1. THEMIS multi-spacecraft observations of a 3D magnetic
flux rope flanked by two active reconnection
X-lines at the Earth’s magnetopause
Marit Øieroset (UC Berkeley)
Collaborators: Tai Phan, Jonathan Eastwood, Masaki Fujimoto,
Bill Daughton, Mike Shay, Vassilis Angelopoulos, Forrest Mozer,
Jim McFadden, Davin Larson, Karl-Heinz Glassmeier
This event reveals:
A rare spacecraft encounter with an “active” flux rope
flanked by two active X-lines
3D effects
Super-thermal electron heating in the flux rope core
Øieroset et al. [Phys. Rev. Lett., 2011]

2. Outline Theoretical predictions of reconnection-generated flux ropes:
2D versus 3D
THEMIS multi-spacecraft observations of an active 3D flux rope

3. Thin current sheets are prone to multiple X-lines
Daugton et al. (2011)
Nakamura et al. (2010)
2D: magnetic islands are formed
between X-lines
3D: magnetic islands become
magnetic flux ropes

4. Basic properties of 2D magnetic islands (with a finite guide field)
Strong core field
Enhanced density in the core region of the island: 2D effect?
Out-of-plane By
Island at magnetopause
Enhanced core field
Enhanced density
Density
Density
Enhanced density
Omidi and Sibeck (2007)
(Bill Daughton)

5. Electron energization in 2D islands
2D islands: electrons are trapped
Electrons can be energized via:
Acceleration at X-line (e.g., Pritchett, 2006)
Island contraction (Drake et al., 2006)
Island coalescence (e.g., Pritchett, 2008; Oka et al., 2010, Tanaka et al., 2010)
Island contraction
Coalescing islands
Oka et al. (2010)
Drake et al. (2006)

6. In 3D the islands become flux ropes and particles are no longer trapped
Daugton et al. (2011)
Flux ropes are not uniform in the 3rd dimension
Formation, properties, and evolution much more complex
It is not clear how processes seen in 2D simulations are modified by 3D effects

7. Outline Theoretical predictions of reconnection-generated flux ropes:
2D versus 3D
THEMIS multi-spacecraft observations of an active flux rope
Establish the flux rope encounter using multi-spacecraft (non-trivial)
3D effects:
- Density depletion in flux rope core
- Electrons are not trapped
Super-thermal electron energization

8. THEMIS multiple magnetopause crossings: Reconnection jets
Z (north)
THEMIS-D BX
(nT) X Y
jet BY
(nT)
magnetopause BZ
(nT)
m’sphere
jet
m’sheath
Jet reversal V
(km/s) VZ
Guide field across the magnetopause = 0.3 of reconnecting field

9. Reconnection Jet Reversal: X-line or O-line Crossing?
BN reversal
THEMIS-D BX
(nT)
Z (north) BY
(nT) X Y BZ
(nT)
Jet reversal V
(km/s) VZ
With single spacecraft observations it is often difficult to
distinguish between an X-line and an O-line

10. THEMIS in 2010-2011: ZGSM separation
Z separation = km
= ion skin depths
:00:00
THE
THD
THA X Z

11. All three spacecraft observed the flow reversal
→ Can determine conclusively whether this is an X-line or an O-line crossing
TH-D
Z (north)
TH-A
TH-E BY
(nT) X y
TH-D
TH-E
TH-E VZ
(km/s)
TH-A
TH-A
TH-D
DZ= 354 km
DZ= 1094 km
Flow reversal sequence:
If southward moving X-line: TH-D, TH-E, TH-A
If northward moving O-line: TH-A, TH-E, TH-D → this is a flux rope!

12. Spatial dimension of flux rope along Z (outflow direction):
15,000 km = 274 ion skin depths
TH-D
Z (north)
TH-A
TH-E BY
(nT) X y
TH-D
Vz m’sheath
TH-E
TH-E VZ
(km/s)
TH-A
TH-A
TH-D
DZ= 354 km
DZ= 1094 km
Propagation speed of flow reversal: 21 km/s
(comparable to the external magnetosheath flow)

13. Flux rope consists roughly of an outer and an inner (core) region
outer core outer
|B|
(nT) BY B
(nT)
BX (BN) BZ V
(km/s) VZ
Outer region: converging bi-directional jets
Core region: nearly stagnant and enhanced core field (BY)

14. Outline Theoretical predictions of reconnection-generated flux ropes:
2D versus 3D
THEMIS multi-spacecraft observations of an active flux rope
Establish the flux rope encounter using multi-spacecraft (non-trivial)
3D effects:
- Density depletion in flux rope core
- Electrons are not trapped
Super-thermal electron energization

15. Density variations in the flux rope
Outer region: The density is enhanced
Core: The density is reduced compared to outer region → 3D effect
outer core outer
|B|
(nT) BY B
(nT)
BX (BN) BZ V
(km/s) VZ
magnetosheath NP
(cm s-3)

