From Aberystwyth University [Prifysgol Aberystwyth](WLS) via EarthSky : “Why is the sun’s atmosphere hotter than its surface?”

From Aberystwyth University [Prifysgol Aberystwyth](WLS)

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EarthSky

May 30, 2021
Marianna Korsos
Huw Morgan

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The sun’s atmosphere via Mongta Studio/ Shutterstock.

Burning questions about the sun’s atmosphere

The visible surface of the sun or the photosphere, is around 6,000 degrees Celsius (11,000 degrees Fahrenheit). But a few thousand kilometers above it – a small distance when we consider the size of the sun – the solar atmosphere also called the corona is hundreds of times hotter. The corona reaches a million degrees C or higher (over 1.8 million degrees F).

This spike in temperature, despite the increased distance from the sun’s main energy source, has been observed in most stars. It represents a fundamental puzzle that astrophysicists have mulled over for decades.

In 1942, the Swedish scientist Hannes Alfvén proposed an explanation [Nature]. He theorized that magnetized waves of plasma could carry huge amounts of energy along the sun’s magnetic field from its interior to the corona. The energy bypasses the photosphere before exploding with heat in the sun’s upper atmosphere.

Modern proof for an old theory

Scientists had tentatively accepted the theory, but we still needed proof. We needed empirical observation that these waves existed. Our recent study [Nature Astronomy] has finally achieved this. It has validated Alfvén’s 80-year-old theory and taking us a step closer to harnessing this high-energy phenomenon here on Earth.

The coronal heating problem has been established since the late 1930s. That’s when the Swedish spectroscopist Bengt Edlén and the German astrophysicist Walter Grotrian first observed phenomena in the sun’s corona that could only be present if its temperature was a few million degrees Celsius [Frontiers in Astronomy and Space Sciences].

This represents temperatures up to 1,000 times hotter than the photosphere beneath it, which is the surface of the sun that we can see from Earth. Estimating the photosphere’s heat has always been relatively straightforward. We just need to measure the light [Astronomy] that reaches us from the sun, and compare it to spectrum models that predict the temperature of the light’s source.

Over many decades of study, scientists have consistently estimated the photosphere’s temperature at around 6,000 degrees C (11,000 degrees F). Edlén and Grotrian’s finding that the sun’s corona is so much hotter than the photosphere – despite being further from the sun’s core [Space.com], its ultimate source of energy – has led to much head scratching in the scientific community.


Science@NASA. ScienceCasts: The Mystery of Coronal Heating.

Convection and sun’s atmosphere

Scientists looked to the sun’s properties to explain this disparity. The sun is composed almost entirely of plasma, which is highly ionized gas that carries an electrical charge. The movement of this plasma in the convection zone – the upper part of the solar interior – produces huge electrical currents and strong magnetic fields.

Convection drags these fields up from the sun’s interior. They burble onto its visible surface in the form of dark sunspots [Advances in Space Research], which are clusters of magnetic fields that can form a variety of magnetic structures in the solar atmosphere.

This is where Alfvén’s theory comes in. He reasoned that within the sun’s magnetized plasma any bulk motions of electrically charged particles would disturb the magnetic field. It would create waves that can carry huge amounts of energy along vast distances – from the sun’s surface to its upper atmosphere. The heat travels along what are called solar magnetic flux tubes [The Astrophysical Journal] before bursting into the corona, producing its high temperature.

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Sunspots are darker patches on the sun’s surface.. This diagram shows multiple layers, including the sun’s atmosphere. Image via Siberian Art/ Shutterstock.

These magnetic plasma waves are now called Alfvén waves, and their part in explaining coronal heating led to Alfvén being awarded the Nobel Prize in Physics in 1970.

Observing Alfvén waves

But there remained the problem of actually observing these waves. There’s so much happening on the Sun’s surface and in its atmosphere – from phenomena many times larger than Earth to small changes below the resolution of our instrumentation – that direct observational evidence of Alfvén waves in the photosphere has not been achieved before.

But recent advances in instrumentation have opened a new window through which we can examine solar physics. One such instrument is the Interferometric Bidimensional Spectropolarimeter (IBIS) for imaging spectroscopy, installed at the Dunn Solar Telescope in the U.S. state of New Mexico.

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Interferometric Bidimensional Spectropolarimeter at The Dunn Solar Telescope Sunspot New Mexico located at Sacramento Peak New Mexico US at the Sunspot Solar Observatory operated by New Mexico State University in partnership with the National Solar Observatory.

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The Dunn Solar Telescope Sunspot New Mexico located at Sacramento Peak New Mexico US at the Sunspot Solar Observatory (US) operated by New Mexico State University (US) in partnership with the National Solar Observatory (US).

This instrument has allowed us to make far more detailed observations and measurements of the Sun.

Combined with good viewing conditions, advanced computer simulations, and the efforts of an international team of scientists from seven research institutions, we used the IBIS to finally confirm, for the first time, the existence of Alfvén waves in solar magnetic flux tubes.

