15 th CAA Cross-Calibration Workshop, 17th – 19th April 2012, UCL, London PEACE OPS TEAM Presented by Natasha Doss UCL Department of Space and Climate Physics Mullard Space Science Laboratory Calibration of the PEACE sensors using density comparisons with WHISPER - Alpha Factors

Since the last meeting... Alpha factors for Jan 2010 – Jun 2010 determined for all sensors - 2 nd attempt Alpha factors for Nov 2010 – Mar 2011 determined for all sensors Alpha factors interpolated for Jul 2010 – Oct 2010 for all sensors

1. Jan 2010 – Mar 2011 Alpha Factors LEEA sensors

The following slides show the degradation for each LEEA sensor between Jan 2010 to Jun 2010 & Nov 2010 to Mar The plots show weekly averaged density ratios of PEACE LEEA / WHISPER (ACTIVE) in ‘good’ sheath intervals. The PEACE moments used in these comparisons were calculated using only the ground calibration geometric factor. Thus they do not include: - post launch geometric factor corrections needed to correct densities (include α- factors) - inter-anode calibration correction factors needed to correct velocity The LEEA MCP operational level was raised on all 4 SC on 20 th Feb for operational MCP level. The effect of raising the MCP operational levels is clearly seen. The lowered MCP levels (applies to SC1 & 2 only) was increased one week later. The effect of some of the thruster firings are also visible in these plots. LEEA degradation Jan 2010 – Mar 2011

For many years we have lowered the gains of our MCPs when in the sheath/wind to protect the MCP’s as we believed the performance decline was more gradual when they operated at lower gain when in high flux environments. As of November 2009 we no longer lower the MCP gain for SC3 & 4 when in the sheath/wind, as the performance at operational level now counts as “low gain” so there is no need to lower them further. Plus very low gain translates to bad signal to noise ratio which will become an increasing problem on all sensors as the mission carries on in the future. In the following slides the ratios for SC1 & 2 are for lowered MCP level and for SC3 & 4 the ratios are for operational MCP level. LEEA degradation Jan 2010 – Mar 2011

LEEA (ground g.f. only) vs WHI density CLUSTER-1 CLUSTER-2 Density ratio MCP - lowered Thruster firingsMCP level raise

LEEA (ground g.f. only) vs WHI density CLUSTER-3 CLUSTER-4 Density ratio MCP - operational Thruster firingsMCP level raise

LEEA alpha factors v6.0 Jan 2010 – Mar 2011 v6.0 alpha factors Thruster firings MCP level raise C1 C2 Alpha interpolated

LEEA alpha factors v6.0 Jan 2010 – Mar 2011 C3 C4 v6.0 alpha factors Thruster firings MCP level raise Alpha interpolated

LEEA (v6.0 cal.) vs WHI density CLUSTER-1 CLUSTER-2 Density ratio MCP - lowered

LEEA (v6.0 cal.) vs WHI density CLUSTER-3 CLUSTER-4 Density ratio MCP - operational

Alpha factors for the LEEA sensors looks good. Small corrections may be required. It is becoming increasingly difficult to determine alpha due to availability of ‘good’ sheath intervals. SC1,3,4 are routinely turned off in the sheath. We sometimes capture some sheath if predicted timings are not accurate but sensors may not be in the best instrument mode and so not see all the plasma giving underestimated densities.

2. Jan 2010 – Mar 2011 Alpha Factors HEEA sensors

The following slides show the degradation for each HEEA sensor between Jan 2010 to Jun 2010 & Nov 2010 to Mar The plots show the weekly averaged density ratios of PEACE HEEA / PEACE LEEA in the energy overlap region. The HEEA moments used in these comparisons were calculated using only the ground calibration geometric factor. The LEEA moments use the determined v6.0 alpha factors. The HEEA MCP operational level was raised on SC1, 3 & 4 on 20 th Feb. The effect of raising the MCP operational level is clearly seen. The lowered MCP level was increased one week later (applies to SC1 only). There was no MCP level raise for SC2 HEEA. The effect of some of the thruster firings are also visible in these plots. The HEEA sensors have been harder to calibrate then the LEEA sensors. HEEA degradation Jan 2010 – Mar 2011

HEEA (ground g.f. only) vs LEEA (v6.0) CLUSTER-1 Density ratio MCP - operational MCP - lowered Thruster firingsMCP level raise

HEEA (ground g.f. only) vs LEEA (v6.0) CLUSTER-2 Density ratio MCP - lowered MCP - operational Thruster firingsMCP level raise

