FAST Spacecraft Reveals Fundamental Plasma Wave Emission
Mechanism
NASA’s Fast Auroral Snapshot (FAST) Small Explorer has traveled to the source
region of the Earth’s most powerful radio emission – Auroral Kilometric Radiation
(AKR).
FAST’s high resolution particles and fields measurements have revealed the mechanism
responsible for these waves, with major implications for other planetary and
astrophysical radio emissions.
FAST has demonstrated that AKR is generated by
the Electron Cyclotron Maser, and that the
maser requires parallel electric fields to create
the population inversion in the plasma that
drives the instability.
Background:
AKR discovered by space probes in the 1960’s and 1970’s
• Intense (1 GW) radio emission.
• Makes the Earth a radio-astronomical object.
AKR has astronomical and astrophysical counterparts:
• Jovian Decametric Radiation from Jupiter.
• Solar Type V radio bursts.
• Solar microwave bursts.
• Radio emissions from binary systems.
• Radio emissions from dwarf M stars.
AKR SOURCE
REGION
Sketch of auroral acceleration region:
Source of Auroral Kilometric Radiation
AKR generation is a fundamental plasma process:
• AKR escapes from the source region – escaping waves have phase speeds faster than light.
• In a plasma, waves are usually generated by resonance between particles and waves travelling at
the same velocity – such waves are trapped.
• The cyclotron maser allows electrons to resonate with faster-than-light waves – the resonant
particles see waves oscillating at their gyration frequency.
The cyclotron maser requires low plasma densities. It was thought that parallel electric
fields played a minor role for the maser, mainly acting to reduce the density.
FAST has demonstrated that the parallel electric fields which accelerate the
precipitating electrons to auroral energies do more than allow the instability to
occur – they are the energy source for the waves.
FAST Spacecraft Reveals Fundamental Plasma Wave Emission
Mechanism
FAST Orbit 1843
-5
Log (V/m)2/Hz
AKR PWT E
(kHz)
370
365
360
355
-11
9
Plasma Wave Tracker (PWT) measures AKR
in the source region – waves below gyro
frequency (white line) – relativistic dispersion
Loss-cone (dark blue at 180°) present
throughout interval – standard loss-cone driven
maser insufficient to generate AKR
Energetic ion beam (~ 1 keV) marks AKR
source region – evidence that parallel electric
field blocks cold ionospheric electrons
Ions strongly field-aligned – confirms presence
of parallel electric field below FAST
6
9
360
270
180
90
0
6
7
Ions
Energy (eV)
10000
1000
100
10
4
7
Ions
Angle (Deg.)
360
270
180
90
0
UT
ALT
ILAT
MLT
4
:49:45 :49:50 :49:55 :50:00 :50:05 :50:10 :50:15
3956.6 3953.8 3950.9 3948.1 3945.2 3942.3 3939.3
65.7
65.8
65.9
66.0
66.1
66.2
66.3
22.8
22.8
22.8
22.8
22.8
22.9
22.9
Minutes from 1997-02-07/20:49:45
Electron Distribution in
Density Cavity
-1•105
-12.0
Upgoing to
Magnetosphere
The parallel electric field therefore is the free energy
source for the waves. The sketch, using an
observed distribution, shows how energy flows
from the electric field to the waves.
Loss Cone
-5•104
Para. Velocity (km/s)
The absence of cold electrons allow relativistic
effects to modify the wave dispersion, with the
waves occurring below the cold electron gyrofrequency. This allows the accelerated and
mirroring electrons to resonate with the waves.
-13.4
3
-14.9
0
1
2
5•10 4
-16.3
Downgoing to
Ionosphere
1•10 5
-1•10 5
-5•10 4
0
Perp. Velocity (km/s)
5•10 4
1•10 5
-17.8
Energy Flow
FAST results have major implications for
radio astronomy
1. Acceleration by Electric Field
2. Mirroring by Magnetic Mirror
3. Diffusion through Auroral Kilometric Radiation
• Parallel electric fields resupply free energy to the electron distribution, increasing
the efficiency of the maser.
• The maser is unstable over extended regions, not quenched through filling of the
loss-cone.
Log10 (Phase Space Density)
Electrons
Angle (Deg.)
Log eV
/cm2-s-sr-eV
10
Energetic electrons (~ 5 keV) – auroral
electrons accelerated by parallel electric field
Log eV
/cm2-s-sr-eV
100
Log eV
/cm2-s-sr-eV
1000
Log eV
/cm2-s-sr-eV
Electrons
Energy (eV)
10000