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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 6000 Kelvin. 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 6000 K Sun 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 actually come from the solar corona, not the solar surface. The corona, the upper layer of the Sun's atmosphere, is very, very hot (over a million degrees!). Thus, it is an excellent source of X-rays.

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

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 6000 Kelvin heat an atmosphere to a million Kelvin? The mechanism via which the solar corona is heated is not fully understood, even though the issue has been known and investigated for over 50 years. Many very smart and very talented people have worked on this problem, including Nobel prize winners. However, the mystery continues.

What We Do Know

X-rays from the Sun through the entire 11 year solar cycle
There are cycles of behavior seen in the coronal structure with time scales on the order of 11 and 22 years. The 12 X-ray images of the Sun's atmosphere seen here, obtained between 1991 and 1995 at 90 day increments, provide a dramatic view 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, doesn't it?

Only very hot gases can emit X-rays. The Sun's atmosphere, at millions of degrees, is hot enough to emit X-rays, while the much cooler surface of the Sun is not. As a result, 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 which 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. They are thus brighter in X-rays.

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 "movie" shows you something about the variation of the X-ray corona.

Sun in X-rays
Sun movie (These are BIG files, but worth the time they take to load!)

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

The Imagine Team
Project Leader: Dr. Jim Lochner
Curator:Meredith Bene
Responsible NASA Official:Phil Newman
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