POLAR Scientists Discover the Energy Source Powering the Aurora

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POLAR Scientists Discover the Energy Source
Powering the Aurora
It is known that the aurora is created by intense
electron beams which impact the upper atmosphere
of the Earth.
Until POLAR, it hasnt been clear where the
energy comes from to create those energetic
particle beams.
Measurements from POLAR in the geomagnetic tail
show that energy is transmitted towards the Earth
along magnetic field lines by intense
low-frequency Alfven waves.
Wygant et al.,Polar spacecraft based comparisons
of intense electric fields and Poynting flux near
and within the plasma sheet-tail lobe boundary to
UVI images An energy source for the aurora, J.
Geophys. Res., 105, 18,675, August, 2000
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POLAR Scientists Discover the Energy Source
Powering the Aurora
MEASUREMENTS POLAR carries the correct
instrumentation to resolve details of the field
and particle interactions leading to particle
acceleration and the aurora
EFI, the only three-axis electric field
detector flown in the magnetosphere, resolves
without assumptions the field structures parallel
and perpendicular to B. With magnetometer data
this allows the unambiguous determination of the
direction of propagation of the energy of waves
which is crucial to determine the energy
generation region.
UVI images show when the spacecraft is
magnetically linked to intense auroral displays
and provide a measurement of the energy flux
producing the aurora.
At least five mechanisms have been proposed to
explain the energy transfer. Until POLAR no data
have been available to address the fundamental
questions.
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POLAR Scientists Discover the Energy Source
Powering the Aurora
Wygant et al. find that the dominant form of
energy incident on auroral acceleration regions
is due to intense Alfven waves propagating down
magnetic field lines
Observations Shown are the Alfven wave E and B
fields along with the Poynting Flux due to the
wave as it flows towards Earth at 10,000 km/s.
As the wave propagates toward earth, it becomes
more intense as the magnetic field lines
converge. It is this energy that powers the
aurora. Peak values of the measured energy flux
are 1 erg/cm2-sec at the altitude of the
spacecraft. Mapped to ionospheric altitudes, the
energy flux is greater than 100 ergs/cm2-sec. A
barely visible aurora requires only about 1
erg/cm2-sec.
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Wygant et al. Discovers the Energy Source
Powering the Aurora
This UV image of the aurora was obtained at the
same time that POLAR measured the intense Alfven
wave associated energy flux.

The aurora was most intense near the foot of the
POLAR magnetic field line and has an energy of
gt30 ergs2s-1 a very intense aurora.
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Wygant et al. Discovers the Energy Source
Powering the Aurora
These waves carry enough energy to power all
known candidate acceleration mechanisms in the
auroral region and appear to carry most of the
energy released during substorms

• Electron and ion beams coinciding with the Alfven
  waves on the same magnetic field line indicate
  direct acceleration by the parallel E Field and
  suggest that POLAR has observed the earliest
  stages of auroral particle acceleration with
  further acceleration occurring at low altitudes.

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Wygant et al. Discovers the Energy Source
Powering the Aurora
The energy flux in auroral electron beams scales
with the energy flux in Alfven waves propagating
towards the Earth.

• POLAR observations show that the Poynting flux is
  often radiated towards the Earth along magnetic
  field lines during periods of explosive energy
  release associated with magnetospheric substorms.

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Wygant et al. Discovers the Energy Source
Powering the Aurora

• Particle data show that the waves and their
  energy flux are strongest along the boundary
  between the low density tail lobes and the higher
  density plasma sheet.
• This boundary maps to the reconnection regions
  associated with the release of magnetic energy
  stored in stressed configurations of the
  geomagnetic tail.

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Wygant et al. Discovers the Energy Source
Powering the Aurora

• Implications
• Understanding the sequence of physical processes
  involving Alfven wave auroral acceleration is
  important to unraveling the complex mechanisms
  underlying other, larger scale acceleration
  processes in space and astrophysical systems.
• Similar processes involving Alfven waves can
  explain the heating of the solar corona, the
  energy release during solar flares, and the
  acceleration of the solar wind.
• Alfven waves have been thought to generate the
  stellar winds from several classes of star
  systems and are candidate mechanisms for the
  acceleration of jets from galactic nuclei powered
  by accretion disks around rotating black holes.