Solar dynamic loops reveal a simultaneous explosion and implosion, plus evidence for magnetic reconnection
Movies of giant loops projecting from the surface of the Sun are giving new insights into the complex mechanisms that drive solar flares and Coronal Mass Ejections (CMEs). These eruptions release vast energy and electrically charged particles that can affect the Earth through space weather. Imagery from NASA's Solar Dynamics Observatory (SDO), used in two separate studies, shows the dynamics of loops before, during and after eruptions. Results have been presented at the National Astronomy Meeting in St Andrews.
Coronal loops are giant magnetic arches filled with hot plasma at temperatures of over a million degrees Celsius. The structures are anchored in the dense photosphere, the visible surface of the Sun. The loops form the building blocks of the corona, the halo surrounding the Sun that can be seen during a total eclipse. They are dynamic structures that oscillate back and forth after explosive events such as solar flares.
Researchers from the University of Glasgow observed four groups of loops that contracted rapidly during a flare on 9 March 2012. The loops had a 'staggered start' to their collapse, showing delays of 60–80 seconds from the inner to the outer loops.
"This event is a great example of a simultaneous implosion and explosion," said Dr Paulo Simões. "Our interpretation is that energy is transferred from the magnetic field to power the flare, leaving a pocket of reduced magnetic support that causes an implosion. The staggering between the loop contractions is caused by the time delay needed for the 'information' about the loss of support to travel outwards."
The loop contractions are triggered at the same time as the flare begins emitting intense X-rays and microwaves. The three outer loops show clear oscillations even as they contract, with distinct periods and phases. After being compressed by the collapsing loops, the flaring loops oscillate until they find a new equilibrium, as indicated by the X-ray emission from the hot plasma. During the contraction a wave blast revealed by extreme ultraviolet radiation spreads away from the source of the flare.
"This presents an intriguing picture of how magnetic energy is moved rapidly around the solar corona during a flare," said Dr Simões.
Flares and CMEs are thought to be driven by a process called magnetic reconnection, in which magnetic field lines in plasma break and then re-join to field lines flowing in the opposite direction. Energy that has built up over days or months is released in just a few minutes.
In a separate study, a team from the University of Warwick has observed the first evidence that loop oscillations are driven directly by magnetic reconnection processes.
"The structure and dynamics of the solar corona can be imaged in exquisite detail and over an unprecedented range of temperatures by SDO. Oscillating loops are a useful tool for probing conditions in the corona. This offers a unique opportunity to discover the tell-tale signatures of magnetic reconnection," said Rebecca White, who presented the findings on Tuesday 2nd July.
The Warwick team used SDO data to study the behaviour of loops following two eruptions: a CME on 3 November 2010 and a solar flare on 8 May 2012. With the first eruption, they saw a coronal loop form below the bubble of material ejected during the CME. There appeared to be a strand connecting the CME with the top of the loop. Unusually, parts of the loop were observed to oscillate in different directions about a central pivot point.
"The loop appears to twist about a fixed point along its length. Not only is the form of this oscillation highly unusual but the coronal loop has a temperature of between 9 and 11 million degrees - this is much hotter than most loops we see, which are generally between 1 and 3 million degrees. This extreme heat has been generated by the reconnection processes," said White. "For the first time we can see a direct link between the reconnection process itself, causing the formation of the loop below the ejected bubble, and the oscillations of the loops."
The second observation showed two separate but adjacent loops oscillating in opposite directions to one another. Previous observations have shown loop oscillations caused by blast waves emanating from the flare, however this pushes the loops in a single direction.
"Again, this cannot be explained by a blast wave since this would push both loops in the same direction. We think that the oscillations here are a direct result of the flare reconnection process changing the structure of the corona between the loops and sucking them towards each. These observations demonstrate that loop oscillations are a valuable tool for studying 3D reconnection processes at work," said White.
