Sun's coronal hole
By Mark Prigg
There were no fireworks on the sun to welcome in the New Year - and in fact, scientists say the end of the year was relatively quiet on the solar surface.
However, the sun has started 2015 with a mysterious event - a huge hole has appeared.
Known as a coronal hole, the phenomenon occurred near the south pole - and is seen as a dark area covered all of its base in these stunning images.
The incredible image was captured on Jan. 1, 2015 by the Atmospheric Imaging Assembly (AIA) instrument on NASA's Solar Dynamics Observatory, shows the coronal hole as a dark region in the south.
Coronal holes are regions of the corona where the magnetic field reaches out into space rather than looping back down onto the surface.
Particles moving along those magnetic fields can leave the sun rather than being trapped near the surface. Those trapped particles can heat up and glow, giving us the lovely AIA images.
In the parts of the corona where the particles leave the sun, the glow is much dimmer and the coronal hole looks dark.
Coronal holes were first seen in images taken by astronauts on board NASA's Skylab space station in 1973 and 1974. They can be seen for a long time, although the exact shape changes all the time. The polar coronal hole can remain visible for five years or longer.
Each time a coronal hole rotates by the Earth we can measure the particles flowing out of the hole as a high-speed stream, another source of space weather.
Charged particles in the Earth's radiation belts are accelerated when the high-speed stream runs into the Earth's magnetosphere.
The acceleration of particles in the magnetosphere is studied by NASA's Van Allen Probes mission.
As Solar Cycle 24 fades, the number of flares each day will get smaller, but the coronal holes provide another source of space weather that needs to be understood and predicted.
Coronal holes are a typical feature on the sun, though they appear at different places and with more frequency at different times of the sun's activity cycle.
The holes are important to our understanding of space weather, as they are the source of a high-speed wind of solar particles that streams off the sun some three times faster than the slower wind elsewhere.
While it's unclear what causes coronal holes, they correlate to areas on the sun where magnetic fields soar up and away, failing to loop back down to the surface, as they do elsewhere.
The material constantly flowing outward is called the solar wind, which typically 'blows' at around 250 miles (400 km) per second.
When a coronal hole is present, though, the wind speed can double to nearly 500 miles (800 km) per second.
Late last year one of Nasa's most powerful space telescopes has turned its gaze on the Sun for the first time to capture this stunning image.
X-rays stream off the sun in this image showing observations from by NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, overlaid on a picture taken by NASA's Solar Dynamics Observatory (SDO). The NuSTAR data, seen in green and blue, reveal solar high-energy emission (green shows energies between 2 and 3 kiloelectron volts, and blue shows energies between 3 and 5 kiloelectron volts).Nasa's Nuclear Spectroscopic Telescope Array, or NuSTAR, has produced the most sensitive solar portrait ever taken in high-energy X-rays. The mission is primarily designed to look at black holes and other objects far from our solar system.
'NuSTAR will give us a unique look at the sun, from the deepest to the highest parts of its atmosphere,' said David Smith, a solar physicist and member of the NuSTAR team at University of California, Santa Cruz.
Solar scientists first thought of using NuSTAR to study the sun about seven years ago, after the space telescope's design and construction was already underway, but before the telescope launched into space in 2012.
Smith had contacted the principal investigator, Fiona Harrison of the California Institute of Technology in Pasadena, who mulled it over and became excited by the idea.
'At first I thought the whole idea was crazy,' says Harrison.
'Why would we have the most sensitive high energy X-ray telescope ever built, designed to peer deep into the universe, look at something in our own back yard?' Smith eventually convinced Harrison, explaining that faint X-ray flashes predicted by theorists could only be seen by NuSTAR.
While the sun is too bright for other telescopes such as NASA's Chandra X-ray Observatory, NuSTAR can safely look at it without the risk of damaging its detectors.
The sun is not as bright in the higher-energy X-rays detected by NuSTAR, a factor that depends on the temperature of the sun's atmosphere.
This first solar image from NuSTAR demonstrates that the telescope can in fact gather data about sun.
And it gives insight into questions about the remarkably high temperatures that are found above sunspots -- cool, dark patches on the sun.
Future images will provide even better data as the sun winds down in its solar cycle.
'We will come into our own when the sun gets quiet,' said Smith, explaining that the sun's activity will dwindle over the next few years.
With NuSTAR's high-energy views, it has the potential to capture hypothesized nanoflares -- smaller versions of the sun's giant flares that erupt with charged particles and high-energy radiation.
Nanoflares, should they exist, may explain why the sun's outer atmosphere, called the corona, is sizzling hot, a mystery called the 'coronal heating problem.'
