Remote sensing magnetic Remote sensing magnetic reconnection reconnection

in the magnetosphere.in the magnetosphere.

Mervyn FreemanMervyn Freemanand the Magnetic Reconnection project teamand the Magnetic Reconnection project team

British Antarctic Survey, CambridgeBritish Antarctic Survey, Cambridgewww.antarctica.ac.uk/BAS_Science/Programmes/MRS/mrproject/www.antarctica.ac.uk/BAS_Science/Programmes/MRS/mrproject/

index.htmlindex.html

Magnetic Reconnection Theory Workshop, Cambridge

17 August 2004

This talkThis talk• Geospace is the only Geospace is the only

natural environment in natural environment in which reconnection can which reconnection can be sensed bothbe sensed both– in-situ (local)in-situ (local)

• by spacecraftby spacecraft– remotely (local - global)remotely (local - global)

• by instruments on earthby instruments on earth• using dipole magnetic using dipole magnetic

field as a lensfield as a lens

• Explain ground-based radar signatures of Explain ground-based radar signatures of reconnectionreconnection

• Show examples of reconnection studies and Show examples of reconnection studies and results.results.

Why remote sensing?Why remote sensing?

• In-situ spacecraft In-situ spacecraft observationsobservations– Direct but localised in time Direct but localised in time

and spaceand space

• Remote sensingRemote sensing– Indirect but continuous Indirect but continuous

observations of regions observations of regions across the MHD scalesacross the MHD scales

• When?When?– Continuous or intermittent?Continuous or intermittent?– Driven or spontaneous?Driven or spontaneous?

• Where?Where?– Anti-parallel or component Anti-parallel or component

reconnection?reconnection?– Homogeneous or patchy?Homogeneous or patchy?

Principles of Principles of ground-based ground-based

remote sensingremote sensing• Reconnect magnetic field line Reconnect magnetic field line

with at least one footpointwith at least one footpoint

• Reconnected magnetic field Reconnected magnetic field lines near separatrices lines near separatrices communicate signatures to low communicate signatures to low altitude detectable from altitude detectable from ground such thatground such that

• separatrices can be identifiedseparatrices can be identified– distinct magnetic topologies distinct magnetic topologies

with different environments with different environments and/orand/or

– distinct separatrix surface distinct separatrix surface signaturessignatures

• reconnection rate can be reconnection rate can be estimatedestimated– from measurement offrom measurement of E E x x BB

plasma motion across plasma motion across separatricesseparatrices

Solar exampleSolar example

• Separatrix between open Separatrix between open and closed magnetic field and closed magnetic field lines identified by super-lines identified by super-hot particleshot particles

• Reconnection rate derived Reconnection rate derived fromfrom– footpoint motion of footpoint motion of

separatrix, useparatrix, ubxbx

– normal magnetic field Bnormal magnetic field Bzz

in photospherein photosphere

– plasma velocity is zero in plasma velocity is zero in photospherephotosphere

.

ribbon

fast shock

evaporation

Mach 2 outflow

H loops (104 K)

isothermal slow shock

conductionfront

x-ray

loops

(107 K)

chromosphere

current sheet

super hot ornonthermal

CME/Flare Loop Structures

zbxBuE

x

z

ubx

Magnetosphere exampleMagnetosphere example

Earth

• Separatrix between open Separatrix between open and closed magnetic field and closed magnetic field lines identified by lines identified by accelerated accelerated magnetosheath particles in magnetosheath particles in cuspcusp

• Reconnection rate derived Reconnection rate derived fromfrom– footpoint plasma velocity footpoint plasma velocity

normal to separatrix, vnormal to separatrix, vxx

– footpoint motion of footpoint motion of separatrix, useparatrix, ubxbx

– normal magnetic field Bnormal magnetic field Bzz

in ionospherein ionosphere

• Similarly on nightsideSimilarly on nightside

zxbx BvuE )( ubx

vx

Bz

Radar remote Radar remote sensingsensing

• Use Super Dual Auroral Use Super Dual Auroral Network (SuperDARN) of HF Network (SuperDARN) of HF radarsradars– to identify separatrixto identify separatrix

andand– to measure to measure EE x x B B velocityvelocity

• 8 radars in northern 8 radars in northern hemispherehemisphere

• 6 radars in southern 6 radars in southern hemispherehemisphere

• more coming!more coming!

