1 CSSAR Center for Space science and Applied Research Chinese academy of Sciences FAC in magnetotail observed by Cluster J. K. Shi (1), Z. W. Cheng (1), T. L. Zhang (2), M. Dunlop (3), Z. X. Liu (1) A. Fazakerley (4), E. Lucek (5), H. R´eme (6), M. Taylor(7), I. Dandouras (6) (1) CSSAR / CAS, China (2) Space Research Institute, AAS, Austria (3) SSTD, Rutherford Appleton Laboratory, UK. (4) MSSL, UCL, UK (5) Imperial College London, UK (6) Univ. of Toulouse, UPS, CESR, and CNRS, France (7) ESTEC/ESA, The Netherlands 19th Cluster Workshop, Romania, May, 2010

2 Outline 1.Introduction 2.Principle and Method 3.FAC properties 3.1 FAC’s asymmetry 3.2 FAC’s distribution function 3.3 Relationship between FAC and geo-magnetic activity 3.4 FAC in storm times 4. Summary

3 1. Introduction M-I Coupling is important to understand dynamics in the Geo- space. It concerns: Up-flowing ions Precipitation particles FAC Field aligned electric field Wave and instability, and so on

4 FACs flowing between magnetosphere and ionosphere A Sketch map of the FACs going into polar region and out from polar region FACs flow Into and out of the ionosphere

5 Cluster 4-ponit measurement have an advantage to provide data for the study of FACs Cluster TC-2

6 Before Cluster, the authors used By from one or two satellite to estimate the FAC. It may ambiguous because time and space variations can not be fully distinguished. With Cluster 4-point magnetic measurement, the "Curlometer Technique" was propsed to calculate the current --- Vallat et.al. [2005] studied the current density in the ring current region. --- Draper et. al. [2005] studied FACs in the PSBL associated with periods of reconnection. --- Snekvik et. al. [2007] studied FAC at the dawn flank of a burst bulk flow in the outer central plasma sheet --- Forsyth et. al. [2008] determined that the current density of the FACs in a substorm is about 5 nAkm−2.

7 In this study, we use magnetic field data from the Cluster 4-point measurement to study FACs in Plasma Sheet Boundary Layer (PSBL) To research FACs with Cluster data in the PSBL Lobe PlasmaSheet Lobe The data from 4 satellites The data were obtained in 2001

8 2. Principle and Method

9 The current was calculated with the so called “Curlometer Technique” with the 4-point data through Ampere’s law: The tetrahedron consisted by the four-spacecraft Cluster mission In integral form: Using the 4-point data : The FACs: The data quality: and we take Q< 0.3

10 4 second data to be used to calculated the FAC in the PSBLs

11 FAC cases selection: and time separation > 10 minutes Fig. 1b, FAC cases selected according to Bxy, FAC density and interval. All cases A, B, C, and D lie in the PSBLs because Bxy > 15 nT, but the case C should not be selected because the current density is less than 3 pT / km and the interval of the case C and D is less than 10 minutes. Bxy > 15 nT

12 Jx By Bx Earthward and Tailward FACs

13 Fig. 2 FAC distribution on the Bx–By plane (left panel) that shows the FAC cases were only selected in the plasmasheet boundary layers, and the calculated FAC density distribution for all the cases (right panel) that shows FAC have a larger range of density values in the north hemisphere There are 172 FAC cases selected for this study:

14 3. FAC properties 3.1 FAC’s asymmetry

15 Projection of FACs in X-Y plane Red: Earthward, Blue: Tailward Arrow Length: Density Fig.3 FAC cases on the X-Y plane in the GSM system. It shows the FACs’s: Dawn-Dusk asymmetry Earthward –tailward asymmetry.

