Copyright © National Academy of Sciences. All rights reserved.
Solar and Space Physics: A Science for a Technological Society
56
SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY
Active regions are locations where these explosive events are concentrated. There, magnetic energy is
released in the form of ejected plasma, electromagnetic radiation, and heat that energize the local plasma.
Figure 1.3 shows a series of active regions seen in EUV light from the Solar Dynamics Observatory (SDO)
that form a chain across the upper half of the Sun. These arrays of loops emerge from the churning solar
atmosphere below and are embedded in plasmas with temperatures of around 10 7 K. The photosphere,
by comparison, is relatively cold at 6,000 K. The mechanisms that produce the hot corona of the Sun and
other stars still defy definitive explanation, and determining how this occurs is a high-priority science goal
of NASA’s Solar Probe Plus (SPP) mission and also of the Solar Orbiter ESA/NASA joint mission.
How the corona is generated and what physical processes heat the coronal plasma and control its
dynamics are not yet understood, thereby defining the second major challenge: SHP-2. Determine how
the Sun’s magnetism creates its hot, dynamic atmosphere.
An important result of recent research is the discovery of the critical role that magnetic reconnection
plays in modulating the energy flux from the Sun. The turbulent flows of the Sun’s surface twist and distort the coronal magnetic fields, thereby increasing their energy. The magnetic energy accumulates over
days, weeks, or perhaps longer. When adjacent magnetic fields pointing in opposite directions become
sufficiently strong, the magnetic fields explosively annihilate each other during magnetic reconnection (see
Figure 1.3). The released magnetic energy drives high-speed flows, heats the local plasma, and contributes
in complex ways to accelerating particles to relativistic energies, producing the intense bursts of energized
particles that characterize solar flares. This process occurs almost continuously in the active regions in the
corona (see Figure 1.3). As a result, the corona and heliosphere are filled with high-energy radiation, both
particle and electromagnetic (UV, X rays, and gamma rays).
The strongest of these reconnection events propel CMEs into the solar wind, and the CMEs steepen
into shocks that accelerate ions and electrons to high energy. Figure 2.8 shows a numerical simulation of
a CME, illustrating the scale of the ejected field and plasma. When directed Earth-ward, CMEs generate
large geomagnetic storms and intense energetic particle events in near-Earth space. The energetic particles
from these shocks pose significant threats to human and robotic space exploration. 5
The success of simulations in reproducing many of these observations testifies to the maturity of
scientific understanding of these significant events. However, even though it is now possible to predict
where on the Sun a CME will originate, it is not yet possible to predict CMEs’ timing, speed, energy, or
momentum, nor is there full scientific understanding of how a CME converts so much of its energy into
particle radiation. The planned SPP and SO missions will provide crucial information related both to the
reconnection process and to CME initiation. These issues present a third challenge: SHP-3. Determine
how magnetic energy is stored and explosively released and how the resultant disturbances propagate
through the heliosphere.
The heliopause, where the Sun’s extended atmosphere ends and the galactic medium begins, is a
region that is rich in unique and unexplored physics. It is also the boundary that, in part, controls the
penetration of high-energy galactic cosmic rays into near-Earth space. Interstellar neutrals are crucial to
the outer heliosphere because they stream into the heliosphere unimpeded by the heliospheric magnetic
field and dump energy into the solar wind. They are the dominant energy source of the outer heliosphere.
A revolution in understanding of the outer heliosphere is unfolding as the Voyager spacecraft provide the
first in situ data from this region and NASA’s Interstellar Boundary Explorer (IBEX) and Cassini missions
use energetic neutral atoms to remotely sense processes occurring in the same region (see Figure 2.3).
During the next decade, the Voyager spacecraft are expected to exit Earth’s heliosphere, entering
interstellar space. For the first time, operating spacecraft will enter into our local galaxy and gather local
5 National Research Council, Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop, The National
Academies Press, Washington, D.C., 2006.
