A MEETING WITH THE UNIVERSE

Chapter 3-3


The Sun in Space:
Magnetic Fields and the Solar Wind

The Sun as a Magnet

We have known since 1912 that there are powerful magnetic fields in sun spots. In fact, the sunspot cycle could just as well be described as a periodic increase and decrease in the amount of solar magnetism. The magnetic fields are generated deep within the Sun by a natural dynamo that reverses itself every eleven years. This dynamo action may result from an interaction between the Sun's 27-day rotation and the rising and falling of huge blobs of gas in a layer just below the solar surface.

When the Sun was photographed from Skylab, we learned that the whole atmosphere above the surface of the Sun is structured by the presence of changing magnetic fields. The solar corona often appears smooth when glimpsed from the ground during total eclipses of the Sun. However, Skylab X-ray photographs proved that the corona is composed almost entirely of individual loop structures, formed by streams of hot gas channeled along lines of magnetic force.

Some loops are small and isolated; others are so large that their two feet may be separated by half the solar surface. The majority of the loops are arranged in long rows or arcades, which frequently cover a large fraction of the surface.

image of the swirling surface of the Sun
Mysterious source of the Solar Wind.
A long, dark coronal hole, here seemingly parting a glowing "Red Sea" of million-degree coronal gas, separates magnetically-shaped coronal arches to expose the Sun's cooler chromosphere layer below. Invisible in ordinary photographs, coronal holes appear prominently in images made, as here, in so called "soft", or low-energy, X-rays.
 


In addition to the prominent loops in the corona, many very small magnetic loops appear suddenly at random locations on the solar surface, where they emit intense X-rays for a few minutes, and then fade away in an hour or two. These loops, identified as "X-ray bright points" in Skylab photos, are generated when magnetic fields emerge from inside the Sun. For some unknown reason, this type of magnetic activity grows to a maximum when sunspots are at a minimum. An explanation of this unexpected anticorrelation remains as a challenge to solar researchers.

Perhaps the Sun's most remarkable magnetic features are the coronal transients. These objects were discovered by the seventh Orbiting Solar Observatory satellite (OSO-7) and later were studied intensively by Skylab. A typical transient begins as a gigantic bubble that appears near the solar surface and flies out through the corona at a velocity of hundreds of kilometers per second. These transients usually are caused by solar flares or by the eruption of a type of solar prominence known as a cool loop. The bubbles, consisting of high temperature plasma, carry along magnetic fields from the lower corona and disrupt the upper corona as they expand outward.

The solar wind blows on the Earth

Before the Space Age, people thought that the material that makes up the Sun stays with it. The solar corona was thought to merge gradually into interplanetary space, a static gas of low density.

There were some problems with this "quiet sun" concept. It was known that the violent solar flare explosions are often followed by brilliant displays of aurorae (the Northern Lights and Southern Lights) in the Earth's atmosphere and by disturbances in the Earth's magnetic field. Some physicists speculated that these effects might be due to streams of atomic particles from the Sun, but even these scientists felt that these events were rare and not part of the day-to-day behavior of the Sun.

Geophysicists who studied the Earth's magnetic field were faced with a mystery. They often found disturbances in the Earth's field that did not coincide with solar flares but which repeated at intervals of 27 days, the period of the Sun's rotation on its axis. Because of the coincidence in timing, the geophysicists thought that some solar influence was at work. The influence was ascribed to so-called "M-regions" ("M" standing for "mystery") on the Sun, but attempts to locate the M-regions as sunspot groups or other specific visible features on the solar surface were unsuccessful.

photo of explosions on the sun's surface
Escape from the Sun.
More than 1. 6 million kilometers (one million miles) in diameter, a glowing white "solar bubble", or coronal transient, races off into space from the solar corona. The Sun itself is eclipsed by the dark disk of a Skylab instrument designed to study these remarkable phenomena that occur unseen from Earth. A series of photographs made on June 10, 1973, showed how the bubble grew as it moved rapidly outward. Other Skylab observations confirmed that the bubble was produced when a prominence erupted into the corona from the solar surface below.
 

