The magnetic zenith effect and morphology of striation bunching (MAZE)
M. J. Kosch and A. Senior
University of Lancaster, Lancaster LA1 4WA, UK.
Objective
To investigate the ionospheric magnetic zenith effect as well as the growth and spatial morphology
of large-scale slow-growth bunches of striations. In addition, artificial ion outflow, which may
occur as a consequence of the other activities, will be studied.
Background
Since February 1999 excellent progress has been achieved at the EISCAT, HAARP (Alaska) and
SURA (Russia) facilities regarding pump-induced artificial auroras at high latitudes. The lead
author is involved in experiments at all these facilities. For the sake of brevity, a review of past
successes is not presented, except for the main points of this proposal.
It is now a well established fact that over-dense O-mode pumping of the ionosphere causes 1-10
metre scale plasma irregularities of 5-10% plasma density depletion, which are highly elongated in
the magnetic field aligned direction and are called striations [e.g. 1]. Artificial striations are easily
produced by the EISCAT Heating facility and are effective scatterers of HF probing waves, which
are routinely observed by the CUTLASS SuperDARN radars [2], also during artificial aurora
experiments [3]. Radar frequency-scanning experiments confirm the elongated nature of the
striations, which have vertical scale lengths of 10s km [4]. Striations are caused by the conversion
of the pump electromagnetic wave on pre-existing plasma irregularities into electrostatic waves at
the upper-hybrid resonance height, which is typically ~5-10 km below the HF reflection altitude.
The upper-hybrid waves propagate perpendicular to the magnetic field direction and effectively heat
the plasma. The heated plasma expands, reinforcing the plasma density depletions even further,
which trap the upper-hybrid waves, forming a positive feedback process that goes into saturation
with 1-10 s [1].
Rocket experiments at Arecibo experimentally showed that striations tend to collect in large-scale
bunches some km across [5,6]. The striation bunches formed during long pumping intervals (>10
minutes), much longer than the 1 or 2 minute pump cycles typical for the artificial aurora
experiments. At EISCAT short pump cycles tend to be used because of the variable nature of the
ionosphere, especially at night soon after sunset. Fig. 1 shows ionosonde skymaps of ionospheric
backscatter for long pump on periods (up to 20 minutes) at Platteville [7]. The skymaps do not have
the resolution to resolve individual striations but do observe of bunches of striations. The time
development of these bunches is clearly shown and they all cluster around the magnetic zenith. Fig.
2 shows how the horizontal location of the striation bunches varies with virtual height. The
displacement is consistent with the striation bunches being parallel to the magnetic field direction
[7]. In addition, striation bunching is coincident with spread-F ionogram traces, which is consistent
with our artificial aurora experiments at EISCAT. The bunching of striations is thought to be caused
by self-focusing of the pump beam [8]. Detailed ray tracing has shown this explanation to be
feasible as well as explain the artificial spread-F [9].
It is an unpublished fact that skymap plots of HF backscatter by the Dynasonde at EISCAT tend to
show preferential backscatter from the region close to the magnetic zenith (dip angle 13o South)
during successful artificial aurora experiments. Fig. 3 shows an example from 3 November 2000
where higher Dynasonde frequencies tend to backscatter further southward, a clear indication of the
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dipped magnetic field direction. We now know that the artificial aurora preferentially appears in the
magnetic zenith as confirmed by beam swinging experiments at EISCAT [10, 11], HAARP [12]
and SURA (unpublished). However, the magnetic zenith effect is not always present: Often
preferential ionosonde backscatter comes from the local zenith, as expected from simply geometry,
and sometimes even from the north, which may indicate a tilted ionosphere.
Fig. 4 shows the first optical evidence of magnetic zenith self-focussing (unpublished) from
HAARP on 4 February 2005. Two time sequences of optical images, taken with 30 sec cadence, are
shown for a 1-minute pump period. The colour panels show first difference images; red positive
going, blue negative going, and green is no change. The monochrome images show the raw data.