New energy source

The direct discovery of Alfvén waves in the solar photosphere is an important step towards exploiting their high energy potential here on Earth. They could help us research nuclear fusion, for instance, which is the process taking place inside the sun [Nature]that involves small amounts of matter being converted into huge amounts of energy. Our current nuclear power stations use nuclear fission, which critics argue produces dangerous nuclear waste, especially in the case of disasters including the one that took place in Fukushima in 2011.

Creating clean energy by replicating the nuclear fusion of the sun on Earth remains a huge challenge. We’d still need to generate 100 million degrees Celsius quickly for fusion to occur. Alfvén waves could be one way of doing this. Our growing knowledge of the sun shows it’s certainly possible under the right conditions.

We’re also expecting more solar revelations soon, thanks to new, ground-breaking missions and instruments. The European Space Agency’s Solar Orbiter satellite is now in orbit around the sun, delivering images and taking measurements of the star’s uncharted polar regions.

Terrestrially, the unveiling of new, high-performance solar telescopes are also expected to enhance our observations of the sun from Earth.

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European Solar Telescope to be located at Teide Observatory [Observatorio del Teide] in the Canary Islands(ES).

There are many secrets of the sun still to be discovered, including the properties of the sun’s magnetic field. Thus this is an exciting time for solar studies. Our detection of Alfvén waves is just one contribution to a wider field that’s looking to unlock the sun’s remaining mysteries for practical applications on Earth.

See the full article here.

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Please help promote STEM in your local schools.

Stem Education Coalition

Aberystwyth University [Prifysgol Aberystwyth] (WLS) is a public research university in Aberystwyth, Wales. Aberystwyth was a founding member institution of the former federal University of Wales. The university has over 8,000 students studying across 3 academic faculties and 17 departments.

Founded in 1872 as University College Wales, Aberystwyth, it became a founder member of the University of Wales in 1894, and changed its name to the University College of Wales, Aberystwyth. In the mid-1990s, the university again changed its name to become the University of Wales, Aberystwyth. On 1 September 2007, the University of Wales ceased to be a federal university and Aberystwyth University became independent again.

In 2019, it became the first university to be named “University of the year for teaching quality” by The Times/Sunday Times Good University Guide for two consecutive years. It is the first university in the world to be awarded Plastic Free University status (for single-use plastic items).

Aberystwyth University is placed in the UK’s top 50 universities in the main national rankings. It is ranked 48th for 132 UK university rankings in The Times/Sunday Times Good University Guide for 2019 and the first university to be given the prestigious award “University of the year for teaching quality” for two consecutive years (2018 and 2019).

The Times Higher Education World University Rankings placed it in the 301—350 group for 800 university rankings, compared with 351—400 the previous year, and the QS World University Rankings placed it at the 432th position for 2019, compared with 481—490 of the previous year. In 2015, UK employers from “predominantly business, IT and engineering sectors” listed Aberystwyth equal 49th in their 62-place employability rankings for UK graduates, according to a Times Higher Education report.

Aberystwyth University was rated in the top ten of UK higher education institutions for overall student satisfaction in the 2016 National Student Survey (NSS).

Aberystwyth University was shortlisted in four categories in the Times Higher Education Leadership and Management Awards (THELMAs) (2015).

Aberystwyth University has been awarded the Silver Award under the Corporate Health Standard (CHS), the quality mark for workplace health promotion run by Welsh Government.

The University has been awarded an Athena SWAN Charter Award, recognizing commitment to advancing women’s careers in science, technology, engineering, maths and medicine (STEMM) in higher education and research.

In 2007 the University came under criticism for its record on sustainability, ranking 97th out of 106 UK higher education institutions in that year’s Green League table. In 2012 the university was listed in the table’s “Failed, no award” section, ranking equal 132nd out of 145. In 2013 it ranked equal 135th out of 143, and was listed again as “Failed, no award”.

Following the University’s initiatives to address sustainability, it received an EcoCampus Silver Phase award in October 2014.

In October 2015, the University’s Penglais Campus became the first University campus in Wales to achieve the Green Flag Award. The Green Flag Award is a UK-wide partnership, delivered in Wales by Keep Wales Tidy with support from Natural Resources Wales, and is the mark of a high quality park or green space.

In 2013, the University and College Union alleged bullying behaviour by Aberystwyth University managers, and said staff were fearful for their jobs. University president Sir Emyr Jones Parry said in a BBC radio interview, “I don’t believe the views set out are representative and I don’t recognise the picture.” He also said, “Due process is rigorously applied in Aberystwyth.” Economist John Cable resigned his emeritus professorship, describing the university’s management as “disproportionate, aggressive and confrontational”. The singer Peter Karrie resigned his honorary fellowship in protest, he said, at the apparent determination to “ruin one of the finest arts centres in the country”, and because he was “unable to support any regime that can treat their staff in such a cruel and appalling manner.”