Tuesday, 17 February 2008
SP-UK-Tristatic 6: E- and F-region ion-neutral coupling studies
A.L.Aruliah, Iris Yiu,
University College London
K.Shiokawa, S.Oyama,
STELAB, University of Nagoya,
M.Kosch,
University of Lancaster
Introduction
A new Fabry-Perot Interferometer (FPI) belonging to the Solar-Terrestrial Environment
Laboratory (STELAB) of Nagoya University was installed at Ramfjordmoen in January
2009. The STELAB FPI will now allow tristatic FPI studies of the upper atmosphere to
resume since the Lancaster FPI was decommissioned in March 2005. The UCLLancaster-EISCAT tristatic FPI-incoherent scatter radar configuration was the first of its
kind to allow common volume observations of neutral and ion parameters. Since 2007 it
has been joined by the tristatic arrangement provided by 3 FPIs at Poker Flat, Fort Yukon
and Eagle, combined with the Advanced Modular Incoherent Scatter Radar (AMISR)
radar at Poker Flat. Meso-scale ion-neutral interactions are attracting great interest to
account for a significant energy and momentum transfer deficit in general circulation
models of the auroral regions. Now measurements are required for the spatial and
temporal scale-sizes of ion-neutral interactions on the meso-scale. These may only be
obtained reliably by common volume measurements. For this experiment it will be
possible to monitor both the upper and lower thermosphere using the Japanese FPI and
the KEOPS green line FPI. The flexibility of the pointing direction of the Japanese FPI
will allow identification of the peak emission heights of the 6300nm and 557.7nm
emissions.
The Tristatic FPI-EISCAT Experiment
The EISCAT tristatic UHF facility will end soon due to increasing problems with mobile
phone interference. This gives a limited lifespan for the tristatic FPI-EISCAT experiment.
We request 24 hours of radar time to use in the new moon period in November/December
2009. The charging factor is 1.25 which means 30 accounting hours are requested.
Although we have been granted 5 previous campaigns using 30 accounting hours each
time, we are limited in the number of useful hours obtained by the prevalent geophysical
conditions. The FPIs require predominantly cloud-free skies otherwise light scattering
means that the directional information for the emission is lost. At present we have had 5
nights of clear weather data from the tristatic FPI-EISCAT campaigns as shown in Table
1. The Japanese FPI will introduce a new level to the tristatic studies. It will allow a very
high time resolution for tristatic measurements and the opportunity to monitor both the
lower and upper thermosphere, corresponding to the E- and F-regions, respectively.
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Tuesday, 17 February 2008
Table 1 List of SP-UK-Tristatic Campaigns
Campaign dates
27 Feb-01 Mar 2003
25 and 27 Nov 2003
15-16 Feb 2004
weather
Mainly clear throughout
Common volume location
Tristatic-A and 2 heights (240km
and 300km)
1st night clear, 2nd night looking over Kiruna zenith
cloudy
cloudy throughout
No observations
08-09 Feb 2005
1st night clear, 2nd night Tristatic-A, and scanning the
cloudy
Skibotn FPI in azimuth. Skibotn
FPI decommissioned after March
2005
01-10 Feb 2008 shared only 4 Feb clear, sporadic looking over Kiruna zenith at
time
with
NEMI clear periods on other nights 110km and 240km. Only bistatic
campaign
FPI mode: KEOPS and Sodankyla
FPIs
Figure 1 shows the tristatic experimental geometry where all 3 FPIs and 3 radars point at
what we called the Tristatic-A position. The common viewing volume is at an altitude of
240km, which is the peak emission height of the 630nm emission at high-latitudes. The 3
FPIs were located near the 3 radars at Skibotn, KEOPS and Sodankylä. A new FPI was
installed by STELAB in January 2009 to provide ground support for the DELTA-2 rocket
campaign by observing the green line emission at 557.7nm to measure lower
thermosphere neutral winds and temperatures. The Japanese FPI at Ramfjordmoen now
replaces the Lancaster FPI in the tristatic FPI configuration. There are two other common
volumes: at the bistatic-B position and directly over KEOPS which also give useful
information about the emission height behaviour and allow some estimate of neutral wind
velocities at different locations. UCL have a second FPI at KEOPS which monitors the
green line emission (557.7nm). This line has a peak emission height of around 110km
which allows observation of the lower thermosphere, corresponding to the E-region.
