Search for: All records

The Great Oxidation Event was a period during which Earth’s atmospheric oxygen (O2) concentrations increased from ∼10−5 times its present atmospheric level (PAL) to near modern levels, marking the start of the Proterozoic geological eon 2.4 billion years ago. Using WACCM6, an Earth System Model, we simulate the atmosphere of Earth-analogue exoplanets with O2 mixing ratios between 0.1 and 150 per cent PAL. Using these simulations, we calculate the reflection spectra over multiple orbits using the Planetary Spectrum Generator. We highlight how observer angle, albedo, chemistry, and clouds affect the simulated observations. We show that inter-annual climate variations, as well short-term variations due to clouds, can be observed in our simulated atmospheres with a telescope concept such as LUVOIR or HabEx. Annual variability and seasonal variability can change the planet’s reflected flux (including the reflected flux of key spectral features such as O2 and H2O) by up to factors of 5 and 20, respectively, for the same orbital phase. This variability is best observed with a high-throughput coronagraph. For example, HabEx (4 m) with a starshade performs up to a factor of two times better than a LUVOIR B (6 m) style telescope. The variability and signal-to-noise ratio of some spectral features depends non-linearly on atmospheric O2 concentration. This is caused by temperature and chemical column depth variations, as well as generally increased liquid and ice cloud content for atmospheres with O2 concentrations of <1 per cent PAL.

Energetic electron precipitation leads to increased nitric oxide (NO) production in the mesosphere and lower thermosphere. NO distributions in the wintertime, high‐latitude Southern Hemisphere atmosphere during geomagnetic storms are investigated. NO partial columns in the upper mesosphere at altitudes 70–90 km and in the lower thermosphere at 90–110 km have been derived from observations made by the Solar Occultation For Ice Experiment (SOFIE) on board the Aeronomy of Ice in the Mesosphere (AIM) satellite. The SOFIE NO measurements during 17 geomagnetic storms in 2008–2014 have been binned into selected geomagnetic latitude and geographic latitude/longitude ranges. The regions above Antarctica showing the largest instantaneous NO increases coincide with high fluxes of 30–300 keV precipitating electrons from measurements by the second‐generation Space Environment Monitor (SEM‐2) Medium Energy Proton and Electron Detector (MEPED) instrument on the Polar‐orbiting Operational Environmental Satellites (POES). Significant NO increases over the Antarctic Peninsula are likely due to precipitation of >30 keV electrons from the radiation belt slot region. NO transport is estimated using Horizontal Wind Model (HWM14) calculations. In the upper mesosphere strong eastward winds (daily mean zonal wind speed ~20–30 m s⁻¹at 80 km) during winter transport NO‐enriched air away from source regions 1–3 days following the storms. Mesospheric winds also introduce NO‐poor air into the source regions, quenching initial NO increases. Higher up, in the lower thermosphere, weaker eastward winds (~5–10 m s⁻¹at 100 km) are less effective at redistributing NO zonally.

This study considers the impact of electron precipitation from Earth's radiation belts on atmospheric composition using observations from the NASA Van Allen Probes and NSF Focused Investigations of Relativistic Electron Burst Intensity, Range, and Dynamics (FIREBIRD II) CubeSats. Ratios of electron flux between the Van Allen Probes (in near‐equatorial orbit in the radiation belts) and FIREBIRD II (in polar low Earth orbit) during spacecraft conjunctions (2015–2017) allow an estimate of precipitation into the atmosphere. Total Radiation Belt Electron Content, calculated from Van Allen Probes RBSP‐ECT MagEIS data, identifies a sustained 10‐day electron loss event in March 2013 that serves as an initial case study. Atmospheric ionization profiles, calculated by integrating monoenergetic ionization rates across the precipitating electron flux spectrum, provide input to the NCAR Whole Atmosphere Community Climate Model in order to quantify enhancements of atmospheric HO_xand NO_xand subsequent destruction of O₃in the middle atmosphere. Results suggest that current APEEP parameterizations of radiation belt electrons used in Coupled Model Intercomparison Project may underestimate the duration of events as well as higher energy electron contributions to atmospheric ionization and modeled NO_xconcentrations in the mesosphere and upper stratosphere.

