Cluster and SuperDARN observations during a positive By period

D. Ambrosino, E. Amata, M.F. Marcucci, I. CocoIstituto di Fisica dello Spazio Interplanetario, INAFVia del fosso del cavaliere, 100, 00133 Roma, Italy

Introduction

On January 2, 2003, around 23:00-24:00 UT Cluster crossed the northern dusk high latitude

magnetopause on an outbound orbit. The IMF, observed by ACE, and the magnetosheath magnetic

field, observed by GEOTAIL and by Cluster SC2, first changed from being predominantly northward

to being By dominated and slightly southward; later on the IMF rotated slowly until it became purely

northward.

Before crossing the MP, Cluster SC3 observed a boundary layer of dawnward flowing plasma;

during the same period, SC1 and SC4 stayed mainly in the magnetosheath and crossed the MP

several times observing a similar BL of dawnward flowing plasma. Cluster observations can be

explained in the framework of high latitude dusk reconnection at the cusp.

Simultaneously, SuperDARN observations show that the high latitude convection at noon is

characterized by an extended westward flow, when By > 0 and Bz < 0, and by sunward and

westward flows when By > 0 and Bz > 0. We analyze the reconnection signatures at Cluster and

study SuperDARN and Cluster data to describe the global reconfiguration of reconnection at the

magnetopause driven by the IMF turnings.

Fig. 1

02/01/2003

Fig. 1 shows ACE and WIND positions in interplanetary space and GEOTAIL location in the dawn flank magnetosheath for the event under study.

SC1 SC2 SC3 SC4 Fig. 2

GSM

z

x

xz

yy

Fig. 2 shows the Cluster outbound orbit during the crossing of the northern high latitude dusk magnetopause tailward of the cusp. The spacecraft formed a regular tetrahedron with 1 RE distance. SC3 was deeper in the magnetosphere; SC1, SC2 and SC4 were duskward with respect to SC3; SC2 had the most sunward position.

a b c d e f

IMF clock angleFig. 3

Fig. 3 displays solar wind density, magnetic field components and clock angle between 2003/01/02 23:00 and 2003/01/03 00:24 UT. The ACE and WIND data have been shifted to match the SC2 clock angle in the magnetosheath. At about 23:20 UT the IMF turneds rapidly from a northward to a duskward orientation, afterwards it slowly returned northward. Shaded boxes a through f highlight 2-min intervals further discussed hereafter (see Fig. 4).

ACE

WIND

GEOTAIL

SC2

SC2 MSH

Fig.4 shows six 2-min northern hemisphere SuperDARN convection maps corresponding to the intervals highlighted in Fig. 3. The maps are based on data from the following radars: Hankasalmi, Thykkvibaer, Stokkseyri, Goose Bay, Kapuskasing, Saskatoon, Kodiak, Prince George.

a.23:10 - 23:12 UT. The polar cap convection is sunward and slightly westward. This is also observed during the previous 12 minutes and is compatible with reconnection tailward of the northern cusp. During this period the IMF z and y components are both positive.

b.23:18 – 23:20 UT. Situation similar to panel a. However, the sunward/westward flows are less intense. At this time B, as observed at Cluster, becomes duskward and slightly southward.

c. 23:24 – 23:26 UT. The convection pattern displays a clockwise cell predominantly in the post noon sector with noon flows directed antisunward as expected for a By dominated IMF. The convection

reconfiguration time is ~ 6 min.d.23:36 – 23:38 UT. The westward flows seem to enhance. Note that

between 23:33 and 23:38 UT Bz becomes zero and the IMF is purely

duskward.e.00:10 – 00:12 UT. The IMF is again northward with By > 0:

correspondingly the flow rotates and becomes sunward/westward directed again.

f. 00:20 – 00:22 UT. Two reversed cells typical for a purely northward IMF are observed. This is confirmed by DMSP observations (see Fig. 5).

a

c

e

b

d

f

Fig. 4

Cluster 3

M P

Fig. 6a

Fig. 6a. Energy spectrograms and moments for SC3. BL (with dawnward flow) was entered at about 23:23 UT. MP was crossed at about 00:07 UT.

Cluster 4 M P

Fig. 6b

Fig. 6b. Same as Fig. 6a for SC4.The Boundary Layer (dawnward flow) was entered at about 23:20 UT. The magnetopause was crossed at about 23:40 UT (while SC3 exited later on at about 00:07 UT).

Fig. 6c. Same as Fig. 6a for SC1. BL was entered at 23:22 UT. MP was crossed at 23:38 UT. From 23:23 to about 23:26 UT (shaded area in the V panel) all SC (see also 6a and 6b) observed a duskward, tailward and northward flow at a lower velocity. The BL dawnward flow could be the effect of reconnection due to a duskward magnetosheath field. In the following we will show that this is indeed the case.

