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A New Method For Investigating Reconnection At A Current Sheet by F.S. Mozer 1 and A. Retino 2

A New Method For Investigating Reconnection At A Current Sheet by F.S. Mozer 1 and A. Retino 2 1. Physics Department and Space Sciences Laboratory, University of California, Berkeley, Ca. 94720, USA 2. Swedish Institute of Space Physics, Uppsala, Sweden.

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A New Method For Investigating Reconnection At A Current Sheet by F.S. Mozer 1 and A. Retino 2

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  1. A New Method For Investigating Reconnection At A Current Sheet by F.S. Mozer1 and A. Retino2 1. Physics Department and Space Sciences Laboratory, University of California, Berkeley, Ca. 94720, USA 2. Swedish Institute of Space Physics, Uppsala, Sweden

  2. SPATIAL SCALES ASSOCIATED WITH RECONNECTION • MHD SCALE • Paschmann, G. et al, (1979), Nature, 282:243–246. • Sonnerup, B.U.O. et al, (1981), J. Geophys. Res., 86, 10,049 • Compared plasma changes with B changes • ION SCALE (c/ωpI ~ 100 km) • Mozer F.S. et al, (2002), Phys. Rev. Lett., 89(1), 015002-1 • Vaivads A. et al, (2004), Phys. Rev. Lett., 93(10):105001. • Wygant, J. R., et al. (2005), J. Geophys. Res., 110, A09206, • doi:10.1029/2004JA010708 • Electric and magnetic fields at individual crossings • ELECTRON SCALE (c/ωpe ~ 2 km) • Mozer, F.S., (2004), J. Geophys. Res., 110, A12222, • doi:10.1029/2005JA011258 • Observations and statistics of parallel electric fields

  3. POLAR OBSERVATION OF THE ELECTRON DIFFUSION REGION • Reconnection magnetic field changes in steps • Current filamentary • At largest filament, see 60 mV/m electric field • lasting for ~75 msec (width ~c/ωpe). • Ell ~ 8 mV/m • jperp·Eperp/n >100 keV per particle per second • Major density change at this time. • Electron beta < 1

  4. RECONNECTION ON THE ION SCALE LET X = normal to the current sheet Y = out-of-plane direction Z = direction of reconnecting component of B EX, BY, BZ = the large fields in a reconnecting current sheet eY, eZ, bX = the small fields in a reconnecting current sheet The most important field components are: bX because a normal magnetic field is required for the field lines on the two sides to be connected eY and eZ because a tangential electric field is required for (EXB/B2)X = (eYBZ – eZBY)/B2 to be inward from both sides

  5. PROBLEM The most important fields (eY, eZ, bX), being the smallest fields, are contaminated by the largest fields when the normal to the current sheet is inaccurate. The analysis assumes a normal direction that is constant, whereas the normal oscillates ~5 degrees due to surface waves on the magnetopause. These produce error signals ~ (Big Field)*sin5o which are comparable to the (Small Field). CONCLUSION The parameters that determine that reconnection is occurring and what its magnitude is, are poorly measured.

  6. IN THIS TALK 1. Two Polar satellite magnetopause crossings, of two second and six minute durations, are analyzed to show that reconnection was occurring but that the reconnection rate was uncertain due to oscillations of the normal direction. 2. A method is described for minimizing this uncertainty by using measurements of only the three large field components to obtain the three small components from correlations that result from the parallel electric field being zero on ion scales. 3. Of >100 sub-solar magnetopause crossings, about 40% had correlations that suggest a constant reconnection rate. Detailed statistics are presented for 20 of these crossings.

  7. 7.52 RE, 1130 MLT, 22.30 MLAT S/C crosses current sheet in ~2 seconds Thickness = 300 km =2 c/ωpI Density (panel a)), BZ (panel e)) show passage from sphere to sheath through magnetopause. <bX> <0 (panel c)) NECESSARY CONDITION FOR RECONNECTION Magnitude uncertain due to oscillations <eY>, <eZ> >0 (panels g), h)) NECESSARY AND SUFFICIENT FOR RECONNECTION. Magnitude uncertain due to oscillations (EXB/B2)X inward from both sides (panel i)) <bX> < 0 requires outflow in +Z direction. It is (panel k)). Outflow speed of 500 km/sec agrees with vALFVEN = 340 km/sec (sheath) = 830 km/sec (sphere) BG ~0.2, Btotal ~ constant Unipolar out-of-plane B with parallel j. EX CORRELATES WITH BY (panels d) and f)

  8. 9.2 RE, 1225 MLT, 22.70 MLAT S/C crosses current sheet in ~ 6 minutes. Density (panel a)), BZ (panel e)) show passage from sphere to sheath through magnetopause. <bX> <0 (panel c)) NECESSARY CONDITION FOR RECONNECTION Magnitude uncertain due to oscillations <eY>, <eZ> >0 (panels g), h)) NECESSARY AND SUFFICIENT FOR RECONNECTION. Magnitude uncertain due to oscillations (EXB/B2)X inward from both sides (panel i)) <bX> < 0 requires outflow in +Z direction. It is (panel k)). Outflow speed of 400 km/sec agrees with vALFVEN = 500 km/sec (sheath) = 630 km/sec (sphere) BG ~ 1, Btotal ~ constant Unstructured out-of-plane B. EX CORRELATES WITH BZ (panels d) and f)

  9. REASON THAT EX CORRELATES WITH BY AND/OR BZ ASSUME THAT Ell = 0 ON THE ION SCALE EXbX + eYBY + eZBZ = 0 If bX = eY = eZ = 0 because there is no reconnection, then EX, BY, and BZ have no necessary relationship to each other. If there is reconnection, bX, eY, and/or eZ≠ 0 and EX =– (eY/bX)BY – (eZ/bX)BZ = – aBY – bBZ where a = (eY/bX) and b = (eZ/bX) IF a and b are constant and EX correlates with BY and/or BZ THEN The small fields that determine reconnection may be obtained from the large and easily measured fields, by least-squares fits.

  10. LEAST SQUARES FIT TO APRIL 3, 2002 DATA R2 = 0.94 (94% of the variation of EX is accounted for by (–aBY –bBZ) a = eY/bX = -0.41 b = eZ/bX = -0.005

  11. LEAST SQUARES FIT TO MARCH 20, 2001 DATA R2 = 0.93 a = eY/bX = -0.32 b = eZ/bX = -0.20

  12. SUMMARY OF OBSERVATIONS: • Because constant a and b fit the data so well, reconnection must have been occurring at a constant rate during these crossings. • To the eye, about 40 of 120 sub-solar magnetopause crossings had apparent correlations between EX and the combination of BY and/or BZ. So reconnection at a constant rate may occur for more than 40% of sub-solar magnetopause crossings. • Twenty of these crossings were analyzed quantitatively.

  13. SUMMARY • On the ion scale, reconnection can be directly observed by measuring the normal magnetic field and tangential electric field components. • These measurements are uncertain due to the uncorrected oscillation of the magnetopause normal. • In many cases, the normal magnetic field and tangential electric field can be obtained by analyses of the correlations between the normal electric field and tangential magnetic field that arise from the parallel electric field being zero. • These analyses show that reconnection at a constant rate may occur at the sub-solar magnetopause more than 40% of the time. • They suggest that reconnection at the sub-solar magnetopause may not depend on the guide magnetic field strength (the clock angle between the reconnecting fields) but that the reconnection rate may decrease with increasing guide magnetic field.

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