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Back to 25 Years of Voyager Main Page The Large Scale Solar Wind
How does the solar wind expand outward from the corona?

For a solar corona with a temperature of several million degrees, a static solar atmosphere would have a pressure, far from the Sun, much larger than the pressure of the Local Interstellar Medium.

Local Interstellar Medium:
The Local Interstellar Medium is the gas filling the space between the stars, up to a distance of a couple of hundred parsecs from the Sun.

Thus, no hydrostatic equilibrium can exist, and the solar atmosphere expands away from the Sun.

How does mass actually leave the Sun? The Sun can lose mass either continuously or through discrete events. First we describe how mass is lost continuously.

The solar atmosphere is highly ionized and tends to follow the strong magnetic field in the solar corona, which has a mixture of closed magnetic field regions and open regions (a). In the open regions, called coronal holes (b), the coronal magnetic field opens into the interplanetary region.

a. Image
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b. Image
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(a): Three-dimensional views, from vertical, south, and east, of magnetic potential field lines (yellow lines) calculated with the Sakurai code by extrapolation of a SOHO/MDI magnetogram recorded on 1996 August 30, 20:48 UT (red surface with white and black polarities), the traced 171A loop segments (blue lines), the traced 195A loop segments (green lines), and the traced 284A loop segments (red lines). The three-dimensional coordinates of the traced EIT loops are based on stereoscopic reconstruction. Note some significant deviations between the observed loops and the theoretical magnetic field model. Figure by Aschwanden et al., published in the Astrophysical Journal, Vol. 531, pp. 1129-1149 (2000).

The magnetic field opens up radially or even more rapidly (c), similar to a jet nozzle (d).

c. Image
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d. Image
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(c): A picture of the Sun as seen during a total eclipse, with the magnetic field configuration superimposed upon it. The regions of white light show the equatorial streamer belt associated with the slow stream. Darker regions show coronal holes, associated with the fast stream. In the coronal holes, the magnetic field lines can be seen to stretch out spaceward (hence the term "open magnetic field"), whereas in the streamer belt the field lines mainly have a loop structure ("closed magnetic field"). Credit: Department of Physics, The University of Wales, Aberystwyth.

The hot gas at the base of the nozzle cannot be held by gravity and expands rapidly into the nozzle. The flow speed, which at the base is slow, increases rapidly. Because the pressure of the flow must match the interstellar pressure, it should become small at large distances, so the flow cannot expand at an arbitrary speed. The only way for the coronal atmosphere to expand from a low speed at the base to a very low pressure at a large distance is via a wind that becomes supersonic at some 10-20 solar radii (e).

e. Image
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Family of solutions of the steady-state gas dynamic equation for different boundary conditions. The dashed line traces the transonic solution that starts out subsonic and becomes supersonic.

The very active solar magnetic field during solar maximum produces a slow, unsteady wind that is approximately the same in all directions.

Solar Maximum:
Solar maximum is the month(s) during the solar cycle when the 12-month mean of monthly average sunspot activity is at a maximum.

During solar minimum (f,g), when the magnetic field is steady, large coronal holes exist over the polar region, producing a very long-lived, high speed, steady wind that is possibly the true solar wind. This wind extends down to 30 degrees.

f. Image
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g. Image
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(f): Sketch of the inferred solar magnetic field during solar maximum. Observe the highly complex structure and the absence of any large coronal holes. (g): The Ulysses spacecraft made a latitudinal survey of the solar wind during both solar minimum and solar maximum. The wind velocity and magnetic field strength and polarity are superimposed on an image of the sun during solar maximum. The wind speed, although subject to large fluctuations, is approximately 400 km/s at all latitudes.
Solar Minimum:
Solar minimum is the month(s) during the solar cycle when the 12-month mean of monthly average sunspot activity is at a minimum.

Since 1977, Voyager has been monitoring the solar wind velocity and density (h,i), through two solar maximums and one solar minimum. As we can see, the wind velocity is highly variable on short time scales.

h. Image
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i. Image
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(h): 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. (i): The corresponding plot of solar wind density, measured as the number of particles per centimeter cubed.

The density of the solar wind decreases with increasing distance from the Sun, following an almost perfect R-2 power law. Then, after its initial rapid acceleration, the solar wind simply "coasts" outward at its final supersonic speed, achieved within 10-20 solar radii.

Mass is not only lost continuously via the expansion of the solar wind, but also discretely. Some of the most dramatic expulsions of mass from the solar atmosphere result from "coronal mass ejections," or CMEs (j-l). Deep in the corona, an event, possibly involving massive reconnection, occurs, leading to an enormous amount of material being hurled explosively into the supersonic wind. Sometimes the ejected mass is expelled at such a high speed that it drives a huge shock wave ahead of it. The coronal mass ejection and shock can disrupt the interplanetary medium dramatically, disturbing the magnetic field and heating, slowing, and diverting the solar wind.

Magnetic Reconnection:
Magnetic reconnection is the breaking and reconnecting of oppositely directed magnetic field lines in a plasma, causing magnetic field energy to be converted to plasma kinetic and thermal energy.
Shock Wave:
A shock wave is a pressure wave passing supersonically through a gas, causing dramatic changes in the pressure, density, and velocity of the particles.

j. Image
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k. Image
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l. Image
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These movies show a CME being expelled away from the Sun, as observed by the LASCO (Large Angle Spectrometric Coronagraph) C3 coronagraph on November 4, 2001(j), C2 coronagraph on October 25, 2001(k), and C3 coronagraph on October 1, 2001 (l). LASCO is able to take images of the solar corona by blocking the light coming directly from the Sun with an occulter disk, creating an artificial eclipse within the instrument itself. The position of the solar disk is indicated in the images by the white circle. The most prominent feature of the corona are usually the coronal streamers--the nearly radial bands. The shadow crossing from the lower left corner to the center of the image is the support for the occulter disk. C3 images have a larger field of view than C2 images; they encompass 32 diameters of the Sun. To put this in perspective, the diameter of the images is 45 million kilometers (about 30 million miles) at the distance of the Sun, or half of the diameter of the orbit of Mercury. Many bright stars can be seen behind the Sun. Courtesy of SOHO/LASCO consortium. SOHO is a project of international cooperation between ESA and NASA.

The shock can accelerate particles to high energies, and both the shock and the coronal mass ejection(m) can be enormously disruptive when they collide with the Earth's magnetosphere.

m. Image
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An example of a CME propagating into the interplanetary medium, driving a shock wave ahead of it
The magnetosphere is the region above the ionosphere in which the Earth's magnetic field controls the movement of charged particles.

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