A possible origin for large aspect angle "HAIR'' echoes seen by SuperDARN radars in the E region

International audienceMilan2004 have recently reported on close-range E region decameter size echoes that seem to be relatively weak, have apparently unusually large aspect angles, and possess Doppler shifts that are slow and are clearly consistent with the ion drift of the medium as opposed to, say, its electron drift or its ion-acoustic speed. We argue that these irregularities are the result of a nonlinear wave conversion process triggered by the nonlocal evolution of decameter Farley-Buneman waves. According to this picture, structures which have weak spontaneous growth rates and are initially field-aligned undergo an evolution of their aspect angle that results in a jump in the aspect angle at some point in time and space. When this takes place, a rapid mode conversion must follow, which takes energy away from a standard two-stream signature and converts it either to a strongly damped ion-acoustic mode or to a purely decaying mode, depending on altitude

Local ionospheric electrodynamics associated with neutral wind fields at low latitudes: Kelvin-Helmholtz billows

International audienceThe Gadanki radar observation of plasma irregularities bearing the signature of Kelvin-Helmholtz billows above 100 km altitude raises the question of the electrodynamical mechanism that would allow the structures to drift with the neutral wind. We show that for locally varying neutral wind fields with the right geometry at night, multiple Hall effects in the electron gas lead to a situation where ions, electrons, and neutrals move together along the component of the wind that changes most rapidly in space. The species must not move together along all directions, however. If this were the case the plasma would be stable and a radar would be unable to observe the wind field. We discuss the stability of the plasma itself for Es layers affected by the Kelvin-Helmholtz wind field and show that a variety of factors have to be taken into account beyond the study of the zeroth order mechanism

Origin of type-2 thermal-ion upflows in the auroral ionosphere

International audienceThe origin of thermal ion outflows exceeding 1km/s in the high-latitude F-region has been a subject of considerable debate. For cases with strong convection electric fields, the "evaporation" of the ions due to frictional heating below 400-500km has been shown to provide some satisfactory answers. By contrast, in the more frequent subclass of outflow events observed over auroral arcs, called type-2, there is no observational evidence for ion frictional heating. Instead, an electron temperature increase of up to 6000° K is observed over the outflow region. In this case, field-aligned electric fields have long been suspected to be involved, but this explanation did not seem to agree with expectations from the ion momentum balance. In the present work we provide a consistent scenario for the type-2 ion upflows based on our case study of an event that occurred on 20 February 1990. We introduce, for the first time, the electron energy balance in the analysis. We couple this equation with the ion momentum balance to study the salient features of the observations and conclude that type-2 ion outflows and the accompanying electron heating events are indeed consistent with the existence of a field-aligned electric field. However, for our explanation to work, we have to require that an allowance be made for electron scattering by high frequency turbulence. This turbulence could be generated at first by the very fast response of the electrons themselves to a newly imposed electric field that would be partly aligned with the geomagnetic field. The high frequencies of the waves would make it impossible for the ions to react to the waves. We have found the electron collision frequency associated with scattering from the waves to be rather modest, i.e. comparable to the ambient electron-ion collision frequency. The field-aligned electric field inferred from the observations is likewise of the same order of magnitude as the normal ambipolar field, at least for the case that we have studied in detail. We propose that the field-aligned electric field is maintained by the north-south motion of an east-west arc. The magnetic perturbation associated with the arc itself converts a small fraction of the perpendicular electric field into a field parallel to the total magnetic field, while the north-south motion ensures that the conversion never stops

Velocity shear and current driven instability in a collisional F-region

We have studied how the presence of collisions affects the behavior of instabilities triggered by a combination of shears and parallel currents in the ionosphere under a variety of ion to electron temperature ratios. To this goal we have numerically solved a kinetic dispersion relation, using a relaxation model to describe the effects of ion and electron collisions. We have compared our solutions to expressions derived in a fluid limit which applied only to large electron to ion temperature ratios. We have limited our study to threshold conditions for the current density and the shears. We have studied how the threshold varies as a function of the wave-vector angle direction and as a function of frequency. As expected, we have found that for low frequencies and/or elevated ion to electron temperature ratios, the kinetic dispersion relation has to be used to evaluate the threshold conditions. We have also found that ion velocity shears can significantly lower the field-aligned threshold current needed to trigger the instability, especially for wave-vectors close to the perpendicular to the magnetic field. However the current density and shear requirements remain significantly higher than if collisions are neglected. Therefore, for ionospheric F-region applications, the effect of collisions should be included in the calculation of instabilities associated with horizontal shears in the vertical flow. Furthermore, in many situations of interest the kinetic solutions should be used instead of the fluid limit, in spite of the fact that the latter can be shown to produce qualitatively valid solutions