Halley HF radar Halley HF radar arrayarray

• Electronically steered phased-array antennae

• 16 log-periodic antennae give 16 possible beam azimuths

• 75 ranges with 45 km spatial separation

• Each beam sampled Each beam sampled every 7 severy 7 s

• Full scan every 2 minFull scan every 2 min

Halley

South pole X geographic + magnetic

+

X

HF radar HF radar measurement of measurement of E E x Bx B plasma motion plasma motion

• RAdio Detecting And Ranging

• Radar signal is backscattered off Radar signal is backscattered off targettarget

• Time delay measures range of Time delay measures range of targettarget

• Doppler shift measures line-of-sight Doppler shift measures line-of-sight velocity of targetvelocity of target

• Target is field-aligned electron Target is field-aligned electron density irregularities in F-region density irregularities in F-region ionosphere ionosphere (~300 km altitude)(~300 km altitude)– created by plasma instabilitiescreated by plasma instabilities– backscatter HF radar waves where backscatter HF radar waves where

wave vector is refracted wave vector is refracted perpendicular to magnetic fieldperpendicular to magnetic field

– move at move at EE x x BB drift velocity drift velocity

Target

Antenna

Va

Vcosa

300 km

500-1000 km

B

irregularities

SuperDARN velocity mapsSuperDARN velocity maps

• Combine line-of-sight velocity measurements into global convection Combine line-of-sight velocity measurements into global convection map by spherical harmonic fit to data from different lines of sight, map by spherical harmonic fit to data from different lines of sight, assuming incompressible flow, and using statistical model to fill gapsassuming incompressible flow, and using statistical model to fill gaps

Sun

magnetic pole

Identification of open-closed Identification of open-closed separatrixseparatrix

• Reconnection in earth’s magnetosphere Reconnection in earth’s magnetosphere creates different plasma regions in creates different plasma regions in magnetosphere and at magnetic footpoints in magnetosphere and at magnetic footpoints in ionosphereionosphere

• Identified by flux and energy of magnetic Identified by flux and energy of magnetic field-aligned particles “precipitating” to low field-aligned particles “precipitating” to low altitudealtitude

Radar proxy for Radar proxy for open-closed open-closed separatrixseparatrix

[Chisham and Freeman, Ann. [Chisham and Freeman, Ann. Geophys., 21, 983, 2003; 22, 1187, Geophys., 21, 983, 2003; 22, 1187,

2004;2004;Chisham et al., Geophys. Res. Lett., Chisham et al., Geophys. Res. Lett.,

31, L02804, 2004]31, L02804, 2004]• Compared spacecraft Compared spacecraft measurements of open-measurements of open-closed separatrix with closed separatrix with contemporaneous and co-contemporaneous and co-located radar located radar measurementsmeasurements

• Separatrix is co-located Separatrix is co-located with a boundary in radar with a boundary in radar spectral widthspectral width– measure of small-scale measure of small-scale

velocity structurevelocity structure– increases poleward of increases poleward of

separatrixseparatrix

• Valid on both dayside and Valid on both dayside and nightside (except 2-10 nightside (except 2-10 MLT)MLT)

ocb

ocb

Example 1: Example 1: Reconnection for Reconnection for southward IMFsouthward IMF

[Pinnock et al., Ann. [Pinnock et al., Ann. Geophys., 21, 1647, 2003]Geophys., 21, 1647, 2003]

• Magnetopause reconnection Magnetopause reconnection X-line expected to be on X-line expected to be on equatorequator

• Phan et al. [Nature, 404, Phan et al. [Nature, 404, 848, 2000]848, 2000] report bi- report bi-directional reconnection jets directional reconnection jets centred on equator near centred on equator near dawn flankdawn flank– steady southward steady southward

interplanetary magnetic interplanetary magnetic fieldfield

• How long is X-line?How long is X-line?

