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- Kuznetsov, E. A., et al.
(author)
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Strong space plasma magnetic barriers and Alfvenic collapse
- 2007
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In: JETP Letters. - 0021-3640 .- 1090-6487. ; 85:5, s. 236-241
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Journal article (peer-reviewed)abstract
- High-magnitude magnetic barriers in space and solar plasma are proposed to be attributed to the pile up of magnetic field lines and their Alfvenic collapse for MHD flows. The analysis of experimental data from both the Interball and Cluster spacecrafts shows that high-magnitude magnetic structures found in the Earth magnetosheath and near the magnetopause are supported by a nearly thermal transverse plasma flow, with the minimum barrier width being on the order of the ion gyroradius. The collapse termination at such scales can be explained by the balance between the pile up of magnetic field lines and backward finite-gyroradius diffusion. Comparison between the theoretical, modeling, and experimental data shows that the Alfvenic collapse is, in general, a promising mechanism for magnetic field generation and plasma separation.
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| 3. |
- Savin, S., et al.
(author)
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Experimental study of nonlinear interaction of plasma flow with charged thin current sheets : 2. Hall dynamics, mass and momentum transfer
- 2006
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In: Nonlinear processes in geophysics. - 1023-5809 .- 1607-7946. ; 13:4, s. 377-392
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Journal article (peer-reviewed)abstract
- Proceeding with the analysis of Amata et al. (2005), we suggest that the general feature for the local transport at a thin magnetopause (MP) consists of the penetration of ions from the magnetosheath with gyroradius larger than the MP width, and that, in crossing it, the transverse potential difference at the thin current sheet (TCS) is acquired by these ions, providing a field-particle energy exchange without parallel electric fields. It is suggested that a part of the surface charge is self-consistently produced by deflection of ions in the course of inertial drift in the nonuniform electric field at MP. Consideration of the partial moments of ions with different energies demonstrates that the protons having gyro-radii of roughly the same size or larger than the MP width carry fluxes normal to MP that are about 20% of the total flow in the plasma jet under MP. This is close to the excess of the ion transverse velocity over the cross-field drift speed in the plasma flow just inside MP (Amata et al., 2005), which conforms to the contribution of the finite-gyroradius inflow across MP. A linkage through the TCS between different plasmas results from the momentum conservation of the higher-energy ions. If the finite-gyroradius penetration occurs along the MP over similar to 1.5 R-E from the observation site, then it can completely account for the formation of the jet under the MP. To provide the downstream acceleration of the flow near the MP via the cross-field drift, the weak magnetic field is suggested to rotate from its nearly parallel direction to the unperturbed flow toward being almost perpendicular to the accelerated flow near the MP. We discuss a deceleration of the higher-energy ions in the MP normal direction due to the interaction with finite-scale electric field bursts in the magnetosheath flow frame, equivalent to collisions, providing a charge separation. These effective collisions, with a nonlinear frequency proxy of the order of the proton cyclotron one, in extended turbulent zones are a promising alternative in place of the usual parallel electric fields invoked in the macro-reconnection scenarios. Further cascading towards electron scales is supposed to be due to unstable parallel electron currents, which neutralize the potential differences, either resulted from the ion-burst interactions or from the inertial drift. The complicated MP shape suggests its systematic velocity departure from the local normal towards the average one, inferring domination for the MP movement of the non-local processes over the small-scale local ones. The measured Poynting vector indicates energy transmission from the MP into the upstream region with the waves triggering impulsive downstream flows, providing an input into the local flow balance and the outward movement of the MP. Equating the transverse electric field inside the MP TCS by the Hall term in the Ohm's law implies a separation of the different plasmas primarily by the Hall current, driven by the respective part of the TCS surface charge. The Hall dynamics of TCS can operate either without or as a part of a macro-reconnection with the magnetic field annihilation.
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| 4. |
- Amata, E., et al.
