By Steele Hill
A flare to remember - The largest solar flare ever recorded as observed by the NASA-European Space Agency Solar and Heliospheric Observatory (SOHO) satellite on April 2, 2001. Luckily, the eruption in the sun’s atmosphere, releasing the energy equivalent of a billion megatons of TNT, was not aimed at Earth.
While we do not often stop to think about it, we are living in the atmosphere of the sun, known as the heliosphere. And we creatures on Earth are just beginning to grasp many of the implications this brings. The sun is our source of light and life, but also of radiation. And it challenges our reliance on technology in numerous ways. Over the past 50 years NASA has made significant investments and achieved huge scientific advances in understanding the sun, its processes and impact on the entire solar system. At NASA, the Heliophysics Division is concerned with three major questions: Why and how does the sun vary? How do the Earth and planetary systems respond? What are the impacts on humanity?
From NASA's beginnings, agency scientists realized solar observation from space would create unique research opportunities not possible on Earth. From space, the sun’s layers can be peeled away by observations in multiple wavelengths of light, light that is filtered out by our atmosphere. From space, the solar wind (particles constantly streaming from the sun) can be sampled. In space, weather and a rotating Earth are never an issue, so we can observe the sun every moment of the day.
Research outpost - The Skylab space station, occupied by three astronaut teams in 1973-1974, enabled the first human study of solar phenomenon from space.
The idea of studying the sun, of course, is not new to human history. Native peoples have shown great interest in the sun and its movements throughout history. In ancient China, people reportedly blinded themselves trying to discern sunspots. The great monolithic structure at Stonehenge (begun around 2300 B.C.) has a critical orientation towards the summer solstice. Thousands of smaller stone circles exist throughout Europe and beyond, noting changes in the sun’s angle and position from day to day, repeated year to year. The large Mayan pyramid of El Castillo, built around 800 A.D., had a specific solar orientation.
The scientific record of our life-giving star probably began in 1612 when Galileo Galilei observed, recorded, and wrote about sunspots with a recent invention, the telescope. Others began to do the same. Science had now marked the creation of a consistent record of sunspot observations. Developments in the nascent field came slowly. In 1845 two French physicists took the first photo of the sun. Over the next 100 years or so solar science made incremental progress with ground-based instruments, including the development of radio astronomy. Yet, it was not until the Space Age that major advances ensued, one building upon the other. From initial, sporadic steps, a whole Sun-Earth systemic program emerged at NASA called Heliophysics, which stands for the exploration of the sun, its effects on Earth and the planets and for the study of space environmental conditions and their evolution that will be experienced by human and robotic explorers. This broad charter for NASA’s Heliophysics division emphasizes the understanding of the underlying physics of this complex, coupled dynamic system with the sun at its center. The group’s goal is to understand this system’s behavior to the point that it can be the object of prediction. To advance this end, a remarkable view of solar system events is provided through a set of distributed fleet of spacecraft, managed as one Heliophysics Great Observatory.
A fundamental finding - NASA scientists continue to investigate the Van Allen radiation belts, the regions within Earth’s magnetic field that traps charged subatomic particles, confirmed by America’s first space mission, Explorer 1.
We know that the sun has a fairly regular 11-year cycle of activity. Does the sun change over time? Could it be the main culprit behind global warming? These are science questions, but more immediate, practical concerns came to the fore around 1945. The Army and Navy expressed an early interest in the impact of solar activity on military radar and communications systems. Dr. Richard Fisher, director of the Heliophysics Division of NASA’s Science Mission Directorate, explained, “There is a traceable history from military research dealing with situational awareness, of solar-originating events that cluttered radars, issues of the nature of radiation on the top of the Earth, which was not well known. Almost all long distance UHF communications were sensitive to radio propagation conditions modulated, to a certain extent, by the sun.”
This interest led to Explorer 1, the first space mission to advance solar-terrestrial science. Its instruments for studying space plasmas discovered charged particles from the sun trapped by the geomagnetic field around Earth that we came to call radiation belts. With additional explorations, we found more Sun-Earth interaction surprises. For example, the Mariner 2 (1966) mission to Venus carried instruments that measured magnetic fields and the radiation environment around the spacecraft. As Fisher recalled, “There was a huge surprise with the discovery of a wind that blows straight up off the sun. The magnetic field in the solar system near the Earth was almost as strong as the Earth’s own magnetic field, which was a great surprise, as well. That discovery stimulated a variety of research, showing how the Earth is connected to the sun, not only by radiation – heat and light – but also by solar winds and solar magnetic fields out past the Earth.” The impact of solar wind and solar transient events on Earth came to be known as Space Weather and an entirely new science discipline titled “Heliophysics” emerged to address the growing needs for understanding the physics of space weather.
Here comes the sun - An illustration of a coronal mass ejection blast from the sun and subsequent impact on Earth composed of an actual sun image superimposed on a coronagraph, both from the SOHO spacecraft.
