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Sun-Earth Day 2007 presents: Living in the Atmosphere of the Sun

Sun-Earth Day 2007 presents: Living in the Atmosphere of the Sun

The sun's influence extends beyond Pluto, but must eventually come to an end.

ISSUE #57: THE HELIOSPHERE

Figure 1: A sketch of the heliosphere.

Figure 1: A sketch of the heliosphere.

Despite the fact that it is the largest single physical system in the entire solar system, the heliosphere was only discovered at the dawn of the space age in the late 1950's. It took another decade for satellite measurements to confirm its existence and to determine some of its properties. ( See TTT #47: Discovery of the Heliosphere ).

The heliosphere is defined as the region of space surrounding the sun, in which the solar wind, the solar magnetic field, and all of the ejections of matter from the sun, play a greater role in controlling how plasma inside the solar system behaves, than the magnetic fields and plasmas from the rest of the Milky Way galaxy beyond. To understand the size of the heliosphere, we first need to understand a little about the flow of energy and matter through interplanetary space. To define the heliosphere, we only need to consider the movements of plasma and magnetic fields, not the sun's electromagnetic radiation.

The sun emits a stream of particles from its outermost layers in the corona, and this stream flows out through space as the solar wind. Because the sun rotates once each month, this wind is actually a pinwheel-shaped outflow, mostly within the plane of the solar system. As it flows past the planets, it interacts with their magnetic fields, if they have one, or with the upper atmospheres of the planets if no field is present. Because it is largely unimpeded as it flows through the vacuum of space ,it loses very little speed as it journeys past Jupiter, Saturn and eventually Pluto. Only the enfeebled gravity of the sun slows this wind down as it speeds at nearly 500,000 km/hour past the known outposts of the solar system. Eventually this motion has to stop as the solar wind plasma collides with the vast ocean of interstellar gas through which the sun and solar system are moving.

Figure 2: A sketch of the main parts of the heliopause (Courtesy:  Mike Gruntman 2001 )

Figure 2: A sketch of the main parts of the heliopause (Courtesy: Mike Gruntman 2001 )

In addition to this solar wind, expulsions of matter from the sun called Coronal Mass Ejections also travel through interplanetary space until these clouds of plasma and magnetism also collide with the 'interstellar medium'. For some very large storms, such as the spectacular Halloween 2003 storm, for example, launched a cloud at a speed of eight million km/hr, which raced past spacecraft near Earth, Mars, Jupiter and Saturn. The impulse from this storm was eventually detected by the distant Voyager 2 spacecraft located 11 billion kilometers from the sun seven months later!

The region where the solar wind encounters the interstellar medium is a complicated zone that is in rough pressure balance. The pressure of the expanding 'supersonic' solar wind, pushes against the pressure of the interstellar medium through which the solar system is passing. The two pressures come, not from the temperatures of the two gasses, but from their density and rate of motion much like the plowing of a boat across the water leaves a 'bow wave' ahead if it.

As we leave the solar system behind, this pressure front between the two gases consists of a Termination Shock, where the supersonic solar wind is slowed down rapidly and is pushed backwards and out of the plane of the solar system; a heliosheath region where interstellar and solar wind plasma mixes with magnetic fields in a turbulent froth, a heliopause where the two gas pressures are in equilibrium, and the bow shock, where the interstellar medium slows down suddenly as it first begins to feel the 'blockage' from the solar system. How far away from the sun is this zone? The action starts at a distance of about 100 Astronomical Units or 2.5 times farther from the sun than the orbit of Pluto!

Figure 3: This even more detailed model is based on computer calculations of what to expect near the heliopause, and the different kinds of phenomena we may see there. (Courtesy: IBEX mission )

Figure 3: This even more detailed model is based on computer calculations of what to expect near the heliopause, and the different kinds of phenomena we may see there. (Courtesy: IBEX mission )

The Voyager spacecraft have been our robotic explorers of the distant solar system. At 8.7 billion miles from the sun, Voyager I has entered the heliosheath, a region just beyond termination shock. This happened in August 2006. The termination Shock is believed to be about 4 billion miles inside the heliopause, so it expected that by 2017 Voyager 1 will arrive there, and soon afterwards be in the Bow Shock region. Once Voyager 1 is on the other side of the heliopause, it will be in the zone where interstellar gas and magnetic fields are the main ingredient. For the first time, we will be able to truly say that we have arrived in interstellar space. But wait..there's more!!!

The heliosphere is a vast, comet shaped object. Voyager 1 is in the northern hemisphere of the nose of this 'comet' and Voyager 2 is in the southern hemisphere. Voyager 2 has found that the Termination Shock is about 900 million miles closer to the sun than Voyager 1 did in the north. Voyager scientists think this is caused by an interstellar magnetic field pressing against the heliosphere preferentially in the south. Voyager 2 may reach interstellar space within another few years!

GALLERY

REFERENCES

(M. Gruntman, E. C. Roelof, D. G. Mitchell, H. J. Fahr, H. O. Funsten, and D. J. McComas, Energetic neutral atom imaging of the heliospheric boundary region, Journal of Geophysical Research, 106, 15767-15781, 2001) in 2001

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