Micro-scale current sheets unleash macro-scale space weather

Solar wind can cause global compression of the Earth's magnetosphere. This causes magnetotail current sheets to become thin and unideal. NASA's MMS spacecraft flies through this thin, compressed current sheet. It reveals that a small-scale electron flow forms (blue lines on plot), which drives lower hybrid waves (bright spots on plot), while plasma density is relatively flat (orange dashed lines on plot). It is crucial to understand the dynamics and formation of these current sheets, as well as the small-scale structures within. This knowledge could be used to initiate magnetic reconnection which can lead to intense space weather in Earth's magnetosphere. Credit: Dr. Bill Amatucci (NRL).
Although movies portray Earth as a peaceful, serene place in the universe, the reality is that the near-Earth space environment can be dangerous and dynamic. Hot charged particles and blobs made of plasma (called the solar wind) travel from the sun every day and are deflected and directed by the Earth's magnet field. This creates beautiful aurora around the north and south poles. Solar storms can cause the Earth's magnetic fields to be compressed, which causes the lines of the magnetic field to reorganize and reconnect. This results in hot, dense plasma being pushed back towards the Earth. These processes are often referred to space weather. Understanding these space-based disruptions is as important as understanding ground-based weather because of the potential impact they can have on key components of modern society like telecommunications systems and power grids.

The difficulty of resolving smaller kinetic-scale processes in satellite observation has been a major obstacle to understanding magnetic reconnection in Earth's magnetosphere. NASA's Magnetospheric Multi-Scale spacecraft (MMS), however, has recently allowed detailed analysis of this previously unknown micro-scale physics.

Scientists from the U.S. Scientists at the U.S. Naval Research Laboratory (NRL), Washington, D.C., have used MMS data to study micro-scale physics in the Earth's magnetic tail, a portion of the magnetosphere illustrated in Figure 1. The Earth's magnetosphere compresses into a thin current sheet by the sun, creating the magnetotail. This is a great place to study magnetic reconnection.

NRL scientists made the first observation of plasma waves caused by high-sheared electron flows (velocity ear) in one these compressed current sheets. When the current sheet is compressed, a localized electrical field perpendicularly to the background magnetic fields is created. This creates velocity shear. These waves can be used to trigger the magnetic reconnection process by providing a rich source for local enhanced diffusivities.

NRL scientists used these observations to find a missing component in existing theoretical models for magnetic reconnection and thin current sheets. This is an ambipolar electric force that forms perpendicularly to the current sheet, and intensifies when the current sheet experiences strong compression. New theoretical models have been created that show that the ambipolar electrical field can self-consistently form in response to the global compression of plasma. This creates the velocity shear which can drive the waves seen in spacecraft data. The electron flow current also alters the magnetic field profiles, allowing for the formation non-ideal current sheets. These features are not possible to explain by standard models. The results of the theoretical model shed new light on the connection between micro-scale and macroscale physics.

These discoveries challenge our understanding of the physics behind thin current sheets. The identification of shear-driven plasma wave also establishes both the importance of the localized electric field and highly homogeneous conditions that drive the physical physics. Combining the smaller scale models with this deeper understanding of physics will allow for a better understanding of global dynamics, and in particular the energy flux within the heliosphere that influences near-Earth space weather.

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