Whistlers were first detected during World War I. They are audio
frequency electromagnetic waves produced by lightning. Once produced,
these waves travel along closed magnetic field lines from one
hemisphere to the other in the right-hand polarized, whistler mode of
propagation. The duration of the whistling tone is related to the
length of the propagation path. Because of anisotropies in the index
of refraction, the wave energy is confined within a cone that makes
an angle of 19° 28' with respect to the local magnetic field.
On a high-resolution wideband spectrogram, the whistler's characteristic
spectral feature is a clearly defined tone descending rapidly in frequency
over several seconds. The name "whistler" refers to this characteristic
whistling sound in the audio frequency range.
The first spectrogram is a 48-second wideband spectrogram taken from a
nightside plasmaspheric pass on March 26, 1996. Initially the wideband
receiver was connected to the electric Eu antenna, but was switched to
the Bu magnetic search coil antenna at 07:59:06 UT. A series of brief
whistlers is evident throughout this interval below 1.5 kHz.
The second spectrogram is a 48-second wideband spectrogram taken from a
dayside plasmaspheric pass on May 10, 1996. The wideband receiver was
connected to the magnetic loop antenna throughout this interval. Two
clusters of whistlers of varying duration are seen below 8 kHz at
00:16:25 UT and 00:16:44 UT.
The third spectrogram is a 48-second wideband spectrogram taken from a
nightside plasmaspheric pass on June 12, 1996. The wideband receiver
was again connected to the magnetic loop antenna. Some whistlers can
be seen up to 9 kHz (13:58:24 UT and 13:58:29 UT) and several more
below 4 kHz (13:58:32 UT and 13:58:44 UT).
Saucer emissions are found near the low-latitude boundary of the auroral
precipitation region. Saucers are electromagnetic whistler-mode emissions
characterized by a V-shaped or saucer-shaped signature on high resolution
Saucers are upward-propagating emissions that usually last only seconds.
The short time duration of the saucers is the most significant spectral
difference between these emissions and the broadband auroral hiss found in
the same region. On the audio tape, the saucers have distinct falling and
In a wideband spectrogram from March 27, 1996, two distinct saucers can be
seen over-lapping each other. The V-shaped saucer is centered on
20:05:42 UT and extends in frequency up to 5 kHz. The dish-shaped saucer
is centered on 20:05:47 UT and extends in frequency up to 2.5 kHz. Both
saucers are found on dayside auroral field lines near the poleward edge
of the auroral zone. For this pass, the wideband receiver was connected
to the electric Eu antenna.
Chorus emissions are electromagnetic emissions propagating in the
right-hand polarized whistler mode. They are among the most intense
plasma waves in the outer magnetosphere. Chorus emissions are
observed at intermediate invariant latitudes, between L=4 and L=10,
and over a wide range of local times with a peak in the distribution
near local dawn. Typical wave spectra of chorus emissions show a
characteristic frequency that varies inversely with invariant latitude.
The chorus occurs primarily in two distinct frequency bands, one above
and the other below the equatorial half-gyrofrequency. A characteristic
null in the distribution of these emissions at the half-gyrofrequency
is clearly visible in the low-resolution frequency-time spectrograms.
The spectral characteristic which gives these emissions their name is
the succession of predominantly rising tones which sound like a chorus
of chirping birds. These rising tones are very short in duration,
typically only 0.1-1.0 seconds. Because of their short duration, these
tones can only be distinguished on high-resolution wideband spectrograms.
The wideband spectrogram for May 31, 1996 was taken from a dayside pass
at latitudes just below the dayside auroral zone. The receiver was
connected to the magnetic loop antenna. The discrete tones
characteristic of chorus can be seen as a dense population of short,
very intense rising tones between 500 Hz and 1.2 kHz.
