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A brief overview of magnetosheath physics Maria Federica Marcucci INAF-IAPS. STORM Annual Meeting - Graz, November 25-26, 2013. image credit: ESA. STORM Annual Meeting - Graz, November 25-26, 2013. Large scale properties.
A brief overview of magnetosheath physics Maria Federica Marcucci INAF-IAPS STORM Annual Meeting - Graz, November 25-26, 2013
image credit: ESA STORM Annual Meeting - Graz, November 25-26, 2013
Large scale properties Velocity Density Temperature from Baumjohann and Treumann , 1997
Large scale properties Results of gasdynamicconvected field model (GDCFM) (Spreiter et al., 1966; 1968; Spreiter and Stahara, 1980) The gasdynamic flow solution is calculated ignoring magnetic forces and then the magnetic field lines are computed by convecting them through the fluid. The three components and magnitude of magnetic field is agree with the observations. Such models give reference estimates of magnetosheath properties. Some magnetohydrodynamiceffects, such as the plasma depletionlayer - decrease in density and increase in magnetic field - at the sub-solarmagnetopause, are notpredicted. Zhang, 1995
Large scale properties BATS-R-US 3-D global MHD model (http://ccmc.gsfc.nasa.gov/) and CLUSTER observations for 10 November 2002. Magnetic forces effect: low MA and low magnetosheath magnetic forces become preponderant and act to accelerate flows in spatial quadrants quasi-perpendicular to the IMF direction Lavraud, 2013
Large scale properties from Baumjohann and Treumann , 1997
Large scale properties Statistical mapping of THEMIS measurements in the magnetosheath interplanetary medium (MIPM) reference frame (Dimmock and Nykyri, 2013) Data organized with respect to the shock geometry ( YMIPM>0 -> and YMIPM<0 -> ) - motion of BS and MP taken into account – data set IMF predominately along the Parker spiral • Velocity • slowest velocities at the BS nose-velocity decreases from the BS to the MP • velocity is on average greater on the flank • No asymmetry when considering ortho-Parker spiral • Magnetic field strength • enhanced field close to the MP at the BS nose • greater magnetic field on the flank • Asymmetry conserved when considering ortho-Parker spiral • Density • Increased density behind the BS, region of greatest compression. • No evidence of dawn enhancement previously observed.
Large scale properties • average density and magnetic field strength are higher than in the upstream solar wind (the fast mode shock); • the average flow direction deviates from the anti-solar direction such that the plasma flows around the magnetosphere; • the velocity downstream of the bow shock is lower than the local fast magnetosonic speed and the flow velocity increases again to supersonic speeds at the magnetopause flanks; • the ion temperature of the sheath is higher than in the solar wind while the electron temperature does not increase very much over its upstream value. • the magnetosheath plasma develops a pronounced temperature anisotropy (T⊥ > T ) behind the bow shock that increases toward the magnetopause and is more pronounced in the ions than in the electrons.
