M. Yamauchi1, H. Nilsson1, I. Dandouras2, H. Reme2, R. Lundin3, Y. Ebihara4

(1) Swedish Institute of Space Physics (IRF), Kiruna, Sweden (2) CNRS and Universite de Toulouse, IRAP, Toulouse, France (3) Swedish Institute of Space Physics, Umeå, Sweden (4) Kyoto University, Uji, Japan

Inbound-outbound asymmetry of Cluster perigee traversals as

indicator of substorm dynamics

Ion’s are normally north-south symmetric (except seasonal effect) because these ions are bouncing between southern and northern hemispheres

Cluster (2001-2006): should be inbound-outbound symmetric because they are different only by 1-2 hours in both UT and LT.

Any asymmetry should reflect “event” such as substorms.

We examined all perigee pass during 2001-2006 SC4 (about 670 traversals, with clean data of 460 traversals).

Cluster perigee (L=4~6) ion patterns

Cluster perigee (L=4~6) ion patterns

(1)(2)(3)

(1) Westward drifting > 10 keV

(2) Eastward drifting ~ few keV (mono-energy)

(3) Eastward drifting wide-energy stripes < 1 keV

Three major ion components in the dawn-to-noon sector:

symmetric patterns

(1)(2)(3a)

(3b)

2001-9-4 (12 LT)

2001-7-18 (15 LT)

2002-6-2 (18 LT)

other local times(3b)

statistics for (3b) “wedge”

Howeversometimes inbound-outbound asymmetry.

related to substorms!

Examples of (2) & (3a)

AE

AE

AE

AEAE

Examples of (3b) “wedge-like structure”

Classify asymmetry in terms of basic patterns

(1) Westward drifting > 10 keV

* Intensity & energy @ same L (latitude)

(2) Eastward drifting ~ few keV (mono-energy)

* Intensity & energy @ same L (latitude)

(3) Eastward drifting wide-energy stripes < 1 keV

(3a) Vertical (extend from keV):

* Intensity @ same L (latitude)

(3b) Dispersed (extend from low energy)

* Intensity & energy @ same L (latitude)

* dispersion direction @ same L (latitude)

Today

Today

AEAE

Asymmetry sometimes appears as (3c) internal structure (no model yet)

Statistics for (3a)

2003-3-15 (0 LT)

|AL| > 400nT and the appearance of the vertical structure (type 3a) are related at midnight.

Statistics for (3b)/(3c)

2001-7-9 (16 LT)

2002-9-8 (12 LT)

In many cases, “wedge” is enhanced within 2h of traversals and LT difference.

Hr from substorm onset

cf. Viking

dynamic-statistics

6 MLT

9 MLT

12 MLT

15 MLT

18 MLT

ExB drift (+ co-rotation) is quite strong.

The “wedge” is moving rather fast toward late afternoon

actually, simulation can re-produce the asymmetry!

SC-4

(a) Asymmetric in vertical stripes (1~10 keV ions) * post-midnight peak

* related |AL|>400nT substorm onset

(b) Asymmetric “Wedge” (dispersed stripe at sub-keV)(c) Asymmetric internal structure of “Wedge”

* late-morning peak, but extends wide LT* quite soon (few hours) after substorm activity* right after onset@early morning

Summary of asymmetric ion patternWe examined ion (CIS) data for all Cluster perigee traversals during 2001-2006 when the orbit is relatively north-south symmetric (2006 data is affected by orbit). Quite many cases are north-south asymmetric in the inner magnetosphere.

On can guess the past history of substorm (+ E field) just from the ion spectrogram (if orbit is ideal).

Thank you for your attention

other substorms

VB energy (mass independent)

VExB = constant (energy mass)

dispersed sub-keV ion = “wedge”