The primary mechanism through which energy and momentum are transferred from the lower atmosphere to the upper atmosphere and ionosphere is through the generation and propagation of waves. Wave Coupling in the Atmosphere-Ionosphere System 1 Jeff Forbes, Aerospace Engineering Sciences Dept, University of Colorado Gravity Waves min 10’s – 1000 km Tides 24, 12, 8 hours 1000’s to 10,000 km Planetary Waves 2-20 days 1000’s to 10,000 km Wave Spectrum

2 Important Wave Facts Electric fields are generated through dynamo action around km, in the region of maximum dissipation. These electric fields play a major role in redistributing ionospheric plasma. Ne, 400 km, 12 LT Waves over a broad spectrum are excited in the lower atmosphere, grow exponentially with height, and are dissipated in the thermosphere (> 100 km). The wave spectrum is modified by wave-mean flow, wave-wave interactions and differential (scale-dependent) dissipation up to about 200 km. The wave spectrum imparts significant variability, originating in the lower atmosphere, on the IT system over daily, weekly, monthly, intra-annual and inter- annual scales. Dissipating waves deposit their momentum into the thermosphere, modify the mean circulation, and change the temperature and density structures. For all practical purposes, there are no measurements of the wave spectrum between 110 and 200 km, where the waves exert much of their influence on the IT system.

3 Key Questions How and why does the wave spectrum vary with height, time and geographic location, especially in the dissipation region km? The challenge Measure key parameters (winds, temperatures, neutral and ion composition, plasma drifts) from 100 to 300 km at high spatial and temporal resolution, day and night, over the globe. How does the wave spectrum drive the mean state and variability of the upper atmosphere? How does wave variability translate to ionospheric variability? 400 km 110 km Equatorial, 24h period, ~60d mean Major impediment: Observations between 100 and 200 km are extremely sparse.

4 The Issue of Local Time vs. Latitude Coverage Atmospheric tides constitute a major portion of the large-scale wave spectrum above 100 km, and must be considered in any wave coupling mission. Days to achieve 24h local time coverage, dual-node sampling CHAMP i = 87 o, 130d TIMED i = 74 o, 60d UARS i = 57 o, 36d Atmospheric tides are ordered in local time; 24 hours local time coverage is needed for a full comprehensive analysis. However, high-inclination satellites have slow local time precession rates. Tidal analyses over periods greater than days miss daily, weekly and intra- seasonal variations, and significant averaging occurs. Only climatological views are possible.

5 Efforts Underway to Address the Key Questions The Ionospheric Connection Explorer (ICON) mission will measure winds, temperatures, plasma drifts and densities, and neutral- plasma interactions at low latitudes – see Immel et al. – SA42A-03 The Global-scale Observations of the Limb and Disk (GOLD) mission will globally measure lower thermosphere temperature over the daytime disk at high cadence at an effective altitude of ~160 km – see Eastes et al. – SA42A-04 The Observatory for Atmosphere-Space Interaction Studies (OASIS) will enable observation and understanding of various wave interactions over a range of spatial and temporal scales – see Thayer et al. – SA32A-07 Further development of local, regional and global physics-based models to better understand the physics and ramifications of neutral-plasma interactions and wave- wave interactions over a range of spatial and temporal scales. also O/N 2

understand how tide and PW variability translates to ionospheric variability at low latitudes 6 ICON-GOLD-OASIS Synergy & Discovery in the Context of Wave Coupling Altitude (km) equator -90 o +90 o ICON GOLD OASIS high-cadence 12h & 8h tidal variability at a single key altitude samples the low- latitude ionospheric consequences of GOLD dynamics pole-to-pole view of tide & PW variability, and large-scale GW GW-tide, GW-PW, GW-GW, GW-mean flow, GW-plasma interactions; wave dissipation covers critical height region km at high resolution pole-to-pole propagation of large-scale GW (TADs) originating in polar/auroral regions – provides context for ICON observations Connects lower- atmosphere GW sources with upper atmosphere effects provides altitude information and electrodynamic consequences of TADs resolve space-time ambiguities

7 ICON-GOLD-OASIS Shortcomings in Addressing the Key Questions Key Questions How and why does the wave spectrum vary with height, time and geographic location? How does the wave spectrum drive the mean state and variability of the atmosphere? How does wave variability translate to ionospheric variability? No measurement of the gravity wave spectrum or its interactions with tides and PW on a global scale Observations are very limited in latitude, height and local time coverage No real sense of how the height evolution of the wave spectrum drives the mean circulation and thermal structure. This question only answered in the low-latitude region for tides & PW

8 What Would a WAVES-Focused Mission Look Like? At least 3 polar-orbiting satellites making measurements at 6 local times, enabling day-to-day sensing of gravity waves, tides, planetary waves, the zonal mean state, and the interactions between them Remote sensing of neutral winds and temperatures at high spatial resolution, day and night, between km – instruments exist to do this remote sensing of ionospheric emissions and thus plasma structures, ~ km – instruments exist to do this in-situ measurement of neutral and plasma composition, winds/drifts and temperatures at orbital altitudes (~550 km) – instruments exist to do this physics-based models with higher spatial and temporal resolution, and with data assimilation capabilities Essentially the DYNAMIC mission described in the Solar and Space Physics Decadal Survey

9 FINAL THOUGHTS Troposphere weather is a significant driver of IT space weather. Troposphere variability is imprinted on the IT system through the vertical propagation of waves. A full understanding of exactly how this vertical transfer happens presents a significant challenge. The impact of aurorally-generated waves on the weather and mean state of the IT system remains an unresolved question. ICON, GOLD, and OASIS will synergistically provide new insights and significant advances, but still a largely incomplete picture will remain. Near closure on the problem should be obtainable with a DYNAMIC-like mission. Realizing DYNAMIC affordably requires intentional and strategic further development of the instrument technologies and observing platforms to realize the needed observations. ICON, GOLD and OASIS lay excellent groundwork for such a mission.