Copyright © National Academy of Sciences. All rights reserved.
Solar and Space Physics: A Science for a Technological Society
ENABLING DISCOVERY IN SOLAR AND SPACE PHYSICS
27
The release of greenhouse gases (e.g., carbon dioxide and methane) into the atmosphere is changing the
global climate, warming the lower atmosphere but cooling the upper atmosphere. In the lower atmosphere
the opacity of greenhouse gases traps energy by capturing the radiant infrared energy from Earth’s surface
and transferring it to thermal energy via collisions with other molecules. In the thermosphere, however,
where intermolecular collisions are less frequent, greenhouse gases promote cooling by acquiring energy
via collisions and then radiating this energy to space in the infrared.
Continued cooling of the thermosphere will alter atmosphere-ionosphere coupling, thereby altering
global currents in the magnetosphere-ionosphere system and thus fundamentally altering magnetosphereionosphere coupling. These trends are only now apparent from the long-term set of satellite data and
ground-based ionosonde networks, the latter of which show that the ionosphere is dropping to lower
altitudes over time. This is a remarkable planetary change attributable, at least in part, to human society’s
modification of the atmosphere.
Key Science Goal 3
Key Science Goal 3. Determine the interaction of the Sun with the solar system and the interstellar
medium. The outer regions of the heliosphere remain only sparsely explored. 6 Regions of high intrinsic interest, they are also the location where anomalous cosmic rays are generated. Anomalous and
galactic cosmic rays penetrate into the inner solar system, posing a danger for humans and spacecraft
traveling to locations beyond those afforded protection by Earth’s magnetic field.
The supersonic flow of the solar wind transitions to a subsonic flow that merges with the local interstellar medium at a distance of about 100 AU from the Sun. The heliosheath lies beyond this “termination
shock” and extends out to the heliopause, an as-yet-unexplored region that separates the domain of the
Sun from the local interstellar medium. The two Voyager spacecraft, launched in 1977, crossed the termination shock in 2004 and 2007, and they are now exploring the heliosheath—a region of prodigious
particle acceleration (see Figure 1.5). In terms of the total amount of energy placed into energetic particles,
the heliosheath is a more copious accelerator than the Sun.
The heliosheath and the heliopause are the principal barriers against entry of galactic cosmic rays
(GCRs) into the solar system, with the largest reduction in GCR intensity occurring in these regions. How
this reduction occurs, how it depends on solar activity, and what is the maximum intensity at which GCRs
can penetrate into the inner heliosphere are not known. GCRs generate isotopes in Earth’s atmosphere that
are preserved as an archive of historical solar activity—for example, carbon-14 abundance can be measured
in tree rings. Understanding the production and formation of these unique records of past changes in solar
activity is essential for interpreting changes in Earth’s climate over the past millennium.
The Voyager spacecraft are currently at widely separated locations, and their measurements are limited
by their 35-year-old instrumentation. Nevertheless, recent Voyager measurements have enabled discovery of unanticipated structures in the heliosheath. Measurements from the Interstellar Boundary Explorer
(IBEX) have revealed that the interstellar magnetic field organizes the distribution of energetic ions. These
unique measurements from the Voyagers as they cross the heliopause into the local interstellar medium,
along with measurements from IBEX and the proposed IMAP mission, promise a new understanding of
the important acceleration processes occurring in these unexplored regions. In particular, they will help
6 The Voyager spacecraft continue to make in situ measurements of the outer heliosphere, and their entry into interstellar space
sometime in the next decade will be an historic event. Remote sensing measurements from instruments on near-Earth spacecraft
complement Voyager measurements and facilitate a multipronged attack on fundamental science questions.
Solar and Space Physics: A Science for a Technological Society
ENABLING DISCOVERY IN SOLAR AND SPACE PHYSICS
27
The release of greenhouse gases (e.g., carbon dioxide and methane) into the atmosphere is changing the
global climate, warming the lower atmosphere but cooling the upper atmosphere. In the lower atmosphere
the opacity of greenhouse gases traps energy by capturing the radiant infrared energy from Earth’s surface
and transferring it to thermal energy via collisions with other molecules. In the thermosphere, however,
where intermolecular collisions are less frequent, greenhouse gases promote cooling by acquiring energy
via collisions and then radiating this energy to space in the infrared.
Continued cooling of the thermosphere will alter atmosphere-ionosphere coupling, thereby altering
global currents in the magnetosphere-ionosphere system and thus fundamentally altering magnetosphereionosphere coupling. These trends are only now apparent from the long-term set of satellite data and
ground-based ionosonde networks, the latter of which show that the ionosphere is dropping to lower
altitudes over time. This is a remarkable planetary change attributable, at least in part, to human society’s
modification of the atmosphere.
Key Science Goal 3
Key Science Goal 3. Determine the interaction of the Sun with the solar system and the interstellar
medium. The outer regions of the heliosphere remain only sparsely explored. 6 Regions of high intrinsic interest, they are also the location where anomalous cosmic rays are generated. Anomalous and
galactic cosmic rays penetrate into the inner solar system, posing a danger for humans and spacecraft
traveling to locations beyond those afforded protection by Earth’s magnetic field.
The supersonic flow of the solar wind transitions to a subsonic flow that merges with the local interstellar medium at a distance of about 100 AU from the Sun. The heliosheath lies beyond this “termination
shock” and extends out to the heliopause, an as-yet-unexplored region that separates the domain of the
Sun from the local interstellar medium. The two Voyager spacecraft, launched in 1977, crossed the termination shock in 2004 and 2007, and they are now exploring the heliosheath—a region of prodigious
particle acceleration (see Figure 1.5). In terms of the total amount of energy placed into energetic particles,
the heliosheath is a more copious accelerator than the Sun.
The heliosheath and the heliopause are the principal barriers against entry of galactic cosmic rays
(GCRs) into the solar system, with the largest reduction in GCR intensity occurring in these regions. How
this reduction occurs, how it depends on solar activity, and what is the maximum intensity at which GCRs
can penetrate into the inner heliosphere are not known. GCRs generate isotopes in Earth’s atmosphere that
are preserved as an archive of historical solar activity—for example, carbon-14 abundance can be measured
in tree rings. Understanding the production and formation of these unique records of past changes in solar
activity is essential for interpreting changes in Earth’s climate over the past millennium.
The Voyager spacecraft are currently at widely separated locations, and their measurements are limited
by their 35-year-old instrumentation. Nevertheless, recent Voyager measurements have enabled discovery of unanticipated structures in the heliosheath. Measurements from the Interstellar Boundary Explorer
(IBEX) have revealed that the interstellar magnetic field organizes the distribution of energetic ions. These
unique measurements from the Voyagers as they cross the heliopause into the local interstellar medium,
along with measurements from IBEX and the proposed IMAP mission, promise a new understanding of
the important acceleration processes occurring in these unexplored regions. In particular, they will help
6 The Voyager spacecraft continue to make in situ measurements of the outer heliosphere, and their entry into interstellar space
sometime in the next decade will be an historic event. Remote sensing measurements from instruments on near-Earth spacecraft
complement Voyager measurements and facilitate a multipronged attack on fundamental science questions.