The Three-Dimensional Solar Wind
Ulysses observations contribute to an evolving view of the three-dimensional solar wind from the Sun to the galactic frontier
A Hubble Space Telescope image shows the interaction of an interstellar wind with the "astrosphere" surrounding a newly formed star in the Orion Nebula. Institute scientists are involved in observational and theoretical studies of the Sun's astrosphere, the heliosphere.
Our Sun is a middle-aged, main-sequence star located near the edge of one of the spiral arms of the Milky Way, some 30,000 light years from the center of the galaxy. The Sun's immediate environment is a warm cloud of interstellar gas called the "Local Interstellar Cloud" or, more colorfully, the "Local Fluff." Within the Local Interstellar Cloud, a supersonic outflow of ionized gas (plasma) from the Sun - the solar wind - inflates an enormous bubble-like cavity which has boundaries that lie far beyond the orbit of Pluto. This solar-wind-dominated region of space is known as the heliosphere, which is an example of the fascinating astrophysical structures created by the interaction of a cosmic wind with the ambient interstellar gas.
The heliosphere's structure, dynamics and properties, and its interaction with the interstellar medium, are subjects of vigorous research by the international space science community. With strong experimental and theoretical programs in heliospheric physics, scientists at Southwest Research Institute are playing a leading role in the efforts to understand the evolution of the solar wind and its ultimate interaction with the galactic frontier.
The Inner Heliosphere in Three Dimensions
The solar wind boils out of the Sun's corona (upper atmosphere), which, at temperatures of millions of degrees, is several hundred times hotter than the Sun's visible surface. At a certain critical altitude, the corona begins to expand supersonically and flows out into the vacuum of space as the solar wind, forming the heliospheric cavity and populating it with a tenuous plasma consisting of mostly electrons and protons, some alpha particles and traces of heavier ions. The solar wind continues to expand until the pressure of the interstellar medium, through which the Sun and heliosphere are moving at about 26 kilometers per second, balances the outflow of solar plasma. This pressure balance is achieved at a distance from the Sun of several hundred astronomical units or AU (1 AU = the mean distance from the Sun to the Earth). Embedded in the expanding solar wind is the Sun's magnetic field, which is carried out to the farthest reaches of the heliosphere as the interplanetary magnetic field (IMF).
For the first three decades of the space age, our picture of the solar wind was essentially two-dimensional. It was based solely on measurements made in the ecliptic - that is, the plane in which the planets orbit the Sun. That changed in February 1992, however, when the Ulysses spacecraft, after a 17-month journey to Jupiter, used that planet's powerful gravitational field to "slingshot" itself out of the ecliptic and into polar orbit about the Sun. Ulysses became the first - and to date only - spacecraft to directly observe the solar wind at high solar latitudes, thus supplying the missing third dimension to our knowledge of the structure of the solar wind as it flows through the inner heliosphere.
Ulysses is a joint mission of the European Space Agency and the National Aeronautics and Space Administration. Launched from the space shuttle Discovery in October 1990, Ulysses carries about a dozen instruments to study the solar wind, the IMF and a variety of other solar, heliospheric and astrophysical phenomena, including cosmic rays, solar X-rays, cosmic gamma-ray bursts and cosmic dust. Institute scientists lead the Solar Wind Observations Over the Poles of the Sun (SWOOPS) experiment, which provides the primary observations of the solar wind density and velocity. Now in its thirteenth year of operation, Ulysses is set to complete its second orbit of the Sun in July 2004. Ulysses has flown over both solar poles twice and has measured the solar wind at all heliographic latitudes, from pole to pole, and at heliocentric distances between 1.3 and 5.4 AU. Owing to the mission's long lifetime, Ulysses has been able to study the evolution of the solar wind over all phases of the Sun's 11-year activity cycle, from the declining phase of the previous solar cycle, through solar minimum and solar maximum, into the declining phase of the present cycle.
The solar cycle results from the periodic polarity reversal of the Sun's global dipolar magnetic field. The most familiar manifestation of the solar cycle is the rise and fall in the number of sunspots. As the solar cycle progresses from sunspot minimum to sunspot maximum and back to sunspot minimum, the large-scale structure of the corona changes dramatically. Polar coronal holes, regions of open magnetic field associated with high-speed solar wind flows, shrink, disappear and then re-form soon after the Sun's field reverses. With increasing activity, bright coronal streamers begin to appear at all heliolatitudes. Streamers, closed magnetic structures with high plasma densities, are the source of a slow, dense wind. During the descending part of the sunspot cycle and at solar minimum, they form a "belt" around the Sun's magnetic equator. Not surprisingly, because the corona is the source of the solar wind, the more complex coronal structure around solar maximum produces a more complex solar wind. Another solar cycle effect observed in the solar wind around solar maximum is an increased number of disturbances caused by the passage of coronal mass ejections (CMEs). These transient ejections of billions of tons of coronal material into the heliosphere occur more than 10 times more often at solar maximum than at solar minimum.
