Coronal loop

Coronal loops are often associated with the strong magnetic fields located within active regions and sunspots. The number of coronal loops varies with the 11 year solar cycle.

Origin and physical features

Due to a natural process called the solar dynamo driven by heat produced in the Sun's core, convective motion of the electrically conductive plasma which makes up the Sun creates electric currents, which in turn create powerful magnetic fields in the Sun's interior. These magnetic fields are in the form of closed loops of magnetic flux, which are twisted and tangled by solar differential rotation (the different rotation rates of the plasma at different latitudes of the solar sphere). A coronal loop occurs when a curved arc of the magnetic field projects through the visible surface of the Sun, the photosphere, protruding into the solar atmosphere.

Within a coronal loop, the paths of the moving electrically charged particles which make up its plasma—electrons and ions—are sharply bent by the Lorentz force when moving transverse to the loop's magnetic field. As a result, they can only move freely parallel to the magnetic field lines, tending to spiral around these lines. Thus, the plasma within a coronal loop cannot escape sideways out of the loop and can only flow along its length. This is known as the frozen-in condition of ideal magnetohydrodynamics.

The strong interaction of the magnetic field with the dense plasma on and below the Sun's surface tends to tie the magnetic field lines to the motion of the Sun's plasma; thus, the two footpoints (the location where the loop enters the photosphere) are anchored to and rotate with the Sun's surface. Within each footpoint, the strong magnetic flux tends to inhibit the convection currents which carry hot plasma from the Sun's interior to the surface, so the footpoints are often (but not always) cooler than the surrounding photosphere. These appear as dark spots on the Sun's surface, known as sunspots. Thus, sunspots tend to occur under coronal loops, and tend to come in pairs of opposite magnetic polarity; a point where the magnetic field loop emerges from the photosphere is a North magnetic pole, and the other where the loop enters the surface again is a South magnetic pole.

Coronal loops form in a wide range of sizes, from the minimum observable scale (< 100 km) to 10,000 km. There is currently no accepted theory of what defines the edge of a loop, which is embedded in a general corona that is itself strongly magnetized. Coronal loops have a wide variety of temperatures along their lengths. Loops at temperatures below 1 megakelvin (MK) are generally known as cool loops; those existing at around 1 MK are known as warm loops; and those beyond 1 MK are known as hot loops. Naturally, these different categories radiate at different wavelengths.

A related phenomenon is the open flux tube, in which magnetic fields extend from the surface far into the corona and heliosphere; these are the source of the Sun's large scale magnetic field (magnetosphere) and the solar wind.

Location

Coronal loops have been shown on both active and quiet regions of the solar surface. Active regions on the solar surface take up small areas but produce the majority of activity and are often the source of flares and coronal mass ejections due to the intense magnetic field present. Active regions can produce some 80% of the total coronal heating energy.

Dynamic flows

Many solar observation missions have observed strong plasma flows and highly dynamic processes in coronal loops. For example, SUMER observations suggest flow velocities of 5–16 km/s in the solar disk, and other joint SUMER/TRACE observations detect flows of 15–40 km/s. Very high plasma velocities (in the range of 40–60 km/s) have been detected by the Flat Crystal Spectrometer (FCS) on board the Solar Maximum Mission.