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MkrGeo

An original point of view

Astronomy, Astrophotography

Solar corona visibility before & after totality

Solar corona visibility outside the total solar eclipse

Solar corona visibility at 99% obscuration, 53s after total phase.

 

Seeing a solar corona during the total solar eclipse can be an awesome experience. This view is jaw-dropping for everyone, even for solar eclipse chasers, who used to see it at least a few times before.
There is a lot of pictures of the amazing solar corona on the web, which amount grows up after every single totality. Unfortunately for a long time, there was no decent report of solar corona visibility outside the umbral area, except the short period of Baily’s Beds appearance. Because Baily’s Beads can occur up to 22 sec before and after totality (for people being in the middle of the path of totality) (Pic. 1), so I am considering about 30-50 sec outside of the total phase, when usually solar filters are taken up.

Baily's Beds Nugent comparison

Pic. 1 A Baily’s Beads appearance comparison between a different shadow deep. On the left, near the edge of the umbral path, we can observe Baily’s beads usually much longer (i.e. 50-60s), whereas in the centerline they are visible just more than 20 sec, as you can see on the right image (Sigismondi, 2008).

Having in mind, that solar corona has a brightness nearly equal to full Moon (-11,5 mag), it could be easily seen during the deep partial phase when lunar disk almost fully overlaps with the solar one. Unfortunately the solar corona, that appears on the opposite side of the crescent is literally washed out by its much stronger gleam and remains invisible for the observer for most of the time.
Situation changes, when the umbral region is close to the observer. Then the solar crescent illumination is faint enough to let another part of the solar region being visible on the opposite side of the lunar disk (Pic. 2, 3).

Solar corona outside totality simulation in Stellarium

Pic. 2 Solar corona outside totality displayed in the Stellarium 0.16 software.

Solar corona outside totality visualization

Pic. 3 Solar corona visibility outside the totality pattern, where: A – thin solar crescent; B – lunar disk; C – solar corona outline visibility.

We must be aware, that the brightest part of the solar corona is situated just above the photosphere. So the explicable fact is, that as the solar crescent is thicker, the chances to see a corona outline are smaller, not only because of sunlight strength. Anyway, the solar radiation remains the main factor enabling or not to see something special on the opposite side of the crescent. Another factor, being as an outreach of solar radiation is an atmosphere (medium) presence, which causes intense light scattering (on air molecules or haze) as the light grows, especially within the nearest vicinity of the illumination source.

Watching some footages on Youtube I noticed, that solar corona outline can be visible up to about 3min after totality, which means upon around 96% obscuration. The best proof of this phenomena has been reported by Daniel Fisher during the 2017 annular solar eclipse in Argentina. He captured a solar corona and prominences, being visible just before 2nd contact, when solar obscuration was turning into 97% (Pic. 4, 5). These pictures were a serious inspiration for me to carry out a full observation and set a threshold of solar corona visibility against the obscuration value.

Solar corona seen during 2017 annular solar eclipse

Solar corona seen during 2017 annular solar eclipse in Argentina

Pic. 4, 5 The images of the solar corona and some prominences visible just before the annular solar eclipse on February 26, 2017. 1/5000s, ISO 100 (credits: Daniel Fisher).

The aforementioned images have proven, that both solar corona and solar prominences are to be visible also during annular eclipses, although not in all cases, about which I will explain below.

During the 2017 Great American Eclipse, I was fortunate to manage this kind of observation, which is obviously dangerous. Before I show my results, I would like to assure you, that in spite of direct sunlight hazard there are two tips & tricks, thanks to them you will able to see the solar corona outline:
– If you don’t have a binocular just block the light coming from thin solar crescent by your thumb and try to spot a fuzzy arc next to your thumb. It should work at least 2 min before or after totality.
– When you have a binocular do analogically, but the very important thing is placing the sunlight outside your view because otherwise, it can be dangerous for your eyes! Your hand may shake, so I advise you to use a tripod. A binocular will enable you to see a discernable fuzzy arc even up to 5 min before or after totality. A DSLR long lenses work analogically. A helpful thing here is a light-fall, that can prevent you from direct sunlight, coming from the crescent. The option with binocular is the last one, that can be safe enough. Don’t use equipment with bigger enlargement, because it can be seriously dangerous for your sight.

