The Sun
The Sun

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The Sun

1.4 The invisible Sun

Figure 7 shows an image of the Sun, taken when a huge prominence was visible (bottom left). The image was recorded using instruments that are sensitive to ultraviolet radiation rather than visible light, so the colours that you see are 'false'. They simply indicate different levels of intensity of ultraviolet radiation. The use of such false colour images is unavoidable when dealing with radiation that lies outside the range of visible wavelengths. None the less, the image shows that prominences emit copious amounts of ultraviolet radiation and are therefore observed easily at those wavelengths.

The Sun's radio waves carry much less energy than its visible light but can readily be detected with even a small radio telescope. Fortunately for us, the Earth's atmosphere shields us from the Sun's potentially harmful X-rays, so these can only be studied using telescopes put into orbit above the atmosphere.

Figure 7
Figure 7 A 'false colour' image of the Sun, taken at ultraviolet wavelengths.

A combination of ground-based and space-based instruments has enabled astronomers to observe the Sun over a wide range of wavelengths and to build up a clear picture of its various emissions.

Many solar observers are particularly interested in the Sun's active regions (see Figure 8), the nature and appearance of which depend on the locality and the wavelength being observed. When seen in the photosphere at visible wavelengths, active regions are often associated with groups of sunspots. Their counterparts in the chromosphere, sometimes observed at specific red, blue and ultraviolet wavelengths, are bright regions known as plage (which is French for 'beach'), while in the corona, X-ray astronomers can see transient regions of higher than usual temperature and pressure called coronal condensations. Active regions are generally caused by the Sun's magnetic field, which influences the flow of hot gaseous material on the Sun, and can sometimes rearrange itself on very short time-scales (seconds or minutes). Such sudden changes to the magnetic field in the corona are thought to be responsible for flares, one of the most energetic of all solar phenomena. Flares emit radiation of all wavelengths, from radio waves to gamma rays. Much of the energy is emitted very quickly at the start of the flare, although the flare will typically continue to radiate for several hours, even lasting a day in exceptional cases. Energetic particles are also emitted during a flare (fast-moving protons and electrons for instance), which certainly reach the Sun's surface and, sometimes, even the Earth.

Figure 8
Figure 8 An active region of the Sun, represented by a powerful flare (seen here as X-rays) associated with a large sunspot group.

Activity 2 Examining images of the invisible Sun

0 hours 20 minutes

Images from a variety of telescopes, representing various wavelengths of 'invisible' radiation, are included below. Some of these images enhance astronomers' knowledge of particular solar features (such as prominences or sunspot groups), while others help them to observe particular regions (such as X-ray images of the corona or ultraviolet images of the chromosphere). You should examine those images now, taking care not to be misled by their use of false colours.

Figure 9
Figure 9 The Sun at radio wavelengths

Click below to view a larger version of this image.

https://www.open.edu/ openlearn/ ocw/ mod/ resource/ view.php?id=26568 [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)]

An image of the Sun recorded at the Nobeyama Radioheliograph in Japan, at a wavelength of 1.76 centimetre (which corresponds to a frequency of 17 gigahertz). The radioheliograph takes the form of a T shaped arrangement of 84 individual dish shaped antennas. The use of high speed computer analysis allows the radio heliograph to provide up to 20 images of the Sun per second. enough to follow rapidly developing eruptive processes. This image was recorded in 2001 at around the time of maximum activity in solar cycle 23. Signs of activity are readily apparent across the solar disc.

The image in Figure 7 was captured in September 1977 by the Extreme Ultra-violet Imaging Telescope (EIT) onboard the Solar and Heliospheric Observatory (SOHO) spacecraft. The image used radiation with a wavelength of 30.4 billionths of a metre, emitted by helium atoms that have lost one of their orbiting electrons. These emissions come mainly from the upper part of the chromosphere (the transition region), where the temperature is around 60 000 degrees. In addition to some bright plage regions on the disc, there are also some striking prominences extending out into the corona. Particularly noticeable is the enormous erupting prominence at the 7 o'clock position. The material in this prominence will be at a temperature of 60,000 - 80,000 degrees; much cooler than the surrounding corona, which is typically at temperatures above 1 million degrees.

Figure 10
Figure 10 A coronal hole in extreme ultra-violet radiation

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https://www.open.edu/ openlearn/ ocw/ mod/ resource/ view.php?id=26569

The dark shape sprawling across the face of the active Sun in this extreme ultraviolet image is a coronal hole - a low density region extending above the surface where the solar magnetic field opens freely into interplanetary space. Studied extensively from space since the 1960s at ultraviolet and x-ray wavelengths, coronal holes are known to be the source of the high-speed solar wind, atoms and electrons which flow outward along the open magnetic field lines. During periods of low solar activity, coronal holes typically cover regions just above the Sun's poles. But this large coronal hole extends from the Sun's South Pole (bottom) well into northern hemisphere. Coronal holes like this one may last for a few solar rotations before the magnetic fields shift and change configuration. Shown in false-colour, this picture of the Sun was recorded by the EIT instrument on board the space-based SOHO observatory using radiation with a wavelength of 28.4 billionths of a metre, from iron atoms that have lost fourteen of their orbiting electrons.

Figure 11
Figure 11 The Sun at soft X-ray wavelengths

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https://www.open.edu/ openlearn/ ocw/ mod/ resource/ view.php?id=26570

An image of the Sun recorded in soft X-rays, using the Soft X-ray Telescope (SXT) on the Japanese Yohkoh satellite. The SXT gathers radiation in the wavelength range from 0.8 to 5 billionths of a metre. Only the corona emits in this range. The large dark region at the top of the image is a coronal hole. where the Sun's magnetic field opens out into interplanetary space. High temperature regions of the corona can be seen as bright spots in this false colour image. Accounting for the high temperature of the corona is a major challenge. The average temperature of the photosphere is between 5000 and 6000 degrees Celsius, so the million degree temperatures observed in the corona cannot be due to radiation from the photosphere, other heating mechanisms must be involved. Sound waves and magnetic effects are plausible candidates, but the details are still disputed.

Figure 12
Figure 12 An active region

Click below to view a larger version of this image.

https://www.open.edu/ openlearn/ ocw/ mod/ resource/ view.php?id=26571

This false-colour ultra-violet image shows an active region near the edge of the Sun. Hot material is seen travelling along loops defined by the Sun's magnetic field. The red regions are particularly hot, indicating that some magnetic field loops carry hotter gas than others. These active loops are so large that the Earth could easily fit under them. Active regions are often associated with sunspot groups in the photosphere, with plage in the chromosphere, and with coronal condensations in the corona. They are also sometimes associated with flares, one of which was responsible for the outburst shown in this image. This image was obtained from space using the telescope onboard the Transition Region and Coronal Explorer. The transition region is a thin and very irregular layer of the Sun's atmosphere that separates the hot corona from the much cooler chromosphere. Heat flows down from the corona into the chromosphere and in the process produces the transition region where the temperature changes rapidly from 1,000,000°C down to about 20,000°C.

Using the internet for updates

The Sun is constantly being watched from a variety of observatories. You can usually find recent images by searching the internet, using terms such as 'solar image' or modifications such as 'solar image, X-ray'. Some particularly useful websites are given below. These sites have been chosen partly because of their reliability. By all means look for other websites but be aware that there are few guarantees of quality or reliability on the internet. Always ask yourself how much you should rely on any particular source. University websites are generally fairly reliable but even there you should exercise caution.

University of Hawaii Institute of Astronomy

Solar and Heliospheric Observatory

The European Space Agency

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