Remote sensing magnetic reconnection in the magnetosphere. Mervyn Freeman and the Magnetic Reconnection project team British Antarctic Survey, Cambridge Magnetic Reconnection Theory Workshop, Cambridge 17 August 2004

This talk Geospace is the only natural environment in which reconnection can be sensed bothGeospace is the only natural environment in which reconnection can be sensed both –in-situ (local) by spacecraftby spacecraft –remotely (local - global) by instruments on earthby instruments on earth using dipole magnetic field as a lensusing dipole magnetic field as a lens Explain ground-based radar signatures of reconnectionExplain ground-based radar signatures of reconnection Show examples of reconnection studies and results.Show examples of reconnection studies and results.

Why remote sensing? In-situ spacecraft observationsIn-situ spacecraft observations –Direct but localised in time and space Remote sensingRemote sensing –Indirect but continuous observations of regions across the MHD scales When?When? –Continuous or intermittent? –Driven or spontaneous? Where?Where? –Anti-parallel or component reconnection? –Homogeneous or patchy?

Principles of ground- based remote sensing Reconnect magnetic field line with at least one footpointReconnect magnetic field line with at least one footpoint Reconnected magnetic field lines near separatrices communicate signatures to low altitude detectable from ground such thatReconnected magnetic field lines near separatrices communicate signatures to low altitude detectable from ground such that separatrices can be identifiedseparatrices can be identified –distinct magnetic topologies with different environments and/or –distinct separatrix surface signatures reconnection rate can be estimatedreconnection rate can be estimated –from measurement of E x B plasma motion across separatrices

Solar example Separatrix between open and closed magnetic field lines identified by super-hot particlesSeparatrix between open and closed magnetic field lines identified by super-hot particles Reconnection rate derived fromReconnection rate derived from –footpoint motion of separatrix, u bx –normal magnetic field B z in photosphere –plasma velocity is zero in photosphere x z u bx

Magnetosphere example Earth Separatrix between open and closed magnetic field lines identified by accelerated magnetosheath particles in cuspSeparatrix between open and closed magnetic field lines identified by accelerated magnetosheath particles in cusp Reconnection rate derived fromReconnection rate derived from –footpoint plasma velocity normal to separatrix, v x –footpoint motion of separatrix, u bx –normal magnetic field B z in ionosphere Similarly on nightsideSimilarly on nightside u bx vxvx BzBz

Radar remote sensing Use Super Dual Auroral Network (SuperDARN) of HF radarsUse Super Dual Auroral Network (SuperDARN) of HF radars –to identify separatrix and –to measure E x B velocity 8 radars in northern hemisphere8 radars in northern hemisphere 6 radars in southern hemisphere6 radars in southern hemisphere more coming!more coming!

Halley HF radar array Electronically steered phased-array antennae 16 log-periodic antennae give 16 possible beam azimuths 75 ranges with 45 km spatial separation Each beam sampled every 7 sEach beam sampled every 7 s Full scan every 2 minFull scan every 2 min Halley South pole X geographic + magnetic + X

HF radar measurement of E x B plasma motion RAdio Detecting And Ranging Radar signal is backscattered off targetRadar signal is backscattered off target Time delay measures range of targetTime delay measures range of target Doppler shift measures line-of-sight velocity of targetDoppler shift measures line-of-sight velocity of target Target is field-aligned electron density irregularities in F-region ionosphere (~300 km altitude)Target is field-aligned electron density irregularities in F-region ionosphere (~300 km altitude) –created by plasma instabilities –backscatter HF radar waves where wave vector is refracted perpendicular to magnetic field –move at E x B drift velocity Target Antenna V a Vcosa 300 km km B irregularities

SuperDARN velocity maps Combine line-of-sight velocity measurements into global convection map by spherical harmonic fit to data from different lines of sight, assuming incompressible flow, and using statistical model to fill gapsCombine line-of-sight velocity measurements into global convection map by spherical harmonic fit to data from different lines of sight, assuming incompressible flow, and using statistical model to fill gaps Sun magnetic pole

