In the Space Age the Space Cadets provide a wealth of high resolution solar images [in a whole range of sizes and eye catching colours] snapped by satellites circling somewhere above our sometimes sunny skies.
A rainbow of lunar transits as seen by NASA’s Solar Dynamics Observatory.
The observatory watches the sun in many different wavelengths of light, which are each colorized in a different color.
Dramatic Sun Storm, Partial Solar Eclipse Spied by NASA Spacecraft
Megan Gannon- 4 February 2014 – Space.com
Images from the Black and White Era are hardly given a second glance in the Information Age especially when Wikipedia provides little more than thumbnail smudges.
However, it’s worth giving these old Black and White Era images a second glance because early Photographic Plates were “usefully sensitive only to blue, violet and ultraviolet light”.
Photographic plates were also an important tool in early high-energy physics, as they get blackened by ionizing radiation.
Early photographic plates and films were usefully sensitive only to blue, violet and ultraviolet light
However, photographic plates were reportedly still being used by one photography business in London until the 1970s, and they were in wide use by the professional astronomical community as late as the 1990s.
Therefore, although these early Photographic Plate cameras were very clunky and unsophisticated [especially compared to modern digital cameras] they did manage to capture images that included ultraviolet artefacts which are rarely seen in modern photographs.
These ultraviolet artefacts are especially evident in the Solar Eclipse images captured at the end of the 19th century with their large Halos [extending to over one Solar radius – see the 1898 eclipse example below] when compared to modern images.
Interpreting these early Photographic Plates [that were usefully sensitive to ultraviolet light] is facilitated by referencing the ultraviolet spectrum and blue spectrum images of the January 1925 Solar Eclipse published by the Harvard College Observatory.
Harvard Results from Eclipse of January 24, 1925
Harvard College Observatory Circular, vol. 286, pp.1-11
In the negative image [Fig 2 above] ultraviolet light is shown in black.
Perhaps the most striking aspect of this negative image is the black ring of ultraviolet light that surrounds the Solar Eclipse.
The mainstream suggests that this dark ring of ultraviolet light originates in the Sun’s Corona.
A corona (Latin, ‘crown’) is an aura of plasma that surrounds the Sun and other celestial bodies.
The Sun’s corona extends millions of kilometres into space and is most easily seen during a total solar eclipse, but it is also observable with a coronagraph.
The corona is 10−12 times as dense as the photosphere, and so produces about one-millionth as much visible light.
However, this dark ring of ultraviolet light could also be generated by the Earth’s upper atmosphere fluorescing as it is penetrated by the Solar Wind plasma [travelling at about 400 kilometres per second] which has [strangely enough] “a composition that is a close match to the corona”.
The solar wind is a stream of plasma released from the upper atmosphere of the Sun.
It consists of mostly electrons, protons and alpha particles with energies usually between 1.5 and 10 keV
The slow solar wind has a velocity of about 400 km/s, a temperature of 1.4–1.6×106 K and a composition that is a close match to the corona.
From the European Space Agency’s Cluster mission, a new study has taken place that proposes it is easier for the solar wind to infiltrate the magnetosphere than previously believed.
A group of scientists directly observed the existence of certain waves in the solar wind that were not expected.
A recent publication in the Journal of Geophysical Research shows that these waves enable incoming charged particles of solar wind to breach the magnetopause.
This suggests that the magnetic bubble forms more as a filter than a continuous barrier.
This latest discovery occurred through the distinctive arrangement of the four identical Cluster spacecraft, which fly in a strictly controlled configuration through near-Earth space.
As they sweep from the magnetosphere into interplanetary space and back again, the fleet provides exceptional three-dimensional insights on the processes that connect the sun to Earth.
Earth itself is largely protected from the solar wind by its magnetic field, which deflects most of the charged particles; however some of the charged particles are trapped in the Van Allen radiation belt.
A smaller number of particles from the solar wind manage to travel, as though on an electromagnetic energy transmission line, to the Earth’s upper atmosphere and ionosphere in the auroral zones.
Given that the Solar Corona is “10−12 times as dense as the photosphere, and so produces about one-millionth as much visible light” whilst the Solar Wind is concentrated into a narrow ring as it streams around the Moon [which has a mean diameter of about 3,474 kilometres] it is far more likely that this dark ring of ultraviolet light is caused by the Earth’s upper atmosphere fluorescing as it is penetrated by the Solar Wind.
The other striking aspect of this negative image [Fig 2 above] is the heavily speckled black dots [representing ultraviolet light] that dapple the remainder of the image.
This clearly confirms that some ultraviolet light reaches the surface of the Earth.
Moving on to the second Harvard College Observatory image [Fig 1 above], which is a positive [blue spectrum] photographic print of the Solar Eclipse, it is very interesting to note that this image is only lightly speckled with white dots [representing blue light].
This implies that the blue light is generated by the Earth’s upper atmosphere fluorescing when it absorbs [the more intense] ultraviolet light.
This implication is confirmed [in Fig 1 above] by the bright halo of white dots [i.e. a halo of blue light surrounding the Solar Eclipse] which is far wider that the dark ring of ultraviolet light in the negative image [Fig 2 above].
This explosive cascade of fluorescing visible light [triggered by the partial absorption of ultraviolet light in the Earth’s upper atmosphere] is clearly visible in the following early Photographic Plate image [that was “usefully sensitive only to blue, violet and ultraviolet light”] of the Solar Eclipse on the 22nd December 1889.
The American Annual of Photography and Photographic Times Almanac – 1891
Unsurprisingly, this explosive Solar Eclipse image [with its very large halo] isn’t referenced by the Wikipedia entry for this eclipse [see image and link above].
The American Annual of Photography and Photographic Times Almanac  contained the explosive image of the Solar Eclipse on the 22nd December 1889 [above] and the intriguing image of Dark Lightning referenced in an earlier posting.
Page 129 – Photographs of Lightning – W N Jennings
The American Annual of Photography and Photographic Times Almanac – 1891
For more information of the Earth’s fluorescing sky please reference the following posting:
Why The Sky Is Blue
Another large Halo Solar Eclipse image can be found in the January 1902 edition of Popular Science Monthly
Recent Total Eclipses of the Sun – S. I. Bailey – Harvard College Observatory
Popular Science Monthly- Volume 60 – January 1902
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Thank you for pursuing quantitative information on the Sun.
Precise information on solar radiation might settle a 40-year debate over the Sun’s origin, composition and source of energy:
“Strange xenon, extinct super-heavy elements, and the solar neutrino puzzle,” Science 195, 208-210 (1977):
Click to access StrangeXenon.pdf
Here are a few additional references to the debate:
01. Peter Toth, “Is the Sun a pulsar?” Nature 270, 159-160 (1977): http://www.nature.com/nature/journal/v270/n5633/abs/270159a0.html
02. Oliver K. Manuel and Alberto Boretti, “Yes, the Sun a pulsar,” Nature (submitted 12 Dec 2012) http://dl.dropbox.com/u/10640850/Yes_the_Sun_is_a_pulsar.pdf
03. “The sun’s origin, composition and source of energy,” in Lunar and Planetary Science XXIX, 1041 (CD-ROM, March 16-21, 2001) http://www.omatumr.com/lpsc.prn.pdf
04. “Neutron repulsion,” The Apeiron Journal 19, 123-150 (2012) http://tinyurl.com/7t5ojrn