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"FIELDS OF DREAMS": MEASURING
MAGNETIC FIELDS IN SPACE
Alyssa Goodman, CfA astronomer and Associate Professor of Astronomy at Harvard, was recipient of the Newton Lacy Pierce Prize for outstanding achievement by a young (under 36) astronomer. Goodman presented the Pierce Prize lecture on January 9 at the AAS meeting in Washington. A summary of her remarks follows:
On Earth, when one wants to measure a magnetic field, all one needs are a few simple hand-held instruments. Even a 10-cent compass will suffice to measure the field's direction to very high precision! However, in interstellar space, where the use of a dime-store compass is at present very impractical, it is a good deal harder--and more expensive--to measure magnetic fields accurately.
Painstaking measurements of fields in our own Galaxy carried out over the past thirty years have shown that the fields are often strong enough to affect the structure and evolution of the gaseous material between the stars known as the "interstellar medium." For example, measurements of a relatively obscure quantum mechanical phenomenon known as the "Zeeman Effect" have shown that magnetic fields in clouds of gas surrounding new stars are absolutely critical to the star-formation process. If not for the constraints imposed by the magnetic fields, new stars would form too quickly--and our Galaxy would begin to get very crowded.
The Zeeman Effect, as well as other measurements, all seem to point to a dynamic, vital role for magnetic fields in most interstellar dramas. Unfortunately, it is very difficult to get a clear picture of this key player. Magnetic fields, like gravitational or electric fields, are invisible, so we can only map their structures by recording their effects on a surrounding environment. In classic laboratory experiments, for example, iron filings on a flat surface react to and trace out a magnetic field's presence. Interstellar space is inherently three dimensional, however; so, even if it were somehow filled with iron filings, we would still have to disentangle the intersecting and overlapping effects of many fields lying along any line of sight.
Despite the difficulties, remarkable progress has been made recently toward revealing the structure of interstellar magnetic fields. Many of the techniques that have proved most successful rely on an idea related to the laboratory "iron filings" method. Interstellar space is filled with tiny, solid particles known as "dust grains," which may become aligned in the presence of a magnetic field. This alignment can be detected through astronomical observations which measure polarization similar to the polarization produced by "Polaroid" sunglasses. In sunglasses, tiny iodine molecules are lined up in rows, which causes the electric vector of transmitted light to pass through more easily when it is parallel to the rows than when it is perpendicular. In space, when the light from a background star passes through a sea of elongated magnetically aligned dust grains, its electric vector will pass through the dusty region most easily when it is parallel to the short axis of the aligned grains, which is the direction of the magnetic field. Thus, the "polarization of background starlight" can give the (projected) direction of an interstellar magnetic field.
Background starlight polarimetry has been used to map interstellar magnetic fields for nearly half a century. Unfortunately, the author's recent research has shown that this kind of polarimetry is not always a reliable probe of magnetic fields in the cold, dense regions of interstellar space where stars like our Sun are born. Apparently, these regions are filled with grains which are unable to polarize background starlight. Nonetheless, since magnetic fields are believed to play a key role in star formation in these regions, new techniques for mapping fields must be sought out and used.
The polarization of background starlight is usually described as "selective absorption," that is, aligned grains block out certain orientations of the light's electric vector. The new mapping techniques utilize the grains as "selective emitters" rather than as "selective absorbers," with the emission produced as the dust radiates away its own heat at far-infrared through millimeter wavelengths. Because the Earth's atmosphere is opaque over much of this wavelength range, the new observations must be carried out above most or all of the atmosphere from very high mountains, airplanes, and satellites. For example, observations of polarized "thermal" radiation from magnetically-aligned dust, carried out primarily from NASA's Kuiper Airborne Observatory, have revealed field structures very different from those implied by the old background starlight polarimetry maps of star-forming regions. The same structures are just now beginning to be understood theoretically, but they do appear to confirm the magnetic field's "starring" (or, at least, "supporting") role in the interstellar drama.
