Browsing by Author "Oksavik, Kjellmar"

Now showing 1 - 2 of 2
  • Thumbnail Image
    A comparison of SuperDARN ACF fitting methods
    Ribeiro, A. J.; Ruohoniemi, J. Michael; Ponomarenko, Pavlo V.; Clausen, Lasse B. N.; Baker, Joseph B. H.; Greenwald, R. A.; Oksavik, Kjellmar; de Larquier, S. (American Geophysical Union, 2013-05-01)
    The Super Dual Auroral Radar Network (SuperDARN) is a worldwide chain of HF radars which monitor plasma dynamics in the ionosphere. Autocorrelation functions are routinely calculated from the radar returns and applied to estimate Doppler velocity, spectral width, and backscatter power. This fitting has traditionally been performed by a routine called FITACF. This routine initiates a fitting by selecting a subset of valid phase measurements and then empirically adjusting for 2 phase ambiguities. The slope of the phase variation with lag time then provides Doppler velocity. Doppler spectral width is found by an independent fitting of the decay of power to an assumed exponential or Gaussian function. In this paper, we use simulated data to assess the performance of FITACF, as well as two other newer fitting techniques, named FITEX2 and LMFIT. The key new feature of FITEX2 is that phase models are compared in a least-squares fitting sense with the actual data phases to determine the best fit, eliminating some ambiguities which are present in FITACF. The key new feature of LMFIT is that the complex autocorrelation function (ACF) itself is fit, and Doppler velocity, spectral width, and backscatter power are solved simultaneously. We discuss some of the issues that negatively impact FITACF and find that of the algorithms tested, LMFIT provides the best overall performance in fitting the SuperDARN ACFs. The techniques and the data simulator are applicable to other radar systems that utilize multipulse sequences to make simultaneous range and velocity determinations under aliasing conditions.
  • Thumbnail Image
    Seasonal and Solar Cycle Variations of Thermally Excited 630.0 nm Emissions in the Polar Ionosphere
    Kwagala, Norah Kaggwa; Oksavik, Kjellmar; Lorentzen, Dag A.; Johnsen, Magnar G.; Laundal, Karl M. (2018-08)
    Solar cycle and seasonal variations have been found in the occurrence of strong thermally excited 630.0 nm emissions in the polar ionosphere. Measurements from the European Incoherent Scatter Svalbard Radar have been used to derive the thermal emission intensity. Thermally excited emissions have been found to maximize at solar maximum with peak occurrence rate of similar to 40% compared to similar to 2% at solar minimum. These emissions also have the highest occurrence in equinox and the lowest occurrence rate in summer and winter. There is an equinoctial asymmetry in the occurrence rate which reverses with the solar cycle. This equinoctial asymmetry is attributed to variations of the solar wind-magnetosphere coupling arising from the Russell-McPherron effect. The occurrence rate of thermal excitation emission on the dayside, at Svalbard, has been found to be higher in autumn than spring at solar maximum and the reverse at solar minimum. Enhanced electron temperatures characterize the strong thermal component for solar minimum and winter, whereas enhanced electron densities characterize the thermal component for solar maximum. The results point to solar wind-magnetosphere-ionosphere coupling as the dominant controlling process.