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Venus and Jupiter

OliUsher30 June 2015

If you get a chance tonight (or in the next few nights), take a look at the Western horizon just before or after sunset. Two bright ‘stars’ will be following the Sun down. These are Jupiter and Saturn, and they are unusually close together just now from our perspective. (In reality, Venus is close to us, between Earth and the Sun, while Jupiter is in the outer Solar System far beyond).

Jupiter and Venus as they look to the naked eye. Photo: Francisco Diego (UCL Physics & Astronomy)

(Click to enlarge picture)
Jupiter and Venus as they look to the naked eye. Photo by Francisco Diego (UCL Physics & Astronomy), taken from Ruskin Park, London, last night.

The picture above (click to enlarge), taken by UCL’s Francisco Diego, shows roughly what the view looks like to the naked eye.

Francisco is a star of astronomy outreach at UCL – as well as teaching our students, he works closely with schools and runs the annual Your Universe festival.

Through a pair of binoculars or a low-powered telescope it’s even more impressive: you should be able to see Venus as a crescent (like a miniature moon), and you may see a few faint dots either side of Jupiter: the gas giant’s four largest moons, Io, Europa, Ganymede and Callisto.

IMG_7940r

Jupiter and Venus as seen during daylight at 7pm by Francisco Diego (UCL Physics & Astronomy) with magnification similar to an ordinary pair of binoculars – they should be even clearer once the Sun has set.

Added 1 July:

There was a bit of cloud last night, but the planets were visible for a time. Here’s an amazing shot captured by Francisco on the night of 30 June (click the photo to enlarge):

Venus and Jupiter by Big Ben on 30 June. Photo credit: Francisco Diego (All rights reserved, not to be reproduced without permission)

Venus and Jupiter by Big Ben on 30 June. Photo credit: Francisco Diego (All rights reserved, not to be reproduced without permission)

Tonight, they will still be visible but a touch further apart, and crescent Venus and the disk of Jupiter will have swapped positions as they continue their journey through the Solar System.

The LHC is back in operation at record energy

OliUsher3 June 2015

After two years of repairs and upgrades, the Large Hadron Collider (LHC) is back in operation – and UCL scientists are at the heart of the action. Engineers at CERN confirmed today that the beams of protons that circle in the 27km tunnel near Geneva are stable, and scientific data is once again being collected.

The ATLAS experiment is made of concentric rings of detectors (the particle beam passes through the centre), as seen here during shutdown in 2008. Credit: CERN (CERN licence)

The ATLAS experiment is made of concentric rings of detectors (the particle beam passes through the centre), as seen here during shutdown in 2008. Credit: CERN/Claudia Marcelloni De Oliveria (CERN licence)

UCL scientists have been closely involved in the design, construction and operation of ATLAS, one of the giant detectors that track the high-energy particle collisions in the LHC, so this is an important milestone for the university’s High Energy Physics group.

Until now, the LHC has not been operating at full power. The faults that led to its shutdown shortly after it was inaugurated in 2008 meant that it could only be used to accelerate particles to around half the energy it was designed for.

* * *

Einstein’s equation E=mc2 states that energy and matter are interchangeable.

Atom bombs, famously, create vast amounts of energy by destroying small amounts of matter.

Particle accelerators like the LHC do the opposite, pumping vast amounts of energy into tiny particles, making them move at close to the speed of light. When they collide together, some of that energy is converted into extra matter, in the form of new particles flung out from the site of the collision. The greater that energy, the heavier the particles that can be generated.

Physicists describe the world around us using the ‘standard model’ of particle physics, a set of a handful of particles which can explain the properties and makeup of all the matter and energy we see around us.

The last of those particles to be detected in the lab was the Higgs boson, discovered at CERN in 2012 shortly before the upgrade began.

This doesn’t mean there is nothing left to discover, though. Scientists, including CERN’s director, have begun speaking of a tantalising ‘new physics’ – whole new uncharted areas of science that are currently totally unknown, but which might be explored with higher-energy collisions and heavier particles generated.

One of these areas could be a solution to the riddle of dark matter.

Dark matter can be detected by astronomers (as seen in this Hubble image of a galaxy cluster), but it has not been spotted on Earth, and is known to not be made out of any of the particles in the standard model. Credit: NASA/ESA (CC-BY)

Dark matter can be detected by astronomers (as seen in this Hubble image of a galaxy cluster), but it has not been spotted on Earth, and is known to not be made out of any of the particles in the standard model. Credit: NASA/ESA (CC-BY)

Dark matter has been detected in distant galaxies, thanks to its gravitational effects, but astronomers have determined that it cannot be made of any of the particles described in the standard model.

