1. CHAMP Observations of Multiple Field-Aligned Currents Dimitar Danov 1, Petko Nenovski 2 Solar-Terrestrial Influences Laboratory, Bulgarian Academy of Sciences, Sofia, Bulgaria 1 Solar-Terrestrial Influences Laboratory, Bulgarian Academy of Sciences, Sofia, Bulgaria Geophysical Institute, Bulgarian Academy of Sciences, Sofia, Bulgaria 2 Geophysical Institute, Bulgarian Academy of Sciences, Sofia, Bulgaria

2. Introduction We analyze FAC measurements on board the CHAMP satellite, conducted on days 30.06-02.07.2008, under quiet geomagnetic conditions. In general, there are 91 crossings over polar regions (46 in the Northern and 45 – in the Southern hemisphere). A typical picture of measurements is shown on Fig.1. Figure 1

3. What in the data is unusual? A) too many FAC sheets B) too intense FAC in the polar cap The procedure of determination of FAC density and distribution by single satellite measurement is accompanied with the some uncertainties Principal objective of our work is identification of ‘false’ FAC sheets

4. Possible sources of false FAC sheets. 1.Associated with calculations. The formula used is: We see that at small angles of attack ( big α ), the calculation error becomes unacceptably large.

5. Possible sources of false FAC sheets. 2.Errors independent of the instrument They are related to departure from the basic assumption for unlimited in azimuthal direction layers (false sheets) - Illustrated in Fig.2 and 2b. Figure 2.a Two cases of satellite crossing square to finite current sheet On both figures current density is 1 (to us), the finite dimensions of the sheet are 20x2 and it is centered on (0,0). (in conventional units). The measured tangential B-field component is parallel to the X-axis, the normal component - to Y-axis. With blue squares are shown the areas of false currents.

6. Possible sources of false FAC sheets. 2.Errors independent of the instrument Figure 2.b Three cases of satellite crossing to finite current sheet, at angles of attack different from 90 0. With blue squares are shown the areas of false currents.

7. Possible sources of false FAC sheets. 2.Errors independent of the instrument Summary: In cases shown on figures 2a and 2b, a “three sheet” structure appears – one true (in the middle) and two “false” on both sides. Errors in FAC estimation…. (summary) Errors in FAC estimation…. (summary) In case of finite (azimuthally) FAC sheets, the measurements are highly dependent on the way in which the satellite encounters them.  False layers appear and / or  Actual FAC values are greater than measured  At small angles of attack ( big α ), the calculation error becomes unacceptably large.

8. Processingof CHAMP’s Data Processing of CHAMP’s Data We define single current sheet, as measurements spacing between two zero crossings of the curve shown in Figure 1. 1084 sheets are numbered around the north pole and 1176 - around the south pole; We believe that an averaged FAC would presents layers better than the maximum value of the FAC in layers In order to reduce the number of false FACs, we impose the following conditions: 1/ the average density must be «significantly» different from zero (Student criterion) (56 sheets were removed in North, 76 – in SUD) 2/ all FAC obtained at attack angles (the angle between the satellite trajectory and polar oval) less than 650 were discarded (136 - in North, 720 – in SUD) (thus whole séances were omitted) Knowing the coordinates (LAT, MLT) of the points where the satellite encounters the boundaries of the layers, we can determine "thickness" δLAT and “width” δLong=15*δMLT of the layers and angle of attack 3/ impose the condition that α must be larger than 650 (311 sheets were removed in North, 155 – in SUD) These conditions are sufficient to exclude all layers in the DAWN-DUSK sectors. All satellite passes are in Noon-Night direction.

9. Results Most part of the measured FAC are weak. They are positioned equatorward or poleward from the main area of observation. Strong FACs (>0.5μA/m -2 ) were noted in the following intervals: [77 0 ;83 0 ]NORD/NOON, [67 0 ;74 0 ]NORD/NIGHT and [68 0 ;73 0 ]SUD/NIGHT

10. Results continuation 1.Most part of the measured FAC are weak. They are positioned equatorward or poleward from the main area of observation. Strong FACs (>0.5μA/m-2) were noted in the following intervals: [770;830]NORD/NOON, [670;740]NORD/NIGHT and [680;730]SUD/NIGHT 2.The stronger ones are associated with open field lines on dayside, and prolonged (L > 7) on night side. ( Tsyganenko GEOPAC2005 ) 3.The number of simultaneously measured FAC sheets is time dependent. It have a peak in the intervals (UT) 01:28÷06:25 and 16:48÷21:34 on 30 June, 00:28÷05:12 and 17:07÷21:51 on 01 July... 4.In other time intervals we measure 1÷3 sheets per sector. It exists passes with zero sheets in one sector and 2÷3 sheets in other.

11. Results continuation Figure 3 An example of good agreement of measurements with the model, perhaps because the satellite crosses the layers perpendicular and away from their edges.

12. Conclusions There are no regions where the sign of current remains unchanged, therefore, it is not possible to discriminate regions similar to R1/R2 We were unable to find a relationship between SW-parameters and observed FAC structures. Possible reasons include: 1.Within the time resolution of SW-parameters data ( 1 min sampling rate in OMNI-WEB data base ) the FAC changes its sign several times. 2.Effects caused by the FAC structure crossings ( especially after the manner of that crossing ) distort the measurements in such a way, that the relationship of FAC with the solar wind parameters is lost.

13. Acknowledgments: Dr Patricia RitterDr Hermann Luehr The authors thank Dr Patricia Ritter and Dr Hermann Luehr ( from Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Germany ) for providing CHAMP data, consulting and their recent publications on FAC.

14. Thank you for your attention