ভূচৌম্বকত্ব

Introduction

Alright, so today we will be introducing one of the most fascinating features of our mother Earth: Geomagnetism. Before you jump in, I will give you one warning:

If you haven't learned about and understand magnetism then this topic can be difficult to understand, and it may be better if you learn about magnetism before you continue with this topic.

Definition of Geomagnetism and the Geomagnetic Field

Geomagnetism is defined as the study of the Earth's magnetism. (Merriam-Webster Dictionary). Pop quiz!

The Earth's magnetism exists on the geomagnetic field. The geomagnetic field, is the magnetic field that exists within the boundaries of the Earth's interior to where the field meets solar wind, a stream of charged particles emanating from the Sun. It's also defined as the field of a magnetic dipole that's tilted with respect to Earth's rotational axis, as if there were a bar magnet placed at that angle at the center of the Earth (although unlike a bar magnet, the magnetic force is not quite constant). Here is a computer simulation model of the geomagnetic field .

Why is it so important (to us)?

One of the reasons why our geomagnetic field is so important is since it deflects most of the solar wind, which would otherwise rip apart the ozone layer which protects the Earth from harmful UV-radiation. Furthermore, if it weren't for our geomagnetic field, then our compasses wouldn't be of any use to us (and most of our geographical discoveries probably wouldn't have occurred!)

How was it made?

One of the mysteries of geomagnetism is figuring out exactly how the geomagnetic field was made. Most of the planetary bodies (including the sun) in the our solar system generate magnetic fields through the motion of certain highly conductive fluids.Thus we presume that the geomanetic field originates in its core. This is a region of iron alloys extending to about 3400 km which is a little over half of the Earth's radius. We can divide this region into a solid inner core and a liquid outer core. The motion of the liquid in the outer core is driven by heat flow from the inner core to the core-mantle boundary. The pattern of flow is organized by the rotation of the Earth and the presence of the solid inner core.

The mechanism by which the geomagnetic field is generated is called a dynamo. The effects from Ampère's circuital law, Faraday's law, and the Lorentz force can be combined in a partial differential equation for the magnetic field called the magnetic induction equation:

$$\frac{\partial \mathbf{B}}{\partial t} = \eta \nabla^2 \mathbf{B} + \nabla \times (\mathbf{u} \times \mathbf{B})$$

where $\mathbf{u}$ is the fluid's velocity, $\mathbf{B}$ is the magnetic B-field (a magnetic field defined in terms of the Lorentz force it exerts on moving electric charges), and $\eta = \frac{1}{\sigma \mu}$ is the product of the electrical conductivity $\sigma$ and the permeability $\mu$. $\frac{\partial \mathbf{B}}{\partial t}$ defines the time derivative of the field, $\nabla^2$ is the Laplace operator and $\nabla \times$ is the curl operator.

Concluding Remarks

So there you have it, a somewhat simple introdction to geomagnetism. We talked a lot about the geomagnetic field, but that was since that's what those involved in the field do. There are still a lot of big gaps in the study (just like any geophyical study) which have yet to be filled. Hope you enjoyed, and could understand!