Mapping ionospheric backscatter measured by the SuperDARN HF radars - Part 1: A new empirical virtual height model

Accurately mapping the location of ionospheric backscatter targets (density irregularities) identified by the Super Dual Auroral Radar Network (SuperDARN) HF radars can be a major problem, particularly at far ranges for which the radio propagation paths are longer and more uncertain. Assessing and increasing the accuracy of the mapping of scattering locations is crucial for the measurement of two-dimensional velocity structures on the small and meso-scale, for which overlapping velocity measurements from two radars need to be combined, and for studies in which SuperDARN data are used in conjunction with measurements from other instruments. The co-ordinates of scattering locations are presently estimated using a combination of the measured range and a model virtual height, assuming a straight line virtual propagation path. By studying elevation angle of arrival information of backscatterred signals from 5 years of data (1997-2001) from the Saskatoon SuperDARN radar we have determined the actual distribution of the backscatter target locations in range-virtual height space. This has allowed the derivation of a new empirical virtual height model that allows for a more accurate mapping of the locations of backscatter targets

Comment on 'Concerning the generation of geomagnetic giant pulsations by drift-bounce resonance ring current instabilities' by K.-H. Glassmeier et al., Ann. Geophysicae, 17, 338-350, (1999)

International audienceNot availabl

Magnetic local time variation and scaling of poleward auroral boundary dynamics

The balance of dayside and nightside reconnection processes within the Earth's magnetosphere, and its effect on the amount of open magnetic flux threading the ionosphere is well understood in terms of the expanding-contracting polar cap model. However, the nature and character of the consequential fluctuations in the polar cap boundary are poorly understood. By using the poleward auroral luminosity boundary (PALB), as measured by the FUV instrument of the IMAGE spacecraft, as a proxy for the polar cap boundary we have studied the motion of this boundary for more than two years across the complete range of magnetic local time. Our results show that the dayside PALB dynamics are broadly self-similar on timescales of 12 minutes to 6 hours and appear to be monofractal. Similarity with the characteristics of solar wind and interplanetary magnetic field (IMF) variability suggest that this dayside monofractal behaviour is predominantly inherited from the solar wind via the reconnection process. The nightside PALB dynamics exhibit scale-free behaviour at intermediate timescales (12-90 minutes) and appear to be multifractal. We propose that this character is a result of the intermittent multifractal structure of magnetotail reconnection

IMF clock angle control of multifractality in ionospheric velocity fluctuations

We present an analysis of 8 years of meridional line-of-sight ionospheric plasma velocity measurements from the Halley SuperDARN radar which investigates the effect of the interplanetary magnetic field (IMF) clock angle on the scaling exponents of the first three order velocity structure functions. We only use velocity measurements made poleward of the open/closed magnetic field line boundary in the nightside ionosphere. The measured scaling exponents are consistent with multifractal Kraichnan-Iroshnikov turbulence for all clock angles but with varying intermittency that decreases to zero during purely northward IMF conditions. We thus propose that intermittency is inherited from the solar wind but also discuss other possible reasons for this relationship. Citation: Abel, G. A., M. P. Freeman, and G. Chisham (2009), IMF clock angle control of multifractality in ionospheric velocity fluctuations, Geophys. Res. Lett., 36, L19102, doi:10.1029/2009GL040336

A statistical model of vorticity in the polar ionosphere and implications for extreme values

Measurements of vorticity in the Earth’s ionosphere enable the characterisation of turbulent structure in the ionospheric plasma flow and how it varies spatially in relation to large-scale magnetic field-aligned current (FAC) systems. We have determined the spatial variation of the probability density function (PDF) of ionospheric vorticity measurements made by the Super Dual Auroral Radar Network (SuperDARN) across the northern polar ionosphere for the 6-year interval 2000-2005 inclusive. These PDFs are highly leptokurtic, with heavy tails, and are well-modelled by Tsallis q-exponential probability distributions. The parameters of the model q-exponential distributions have been determined using maximum likelihood estimation (MLE), resulting in a statistical model of ionospheric vorticity that covers the polar ionosphere. The spatial variation of the model parameters is highly variable, with the shape and scale of the model distributions varying systematically in relation to the well-known FAC regions, showing that FACs have a major influence on the character of ionospheric plasma vorticity. From the model distributions we estimate the probability of observing extreme vorticity values with the SuperDARN radars (beyond thresholds of 5, 10, 20, and 40 mHz) across the northern polar ionosphere

Data‐driven basis functions for SuperDARN ionospheric plasma flow characterisation and prediction

The archive of plasma velocity measurements from the Super Dual Auroral Radar Network (SuperDARN) provides a rich dataset for investigation of magnetosphere-ionosphere-thermosphere coupling. However, systematic gaps in this archive exist in space, time, and radar look-direction. These gaps are generally infilled using climatological averages, spatially smoothed models, or a priori relationships determined from solar wind drivers. We describe a new technique for infilling the data gaps in the SuperDARN archive which requires no external information and is based solely on the SuperDARN measurements. We also avoid the use of climatological averaging or spatial smoothing when computing the infill. In this regard, our approach captures the true variability in the SuperDARN measurements. Our technique is based on data-interpolating Empirical Orthogonal Function (EOF) analysis. This method discovers from the SuperDARN data a series of dynamical modes of plasma velocity variation. We compute the modes of a sample month of northern hemisphere winter data, and investigate these in terms of solar wind driving. We find that the By component of the Interplanetary Magnetic Field (IMF) dominates the variability of the plasma velocity. The IMF Bz component is the dominant driver for the background mean field, and a series of non-leading modes which describe the two-cell convection variability, and the substorm. We recommend our new technique for reanalysis investigations of polar-scale plasma drift phenomena, particularly those with rapid temporal fluctuations and an indirect relationship to the solar wind

AMPERE Polar Cap Boundaries

The high-latitude atmosphere is a dynamic region with processes that respond to forcing from the Sun, magnetosphere, neutral atmosphere, and ionosphere. Historically, the dominance of magnetosphere–ionosphere interactions has motivated upper atmospheric studies to use magnetic coordinates when examining magnetosphere–ionosphere–thermosphere coupling processes. However, there are significant differences between the dominant interactions within the polar cap, auroral oval, and equatorward of the auroral oval. Organising data relative to these boundaries has been shown to improve climatological and statistical studies, but the process of doing so is complicated by the shifting nature of the auroral oval and the difficulty in measuring its poleward and equatorward boundaries. This study presents a new set of open–closed magnetic field line boundaries (OCBs) obtained from Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) magnetic perturbation data. AMPERE observations of field-aligned currents (FACs) are used to determine the location of the boundary between the Region 1 (R1) and Region 2 (R2) FAC systems. This current boundary is thought to typically lie a few degrees equatorward of the OCB, making it a good candidate for obtaining OCB locations. The AMPERE R1–R2 boundaries are compared to the Defense Meteorological Satellite Program Special Sensor J (DMSP SSJ) electron energy flux boundaries to test this hypothesis and determine the best estimate of the systematic offset between the R1–R2 boundary and the OCB as a function of magnetic local time. These calibrated boundaries, as well as OCBs obtained from the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) observations, are validated using simultaneous observations of the convection reversal boundary measured by DMSP. The validation shows that the OCBs from IMAGE and AMPERE may be used together in statistical studies, providing the basis of a long-term data set that can be used to separate observations originating inside and outside of the polar cap