Powerful geomagnetic storms have been buffeting the earth over the past week, due to a huge cluster of sunspots which have formed on the sun's surface. The sunspots have launched a number of very powerful flares, ejecting clouds of particles that outpace the solar wind and deliver the aurora borealis.
There are about 320 sunspots on the solar surface but the large group causing the storms covers an enormous area, measuring 22 earth diameters across. It crossed the solar meridian earlier this week and at this stage should just have passed the sun's western edge, explained Dr Ian Elliott, a staff member at Dunsink Observatory.
A large explosion or "coronal mass ejection" (CME) occurred last Sunday and sent a cloud of high-energy particles slamming into our magnetosphere on Tuesday. This event was ranked as an "M-5", at the upper end of the medium-energy CMEs.
The cluster has kicked out two other M-class CMEs but also a much larger X-class explosion a week ago that delivered an aurora borealis display seen as far south as Mexico.
The peak of sunspot activity was meant to have occurred last year as the 11-year cycle reached "solar maximum", but there is still plenty of energy left as the process approaches its downward slope towards "solar minimum".
The European Ulysses spacecraft, the only satellite that flies over the solar poles, has been keeping watch and the latest results from Ulysses were presented in Cambridge yesterday during the UK Solar Physics Meeting. A key experiment on board the satellite is a magnetometer developed by Imperial College, London, which measures magnetic fields in space. This device has been watching as a key element of the sunspot cycle takes place, when the sun reverses the direction of its magnetic field.
The latest data shows that the transition from a northward oriented magnetic field to a south oriented one is currently underway. It can take several months and has implications for the severity of the magnetic storms discharged from the solar surface. It is the completion of this reversal that marks the next phase of the solar activity cycle.
Sunspots form above distortions in the complex solar magnetic field. The powerful fields block the normal circulation of heat from the sun's interior and allow cooler patches to form. These are still intensely bright but appear dark compared to hotter surrounding areas.
Sudden and gigantic releases of magnetic energy deliver the CMEs and the billions of tonnes of particles that accompany these discharges. The flares occur within a period of seconds, accelerating particles to very high velocities and generating phenomenal heat.
First to leave the pack are Xrays and ultraviolet wavelengths, Dr Elliott explained, and these reach the earth in just eight minutes. They disturb the ionosphere, in the process affecting shortwave radio transmissions.
These are followed in about an hour by protons, heavy particles carrying energies measuring millions of electron volts. The next wave is the electrons and ions which reach us 30 hours after a CME and which deliver the magnetic storms associated with sunspot activity.
Their large particle cloud has a powerful magnetic field that interacts with the earth's field, inducing potentially damaging currents in power lines and the delicate electronics of orbiting satellites. These particles also bring the beautiful Northern Lights.