Tuesday, January 24, 2012

From the Sun to the Earth

NASA STEREO-A COR-2 image
Over the past few weeks the Sun has been unleashing a huge number of particles toward the Earth and no doubt in the far north and south (clear skies permitting) they are putting on quite a show. This is a story of the journey from the Sun to the night sky and our eyes.

The journey starts at the heart of our star, where it is an unimaginable 15 million °C and 150,000kg of matter is packed into each cubic metre; that is as if 30 African elephants occupied a 1 meter cubed box. The particles at the heart of our Sun are Hydrogen nuclei; these particles collide together with enough force to combine through nuclear fusion to produce Helium. This process converts a small amount of mass into a large amount of energy, via the famous equation E=mc2. Every second the Sun converts around 4.3 thousand million kg of matter into pure energy. This energy is radiated out, in the form of photons, 70% of the way through the Sun in a process which takes tens to hundreds of thousands of years.
The final 30% of the journey to the sun’s surface is through a process of convection where the hot plasma rises to the surface as cool plasma descends down.  The ‘surface’ of the sun is covered in convective cells up to 600 miles across making it look grainy. The temperature at the surface is only 5,500 °C and the density has dropped to 0.001 kg per cubic meter.
The Sun’s surface is dominated by a host of different activities from dark sunspots to bright faculae. The most violent events that occur on the sun’s surface are where the journey of our particles continues.

A coronal mass ejection, or CME, is one of the most explosive events on the Sun where up to 1013kg (that is 10,000,000,000,000 kg) of solar plasma breaks away from the surface at speeds between 100 and 2,000 km/s. These particles form a dense loop of plasma that moves away from the Sun with each particle aligned to the magnetic field of the Sun.  On occasion these CMEs cane be directed towards the Earth and take between 17 days to 20 hours to travel just under 150,000,000 km to reach the Earth.
The Earth itself is surrounded by a magnetic field, which can be compared to that of a bar magnet, where lines of force flow out from the south magnetic pole and loop inwards at the north magnetic pole. The magnetic field of the particles in the CME loop (aligned to the suns magnetic field at time of release) interact with the magnetic field of the Earth. If these two magnetic fields are in opposition, one pointing upwards and the other pointing downwards, a strong interaction will occur creating a reconnection point (where the magnetic fields join together).  At this point the particles in the loop of the CME flow along the field toward the poles. The continuous pressure from the solar wind pushes the field carrying the particles around the Earth to a region where the magnetic field is stretched out away from the Sun. The pressure increases and pushes the field lines from the north and the south together in the tail where the fields once again reconnect channelling particles towards the poles.
It is this inflow of particles to the poles which generates the aurora as they collide with the atmosphere. The particles have been accelerated by magnetic reconnection along the Earth’s magnetic field to the upper atmosphere at the poles, when these particles collide with the atmosphere they produce photons that we then see as the aurora.
The aurora is often viewed as a green illumination of the sky, which is a result of the particles interacting with Oxygen. This process also produces Red light, however, this occurs higher up in the atmosphere where there is less oxygen and collisions less frequent allowing the time for the reaction to take place. The process where green light is produced is over 100 times faster than that for red light so is more widely seen (our eyes are also more sensitive to green light).  It is also possible to get Blue aurora which is when the solar particles interact with nitrogen in the atmosphere this is at even greater altitudes than that of the Red aurora and the transition between green to red to blue can produce a beautiful array of colors in the night sky. 

The light we see from the aurora is a beautiful consequence of a monumental journey from the heart of our Sun, to a huge explosion on its surface that sends it hurling through space interacting with any object in its path. These events can cause some serious ramifications, but they also have the benefit of displaying their beauty, something which I was lucky enough to experience last year when this picture was taken. 

For more information on space weather check out the NOAA Space Weather Prediction Center website.

Thursday, January 19, 2012

Fireside Planets

Imagine you are standing on a beach and there is a bonfire at the other end. If someone facing the fire were to walk between you and the fire, the amount of light you would be able to see would decrease; this is similar to the primary eclipse or transit of a planet across a star. If that person, still facing the fire, were then to walk around the fire so that they passed behind it. Just before they disappear behind the flames you will be able to see the fire and the face of the person due to the light that is reflected back. This is similar to what is called the secondary eclipse, when a planet passes behind a star.
Now if this were all viewed with an infrared camera you would also be able to detect any thermal heat from the person just before they walked behind the fire.

That may sound like a long winded analogy, but thus far it has proved to be the best way to try and explain what I do to the rest of my family. What I just describe was the Secondary eclipse which is on the opposite side of the orbit to the transit. It is the period where the planet passes behind the host star and its heated face is eclipsed. During the transit the night side hemisphere is being viewed as the planet proceeds through its orbit towards the secondary eclipse the dayside hemisphere becomes gradually more visible. Just before the secondary eclipse our view is almost entirely of the dayside hemisphere. The observed light is then a combination of the star light and light reflected by and energy generated within the planet.

