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More than sunshine

Lucie Green, Dr. of solar physics & Royal Society University Research Fellow
Apr 26, 2012, 7:00 UTC

The Sun’s influence reaches billions of kilometres to interstellar space, and its effect on the solar system is more complex than its gravitational forces.

Beyond its gravitational influence the Sun affects the atmospheres and magnetic fields of all the planetary bodies in its empire as the solar wind blasts past the planets at mind-blowing speeds toward interstellar space.

Here on Earth, the constant visual luminescence of our life-giving Sun hides a darker side of violent activity that is capable of damaging our technological infrastructure.

To understand the impact of the Sun on Earth it’s important to describe the different types of solar activity.

Solar wind
We have known since the early days of the space age that the atmosphere of the Sun is much hotter than its surface, meaning that the hot gases are constantly expanding out into space forming a solar wind. The flow of gas takes with it magnetic fields that fill the solar atmosphere. The wind only stops blowing when it encounters the interstellar medium and forms a vast bubble in space in which the Solar System resides.

The Earth is constantly being buffeted by this gusty solar wind that blows with speeds of several hundred kilometres per second.

The vast bubble of solar wind is known as the heliosphere and NASA’s Voyager spacecraft are now exploring its far-reaches almost 40 years after they were launched.

Coronal Mass Ejections (CMEs)
More recently we discovered that the solar wind isn't the only kind of outflow from the Sun that the Solar System gets subjected to. In 1971, ejections of immense bubbles of magnetic field containing charged particles were discovered blasting away from the Sun. These eruptions travel with speeds of up to 2000 kilometres per second, quickly expand to become many times larger than the Sun itself and are referred to as coronal mass ejections (CMEs).

CMEs are bulk eruptions of bubbles of magnetic field and gas from the solar atmosphere. The gas and the magnetic field are tied together. These eruptions take anywhere between 1 and 4 days to reach the Earth. We see them leave and have some time to prepare. When they reach us the magnetic field of the CMEs interacts with that of the Earth, and the particles of the solar gas spiral onto the Earth's magnetic field lines, driving space weather effects such as problems with satellites, power lines, changes to the ionosphere and more. If the particles that spiral along the Earth's field lines make it all the way down to the atmosphere, they can energise the atmosphere gases and make them glow producing the aurora.

CMEs can head in any direction, including toward the Earth. We have been hit by coronal mass ejections many times in the past and will continue to be hit in the future. 

CME affecting Earth's magnetosphere
The Sun’s magnetic field and releases of plasma directly affect Earth and the rest of the solar system. Solar wind shapes the Earth’s magnetosphere and magnetic storms are illustrated here as approaching Earth. The white lines represent the solar wind; the purple line is the bow shock line; and the blue lines surrounding the Earth represent its protective magnetosphere. The magnetic cloud of plasma can extend to 30 million miles wide by the time it reaches earth. Credit: SOHO/ESA/NASA

Solar prominence
A solar prominence is the name given to clouds of relatively cool gas that are held aloft in the Sun's hot atmosphere. We think today that the gas is held up by dips in the magnetic field - everything comes back to magnetic fields! Prominences are the name given to these features when they are observed at the edge of the Sun. Sometimes the magnetic field of the prominence becomes unstable and it erupts upward away from the Sun carrying the gas with it - a coronal mass ejection is born!

So, prominence eruptions are a subset of all coronal mass ejections. If these reach the Earth the resulting space weather can be strong because they carry so many particles from the Sun's atmosphere.

NASA SDO image of a solar prominence 16 April 2012 
Solar activity on April 16, 2012 was captured here by NASA's Solar Dynamics Observatory in the 304 Angstrom wavelength, which is typically colored in red. Credit: NASA/SDO/AIA

Solar Flares 
Solar flares are the result of a release of energy that has been stored in the magnetic fields in the Sun's atmosphere. The energy goes into accelerating particles and heats the gases which results in the production of radiation across the electromagnetic spectrum, from gamma to radio.

The UV and X-ray radiation produced cause changes to the Earth’s upper atmosphere - specifically it causes the ionosphere to change which leads to space weather effects such as loss of radio communications which rely on the ionosphere to be in the right state to reflect radio waves.

This electromagnetic radiation takes about 8 minutes to reach the Earth. When we see it is already here!

The impact of coronal mass ejections on Earth
When a CME reaches the Earth it encounters the Earth’s magnetic field, the magnetosphere, that acts as our natural shield. However, under the right conditions the magnetic field in the coronal mass ejection joins together with the Earth’s allowing the charged particles it carries to stream in. Our magnetosphere is a leaky shield. The disruption to the magnetosphere that follows creates a complex system of electric currents and flow of charged particles that impacts our technological infrastructure in a multitude of ways. Luckily for us an international team of scientists has developed a good understanding of how the Earth responds when this happens using a fleet of spacecraft including NASA’s Solar Dynamics Observatory, the joint ESA/NASA SOHO spacecraft and ESA’s Cluster satellites.

The currents created high above our heads accelerate electrons downward along the Earth’s magnetic field lines toward the poles. When the particles come crashing down into our atmosphere, they excite the gases there and cause them to glow producing the aurora. However, the particles can also damage our satellites, even leading to the total loss of the satellite.

Currents created at ground level can flow through our power lines causing problems for national power grids. Most notably, in 1989 a transformer in the Canadian national grid failed due to such currents and several million people lost their electricity for nine hours. But the largest solar event on record took place in 1859 when the cutting edge technology was the telegraph system and so only resulted in electric shocks and fires in the offices. A repeat of a storm this size today would have significant consequences across the developed world and lead to trillions of US dollars of damage.

Awareness of the 1859 event has spread rapidly in the last few years and has sparked a fear that the coming solar maximum will spark the perfect solar storm. However, many aspects need to come together to produce the ‘perfect solar storm’ which makes such an event hard to forecast. The coronal mass ejections need to have the right magnetic configuration and it needs to hit us at high speed.  

So, coronal mass ejections are very relevant to us on a day-to-day basis but they are also important for the global evolution of the Sun’s magnetic field.

The Sun has a magnetic cycle that lasts roughly 11 years during which time its magnetic field pulses in size and complexity.Coronal mass ejections act as a valve to release energy stored in the magnetic field and so the number of coronal mass ejections varies along the solar cycle. As many as five CMEs could be launched each day at cycle maximum and maybe one at minimum. At the moment we are approaching solar maximum (expected to occur around 2013) which means that the number of eruptions is on the rise and so too is the likelihood that we feel some effects here on Earth.  

The Sun has been around for 4.5 billion years though and has variability over timescales many times longer than the 11 year cycle. In fact, the Sun has only recently come out of the deepest solar minimum for 100 years. Things were very quiet indeed. Throughout the highs and lows of solar activity the Sun will continue to exert its dominance in the Solar System and we should be studying its every move.