1. Image from NASA’s Solar Dynamic Observatory (SDO) at 17.7 nanometres showing the flaring active region on 9 March 2012. The coronal loops that contract during the flare are indicated by the labels L1 to L4, outer to inner. Credit: NASA/SDO/University of Glasgow https://www.ras.org.uk/images/stories/NAM2013/2July/simoes1.jpg
2. Version of same image of active region with frame superimposed showing time evolution of the heights of the collapsing loops (taken through the white line drawn across the imploding loops) showing the cleat collapse and oscillations of the bright structures around 03:40 UT on 9 March 2012. Credit: NASA/SDO/University of Glasgow https://www.ras.org.uk/images/stories/NAM2013/2July/simoes2.jpg
3. Animation showing time evolution of the collapsing loops observed by SDO on 9 March 2012 Credit: NASA/SDO/University of Glasgow
4. Left: Image of the hot loop on 3 November 2010. Right: Difference image (one image subtracted from another at a different point in time) highlighting the unusual oscillation. Credit: NASA/SDO/University of Warwick http://www.ras.org.uk/images/stories/NAM2013/2July/white1.jpg
5. SDO movie of loop oscillations after the flare/CME eruption on 3 November 2010. Credit: NASA/SDO/University of Warwick
R.S. White, E. Verwichte & C. Foullon, First observation of a transverse vertical oscillation during the formation of a hot post flare loop, A&A, 545, A129 (2012)
Dr Paulo Simões
School of Physics & Astronomy
University of Glasgow
Tel: +44 (0)141 330 4502
Rebecca White University of Warwick
Dr Robert Massey
Royal Astronomical Society
Mob: +44 (0)794 124 8035
Ms Anita Heward
Royal Astronomical Society
Mob: +44 (0)7756 034 243
Ms Emma Shea
Head of Development Communications
University of St Andrews
Tel: +44 (0)1334 462 167
Mob: +44 (0)785 090 0352
Landline numbers in NAM 2013 press room (available from 9 a.m. to 5 p.m. from 1-4 July, 9 a.m. to 3 p.m. 5 July): Tel: +44 (0)1334 462231, +44 (0)1334 46 2232
NOTES FOR EDITORS
Bringing together more than 600 astronomers and space scientists, the RAS National Astronomy Meeting (NAM 2013) will take place from 1-5 July 2013 at the University of St Andrews, Scotland. The conference is held in conjunction with the UK Solar Physics (UKSP: www.uksolphys.org) and Magnetosphere Ionosphere Solar Terrestrial (MIST: www.mist.ac.uk) meetings. NAM 2013 is principally sponsored by the RAS, STFC and the University of St Andrews and will form part of the ongoing programme to celebrate the University’s 600th anniversary.
Meeting arrangements and a full and up to date schedule of the scientific programme can be found on the official website at http://www.nam2013.co.uk The Royal Astronomical Society (RAS: www.ras.org.uk, Twitter: @royalastrosoc), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organises scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3500 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.
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Founded in the 15th century, St Andrews is Scotland’s first university and the third oldest in the English speaking world. Teaching began in the community of St Andrews in 1410 and the University was formally constituted by the issue of Papal Bull in 1413. The University is now one of Europe’s most research intensive seats of learning – over a quarter of its turnover comes from research grants and contracts. It is one of the top rated universities in Europe for research, teaching quality and student satisfaction and is consistently ranked among the UK’s top five in leading independent league tables produced by The Times, The Guardian and the Sunday Times. The University is currently celebrating its 600th anniversary and pursuing a £100 million fundraising campaign, launched by Patron and alumnus HRH Prince William Duke of Cambridge, including £4 million to fund the creation of an ‘Other Worlds’ Think Tank and Observatory. The new think tank and Observatory project will extend the University of St Andrews’ flagship work on extra-solar planets, and provide a creative environment for problem-focused research, education and continuing public engagement. For further information go to: www.st-andrews.ac.uk/600/