Coincident multi-point observations of the E- and F-region decametre-scale plasma waves at high latitudes
Highlights
► The E-region backscatter observed by the PolarDARN radars is presented. ► Most echoes are detected at locations where geometric magnetic aspect angles are in excess of 10°. ► The spectral echo types are very similar to that detected by the auroral SuperDARN radars. ► A new radar setup that has coverage of the E and F regions on the same magnetic field lines is used. ► The E-region phase velocity dependence on the electric field strength and the flow angle is studied.
Introduction
The magnetic-field-aligned irregularities or waves in the electron density are a common feature of the Earth's ionosphere. E-region irregularities are routinely observed using UHF, VHF and HF radar systems in the equatorial and auroral regions and as such have been extensively studied in recent decades. The auroral irregularities also known as the radar aurora have been found to occur at a multitude of scales/wavelengths from a few centimetres corresponding to UHF frequencies to decametre scales for HF radar observations. The overall goal of this research has been to gain a better understanding of the E-region irregularities in the context of the plasmaphysical and geophysical conditions that result in their generation.
There are two plasma instabilities that are thought to generate most of the irregularities observed in the E region: the Farley–Buneman instability (FBI) and the gradient-drift instability (GDI) (e.g. Fejer and Kelley, 1980). The FBI operates when a substantial electric field exists (i.e. ), which results in a strong differential plasma flow between the collisionally dominated ions and the fully magnetised electrons. The GDI operates when a background plasma density gradient exists (Simon, 1963), with the largest irregularity growth rates achieved when the gradient is aligned in the direction of the background electric field and perpendicular to the magnetic field (e.g. Fejer et al., 1984).
The vast majority of the E-region plasma wave observations have been conducted in the equatorial and auroral regions using coherent backscatter radar systems, with the polar cap irregularities much less investigated. Much of the work in the polar cap has been done using oblique sounders (Olesen et al., 1975, Iverson et al., 1975, Tsunoda et al., 1976), with slant E condition or slant E echoes attributed to scattering processes from FBI-generated irregularities. The coherent scatter radars, on the other hand, were usually used on a campaign basis (e.g. Kustov et al., 1996, Kustov et al., 1997). The reported backscatter properties were such that it was suggested that E-region irregularities were solely generated by the FBI (Hanuise, 1983). As discussed by Hanuise (1983), the orientation of the magnetic field in the polar cap is close to vertical, implying that in order for GDI waves to be generated the plasma density gradient must have a large component in the horizontal direction. Therefore, the strong vertical gradients that exist due to particle precipitation would not cause the observations of GDI waves in the polar cap region. Hence the radar backscatter should only be associated with the FBI in the polar region; i.e. it should only be observed during periods of strong electric field with the backscatter power peaking in the direction of the current (Hanuise, 1983). This idea has been largely supported by observations (e.g. Iverson et al., 1975, Tsunoda et al., 1976). However, it was later put forward by Kustov et al. (1994) that the electric field, or equivalently the plasma flow velocity, is not the only factor that controls the E-region irregularity production and detection in the polar cap. Kustov et al. (1994) demonstrated that other factors such as electrical conductance, refraction effects in the ‘inclined’ E layer and the field-aligned currents (FACs) also affect the detection of E-region irregularities in the polar cap.
At E-region altitudes, where the electrons are magnetised and the ions are collisionally bound to the neutrals, the differential plasma drift between the ions and electrons, Vd, is approximately equal to the electron drift velocity, where and are the background electric and magnetic fields, respectively. From the linear fluid theory of the FBI and GDI, the E-region irregularity phase velocity is known to vary with the cosine of the plasma flow angle . It is also a strongly decreasing function of the off-perpendicular aspect angle maximising in the direction perpendicular to the local magnetic field direction, i.e. at (e.g. Fejer and Kelley, 1980, Sahr and Fejer, 1996). The observations in the auroral region showed though that these relationships are far more complicated, with different spectral echo types exhibiting different dependencies (e.g. Villain et al., 1987, Villain et al., 1990, Uspensky et al., 2001, Milan and Lester, 2001, Makarevich, 2010). In particular, the so-called Type I echoes that are believed to be generated by the FBI have been found to move with velocities that were smaller than the plasma drift component being limited by the ion acoustic speed Cs (e.g. Nielsen and Schlegel, 1983, Nielsen and Schlegel, 1985). The phase velocity of the Type II echoes associated with the GDI was sometimes much smaller than the cosine component of the background flow (Milan et al., 2003, Makarevitch et al., 2004, Makarevich, 2010). The possible reasons as to why the E-region phase velocity was smaller than the plasma drift component were proposed to be the aspect angle attenuation of the phase velocity (Kustov et al., 1997, Koustov et al., 2002, Makarevich et al., 2006b, Makarevich et al., 2007), neutral wind effects (Koustov et al., 2002), and enhanced collision frequencies in the lower E region (Makarevitch et al., 2004, Koustov et al., 2005). More recently, the ion drift contribution has been demonstrated to be important for large aspect and flow angle observations (Uspensky et al., 2003, Uspensky et al., 2004; Makarevich et al., 2006a, Makarevich et al., 2006b, Makarevich et al., 2007). In the nonlocal generalisation of the FBI given by Drexler et al. (2002) it was demonstrated that the growth ofdecametre-scale waves is dominated by convective processes. The implication for HF radar observations is that for strong electric fields (e.g. ), the growth rates for the decametre-scale waves are relatively high and their phase speeds as measured on the ground should be between Cs and the drift component. However, for weaker electric fields the growth rates are smaller, which results in the dominance of convective effects and to a ground-based observer the phase speeds would approach the drift.
