Flares and Coronal Mass Ejections (CMEs) in the Sun: Coronal and Heliospheric investigations with STEREO (Full version)

There are two important phenomenons associated with the solar plasma. The solar flares and the Coronal Mass Ejections (CMEs). The solar flares are active regions of magnetic reconnection, in which strong and opposite magnetic field bound together, radiating huge quantities of energy. The solar flares could be the possible cause of the CMEs. The CMEs are plasma emissions to space at high speeds and high temperatures. These CMEs can affect the Earth environment when they collide with the planet magnetosphere (causing problems in the satellites and telecommunications). An important solar observing instrument in the NASA STEREO mission. STEREO mission use two spacecrafts that observe the Sun from different points-of-view, which allow observe the Sun in 3 dimensions. STEREO has made great discoveries: the helical structure of the coronal jets (caused by the twisted magnetic fields) and the origin of the solar tsunamis (caused by plasma waves traveling along the surface).

Sun photosphere in UV. Source: Space Observatory TRACE
1. Introduction
At high temperatures the hydrogen and the helium are totally ionized. The nucleus and the electrons are not bounded and they move independently, which prevent that the atoms bound together. This state is the fourth state of the matter and is called plasma. It is a very frequent states in the Universe and the most part of the Sun consists in gas or plasma, with ions negative or positively charged. Several phenomenons that occurs in the Sun are related with this state of the matter and they are of special interest for us. The solar flares and the Coronal Mass Ejections (CMEs), are the object of study of the first part of this report. The second part will be focused in the STEREO space-based observatory, launched by NASA in 2006, and which objective is the study of the Sun in 3 dimensions.

2. The plasma
In the plasma the ions move in opposite direction to the electrons. The total I 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.
It is called plasma frequency to the natural frequency of oscillation in a ionized medium. The electrons experiment a force that reduces their motion, due to the attraction from the ions. This causes a motion similar to the pendulum one. This frequency is defined by:
       f(n)=(80,5 x N x e)^(0.5) Hz
where Ne is the electrons density.
The magneto-hydrodynamics (MHD) is the study of the interaction between electromagnetic fields and the motion of the charged particles. The mass, energy and momentum conservation laws are conserved.
The Lorentz force is the resulting force from the motion of a charged particle relative to the magnetic field. It is only considered the motion component that it is in a perpendicular plane to the field lines. This causes that the particle changes its motion from a lineal shape to a circular one, around a B magnetic field. The gyroradius is defined as:
       r=(m x v)/(B x|q|)
and the gyrofrequency as:
       f=(B x |q|)/(2 x π x m)
The proton gyroradius is 45 times greater than the electron one. The gyroradius of oxygen ion is 180 times greater than the electron one. In the Sun, which magnetic field is around 0.03 T, the oxygen ion has a gyroradius of 120 meters. Also the collisions between ionized particles and electrons generate additional emission of electromagnetic radiation. The plasma is considered as a magnetic fluid.
It is called magnetic pressure to the pressure that acts in a magnetic field in all directions, and is defined as:
       P(m)=(B^2)/(2 x µ(0))
It is called plasma beta (β) is the ratio of gas pressure to the magnetic pressure. It indicates which pressure dominates. If it is greater than 1, the magnetic field follows the motion of the plasma, and if it is lower than 1, the plasma motion follows the movement of the magnetic field.

3. The plasma in the Sun
In the solar Corona a lot of ions are highly ionized. The loops that are seen in the solar surface are the effect of ions looping around the magnetic loops.
The Sun’s magnetic field is dipole with poles near to the rotation poles. Probably the field is generated by a dynamo effect, due to the rotation of magnetic material with respect to a second magnetic material. The origin could be the interaction between the convection zone and the radiative zone. The magnetic field varies with a period of approximately 22 years. The polarity of sunspot polarity reverses each 11 years. If the solar cycle starts with positive polarity in northern hemisphere, after 11 years it will be negative. Each solar cycle length is the difference between two minima.
In the Sun outer region, above the cromosphere, there is the Corona. The Corona is the outer region that extends several millions of kilometers. Starting in the transition zone, the temperature increases from 25,000 K to 2,000,000 K at 30,000 kilometers from the surface. This temperature increment would be related with the Sun magnetic field. In the corona the magnetic field varies between 0.00002 and 0.00008 T, and it is expected that plasma beta parameter is greater than one, so plasma dominates the motions. In the solar Corona there are coronal holes, which are dark zones where born the fast solar wind, which travel at 700 kilometers per second.
It is called active region to those where interacts different parts of the magnetic field, and they are the origin of two important phenomenons, the solar flares and the Coronal Mass Ejections (CMEs), and are the omen of a new sunspot. They are dominated by loop that emerge and looping back into the Sun. The motion of the magnetic fields due to convection under the surface causes changes in magnetic field shape in active regions.
The solar wind particle density is low, 1 particle in each 5000000 m3, traveling at 300 kilometers per second at quiet times. The dynamic pressure due to solar wind is defined as:
       P(d)=n x m x v^2
where n is the particle density, m the particle mass and v the wind velocity. The solar wind is dominated by hydrogen plasma.

