The Solar

The Sun makes itself known throughout much of the Solar System by the influence the solar wind of high-speed charged particles constantly blowing off the Sun. The solar wind may be viewed as an extension of the outer atmosphere of the Sun (the corona) into interplanetary space.

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 Rochester to New York in seconds! This wind leads to a mass loss of about 10 million tons of material per year, 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

Magnetic field lines
in the solar corona
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. 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

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 the Earth's magnetic field, the IONOSPHERE, and 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.

Earth's Ionosphere and the Sun

The level of activity on the Sun can have a large influence on the Earth's ionosphere, as illustrated in the following animations. Much stronger atmospheric ionization (and thus higher electron density) occurs during an active sun (e.g. 1990, with 150 sunspots) than in a quiet sun (e.g. 1996, with 10 sunspots).

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 Feb 26, 2001:

Space Weather Outlook

IA. Analysis of Solar Active Regions and Activity from 25/2100Z to 26/2100Z: Solar activity was low. An optically uncorrelated C1 x-ray flare occurred at 26/0741 UTC. The probable source for this event was observed by SOHO/LASCO from behind the west limb. IB. Solar Activity Forecast: Solar activity is expected to be very low to low. IIA. Geophysical Activity Summary 25/2100Z to 26/2100Z: The geomagnetic field was quiet to unsettled. An isolated period of active conditions occurred at 26/1800 UTC. A slight proton enhancement was observed which was probably associated with the event which occurred behind the limb earlier today. IIB. Geophysical Activity Forecast: The geomagnetic field is expected to be quiet to unsettled for the forecast period.

In this report, the region numbers refer to active areas on the Sun and C is a classification of solar flares. (Note: M is also a classification and CME is a coronal mass ejection in such reports.) As noted above in conjuction with the ill-fated Telstar 401 satellite, "space weather" may have non-trivial practical consequences.