6. Perpetual Change

Termination of the solar wind

All of the planets are immersed in the solar wind that becomes increasingly rarefied as it spreads out into space. It moves past the planets and beyond the most distant comets. Thus, the entire solar system is bathed in the hot gale that blows frsom the Sun, creating a large cavity in interstellar space called the heliosphere.Within the heliosphere, physical conditions are dominated, established, maintained, modified and governed by the magnetized and electrified solar wind.

The size of the heliosphere has been inferred from the twin Voyager spacecraft, cruising far beyond the outermost planets. At the time of writing they are more than 21 years old and approaching 71 AU. Strong shock waves, associated with intense explosions on the Sun, have plowed into the cold interstellar gas at the heliopause, generating a hiss of radio noise detected by the remote Voyagers. Thirteen months before the spacecraft detected the radio hiss, unusually intense eruptions on the Sun generated one of the largest interplanetary disturbances ever observed. From the measured speed of the disturbance, and the time it took to travel to the heliopause and generate the radio signals, the outer edge of our solar system has been located somewhere between 110 and 160 AU, or roughly a hundred times further from the Sun than the Earth. That is where the solar system ends, in a gigantic distant wall of compressed gas that fences off our Sun from the rest of the cosmos.

Solar Winds at Minimum and Maximum Solar Activity

Fig. .. 

In more than 17 years of Ulysses operations, the spacecraft has completed three polar orbits, providing a unique perspective from which to study the Sun and its effect on surrounding space.It has followed the complete course of the 11-year solar activity cycle, first during a minimum in the number of sunspots (in 1994 to 1995), then when the sunspot number was at its maximum (2000 to 2001).

Ulysses thus measured the distribution of solar wind velocities at both the minimum and maximum of the solar cycle (Fig. 6.29).During its first polar orbit, at sunspot minimum, it found fast wind over the poles, and slower, variable wind confined near the solar equator.The second polar orbit, performed near solar maximum, revealed variable solar wind at all latitudes including near the poles.During this second polar orbit at sunspot maximum, the Sunís polar fields disappeared and then reappeared with the opposite polarity or direction.The third orbit over the solar poles is also at an activity minimum (2007 to 2008), but under very different circumstances after a reversal of the magnetic poles of the Sun, permitting studies of the changed magnetic field and its effect on the solar wind, galactic cosmic rays and solar energetic particles.

Source of Solar Wind at Activity Maximum

Fig. .. 

At activity maximum, the large polar coronal holes shrink and disappear, smaller coronal holes appear at all solar latitudes, the slow and fast winds seem to emanate from all over the Sun, and the high-speed winds abate. The slow winds seem to be associated with closed magnetic structures, such as active regions and their associated streamers, or small coronal holes in their vicinity, while the fast winds rush out of the interiors of the largest of the smaller coronal holes (Fig. 6.30). Coronal mass ejections briefly provide a noticeable third flow as they pass through interplanetary space near the maximum of the activity cycle.

Outer Boundary of the Solar System

Fig. .. 

How far does the Sunís influence extend, and where does it all end?The relentless solar wind streams out in all directions, rushing past the planets and carving an immense heliosphere in interstellar space (Fig. 6.31).But since the solar wind thins out as it expands into a greater volume, it eventually becomes too dispersed to repel interstellar forces.The winds are no longer dense or powerful enough to withstand the pressure of gas and magnetic fields coursing between the stars. The radius of this celestial standoff distance, in which the pressure of the solar wind falls to a value comparable to the interstellar pressure, has been estimated at about 100 AU, or one hundred times the mean distance between the Earth and the Sun.

Fig. .. 

Instruments aboard the twin Voyager 1 and 2 spacecraft, launched in 1977 and now cruising far beyond the outermost planets, have approached this edge of the solar system from different directions, Voyager 1 moving in the northern hemisphere of the heliosphere and Voyager 2 in the southern hemisphere (Fig. 6.32).Voyager 1 crossed the termination shock of the supersonic flow of the solar wind on 16 December 2004 at a distance of 94 times the mean distance between the Earth and the Sun, or at 94 AU from the Sun.Voyager 2 crossed the termination shock on 30 August 2007 at a distance of 84 AU from the Sun. It appears that there is a significant north/south asymmetry in the heliosphere, likely due to the direction of the local interstellar magnetic field.

Both Voyager 1 and 2 have therefore now crossed into the vast, turbulent heliosheath, the region where the interstellar gas and solar wind interact, due to the reflection and deflection of the solar-wind ions by the magnetized wind beyond the heliosheath.In technical terms, the solar-wind ions in the heliosheath are deflected by magnetosonic waves reflecting off of the heliopause, causing the ions to flow parallel to the termination shock toward the heliotail.

Fig. .. 

The motion of the interstellar gas, with its own wind, compresses the heliosphere on one side, producing a teardrop-like, non-spherical shape with an extended tail.A bow shock is formed when the interstellar wind first encounters the heliosphere; just as a bow shock is created when the solar wind strikes the Earthís magnetosphere. And the graceful arc of a bow shock, created by an interstellar wind, has been detected around the young star LL Orionis (Fig. 6.33).

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Copyright 2010, Professor Kenneth R. Lang, Tufts University