For my observation purposes, I used a long lenses Nikkor f3.5-5.6 55-300mm for the solar corona observation outside the totality. In order to capture the solar corona next to the crescent sun, I used the light-fall feature of the Nikkor 55-300mm lens, that could save my sight and camera a bit diminishing a major light at once. At 300mm the fuzzy arc was looking big likewise you could watch it through a good binocular.

The images below show the sequence of solar corona visibility between 94% and 92% obscuration, as has been captured during 2017 Greatest American Eclipse (Pic. 6 – 15.

Solar corona visibility during 94,3% obscuration

Pic. 6 Barely visible solar corona outline at more than 4 mins before totality. 1/4000s ISO 100, f/5.6.

Solar corona visibility at 96,8% obscuration

Pic. 7 When obscuration is higher than 96%, the corona outline is quite easy discernable. Moreover, through the binoculars, you can see a pinkish solar photosphere at both horns of the solar crescent. 1/4000s ISO 100, f/5.6.

I took more attention to capturing a solar corona outline after the totality when I carried out the observation more or less between 3 and 5 min after the 3rd contact.

Solar corona visibility at 99% obscuration

Pic. 9 The moment of 99% obscuration is typical for taking the solar filters down before the totality and covering the instruments again after.  In Wyoming (near Riverton), the 99% obscuration moment occurred about 53 sec before and 52 sec after totality. 1/125s ISO 100, f/5.6.

Solar corona visibility at 96,4% obscurationSolar corona visibility at obscuration 96%

Solar corona outline visibility at 94,9% obscuration

Solar corona outline visibility at obscuration 94,8%

Pic. 13, 14 – At about 95% obscuration both solar corona outline (2) and pinkish chromosphere (1) is less pronounced. The chromosphere is almost invisible at this moment. 1/4000s, ISO 100, f/5.6

Solar corona outline visibility at obscuration 94,3% magnitude about 0.95
Solar corona outline barely visible at 93,9% obscurationSolar corona outline barely visible at obscuration 93,7%

Near treshold of solar corona outline visibility at 93,3% obscuration, near magnitude 0.94

Pic. 15 – 18 Between 95 and 93% the solar corona is getting fainter rapidly, being almost gone about 5 min after totality.

Basically, I have proven a solar corona observation up to 93% obscuration, which corresponds to about 5 min after totality. Possibly this is still not last time when we would be able to see something bright at the opposite side of the solar crescent, but the sunlight is too bright for taking pictures, even with the light-fall feature. Moreover, as the solar crescent grows, the bright gleam (caused by forward light scattering on atmospheric molecules) spreads further across the sky, drawing out completely the objects emitting light in the vicinity. Hence the solar corona is no longer visible for observer, at least in naked eye-binocular conditions (Pic. 19).

Sunlight too much bright to reveal the solar corona outline near magnitude 0.93

Pic. 19 Later, the solar corona was no longer visible for me through the long lens. The 92% obscuration is really dangerous both for eyes and the instrument, so… next time I must find another way to study the solar corona visibility outside the totality.

As a conclusion of this article, I would like to announce, that finally, I managed the solar corona observation outside the totality. I have gathered all my pictures together, what you can see below (Pic. 20).

Solar corona visibility outside totality

Pic. 20 Solar corona visibility before & after totality – a photo sequence from 2017 Greatest American Eclipse. Click to enlarge or see the image in original size here.

I discovered, that solar corona can be visible upon 93% obscuration, which is amazing! Interesting can be also the point of lunar and solar disk cross, where some prominences and photosphere are to be visible! This phenomenon appears to be visible upon 96% obscuration. This information is pivotal also for annular eclipse observations. Knowing, that 93% obscuration enables us to see the solar corona outline (throughout the binoculars) we can see it in case of 80% annularities. However, is important to remember, that under annular eclipse conditions the solar crescent is more circular, engulfing much more lunar disk, that may disrupt this kind of observation. A smaller lunar disk against the Sun means, that the prominences and corona features can be possible to see even for a bit smaller obscuration, especially just before 2nd and after 3rd  contact. This issue definitely requires further study and observation.

Mariusz Krukar

Links:

  1. 2017 annular solar eclipse in Argentina – a first reports

References:

  1. Sigismondi C., 2008, Measures of solar diameter with eclipses: Data analysis, problems and perspectives, (in:) AIP Conference proceedings
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