July 25, 2011

Since 1996 the mass coronal ejections have been studied by SOHO

Update: 07/25/2011
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The Sun permanently emits a flux of particles - the solar wind - mainly made up from ions and electrons which are ejected from the Sun's upper atmosphere. This plasma flux ranges in speed and temperature over time according to the solar activity.

The SOHO satellite contributed a lot to our knowledge about the Sun's structure and Sun-Earth relations.

The systematic observations of the coronal mass ejections (CME) and associated phenomena in the low corona made significant progress thanks to SOHO and especially the EIT instrument and C2 coronograph of the LASCO instrument.
A coronal mass ejection is a bubble of plasma produced in the solar corona. It is often related to a solar flare or extrusion but not systematically.

This burning plasma is then ejected at a considerable speed. The solar wind speed ranges from 400 to 800 km/s (from 1,440,000 to 2,880,000 km/h) with an average speed of 450 km/s (1,620,000 km/h).

The CMEs are large scale phenomena: their sizes can reach several tens of solar radius (the Sun's radius is 700.000 km). They alter the solar wind characteristics by moving so quickly in the interplanetary medium (between 100 km/s and 2,500 km/s) and can travel the Sun-Earth distance within a few days (typically three days).

The CMEs' magnetic fields are very strong: a CME close to the Earth can thus trigger magnetic storms by interacting with the terrestrial magnetic field. Magnetic reconnection phenomena can thus be observed and some field lines can be opened, weakening the Earth's magnetic "shield". The CMEs play a major role in the space meteorology.

In order to detect these CMEs, coronographs are used to pinpoint the propagation speed and direction as well as the CMEs' extents.

The emission frequency varies according to the solar cycle. One CME occurs on average every week during the solar minimum and two to three times per day during the solar maximum. Nevertheless, only a few CMEs are pointed towards the Earth and the number of magnetic storms depends on these CMEs.

In the solar system, the solar plasma composition is similar to the solar corona's: 73 % of hydrogen and 25 % of helium. Every second, approximately 1 x 109kg (i.e. 1 million tons) of Sun's material is turned into solar wind. In the overheated Sun's corona (1 million degrees), hydrogen atoms are being ionized giving them an electrical load.

Since the solar wind is a plasma, it is influenced by the solar magnetic field (near the Sun, where the magnetic field is strong) but, because of its motion, it also alters the interplanetary magnetic field lines (where the magnetic field is weak). Because of the combination of the particles' radial motion and Sun's rotation, the solar magnetic field lines look like a spiral: the Parker spiral field lines.

The pressure of the solar wind creates a "bubble" in the interstellar medium. The limit to which the solar wind cannot push the interstellar medium anymore is called heliopause. It is often considered to be the solar system's "frontier".

The heliopause distance is not precisely known and is probably sharply varying according to the solar wind speed and interstellar medium's local density but we do know that it located much further than the Pluton's orbit.

The solar wind particles which are trapped in the terrestrial magnetic field tend to accumulate in the radiation belts and enter the Earth's atmosphere, near the poles, producing the polar aurora Borealis.

Other planets with magnetic fields also have their own Aurora: Jupiter, Saturn, Uranus, Neptune for example.

The specifically energetic solar wind bursts produced by solar flares, coronal mass ejections and other phenomena are called solar storm. The Earth's magnetosphere is opposed to the solar wind just like a bridge abutment against the river current. It protects us against the solar wind and acts like a shield. The magnetosphere, which is supposed to act like a dipole, is distorted by the solar wind. The magnetosphere is thus compressed on its day side and extended to large distances on its night side.

During the solar flare, 10,000 particles reaches the terrestrial atmosphere (compared to 10 in the absence of flare). These flares can submit space probes and satellites to high doses of radiations and thus accelerate their ageing and affect their operations. Terrestrial ionosphere (ionized region located at an altitude of approximately 300 km) disruptions are also likely to appear producing strong alterations of electromagnetic signal transmissions and deteriorating the global positioning system's performance.


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