Answer the following questions .
a. The earth's magnetic field varies from point is space. Does it also change with time ? It so , on what time scate does it change appreciably ?
b. The earth's core is known to contain iron. Yet geologists do not regard this as a source of the earth's magnetism why ?
c. The charged currents in the outer conducting regions of the earth's core are thought to be responsible for earth's magnetism . What might be the ' battery ' (i.e., the source of energy) to sustain these currents ?
d. The earth may have even reversed the direction of its field several times during its history of 4 to 5 billion years. How can geologists know about the earth's field in such distant past ?
e. The earth's field departs from its dipole substantially at large distances (greater than about 30,000 km) what agencies may be responsible for this distortion ?
f. Interstellar space has an extremely weak magnetic field of the order of `10^(-12)` T, Can such a weak field be of any significant consequence ? Explain.
[Note : Exercise 2 is meant mainly to arouse your curiosity . Answers to some question above are tentative or unknown . Brief answers wherever possible are given at the end . For details , you should consult a good text on geomagnetism.]

Explanation

a. Yes Yearly variations or variations in few years . For appreciable change , it may take a few hundred years.
b. Molten iron is ferromagnetic
c. Not sharply known, possibly the radioactivity in the interior core.
d. From the magnetic properties of some rocks.
e. the motion of ions in ionosphere.
f. Even if the is weak, it has cosmological importance. The new particles created in interstellar space will be accelerated by this field .