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The Institute of Geophysics and Planetary Physics

Back to 25 Years of Voyager Main Page The Large Scale Solar Wind
How and into what does the solar wind expand?

The solar wind expands into the interstellar medium, or ISM, carving out a gigantic bubble(a). The creation of the cavity continues until the pressure of the ISM becomes large enough to halt the expansion of the supersonic solar wind. Clearly, different interstellar environments can therefore affect the extent of the cavity and, as we describe later, even introduce fundamentally different physical processes into the solar wind. (b,c)

a. Image
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b. Image
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c. Image
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(b): Mosaic of 15 pictures of the Great Nebula in Orion, from the Hubble Space Telescope. In addition to housing a bright open cluster of stars known as the Trapezium, the Orion Nebula contains many stellar nurseries containing hydrogen gas, hot young stars, proplyds, and stellar jets spewing material at high speeds. Most of the filamentary structures visible in this image are actually shock waves - fronts where fast moving material encounters slow moving gas. Shocks are particularly apparent near the bright stars in the lower left of the picture. The Orion Nebula is about 1500 light years distant, located in the same spiral arm of our Galaxy as the Sun. Credit: C. O'Dell and S. Wong (Rice University), NASA.
(c): Interstellar space if filled with extremely tenuous clouds of gas, consisting mainly of hydrogen. This false color image represents an all-sky neutral hydrogen atom survey, with the plane of our Milky Way Galaxy running horizontally through the center. No stars are visible, just diffuse clouds of gas that seem to form arching, looping structures, stirred up by stellar activity in the galactic disk. Credit: J. Dickey (Umn), F. Lockman (NRAO), SkyView.

At 1 AU, the solar wind has a velocity(d) of about 400 km/s and density of around 5-10 protons per cubic centimeter. As the solar wind expands, its density(e) decreases as the inverse of the square of its distance from the Sun. Eventually the solar wind can no longer hold back the Local Interstellar Medium particles and fields, and so it slows from supersonic to subsonic speeds, which causes a termination shock to form. At the termination shock there are substantial changes in the direction of the plasma flow and the orientation of the magnetic field. We don't yet know the location of the termination shock, but the Voyager spacecraft, which are now at about 82 AU, will soon make this their next momentous discovery! Current models predict that the termination shock is unlikely to lie beyond 120 AU, so Voyager should have the answer for us within the next five years. (f)

d. Image
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e. Image
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f. Image
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(d): The solar wind velocity, measured in km/s, as measured by Voyager since the inception of the mission. The bottom axis corresponds to time and the top to heliocentric distance, measured in AU.
(e): The corresponding plot of solar wind density, measured as the number of particles per centimeter cubed.
(f): A simulation of the large-scale heliosphere. The color denotes the plasma temperature, with highest temperatures (red) being found in the inner heliosheath. The three principal boundaries can be seen: the bow shock, the heliopause separating the solar wind plasma from the interstellar plasma, and the termination shock. The extended heliotail is clearly visible.

The solar wind does not cross the heliopause, which is the boundary between the heliosphere and the Local Interstellar Medium. The Voyager spacecraft should reach the heliopause, which is believed to be about 130-150 AU from the Sun, by the year 2017.(g)

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Our Sun is constantly in motion. It oscillates through the plane of the galaxy(h) with an amplitude of approximately 230 light-years and crosses the galactic plane every 33 million years. It takes the Sun 250 million years to orbit the center of the galaxy.

h. Image
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(h): The orange dot in the above false-color drawing represents the current location of the Sun among local gas clouds in the spiral Milky Way Galaxy. These gas clouds are so thin that we usually see right through them. Nearly spherical bubbles surround regions of recent star formation. The purple filaments near the Sun are gas shells resulting from star formation 4 million years ago in the Scorpius-Centaurus Association, located to the Sun's lower left. The Sun has been between spiral arms moving through relatively low density gas for the past 5 million years. In contrast, the Sun oscillates in the Milky Way plane every 66 million years, and circles the Galactic Center every 250 million years. Courtesy of P. C. Frisch (University of Chicago).

Our galaxy is not static, but ever changing. Over millions of years, interstellar clouds form and sometimes become stars. Supernovae whisk up the interstellar matter and create the hot interstellar medium, between which there is typically a cooler interstellar medium, often populated by interstellar clouds.

There are two types of supernova. A Type I supernova occurs when a star explodes as a result of sudden nuclear burning. A type II supernova is the result of gravitational collapse of a star, leading to an exceedingly energetic shock wave.

