ASK science case - UK EISCAT Support Group

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EISCAT Svalbard Radar joint optical/radar winter campaign in 2005/06

Betty Lanchester

(University of Southampton, UK)

Nickolay Ivchenko

(Alfvén Laboratory, KTH, Sweden)

Tom Grydeland

(University of Tromsø, Norway)

Introduction

Coordinated optical and radar studies of the aurora provide versatile datasets for studies of the physical mechanisms behind the phenomenon, and its effects in the ionosphere. Location at Svalbard provides a unique site, from which high latitude aurora (e.g. cusp and polar cap boundary) can be observed with optical instruments around the clock during the polar night.

The interests of the three contributing groups have had a close overlap in recent years, and our ESR observations have been carried out with varying degrees of coordination. The contacts between the groups have strengthened, and now we plan to coordinate efforts in a common campaign, with a joint application for radar time.

This winter new experimental equipment is being installed on Svalbard, widening the observational capabilities of the facilities already in place. One crucial addition to the ESR is a third antenna, providing the extension to the interferometric capabilities with a second baseline. On the optical side, a new multispectral imager will be installed, named ASK

(auroral structure and kinetics). The ASK project is a joint initiative of the University of

Southampton Solar-Terrestrial Physics group and Alfvén laboratory, KTH, and is funded by

PPARC in the UK and Swedish Science Council in Sweden. The instrument will take images of aurora in three different spectral lines, with a field of view of 3 degrees in the magnetic zenith. The existing Spectrographic Imaging Facility (SIF), run by the University of

Southampton and UCL, is being upgraded with a new detector on the imaging spectrograph, and new photometer channels. The Odin imager of the University of Tromsø will also be running at the ESR site.

Science Case

The objectives of our studies fall into two major interrelated categories: understanding the mechanisms behind small scale structuring of the aurora, and understanding the ionospheric effects of various kinds of aurora (including that caused by proton precipitation).

It has been known for several decades that auroral emissions may have spatial structures of order 50 m or less in the direction perpendicular to the magnetic field. Hence, understanding the appearance of the aurora requires knowledge of the origin of such small-scale features and the physical processes taking place within these thin sheets or filamentary structures.

Precipitation of energetic electrons as well as intense field-aligned currents (which are carried by low-energy electrons) inside these structures may create conditions favourable for growth of plasma instabilities. Hence the study of these auroral structures must be related to the study of plasma instabilities. The electron beams and resulting instabilities disturb the local thermodynamic equilibrium. ESR observations frequently show greatly enhanced spectra during such conditions. These enhanced echoes are coherent, as has been demonstrated by interferometry using both ESR antennas, and originate from regions a few hundred meters

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across the magnetic field. Our groups have initiated a study into the relation of these phenomena to the properties of the optical aurora . The naturally enhanced ion acoustic lines

(NEIALs) occur in connection with a specific type of dynamic rayed aurora [Grydeland et al., 2003; Blixt, 2005].

The auroral spectrum, consisting of a large number of emissions from various atmospheric constituents, contains information about both the energy distribution of precipitating particles, and the state of the high altitude atmosphere. This information can be revealed by analysing the spectrum of the aurora. This is one of the central topics of the research being done with the Spectrographic Imaging Facility (SIF), located at the auroral station in

Adventdalen [Ivchenko et al., 2004a; 2004b]. SIF consists of the High Throughput Imaging

Echelle Spectrograph (HiTIES), four photometers with narrow passband filters, and an auroral imager operating at video-rate with the long pass RG645 cutting out the long-lived oxygen emissions. Use of the Odin imager (operating with the same filter) at the ESR location allows bistatic observations to be made. The altitude distribution of the emissions also provides information on the energy distribution of the precipitating electrons.

The natural development in the field of spectral studies of the aurora is multispectral imaging.

