Copyright © National Academy of Sciences. All rights reserved.
Solar and Space Physics: A Science for a Technological Society
SOLAR AND SPACE PHYSICS: RECENT DISCOVERIES, FUTURE FRONTIERS
41
for nearly a century. Cycle 24 had been predicted to be more active than cycle 23, and the unexpected
deep minimum focused attention on the need to improve understanding of the solar dynamo. Ground-based
and SOHO space-based measurements prior to the activity minimum revealed unusually low magnetic
flux near the poles of the Sun, and these low flux levels suggest that solar activity at the maximum of the
current solar cycle will be low relative to that of recent past cycles.
Poleward meridional flows in the solar convective zone may be responsible for concentrating solar
magnetic flux at the poles. Observations of this flow were made possible with great improvements in spacebased (SOHO and SDO) and ground-based (GONG) helioseismic measurements of the solar interior. These
observations have revealed changes in zonal and meridional flows consistent with the low polar flux and
have shown that solar active regions exhibit subsurface helical flows whose strength is closely related to
flare activity. Helioseismic observations are needed to firmly establish if scientists have indeed found a
key to understanding the engine of solar activity.
The deep solar minimum in 2008-2009 provided an opportunity to study the heliosphere under conditions not present since the dawn of the space age and—enabled by STEREO—to study it for the first time
in a truly global fashion: cosmic-ray fluxes near Earth reached the highest levels on record; reduced heating of Earth’s upper atmosphere by solar ultraviolet radiation led to unprecedented low drag on satellites;
and the radiation belts reached historically low levels of intensity. This enhanced galactic cosmic-ray flux
was caused by reduced modulation in a historically weak solar wind with slower speeds, lower magnetic
field, and historically low activity.
The extended solar minimum, prolonged period of low sunspot numbers, and record cosmic-ray
intensity led to suggestions that the Sun might be entering an extended period of minimum activity such
as that observed (in sunspot, 14 C and 10 Be data) during the Dalton minimum (1800-1820) or the Maunder
minimum (1645-1715). Recent low activity was used to set a lower limit for total solar irradiance (TSI), a
key factor in climate change. Measurements of TSI have consistently shown a cycle variation on the order
of 0.1 percent but give conflicting results about its absolute value. These conflicts have recently been
resolved, in favor of the lower values shown in Figure 3.1. It remains uncertain whether the TSI levels during the recent solar minimum are indicative of levels expected for a prolonged cessation of solar activity.
The past decade has seen spectacular advances in understanding of the structure of the solar magnetic
field. Increases in processor speed and massively parallel computational techniques have enabled greater
than 100-fold improvements in the spatial resolution of simulations. The resolution of observations has
improved with data from the 0.5-m-aperture telescope of the Hinode satellite and through image processing techniques applied to ground-based data from the 1-m-class apertures and the new 1.6-m New
Solar Telescope (NST). Researchers have now identified the main physical processes at work in sunspot
penumbral filaments, bright umbral dots, bright faculae, and small-scale magnetic structures. Figure 2.2
is a side-by-side comparison of the results of a numerical simulation of a sunspot and an image from the
NST; it shows astonishing correspondence in the background circulation pattern (granulation), the umbra
fibrils, and the central spot.
Solar Wind Origins
New observations of the photosphere and lower corona have revealed significant information on the
mechanisms of coronal heating, which is ultimately the driver of the solar wind. High-resolution chromospheric images from Hinode’s Solar Optical Telescope unveiled relentless dynamics and contorted
structures. A new type of spicule (a radial jet of plasma) was discovered that may play a critical role in
transferring mass and energy to the corona. The narrowband EUV images from the SDO Atmospheric
Imaging Assembly have revealed that coronal loops cannot be in a steady state as previously believed.
Solar and Space Physics: A Science for a Technological Society
SOLAR AND SPACE PHYSICS: RECENT DISCOVERIES, FUTURE FRONTIERS
41
for nearly a century. Cycle 24 had been predicted to be more active than cycle 23, and the unexpected
deep minimum focused attention on the need to improve understanding of the solar dynamo. Ground-based
and SOHO space-based measurements prior to the activity minimum revealed unusually low magnetic
flux near the poles of the Sun, and these low flux levels suggest that solar activity at the maximum of the
current solar cycle will be low relative to that of recent past cycles.
Poleward meridional flows in the solar convective zone may be responsible for concentrating solar
magnetic flux at the poles. Observations of this flow were made possible with great improvements in spacebased (SOHO and SDO) and ground-based (GONG) helioseismic measurements of the solar interior. These
observations have revealed changes in zonal and meridional flows consistent with the low polar flux and
have shown that solar active regions exhibit subsurface helical flows whose strength is closely related to
flare activity. Helioseismic observations are needed to firmly establish if scientists have indeed found a
key to understanding the engine of solar activity.
The deep solar minimum in 2008-2009 provided an opportunity to study the heliosphere under conditions not present since the dawn of the space age and—enabled by STEREO—to study it for the first time
in a truly global fashion: cosmic-ray fluxes near Earth reached the highest levels on record; reduced heating of Earth’s upper atmosphere by solar ultraviolet radiation led to unprecedented low drag on satellites;
and the radiation belts reached historically low levels of intensity. This enhanced galactic cosmic-ray flux
was caused by reduced modulation in a historically weak solar wind with slower speeds, lower magnetic
field, and historically low activity.
The extended solar minimum, prolonged period of low sunspot numbers, and record cosmic-ray
intensity led to suggestions that the Sun might be entering an extended period of minimum activity such
as that observed (in sunspot, 14 C and 10 Be data) during the Dalton minimum (1800-1820) or the Maunder
minimum (1645-1715). Recent low activity was used to set a lower limit for total solar irradiance (TSI), a
key factor in climate change. Measurements of TSI have consistently shown a cycle variation on the order
of 0.1 percent but give conflicting results about its absolute value. These conflicts have recently been
resolved, in favor of the lower values shown in Figure 3.1. It remains uncertain whether the TSI levels during the recent solar minimum are indicative of levels expected for a prolonged cessation of solar activity.
The past decade has seen spectacular advances in understanding of the structure of the solar magnetic
field. Increases in processor speed and massively parallel computational techniques have enabled greater
than 100-fold improvements in the spatial resolution of simulations. The resolution of observations has
improved with data from the 0.5-m-aperture telescope of the Hinode satellite and through image processing techniques applied to ground-based data from the 1-m-class apertures and the new 1.6-m New
Solar Telescope (NST). Researchers have now identified the main physical processes at work in sunspot
penumbral filaments, bright umbral dots, bright faculae, and small-scale magnetic structures. Figure 2.2
is a side-by-side comparison of the results of a numerical simulation of a sunspot and an image from the
NST; it shows astonishing correspondence in the background circulation pattern (granulation), the umbra
fibrils, and the central spot.
Solar Wind Origins
New observations of the photosphere and lower corona have revealed significant information on the
mechanisms of coronal heating, which is ultimately the driver of the solar wind. High-resolution chromospheric images from Hinode’s Solar Optical Telescope unveiled relentless dynamics and contorted
structures. A new type of spicule (a radial jet of plasma) was discovered that may play a critical role in
transferring mass and energy to the corona. The narrowband EUV images from the SDO Atmospheric
Imaging Assembly have revealed that coronal loops cannot be in a steady state as previously believed.