Earth has a substantial magnetic field called the magnetosphere, which acts as a buffer that protects the planet from sudden, incoming bursts of energy from the Sun and outer space. The magnetosphere is the result of the interaction between Earth’s magnetic field and solar wind, and its charged particles are controlled by Earth’s magnetic field rather than by the whims of the solar system. However, solar winds can draw out the magnetosphere so that the front gets compressed, forming a taper—a magnetotail—on the opposite end.
Within the magnetosphere, plasma—a highly ionized gaseous substance—flows freely. This plasma can interact with dipolarization fronts—a thin sheet of electrical current associated with coherently structured disturbances—found at the leading edge of a magnetotail. The physics of how these dipolarization fronts interact with the plasma sheets within the magnetosphere are still largely unknown. What’s more, the area around dipolarization fronts can affect the entire global magnetosphere. Here Lu et al. use observations from NASA’s Time History of Events and Macroscale Interactions (THEMIS) spacecraft to identify the electron density around a depolarization front and what contributes to that environment.
The goal of the THEMIS program is to examine the physical processes that govern the formation of colorful auroras that spawn from storms in Earth’s magnetosphere. The scientists assessed observations between October and December 2015 from three THEMIS spacecraft that hovered around the magnetotail. These spacecraft measured the magnetic field in two dimensions—this, combined with data on electrons and ions present, allowed scientists to determine the average flow of electric charge density in the region.
Observations indicated that the area preceding the dipolarization fronts had a reduced current. To understand why this might be true, researchers simulated the front using a two-dimensional particle-in-cell model. From this simulation, the scientists found that positive charge builds up ahead of depolarization fronts and deflects electrons away, which explains the current reduction observed by THEMIS. This research contributes to a better scientific understanding of Earth’s magnetosphere and the role it plays as an intermediary between our planet and infinite space. (Journal of Geophysical Research: Space Physics, doi:10.1002/2016JA022754, 2016)