Roman A. Makarevich

PCN magnetic index and average convection velocity in the polar cap inferred from SuperDARN radar measurements

[1] The relationship between the polar cap north (PCN) magnetic index and the average convection velocity of the plasma flow across the polar cap is investigated using data from both the Rankin Inlet (RKN) polar cap Super Dual Auroral Radar Network (SuperDARN) radar and the entire SuperDARN network. Correlation between the PCN index and the average velocity, determined from the median RKN line of sight (LOS) velocity, maximizes near magnetic noon and midnight when the radar field of view is roughly aligned with the noon-midnight meridian. For observations between 1000 and 1100 MLT, a roughly linear increase of the average velocity was found for a PCN index between 0 and 2, but the rate of increase is � 2 times faster than in previous publications in which the average velocity was estimated from DMSP ion drift measurements. Comparisons between the PCN index with the cross-polar cap velocity estimated from (1) SuperDARN convection maps and (2) median RKN LOS velocities show similar trends. Both the average cross-polar cap velocity (estimated by two methods) and the cross-polar cap potential show a tendency for saturation at PCN > 2. No significant seasonal change in the nature of the relationships was found.

On the occurrence of high-velocity E-region echoes in SuperDARN observations

[1] A comprehensive data set comprising 3 years of E‐region backscatter observations by the entire Super Dual Auroral Radar Network (SuperDARN) of coherent high‐frequency (HF) radars is analyzed statistically. The average spectral characteristics are examined for both morning and evening short‐range echoes to develop a simple criterion for separating E‐ and F‐region HF backscatter. Different E‐region echo populations are considered for all 16 auroral radars with half of the radars detecting substantial numbers of echoes with Doppler velocities above the nominal value of the ion‐acoustic speed Cs of 350–450 ms −1 . The high‐velocity echoes with Doppler velocity distributions that are asymmetric about zero velocity are examined separately for velocities near and well above the nominal Cs. Indicators of the total numbers of echoes near the Cs are shown to decrease from radar to radar as the radar’s boresite direction approaches an L‐shell angle of 90°. The very high‐velocity echoes, on the other hand, are predominantly seen by the two most zonally looking radars. These echoes are typically observed at greater slant ranges than echoes both below and near the Cs and both their occurrence and velocity are independent of the direction of observation. Moreover, their velocity polarity is inconsistent with that of the plasma convection component. The observations provide further support for the idea that the auroral decameter‐scale irregularities with unusually high phase velocities independent of the flow angle are generated through nonlinear mode coupling processes.

HF radar observations of high‐velocity E region echoes from the eastward auroral electrojet

[1] We present a statistical analysis of short-range E region echoes observed by the Super Dual Auroral Radar Network (SuperDARN) HF radars in the evening sector (16-22 MLT) over 3 years. Significant populations of the high-velocity (350-450 m/s) E region echoes similar to the classical Type 1 echoes are observed by 4 zonally-looking SuperDARN radars at small magnetic L-shell angles. The spatial occurrence pattern of Type 1 echoes is investigated. It is shown that the latitudinal (slant range) extent of the region where Type 1 echoes occur increases as the L-shell angle decreases, which is interpreted as widening of the aspect angle instability cone with the flow angle decrease. The echoes with unusually high velocities (500-600 m/s) observed by the Syowa East HF radar are also investigated. These echoes are seen at all L-shell angles (15°-75°) and their Doppler velocity increases with range and exhibits little variation with L-shell angle. The echoes occur at ranges 360-495 km when the strong low-velocity echoes (P > 30 dB, V < 200 m/s) are observed at ranges 225-360 km. We analyze the echo occurrence and velocity dependencies on the simultaneously observed F region echo velocity (plasma drift velocity) and E region echo power. It is suggested that the auroral E region echoes with unusually high velocities observed at large flow angles are similar to the vertically propagating Type 1 echoes reported previously in observations at the magnetic equator and are likely to be secondary waves generated through nonlinear mode coupling processes.

