STORM Annual Meeting - Graz, November 25-26, 2013 A brief overview of magnetosheath physics Maria...

STORM Annual Meeting - Graz, November 25-26, 2013

A brief overview of magnetosheath physics

Maria Federica Marcucci

INAF-IAPS


STORM Annual Meeting - Graz, November 25-26, 2013STORM Annual Meeting - Graz, November 25-26, 2013

image credit: ESA


Large scale properties

from Baumjohann and Treumann , 1997

Velocity Density Temperature


Results of gasdynamic convected 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 magnetohydrodynamic effects, such as the plasma depletion layer - decrease in density and increase in magnetic field - at the sub-solar magnetopause, are not predicted.


Zhang, 1995


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 becomepreponderant and act to accelerate flowsin spatial quadrants quasi-perpendicular to the IMF direction


Lavraud, 2013

http://ccmc.gsfc.nasa.gov/



from Baumjohann and Treumann , 1997


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.



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.



At a quasi-perpendicular shockthe 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 counterstreaming beams -> is a very disturbed region due to streaming instabilities

Adjacent regions to the MS proper: bow shock

Grensstadt and Fredericks, 1979


Adjacent regions to the MS proper: magnetopauseProperties, both large-scale mesoscale and small scale, are variable and depend on the interaction with the solar wind and IMF.

Reconnection involves small (diffusion region) to large scale.

Component merging demostrated together with long and stable X line along the MP.

Importance of and shear angle .

Still to asses the relative importance of transient and patchy reconnection.

Flux ropes identified and reconstucted(aid by multispacecraft mission +Grad- Shafarnov)

A lot of progress in the last years regarding phisycs in the diffusion region.

Trattner et al. 2012

Lui et al. 2007


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 evidence of plasma trasport through rolled-up Kelvin-Helmholtz vortices.

Adapted from Hasegawa et al., Nature, 2004

Plasma transport across the magnetopauseAdjacent regions to the MS proper: magnetopause


Lavraud et al., JGR, 2005

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;

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 magneticfield 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.

Adjacent regions to the MS proper: cusp

Fairfield and Hones, 1978


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 observedupstream 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

Some processes in the MS: reconnection

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 fastand electromagnetic energy is converted into heating and acceleration of particles.


Jets NOT associated with magnetic reconnection

Anomalous high kinetic energy densities in the magne tosheath whose 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 jet is 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


Statistical study of Cluster 1 data demonstratethat 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 magnetosphereThe largest flows are adjacent to the magnetopause.

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

Magnetosheath enhanced flows during low Alfvén Mach number solar wind


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 shockfront through 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 reflectedfast shock causes these boundaries to move outward.

Pallocchia et al. 2010, 2013


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 perturbations generated by the impacts of IP shocks on magnetosphere


Fluctuations may arise from solar wind turbulence transmitted through the bow shock and can be generated at the bow shock itself.

Fluctuations are 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 magnetopausethe 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. Alfvén Ion Cyclotron waves will grow when the temperature anisotropy is high and the proton plasma β 1 ∼ (e.g.Schwartz et al., 1996).

Magnetosheath fluctuations


Czaykowska et 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 gyrofrequency and 2.6 above the ∼proton gyrofrequency.

No relevant difference between the quasi-parallel and quasi-perpendicular cases.

Quasi parallel

Quasi perpendicular



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 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



Spectrum of the magnetic field fluctuations observed in a quiet interval of magnetosheath proper .

Nearly a Kolmogorov spectrum is observed for the fluctuations at 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 methodThe PSD has been used using the Welch periodogram method combined with a Hanning window. 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 YouMuch 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., Magnetic Turbulence in the Geospace Environment, Space Science Reviews, 1-46, http://dx.doi.org/10.1007/s11214-010-9692-5

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