earth magnetotail current sheet: observations vs. model
TRANSCRIPT
Earth magnetotail current sheet: observations vs. models
Anton Artemyev
IGPP/UCLA
in collaborations with: Vassilis Angelopoulos (IGPP/UCLA), San Lu (IGPP/UCLA), Anatoliy Petrukovich (IKI/RAS), Andrei Runov (IGPP/UCLA), Ivan Vasko (SSL/UCB), Lev Zelenyi (IKI/RAS).
Solar wind: b~1, M>>1
magnetotail: b>>1, M≤1
Inner magnetosphere: b<<1, M<<1
solar flare: b≥1, M>1
Location and plasma parameters
Plasma b – ratio of plasma and magnetic field pressuresMach number M – ratio of plasma bulk velocity and magnetosonic speed
The simplest current sheet configuration
x
z
Diamagnetic plasma current
y
x
T dnj c
B dz
z
plasma density (pressure)
magnetic field
Self-consistent fields and plasma motion
4 1
4
1
0
c c t
c t
EB j
E
BE
B
Stationary Maxwell equations
,
,
q f r d
q f r d
j v v v
v v
Currents and charged density
+0
f f f
t m
Fv
r v
+
Vlasov equation
Lorentz force
1
qc
F E v B
Nonlinear equations for electromagnetic fields
An example of solutions: 1D Harris CS An example of solutions: 2D Fadeev CS An example of solutions: 2D Kan CS
x x x
zMagnetic field lines (black) and plasma pressure (color)
0
Harris 1962 Fadeev et al. 1965 Kan 1973
ions (+) and electrons (-)
Cluster 2001-2004
THEMIS 2008-2009
ARTEMIS 2010
Geotail 1993-1994
Spacecraft missions and available datasets
r~18RE
r~10-30RE
r~50-200RE
THEMIS 2010-2017
r~10-12RE
r~30-200RE
ARTEMIS 2011-2016
r~55RE
MMS 2017
r~30RE
Outline:
•Current sheet structure: distributions of currents and plasma, embedding, main gradients
•Current carriers in the magnetotail: electron and ion contributions
•Pressure balance: role of plasma anisotropy/nongyrotropy in current sheet balancing
• Electric fields: mechanisms of generation, typical amplitudes
•Current sheet thinning: pre-reconnection conditions in the magnetotail
Current sheet structure: thin current sheets
i~1000 kmRunov et al., 2006Runov et al., 2006; Petrukovich et al. 2015; Vasko et al. 2015; Artemyev et al., 2016.
Typical CS thickness is about 2000 km << RE
x
Current sheet structure: embedding
0N Runov et al., 2006
Absence of significant density variation along z-axis
Magnetic field as a distance along z-axis: Bx/Blobe~z/LCS
Bx/Blobe
,P N T P N T
Petrukovich et al., 2015
Bx/Blobe Bx/Blobe
T e/T e,
max
T i/T i,m
ax
Electron and ion pressure gradients are mainly supported by temperature gradients
CS structure: observations vs. simple kinetic models
Harris CS model with 10-20% of cold background plasm
Ni
z
jy
Ti
Ni
z
jy
Ti
Observed current sheets
• Lobes are filled by rarified cold plasma, but CS boundaries are hot due to uniform temperature.
• Plasma density profile is very close to the current density profile
• Lobes and CS boundaries are filled by dense cold plasma.
• Current density profile is embedded into plasma density profile
CS structure: what model can reproduce CS structure?
Kinetic models do not describe strongtemperature gradients for 2D (x,z) CSconfigurations, but can reproduce embeddedcurrent density profile
Hybrid simulations (MHDelectrons + kinetic ions) areflexible enough to reproduceboth embedding andtemperature distributions.
see also Schindler & Birn 2002;Sitnov et al., 2006; Zelenyi et al.2011
observations
Birn et al. 2004
Yoon&SLui 2004
Sitnov et al. 2000Zelenyi et al. 2000
Lu et al., 2016
see also Lin et al. 2015, Lu et al. 2016
Current carriers: absence of ion currentsWang et al., 2012
Ti>>Te thus wewould expect toobserve strongion diamagneticcurrents.
j yn
A/m
2
Bx/Blobe Bx/Blobe
Examples of ion current observations in CSs
Runov et al., 2006
currents from B-gradients
Ion currents C4
Ion currents C1
jy/ene km/s
Israelevich et al., 2008
Artemyev et al., 2015
Comparison of B-gradient currentsand ion currentsfor subset of CSs
Current carriers: unexpectedly strong electron currents
Ion currents
electron currents
Ion currents
electron currentselectron currents
Ion currents
Geotail Cluster THEMIS
Three data sets show the same effect of strongelectron currents and weak ion currents. Plasma(ion + electron) currents are close to B-gradientcurrents derived from multispacecraftmeasurements (Cluster) or from CSoscillations/flapping (Geotail, THEMIS).
