High-Energy Particles
Because of the speed with which flares and CMEs proceed, it is generally believed that their energy comes from magnetic fields. However, even in Yohkoh images one cannot see small details, nor do those images tell enough about local magnetic fields or about 3-dimensional magnetic structure, and in the absence of better data, a detailed understanding is still lacking.
Physicists on Earth use electromagnetic devices--high-energy accelerators--to accelerate electrons, protons and other electrically charged particles to great velocities, in order to study their collisions with matter and so learn about their make-up and about the forces that bind them. Some pretty sophisticated accelerator tools is needed for this, but it appears that Nature, too, can do so. This is shown when flare events, once or twice per year during active parts of the solar cycle, emit streams of high-energy ions and electrons, which can flood interplanetary space for some hours, even at the Earth's orbit and beyond it. CME shocks may also do so, and the relative roles of CME and flares is still being debated. For more about such events, see here.
NASA is rightly worried about such particles. They do not threaten life on Earth, which is shielded by a thick atmosphere. Even astronauts in space stations near the Earth's equator are shielded, by the Earth's magnetic field. However, any humans beyond the Earth's inner magnetosphere--for instance, in transit between Earth and Mars--would need to be sheltered in some way.
The way those accelerations occur is still unclear, but it is widely held that they are associated with small regions in space in which magnetic fields from adjoining sources (e.g. sunspot groups) cancel each other, creating "neutral points" of zero field intensity. Such points--unfortunately for experimenters--are usually well above the photosphere, in region whose magnetic fields are difficult to measure. Acceleration may also occur at shocks associated with CMEs.
Some information about accelerated particles may however be deduced from the radiation they emit: fast electrons, in particular, excel in producing x-rays. Medical x-rays are produced when beams of fast electrons, created inside a tube from which all air has been evacuated, come to a sudden stop against a metal target; on the Sun, when fast electrons are stopped by the surrounding gas, a similar process takes place. Such x-rays can rise much faster than other flare emissions--a minute in some cases, but only a second or two in others.
In one such event (picture on right), Yohkoh actually observed the position of an x-ray burst, localized at the top of a magnetic arch, well above the limb (edge) of the visible Sun. Note that in the picture, two places are particularly bright--the top of the arch, where the acceleration (presumably) took place, and one "foot" at the bottom, where such electrons enter the denser layers of the Sun's atmosphere.
Radio and Microwaves
Emissions in which individual atoms of ions contribute a large part of their energy are not the only way the Sun produces electromagnetic radiation. There also exist plasma waves, oscillations and turbulence, in which many electrons or ions act in unison, creating waves in the radio and microwave range. The energy lost by each particle is small (and so is the photon produced), but with so many acting in unison, an observable signal is emitted.
For instance, waves emitted by electron and ion beams traveling outwards from the Sun are regularly tracked. Also, rising microwave radiation from above sunspot groups is often a good warning that "something is brewing. "
Electromagnetic Waves from the Universe
Astronomical objects, in our galaxy and beyond, radiate electro-magnetic waves across the entire spectrum, from radio to gamma rays. In a 1981 book "Cosmic Discovery, " Martin Harwit--astronomer and historian--addressed the question of what brings new discoveries in astronomy. He first noted that almost all our data about the universe come from electromagnetic radiations of objects in the sky.
He then showed that a large fraction of the discoveries in astronomy were associated with some sort of improved coverage of the electromagnetic spectrum: new wavelength ranges (e.g. radio, x-rays) or better resolution (e.g. larger, better telescopes). He therefore recommended to NASA to concentrate its space astronomy efforts on extending that coverage, and NASA has largely followed his lead. Each of NASA's "great observatories"--e.g. Compton for gamma rays, Hubble for the visible and near-visible spectrum, Chandra for x-rays--has targeted a certain spectral region and tried to extend its coverage. The results have been very rewarding, but they are beyond the scope of this presentation, which is focused on the Sun.
Further reading: The Sun from Space by Kenneth R. Lang, 373 pp, $64.95, publ. by Springer, New York, 2000.
Reviewed in "Science" vol. 292, p. , 27 April 2001.
"Astronomy picture of the day" for 16 May 2002, with an example of coronal mass ejections and some relevant links
Questions from Users:
The best-known early observations of the Sun from space were the ones made in 1973 from the 