Copyright © National Academy of Sciences. All rights reserved.
Solar and Space Physics: A Science for a Technological Society
54
SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY
28, 2003, topped −400. Fortunately that same active region had rotated past Earth when the largest flare
ever measured by spacecraft erupted on November 4 with an energy index of X28.
During these great storms, the atmosphere responded with dramatic changes in neutral composition,
winds, temperature, and mass density. Thermosphere mass density at 400-km altitude increased by more
than 400 percent and recovered to pre-storm levels exceptionally rapidly, indicating a strong overcooling
mechanism. Although many of the responses of the atmosphere-ionosphere-magnetosphere (AIM) system
to these storms have been documented, the mechanisms responsible for producing these effects are poorly
understood—scientists have not been able to emulate these effects in simulations. In particular, scientists
cannot yet predict the impacts of so-called superstorms, storms comparable in magnitude to the Carrington
event of 1859 that had an astounding estimated Dst of −850.
Tropospheric Driving
One of the most exciting developments in recent years has been the realization that tropospheric
weather and climate can strongly affect the upper atmosphere and ionosphere. Ultraviolet imaging of
Earth by the NASA IMAGE and TIMED satellites in the 2000-2003 time frame provided an unprecedented
new view of the equatorial ionosphere that revealed a large, longitudinal variation in density, with peaks
over rainforests. In the same period, atmospheric models developed at NCAR were gaining new capability
showing that atmospheric tides driven by tropospheric heat released in thunderstorms would propagate well
above 100 km and potentially modify the ionosphere-thermosphere (IT) system. Large-scale changes in the
structure of the ionosphere on seasonal timescales were also revealed, which also matched the seasonal
changes in tropical weather conditions. Since its launch in 2006, the COSMIC mission has observed a
number of ionospheric features that point to forcing from below: tidal influence on total electron content
and the F region of the ionosphere; wave signatures in the ionosphere and plasmasphere; a geographically
fixed (with the Weddell Sea) ionospheric anomaly; and complex structure in ionosphere F-region density
potentially attributable to tropospheric storm systems. These results have been matched by extensive
numerical modeling efforts (e.g., the Whole Atmosphere Community Climate Model; WACCM) focused
on understanding how atmospheric waves and tides of tropospheric origin propagate through the lower
and middle atmosphere, and with the upper-atmospheric general circulation models now also being driven
by stratospheric lower-boundary forcing that mirrors the tropospheric inputs, or with input of fitted wave
data at approximately 100 km, the boundary of space. Further, the signature of tropospheric forcing has
subsequently been observed in upper-thermospheric composition and temperature.
These and other observations and model studies have unequivocally revealed that Earth’s IT system
owes a considerable amount of its longitudinal, local-time, seasonal-latitudinal, and day-to-day variability
to atmospheric waves that begin near Earth’s surface and propagate into the upper atmosphere. Current
estimates indicate that waves propagating upward from the lower atmosphere contribute about as much to
the energy transfer in the IT system as does forcing from above in the forms of solar EUV and UV radiation,
precipitating particles, resistive heating, and winds driven by magnetospheric convection.
Thermospheric Climate Change
A systematic decrease by several percent per decade in thermosphere mass density is now evident in
the record of satellite orbit decay measured since the beginning of the space age. An effect predicted in
the 1980s, this change is thought to be largely in response to the increase in atmospheric CO 2 , which,
although it acts to trap infrared heat in the lower atmosphere, acts as a radiative cooler in the upper atmosphere. Thermospheric cooling is therefore an unambiguous signature of a human-influenced change in
Solar and Space Physics: A Science for a Technological Society
54
SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY
28, 2003, topped −400. Fortunately that same active region had rotated past Earth when the largest flare
ever measured by spacecraft erupted on November 4 with an energy index of X28.
During these great storms, the atmosphere responded with dramatic changes in neutral composition,
winds, temperature, and mass density. Thermosphere mass density at 400-km altitude increased by more
than 400 percent and recovered to pre-storm levels exceptionally rapidly, indicating a strong overcooling
mechanism. Although many of the responses of the atmosphere-ionosphere-magnetosphere (AIM) system
to these storms have been documented, the mechanisms responsible for producing these effects are poorly
understood—scientists have not been able to emulate these effects in simulations. In particular, scientists
cannot yet predict the impacts of so-called superstorms, storms comparable in magnitude to the Carrington
event of 1859 that had an astounding estimated Dst of −850.
Tropospheric Driving
One of the most exciting developments in recent years has been the realization that tropospheric
weather and climate can strongly affect the upper atmosphere and ionosphere. Ultraviolet imaging of
Earth by the NASA IMAGE and TIMED satellites in the 2000-2003 time frame provided an unprecedented
new view of the equatorial ionosphere that revealed a large, longitudinal variation in density, with peaks
over rainforests. In the same period, atmospheric models developed at NCAR were gaining new capability
showing that atmospheric tides driven by tropospheric heat released in thunderstorms would propagate well
above 100 km and potentially modify the ionosphere-thermosphere (IT) system. Large-scale changes in the
structure of the ionosphere on seasonal timescales were also revealed, which also matched the seasonal
changes in tropical weather conditions. Since its launch in 2006, the COSMIC mission has observed a
number of ionospheric features that point to forcing from below: tidal influence on total electron content
and the F region of the ionosphere; wave signatures in the ionosphere and plasmasphere; a geographically
fixed (with the Weddell Sea) ionospheric anomaly; and complex structure in ionosphere F-region density
potentially attributable to tropospheric storm systems. These results have been matched by extensive
numerical modeling efforts (e.g., the Whole Atmosphere Community Climate Model; WACCM) focused
on understanding how atmospheric waves and tides of tropospheric origin propagate through the lower
and middle atmosphere, and with the upper-atmospheric general circulation models now also being driven
by stratospheric lower-boundary forcing that mirrors the tropospheric inputs, or with input of fitted wave
data at approximately 100 km, the boundary of space. Further, the signature of tropospheric forcing has
subsequently been observed in upper-thermospheric composition and temperature.
These and other observations and model studies have unequivocally revealed that Earth’s IT system
owes a considerable amount of its longitudinal, local-time, seasonal-latitudinal, and day-to-day variability
to atmospheric waves that begin near Earth’s surface and propagate into the upper atmosphere. Current
estimates indicate that waves propagating upward from the lower atmosphere contribute about as much to
the energy transfer in the IT system as does forcing from above in the forms of solar EUV and UV radiation,
precipitating particles, resistive heating, and winds driven by magnetospheric convection.
Thermospheric Climate Change
A systematic decrease by several percent per decade in thermosphere mass density is now evident in
the record of satellite orbit decay measured since the beginning of the space age. An effect predicted in
the 1980s, this change is thought to be largely in response to the increase in atmospheric CO 2 , which,
although it acts to trap infrared heat in the lower atmosphere, acts as a radiative cooler in the upper atmosphere. Thermospheric cooling is therefore an unambiguous signature of a human-influenced change in