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Solar and Space Physics: A Science for a Technological Society
44
SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY
efficiency of energetic particle production in flares. Unexpectedly, most of these reconnection sites have
been found away from the heliospheric current sheet. The observations have also emphasized the importance of observations nearer to the Sun to enhance understanding of the roles of waves, wave turbulence,
and reconnection physics in driving solar wind dynamics.
Solar Energetic Particles
New observations of solar energetic particles have yielded a number of surprises. Solar-cycle 23 produced 16 ground-level events in ground-based neutron monitors, which allowed researchers to establish
that most large SEP events have a recent, preceding CME from the same active region. This finding indicates
that the most intense events may involve the acceleration of particles in one or more flares that produce
a seed population of energetic ions that can then reach very high energy through classical diffusive shock
acceleration at the CME-driven shock. The measured enrichments by ACE of 3 He and Fe in many large SEP
events are consistent with this picture. Continuing observations from STEREO, ACE, and other platforms
as well as upcoming Solar Orbiter and Solar Probe Plus missions will provide key measurements in the
source regions of these events and data on their spatial extent and evolution so that the complex dynamics
of SEP acceleration and transport to the geospace environment can be unraveled.
Exploring the Heliosphere’s Outer Limits
A series of groundbreaking discoveries were made as the Voyager spacecraft approached and crossed
the termination shock (TS) and entered the heliosheath on their way to the heliopause, the outer boundary of the Sun’s domain in the universe. These measurements and results from the Interstellar Boundary
Explorer (IBEX) and Cassini have significantly altered understanding of how the solar system interacts
with the interstellar medium and have also quantitatively confirmed a number of scientific predictions
about the heliospheric boundary region. The TS, which is where the solar wind can no longer maintain its
supersonic velocity as it pushes against the interstellar medium, had long been accepted as the driver of
anomalous cosmic ray (ACR) acceleration, but when the two Voyager spacecraft crossed the TS, neither
found evidence that the local TS is the source of ACRs. The source of the ACRs is now a subject of fierce
scientific debate. In addition, consistent with earlier theoretical predictions, most of the supersonic-flow
energy did not heat the ambient solar wind but likely went into supra-thermals (not measureable with the
Voyager instruments). The most recent observations may indicate the presence of an unexpected transition
region in which the outward solar wind flow stagnates.
Energetic neutral atom (ENA) maps by IBEX and Cassini show an unpredicted “ribbon” of emissions
from the outer heliosphere, apparently ordered by the local interstellar magnetic field (Figure 2.3). The
ribbon evolves on timescales as short as 6 months, demonstrating that the heliosphere/interstellar-medium
interaction is highly dynamic. The role of the interstellar magnetic field in shaping the outer heliosphere
is stronger than was expected prior to the recent influx of new data. Models based on these observations
suggest that the local interstellar magnetic field provides most of the pressure in the local cloud. The unexpected results from Voyager, IBEX, and Cassini observations demonstrate how little is really understood
about the interactions of stars with their interstellar environments.
Solar Wind-Magnetosphere Interactions
Advances in the physics of magnetospheres, their dynamics, and their coupling with the solar wind and
ionospheres were made on a number of fronts. Global imaging and in situ observation networks revealed
Solar and Space Physics: A Science for a Technological Society
44
SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY
efficiency of energetic particle production in flares. Unexpectedly, most of these reconnection sites have
been found away from the heliospheric current sheet. The observations have also emphasized the importance of observations nearer to the Sun to enhance understanding of the roles of waves, wave turbulence,
and reconnection physics in driving solar wind dynamics.
Solar Energetic Particles
New observations of solar energetic particles have yielded a number of surprises. Solar-cycle 23 produced 16 ground-level events in ground-based neutron monitors, which allowed researchers to establish
that most large SEP events have a recent, preceding CME from the same active region. This finding indicates
that the most intense events may involve the acceleration of particles in one or more flares that produce
a seed population of energetic ions that can then reach very high energy through classical diffusive shock
acceleration at the CME-driven shock. The measured enrichments by ACE of 3 He and Fe in many large SEP
events are consistent with this picture. Continuing observations from STEREO, ACE, and other platforms
as well as upcoming Solar Orbiter and Solar Probe Plus missions will provide key measurements in the
source regions of these events and data on their spatial extent and evolution so that the complex dynamics
of SEP acceleration and transport to the geospace environment can be unraveled.
Exploring the Heliosphere’s Outer Limits
A series of groundbreaking discoveries were made as the Voyager spacecraft approached and crossed
the termination shock (TS) and entered the heliosheath on their way to the heliopause, the outer boundary of the Sun’s domain in the universe. These measurements and results from the Interstellar Boundary
Explorer (IBEX) and Cassini have significantly altered understanding of how the solar system interacts
with the interstellar medium and have also quantitatively confirmed a number of scientific predictions
about the heliospheric boundary region. The TS, which is where the solar wind can no longer maintain its
supersonic velocity as it pushes against the interstellar medium, had long been accepted as the driver of
anomalous cosmic ray (ACR) acceleration, but when the two Voyager spacecraft crossed the TS, neither
found evidence that the local TS is the source of ACRs. The source of the ACRs is now a subject of fierce
scientific debate. In addition, consistent with earlier theoretical predictions, most of the supersonic-flow
energy did not heat the ambient solar wind but likely went into supra-thermals (not measureable with the
Voyager instruments). The most recent observations may indicate the presence of an unexpected transition
region in which the outward solar wind flow stagnates.
Energetic neutral atom (ENA) maps by IBEX and Cassini show an unpredicted “ribbon” of emissions
from the outer heliosphere, apparently ordered by the local interstellar magnetic field (Figure 2.3). The
ribbon evolves on timescales as short as 6 months, demonstrating that the heliosphere/interstellar-medium
interaction is highly dynamic. The role of the interstellar magnetic field in shaping the outer heliosphere
is stronger than was expected prior to the recent influx of new data. Models based on these observations
suggest that the local interstellar magnetic field provides most of the pressure in the local cloud. The unexpected results from Voyager, IBEX, and Cassini observations demonstrate how little is really understood
about the interactions of stars with their interstellar environments.
Solar Wind-Magnetosphere Interactions
Advances in the physics of magnetospheres, their dynamics, and their coupling with the solar wind and
ionospheres were made on a number of fronts. Global imaging and in situ observation networks revealed