ATLAS, the building-sized instrument that UCL participates in, played a key role in the Higgs boson discovery, and will play a starring role in the future work of the LHC, and could help explain how dark matter relates to the standard model of particle physics.

* * *

The LHC consists of two 27km-long pipes that use powerful magnets to accelerate beams of protons in opposite directions.

Inside ATLAS, the two beams are brought together on a collision course. The beams are not continuous: the protons come in pulses (“bunches”) about 10cm long and the width of a human hair, each containing around a hundred billion particles. When these bunches cross each other, protons collide and new particles cascade out through the concentric rings of detectors that make up the ATLAS experiment.

One of the detections made by ATLAS today. This picture is a cross-section of the instrument, with each concentric ring detecting particles' location or energy, and the particles' tracks (shown as multi-coloured curved lines) inferred from this data. Credit: CERN

One of the detections made by ATLAS today. This picture is a cross-section of the instrument, with each concentric ring detecting particles’ location or energy, and the particles’ tracks (shown as multi-coloured curved lines) inferred from this data. Credit: CERN (CC BY SA)

Scientists can then trace the path that the particles took – and determine their energy, mass and electrical charges. And from those, they can infer the process that take places in each proton-proton collision.

During the two-year LHC shutdown, the ATLAS scientists also made several improvements to their detector, most notably with the installation of an extra ring of detectors close to the beam pipe, making it more precise than ever before, and part of the team’s work now that the LHC is running again is to ensure that this is all properly calibrated and working as expected.

When it runs at full capacity, ATLAS detects 40 million particle collisions every second, far more than could ever be studied.

Part of the challenge is to discard the unimportant data so that scientists can focus on what’s important. One of the major contributions UCL scientists have made to ATLAS is to the design and operation of the hardware and software algorithms used to discard trivial events in real time and select only the interesting ones – reducing 40 million collisions per second down to a far more manageable 1,000 that are recorded offline.

Another challenge is the simple matter of timing and synchronisation.

With millions of events per second, and everything moving at close to the speed of light, untangling the data from different collisions is challenging. ATLAS is still detecting particles ejected by one collision while another is already taking place.

Particles from multiple events cascade through the detectors at one time, and synchronising them is not straightforward. Credit: CERN (CC-BY-SA)

Particles from multiple events cascade through the detectors at one time, and synchronising them is not straightforward. Credit: CERN (CC-BY-SA)

UCL scientists played key roles in developing the electronics that ensure that the data is accurately recorded and readouts from the different components of ATLAS are all kept properly synchronised.

* * *

So what’s next for the LHC?

It’s very hard to say – and that’s what is so exciting about particle physics today.

The standard model is complete. There could be a radical departure, revealing areas of physics never explained before.

Equally, there might just be further confirmation of the dramatic discoveries of the past few decades, giving more precision and certainty to the standard model.

In either case, there is now a chance to explore physics without the constraint of theoretical preconceptions – an unusual and liberating place for a physicist to be.

With thanks to Prof Nikos Konstantinidis for help with this article

 

 

 

 

The coolest place in London

OliUsher18 May 2015

Quantum refrigerator. Photo: O. Usher (UCL MAPS)

Quantum refrigerator. Photo: O. Usher (UCL MAPS)

This photo shows a specialised refrigerator, used for cooling objects to within a fraction of absolute zero, located in the Physics building on UCL’s main campus.

When in operation, the refrigerator is entirely enclosed in a sealed and insulated housing, which has been removed here for maintenance.

As with a household fridge, the temperature drops as you go down – with the highest of the four shelves being at a temperature of about 50 Kelvins (-223 Celsius) and the bottom one at just 0.03 K (-273 C).

The refrigerator is used to cool small objects, such as transistors, down to levels where thermal effects (such as the vibration of the atoms in them) are eliminated, allowing quantum effects to be observed.

Samples can be raised or lowered through each shelf via a circular hole in the centre of the refrigerator.

The refrigerator is cooled by a mixture of liquid Helium-3 and Helium-4. The lowest shelf of this refrigerator has a strong claim to being the coldest place in London (alongside a handful of similar facilities at other London universities).

