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(S-6) Seeing the Sun in a New Light

Coronal Holes

    The best-known early observations of the Sun from space were the ones made in 1973 from the space station Skylab. Skylab carried several solar telescopes, including the "Apollo Telescope Mount" (ATM) which observed the Sun in EUV (=Extreme Ultra Violet, wavelengths shorter than UV but longer than those of X-rays) and in the longer wavelengths of the X-ray range. Since these wavelengths preferentially observe the corona, it was able to track and map coronal features.

On right: Yohkoh X-ray image of the Sun. For bigger image, click here.

    As might have been expected, the brightest coronal emissions came from above active sunspot regions. Separating such bright patches were large dark areas, named "coronal holes"; they were appparently dark because of lower density and less heat dissipation in the lower corona.

    The discovery of coronal holes helped solve a long-outstanding puzzle. Long before Skylab, space probes such as Mariner 2 in 1962 detected fast streams in the solar wind, flowing not at 400 km/s but perhaps at 600 km/s or more. They tended to recur at intervals of 27 days--the rotation period of the low-latitude Sun--suggesting that whatever their source was, it rotated with the Sun. Even earlier, around the turn of the century, series of moderate magnetic storms were observed which tended to recur at 27-day intervals, prevalent not near the peak of the sunspot cycle but, perversely, near its minimum.

    Skylab showed that both phenomena were associated not with sunspots but with the dark areas of coronal holes, which also seemed to contribute much of the solar wind. Apparently loops of magnetic field lines above sunspots help trap plasma (a bit like they do in the Earth's radiation belt) and hold it back.

    Field lines of coronal holes, on the other hand, seem to extend far outwards, their ends dragged by the solar wind to the Earth's orbit and far beyond it. Since plasma motion tends to be guided by magnetic field lines, such lines provide an easy exit to solar wind plasma. Above the poles of the Sun, as already noted, field lines stick nearly straight out, creating two large, permanent "coronal holes. " As expected, the space probe Ulysses found those regions filled with fast-moving solar wind, similar to the kind observed in solar wind streams.

Coronal Mass Ejections

    Skylab's ATM also observed huge bubbles of plasma rising occasionally (every 2 days or so) from the Sun. It was immediately suspected that such bubbles--named "Coronal Mass Ejections" (CMEs) marked the first stage of the development of interplanetary plasma clouds, some of which initiated magnetic storms when they reached Earth.

    The problem was that CMEs are best observed when they move across the line of view, rising above the flanks of the Sun, and clouds moving that way will not hit Earth. At the end of 1983, however, the magnetospheric probe ISEE-3 (International Sun-Earth Explorer 3) pulled away from Earth towards comet Giacobini-Zinner, and some time later was sufficiently far from Earth to intercept such CMEs. It confirmed that their signatures were similar to those of plasma clouds near Earth.

    In the future NASA plans to conduct such observations from twin sun-observing spacecraft of the "Solar Stereo" mission. Located on the Earth's orbit but 60° ahead and behind it (at the L4 and L5 Lagrangian points of the Earth-Sun system), the cameras on these spacecraft will be able to observe CMEs heading for Earth and even (because of their different vantage points) obtain some information about their 3-dimensional structure.

    More recently the Sun has been monitored in EUV and X-rays by Yohkoh, a highly successful Japanese satellite. Its images give a clear and detailed view of coronal holes, coronal bright spots and CMEs.

    Another notable observer of CMEs has been the LASCO instrument aboard the SOHO spacecraft, stationed at the Lagrangian L1 point, sunward of Earth. For some SOHO images, see here and here. Sophisticated image-enhancement procedures on LASCO images made it possible for SOHO investigators to see CMEs even when headed towards Earth; an example can be seen here.

    With all these modes of observation, a great deal was learned about CME since 1973, and it is now believed that much of the magnetic storm activity at Earth formerly credited to flares is actually associated with CMEs. Their energy apparently comes from the coronal magnetic field, and their material from prominences which are blown away by the process. They need not arise in sunspot regions.

   Exploring Further: Click here to reach a web site which covers CMEs in greater details than is done here.

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:   What if the Sun were a much hotter star?
                      ***     Consequences of no more solar X-rays