Discover the vital role sunlight plays in human health and how to harness its benefits safely.
Nothing matters more to life on Earth than the Sun. Without its heat and light, our planet would be a frozen, lifeless rock. The Sun warms the oceans, drives the atmosphere, shapes weather patterns, and fuels the photosynthesis that supplies food and oxygen.
We usually notice the Sun through heat and light, yet subtler emissions also affect Earth and society. Bursts of energetic particles and X-rays from solar flares can disturb the ionosphere, disrupting or even blacking out long-distance radio communications. Magnetic storms triggered by solar events may induce large voltage swings in power grids, threatening urban blackouts. Activities as varied as homing-pigeon flights, transatlantic cable traffic, and oil-flow control in the Alaska pipeline can be influenced by such disturbances. Understanding these solar-driven changes is therefore important for science, society, and the economy.
Humans have long watched the Sun with reverence and fear, worshiping it and dreading eclipses. Since the early 1600s, telescopes have let scientists study the light and heat that penetrate our turbulent atmosphere. In recent decades, instruments and astronauts have ventured above the atmosphere to observe the Sun and its eruptions directly.
Viewed only in visible light from the ground, the Sun appeared to be a fairly steady star, waxing and waning through an roughly eleven-year sunspot cycle. Space-based observations in ultraviolet, X-ray, and gamma radiation—wavelengths blocked by the atmosphere—reveal a far more dynamic object, highly responsive to flares and other solar activity.
We now regard the Sun as a site of violent disturbances, with sudden, dramatic motions above and below its visible surface. Evidence is growing that solar activity can influence space conditions well beyond Earth. Historical records hint at unexplained solar variations in the past, prompting questions about how such changes might modulate future climate.
NASA: The Sun NOAA Space Weather Prediction Center
We now have a clearer picture of the Sun’s reach. Its magnetic field fills interplanetary space all the way to the edge of the solar system. Steady solar wind and occasional bursts of charged particles sweep outward, frequently striking Earth’s atmosphere and those of the other planets. Spectacular photographs from space show only part of the story; satellite instruments reveal invisible features—lines of magnetic force, swarms of atomic particles, electric currents, and a vast geocorona of hydrogen atoms—encircling our planet. Each component is as dynamic as the cloud patterns seen from below. Earth’s magnetic field stretches tens of thousands of miles into space, trapping diverse streams of electrons and protons, while large electric currents flow above the planet, influencing both high-altitude regions and ground-level conditions.
Space-based observations have greatly expanded our view of the Sun, interplanetary space, and Earth’s neighborhood. We can now detect phenomena completely invisible from the surface, assembling a more complete and coherent picture of how activity in one region of the solar system affects another.
We often overlook the one star visible by day: the Sun. It is the only star close enough to study in detail, yet the physical processes we observe must also operate in billions of distant stars. Understanding the Sun is therefore essential for interpreting the nature and behavior of other stars, while surveys of different stellar types place the Sun in context.
The Sun is a fairly typical star among the roughly 100 billion in the Milky Way. Most observable stars have masses between about 0.1 and 30 times that of the Sun, and surface temperatures ranging from roughly 2 000 °C to 40 000 °C. At about 6 000 °C the Sun lies on the cool side of this range; hot stars are rare, and most ordinary stars are cooler. Compared with explosive variables such as novae and supernovae, the Sun is notably stable and ordinary.
This long-term stability of our Sun was probably crucial for the development of life on Earth. Biologists believe that a relatively stable average temperature had to prevail on Earth during the past 3 billion years for life to evolve to its present state. The relative stability of the Sun is also important to astronomers trying to understand the basic nature of it and other stars. Violent activity in the Sun could mask the more subtle and long-enduring processes, which are the basic energy-transport mechanisms of our star. Fortunately, they are not hidden, and we have been able to map the trend in solar properties with height above the visible surface.
Above the minimum-temperature region in the photosphere, we have measured how the gas gets hotter as it thins out with height. The chromosphere and corona, each hotter than the layer below, are warmed by the transfer of energy from below through processes that are still not fully understood.
