Hamrin, M., Norqvist, P., Marghitu, O., et al.

e-mail: hamrin@space.umu.seDepartment of Physics, Umeå University, Sweden

Nordic Cluster Meeting, Uppsala, August 2011

Hamrin, Nordic Cluster Meeting, 2011

Cluster observations of energy conversion regions

in the plasma sheet

• Our previously published papers on plasma sheet (PS) energy conversion.

• Recently we have obtained experimental evidence of the relation between PS energy conversion and1. the energy input at the magnetopause2. the energy flow towards the ionosphere

• Work in progress…

• Now I’m looking for advice from you!

• Splinter?

- BACKGROUND -

Hamrin, Nordic Cluster Meeting, 2011

• ~20 years ago, the view on PS transport processes changed by the discovery of BBFs [Baumjohann et al., JGR, 1990; Angelopoulos et al., JGR, 1992].

• PS plays a fundamental role for the energy conversion and transport processes in the Earth’s magnetosphere (Msp).

• Primary energy source: Solar wind kinetic energy, transferred into the Msp by means of reconnection [Paschmann et al., Nature, 1979].

• Tail reconnection releases energy stored in the tail magnetic field. Magnetic energy converted to kinetic energy (BBFs).

• Energy is transported away as Poynting flux and particle flux. Possibly recurring conversions in the PS between electromagnetic energy and kinetic energy (plasma bulk and thermal) forms [Hamrin et al., JGR, 2011].

• Some energy dissipated in the auroral ionosphere [Hamrin et al., Ann Geophys,

2006, Hamrin et al., Ann Geo, JGR, 2011].

• Appealing to explore the structure of the PS energy conversion and transport in relation to the SW energy input and auroral dissipation!

- INTRODUCTION: THE PLASMA SHEET -

Hamrin, Nordic Cluster Meeting, 2011

• Poynting’s theorem:

• Energy of bulk motion:

• Plasma acc./decel. by Lorenz force, JB

• Study E∙J in the PS with Cluster! (JB)v= (-vB)J=EJ

E∙J<0 (generator)

E∙J>0 (load)

Mechanicalenergy

Electromag.energy

JEvv

pvv

t 22

22

JES

0

2

2B

t

- TOOLS -

Power density

Hamrin, Nordic Cluster Meeting, 2011

• J=B from FGM

• <E>=Esc/n from CIS -vB

• Reference frame of irreversible energy dissipation: neutral wind system ≈ GSE (GSM)

• We study clearly identifiable regions of energy conversion: “ECR” = Energy Conversion Region: “CLR” = Concentrated Load Region “CGR” = Concentrated Generator Region

• ‘Automatic’ ECR database: 2001, 2002, 2004, 15-20RE.

428/127 CLRs/CGRs from 220 Cluster PS passages (2730h)

E∙J

∑E∙J

- CLUSTER OBSERVATIONS -

Hamrin, Nordic Cluster Meeting, 2011

1. Concentrated regions (time, space) of energy conversionConsistent with high level of fine structure in PS

2. CLRs are stronger and more frequent than CGRs Consistent with PS on the average behaving as a load

3. CLRs/CGRs preferring locations towards CPS/PSBL (high/low ) Consistent with MHD simulation [Birn and Hesse, Ann Geo, 2005]

4. ECR life time and (cross-tail) scale size = 1-10 min and a few RE

Consistent with BBF scale size [Nakamura et al., Adv Space Res, 2005; Walsh et

al., Ann Geo, 2009], MHD simulation [Birn and Hesse, Ann Geo, 2005] and BBF duration [Juusola et al., JGR, 2009]

5. Energy conversion increases with the geomagnetic activity (AE, Kp)

6. Correlated with auroral activity (AE and event investigations)

• Consistent picture of the PS energy conversion!

- (STATISTICALLY) OBSERVED ECR CHARACTERISTICS -

Hamrin, Nordic Cluster Meeting, 2011

Now heading towards the new results linking the PS energy conversion to…

1. …SW input into the Msp

2. …transport of energy to the ionosphere(BBFs, Poynting flux)

- NEW STUFF -

Hamrin, Nordic Cluster Meeting, 2011

• Dayside reconnection (SW energy input) is favorable towards southward IMF Bz

• ECRs are more common after periods of stable southward IMF

• Expected but now we have verification!

• More significant result for CLRs. Perhaps because of differences between the spatial distributions of CLRs and CGRs with respect to the Cluster orbit (?)

- (1.) ECR VS. SW INPUT -

Hamrin, Nordic Cluster Meeting, 2011

#ECRs

#ECRs

Bz(T-20min)<0 Bz(T-20min)>0

Bz(T-20min)>0Bz(T-20min)<0

• Some extra material…

• Number of ECRs observed t hours after stable 1h-intervals of southward or northward IMF.

• The memory of the IMF direction remains up to ~5h.

• Why?

- (1.) ECR VS. SW INPUT -

Hamrin, Nordic Cluster Meeting, 2011

’Stable level’ of fluctuations

• ECRs are associated with high speed flows (cf. background data – gray)

• ECRs are more common during GSM Vx>0 than Vx<0 (not shown)

• Vx generally gives the most dominant contribution to Vtot, but there are many ECRs where the Vyz contribution cannot be neglected (not shown)

• Manuscript in preparation, Marghitu et al.

- (2.1) ECR VS. BBF -

Hamrin, Nordic Cluster Meeting, 2011

Vtot [km/s]

Norm # ECRs

|Vtot| [km/s]

• Main conclusion 1: There is a strong correlation between high speed flows and ECRs.

• (Future research: Linking generators to the breaking of the plasma)

- (2.1) ECR VS. BBF -

Hamrin, Nordic Cluster Meeting, 2011

• Sx within ECRs and RAND intervals (background reference data)

• Observation 1. Background data Poynting flux is small and directed towards the Earth.

Interpretation: Cluster is generally earthward of NENL

• Observation 2. CLRs and CGRs are correlated with a strong Poynting flux towards the Earth (consistent).

Interpretation: During times of increased PS energy conversion (ECRs observed), we observe increased Poynting flux (superposition) towards the Earth/ionosphere, where some energy may be dissipated in auroral processes (e.g. correlation between ECRs and AE, Kp).

- (2.2) ECR VS. POYNTING FLUX -

Hamrin, Nordic Cluster Meeting, 2011

Norm # ECRs

Sx~0 Sx>0, towards Earth

Sx<0, away from Earth

Sx (GSM)

• Observational evidence: Verifying the role of the PS as an important link in the chain of processes controlling energy conversion and transfer from the SW and to final auroral dissipation

1. Dayside reconnection is expected to be efficient during southward IMF. PS energy conversion should therefore correlate with IMF direction. Now this is experimentally verified!

2. Also verified that PS energy conversion correlates well with

2.1 earthward plasma flows and 2.2 earthward Poynting flux.

Further discussions and suggestions? Splinter?

- SUMMARY AND CONCLUSIONS -

Hamrin, Nordic Cluster Meeting, 2011