This paper discusses the solar cycle variation of the DE3 and DE2 nonmigrating tides in the nitric oxide (NO) 5.3 μm and carbon dioxide (CO₂) 15 μm infrared cooling between 100 and 150 km altitude and ±40° latitude. Tidal diagnostics of SABER NO and CO₂cooling rate data (2002–2013) indicate DE3 (DE2) amplitudes during solar maximum are on the order of 1 (0.5) nW/m³in NO near 125 km, and on the order of 60 (30) nW/m³in CO₂at 100 km, which translates into roughly 15–30% relative to the monthly zonal mean. The NO cooling shows a pronounced (factor of 10) solar cycle dependence (lower during solar minimum) while the CO₂cooling does not vary much from solar min to solar max. Photochemical modeling reproduces the observed solar cycle variability and allows one to delineate the physical reasons for the observed solar flux dependence of the tides in the infrared cooling, particularly in terms of warmer/colder background temperature versus smaller/larger tidal temperatures during solar max/min, in addition to cooling rate variations due to vertical tidal advection and tidal density variations. Our results suggest that (i) tides caused by tropospheric weather impose a substantial—and in the NO 5.3 μm case solar cycle dependent—modulation of the infrared cooling, mainly due to tidal temperature, and (ii) observed tides in the infrared cooling are a suitable proxy for tidal activity including its solar cycle dependence in a part of Earth's atmosphere where direct global temperature observations are lacking.

The capability to forecast conditions in the mesosphere and lower thermosphere is investigated based on 30‐day hindcast experiments that were initialized bimonthly during 2009 and 2010. The hindcasts were performed using the Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (WACCMX) with data assimilation provided by the Data Assimilation Research Testbed (DART) ensemble Kalman filter. Analysis of the WACCMX+DART hindcasts reveals several important features that are relevant to forecasting the middle atmosphere. The results show a clear dependence on spatial scale, with the slowest error growth occurring in the zonal mean and the fastest error growth occurring for small‐scale waves. The error growth rate is also found to be significantly greater in the upper mesosphere and lower thermosphere compared to in the upper stratosphere to lower mesosphere, suggesting that the forecast skill decreases with increasing altitude. The results demonstrate that the errors in the lower thermosphere reach saturation, on average, in less than 5 days, at least with the current version of WACCMX+DART. A seasonal dependency to the error growth is found at high latitudes in the Northern and Southern Hemispheres but not in the tropics or global average. We additionally investigate the error growth rates for migrating and nonmigrating atmospheric tides and find that the errors saturate after ∼5 days for tides in the lower thermosphere. The results provide an initial assessment of the error growth rates in the mesosphere and lower thermosphere and are relevant for understanding how whole atmosphere models can potentially improve space weather forecasting.

The atmospheric effects of precipitating electrons are not fully understood, and uncertainties are large for electrons with energies greater than ~30 keV. These electrons are underrepresented in modeling studies today, primarily because valid measurements of their precipitating spectral energy fluxes are lacking. This paper compares simulations from the Whole Atmosphere Community Climate Model (WACCM) that incorporated two different estimates of precipitating electron fluxes for electrons with energies greater than 30 keV. The estimates are both based on data from the Polar Orbiting Environmental Satellite Medium Energy Proton and Electron Detector (MEPED) instruments but differ in several significant ways. Most importantly, only one of the estimates includes both the 0° and 90° telescopes from the MEPED instrument. Comparisons are presented between the WACCM results and satellite observations poleward of 30°S during the austral winter of 2003, a period of significant energetic electron precipitation. Both of the model simulations forced with precipitating electrons with energies >30 keV match the observed descent of reactive odd nitrogen better than a baseline simulation that included auroral electrons, but no higher energy electrons. However, the simulation that included both telescopes shows substantially better agreement with observations, particularly at midlatitudes. The results indicate that including energies >30 keV and the full range of pitch angles to calculate precipitating electron fluxes is necessary for improving simulations of the atmospheric effects of energetic electron precipitation.