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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
penetrate to high altitudes can significantly modify ionospheric peak heights at low latitudes. Variations
in composition also accompany tidal dynamics, thereby introducing chemical influences on ionospheric
production and loss with large effects in scale and magnitude. Finally, breaking gravity waves are thought
to provide the turbulent mixing at the base of the thermosphere (ca. 90-100 km) that determines the geographical and temporal variation of the turbopause altitude, and hence that of the O/N 2 ratio at higher
altitudes. How does the turbopause vary in space and time, and what are the causes and consequences?
remains one of the outstanding fundamental questions in aeronomy, and one that can conceivably be
addressed in the next decade.
Gravity waves have often been cited as the source for small-scale plasma variability, but the absence
of coordinated observations of neutral waves and ionospheric perturbations in the right altitude regions has
greatly impeded progress. In particular, a long-standing question that must be answered in the next decade
if significant progress is to be made in understanding and predicting how small-scale plasma structures
interfere with radio propagation is, What is the role of gravity waves in “seeding” equatorial Rayleigh-Taylor
instabilities that lead to plasma bubbles (depletions)?
One hypothesis suggests that the interaction between in situ gravity waves and the steep bottom-side
plasma gradient of the post-sunset equatorial ionosphere generates alternating east and west electric fields
that can excite this instability. Another theory requires gravity-wave winds only in the E region, which
generate electric fields that couple to the F layer. In addition, the tidal and mean wind fields modulate
the accessibility of gravity waves to these ionosphere regions, and moreover contribute to instability onset
and suppression criteria, and to instability growth rates. Thus, the interactions between small, local, and
regional-scale plasma-neutral coupling phenomena are all involved in this complex but highly relevant
emergent behavior in the system. Resolving this problem requires high-resolution measurements of neutral
and plasma parameters with high spatial and temporal resolution over the 100- to 300-km height region,
and further development of the relevant theories and models.
Finally, lightning is known to generate low-frequency electromagnetic waves called whistlers, which
can induce precipitation of radiation belt particles into the opposite hemisphere and enhance lower
ionosphere densities there. Lightning events also accelerate electrons to very high energies and create
strong electric fields in the mesosphere. Gamma-ray flashes observed from space (e.g., from RHESSI) may
indeed result from the deceleration of very energetic electrons due to collisions with atmospheric molecules. Luminous optical manifestations of these events are referred to variously as sprites, elves, or blue
jets (Figure 8.13). All of these processes raise questions about chemical modification of the mesosphere
and electrodynamic coupling between the troposphere, the ionosphere, and all of geospace through these
energetic lightning events.
The AIMI panel concluded that a major goal of the coming decade is to understand how tropospheric
weather drives space weather.
8.4.3 AIMI Science Goal 3. Ionosphere-Thermosphere-Magnetosphere Coupling
How do high-latitude electromagnetic energy and particle flows impact the geospace system?
What are the origins of plasma and neutral populations within geospace?
The IT-magnetosphere interaction at high latitudes is catalyzed by convective flows, which transport
and mix plasma and neutral gases across subauroral, auroral, and polar regions, and by magnetic fieldaligned flows of plasma and electromagnetic energy, which couple the collisionless magnetosphere to the
collisional ionosphere-thermosphere boundary layer. Researchers now recognize that the active response
Solar and Space Physics: A Science for a Technological Society
172
SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY
penetrate to high altitudes can significantly modify ionospheric peak heights at low latitudes. Variations
in composition also accompany tidal dynamics, thereby introducing chemical influences on ionospheric
production and loss with large effects in scale and magnitude. Finally, breaking gravity waves are thought
to provide the turbulent mixing at the base of the thermosphere (ca. 90-100 km) that determines the geographical and temporal variation of the turbopause altitude, and hence that of the O/N 2 ratio at higher
altitudes. How does the turbopause vary in space and time, and what are the causes and consequences?
remains one of the outstanding fundamental questions in aeronomy, and one that can conceivably be
addressed in the next decade.
Gravity waves have often been cited as the source for small-scale plasma variability, but the absence
of coordinated observations of neutral waves and ionospheric perturbations in the right altitude regions has
greatly impeded progress. In particular, a long-standing question that must be answered in the next decade
if significant progress is to be made in understanding and predicting how small-scale plasma structures
interfere with radio propagation is, What is the role of gravity waves in “seeding” equatorial Rayleigh-Taylor
instabilities that lead to plasma bubbles (depletions)?
One hypothesis suggests that the interaction between in situ gravity waves and the steep bottom-side
plasma gradient of the post-sunset equatorial ionosphere generates alternating east and west electric fields
that can excite this instability. Another theory requires gravity-wave winds only in the E region, which
generate electric fields that couple to the F layer. In addition, the tidal and mean wind fields modulate
the accessibility of gravity waves to these ionosphere regions, and moreover contribute to instability onset
and suppression criteria, and to instability growth rates. Thus, the interactions between small, local, and
regional-scale plasma-neutral coupling phenomena are all involved in this complex but highly relevant
emergent behavior in the system. Resolving this problem requires high-resolution measurements of neutral
and plasma parameters with high spatial and temporal resolution over the 100- to 300-km height region,
and further development of the relevant theories and models.
Finally, lightning is known to generate low-frequency electromagnetic waves called whistlers, which
can induce precipitation of radiation belt particles into the opposite hemisphere and enhance lower
ionosphere densities there. Lightning events also accelerate electrons to very high energies and create
strong electric fields in the mesosphere. Gamma-ray flashes observed from space (e.g., from RHESSI) may
indeed result from the deceleration of very energetic electrons due to collisions with atmospheric molecules. Luminous optical manifestations of these events are referred to variously as sprites, elves, or blue
jets (Figure 8.13). All of these processes raise questions about chemical modification of the mesosphere
and electrodynamic coupling between the troposphere, the ionosphere, and all of geospace through these
energetic lightning events.
The AIMI panel concluded that a major goal of the coming decade is to understand how tropospheric
weather drives space weather.
8.4.3 AIMI Science Goal 3. Ionosphere-Thermosphere-Magnetosphere Coupling
How do high-latitude electromagnetic energy and particle flows impact the geospace system?
What are the origins of plasma and neutral populations within geospace?
The IT-magnetosphere interaction at high latitudes is catalyzed by convective flows, which transport
and mix plasma and neutral gases across subauroral, auroral, and polar regions, and by magnetic fieldaligned flows of plasma and electromagnetic energy, which couple the collisionless magnetosphere to the
collisional ionosphere-thermosphere boundary layer. Researchers now recognize that the active response