1. Possible long-term trend in the relationship between the ASPOC ion current and spacecraft potential measurements Klaus Torkar IWF/OAW, Graz, Austria and the ASPOC and EFW Teams LPCE, February 12 th, 2007

2. Reminder of Concept  Look for a method to reconstruct the uncontrolled s/c potential (C1) from the residual, small variations of the controlled potential (C4) when ASPOC is turned on V1 V4

3. Reminder of Concept  Look for a method to reconstruct the uncontrolled s/c potential (C1) from the residual, small variations of the controlled potential (C4) when ASPOC is turned on V1 V4

4. Simulation of spacecraft potential free- floating controlled  The potential controlled by a constant ion beam:  approaches a constant value at low n, independent of plasma  follows the uncontrolled potential at high density.  Curves shown are valid for 10 µA ion current on Cluster.

5. Estimation of free floating potential from the controlled potential  The relation between controlled and uncontrolled potential is almost independent of electron temp. T e.  The correction term, f, for the potential V uncontrolled = V controlled + f(V controlled ) may be approximated over a wide range of spacecraft potentials by a simple exponential function.  Relation is still non- linear near V max (low density)  Results are less accurate in the steep part of the curve (near V max ).

6. Measured Correlation, Example  Indeed, good correlation of log(  V) with V(controlled) in this interval  Beam current dependance  ASPOC (C3) operated with 12 µA, ASPOC (C4) with 10 µA

7. More "good" examples mainly from 2001  Good correlation between controlled and uncontrolled potential has been found in several characteristic intervals  2001-03-21 2001-09-15 2001-09-24 all beam turn-on times in 2001 2002-04-30 2002-05-10

8. More "good" examples mainly from 2001

19. In late 2001 the good correlation disappeared sometimes (2000 km separation)

22. In 2003 the correlation is often bad despite 300 km separation

27. or the controlled potential is smaller than in 2001, and less variable

32. C3, 2001 C3, 2003 C4, 2001 C4, 2003

33. or the controlled potential is smaller than in 2001, and less variable

34. Sometimes (17 Sep 2003) the controlled potential is "noisy"

35. Noisy potential data correspond to spiky E-field data

37. Discussion  There is a tendency towards higher ASPOC ion currents in 2003: 12-16 µA, compared to 10-12 µA in 2001. Some differences in the potential observations may result.  However, even for selected cases with equal ion currents there is a decreasing trend of the controlled potential  Maybe two different effects:  Solar cycle trend of photo-emission  Disturbance of probe measurements for certain conditions (ASPOC current, plasma density, solar flux, probe bias setting)  EFW bias current is unchanged since June 2001 (140 nA on all s/c), also the puck and guard voltages  Is the bias current too high for low solar activity?  Is the bias current during adequate also for low solar activity, but without ASPOC only?

38. F10.7cm Flux  Steep decrease of solar activity in early 2003

39. Discussion  Reconstruction of the free floating potential is possible under favourable conditions, using a simple expression.  c 1 varies mainly with the known ASPOC beam current  c 2 has been derived for some periods in 2001 and 2002:  If a correlation can be established in 2003 data, c 2 is higher than in 2001 (e.g., for C4 summary: 0.70 => 0.92) Conditionc2c2 Range of controlled potential Theoretical model0.711... 6 V C3, summary of 20010.831... 8 V C4, summary of 20010.701... 8 V C3, 2002-04-301.194.5... 6.5 V C3, 2001-04-021.056.3... 7.6 V

40. Conclusion  Reconstruction of the free floating potential is possible under favourable conditions, using a simple expression.  In contrast to earlier expectations there are conditions when a correlation cannot be established.  The occurrence of such cases increases in 2003.  More work is needed to come to firm conclusions.