SOLAR STORMS, described above, are spectacular outbursts due to the sudden transformation of a considerable amount of energy (about 1026 joules) into electromagnetic and charged-particle radiation within an hour or so. A typical event is a solar flare, which, apart from the burst of radio, optical and X-radiation, releases electrons and protons with energies that can range up to l00GeV. The highest energy, relativistic particles, arrive at the Earth almost simultaneously with the visible activity and they can be recorded by neutron monitors located below ground. These neutrons do not come directly from the Sun; they are produced when the solar cosmic-ray protons smash into the terrestrial atmosphere at height of about 20 km. Twentyfold increases in counting rate have
been recorded in intervals of a quarter of an hour or so.

The sudden local injection of energy into the solar wind above a flare creates a shock wave that propagates out to the distance of the Earth in about two days. The resultant sudden compression of the magnetosphere is recorded on the ground by a sudden change in the magnetic field at the surface. This is the sudden commencement of a magnetic storm. This change reverses itself later as the plasma sweeps over the magnetosphere, and partially replenishes the radiation belts and changes the currents flowing in them. The MAGNETIC STORM, as this sequence of events is termed, takes several days to run its course, after which Earth’s magnetism returns to normality.

Cosmic rays are reflected by the distortion of the interplanetary magnetic field associated with the shock wave spreading out in the interplanetary space and this gives rise to a decrease in detection rate. Changes occur in the ionosphere as well as radio fadeouts over the sunlit hemisphere. Polar blackouts, in which cosmic radio noise is strongly attenuated at latitudes above the arctic circles, are frequently observed.

Disturbances of the geomagnetic field are extremely complicated and have been recorded for centuries. A measure of worldwide activity is given by the KP INDEX ; this is a three-hourly mean of fluctuation in the magnetic field observed at twelve stations between latitudes 40° and 63°. Many of these disturbances are caused by currents flowing in the magnetosphere and ionosphere, among which is that due to the opposing drifts of positively and negatively charged particles in the radiation belts.

A large variety of everyday phenomena have been correlated with solar activity, and the 11-year solar cycle. These include the prices of shares and certain commodities, political upheavals, geophysical effects such as earthquakes, and the weather. Many of the claimed relationships are undoubtedly spurious, but there is an increasing body of evidence to suggest that some aspects of the weather are linked to activity on the Sun. The precise mechanism is unknown, but the cycle is evident in counts of tree rings. More recently it has been found that the large-scale movements of air masses over the Earth are regulated by the sectorial structure.Changes are noted a few days after a sector boundary has swept across the Earth’s magnetosphere.

There is a notable similarity between solar and magnetic storms in that magnetic field energy is converted into particle energy. 1 his occurs in a solar flare, which later triggers a similar conversion in the Earth’ magnetotail. Satellite measurements in this latter region can provide direct observations of a universally occurring phenomenon, the conversion of magnetic energy to kinetic energy, that is currently little understood.

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