$A$ line passing through places having zero value of magnetic dip is called

Due to the earth's magnetic field,charged cosmic ray particles

The magnetic compass is not useful for navigation near the magnetic poles because

The correct relation is:
$B_H$ = Horizontal component of earth's magnetic field; $B_V$ = Vertical component of earth's magnetic field and $B$ = Total intensity of earth's magnetic field.

The ratio between the total intensity of the magnetic field at the equator to the poles is:

$A$ magnetic needle free to rotate in a vertical plane parallel to the magnetic meridian has its north tip pointing down at $30^{\circ}$ with the horizontal. The horizontal component of the earth's magnetic field at the place is $0.3 \ G$. Then the magnitude of the earth's magnetic field at the location is