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The Sun as an X-ray Source

Animation of the Sun in
The Sun as seen in X-rays
(from the Yohkoh satellite)

The Sun has a surface temperature of approximately 6,000 Kelvin, or around 10,000 degrees Fahrenheit. The solar surface emits most of its electromagnetic radiation in the "visible spectrum," or the portion of the electromagnetic spectrum we can see with our eyes. A 6,000 K star should be an extraordinarily weak source of X-rays. However, we have known since the 1940s that the Sun is, in fact, a very strong X-ray emitter. So what is going on here?

The X-rays we detect from the Sun do not come from the Sun's surface, but from the solar corona, which is the upper layer of the Sun's atmosphere. Only very hot gases can emit X-rays, and the corona, at millions of degrees, is hot enough to emit X-rays, while the much cooler surface of the Sun is not. Thus, the Sun's atmosphere is an excellent source of X-rays.

However, the discovery of the hot corona created a big problem for astronomers and physicists.

The coronal heating problem

Simply stated, the problem is this: The corona is hot, the Sun is not (relatively speaking). So how does a surface that is about 6,000 Kelvin heat an atmosphere to a million Kelvin? The mechanism by which the solar corona is heated is still not fully understood, though the problem has been addressed by some of the sharpest minds in the field over the past 50 years. The mystery persists.

What we know

X-rays from
 the Sun from 1991 to 1995
Click for larger image

There are cycles of behavior seen in the coronal structure with time scales of 11 and 22 years. The 11 X-ray images of the Sun's atmosphere seen here obtained between 1991 (at solar maximum) and 1995 (near solar minimum), provide a dramatic illustration of how the corona changes during the waning part of the solar cycle. The X-ray Sun looks completely different from the Sun we see in the sky.

An X-ray image reveals a bright glow for the corona and a black disk for the surface of the Sun. In the corona, the shape and character of the hot gases are controlled by the solar magnetic fields. There is a clear structure to the bright areas. Many structures have a filamentary (or thread-like) appearance that seems to link two regions. These filamentary structures are called coronal loops. In general, the loops are hotter and denser than the areas around them. For this reason, they appear brighter in the X-rays spectrum.

As the solar activity cycle progresses from maximum to minimum, the Sun's magnetic field changes from a complex structure to a simpler configuration with fewer fields. Since the Sun's hot gases are controlled by these fields, the X-ray images reflect this global change, with an overall decrease in brightness by a factor of 100.

The hot corona and its structure are not some short-lived, temporary phenomena. Although the corona is very dynamic with events occurring on time scales of minutes (flares), there are also long-lived structures that have time scales of months. In fact, the general features of the X-ray-emitting corona have never ceased in the many years we have been observing them. This video clip illustrates the variation of the X-ray corona.

Sun in X-rays
Last Modified: December 2010

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

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