A Tale From Our Leader
Once upon a new Moon the Sun finds itself slowly getting obscured, a shadow several hundred miles wide starts to darken the Earth in mid-day. An hour later, the darkness is total. The temperature drops by a few degrees. Nature becomes very quiet. Not a sound can be heard, not a bird sings. A feeling of suspense and awe, sometimes fear, electrifies the air. Those lucky to have reasonably clear skies in the direction of the Sun suddenly cheer, and yell, others are dumbfounded and scared, as they witness a cosmic spectacle beyond compare. The Sun’s beauty is suddenly revealed. A diamond ring glitters in the darkened sky. Red flames protrude beyond the edge of the Moon. Long ago someone named them prominence. The mystery of their pinkish color was solved when an invention called the spectrograph, revealed that it was none other than hydrogen, the element essential for life, but at a temperature of 10,000 degrees. A crown appears beyond these flames. It shimmers as it fills space, with streamers whishing outwards any which way. Cameras snap with hopes to capture the sight they behold. Alas, when the photos are produced the eye-witnessed beauty is subdued. We have solutions say Milos Druckmüller and Huw Morgan. They come up with different techniques to process the images to the level of what the eye can see. An even more beautiful corona is now revealed. Rays upon rays are everywhere, streaming, merging and diverging; curling in fiery beauty. They move across arches of all sizes, reaching out into the infinity of space. Every eclipse is a different magical surprise, revealing the intriguing beauty of coronal features revealed by the image processing. In the 2010 total solar eclipse, a clear wedge appeared to the west edge of the Sun, produced by the passage of a solar tornado. An elongated helical sheet to its east emerged from the snapping of a prominence. In 2012, the corona was littered with balloon-shaped bubbles. One bubble to the northeast seemed attached by a long tether. Streamers were distributed evenly around the Sun. Some ripples were seen here and there. This was at a time when the number of sunspots was at its highest within the approximately 11 year cycle of rise and decline. So, the Sun is changing, but we do not know why. Peer up at the Sun and you will see black spots. They reveal the presence of the mighty magnets created deep below its surface. As eclipses reveal, the corona changes in tandem with their number in an unpredictable way. Not only are bright rays and streamers ubiquitous in all eclipse images, so are faint bubbles revealed by image processing. Some of them are tiny, others huge, some of them like smoke rings. They expand outwards to the edge of the images. Where do they come from? Do they continue their voyage into space unscathed? Are these faint features a fluke of the image processing? Before photography, people drew what they saw. During the 1860 eclipse Tempel drew rays upon rays, with one bundle strangely twirled in a ball. By comparison, the processing reveals not only features similar to what Tempel had seen, but far more fainter details that are hidden to the eye. With special filters attached to the cameras, colors are revealed that no eye can see. In 1945 Edlen and Grotrian discovered that the red was Fe XI stripped of 10 of its 26 electrons, while green was Fe XIV that lost 13 electrons. Such a massive electron loss can only happen if the corona is at a million degrees or more, a mystery yet unsolved. With this high temperature, the corona is swept into space, by winds of 300 to 800 km/s. When comparing the relative brightness of Fe XI, with that of the white light structures, red patches of Fe XI show up in a few places, where these ions can be 0ccasionally trapped in the corona unable to escape with the wind. More mysteries also emerged when we discovered that the relatively cool prominences were embedded in the hottest material in the corona. A close-up view reveals that prominences can stretch away from the solar surface as they become intricately intertwined with the filamentary structures of the corona. On the other hand, at a million degrees or more, the corona can emit x-rays and extreme ultraviolet radiation; this property provides new tools for observing the corona outside eclipses from space. One of the colors in this space image is from helium, discovered by Lockyer and Janssen during the eclipse of 1868. It reveals exquisite details of highly filamentary material in prominences. 2013 was the first time when two plasmoids were detected during an eclipse so far from the Sun. They seemed to be desperate to escape, while still tethered to the surface. This eclipse also revealed how their passage shook the corona, pushing streamers aside, with rays wriggling in response to this bullying approach. They are the counterparts of coronal mass ejections, faint huge bubbles discovered from space with manmade eclipses. They expand outwards at speeds sometimes reaching a few thousand km/s, producing shock waves in space. The solar wind and these CMEs sweep past the planets, as the Sun tries to catch up with the stars. What controls their speed remains a mystery. Through its wind’s voyage the Sun’s fickle magnets tease the Earth’s molten core to produce its own show. In the deep clear winters of the arctic nights, drapes of yellow, red and green rays suddenly adorn the sky They reveal an eerie landscape of strange features and more, as they shimmer and swirl. Back to Tatakoto to 2010, the eclipse is over all too soon. The Sun starts to peak behind the Moon, its brightness blinding, its beauty gone. All what is left is a bright yellow ball with a few beauty spots, mind you. As if nothing happened, birds start to sing and dogs bark. But spectators are still in a daze. The evening after at dusk, a sliver from the new Moon smiles mischievously, proud to have put on a magnificent show. It lingers asking for attention and thankfulness. In the day after, all will be forgotten, but not for the Moon, who knows that it can repeat its trick, even if it has to wait a year or so. So, here we have descended upon this beautiful island beyond the arctic circle at more than 78 degrees north latitude. Some of us have traveled half way around the world, loaded with cameras, dodging the wrath of airport security. We have been preparing for a unique event with corona, aurora and polar bears. Let’s hope the Sun will not be shy, and will reveal the beauty of our own star and the wonders of the clockwork of planetary motion.
