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Solar and Space Physics: A Science for a Technological Society
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SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY
thought to arise from winds blowing across Pedersen conductivity gradients associated with the waves. It
also appears that the physics behind the ionospheric manifestation of the waves may be different during
nighttime and daytime, and that their directionality varies with season. This may reflect different generation
mechanisms and/or the influence of the background large-scale wind circulation on the wave propagation. Note that explanation of this phenomenon involves coupling between instability, local, regional, and
global-scale processes in ways that scientists do not understand, leading to the question, How do plasmas
and neutrals interact across local, regional, and global scales to produce the operationally important density
variations referred to as space weather?
Plasma-neutral interactions at high latitudes are strongly coupled to solar wind and magnetospheric
dynamics. This coupling is regulated to a large extent by the ionospheric conductance, which is dependent
on the ion and neutral gas densities. Despite their importance in the AIM interaction, the spatial distributions of these densities and their time variability are among the most poorly measured parameters of the
IT system. Consequently, the interplay between neutral and ionized gas constituents and electromagnetic
activity are not well understood. At high latitudes, ion motions driven by the interaction of Earth with the
solar wind provide the strongest forcing to the neutral atmosphere. The temporal and spatial scales of the
neutral atmosphere response are quite different at different altitudes and quite different from those imposed
by the driver. The driven neutral gas motions persist long after the driving fields change, and the neutral
gas convects and diffuses well beyond the region of ion forcing. This complex interaction changes the
energy deposited in the atmosphere, which causes changes in the global temperature, composition, and
density that cannot yet be predicted. For example, in both the cusp region and its counterpart in the nightside ionospheric convection throat, the average mass density of the neutral atmosphere near the F-region
peak is observed to be significantly higher than that predicted by the empirical reference thermosphere
(MSIS90) (Figure 8.17).
This discrepancy has stimulated a search for the causative mechanisms. The neutral density enhancements are statistically collocated with observed regions of soft electron precipitation, Joule heating, ion
upflows and outflows, dispersive Alfvén waves, and small-scale field-aligned currents. Determining cause
and effect among these variables and advancing an operational capability to predict regional enhancements
in neutral density will require simultaneous, multivariable measurements in the topside and bottomside
ionosphere. Model results indicate that thermospheric upwelling strongly influences the scale height of O +
ions in the topside ionosphere and their escape flux into the magnetosphere. Thus the population of the
magnetosphere by ionospheric outflows is also dependent on plasma-neutral interactions. Breakthroughs in
the next decade in understanding the dynamic interaction between the magnetosphere and the IT system
must therefore confront the question, How do activities in neutral and ionized gases and electromagnetic
fields interact to produce observed magnetic field-aligned structure and motions of the thermosphere and
ionosphere?
The AIMI panel concluded that a major goal of the 2013-2022 decade is to understand the plasmaneutral coupling processes that give rise to local, regional, and global-scale structures in the AIM system,
particularly those relevant to society.
8.4.5 AIMI Science Goal 5. Planetary Change
How is our planetary environment changing over multidecadal scales, and what are the underlying
causes?
The preceding discussions indicate why achieving an understanding of how the whole atmosphere
system is coupled to the geospace environment remains a singular challenge for AIM research. Addressing
Solar and Space Physics: A Science for a Technological Society
178
SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY
thought to arise from winds blowing across Pedersen conductivity gradients associated with the waves. It
also appears that the physics behind the ionospheric manifestation of the waves may be different during
nighttime and daytime, and that their directionality varies with season. This may reflect different generation
mechanisms and/or the influence of the background large-scale wind circulation on the wave propagation. Note that explanation of this phenomenon involves coupling between instability, local, regional, and
global-scale processes in ways that scientists do not understand, leading to the question, How do plasmas
and neutrals interact across local, regional, and global scales to produce the operationally important density
variations referred to as space weather?
Plasma-neutral interactions at high latitudes are strongly coupled to solar wind and magnetospheric
dynamics. This coupling is regulated to a large extent by the ionospheric conductance, which is dependent
on the ion and neutral gas densities. Despite their importance in the AIM interaction, the spatial distributions of these densities and their time variability are among the most poorly measured parameters of the
IT system. Consequently, the interplay between neutral and ionized gas constituents and electromagnetic
activity are not well understood. At high latitudes, ion motions driven by the interaction of Earth with the
solar wind provide the strongest forcing to the neutral atmosphere. The temporal and spatial scales of the
neutral atmosphere response are quite different at different altitudes and quite different from those imposed
by the driver. The driven neutral gas motions persist long after the driving fields change, and the neutral
gas convects and diffuses well beyond the region of ion forcing. This complex interaction changes the
energy deposited in the atmosphere, which causes changes in the global temperature, composition, and
density that cannot yet be predicted. For example, in both the cusp region and its counterpart in the nightside ionospheric convection throat, the average mass density of the neutral atmosphere near the F-region
peak is observed to be significantly higher than that predicted by the empirical reference thermosphere
(MSIS90) (Figure 8.17).
This discrepancy has stimulated a search for the causative mechanisms. The neutral density enhancements are statistically collocated with observed regions of soft electron precipitation, Joule heating, ion
upflows and outflows, dispersive Alfvén waves, and small-scale field-aligned currents. Determining cause
and effect among these variables and advancing an operational capability to predict regional enhancements
in neutral density will require simultaneous, multivariable measurements in the topside and bottomside
ionosphere. Model results indicate that thermospheric upwelling strongly influences the scale height of O +
ions in the topside ionosphere and their escape flux into the magnetosphere. Thus the population of the
magnetosphere by ionospheric outflows is also dependent on plasma-neutral interactions. Breakthroughs in
the next decade in understanding the dynamic interaction between the magnetosphere and the IT system
must therefore confront the question, How do activities in neutral and ionized gases and electromagnetic
fields interact to produce observed magnetic field-aligned structure and motions of the thermosphere and
ionosphere?
The AIMI panel concluded that a major goal of the 2013-2022 decade is to understand the plasmaneutral coupling processes that give rise to local, regional, and global-scale structures in the AIM system,
particularly those relevant to society.
8.4.5 AIMI Science Goal 5. Planetary Change
How is our planetary environment changing over multidecadal scales, and what are the underlying
causes?
The preceding discussions indicate why achieving an understanding of how the whole atmosphere
system is coupled to the geospace environment remains a singular challenge for AIM research. Addressing