Embedded current sheet in the Earth’s magnetotail

A.A. Petrukovich, A.V. Artemyev, L.M. Zelenyi Space Research Institute, Moscow

R. NakamuraSpace Research Institute, Graz

Outline

1. Thin embedded sheets are frequently observed

2. How sheets can be quantified ?

3. Embedded sheets in the magnetotail context

4. Implications for substorms

Вложенный токовый слой в хвосте магнитосферы

A.A. Petrukovich, A.V. Artemyev, L.M. Zelenyi Space Research Institute, Moscow

R. NakamuraSpace Research Institute, Graz

1. Тонкие вложенные слои – характерный объект в хвосте

2. Количественное описание вложенных слоев

3. Вложенные слои в контексте плазменного слоя

4. Вложенные слои в контексте суббури

Runov et al., 2006, AnGeo

Embedding: spatial scale of current is smaller than

spatial scale of density

Asano et al 2005, GRL

ISEE-1,2: thin current sheet (with small Bz) is embedded in a thicker plasma sheet

Mitchell et al, 1990, Sergeev et al, 1993

Cluster:

Observations

Embedded sheet profile

Harris sheet: Be and Je: He=Be/Je

Embedded sheet: J0 and B0: H0=B0/J0

Total: Jmax, B0, BeJe<<J0 , Je is not resolved by Cluster

at distances 15-20 Re

Jmax ~J0

Jmax=J0+Je, Je<<J0

B0 Be

Harris profile

Je

Embedded sheet in theory

essentially embedded sheet in thin anisotropic current sheet

model by Zelenyi et al.

B0 is less definite:

embedding is often created just adding background plasmas.

Thin current sheets have typical thickness ~ Larmor radiusin boundary magnetic field B0

Cluster 2001-2004 cases: thin (almost) horizontal single-peaked symmetric sheets

Cluster view on embedding

Theory predicts thickness of the order of ion Larmor in B0!

L=B0/J0~B0/Jmax

Cluster view on embedding

1. Large current densities areunderestimated

2. Small current densitiesare overestimated: J0~Je

3. Particle sorts add scatter

Cluster view on embedding: uncertainties

1. L0 ~ 1-3 * RL0

2. Range of B0

=> J0 ~ B0^2 with larger B0 sheets are thinner!

=> Magnetic flux F0 ~ B0 * RL0 = pc/e/B0 * B0 = const (B0) - “quantum” flux

Most of the flux is in wings but

Harris sheet has infinite range!

Make J=0 at finite Z“finite” Harris (Veltri et al 1998)

F0=(0.5-2)B0*L0 ~ 1-5 *B0*RL0

Quantify embedded sheet

Embedding in the magnetotail context

F0 ~ (1-5) * 0.02 Wb/m (4000 eV) << Fe ~ 0.5 Wb/m 10 times!

1. Extreme case smallest possible B0 so that J0 >= Je

B0/Be >= sqrt(F0/Fe) ~ .3-.25

2. Extreme case B0 ~ Be: if F0<<Fe

singular thin sheet in empty plasma sheet Drop all plasma from plasma sheet

Or much smaller Fe~F0

At 15 Re – post-plasmoid plasma sheet ?

Total plasma sheet flux Fe is much larger than F0 at 15-20 Re

Embedding in the magnetotail context

Minimum embeddingis indeed B0 > 0.25-0.3 Be

8 of 10 cases of B0 > 0.5 Be are after tailward plasmoids!

Example->

Depleted magnetic flux in the post-plasmoid plasma sheet Hones, 1984

Embedding in the magnetotail context

Distant tail

Fe~F0

B0~Be

Be – small

thin sheets are almost Harris

Near tailBoundaryWhen Bz ~ B0

1D approximationIs not valid

Middle tailFe is growing to E

Be is growing

B0 and J ?

Embedding in the magnetotail context

Embedding parameters inTsyganenko modelsdepend on tail stretching

Latitude 70 used as boundary of PS

Embedding in the magnetotail context

Implications for substorms

1. Criterion of being inside embedded sheet when considering onset flows:in a regular quiet sheet: F0/Fe < 1/10 B0/Be ~ 1/3, => 0 ~ 10

2. Plasma sheet with embedding is thinner for the same Fe and Be

than Harris sheet (it contains more flux and less plasma):

Re_new = Re_harris * (1+aF0/Fe)/(1+aB0/Be) = Re_harris *0.9

Leaving more space for increase of open magnetic flux even when Be=const

3. Total cross-tail current I grows towards Earth and during growth phase, but local current density J is controlled by embeddingwhere and why current density peaks (at onset)?

4. During growth phase embedded sheet intensifies.Flux is removed or plasma is removed to allow larger B0

Sergeev et al, 1993

Implications for substorms

Petrukovich et al, 2007

5. Stability

Burkhart, 1992, suggestedfor an embedded sheet

The growth rate scales asB0/Be *(1- (B0/Be)2)

Zelenyi et al model predicts instabilityzone related with Bz

Implications for substorms

Conclusions

Thin ion scale embedded sheets are frequent especially during substorms

Thickness is controlled by local larmor (as in theory) and plasma properties

Crucial internal parameter is B0, magnetic flux is constant.

Sheet in the magnetotail is controlled by B0/Be and F0/Fe

Quiet sheets have deep embedding B0<< Be (at 15-20 Re) and slowly

evolve with increase of B0 during growth phase

Largest embedding B0~Be requires plasma sheet magnetic flux drop –

after onset or in the distant tail

Plasma sheet is thinner when embedded and less stable.

Embedding should be taken into account in analyses of ion kinetics,global convection, stability, etc.