Cluster 1

Fig. 6c

M P

ΔBΔVμρ

1th

Changes in B and V across a reconnected MP must satisfy the Walén relation (Hudson, P.D., Planet. Space Sci., 1970, Paschmann, G. et al., Nature, 1979):

The plus (minus) sign is for flow parallel (antiparallel) to B, i.e. , in our case, for observations sunward (tailward) of the reconnection site.Fig. 7 shows the Walén test result relative to the SC3 MP crossing of 00:07 UT, marked in Fig. 6a by a horizontal line between vertical arrows. The quality of the test is indicated by the red vector. Its length is the ratio between Vobs observed across the MP

and the theoretical value; the angle between its direction and the horizontal reference line is the angle between Vobs and Vth. Thus, ideally the red vector should be of

length one and parallel (antiparallel) to the positive horizontal axis for sunward (tailward) flow. Figs. 8a and 8b similarly show the Walén test results for two SC1 MP crossings, at 23:27 and 23:36 UT, marked in Fig. 6c by horizontal lines between vertical arrows.

Walén test

Fig. 7 Fig. 8a Fig. 8b

00:07 UT 23:27 UT 23:36 UT

SC4 23:22:10 UT

Kinetic signatures of reconnection are the so called D-shaped distribution functions of transmitted magnetosheath ions in the MP/BL. D-shaped distribution functions are expected in the MP/BL with a low energy cutoff at a velocity along the magnetic field equal to the parallel component of the deHoffmann-Teller (dHT) velocity (Cowley, 1982).

Fig. 9

During this event, D-shaped distribution functions were observed almost continuously in the BL by SC1, SC2 and SC3. For one case (not shown herein) we verified that the cutoff corresponds to the parallel component of the MP dHT velocity.

SC1 23:21:59 UT SC3 23:23:11 UT

Fig. 9 displays three D-shaped distributions in the V// - Vplane. We notice that: a) their flow is parallel to the BL field, b) their cutoff ranges from a few km/s to about 100 km/s,c) they are mixed with the magnetospheric plasma,d) at times a second antiparallel population appears (see e.g. the SC4 observations of 23:22:10

UT).The second antiparallel population can be interpreted as the transmitted m’sheath population reflected back from the ionosphere. When such a population is present (see SC4 distribution function at 23:22:10) the computed velocity results to be very low.

From the analysis of the CLUSTER data we conclude that the dawnward jets

observed in the BL are due to reconnection.

Fig

. 10

In F

ig.

10

we s

how

a z

oom

of

the C

lust

er

velo

citi

es

and m

agneti

c field

s betw

een

23

:10

and 2

3:4

0 U

T.

The 2

3:2

0 –

23

:30

UT p

eri

od is

hig

hlig

hte

d.

Note

: w

e c

heck

ed

that

the V

x n

egati

ve jets

seen b

y S

C1

at

23

:28

:30

and a

t 2

3:2

9 U

T a

re n

ot

due t

o

reco

nnect

ion b

ut

corr

esp

ond t

o p

art

ial M

P c

ross

ings.

SC

1S

C3

SC

3

Bew

teen 2

3:1

8 a

nd 2

3:2

3 a

ll th

e C

lust

er

sate

llite

s obse

rve D

-shaped d

istr

ibuti

ons.

The

transm

itte

d p

opula

tions

are

dir

ect

ed d

aw

nw

ard

, su

nw

ard

and s

outh

ward

(note

that

com

pute

d v

elo

city

for

SC

1 a

nd S

C4

in t

he last

part

of

this

inte

rval is

low

sin

ce a

seco

nd

reflect

ed p

opula

tion is

pre

sent)

. Fro

m 2

3:2

3 t

o a

bout

23

:26

(sh

aded a

rea)

all

the

space

craft

obse

rve a

pla

sma w

hic

h is

movin

g d

usk

ward

, ta

ilward

and n

ort

hw

ard

at

a

much

low

er

velo

city

. A

fter

23

:26

, a r

evers

al in

the p

lasm

a fl

ow

dir

ect

ion o

ccurs

: th

e fl

ow

bein

g d

irect

ed d

aw

nw

ard

, but

anti

sunw

ard

and n

ort

hw

ard

.Sin

ce a

t about

23

:20

the IM

F ch

anged f

rom

nort

hw

ard

to d

usk

ward

and s

lightl

y

south

ward

we inte

rpre

t th

ese

obse

rvati

on a

s re

connect

ion fi

rst

occ

urr

ing t

ailw

ard

of

Clu

ster,

then c

easi

ng a

nd init

iati

ng a

t hig

h lati

tude d

usk

ward

of

the t

hre

e s

pce

craft

.

Fast

Clu

ste

r

Fig

. 11

As

the c

onvect

ion r

eco

nfigura

tion

occ

urr

ed b

etw

een 2

3:2

2 a

nd 2

3:2

6

UT (

cf.

Fig.

4),

in t

he f

ollo

win

g w

e

conce

ntr

ate

on t

he 2

3:2

0-2

3:3

0 U

T

peri

od.

Fig

. 1

0 s

how

s C

lust

er

and

FA

ST f

ootp

rints

dra

wn o

n

SuperD

AR

N m

aps.

23

:20

– 2

3:2

4 U

T m

ap

s.

