Where do the Sun's winds come from? Since spacecraft generally measure the solar wind streams near the Earth, far from their source on the Sun, scientists have to use other measurements to extrapolate back to the place of their origin. Comparisons of wind-velocity measurements with X-ray images of the Sun indicated, in the 1970s, that whenever a high-speed stream in the solar wind swept by Earth, a coronal hole rotated into alignment with our planet, ttis naturally suggested that at least some of the highspeed wind is gushing out of coronal holes on the Sun. Observations of the scintillation, or twinkling, of radio signals from remote radio sources indicated that the gusty, slow-speed wind comes from a different place, confined to low latitudes near coronal streamers when the Sun is near the minimum of its activity cycle.
Nevertheless, until recently spacecraft have only been able to directly measure a limited, two-dimensional section, or slice, of the solar wind within the plane in which the Earth orbits the Sun. ttis plane, called the ecliptic, is tilted only 7 degrees from the plane of the solar equator. Interplanetary spacecraft usually travel within the ecliptic, both because they normally rendezvous with another planet and also because their launch vehicles obtain a natural boost by traveling in the same direction as the Earth's spin and in the plane of the Earth's orbit.
tten in 1994-95, the Ulysses spacecraft, a collaborative project of NASA and the European Space Agency, abbreviated ESA, traveled outside the ecliptic plane for the first time to sample the Sun's wind over the full range of solar latitudes, including previously unexplored regions above the Sun's poles.
Ulysses' velocity data indicated that a relatively uniform, fast wind pours out at high latitudes, both near the solar poles and far outside them, and that a capricious, gusty, slow wind emanates from the Sun's equatorial regions.
As the winds blow away, they must be replaced by hot gases welling up from somewhere on the Sun. However, since Ulysses never passed closer to the Sun than the Earth does, simultaneous observations with other satellites were required to tell exactly where the winds come from. Fortunately, the Ulysses data were obtained near a minimum in the Sun's 11-year activity cycle, with a particularly simple corona characterized by marked symmetry and stability, ttere was a pronounced coronal hole at and near the Sun's north and south poles, and coronal streamers encircled the solar equator.
Comparisons of Ulysses' high-latitude passes with Yohkoh soft X-ray images showed that coronal holes were then present at the poles of the Sun, as they usually are during activity minimum. Much, if not all, of the high-speed solar wind therefore seems to blow out from polar coronal holes along open magnetic field lines, whereas the slow wind emanates from the stalks of coronal streamers, above closed magnetic field lines, at least during the minimum in the 11-year cycle of magnetic activity.
But the fast wind is not only found above polar coronal holes during solar activity minimum. At roughly the Earth's distance from the Sun, Ulysses found that the fast wind is almost everywhere outside the ecliptic, even at low latitudes near the solar equator and outside the radial projection of the coronal hole edges, tte charged particles could be guided to low latitudes by magnetic fields that originate in coronal holes and bend outward toward the equator.
But the polar coronal-hole boundaries might extend radially out from the Sun, instead of diverging at large distances, tte fast winds would not then be bent to lower latitudes, ttey might instead emanate radially from all parts of the inactive Sun, outside coronal holes, streamers or active regions. Or they could sometimes shoot straight out from coronal holes close to the equator.
Comparisons of Ulysses data with coronagraph images pinpointed the equatorial streamers as the birthplace of the slow and sporadic wind during the minimum in the 11-year activity cycle. Hot gas is bottled up in the closed coronal loops at the bottom of the helmet streamers, tte capricious slow wind can therefore only leak out along the elongated, stretched-out streamer stalks, tte part that manages to escape varies in strength as the result of the effort.
Ulysses has now completed a second orbit around the Sun, during a maximum in the 11-year cycle of solar activity, permitting a comparison of solar wind speed at activity minimum and maximum (Fig. 6.14). Near the solar activity minimum, a persistent, fast, tenuous and uniform solar wind was found at high solar latitudes, arising from coronal holes that covered both solar poles during these portions of the solar cycle. During more active parts of the 11-year cycle, fast, low-latitude flows originated from multiple low and mid-latitude coronal holes. Highly variable flows were observed at all latitudes near solar maximum, arising from a mixture of sources including coronal streamers, coronal mass ejections, coronal holes and possibly active regions.
