The storm response of the ionosphere and thermosphere produces structures over a wide range of time and spatial scales. To understand the storm-time behavior of this system, researchers must address the following science challenge: AIMI-1. Understand how the ionosphere-thermosphere system responds to, and regulates, magnetospheric forcing over global, regional, and local scales.
An important element of the dynamics of the IT system is the transfer of energy and momentum between the plasma and neutral components of the system and the role that electric and magnetic fields serve in accentuating and sometimes moderating this interchange. The pathways through which ions and neutrals interact are of course fundamental to space physics, given that they occur at all planets with atmospheres, at comets, and within the magnetospheres of Jupiter and Saturn. For example, in Earth’s ionosphere at an altitude from 100 to 130 km the collisions between ions and electrons and neutrals enable current to flow across the local magnetic field, which facilitates closure of currents flowing along magnetic fields from the magnetosphere. The proper description of these cross-field currents requires the development of an accurate model of the plasma “conductivity,” yet the dynamics of ionospheric conductivity are among the most poorly quantified parameters of the IT system. Earth’s equatorial region is a rich laboratory for the investigation of plasma-neutral coupling in the presence of a magnetic field. The behavior can be extraordinarily complex: plasma-neutral collisions and associated neutral winds drive turbulence that cascades to very small spatial scales and regularly disrupts communications. The chemical interaction of a variety of ion species further complicates the dynamics.
A different suite of interactions occurs at middle latitudes. Spontaneous airglow emissions at 6,300 Å exhibit waves propagating to the south-west. They are thought to originate as neutral density waves at high latitudes which then interact with the mid-latitude ionosphere to create the structures, but their occurrence is curiously unrelated to levels of magnetic activity.
Thus, plasma-neutral coupling plays a critical role in ionospheric dynamics across the full range of latitudes. Researchers must therefore address the following challenge: AIMI-2. Understand the plasma-neutral coupling processes that give rise to local, regional, and global-scale structures and dynamics in the AIM system.
Numerous recent observations and simulations show that the IT system owes much of its longitudinal, local-time, seasonal, and even day-to-day variability to meteorological processes in the troposphere and stratosphere. 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. The absorption of solar radiation (e.g., by tropospheric H2O and stratospheric O3) excites a spectrum of thermal tides. Figure 2.11 shows the spatial structure in daytime convective clouds that is believed to introduce longitudinal structure in the ionosphere, seen in Figure 2.11 in ultraviolet emissions. Surface topography and unstable shear flows excite planetary waves and gravity waves extending from planetary to very small (~tens to hundreds of kilometers) spatial scales and having periods from tens of days down to minutes. Convective tropospheric weather systems radiate additional thermal tides, gravity waves, and other classes of waves.
Those waves that propagate vertically grow exponentially with height into the more rarified atmosphere. Some of the waves spawn additional waves and turbulence. Figure 2.12 shows sodium layer observations revealing amazing wave structures at the base of the thermosphere, illustrating the rich spectrum of dynamics that occurs. Although the presence and the importance of waves are not in dispute, the relevant coupling processes operating between the neutral atmosphere and ionosphere involve a host of multiscale dynamics that are not understood at present. This leads to another major scientific challenge: AIMI-3. Understand how forcing from the lower atmosphere via tidal, planetary, and gravity waves influences the ionosphere and thermosphere.