High resolution images

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New space photos from UCL’s observatory

OliUsher29 April 2015

Messier 51, from the University of London Observatory. Credit: UCL/ULO/Ian Howarth

Messier 51, from the University of London Observatory. Credit: UCL/ULO/Ian Howarth

The University of London Observatory – UCL’s astronomical observatory in Mill Hill, North London – has to deal with England’s murky skies and London’s bright lights, but it can still make some impressive images. Messier 51, seen in the picture above, is actually not one galaxy but two – a large spiral galaxy (Messier 51a) interacting with a smaller dwarf galaxy (NGC 5195). Over the next few hundred million years, they will merge together into one larger galaxy.

Such mergers are quite common. Large spiral galaxies can absorb dwarf galaxies without major disruption to their shapes, though the (rarer) mergers between similarly-sized galaxies tend to destroy all structure, leaving a largely featureless elliptical galaxy. This will be the fate of the Milky Way when it merges with the Andromeda Galaxy in a few billion years time.

The Messier 51 pair are a popular target for amateur astronomers – on a dark night, even relatively basic telescopes can pick out the very faint comma-shape of the galaxy pair, visible near one end of the Plough (Ursa Major).

The picture is one of several newly processed images just published by UCL’s observatory, based on data gathered by astronomy students. The observatory now routinely archives all the digital data gathered with its Celestron telescopes, which are used intensively for undergraduate teaching. This growing archive of data means that multiple observations can be easily combined into a single image, improving contrast and revealing faint details that would otherwise be invisible.

A selection of several dozen of these images from the observatory, with multiple observations processed and combined to form colour composites, is available online to the public. They are free to reuse and reproduce.

What’s next for Hubble?

OliUsher24 April 2015

This is the third and final in a series of posts marking the 25th anniversary of the Hubble Space Telescope. Read the first here, and the second here.

Hubble won’t last forever – electrical faults could render any of its instruments inoperable at any time. Though this has happened before, there is no longer a Space Shuttle that can be launched to send astronauts on a repair mission, so any future instrument failures are likely to be permanent.

Moreover, the telescope needs to reliably and steadily lock onto the position of the astronomical objects it is observing. It does this thanks to gyroscopes dotted around the spacecraft – but these will eventually wear out and fail too. Engineers are quietly confident that Hubble can last till at least 2015, but beyond that, the observatory’s future is unclear.

Artist's impression of the James Webb Space Telescope. Credit: ESA/C. Carreau

Artist’s impression of the James Webb Space Telescope. Credit: ESA/C. Carreau

By the end of 2018, the James Webb Space Telescope should join Hubble in orbit – with hardware built at UCL on board. The Webb telescope is not a like-for-like replacement. Webb will have far more powerful capabilities in infrared light, allowing it to peer deep into dust clouds, observe planetary systems being formed, and the see distant redshifted light of the first galaxies.

JWST NIRSpec calibration assembly. Photo credit: UCL MSSL

UCL’s contribution to the James Webb Space Telescope: the NIRSpec (Near Infrared Spectrograph) calibration assembly. This helps maintain accurate scientific observations. Photo credit: UCL MSSL

But it will not have Hubble’s abilities in ultraviolet and visible light. A new breed of telescopes on the ground, such as the European Extremely Large Telescope and the Thirty Meter Telescope will partly replace Hubble’s visible light capabilities (although not with Hubble’s sharpness).

But when Hubble fails, no telescope in operation or in development will replace its ability to observe ultraviolet light, which is blocked by the Earth’s atmosphere.

 

 

 

UCL’s science with the Hubble Space Telescope

OliUsher22 April 2015

This is the second in a series of posts marking the 25th anniversary of the Hubble Space Telescope. Read the first here.

UCL astronomers have been involved with the full range of Hubble science over the years.

Here are just a few highlights.

Detecting the first organic molecule on an extrasolar planet

Artist's impression of HD 189733b passing in front of its star. The small amount of starlight that passes through the planet's atmosphere carries with it the fingerprint of the gases present there. Credit: ESA/Hubble (Martin Kornmesser)

Artist’s impression of HD 189733b passing in front of its star. The small amount of starlight that passes through the planet’s atmosphere carries with it the fingerprint of the gases present there. Credit: ESA/Hubble (Martin Kornmesser)

In 2008, a team including UCL’s Giovanna Tinetti (now working on the proposed Twinkle mission) made the first detection of an organic molecule on a planet outside the Solar System, using Hubble. Organic molecules – ones based on carbon – are thought to play a key role in the emergence of complex chemistry and the appearance of life.