The first step in solving these puzzles was taken in 1957, when a University of Chicago physicist, Eugene Parker, demonstrated that the solar corona, which has a temperature of two million degrees, cannot be static. The corona is so hot, Parker explained, that its gas is constantly evaporating away from the Sun at supersonic speed. He predicted that there must be a solar wind, with a density of about 10 atoms per cubic centimeter (160 atoms per cubic inch), continuously blowing away from the Sun and passing the Earth at the high speed of several hundred kilometers per second. The theory would also account for the behavior of certain comet tails, which acted as though they were blowing in an electrified wind.

Several early interplanetary spaceprobes detected the solar wind around 1959-1961. In 1962, Mariner 2 made a detailed survey that showed that the properties of the solar wind agreed with Parker's predictions. Instruments carried on Mariner and other spacecraft also found that the solar wind actually is guided along an interplanetary magnetic field which originates at the Sun. The magnetic field is stretched outward by the flowing wind and warped by the turn ing of the Sun, so that it has the spiral shape of a gigantic pinwheel. The field begins to be stretched at a height of about one solar diameter above the surface of the Sun. This stretching explains the elongated and nearly radial appearance of the streamers seen in the outer corona during eclipses of the Sun and which also have been photographed by solar telescopes on Skylab and other satellites.

diagram illustrating the rotating magnetic field that surrounds the sun
Spiral patterns in interplanetary space.
The interplanetary magnetic field makes a spiral pattern as the Sun turns. Pie-shaped "sectors" between the curved black lines are regions where the magnetic field has a consistent direction, its lines of force pointing either away from or toward the Sun (denoted by plus and minus signs, respectively). Illustrated here are measurements made by the IMP-1 (Interplanetary Monitoring Platform 1) spacecraft during three 27-day solar rotation periods beginning in late 1973. Each plus or minus denotes the direction of the field according to three hours of IMP-1 data. Outermost circle of plus and minus signs (with December dates) represents the first 27-day period; the two circles within represent the next two periods in succession. Comparison of data from one circle to the next shows that the sector structure persisted over most of the long interval of observation.
 

In the plane of the Earth's orbit, the interplanetary magnetic field often is divided into sectors of alter nating inwardly (i.e., toward the Sun) and outwardly directed fields. This magnetic pattern will persist for months as the Sun rotates and some times lasts for as long as two years.

We have recently discovered that the magnetic fields which originate in the northern hemisphere of the Sun will point in one direction (inward or outward) while fields originating in the southern hemisphere point in the opposite direction. The boundary between the two magnetic hemispheres consists of a thin neutral sheet, in which the magnetic directions are not consistent. The neutral sheet is slightly warped, so that it does not lie quite flat in the plane of the Earth's orbit. As the Sun rotates, the sheet turns too, so that the Earth is alternately on one side of the warped region and the other. As this happens, satellites near the Earth observe the change in the direction of the interplanetary magnetic field as the sector boundaries pass the Earth.

Study of the solar wind revealed the identity of the mysterious M-regions on the Sun, which cause the recurrent disturbances (geomagnetic storms) in the Earth's magnetic field. The geomagnetic storms are found to coincide with streams that are much faster than the normal solar wind. By comparing the arrival times of these high-velocity streams with pictures of the Sun's corona taken by Skylab X-ray telescopes on known dates, the high speed streams were traced to parts of the corona which emit no X-rays, the so-called coronal holes.

The temperatures and densities of coronal holes are much lower than those of other parts of the corona. Investigations show that in the holes, the magnetic field has no loops, but extends directly out into the solar wind. We do not yet know how and why coronal holes form, but we do know that they are a major source of the solar wind. Two apparently permanent coronal holes exist at the north and south poles of the Sun, and it may be that much of the solar wind that leaves the Sun originates in these two polar coronal holes.


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