The sun gives off radio waves at many different wavelengths. Different layers of the sun tend to give off waves a different wavelengths, so we can observe individual layers by focusing on certain wavelengths. This allows us to compare different phenomena on different layers of the sun to find patterns and connections. Here are some of the most common ways to look at the sun:
Visible light is the most obvious type to use. This light that we can see is typical of matter heated to a few thousand kelvins. The photosphere, from which comes the visible light, is not hot enough, however, for the plasma to be completely ionized. It is still in an atomic state, which means that nuclei do have electrons orbiting them, though they are missing outer electrons. The light given off by the photosphere is produced mainly by electrons jumping to lower energy levels. By analyzing this light, we can see absorption lines which tell us that the solar atmosphere is about 92% hydrogen and 7.8% helium, which is typical of the stars we can see. We can also see the granules which mark convection patterns of rising and sinking plasma, and sunspots.
An important picture of the sun emerges when we look at it at the hydrogen-alpha wavelength, which is 656 nm. This is the most prominent wavelength that hydrogen emits, and looking at the sun using this wavelength gives us a picture of the lower chromosphere. In the lower chromosphere we can see filaments, plages, and evidence of sunspot groups. This is one of the most useful views for prediction because it tells much about the magnetic state of the sun, which can help predict flares and eruptions.
UV light (or ultraviolet light) is produced mostly by the upper chromosphere. The plasma in the chromosphere exhibits a degree of blackbody radiation, which appears in the form of UV rays. The pictures above were actually taken in more energetic extreme UV light, which is more typical of the higher-temperature corona. UV pictures of the upper chromosphere show features which are otherwise difficult to observe, such as spicules (considered to be quiet and small, though they are about the size of the Earth!) and the chromospheric network, which is caused by convective cells.
Looking at x-rays coming from the sun gives us a good picture of the corona. It is hard to see the corona in visible light because its atoms are so sparse and the underlying photosphere so bright that the corona is drowned out. X-rays, however, are produced when electrons are accelerated by positive ions. Large quantities are also sent out when electrons crash into the denser matter in the chromosphere. These x-rays produced by the rapid deceleration are called Bremsstrahlung (braking, in German) x-rays. Flares are usually discovered first by monitoring the x-ray flux of the corona, so x-rays are very useful for predicting flares.
Radio wavelength waves can penetrate the Earth's atmosphere, so they can be monitored by ground-based observation equipment. When a flare occurs, the magnetic fields accelerate charged particles. Electrons accelerate in spiral paths and send out synchotron radiation, which is simply radiation produced by circular acceleration. The level of activity at the radio wavelength 10.7 cm seems to be intimately connected to the sunspot cycle, so it may be useful in the future to help forecast flares.
Magnetogram images give us pictures of the dynamic and complex magnetic fields in the sun. These pictures suggest that magnetic fields are actually the most prominent driving force in the solar atmosphere. The magnetogram images add one more layer to our knowledge of the sun, yet expose many degrees of complexity which we have no way of understanding at the present.
Infrared light is often considered heat, much like the heat a human body gives off. More than half of the Sun's power output is in the form of infrared light, though much of it is absorbed by the Earth's atmosphere. Through infrared light, some features of the Sun's chromosphere, and some in the corona can be seen. The markings in this picture are caused by absorption of the infrared light. Some of the light is absorbed wherever it runs into gas in the Sun's atmosphere, and so the darker features in the picture show where the gas is more dense. Filaments, and loops near active regions show up dark; coronal holes in the north and south poles show up as slightly brighter than the rest of the solar disk.
Infrared passage based on information from Caleb and Al's Solar Physics Home Page