So, with the help of other spacecraft, Ulysses has located the general place of origin for both the fast and slow components of the solar wind. Like its namesake, the great explorer in Greek mythology, Ulysses chose
To venture the uncharted distances;
to feel life and the new experience
Of the uninhabited world behind the Sun____
To follow after knowledge and excellence.27
and we would therefore agree with Joachim du Bellay(1522-1560) that:
Happy he who like Ulysses has made a glorious voyage.28
Oppositely directed magnetic fields run in and out of the Sun on each side of the long, narrow stalks of coronal streamers, providing an open channel for the slow flow once it gets out. SOHO's instruments have observed spurt-like blobs of material moving out along the stalks, like water working its way down a clogged pipe in your bathtub or sink. Observations of scintillating radio signals from spacecraft confirm the localized nature of the slow wind at activity minimum, suggesting that it is associated with the narrow stalks of coronal streamers near the solar equator.
A streamer could get so stretched out and constricted that it pinches itself off and snaps at just a few solar radii from Sun center, tte lower parts of the streamer would
FIG. 6.14 Distributions of solar wind speeds at solar minimum and maximum
Plots of the solar wind speed as a function of solar latitude, obtained from two orbits of the Ulysses spacecraft (toppanel). The north and south poles of the Sun are at the top and bottom of each plot, the solar equator is located along the middle, and the velocities are in units ofkilometers per second, abbreviated km s_1. The sunspot numbers (bottom panel) indicated that the first orbit (top left) occurred through the declining phase and minimum of the 11-year solar activity cycle, and that the second orbit (top right) spanned a maximum in activity. Ultraviolet images of the solar disk and white-light images of the inner solar corona form a central backdrop for the wind speed data, and indicate the probable sources of the winds. Near solar minimum (top left), polar coronal holes, with open magnetic fields, give rise to the fast, low-density wind streams, whereas equatorial streamer regions of closed magnetic field yielded the slow, gusty, dense winds. At solar maximum (top right), small, low-latitude coronal holes gave rise to fast winds, and a variety of slow-wind and fast-wind sources resulted in little average latitudinal variation. (Courtesy of David J. McComas and Richard G. Marsden. The Ulysses mission is a project of international collaboration between ESA and NASA. The central images are from the Extreme ultraviolet Imaging Telescope and the Large Angle Spectrometric Corona-graph aboard the Solar and Heliospheric Observatory, abbreviated SOHO, as well as the Muana Loa K-coronameter.)
then close down and collapse, and the outer disconnected segment would be propelled out to form a gust in the slow solar wind.
Coronal loops are found down at the very bottom of streamers, and the expansion of these magnetized loops may provide the energy and mass of the slow component of the solar wind. Sequential soft X-ray images, taken from the Yohkoh spacecraft, have shown that magnetic loops expand out into space, perhaps contributing to the slow wind. And SOHO instruments have shown that small magnetic loops are nearly continuously emerging into the Sun's equatorial regions at activity minimum, where they reconnect with the Sun's global magnetic field, perhaps becoming the engine that drives the slow solar wind, tte small expanding loops also carry material with them, perhaps accounting for the running blobs detected further out in the low-latitude, slow-speed solar wind.
Instruments aboard the SOlar and Heliospheric Observatory, abbreviated SOHO, have demonstrated that the fast winds accelerate rapidly, like a racehorse breaking away from the starting gate, reaching high speed very low in the corona.
Some of the high-speed outflow from polar coronal holes is apparently concentrated at the boundaries of the magnetic network formed by underlying supergranular convection cells, ttese edges are places where the magnetic fields are concentrated into inverted magnetic funnels that open up into the overlying corona, tte strongest highspeed flows apparently gush out of the crack-like edges of the network.
tte fast wind emanating from coronal holes may be accelerated to high speeds at higher altitudes, in the magnetic funnels. Closed magnetic loops maybe swept by underlying convection into the funnel regions where they undergo magnetic reconnection with existing open magnetic field lines, releasing energy to power the fast wind in polar coronal holes.
Another SOHO instrument has demonstrated that heavier particles in polar coronal holes move faster than light particles in coronal holes (Fig. 6.15). Above two solar radii from
FIG. 6.15 Heavier ions move faster than lighter ones in coronal holes Outflow speeds at different distances over the solar poles for hydrogen atoms, denoted by H° or H I, and ionized oxygen, designated 05+ or O VI. Here the distances are given in units of the solar radius, denoted R©. These data were taken in late 1996 and early 1997 with the Ultraviolet Coronagraph Spectrometer, abbreviated UVCS, aboard the SOlar and Heliospheric Observatory, or SOHO for short. They show that the heavier oxygen ions move out of coronal holes at faster speeds than the lighter hydrogen, and that the oxygen ions attain supersonic velocities within 2.5 solar radii from the Sun center. The dark shaded area denotes the hydrogen outflow speed derived from mass flux conservation; for a time-steady flow, the product of the density, speed and flow-tube area should be constant. (Courtesy of the SOHO UVCS consortium. SOHO is a project of international collaboration between ESA and NASA.)
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