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Solar and Space Physics: A Science for a Technological Society
REPORT OF THE PANEL ON SOLAR AND HELIOSPHERIC PHYSICS
289
observations with expected advances in theory and modeling, 6 researchers will understand the structure
of the heliosheath in detail, including how time-dependent effects propagate in the heliosheath and affect
heliospheric structures. IMAP will make it possible to solve the mystery of the IBEX ribbon and INCA belt
and to discover the implications of these vast structures for the heliosphere and the local galactic medium
(motivation M1).
• Discover whether particles are accelerated in the heliosheath. When Voyager 1 crossed the TS,
particles up to about 1 MeV/nucleon peaked at the shock, but, surprisingly, the intensity of higher-energy
ACRs continued to rise well after the shock. There are several theories about the location and mechanism
of acceleration of these high-energy ACRs. One theory suggests that the highest-energy ACRs originate at
the flanks of the heliosphere rather than at the nose because of the blunt TS shape. Another idea invokes
random “hot spots” along the TS due to large-scale turbulence. It is also proposed that acceleration occurs
in the heliosheath as particles move within random plasma compressions. Still another theory suggests
that particle acceleration arises from contracting magnetic islands that result from magnetic reconnection
in the heliosheath. The last two ideas predict that the main acceleration occurs near the heliopause.
Distinguishing among those theories requires Voyager energetic-particle, solar wind, and magnetic-field
measurements through the heliosheath to the heliopause. In addition, current global-MHD models need
to be expanded to include turbulence, magnetic reconnection, and feedback from suprathermal particles.
Hybrid and kinetic codes are needed to complement global-MHD studies. Voyager data will test quantitative predictions from the models.
The Voyagers are exploring a new region, the heliosheath, which, unlike the region inside the TS, is not
dominated by supersonic solar wind. Compressive magnetic structures and possibly turbulence or magnetic
reconnection dominate this region. Heliosheath physics is not yet well understood, but new measurements
and models will spark new advances. Understanding heliosheath physics is also important for interpreting
IBEX, Cassini, STEREO, and IMAP ENA observations. Finally, understanding the heliosheath is important
because of the role it plays in modulating the intensity of galactic cosmic rays that penetrate into the inner
solar system and reach Earth. Figure 10.19 shows a schematic of the heliosphere, the heliosheath, and the
interstellar medium.
• Explore the properties of the heliopause and surrounding interstellar medium. The Voyagers are
expected to cross the heliopause into the local interstellar medium (LISM) within the next decade and will
provide the first measurements of interstellar magnetic-field strength and orientation and, it is hoped, measurements of interstellar plasma properties (motivation M3). They will also measure interstellar cosmic-ray
spectra, which may distinguish among acceleration-transport models or reveal contributions from nearby
sources. Interstellar cosmic-ray measurements have other consequences: they represent the maximum
cosmic-ray intensity that Earth has experienced (important for interpreting 10 Be archives) and they establish the maximum intensity of the radiation to which future space travelers can be exposed (constraining
interplanetary environmental models). Simultaneous near-Earth measurements will establish the absolute
intensity drop from interstellar space to Earth.
The Voyagers will provide the first measurements of the structure of the heliopause. What is the role
of instabilities and reconnection near the heliopause? How are cosmic rays modulated? How thick is
the heliopause? Heliopause models exist, but surprises are inevitable, such as when the TS was crossed.
In addition, if the Voyagers enter interstellar space roughly coincidentally with increased solar activity,
plasma-wave data may constrain LISM kinetic properties (such as the turbulence level). IBEX and IMAP
6 J.D. Richardson et al., The Heliospheric Interaction with the LISM: Observations and Models, white paper submitted to the Decadal
Strategy for Solar and Space Physics (Heliophysics), Paper 227; V. Florinski et al., The Outer Heliosphere-Solar System’s Final Frontier,
white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 76.
