You didn’t have to be a solar expert to see stunning beauty in the images that satellite-borne instruments sent to Earth in the late 1990s. The sophisticated new tools sampled the Sun’s electromagnetic energy at a variety of wavelengths, resolutions, and intervals, producing data that were both visually compelling and rich in science-relevant detail.
Solar modelers at NCAR’s High Altitude Observatory (HAO), and their colleagues elsewhere, drew on these and other observations as they developed a new generation of models that aimed to simulate the Sun’s behavior in three dimensions. Modelers also made big strides in simulating the impact of solar magnetism on Earth’s atmosphere, where electrodynamics and thermodynamics mingle in complex ways.
The Sun’s coronal mass ejections, a topic of NCAR research since the center’s earliest days (see page 9), remained front and center. A CME typically accelerates through the corona in only a few hours. If pointed at Earth, it can irradiate astronauts, disable the circuitry in satellites, knock out surface power grids, degrade the accuracy of the Global Positioning System by up to a factor of five, and paint the high-latitude sky with shimmering auroras.
Discovered by NASA’s pioneering Skylab satellite in 1973, CMEs were soon linked to auroras and other Earth-based impacts. Today, forecasts issued shortly after a CME emerges—based on reports from NCAR’s Mauna Loa Solar Observatory and elsewhere—provide warnings from hours to several days in advance of a potential geomagnetic storm. To provide even more lead time, scientists still need to learn more about how magnetic fields trigger CMEs and related coronal activity.
Starting in 1998, NASA’s TRACE satellite (Transition Region and Coronal Explorer) captured arching structures across tightly focused regions from the photosphere up through the corona (see image at top right). But the arches represented only a few of the corona’s intricately nested and evolving magnetic field lines. To better envision these multilayered structures, NCAR’s Sarah Gibson used visualization routines based on modeling by colleague Yuhong Fan to see how an idealized twisted tube of magnetic flux—a CME in the making—might appear in observations.
By 2008, Gibson and Fan had demonstrated the importance of partially ejected “ropes” of flux for space weather predictions. “If we can figure out the science behind the eruptions, we’ll be in a much better position for making future forecasts,” said Gibson.
Meanwhile, Mausumi Dikpati led development of a groundbreaking model that simulates the evolution of magnetic fields in the outer third of the Sun’s interior (the solar convection zone). With colleagues at NCAR and elsewhere, Dikpati showed that the duration of a solar cycle is probably determined by the strength of the Sun’s meridional flow, and that the combination of this flow and the lifting and twisting of magnetic fields near the bottom of the convection zone generates the observed symmetry of the Sun’s global field with respect to the solar equator. The model provides a physical basis for projecting the nature of an upcoming cycle from the properties of previous cycles, as opposed to statistical models that emphasize correlations between cycles. “The results compare well with many features of actual solar cycles,” says Dikpati.
With an eye toward Earth impacts, NCAR partnered with several other institutions in 2002 to launch the NSF-funded Center for Integrated Space Weather Modeling, based at Boston University. The center is building a complete Sun-to-Earth model that could provide advance notice of solar disturbances and their effects. Toward that end, NCAR produced the Coupled Magnetosphere Ionosphere Thermosphere model, which is used to investigate how geospace is affected by CMEs and other types of space weather. “Our model shows the importance of two-way interactions between the magnetosphere and ionosphere,” notes CMIT leader Michael Wiltberger.
As NCAR’s solar modeling rose to new prominence in the 2000s, the center kept an eye on the actual Sun through its array of instruments at the Mauna Loa Solar Observatory (see page 10) and its involvement in satellite instruments, including a spectropolarimeter developed by Bruce Lites and colleagues for Japan’s 2006 Hinode mission. From a spaceborne perch, the instrument—part of the Solar Optical Telescope—measures magnetism on the solar surface over far lengthier periods than its Earth-bound predecessors could. “You really need long time sequences to follow the evolution of the magnetic fields responsible for heating the upper layers of the solar atmosphere and for variables that affect our climate,” said Lites.
With data and simulations now extending from the solar interior to terra firma, the Earth-Sun system is ripe for multidisciplinary, boundary-crossing research, says NCAR deputy director and upper-atmosphere researcher Maura Hagan. As Hagan puts it: “I see the 21st century as the epoch of system science.”
"The solar wind can hit Earth like a fire hose even when there are virtually no sunspots."
—Sarah Gibson, NCAR
Like an actor missing his cue, Solar Cycle 24 made more than a few people nervous. In 2006, many solar scientists expected the now-unfolding cycle to rise toward a peak sometime in 2011 or 2012. But the cycle didn’t officially start until 2008, according to NASA, and activity remained at very low levels into 2010, pushing the expected peak to at least 2013.
The cycle’s delayed onset came as no surprise to NCAR’s Mausumi Dikpati. In 2004, using a model of processes within the solar convection zone (see page 54), Dikpati and colleagues were the only solar scientists to successfully predict the protracted solar minimum. Improved observations of flow below the Sun’s surface have led to further insight. In a 2010 paper, the NCAR team showed that the duration of the minimum is linked to the span of the conveyor belt that transports magnetic flux away from the Sun’s equator (see graphic, page 55). When the conveyor extends all the way to the Sun’s pole, as it did in Solar Cycle 23, Dikpati’s model produces a longer-duration cycle.
The prolonged quiet spell was a treasure trove for those studying the Sun and its interactions with Earth. The solar minimum set space-age record lows for sunspot activity (the least observed since 1913), extreme ultraviolet radiation, and the magnetic field produced by the solar wind. Cosmic rays, partially blocked from reaching Earth by solar activity, hit a record high in mid-2009. And Earth’s thermosphere chilled to record cold temperatures, with its density reduced by about 30% at a height of 400 kilometers (250 miles), according to NCAR’s Stan Solomon. The weakened solar influence also allowed more subtle features to become evident, such as the imprint of tropical cloud systems pushed upward by atmospheric tides into the thermosphere and ionosphere.
Two special observing periods—the Whole Sun Month in 1996 and the Whole Heliosphere Interval (WHI) in early 2008—revealed much about what happens when the Sun temporarily powers down. “We’ve learned that what we think of as the quiet Sun can change from one cycle to the next in ways that alter space weather disturbances and their effects at Earth,” said Janet Kozyra (University of Michigan). However, she added, “We don’t yet know why these differences in the quiet Sun occur or how to predict them.”
While the intense solar storms produced by coronal mass ejections (CMEs) are least likely to occur during solar minima, the Sun tends to unleash another form of magnetic havoc just as it’s moving into a new minimum. Working with Kozyra, NCAR’s Giuliana de Toma and Barbara Emery, and several other colleagues, NCAR’s Sarah Gibson used WHI data to analyze streams of charged particles shooting toward Earth from “holes” in the solar corona’s magnetic field at low latitudes. Although these low-latitude streams are more dilute than the bursts of magnetism and plasma found in a CME, they can last several times longer: up to 7–10 days. As a result, the collective energy they send Earth over a year’s time can rival that delivered by CMEs during a year of solar maximum.
The University Corporation for Atmospheric Research manages the National Center for Atmospheric Research under sponsorship by the National Science Foundation. Any opinions, findings and conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.