Although this planet, known as HD 189733b, is so hot that it is almost certainly sterile, the team proved it has traces of methane in its atmosphere. This shows not only that organic molecules exist outside our Solar System, but that they can be detected from Earth – and that one day we might detect signs of life on another planet using the same methods.

Hubble observed the light coming from HD 189733b’s parent star as the planet passed between Hubble and the star. The gases in the atmosphere leave a faint fingerprint in the light that passes through the atmosphere, letting the scientists deduce what gases were present.

Understanding the superwinds of Messier 82

Composite image of Hubble and WIYN observations of M 82. Credit: Mark Westmoquette (University College London), Jay Gallagher (University of Wisconsin-Madison), Linda Smith (University College London), WIYN//NSF, NASA/ESA

Composite image of Hubble and WIYN observations of M 82. Credit: Mark Westmoquette (University College London), Jay Gallagher (University of Wisconsin-Madison), Linda Smith (University College London), WIYN//NSF, NASA/ESA

Messier 82 (or M 82 for short) is a peculiar-looking galaxy – in the news last year thanks to a UCL lecturer and his students discovering a bright supernova there. It is in the midst of a sustained period of star formation known to astronomers as a ‘starburst’ – and this has dramatic effects on the appearance of the galaxy. It shines brightly with bright blue newly-formed stars, with noticeable regions of disrupted gas and dust clouds. It is an easy and popular target for amateur astronomers to find with mid-sized telescopes.

But the most dramatic aspect of M 82’s appearance has to be the powerul winds of glowing gas ejected out of the galaxy.

In 2004, a team of scientists including Linda Smith and Mark Westmoquette (both then at UCL) used archival Hubble images, alongside images from the WIYN telescope, to trace these ‘superwinds’. They found multiple streams of gas expanding at different rates, creating a shower of hot gas expelled from the galaxy. Some of these were travelling at more than a million miles per hour.

The team believes that the burst of star formation was triggered by a near-miss with nearby spiral galaxy M 81, which disrupted the gas clouds in M 82.

Probing the dark universe using galaxy clusters

The discovery of the accelerating expansion of the Universe was one of the most startling scientific breakthroughs of the past century. Astronomers had long known that the universe was expanding, but assumed that this expansion was gradually slowing over time. Instead, in 1998, two teams of astronomers discovered that the universe’s expansion is speeding up, with an unknown force they call dark energy driving it.

The amount of energy involved in this mysterious process is enormous – calculations show it makes up around three quarters of the total mass and energy content of the universe. This is on top of dark matter – matter whose gravity can be seen, but which is totally invisible to telescopes – which also outweighs all the ordinary matter in the universe. This means that the matter we can actually see – stars, planets, nebulae, dust clouds, galaxies – only makes up about one part in 20 of the whole universe. The rest is dark; it neither emits, reflects nor absorbs light.

A team of astronomers including UCL’s Ofer Lahav proposed to carry out a huge programme of observations called the Cluster Lensing and Supernova Survey with Hubble (CLASH), in order to gather as much data as possible about the mysteries of the dark universe.

Cluster MACSJ1206, one of 25 observed by the CLASH programme. The distorted, magnified shapes of background galaxies can clearly be seen in the image. Credit: NASA, ESA, the CLASH team and Marc Postman

Cluster MACSJ1206, one of 25 observed by the CLASH programme. The distorted, magnified shapes of background galaxies can be faintly seen in the image. Credit: NASA, ESA, the CLASH team and Marc Postman

Between 2010 and 2013, the survey made detailed images of 25 massive galaxy clusters. These clusters, which are among the largest structures in the universe, have so much gravity – largely thanks to the dark matter they contain – that they warp space-time. This means that they bend the path of light that passes through them, in a similar way to a lens.

The lensing of light reveals the location of dark matter, which is otherwise invisible. It also amplifies the light coming from galaxies in the background, enabling astronomers to see distant supernovae that would otherwise be too faint to observe. The apparent brightness of these supernovae is one of astronomers’ key tools for measuring the expansion rate of the universe, and hence the nature of dark energy.