Solar and Space Physics: A Science for a Technological Society
56
SOLAR AND SPACE PHYSICS: A SCIENCE FOR A TECHNOLOGICAL SOCIETY
Active regions are locations where these explosive events are concentrated. There, magnetic energy is
released in the form of ejected plasma, electromagnetic radiation, and heat that energize the local plasma.
Figure 1.3 shows a series of active regions seen in EUV light from the Solar Dynamics Observatory (SDO)
that form a chain across the upper half of the Sun. These arrays of loops emerge from the churning solar
atmosphere below and are embedded in plasmas with temperatures of around 10 7 K. The photosphere,
by comparison, is relatively cold at 6,000 K. The mechanisms that produce the hot corona of the Sun and
other stars still defy definitive explanation, and determining how this occurs is a high-priority science goal
of NASA’s Solar Probe Plus (SPP) mission and also of the Solar Orbiter ESA/NASA joint mission.
How the corona is generated and what physical processes heat the coronal plasma and control its
dynamics are not yet understood, thereby defining the second major challenge: SHP-2. Determine how
the Sun’s magnetism creates its hot, dynamic atmosphere.
An important result of recent research is the discovery of the critical role that magnetic reconnection
plays in modulating the energy flux from the Sun. The turbulent flows of the Sun’s surface twist and distort the coronal magnetic fields, thereby increasing their energy. The magnetic energy accumulates over
days, weeks, or perhaps longer. When adjacent magnetic fields pointing in opposite directions become
sufficiently strong, the magnetic fields explosively annihilate each other during magnetic reconnection (see
Figure 1.3). The released magnetic energy drives high-speed flows, heats the local plasma, and contributes
in complex ways to accelerating particles to relativistic energies, producing the intense bursts of energized
particles that characterize solar flares. This process occurs almost continuously in the active regions in the
corona (see Figure 1.3). As a result, the corona and heliosphere are filled with high-energy radiation, both
particle and electromagnetic (UV, X rays, and gamma rays).
The strongest of these reconnection events propel CMEs into the solar wind, and the CMEs steepen
into shocks that accelerate ions and electrons to high energy. Figure 2.8 shows a numerical simulation of
a CME, illustrating the scale of the ejected field and plasma. When directed Earth-ward, CMEs generate
large geomagnetic storms and intense energetic particle events in near-Earth space. The energetic particles
from these shocks pose significant threats to human and robotic space exploration. 5
The success of simulations in reproducing many of these observations testifies to the maturity of
scientific understanding of these significant events. However, even though it is now possible to predict
where on the Sun a CME will originate, it is not yet possible to predict CMEs’ timing, speed, energy, or
momentum, nor is there full scientific understanding of how a CME converts so much of its energy into
particle radiation. The planned SPP and SO missions will provide crucial information related both to the
reconnection process and to CME initiation. These issues present a third challenge: SHP-3. Determine
how magnetic energy is stored and explosively released and how the resultant disturbances propagate
through the heliosphere.
The heliopause, where the Sun’s extended atmosphere ends and the galactic medium begins, is a
region that is rich in unique and unexplored physics. It is also the boundary that, in part, controls the
penetration of high-energy galactic cosmic rays into near-Earth space. Interstellar neutrals are crucial to
the outer heliosphere because they stream into the heliosphere unimpeded by the heliospheric magnetic
field and dump energy into the solar wind. They are the dominant energy source of the outer heliosphere.
A revolution in understanding of the outer heliosphere is unfolding as the Voyager spacecraft provide the
first in situ data from this region and NASA’s Interstellar Boundary Explorer (IBEX) and Cassini missions
use energetic neutral atoms to remotely sense processes occurring in the same region (see Figure 2.3).
During the next decade, the Voyager spacecraft are expected to exit Earth’s heliosphere, entering
interstellar space. For the first time, operating spacecraft will enter into our local galaxy and gather local
5 National Research Council, Space Radiation Hazards and the Vision for Space Exploration: Report of a Workshop, The National
Academies Press, Washington, D.C., 2006.