One of the persistent problems in solar physics has been how the solar corona achieves such incredibly high temperatures (2 million K) relative to the solar photosphere, chromosphere. High-temperature solar plasma emits radiation primarily at EUV and x-ray wavelengths, which must be observed from space, as revealed in the solar corona photograph above.
However, no conventional reflecting telescope has been manufactured with the extreme accuracy and smoothness necessary to achieve diffraction-limited imaging at those wavelengths. EUV wavelengths would require sub-nanometer accuracy, which is extremely difficult to achieve for meter-scale mirrors. Why is this important to note?
Recent and considerable indirect evidence suggests that the corona is heated by releases of energy on scales smaller than 100 km, likely in solar spicules. But these would exceed the current limits of optical resolution, given scales of about 0.15 arcseconds. One remedy suggested (in a recent issue of Physics Today, August, p. 40) is the development of advanced optical systems based on Fresnel zone plates. These would be one way to image the Sun near the diffraction limit.
Readers may recall my series of posts to do with Fresnel diffraction from 2022, and especially the first (in May). Therein I noted that central to Fresnel's approach to most diffraction problems were his "half period zones" whereby he found that a slightly divergent spherical wave will produce a point ahead of the wave and the resulting paths could be quantified in terms of the wavelength l, e.g.
Here we have a spherical wave segment BCDE of monochromatic light traveling to the right to point P. Each point on the spherical segment can be thought of as an origin for secondary wavelets for which the resultant is found to terminate at point P. The problem then becomes how to find the resultant effect. Fresnel's approach prescribed dividing the wave front into a series of concentric "zones" as are visible in the image as something like circles on a target.
A flat diffractive optic, such as a Fresnel zone plate (see lower graphic above), has been demonstrated to produce nearly diffraction limited images with a tolerance at least an order of magnitude better than conventional optics. A conventional optical surface such as a mirror or lens is a powerful way to bend light rays. But each surface must have just the right tilt with respect to each incoming ray to produce a focal point. A zone plate, by contrast, can transform each incoming ray not into a single deflected ray but instead into a cone of outgoing rays, only some of which interfere constructively to produce a focal point. An EUV space observatory with a diffractive optic of modest aperture (less than 1 m in diameter) could probe the small spatial scales at which coronal heating is believed to take place. In other words, we'd achieve a resolution breakthrough.
In the graphic of solar zone plates at top, panel (a) shows a classical Fresnel zone plate with concentric rings and decreasing widths. In (b) The photon-sieve variant has concentric rings of circles with decreasing radii. In panel (c) a fabricated photon sieve needs to be precise enough to achieve the high-resolution images desired. The one seen here in (c) has holes as small as 2 µm in diameter.
Recent progress suggests a suitable diffractive optic should be available in the near future. A variant of a zone plate is the photon sieve. A photon sieve 80 mm in diameter has been shown in the lab to produce nearly diffraction-limited EUV images. A mission concept in development calls for 170-mm-diameter sieves to achieve the angular resolution required for its scientific goals.
The most challenging aspect of that coronal microscale observatory arises from an intrinsic property of the Fresnel zone plate and its cousins: The focal length is so long at short wavelengths that the telescope must be distributed between two spacecraft separated by 0.1–1 km. With the optics on one and the detector on the other, the relative positions of the two spacecraft must be controlled to millimeter accuracy. The technology for such precision formation flying is developing rapidly. Thus, a coronal microscale observatory that implements a classical optical concept using state-of-the-art technology may solve a solar mystery that has endured for the better part of a century.
See Also:
And:
'Magnetic Switchbacks' Now The Focus Of Parker Solar Probe's Plumbing of Solar Corona
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