UCL’s work in CLASH has focused on ‘photometric redshifts’, a means of deducing the distance of faraway galaxies from the colour profile they have. This work was led by Ofer Lahav, along with researchers Stephanie Jouvel and Ole Host.

On Friday, the UCL Science blog will explain what comes next for the ageing space telescope – and how UCL is helping to build its successor.

Happy birthday Hubble

OliUsher20 April 2015

This week marks the NASA/ESA Hubble Space Telescope’s 25th birthday. Since its launch on 24 April 1990, it has revolutionised astronomy, playing a role in huge scientific events including the first images of exoplanets and the measurement of the rate of expansion of the universe.

Along the way, it has taken stunning, sharp images of space that are now icons of popular culture.

One of Hubble's famous images, the 'Pillars of Creation' in the Eagle Nebula. Credit: NASA, ESA and the Hubble Heritage Team

One of Hubble’s famous images, the ‘Pillars of Creation’ in the Eagle Nebula. Credit: NASA, ESA and the Hubble Heritage Team

At 25, the telescope has lasted far longer than was ever planned. But thanks to regular servicing over the years, most recently in 2009, NASA’s engineers calculate the telescope still has a few years of operation left before its hardware begins to wear out. It should even last long enough to see the first few years of operations of its successor, the James Webb Space Telescope, which launches in 2018.

Hubble had a long and difficult gestation – the idea dates back to the 1940s, with design work beginning in the ‘70s. By the early 1980s, amid rising costs and political controversy, it was clear that the US couldn’t deliver Hubble alone, so the European Space Agency was brought in as a partner.

ESA astronaut Claude Nicollier during the third Hubble servicing mission in 1999. Credit: NASA/ESA

ESA astronaut Claude Nicollier during the third Hubble servicing mission in 1999. Credit: NASA/ESA

Since then, Hubble has been an international project, with staff from around the world working on the telescope, and observing time awarded to astronomers from around the world in annual competitions. Among them have been a fair few from UCL.

Hubble science comes in different flavours.

Most of the telescope’s time is devoted to observations carried out on behalf of small teams of astronomers. If their proposal is considered to be scientifically interesting, they will get to observe their chosen targets and use the data for their research. After a year, the data is uploaded to an online archive for anyone – other scientists, or even members of the public – to view. Surprisingly, perhaps, these archival observations are still incredibly useful, and they end up being used in huge numbers of studies, often making important discoveries that are totally unrelated to the original plan.

As well as these small projects, a proportion of Hubble’s time is set aside for bigger projects. These might be a systematic survey of dozens of galaxy clusters, or a highly detailed map of a large, nearby galaxy. These surveys are carried out in order to create large, comprehensive archives that can be used for a wide range of different scientific goals.

UCL astronomers have been involved with the full range of Hubble science over the years.

On Wednesday, the UCL Science blog will cover a few of the research highlights from UCL’s work with Hubble over the years.

First dark matter map from DES

OliUsher14 April 2015

The first dark matter map to come out of the Dark Energy Survey. Credit: Fermilab/Dark Energy Survey

The first dark matter map to come out of the Dark Energy Survey. Credit: Fermilab/Dark Energy Survey

The Dark Energy Survey, an international collaboration to probe the history, evolution and large-scale structure of the cosmos, has produced its first dark matter map. Dark matter is a transparent form of matter that is distributed in vast filaments throughout the known universe. Galaxies are located along these structures, and galaxy clusters lie where they meet. Because dark matter is fully transparent, it cannot be observed directly – its presence must be inferred from the gravitational effects it has on light, in particular, the way it distorts the shape of galaxies which lie in the background.

The map is the first of a series of dark matter maps that will be published by the survey team, and is part of a batch of research that is being released to coincide with the April meeting of the American Physical Society this week.

UCL is heavily involved with the Dark Energy Survey, and has several researchers involved in the newly-published research. The university is also involved in the project through the scientific instrumentation (UCL built some of the lenses that are used by the Dark Energy Camera), and UCL’s Ofer Lahav is co-chair of the DES science board.

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Observing the eclipse in Svalbard

OliUsher1 April 2015

The observatory during the solar eclipse. Photo: : Noora Partamies / Kjell Hendriksen Obsevatory, The University Centre in Svalbard (free for non-commercial use)

The observatory during the solar eclipse. Photo: : Noora Partamies / Kjell Henriksen Observatory, The University Centre in Svalbard (free for non-commercial use)

The eclipse might have been underwhelming in London. But in Svalbard, a Norwegian archipelago far north of the Arctic Circle, the skies were clear, and the eclipse was total. Sitting on a hillside ten kilometres outside Longyearbyen – the northernmost town in the world – is Kjell Henriksen Observatory.

