Copyright © National Academy of Sciences. All rights reserved.
Solar and Space Physics: A Science for a Technological Society
SOLAR AND SPACE PHYSICS: RECENT DISCOVERIES, FUTURE FRONTIERS
53
changes propagating upward from the atmosphere below. The extended epoch of low solar EUV and the
reduced neutral densities at low-Earth-orbit altitudes led to the unexpectedly long mission life for the German Challenging Mini-Satellite Payload (CHAMP) satellite. But despite the extended solar quiet period, the
ionosphere displayed a surprising array of dynamics, including complex density structures in the morning
hours near local dawn that were documented by the USAF C/NOFS mission and NASA’s CINDI experiment and other space- and ground-based assets. It is now known that a quiet Sun does not correspond to
a calm, benign ionosphere and that deleterious impacts on navigation and communications occur under
these conditions in unexpected ways.
Global Density Structures and Reactive Feedback
Global GPS maps of ionospheric density showed, for the first time, large-scale dense plumes of plasma
extending from middle latitudes to the auroral zone at the onset of magnetic storms (see Figure 3.3). During
such events, plasmaspheric imaging of He + ions by IMAGE showed corresponding structures in the inner
magnetosphere, where plasma was sheared away from the plasmasphere and advected toward the magnetopause (see Figure 2.5). The plasmaspheric structure was never expected to appear in the ionosphere,
and the discovery points to a process critical to enhancing auroral ion outflow during storms.
Localized structures in the neutral density were discovered by international geodesy programs. The
CHAMP and NASA/German Gravity Recovery and Climate Experiment (GRACE) missions led to the discovery of localized neutral upwelling very near the poles associated with strong Joule heating that occurs
during geomagnetically calm or moderate conditions. This result demonstrated the surprising range of
conditions wherein neutral densities are sufficiently altered to modify the decay rates of satellites in low
Earth orbit. Understanding of the generation of these localized densities is not yet mature enough to predict
their occurrence.
Recent results from NASA’s FAST and IMAGE satellites revealed intense outflows of ionospheric ions
during storms. The solar wind-magnetosphere interaction on the dayside, that is, magnetopause reconnection, is a copious source of electromagnetic energy that propagates along the magnetic field into the
ionosphere at high latitudes near noon. This energy is converted to heat and momentum through ion-neutral
interactions and promotes resonant heating of O + that drives outflows. The O + flows upward and is carried into the magnetotail by the reconnection-convection cycle. The resultant large O + densities in the tail
plasma sheet appear to change reconnection dynamics in the tail, leading to the ~3-hour planetary-scale
(sawtooth) oscillations or quasi-periodic substorms in the magnetosphere. The influence of the O + outflow
on global dynamics is only one of a number of instances in which nonlinear reactive feedback leads to
nonlinear dynamics.
Storm Dynamics
Several of the geomagnetic storms driven by CMEs during solar cycle 23 were considered “great”
storms that led to highly nonlinear dynamics. Ionosphere observations indicated the emergence of a daytime superfountain effect, lifting the ionosphere to new heights and increasing its total electron content
by as much as 250 percent. Other extreme responses included very-large-amplitude traveling ionospheric
disturbances and modifications in the equatorial plasma irregularities that impact communications.
During cycle 23, there were 89 great storms that drove the geomagnetic storm index Dst 4 below −100,
but only one, associated with the extremely fast CME launched by the spectacular X17 flare of October
4 The Dst (disturbance–storm time) index is used to define geomagnetic storms. Quiet times usually have a Dst of between +20
and −20 nanoteslas.
Solar and Space Physics: A Science for a Technological Society
SOLAR AND SPACE PHYSICS: RECENT DISCOVERIES, FUTURE FRONTIERS
53
changes propagating upward from the atmosphere below. The extended epoch of low solar EUV and the
reduced neutral densities at low-Earth-orbit altitudes led to the unexpectedly long mission life for the German Challenging Mini-Satellite Payload (CHAMP) satellite. But despite the extended solar quiet period, the
ionosphere displayed a surprising array of dynamics, including complex density structures in the morning
hours near local dawn that were documented by the USAF C/NOFS mission and NASA’s CINDI experiment and other space- and ground-based assets. It is now known that a quiet Sun does not correspond to
a calm, benign ionosphere and that deleterious impacts on navigation and communications occur under
these conditions in unexpected ways.
Global Density Structures and Reactive Feedback
Global GPS maps of ionospheric density showed, for the first time, large-scale dense plumes of plasma
extending from middle latitudes to the auroral zone at the onset of magnetic storms (see Figure 3.3). During
such events, plasmaspheric imaging of He + ions by IMAGE showed corresponding structures in the inner
magnetosphere, where plasma was sheared away from the plasmasphere and advected toward the magnetopause (see Figure 2.5). The plasmaspheric structure was never expected to appear in the ionosphere,
and the discovery points to a process critical to enhancing auroral ion outflow during storms.
Localized structures in the neutral density were discovered by international geodesy programs. The
CHAMP and NASA/German Gravity Recovery and Climate Experiment (GRACE) missions led to the discovery of localized neutral upwelling very near the poles associated with strong Joule heating that occurs
during geomagnetically calm or moderate conditions. This result demonstrated the surprising range of
conditions wherein neutral densities are sufficiently altered to modify the decay rates of satellites in low
Earth orbit. Understanding of the generation of these localized densities is not yet mature enough to predict
their occurrence.
Recent results from NASA’s FAST and IMAGE satellites revealed intense outflows of ionospheric ions
during storms. The solar wind-magnetosphere interaction on the dayside, that is, magnetopause reconnection, is a copious source of electromagnetic energy that propagates along the magnetic field into the
ionosphere at high latitudes near noon. This energy is converted to heat and momentum through ion-neutral
interactions and promotes resonant heating of O + that drives outflows. The O + flows upward and is carried into the magnetotail by the reconnection-convection cycle. The resultant large O + densities in the tail
plasma sheet appear to change reconnection dynamics in the tail, leading to the ~3-hour planetary-scale
(sawtooth) oscillations or quasi-periodic substorms in the magnetosphere. The influence of the O + outflow
on global dynamics is only one of a number of instances in which nonlinear reactive feedback leads to
nonlinear dynamics.
Storm Dynamics
Several of the geomagnetic storms driven by CMEs during solar cycle 23 were considered “great”
storms that led to highly nonlinear dynamics. Ionosphere observations indicated the emergence of a daytime superfountain effect, lifting the ionosphere to new heights and increasing its total electron content
by as much as 250 percent. Other extreme responses included very-large-amplitude traveling ionospheric
disturbances and modifications in the equatorial plasma irregularities that impact communications.
During cycle 23, there were 89 great storms that drove the geomagnetic storm index Dst 4 below −100,
but only one, associated with the extremely fast CME launched by the spectacular X17 flare of October
4 The Dst (disturbance–storm time) index is used to define geomagnetic storms. Quiet times usually have a Dst of between +20
and −20 nanoteslas.