Adjacent regions to the MS proper: bow shock At a quasi-perpendicular shock the shock transition is much thinner and smooth. Reflected ions re-enter the shock after gyrating in the upstream magnetic field At a quasi-parallel shock the transition is characterized by strong fluctuations (upstream and downstream) Reflected ions and electrons propagate upstream of the shock and the foreshock is populated by counterstreamingbeams-> is a very disturbed region due to streaming instabilities Grensstadt and Fredericks, 1979
Adjacent regions to the MS proper: magnetopause Properties,bothlarge-scalemesoscale and smallscale, are variable and depend on the interactionwith the solarwind and IMF. Reconnectioninvolvessmall (diffusionregion) tolarge scale. Componentmergingdemostratedtogetherwith long and stable X linealong the MP. Importanceof and shear angle . Stilltoasses the relative importanceoftransient and patchyreconnection. Fluxropesidentified and reconstucted (aidbymultispacecraftmission + Grad-Shafarnov) A lotof progress in the last yearsregardingphisycs in the diffusionregion. Trattner et al. 2012 Lui et al. 2007
Plasma transport across the magnetopause Adjacent regions to the MS proper: magnetopause The KH instability on the flanks of the magnetopause is considered one of the mechanisms for populating the low latitude boundary layer (LLBL) during periods of northward interplanetary The condition for onset of KHI is a large velocity shear across the MP. First evidenceof plasma trasportthroughrolled-upKelvin-Helmholtzvortices. Adapted from Hasegawa et al., Nature, 2004
Adjacent regions to the MS proper: cusp • 3-year statistical study on the plasma flows IMF dependence in the NH high-altitude cusp. • Southward IMF: • plasma penetration occurs preferentially at the dayside low-latitude magnetopause. • Northward IMF: • plasma penetration from the poleward edge of the cusp; Lavraud et al., JGR, 2005
Adjacent regions to the MS proper: cusp Due to the topology of the Earth’s magnetic field the cusp is influenced by processes occurring at the surrounding magnetopause ( e.g. reconnection) Moreover, at the cusp, the Earth’s magnetic field is weak , so it is expected that the magnetopause presents an indentation which disturbs the magnetosheath flow. Observations (HEOS, Interball, Polar, Cluster) show high variabilty. Variable position with the IMF – double cusp observed. Fluctuations observed on Cluster are suggestive of very localized and filamentary structure. Fairfield and Hones, 1978
Some processes in the MS: reconnection • Reconnection exhaust embedded in the magnetosheath flow observed at 06:12 UT • (time interval of the exhaust 15 s) • accelerated plasma outflows, interpenetrating ion beams • reconnection inflows and tangential reconnection electric field. • The same current sheet was observed • upstream in the solar wind by the ACE and Wind without the reconnection signatures. • Reconnection initiated in the magnetosheath due to compression of the solar wind current sheet at the bow shock and against the dayside magnetopause. • A super-Alfvenic outflow jet of electrons Phan et al. 2009
Some processes in the MS: First in situ evidence of magnetic reconnection in a turbulent plasma Evidence for ongoing reconnection comes from the measurements of tangential electric field, normal magnetic field, plasma inflow and outflow in the reconnection region and the plasma heating during reconnection. The evidence for crossing the ion diffusion region is the Hall magnetic and electric fields. It is shown that reconnection is fast and electromagnetic energy is converted into heating and acceleration of particles.
Jets NOT associated with magnetic reconnection Anomalous high kinetic energy densities in the magnetosheathwhose kinetic energy density is comparable to or higher than that of the upstream solar wind No fluid or kinetic signatures related to reconnection are observed. The B jetis observed fully in the magnetosheath it is directed towards the magnetopause; The A jet is associated with an indentation of the magnetopause. See also Shue et al. 2009. Amata et al., 2011
Magnetosheath enhanced flows during low Alfvén Mach number solar wind Magnetic forces effect: low MA and low magnetosheath magnetic forces become preponderant and act to accelerate flows in spatial quadrants quasi-perpendicular to the IMF direction Statistical study of Cluster 1 data demonstrate that enhanced flows in the magnetosheath are expected at locations quasi-perpendicular to the IMF direction in the plane perpendicular to the Sun-Earth line: for northward IMF, enhanced flows are observed on the dawn and dusk flanks of the magnetosphere The largest flows are adjacent to the magnetopause.