(author)
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Experimental study of nonlinear interaction of plasma flow with charged thin current sheets : 1. Boundary structure and motion
- 2006
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In: Nonlinear processes in geophysics. - 1023-5809 .- 1607-7946. ; 13:4, s. 365-376
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Journal article (peer-reviewed)abstract
- We study plasma transport at a thin magnetopause (MP), described hereafter as a thin current sheet (TCS), observed by Cluster at the southern cusp on 13 February 2001 around 20:01 UT. The Cluster observations generally agree with the predictions of the Gas Dynamic Convection Field (GDCF) model in the magnetosheath (MSH) up to the MSH boundary layer, where significant differences are seen. We find for the MP a normal roughly along the GSE x-axis, which implies a clear departure from the local average MP normal, a similar to 90 km thickness and an outward speed of 35 km/s. Two populations are identified in the MSH boundary layer: the first one roughly perpendicular to the MSH magnetic field, which we interpret as the "incident" MSH plasma, the second one mostly parallel to B. Just after the MP crossing a velocity jet is observed with a peak speed of 240 km/s, perpendicular to B, with M-A=3 and beta> 10 (peak value 23). The magnetic field clock angle rotates by 70 degrees across the MP. E-x is the main electric field component on both sides of the MP, displaying a bipolar signature, positive on the MSH side and negative on the opposite side, corresponding to a similar to 300 V electric potential jump across the TCS. The E x B velocity generally coincides with the perpendicular velocity measured by CIS; however, in the speed jet a difference between the two is observed, which suggests the need for an extra flow source. We propose that the MP TCS can act locally as an obstacle for low-energy ions (<350 eV), being transparent for ions with larger gyroradius. As a result, the penetration of plasma by finite gyroradius is considered as a possible source for the jet. The role of reconnection is briefly discussed. The electrodynamics of the TCS along with mass and momentum transfer across it are further discussed in the companion paper by Savin et al. (2006).
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| 5. |
- Marcucci, M. F., et al.
(author)
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Extended SuperDARN and IMAGE observations for northward IMF : Evidence for dual lobe reconnection
- 2008
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:A2, s. A02204-
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Journal article (peer-reviewed)abstract
- We present observations of ionospheric convection in the Northern Hemisphere made by the SuperDARN radar network during a 3 h period on 3 December 2001. The interplanetary magnetic field (IMF) during the time of observations is predominately northward with the By component changing from positive to slightly negative. During this period Cluster is skimming the southern high latitude dusk magnetopause and reveals that reconnection is going on quasi-continuously with the reconnection site being most of the time tailward of the southern cusp and always near the satellite location (Retino, et al., 2005). Detailed analysis of the three dimensional distribution function indicates that Cluster samples magnetosheath lines connected with geomagnetic field lines tailward of the cusps in both hemispheres (Bavassano Cattaneo et al., 2006). The evolution of the ionospheric convection measured by SuperDARN, together with IMAGE FUV observations of aurorae and DMSP particle precipitation data, confirms Cluster observations and shows that simultaneous reconnection poleward of both the northern and southern cusps occurs at a variable rate on the dusk part of the magnetosphere when the IMF clock angle is small.
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| 6. |
- Betta, G., et al.
(author)
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Characterization of 3D Image-Based Biometric Systems in Dynamic Acquisition Conditions
- 2021
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In: Conference Record - IEEE Instrumentation and Measurement Technology Conference. - : Institute of Electrical and Electronics Engineers Inc.. - 9781728195391
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Conference paper (peer-reviewed)abstract
- The paper focuses on the analysis of biometric measurements in dynamic acquisition conditions and their impact on the reliability of the recognition judgments. To this aim, a suitable simulator of stereoscopic systems has been designed and realized. It relies on a fully simulated procedure based on the following steps: (i) generation of a set of realistic 3D face models through a proper face simulator software; (ii) definition of an arbitrary trajectory for the face models and stereo images to simulate a set of images acquired in different poses (positions and orientations) of the subject during the movement; (iii) addition of selectable levels of motion blur in a controlled environment, to simulate critical acquisition conditions. This procedure allows ensuring that the recognition results are not due to the natural change of expression of real faces or an imperfect image acquisition device. Moreover, every face model is moved exactly with the same trajectory in front of the stereoscopic system, allowing compare the recognition performances all along the trajectory, also in controlled and under repeatable blur levels. A face biometrics procedure, based on a popular recognition algorithm, is then run on the generated images and the recognition performances are analyzed in detail. The achieved results demonstrated how the motion blur and also the slight differences between the acquired images and the reference ones significantly affect the performance in the recognition of such kinds of systems, thus confirming the usefulness of the proposed simulator. © 2021 IEEE.
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