Until space observations became possible, we knew nothing about coronae in any other stars and had only marginal information about stellar chromospheres. Now, space observations have shown us that a large fraction of the stars in the sky have chromospheres and coronae.
On several dozen stars, we have even detected activity that may be connected with sunspot (or “starspot”) cycles like those of our own Sun. X-ray telescopes carried on satellites have recorded flares in other stars that are far more powerful than the already impressive flares of the Sun. By observing the strength and frequency of these events on stars with masses, ages, and rotation rates that differ from those of the Sun, we search for answers to such basic questions as: “How does the sunspot-cycle period depend on the star’s rotation rate?” or “What is the relation between the temperature of a star’s corona and the strength of its magnetic field?” By deciphering the general pattern of stellar properties, we can better understand what drives activity on the Sun.
The Sun presents us with a bewildering variety of surface features, atmospheric structures, and active phenomena. Sunspots come and go. The entire Sun shakes and oscillates in several different ways at the same time. Great eruptions called prominences hang high above the Sun’s surface for weeks, suspended by magnetic force, and sometimes shoot abruptly into space from the corona. The explosions called solar flares emit vast amounts of radiation and atomic particles in short periods, often with little or no warning.
Space observations have revealed many solar features hidden from ground-based telescopes—Sunshine’s hottest spots shine mainly in ultraviolet and X-rays, not visible light. Only from orbit can we map high-temperature flares in detail and measure their physical conditions. Satellites expose the higher, hotter layers of the solar atmosphere that are normally invisible from Earth. During flares and other violent events, the Sun behaves like a natural particle accelerator, pushing electrons and protons to near-light speeds. These fast particles emit the high-energy X-rays and gamma rays our spacecraft record, and can even trigger nuclear reactions on the solar surface.
Two advances deserve emphasis: the discovery that magnetic fields shape almost every aspect of the Sun’s upper atmosphere, and the identification of the solar wind—a steady outflow of atomic particles that evaporates from the solar atmosphere, accelerates to hundreds of kilometers per second, and streams into space in all directions.
For any solar particle to reach Earth it must first cross our planet’s magnetic field. Before the solar wind was known, Earth’s field was pictured as a simple bar-magnet pattern fading indefinitely into space. We now know the solar wind compresses the outer field into a sharp boundary. Outside this boundary, the solar wind and interplanetary magnetic field dominate; inside lies the magnetosphere, controlled by Earth’s field. Data from many missions show the wind molds the magnetosphere into a teardrop: the nose sits about 10 Earth radii (≈ 65,000 km / 40,000 mi) sunward, while the tail stretches more than 600,000 km (370,000 mi) downstream—well past the Moon’s orbit.
At the boundary of the magnetosphere, a constant tug-of-war unfolds between Earth’s magnetic field and the Sun’s forces. Buffeted by changes in solar-wind speed and density, the magnetosphere’s size and shape shift continuously. When the solar wind hits this boundary, it creates shock waves akin to the sonic boom ahead of a supersonic jet. Just inside this frontier, the magnetosphere remains dynamic, hosting two belts of highly energetic charged particles trapped hundreds of miles above the atmosphere. Professor James Van Allen and his University of Iowa team discovered these belts in 1958 with simple radiation detectors aboard Explorer 1, the first U.S. satellite.
The magnetosphere’s structure also governs aurorae. Pre-space-age textbooks suggested that aurorae arose when protons from the Sun slipped through supposed gaps near the magnetic poles, struck oxygen atoms, and produced the familiar glow of the Northern Lights.
Space-age measurements show a more intricate picture. Particles from both the solar wind and Earth’s atmosphere appear to accumulate in the magnetotail, then periodically surge into the polar regions along magnetic field lines. Some mechanism—still not fully understood—accelerates them to high speeds. The magnetotail thus acts as a reservoir that refills and empties in cycles. When solar activity peaks near sunspot maximum, the process intensifies, aurorae brighten, and their reach can extend closer to the equator.
