Friday, April 23, 2004

Solar Radiation

Now my Easter break is over , I'm going to pick up with a more detailed analysis of the Solar radiation environment [1].

The Sun's outer three layers are responsible for the majority its electromagnetic radiation. The photosphere, at about 5900 K, emits visible wavelengths; the chromosphere above emits more intense ultra-violet radiation; and the corona, the Sun's outer layer, gives out X-rays. The chromosphere has temperatures up to 10,000 K, and the corona reaches temperatures greater than 2x106 K.

The radiation pressure of the Sun's electromagnetic emissions is sufficient to drive of huge amounts of matter in the form of the solar wind. At one AU, the solar wind has a flux of about 9 protons cm-3, travelling at an approximate mean velocity of 450 kms-1.

Disturbances in the Sun's atmosphere are the cause of solar flares, large plumes of material thrust out from the Sun's surface. Associated with flares are increases both in radiation and high energy particle fluxes. Near the Earth the first observable change is a sudden and relatively brief increase in solar radiation about 20 minutes after the flare occurs, and then about a day later a longer burst of high energy particles, similar to the solar wind but more intense and at higher velocities, typically 103 kms-1.

As discussed previously, the enhanced electromagnetic and particle emissions pose a substantial risk to exposed equipment and personnel.


J. P. W. Stark (2003) The Spacecraft Environment and its Effect on Design. In Spacecraft Systems Engineering (J. P. W. Stark, G. G. Swinerd & P. W. Fortescue, ed.), pp. 11-47. John Wiley & Sons Ltd, Chichester.

No comments: