1. Magnetic cloud erosion by magnetic reconnection
and impact on their geo-effectiveness
A. Ruffenach,1 B. Lavraud,1 M. J. Owens,2 J.-A. Sauvaud,1 N. P. Savani,3,4 A. P. Rouillard,1 P. Démoulin,5 C. Foullon,6 A. Opitz,1 A. Fedorov,1 C. J. Jacquey,1 V. Génot,1 P. Louarn,1 J. G. Luhmann,7 C. T. Russell,8 C. J. Farrugia,9 and A. B. Galvin9
IRAP, UPS-CNRS, Toulouse, France,
Space Environment Physics Group, Reading, UK,
Naval Research Laboratory, Washington, USA,
College of Science, George Mason University, Fairfax, USA
LESIA, Observatoire de Meudon, Paris, France
University of Warwick, Warwick, UK
Space Sciences Laboratory, University of California, Berkeley, USA,
Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, USA,
Space Science Center, University of New Hampshire, Durham, New Hampshire, USA

2. Outline Introduction Event overview: 19-20 November 2007
Methods and signatures
- Direct method
- Signatures of reconnection
- Topological signatures
Expected impact on geo-effectiveness
Conclusions

3. Introduction : Magnetic cloud characteristics
Basic properties:
Magnetic cloud can be distinguished
by : [Burlaga et al. 1981]
Relatively strong magnetic field
Large and smooth B-field rotation
Lower temperature than average
Magnetic structure:
Toroidal structure, different models :
Cylindrical models (locally described as cylinder) [e.g., Lepping et al., 1990]
non-cylindrical models (expansion during propagation), …
Owens et al. (2011)

4. Introduction : Erosion by magnetic reconnection
Many recent observations of reconnection exhausts in solar wind (Gosling et al., 2005 ; Phan et al., 2006), some associated with CMEs
Magnetic reconnection proposed as a mechanism to erode away MC flux (Dasso et al., 2006):
Impact on MC geo-effectiveness?
MC without erosion
MC eroded
Purpose of this study :
Multi-spacecraft analysis of several potential MC erosion signatures

5. Overview of the event 19-20 Nov 2007
ACE, ST-A, ST-B locations and orientation of the axis derived from MVA
Event seen at ACE
 3 nicely-separated spacecraft observation of a clean magnetic cloud

6. Methods and signatures : 1- Direct Method (Dasso et al., 2006)
Calculation of accumulated azimuthal magnetic flux along spacecraft trajectory (in proper MC frame).
Asymmetry in the flux balance with excess flux at the back of the MC proposed to signal erosion by magnetic reconnection at its front.
Accumulative flux per unit length:

7. Methods and signatures : 1- Direct Method (Dasso et al., 2006)
Calculation of accumulated azimuthal magnetic flux along spacecraft trajectory (in proper MC frame).
Asymmetry in the flux balance with excess flux at the back of the MC proposed to signal erosion by magnetic reconnection at its front.
Accumulative flux per unit length:

8. RESULTS
BY component (MC coordinate system)
End of cloud
« Back » region of MC
Two MC axis determinations :
1) MVA (minimun variance analysis) AND bootstrap error estimates.
2) Cloud-fitting method : MC is described by a force-free model
 Presence of back region with excess flux at both ST-A and ACE  Erosion of % of MC magnetic flux at both SC
Accumulated Flux (BY)

9. PARAMETRIC STUDY MC eroded by ~half 10°
Variation of accumulated flux balance when alternate MC orientations are used
If contour value = 0%,
accum. flux = 0 at cloud end;
If contour value = 50%,
accum. flux = 0 at half cloud.
Only 20°specifically in latitude could explain imbalance, while latitude of axis similar at all S/C
MC eroded by ~half
10°
NO EROSION
 The imbalance is very unlikely due to an error in axis orientation

10. Methods and signatures 2- Local reconnection
Dashed lines: Walén test for rotational discontinuity
Reconnection signatures N
ST-A
Presence of bifurcated current sheets  At all spacecraft
Valid Walén test  At ST-A and THEMIS (i.e., Earth) (ACE and ST-B too coarse resolution) B V
|V|
Exhaust
 Reconnection signatures found at front of MC at all spacecraft

11. Methods and signatures 3- Topological Signatures
Electron PADs are unidirectional throughout the last part of the MC
BUT
There is a distinct change at the expected boundary between the core of the MC and the back region (ACE and ST-A)
Magnetic cloud
Start of « back » region
Suprathermal (250 eV) electron pitch angle distributions
 Signature consistent with a different topology in back region

12. Expected impact on geo-effectiveness
Here we use a Lundqvist MC with South-North B-field and model the impact on the DST index as a function of erosion (using the semi-empirical model of O’Brien and McPherron, 2000)
Half
erosion
No erosion
We are still studying several MCs to illustrate this impact from data
 50% erosion may lead to almost twice lower Dst

13. Conclusion Summary of signatures compatible with MC erosion
Ruffenach et al., JGR, 2012, in press
Summary of signatures compatible with MC erosion
Azimuthal flux integration (Dasso et al direct method)
Error/parametric assessment of axis orientation
Alpha parameter from MC fitting (< 2.4  erosion)
Reconnection signatures observed at all MC fronts
Suprathermal electrons in MC back show clear changes as compared to the core of the MC.
Observed at three widely separated S/C  cannot be the result of crossing the leg at (all) these locations
Significant impact on geo-effectiveness is to be expected
 Future work: Statistical study of all MCs of solar cycle 23

14. Additional Material

15.  Similar issue as X-line models at Earth’s magnetopause
X line orientation (MVA) at ACE and ST-A
One or several X-lines?
 Similar issue as X-line models at Earth’s magnetopause

17.  All complex topologies and electron signatures possible
Expectations for topologies?
 All complex topologies and electron signatures possible

18. Expected impact on geo-effectiveness
Here we use a Lundqvist MC with South-North B-field and model the impact on the DST Index as a function of erosion (using the semi-empirical model of O’Brien and McPherron, 2000)
 Compression and lack of erosion (if parallel fields) should conspire to render the MC more geo-effective