16 (a) (b) Fig. 4 FAC distribution versus Y GSM, gray and black colour mark the Earthward and tailward FACs respectively, (a) for the FAC cases number distribution and, (b) for the FAC fractional occurrence. Dawn-Dusk asymmetry, Tailward-Earthward asymmetry Shi et. al., 2006, 2009

17 The positions of FACs in magnetotail are likely to connect both to the inner R1 current system and to currents at the poleward boundary. It is also possible that, in tail reconnection, a NENL may be either Earthward or tailward of the Cluster location. In order to consider the R1 current system more directly, we use Bz to filter out any reconnection driven currents, which arise from a NENL located Earthward of the spacecraft. Bz > 0, field lines connecting to the Earth. Bz<0, not. 146 cases whose with condition Bz>0 were selected for the analysis.

18 North-South asymmetry Shi et.al.,JGR, 2010

19

20 Fig. 6a The footprints of the mapping of the FACs along the field line to the polar region. The left panel is for the northern hemisphere and the right panel is for the south hemisphere, respectively. The FAC cases in this figure exclude those with the condition Bz<0.

21 Fig.6b FAC footprint distribution versus latitude in the two hemispheres corresponding to Fig 6a, The left panel is for the northern hemisphere and the right panel is for the southern hemisphere, respectively. The FAC cases in this figure, of course, exclude those with the condition Bz<0.

FAC’s distribution function

23

24

25 F T (J) = F 1 (J) + F 2 (J) ( Cheng and Shi et.al., 2008 ) (4)

26 FT(J) = F3(J) + F4(J) (Cheng and Shi et.al., 2008)

27 FACs occurance also appeared two peaks at PSBL on Aug.17, 2001 Peaks: Tailward, -7pT/km; Earthward, 12pT/km

Relationship between FAC and geo-magnetic activity

29 Fig.2. Normalized frequency of FACs cases as a function hourly geomagnetic activity level (They were defined by Kp). The number of cases in each geomagnetic activity level bin, which is indicate above the bar, is normalized by total hours when Kp was in each geomagnetic activity level during the period of crossing.

30 Fig.3. The FACs density as a function of Kp. The curve shows the FACs average density variation.

31 Fig.4. The density of field-aligned currents as a function of AE index.

FAC in storm time

33 ► Only two cases were found for this research (Cheng and Shi et.al., 2007) * Case 1: on Aug. 17, 2001 Sudden storm commencement to main phase * Case 2: on Oct.1, 2001 Main phase and recovery phase ► In the each case, Cluster just crosses the plasma sheet which is with two sheet-lobe boundary lays. ► FAC sample selection Separation is more than 5 min.

34 (1) The storm starting on August 17, 2001 Gray area, , Cluster crossing

35 Aug.17, 2001 （ Aug.17, 2001 （ sudden commencement and main phase ） FACs versus AE （ including Earthward and Tailward)

36 Earthward FACs versus AE Aug.17, 2001 （ Aug.17, 2001 （ sudden commencement and main phase ）

37 versus Tailward FACs versus AE Aug.17, 2001 （ Aug.17, 2001 （ sudden commencement and main phase ）

38 (2) The storm starting on Oct. 1, 2001 Gray area, , Cluster crossing

39 Oct. 1, 2001 （ later Oct. 1, 2001 （ later main phase and recovery phase ） FACs versus AE （ including Earthward and Tailward)

40 It seems as in the storm time, In SC, FAC havs an inversed variation with AE In the main phase, FAC has same variation with AE In the recovery phase, FAC has no relation with AE This is only from two csaes and need to be further studied.

41 Summary The FACs in PSBL was studied by analyzing the data from Cluster 4-point measurement in The results are: ► FACs not only have earth-tailward asymmetry, dawn-dusk assymmetry, but also north-south hemisphere asymmetry ► FACs distribution function consits of a Gauss distribution and a exponent distribution. ► No storm time, FACs variation is consistant with Kp not AE. ► In storm time: in SC, FACs inversed to AE, in main phase, FACs' variation is consitant with AE in recovery phase, FAC's has nothing to do with AE

42 CSSAR Center for Space science and Applied Research Chinese academy of Sciences