Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from the Earth's interior to where it meets the solar wind, a stream of charged particles emanating from the Sun. Its magnitude at theEarth's surface ranges from 25 to 65 microteslas ( (0.25 to 0.65 gauss). The field of a magnetic dipole currently tilted at an angle of about 10 degrees with respect to Earth's rotational axis, as if there were a bar magnet placed at that angle at the center of the Earth. Unlike a bar magnet, however, Earth's magnetic field changes over time because it is generated by a geodynamo (in Earth's case, the motion of molten iron alloys in its outer core).

The North and South magnetic poles wander widely, but sufficiently slowly for ordinary compasses to remain useful for navigation. However, at irregular intervals averaging several hundred thousand years, the Earth's field reverses and the North and South Magnetic Poles relatively abruptly switch places. These reversals of the geomagnetic poles leave a record in rocks that are of value to paleomagnetists in calculating geomagnetic fields in the past. Such information in turn is helpful in studying the motions of continents and ocean floors in the process of plate tectonics.

Photo Credit: NASA and  Peter Reid, The University of Edinburgh

And how did Earth get its magnetic field? Magnetic fields surround electric currents, so we surmise that circulating electic currents in the Earth's molten metalic core are the origin of the magnetic field.

The Earth's magnetic field is attributed to a dynamo effect of circulating electric current, but it is not constant in direction. Rock specimens of different age in similar locations have different directions of permanent magnetization. Evidence for 171 magnetic field reversals during the past 71 million years has been reported. Although the details of the dynamo effect are not known in detail, the rotation of the Earth plays a part in generating the currents which are presumed to be the source of the magnetic field. It was found that Venus does not have such a magnetic field although its core iron content must be similar to that of the Earth. Venus's rotation period of 243 Earth days is just too slow to produce the dynamo effect. The magnetic field of Eartht depends upon the rotation of the fluid metallic iron which makes up a large portion of the interior, and the rotating conductor model leads to the term "dynamo effect" or "geodynamo", evoking the image of an electric generator.

Why does magnetic north sits off the coast of Canada, rather than at the North Pole? It is because of lopsided nature of Earth's inner core. The inner core is a ball of solid iron about 760 miles (1,220 kilometers) wide. It is surrounded by a liquid outer core (mostly iron and nickel), a rocky, viscous mantle layer and a thin, solid crust. As the inner core cools, crystallizing iron releases impurities, sending lighter molten material into the liquid outer core. This upwelling, combined with the Earth's rotation, drives convection, forcing the molten metal into whirling vortices. These vortices stretch and twist magnetic field lines, creating EarthÕs magnetic field. Currently, the center of the field, called an axis, emerges in the Arctic Ocean west of Ellesmere Island, about 300 miles (500 kilometers) from the geographic North Pole.

Convection (form of heat transfer -  convective heat transfer involves the combined processes of conduction -heat diffusion- and advection - heat transfer by bulk fluid flow) drives the outer-core fluid and it circulates relative to the earth. This means the electrically conducting material moves relative to the Earth's magnetic field.  If it can obtain a charge by some interaction like friction between layers, an effective current loop could be produced. The magnetic field of a current loop  could sustain the magnetic dipole type magnetic field of the Earth. Large-scale computer models are approaching a realistic simulation of such a geodynamo.

Interaction of the terrestrial magnetic field with particles from the solar wind sets up the conditions for the aurora phenomena near the poles.

Although generally Earth's field is approximately dipolar and its magnetic moment is nearly aligned with the rotational axis, occasionally the North and South geomagnetic poles trade places. Evidence for these geomagnetic reversals can be found worldwide in basalts, sediment cores taken from the ocean floors, and seafloor magnetic anomalies. Reversals occur at apparently random intervals ranging from less than 0.1 million years to as much as 50 million years.

The Earth's magnetosphere has weakened by 15 per cent over the last 200 years. 

Scientists found that changes in the Earth's magnetic field are more relevant for climatic changes in the upper atmosphere (about 100-500 km above the surface). Understanding the cause of long-term change in this area helps scientists to predict what will happen in the future (1). The increase in atmospheric CO2 concentration has been thought to be the main cause of climatic changes at these high altitudes. But magnetic field changes that have taken place over the past century are as important.

Both increasing levels of CO2 and changes in the Earth's magnetic field affect the upper atmosphere, including its charged portion, also known as the ionosphere. While CO2 causes heat to be trapped in the lower atmosphere, it actually cools the upper atmosphere. The simulations show that the increase in CO2 concentration over the past 100 years has caused the upper atmosphere, at around 300 km altitude, to cool by around 8 degrees. At the same altitude, changes in the Earth's magnetic field caused a similar amount of cooling over parts of North America, but caused a warming over other parts of the world, with the strongest warming, of up to 12 degrees, located over Antarctica.

Earth’s magnetic field is vital for keeping our atmosphere in place. Mars lost its atmosphere because it lost its magnetic field. It was found that while the pressure of the solar wind increased at each planet by similar amounts, the increase in the rate of loss of martian oxygen was ten times that of Earth’s increase. Such a difference would have a dramatic impact over billions of years, leading to large losses of the martian atmosphere, perhaps explaining or at least contributing to its current tenuous state. This proves the efficacy of Earth’s magnetic field in deflecting the solar wind and protecting our atmosphere. Without our magnetic field all the water on our planet would have evaporated and life wouldn't have survived and thrived in the way it is doing now! So no magnetic field means no atmosphere, no oxygen, no water and no life!

This has another significance too : Earth's magnetic field serves to deflect most of the solar wind, whose charged particles would otherwise strip away the ozone layer that protects the Earth from harmful ultraviolet radiation. It protects us from solar winds and excess radiation as it could be potentially dangerous to humans and animals alike.

Animals including birds and turtles can detect the Earth's magnetic field, and use the field to navigate during migration. Disablement of the magnetosphere would interrupt whale navigation and bird flight. Possibly the most closely studied of the variable Sun's biological effects has been the degradation of homing pegions' navigational abilities during geomagnetic storms. Pigeons and other migratory animals, such as dolphins and whales, demonstrate magneto-sensitive behavioral responses that were once thought to be mediated by neurons that contained the mineral magnetite located in the beak. The basis of sensory perception of magnetic fields had been unknown for a long time. However, in 2015 Chinese scientists reported they found a protein complex that may form the basis of magneto-reception in animals. 

These storms cause radiation effects to human beings. Intense solar flares release very-high-energy particles that can cause radiation poisoning to humans (and mammals in general) similar to low-energy radiation from nuclear blasts.

Many communication systems use the ionosphere to reflect radio signals over long distances. Ionospheric storms can affect radio communication at all latitudes. Some frequencies are absorbed and others are reflected, leading to rapidly fluctuating signals and unexpected propagation paths. TV and commercial radio stations are little affected by solar activity, but ground-to-air, ship-to-shore, shortwave broadcast and amateur radio  (mostly the bands below 30 MHz) are frequently disrupted. Radio operators using HF bands rely upon solar and geomagnetic alerts to keep their communication circuits up and running.

Some military detection or early warning systems are too affected by unusual solar activity.

Damage to communications satellites can disrupt non-terrestrial telephone, television, radio and Internet links.

 Systems such as GPS, LORAN  are adversely affected when solar activity disrupts their signal propagation.