The

convect

ion r

eco

nfigura

tion

begin

s at

low

lati

tudes

as

the

anti

sunw

ard

flow

begin

s to

appear

at

about

75

° M

LAT 1

1-1

2

MLT

. A

t th

is t

ime t

he C

lust

er

footp

rints

are

slig

htl

y a

bove 8

0°

MLA

T a

nd 1

2 M

LT,

where

the

convect

ion is

still

sunw

ard

. This

is

in a

gre

em

ent

wit

h t

he

sunw

ard

flow

s obse

rved b

y

Clu

ster

at

the m

agneto

pause

.

23

:24

-23

:26

UT m

ap

. The

Clu

ster

footp

rint

is

in t

he c

ente

r of

the d

evelo

pin

g w

est

ward

cell

in a

regio

n o

f lo

w c

onvect

ion.

We r

eca

ll th

at

at

this

tim

e

reco

nnect

ion s

eem

s to

sto

p a

t th

e M

P c

lose

to C

lust

er.

23

:26

-23

:28

UT m

ap

. The

Clu

ster

footp

rint

is e

mbedded

in t

he w

est

ward

and

anti

sunw

ard

flow

, again

in

agre

em

ent

wit

h o

bse

rvati

ons

made b

y C

lust

er

at

the M

P.

As

regard

s FA

ST,

we n

oti

ce t

hat

its

footp

rint

moves

from

daw

n t

o

dusk

and t

ow

ard

s lo

wer

lati

tudes

in a

regio

n w

here

SuperD

AR

N o

bse

rves

tailw

ard

-w

est

ward

convect

ion.

a b c d

Fig

. 12

Fig

.12

show

s th

e A

CE

IM

F co

mponents

for

23:1

0 -

23

:30 U

T a

nd

the F

AST

pro

ton e

nerg

y s

pect

rum

for

23

:20 –

23

:30 U

T. Fr

om

23

:24 U

T o

nw

ard

s pro

ton e

nerg

y in

crease

s as

lati

tude d

ecr

ease

s, s

uggest

ing t

hat

a

reco

nnect

ion

sit

e is

bein

g a

ppro

ach

ed. A

ctually

, w

e s

how

ed t

hat

reco

nnect

ion

sta

rts

dusk

ward

of

the c

usp

betw

een 2

0:2

0 a

nd 2

0:2

4 U

T

(cf.

Fig

. 1

1).

Severa

l energ

y d

ecr

ease

s occ

urr

ing

on t

ime s

cale

s of

10-2

0 S

need t

o b

e f

urt

her

stu

die

d.

Fig

.5

At

00

:20 U

T,

when IM

F B

z >

0,

the S

uperD

AR

N m

ap

s sh

ow

a

sunw

ard

convect

ion o

ver

the

pola

r ca

p.

Fig

. 5

show

s th

e D

MSP F

13

traje

ctory

(east

to w

est

) and

th

e p

ola

r plo

t of

Cro

ss-T

rack

Pla

sma D

rift

. W

e c

learl

y s

ee a

su

nw

ard

flow

aro

un

d n

oon

.

a) Vy < 0, Vx > 0 (t < 23:22 UT). Reconnection tailward of the cusp.

b) Vy 0 (23:22 UT < t < 23:26 UT). No reconnection signatures at Cluster. “By reconnection” is approching from lower latitudes.

c) Vy < 0, Vx < 0 (23:26 UT < t) By > 0, Bz < 0 reconnection duskward of the cusp.

Summary

Fig. 13a Fig. 13c

Cluster “X line” Reconnection jets and correspondent ionospheric flows.

Fig. 13b

Fig. 14a Fig. 14b Fig. 14c

WARNING: in Fig. 15 dawn is on the left and the northern polar cap on the top; in Fig. 16 dawn is on the right and low latitude is on the top.

23:20:00 – 23:22:00 UT 23:22:00 – 23:24:00 UT 23:26:00 – 23:28:00 UT

ConclusionsBetween 23:00 UT, 2 January, and 01:00 UT, 3 January, 2006, the IMF underwent several orientation changes. The SuperDARN convection maps, the DMSP and FAST observations and the observations made by Cluster (in its outbound orbit) at the MP, in the MS and in the BL all agree on a global reconfiguration of MP reconnection and ionospheric convection driven by IMF changes.

The period between 23:20 and 23:30 UT was analysed in detail. At about 23:20 UT the IMF turned rapidly from a northward to a duskward and slightly southward orientation. At the time of the IMF turning reconnection tailward of the cusp was ongoing.

The 23:20 UT IMF rotation drove a reconfiguration of reconnection at the MP (Cluster) and of ionospheric convection (SuperDARN).

At the IMF rotation, the cusp dusk side reconnection started, while reconnection tailward of the cusp was still going on (23:20-23:24 UT; cf. Figs. 11a,b). Within 2 min tailward reconnection stopped. From 23:26 UT onwards the cusp dusk side reconnection extended to higher latitudes reaching Cluster.

FAST observed on average an energy dispersion (energy increased as latitude decreased) suggesting that a reconnection site was being approached. At the FAST footprint (9-13 MLT and 80°-75° magnetic latitude) SuperDARN observed anti-sunward flows. Such observations are reconciled under the hypothesis that the flows came from a By dominated reconnection line duskward of the cusp.