Bryers et al.: Artificial aurorae
DIY Northern Lights
Carl Bryers, Michael Kosch, Andrew Senior, Tim Yeoman and Michael Rietveld find out whether more
powerful radio emissions produce brighter artificial aurorae – a Rishbeth Prize winner in 2012.
T
he Northern Lights, or aurora borealis,
have been observed for thousands of
years and their presence still fascinates
us today. Cave paintings in France dating back
30 000 years are thought to depict the Northern
Lights; early descriptions of aurorae mentioned
“burning clouds”; some scientists proposed they
were due to reflected sunlight; and they have
been associated with the spread of both disease
and fertility. It wasn’t until the early 1950s that
the cause of aurorae was established as the
excitation of neutral gas in the atmosphere by
energetic particles. In 1958, a rocket was fired
through the emission region; the resulting data
indicated that these energetic particles were in
fact electrons originating from the solar wind,
guided into the atmosphere from space along the
Earth’s magnetic field lines where they converge
at the magnetic poles.
Auroral colours
Aurorae arise from the decay of excited oxygen and nitrogen in the atmosphere back to the
ground state; oxygen typically emits photons
that are observed as green or red by the human
eye. The excited states of oxygen have lifetimes
ranging from approximately 1 second for green
light or up to 100 seconds for red. Because the
lifetime of the red O(1D) state is relatively long,
the oxygen atom can collide with other gases
and become de-excited; a photon is not emitted.
This process is known as quenching and occurs
to a greater extent at lower altitudes where the
neutral-gas density is higher. The green excited
O(1S) state of oxygen does not suffer quenching to a high degree because its lifetime is very
short. The neutral-gas density decreases with
increasing altitude, so that the maximum intensity of the red emissions takes place at higher
altitudes than that of the green. A third type
of emission, which looks blue, results from
excited ions of molecular nitrogen. This emission is generated at low altitudes where nitrogen
is found in higher concentrations. Typically the
auroral emissions are generated around 100–
200 km altitude.
The energetic electrons arrive at Earth from the
Sun. During solar storms, when the Sun’s surface
is highly active, more energetic electrons can be
emitted, bringing heightened auroral activity in
the polar regions. Because the mechanism for
creating aurorae relies on the existence of highly
energetic electrons, electrons accelerated by radio
A&G • December 2013 • Vol. 54 waves can create the same effect, artificially.
High-power, high-frequency radio-wave
transmitters exist at various locations on Earth;
two of the most well known are HAARP and
EISCAT. These ionospheric heaters comprise
arrays of antennas which heat the plasma in the
ionosphere in order to further understand its
behaviour and properties. They are often used
in conjunction with incoherent-scatter radars,
which measure the plasma temperature, velocity and density, among other parameters. The
action of the heater can then be studied and
its properties varied to determine the plasma
response under different conditions. The EISCAT heater, located in Norway, can transmit
radio waves at frequencies of 4–8 MHz, at up to
1.2 GW effective radiated power (ERP). Located
50 km away is DASI, an all-sky camera with
filters in order to observe aurorae.
Artificial aurorae were detected for the first
time in 1999, following a heating experiment
performed at EISCAT by Brändström et al.
(1999) using the ALIS camera system in northern
Scandinavia. A second observation was made
by Kosch et al. (2000) less than one week later
using DASI and it was noticed that even when
the heating took place in a vertical direction, the
aurora was displaced towards the magnetic field
line, which points 13° to the south. This effect
was termed the magnetic zenith effect. It is now
well established that the brightest aurora can be
generated when heating along the magnetic field
line. The primary mechanism for the generation of the artificial aurora is the coupling of the
heater’s electromagnetic wave to plasma waves,
which accelerate electrons in the region close to
where the heater wave reflects back down. These
plasma waves are upper hybrid waves, created
in a process known as upper hybrid resonance
(UHR), and they propagate perpendicular to
the magnetic field. Small-scale electron density
irregularities (striations) form along the magnetic field line and trap these plasma waves. This
self-focusing of the waves leads to the most efficient acceleration of the electrons when heating
in this direction. These accelerated electrons
then collide with neutral gases which can then
be excited and go on to emit photons in the same
way that occurs naturally.
This paper describes an experiment to determine whether increasing the power of the heater
radio wave would create a more intense aurora.
This seemed intuitively likely, but the hypoth-
esis had not been tested and there are processes
involved in producing an aurora that could
have counterintuitive effects. The ionosphere
is highly active and dynamic. As the Sun sets,
fewer solar photons are available to ionize the
neutral gas, resulting in a decrease in the plasma
density. Solar activity can also make particles
precipitate to low altitudes (the D-region), ionizing neutral species there which can lead to the
strong absorption of the radio wave as it propagates through this region. Choosing the correct
heater frequency is also important because at
certain frequencies UHR, the electron acceleration process, can be suppressed. Accounting
for these many factors, an estimate was made
at determining how much energy reaches the
UHR altitude, the altitude where the electrons
are accelerated, to relate this to the artificial
auroral optical intensity.
Effects of altitude
When the Sun sets and the electron density
decreases, the height at which the electrons are
accelerated increases if the heater frequency is
kept constant. Because the aurora is generated
at different altitudes throughout the experiment, it is incorrect to compare the intensity
of the aurora at one altitude directly to that at
another because quenching varies with altitude.
