The Earth’s magnetic field is distorted by the solar wind to a droplet-shaped magnetic bubble called the magnetosphere. On the nightside, the Earth’s magnetic field is stretched into a long tail, the magnetotail, much like the tail of a comet. The blue cavity represents the magnetosphere. The red area denotes the region where a large amount of charged particles reside and intense electric currents flow within the magnetosphere. The four Cluster satellites encountered a flow reversal region in the magnetotail.
Credit: Tony Lui, JHU/APL
New satellite data are providing insights into space "tsunamis" that disrupt the northern lights and result in auroral dances in the sky.
Generally seen in high-latitude regions, aurorae are colorful light curtains in the sky, caused by high-energy particles that are carried on the solar wind and interact with Earth's magnetic field.
Early in the evening, the aurora often forms a motionless green arc stretching across the sky in an east-west direction. Colorful dancing auroral forms result from disturbances known as "substorms" or space tsunamis in Earth's magnetosphere, a droplet-shaped magnetic bubble created when the solar wind distorts Earth's magnetic field.
These substorms typically last one to two hours, and are three-dimensional physical phenomena spread across distances ranging from 62,000 to 93,000 miles.
Understanding such complex physical processes using a single scientific spacecraft would be like trying to predict the behavior of a tsunami with a single ocean buoy, necessitating the simultaneous use of several satellites like the Cluster constellation.
Currently, two competing theoretical models describe these space tsunamis, the "Current-Disruption" model and the "Near Earth Neutral Line Model." Using Cluster spacecraft data, scientists confirmed that the behavior of some substorms is consistent with the Current Disruption model.
Study of one of the stages of a substorm helps determine which model applies. For example, in the late stage of substorm development, auroral disturbances move towards the poles, suggesting that the energy source for auroras and substorms moves away from Earth.
Previous satellite observations have found that, during this late stage, the plasma or superhot gas flows in the the tails of the magnetosphere exhibit a reversal in direction. In recent years it was generally thought that a flow reversal region is where magnetic reconnection takes place, where the magnetic field's energy is converted into particle energy (dissipation effect), resulting in high-speed plasma flows hurtling towards Earth, like space tsunamis.
Tony Lui, a scientist at John Hopkins University analyzed Cluster satellite data measured while crossing such a region in the magnetotail, where flows of plasma reverse direction. Owing to Cluster's ability to perform simultaneous multipoint measurements, the scientists were able to mathematically describe some energy movement there that had never before been estimated for such a flow reversal region.
By comparing the directions of the electric current and the electric field in the magnetosphere, it is possible to understand whether the flow reversal is a dissipation effect (where magnetic field energy converted to particle energy) or a dynamo effect (where particle energy is converted to magnetic field energy). The Cluster scientists observed that features associated with flow reversal are actually very complex, consisting of both effects in localized sites.
This result shows that the plasma turbulence disrupts the local electric current. "The features we observed are consistent with the current disruption model," Lui said "However, it is unclear how general these findings are."
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