• Is reconnection continuous Is reconnection continuous or intermittent?or intermittent?

• Driven or spontaneous?Driven or spontaneous?

SuperDARN SuperDARN remote sensingremote sensing

• Analyse SuperDARN Analyse SuperDARN data over Phan et al. data over Phan et al. intervalinterval

• Convection consistent Convection consistent with standard 2-cell with standard 2-cell convectionconvection

• Separatrix and Separatrix and EE x x BB velocity measured velocity measured continuously continuously (2 min resolution)(2 min resolution)

• over much of dayside over much of dayside magnetopause magnetopause (9-16 MLT)(9-16 MLT) O Equator-S O Geotail

- DMSP

Reconnection Reconnection rate rate

– spatial – spatial structurestructure

• Reconnection electric Reconnection electric field shows stable field shows stable large-scale structurelarge-scale structure

• Reconnection Reconnection signature seen at signature seen at Equator-S footpoint Equator-S footpoint (~0.5 mV/m)(~0.5 mV/m)

• Reconnection extends Reconnection extends beyond dawn and dusk beyond dawn and dusk flanks on equatorflanks on equator

• Length >~ 38 RLength >~ 38 REE

Equator-S

Reconnection rate – temporal Reconnection rate – temporal structurestructure

• Reconnection is Reconnection is continuous during continuous during continuous IMF Bz < continuous IMF Bz < 00

• Some fluctuationSome fluctuation– not clearly related to not clearly related to

IMF driverIMF driver

Example 2: Example 2: Reconnection for Reconnection for

northward IMFnorthward IMF[Chisham et al., Ann. [Chisham et al., Ann.

Geophys., in preparation, Geophys., in preparation, 2004]2004]

• Expect closure of open Expect closure of open magnetic flux poleward magnetic flux poleward of cuspof cusp

• Reverse two-cell Reverse two-cell convection observedconvection observed

• Spatial extent of X-line Spatial extent of X-line footpoint much smaller footpoint much smaller than southward IMFthan southward IMF

Reconnection Reconnection rate rate

– spatial – spatial structurestructure

• Reconnection electric Reconnection electric field shows stable field shows stable large-scale structurelarge-scale structure

• Reconnection rate at Reconnection rate at magnetopause magnetopause similar to southward similar to southward IMF (~0.5 mV/m)IMF (~0.5 mV/m)

• Reconnection Reconnection footpoint maps footpoint maps poleward of cusppoleward of cusp

• Length <~ 10 RLength <~ 10 REE

Reconnection rate – temporal Reconnection rate – temporal structurestructure

• Reconnection is Reconnection is continuous during continuous during continuous IMF Bz > 0continuous IMF Bz > 0

• Some fluctuationSome fluctuation– not clearly related to not clearly related to

IMF driverIMF driver

• Global reconnection Global reconnection rate much smaller rate much smaller than for southward than for southward IMFIMF– limited by X-line extentlimited by X-line extent

Example 3: Reconnection for Example 3: Reconnection for arbitrary IMFarbitrary IMF

[Coleman et al., J. Geophys. Res., 2000; J. Geophys. Res., 2001;

Chisham et al., J. Geophys. Res., 2002]• How do macroscopic How do macroscopic

magnetic fields and magnetic fields and plasma flows control plasma flows control reconnection?reconnection?

• Where does Where does reconnection occur?reconnection occur?– Is it sub-solar?Is it sub-solar?

• slow flowslow flow

– Is it anti-parallel?Is it anti-parallel?• high magnetic shearhigh magnetic shear

• Is there a critical Is there a critical experimental test?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 Anti-parallel reconnection reconnection

testtest• Test if magnetic Test if magnetic

shear is most shear is most important factor important factor controlling controlling reconnection.reconnection.

