Magnetic reconnection in the magnetotail: Geotail observations

T. Nagai

Tokyo Institute of Technology

　　　 World Space Environment Forum 2005

May 4, 2005 Wednesday 09:00 – 09:20 AM

Geotail

distant tailnear tail

Geotail Near-Tail Observations

10 x 31 RE orbits

Solar Wind

Magetosheath

Plasma Sheet

Bow Shock

Magnetopause

Nagai et al. (JGR 1998)

Ion Electron

High-Speed Tailward Flowing Ions

High Accelerated Electrons

V=3000km/s

Inflow Ions

Outflow Ions

Outflow IonsConvection

Nagai et al. (1998)

Electrons

Ions

1406:12 UT January 27, 1996Outer Boundary Region in Tailward Flows

Tailward Field-Aligned Flows

Inflowing Ions

Inflowing Electrons

Tailward Escaping Electrons

Hall Currents in Magnetic Reconnection

December 10, 1996

Nagai et al. (JGR 2001)

Electrons for Hall currentsin the December 10, 1996, event

Hall Current density ・・・・ 6 ～ 13 nA/m2

In Situ Observations of Magnetic Reconnection

in 1995 - 1997

Solar wind control of the radial distance of the magnetic reconnection site in the magnetotail

T. Nagai and M. Fujimoto (Tokyo Institute of Technology)R. Nakamura and W. Baumjohann (OEAW)A. Ieda (Solar-Terrestrial Environment Laboratory)I. Shinohara (ISAS)S. Machida (Kyoto University)Y. Saito and T. Mukai (ISAS)

In 1995-1997

Solar Minimum Low B and Low V

Solar Maximum High B and High V

Solar Wind

IMF Bt

Velocity

Geotail Near-Tail Observations

10 x 31 RE orbits

Plasma Sheet (β> 1) XGSM = -10 to -31 RE and YGSM = -5 to +10 RE

Solar Minimum Solar Maximumin 1996-1998 in 1999-2003

Even Coverage of XGSM = -10 to -31 RE

1996/02/18 Reconnection Event

highly accelerated electrons

thermal

accelerated

electron energy spectra

flux

energy

fast tailward flows with Bz < 0

magnetic field

Solar Minimum Solar Maximumin 1996-1998 in 1999-2003

XGSM = -25 to -31 RE XGSM = -17 to -31 RE

Average Solar Wind Parameters Prior to Each Reconnection Event

Total 34 Events

SW parameters during 60 minutesprior to each reconnection event

Solar Wind Velocity ?

circles 1996-1998 eventsblack dots 1999-2003 events

Solar Wind Dynamic Pressure ?

Southward IMF Bz

Southward IMF Bz

X = -25 RE

circles 1996-1998 eventsblack dots 1999-2003 events

Solar Wind Energy Input

Solar Wind Energy Input

X = -25 RE

Near-TailMidtail

Average solar wind parameters for the period from -6 to +2 hours

Reconnection Start Time

9 events

25 events

Near-tail

Midtail

Solar Wind Energy Input

Near-tail

Midtail

Near-Tail

Midtail

Accumulation of Solar Wind Energy Input

Solar Wind Velocity

Near-Tail

Midtail

High Velocity520 km/s

Low velocty460 km/s

Solar Wind Dynamic Pressure

Near-Tail

Midtail

2 nPa

high velocity andlow density

2 nPa

low velocity andhigh density

Near-Tail

Midtail

Accumulation of Solar Wind Energy Input

Solar Wind Input Energyx 2

High Efficiency of Solar Wind Energy Input

Near-Tail

Low Efficiency of Solar Wind Energy Input

Midtail

Solar Wind Energy Input

High Efficiency of Solar Wind Energy Input

Low Efficiency of Solar Wind Energy Input

Near-Tail

Midtail

1. Reconnection event analysis 34 eventsTailward flow with Bz < 0Electron acceleration

2. Tailward flow event analysis 174 eventsTailward flow with Bz < 0No electron acceleration

Cluster events at 15-19 RE (from Nakamura et al., GRL 2004)

Cluster

Near-tail

Midtail

15 tailward flows

56 tailward flows

118 tailward flows

IMF Bz

Cluster

Near-tail

Midtail

Solar Wind Energy Input

Cluster

Near-tail

Midtail

Accumulation ofSolar Wind Energy Input

Past Spacecraft Observations

Vela 1967-1968 Hones et al., 1971; 1973

IMP 6 1971-1973 Hones and Schindler, 1979

ISEE-1 1980 Cattell and Mozer, 1984

ISEE-2 1978-1979 Angelopoulos et al. 1994CDAW-6 McPherron and Manka, 1985

IRM 1985-1986 Baumjohann et al., 1988,1989,1990,1991

Geotail 1996-1998 Nagai et al. 19981999-2003 This study

Cluster 2001-2002 Nakamura et al., 2004

Past Spacecraft Observations

Vela 1967-1968 Hones et al., 1971; 1973

at 18 RE

15-18 RE

Nishida and Nagayama (1973)…somewhere between X = -10 RE to -25 RE

Past Spacecraft Observations

ISEE-1 1980 Cattell and Mozer, 1984ISEE-2 1978-1979 Angelopoulos et al. 1994

CDAW-6 McPherron and Manka, 1985IRM 1985-1986 Baumjohann et al., 1988,1989,1990,1991

CDAW-6 13-14 RE

Earthward Flows inside 20 RE

VELA15RE

IMP-6

ISEE-2> 22RE

ISEE-1> 21 RE

IRM> 18 RE

Geotail 1996-1998> 25 RE

Geotail 1999-Cluster

15-30RE

Conclusions

1. Reconnection takes place in the near-Earth tail (midtail) under the high (low) solar wind energy input.

2. Controlling factor is efficiency of solar wind input energy, not the total amount of input energy.

3. Tail reconnection site can be controlled by solar cycle variations in the solar wind.

1996 1999 2002

1997 2000 2003

1998 2001 2004

Near-Tail MidtailXGSM = -15 to -25 RE XGSM = -25 to -30 RE

High Efficiency of Low Efficiency of Solar Wind Energy Input Solar Wind Energy Input

VELA15RE

IMP-6

ISEE-2> 22RE

ISEE-1> 21 RE

IRM> 18 RE

Geotail 1996-1998> 25 RE

Geotail 1999-Cluster

15-30RE

Nagai et al. (1998)

Structure of Magnetic Reconnection

Ions Electrons

Type 　 B

Type 　 A

Type 　 N

a part of the Hall current system

Nagai et al. (2001)

Reconnectionevent

Hall Current density ・・・・ 6 ～ 13 ｎＡ /mNagai et al. (2001)

2

the 3-min interval the 90-s interval

Bz = -36 nT

Bt = 36 nTtail lobeBt = 24 nT

10 sec

Event 3 1996/02/18

1996/02/18 Event

48 sechighly acceleration of electrons

Hall Currents

170934UT

.

174014UT

.

180446UT

.

.

.

Counterstreaming Ions

Magnetic Field

Nagai et al., Phys. Plasmas 9, 3705, 2002

M. S. NakamuraHybrid-code Simulation

Counterstreaming Ions

Magnetic Field

Magnetic Reconnection in the Distant Tail

Nagai et al., Phys. Plasmas 9, 3705, 2002

Tailward flows(< -300 km/s)with Bz < 0

882 eventsin 1995-2003

Tailward flows(< -300 km/s)with Bz < 0

882 eventsin 1995-2003

90% 90%