To make significant progress toward understanding the fascinating and varied roles interstellar magnetic fields can play, a "Milky Way Magnetic Field Mapping Mission" is being proposed to NASA. The "M4" satellite experiment would map out polarized thermal emission over a large fraction of the Milky Way, as well as in several star-forming regions of interest. The "dream plan" of M4 and/or its equivalents could provide us with the first clear and comprehensive view of the Galaxy's biggest "stars"--its interstellar magnetic fields!
NEW UNDERSTANDING OF THE SOLAR WIND: THE IMPACT OF ULYSSES AND SOHO MEASUREMENTS
CfA's Shadia Habbal was one of two solar physicists invited by the American Astronomical Society to provide updates on the extraordinary discoveries from recent solar space experiments. A summary of her talk , presented January 9 in Washington, DC, follows.
In late 1990, the Ulysses spacecraft set off on an unprecedented journey to orbit the Sun in an elliptical trajectory over its southern and northern poles. Bathed in the wind of particles known as the "solar wind" that streams away from the Sun's surface, Ulysses confirmed that the solar wind fills the entire heliosphere, that is, the vast volume of interplanetary space stretching from the Sun to the farthest edge of the Solar System.
The spacecraft also confirmed that the solar wind flows across this space in two remarkably distinct states: as a steady, fast wind that blows out at speeds up to 850 km/s, and as a more capricious, more turbulent, slower component puffing out in gusts of 300 to 400 km/s. The chemical composition of the two types of wind is also clearly different.
Surprises also awaited the mission. The fast wind occupied a significantly larger fraction of the heliosphere, extending to about 30 degrees in latitude above and below the equator. Around the equator, a pattern of alternating fast and slow winds was present. While generally steady in nature, the fast wind also showed some signs of fine structure, mainly in the form of microstreams coexisting alongside each other but flowing at slightly different speeds.
The puzzle regarding the origin of the fast and slow solar wind at the Sun was eventually unraveled by two other spacecraft: the Solar and Heliospheric Observatory (SOHO) launched in 1995, which observes the Sun from near Earth; and the Galileo space probe, whose radio signal was transmitted through the solar wind while in orbit around Jupiter.
These spacecraft observed the slow wind originating along narrow structures called "stalks," which cap the arched magnetic fields of streamers. The measurements also hinted that the fast wind emerges not only from near the poles, but from patches of open magnetic field lines all over the Sun.
Images from the Large Angle Spectroscopic Coronagraph (LASCO) on SOHO also revealed that the magnetic structure of the corona extending out to the edge of the field of view of 30 solar radii is highly filamentary, thus tracing the microstreams observed in interplanetary space by Ulysses.
Further stunning discoveries about the nature of the solar wind very close to its origin at the Sun were made by SAO's Ultraviolet Coronagraph Spectrometer (UVCS) experiment on SOHO. For example, oxygen ions in the solar wind were found to have temperatures of over 100 million degrees Celsius, many times hotter than the protons and electrons which are the dominant species in the solar wind. The oxygen ions were also found to be traveling faster than the protons and electrons, suggesting a vital role in the origin and acceleration of this supercharged stream. [See next story.]
With the Sun holding many more secrets, additional surprises may be expected from the continuing journeys of both Ulysses and SOHO into the solar wind.
SOLAR WIND APPARENTLY ACCELERATED
BY HIGH FREQUENCY WAVES
The discovery of extremely high temperatures in the solar corona by the SAO-designed Ultraviolet Coronagraph Spectrometer (UVCS) aboard the SOHO satellite has led researchers to an explanation of how electrically charged particles are accelerated in the solar corona away from the Sun at over 1.5 million kilometers per hour to fill the Solar System with the supercharged gas called the solar wind. The random motions of coronal particles in the vertical direction were found to correspond to temperatures of about 1 million degrees centigrade, but the motions in the horizontal direction are much larger, corresponding to 300 million degrees centigrade.
The spectacular and unexpected discovery that the temperature is both extremely high and dependent on direction in the solar corona cries out for an explanation. Ian Axford and co-workers at the Max Planck Institute for Aeronomy in Lindau, Germany, suggested that the asymmetric temperatures might be induced by high-frequency waves that preferentially accelerate the charged particles around magnetic field lines, which extend outward from the Sun.
