Abstract

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.

1. Virtanen, I. I., Tesfaw, H. W., Roininen, L., Lasanen, S., & Aikio, A. (2021). Bayesian filtering in incoherent scatter plasma parameter fits. Journal of Geophysical Research: Space Physics, 126(3), e2020JA028700. https://doi.org/10.1029/2020JA028700

   Rinnakkaistallennettu versio / Self-archived version

2. Tesfaw, H. W., Virtanen, I. I., Aikio, A. T., Nel, A., Kosch, M., & Ogawa, Y. (2022). Precipitating electron energy spectra and auroral power estimation by incoherent scatter radar with high temporal resolution. Journal of Geophysical Research: Space Physics, 127(4), e2021JA029880. https://doi.org/10.1029/2021JA029880

   Rinnakkaistallennettu versio / Self-archived version

3. Tesfaw, H. W., Virtanen, I. I., & Aikio, A. (2023). Characteristics of auroral electron precipitation at geomagnetic latitude 67° over Tromsø. Manuscript submitted for publication.