UCL’s Atmospheric Physics Laboratory has instruments at the observatory – and during the eclipse, three UCL scientists (Anasuya Aruliah, Ian McWhirter and PhD student Amy Ronksley) operated them remotely from overcast London.

The instruments study the properties of the uppermost reaches of the Earth’s atmosphere. Near the north and south magnetic poles, Earth’s magnetic field plunges into the atmosphere. The charged particles of the solar wind which are deflected before reaching the atmosphere in the rest of the world are able to interact with atmospheric gases here.

These interactions lead to the phenomenon known as the aurora borealis – or northern lights – and their southern counterparts.

Aurora observed from the observatory during night-time hours on 6 January. Credit: UCL Atmospheric Physics Group

Aurora observed from the observatory during night-time hours on 6 January. Credit: UCL Atmospheric Physics Group (CC-BY)

UCL’s instruments at the observatory measure winds and temperatures at an altitude of 240km, where the air is only one ten billionth the density of that at ground level. They do this by detecting the wind’s effect on the aurora.

The two main components of the aurora are green light, produced at an altitude of 90km, and red light, at 240km altitude. Both come from oxygen atoms whose electrons have been excited by the solar wind. As the electrons return to their usual orbits, they produce light of extremely precisely defined colours – much like neon signs or lasers do.

These colours let the scientists determine the speed and temperature of the gases in the upper atmosphere.

They determine the speed by observing how the colour shifts almost imperceptibly towards blue or red, when the wind is blowing towards or away from the observatory. (This phenomenon, known as the Doppler Effect, is also what causes the shift in the pitch of an ambulance siren as it drives past.)

They determine the temperature thanks to the oxygen atoms’ vibration. As the atoms vibrate to and fro, a miniature Doppler Effect makes the light appear slightly redder for the time the atom is moving away, and slightly bluer for the time it is moving towards the observer. This means the light is no longer a perfectly pure shade of red, but encompasses a tiny range of colours – a process known as spectral broadening. The hotter a gas is, the greater the movement, and the greater the spread of colours.

By measuring Doppler shifts and spectral broadening, the UCL instruments produce a complete weather map for the atmosphere at the edge of space.

Alongside the instruments is a colour camera with a fisheye lens, which can see from horizon to horizon, and which produces photos such as the one shown above.

Such measurements can only be done at night, not just because the auroral emissions are swamped by sunlight during the day, but because the sensitive cameras in the instruments would be damaged by sunlight. The eclipse provided a unique, albeit brief opportunity to observe during daytime. Preliminary analysis of the data has indicated strong daytime winds in the upper atmosphere during totality.

And although the scientists weren’t able to see the aurorae they hoped to catch a glimpse of, a fisheye picture they took during the eclipse does show how a small ‘umbrella’ of darkness sat directly above Svalbard, while bright sunlight was visible on the horizon in all directions.

The sky above the observatory during the eclipse - a ring of sunlight can be seen around the horizon. Photo: UCL Atmospheric Physics Group (CC-BY)

The sky above the observatory during the eclipse – a ring of sunlight can be seen around the horizon. Photo: UCL Atmospheric Physics Group (CC-BY)

As well as simple scientific curiosity, scientists study the uppermost reaches of the atmosphere (known as the thermosphere) for some very practical reasons. Most satellites orbit within this region, and better data on its properties can help engineers plan their orbital corrections and manoeuvres. Moreover, the interaction between the thermosphere and the solar wind is also a key element of ‘space weather’, which can disrupt electrical and communications infrastructure on the ground.

The information gathered by atmospheric physicists at UCL and elsewhere is helping to improve the computer models of how the upper atmosphere behaves, so that we can better understand these phenomena.

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Darkness falls across the land

Dewi WLewis24 March 2015

 

eclipse

These two photographs were taken in the quad during the eclipse. The first at 9:25 the second at 9:31am.

During this time the light dropped by a factor of 16: dropping from an Exposure Value (EV) value of 11 to 7, or 4 stops, to the photographers amongst you.

My eyes told me it got “a bit darker”, the reality was far more!