Incoherent scatter radar studies of electron precipitation
|Author:||Tesfaw, Habtamu Wubie1,2|
1University of Oulu, Faculty of Science, Physics, Space physics and astronomy (SpaceAstro)
2University of Oulu Graduate School
|Online Access:||PDF Full Text (PDF, 6 MB)|
|Persistent link:|| http://urn.fi/urn:isbn:9789526237084
Oulu : University of Oulu,
|Publish Date:|| 2023-05-31
|Thesis type:||Doctoral Dissertation
|Defence Note:||Academic dissertation to be presented with the assent of the Doctoral Training Committee of Technology and Natural Science of the University of Oulu, for public discussion in the Auditorium L6, on 16 June, 2023, at 12 o’clock noon
Docent Ilkka Virtanen
Professor Anita Aikio
Professor Betty Lanchester
Associate Professor Juha Vierinen
Professor Noora Partamies
Docent Ilkka Virtanen
In the studies presented in this thesis, we use the EISCAT UHF incoherent scatter radar (ISR) to study electron precipitation. A new ISR data analysis technique called BAFIM (BAyesian FIltering Module) is developed to calculate plasma parameters (electron density, electron temperature, ion temperature and line of sight ion velocity) with high time and range resolutions from incoherent scatter radar autocorrelation function (ACF) data. BAFIM adds properties of the so-called full-proﬁle analysis to the standard EISCAT data analysis tool, GUISDAP, and extends the concept of full-proﬁle analysis from range direction to both range and time.
BAFIM-ﬁtted electron density is used to study a rapidly varying electron precipitation event with high time resolution (4 s). Using a method called ELSPEC, diﬀerential number ﬂuxes of precipitating electrons are inverted from electron density altitude proﬁles measured along the geomagnetic ﬁeld line by the EISCAT UHF incoherent scatter radar. We show that the raw electron density, that was previously used in high time resolution works, may signiﬁcantly underestimate the true electron density, when auroral electron precipitation heats the electron gas. The bias aﬀects also electron energy spectra inverted from the raw density proﬁles, as well as auroral powers and ﬁeld-aligned currents integrated from the spectra. Temporal variations of the auroral power derived from the ﬁtted electron density show a very good agreement with variations of auroral emission intensity at 427.8 nm.
Using more than 20 years of EISCAT UHF radar data, we study statistical characteristics of 1–100 keV electron precipitation at 66.7° magnetic latitude over Tromsø, Norway. Peak energy, auroral power and number ﬂux of electron precipitation are derived from the radar data using the ELSPEC method. We ﬁnd that 1–5 keV electrons dominate the precipitation from evening until morning in magnetic local time (MLT), while 5–10 keV electrons dominate the late morning hours (06–09 MLT). The average peak energy of precipitating electrons increases almost monotonically from evening (18 MLT) to morning hours (09 MLT). Energetic 30–100 keV electrons, which have been poorly covered in previous studies, are observed most frequently in the post midnight and morning hours. The 30–50 keV electrons dominate the energetic electron precipitation before 06 MLT, after which the 50–100 keV precipitation becomes dominant. Auroral power of the precipitating electrons is mostly in the 2–10 mWm−2 range at night (18–09 MLT), and average auroral powers measured in the pre-midnight hours are all larger than the corresponding measurements in the post-midnight hours. Auroral powers larger than 30 mWm−2 are observed most frequently in the pre-midnight side of the main auroral oval. Number ﬂux of precipitating electrons has similar characteristics with auroral power. Occurrence rate of auroral electron precipitation as observed by the radar maximizes during declining phases of solar cycles 23 and 24, and during September and March equinoctial months. The occurrence frequency increases with MLT from evening to morning hours, partially due to motion of the auroral oval relative to the radar location.
The analysis tools developed and used in this work can be applied to data analysis of the next-generation EISCAT_3D radar, which is currently under construction in Finland, Norway, and Sweden. The tools will allow the radar to reach its full potential, and reveal small-scale and rapidly varying auroral structures with unprecedented temporal and spatial resolutions. The techniques could be applied also to other ISR systems and ELSPEC could be further developed to enable its use for day-time observations of electron precipitation.
Osajulkaisut / Original papers
Osajulkaisut eivät sisälly väitöskirjan elektroniseen versioon. / Original papers are not included in the electronic version of the dissertation.
Report series in physical sciences
|Type of Publication:||
G5 Doctoral dissertation (articles)
|Field of Science:||
115 Astronomy and space science
I would like to express my gratitude to the Kvantum Institute of the University of Oulu and the Vilho, Yrjö and Kalle Väisälä foundation of the Finnish Academy of Science and Letters for their financial support and funding.
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