Solar wind

The solar wind is the continuous stream of energetic and ionized particles from the Sun that extends far into interplanetary space following the interplanetary magnetic field (IMF). The solar wind strongly interacts with magnetic fields of the planets and will often manifest itself as aurorae in the upper atmospheres of planets with magnetic fields. The solar wind is a day to day event, and is not caused by Coronal Mass Ejections (CMEs). The density of the solar wind is effected by solar flares and coronal holes.

In the case of Mars, with a weak (often considered to be non-existent) global magnetic field, solar wind particles directly impact the atmosphere, where as on Earth, charged particles are directed to polar regions. There they usually have no effect, but might produce faint aurora. (Strong aurora are caused by solar storms (CMEs).)

The normal solar wind is little danger, but a powerful CME is deadly dangerous, since of Mars' thin atmosphere, radiation from CME's will reach ground level. Large CME typically happen a couple times a year, so the plans to shield against radiation are concerned with handling these.

Note that the solar wind consist of the following particles: electrons, protons, and alpha particles (helium nuclei). This is bad (they are all forms of ionizing radiation), and good (this type of radiation is much easier to shield against than gamma rays).

Characteristics

The solar wind is characterised as a constant flow of plasma from the chromosphere into interplanetary space. Understood to be bi-modal, the solar wind consists of two distinct streams. The fast solar wind travels at velocities of between 700-900 km/s and is associated with open magnetic flux (i.e. coronal holes located in solar polar regions). The slow solar wind travels at velocities of between 300-400 km/s and is located above equatorial closed magnetic flux regions (i.e. the streamer belt).

Effect on Mars

It is believed that the solar wind is partly responsible for the loss of up to one third of Mars' atmosphere into space. (It is thought that the solar wind has stripped away a significant fraction of Mars' atmosphere, tho why this has not happened on Venus is unclear.) This process is very slow, if humans were to give Mars a 1 bar atmosphere, it would take 500 million years for the solar wind to reduce it back to a near vacuum. See atmospheric loss for more information.

Counter Measures

Artificial magnetic field

The threat from solar wind can be managed through the generation of local magnetic fields. Recent research has shown that the size of the field need only be several hundred meters wide to provide protection to humans in space. This works for low energy particles, high energy ones will punch thru. Also note, that low energy particles will be trapped in the magnetic field lines, and approach the ship from two directions (the north and South Pole of the magnet). Shielding will be required at these two locations, but this takes less mass than surrounding the full ship.

This magnetic shield could be scaled up to protect an entire settlement on the surface. This is easier than in space, as the field lines can be designed to intersect the ground some distance away from the base.

Shielding

While shielding a space ship increases its mass and fuel consumption, the shielding of a Martian colony is much easier, since the mass is only limited by structural stability constraints. Space craft will likely have a Storm shelter, a small volume where there is much higher radiation protection. The crew will have to crowd inside the storm shelter for the duration of the solar storm (~2 to 5 hours). Small ground bases can either use this storm shelter, or use local resources (such as water or sand bags) to increase the radiation protection for the entire habitat, or both.