A statistical analysis of ionospheric velocity and magnetic field power spectra at the time of pulsed ionospheric flows

Abstract

[1] We analyze the power spectrum of the line-of-sight ionospheric velocity measured by the SuperDARN CUTLASS Finland radar and of contemporaneous magnetic field perturbations measured by nearby IMAGE magnetometers. The measurements come from 69-87°AACGM latitude and 0415-2230 MLT during 58 intervals of pulsed ionospheric flow between March 1995 and September 1996. The median power spectrum of both the velocity and magnetic field is of power law form with a best fit exponent of À0.5 and À1.3, respectively. By simulating the effect of the finite sample window on the measured spectral exponent and on the structure seen in the power spectral density of individual spectra, we show that the measured spectral exponent actually corresponds to a true exponent of À0.95 for the velocity and À1.8 for the magnetic field. Furthermore, it is shown that the difference in the exponents of the velocity and magnetic field spectra is at least in part due to a spatial smoothing effect that causes the spectral exponent of the ground magnetic spectrum to be less than that of the ionospheric current spectrum producing it. The power law power spectrum implies that there is no preferred timescale for ionospheric velocity and current fluctuations in the observed frequency range. It is proposed that this scale-free nature may arise from the intermittent, turbulent nature of the interplanetary magnetic field causing magnetic reconnection to occur at the magnetopause on a wide range of spatial and temporal scales.

Dynamical critical scaling of electric field fluctuations in the greater cusp and magnetotail implied by HF radar observations of F-region Doppler velocity

“…Although the observed v LOS fluctuations were mostly due to temporal variations in the electric field (Abel and Freeman, 2002), the radars rotated through their respective time sectors once per day. Hence the observed fluctuations were influenced by spatial variations in the underlying statistical convection pattern, especially at the largest time scale τ .…”

A decade of the Super Dual Auroral Radar Network (SuperDARN): scientific achievements, new techniques and future directions

“…Now, with over 10 years of SuperDARN data already collected, and with the increased computational power and data storage facilities available, it is possible to address science questions with large-scale statistical studies of SuperDARN data. There have been some instances of large statistical studies with SuperDARN which have proved very successful: defining statistical maps of ionospheric convection Greenwald 1996, 2005), determining the distribution of recurrence times of ionospheric flow burst signatures of FTEs (McWilliams et al 2000;Abel and Freeman 2002), and determining the global relationship between the SuperDARN SWB and the open-closed magnetic field line boundary (Chisham et al 2004a(Chisham et al , 2005a. One area where large-scale statistical analysis may have a large impact in the future is in the emerging area of complexity science.…”

“…McWilliams et al (2000) further compared their measured distribution with the distributions of inter-FTE intervals deduced from the ISEE spacecraft (Lockwood and Wild 1993;Kuo et al 1995), and the distribution of the recurrence time of PMAFs in optical data (Fasel 1995), showing that all the distributions were very similar. Abel and Freeman (2002) argued that the functional form of these occurrence distributions was a power law, implying that there is no preferred timescale for these convection velocity fluctuations. They proposed that this scale-free nature could arise from the intermittent, turbulent nature of the IMF causing magnetic reconnection to occur at the magnetopause across a wide range of spatial and temporal scales (Coleman and Freeman 2005).…”

Investigating turbulent structure of ionospheric plasma velocity using the Halley SuperDARN radar

“…Abel (gaab@bas.ac.uk) northward IMF, with modifications of these patterns by the east-west component of the IMF (Ruohoniemi and Greenwald, 2005). However, at scales smaller than the global scale, ionospheric convection is less well understood and has been shown to be complex and structured both temporally (Abel and Freeman, 2002) and spatially (Abel et al, 2006).…”

“…a measure of fluctuations with no characteristic scales (power law), over a wide range of scales. Examples include (a) power spectra of ground magnetic fields (Cambell, 1976;Francia et al, 1995;Consolini et al, 1998;Weatherwax et al, 2000;Abel and Freeman, 2002), and ionospheric electric fields (Kintner, 1976;Weimer et al, 1985;Bering et al, 1995;Buchert et al, 1999;Abel and Freeman, 2002;Golovchanskaya et al, 2006), (b) probability density functions (PDFs) of durations between threshold crossings of the auroral electrojet indices AU and AL (Freeman et al, 2000), (c) PDFs of durations, areas, and other quantities of auroral bright patches (Lui et al, 2000;Uritsky et al, 2002;Kozelov et al, 2004) and (d) structure functions of the auroral electrojet indices AU, AL and AE, the polar cap index PC (Takalo et al, 1993;Takalo and Timonen, 1998;Hnat et al, 2002), ground magnetic fields (Pulkkinen et al, 2006) and ionospheric convection (Parkinson, 2006;Abel et al, 2006).…”