Nonlinear model of short-scale electrodynamics in the auroral ionosphere

International audienceThe optical detection of auroral subarcs a few tens of m wide as well as the direct observation of shears several m/s per m over km to sub km scales by rocket instrumentation both indicate that violent and highly localized electrodynamics can occur at times in the auroral ionosphere over scales 100 m or less in width. These observations as well as the detection of unstable ion-acoustic waves observed by incoherent radars along the geomagnetic field lines has motivated us to develop a detailed time-dependent two-dimensional model of short-scale auroral electrodynamics that uses current continuity, Ohm's law, and 8-moment transport equations for the ions and electrons in the presence of large ambient electric fields to describe wide auroral arcs with sharp edges in response to sharp cut-offs in precipitation (even though it may be possible to describe thin arcs and ultra-thin arcs with our model, we have left such a study for future work). We present the essential elements of this new model and illustrate the model's usefulness with a sample run for which the ambient electric field is 100 mV/m away from the arc and for which electron precipitation cuts off over a region 100 m wide. The sample run demonstrates that parallel current densities of the order of several hundred µA m-2 can be triggered in these circumstances, together with shears several m/s per m in magnitude and parallel electric fields of the order of 0.1 mV/m around 130 km altitude. It also illustrates that the local ionospheric properties like densities, temperature and composition can strongly be affected by the violent localized electrodynamics and vice-versa.Key words: Ionosphere (auroral ionosphere, electric fields and currents, ionosphere-magnetosphere interactions)</p

Mechanisms underlying the prereversal enhancement of the vertical plasma drift in the low-latitude ionosphere

The evening prereversal enhancement (PRE) of the vertical plasma drift has important consequences for the Appleton density anomaly and the stability of the nighttime ionosphere. Simplified simulations were used to review the three competing theories of the PRE origin, to explore their relative importance, and to indentify their interdependence. The mechanisms involved in the generation and climatology of the PRE are, first, a curl-free electric field response to rapid changes in the vertical electric field associated with the nighttime F region dynamo; second, a divergence of Hall currents in the E region away from the magnetic equator; and, third, the moderating effect of the large Cowling conductivities in the equatorial E region. The simulations indicate that the equatorial Cowling conductivity creates an important current path that limits the other two mechanisms prior to equatorial sunset and releases them after equatorial sunset. The curl-free mechanism is the dominant mechanism when the terminator and magnetic meridian are aligned in part due to the accelerating F region zonal wind. When the solar terminator is not aligned with the magnetic meridian, there is an interaction involving all three mechanisms contributing to the magnitude and timing of the PRE. Finally, the altitude profile of the PRE decays more quickly with altitude when the curl-free mechanism dominates as compared to when the Hall current mechanism dominates. ©2015. American Geophysical Union. All Rights Reserved

Elevated Electron Temperatures Around Twin Sporadic E Layers at Low Latitude: Observations and the Case for a Plausible Link to Currents Parallel to the Geomagnetic Field

We present data from nighttime sounding rocket flights in the low latitude E region. The payloads carried a sweeping Langmuir probe, a plasma impedance probe, and electric field probes. A detailed examination of the plasma density, temperature, and electric field measurements show two strong sporadic E (Es) layers with very high electron temperatures (∼1000 K) on each side of the upper layer. The lower layer was consistent with the presence of a strong zonal neutral wind shear. The upper layer was strongly influenced by the presence of a strongly negative vertical electric field, with zonal winds and their shears also contributing. A strong downward motion of the plasma from the combined action of the downward electric field and negative zonal wind advected the upper layer far below the region of maximum growth. We have attributed the more puzzling high electron temperatures to frictional heating from parallel currents and shown that the F region nighttime dynamo could easily generate the necessary parallel current densities (1 μA m−2) near the electron density troughs. The electron temperature was also elevated in the Es layers themselves, implying parallel current densities of the order of 15 μA m−2 around the Es peaks. Those parallel currents were attributed to strong Hall current divergences driven by the zonal electric field around the Es peaks

The effects of mesoscale regions of precipitation on the ionospheric dynamics, electrodynamics and electron density in the presence of strong ambient electric fields

We have developed a new high resolution two-dimensional model of the high latitude ionosphere in which nonlinear advection terms are closely coupled with the electrodynamics. The model provides a self-consistent description of the ionospheric feedback on the electrodynamical perturbations produced by auroral arc-related particle precipitation in regions with strong ambient electric fields. We find in particular that a heretofore neglected ion Pedersen advection term can introduce considerable changes in the electron density profile, the current density distribution, the conductivities and the electron temperatures. We find that the convective effects can carry the ionisation more than 150 km outside the precipitation region in a few minutes, with attendant large changes in the current distribution and E-region densities that become enhanced outside the region of particle precipitation. The production of a tongue of ionisation that slowly decays outside the auroral boundaries contrasts with the sharp geometric cut-off and associated stronger current densities found in previous studies