This technique which has been rapidly growing in the last 3-5 years uses imagers with narrow band filters, rather than photometers, thus providing spatial information on the morphology of different emissions [Ivchenko, 2005]. Our new instrument, ASK (auroral structure and kinetics) will provide simultaneous imaging in three spectral intervals. The instrument has a narrow field of view (3 x 3 degrees), and will be deployed at the ESR site. The time resolution of the instrument will allow studies of the dynamic aurora to be carried out, providing spatially and temporally resolved imaging inside the radar beam.

The subjects with questions to be addressed by the ASK data are as follows.

Mechanisms leading to the formation of fine structure

Why are auroral arcs thin [Borovsky, 1993]?

Is it related to dispersive Alfvén waves [Stasiewicz, 2001]?

Does the structuring take place in the generator region, in the acceleration region, or below it?

What is the role of the lower ionosphere?

First multispectral observations [Semeter, 2001; Ivchenko, 2005] indicate that there are clear differences between the energy distributions of particles producing auroral forms such as rays and curls. What are the sources of these different distributions?

Plasma drifts in the F-layer

Imaging of active structured aurora in the forbidden ion line can provide a direct observation of plasma drifts in the topside ionosphere, thus giving the electric fields.

What is the role of ‘proper motion’ of auroral arcs [Haerendel, 1993]?

It has been suggested that thin ionization layers in the aurora may be produced by horizontal drift of the plasmas [Palmer, 1995; Lanchester et al., 1998]. Imaging in the forbidden line of the oxygen ion will be able to reveal such drifts, resolving this standing question in the radar community.

Ionospheric effects of the fine auroral structure

Filamentary structures affect the macroscopic properties of the aurora (e.g. efficient electron and ion heating). Averaging of nonlinear quantities yields results that depend on the variances [Semeter, 2002].

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An intriguing effect is the naturally enhanced ion acoustic echo in ISR data, related to structured active aurora [Blixt, 2005]. ASK data will provide characterisation of the energies of the auroral electrons with an unprecedented spatial resolution for ground based instruments, allowing the identification of the auroral structure in which the

NEIAL-producing instability operates.

Spatio-temporal characteristics of the structuring

Are statistical distributions of auroral scale sizes related to characteristic energies?

What do the apparent “optical flow” [Blixt, 2005] and vorticity [Vogt, 1999] tell us?

Time of flight energy dispersions are a common feature in particle data from rockets and satellites [e.g. Andersson, 2002]. What are their spatial properties?

Doppler-shifted hydrogen emissions observed in high latitude regions are a unique signature of proton precipitation. This particle source provides a significant energy input to ionospheric regions such as the cusp. In addition, it is a key tracer of magnetospheric regions and processes. SIF measures the line profile of Hydrogen beta from the ground in the magnetic zenith. By combining these measured profiles with modelling we can investigate the energy and angular distribution of the incoming protons in detail [Lanchester et al., 2003]. This is of particular interest during different geomagnetic conditions, and when Svalbard is in different locations relative to the open/closed field line boundary. Radar measurements provide valuable information about the ionosphere during such conditions.

The relationship between the signatures of electron precipitation and the proton-induced emissions is complex. Although radar measurements do not distinguish between the effects of protons and electrons, by combining radar and optical measurements with modelling, we can begin to determine what the different ionospheric effects might be. The high spectral resolution of the SIF measurements allows us to study many of the reactions responsible for the emissions, and to estimate their relative importance under different auroral conditions.

Among other ionospheric effects of the aurora to be studied are the mechanisms for electron and ion heating, and associated ion outflows. This coordinated campaign will give an opportunity to study events which may or may not have optical signatures.

Experimental modes

It is suggested to have joint time, with the experiment selectable by the observer at the ESR.

The two experiments to be used are interferometry, and ‘arc’.

For the interferometry experiment, a long pulse experiment will be used, with reception on all

3 antennas. The major target is NEIALS (E-region is not really covered by the long-pulse experiment).

The arc experiment is the highest time resolution experiment available on the ESR for E and

F region studies. It is an alternating code experiment, with a decoded range resolution of 0.9 km, and basic time resolution of 0.505 seconds. Several modifications are being introduced to the arc experiment to make it more versatile, and suitable for inter-antenna comparisons and interferometry tests.

