polar scientists discover the energy source powering the aurora

Space Science MO&DA Programs - September 2000 -

SSIt is known that the aurora is created by intense electron beams

which impact the upper atmosphere of the Earth.

POLAR Scientists Discover the Energy Source Powering the Aurora

Until POLAR, it hasn’t been clear where the energy comes from to create those energetic particle beams.

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

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.


SSMEASUREMENTS:

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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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.

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



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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 >30 ergs2s-1 – a very intense aurora.

Wygant et al. Discovers the Energy Source Powering the Aurora


SSWygant et al. Discovers the Energy

Source Powering the Aurora

• 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.

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


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• 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.

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.




• 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.




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.

polar scientists discover the energy source powering the aurora

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