Original material on asymmetric spectra for the research grant application.
It has long been known that “anomalous echoes” are a pervasive feature in ion line
data from incoherent scatter radars such as EISCAT. The term is basically used to
describe any received spectrum whose distribution of power with respect to frequency
differs from the classical symmetric single or double-humped forms predicted by the
standard scattering theory applicable to ion-acoustic waves in plasmas with a
Maxwellian thermal velocity distribution.
Because departures from this equilibrium situation can be produced by a number of
different mechanisms, the spectra classified as being “anomalous” originate from
many different causes. For example, a well-defined type of spectral deformation
arises in the case of high ion-neutral relative velocities, when charge exchange
collisions between ionised and neutral oxygen atoms transform the thermal velocity
distribution from a Maxwellian to a toroidal form. EISCAT data have been of
enormous significance in enabling such spectra to be identified and studied.
In the last few years, intense interest has focused on anomalous echoes which
apparently occur in association with intense auroral precipitation. Such spectra can
apparently occur over a wide range of altitudes, and are characterised by strong
enhancements in either the upshifted or downshifted ion acoustic lines. Their cause is
still a matter of some debate. One possible speculation is that they arise as a result of
instablilities driven by field-aligned currents. However, the current densities required
to achieve the observed effects are much higher than the levels generally accepted for
auroral currents. Another possibility is that they arise as a result of some kind of
wave coupling, with modes such as Alfven waves providing the primary energy
source. Such an explanation has the attractive feature that it does not demand
apparently unfeasible current densities; however, some elaboration of the basic theory
is needed to explain the whole range of observed spectral phenomena.
Latest research using novel interferometric experiments on the EISCAT Svalbard
Radar in conjunction with high temporal resolution data from ground-based optical
systems and from the Oersted satellite, strongly suggests that at least some of the
anomalous echoes are co-incident with very narrow, but highly dynamic, filamentary
current structures within active aurora. This may hint at a current-driven origin for
these echoes. If this is confirmed, it would imply that spatially small field-aligned
currents can support much higher current densities than are conventionally measured,
suggesting that the commonly assumed range of current densities is an underestimate
arising from measurements which are highly averaged.
In order to make further progress on this topic, a combination of (at least) three
technologies is needed. Firstly, the interferometric techniques developed by the
Norwegians are essential to resolve structures with scale sizes appreciably smaller
than the ESR beamwidth (about 1.5 km at 100 km altitude). These techniques need to
be implemented in conjunction with radar experiments having very high temporal
resolution (resolutions of less than half a second are possible with the latest modes).
These observations in turn need to be supported by state-of-the-art optical data from
an instrument which combines both high spatial and temporal resolution with imaging
capabilities at multiple wavelengths. The resulting data sets will produce a substantial
data processing challenge, because of their very large size and the relative
infrequency of suitable events.
Members of the EISCAT group at RAL already have experience of highly timeresolved modes on the ESR, as these have been used by a number of UK groups. RAL
staff members also have the possibility of accessing data from instruments on the
UCL/Southampton optical platform, including a multi-frequency spectrograph and a
high-resolution imager. We do not, however, have direct experience of interferometric
work (though the correlation techniques needed are not in principle very complex) nor
do we have access to the receiver hardware which is needed in order to make the
necessary measurements (basically an independent digital signal processor for the
second ESR antenna, as the existing DSP at the ESR cannot process signals from two
antennas simultaneously).
The receiver hardware needed to achieve this could be obtained from a number of
commercial sources. The most suitable would probably be the MIDAS-W receiver
system, already used at the Millstone Hill incoherent scatter radar in the USA. This
system is essentially the same as that used by the Tromso group in their existing
interferometry experiments. It uses open-source software available under the auspices
of the international Open Radar Initiative (ORI), so that the required signal processing
code is essentially already available, and has been extensively tested for incoherent
scatter radar applications. The basic hardware configuration consists of a fast ADC
card plus two CPUs with appropriate adapters and switching. A MIDAS-W receiver
system which could process two incoming signals simultaneously has been costed at
$27,570 = £17,093. The performance of the system could be further enhanced by the
additional purchase of high-speed networking, though the networking infrastructure
already available at the ESR is probably adequate for the present.
Even if we were not funded to develop our own interferometry programme, there is
much useful work which could be done on asymmetric spectra using conventional
EISCAT/ESR data. Previous studies have suggested that many of the data sets which
appear to show strong field-aligned flows at high-altitude are actually instances of
anomalous spectra which, although not as dramatic as those associated with auroral
arcs, nonetheless display measurable asymmetries which are not simply the result of
Doppler shifts caused by ion drift in a Maxwellian plasma but more probably arise
from processes such as ion-acoustic turbulence (see, for example, the results of
Wahlund et al, JATP, 55, 623-645, 1993).
To the best of our knowledge, all of the reported examples of such “turbulence
spectra” have been case studies, and there has never been a systematic attempt to
investigate their occurrence as a function of the auroral processes which may cause
them, or the electron heating to which they are expected to give rise. A proper
statistical study of spectral asymmetry and its relationship to auroral precipitation and
plasma temperature is long overdue. The large size of the EISCAT and ESR raw data
sets available at RAL, and our previous experience in developing tools to search for
asymmetric spectra, make us ideally placed to carry out such a study. The results may
have important consequences for our understanding of the physics of topside electron
temperature enhancements and ion outflows.
There are also a number of unresolved questions relating to the study of anomalous
spectra arising from non-Maxwellian ion thermal velocity distributions. It is now
well established that the shape of the observed spectrum is strongly dependent on
aspect angle, and the extent of this anisotropy gives important information on the
dominant ion-neutral collision process. A vast amount of work has been done on
studying this phenomenon at altitudes close to the F-region peak, where charge
exchange collisions dominate. Some work on E-region temperature anisotropies has
also been done, demonstrating the importance of polarisation collisions involving
molecular species. However, the nature and extent of temperature anisotropies in the
topside F-region has hardly been studied at all, due to the practical difficulties
involved in making the required measurements. The understanding of topside ion
temperature anisotropy remains important, however, as it will shed light on the
relative importance of ion-ion collisions, the effects of which are almost impossible to
measure by any other means.
Only now, after more than 20 years of operation, has the EISCAT system developed
to a state where topside ion temperature anisotropies can be properly studied using
simultaneous field-aligned observations from the ESR 42m dish and a low-elevation
northward-looking beam from the EISCAT UHF to achieve a common volume at an
altitude of 600 km above Svalbard. The long range from Tromso (~1200 km) means
that a high transmitter power will be needed to achieve the required accuracy of
measurement, and this should become possible following the installation of new
rotary joints which will, for the first time ever, allow dual-klystron transmission on
the UHF system. This is expected in early 2004.
Members of the RAL EISCAT group have, more than a year ago, been allocated
EISCAT time to carry out the experiment proposed above, but it has not yet been
possible for hardware reasons. Results from such experiments which could be run in
the next two years, will confirm or refute our present understanding of ion-ion
collisions in terms of their importance in upper atmosphere energy transport.
Naturally-enhanced ion-acoustic spectra observed by the EISCAT Svalbard Radar
(top panel), and a simultaneous narrow-angle auroral image (bottom panel) (from
Sedgemore-Schulthess and St. Maurice, Surveys in Geophysics, 22, 55-92, 2001)
Anomalous ion line spectra recorded simultaneously on both dishes of the ESR, using
a MIDAS-W receiver (left) and the standard ESR receiver (right) with a two-second
integration. (T. Grydeland, private communication).