see also Vasko et al., 2015; Petrukovich et al.,2015; Artemyev et al., 2016
Current carriers: observations vs. modelsConservation of total plasma momentum in volume V
total current
rate of total plasma momentum change
ion (+) and electron (-) currents
Hesse et al., 1998
factor ~m-/m+<<1 for ions. Thus, ifthe total momentum is conserved
(M = 0), the change of the total
current J is mainly due to change
of the electron flow nv-
-vy/
v Ti electron velocity, v-
ion velocity, v+
Dv=v+-v-
Pritchett et al., 1995
Increase of electron flow
Kinetic thin CS formation
CS thinning and intensification of jy~enDv current density in PIC simulation
Pressure balance: expected gradients
21
0,8
pxp z y
pBp B j
z x c
x
z
magnetic field line Bx
Bzjy
1p̂
c
j BIsotropic plasma pressure pp=p++p-
, 84
lobe lobey lobe P
z
B Bcj B p
B x
static pressure balance
Nakai et al. 1991
Blo
be
nT
r/RE
Balance along z Balance along x
Current density magnitude is defined by the lobe magnetic field distribution
Blobe distribution along the tail
Pressure balance: statistical observations
22 nA/myj upper limit for jy at r~18RE
observed jy are much larger than expected values
Nakai et al. 1991;Shukhtina et al. 2004
0.8
20nT / 25lobe EB r R
THEMIS measurements
( )4
lobe lobey
z
B Bcj x
B x
B-gradient current
P-gradient current
Pressure balance: electron anisotropy
2
4
4 ( ),
ep
e e
p i e e
pB c
p pp p p
B
B B B j B
curvature force
1
4 1
lobe lobey
e z
B Bcj
B x
at the equatorial plane (Bx=0)
Field-aligned electron anisotropy helps balancecurrent sheet. For e=1 the entire current densitygenerated by curvature electron drifts (d/dx=0)
THEMIS observations
There is a significant populationof CSs with almost isotropicelectrons
Pressure balance: observations vs. models
1xzy z
pj B
z c
Ion nongyrotropy can balance CS
Burkhart et al., 1992
see also Eastwood 1972; Pritchett and Coroniti 1992; Ashour-Abdalla et al., 1994;
PIC simulation of thin CS
distribution function of ions supporting pxz≠0
Artemyev & Zelenyi 2013
Ob
serv
atio
ns
mo
de
ls
see also Sitnov et al., 2004; Zhou et al., 2009; Zelenyi et al. 2011
vx
vx
vy
vy
Electric fields: Hall field in thin CS
Dominance of electron currents requires strong electric fields Ex, Ez
2 2
y z x x zE B
E B E Bv c c
B B
E BWygant et al., 2005
time~30s
Bx<0 Bx>0
-E m
V/m
Cluster crossing of very intense CS
Statistics of quiet CSs
E m
V/m
Bx/Blobe Bx/BlobeBx/Blobe
Vasko et al., 2014
~ ~ ~1mV/m
~ 0.1mV/m
yE Bz x x
zx z
lobe
jvE B B
c qn c
BE E
B
Cluster & THEMIS can provide reliable electricfield measurements only in CS (x,y) plane.Therefore, observed electric fields can beestimated only for strongly titled CS with z↔y
Electric fields: field-aligned field
2
e e
e e
p p Ben E p
B B
B B
Electron stress balance along magnetic filed lines
peII/pepeII/pe
Cluster statistics THEMIS statistics
1.1 1.2e
e
e
p
p
1ln
e
e
ee
TeE T
B
neconst
0.1 mV/mE
Electric fields: observations vs. modelsTransverse electric field (Ex, Ez)
Lu et al., 2016
Ez>0
Ez<0
Hybrid simulations
PIC simulations
see also Pritchett 2005; Zelenyi et al. 2011;Schindler et al. 2012;
Birn & Hesse, 2014
Ez>0
Ez<0
Field-aligned electric field (EII)
eFII/Te
Egedal et al., 2013
PIC simulations
Artemyev et al., 2016eEIIL/Ti
EII>0
EII>0
Analytical CS model
Current sheet dynamics: thinning McPherron et al. 1972 An example of thinning CS observed by THEMIS at x~-12RE
Bz decreases
Blobe increases
jy increases
LCS decreases
Current sheet dynamics: plasma cooling
An example of thinning CS observed by THEMIS; plasma data are collected around the equatorial plane
growth of the plasma density
2/3
1/4
,
~ 1
~
i z
i e z
n B
T B
Electrons are cooling faster than ions!+
Current sheet dynamics: observations vs. model
• Current sheet thinning is accompanied by (driven) pressure increase.
