There are two important phenomenons that occurs in the Sun and are related to the plasma: The Coronal Mass Ejections or CMEs, and the solar flares. The NASA-STEREO project studies the Sun in three dimensions.
The plasma is the fourth state of the matter, in which the ions move in opposite direction to the electrons. The total current is the sum of the positive and negative currents. As the electrons and ions have opposite charge, the gas is neutral and its behavior is like a fluid. The magneto-hydrodynamics (MHD) is the study of the interaction between electromagnetic fields and the motion of the charged particles, and in MHD Theory the mass, energy and momentum conservation laws are conserved. There are five fundamental concepts that we need to know about MHD Theory related with the plasma:
– The first is the plasma frequency, which it is the natural frequency of oscillation in a ionized medium.
– The second and third are the gyroradius and gyrofrequency of the particle, which describe the motion of a charged particle inside the plasma.
– The fourth is magnetic pressure, which it is the pressure that acts in a magnetic field in all directions.
Finally, the plasma Beta parameters, which it is the ratio of gas pressure to the magnetic pressure. If it is greater than one, the magnetic field follows the motion of the plasma, and if it is lower than one, the plasma motion follows the movement of the magnetic field.
The plasma is a very common state of the matter in the Sun. In particular, the solar Corona consist in highly ionized gas at extremely high temperatures: the temperature increases from 25,000 K to 2,000,000 K at 30,000 kilometers from the surface. it is expected that plasma beta parameter is greater than one, so plasma dominates the motions. The Sun’s magnetic field is dipole with poles near to the rotation poles. Probably the solar magnetic field is generated by a dynamo effect and causes a solar cycles of eleven years.
An active regions are those where interacts different parts of the magnetic field. In these active regions are originated two important phenomenons, the Coronal Mass Ejections and the Solar flares.
A magnetic reconnection are the regions where the change of the magnetic field is very high in small distances. It occurs in regions where two opposed magnetic field lines and plasma are forced to become nearer. The solar flares are associated to active regions where magnetic reconnections happen. They are flashes that can be seen along all the electromagnetic spectrum, radiating high energy particles as electrons and protons.An important discovery was done by SOHO space-based observatory: the flares are generated where the shape of the magnetic field changes very fast.
A Coronal Mass Ejection or CME is matter emitted through the solar Corona, traveling away from the sun at high speeds. A lot of flares are the symptom of a new CME. During the solar maximum there are typically 2 or 3 CMEs per day, meanwhile during the solar minimum there is 1 CME per week.
A CME that travels towards us is detected when we observe an halo CME. When CME arrives to the Earth environment, it collides with the Earth’s magnetosphere. The energetic particles are redirected following the magnetic field lines towards the Earth’s magnetic poles, causing a well known phenomenon, the aurora.
The STEREO mission is a solar observation mission, launched by NASA in 26th October of 2006. Its main objective is imaging stereoscopic the Sun, study the solar phenomenons as CMEs and solar flares, and improve the understanding of the space weather. The mission consists in two identical spacecraft, each one of 620 kilograms, one traveling Earth ahead and orbiting nearer to the Sun, and the second one behind.
In January of 2009 they have been separated by 90 degrees, in the quadrature, in which the observed CMEs in the limb by one spacecraft, can be studied with the particle detectors of the other spacecraft. Also is remarkable that in 2011 they will be separated 180 degrees and it will be the first time that we could observe all the Sun.
The STEREO mission has 4 instruments. The first one, called SECCHI
is instrument is dedicated to the coronal and heliospheric investigation. It consists in a extreme UV imager, two white-light coronagraphs (COR1
) and two heliospheric imagers
There are other three instruments in the STEREO spacecrafts. The first one is the IMPACT instrument, which it is centered in the study of energetic particles and the distribution in three dimensions of the solar wind and the interplanetary magnetic field. The second one is the PLASTIC instrument, which it is centered in the study of the plasma characteristics.Finally the SWAVES instrument
which it is used to study the radio disturbances that travel from the Sun towards the Earth.
One of the most important discovery of the STEREO mission with the instrument SECCHI it the geometry of the coronal jets, discovering their helical structure, thanks to the two viewpoint of STEREO mission. In the photograph it is easy to appreciate its shape.
Other important study developed with STEREO instruments is the study of the hot plasma waves that travel along the solar surface. This phenomenon is commonly known as solar tsunamis.
There a lot of important studies developed by STEREO mission, such as the detection of the hydrogen atoms first and lately the ions, when a solar flare of X-Class happened. The most important benefit that the science has obtained from this mission is that this is the first time that we can observe the Sun in three dimensions.
As conclusion, the space-based observatories fleet that study the Sun without the interference of the Earth atmosphere and observe the full range of electromagnetic radiation, are allowing a better understanding of the Sun and the space weather.