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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