4. Solar flares
It is called 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. It is possible that the magnetic reconnections are the responsible of the solar flares and the Coronal Mass Ejections (CMEs), in which a lot of Coronal gas is ejected into the space.
The solar flares are associated to active regions and they are flashes that can be seen along all the electromagnetic spectrum, radiating high energy particles as electrons and protons. They emit high quantities of energy in electromagnetic spectrum, from the radio to x-ray, and with a 1020 W power (around 10% of the energy released by the Sun in 1 second). The plasma is so hot that it can be studied in x-rays.
There are more quantity and more intense flares during the solar maximums. The SOHO space-based observatory demonstrated 1 that the flares are generated where the shape of the magnetic field changes very fast.

5. Coronal Mass Ejections (CMEs)
The Coronal Mass Ejections (or CMEs) are matter emitted through the solar Corona. A lot of flares are the symptom of a new CME. The CMEs were discovered thanks to space-bases observatories. During the solar maximum there are typically 2 or 3 CMEs per day, meanwhile during the solar minimum there is 1 CME per week. Near the solar minimum they are generated near the equator, but during the solar maximum they can occurs in any latitude. It can occur in any direction, but when they are orientated toward us, and the Earth is in their path, we observe the halo CMEs.
The CMEs travel away from the sun at high speeds (up to 2,000 kilometers per second) and can create disturbances in the interplanetary medium and can cause magnetic storms when they collide with Earth magnetosphere (The space where the Earth’s magnetic field dominates and typically extends around 10 Earth radii).
The solar flares and the CMEs produce highly energetic protons. It is defined the proton event as the sequence that starts when 3 consecutive data points (taken 5 minutes apart) have a flux equal to or greater than 10 pfu (particle flux unit). It ends when the value is lower than 10 pfu.
The solar energetic particles (SEP) are electrons, protons and ions, and they are accelerated to high energies during solar flares and CMEs. They could be dangerous for spacecrafts, satellites and humans in the space.

6. STEREO mission
The STEREO mission (Solar TErrestrial RElations Observatory) 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 (CMEs, 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 (STEREO A) and orbiting nearer to the Sun, and the second one behind (STEREO B). The continue separation between both spacecraft will finally case that it will not be possible to create 3 dimension images for human eye, although the mission will continue producing very interesting scientific data.
In January of 2009 they have been separated by 90 degrees, in the quadrature: in this situations the observed CMEs in the limb by one spacecraft, can be studied with the particle detectors of the other spacecraft. In 2011 they will be separated 180 degrees and it will be the first time that we could observe all the Sun. This fact, with observations of Earth-based observatories will allow the full-Sun observation during year. In 2023 they will approximate again to the Earth.
The STEREO mission has 4 instruments:
- SECCHI (Sun Earth Connection): The instrument is dedicated to the coronal and heliospheric investigation. Its objective is the study in 3 dimensions of the CMEs evolution, since their emission until their impact in the Earth. It has 5 cameras:
- Extreme UV imager (EUVI) 2. It is a telescope of 98 mm of aperture, which can be observed the low corona in 4 different EUV emission lines. The EUV radiation enters in the telescope through a thin metal filter of 150 nm that removes the UV, visible and infrared radiation.
- Two white-light coronagraphs. The inner coronagraph 3, called COR1, is based on the classical refractive internally-occulted Lyot coronagraph, for observing the inner corona, from 1.3 to 4 solar radii. It uses a linear polarizer to suppress scattered light. The outer coronagraph 4, called COR2, has an observing range from 2 to 15 solar radii.
- Two heliospheric imagers (HI) 5, for taking images of the space between the Sun and the Earth. It has 2 cameras 6 of 20 degrees and 70 degrees of fields-of-view and they are off-pointed from the Sun 14 degrees and 53.7 degrees respectively.
- IMPACT (In-situ Measurements of Particles And CME Transits): With 7 instruments, it is centered in the study of energetic particles and the distribution in 3 dimensions of the solar wind and the interplanetary magnetic field.
- PLASTIC (PLAsma and SupraThermal Ion Composition): It is centered in the study of the plasma characteristics. It consists in a input system with 3 apertures and an electrostatic analyzer connected with a mass spectrometer with solid state detector, to measure the distribution of the solar wind particles.
- SWAVES 7: It studies the radio disturbances that travel from the Sun towards the Earth. It uses 3 orthogonal monopole antennae and five radio receivers.

