Magnetosphere & Solar Protons
Occasionally, very intense bursts of activity occur on the Sun, known as solar flares, which cause vast quantities of radiation to be ejected. This radiation from the sun consists of both photons of electromagnetic radiation component (x-rays, gamma rays, UV-light, etc.) and particulate radiation (neutrons, protons, electrons etc). The electromagnetic part of the radiation travels at the speed of light and so reaches the Earth's orbit at 150 million km from the Sun approximately 8.5 minutes after the occurrence of the flare. This electromagnetic radiation component usually gives the first warning that a flare has occurred.
The particle component of the radiation (i.e. the solar cosmic-rays) follows on some time later, depending on the energy of the particles, which are mostly travelling at relativistic energies close to the speed of light. The neutrons, being electrically neutral, travel in straight lines from the Sun and are unaffected by magnetic fields. Only the very energetic (ie. The fastest) neutrons will actually reach the Earth, most having decayed into protons in the time taken to travel between the Sun and the Earth (the half life for a free neutron is 12.9 minutes).
Solar protons, being charged particles, will follow a path to the Earth along the magnetic field lines from the Sun. If the Earth happens to lie on a field line which is connected to the flare region on the Sun, the flare particles will arrive at the Earth promptly causing a solar-particle event (SPE). If the Earth is not magnetically connected to the flare region, the particles may diffuse through space and still reach the Earth, or alternatively, they may miss the Earth altogether. Therefore, even if a flare is observed on the Sun, the particles associated with it will not necessarily arrive at the Earth. Similarly, particles can arrive at the Earth associated with solar flares occurring on the side of the Sun hidden from view - i.e. with no prior warning.
When flare particles arrive, they can cause a dramatic increase in the radiation dose received by spacecraft and high-altitude aircraft. Intense geomagnetic storms may be associated with such events producing highly visible aurorae at middle to low latitudes. Such storms can also disrupt radio communications and can even cause overloads in the national grid. However, the precise effect of the event will depend upon the maximum intensity and duration of the solar flare, together with the relative intensity of the radiation reaching Earth.
The frequency of occurrence of solar flares is well correlated with the sun-spot (or solar) cycle, which has traditionally been observed to vary on an approximately 11 year cycle. The last complete solar cycle (called cycle number 22) reached its maximum in 1989 and a minimum around 1996-1997. We are currently (2002) at the solar maximum in cycle 23, and solar activity is expected to remain high through this year. However the occurrence of major solar flare events is unpredictable, although the likelihood rises during the seven year period around the solar cycle maximum.
The data in the graph show a real solar proton event recorded in the University's PoSat-1 during mid July 2000. This satellite contains a dedicated proton radiation detector, the Cosmic Ray Experiment (CRE), that records the number of energetic protons passing through the satellite as a function of time. The image shows some of these data as a histogram of events per unit time, during July 14-15 2001, just at the time when a particularly powerful solar proton burst reached the Earth.
When particles from a solar flare reach the Earth, the highly relativistic particles (those which have kinetic energies much greater than their rest mass energy), which are travelling at speeds close to the speed of light, will arrive at the Earth several minutes after the flare has erupted. Lower energy particles travelling at over 1,000 km/s will arrive at the Earth's orbit within a matter of hours. The flux of solar particles seen at the Earth will thus increase dramatically over a period of hours, before decaying to background levels, usually over the next few days.