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Radar Signatures of Auroral Plasma Instability
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.ORCID iD: 0000-0001-6802-1842
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 [en]
Ionosphere, Particle Precipitation, Instability, Plasma Turbulence
National Category
Fusion, Plasma and Space Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:kth:diva-160894ISBN: 978-91-7595-442-4 (print)OAI: oai:DiVA.org:kth-160894DiVA: diva2:792110
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
List of papers
1. Radar interferometer calibration of the EISCAT Svalbard Radar and a additional receiver station
Open this publication in new window or tab >>Radar interferometer calibration of the EISCAT Svalbard Radar and a additional receiver station
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2013 (English)In: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 105-106, 287-292 p.Article in journal (Refereed) Published
Abstract [en]

The EISCAT Svalbard Radar has two parabolic dishes. In order to attempt to implement radar aperture synthesis imaging methods three smaller, passive receive array antennas were built. Several science goals for this new receiver system exist, the primary of which is to study so called naturally enhanced ion acoustic lines. In order to compare radar aperture synthesis imaging results with measurements from optical imagers, calibration of the radar interferometer system is necessary. In this work we present the phase calibration of the EISCAT Svalbard interferometer including one array antenna. The calibration was done using the coherent scatter from satellites passing through the radar beam. Optical signatures of the satellite transits provide accurate position for the satellites. By using transits of a number of satellites sufficient for mapping the radar beam, the interferometric cross-phase was fitted within the radar beam. The calibration technique presented in this work will be applied to all antenna pairs of the antenna configuration for future interferometry studies.

Keyword
Radio science (interferometry), Space plasma physics (instruments and techniques)
National Category
Natural Sciences Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-108331 (URN)10.1016/j.jastp.2012.11.017 (DOI)000328913100032 ()2-s2.0-84889594499 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20140122

Available from: 2012-12-20 Created: 2012-12-20 Last updated: 2017-12-06Bibliographically approved
2. Observation of a NEIAL event with a radar interferometer
Open this publication in new window or tab >>Observation of a NEIAL event with a radar interferometer
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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
Auroral Ionosphere, Instability, NEAIL
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-159735 (URN)
Note

QC 20150210

Available from: 2015-02-09 Created: 2015-02-09 Last updated: 2015-03-03Bibliographically approved
3. Enhanced EISCAT UHF backscatter during high-energy auroral electron precipitation
Open this publication in new window or tab >>Enhanced EISCAT UHF backscatter during high-energy auroral electron precipitation
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2013 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 31, no 10, 1681-1687 p.Article in journal (Refereed) Published
Abstract [en]

Natural enhancements in the backscattered power of incoherent scatter radars up to 5 orders of magnitudes above the thermal backscatter are sometimes observed at high latitudes. Recently observations of enhancements in the backscattered power including a feature at zero Doppler shift have been reported. These enhancements are limited in altitude to tens of kilometers. The zero Doppler shift feature has been interpreted as a signature of electron density cavitation. Enhanced plasma lines during these observations have also been reported. We report on the first EISCAT UHF observations of enhanced backscattered radar power including a zero Doppler shift feature. The enhancements originated from two distinct and intermittent layers at about 200 km altitude. The altitude extent of the enhancements, observed during auroral high-energy electron precipitation, was < 2 km.

Keyword
Ionosphere, Auroral ionosphere
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:kth:diva-116667 (URN)10.5194/angeo-31-1681-2013 (DOI)000326548600007 ()2-s2.0-84885789204 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20131129. Updated from manuscript to article in journal.

Available from: 2013-01-24 Created: 2013-01-22 Last updated: 2017-12-06Bibliographically approved
4. On the relation of Langmuir turbulence radar signatures to auroral conditions
Open this publication in new window or tab >>On the relation of Langmuir turbulence radar signatures to auroral conditions
2014 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 119, no 10, 8499-8511 p.Article in journal (Refereed) Published
Abstract [en]

We present a statistical study of anomalous radar echoes observed in the auroral ionosphere thought to be signatures of Langmuir turbulence (LT). Data obtained with the European Incoherent Scatter Svalbard radar during the international polar year (IPY) were searched for these anomalous echoes in the auroral Fregion. In incoherent scatter radar experiments LT may in certain circumstances be observed as enhanced backscattered radar power at the ion line frequencies, plasma line frequencies, and at zero Doppler shift. The power enhancement at zero Doppler shift could arise due to Bragg scattering from nonpropagating density fluctuations caused by strong LT. In the IPY data set, around 0.02% of the data comply with our search criteria for altitudes above 190 km based on the ion line spectrum including enhancement at zero Doppler shift. The occurrence frequency of the identified events peaks in the premidnight sector and increases with local geomagnetic disturbance. Enhanced backscattered power is observed with limited altitude extent (below 20 km in 70% of the events), and the altitude distribution of identified radar signatures in the ion line channel has a peak at about 220 km. Enhancement of the plasma line is observed with the ion line enhancements in more than 60% of the events. Two classes of enhanced plasma lines occur. In the first class, plasma lines are limited in frequency and altitude and occur at altitudes of ion line enhancements. In the second class, the plasma lines are spread in frequency and range and are observed at lower altitudes than the first class (at about 170 km) with frequencies close to 3 MHz. Available optical data available indicate that the identified events to occur during auroral breakup with high-energy electron precipitation.

Keyword
Langmuir turbulence, electron precipitation
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-158328 (URN)10.1002/2013JA019457 (DOI)000345445400037 ()2-s2.0-84925356757 (Scopus ID)
Note

QC 20150126

Available from: 2015-01-26 Created: 2015-01-07 Last updated: 2017-12-05Bibliographically approved

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