106 Lee and Fu.- Magnetic Flux Transfer

in general has a finite B. component, the X-lines will not be in the y-direYctionof the solar- magnetospheric coordinates, as shown in Figure 3. Let 0 be the angle between the z-axis and the projected IMF in the y-z plane. The X-lines will make an angle 0/2 with the z-axis. It can be seen from Figures 2a and 3 that the flowing / directionof the field-alignedcurrent (JII) associated with the twisted field lines in the magnetopauseregion depends on the sign of By. "' , For B > o the current flows along the guide'd X•C1 D1B2 E1C2 .D2, E2' >2 Basedy on t =present model >o;t•e tl --- X-line Figure 3. FluxThe X-line transferis tilted counterclockwise event observation at ’s dayside fromthe y-axis for By > o and is tilted :Z -•--•-•--•--5-x-line clockwise for B < 0. This effect has been observedbyby Dal• the et al. (1984,CassiniFigure 3). Itspacecraft is noted that even when the IMF turns exactly southward(By-- 0), the magnetic1,2,3, fields on the 1 4 1 1 5 2,3 2,3 6 F2 G2 H2G1 12 H1 J211 J• F1 two sides J. of M.the Jasinskimagnetopause are *,no J. A. Slavin , C. S. Arridge , G. Poh , X. Jia , N. Sergis , A. J. Coates , G. H. Jones , J. H. Waite Jr . antiparallel 1Climatein most and regions Space of Sciences the dayside and Engineering, University of Michigan, MI, USA; 2MSSL, UCL, UK; 3Centre for Planetary Sciences, UCL/Birkbeck, UK; magnetopause. The magnetic flux tubes can still A1•5 A2•/A• 4 5 6 be formed byDept. the ofmultiple Physics, X-line Lancaster reconnectton University, UK; Office for Space Research and Technology, Academy of Athens, Greece; Southwest Research Institute, TX, USA. 106 Lee process,and Fu.- Magnetic Flux Transfer _%% Along each*Email: open [email protected] only the portion (c) t=t3>t2 Introduction that is embedded in the magnetopause has a Observations MVA and flux rope modeling transverse magneticin general field has componenta finite B. whichcomponent, is the X-lines • The process of reconnectionassociated occurringwithwill the not athelical bemultiple in thefield y-direYctionX-lines lines. at of Itthe is solar-• Cassini plasma observations are shown below with • Minimum variance analysis (MVA) shows the FTE expected that magnetosphericthese transverse coordinates, fields aswill shown in Figure 3. Let 0 be the angle between the z-axis and the F• the daysideF• magnetopause,propagate results as Alfv'en in the waves creation away offrom flux the Cassinia) in the at the startIons where - FIPS it structure is consistent with a flux rope (black). projected IMF in the y-z plane. The X-lines will ! •8- L. •o•aa•oa o• aa•ae•c [Luxtransfer •ubes events as a (FTEs). make an angle 0/2 with the z-axis. It canobserved be 5 TCRs, before it entered a region of open flux, ! λint/λmin = 18.7; λmax/λint = 4.5 ßesuL• o• auL•pLe X-L•ae •ecoaaec•oa. (a) seen from Figures 2a and 3 that the flowing 4 / • The FTE was a b followed by a crossing into the and finally 2 e directionof the field-alignedcurrent (JII) Cusp Energy Dispersion e associated with Magnetopausethe twisted field lines in the compared to a force- 2 ! ] the SW. ] 0 ! min int max nT magnetopauseregion depends on the sign of By. nT [ free flux rope model [ 0 Eigenvalue For B > o the current flows along the guide'd ! 0.03 0.56 2.52 max max -2 ! B •3• •he heLicaL •eL8 L•aes kLk2"' aa8 •2•L a•e , first put forward by B Eigenvector -0.72 -0.15 -0.68 , ' s -2 ! R X•C1 D1B2 E1C2 .D2E2 >2 • Plasma observations show a mixed magnetosheath and ! -0.69 -0.31 -0.65θ •appe8 by •he ae•Ly •o•ae8 •eL8 L•aes Basedy on t =present model >o;t•e = -2.11 nT = -4.80 nT southward convected flux tubes as illustrated in developed by Lepping ! min int Bint [nT] Bmin [nT] = -0.83 nT ΔB = 9.14 nT (b) t=t2 >tl --- X-line Figure 3. The X-line is tilted counterclockwisethe FTE is observed (blue dotted line). ! max! et al. [1990] (model 6 c x = (0.72, -0.69, 0.11) vSW ] 4 min λ = 0.03 Eigenvalue ratio int/min = 17.3 formation of the flux tube with helical fields. fromthe y-axis for By > o and is tilted nT 2 min max/int = 4.5 [ :Z -•--•-•--•--5-x-line clockwise for B < 0. This effect has been Magnetosphere ‘Open’ Region M’sheath SW shown in blue). min 0

