On the seasonal variation of electric and magnetic turbulence at high latitudes

Abstract

[1] High-pass filtered field measurements from the lowaltitude polar-orbiting Dynamic Explorer 2 (DE2) spacecraft are used to investigate seasonal variation in the high-latitude electric and magnetic fields on the scales from 3.8 km to 100 km. It is shown that the seasonal asymmetry has an opposite sense for the electric and magnetic components. The electric fields are by factor 3 enhanced and the magnetic fields are by factor 1.5 reduced under winter conditions as compared to summer conditions. The effect persists at very high polar latitudes, where the magnetic field lines are strongly stretched or open, so that the time required for the Alfvén-wave communication between the ionosphere of summer and winter hemispheres is very large (>1 hour) or infinite. In this case, the earlier interpretations of the seasonal effect, which suggest that the magnetospheric generator of the turbulent fields is linked by the field-aligned currents (FACs) to the high-latitude ionosphere of both hemispheres, may be not applicable. A new explanation is proposed, in which it is assumed that the FACs from the generator region close through the Pedersen current in the ionosphere of only one hemisphere, either wintertime or summertime, and through the inertial current in the front of an outgoing Alfvén wave. It is demonstrated that the values of conductances derived within such an assumption are in a reasonable agreement with observations.

Statistical maps of small‐scale electric field variability in the high‐latitude ionosphere

“…As described in section 3, the primary differences are that a larger tilt dependence and slightly larger average variability magnitudes are observed in the Southern Hemisphere than in the Northern Hemisphere. These results are somewhat consistent with those of Golovchanskaya [2007], who, using DE‐2 electric field data on scales from 3.8 to 100 km, observed slightly larger average variability in the Southern Hemisphere than in the Northern Hemisphere. The seasonal dependence of variability, however, was observed to be approximately the same in the two hemispheres in that study.…”

“…The techniques used to calculate small‐scale electric field variability and to derive statistical maps of variability are described in this section. These techniques are similar to those used by Golovchanskaya et al [2006], Golovchanskaya [2007], and Matsuo and Richmond [2008]with DE‐2 data in that small‐scale variability data is calculated, sorted by seasonal and solar wind conditions, and averaged onto a grid in geomagnetic coordinates. The method of calculating variability, however, is different in this study (velocity differences are measured over small spatial scales as opposed to the high‐pass filtering of time series) and the amount of data available is much larger (6 years of data from 10 to 16 radars as opposed to 1.5 years of data from a single satellite), capturing many seasons over almost half a solar cycle and enabling, for example, independent treatment of the two hemispheres.…”

“…The statistical properties of small‐scale electric field or velocity variability have been investigated using data from the Dynamics Explorer (DE) 2 low‐altitude satellite [ Heppner et al , 1993; Golovchanskaya et al , 2002; Johnson and Heelis , 2005; Golovchanskaya et al , 2006; Golovchanskaya , 2007; Matsuo and Richmond , 2008] and from ground‐based coherent backscatter radars [ Abel et al , 2007, 2009; Parkinson , 2008; Cousins and Shepherd , 2012]. In these studies, the observed variability is found to depend on scale size, on season or dipole tilt angle, on gradients or shears in the background plasma drift and on location in geomagnetic coordinates.…”

“…In these studies, the observed variability is found to depend on scale size, on season or dipole tilt angle, on gradients or shears in the background plasma drift and on location in geomagnetic coordinates. Several studies have also derived maps of the magnitude of small‐scale electric field variability in the high‐latitude region [ Golovchanskaya et al , 2006; Golovchanskaya , 2007; Matsuo and Richmond , 2008], finding that the spatial distribution of variability magnitudes changes with season and with the orientation of the interplanetary magnetic field (IMF). Note that these variability maps were all derived using satellite data (from DE 2), which cannot distinguish between spatial and temporal variations in the observed electric fields.…”

Statistical characteristics of small‐scale spatial and temporal electric field variability in the high‐latitude ionosphere

“…With the aim of characterizing the turbulent behavior and identifying the origin of the turbulence, the scaling properties of electric field (or equivalently, velocity) fluctuations have been the subject of many studies [e.g., Kintner , 1976; Weimer et al , 1985; Ishii et al , 1992; Earle and Kelley , 1993; Heppner et al , 1993; Tam et al , 2005; Golovchanskaya et al , 2006; Parkinson , 2006; Abel et al , 2007]. To estimate the contribution to the total electric field in the ionosphere and to the amount of energy input to the atmosphere, several statistical studies have also investigated the absolute magnitudes of small‐scale electric field variability observed in the ionosphere [ Heppner et al , 1993; Johnson and Heelis , 2005; Golovchanskaya et al , 2006; Golovchanskaya , 2007; Matsuo and Richmond , 2008]. These statistical studies were all based on data from the Dynamics Explorer (DE) 2 spacecraft, which operated for ∼1.5 years (August, 1981 to February, 1983) during the declining phase of solar cycle 21.…”