The Japanese FPI has a state-of-the-art EMCCD detector, with a fully flexible pointing
direction mechanism and a 5 degrees field-of-view, which allows high light throughput. It
can observe both 557.7nm and 630nm, and runs automatically, with remote control from
STELAB. This combined with the UCL FPIs will allow very high time resolution
measurements of thermospheric winds and temperatures, and emission intensities, down
to a few 10s seconds resolution depending on the emission wavelength. During this
extended period of solar minimum, although the airglow emissions have been very low,
UCL have been operating the KEOPS green line FPI at 10s resolution with quite good
signal to noise. The Japanese FPI has a field-of-view that is a factor of 5 larger than the
UCL FPIs, which gives confidence that they will achieve a much higher time resolution
than the UCL FPIs. Potentially the Japanese FPI has the capability of monitoring both the
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Tuesday, 17 February 2008
upper and lower thermosphere in tandem with the 2 KEOPS FPIs. We will conduct
experiments with all the FPIs over the next month in preparation for a radar campaign.
The Tromsø radar will be in a CP1-like mode (a modified cp1lt code) which we have
used successfully in the previous experiments. The aim will be to point at a volume
240km above KEOPS. This is close to a field-aligned position that will allow us to
monitor E- and F-region heating by using the assumption that the F-region plasma
vectors are a good approximation of the E-region vectors. The 2 FPIs at KEOPS will be
monitoring the red and green line, while the single FPI at Sodankylä will monitor the red
line. The Japanese FPI is capable of switching between red and green line filters. For this
experiment we will use the opportunity to look at both the E- and F-region ion-neutral
coupling behaviour. There will also be a contextual field of neutral temperatures,
intensities and winds from all the look directions of all the FPIs.
It is proposed that the Japanese FPI can be set to scan in elevation along the zenith look
direction of the KEOPS FPI. This can be done with both the red and the green filter. The
elevation that produces the best match with the KEOPS FPI intensities will identify the
peak emission altitude. The EISCAT measurements will provide ion temperatures and
electron density profiles from which emission profiles can be modelled for comparison
with the FPI observations. The ALIS tomography network will be used in parallel to
identify the emission peak height.
The original Lancaster FPI had an intensified CCD with the poorest sensitivity. This
limited the time resolution of the tristatic neutral wind parameter measurements and
comparisons to around 15 minutes. In 2005 we attempted to scan the Lancaster FPI along
the KEOPS FPI line-of-sight to find the emission peak height. However, the time
resolution was too poor to produce significant results from a study of the correlation of
intensities. We hope to have a far better time resolution with this new set of FPIs, of the
order of minutes with which we can clearly identify a match with the KEOPS intensities.
This time resolution is close to the apparent temporal scale size of the neutral atmosphere
as proposed by Ford et al. [2007], from high time resolution FPI measurements of the
auroral thermosphere. The large-scale temporal scale size of the thermosphere has been
found to be of the order of a few hours [e.g. Rishbeth and Garriot, 1969], but recent
meso-scale observations have shown that the thermosphere is far quicker to respond over
spatial distances of a few 10s kilometres. Griffin et al [to be submitted] have shown a
rapid and localised response of thermospheric winds and temperatures to a localised burst
of ionospheric currents lasting 10 minutes and localised in a region a few 100km wide,
lying to the north-west of Svalbard. The burst of current occurred at 0800UT on 05/12/07
(Figure 3), and the neutral temperature rose by 100K (Figure 4), reaching a peak about 20
minutes later. Simultaneously a sharp northward enhancement of the wind by 100m/s was
seen by the FPI and SCANDI only to the north of Svalbard (Figure 5). The CUTLASS
radar showed a localised burst of ion velocities greater than 800m/s in this region as
shown in Figure 2. These observations of the neutral parameters were made by the UCL
all-sky FPI called the Scanning Doppler Imager (SCANDI), which makes simultaneous
measurements over the whole sky, unlike the standard FPI which has a mirror to point
consecutively in different look directions. Thus we know for sure with the SCANDI data
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Tuesday, 17 February 2008
that any differences seen in parameters observed in different directions are real, rather
than due to a time evolution of that parameter while the FPI mirror is turning to the new
look direction.
The importance of the 630nm emission altitude for interpreting meso-scale structure
Measurements of neutral parameters have either been derived from ionospheric
measurements or taken from passive optical observations of airglow and auroral
emissions. Ionospheric measurements have the advantage of active radar measurements
where the range is well-known, but passive measurements rely on a few limb scanning
measurements and models [e.g. Solomon et al., 1988]. These provide little knowledge of
meso-scale variability of emission height profiles in the auroral regions. The tristatic FPI
observations have already indicated that the 630nm emission height depends on the
location of the observing volume with respect to the auroral oval. Comparison of many
nights of intensities from KEOPS and Sodankylä at positions A (usually within the
auroral oval) and B (usually just outside the auroral oval) indicate that the peak emission
heights are different. The match between the two FPI intensities at position A is
consistently much closer than at position B. Positions A and B have been determined
from the assumption that the 630nm peak emission height is 240km. It is not possible
with a single point bistatic measurement to determine whether the Sodankylä FPI has
overshot or undershot the KEOPS FPI volume, in other words, whether the peak emission
height at position B is above or below 240km. This is the reason for scanning the
Japanese FPI along the KEOPS line-of-sight to find the optimum match of intensities and
hence the emission peak height.
