Electric currents are a net flow in charge from one location to another. Nearly everyone in the world comes into regular contact with electric currents, such as when an electrical plug is inserted into a live socket. Near-Earth space is a place to find electric currents too, from the upper atmosphere 100 kilometers above our heads, to the solar wind tens of thousands of kilometers from the planet. In an article recently published in Reviews of Geophysics, Ganushkina et al  describe the structure and dynamics of the main electric current systems in near-Earth space. Here, one of the authors gives an overview of the nature of these currents and describes how our knowledge of them has improved and could be further advanced.
What electric currents flow in near-Earth space?
There are various different types of electric current flowing around the Earth. Geospace, the region of near-Earth controlled by Earth’s internally-generated magnetic field, is a fairly hard vacuum compared to the air we breathe. It still has some particles in it, though; specifically, it contains plasma, a rarified electrically charged gas. Plasma distribution is by no means uniform and there are sharp boundaries separating types of plasmas with totally different characteristics. Electric currents tend to form at these boundaries, and our review summaries the typical structure and motion of the major current systems.
The Sun constantly emits charged particles called solar wind. The Earth, with its magnetic field, is an obstacle in the flow of the solar wind. The kinetic pressure of the solar wind compresses the terrestrial magnetic field on the dayside, in front of the Earth, and a current flows across the magnetopause, a surface boundary separating Earth’s field and the interplanetary magnetic field (IMF). On the nightside, behind Earth, the magnetic field is stretched and this is where the magnetotail current exists.
Near-Earth space is filled with ions and electrons which come from the solar wind and the terrestrial ionosphere. Opposite drifts of these differently-charged particles result in a net charge transport with the ring current flowing around the Earth. There exist also currents flowing along magnetic field lines, called field-aligned currents, which are mainly carried by electrons and connect the magnetospheric currents with ionospheric currents.
Some of these current systems have unusual structure, like the cut ring current. The currents in the inner magnetosphere are often centered around the local minimum of the magnetic field, in the magnetic equatorial plane around Earth.
In the dayside region, close to the boundary with the solar wind, the magnetic field minimum shifts away from the equatorial plane, splitting the current for a segment of its path around the planet.
How do these currents vary over space and time?
As the Earth’s magnetosphere responds to changes in solar activity, the main magnetospheric current systems can undergo dramatic changes with new transient current systems being generated. The size of the Earth’s magnetosphere is huge: the distance from the Earth’s surface to its end on the side which looks to the Sun is about 40,000 miles. Away from the Sun, the magnetosphere stretches very far into space, hundreds of thousands of kilometers beyond the Moon’s orbit. Currents vary over these very large distances and by orders of magnitude on the time scales from minutes to hours.
One typical shift in currents is when the solar wind increases in intensity and squeezes the dayside region of near-Earth space. This squeeze systematically changes which current system dominates the boundary region between the solar wind and geospace, which has consequences for the convective motion within near-Earth space.
What do the characteristics of these currents reveal about the magnetosphere?
Magnetospheric currents generated as a result of distortion of the terrestrial internal magnetic field due to the interaction with the solar wind and formation of the magnetosphere are important constituents of the dynamics of plasma around the Earth. They transport charge, mass, momentum and energy, and they themselves generate magnetic fields which distort significantly the pre-existing fields.
Understanding the relative strength and location of each electric current system is vital to accurately predicting the variations of the magnetic field related to them and the associated space weather effects.
One example is alterations of the drift paths of the relativistic electrons changing the location and intensity of radiation belts, which are the major source of damaging space weather effects on satellites.
Another example is variations in the strength of Geomagnetically Induced Currents responsible for disruptions of the transmission system operations with voltage collapse or damage to transformers on the ground.
These effects are controlled by the magnetospheric and ionospheric currents and by the Earth’s conductivity.
What have been some of the most significant research advances in this field?
It is very difficult to distinguish current systems from single-point spacecraft measurements, but a constellation of satellites can provide the necessary observations to identify and classify local current density values into large-scale current systems. A specific method to obtain the current densities called the curlometer technique was successfully used in the Cluster mission with four identical spacecraft launched on similar elliptical polar orbits. The tetrahedron formed by the four spacecraft was used to study various plasma structures with characteristic sizes ranging between tens of kilometers and a few Earth radii (Earth’s radius RE is equal to 6371 km).
Another significant advancement is the advent of realistic global magnetosphere models that are capable of reproducing the structure and dynamics of the magnetosphere, including these current systems. While there is still a long way to go to fully capture all of the physics happening in near-Earth space, these high-end computing tools have led to many insights in how charged particles flow through outer space and how they interact with Earth’s magnetic field.
What are some of the unresolved questions where additional research, data or modeling is needed?
As noted above, the identification of specific current systems from in situ spacecraft measurements is extremely difficult. Taken alone, a current density value at a single point in the magnetosphere cannot be identified as part of a particular current system. Current density values at multiple locations must be synthesized into a regional or global scenario of possible current closure. Even this may not produce a unique current system pattern, and numerical models can help connect the localized current density values into a synoptic mapping of current flow through geospace.
At the same time, there are several multi-spacecraft missions with different orbits that can provide the right distribution of measurements for global current system analysis: the Cluster mission with four spacecraft regularly passing through the inner, outer and high-latitude magnetosphere, the THEMIS mission originally with five spacecraft in a highly elliptical low-inclination orbit, the two Van Allen Probes in the inner magnetosphere, and the four MMS spacecraft with an apogee of 12 RE, eventually moving to 25 RE.
The low-Earth orbiting satellites, such as AMPERE and Defense Meteorological Satellite Program (DMSP) spacecraft can provide a highly complementary data set to the magnetospheric missions, allowing for analysis of the current system connections and interplay between the ionosphere and magnetosphere.
—Natalia Yu Ganushkina, Department of Climate and Space Sciences and Engineering, University of Michigan and Finnish Meteorological Institute; email: [email protected]