Solar Storms

21 04 2009

The sun is our constant companion; it supplies heat and light to us and keeps our small blue/green world from freezing. However, as serene as the sun seems on a lazy summers day, a close look at its surface tells a very different story.

Sunspot (with Earth for size)

Sunspot (with Earth for size)

The surface of the sun is a roiling, turbulent, ever-changing environment. Pockets of superheated gas larger than the Earth rise to the surface and sink again within a matter of hours. The colossal magnetic field of the sun can even physically lift some of the superheated gas right off the surface, wrap it into coils and fling it off into space at hundreds, if not thousands of miles per hour. If this ejecta is flung at the Earth, things down here on the surface can get very interesting indeed. Space weather is now a full time area of study for astronomers – trying to predict when the next major solar flare will happen and what we could possibly do to mitigate its effects.

New Scientist magazine ran an article entitled ‘Space storm alert: 90 seconds from catastrophe’ in their 21st March issue. It was full of dire predictions of death, disaster and the collapse of society if such an event were to occur today. But how true is this scenario? For example, if the next solar cycle, due to reach maximum in 2012, were to produce such a space storm, what would happen to our everyday lives?

Magnetosphere

Magnetosphere

A truly massive solar event would swamp out radio emissions, as the sun increased its output of radio frequencies, followed shortly after by satellites’ circuitry shorting out due to induced current because of the increase in electromagnetic radiation. The Earths magnetic field would warp, bend and reconfigure, causing showers of highly energetic particles to shower down into the upper atmosphere, causing brilliant aurorae that could be seen as far south as the tropics. Induced currents along high-voltage power lines would flood transformer stations, causing the breakers and transformers to melt, creating blackouts across entire cities. Any humans in space would, if inadequately shielded, be exposed to radiation that could kill them very quickly.

Following on from this, if we cannot get the grid back online, there will be no power, no heating; without the ability to pump gas, water or petrol, essential services will vanish within a couple of days, hospitals will run dry of essential medicines and starvation and disease will ramp up, as food and medical supplies cannot be transported.

Sounds like a pretty doomsday scenario, and it is something that we should be concerned about; after all, the sun is well known as a troublemaker. In both 1972 and 1989, telecommunications, electricity grids and other services were severely disrupted. On 13 March 1989 a severe geomagnetic storm caused the collapse of the Hydro-Québec power grid in a matter of seconds as equipment protection relays tripped in a cascading sequence of events. Six million people were left without power for nine hours, with significant economic loss. The storm even caused auroras as far south as Texas. The geomagnetic storm causing this event was itself the result of a Coronal Mass Ejection, ejected from the Sun on March 9, 1989. When magnetic fields move about in the vicinity of a conductor such as a wire, a geomagnetically induced current is produced into the conductor. This happens on a grand scale during geomagnetic storms (the same mechanism also influences telephone and telegraph lines). Power companies transmit alternating current to their customers via long transmission lines. The nearly direct currents induced in these lines from geomagnetic storms are harmful to electrical transmission equipment, especially to the transformers; it overheats their coils and causes saturation of their cores, constraining their performance; it also tends to trip various protective devices. Potentially the heat generated in the iron cores of the generators can destroy them and chain reaction could blow transformers throughout a system.

The solar wind also carries with it the magnetic field of the Sun. This field will have either a North or South orientation. If the solar wind has energetic bursts, contracting and expanding the magnetosphere, or if the solar wind takes a southward polarisation, geomagnetic storms can be expected. The southward field causes magnetic reconnection of the dayside magnetopause, rapidly injecting magnetic and particle energy into the Earth’s magnetosphere.

During a geomagnetic storm, the ionosphere will become unstable, fragment, and may even disappear. In the Northern and Southern pole regions of the Earth, auroras will be observable in the sky.

Solar Flare (Hinode)

Solar Flare (Hinode)

Intense solar flares release very-high-energy particles that can cause radiation poisoning to humans in the same way as low-energy radiation from nuclear blasts. Earth’s atmosphere and magnetosphere allow adequate protection at ground level, but astronauts in space are subject to potentially lethal doses of radiation. The penetration of high-energy particles into living cells can cause chromosome damage, cancer, and a host of other health problems. Large doses can be fatal immediately. Solar protons with energies greater than 30 MeV are particularly hazardous. In October 1989, the Sun produced enough energetic particles that an astronaut on the Moon, wearing only a space suit and caught out in the brunt of the storm, would probably have died; the expected dose would be about 7000 rem. (Astronauts who had time to gain safety in a shelter beneath moon soil would have absorbed only slight amounts of radiation.) The cosmonauts on the Mir station were subjected to daily doses of about twice the yearly dose on the ground, and during the solar storm at the end of 1989 they absorbed their full-year radiation dose limit in just a few hours.

Solar proton events can also produce elevated radiation aboard aircraft flying at high altitudes. Although these risks are small, monitoring of solar proton events by satellite instrumentation allows the occasional exposure to be monitored and evaluated, and eventually the flight paths and altitudes adjusted in order to lower the absorbed dose of the flight crews.

Geomagnetic storms and increased solar ultraviolet emission heat Earth’s upper atmosphere, causing it to expand. The heated air rises, and the density at the orbit of satellites up to about 1000 km increases significantly. This results in increased drag on satellites in space, causing them to slow and change orbit slightly. Unless Low Earth Orbit satellites are routinely boosted to higher orbits, they slowly fall, and eventually burn up in Earth’s atmosphere.

Should we really be worried?

The worst solar storm on record, now known as the Carrington Event after the man who witnessed it, was an unprecedented eruption from the surface of the sun. It happened without warning and it was only because Richard Carrington was studying the sun that morning that we even knew that what was to unfold was caused by the sun.

On the morning of the 1st September 1859, Carrington was observing the sun. He was an amateur astronomer, widely regarded as one of the finest solar astronomers alive. Using the projection method of viewing the sun, he was tracing the outlines of sunspots; in particular, a large group of sunspots near the equator of the sun. As he was tracing these sunspots, he saw a brilliant white flash of light, quickly followed by another bright flash that lingered for almost five minutes.

Within 24 hours, the Earth was subjected to a geomagnetic storm the likes of which hasn’t been since. Skies all over Earth erupted in auroras that were seen as far south as Hawaii, Mexico, Cuba, and Italy. Such solar outbursts normally take between 3 to 4 days to reach Earth; this one managed the 93 million mile journey in less than 24 hours.

Even more worrying, telegraph systems worldwide went berserk. Electrical discharges sparked along the wires and caused fires. In some cases, the telegraph wires melted from the increased current.

In the 150 years since the Carrington Event, there has been nothing even close to it. It’s now possible to look back in time by examining arctic ice as energetic particles leave a nitrate record in arctic ice cores. The Carrington Event was the biggest event in 500 years and nearly twice as big as the next largest. Are we in danger? Potentially. The current solar cycle appears to be fairly quiet compared with the past few cycles, so the sun may be entering a ‘quiet phase’. Ice cores show these events seem to have a half-millennium cycle, so we could be safe for another 350 years, but it’s always wise to be prepared.

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