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Observation of a NEIAL event with a radar interferometer
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.ORCID iD: 0000-0001-6802-1842
University of Tromso, Norway. (Department of Physics)
University of Tromso, Norway. (Department of Physics)
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.ORCID iD: 0000-0003-2422-5426
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

We present the first measurements of Naturally Enhanced Ion Acoustic Line (NEIAL) echoes observed with 5 antennas. The observations were obtained with the European Incoherent CATter (EISCAT) radar and the EISCAT Aperture Synthesis Imaging receivers (EASI) installed at the EISCAT Svalbard site. Four baselines of the interferometer are used in the analysis. Based on the coherence estimates derived from the measurements we show that the enhanced backscattering region is limited in the plane perpendicular to the geomagnetic field. Previously it has been argued that the enhanced backscatter region is limited in size, however, here the first unambiguous observations are presented. The size of the enhanced backscatter region is determined to be less than 900500 m, and at times less than 160 m in the direction of the longest antenna separation, assuming the scattering region to have a Gaussian scattering cross section in the plane perpendicular to the geomagnetic field. Using aperture synthesis imaging methods volumetric images of the NEIAL echo are obtained showing the enhanced backscattering region to be aligned with the geomagnetic field. Although optical auroral emissions are observed outside the radar beam we show that the enhanced backscatter region and optical emission likely occur on the same geomagnetic field lines.

Keyword [en]
Auroral Ionosphere, Instability, NEAIL
National Category
Physical Sciences
Research subject
Physics
Identifiers
URN: urn:nbn:se:kth:diva-159735OAI: oai:DiVA.org:kth-159735DiVA: diva2:787158
Note

QC 20150210

Available from: 2015-02-09 Created: 2015-02-09 Last updated: 2015-03-03Bibliographically approved
In thesis
1. Radar Signatures of Auroral Plasma Instability
Open this publication in new window or tab >>Radar Signatures of Auroral Plasma Instability
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Incoherent scatter radars are powerful ground based instruments for ionospheric measurements. By analysis of the Doppler shifted backscatter spectrum, containing the signature of electrostatic plasma waves, plasma bulk properties are estimated. At high latitudes the backscattered radar power is occasionally enhanced several orders of magnitude above the thermal backscatter level. These enhancements occur during geomagnetic disturbed conditions and are referred to as naturally enhanced ion acoustic echoes (NEIALs). NEIALs are linked to auroral activity with optical auroral emission observed in the vicinity of the radar measurement volume simultaneously to NEIALs. The backscatter enhancements are thought to be caused by wave activity above thermal level due to instability. A number of theories have been put forward including streaming instabilities and Langmuir turbulence to explain NEIAL observations. NEIALs occur in two classes distinct by their Doppler features. Observations of the first type, which has been studied more extensively, are generally modelled well by the Langmuir turbulence model. The difficulty in trying to understand the driving mechanism of the instability is the limited spatial resolution of the radar measurements. Observations of the second type, reported on more recently, have been interpreted as evidence for naturally occurring strong Langmuir turbulence by means of their Doppler features.

Aperture synthesis is a technique to increase the spatial resolution of the radar measurements to below beam width of the single receiver antennas. The technique is employed to investigate the structure of NEIALs in the plane perpendicular to the magnetic field at sub-degree scale corresponding to hundreds of meters to a few kilometres at ionospheric altitudes. Calibration of the radar interferometer is necessary and a calibration technique is presented in paper I. Interferometry observations of a NEIAL event with receivers deployed at the EISCAT incoherent scatter radar on Svalbard are presented in paper II. The size of the enhanced backscatter region is found to be limited to 900 x 500m in the plane perpendicular to the geomagnetic field. These observations constitute the first unambiguous measurements giving evidence for the limited size of the enhanced backscatter region.

In paper III observations of strong Langmuir turbulence signatures are presented. The apparent turbulent region in these observations is limited to two narrow altitude regions, 2km extent, and electron density irregularities caused by the turbulence are thought to reach down to decimeter scale length. The turbulence observations were obtained during energetic electron precipitation thereby differing from other observations during which a low energy component in the electron precipitation is reported. In paper IV a statistical study of strong Langmuir turbulence radar signatures is presented. The study reveals differing local time distributions for these signatures from type I NEIALs indicating di_ering driving conditions for the two types of NEIALs. It is found that strong Langmuir turbulence signatures are predominantly observed in the pre-midnight sector where auroral break-up aurora prevails.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. xv, 65 p.
Series
TRITA-EE, ISSN 1653-5146 ; 2015:007
Keyword
Ionosphere, Particle Precipitation, Instability, Plasma Turbulence
National Category
Fusion, Plasma and Space Physics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-160894 (URN)978-91-7595-442-4 (ISBN)
Public defence
2015-03-27, F3, Lindstedtsvägen 26, KTH, Stockholm, 09:56 (English)
Opponent
Supervisors
Note

QC 20150303

Available from: 2015-03-03 Created: 2015-03-03 Last updated: 2015-03-03Bibliographically approved

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Schlatter, NicolaIvchenko, NickolayDahlgren, Hanna

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