Solar Wind Characteristics of the Heliosphere

The upper atmosphere of the sun ejects a charged particle stream known as solar wind. Solar wind is energy in the form of electrons and protons that average around 10 and 100 keV or electronvolts. The particles have such a high temperature and high kinetic energy that the particles are able to escape the gravitational pull of the sun. The heliosphere is a giant “bubble” that surrounds the solar system and is created by the solar wind.

The corona of the Sun (the outer layer) is a vast expanse of plasma at over one million degrees Celsius. Particles inside the inner corona move at speeds of approximately 145km/s, which is below the escape velocity of 618km/s for a particle to escape the sun. Some particles in the lower corona sometime reach excessive speeds up to 400km/s, pushing outward toward the solar wind. Electrons with smaller mass can reach escape velocity at the same temperature because their mass is much smaller than most of the ionized particles. An electric field is slowly built up and the accelerated ions, or charged atoms, fly away from the sun. These atoms make up the heliosphere of the solar system.

Scientists estimate the number of particles (charged atoms) that are carried away by solar wind into the heliosphere to be 1.3 x 10^36. The amount of the sun’s mass lost to solar wind is about 6.7 billion tons an hour. While this seems like an insanely large amount, scientists estimate that the sun has only lost 0.01% of its mass since its formation. Other stars besides the sun are known to have stronger solar wind that does result in more significant losses of mass in the star.

There are actually two types of solar wind, termed the slow solar wind and the fast solar wind. Slow solar wind has the previously mentioned velocity of 400km/; while fast solar wind has a velocity of around 750km/s, which is more than enough to beat the sun’s escape velocity entering the solar system’s heliosphere. The density of the slow solar wind is much larger even having its own “wind weather patterns.”

During the lifetime of the sun, the surface rotation rate has decreased since its beginning. Solar wind is believed to be the culprit with its high density and randomly-patterned winds. Solar wind is also responsible for the tails of comets and the sun’s radiation field.

The heliosphere changes constantly as more charged particles from solar wind exit the atmosphere of the sun. The heliospheric current sheet is a ripple in the heliosphere by the magnetic field of the Sun. It is considered to be a structure in and of itself, and the largest part of the solar system. The heliosheath is located beyond the termination shock, where wind is slowed down and compressed.

Lastly there is the heliopause, which is currently a theoretical block that stops the strength of solar wind from pushing the stellar winds of other stars. NASA expects Voyager 1 to cross the theoretical heliopause in 2014. If scientists are indeed correct regarding the heliopause, then a drastic drop in the temperature of charged particles will follow.


NASA/Marshall Solar Physics

The Solar Wind

The Space Environment