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  • 1.
    Axelsson, Katarina
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Studies of auroral processes using optical methods2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The Aurora is a visual manifestation of the complex plasma processes that occur as the solar wind interacts with the Earth’s magnetosphere and ionosphere. Therefore, studies of the aurora can lead to better understanding of the near-Earth space environment and of fundamental physical processes.This thesis focuses on optical studies of the aurora, both ground-based observations using the Auroral Large Imaging System (ALIS) and measurements from instruments onboard the Japanese micro-satellite Reimei. Various properties of the aurora are studied, such as the characteristic energy of precipitating electrons and scale sizes of diffuse auroral structures. Our understanding of the ionospheric physical processes involved in a particular auroral emission is improved using conjugate particle and optical data.Auroral light is a result of radiative transitions between excited states of the ionospheric gases. These excited states are formed either by direct electron impact or by a series of more complicated processes, involving chemical reactions, where part of the energy is converted into auroral light. Studies of auroral emissions can therefore give information about primary particle fluxes, ionospheric composition, and the magnetospheric and ionospheric processes leading to auroral precipitation. One way of deducing the characteristic energy of the precipitating particles is by using intensity ratios of auroral emissions. To be reliable, this method requires a good understanding of the processes involved in the auroral emissions used. The method works well if the measurements are made along the geomagnetic field lines. Using data from ALIS, both in magnetic zenith and off magnetic zenith, this method is tested for angles further away from the direction of the magnetic field lines. The result shows that it is possible to use this technique to deduce the characteristic energy for angles up to 35 degrees away from magnetic zenith.Using ALIS we have also been able to study structures and variations in diffuse aurora. When mapped to the magnetosphere, this provides information about the characteristics of the modulating wave activity in the magnetospheric source region. A statistical study of the scale sizes of diffuse auroral structures was made and the result shows widths and separation between structures of the order of 13-14 km. When mapped to the magnetosphere, this corresponds to 3-4 ion gyro radii for protons with a typical energy of 7 keV. Magnetometer data show that the structures move southward with a speed close to zero in the plasma convection frame. Stationary mirror mode structures in the magnetospheric equatorial plane are a likely explanation for these diffuse auroral structures. In another study we use measured precipitating electron energy spectra to improve our understanding of how the auroral process itself relates to the 427.8 nm auroral emission, which is often used when studying intensity ratios between different emission lines. The 427.8 nm emission is a fairly simple emission to model, with only a few processes involved, but still has some uncertainties, mostly due to the excitation cross section. Simultaneous measurements of the intensity of this emission from ALIS and the intensity and electron flux from Reimei provide a way to evaluate different sets of cross sections in order to find the best fit to the experimental data. It also allows a comparison of the absolute calibration of ALIS and Reimei imagers, improving the possibility to use the space-borne data for other detailed quantitative studies.In order to compare absolute measurements of aurora using different imagers, optical instruments are usually absolute calibrated by exposing them to a calibration light source. In 2011 an intercalibration workshop was held in Sodankylä, Finland, where nine low light sources were compared to the radioactive Fritz Peak reference source. The results were compared with earlier calibration workshop results and show that the sources are fairly stable. Two sources were also calibrated with the calibration standard source at UNIS, Svalbard, and the results show agreement with the calibration workshop in Sodankylä within 15 to 25%. This confirms the quality of the measurements with ALIS and in turn also of the the Reimei imagers.

