The Solar

The solar corona

The extended outer atmosphere of the Sun is called the corona. It has a temperature of millions of degrees, but it is 10 billion times less dense than the atmosphere of the Earth at sea level.

Observing the Corona

The glow of the corona is a million times less bright than that of the photosphere, so it can only be seen when the disk of the Sun is blocked off in a total solar eclipse (adjacent image), or by using a special instrument called a coronagraph (or coronameter) that artfically blocks the disk of the Sun so that it can image the regions surrounding the Sun. The following two images were obtained using such an instrument.

Solar corona viewed using a coronagraph (more info)

These two images, taken using the coronameter of the Mauna Loa Solar Observatory, shows the corona 27 days apart (approximately one solar rotation).

Coronal Holes and the Solar Wind

The red-orange images superposed on the central disk in the above two figures are corresponding X-ray images taken of the Sun on the same days as the corona images. The dark regions in the X-ray images correspond to coronal holes. The coronal holes also are in evidence in the corona as regions with little bright structure, such as near the north solar pole (top).

As we shall discuss further in the section on the solar wind, the coronal holes are regions where the magnetic field lines of the Sun are open, allowing coronal gas to flow outward into space and producing the solar wind. In contrast, the capped looking streamers in the corona (helmet streamers) correspond to regions in which the magnetic field lines are closed in a loop, trapping the coronal gas and leading to enhanced X-ray emission because of the increased gas densities.

The sun through a coronagraph (more info)

The preceding image (in false color) was taken with the Large Angle and Spectrometric Coronagraph on board the SOlar and Heliospheric Observatory (SOHO). It shows rather graphically the streamers in the solar corona.

The Corona and Solar Activity

We will discuss shortly that the Sun goes through an activity cycle that is correlated with observables like the number of sunspots. The appearance of the corona has a very strong dependence on this cycle, as illustrated in the following two images.

The solar corona during the 1980 solar eclipse (left) and 1988 solar eclipse (right). The 1980 eclipse was near a solar activity maximum and the 1988 eclipse was near a minimum (Source)

The left image was obtained during a period of high solar activity and the right during a period of relatively low solar activity. Notice the fundamentally different appearance of the corona. The corona during the active Sun period shows many streamers at all angles around the disk of the Sun, while the corona during the quiet Sun period shows larger bottle-shaped streamers (the helmet streamers mentioned earlier) concentrated in latitudes near the equator.

You may monitor the current appearance of the Solar corona with this link to the Mauna Loa Coronameter

The Energy of the Corona

It is clear that the corona is very hot because of the electromagnetic radiation that it emits. We observe emission lines from the corona corresponding to very highly ionized atoms (for example, iron atoms in the +16 charge state). Such highly ionized atoms can only be produced at temperatures in the million degree range. The extremely high temperature of the corona is thought to be associated with effects of the solar magnetic field, which can store and transport energy from lower regions of the Sun to the corona. However, the details of how this heating takes place are yet to be fully understood.