The corona is, on average, 1.8 million degrees Fahrenheit (1 million degrees Celsius), while the surface of the sun is relatively cooler at 10,800 Fahrenheit (6,000 degrees Celsius). It is like a flame coming out of an ice cube. Nanoflares, in combination with flares, may be sources of the intense heat.
If NuSTAR can catch nanoflares in action, it may help solve this decades-old puzzle.
'NuSTAR will be exquisitely sensitive to the faintest X-ray activity happening in the solar atmosphere, and that includes possible nanoflares,' said Smith.
What's more, the X-ray observatory can search for hypothesized dark matter particles called axions. Dark matter is five times more abundant than regular matter in the universe.
Everyday matter familiar to us, for example in tables and chairs, planets and stars, is only a sliver of what's out there. While dark matter has been indirectly detected through its gravitational pull, its composition remains unknown.
It's a long shot, say scientists, but NuSTAR may be able spot axions, one of the leading candidates for dark matter, should they exist.
The axions would appear as a spot of X-rays in the center of the sun.
Meanwhile, as the sun awaits future NuSTAR observations, the telescope is continuing with its galactic pursuits, probing black holes, supernova remnants and other extreme objects beyond our solar system.
This 04 November, 2003 image shows giant sunspot 486 (lower-R) 486 unleashing another powerful solar flare. Ionizing radiation hit Earth's atmosphere soon after the explosion and caused a severe radio blackout, which radio listeners noticed across North America.
The images comes weeks after warnings that Earth could be hit by a series of damaging solar flares after the largest sunspot to be seen on the star for 24 years aligns with our planet.
The sunspot, previously known as Active Region 12192, began facing Earth in October but did not produce any coronal mass ejection (CMEs).
CMEs are the most energetic events in our solar system, involving huge bubbles of plasma and magnetic fields being spewed from the sun's surface into space.
The region, renamed Active Region 12192, has now rotated around to face Earth again, and is likely to create CMEs, Nasa scientist Holly Gilbert told Space.com during a video interview.
'This time around, it's more likely to have some coronal mass ejections associated with it, even though the solar flares might be smaller,' she said.
'We have a good idea, based on the structure of that magnetic field and the sunspot, that it's very possible that it will create some mid-level flares.'
Magnetic fields in sunspots can store vast amounts of energy, but looping magnetic field lines can get tangled up and snap, releasing their energy as explosions called flares.
According to Dr Gilbert, the sunspot is still large enough for 10 Earths to fit inside it, and is believed to be the 33rd largest of 32,908 active regions recorded since 1874.
The Jupiter-sized sunspot produced six eruptions in October and early November, before disappearing for two weeks.
Earlier this year, Ashley Dale, who is a member of an international task force, dubbed Solarmax, warned that solar 'super-storms' pose a 'catastrophic' and 'long-lasting' threat to life on Earth.
A solar superstorm occurs when a CME of sufficient magnitude tears into the Earth's surrounding magnetic field and rips it apart.
Such an event could induce huge surges of electrical currents in the ground and in overhead transmission lines, causing widespread power outages and severely damaging critical electrical components.
Earth could be about to be hit by a series of damaging solar flares as a huge sunspots aligns with the planet
Mr Dale, carrying out doctoral research in aerospace engineering at Bristol University, said it is only a 'matter of time' before an exceptionally violent solar storm is propelled towards Earth.
He says such a storm would wreak havoc with communication systems and power supplies, crippling vital services such as transport, sanitation and medicine.
Without power, people would struggle to fuel their cars at petrol stations, get money from cash dispensers or pay online,' he said.
'Water and sewage systems would be affected too, meaning that health epidemics in urbanised areas would quickly take a grip, with diseases we thought we had left behind centuries ago soon returning.'
The largest ever solar super-storm on record occurred in 1859 and is known as the Carrington Event, named after the English astronomer Richard Carrington who spotted the preceding solar flare.
This massive CME released about 1022 kJ of energy - the equivalent to 10 billion Hiroshima bombs exploding at the same time - and hurled around a trillion kilos of charged particles towards the Earth at speeds of up to 3000 km/s.
However, its impact on the human population was relatively benign as our electronic infrastructure at the time amounted to no more than about 124,000 miles (200,000 km) of telegraph lines.
Mr Dale says these types of events are not just a threat, but inevitable.
Nasa scientists have predicted that the Earth is in the path of a Carrington-level event every 150 years on average.
This means that we are currently five years overdue - and that the likelihood of one occurring in the next decade is as high as 12 per cent.