50 research outputs found
Ionospheric Plasma Transport and Loss in Auroral Downward Current Regions
A detailed study of the effects of auroral current systems on thermal ionospheric plasma transport and loss is conducted using a new ionospheric model. The mathematical formulation of the model is a variation on the 5âmoment approximation which describes the temporal evolution of density, drift, and temperature for five different ion species in two spatial dimensions. The fluid system is closed through a 2âD electrostatic treatment of the auroral currents. This model is used to examine the interplay between ion heating, perpendicular transport, molecular ion generation, and typeâ1 ion upflows in a selfâconsistent way for the first time. Simulations confirm that the depletion of Eâregion plasma due to current closure occurs on extremely fast time scales (5â30 s), and that it is dependent on current system scale size. Near the Fâregion peak, the loss is mostly due to enhanced recombination from the conversion of the plasma to molecular ions. The Fâregion loss process is fairly slow (120â300 s) by comparison to lower altitude processes and is highly electric field dependent. On similar time scales, transient ion upflows from frictional heating move plasma from the near topside ionosphere (âŒ500 km) to higher regions, leaving depletions and enhancing plasma densities at very high altitudes. Results indicate the existence of large molecular ion upflows near the Fâregion peak and may shed some light on ionospheric source regions for outflowing molecular ions. Neutral atmospheric winds and densities are also shown to play an important role in modulating molecular ion densities, frictional heating, and currents
Dynamics of Density Cavities Generated by Frictional Heating: Formation, Distortion, and Instability
A simulation study of the generation and evolution of mesoscale density cavities in the polar ionosphere is conducted using a time-dependent, nonlinear, quasi-electrostatic model. The model demonstrates that density cavities, generated by frictional heating, can form in as little as 90 s due to strong electric fields of âŒ120 mV/m, which are sometimes observed near auroral zone and polar cap arcs. Asymmetric density cavity features and strong plasma density gradients perpendicular to the geomagnetic field are naturally generated as a consequence of the strong convection and finite extent of the auroral feature. The walls of the auroral density cavities are shown to be susceptible to large-scale distortion and gradient-drift instability, hence indicating that arc-related regions of frictional heating may be a source of polar ionospheric density irregularities
Incoherent Scatter Radar Estimation of F Region Ionospheric Composition During Frictional Heating Events
A method is developed for estimating F region ion composition from incoherent scatter radar (ISR) measurements during times of frictional ion heating. The technique addresses ion temperatureâmass ambiguities in the IS spectra by selfâconsistently modeling ion temperature profiles, including the effects of ion temperature anisotropies and altitudeâindependent neutral winds. The modeled temperature profiles are used in a minimization procedure to estimate ion composition consistent with the recorded IS spectra. The proposed method is applicable to shortâintegration (min) data sets from either singleâbeam or multipleâbeam experiments. Application of the technique to Sondrestrom ISR measurements shows increases in F region molecular ions in response to frictional heating, a result consistent with previous theoretical and observational work. Estimates of ion composition are shown to be relatively insensitive to moderate variations in the neutral atmospheric model, which serves as input to the method. The technique developed in this work is uniquely qualified for studying highly variable ion composition near auroral arcs and associated processes such as molecular ion upflows. It also addresses a systematic source of error in standard ISR analysis methods when they are applied in such situations
Auroral Ion Outflow: Low Altitude Energization
The SIERRA nightside auroral sounding rocket made observations of the origins of ion upflow, at topside F-region altitudes (below 700 km), comparatively large topside plasma densities (above 20 000/cc), and low energies (10 eV). Upflowing ions with bulk velocities up to 2 km/s are seen in conjunction with the poleward edge of a nightside substorm arc. The upflow is limited within the poleward edge to a region (a) of northward convection, (b) where Alfvenic ÂŽ and Pedersen conductivities are well-matched, leading to good ionospheric transmission of Alfvenic power, and (c) of ÂŽ soft electron precipitation (below 100 eV). Models of the effect of the soft precipitation show strong increases in electron temperature, increasing the scale height and initiating ion upflow. Throughout the entire poleward edge, precipitation of moderate-energy (100s of eV) protons and oxygen is also observed. This ion precipitation is interpreted as reflection from a higher-altitude, time-varying field-aligned potential of upgoing transversely heated ion conics seeded by the low altitude upflow
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Do magnetospheric shear Alfvén waves generate sufficient electron energy flux to power the aurora?
