Browsing by Author "Viñas, A. F."
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- ItemDirect observations of the formation of the solar wind halo from the strahl(EGU, 2012-01-17) Gurgiolo, C.; Goldstein, Melvyn; Viñas, A. F.; Fazakerley, A. N.Observations of a continual erosion of the strahl and build up of the halo with distance from the sun suggests that, at least in part, the halo may be formed as a result of scattering of the strahl. This hypothesis is supported in this paper by observation of intense scattering of strahl electrons, which gives rise to a proto-halo electron population. This population eventually merges into, or becomes the halo. The fact that observations of intense scattering of the strahl are not common implies that the formation of the halo may not be a continuous process, but one that occurs, in part, in bursts in regions where the conditions responsible for the scattering are optimum.
- ItemFirst measurements of electron vorticity in the foreshock and solar wind(EGU, 2010-12-21) Gurgiolo, C.; Goldstein, Melvyn; Viñas, A. F.; Fazakerley, A. N.We describe the methodology used to set up and compute spatial derivatives of the electron moments using data acquired by the Plasma Electron And Current Experiment (PEACE) from the four Cluster spacecraft. The results are used to investigate electron vorticity in the foreshock. We find that much of the measured vorticity, under nominal conditions, appears to be caused by changes in the flow direction of the return (either reflected or leakage from the magnetosheath) and strahl electron populations as they couple to changes in the magnetic field orientation. This in turn results in deflections in the total bulk velocity producing the measured vorticity.
- ItemNonlinear evolution of Alfvénic wave packets(AGU, 1998-07-01) Buti, B.; Jayanti, V.; Viñas, A. F.; Ghosh, S.; Goldstein, Melvyn; Roberts, D. A.; Lakhina, G. S.; Tsurutani, B. T.Alfvén waves are a ubiquitous feature of the solar wind. One approach to studying the evolution of such waves has been to study exact solutions to approximate evolution equations. Here we compare soliton solutions of the Derivative Nonlinear Schrödinger evolution equation (DNLS) to solutions of the compressible MHD equations. We find that the soliton solutions of the DNLS equation are not stable solutions of Hall-MHD—they evolve and dissipate with time. Although such solitons may serve as approximate initial conditions to the Hall-MHD equations, they are not stationary solutions. This may account for the absence of soliton-like wave forms in the free-flowing solar wind.
- ItemObservations of electron vorticity in the inner plasma sheet(EGU, 2011-09-01) Gurgiolo, C.; Goldstein, Melvyn; Viñas, A. F.; Matthaeus, W. H.; Fazakerley, A. N.From a limited number of observations it appears that vorticity is a common feature in the inner plasma sheet. With the four Cluster spacecraft and the four PEACE instruments positioned in a tetrahedral configuration, for the first time it is possible to directly estimate the electron fluid vorticity in a space plasma. We show examples of electron fluid vorticity from multiple plasma sheet crossings. These include three time periods when Cluster passed through a reconnection ion diffusion region. Enhancements in vorticity are seen in association with each crossing of the ion diffusion region.
- ItemThe first in situ observation of Kelvin-Helmholtz waves at high-latitude magnetopause during strongly dawnward interplanetary magnetic field conditions(AGU, 2012-08-29) Hwang, Kyoung-Joo; Goldstein, Melvyn; Kuznetsova, M. M.; Wang, Y.; Viñas, A. F.; Sibeck, DavidWe report the first in situ observation of high-latitude magnetopause (near the northern duskward cusp) Kelvin-Helmholtz waves (KHW) by Cluster on January 12, 2003, under strongly dawnward interplanetary magnetic field (IMF) conditions. The fluctuations unstable to Kelvin-Helmholtz instability (KHI) are found to propagate mostly tailward, i.e., along the direction almost 90° to both the magnetosheath and geomagnetic fields, which lowers the threshold of the KHI. The magnetic configuration across the boundary layer near the northern duskward cusp region during dawnward IMF is similar to that in the low-latitude boundary layer under northward IMF, in that (1) both magnetosheath and magnetospheric fields across the local boundary layer constitute the lowest magnetic shear and (2) the tailward propagation of the KHW is perpendicular to both fields. Approximately 3-hour-long periods of the KHW during dawnward IMF are followed by the rapid expansion of the dayside magnetosphere associated with the passage of an IMF discontinuity that characterizes an abrupt change in IMF cone angle, ϕ = acos (Bₓ / |B|), from ∼90° to ∼10°. Cluster, which was on its outbound trajectory, continued observing the boundary waves at the northern evening-side magnetopause during sunward IMF conditions following the passage of the IMF discontinuity. By comparing the signatures of boundary fluctuations before and after the IMF discontinuity, we report that the frequencies of the most unstable KH modes increased after the discontinuity passed. This result demonstrates that differences in IMF orientations (especially in ϕ) are associated with the properties of KHW at the high-latitude magnetopause due to variations in thickness of the boundary layer, and/or width of the KH-unstable band on the surface of the dayside magnetopause.
- ItemWhistler Waves Driven by Anisotropic Strahl Velocity Distributions: Cluster Observations(AIP, 2010-03-25) Viñas, A. F.; Gurgiolo, C.; Nieves‐Chinchilla, T.; Gary, S. P.; Goldstein, MelvynObserved properties of the strahl using high resolution 3D electron velocity distribution data obtained from the Cluster/PEACE experiment are used to investigate its linear stability. An automated method to isolate the strahl is used to allow its moments to be computed independent of the solar wind core+halo. Results show that the strahl can have a high temperature anisotropy (T⊥/ T ₗₗ ≳ 2) This anisotropy is shown to be an important free energy source for the excitation of high frequency whistler waves. The analysis suggests that the resultant whistler waves are strong enough to regulate the electron velocity distributions in the solar wind through pitch‐angle scattering.