The warm, tenuous, partly ionized interstellar cloud currently surrounding the solar system is called the Local Interstellar Cloud(i). It is composed of about 90 percent hydrogen, 9.99 percent helium, and 0.01 percent heavier elements and dust.

i. Image
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j. Image
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(i): A three-dimensional animation of the Local Interstellar Cloud (LIC) using spherical harmonics, based on data from HST/GHRS, EUVE, and ground-based CaII observations. Courtesy of Jeff Linsky and Seth L. Redfield, University of Colorado.
(j): A view of the Local Interstellar Cloud, looking down from the North Galactic Pole showing that the Sun (located at 0,0) lies barely inside the LIC. Courtesy of Jeff Linsky and Seth L. Redfield, University of Colorado.

The Sun is on the boundary of an enormous vacuum, known as the Local Bubble (k). Apart from being almost empty, with a density of less than 0.001 atoms per cubic centimeter, the Local Bubble, at one million degrees Kelvin, is fairly hot compared with the Local Interstellar Cloud, which has a temperature of only 7,000 degrees.

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(k): A view of the Local Bubble as formed by the Scorpius-Centaurus superbubble expanding into the interarm region surrounding the Sun.

Neutral atoms and dust grains from the interstellar medium are able to cross the heliopause, where some of the atoms collide with the solar wind and become electrically charged. They are then picked up by the solar wind's magnetic field and carried off as "pickup ions." (l)

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(l): Heliospheric boundary features are mapped according to their relative temperatures. Charged particles, represented by the white lines, are deflected around the heliopause, while interstellar neutrals, represented by the pink arrow, enter the heliosphere.

Within the heliosphere the pickup ions are accelerated by interplanetary shocks but, at the termination shock, they gain so much energy through shock acceleration that they are able to escape from the shock and diffuse toward the inner heliosphere, where they become known as anomalous cosmic rays(m).

m. Image
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(m): Schematic of the life cycle of a neutral atom. Interstellar neutral atoms stream into the supersonic solar wind, where some experience charge exchange to produce fast, outwardly streaming neutrals and pickup ions. The pickup ions are convected to the shock, where some are energized at the termination shock and become anomalous cosmic rays.

The creation of pickup ions in the outer heliosphere acts to decelerate the solar wind, which has profound implications for the global structure of the heliosphere.

When no neutral atoms, and hence pickup ions, are present, the global heliosphere acquires a characteristic bullet shape and is large. By contrast, the inclusion of neutrals, and hence pickup ions, yields a smaller, much smoother heliosphere. (n,o)

n. Image
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o. Image
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(n): Temperature plot from a simulation of the global heliosphere, with neutral hydrogen neglected.
(o): Temperature plot from a simulation of the global heliosphere, now including interstellar neutrals self-consistently.

When the neutral hydrogen crosses the bow shock, it experiences charge exchange with heated, slowed, and diverted interstellar protons, which acts gradually to slow, heat, and divert the neutral hydrogen, but over a much greater distance. As a result, an enormous wall of compressed neutral hydrogen forms. By measuring the amount of light absorbed between the earth and a nearby star emitting the light, scientists have been able to detect the hydrogen wall. (p)

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(p): A plot of the neutral interstellar hydrogen density, showing the "hydrogen wall" that is formed roughly between the heliopause and the bow shock.

The heliosphere is highly variable, with the dynamic pressure of the solar wind increasing and decreasing with solar cycle. The high-speed wind over the solar poles appears during solar minimum and disappears during solar maximum. During solar minimum the higher speed wind over the poles implies a higher dynamic pressure there than in the ecliptic, which means that the heliosphere is asymmetric. Thus, the polar asymmetry of the heliosphere varies with solar cycle. The heliosphere is constantly subjected to large shock waves that propagate through the solar wind and eventually collide with the boundaries of the heliosphere.

This simulation shows the effect of varying the solar wind dynamic pressure with solar cycle, illustrating how the heliosphere breathes in and out like an enormous lung. (q)

q. Image
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In the picture we see how different the 3D heliosphere appears during solar minimum. During solar minimum the heliosphere has a distinct pinching at the waist, whereas, during solar maximum, the heliosphere is more uniformly spherical. During solar minimum, the heliosphere is more elongated over the poles than during solar maximum. (r)

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(r): The asymmetric heliosphere that results from solar minimum conditions. The cyclonic spiral character of the interplanetary figure is clearly illustrated.

This simulation shows how a GMIR propagates in the solar wind, to eventually collide with the termination shock in a highly asymmetric fashion, setting the large-scale heliosphere in motion, rather like Jell-O on a plate. (s)

s. Image
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