In 1973-74 NASA conducted its first sustained long-term manned presence in space with Skylab, a rudimentary space station manned by three crews for missions up to 80 days. Skylab had six solar study instruments. “One reason you go into space, as an astronomer, is to see wavelengths stopped by the Earth’s atmosphere,” Fisher explained. “A number of Skylab instruments looked at the sun in extreme ultraviolet and X-ray wavelengths, allowing us to see that the upper atmosphere of the sun is quite hot – a couple of million degrees. Skylab was responsible for verifying one of the most important discoveries about solar activity – coronal mass ejections [CMEs].”
Fisher recalled, “Skylab’s efforts showed occasional disturbances, kind of an arch-shaped bubble, strongly modulated by the sun’s magnetic field. This was a way in which the sun sent charged particles and magnetic fields to Earth.” It had long been known that solar flares hit the Earth with excessive UV radiation, but this was something new and a major step forward in understanding Sun-Earth connections. And its discovery of coronal holes was transformative for solar physics.
Gateway between Earth and space - View of Earth’s ionosphere as seen by NASA’s TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics) spacecraft. TIMED studies the influences of the sun and humans on the least explored and understood region of Earth’s atmosphere – The Mesophere and Lower Thermosphere/Ionosphere – where the sun’s energy is first deposited into Earth’s environment.
Other solar missions soon followed. The Solar Maximum Mission spacecraft (1980-89) enabled the solar physics community to examine, in more detail than ever before, the most violent aspect of solar activity: flares. It first detected variations in total solar irradiance due to the passage of active regions across the solar disk. Ironically, increased atmospheric drag caused by a solar storm slowly caused altitude loss that led to the spacecraft's fiery demise.
Not long after that came Yohkoh (1991-2001), a significant solar mission involving Japanese-American-British cooperation. It observed solar flares in X-ray and gamma rays, while producing a total of 6 million solar images. Yohkoh was the first spacecraft to continuously observe the sun’s corona in X-rays over nearly an entire sunspot cycle, completely changing our understanding of coronal activity.
Polar view - NASA’s Polar spacecraft provides high resolution global imaging to determine solar influenced controls of the upper atmosphere. This aurora image is part of Polar’s mission to measure the entry of plasma into the polar magnetosphere and the geomagnetic tail, the flow of plasma to and from the ionosphere, and the deposition of particle energy in the ionosphere and upper atmosphere. In using Polar, scientists have captured the first-ever movie of auroras dancing simultaneously around both of Earth’s polar regions.
O.C. St. Cyr, a NASA heliophysics scientist, asserted (and many echo this) that the joint European Space Agency (ESA)-NASA Solar and Heliospheric Observatory (SOHO) mission (1995) really let us begin to observe, understand, and begin to predict space weather. SOHO is at the Lagrangian (L1) point, almost a million miles sunward from Earth (where the Earth-Sun gravitational pulls are at an equilibrium). With its 12 instruments, SOHO was the largest and most sophisticated solar observatory ever developed. St. Cyr observed, “With SOHO’s coronagraphs and Extreme Ultraviolet Imaging Telescope, we demonstrated that we could detect coronal mass ejections at the sun that were headed toward Earth, and thus provide two to three days notice. With the Advanced Composition Explorer spacecraft, we get the final 30-60 minute warning telling us the severity of any ensuing geomagnetic storm.” Dr. Lika Guhathakurta, NASA’s Living With a Star & STEREO Program Scientist, added that, “The LASCO coronagraph images of coronal mass ejections out to 30 solar radii changed the way we viewed the sun’s corona forever.”
SOHO’s importance became immediately apparent. In early January 1997, Goddard Space Flight Center in Greenbelt, Md., (where SOHO is commanded and managed) became ground-zero for a landmark event when, to the surprise and delight of a solar-terrestrial physics workshop being held there, SOHO’s instruments recognized a small area on the sun as the source of a “halo” coronal mass ejection (i.e., a coronal mass ejection headed directly toward or away from Earth) aimed at Earth and followed its progress until impact registered at our magnetosphere. Joe Gurman, U.S. Project Scientist for SOHO noted, “The direct observation of this geo-effective event and the ensuing news coverage really marked the beginning of ‘space weather.’ Up to that point, about half the solar scientists in the world did not even believe there were halo coronal mass ejections.” He pointed out that when SOHO was lost for several months the following year, NASA realized “that a national need existed for this kind of information and immediately made it a top priority to build replacements for the SOHO space weather instruments.”
Magnetospheric substorm - Image from the MENA (Medium Energy Neutral Atom) imager showing the injection of energetic particles during a magnetospheric substorm on June 10, 2000. Substorms events, which occur on average 6 times a day and more often during the solar maximum, are magnetospheric disturbances during which the magnetic flux that has built up in the magentotail is explosively released through a reconfiguration of the Earth’s magnetic field.
Fisher expanded on another research area: “SOHO, still in operation, has a new kind of instrumentation, tuned to detect acoustic pressure waves on the sun, which it turns out are organized in a way that can be understood. And you can begin to diagnose what is underneath the visible surface of the sun, interpreting the pattern of seismic activity – pressure waves – on the sun. This technique [called helioseismology] lets us look down to about a third of the way into the sun and get an idea how material is circulated in there to create a solar magnetic cycle and calculate how the sun makes a solar cycle.” The technique allows scientists to deduce what is occurring on the far side of the sun, an achievement that only a handful of scientists thought possible. It also let scientists define at what level the dynamo of active regions is tied to the sun’s differential rotation. Today, SOHO’s lengthy and comprehensive record of solar observation continues to inspire solar physicists, students, and educators around the world.
