The Solar Wind

What is solar wind?

The solar wind is a stream of energized, charged particles, primarily electrons and protons, flowing outward from the Sun, through the solar system at speeds as high as 900 km/s and at a temperature of 1 million degrees (Celsius). It is made of plasma.

The sun gradually loses mass in the form of high speed protons and electrons leaking away from the sun's out layers. This flux of particles is called the solar wind. It can be thought of as a kind of "evaporation" of particles from the corona. The corona reaches a temperature of about a million Kelvin at a distance of 10,000 km above the photoshpere. Such a hot gas would have a thermal energy of about 130 electron volts, and the mean speed for hydrogen nuclei in such a gas if viewed as having a Maxwellian speed distribution is about 145 km/s. The escape velocity from the surface of the sun is about 618 km/s, so those hydrogen atoms with average speed would not escape. Considering the nature of the speed distribution would show that there will be a few with speed above the escape velocity. Chaisson & McMillan characterize the mass loss as being about a million tons of solar matter per second. They note that at this rate, less than 0.1% of the Sun has been lost through this mechanism in its 4.6 billion year lifetime.

If a planet has a magnetic field, it will interact with the solar wind to deflect the charged particles and form an elongated cavity in the solar wind. This cavity is called the magnetosphere of the planet.

In the vicinity of the earth, the particles of the solar wind are traveling about 400 km/s. They are slowed by the interaction with the earth to produce a bow shaped shock wave around the earth.

Inside a boundary called the magnetopause, the earth's magnetic field is dominant over the effects of the solar wind. The small fraction of the charged particles which do leak through the magnetopause are trapped in two large doughnut-shaped rings called the Van Allen radiation belts.

The solar wind was first detected directly by the spacecraft Mariner 2. It has been studied in more detail by the SOHO satellite.

Components of the Solar Wind

The solar wind contains roughly equal number of electrons and protons, along with a few heavier ions, and blows continously from the surface of the Sun at an average velocity of about 400 km/second. This is a remarkable velocity: particles in the solar wind from the Sun's surface travel at a speed that would allow them to go from Knoxville to Memphis in less than 2 seconds! This wind leads to a mass loss of more than 1 million tons of material per second, which may seem like a large number, but is insignificant relative to the total mass of the Sun.

The Role of the Coronal Magnetic Field

The solar wind escapes primarily through coronal holes, which are found predominantly near the Sun's poles; in the equatorial plane the magnetic field lines of the Sun are more likely to close on themselves, particularly in periods of low solar activity. These closed field lines trap the hot coronal gases, leading to enhanced X-ray emissions from these hotter regions, but suppressing contributions to the solar wind.

The adjacent image shows an enhanced image of the solar corona. In this image the magnetic field lines of the corona correspond approximately to the boundaries between regions of different color (more info). Notice that in the equatorial regions the field lines traced by these color boundaries tend to form closed loops, indicating the trapping of coronal gas.

Influence of the Solar Wind on the Earth

As we have already discussed in the section on the Earth, the solar wind can have a large influence on our planet, particularly in times of the active Sun (near sunspot maximum) when the wind is strong and can contain bursts corresponding to flares and coronal mass ejections from the Sun. The solar wind has a significant influence on our ionosphere, the Earth's magnetic field, on Earth's auroras, and on telecommunication systems. For example, there is reason to believe that a burst of particles from a coronal mass ejection detected 5 days earlier by SOHO may have killed the Telstar 401 communications satellite on January 11, 1997 (News Story).

Earth's Ionosphere and the Sun

 Electron Density

The adjacent animations simulate the variation by month of the ionosphere for two different years:

1. The year 1990 (upper image), which was a period of high solar activity with many (150) sunspots.
2. The year 1996 (lower image), which was a period of low solar activity with few (10) sunspots.

The plots show electron density contours, which are an indication of the amount of ionization in the atmosphere. Yellows and reds indicate larger ionization and blues and greens indicate smaller ionization. Notice the substantial differences in these two animations, with much stronger atmospheric ionization in the upper image (the active Sun of 1990) than the lower image (the quiet Sun of 1996).

The adjacent images are based on these electron density contour maps of the ionosphere for months in the year 1957 to the present. Additional animations may be found in this NOAA directory.

The level of solar activity has similar effects on related phenomena such as Earth's auroras.

The "Space Weather" Report

One can monitor solar data for the last 30 days. This data gives information on solar flares, sunspots, X-ray and radio-frequency fluxes. One can even tune into Today's Space Weather, which gives a "weather report" of current and predicted conditions in space with respect to the solar wind, solar activity, X-ray activity, and related phenomena. For example, here is the space weather outlook that was reported on January 27, 1998:

Space Weather Outlook

SOLAR ACTIVITY IS EXPECTED TO REMAIN LOW. REGIONS 8142 AND 8143 COULD PRODUCE ISOLATED C-CLASS FLARES. THERE IS ALSO A SLIGHT CHANCE FOR AN M-CLASS FLARE FROM EITHER REGION.

THE GEOMAGNETIC FIELD IS EXPECTED TO BE MOSTLY QUIET THROUGH 28 JANUARY. A DISTURBANCE IS EXPECTED DURING 29 - 30 JANUARY IN RESPONSE TO THE PARTIAL-HALO CME THAT OCCURRED ON 25 JANUARY. UNSETTLED TO MINOR STORM LEVELS ARE EXPECTED DURING THE DISTURBANCE.

In this report, the region numbers refer to active areas on the Sun, C and M are classifications of solar flares, and CME stands for a coronal mass ejection. As noted above in conjuction with the ill-fated Telstar 401 satellite, "space weather" may have non-trivial practical consequences.