Solar physics

Waves galore


Wave energy has long been proposed to be a source of the hot solar corona and fast solar wind. Direct measurements made by spacecraft have finally established that coronal waves are ubiquitous and can have the required energy. See Letter p.477

The Sun's outer atmosphere, the corona, is known to have a temperature in excess of 1 million kelvin — hotter than the visible surface by a factor of more than 200. The corona also expands into interplanetary space as a high-speed flow, the solar wind, which is supersonic, reaching speeds of several hundred kilometres per second near Earth. The existence of the corona and wind requires in situ deposition of energy, with the likely source being the coronal magnetic field. It has long been conjectured that a class of magnetic waves called Alfvén waves can contain the required energy, but direct detection has proved challenging. On page 477 of this issue, McIntosh et al.1 identify Alfvén waves in the corona in such profusion that their role needs urgent reassessment.

On the basis of measurements of coronal radiation at extreme ultraviolet (EUV) and X-ray wavelengths, sustenance of the corona requires an energy input of 102–104 W m−2, depending on location2. Alfvén waves are named after their proposer, the Nobel laureate Hannes Alfvén, and are analogous to waves on a string, with the string tension being replaced by a tension force associated with bent magnetic field lines, and the string mass density being replaced by the density of the ionized coronal gas (or plasma). They have well known properties3, are thought to be generated in the cool low solar atmosphere, to propagate into the corona, and have been directly measured by spacecraft in the interplanetary medium4. Direct detection of waves in the corona itself has been challenging, because it requires very high time-cadence EUV observations with high spatial resolution. However, sporadic detections have been made at high altitudes off the solar limb5.

McIntosh et al.1 present results that, for the first time, detect coronal Alfvén waves in such profusion that they must now be regarded as ubiquitous. The key to this fundamental result is the remarkable time cadence, spatial resolution and sensitivity possible with the new Atmospheric Imaging Assembly on NASA's Solar Dynamics Observatory (SDO). EUV images of structures and motions on the Sun can be generated, down to 1,000-km resolution, every 12 seconds, and this is fast and fine-scale enough to identify the presence of small-amplitude oscillatory displacements of structures throughout the corona. These oscillatory displacements have been interpreted as 'Alfvénic' waves on the basis of the large-scale (bulk) transverse oscillations of coronal structures that have been observed. (The term Alfvénic, rather than Alfvén, is used because the original definition by Alfvén applies only to oscillations in a uniform medium, which the highly structured corona is not.)

The second fundamental result concerns the estimate of the wave energy. McIntosh and colleagues divide the corona into three regions (Fig. 1). The first includes areas, known as coronal holes, in which the solar wind is believed to originate. These are associated with a magnetic field whose lines of force close in interplanetary space. The second comprises regions that are faint when viewed in EUV — known as the quiet Sun — but in which the lines of force close below the solar surface (Fig. 2). The third includes regions that are magnetically like the quiet Sun, but are sources of intense emission and are known as active regions.

Figure 1: The solar corona as seen in emission at 171 ångströms.

AIA/SDO team

This image, taken from the Atmopsheric Imaging Assembly (AIA) instrument on the Solar Dynamics Observatory (SDO), shows coronal plasma at a temperature of about 1 million kelvin. McIntosh and colleagues1 analyse three different types of region: coronal holes (source of the solar wind); quiet Sun; and active regions (sites of intense emission). In the coronal holes and quiet Sun, the power measured in magnetic oscillations known as Alfvén waves is adequate to account for the observed radiation.

Figure 2: Alfvén waves in two coronal geometries.

The solid lines represent magnetic field lines. On the left, the open magnetic field lines extend into interplanetary space, closing at a large distance from the Sun, and are representative of coronal holes. On the right, the field lines create closed structures in the corona, as occurs in the quiet Sun and active regions, with the lines closing beneath the surface. In both cases, the wavy blue lines represent upward-propagating transverse displacements (Alfvén waves), with the solid blue arrows showing the direction of energy transport. The dashed lines with red arrows represent the oscillatory displacements observed by McIntosh et al.1 in active-region loops, interpreted as Alfvén waves.

In the coronal hole and quiet Sun regions, the wave power estimated by McIntosh et al. is adequate to account for the coronal energy losses. In the observed active region, the measured power is inadequate by some margin. However, superposition of random motions along the line of sight (between the observer/spacecraft and the observed coronal structures) can lead to a large underestimation of the wave power (I. D. M. and D. J. Pascoe, unpublished work), so the authors' results can be considered as minimum values. In addition, higher time resolution may reveal more power at higher frequencies than currently observed.

Where does this leave things? A significant difficulty for coronal-heating mechanisms based on Alfvén waves is the weak damping of the waves, which can be likened to trying to damp the motion of a pendulum in a near-vacuum. Efficient damping is required to convert the kinetic and magnetic energy in the wave into heat. Idealized theories have suggested ways of enhancing the damping through the creation of strong, localized currents6, but one can see in movies from the SDO and the Japanese Hinode spacecraft that the solar corona is highly dynamic and structured, factors not accounted for in the basic theoretical models. The results of McIntosh et al. must lead to a thorough reassessment of the theory of waves in the solar atmosphere, with a focus on the (computational) modelling of complex, dynamic structures.

Finally, the argument about the origin of the corona has been framed for decades as a competition between the dissipation of small-scale currents giving rise to impulsive heating7,8, direct plasma injection9 and the dissipation of wave energy, with the first becoming more favoured. The new detection of 'waves galore' by McIntosh et al. suggests that this argument needs to be reconsidered as a matter of urgency. We suggest that the dynamic complexity revealed by Hinode and the SDO indicates that both processes are almost certainly at work, and hence a more pragmatic approach is called for to assess the relative contribution of each process in different regions.


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Correspondence to Peter Cargill or Ineke De Moortel.

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Cargill, P., De Moortel, I. Waves galore. Nature 475, 463–464 (2011).

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