Auroral hiss emissions are broad, intense electromagnetic emissions
which occur over a wide frequency range from a few hundred Hz to
several tens of kHz. At low frequencies, auroral hiss occurs in a
narrow latitudinal band, typically only 5-10 degrees wide, centered
on the auroral zone. At high frequencies, the emission spreads out
over a broad region, both toward the polar cap, and to a lesser
extent toward the equator. This spreading at high frequencies is
caused by the anisotropic character of whistler mode propagation.
Auroral hiss is emitted in a beam around an auroral magnetic field
line at altitudes of 2-4 RE. The beam width increases with
increasing frequency. As the spacecraft approaches the source
field line, the higher frequencies are detected first, thereby
producing the "funnel-shaped" frequency-time signature that is the
characteristic feature of the auroral hiss spectrogram. At high
altitudes, the auroral hiss often has a sharp high frequency cutoff.
This cutoff is a propagation effect that occurs because the whistler
mode has an upper frequency limit of either the electron plasma
frequency or the electron cyclotron frequency, whichever is smaller.
Poynting flux measurements have shown that auroral hiss propagates
both upward and downward along auroral field lines. Typically above
10,000 km, the emissions are propagating upward and at low altitudes.
Below 1000 km, the radiation is usually propagating downward. The
source of the auroral hiss emissions is in the intermediate region,
between 2 and 4 RE. Downward propagating auroral hiss emissions are
closely correlated with intense, downgoing 100 eV to 1 keV inverted-V
electron beams. Upward propagating auroral hiss is correlated with
upgoing ~50 eV electron beams.
Because the auroral hiss emissions appear as a uni-directional signal
to the spacecraft antennas, the continuous, featureless spectrum of
the hiss emissions is strongly spin-modulated when observed on high-
resolution wideband spectrograms. Well-defined nulls in the signal
occur every half-spin when the electric antennas are aligned
perpendicular to wave propagation direction. The resulting tones on
the audio tape are strongly modulated hiss-like tones.
The first wideband spectrogram is taken from a nightside auroral zone
pass in the northern hemisphere on May 28, 1996. The wideband
receiver is connected to the electric Eu antenna during this pass.
The strongly spin-modulated hiss signal is found below 3 kHz.
The second wideband spectrogram is also taken from a nightside
auroral zone pass in the northern hemisphere. For this pass, on
June 11, 1996, the wideband receiver is again connected to the
electric Eu antenna. The strongly spin-modulated hiss signal
is found below 1 kHz.
Auroral Kilometric Radiation (AKR)
Auroral Kilometric Radiation (AKR) is an intense radio emission
escaping outward from the earth's auroral regions at frequencies
above the local electron plasma frequency. AKR usually consists
of a very intense band of emission in the frequency range of about
50-500 kHz. The AKR intensity is usually highly variable, often
changing by as much as 60-80 dB on time scales of ten minutes or
less. The periods of high intensity tend to occur in storms
lasting from a fraction of an hour to days and are closely
correlated with global auroral displays, particularly with discrete
auroral arcs in the evening sector. The occurrence of intense
bursts of AKR is closely associated with the occurrence of
inverted-V electron precipitation events. Direction-finding
measurements have shown that the most intense bursts of AKR come
from a source region on the nightside auroral field lines at radial
distances ranging from 2-4 RE. Dayside sources are also observed
and are associated with the dayside cusp region. These dayside
sources are typically less intense than the nightside sources.
AKR is observed in both hemispheres and has been found to propagate
in both the right- and left-hand polarization R-X and L-O free
High-resolution wideband AKR spectrograms consist of many narrowband
emissions with rapidly varying center frequencies. This spectral
structure is responsible for the rapid combinations of rising and
falling tones that are heard on the audio tape.
The wideband spectrogram for May 10, 1996 is taken from a pass
through the nightside auroral zone in the southern hemisphere.
The wideband receiver is configured to obtain data in the
frequency range of 250-340 kHz and is connected to the electric
Eu antenna. The multiple discrete spectral features are
predominantly rising tones of varying frequency dispersions between
270 kHz and 340 kHz.