Ulysses' First Orbit: Heliospheric Order
Ulysses completed its first orbit of the Sun in December 1997. During the nearly six years since it headed south out of the ecliptic, Ulysses observed the solar wind at periods of decreasing and low solar activity. When the spacecraft first left the ecliptic until it reached a southern heliolatitude of about 13 degrees, it measured the dense, relatively slow (350-550 kilometers per second) wind flowing from the streamer belt, which was typically observed by previous, in-ecliptic spacecraft. Once Ulysses climbed to around 36 degrees, however, SWOOPS revealed the presence of a very fast, steady wind with a speed of around 750 kilometers per second. Ulysses remained immersed in this fast solar wind for the next 20 months, as it made its first pass over the Sun's south pole and returned down to near the ecliptic. The source of the high-speed flow was the large circumpolar coronal hole that had formed in the Sun's southern hemisphere. The polarity of the post-reversal hole was negative, as indicated by Ulysses' magnetometer, which measured an inward-directed IMF.
During the declining phase of the solar cycle, the axis of the dipole field was strongly tilted relative to the Sun's rotation axis, and thus the magnetic equator and streamer belt were significantly offset from the rotational equator. Traveling southward from 13 to 36 degrees, Ulysses encountered, with each solar rotation, alternating streams of slow solar wind from the tilted streamer belt and fast solar wind from an equatorial extension of the southern polar coronal hole. As these streams propagate into the heliosphere, the fast solar wind overtakes the slower wind, forming a high-pressure region known as a "co-rotating interaction region" or CIR. At distances beyond 2-3 AU, shock waves develop at the leading and trailing edges of the CIRs. While these regions and their associated shocks have been studied for more than three decades, it was not until Ulysses that researchers were able to determine their three-dimensional structure. Ulysses' out-of-the-ecliptic data revealed that CIRs are tilted relative to the ecliptic plane, with opposite tilts in the northern and southern hemispheres.
The overall structure of the solar wind during the second (northern hemisphere) half of Ulysses' first orbit was virtually identical to that seen during the first half: a fast steady wind at high latitudes from the northern polar coronal hole, alternating streams of slow and fast wind at mid-latitudes, and slow and variable solar wind at low latitudes. Just as it did during its traversal of the southern hemisphere, Ulysses observed a number of CIR-associated shocks as it flew through the mid-latitudes of the northern hemisphere on its return to the ecliptic.
Ulysses' Second Orbit: Heliospheric Chaos
Ulysses' second orbit began in December 1997, during the rising phase of the solar cycle. Ulysses performed its second pass over the Sun's south pole in 2000, at the time of maximum solar activity, and completed its second pass over the north pole in December 2001, following the reversal of the solar magnetic field and the reformation of the polar coronal hole in the north. Ulysses is now heading back toward the ecliptic, gathering data on the solar wind during the descending phase of the activity cycle.
The solar wind Ulysses observed during the increased solar activity of Solar Cycle 23, which began in September 1996, was strikingly different from the wind it saw during its first journey around the Sun. Whereas Ulysses was immersed in a fast, steady wind from the southern and northern polar coronal holes during the high-latitude segments of its first orbit, it encountered no such persistent high-speed wind during its second traversal of the southern high latitudes, which took place during solar maximum. Only after Ulysses reached northern heliolatitudes above about 70 degrees did it again observe relatively steady fast flows like those seen at high latitudes during the first orbit. As it dropped back down to 65 degrees, it began to observe slower wind again, indicating a much smaller polar coronal hole and a larger dipole tilt than previously observed.
The fact that Ulysses did not detect a steady fast wind as it climbed to higher and higher southern latitudes meant that the southern polar coronal hole, the source of the high-speed flows seen in the southern hemisphere during the first orbit, was disappearing and its boundary was receding to higher latitudes before the spacecraft could enter into the fast wind originating from it. The southern hole disappeared entirely by June 2000 and did not re-form before Ulysses had passed over the pole and was heading north. Conversely, the observation of relatively persistent high-speed flows above 70 degrees north indicated that at least a small northern polar coronal hole had re-formed. The direction of the IMF measured in the fast flow was evidence that the reversal of the Sun's field had occurred, at least in the northern hemisphere.