HEEA (ground g.f. only) vs LEEA (v6.0) CLUSTER-3 Density ratio MCP - operational CLUSTER-4MCP - operational Thruster firingsMCP level raise

HEEA alpha factors v6.0 Jan 2010 – Mar 2011 C1 C2 v6.0 alpha factors Thruster firings MCP level raise Alpha interpolated

HEEA alpha factors v6.0 Jan 2010 – Mar 2011 C3 C4 v6.0 alpha factors Thruster firings MCP level raise Alpha interpolated

3. Problems when determining HEEA alpha factors

HEEA saturation CLUSTER-1 Density ratio MCP - operational These intervals include times when we are in the sheath with MCP at op level and the HEEA’s are saturated as shown in the examples below (CAA flag Status_Counts_Stats as produced by Yulia Bogdanova – Note this flag applies relates to data compression in 3DR made onboard so does not apply to 3DX intervals).

CLUSTER-3 Density ratio MCP - operational Jumpy ratios CLUSTER-4 Ratios are very jumpy making it difficult to determine what the correct alpha is. Previous studies made when determining 2009 alphas have shown that this jumpyness appears to be instrument mode dependent. This requires more investigation. Filtering of modes may be required once it is better understood what the issue is. Initial investigation suggests this may be due to low counts resulting in bad statistics.

CLUSTER-3 Density ratio MCP - operational CLUSTER-4 For both SC3 and SC4 we see a jump in alpha in mid February On Feb 19 th 2011 we made changes to the routine operations of the instruments. Before this date we used single sensor (LEEA only) operations for the bow shocks (all SC) and powered off in the solar wind (SC1,3,4 only – SC2 would stay on with single sensor ops). From Feb 19 th we stopped using single sensor ops. For the bow shocks we use mode 76m16-68m08 (all SC) and for the solar wind 58h28-58h28 (SC1,3,4) 76m16-68m08 (SC2) More investigation into this is required. The jump is not seen in the plots for SC1 & 2 due to the raising/lowering of MCP level. Jump in alpha on 19 th February 2011

CLUSTER-1 Density ratio MCP - lowered MCP - operational CLUSTER-1 However from Feb 19 th 2011 we do begin to see disagreement between the alphas for lowered MCP level obtained directly from the ratios and those inferred from the alphas from operational MCP level The inferred alphas are higher than those given by the ratios. Disagreement in alpha for MCP lowered

CLUSTER-2 Density ratio MCP - lowered MCP - operational CLUSTER-2 We also see this disagreement for SC2 but here the inferred alphas for the lowered MCP level are lower than the alphas given by the ratios We suspect this to be due to instrument mode. In solar wind we use 58h28- 58h28 (MCPs operational) for SC1 and 76m16-68m08 (MCPs lowered) for SC2. Disagreement in alpha for MCP lowered

The first iteration of alpha factors for the HEEA sensors looks mostly good. The problem with instrument modes giving jumpy results has become an increasingly more difficult issue as time progresses, perhaps due to the low MCP gains. Other modes need to be filtered out of the study e.g. some solar wind BM ops where we do not return all azimuths and so give bad moments. One possibility is to use the now available Status_Counts_Stats flag to filter out bad intervals with saturation and low counts. Of more concern, although perhaps related to the above problem, is the issue of the jump in alpha (SC 3 & 4) and the disagreement in alpha between operational and lowered level (SC1 &2) after 19 th February This needs more investigation. Corrections may be required.

Thank-you to WHISPER and EFW for all the data they have provided to make this work possible. At present the required WHI and EFW data is available until end of Jun 2011 and Dec 2011 respectively. In order to make PEACE moments we also require CSDS AUX data. At the last meeting we reported that this was the main holdup of extending our calibrations beyond June 2010 as the data was not available. Since the last meeting nearly a years more data has been made available. At the end of March 2012 the availability of CSDS AUX data was until March In the last couple of weeks April and May 2011 have been made available and we will be extending our alphas for these two months after this meeting. Thank-you to Mariella at the CSDS Hungarian Data Centre for the AUX data provided in the last 6 months and we look forward to receiving more data for the rest of 2011 (and 2012) to continue alpha factor work. We also look forward to receiving more WHISPER data beyond June It would also be valuable for operational reasons (current and future operations) to have the most up to date alpha factors

THE END

APPENDIX

Calibrations involve two main steps: 1.MCP efficiency calibration (using comparisons with WHISPER/WBD) => corrected densities (n) 2. Inter-anode calibration => corrected velocities (V z ), and higher order moments improved Calibration: What is Involved?

