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

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

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)

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

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

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:

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:

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 40 - 50% of MC magnetic flux at both SC Accumulated Flux (BY)

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

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

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

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

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. 2006 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

Additional Material

 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

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

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