Holmes et al. [2005] demonstrated sudden changes in lower thermosphere temperatures
seen over the field-of-view of an FPI at Poker Flat. This was explained in terms of
changes in the height profile of the 557.7nm emission. The lower thermosphere
temperatures and winds have a large gradient over the altitude region 100-200km. The
wind speeds increase from a few tens m/s in the lower thermosphere, to a few hundred
m/s in the upper thermosphere, while the temperatures increase from a few hundred
Kelvin to nearly a thousand Kelvin. Thus it is critical to know what altitude is being
sampled for the lower thermosphere in order to judge whether the changes are due to ionneutral coupling, composition change, or only due to a change in the altitude.
In contrast, the very low particle density of the upper thermosphere should mean that it
has a high viscosity and therefore is expected to be isothermal with height, with very
large scale sizes, both temporal and spatial. This would mean that altitude variation in the
630nm emission peak should not matter. However, there are many examples of mesoscale structures seen in the upper thermosphere. Large temperature differences of 300450K across the 500km field-of-view of the FPI have been noted by Aruliah et al.,
[2005]. Conde et al [2001] have used an all-sky FPI to show complicated structures in the
thermosphere that correspond closely with auroral boundaries (Figure 6). Lühr et al.
[2007] have calculated thermospheric wind speeds averaging 500m/s from the cross-track
wind drag on the CHAMP orbit at an altitude of around 400km. This is double the size of
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Tuesday, 17 February 2008
winds observed by ground-based FPIs and implies considerable vertical shear. Thus the
uniformity of the upper thermosphere on meso-scales is being challenged by observations
and must be investigated. It is also imperative to study the emission heights to ensure that
the method of observation is reliable.
References
Aruliah, A. L.;
Griffin, E. M.;
Aylward, A. D.;
Ford, E. A. K.;
Kosch, M. J.;
Davis, C. J.;
Howells, V. S. C.; Pryse, S. E.; Middleton, H. R.; Jussila, J. , First direct evidence of meso-scale variability
on ion-neutral dynamics using co-located tristatic FPIs and EISCAT radar in Northern Scandinavia,
Annales Geophys., vol. 23, Issue 1, pp.147-162, 2005
Conde, M.; Craven, J. D.; Immel, T.; Hoch, E.; Stenbaek-Nielsen, H.; Hallinan, T.; Smith, R. W.; Olson, J.;
Sun, Wei; Frank, L. A.; Sigwarth, J., Assimilated observations of thermospheric winds, the aurora, and
ionospheric currents over Alaska, JGR, 106 (6), 10493-10508, 2001
Ford, E. A. K.; Aruliah, A. L.; Griffin, E. M.; McWhirter, I., High time resolution measurements of the
thermosphere from Fabry-Perot Interferometer measurements of atomic oxygen, Annales Geophys.,
Volume 25, Issue 6, 2007, pp.1269-1278, 2007
Griffin, E.M., A.L.Aruliah, I.McWhirter, H.-C.I.Yiu, A.Charalambous, I.McCrea, Upper thermospheric
ion-neutral coupling from combined optical and radar experiments over Svalbard, to be submitted to
Annales Geophys.
Holmes, J.M., M.Conde, D.Lummerzheim, Morphology of evening sector aurorae in λ557.7-nm Doppler
temperatures, GRL, 32, 10.1029/2004GL021553, 2005
Lühr, H.; Rentz, S.; Ritter, P.; Liu, H.; Häusler, K., Average thermospheric wind patterns over the polar
regions, as observed by CHAMP, Annales Geophys., 25(5), 1093-1101, 2007
Rishbeth H., O.K.Garriott, Introduction to Ionospheric Physics, Academic Press, 1969
Figure 1
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Tuesday, 17 February 2008
Figure 6 SCANDI Tn latitude versus UT on 5th
Dec 2007
Figure 5 Derived current densities from
the MIRACLE magnetometer network
on the 5th Dec 2007
Figure 4 Showing localised high plasma
velocities to north-west of Svalbard
Figure 3 Sharp burst in meridional
wind seen only to north of Svalbard
Figure 2 Poker Flat SDI neutral winds show
alignment with auroral arcs seen in all-sky
camera
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