I THE SOLAR WIND

1 the solar wind

2 the magnetosphere

3 the radiation’s belt

4 the solar eruptions

5 the particles received by the satellite

II ITS INFLUENCE

A) The effect of particles on "materials"

1 interaction of particles with the satellite

2 the spatial "surroundings"

3 charge and discharge of a satellite

4 effect of radiation, protections

B) How to minimise its effects

Nowadays, satellites play a very important role in our life. However, their environment is not, as one could think, vacuum but contains particles, a majority of which coming from the solar wind. These particles can cause damage to satellites and therefore cause communication problems with them. We will see what is, more precisely, the composition of the solar wind. In a second part, we will tell what damage can those particles cause. In a third part, we will explain the different techniques of solar activity forecast.

The earth, as every other planet in the solar system, is surrounded by a plasma, called heliosphere, which particles are ejected from the sun according to a process discovered by the American Eugene PARKER in 1958.

The high solar atmosphere, called the solar "crown", contents a very hot plasma, of one million of degrees (a hundred of electronvolts). Therefore, the particles from this plasma have a high thermic agitation velocity, around 150 km.s-1 for the protons and bigger than 500 km.s-1 for the electrons. From a certain altitude, the electron’s thermic agitation energy becomes greater than their gravitational link energy with the sun and can therefore escape from it (not very good!!). The charge discrepancy that results carries along protons, alpha particles (7 to 8 % of the total of particles) as well as "very little trace" of ions of heavier elements such as oxygen or carbon. The whole of that constitutes the solar wind, which mean density is, for a calm sun, at the earth height, around 5 protons per cm³ and its mean velocity is around 400 km/s.

The astronomical observations have also "pinpointed" a fast solar wind, up to 800 km/s, which is emitted from ‘holes’ in the solar crown (called holes because they are black areas in the solar crown).

Other particles are emitted from the sun, they are the solar eruptions, and we will talk about them after having defined some elements of the spatial environment, which are necessary for a good understanding.

When the charged particles of the solar wind – protons and electrons- enter the earth surroundings, they interact with its magnetic field which, on the one hand screen their penetration, but on the other hand is being compressed by this flow and is staled in a "space": the magnetosphere. We can see it as a natural cavity in the interplanetary surrounding, in the centre of which "lies" the earth, relatively protected from what goes on on the outside.

They are called ‘Van Allen’s belts’ and are not stable configurations, in either space or time. They are influenced by the local irregularity of the earth magnetic field, the effect of the earth’s rotation and the perturbations due to the compression of the magnetosphere by the solar wind , depending on the sun’s activity. The particles of high energy are trapped in those belts.

The sun’s activity is cyclic, each cycle during 11 years in which one can see two different periods, one is a seven-year-maximum activity period, and the other is a minimum period. During this second part of the cycle, the emission of particles are rare and, in a first approximation, negligible. On the contrary, during the peak activity period, the solar eruptions can be very violent, the ejected plasma having a velocity of more than 1000 km/s, which will cause very intense geomagnetic storms when colliding with the earth’s magnetosphere. Among the thousands of eruption that can be observed each year, some of them, called ‘ protons events’, send protons, called eruption protons, into the interplanetary surroundings. Their can travel at a speed which can be greater than 50000 km^s-1 they are responsible for spectacular geomagnetic storms. About an hour after they have left the sun, the magnetosphere is being bombed: the particles go through Van Allen’s belt. They will be stopped by the magnetopause and are of no danger for the men on earth.

The characteristic of the solar wind are very dependent on the solar activity, moreover they decrease as the square of the particle’s distance from the sun.

The flux of particles coming from the sun can cause damage to satellites because most of them are in the Van Allen’s belt: the high altitude satellites (altitude greater than 15000km) and most particularly geostationnary satellites. The other satellites are protected by the magnetosphere, and they would be subjected to solar wind only if the solar eruption was such that it would compress the magnetopause below six times the earth’s radius.

After having described the plasma in which are the satellites, we are going to set out the main effects of particles on them.

A) PARTICLES’ EFFECT ON MATERIALS

The energy transfer caused by the chocks between the plasma’s particles and the satellite’s particles create various defects and sides effects on materials:

-Ionisation

-Displacements

-Excitation

-Break of chemical links

-Emission of electrons, gamma photons, X and visible rays

-Emission of heat

It is the defects caused by ionisation, displacement, and break of chemical links that damage either for a short period or permanently the materials and components.

They can cause damage to the surface of the satellite as well as the position captors. By causing damage to the latest, the particles can be responsible for a change in the satellite’s attitude (its trajectory and its inclinaison); the antennas or the solar panels can face the wrong direction or the orbit can be modified.

A charged particles can have (?) an elastic collision with a whole atom of the satellite. It is responsible for most of the atom displacement in the materials. The low energy electrons (< 500 KeV) do not penetrate inside the satellite, they are stopped by 0.5 mm thickness of aluminium.

The risk of damage comes essentially from the electrons which energy lies beneath 500 KeV and 1.5 MeV. They can penetrate inside the satellite, charge dielectrics of intern cable.

 Influence of the earth environment:

The earth environment is such that the satellites are "submitted" to radiation that damages its elecronical components. The length of the life of a system and its reliability depends therefore on the amount of radiation received by the satellite during its life.

This radiative background is not homogenous and is made of belts, which characteristics depend on the altitude as well as the inclinaison. It dictates the choice of orbits. The orbits that enable the satellite to have an acceptable life-length are:

- The orbits with an altitude beneath 500 and 2000 km and inclinaison of 50 degrees.

-The orbits with an altitude beneath 10,000 and 20,000 and inclinaison of 50 degrees.

- The altitude of the geostationnary type.

At a macroscopic scale, a world perturbation can be caused by an insignificant change of the characteristic of the solar wind. Theses phenomenon have important practical consequences: perturbation of radioelectic communication, creation of X rays dangerous for high altitude travel.

The geomagnetic storm is the cause of the degradation of the satellite’s semi-conductor, particularly the photovoltaic cells that provide the energy on board.

B) HOW TO MINIMISE THES EFFECTS

To protect the satellite from discharge that damages the electrical components and materials, all the conductor elements of the satellite are linked to the mass of the structure. Moreover, the surface of the satellite has to be made the most conductive, so that the charge can be evacuated by photoemission.