Magnetosphere - Electric Currents in Space

Electric Currents in Space

Magnetic fields in the magnetosphere arise from the Earth's internal magnetic field as well as from electric currents that flow in the magnetospheric plasma: the plasma acts as an electromagnet. Magnetic fields from currents that circulate in the magnetospheric plasma extend the Earth's magnetism much further in space than would be predicted from the Earth's internal field alone. Such currents also determine the field's structure far from Earth, creating the regions described in the introduction above.

Unlike in a conventional resistive electric circuit, where currents are best thought of as arising as a response to an applied voltage, currents in the magnetosphere are better seen as caused by the structure and motion of the plasma in its associated magnetic field. For instance, electrons and positive ions trapped in the dipole-like field near the Earth tend to circulate around the magnetic axis of the dipole (the line connecting the magnetic poles) in a ring around the Earth, without gaining or losing energy (this is known as Guiding center motion). Viewed from above the magnetic north pole (geographic south), ions circulate clockwise, electrons counterclockwise, producing a net circulating clockwise current, known (from its shape) as the ring current. No voltage is needed—the current arises naturally from the motion of the ions and electrons in the magnetic field.

Any such current will modify the magnetic field. The ring current, for instance, strengthens the field on its outside, helping expand the size of the magnetosphere. At the same time, it weakens the magnetic field in its interior. In a magnetic storm, plasma is added to the ring current, making it temporarily stronger, and the field at Earth is observed to weaken by up to 1-2%.

The deformation of the magnetic field, and the flow of electric currents in it, are intimately linked, making it often hard to label one as cause and the other as effect. Frequently (as in the magnetopause and the magnetotail) it is intuitively more useful to regard the distribution and flow of plasma as the primary effect, producing the observed magnetic structure, with the associated electric currents just one feature of those structures, more of a consistency requirement of the magnetic structure.

As noted, one exception (at least) exists, a case where voltages do drive currents. That happens with Birkeland currents, which flow from distant space into the near-polar ionosphere, continue at least some distance in the ionosphere, and then return to space. (Part of the current then detours and leaves Earth again along field lines on the morning side, flows across midnight as part of the ring current, then comes back to the ionosphere along field lines on the evening side and rejoins the pattern.) The full circuit of those currents, under various conditions, is still under debate. Because the ionosphere is an ohmic conductor of sorts, such flow will heat it up. It will also create secondary Hall currents, and accelerate magnetospheric particles—electrons in the arcs of the polar aurora, and singly ionized oxygen ions (O+) which contribute to the ring current.

Two kinds of global-scale magnetospheric electric fields can be identified:

a) a magnetospheric electric convection field, which originates from the interaction between the solar wind plasma and the polar geomagnetic field. It is directed from dawn to dusk, and

b) a co-rotation field, which is generated in a co-rotating frame of reference in order to compensate for the Lorentz force.

The thermal plasma within the inner magnetosphere corotates with the Earth and therefore reacts to the sum of these two fields. The configuration of the sum of both electric potentials has a torus-like inner region of closed electric potential lines in which ionized particles of thermal energy are trapped (plasmasphere). Outside the last closed electric potential shell (the plasmapause), the ionized particles are lost to space.

The electric convection field causes charge separation at the magnetopause. Therefore, discharging currents flow via electric field-aligned currents (Birkeland currents) into the auroral regions of the ionosphere on the morning side and out of the ionosphere on the evening side. The electric circuit is closed within the ionospheric dynamo region (about 100 to 200 km above the ground). These currents are the DP1 current (the auroral electrojet) and the polar DP2 current. Their magnetic manifestations can be observed on the ground. Joule heating due to the varying electric currents heats the neutral gas of the thermosphere causing thermospheric disturbances.