The ominous black square on the surface of the sun isn't a portal to another dimension. It's a coronal hole, a depressingly mundane name for a patch of cooler and darker corona. While the usual bright glow is hot, bright plasma captured by the magnetic field, a coronal hole is a gap in the magnetic field.


Square coronal hole, May 5-7, 2014 in AMA211.


The outside of a hole is ringed with open magnetic field lines stretching into space. In contrast, the very brightest points on the sun's surface trace the magnetic field, plasma dancing along loops of the magnetic field.

Left: Bright coronal loops trace out loops in the magnetic field as magnetic particles spin along the loop from August in extreme ultraviolet light (AMA 171). Image credit: SDO/NASA

Instead of trapping plasma, the gap in the magnetic field allows solar wind to rush out, producing gusts that can impact the interplanetary magnetic field. If a hole is facing us on Earth, the wind buffets our magnetic field, producing aurora.


Left: A pair of coronal holes from January 2010 in extreme ultraviolet light. Image credit: SDO/NASA

A particularly geometrically-striking coronal hole showed up this week. Showing up in the Solar Dynamics Observatory images for May 5 through 7th, 2014, the hole is oriented so far south on the Sun that the gusts of wind are unlikely to impact us.

Those pinpricks of light within the gaping maw on our sun aren't stars twinkling on the far end of a dimensional gate. The sense of depth is an illusion of the stark darkness, and those bright spots are loops of hot plasma in the corona. They trace out the solar magnetic field poking out above the surface but looping back down, unlike the open field lines jutting out around the edge of the hole.


Why is this particular hole square? No particular reason — holes can be all sorts of shapes and sizes. Given the variety of shapes and sizes, coronal holes make for a solar version of cloud-watching, identifying hidden shapes in the abstract patterns. (I see this month's square as a roaring mouth, with two smaller eyes above and a bright-spot nose.)


Two coronal holes in January 2010 [left], a massive coronal hole in May 2013 [center], and coronal holes from February 2011 [right].

Different wavelengths of light probe different layers of the sun. Any image from the Solar Dynamics Observatory colourized purple is the 211 angstrom wavelength. That's the light capturing the emission line marking by iron-14 at 2 million degrees Kelvin, the active region of the sun. Typically, coronal holes are most visible in ultraviolet light and x-rays. For example, check out the sun as seen on Friday night in a variety of ultraviolet and visible wavelengths.


HMI Continuum [left], AMA 4500 [center], and AMA 304 [right].

On the left is the HMI Continuum bandwidth, a compilation of a broad range of visible light. It captures the photosphere of the sun, the solar surface. This is below the corona. Similarly, the center image is 4500 angstroms (blue-violet visible light), also the photosphere. This is the white light continuum at 6000 Kelvin, what we would see with our naked eyes if the sun weren't so bright. The few black freckles are sunspots.


Moving up into the transition region and chromasphere, the 304 angstrom wavelength is for helium-2 at 50,000 Kelvin. The highest density plasma glows brightest in this wavelength, while our hole is marked by a region of particularly low density plasma.


AMA 171 [left], AMA 193 [center], AMA 211 [right].

Iron-9 emission line is marked by the 171 angstrom wavelength in the 600,000 Kelvin, visible on the left. This is the upper transition region and quiet corona, the area of loops and gentle activity. The hole is just barely visible, a faint trace of missing material. The brightest areas in this wavelength mark regions where the magnetic field is particularly strong.


Finally we get into the corona properly, hitting 193 angstroms emission lines for detecting either iron-12 at 1 million Kelvin or iron-24 at 20 million Kelvin in the center image. The iron-12 is the hotter corona region, while the iron-24 marks flare plasma.

To the right is the iron-14 emission line at 211 angstroms tracing out the corona. The coronal hole is clearly visible in both images. Solar flares and coronal mass ejections are extremely bright in this wavelength.


AMA 335 [left], AMA 094 [center], AMA 131 [right].

On the left is 335 angstrom, emission line at iron-16 at 2.5 million Kelvin. These are the active, hot regions of the corona. The most magnetically active regions show up in this wavelength, with a gap in the magnetic field clearly visible.


The center is the emission line of iron-18 heated to 6 million Kelvin, glowing at 94 angstroms. These are solar flares plain and simple. Similarly, 131 angstrom wavelengths mark iron-20 and iron-23 somewhere above 10 million kelvin, once again identifying flaring regions. The hole is once again clearly visible, with an absence of flares within the region.

Credit: Solar Dynamics Observatory/NASA. For more on solar observations, check out this a weirdly-soothing video of a flare erupting, or the unprecedented wealth of data we collected for one flare earlier this year.


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