increases in temperature as a result of ozone loss and gradient-driven transport is subsequently reduced.This effect could slightly reduce the strength of the polar vortex, though they state that the modeled response is small and that the effect of EPPs on climate may be minimal. This finding was surprising to me as previous works (though with less-sophisticated models) predicted stratospheric cooling with resultant strengthening of the polar vortex and a much larger surface warming effect.