Magnetosphere

The shocked particles in the magnetosheath behind the solar wind bow shock cannot easily penetrate the Earth’s magnetic field. Because interplanetary field lines cannot penetrate our terrestrial field lines, and solar wind particles cannot escape the grip of interplanetary field lines. Because the plasma in the solar wind is highly conducting, its particles are ‘frozen’ within the magnetic field lines.

The boundary between the two regions is called the magnetopause and the cavity that the planet’s magnetic field creates in the solar wind is called the magnetosphere. The kinetic pressure of the solar wind distorts Earth’s magnetic field, causing its dayside to contract, and its nightside to expand, forming a magnetotail that extends beyond the Moon’s orbit.

The plasma of the magnetosphere consists mainly of electrons and protons. Some of them come from the solar wind and some from the Earth’s ionosphere. However, some He$^+$ and O$^+$ ions are also obtained from the ionosphere, and some He$^{++}$ ions come from the solar wind. This plasma is not uniform, the density and temperature are different from place to place.

Our radiation belt lies along Earth’s dipolar field lines at two to six Earth-radii. Here many high energy electrons oscillate between the northern and southern hemispheres along the magnetic field lines. Here the electron number density is 1 per cc, the temperature is approximately 50 million Kelvin, and the magnetic field strength is 100 to 1000 nanotesla.

Most of the plasma in the nightside magnetotail coalesces near the magnetic equator to form a single plasma sheet with a thickness of about 10 Earth radii. Near Earth this sheet descends all the way to the auroral ionosphere. Here, the average electron density is 0.5 per cc, the temperature is about 5 million Kelvin, and the magnetic field strength is about 10 nanotesla.

The outer part of the magnetotail is called the lobe. Here the electron density and temperature are very low. The electron number density is only 0.01 per cc, the temperature is 500,000 Kelvin, and the magnetic field strength is 30 nanotesla.

The plasma in the magnetosphere is not static, moving under various external influences. Sometimes electrons and ions move together like the solar wind. But sometimes electrons and ions can move in opposite directions, and this creates electric current, which is very important for transporting charge, mass, momentum, energy, etc. in this region. The magnetic field created from this current can in turn alter the field of the magnetosphere. Some of the currents are shown in the figure below.

A current is associated with each distortion of the Earth’s magnetic field. For example, the current associated with desiderite contraction flows at the surface of the magnetopause, hence the name magnetopause current. Similarly tail current flows on the surface of the nightside tail, and neutral sheet current on the middle plasma sheet. These tail currents and sheet currents are actually connected, and when viewed from the line joining the Earth to the Sun, their combined current looks like the Greek letter $\Theta$ .

The inner part of the magnetosphere plays a major role in forming the ring current. This current flows westward around the Earth at a distance of a few Earth-radii from the Earth. Particles in the radiation belt are carriers of this current. Besides bouncing from north to south, these particles drift east-west. Protons drift west and electrons drift east, so a net transport of charge occurs.

There are many currents in the conductive layer of the Earth’s ionosphere between 100-150 km altitude. Currents worth mentioning include the auroral electrojet moving inside the auroral oval, the SQ current at mid-altitude on the Dayside, and the equatorial electrojet near the magnetic equator.

The direction of the currents discussed so far is perpendicular to the magnetic field. But there is also current flowing along field lines. As such, the various field-aligned currents created from the flow of electrons couple the current in the magnetosphere to the current in the poleward region of the ionosphere.