• Plasma entropy is conserved
Birn & Hesse 2014
pn const
3/5 2/5~ , ~n p T p
Conclusions:1. CS embedding can be reproduced by simulations and analytical models
2. Temperature gradients well seen in observations, but are not included intokinetic simulations
3. Thin CSs are characterized by strong electron currents (both inobservations and simulation)
4. Estimations suggest significant contribution of ion nongyrotropy (not yetobserved!) and electron anisotropy to the pressure balance
5. Strong transverse electric fields are seen in simulations, but more accurateobservations are required to estimate these field in the magnetotail
6. Models and observations of the electron anisotropy indicate on a finitefield-aligned field in the magnetotail (not yet observed!).
7. CS thinning is accompanied by plasma cooling, but this effect is not seen insimulations.
EARTH MAGNETOTAIL CURRENT SHEET
Geotail,
Cluster,
THEMIS,
ARTEMIS
Predicted by models, but
not yet observed
Well observed, but not
yet modeled
• E• EII
• pxz
• T• T
Additional info
Current sheet dynamics: current density growth
jy nA/m2
Petrukovich et al., 2007
~ 1/y zj B
Artemyev et al., 2016
3/2~1/y zj B
-12RE -18RE
TCS properties:- CS thinning is not uniform process:
larger initial Bz in the near-Earth CS requires more significant Bz decrease during thinning
- Bz is smaller at larger r, whereas jy is larger at smaller r
Petrukovich et al., 2013
THEMIS 2015Cluster 2001-2004
j yn
A/m
2
Bz nT
Formation of thin C
Plasma pressure inthe neutral plane
=5/3
1999 JGR
2014 JGR
jy
Modern CS models
Sitnov & Schindler 2010 Sitnov & Merkin 2016
2D solution with magnetic “hump”
2D solution with included dipole!
Sitnov et al., 2007
2D solution with small population of nongyrotropic ions
Recent reviews about CS structure & dynamics• Artemyev, A. V., and L. M. Zelenyi (2013), Kinetic structure of current sheets in the Earth magnetotail, Space Sci. Rev., 178, 419–440,
doi:10.1007/s11214-012-9954-5.
• Baumjohann, W., et al. (2007), Dynamics of thin current sheets: Cluster observations, Ann. Geophys., 25, 1365–1389.
• Birn. J, et al. (2012) Particle acceleration in the magnetotail and aurora Space Sci. Rev. 173 49–102
• Eastwood, J. P., H. Hietala, G. Toth, T. D. Phan, and M. Fujimoto (2015), What controls the structure and dynamics of Earth’s magnetosphere?, Space Sci. Rev., 188, 251–286, doi:10.1007/s11214-014-0050-x.
• Egedal, J., A. Le, and W. Daughton (2013), A review of pressure anisotropy caused by electron trapping in collisionless plasma, and itsimplications for magnetic reconnection, Phys. Plasmas, 20(6), 061201, doi:10.1063/1.4811092.
• Ganushkina, N. Y., et al. (2015), Defining and resolving current systems in geospace, Ann. Geophys., 33, 1369–1402, doi:10.5194/angeo-33-1369-2015.
• Goldstein, M. L. et al. 2015 Multipoint observations of plasma phenomena made in space by cluster J. Plasma Phys. 81 325810301
• Petrukovich, A. A., A. V. Artemyev, I. Y. Vasko, R. Nakamura, and L. M. Zelenyi (2015), Current sheets in the Earth magnetotail: Plasma and magnetic field structure with Cluster project observations, Space Sci. Rev., 188, 311–337, doi:10.1007/s11214-014-0126-7.
• Schindler, K. (2006), Physics of Space Plasma Activity, 522 pp., Cambridge Univ. Press, Cambridge.
• Sitnov, M. I., and V. G. Merkin (2016), Generalized magnetotail equilibria: Effects of the dipole field, thin current sheets, and magnetic flux accumulation, J. Geophys. Res. Space Physics, 121, 7664–7683, doi:10.1002/2016JA023001.
• Yoon, P. H. and Lui, A. T. Y., 2005 A class of exact two dimensional kinetic current sheet equilibria J. Geophys. Res. 110 A01202
• Zelenyi L M, Malova H V, Artemyev A V, Popov V Y and Petrukovich A A (2011) Thin current sheets in collisionless plasma: equilibrium structure, plasma instabilities, and particle acceleration Plasma Phys. Rep. 37 118–60
• Zelenyi, L. M., A. I. Neishtadt, A. V. Artemyev, D. L. Vainchtein, and H. V. Malova (2013), Quasiadiabatic dynamics of charged particles in a space plasma, Phys. Uspekhi, 56, 347–394, doi:10.3367/UFNe.0183.201304b.0365.