Magnetosheath perturbations generated by the impacts of IP shocks on magnetosphere • IS collision on the bow shock and split up in an ensemble of discontinuities or self-similar waves • propagation of the shockfrontthrough the magnetosheath • collision on the magnetopause (poorly understood) • Theoretical study predicts that the impact of a fast shock on the magnetopause produces a fast rarefaction wave moving sunward which starts an oscillating process in which other secondary waves are generated by the reflections upon both the bow shock and the magnetopause • Global MHD simulations (Samsonov et al.,2007): • transmitted fast shock propagating earthward-reflection of the shock at the inner boundary-reflected fast shock propagates sunward. • The transmitted shock causes the bow shock and magnetopause to move inward while the reflected • fast shock causes these boundaries to move outward. Pallocchia et al. 2010, 2013
Magnetosheath perturbations generated by the impacts of IP shocks on magnetosphere • Observations • In the outer MS: • Two-step perturbation : transmitted IP shock and a discontinuity with temperature decrease and plasma density increase. • Inward displacement of the bow shock (BS1) followed by an outward motion (BS2). • In the inner MS a third discontinuity (III) • moving earthward. • The smooth compression is a reverse fast wave (RFW)
Magnetosheath fluctuations Fluctuations may arise from solar wind turbulence transmitted through the bow shock and can be generated at the bow shock itself. Fluctuationsare amplified downstream the quasi-parallel shock and the level of magnetic fluctuations in the is high. Downstream of the quasi-perpendicular shocks the fluctuations are mainly local and their level is smaller. As the magnetosheath plasma convects from the bow shock to the magnetopause the pressure anisotropy increases with (T⊥ >T||) Mirror waves frequently occur in the magnetosheath under conditions of enhanced ion temperature anisotropy (T⊥ >T||) and high plasma β. AlfvénIon Cyclotron waves will grow when the temperature anisotropy is high and the proton plasma β ∼ 1 (e.g.Schwartz et al., 1996).
Magnetosheath fluctuations Quasi parallel Czaykowskaet al. (2001) study 132 samples of magnetosheath fluctuations just downstream of quasi-parallel and quasi-perpendicular bow shocks. Downstream of the shock the spectrum presents a break - the spectral index is ∼ 1 below the proton gyrofrequencyand ∼ 2.6 above the proton gyrofrequency. No relevant difference between the quasi-parallel and quasi-perpendicular cases. Quasi perpendicular
Magnetosheath fluctuations Magnetic field fluctuations observed by Cluster downstream of a quasi-perpendicular bow shock (Alexandrova et al. 2006) The turbulent spectrum presents a spectral break accompanied by a bump usually interpreted as due to Alfvén ion cyclotron waves. The spectral knee corresponds to space-localized coherent magnetic structure identified as Alfvén vortices.
Magnetosheath fluctuations • Magnetosheath turbulence in the high β plasma downstream of a quasi-parallel bowshock(Yordanova et al., 2008) • Level of magnetic fluctuations increases downstream - fluctuations are generated locally, • At ion scales (0.3–2) Hz the spectral index of By and Bz decrease with the distance • the turbulence intermittency and anisotropy increase with the distance from the shock, indicating that a robust nonlinear cascade is going on • intermittency level is stronger than that in the solar wind
Magnetosheath fluctuations Spectrum of the magnetic field fluctuations observed in a quiet interval of magnetosheath proper . Nearly a Kolmogorov spectrum is observed for the fluctuationsat low frequencies A f −2.5 power-law above the spectral break. Alexandrova, 2008
In the framework of WP3 activity at IAPS we computed the Power Spectral Densities (PSDs) of magnetic field for some burst mode intervals of CLUSTER C1 and C3, in the MS as selected by Dr. E. Yordanova PSD method The PSD has been used using the Welch periodogrammethod combined with a Hanningwindow. The sliding time window is of 2^17 points (approx. 32 min) and before computing the Fourier Transform the zero frequency mode (mean value) has been removed. The normalization of the PSD has been done so that for a monochromatic signal of amplitude 1 the integral value of the PSD on the frequency returns the signal Variance.
Thank You Much of this presentation is based on: Lucek, E.A., D. Constantinescu, M.L.Goldstein, J. Pickett, J.L. Pinçon, F. Sahraoui, R.A. Treumann, and S.N. Walker, The Magnetosheath, in Outer Magnetospheric Boundaries: Cluster Results, edited by G. Pashmann, S.J. Schwartz, C. P. Escoubet and S. Haaland, ISSI Space Science Series, Springer, Reprinted from Space Science Reviews, Volume 118, Nos. 1-4, 2005. Zimbardo, G., Greco, A., Sorriso-Valvo, L., Perri, S., Voros, Z., Aburjania, G., Chargazia, K., Alexandrova, O., MagneticTurbulence in the Geospace Environment, Space Science Reviews, 1-46, http://dx.doi.org/10.1007/s11214-010-9692-5