In the past it has been customary for the investigations into the E-region phase velocity dependence on the plasma flow velocity to employ plasma flow measurements obtained from the F region. This technique utilises the fact that the magnetic field lines that join these regions act as equipotential lines. The experimental configurations used to conduct these works have varied greatly. These included measurements from co-located scattering volumes of UHF incoherent and VHF coherent radars (Nielsen and Schlegel, 1983, Nielsen and Schlegel, 1985, Koustov et al., 2002, Uspensky et al., 2003, Uspensky et al., 2004, Makarevich et al., 2006a, Makarevich et al., 2007) and comparisons between the near- and far-range measurements within the field-of-view (FoV) of a single HF radar (Milan and Lester, 1998, Makarevitch et al., 2004, Makarevich, 2008, Makarevich, 2010). Coincident comparisons with the E-region HF velocities were problematic though since the instrumentation is normally positioned to maximise overlaps in the far-range F-region gates. A promising experimental setup has been employed in the study by Kustov et al. (1997) in which the E- and F-region irregularities were observed, respectively, by the stereoscopic VHF and HF radars, on the same magnetic field lines. The benefit of such an experimental setup involving HF radars for studies of the E-region plasma waves was also realised (Chisham et al., 2007), but it has not been until more recently that this sort of radar configuration could actually be used.
In this study, the datasets collected by the PolarDARN component of the Super Dual Auroral Radar Network (SuperDARN) HF radars (Greenwald et al., 1995, Chisham et al., 2007) were analysed. The high-latitude locations of these radars allow the detection of E-region backscatter from the largely unexplored polar region on a routine basis. The time interval studied here covers the recent deep solar minimum providing a good opportunity to establish the quiet-time echo occurrence trends and the associated physical characteristics of the polar cap ionosphere. In addition, the overlap with the auroral SuperDARN radars enables the observations of both E- and F-region backscatter on the same magnetic lines, giving a unique perspective of the ionospheric flow conditions that give rise to the E-region irregularities. The specific objectives of this study are: (1) to determine the typical occurrence patterns for the E-region backscatter from the polar cap and to compare them with those observed by the auroral SuperDARN radars and (2) to examine the plasma flow strength and flow-angle dependencies of the E-region irregularity velocity using the multi-point F-region plasma convection measurements.
Section snippets
Instrumentation
PolarDARN consists of two poleward-facing HF radars located at Rankin Inlet (62.8°N, 266.9°E, geographic) and Inuvik (68.4°N, 226.5°E, geographic) (Koustov et al., 2009), hereinafter referred to as RKN and INV, respectively. As most radars composing the SuperDARN array, the PolarDARN radars sequentially scan through 16 azimuthal beam directions separated by 3.25° in 1 or 2 min, depending on the mode of operation. The range resolution of the SuperDARN radars under the common radar scan modes is
Discussion
In this study, the data collected by the PolarDARN HF radars were employed to analyse the characteristics of the E-region decametre-scale irregularities in the polar region. In the first part, the statistical characteristics of the short-range echoes were analysed including the temporal/spatial variations in occurrence as well as the spectral populations. In the second part, a new HF radar configuration was employed to directly compare the measured phase velocities from the E and F regions
Summary and conclusions
In this study, the characteristics of the E-region echoes received by the PolarDARN RKN and INV radars from magnetic latitudes 75–80°N were analysed both statistically and on a case-by-case basis.