Overlaid are percentage power contours of the pump beam centred on the magnetic zenith. The
first, second and third columns show pump on, steady state and pump off, respectively. Although
the time resolution is poor, it is clearly seen that a large area centred on the magnetic zenith
illuminates (red colour) 0-30 s after pump on (first column). Then the optical intensity continues to
grow in a smaller area centred on the magnetic zenith (red colour) whilst the surrounding area
remains steady (green colour) or even decreases in intensity (blue colour) for 30-60 s after pump on
(middle column). When the pump is turned off (right column) the entire illuminated region
decreases in intensity (blue colour). The additional blue patch to the north is due to an unrelated
HIPAS pump experiment, which happens to be in the all-sky field of view.
In addition to the magnetic zenith effect and striation bunching, we know that long pump pulses (8
minutes) at high power (>200 MW ERP) cause artificial ion outflow above ~500 km altitude at
EISCAT [13]. A previous proposal regarding SPEAR and artificial auroras was intended to prepare
for the Canadian small hybrid satellite CASSIOPE, scheduled for launch in 2007, which includes
the scientific payload ePOP (enhanced polar outflow probe). The ePOP probe will include a suite of
eight scientific instruments, including plasma imagers, radio wave receivers, magnetometers,
cameras and low-energy (<100 eV) particle detectors, which will be used to study the ionosphere.
The satellite will be launched into a precessing polar orbit of ~80o inclination with an apogee of
~1500 km and perigee of ~300 km. A lifetime of 2 years is expected. Not only will the satellite
overfly all high-latitude ionospheric Heaters, but it will also offer a rare opportunity to make (near)
in-situ measurements of the heated volume. The lead author has been asked to participate. It is
anticipated that CASSIOPE will overfly EISCAT making the ePOP payload extremely suited to
(near) in-situ measurements of artificial bunches of striations, ion outflow, optical measurements of
the artificial auroras (night time only) and the energy spectrum of pump-accelerated electrons.
Details of the exact orbit parameters are still undecided, but obviously winter night-time perigee
and apogee conjunctions over EISCAT are highly desirable.
Scientific Justification
1) Magnetic zenith self-focussing of HF pump waves is currently a topic of great interest and may
lead to real practical uses, e.g. the creation of artificial VLF ducts for cyclotron-resonance
experiments aimed at radiation belt remediation projects. However, Dynasonde skymaps do not
always display magnetic zenith self-focussing effects. Hence, it is proposed to perform experiments
similar to those for artificial aurorae and systematically determine when the magnetic zenith selffocussing effect is most likely to occur, and whether this is a function of pump power or duration.
2) It appears that the mid-latitude experiments performed at Platteville in the 1970s have never been
repeated at high latitudes [Herb Carlson, private communication] despite the fact that it has been
technically possible for over 2 decades. Hence, there is a real need to investigate striation bunching
at high altitudes. In addition, the minimum pump power-duration product needed to create bunches
of striations shall be determined as past experiments have mostly been performed at full power over
long pump periods.
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3) In addition, ion outflow remains an unexplained and intriguing phenomenon of topical research.
Since ePOP is specifically designed for such studies, EISCAT can significantly contribute by
artificially inducing ion outflow during a future CASSIOPE overpass. However, artificial ion
outflow has only ever been done once at EISCAT, and then only during an experiment not designed
for this purpose. Hence, the minimum pump duration-power product is to be determined, which will
needed to prepare for any in-situ measurement as the satellite management are concerned about any
adverse effects that flying directly into the pump beam may have on the satellite’s instrumentation.
The relationship, if any, with the pump duration-power product on ion outflow velocity will also be
investigated.
Time Requested
Heating will be operated with O-mode, and the beam pointing vertically up or into the magnetic
zenith, with a nominal cycle of 20-minutes ON, 20-minutes OFF. The pump cycle and power will
be varied according to real time diagnostics. The pump frequency shall be fixed below foF2 away
from any gyroharmonic frequency using the low-gain wide-beam array. The main diagnostic will be
the skymap mode of the Dynasonde. However, the UHF is required to provide electron density
profiles both vertically and along the magnetic field line direction because the ionosonde will be
operated at only a few discrete frequencies (e.g. 2-6 MHz at 1 MHz steps) to provide skymap height
information at various virtual heights but not enough to provide an electron density profile. This is
necessary as the pump wave causes ionosonde interference. This problem will be circumvented by
synchronising the ionosonde to brief pump OFF periods (30-seconds or less every 2.5 minutes)
during the 20-minute Heater ON pulse. The 20-minute Heater OFF period is needed to allow the
ionosphere time to return to a cold unperturbed state. In addition, the UHF is needed to observe
artificial ion outflow, which should occur during the long Heater ON pulses [13]. The UHF will be
operated in a standard mode, e.g. tau1 or tau2pl.
