The solar activity varies constantly following cycles. Solar cycle variability spans from 9 to 13 years, with an average 11 year-cycle. These solar cycles consist of a maximum and a minimum specific to each cycle. However, one cannot quantify their intensity in advance. This so-called solar cycle notion was introduced for the first time in 1843 by the amateur German astronomer Heinrich Schwabe. We currently are in the 24th solar cycle which maximum would have been reached in 2012/2013. NASA and ESA (European Space Agency), which permanently monitor our star’s activity, said, however, that the current solar cycle would be one of the lowest in almost two centuries. A solar cycle maximum, which can vary from simple to threefold, is determined by the appearance of several sunspots on this giant gas sphere’s surface.
Sunspots are the result of multiples phenomena: the solar field and the rotation of the sun on itself. The poles do not rotate at the same speed as the sun’s equator. That’s why equatorial regions move faster, the magnetic field lines tend to wrap around the sun. At some point, they are so tight and twisted they eventually emerge from the Sun’s surface, a spot then appears (see Figure 7).
Sunspots are therefore found on the sun’s surface by pairs (inlet and exit at the surface). Since matter inside the magnetic field lines differs from the rest of the sun (much colder) sunspots therefore appear darker, black.
The loops formed by the coiling magnetic fields are unstable and move in all directions. When two of these collide, there is a magnetic reconnection (as a short-circuit when two electrical wires get in contact, cf. Figure 8), thus releasing incredible amounts of energy and ejecting countless particles into space. It is a solar storm.
Solar storms gather solar flares (X-radiation, gamma and UV emissions and in the visible wavelength range of 400 to 800 nm) and coronal mass ejections or CME (coronal mass ejection). It is, therefore, important to differentiate between solar storms and solar flares.
These coronal mass ejections (CME) are ionized gas spurts (and thus electrically charged) which are composed of energy particles in the form of plasma. A CME also has its own magnetic field. However, this particle flow is not the only one to reach out our planet.
We can also find solar energetic particles (or SEPs) which are high energy particles coming from the sun. Observed for the first time in 1940, these particles are essentially made up of protons, electrons and ions with an energy varying from a few keV to GeV. These are extremely fast particles up to 80% the speed of light that is 240 000 km / s. They therefore reach the Earth in less than an hour. SEPs are objects of interest and importance since they can threaten life in space (particularly particles with an energy higher than 40MeV). They can be formed in two ways: from a solar flare (the particles are accelerated in the magnetic reconnection); either from a coronal mass ejection (coronal ejections quick masses create shock waves that accelerate the protons, electrons and ions from the solar wind).
The coronal mass ejections or CME add up to the permanent solar wind. Indeed, a constant flow of particles (protons and electrons) is emitted by the sun and sweeps over the solar system. This solar wind blows at a speed generally close to 300 or 400 km/s: it generally takes three days for the particles to cover the Sun-Earth distance (about 150 million km). The order of magnitude of the solar wind’s density is about several particles per cm3. It can change very quickly after solar storm. Coronal mass ejections can steeply accelerate the solar wind, sometimes up to 2000 km/s. When it reaches the Earth, this association corresponds to a magnetic or geomagnetic storm.
To counter this phenomenon, Earth has a first, completely natural, line of defence: the terrestrial magnetic field (magnetosphere). This magnetosphere deflects the particles, preventing them from « entering » our atmosphere. A small part reaches the Earth, either by going through the magnetosphere anyway, or arriving by the « polar cusps » where the magnetosphere is less important (around the poles), creating the well-known auroras.
Aurora Borealis (around the North pole) or aurora Australis (around the South pole) form at 80 to 1000 km in altitude. They result from the excitation of atoms in the ionosphere. The stimulated atom can’t stay in this state, so one electron decays to a lower energy shell, releasing a bit of energy at the same time, producing a photon which emits light.
Earth also has a second protection line, also natural, which is the atmosphere, sheltering us from solar storms.
Since a few decades, solar eruptions have been rated according to the maximum intensity of their energy flow, measured in Watt per meter squared by one of the satellites of the GOES program (Geostationary Operational Environmental Satellite). The different categories are named A, B, C, M and X. Each category corresponds to a solar flare, which is 10 times more intense than the last one, where X refers to solar flares with an intensity close to 10-4W/m2. Within a same category, solar flares are classified from 1 to 10 according to a linear scale (thus, a X2 solar eruption is twice as powerful as X1 eruption). Two among the most powerful solar eruptions were recorded by GOES program satellites on August 16th 1989 and April 2nd 2001. They were classified X20 (2mW/m2). However, they were outdone by an eruption on November 4th 2003, the hugest ever registered, estimated at X28. As for the Carrington event, in 1859, it would most certainly have belonged to a class higher than X30 (estimation based on ice core samples, see III)