A significant increase in the number of anthropogenic objects in Earth orbit has necessitated the development of satellite conjunction assessment and collision avoidance capabilities for new spacecraft. Often, the greatest source of uncertainty in predicting a satellite's trajectory in low Earth orbit originates from atmospheric neutral mass density variability caused by enhanced geomagnetic activity and solar EUV absorption. This work investigates the impacts of solar and geomagnetic index forecasting uncertainty on satellite drag and satellite maneuver decision-making. During an averaged point in the solar cycle, accurate index forecasts with reduced uncertainty are shown to provide significantly improved advance notice for dangerous conjunction events above 500 km. Below 500 km, forecast improvements are less impactful. This boundary of utility from forecast improvements shifts upward and downward during solar maximum and solar minimum, respectively. Improved index forecasts are shown to have little impact on making maneuver decisions 12-24 hours from a potential conjunction event, but are demonstrated to be very useful when trying to make maneuver decisions with more lead time. These improved forecasts of the space weather indices help in making actionable, durable conjunction predictions sooner than is currently possible.

Increasing carbon dioxide concentrations in the mesosphere and lower thermosphere are increasing radiative cooling in the upper atmosphere, leading to thermospheric contraction and decreased neutral mass densities at fixed altitudes. Previous studies of the historic neutral density trend have shown a dependence upon solar activity, with larger F10.7 values resulting in lower neutral density reductions. To investigate the impact on the future thermosphere, the Whole Atmosphere Community Climate Model with ionosphere and thermosphere extension (WACCM-X) has been used to simulate the thermosphere under increasing carbon dioxide concentrations and varying solar activity conditions. These neutral density reductions have then been mapped onto the Shared Socioeconomic Pathways (SSPs) published by the Intergovernmental Panel on Climate Change (IPCC). The neutral density reductions can also be used as a scaling factor, allowing commonly used empirical models to account for CO$_2$ trends. Under the“best case’ SSP1-2.6 scenario, neutral densities reductions at 400 km altitude peak (when CO$_2$ = 474 ppm) at a reduction of 13 to 30\% (under high and low solar activity respectively) compared to the year 2000. Higher CO$_2$ concentrations lead to greater density reductions, with the largest modelled concentration of 890 ppm resulting in a 50 to 77 \% reduction at 400 km, under high and low solar activity respectively.

Gravity waves (GWs) play an important role in the dynamics and energetics of the mesosphere. Geomagnetic activity is a known source of GWs in the upper atmosphere. However, how deep the effects of geomagnetic activity induced GWs penetrate into the mesosphere remains an open question. We use temperature measurements from the SABER/TIMED instrument between 2002 - 2018 to study the variations of mesospheric GW activity following intense geomagnetic disturbances identified by AE and Dst indices. By considering several case studies, we show for the first time that the GWs forced by geomagnetic activity can propagate down to about 80 km in the high latitude mesosphere. Only regions above 55° latitudes show a clear response. The fraction of cases in which there is an unambiguous enhancement in GW activity following the onset of geomagnetic disturbance is smaller during summer than other seasons. Only about half of the events show an unambiguous increase in GW activity during non-summer periods and about one quarter of the events in summer show an enhancement in GWs. In addition, we also find that the high latitude mesopause is seen to descend in altitude following onset of geomagnetic activity in the non-summer high latitude region.

An international joint research project, entitled Interhemispheric Coupling Study by Observations and Modelling (ICSOM), is ongoing. In the late 2000s, an interesting form of interhemispheric coupling (IHC) was discovered: when warming occurs in the winter polar stratosphere, the upper mesosphere in the summer hemisphere also becomes warmer with a time lag of days. This IHC phenomenon is considered to be a coupling through processes in the middle atmosphere (i.e., stratosphere, mesosphere, and lower thermosphere). Several plausible mechanisms have been proposed so far, but they are still controversial. This is mainly because of the difficulty in observing and simulating gravity waves (GWs) at small scales, despite the important role they are known to play in middle atmosphere dynamics. In this project, by networking sparsely but globally distributed radars, mesospheric GWs have been simultaneously observed in seven boreal winters since 2015/16. We have succeeded in capturing five stratospheric sudden warming events and two polar vortex intensification events. This project also includes the development of a new data assimilation system to generate long-term reanalysis data for the whole middle atmosphere, and simulations by a state-of-art GW-permitting general circulation model using reanalysis data as initial values. By analyzing data from these observations, data assimilation, and model simulation, comprehensive studies to investigate the mechanism of IHC are planned. This paper provides an overview of ICSOM, but even initial results suggest that not only gravity waves but also large-scale waves are important for the mechanism of the IHC.

Increasing carbon dioxide causes cooling in the upper atmosphere and a secular decrease in atmospheric density over time. With the use of the Whole Atmospheric Community Climate Model with thermosphere and ionosphere extension (WACCM-X), neutral thermospheric densities up to 500 km have been modelled under increasing carbon dioxide concentrations. Only carbon dioxide and carbon monoxide concentrations are changed between simulations, and solar activity is held low at F10.7 = 70 throughout. Using the four Representative Concentration Pathway (RCP) carbon dioxide scenarios produced by the Intergovernmental Panel on Climate Change (IPCC), scenarios of neutral density decrease through to the year 2100 have been modelled. The years 1975 and 2005 have also been simulated, which indicated a historic trend of -5.8% change in neutral density per decade. Decreases in the neutral density relative to the year 2000 have been given for increasing ground-level carbon dioxide concentrations. WACCM-X shows there has already been a 17% decrease in neutral densities at 400 km relative to the density in the year 2000. This becomes a 30% reduction at the 50:50 probability threshold of limiting warming to 1.5 degrees Celsius, as set out in the Paris Agreement. A simple orbital propagator has been used to show the impact the decrease in density has on the orbital lifetime of objects travelling through the thermosphere. If the 1.5 degrees Celsius target is met, objects in LEO will have orbital lifetimes around 30% longer than comparable objects from the year 2000.