16. Density depletion seen by all three spacecraft
→ a robust feature
outer core outer
outer core outer
THEMIS-E
THEMIS-A
|B|
(nT) BY B
(nT)
BX (BN) BZ V
(km/s) VZ NP
(cm s-3)

17. Outline Theoretical predictions of reconnection-generated flux ropes:
2D versus 3D
THEMIS multi-spacecraft observations of an active flux rope
Establish the flux rope encounter using multi-spacecraft (non-trivial)
3D effects:
- Density depletion in flux rope core
- Electrons are not trapped
Super-thermal electron energization

18. Electrons are not trapped in the flux rope → 3D effect
outer core outer
|B|
(nT) B
(nT) BY
BX (BN) BZ V
(km/s) VZ NP
(cm s-3)
180°
Electrons
(eV)
90°
Electrons
(eV)
Electrons are unbalanced
→ flux rope is open-ended 0°
Electrons
(eV)

19. Outline Theoretical predictions of reconnection-generated flux ropes:
2D versus 3D
THEMIS multi-spacecraft observations of an active flux rope
Establish the flux rope encounter using multi-spacecraft (non-trivial)
3D effects:
- Density depletion in flux rope core
- Electrons are not trapped
Super-thermal electron energization

20. Super-thermal electron energization
outer core outer
|B|
(nT) B
(nT) BY
BX (BN) BZ V
(km/s) VZ NP
(cm s-3) Te
(eV)
Te||
Te┴
180°
Electrons
(eV)
The super-thermal (1-4 keV) electron
fluxes significantly enhanced in the core
Te|| is enhanced in the outer region,
but not in the core
90°
Electrons
(eV) 0°
Electrons
(eV)

21. Summary
Three THEMIS spacecraft observed the passage of a 3D flux rope flanked by two active X-lines
3D effects:
Density depletion
Electrons not trapped
Particle heating and energization
Ti┴ is enhanced inside the flux rope core
Te|| is enhanced in the outer region
Super-thermal (1-4 keV) electrons likely energized
somewhere along flux rope core

22. Open questions Active versus non-active flux ropes:
Fact: The majority of flux ropes detected in space
are not flanked by active X-lines [Zhang et al. 2011]
→ X-lines associated with flux ropes die quickly
as they convect away
How does particle energization depend on
the activeness of flux ropes?
2D versus 3D:
How does the fact that particles are not trapped
affect the level of particle energization?
Øieroset et al. [Phys. Rev. Lett., 2011]

23. Ions and electrons are heated differently in the flux rope
outer core outer
Ti┴
Te||
|B|
(nT) B
(nT) BY
BX (BN) BZ V
(km/s) VZ NP
(cm s-3)
Ti┴ is enhanced inside
the flux rope core
Te|| is enhanced in
the outer region
Ti┴ Ti
(eV)
Ti|| Te
(eV)
Te||
Observed Ti and Te not predicted
Te┴

25. Where do the super-thermal (1-4 keV) electrons come from?
not simply leakage of magnetospheric electrons because 1-4 keV electrons not
present inside the magnetosphere
1-4 keV electrons energized somewhere along the flux rope core
m’sphere
m’sheath B
(nT)
jet reversal/flux rope V
(km/s)
1-4 keV electrons
Electrons
(eV)
Void of 1-4 keV electrons

26. Second flux rope event shows (mostly) similar properties as the first event
THEMIS-D
outer core outer
|B|
(nT)
Outer region: converging bi-directional jets
Core region: near stagnant
Core field BY peaks at BNormal reversal
Density depleted inside flux rope
Electrons are not trapped in the flux rope
Ti┴ is enhanced inside the flux rope core (?)
Te|| is enhanced in the outer region B
(nT) V
(km/s) NP
(cm s-3) Ti
(eV) Te
(eV)
180°
electrons
The super-thermal electron fluxes
are enhanced in the core
90°
electrons 0°
electrons

27. Comparison of electron distributions (TH-D)
Inner flux rope region
magnetosphere
Accelerated
Electrons
(>1 keV)
magnetosheath
Phase space density much lower at these energies inside magnetosphere
→ 1-4 keV flux rope electrons are not simply leakage of m’spheric electrons

28. E-field (especially EN) fluctuations are enhanced in the core
outer core outer
|B|
(nT) B
(nT) BY
BX (BN) BZ V
(km/s) VZ EX
(mV/m) EY
(mV/m)
VZ,ExB
(km/s)
VZ,ExB
Vi┴,Z
Ex B velocity does not always agree with measured Vi ┴ inside the core
→ the frozen-in condition is violated in some parts of the core region

29. Peculiar difference between electron thermal and non-thermal heating
outer core outer
|B|
(nT) B
(nT) BY
BX (BN) BZ V
(km/s) VZ NP
(cm s-3) Te
(eV)
Te||
Te┴
180°
electrons
Te|| is enhanced in the outer region,
but not in the core
The super-thermal (1-4 keV) electron
fluxes significantly enhanced in the core
1-4 keV is 3-10 times higher than the
330 eV energy associated with VA,e
90°
electrons 0°
electrons