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Facts of the Matter Richard Brill |
ASSOCIATED PRESS
The photosphere is what we see as the disk of the sun. What appears calm and constant is turbulent, seething with hot plasma driven by convection currents and magnetic fields.
View from Earth belies
sun’s turbulent natureFrom Earth the sun looks calm and constant. It floods us with its gentle heat and white light, keeping our planet at just the right temperature and nourishing Earth's abundant vegetation in its myriad forms. Its energy powers the food chain that sustains all life, including our own.
Despite its outward appearance, our star is anything but calm and far from constant. Although most of its luminosity comes from its predictable and constant day-to-day behavior, there is the much more sporadic and unpredictable behavior that astronomers refer to as the "active" sun that contributes little to its luminosity and has little effect on the sun's evolution as a star, but does affect us directly here on Earth nearly 100 million miles away. Our star is essentially evaporating, losing mass at a million tons a day in the form of charged particles that escape its enormous gravity and radiate into space.
The sun's atmosphere extends a billion miles from its surface and affects all of the planets to varying degrees, depending on their distance from the sun. The interaction of the sun's atmosphere with Earth and its magnetic field, the magnetosphere, results in what astronomers call "space weather."
The sun's huge mass (more than 300,000 Earths) holds its gasses, mostly hydrogen and helium, squeezing them and creating the tremendous pressure (250 billion atmospheres) and temperature (nearly 30 million degrees Fahrenheit) that allow nuclear fusion to take place within its core and release energy in the form of light and heat. The energy slowly works its way to the surface, eventually escaping, but in bursts rather than in a smooth flow.
The outer layer of the sun, called the photosphere, is what we see as the disk of the sun. It is especially turbulent, like a pot of boiling water, seething with currents of hot plasma driven by intense convection currents and fluctuating magnetic fields that are twisted and dragged around by the turbulent motions of the gas and the rapid rotation of the sun.
Since it is a gaseous ball, the sun doesn't have a surface like Earth, but rather, like Earth's atmosphere, it gradually thins outward.
We see what appears to be a sharp edge because the photosphere is so thin. Above the photosphere, which is typically at 10,000 degrees Fahrenheit or so, lies the cooler chromosphere, which is only visible during eclipses. Above that the temperature of the gases increases to around 2 million degrees in the outer atmosphere, the corona. Although solar physicists don't completely understand how the corona is heated to such extremes, it is clear that the temperature of the corona is so high that some particles exceed the escape velocity and radiate outward.
The constant flow of ejected material constitutes the solar wind. It extends outward for several billion miles, getting thinner away from the sun. The solar wind reflects the complexity of the sun itself, changing speed and carrying with it magnetic clouds and compositional variations. The speed of the solar wind varies from just under 1 million miles an hour above sunspots to just under 2 million miles an hour over dark windows in the corona known as coronal holes. As the sun rotates, the streams of particles of different speeds rotate with it, interacting and producing patterns in the solar wind much like that of a rotating lawn sprinkler. The resulting collisions and electromagnetic interactions produce shock waves that can accelerate the particles to even higher speeds.
A solar flare is an extreme fluctuation in the solar wind that is the result of the release of magnetic energy that has built up in the solar atmosphere, usually above a sunspot. The release of magnetic energy blasts electrons, protons, and the nuclei of heavy atoms from the photosphere like oatmeal blown from bursting bubbles in a boiling pot. Unlike the steam bubbles in the oatmeal, however, bursting magnetic bubbles cause solar flares to eject billions of tons of material into space from the photosphere. The ejected particles speed outward at five million miles an hour, which means that they reach earth in about 18 hours.
The amount of energy and matter released in solar flares is huge. During a single eruption, lasting a few minutes to several hours, a million tons of particles are ejected into space every second. The energy released in a single event is typically 10 million times greater than the energy released during a volcanic eruption here on Earth, but is still only a small fraction of the total energy emitted by the sun.
Solar flares are connected with the 11 year cycle of sunspots, the nature and causes of which remain one of the great mysteries of solar astronomy.
The sunspots themselves are located within areas of strong magnetic fields at the sun's surface, but limited understanding of the dynamics of their production does not allow for prediction of future occurrence or severity of sunspot activity.
Coronal mass ejections (CME) once were thought to be triggered by solar flares, but now astronomers know that CME events can occur with or without an associated flare. Each CME is a huge bubble of million-degree magnetic plasma that expands away from the sun behind a crescent shaped shock wave, disrupting the flow of the solar wind. Like solar flares, the frequency of CMEs varies with the sunspot cycle, occurring anywhere from once a week to two or three times a day. A single CME can carry up to 10 billion tons of plasma traveling at speeds up to 5 million miles an hour. If a CME collides with Earth, it can generate a geomagnetic storm, which can cause electrical power outages and damage or disrupt communications satellites.
Solar flares and CMEs can have huge consequences here on Earth. The most severe events can trigger planetwide radio blackouts such as the one that disrupted a communications satellite carrying pager signals in 1998, or cause power outages such as the geomagnetic-induced outage in Quebec in March 1989, which left 6 million customers in the dark for nine hours and interfered with radio communication all over the globe for days. In the latter episode, there were reports of California Highway Patrol messages overpowering local transmissions in Minnesota, 2,000 miles away, and spectacular aurora that were seen as far south as Florida. Smaller events may cause brief radio interferences that go unnoticed by all but those who are locally affected.
The storms of particles and radiation present health problems for astronauts in Earth orbit. Earth's atmosphere normally protects surface-dwellers from high energy X-ray and gamma radiation associated with solar flares, and its magnetic field channels charged particles to the polar regions, protecting those of us who reside in low and mid-latitudes. But in orbit, above the atmosphere in the outer reaches of Earth's magnetic field, there is little to shield human occupants in their fragile spacecrafts.
Radiation from a solar flare or CME can disturb the orbit of a satellite when it ionizes the Earth's upper atmosphere and expands it, thereby increasing the drag on the satellite from the expanded atmosphere.
The sun, once thought to be static and stable, is now known to be quite complex and we are just beginning to understand its intricacies. As a result, space weather is equally complex, with the multitude of particles and range of electromagnetic radiation ejected from the sun and interacting with Earth's magnetic field. It provide us with ample opportunities to observe and study the processes and products of nuclear fusion that power the sun, the solar atmosphere in which we live protected by our own planetary magnetic field, and the interactions between the two.
Space weather also can impact us in other ways, especially as we expand our technology both on the ground and in space and become increasing reliant on it for communications and data transmission.
Richard Brill picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be contacted by e-mail at rickb@hcc.hawaii.edu