7. STEREO main observations
December of 2006 8: It is detected a solar flare of X-Class (the maximum class), detecting first the hydrogen atoms and lately the ions. The possible cause would have been the act of the solar magnetic field, that could have affected the ions but not the neutral hydrogen atoms. The atoms of hydrogen arrived to the Earth 2 hours before than the ions.
April-May of 2007 9: It obtained a solar sequence in 3 dimensions for a period of one week in extreme ultra-violet (EUV).
June of 2007 10: With the instrument SECCHI it was studied the geometry of the coronal jets, discovering their helical structure, thanks to the two viewpoint of STEREO mission. The images were taken in extreme ultra-violet (EUV) and both spacecraft were separated 11.7 degrees. The possible cause could be the twisting of the solar magnetic field. Also it had been discovered that the jet contains both cool plasma (at 80,000 K) and warm plasma (1-1,5x106 K).
January of 2009 9: STEREO mission has both spacecraft at 90 degrees, in the quadrature, allowing a better study of the CMEs, because when they are observed in the limb from one spacecraft, the other could be in from of them, detecting the particles emitted.
February of 2009 9: STEREO develops the study of the hot plasma waves that travel along the solar surface (solar tsunamis). STEREO confirmed their existence when the sunspot #11012 unexpectedly erupted, creating a CME and a plasma wave that traveled along the solar surface. This wave was observed from 2 points-of-view.
April of 2009 9: PLASTIC instrument made a measurement in 3 dimensions of the velocity, trajectory and shape of the solar storms, allowing the study of the CMEs days before, compared with other techniques.
May of 2009 9: STEREO made the first detection of the 24th solar cycle, observing a CME at a 30 degrees latitude. This observation indicated the near end of the previous solar cycle.
September of 2009 9: The STEREO mission captures a large prominence eruption over about 30 hours. This is the first time that this phenomenon is observed for such long period.
April of 2010 9: It is observed the largest prominence eruption seen.

8. Conclusion
The revolutionary idea of using two spacecraft separated in the space in the STEREO mission, to obtain images of the Sun in 3 dimension, has allowed to obtain a new way of understanding the Sun and the solar phenomenons so important as CMEs and the solar flares. Little by little, the space-based observatories fleet (SOHO, STEREO, Yohkoh, GOES,…), 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, especially in which way the terrestrial environment is affected.

Harvard 2002, Nearest Star: The surprising science of our Sun, Leon Golub & Jay M. Pasachoff, United States of America
University of Central Lancashire 2009, Sun, Earth and Climate, Course Notes, Version 3, Preston, UK
Springer  2007, The Sun and Space Weather, 2nd edition, Arnold Hanslmeier, The Netherlands
Freeman 2007, Universe, 8th edition, Roger A. Freedman & William J. Kaufmann III, United States of America

1 'Observations of the Sun and the Earth' in Sun, Earth and Climate, Course Notes 2009, version 3, University of Central Lancashire, Preston, UK
2 LMSAL SECCHI Science Team, STEREO-SECCHI-EUVI (LMSAL), viewed 4 April 2010, http://secchi.lmsal.com/EUVI/
3 NASA STEREO COR1, COR1 Instrument home page, viewed 4 April 2010, http://cor1.gsfc.nasa.gov/instrument/
4 U.S. Navy, SECCHI-Solar Physics Branch-Naval Research Laboratory-Welcome to Cor2, viewed 4 April 2010, http://secchi.nrl.navy.mil/index.php?p=cor2
5 Science & Technology Facilities Council, STEREO-About HI,  viewed 4 April 2010, http://www.sstd.rl.ac.uk/stereo/about-hi.html
6 C.J. Eyles, R.A. Harrison, et al. The Heliospheric Images On-board the STEREO Mission, Space Science and technology department, STFC Rutherford Appleton Laboratory, UK.
7 National Aeronautics and Space Administration, STEREO/WAVES Experiment, viewed 4 April 2010, http://swaves.gsfc.nasa.gov/
8 SAO/NASA ADS-Astronomy Abstract Service, STEREO Observations of energetic neutral atoms during the 5 December 2006 Solar, viewed 17 April 2010, http://adsabs.harvard.edu/abs/2008AGUFMSH11A..02M
9 NASA 16 April 2010, NASA - Archive, viewed 17 April 2010, http://www.nasa.gov/mission_pages/stereo/main/feature_news_archive_1.html
10 arXiv.org 30 April 2008, [0804,4862] STEREO/SECCHI Stereoscopic Observation Constraining the Initiation of Polar Corona, viewed 15 April 2010, http://arxiv.org/abs/0804.4862

(This article was presented at University of Central Lancashire in 24/04/2010 by Francisco José Sevilla)

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