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The Institute of Geophysics and Planetary Physics

Back to 25 Years of Voyager Main Page The Large Scale Solar Wind

Here we delve into the origin and basic structure of the solar wind. In its core, the Sun generates light and heat by converting hydrogen to helium via a proton-proton, or pp, chain. Neutrons and protons approach one another and collide. There is a flash of light plus Helium 2 (a).

a. Image
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Light then tries to diffuse through the dense inner 70% of the Sun, the Radiation Zone (b). In the outer 30%, the Convection Zone, the Sun begins to boil, bubbling like water in a pot on the stove. Above the Convection Zone, light and heat escape into the region above, called the atmosphere (c).

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c. Image
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Because the Sun is heated at the core, we expect the temperature to decrease as we move outwards and, indeed, this is what happens, until we reach the photosphere, which is the visible surface of the Sun. Just after the photosphere, however, the temperature begins to increase in the chromosphere.

The chromosphere is the layer of the Sun above the photosphere and beneath the outermost layer, the corona.

Then, suddenly, in a region less than 100 km thick, which we call the Transition Zone, the temperature rises dramatically to 1,000,000 K (d-f). Beyond the Transition Zone, in the corona, the gas is so hot that atoms lose electrons and the gas becomes a collection of distinct protons, electrons, and some other nuclei.

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e. Image
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f. Image
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The extremely hot corona leads to intense emission of x-rays (g),

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Left: TRACE image of a magnetic loop, or prominence on the Sun. Magnetic field structures appear light blue. The green, blue, and red color tables in this ``true color'' image represent the 171 A (1 MK), 195 A (1.5 MK), and 284 A (2 MK) channels, respectively. Right: TRACE image of a solar flare

but, most importantly, solar gravity can no longer bind the hot atmosphere to the Sun and it "boils off" as a wind, the Solar Wind (h,i), losing 106 metric tons/second of mass.

h. Image
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i. Image
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Left: EIT 304 image captures a sweeping prominence. Prominences are huge clouds of relatively cool dense plasma suspended in the Sun's hot, thin corona. At times, they can erupt, escaping the Sun's atmosphere. Emission in this spectral line shows the upper chromosphere at a temperature of about 60,000 degrees K. Every feature in the image traces magnetic field structure. The hottest areas appear almost white, while the darker red areas indicate cooler temperatures. Right: YOHKOH Soft X-ray Telescope x-ray emission image

So now we are led to one of the major unsolved mysteries of Space Physics: Why did the Sun's temperature not continue to decrease beyond the photosphere? Since the temperature at the surface of the Sun is not sufficiently high, how is the corona heated? We think we have some of the answers.

The Sun was formed from the collapse of an interstellar cloud.

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In nearby galaxy NGC 6822, this glowing emission nebula complex surrounds bright, massive, newborn stars. A mere 4 million years young, these stars condensed from the galaxy's interstellar gas and dust clouds. Credit: C. R. O'Dell (Vanderbilt University) et al., Hubble Heritage Team, NASA

During the collapse, primordial magnetic field was captured and now lies in the Radiation Zone. The magnetic field is constantly bubbling to the solar surface, carried by the churning fluid motions of the Convection Zone, rather like spaghetti in boiling water.

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The magnetic field carries energy, which is liberated as heat when it emerges into the atmosphere. How magnetic field energy is transferred to the coronal plasma remains a major unsolved mystery in solar wind physics, although theories such as reconnection or turbulence have been proposed.

A plasma is any ionized gas, that is, any gas with ions and electrons.
Magnetic Reconnection:
Magnetic reconnection is the breaking and reconnecting of oppositely directed magnetic field lines in a plasma process, causing magnetic field energy to be converted to plasma kinetic and thermal energy.

This brings us to another major unsolved mystery of space and solar physics-what is the origin of the Sun's magnetic field? We think the partial answer is that the solar dynamo generates the magnetic field.

The Sun has a 22-year magnetic cycle. Originally the magnetic field lines run from north to south. This is the period of minimal magnetic activity, called the "solar minimum" (l,m).

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m. Image
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Left: Sketch of the inferred solar magnetic field during solar minimum. The magnetic field structure is significantly simpler than that during solar maximum. Two large, long-lived coronal holes are present at the poles. Right: 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 minimum. The fast solar wind (~800 km/s) emerges from the coronal holes at the poles and exhibits only small variations. In the ecliptic, the solar magnetic field tends to remain closed, and any emerging wind is primarily slow.

However, as the Sun rotates, the Convection Zone spins faster at the equator than at the poles while, below it, the Radiation Zone spins as a solid mass. The two different movements cause the field lines to stretch in the east-west direction and to twist. Eventually, as with a rubber band that is stretched and twisted too much, the magnetic field begins to buckle and the magnetic force breaks the surface of the Sun. The effect is similar to what we see when a puff of smoke emerges from a chimney and is drawn into filaments as it escapes (n-p).

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0. Image
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p. Image
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At the peak of this activity, called the "solar maximum" (q,r), when magnetic "storms" are at their height, we see sunspots forming (s), the corona heating up, and solar flares and loops erupting from the Sun's surface (t,u).

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r. Image
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t. Image
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u. Image
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(q): Sketch of the inferred solar magnetic field during solar maximum. Observe the highly complex structure and the absence of any large coronal holes. (r): 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.
Sunspots are magnetic regions on the surface of the Sun, with magnetic field strengths thousands of times stronger than the Earth's magnetic field. They are about 2000 degrees K cooler than the surrounding photosphere and thus appear as dark spots on the Sun's surface. Usually they last for only a few days, but they can remain for weeks.

Cyclonic convection then returns the field lines to their north-south path, but the polarity is reversed. This cycle takes 11 years, so it is 22 years before the magnetic field lines resume their original path and polarity (v).

v. Image
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Sunspot counts for the current solar cycle peaked in mid-2000 and again in late 2001. Image courtesy of David Hathaway, NASA/MSFC

The problem with the solar dynamo model is that the magnetic field is very strong in the lower Convection Zone and, like a taut rubber band, resists being dragged about by the "boiling" solar fluid. Thus the magnetic field is not easily drawn into a filamentary structure, and the origin of the solar magnetic field remains a mystery.

The Voyager mission has provided insight into these mysteries through its exploration of the properties and character of the solar wind at both the largest and the smallest scales. Voyager's discoveries have helped us to answer questions the following questions: (see next section)

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