Also at the L1 point, the Advanced Composition Explorer (ACE, 1997) spacecraft samples low-energy particles of solar origin and high-energy galactic particles with a collecting power 10 to 1000 times greater than past experiments. ACE, which complements SOHO, is the major warning station for geomagnetic storms, providing near-real-time coverage of solar wind and space weather. ACE works hand in hand with SOHO.
Unique Earth image - The Earth’s plasmasphere (region of magnetosphere consisting of low energy plasma) as viewed by NASA’s IMAGE (Image for Magnetopause-to-Aurora Global Exploration) spacecraft. This view toward Earth’s north pole shows an emission from plasmaspheric helium ions appearing in false color as a pale green cloud surrounding the planet.
STEREO (2006) marked another unique advance. Scientists could observe the sun in 3D using two nearly identical STEREO spacecraft, one ahead of Earth and the other trailing behind. Dr. Lika Guhathakurta could not contain her enthusiasm for the program: “With STEREO we are witnessing the solar wind for the very first time, a view from the sun all the way to one Astronomical Unit in 3D, which was unimaginable even when we put mission payload together. The instrument team did a phenomenal job – they made the impossible possible!” As the NASA spacecraft continue to separate, they will be able to observe directly more and more of the sun. In a few years, we will see it all simultaneously, another first!
What are the effects of these solar storms? Fisher noted, “By the time there were telegraphs, it was known that solar flares could impact telegraph lines. So from the standpoint of communications and, later, navigation, there has been an interest in what will happen next.” NASA has a particular interest in studying the sun: Radiation and solar storms can threaten astronauts and satellites in space. The International Space Station, in low Earth orbit, is protected from most of the sun's radiation by Earth’s protective shield, the magnetosphere. However, astronauts traveling to Mars or exploring the moon’s surface face greater danger from solar storms and cosmic radiation from outer space. One major NASA goal: Predict when solar storms may occur, and when they do, be able to calculate the speed, direction and intensity of such storms, and, ultimately, devise new methods to protect the astronauts from these dangers.
Stereo vision - One of the two STEREO spacecraft took this full disk image of the sun in an extreme ultraviolet wavelength on June 9, 2007. The image showcases plasma racing along magnetic field lines arcing high above a series of at least five active regions strung out like pearls on a thread.
Other technological impacts from solar storms include power transformer spikes and burnouts, pipeline corrosion, damage to and even total failure of satellites, problems with GPS systems, and radiation concerns for airline crew and passengers who often fly polar routes (most open to the flow of charged particles). In fact, 6 million people were affected when Hydro-Quebéc lost power for nine hours in 1989 due to the impact of a major solar storm. Scientists now suggest that the “perfect solar storm,” the one in a hundred years kind, could wreak significant damage to our technological infrastructure, which did not exist in 1859 when the last one occurred.
Looking ahead, NASA’s Solar Dynamics Observatory will be the next major solar explorer. Scheduled for launch in late 2008, the Solar Dynamic Observatory essentially will take over SOHO’s role as our solar watchdog. Engineers have ratcheted up its capabilities so that is like “SOHO on steroids.” Its UV images will present four times the resolution of SOHO; the frame rate of images will improve at least tenfold. Again, we see new missions building on and technologically advancing beyond their predecessors.
Coming up on sweet 13 - The SOHO spacecraft, undergoing final inspection in 1995, is approaching its 13th year of operations.
Over the next 10 or so years NASA will launch more spacecraft to investigate the dynamic sun and its interaction with the planetary environments. The Solar Probe mission will explore within a few million miles of the sun, far closer than anything has gone before. It will ultimately dive into the sun, sending back data to the end. Constellations of spacecraft will measure the complex interactions of solar phenomenon the upper atmospheres of Earth, Mars and Jupiter. Observations of regions near Earth will help us understand key physical processes.
Other efforts will push our technological limits. Solar sail technology, using the solar wind for its propulsion, is being tested. The Heliophysics Great Observatory is constantly renewed, upgraded and refocused, and it will expand to predict hazardous events wherever explorers may travel. Longer term, the Heliophysics flight strategy is to deploy modest-sized missions, frequently, to form a small fleet of solar, heliospheric and geospace spacecraft that function in tandem to understand our electrical and magnetic connection to the sun.
Multiple missions - NASA’s current operating heliophysics missions in white and future scheduled launches in yellow.
NASA’s groundbreaking explorations and heliophysical discoveries and over the past 50 years have been breathtaking. Fisher, pointing to the Great Observatory concept, summed up: “Multipoint observations over the solar system, investigations deeper into the sun, a general reconnection investigation and learning how the sun’s magnetic field interacts with the different planets will occupy scientists for a long time. To date, we have just scratched the surface a little bit.”