Earth 4 B ! ! MIMI-LEMMS (electrons) 10 min -2

a) B If By= 0, the multiple X-line reconnectionwill observedby Dal• et al. (1984,Figure 3). It is 3 -4 only lead to the formation of isolated magnetic noted that even when the IMF turns exactly ! ! 10 -6 102 4 d x = (-0.15, 0.31, -0.94) ) 2 int loops. 100 ]

southward(By-- 0), the magneticfields on the -1 • The FTE was estimated ! ! 1 0 λint= 0.56 DNF nT F2 G2 H2G1 12 H1 J211 J• F1 two sides of the magnetopause are no 10 int We propose that a multiple X-line reconnection [ -2 0 B IMF to have a radius of int -4 antiparallel in most regions of the dayside ! ! 10 keV Energy [keV] -1 B process as described above may occur at the -1 magnetopause. The magnetic flux tubes can still 10 -6 A1•5 A2•/A• 4 -8

sr 4600-8300 km and a dayside magnetopause. A schematic sketch of the b) ! ! MIMI-LEMMS (protons) 10 6 be formed by the multiple X-line reconnectton -1 ! e 3 ] 4

1000 10 s reconnected field lines at the magnetopause is ! ! ! 2 process, magnetic flux content of nT [Lee and Fu, 1985] ! ! 2 -2 [ 10 max 0 shown in Figures 2a and 2b, in which three X-_%% Along each open flux tube only the portion B max -2 xmax= (-0.68, -0.65, 0.33) ! 1 (cm 0.2-0.8 MWb.

! DNF (c) t=t3>t2 10 B -4 lines are assumed to exist. Figure 2a is a that is embedded in the magnetopause has a λmax= 2.52 100 0 -6

transverse magnetic field component which is 10 ! ! • a) The FTEs twisted magnetic field structure produces a Energy [keV] ! ! -1 0 5 10 15 20 25 three-dimensional perspective view of the ) associated with the helical field lines. It is 10 10 -1 Time (s) ! ! CAPS-ELS 10 magnetopause and the reconnected bipolarmagnetic signature field observed in the magneticexpected that field these transverse fields will 10000 c) from 23:33:55 eV lines, while Figure 2b is the projectionmeasurements,F• of the as wellF• as an increasepropagate in magneticas Alfv'en waves field away from the 1000 ! ! -1

9 sr

field lines in the noon-midnight meridian -1 Discussion & Summary •8- L. •o•aa•oa o• aa•ae•c [Lux •ubes as a 100 10 ! ! DEF ßesuL• o• auL•pLestrength X-L•ae at the•ecoaaec•oa. centre of (a)the FTE (the typical FTE s plane. It is seen that two magnetic flux tubes -2 10

observational signatures). ! ! 8 m • We have presented the first observation of a flux rope at Saturn’s containing twisted field lines are formed as a Energy/q [eV/q] 1 10 ] eV ( natural consequence of multiple X-line -3 ELS-MOMENT dayside magnetopause. 1.5 d) ! ! reconnection. The flux tubes northwarda) and b) •3• •he heLicaL •eL8 L•aes kLk2 aa8 •2•L a•e 1.0 ! ! southward of the central X-line will be •appe8 by •he ae•Ly •o•ae8 •eL8 L•aes 0.5 ! ! • This is an important result because it shows that the convected, respectively, aa8 toE2 • •Lthe • . north and south Density [cm ! ! 3 Saturnian magnetopause is conducive to multiple X-line cusp regions by the magnetosheath plasma flow and 10000 e) ! ! CAPS-IMS 10 the tension force of the reconnected flux ! ! reconnection and flux rope generation. formation of the flux tube with helical fields. 1000 tubes. The sense of field twisting as shown in ! ! 102 If By= 0, the multiple X-line reconnectionwill ! ! Figure 2a is consistent onlywith lead theto ISEEthe formation of isolated magnetic 100 ! ! Counts / s • MVA shows that the measured magnetic structure is consistent observations (Saunders et al., 1984). The Fig. 2. (a) A perspective view of the open 1 loops. • b) If the spacecraft does not cross through the FTE, but Energy/q [eV/q] 10 10 with a flux rope. The magnetic observations were also well fitted northward flux tube wouldWe show propose a positivethat a multiple X-line reconnectionmagnetic flux tubes and the regular open field 10 f) ! ! passes near the edge,lines then as a atravelling result of compression three X-line regionreconnection at to a constant-α force-free flux rope model. polarity of the observedprocess Bn signatureas described whileabove themay occur at the 5 ! ! dayside magnetopause.(TCR) is observedA schematic duesketch to of the the draping of the magnetic field southward tube would show a negative polarity. the dayside magnetopause. (b) The projection of [nT] 0 reconnected field lines at the magnetopause is R ! ! around the flux rope (redflux shadingtubes in above).the noon-midnight meridian plane. B -5 Since the interplanetaryshown inmagnetic Figures 2afield and 2b,(IMF) in which three X- • Assuming that the five observed TCRs (in the adjacent -10 ! ! lines are assumed to exist. Figure 2a is a Rev fte_fte: 2007-01-29T00:00:00 - 2007-02-11T00:00:00 Rev fte_fte: 2007-01-29T00:00:00XKSM [RS ]- 2007-02-11T00:00:00 Rev fte_fte: 2007-01-29T00:00:00 - 2007-02-11T00:00:00 20 10 20 2020 g) ! 20 ! magnetosphere) are attributed to FTEs, results in an FTE three-dimensional• Previous perspective surveys view of of the 5 occurrence of ~2min (6 FTEs in 9 min), which is less that the magnetopause and the reconnected magnetic field ! !