This means that it may take a certain amount
of energy to excite 100 oxygen atoms at every
altitude yet, because the quenching rate differs
at each altitude, the total intensity measured
will vary, i.e. only 20 of these excited atoms may
go on to emit at lower altitudes compared to 90
at higher altitudes. A better quantity to measure
is the excitation rate, q, which removes the fact
that the lifetime of the excited states differs at
each altitude. This excitation rate can be calculated by measuring the lifetime of the excited
state and knowing the emission intensity. The
lifetime is determined by calculating how long
an emission can be detected by the camera once
the heater is switched off.
To calculate the heater power flux, i.e. the
energy reaching the UHR altitude, the absorption of the radio wave must be considered as it
propagates through the lower ionosphere (the
D-region) where the neutral density is high.
This too differs depending on the power of the
heater wave. As the wave propagates, its electric
field makes the electrons in the plasma oscillate. These electrons collide with neutral gases,
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Bryers et al.: Artificial aurorae
500
11.5
400
11
10.5
300
10
200
9.5
100
9
log10 [electron density (m–3)]
altitude (km)
12
500
2000
400
1500
300
1000
200
500
100
15:23 15:31 15:39 15:47 15:55 16:03 16:11 16:19 16:27 16:35 16:43 16:51 16:59 17:07 17:15 17:23 17:31
electron temperature (K)
2500
altitude (km)
heating them up, and there is a decrease in the
heater wave energy as a result. Because the collision frequency is temperature-dependent, a
change in temperature causes a change in the
absorption. This effect depends on the heater
wave frequency and power: the net effect is that
a high-power wave undergoes more absorption
than a low-power wave throughout the entire
D-region. We estimate this absorption effect
using the model of Senior et al. (2010) and
found it to vary between 3 db and 12 dB for the
lowest and highest heater powers respectively.
Another effect to be considered is the divergence of the beam as it reaches the reflection
altitude. As the heater wave passes through the
ionosphere, which varies in density with altitude, refraction can occur. Using the electron
density measurements from the EISCAT radar, a
model can be used to determine by how much the
energy density decreases as the beam diverges.
Typically this can decrease the energy density
by 20%. Once these effects are accounted for,
a quantitative comparison between the heater
power flux and excitation rate can be made.
0
time (UT)
1: Electron (top) density and (bottom) temperature with altitude from 60 s integrated EISCAT radar
data. The black line (top panel) shows the UHR height. The data gap was due to technical problems.
The EISCAT experiment
On 8 November 2001, an experiment was run
at EISCAT which involved varying the power
of the heater between 12.5% and 100% of
565 MW in order to relate quantitatively the
heater power flux to the optical emission intensity. Electron density and temperature data
from the EISCAT radar (figure 1) show regions
of enhanced temperature when the heater is
switched on. Most of the heater wave energy
is deposited in the vicinity of the UHR altitude
(marked by a black line in the top panel) and the
accelerated electrons propagate up and down
the magnetic field line.
Figure 2 shows the excitation rate of the O(1D)
state plotted against the modelled heater power
flux at the UHR height, taking into acount the
D-region absorption and heater beam divergence. There is an obvious relationship between
the heater power flux and the excitation rate
in that more energy reaching the UHR altitude
causes a greater number of oxygen atoms to be
excited. The red dashed line shows the minimum flux threshold required to excite the atoms
responsible for the red wavelength auroral emissions. The lowest heater power (12.5% of full
power) does not provide enough energy to cause
optical emissions.
By measuring the electron temperature profile
it is possible to estimate the energy required to
raise the background electron temperature to
the enhanced level. This quantity is known as
the height-integrated heating and is the energy
given to the electrons by the heater every second per square metre. This can be calculated
by considering how the electrons flow along the
magnetic field and how they lose energy in collisions. Comparing this to the total energy that is
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2: O(1D) excitation rate as a function of heater
power flux at the UHR height. The data point
colours correspond to the heater power
level, 100% = 565 MW ERP. Linear fits have
been applied to points above and below
the threshold. The dashed line shows the
approximate threshold flux.
3: Modelled height-integrated heat source
as a function of heater power flux at the UHR
height. The black dashed lines represent
efficiency levels and the data point colours
correspond to the heater power level,
100% = 565 MW ERP. The vertical dashed line
shows the threshold flux.
available at the UHR height, an efficiency calculation can be made, i.e. if the energy deposited
into the electrons is equal to the heater power
flux at the UHR height then the process is 100%
efficient. Figure 3 shows the height-integrated
heating plotted against the heater power flux.
For the two highest heater powers, the efficiency
is on average 70%, whereas for the lower powers this is closer to 40%. The question remains,
where does the leftover heater wave energy go?
For the highest powers, around 20% is thought
to go into accelerating electrons to a very
high energy, of which a third go on to collide
with and excite oxygen atoms. The remaining
radio-wave energy will be reflected back to the
ground. For the low heater powers, the remaining energy will simply be reflected back down.
The experiment was designed to test the
hypothesis that increasing heater power produces a brighter optical emission. This has
indeed been shown and a minimum heater
power flux threshold has been measured which
corresponds to the threshold for UHR. Furthermore it was shown that higher heater powers
are more efficient at heating the electrons in the
ionosphere than lower powers, where most of
the energy is reflected and is not absorbed by
the electrons. ●
Carl Byers is a PhD student at the University of
Lancaster; [email protected]. Michael
Kosch and Andrew Senior, University of
Lancaster; Tim Yeomans, University of Leicester;
Michael Rietveld University of Trømso.
The Rishbeth Prize was awarded for the best talk
by a research student at NAM 2012.
References
Brändström B U E et al. 1999 Geophys. Res. Lett. 26 3561.
Kosch M J et al. 2000 Geophys. Res. Lett. 27 2817.
Senior A et al. 2010 J. Geophys. Res. 115 A09318.
A&G • December 2013 • Vol. 54