Using a self-consistent drift-kinetic simulation code, we investigate whether electron acceleration owing to shear AlfvĂ©n waves in the plasma sheet boundary layer is sufficient to cause auroral brightening in the ionosphere. The free parameters used in the simulation code are guided by in situ observations of wave and plasma parameters in the magnetosphere at distances >4 RE from the Earth. For the perpendicular wavelength used in the study, which maps to âŒ4 km at 110 km altitude, there is a clear amplitude threshold which determines whether magnetospheric shear AlfvĂ©n waves above the classical auroral acceleration region can excite sufficient electrons to create the aurora. Previous studies reported wave amplitudes that easily exceed this threshold; hence, the results reported in this paper demonstrate that auroral acceleration owing to shear AlfvĂ©n waves can occur in the magnetosphere at distances >4 RE from the Earth
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A diagnosis of the plasma waves responsible for the explosive energy release of substorm onset
During geomagnetic substorms, stored magnetic and plasma thermal energies are explosively converted into plasma kinetic energy. This rapid reconfiguration of Earthâs nightside magnetosphere is manifest in the ionosphere as an auroral display that fills the sky. Progress in understanding of how substorms are initiated is hindered by a lack of quantitative analysis of the single consistent feature of onset; the rapid brightening and structuring of the most equatorward arc in the ionosphere. Here, we exploit state-of-the-art auroral measurements to construct an observational dispersion relation of waves during substorm onset. Further, we use kinetic theory of high-beta plasma to demonstrate that the shear Alfven wave dispersion relation bears remarkable similarity to the auroral dispersion relation. In contrast to prevailing theories of substorm initiation, we demonstrate that auroral beads seen during the majority of substorm onsets are likely the signature of kinetic Alfven waves driven unstable in the high-beta magnetotail
Relative drift between black aurora and the ionospheric plasma
Abstract. Black auroras are recognized as spatially welldefined regions within uniform diffuse aurora where the optical emission is significantly reduced. Although a well studied phenomenon, there is no generally accepted theory for black auroras. One theory suggests that black regions are formed when energetic magnetospheric electrons no longer have access to the loss cone. If this blocking mechanism drifts with the source electron population in the magnetosphere, black auroras in the ionosphere should drift eastward with a velocity that increases with the energy of the precipitating electrons in the surrounding aurora, since the gradient-B curvature drift is energy dependent. It is the purpose of this paper to test this hypothesis. To do so we have used simultaneous measurements by the European Incoherent Scatter (EISCAT) radar and an auroral TV camera at TromsĂž, Norway
EISCAT radar and optical studies of black aurora: A signature of magnetospheric turbulence?
Black auroras are recognised as spatially well-defined regions within a uniform diffuse auroral background where the optical emission is significantly reduced, or possibly totally absent. Black auroras typically appear post-magnetic midnight and during the substorm recovery phase, but not exclusively so. Their horizontal size is typically 1x5 km, elongated in the east-west direction, and they move predominantly in an eastward direction with a speed of 1-4 km/s. There is no accepted theory for the phenomenon of black aurora, although they seem associated with substorms. We report on the first incoherent scatter radar observations of black aurora by EISCAT, coupled to white-light TV recordings of the phenomenon. From a 2002 observation, we show that non-sheared black auroras are most probably not associated with field-aligned currents. From 2002 and 2003 observations, we show that the apparent motion of the black aurora is most probably controlled by the drift of particles in the magnetosphere and not ExB drift in the ionosphere. The drift speed is therefore dependent on the energy of the precipitating particles forming the diffuse background. From 2005 bi-static observations, we attempt to confirm this by relating the height and propagation speed of the black aurora to precipitating particle energy within the surrounding background diffuse aurora. Hence, the mechanism for black aurora is most probably active within the magnetosphere and substorm associated plasma turbulence within the magnetosphere may account for the optical morphology of the black aurora, in particular the lack of pitch angle diffusion into the loss cone