Identification of open-closed separatrix Reconnection in earth’s magnetosphere creates different plasma regions in magnetosphere and at magnetic footpoints in ionosphereReconnection in earth’s magnetosphere creates different plasma regions in magnetosphere and at magnetic footpoints in ionosphere Identified by flux and energy of magnetic field- aligned particles “precipitating” to low altitudeIdentified by flux and energy of magnetic field- aligned particles “precipitating” to low altitude

Radar proxy for open- closed separatrix [Chisham and Freeman, Ann. Geophys., 21, 983, 2003; 22, 1187, 2004; Chisham et al., Geophys. Res. Lett., 31, L02804, 2004] Compared spacecraft measurements of open- closed separatrix with contemporaneous and co- located radar measurementsCompared spacecraft measurements of open- closed separatrix with contemporaneous and co- located radar measurements Separatrix is co-located with a boundary in radar spectral widthSeparatrix is co-located with a boundary in radar spectral width –measure of small-scale velocity structure –increases poleward of separatrix Valid on both dayside and nightside (except 2-10 MLT)Valid on both dayside and nightside (except 2-10 MLT) ocb

Example 1: Reconnection for southward IMF [Pinnock et al., Ann. Geophys., 21, 1647, 2003] Magnetopause reconnection X-line expected to be on equatorMagnetopause reconnection X-line expected to be on equator Phan et al. [Nature, 404, 848, 2000] report bi- directional reconnection jets centred on equator near dawn flankPhan et al. [Nature, 404, 848, 2000] report bi- directional reconnection jets centred on equator near dawn flank –steady southward interplanetary magnetic field How long is X-line?How long is X-line? Is reconnection continuous or intermittent?Is reconnection continuous or intermittent? Driven or spontaneous?Driven or spontaneous?

SuperDARN remote sensing Analyse SuperDARN data over Phan et al. intervalAnalyse SuperDARN data over Phan et al. interval Convection consistent with standard 2-cell convectionConvection consistent with standard 2-cell convection Separatrix and E x B velocity measured continuously (2 min resolution)Separatrix and E x B velocity measured continuously (2 min resolution) over much of dayside magnetopause (9-16 MLT)over much of dayside magnetopause (9-16 MLT) O Equator-SO Geotail - DMSP

Reconnection rate – spatial structure Reconnection electric field shows stable large- scale structureReconnection electric field shows stable large- scale structure Reconnection signature seen at Equator-S footpoint (~0.5 mV/m)Reconnection signature seen at Equator-S footpoint (~0.5 mV/m) Reconnection extends beyond dawn and dusk flanks on equatorReconnection extends beyond dawn and dusk flanks on equator Length >~ 38 R ELength >~ 38 R E Equator-S

Reconnection rate – temporal structure Reconnection is continuous during continuous IMF Bz < 0Reconnection is continuous during continuous IMF Bz < 0 Some fluctuationSome fluctuation –not clearly related to IMF driver

Example 2: Reconnection for northward IMF [Chisham et al., Ann. Geophys., in preparation, 2004] Expect closure of open magnetic flux poleward of cuspExpect closure of open magnetic flux poleward of cusp Reverse two-cell convection observedReverse two-cell convection observed Spatial extent of X-line footpoint much smaller than southward IMFSpatial extent of X-line footpoint much smaller than southward IMF

Reconnection rate – spatial structure Reconnection electric field shows stable large- scale structureReconnection electric field shows stable large- scale structure Reconnection rate at magnetopause similar to southward IMF (~0.5 mV/m)Reconnection rate at magnetopause similar to southward IMF (~0.5 mV/m) Reconnection footpoint maps poleward of cuspReconnection footpoint maps poleward of cusp Length <~ 10 R ELength <~ 10 R E

Reconnection rate – temporal structure Reconnection is continuous during continuous IMF Bz > 0Reconnection is continuous during continuous IMF Bz > 0 Some fluctuationSome fluctuation –not clearly related to IMF driver Global reconnection rate much smaller than for southward IMFGlobal reconnection rate much smaller than for southward IMF –limited by X-line extent

