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The corona is the Sun's outer atmosphere. It is extremely hot, with a temperature of about 2 million degrees Celsius, but the density of its plasma is quite low, so the corona is not very bright. Until the introduction of the coronagraph in 1930, it could only be seen during total eclipses of the Sun (Figure 2.6).⁷

The corona displays a variety of transient features, including streamers, plumes, and loops, as well as huge eruptions known as coronal mass ejections. The overall shape of the corona also changes during the 11‐year sunspot cycle. Dark coronal holes are visible in UV and X‐ray images of the polar regions, often extending toward the solar equator during times of solar minimum (see Figure 2.34). These holes are associated with “open” magnetic field lines which allow the high speed solar wind to escape into space. Long, thin streamers known as polar plumes also project outward along the open magnetic field lines at the poles.

Coronal loops are another common feature, sometimes rising over a million kilometers above the photosphere. These are flows of trapped plasma moving along “alleys” in the arch‐shaped magnetic fields of the corona at speeds up to 320,000 km/h. Some loops are extremely hot, with temperatures of well over 1 million degrees Celsius.

Coronal loops are more common around solar maximum, when the Sun's magnetic field is highly disturbed and sunspots are numerous. Indeed, the loops are often found close to sunspots and active regions. The loops run between the north and south poles of a localised magnetic field, such as occurs in pairs of sunspots. Many coronal loops last for days or weeks, but most change quite rapidly.

One explanation is that the flows are caused by uneven heating at either extremity of a loop, with plasma racing from the hotter end to the cooler end. The reason for such heating remains uncertain (see Coronal Heating).

Figure 2.19 A series of coronal loops seen in ultraviolet light by the TRACE spacecraft. Shaped by magnetic field lines, the loops of extremely hot plasma extend 120,000 km above the photosphere.

(NASA)

Observations of the corona's visible spectrum in the late 19^th century revealed mysterious, bright emission lines at wavelengths that did not correspond to any known elements. The true nature of the corona was revealed in 1939, when it was shown that the lines were produced by highly ionized iron and calcium.

Such strong ionization could only happen in an environment where temperatures exceed 1 million degrees Celsius. At these temperatures, both hydrogen and helium (the two dominant elements) are completely stripped of their electrons. Even the less common solar constituents, such as carbon, oxygen, and nitrogen, are reduced to bare nuclei. Only the heavier elements are able to retain a few of their electrons.

The corona's high temperature means that it emits energy mainly at ultraviolet and X‐ray wavelengths. Many active regions, flares, and other coronal features are clearly visible in X‐ray images taken by space observatories such as SOHO, Hinode, IRIS, and the Solar Dynamics Observatory.