Magnetotail reconnection and flux circulation: Jupiter and Saturn compared

C.M. Jackman, M.F. Vogt, J.A. Slavin, S.W.H. Cowley, S.A. Boardsen

Magnetospheres of the Outer Planets, Boston, July 2011

Cowley et al. [2004]

Cowley et al. [2003]

Jupiter

Saturn

In situ reconnection signatures

In situ reconnection signatures

Plasmoid formation and ejection compresses adjacent regions of the magnetosphere to create Travelling Compression Regions (TCRs).

Vogt et al. [2010]

Slavin et al. [1984]

Stretched magnetotail

After reconnection, B component deflected northward/southward

Dipolarization Plasmoid ejection

Plasmoids at Jupiter

249 events found [Vogt et al., 2010]Inferred x-line position ~90 RJ post-midnight from inward/outward flowsSome events in isolation, but also observe “chain” events

Vogt et al. [2010]

Plasmoids and TCRs at Saturn

34 plasmoids, 16 TCRs

Most during deep tail orbits in 2006

Most events occur in isolation but some in “chains”, associated with same reconnection event.

Jackman et al. [in preparation, 2011]

Localized compressions

Northward turnings

Average reconnection signatures

Average reconnection signatures - Jupiter

Vogt et al. [in preparation, 2011]

Plasmoid duration: ~8 min

Field remains northward for ~90 min

Field remains northward for ~ 58 min

Jackman et al. [in preparation, 2011]

Average reconnection signatures - Saturn

No evidence of core field

Plasmoid duration: 8 min

Flux transport estimates - Saturn

Role of reconnection in flux transport

Jackman et al. [in press, 2011]

Jackman et al. [in press, 2011]

PPPS/Ly = BVPPPSdt Ly

VPPPSdt

VPPPS = 800 km/s, dt = 58 min, PPPS/Ly = 0.555 Wb/m

For Ly=90 RS, Each event closes ~3 GWb

southward

northward

PPPS: ~58 min

Flux closure at Saturn

Changes in area enclosed by auroral emission indicate changes in magnetosphere flux content.

Southern pole open flux content can range between ~15-50 GWb, with an average flux content of 40 GWb.

Thus it would take ~13 “average” reconnection events to cycle all the flux in Saturn’s tail.

Milan [2009]Badman et al. [2005]

Recurrence rates of reconnection

Plasmoid recurrence rates - Saturn

Derivation 1:The overall average flux closure rate:RC = (PPPS/Ly) × Dy × (1/TP)

TP = (PPPS/Ly) × Dy × (1/RC) ≥ (PPPS/Ly) × Dy × (1/RO)

RO is the dayside reconnection rate, ~45 kV [Jackman et al., 2004; McAndrews et al., 2008]. RO ≥ RC

Full tail width at ~45 RS is ~90 RS = Dy

TP, time between plasmoid events ~ 19 hours

TP independent of cross-tail extent of reconnection region, Ly

Derivation 2:Average flux closed ~0.555 Wb/s. Gives ~3 GWb (for 90 RS width)

40 GWb average tail flux content~13 plasmoids to cycle tail flux

For Dungey-cycle timescale of ~6-10 days, mean recurrence rate of ~1.3-2.2 plasmoids per day

Observational rate:In 2006, Cassini spent ~84 days in region: R>20 RS, |BR| < 1.5 nT, and found 34 plasmoids

Observational recurrence rate of 1 plasmoid every ~2.4 days

Jupiter: Saturn:Number of in situ reconnection events

~250 ~50 (34 plasmoids, 16 TCRs)

Duration of plasmoid observation

~8 minutes ~8 minutes

Duration of PPPS observation

~90 minutes ~58 minutes

Flux transport per event ?? Up to 3 GWb

Likely recurrence rate ?? ~19 hours

Summary

For more information, see:Vogt et al. Reconnection and flows in the Jovian magnetotail as inferred from magnetometer observations, [JGR, 2010]

Jackman et al. Cassini observations of plasmoid structure and dynamics: Implications for the role of magnetic reconnection in magnetospheric circulation at Saturn, [in press, JGR, 2011]