Observing the Corona:
Anyone witnessing a total solar eclipse is struck by the filamentary structures defining the corona. These trace the magnetic fields that originate from the solar surface entraining charged particles as they expand outwards. The charged particles are responsible for the emission that the eye and cameras capture. What the eye sees is the Thompson-scattered emission from electrons that reflect the full spectrum of light emitted by the photosphere towards the observer. This emission is commonly known as continuum or white light radiation. The other component, indistinguishable to the eye, because it occupies a minute fraction of the coronal radiation, is produced by highly ionized heavier elements in the corona, with the dominant one being iron (Fe). It can only be captured by cameras or detectors retrofitted with filters that isolate the select ‘colors’ of the different stages of ionization of Fe, the dominant ones being Fe X 5303 A, Fe XI 7892 A, Fe XIII 10747 A and Fe XIV 5303 A, stripped of 9, 10, 12 and 13 electrons respectively. This can be achieved with narrow-band pass filters, tuned to the wavelength of the radiation emitted by these different charged states, or with spectrographs.
While most of these field lines, made ‘visible’ by the emission from the charged particles attached to them, escape into interplanetary space, some field lines form arches of all sizes that remain anchored to the Sun at both ends. The different ionization states of Fe and other elements also emit in the extreme ultraviolet part of the spectrum, as well as in X-rays. However, what is unique to the visible wavelength range, is that the emission can be captured to almost ten times further away from the Sun. This is a critical property as it enables one to study the expansion of the field lines to much larger distances from the Sun, and to distinguish between those field lines that escape into interplanetary space, and those that remain anchored to the Sun.
The ionization state is a direct reflection of the temperature of these ions. Our earlier eclipse observations have shown that the field lines escaping from the Sun, are characterized by Fe X/Fe XI emission, while the arch-like field lines are dominated by the twice as hot Fe XIII and Fe XIV emission. These observations thus indicate that there are two distinct temperatures characterizing the coronal plasma: a million degree expanding plasma, and a two million degree confined one. Hence, we decided to concentrate on imaging in Fe XI and Fe XIV for this eclipse.
High spatial resolution images in white light, which are essential for getting the details of the filamentary structures were obtained with different focal length lenses to cover the corona from the solar surface out to 20 radii.
In addition to imaging, we also used a spectrograph, specifically designed for the observations, to study the intensity and shape of the Fe XI and Fe XIV spectral lines as a function of distance from the Sun, and in different filamentary structures. The observations will give us information about the evolution of any plasma wave motion as the plasma propagates outwards.
Joe Hutton (Aberystwyth University) will be measuring the radiation before, during and after the solar eclipse. Working with the CERN@school science outreach program, we will be using a high-tech particle camera to measure any change in the amount of radiation detected during the solar eclipse. Unlike the traditions radiation counters, such as Geiger-Müller tubes, which would just click a lot when particles are detected, the Jablotron MX-10 particle camera is able to record and recognize different types of ionizing particles (alpha, beta, gamma), as well as providing lots of information to study. Depending on where you are in the UK, the eclipse could range from 80-95% totality, whereas here on Svalbard the Moon will be blocking 100% of the solar surface. CERN@school have provided these detectors to many schools across the UK to measure changes in the background radiation during the solar eclipse at varying degrees of totality.