The late Transit of Mercury (and write up)

[johnfosteruk]

Here, courtesy of data pulled from Helioviewer is a short animation showing the first contact of the transit of Mercury on May 9th 2016. The writeup is by no means comprehensive, being a general summary of things I find interesting, hopefully it's all relevant and accurate - I'd like to think of this being a useful and interesting introduction to the subject for a novice. I've not included citations, references or recommended reading but if I develop it further (I'd like to) I will.

This animation was compiled using data captured at 193 Ångstroms wavelength (19.3 nanometers).

193Å highlights an emission line from iron atoms that have lost 11 electrons (known as iron-12 or Fe XII) at temperatures of 1,000,000 K as well as iron atoms that have lost 23 electrons (also known as iron-24 or FeXXIV) at temperatures of 20,000,000K.

Iron-12 represents a slightly higher region of the corona and iron-24 represents the much hotter material of a solar flare. Solar mass ejections & hot active regions appear bright at this wavelength which also makes coronal holes (which appear as dark regions near the solar surface) more visible.

Coronal holes are caused by anomalies in the sun's magnetic field which leave that area of the field open to space, the darker area as seen here at bottom left is abundant with cooler, less dense low energy gas and it is the temperature difference between this area and it's surroundings that gives the impression of a hole. Instead of trapping plasma, the 'hole' allows it to escape as solar wind, if the hole faces the earth this wind interacts with the planet's magnetosphere, leading to the appearance of 'Aurora' seen near the Earth's geomagnetic poles.

This animation also shows a portion of the transit of mercury from 11:00 - 11:33 UT on May 9th 2016, the shadow of Mercury travels across the disc as the planet passes between the sun and the earth. Due to Mercury's small angular diameter of approximately 12 arcseconds it is dwarfed by the sun which has an angular diameter of 1900 arcseconds, thus it appears 1/158th of the suns apparent diameter.

Mercury transits only occur approximately 13 times a century in either May or November and the events are significant, as well as providing interesting views for astronomers they have been used in the past to estimate the Sun-Earth distance or Astronomical Unit (AU). In 1716, Edmond Halley published a paper describing how this could be done, although the method is inaccurate because making contact timings is difficult due to turbulence in the earth's atmosphere known by astronomers as 'seeing'. However the method did prove in some cases to be reasonably accurate, giving a measurement of 24000 Earth Radii in 1771. The AU is now accepted as 23455 Earth Radii or 149,597,870.700 kilometres, in modern times it has been measured by space probes carrying radar and telemetry.

The data used in this animation was captured by the Solar Dynamics Observatory. The Mission of the SDO is to determine how the Sun's magnetic field is generated and structured & how this stored magnetic energy is converted and released into the heliosphere and geospace in the form of solar wind, energetic particles, and variations in the solar irradiance.

It was launched on February 11, 2010, 10:23 am EST on an Atlas V from SLC 41 from Cape Canaveral, the three main instruments, the Atmospheric Imaging Assembly (AIA), EUV Variability Experiment (EVE) and Helioseismic and Magnetic Imager (HMI) each perform observations simultaneously and near continuously from its geosynchronous orbit around earth. Science data is sent to the ground at a rate of 130 Mbps on a continuous, high rate data stream at a Ka-Band frequency of 26 GHz and it produces more than a terabyte of data per day. 

It is interesting to note that the SDO sits at the outer reaches of the Earth's radiation belt so is subject to a higher than optimal dose of radiation in comparison to low earth orbit positions. Additional shielding had to be added to the instruments and electronics to protect them from the very phenomena that the SDO was designed to study.

[Pete Presland]

Nice animations and write up, imagine having that view!

[laudropb]

Very nice write up. Love the animation.

[David Smith]

Nice write up and a great little animation.

[cshahar]

Wow. That is all that can be said!

[johnfosteruk]

oops. Double post

[johnfosteruk]

Thanks chaps. it does feel a bit like cheating, but that said it is a great way to learn new processing techniques while I wait 20 years to save for some good solar gear 

Also, knowing how this data was acquired certainly makes you appreciate the effort that goes into capturing solar data from the ground with all its challenges.