The CfA's Steven Cranmer and George Field are producing a theoretical model that appears to explain the observations. In their model, the crests and troughs of the waves move outward at nearly the same rate as the charged particles spiral about the field lines, and the waves efficiently accelerate the particles faster and faster in their helical orbits. This creates a magnetic mirror force that drives the particles away from the Sun toward weaker magnetic fields. The magnetic mirror force is greater the faster the particles move around the magnetic field.
The energy source that produces such high-frequency waves is unknown, but it is likely to be related to the constantly evolving "magnetic carpet'' that was discovered by other SOHO instruments in November. Those instruments observed positive and negative magnetic fields annihilating each other and converting their energies into some other form. It appears from the UVCS/SOHO observations that some of this energy takes the form of high frequency waves that propagate into the corona where they cause the high observed velocities and temperatures.
--Steven Cranmer and George Field
U of A MIRROR LAB COMPLETES PRECISION POLISHING 6.5-METER MIRROR FOR MMT CONVERSION PROJECT
In late October, technicians at the University of Arizona's Steward Observatory Mirror Laboratory marked completion of the 6.5-meter-diameter mirror by lifting the 10-ton behemoth off the automatic polishing table and into its support cell in the vast workshop underneath the east wing of the university's football stadium.
Billed as the lab's biggest and best-yet achievement, the 6.5-meter mirror is destined for installation later this year in the joint Arizona-Smithsonian Multiple Mirror Telescope on Mount Hopkins. Over the next several months, members of the combined staffs will develop support systems and thermal control systems, with the mirror sitting in its cell, and continue to test the mirror's shape throughout this "integration" phase.
Successful integration of mirror and mirror cell is critical: If the support forces that maintain the shape of the mirror in the telescope fail, the mirror will distort and the exquisite figure polished into the mirror could be lost.
The university's mirror experts used stressed-lap polishing and hologram-verified measurements to produce a surface of unprecedented smoothness. The mirror has been polished to a figure that should produce better than one-tenth of an arc second "seeing" once on line. That's a theoretical level of "seeing" about three times better than occurs under optimal conditions on Mount Hopkins, which is an exceptional viewing site.
"We wanted the mirror to be two or three times better than the best wavefront that will ever reach the telescope," said Buddy Martin, head of the Mirror Lab polishing team. The mirror must be that much better to ensure it doesn't add significantly to the distortion of starlight that is slightly bent and blurred as it travels through Earth's atmosphere.
Astronomers used to consider one arc second, or one-3,600th of a degree, which is about the diameter of a quarter seen from the distance of three miles, to be "good" seeing. At its best, Mount Hopkins seeing is one-third arc second. The specifications on the 6.5-meter MMT mirror are .09 arc seconds, according to Martin. "We had to work hard to meet that goal," he says.
The surface of the new 6.5-meter mirror has been polished with such high precision that, if astronomers were to use it in space, they could read newsprint from a distance of three miles. Indeed, if the mirror were the size of North America, the average bump on its surface would be only a centimeter or two high.
The new MMT mirror is a stiff, lightweight "honeycomb" structure of borosilicate glass cast in the Mirror Lab's large rotating furnace in 1992. The casting marked the first time in nearly 60 years that U.S. astronomers had attempted to make a single telescope mirror larger than five meters. It also marked the first time anyone had attempted to make a lightweight mirror larger than 3.5 meters.
The new mirror will replace the MMT's current array of six 1.8m-diameter mirrors. "The $20 million conversion project will increase the telescope's light-collecting area by 2.5 times and its field-of-view by about 400 times," said Craig B. Foltz, director of the MMTO.
"What makes this mirror so spectacular is the image quality it can produce," Foltz said. "It is extraordinary because it is so optically 'fast.' At a focal length of 1.25, it focuses starlight at about eight meters from its surface. That is a really deep dish. Buddy [Martin] and his crew have done miracles in polishing."
--Lori Stiles, University of Arizona