Aurora Borealis
Why are aurora visible only at the north and south poles and not in other places?
J. JEYAKRISHNAN,
Tiruchi, Tamil Nadu
The physics and the mechanism of this phenomenon is now understood to be due to the charged particles emitted by the Sun in the direction of earth which in turn is endowed with a magnetic field of its own.
The systematic features of the observed aurora elucidate the different processes involved. The observation of auroras only in the dayside directly points to the involvement of the Sun in the phenomenon.
The auroras appear as sheets of illuminated curtains with streaks or as bright arcs in the sky. These lines or arcs are in the form and directions of the magnetic lines of force of the earth; indicating the involvement of moving charge particles under the influence of concentrated magnetic field of the earth.
Very recently intelligent and pointed experiments have been conducted; and the results reveal that the charged particles produced in the solar atmospheric activities occasionally escape, especially through the magnetic holes near the sunspots. Some of these travel in the direction of the earth under the influence of the interplanetary magnetic field.
When they reach the earth, they get concentrated near the poles because of the strong axial magnetic fields there. In other regions of the earth, the magnetic field is weaker and so the aurora does not form although the emission of the solar wind particles takes place uniformly.
H.K.SAHU
Indira Gandhi Centre for Atomic Research