A study of aspect angle effects in the E‐region irregularity velocity using multi‐point electric field measurements

E-region irregularity velocity measured by the STARE Norway VHF radar is considered as a function of magnetic aspect angle α and EISCAT-derived electron drift velocity at 7 locations with α between 0.38° and 2.64°. It is shown that the irregularity line-of-sight (l-o-s) velocity normalized to the electron velocity component V e0 comp exhibits a decrease with an increasing aspect angle for V e0 comp exceeding 500 m/s. The rate of velocity decrease is greater than those reported previously and is close to that predicted by the linear theory of electrojet irregularities without assuming anomalously large collision frequencies.

First E region observations of mesoscale neutral wind interaction with auroral arcs

We report the first observations of E region neutral wind fields and their interaction with auroral arcs at mesoscale spatial resolution during geomagnetically quiet conditions at Mawson, Antarctica. This was achieved by using a scanning Doppler imager, which can observe thermospheric neutral line-of-sight winds and temperatures simultaneously over a wide field of view. In two cases, the background E region wind field was perpendicular to an auroral arc, which when it appeared caused the wind direction within ∼50 km of the arc to rotate parallel along the arc, reverting to the background flow direction when the arc disappeared. This was observed under both westward and eastward plasma convection. The wind rotations occurred within 7–16 min. In one case, as an auroral arc propagated from the horizon toward the local zenith, the background E region wind field became significantly weaker but remained unaffected where the arc had not passed through. We demonstrate through modeling that these effects cannot be explained by height changes in the emission layer. The most likely explanation seems to be the greatly enhanced ion drag associated with the increased plasma density and localized ionospheric electric field associated with auroral arcs. In all cases, the F region neutral wind appeared less affected by the auroral arc, although its presence is clear in the data.

Symmetry considerations in the two‐fluid theory of the gradient drift instability in the lower ionosphere

A general dispersion relation for the gradient drift instability (GDI) in the lower ionosphere is derived and solved analytically for the oscillation frequency and growth rate of unstable GDI waves. The approach presented is applicable in the broad range of altitudes, both within the E region and the lower F region, and for an arbitrary background density gradient. The ion and electron fluids are treated in the same way, and linearized system of fluid equations is solved exactly, with no geometry- or altitude-specific approximations required. It is demonstrated that, in the short-wavelength limit, the GDI growth rate is maximized along the bisector between the electric current and the cross product of the gradient vector and magnetic field. This result holds at all considered altitudes, including a transitional region between the E and F regions. Symmetries of the resulting expression for the growth rate are discussed, and numerical calculations for representative gradient and current configurations are presented to illustrate and validate analytical results.

Three‐dimensional data assimilation and reanalysis of radiation belt electrons: Observations of a four‐zone structure using five spacecraft and the VERB code

Obtaining the global state of radiation belt electrons through reanalysis is an important step toward validating our current understanding of radiation belt dynamics and for identification of new physical processes. In the current study, reanalysis of radiation belt electrons is achieved through data assimilation of five spacecraft with the 3-D Versatile Electron Radiation Belt (VERB) code using a split-operator Kalman filter technique. The spacecraft data are cleaned for noise, saturation effects, and then intercalibrated on an individual energy channel basis, by considering phase space density conjunctions in the T96 field model. Reanalysis during the CRRES era reveals a never-before-reported four-zone structure in the Earths radiation belts during the 24 March 1991 shock-induced injection superstorm: (1) an inner belt, (2) the high-energy shock-injection belt, (3) a remnant outer radiation belt, and (4) a second outer radiation belt. The third belt formed near the same time as the second belt and was later enhanced across keV to MeV energies by a second particle injection observed by CRRES and the Northern Solar Terrestrial Array riometer network. During the recovery phase of the storm, the fourth belt was created near L*=4RE, lasting for several days. Evidence is provided that the fourth belt was likely created by a dominant local heating process. This study outlines the necessity to consider all diffusive processes acting simultaneously and the advantage of supporting ground-based data in quantifying the observed radiation belt dynamics. It is demonstrated that 3-D data assimilation can resolve various nondiffusive processes and provides a comprehensive picture of the electron radiation belts.