Secondary eclipse measurements have been made on binary systems for a long time but it was not until 2000 that David Charbonneau from Harvard University suggested that the technique be applied to exoplanets. It was, however, only put to use in 2002 by L. Jeremy Richardson and his team who observed the secondary eclipse of HD209458b using the VLT in Chile. Since then a number of secondary eclipse observations have been made of different exoplanets, but they total in the 10’s and the number of known exoplanets is entering into the 1000’s. Over the next few years, working in the Astrophysics group at Exeter University, using both ground and space based telescopes; I hope to make observations of the secondary eclipse of a number of exoplanets to reveal the dynamics of their atmospheres.

What I also described in the beach fire analogy is something called tidal locking or gravitational locking. This is where two astronomical objects are always facing each other, like the person walking around the fire always facing inwards. There are a number of examples of tidal locking that can be seen in our own Solar System. The moon is tidally locked to the Earth which is why we only ever see one side of it; Jupiter’s five innermost moons are also tidally locked to Jupiter.
A large number of known exoplanets are thought to be tidally locked to their star meaning one side of the planet is in constant sunlight while the other is in relative darkness. The darkside or nightside of the planet is viewed during the transit, while the secondary eclipse gives us a chance to measure the dayside of the planet. From observations of an exoplanets dayside, the temperature can be calculated. This can lead to information about the transport of heat from the dayside to the nightside and potentially the density of cloud structures in the atmosphere. The amount of light that is reflected by the atmosphere can also show us the chemical composition of the upper atmosphere as different elements have different reflective indices at different temperatures.

The depth of the secondary eclipse, however, is very small (roughly 0.2%) making it very difficult to determine without a large number of measurements with high precision instruments. I hope that using both ground and space-based telescopes, I can contribute to the growing number of secondary eclipse observations and expand (if just in a small way) our knowledge of what it might be like on these planets and what it might mean for the evolution of our own solar system.

Monday, January 9, 2012

Going Dark


exmoor-nationalpark.gov.uk
This month in the UK belongs to the sky. BBC Stargazing Live, which airs on Monday 16th at 20:30 and at 20:00 on Tuesday 17th and Wednesday 18th, has sparked a number of events all over Great Britain. 

On Wednesday 18th live on air all the lights in the town of Dulverton, Somerset will be turned off in an attempt to become the first town to turn all of its lights off simultaneously. This is all part of Stargazing Lives’ aim to showcase the beauty of the night sky without air pollution.  All 117 streetlights in the town will be switched off, along with every light in all residences, at around 20:15 on Wednesday night at the sound of the Church bells. 

The switching off of the Dulverton lights will round off a three night spectacular, where the stargazing live team (headed by presenters Dara O’Briain and Prof. Brian Cox) will switch off the lights of many well-known landmarks, Like the Eden Project in Cornwall, to show the magic the sky has without the effects of light pollution. 

As part of Exeter Universities Astrophysics department outreach, a number of colleagues and I, lead by Dr Jenny Hatchell will be taking part in events over the three days and around Exeter in February.  Over the three live events and throughout the month amateur astronomers, stargazers and novices will be welcomes to ‘star parties’ all over the country.  Check out events near you on their website bbc.co.uk/stargazing. Also check out Exmoor National Parks website for some stargazing information http://www.exmoor-nationalpark.gov.uk/visiting/things-to-do/stargazing

Jan 17th (17:00 – 20:30): Dunster Tithe Barn, Exmoor National Park, public event. 
A group from Exeter Astrophysics Department will have a number of telescopes 
and Binoculars, meteorites, and hands on planets for the public to use and ask 
questions with the experts, as well as a from Earth to the Universe exhibit.
 
Jan 18th (18:00-20:00): Community lights out event in Dulverton, Somerset for 
the BBC stargazing live broadcast star trail event. As with the previous event 
at Dunster there will be the Earth to the Universe exhibit and a number of 
hands on activities showcased by Exeter Universities Astrophysics department.   
 
(Both these events are expected to attract a large number of amateurs and all who wish to attend are more than welcome).

Feb 10th (18:30-20:00): St Luke’s Science and Sports College, Exeter are hosting an evening with the stars. The event is open to the community as well as school students with the astronomy club holding rocket building competition as well as opportunities to visit their ‘Space Odyssey’ mobile planetarium. A number of Astrophysicists from the University will also be there to give presentations and answer any questions.

Hope to see you there and feel free to comment about your own stargazing events :)