  • 2.
    Axelsson, Katarina
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering.
    Sergienko, T.
    Swedish Institute of Space Physics / Institutet för rymdfysik.
    Nilsson, H.
    Swedish Institute of Space Physics / Institutet för rymdfysik.
    Brändström, U.
    Swedish Institute of Space Physics / Institutet för rymdfysik.
    Asamura, K.
    Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara.
    Sakanoi, Takeshi
    Tohoku University, Graduate School of Science, Sendai, Japan.
    First negative system of N2 + in aurora: Simultaneous space-borne and ground-based measurements and modeling results2014In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 32, no 5, 499-506 p.Article in journal (Refereed)
    Abstract [en]

    The auroral emission of the first negative system of N2 + at 427.8 nm is analyzed using simultaneous measurements from the ground with ALIS (Auroral Large Imaging System) and from space with optical (MAC) and particle (ESA) instruments of the Reimei satellite. The study has two main objectives. The first is validation of the absolute calibration of the ALIS and the Reimei MAC cameras. The other task is to evaluate different cross sections of the electron excitation of N2 + that are used for the modeling of the auroral 1N system emissions. The simultaneous measurements of the 427.8 nm emission by ALIS and Reimei imagers show excellent agreement, indicating that the calibration of the two instruments is correct. Comparison of the 427.8 nm emission intensity calculated using the incident electron flux measured by the Reimei particle instruments with intensities measured by the optical imagers show that the best match is reached with the cross section from Shemansky and Liu (2005).

  • 3.
    Axelsson, Katarina
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Space Technology.
    Sergienko, T.
    Swedish Institute of Space Physics / Institutet för rymdfysik.
    Nilsson, H.
    Swedish Institute of Space Physics / Institutet för rymdfysik.
    Brändström, U.
    Swedish Institute of Space Physics / Institutet för rymdfysik.
    Ebihara, Y.
    Research Institute for Sustainable Humanosphere, Kyoto University.
    Asamura, K.
    Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara.
    Hirahara, M.
    Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo.
    Spatial characteristics of wave-like structures in diffuse aurora obtained using optical observations2012In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 30, no 12, 1693-1701 p.Article in journal (Refereed)
    Abstract [en]

    We present the results of a statistical study using optical images from ALIS (Auroral Large Imaging System) to investigate the spatial and temporal variations of structures in diffuse aurora. Analysis of conjugate Reimei data shows that such fine structures are a result of modulation of high-energy precipitating electrons. Pitch angle diffusion into the loss cone due to interaction of whistler mode waves with plasma sheet electrons is the most feasible mechanism leading to high-energy electron precipitation. This suggests that the fine structure is an indication of modulations of the efficiency of the wave-particle interaction. The scale sizes and variations of these structures, mapped to the magnetosphere, can give us information about the characteristics of the modulating wave activity. We found the scale size of the auroral stripes and the spacing between them to be on average 13-14 km, which corresponds to 3-4 ion gyro radii for protons with an energy of 7 keV. The structures move southward with a speed close to zero in the plasma convection frame.

  • 4.
    Axelsson, Katarina
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering.
    Sergienko, T.
    Swedish Institute of Space Physics / Institutet för rymdfysik.
    Sandahl, Ingrid
    Brändström, U.
    Swedish Institute of Space Physics / Institutet för rymdfysik.
    A study on the possibility to deduce the 2D distribution of the auroral electron precipitation from multi wavelength optical measurements with auroral imagers2011In: Óptica Pura y Aplicada, ISSN 0030-3917, E-ISSN 2171-8814, Vol. 44, no 4, 605-609 p.Article in journal (Refereed)
    Abstract [en]

    The intensity ratios of auroral emissions at different wavelengths are widely used for reconstruction of auroral electron parameters. This method works quite well if the measurements of the auroral emissions are conducted in the magnetic zenith direction. In this study we want to investigate the possibility to use the intensity ratio method in the case where the observations are made in a direction not parallel to the magnetic field. In particular, we want to check the possibility of using auroral data for deducing the 2D distribution of the auroral electron precipitation. We use ALIS multi-station measurements of the auroral red and green line emissions (6300 Å and 5577 Å) to get data in zenith and non-zenith directions. We also take into account that the red line emission peak and the green line emission peak are at different altitudes. The results of this investigation show we can obtain reliable results for angles up to 35º away from magnetic zenith.

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