Local influence of mirror mode fluctuations on magnetopause reconnection

1. Local influence ofmirror mode fluctuations onmagnetopause reconnection Tiera V. Laitinen, Nick Omidi, Yuri V. Khotyaintsev, Mats André, Andris Vaivads, Elizabeth A. Lucek, Henri Rème Swedish Institute for Space Physics, Box 537, SE-751 21 Uppsala, Sweden Solana Scientific, Solana Beach, California, USA Blackett Laboratory, Imperial College, London, UK University of Toulouse, UPS, CESR, & CNRS, Toulouse, France Photo: Juha Holma

2. B N P Mirror modes • Compressional non-oscillatory magnetic variations • Decrease of |B| ~ 50 % • No change in B direction • Enhancement of density and temperature • Local pressure equilibrium • Created spontaneously by mirror instability • When Tperp > Tpar • Size: a few ion gyroradii • Co-moving • Observed in the solar wind and in the magnetosheath, sometimes even in the magnetosphere • Here seen by Cluster advecting toward m’pause reconnection region: appear as magnetic variations at 1 min timescale.

3. z x Event 1 • 27 March 2007 • Cluster was located near the subsolar point: • GSE: X = 9, Y = 0, Z = 3,5 • Δ(3, 4) ~ 1000 km, other distances ~ Re • Magnetic field in the sheath was pointing southward. From the Sun

4. Quasi-continuous reconnection Mirror modes observed in the magnetosheath. B and V from C1 and C3. LMN coordinates adjusted to MP orientation. e.g. B Outflow jet is seen whenever a s/c comes near the MP. Typical duration of jet observations ~ 1 min, consistent with the incoming mirror mode timescale. V When C3 stays three min at the MP, the jet speed appears to vary. However, it is not possible to determine whether this reflects true variations in jet speed or movement of the s/c in and out of the jet.

5. Fastest jet has limited duration Inbound partial crossing. Jet still observed. Jet speed increases, no signature of crossing in B or N. Outbound partial crossing. No jet observed. Outbound crossing. No fast jet observed. B V If reconnection were continuous and steady, we should observe the jet at all three crossings. N

6. Observed jets (different time interval) Modulation of single x-line by mirror modes? • Mirror modes have smaller B and larger N than ambient values • > smaller vA in reconnection inflow • > smaller predicted outflow jet speed Equation based on Cassak & Shay 2007)

7. z x Event 2 • 3 March 2008 • Cluster is located close to the subsolar point • GSE: X = 13, Y = 4, Z = 0 • Δ(3, 4) ~ 35 km, s/c 1 and s/c 2 separated by about 8000 km. • Magnetic field in the magnetosheath south-west From the Sun

8. B C1 V N B C3 V N Patchy reconnection Northward jets Timescale of jet variations again same as that of incoming mirror modes. Jets from different directions -> not single x-line. Southward jets

9. Simulation • 2.5D hybrid simulation w/ plasma parameters from event 1. • Magnetic variations arise in the m’sheath. • Magnetic islands form on the m’pause guided by the sheath variations.

10. Conclusions • Mirror modes may either • modulate single x-line reconnection or • trigger and quench bursts of patchy reconnection. • We presented one event for each hypothesis. • Similarity of timescales suggests that mirror modes indeed are the cause for variations in reconnection outflow.