Instead of a steady fast wind, what Ulysses observed at high latitudes during the southern leg of its second orbit was a variable wind composed of slow- and intermediate-speed (350-600 kilometers per second) streams from small, transient coronal holes and coronal streamers, which occurred at increasingly high latitudes as the southern polar coronal hole shrank. This same mix of slow and intermediate-speed wind at mid-latitudes was also seen during the earlier portion of Ulysses' southward excursion. Ulysses observed stream interaction regions and the associated shocks at both mid- and high-latitudes; however, the interaction regions were much less periodic than the CIRs observed during the first orbit. Ulysses also observed numerous CMEs and CME-driven shocks during its second orbit. Curiously, although CMEs are usually associated with coronal streamers, few CMEs were observed at heliolatitudes above 30 degrees even though streamers occurred at much higher latitudes during this active period.
The Galactic Frontier
Ulysses has provided a historic look at the three-dimensional structure of the inner heliosphere as it evolves in response to the cyclical decay and regeneration of the Sun's global magnetic field. Beyond the orbit of Jupiter, the heliosphere has been surveyed over the past two decades by Pioneers 10 and 11 and Voyagers 1 and 2, which, their planetary missions complete, are headed into the outer heliosphere and toward the galactic frontier. These probes have sent back invaluable information on the low-latitude structure of the solar wind in the outer heliosphere.
Pioneers 10 and 11 are no longer operational. Voyagers 1 and 2 continue to provide data, however, and are in a race against time to reach the boundaries of the heliosphere before their aging power supplies can no longer support scientific operations. Voyager 1 is now some 89 AU from the Sun, and has just made controversial but tantalizing observations that some scientists believe may be the first, limited observations of the innermost heliospheric boundary, the termination shock.
The termination shock is thought to form where the pressure of the local interstellar medium causes the supersonic solar wind flow to become subsonic. The "shocked" solar wind is turned and flows back around the region bounded by the termination shock, forming an elongated tail-like structure, the "heliotail." Separating the region of shocked solar wind plasma from the magnetized plasma of the Local Interstellar Cloud is a discontinuity called the "heliopause." Depending on the speed of the interstellar medium relative to the heliosphere, a bow shock (analogous to the bow wave formed in front of a boat going through water) may form upstream of the nose of the heliosphere.
These heliospheric boundaries are not stationary but vary according to changes in the solar wind. For example, numerical simulations using Ulysses solar wind data have shown that the location of the termination shock should move by several tens of AU as the structure of the heliosphere evolves from solar minimum to solar maximum and latitudinal differences in solar wind structure strongly affect the global configuration.
Interstellar neutral atoms pass freely into the heliosphere, where they can become ionized through photoionization and through exchange of charge with solar wind protons. The newly created ions are "picked up" by the solar wind and swept outward toward the termination shock, where some of them are accelerated to extremely high energies, creating a population of energetic particles known as anomalous cosmic rays (see Anomalous Cosmic Rays). As samples of interstellar material that can be measured within the heliosphere, interstellar neutrals, pick-up ions and ACRs provide valuable information about the properties of the Local Interstellar Cloud.
Our picture of the outer heliosphere is based largely on theoretical models and computer simulations. While the theories and models are quite sophisticated, they are no substitute for actual observations. A dedicated interstellar probe mission would be needed to chart the boundaries of the heliosphere and directly sample the stuff of our immediate galactic environment. What we would learn from such a mission would be of profound importance - for our understanding of the heliosphere, for our knowledge of the nature and history of the local galactic environment, and for our understanding of other astrospheres.
Unfortunately, before an interstellar probe mission can be implemented, there are a number of technological problems that must be solved, primarily the problem of propulsion. Voyager 1 has taken 26 years to reach 89 AU. What kind of propulsion system can reduce the time required to travel to the boundaries of the heliosphere and beyond? Solar sails or nuclear electric propulsion may provide the needed capability at some time in the future. In the meantime, Institute scientists and engineers, in collaboration with colleagues around the world, have developed a concept for a mission that will use energetic neutral atom imaging, a technique successfully employed on the magnetospheric IMAGE mission, to remotely observe the global interaction between the solar wind and the interstellar medium. By detecting emissions from around the heliospheric boundaries, a suitably instrumented spacecraft near Earth can make the first "pictures" of the three-dimensional structure of the outer heliosphere.
The space science community stands poised at the threshold of a new era in humankind's efforts to understand the place of the Sun and its system of planets in the galaxy. The Institute's heliospheric research program is an essential part of these efforts and offers a preview of the exciting discoveries that await us as we extend the reach of our knowledge and understanding into that mysterious boundary region where the solar wind meets the enveloping cloud of interstellar plasma, neutral gas and dust that is our galactic environment.
Comments about this article? Contact McComas at (210) 522-5983 or firstname.lastname@example.org, Schwadron at (210) 522-5161 or email@example.com, and Lewis at (210) 522-5261 or firstname.lastname@example.org.
Published in the Fall 2003 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.