[nT] 0 lines, while Figure 2b is the projection of the 1010 θ 10 ~8min and more than the 8s observed at and , Cassini data for FTEs B field lines in the noon-midnight meridian -5 ! ! ] S ] -10 ] respective [Rijnbeek et al., 1984; Slavin et al., 2012]. S at Saturn found none S plane. It is seen that two magnetic flux tubes10 [R [R 10 [R 00 10 h) ! 0 ! KSM KSM Z containing twisted field lines are formed as a Y KSM 5 [Lai et al., 2012]. Y ! ! natural consequence of multiple X-line • Six FTEs in 9 min would result in a reconnection voltage of ~2-9 reconnection. The flux tubes northward and [nT] 0 -10-10 φ ! -10 ! ] B southward of the central X-line will be S -5 kV (attributed solely to FTE generation). ] S [R • Cassini observed the [R -10 ! ! convected, respectively, to the north and south00 KSM Z -20-20 10 i) -20 cusp regions by the magnetosheath plasma flow KSM and Rev fte_fte: 2007-01-29T00:00:00 - 2007-02-11T00:00:00 Rev-20-20 fte_fte: 2007-01-29T00:00:00-10-10 00 - 2007-02-11T00:00:001010 2020 -20 -10 0 10 20 FTE near the subsolar Z 20 20 8 X [R ] ! ! Y [R ] the tension force of the reconnected flux 20 KSM S KSM S • With an estimated dayside reconnection voltage of ~100-200 kV 6 ! ! tubes. The magnetopausesense of field twisting (at heas shown in 4 during solar pressure enhancements [Masters, 2015; Figure 2a is consistent with the ISEE 10 1010 |B| [nT] 2 ! ! red arrow) and was Fig. 2. (a) A perspective view of the open T T T T T F Badman et al., 2013], it could be estimated that FTEs contribute observations (Saunders et al., 1984). The -10-10 0

]

magnetic flux tubes and the regular openS field ] ] S northward fluxtraveling tube would equatorward show a positive S UT 23:2223:22 23:3223:32 23:4223:42 23:5223:52 ~1-9% to the opening of flux at Saturn’s dayside magnetopause [R [R lines as a result of three X-line reconnection [R at 0 00 KSM polarity of the observed Bn signature while the KSM RS 17.3 17.3 17.3 17.3 Z Y KSM (during SW compressions). southward tube(blue would arrow show ais negative spaceraft polarity. the dayside magnetopause. (b) The projectionZ Lat (º) -24.8of -24.7 -24.5 -24.3 Since the interplanetary magnetic field (IMF) flux tubes in the noon-midnight meridian LTplane.12:49 12:49 12:50 12:50 Background model from Khurana et al., (2006). -10 -10-10 Badman et al., 2013 (Icarus); Jasinski et al., 2016 (GRL); Lai et al., trajectory direction). -20-20 References: -20 -10 0 10 20 -20 -10 X 0 [R ] 10 20 KSM S 2012 (JGR); Lee and Fu, 1985 (GRL); Lepping et al., 1990 (JGR); Lundquist, 1950 (Arkiv X [R ] -20 KSM S -20-20 fur Fysik); Masters, 2015 (GRL); Rijnbeek et al., 1984 (JGR); Slavin et al., 2012 (JGR).

! -20 -10 0 10 20 -20! -10 0 10 20 ! ! Y [R ] ! TCRs FTE KSM S XKSM [RS] ! YKSM [RS]