The E-region echo occurrence patterns were found to be very different from those of the auroral SuperDARN radars in previous studies. Most of the E-region echoes were detected in the midnight sector at the very short ranges, where the geometric aspect angles are quite large for both PolarDARN radars.
Acknowledgements
The RKN and INV SuperDARN radars operations are funded by a MRS Grant from Natural Sciences and Engineering Research Council of Canada (NSERC) and a Canadian Space Agency (CSA) contract. The CADI data and the ionogram analysis software was retrieved from the CHAIN website. The authors would like to thank A.C. Kellerman for his help with the SuperDARN plasma flow vector calculations. Fruitful discussions with A.V. Koustov and J.P. St.-Maurice are also kindly acknowledged.
References (62)
- et al.
On the diurnal variation of the E-region coherent HF echo occurrence
Journal of Atmospheric and Terrestrial Physics
(2010) - et al.
The 10th generation international geomagnetic reference field
Physics of the Earth and Planetary Interiors
(2005) International reference ionosphere 2000
Radio Science
(2001)- et al.
E-region decameter-scale plasma waves observed by the dual TIGER HF radars
Annales Geophysicae
(2009) - et al.
A decade of the Super Dual Auroral Radar Network (SuperDARN): scientific achievements, new techniques and future directions
Surveys in Geophysics
(2007) - et al.
Mapping ionospheric backscatter measured by the SuperDARN HF radars—part 1: a new empirical virtual height model
Annales Geophysicae
(2008) - et al.
New insights from a nonlocal generalization of the Farley–Buneman instability problem at high latitudes
Annales Geophysicae
(2002) - et al.
Doppler spectrum statistics obtained from three different-frequency radar auroral experiments
Annales Geophysicae
(1995) - et al.
Ionospheric irregularities
Reviews of Geophysics
(1980) - et al.
Theory of plasma waves in the auroral E region
Journal of Geophysical Research
(1984)
Aspect angle variations in intensity, phase velocity and altitude for high-latitude 34 cm E region irregularities
Journal of Geophysical Research
Comparison of plasma flow velocities determined by the ionosonde Doppler drift technique, SuperDARN radars, and patch motion
Radio Science
DARN/SuperDARN: a global view of the dynamics of high-latitude convection
Space Science Reviews
Radar observations of auroral electrojet currents
Journal of Geophysical Research
A review on radio studies of auroral E region ionospheric irregularities
Annales Geophysicae
High latitude ionospheric irregularities
Radio Science
Extraction of polar mesosphere summer echoes from SuperDARN data
Geophysical Research Letters
Statistics of Antarctic mesospheric echoes observed with the SuperDARN Syowa radar
Geophysical Research Letters
Further evidence for the Farley–Buneman instability in the polar cap ionosphere
Journal of Geophysical Research
On the relationship between the velocity of E-region HF echoes and ExB plasma drift
Annales Geophysicae
Velocities of auroral coherent echoes at 12 and 144 MHz
Annales Geophysicae
Observations of 50- and 12-MHz auroral coherent echoes at the Antarctic Syowa station
Journal of Geophysical Research
Three-way validation of the Rankin Inlet PolarDARN radar velocity measurements
Radio Science
Electrodynamics of the upper atmosphere and radio aurora in the northern polar cap
Geomagnetism and Aeronomy
The SAPPHIRE–North radar experiment: observations of discrete and diffuse echoes
Journal of Geophysical Research
Relationship of the SAPPHIRE-North merged velocity and the plasma convection velocity derived from simultaneous SuperDARN radar measurements
Journal of Geophysical Research
HF radar observations of high-velocity E-region echoes from the eastward auroral electrojet
Journal of Geophysical Research
On the occurrence of high-velocity E-region echoes in SuperDARN observations
Journal of Geophysical Research
HF echo types revisited: aspect angle attenuation effects
Annales Geophysicae
A first comparison of irregularity and ion drift velocity measurements in the E region
Annales Geophysicae
Aspect angle dependence of the E-region irregularity velocity at large flow angles
Journal of Geophysical Research
Cited by (5)
Statistical Analysis of the Electron Density Gradients in the Polar Cap F Region Using the Resolute Bay Incoherent Scatter Radar North
2018, Journal of Geophysical Research: Space PhysicsDual radar investigation of e region plasma waves in the southern polar cap
2015, Journal of Geophysical Research: Space PhysicsSignature of polar cap inhomogeneities in vertical sounding data
2013, Radio Science
- 1
Now at: SPACE Research Centre, RMIT University, Melbourne, Victoria 3001, Australia.