• 2 reconnection sites 2 reconnection sites on magnetopause on magnetopause with gap between with gap between them in winter them in winter hemisphere.hemisphere.

• Stagnant flow in Stagnant flow in gapgap

December Solstice

Bz< 0

| By | | Bz |

N S

VA< V

VA> V

Stagnant Flow

2 hr merging gap

Critical test for reconnection Critical test for reconnection locationlocation

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 Reconnection preferentially occurs where magnetic fields are anti-parallel, irrespective of magnetic fields are anti-parallel, irrespective of other conditionsother conditions

Transient Transient reconnectionreconnection

• Transient and localised Transient and localised reconnection at the reconnection at the magnetopause has been magnetopause has been associated with transient associated with transient convection and aurora:convection and aurora:– flux transfer events (FTEs)flux transfer events (FTEs)– flow channel events (FCEs)flow channel events (FCEs)– poleward moving auroral forms poleward moving auroral forms

(PMAFs)(PMAFs)

• Similarly for reconnection in Similarly for reconnection in the magnetotail:the magnetotail:– bursty bulk flows (BBFs)bursty bulk flows (BBFs)– poleward boundary poleward boundary

intensifications (PBIs)intensifications (PBIs)

• Transients may transport Transients may transport majority of magnetic flux and majority of magnetic flux and energy.energy.

Lockwood and Wild [1993]

Neudegg et al. [1999]

Complex Complex reconnection?reconnection?

[Abel and Freeman, J. Geophys. Res., 2002]

• Similar distributions of Similar distributions of waiting times forwaiting times for– Flux Transfer Events Flux Transfer Events

(solid, dotted)(solid, dotted)– Pulsed Ionospheric Flows Pulsed Ionospheric Flows

(dash-dot)(dash-dot)– Poleward Moving Auroral Poleward Moving Auroral

Forms (dashed)Forms (dashed)

• No characteristic scale No characteristic scale (power law) above minimum (power law) above minimum resolvable scale (2-3 min)?resolvable scale (2-3 min)?

• Velocity fluctuations are Velocity fluctuations are self-similar on different self-similar on different temporal and spatial scalestemporal and spatial scales

SOC Reconnection?SOC Reconnection?

• Distributions of areas and Distributions of areas and durations of auroral bright durations of auroral bright spots are power law (scale-spots are power law (scale-free) from kinetic to system free) from kinetic to system scales scales [Uritsky et al., JGR, [Uritsky et al., JGR, 2002; Borelov and Uritsky, 2002; Borelov and Uritsky, private communication]private communication]

• Could this be associated with Could this be associated with multi-scale reconnection in multi-scale reconnection in the magnetotail?the magnetotail?

• Self-organisation of Self-organisation of reconnection to critical state reconnection to critical state (SOC) (SOC) [e.g., Chang, Phys. [e.g., Chang, Phys. Plasmas, 1999]Plasmas, 1999]

• cf SOC in the solar corona cf SOC in the solar corona [Lu, Phys. Rev. Lett., 1995][Lu, Phys. Rev. Lett., 1995]

ConclusionsConclusions• Magnetic reconnection is a universal Magnetic reconnection is a universal

phenomenonphenomenon– Sun, stars, accretion disks, geospace, etc.Sun, stars, accretion disks, geospace, etc.

• Geospace is the only natural environment in Geospace is the only natural environment in which reconnection can be observed both which reconnection can be observed both remotely (globally) and in-situ (locally).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 reconnection preferentially occurs where magnetic

fields are anti-parallel, irrespective of other conditionsfields are anti-parallel, irrespective of other conditions– magnetopause reconnection is drivenmagnetopause reconnection is driven– reconnection may occur on many time and space reconnection may occur on many time and space

scales with no characteristic scalescales with no characteristic scale