Satellites have revealed that the Earth is bombarded by swarms of particles ejected from the Sun at supersonic speeds. Most of these cosmic “bullets” in the solar wind are deflected by the planet’s magnetic shield—the magnetosphere. The solar wind squeezes the magnetic envelope and pushes in back towards the planet. Some energetic particles penetrate the envelope through its tail end and appear as aurora, the curtains of light over the polar areas.
It is truly said that our Earth is cradled in the solar corona! In other words, the corona extends to envelope the Earth’s magnetic envelope, shaping up as a giant tear drop in the sky. Called the coronal mass ejections (CMEs), the charged particles from the Sun—the solar wind—constantly flow out of the corona at 500 km/second and warp the magnetic field of the Earth, and enter its upper atmosphere.
The temperature of the hot corona—an ionized gas mainly confined by the Sun’s magnetic field—is several million Kelvin, some three orders of magnitude higher than the temperature of the visible solar surface. Several questions arise as a result. Where is the energy generated? How does the generated energy propagate? How is the solar wind generated and accelerated to envelope the Earth’s magnetosphere? One possible explanation is provided by the Alfven waves (electromagnetic waves propagated along lines of magnetic force in a plasma) predicted by Hannes Alfen for which he received a Nobel Prize in 1970. It is now thought that the chromosphere, the region between the solar surface and the corona, is permeated by Alfen waves estimated at 10 to 25 km a second with periods varying from 100 to 500 seconds. Such waves are energetic enough to speed up the solar wind and heat up the corona.
The strongest solar storms occur during the crests in the 11-year solar cycle of activity. Severe storms can disrupt satellites and short-circuit their electronics. Astronauts on the Moon or Mars would face dangerous particles. Recently launched satellites provide new insights into the interaction between the electrically charged particles of the solar wind and the Earth’s magnetic field. The key to this scientific revolution is Cluster’s ability to fly in close formation along elongated orbits which take them between 19,000 and 119,000 km from our planet—almost one-third the way to the Moon fro the Earth. During their passage around the Earth, the spacecraft are sometimes inside the Earth’s magnetic shield and sometimes outside, fully exposed to the supersonic solar wind.
In 1996, the Solar and Heliospheric Observatory (SOHO)—a joint NASA and ESA mission—was in orbit of the Lagrangian point, the L1. From that vantage point, SOHO observed the Sun and in 2003, gave several hours of warning of a major solar storm. However, several solar storms cannot be predicted. A continuous close watch of the Sun with modern instrumentation has become essential. Solar scientists hope to increase the one-hour of warning (now believed possible) of an impending storm of energetic ions.
Efforts are on to launch a telescope in a balloon in 2009 to about 40 km above the Earth and photograph solar features as small as 30 km across, which is four times better than the best solar telescope in space and twice as good as the best on Earth. The High Altitude Observatory at the National Centre for Atmospheric Research in Boulder, Colorado (USA) and several European institutions are planning to study particularly solar faculae—strong magnetic bright spots on the solar surface—which push out the plasma towards the Earth.
According to Prof. Eugene Parker of the University of Chicago, well-known for his study of the solar wind, there is much physics that has to be still learnt about the generation of magnetic fields on the Sun. The magnetic fields are carried with the streaming ionized gas of the Sun. the heliosphere is the region created by the supersonic solar wind. It is an expansion of the solar corona, carrying the magnetic field along with it. Interestingly, the magnetic field substantially reduces the cosmic ray intensity near the Earth. Thus increased solar activity reduces the cosmic ray intensity. This is an area worthy of further research, as the full impact of cosmic rays on astronauts and satellites in space has yet to be known.
Thanks to new techniques like adaptive optics, the resolution that can be obtained from optical telescopes on the ground is almost equal to that of satellite-based telescopes. Given this new advantage, ground-based telescopes, working in the optical region of the spectrum can be free of the constraints of a satellite including the need to ensure a stable orbit and limited payload capacity. Moreover, permanent telescopes on the ground would be useful to follow up the phenomena discovered by satellites and by missions to Antarctica. And several new phenomena like solar oscillations are being discovered, awaiting a permanent facility for observation.