The line on the earth's surface joining the points where the magnetic field is horizontal is known as:

At the magnetic poles of the earth,a compass needle will be

Assume the dipole model for Earth's magnetic field $B$,which is given by:
$B_v = \text{vertical component of magnetic field} = \frac{\mu_0}{4\pi} \frac{2m \cos \theta}{r^3}$
$B_H = \text{horizontal component of magnetic field} = \frac{\mu_0}{4\pi} \frac{m \sin \theta}{r^3}$
where $\theta = 90^\circ - \text{latitude}$ as measured from the magnetic equator.
$(a)$ Find the loci of points for which the dip angle is zero.

$A$ compass needle oscillates $20$ times per minute at a place where the dip is $30^{\circ}$ and $30$ times per minute where the dip is $60^{\circ}$. The ratio of total magnetic field due to the earth at the two places respectively is $\frac{4}{\sqrt{x}}$. The value of $x$ is $............$.

At a place,the magnitudes of the horizontal component and total intensity of the magnetic field of the earth are $0.3 \; Oersted$ and $0.6 \; Oersted$ respectively. The value of the angle of dip at this place will be.....$^o$

What are the Earth's magnetic elements? Define them and list them.