Dual radar investigation of E region plasma waves in the southern polar cap

Origins and characteristics of small-scale plasma irregularities in the polar ionosphere are investigated using a dual radar setup in which the E region is probed from opposite directions by two Super Dual Auroral Radar Network (SuperDARN) facilities at the McMurdo and Dome Concordia Antarctic stations. In certain time intervals, velocity agreement is observed when velocities are compared at the same physical location in the horizontal plane. Such an agreement is widely expected if velocity at a given location is largely controlled by the convection electric field. In other cases, however, velocity agreement is unexpectedly observed when measurements are considered at the same slant range (distance along the radar beam) for both radars. This implies that it is not the electric field at a given location that is a controlling factor. Raytracing results show that the same range agreement may be explained for certain E-region density conditions when echo altitude increases with radar range. Backscatter observations under generally unfavorable conditions for irregularity generation and the critical role of propagation conditions in the polar cap are discussed. The observed E-region velocity in the polar cap is demonstrated to depend indirectly on the plasma density distribution, which is important for establishing the fundamental dependence on the convection electric field.

Solar control of F region radar backscatter: Further insights from observations in the southern polar cap†

The role of solar wind and illumination in production of small-scale F region plasma irregularities is investigated using a 4-year data set collected by the Super Dual Auroral Radar Network (SuperDARN) facility at the McMurdo station, Antarctica (MCM). Statistical analysis of ionospheric echoes detected by MCM shows that radar backscatter from the polar F region occurs in wide and persistent bands in range that exhibit systematic changes with local time, season, and solar cycle. It is demonstrated that all variations considered together form a distinct pattern. A comparison with the F region model densities and raytracing simulations shows that this pattern is largely controlled by the F region solar-produced ionization during the day. During the night, however, MCM observations reveal a significant additional source of plasma density in the polar cap as compared with the model. An example of conjugate radar observations is presented that supports the idea of polar patches being this additional source of ionization on the nightside. Echo occurrence exhibits a clear peak near the solar terminator, which suggests that small-scale irregularities form in turbulent cascade from large scales. Further, echo occurrence is enhanced for particular IMF orientations during the night. Observations indicate that solar illumination control of irregularity production is strong and not restricted to the nightside. Indirect solar wind control is also exerted by the IMF-dependent convection pattern, since the gradient-drift instability favors certain orientations between the plasma density gradients and convection velocity.

Fine structure of subauroral electric field and electron content

Small-scale structure of the plasma convection and electron content within the subauroral polarization stream (SAPS) is investigated. We present ionospheric observations during the main phase of the geomagnetic storm on 17 March 2013, during which a sequence of intense, highly localized, and fast-moving electric field (EF) structures within SAPS was observed by the Super Dual Auroral Radar Network Christmas Valley West (CVW) radar. The CVW EF measurements at 60 s resolution are analyzed in context of coincident GPS measurements of the total electron content (TEC) at 30 s resolution. The strong and narrow feature of the subauroral ion drift (SAID) was observed poleward of the TEC trough, with a TEC enhancement (peak) seen in the SAPS (SAID) region. The SAPS wave activity commenced ~2 h (15 min) after first appearance of SAPS (SAID). The SAPS structures appeared near the poleward edge of the trough, propagated westward, and merged with SAID near TEC peak. The propagation velocity was comparable with convection velocity within each EF structure. The SAPS TEC exhibited a general decrease toward the end of the period. On a smaller time scale, TEC exhibited a small but appreciable decrease within EF structures. The wavelet spectra of EF and TEC showed similar variations, with wave period of ~5 min period near onset and increasing to 8–10 min toward the end of the period with significant wave activity. A scenario is discussed, in which the SAPS wave activity may modify the ionospheric conductance and TEC at small scales, with large-scale magnetosphere-ionosphere feedback acting to continuously deplete TEC where/when such activity does not occur.