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Solar and Space Physics: A Science for a Technological Society
280
SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY
and heliosheath commonly have a power-law index and gradual rollover at higher speeds. Observations
reveal that local acceleration occurs in solar wind compression regions and that interplanetary transport
modifies spectra of remotely accelerated particles. The variability of the power law and its implications
for acceleration models have been topics of focused study.
10.3.4 Discovering How the Sun Interacts with the Local Galactic Medium and Protects Earth
The past decade produced one of the most dramatic advances in space physics. As the Voyager
spacecraft approached the termination shock (TS) and entered the heliosheath, a series of groundbreaking
discoveries were made. These in situ measurements, combined with all-sky heliospheric images by the
Interstellar Boundary Explorer (IBEX) and Cassini mission, led to major advances in our understanding of
how the solar system interacts with the interstellar medium (SHP science goal 4; decadal survey key science goal 3).
Voyager 1 (V1) crossed the TS in December 2004 at 94 AU in the Northern Hemisphere, and Voyager
2 (V2) crossed in the Southern Hemisphere in August 2007 at 84 AU. On their locations on the shock,
neither Voyager found any evidence of the acceleration of the higher-energy anomalous cosmic rays (ACRs)
observed in interplanetary space. That puzzle inspired several new ideas of where and how particles may
be accelerated (motivation M3).
In addition, the solar wind did not get heated at the TS nearly as much as expected (Figure 10.14);
apparently, 80 percent of the supersonic-flow energy went into suprathermal particles (Voyager measures
only thermal plasma). Earlier calculations suggested that acceleration of pickup ions may be the primary
dissipation mechanism at the TS. However, that had not been explicitly incorporated into heliospheric
models, and so the V2 observation that heating of solar wind protons accounted for only about 20 percent
of the dissipation was generally surprising. Remarkably, suprathermal-tail energy spectra have remained
power laws with index around −1.5, which is consistent with suprathermal neutral-atom distributions
observed in Cassini/INCA images.
Those discoveries affect other disciplines, including astrophysics and plasma physics; the heliosphere
has become a test bed for other astrospheres and plasma sheaths. Moreover, the heliosheath plasma environment is unlike any other plasma in the heliosphere.
Beginning about April 2010, V1 observed the heliosheath plasma to become nearly stagnant (zero
speed). The V1 plasma instrument is not functional, but the Low-Energy Charged-Particle (LECP) instrument can determine two components of the local plasma velocity by observing its effect on energetic ions.
Voyager scientists reported that the flow velocity near the ecliptic plane was near zero, and the north-south
component was also near zero on the basis of higher-energy observations. There is no consensus interpretation of those unexpected observations, but one idea is that V1 entered a “transition region,” possibly a
precursor of the heliopause, starting in April 2010.
To determine the plasma-flow direction at the Voyager locations more precisely, both spacecraft were
commanded to execute a series of “roll” maneuvers, allowing the LECP instruments to determine the
plasma velocity normal to the ecliptic plane. The rolls have occurred, but results are not yet available.
Such maneuvers may also be used after the Voyagers cross the heliopause.
Another major surprise came from global energetic neutral-atom (ENA) maps by IBEX and Cassini.
IBEX discovered a completely unpredicted, narrow (about 20º) ribbon of ENA emissions from the outer
heliosphere, apparently ordered by the local interstellar magnetic field (ISMF), as indicated by comparison
with global MHD models. More than a half-dozen theories have tried to explain the ribbon origin. The
hypothesized physical mechanisms operate in disparate regions from the TS to beyond the heliopause and
out to the local bubble. CASSINI found a similar, but much broader, feature at higher energies ( Figure 10.15).
Solar and Space Physics: A Science for a Technological Society
280
SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY
and heliosheath commonly have a power-law index and gradual rollover at higher speeds. Observations
reveal that local acceleration occurs in solar wind compression regions and that interplanetary transport
modifies spectra of remotely accelerated particles. The variability of the power law and its implications
for acceleration models have been topics of focused study.
10.3.4 Discovering How the Sun Interacts with the Local Galactic Medium and Protects Earth
The past decade produced one of the most dramatic advances in space physics. As the Voyager
spacecraft approached the termination shock (TS) and entered the heliosheath, a series of groundbreaking
discoveries were made. These in situ measurements, combined with all-sky heliospheric images by the
Interstellar Boundary Explorer (IBEX) and Cassini mission, led to major advances in our understanding of
how the solar system interacts with the interstellar medium (SHP science goal 4; decadal survey key science goal 3).
Voyager 1 (V1) crossed the TS in December 2004 at 94 AU in the Northern Hemisphere, and Voyager
2 (V2) crossed in the Southern Hemisphere in August 2007 at 84 AU. On their locations on the shock,
neither Voyager found any evidence of the acceleration of the higher-energy anomalous cosmic rays (ACRs)
observed in interplanetary space. That puzzle inspired several new ideas of where and how particles may
be accelerated (motivation M3).
In addition, the solar wind did not get heated at the TS nearly as much as expected (Figure 10.14);
apparently, 80 percent of the supersonic-flow energy went into suprathermal particles (Voyager measures
only thermal plasma). Earlier calculations suggested that acceleration of pickup ions may be the primary
dissipation mechanism at the TS. However, that had not been explicitly incorporated into heliospheric
models, and so the V2 observation that heating of solar wind protons accounted for only about 20 percent
of the dissipation was generally surprising. Remarkably, suprathermal-tail energy spectra have remained
power laws with index around −1.5, which is consistent with suprathermal neutral-atom distributions
observed in Cassini/INCA images.
Those discoveries affect other disciplines, including astrophysics and plasma physics; the heliosphere
has become a test bed for other astrospheres and plasma sheaths. Moreover, the heliosheath plasma environment is unlike any other plasma in the heliosphere.
Beginning about April 2010, V1 observed the heliosheath plasma to become nearly stagnant (zero
speed). The V1 plasma instrument is not functional, but the Low-Energy Charged-Particle (LECP) instrument can determine two components of the local plasma velocity by observing its effect on energetic ions.
Voyager scientists reported that the flow velocity near the ecliptic plane was near zero, and the north-south
component was also near zero on the basis of higher-energy observations. There is no consensus interpretation of those unexpected observations, but one idea is that V1 entered a “transition region,” possibly a
precursor of the heliopause, starting in April 2010.
To determine the plasma-flow direction at the Voyager locations more precisely, both spacecraft were
commanded to execute a series of “roll” maneuvers, allowing the LECP instruments to determine the
plasma velocity normal to the ecliptic plane. The rolls have occurred, but results are not yet available.
Such maneuvers may also be used after the Voyagers cross the heliopause.
Another major surprise came from global energetic neutral-atom (ENA) maps by IBEX and Cassini.
IBEX discovered a completely unpredicted, narrow (about 20º) ribbon of ENA emissions from the outer
heliosphere, apparently ordered by the local interstellar magnetic field (ISMF), as indicated by comparison
with global MHD models. More than a half-dozen theories have tried to explain the ribbon origin. The
hypothesized physical mechanisms operate in disparate regions from the TS to beyond the heliopause and
out to the local bubble. CASSINI found a similar, but much broader, feature at higher energies ( Figure 10.15).