Solar and Space Physics: A Science for a Technological Society
REPORT OF THE PANEL ON SOLAR AND HELIOSPHERIC PHYSICS
289
observations with expected advances in theory and modeling, 6 researchers will understand the structure
of the heliosheath in detail, including how time-dependent effects propagate in the heliosheath and affect
heliospheric structures. IMAP will make it possible to solve the mystery of the IBEX ribbon and INCA belt
and to discover the implications of these vast structures for the heliosphere and the local galactic medium
(motivation M1).
• Discover whether particles are accelerated in the heliosheath. When Voyager 1 crossed the TS,
particles up to about 1 MeV/nucleon peaked at the shock, but, surprisingly, the intensity of higher-energy
ACRs continued to rise well after the shock. There are several theories about the location and mechanism
of acceleration of these high-energy ACRs. One theory suggests that the highest-energy ACRs originate at
the flanks of the heliosphere rather than at the nose because of the blunt TS shape. Another idea invokes
random “hot spots” along the TS due to large-scale turbulence. It is also proposed that acceleration occurs
in the heliosheath as particles move within random plasma compressions. Still another theory suggests
that particle acceleration arises from contracting magnetic islands that result from magnetic reconnection
in the heliosheath. The last two ideas predict that the main acceleration occurs near the heliopause.
Distinguishing among those theories requires Voyager energetic-particle, solar wind, and magnetic-field
measurements through the heliosheath to the heliopause. In addition, current global-MHD models need
to be expanded to include turbulence, magnetic reconnection, and feedback from suprathermal particles.
Hybrid and kinetic codes are needed to complement global-MHD studies. Voyager data will test quantitative predictions from the models.
The Voyagers are exploring a new region, the heliosheath, which, unlike the region inside the TS, is not
dominated by supersonic solar wind. Compressive magnetic structures and possibly turbulence or magnetic
reconnection dominate this region. Heliosheath physics is not yet well understood, but new measurements
and models will spark new advances. Understanding heliosheath physics is also important for interpreting
IBEX, Cassini, STEREO, and IMAP ENA observations. Finally, understanding the heliosheath is important
because of the role it plays in modulating the intensity of galactic cosmic rays that penetrate into the inner
solar system and reach Earth. Figure 10.19 shows a schematic of the heliosphere, the heliosheath, and the
interstellar medium.
• Explore the properties of the heliopause and surrounding interstellar medium. The Voyagers are
expected to cross the heliopause into the local interstellar medium (LISM) within the next decade and will
provide the first measurements of interstellar magnetic-field strength and orientation and, it is hoped, measurements of interstellar plasma properties (motivation M3). They will also measure interstellar cosmic-ray
spectra, which may distinguish among acceleration-transport models or reveal contributions from nearby
sources. Interstellar cosmic-ray measurements have other consequences: they represent the maximum
cosmic-ray intensity that Earth has experienced (important for interpreting 10 Be archives) and they establish the maximum intensity of the radiation to which future space travelers can be exposed (constraining
interplanetary environmental models). Simultaneous near-Earth measurements will establish the absolute
intensity drop from interstellar space to Earth.
The Voyagers will provide the first measurements of the structure of the heliopause. What is the role
of instabilities and reconnection near the heliopause? How are cosmic rays modulated? How thick is
the heliopause? Heliopause models exist, but surprises are inevitable, such as when the TS was crossed.
In addition, if the Voyagers enter interstellar space roughly coincidentally with increased solar activity,
plasma-wave data may constrain LISM kinetic properties (such as the turbulence level). IBEX and IMAP
6 J.D. Richardson et al., The Heliospheric Interaction with the LISM: Observations and Models, white paper submitted to the Decadal
Strategy for Solar and Space Physics (Heliophysics), Paper 227; V. Florinski et al., The Outer Heliosphere-Solar System’s Final Frontier,
white paper submitted to the Decadal Strategy for Solar and Space Physics (Heliophysics), Paper 76.