A stable, smooth ionosphere is needed. Also, despite solar minimum approaching, foF2 must be
greater than 4 MHz, which is the Heater’s minimum operating frequency. Hence, the runs shall be
executed in the summer or autumn during the daytime. 4 runs of 4 hours each are requested making
a total of 16 hours Heater time, and 16 hours UHF time, i.e. 24 radar accounting hours. The 4
hour runs are necessary because the low cadence of the pump cycles (nominally 40 minutes) and the
EISCAT 4-hour rule.
References
[1] Robinson, The heating of the high latitude ionosphere by high power radio waves, Physics
Reports, 179, 79-209, 1989.
[2] Bond, Robinson, Eglitis, Wright, Stocker, Rietveld and Jones, Spatial observations by the
CUTLASS coherent scatter radar of ionospheric modification by high power radio waves, Ann.
Geophys., 15, 1412-1421, 1997.
[3] Kosch, Rietveld, Yeoman, Cierpka and Hagfors, The high-latitude artificial aurora of 21
February 1999: An analysis, Adv. Polar Upper Atmos., 16, 1-12, 2002.
[4] Senior, Borisov, Kosch, Yeoman, Honary and Rietveld, Multi-frequency HF radar
measurements of artificial F-region field-aligned irregularities, Ann. Geophys., 22, 3503-3511,
2004.
[5] Kelley, Arce, Salowey, Sulzer, Armstrong, Carter and Duncan, Density depletions at the 10-m
scale induced by the Arecibo Heater, J. Geophys. Res., 100, 17367-17376, 1995.
[6] Peria, Kelley and Franz, Double-probe measurements in field-aligned irregularities produced by
intense electromagnetic radiation, J. Geophys. Res., 104, 6797-6804, 1999.
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[7] Allen, Thome and Rao, HF phased array observations of heater-induced spread-F, Radio Sci., 9,
905-916, 1974.
[8] Thome and Perkins, Production of ionospheric striations by self-focussing of intense radio
waves, Phys. Rev. Lett., 32, 1238-1240, 1974.
[9] Georges, Amplification of ionospheric heating and triggering of ‘spread F’ by natural
irregularities, J. Geophys. Res., 75, 6436-6438, 1970.
[10] Kosch, Rietveld, Hagfors and Leyser, High-latitude HF-induced airglow displaced
equatorwards of the pump beam, Geophys. Res. Lett., 27, 2817-2820, 2000.
[11] Kosch, Rietveld, Honary, Hagfors, High-latitude artificial aurora from EISCAT: A unique
phenomenon?, Proceedings 28AM Optical Workshop (Oulu, Finland, 2001), Sodankyla
Geophysical Observatory Publications, 92, 7-10, 2003.
[12] Pedersen, McCarrick, Gerken, Selcher, Sentman, Carlson, and Gurevich, Magnetic zenith
enhancement of HF radio-induced airglow production at HAARP, Geophys. Res. Lett., 30(4), 11691172, 2003.
[13] Rietveld, Kosch, Blagoveshchenskaya, Komienko, Leyser and Yeoman, Night-time
observations of ionospheric electron heating by HF waves at high latitudes: Aspect angle
dependence, J. Geophys. Res., 108, 1141, 2003.
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Fig. 1: Time sequence of skymaps, showing the morphology of large-scale bunches of striations,
taken during long CW pumping pulses [7]. The Doppler spectra are unimportant for this proposal.
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Fig. 2: An ionogram (top) and skymaps (middle) taken at different virtual heights [7]. The Doppler
spectra (bottom) are unimportant for this proposal.
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Fig. 3: Dynasonde skymaps from the artificial aurora experiment of 3 November 2000 at Tromso.
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Fig. 4: Two time sequences of all-sky optical examples of the magnetic zenith effect taken from
HAARP on 4 February 2005. The colour panels show first difference images, red positive going,
blue negative going, and green is no change. The monochrome images show the raw all-sky data.
Overlaid are power contours of the pump beam centred on the magnetic zenith. The first, second
and third columns show pump on, steady state and pump off, respectively.
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