Example 3: Reconnection for arbitrary IMF Example 3: Reconnection for arbitrary IMF [Coleman et al., J. Geophys. Res., 2000; J. Geophys. Res., 2001; Chisham et al., J. Geophys. Res., 2002] How do macroscopic magnetic fields and plasma flows control reconnection?How do macroscopic magnetic fields and plasma flows control reconnection? Where does reconnection occur?Where does reconnection occur? –Is it sub-solar? slow flowslow flow –Is it anti-parallel? high magnetic shearhigh magnetic shear Is there a critical experimental test?Is there a critical experimental test? View from Sun of geomagnetic field (black) and interplanetary magnetic field (blue). Regions of 180 degree magnetic shear shown in red. Subsolar region of slow flow shown in green.

Anti-parallel reconnection test Test if magnetic shear is most important factor controlling reconnection.Test if magnetic shear is most important factor controlling reconnection. 2 reconnection sites on magnetopause with gap between them in winter hemisphere.2 reconnection sites on magnetopause with gap between them in winter hemisphere. Stagnant flow in gapStagnant flow in gap December Solstice B z < 0 | B y |  | B z | NS V A < V V A > V Stagnant Flow 2 hr merging gap

Critical test for reconnection location As predicted, for Northern hemisphere midwinter, two regions of strong poleward flow either side of noon With stagnation and weaker equatorward flow at noon Observed convection at noon from SuperDARN Reconnection preferentially occurs where magnetic fields are anti-parallel, irrespective of other conditions

Transient reconnection Transient and localised reconnection at the magnetopause has been associated with transient convection and aurora:Transient and localised reconnection at the magnetopause has been associated with transient convection and aurora: –flux transfer events (FTEs) –flow channel events (FCEs) –poleward moving auroral forms (PMAFs) Similarly for reconnection in the magnetotail:Similarly for reconnection in the magnetotail: –bursty bulk flows (BBFs) –poleward boundary intensifications (PBIs) Transients may transport majority of magnetic flux and energy.Transients may transport majority of magnetic flux and energy. Lockwood and Wild [1993] Neudegg et al. [1999]

Complex reconnection? Complex reconnection? [Abel and Freeman, J. Geophys. Res., 2002] Similar distributions of waiting times forSimilar distributions of waiting times for –Flux Transfer Events (solid, dotted) –Pulsed Ionospheric Flows (dash-dot) –Poleward Moving Auroral Forms (dashed) No characteristic scale (power law) above minimum resolvable scale (2-3 min)?No characteristic scale (power law) above minimum resolvable scale (2-3 min)? Velocity fluctuations are self- similar on different temporal and spatial scalesVelocity fluctuations are self- similar on different temporal and spatial scales

SOC Reconnection? Distributions of areas and durations of auroral bright spots are power law (scale-free) from kinetic to system scales [Uritsky et al., JGR, 2002; Borelov and Uritsky, private communication]Distributions of areas and durations of auroral bright spots are power law (scale-free) from kinetic to system scales [Uritsky et al., JGR, 2002; Borelov and Uritsky, private communication] Could this be associated with multi-scale reconnection in the magnetotail?Could this be associated with multi-scale reconnection in the magnetotail? Self-organisation of reconnection to critical state (SOC) [e.g., Chang, Phys. Plasmas, 1999]Self-organisation of reconnection to critical state (SOC) [e.g., Chang, Phys. Plasmas, 1999] cf SOC in the solar corona [Lu, Phys. Rev. Lett., 1995]cf SOC in the solar corona [Lu, Phys. Rev. Lett., 1995]

Conclusions Magnetic reconnection is a universal phenomenonMagnetic reconnection is a universal phenomenon –Sun, stars, accretion disks, geospace, etc. Geospace is the only natural environment in which reconnection can be observed both remotely (globally) and in-situ (locally).Geospace is the only natural environment in which reconnection can be observed both remotely (globally) and in-situ (locally). Can address universal questions:Can address universal questions: Remote sensing observations suggest that:Remote sensing observations suggest that: –reconnection preferentially occurs where magnetic fields are anti-parallel, irrespective of other conditions –magnetopause reconnection is driven –reconnection may occur on many time and space scales with no characteristic scale