A little over a year ago I was in Oregon (as were thousands of other people!) to observe one of nature’s most spectacular shows, a full solar eclipse. It was astounding, almost like being transported to a parallel black-and-white world for those 125 seconds of totality.
Many things contributed to this other-worldly impression - the sudden drop in temperature, the abrupt change from bright sunlight to twilight and back again, seeing stars bright in the dim sky mid-morning - but what topped it all for me was the appearance of the ghostly white corona shimmering like gossamer around the sun, as you can see in this image (my own photographic efforts didnt turn out so well, unfortunately).
A rare opportunity for solar science
An unforgettable visual spectacle indeed - but more than that, an opportunity for serious science. In normal circumstances the corona is invisible, blotted out by the much brighter disc of the Sun, but eclipses, by allowing direct observation of the corona, also offer scientists the chance to investigate the nature of this ghostly feature.
For example, a team of solar physicists led by Zoran Mikic have been working on a system to model the activity in the corona and how this should be reflected in its shape, and they recently announced that their model was able to predict the coronal’s actual shape as seen during the 21st August 2017 eclipse – in other words, it is now possible to broadly predict the effects of solar activity on the corona around a week in advance.
The corona is the upper atmosphere of the Sun, beyond the visible disc known as the photosphere and it is composed of plasma (gas where the electrons and nuclei of the atoms are no longer bound to each other). It is not usually visible to us because of its very low density (very few particles) – it is much more tenuous than our own atmosphere. As it dissipates away from the Sun it turns into the solar wind, which travels outwards in all directions, including towards Earth.
One of the greatest mysteries about the corona is its extremely high temperature – over a million degrees centigrade, vastly higher than the photosphere’s temperature of around 5500 C. Clearly some process is occurring in the plasma to heat it so intensely and scientists believe it this process is at least partly related to magnetic field activity, but this topic is still not well understood and is the subject of active research.
This is of critical importance to us Earthlings, as solar magnetic field activity is responsible for ‘Space Weather’ – sudden releases of magnetic energy from the Sun that can be hugely powerful and potentially cause serious damage and disruption to the technology on which so much of our modern life relies. (See my earlier blog post, stormy-weather.html)
It’s therefore no surprise that a lot of scientific effort is currently directed at understanding the workings of the corona and the solar wind, and a great step was taken with the launch of NASA's Parker Space Probe on 12th August this year.
Thanks to advanced thermal protection technology, this spacecraft will fly closer to the Sun than any before, ten times closer than Mercury, facing temperatures of nearly 1400 C. Travelling into the corona itself, the aim of the mission is to meausre the movements of heat and energy there, to help advance our understanding of the processes involved.
In addition, the European Space Agency (ESA)'s Solar Orbiter mission is currently being built and is due to launch in 2020 - this mission has similar objectives and will carry out joint science with Parker. Unlike Parker, Solar Orbiter will carry telescopes so will be able to take images of the Sun, including the currently unobserved polar regions, which play a vital role in the Sun's magnetic activity
The Parker probe will make its closest approach to the Sun in 2025, and Solar Orbiter should reach its closest orbit of the Sun in 2023 – will the mysteries of the Sun’s gossamer halo finally be revealed within the next decade?
1. Rick Fienberg / TravelQuest International, via Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. https://eclipse.aas.org/sites/eclipse.aas.org/files/TSE2017Aug21_Composite_RTF_TQI.jpg
2. NASA/Bill Ingalls https://www.nasa.gov/image-feature/parker-solar-probe-launches-to-touch-the-sun
3. NASA’s Goddard Space Flight Center/Lisa Poje/Genna Duberstein
https://www.nasa.gov/feature/goddard/2018/nasa-s-parker-solar-probe-and-the-curious-case-of-the-hot-corona
https://www.predsci.com/corona/aug2017eclipse_20170731_prediction/
https://www.nasa.gov/feature/goddard/2018/how-scientists-predicted-corona-s-appearance-during-aug-21-2017-total-solar-eclipse
https://www.nasa.gov/feature/goddard/2017/eclipse-2017-science-from-the-moon-s-shadow
https://scied.ucar.edu/solar-corona
http://solar-center.stanford.edu/FAQ/Qcorona.html
https://www.nasa.gov/content/goddard/parker-solar-probe-humanity-s-first-visit-to-a-star
https://www.esa.int/Our_Activities/Space_Science/Solar_Orbiter
https://www.ncei.noaa.gov/news/what-is-space-weather