Switching between the experiments is estimated to take under 10 minutes at most, so could be done once or twice during each observing session, depending on the conditions.

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Time budget, preliminary schedule, observers at ESR

Two periods most suitable for coordinated optical and radar observations:

Nov 30 - Dec 10 2005 inclusive, 11 days

Jan 3 - Jan 8 2006 inclusive, 6 days

The arc data are desirable at different times of day, including the cusp (e.g. 4-9 UT), the premidnight (19-23 UT), and afternoon (11-13 UT). This is in order to provide radar data for various types of aurora observed with ASK.

Schedule will be decided between experimenters nearer the campaign dates (total 100 hours).

During the 2005 run the following people will be involved in the campaign on Svalbard

(asterisk marks people trained to operate ESR):

UiT: Tom Grydeland (*), Vasyl Belyey (*) TBC, Bjorn Gustavsson

KTH: Nickolay Ivchenko (*), Josef Höök

Soton: Joanna Sullivan (*), Olli-Pekka Jokiaho

The January run in 2006 is shorter, and the supporting optical instrumentation will be already in place and set up, so a smaller number of experimenters will be involved:

UiT: Tom Grydeland (*), Mårten Blixt (*) TBC

Total time application 100 hours, divided as follows:

SWE 60 hours

UK 20 hours

NO 20 hours

References

Andersson, L.; Ivchenko, N.; Clemmons, J.; Namgaladze, A. A.; Gustavsson, B.;

Wahlund, J.-E.; Eliasson, L.; Yurik, R. Yu., Electron signatures and Alfvén waves,

JGR, 107, 15-1, 2002

Blixt, E. M., T. Grydeland, N. Ivchenko, T. Hagfors, C. La Hoz, B. S. Lanchester, U-

P. Lauvhaug, and T. S. Trondsen, Dynamic rayed aurora and enhanced ion-acoustic radar echoes, Annales Geophys. 23, 3-11, 2005.

Borovsky, J. E., Auroral arc thicknesses as predicted by various theories, J. Geophys.

Res. 98, 6101, 1993.

Grydeland, T.; La Hoz, C.; Hagfors, T.; Blixt, E. M.; Saito, S.; Strømme, A.; Brekke,

A. Interferometric observations of filamentary structures associated with plasma instability in the auroral ionosphere Geophys. Res. Lett., 30,. 71-1, 1338, DOI

10.1029/2002GL016362, 2003.

Haerendel, G. et al., On the proper motion of auroral arcs, JGR, 98, 6087-6099, 1993.

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Ivchenko, N., M. Galand, B.S. Lanchester, M. H. Rees, D. Lummerzheim, I. Furniss and J. Fordham, Observation of O

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Furniss, L. M Peticolas, H. U. Frey, M. Mendillo, and J. Baumgardner High resolution measurements and modeling of auroral hydrogen emission line profiles,

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Palmer, J. R., Plasma density variations in the aurora, PhD thesis, Univ. of

Southampton, 1995

Semeter, J., D. Lummerzheim, and G. Haerendel, Simultaneous multispectral imaging of the discrete aurora, J. of Atm. Sol-Terr. Phys., 63, 1981-1992, 2001.

Semeter, J. and Doe, R., On the proper interpretation of ionospheric conductance estimated through satellite photometry, JGR, 107, 19-1, 2002.

Stasiewicz, K., P. Bellan, C. Chaston, C. Kletzing, R.Lysak, J. Maggs, O. Pokhotelov,

C. Seyler, P. Shukla, L. Stenflo, A. Streltsov, J.-E. Wahlund, Small Scale Alfvénic

Structure in the Aurora, Space Sci. Rev., 92, 423-533, 2000.

Vogt, J. et al., Shear velocity profiles associated with auroral curls, JGR, 104, 17277-

17288, 1999.

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