Thermospheric Nitric Oxide Response to Shock-led Storms.

We present a multi-year superposed epoch study of the Sounding of the Atmosphere using Broadband Emission Radiometry nitric oxide (NO) emission data. NO is a trace constituent in the thermosphere that acts as cooling agent via infrared (IR) emissions. The NO cooling competes with storm time thermospheric heating resulting in a thermostat effect. Our study of nearly 200 events reveals that shock-led interplanetary coronal mass ejections (ICMEs) are prone to early and excessive thermospheric NO production and IR emissions. Excess NO emissions can arrest thermospheric expansion by cooling the thermosphere during intense storms. The strongest events curtail the interval of neutral density increase and produce a phenomenon known as thermospheric 'overcooling'. We use Defense Meteorological Satellite Program particle precipitation data to show that interplanetary shocks and their ICME drivers can more than double the fluxes of precipitating particles that are known to trigger the production of thermospheric NO. Coincident increases in Joule heating likely amplify the effect. In turn, NO emissions more than double. We discuss the roles and features of shock/sheath structures that allow the thermosphere to temper the effects of extreme storm time energy input and explore the implication these structures may have on mesospheric NO. Shock-driven thermospheric NO IR cooling likely plays an important role in satellite drag forecasting challenges during extreme events.

Correlations Between the Thermosphere's Semiannual Density Variations and Infrared Emissions Measured With the SABER Instrument.

This paper presents measurements of the amplitudes and timings of the combined, annual, and semiannual variations of thermospheric neutral density, and a comparison of these density variations with measurements of the infrared emissions from carbon dioxide and nitric oxide in the thermosphere. The density values were obtained from measurements of the atmospheric drag experienced by the Challenging Minisatellite Payload, Gravity Recovery and Climate Experiment A, Gravity field and Ocean Circulation Explorer, and three Swarm satellites, while the optical emissions were measured with the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite. These data span a time period of 16 years. A database containing global average densities that were derived from the orbits of about 5,000 objects (Emmert, 2009, https://doi.org/10.1029/2009JA014102, 2015b, https://doi.org/10.1002/2015JA021047) was employed for calibrating these density data. A comparison with the NRLMSISE-00 model was used to derive measurements of how much the density changes over time due to these seasonal variations. It is found that the seasonal density oscillations have significant variations in amplitude and timing. In order to test the practicality of using optical emissions as a monitoring tool, the SABER data were fit to the measured variations. Even the most simple fit that used only filtered carbon dioxide emissions had good correlations with the measured oscillations. However, the density oscillations were also well predicted by a simple Fourier series, contrary to original expectations. Nevertheless, measurements of the optical emissions from the thermosphere are expected to have a role in future understanding and prediction of the semiannual variations.

High correlations between temperature and nitric oxide in the thermosphere.

Obtaining accurate predictions of the neutral density in the thermosphere has been a long-standing problem. During geomagnetic storms the auroral heating in the polar ionospheres quickly raises the temperature of the thermosphere, resulting in higher neutral densities that exert a greater drag force on objects in low Earth orbit. Rapid increases and decreases in the temperature and density may occur within a couple days. A key parameter in the thermosphere is the total amount of nitric oxide (NO). The production of NO is accelerated by the auroral heating, and since NO is an efficient radiator of thermal energy, higher concentrations of this molecule accelerate the rate at which the thermosphere cools. This paper describes an improved technique that calculates changes in the global temperature of the thermosphere. Starting from an empirical model of the Poynting flux into the ionosphere, a set of differential equations derives the minimum, global value of the exospheric temperature, which can be used in a neutral density model to calculate the global values. The relative variations in NO content are used to obtain more accurate cooling rates. Comparisons with the global rate of NO emissions that are measured with the Sounding of the Atmosphere using Broadband Emission Radiometry instrument show that there is very good agreement with the predicted values. The NO emissions correlate highly with the total auroral heating that has been integrated over time. We also show that the NO emissions are highly correlated with thermospheric temperature, as well as indices of solar extreme ultraviolet radiation.