Calibration Parameters for electron density The electron density, n, measured by PEACE is related to the calibration factors G and  : n  1 / (G x  ) The geometric factor, G = Go  o: - relates the incoming particle flux to the number of particles reaching the detector - Go = geometric factor determined by instrument geometry (already well known) -  o = energy-independent part of MCP efficiencies The alpha factor,  : function of gain and time - corrects for variations in detector sensitivity due to time varying MCP gain performance at the operational MCP voltage level - the gain of an MCP is the number of electrons that emerge out of the back of the MCP detector for each incident electron G and  also essential for other moments Usual method for obtaining the MCP gain and hence alpha is from weekly MCP tests. This method was used in calibrations v5.0 – v5.2 However on some sensors we can no longer use this method.

Sensor ageing MCP gain declines with use. Causes for reduced sensitivity are: - high electron fluxes (as seen in the sheath & wind) - thruster gases As sensitivity declines, greater voltages are required to get the same gain and thus same performance. Since Dec. 2003, PEACE MCP voltage levels are routinely lowered 1 or 2 levels below normal operational level when in the sheath/wind in order to protect the MCPs from high fluxes. -> Since November 2009 we no longer lower the MCPs on SC3 & SC4 as their gains are now very low PEACE MCPs have 31 voltage levels MCP THRESHOLD HISTORY Level

PEACE densities (ground calibration G. F. only, using 3DR and 3DX data) compared with CAA WHISPER ACTIVE densities for magnetosheath intervals (requires EFW potential for calculation of PEACE moments) ‘Bad’ intervals filtered out: partial coverage of PEACE energy range & possible errors in WHI PEACE/WHISPER density ratios used to produce new alpha factors for the LEEA sensors for lowered MCP voltage levels for sheath intervals Alpha factors for operational levels inferred from lowered level alpha factors using alpha vs. gain curve (from earlier years MCP test data) and gain vs. voltage curves (from ground tests). When gains are very low we can no longer use this method – then we have to use PEA/WHI density ratios for operational MCP voltage levels. Alpha factors for tail intervals determined using interpolation, taking into account MCP operational level changes or thruster firings that may affect alpha during this period. – Later we will cross- check with WBD/WHI tail densities if available. New alpha factors compared with previous alpha factors from weekly MCP tests New moments produced using new alpha factors and densities again compared with WHI densities New method for determining alpha – LEEA sensors A lot of checking performed. In several instances steps were repeated to improve the alpha factors. This method rests on the assumption that the WHISPER densities are reliable

Partial Coverage & Event Selection Good case: 10 Mar 2002 HEEA – 88m28 LEEA – 68m08 HEEA HEEA & LEEA LEEA Bad case: 10 Nov 2002 HEEA – 88m28 LEEA – 76m16 Potential CAA PEACE Moments use LEEA data in the energy overlap range. This is what we want when comparing with WHISPER as we are considering the LEEA MCPs only. For sheath/wind intervals there are very few counts at high energies measured by the HEEA sensor. Hence we can assume that the LEEA sensor ‘sees’ everything. Example of partial coverage (bad case): PEACE misses part of the plasma above the spacecraft potential, resulting in underestimated densities. Hence we do expect some PEACE/WHISPER ratios < 1 Later in mission we often have LEEA only, or HEEA IN MODE 66h36 => partial coverage This is an occasional problem – we filter these intervals out by manually checking spectrograms

Filtering out possible “bad” WHI points Compromised points filtered out using comparisons between EFW potential and WHI density. Assumption: data should follow a SC potential vs. density curve (c.f. A. Pedersen et al, J Geophys. Res., 113, A07S33, 2008) Points which do not fit this trend were removed. These ‘bad’ points can be due to error in either WHI density or EFW potential and hence the PEACE density (Note the EFW potential is required by PEACE to make moments). WHI density EFW spacecraft potential

PEACE HEEA only densities (ground calibration G.F. only) compared with PEACE LEEA only densities (new v6.0 alpha) for all intervals (i.e. not just PEA/WHI intervals) using only data from the energy overlap region HEEA/LEEA density ratios used to produce new alpha factors for the HEEA sensors for operational MCP voltage levels Alpha factors for lowered HEEA MCP levels either inferred from operational level alphas or using HEEA/LEEA density ratios when available New alpha factors compared with previous alpha factors from weekly MCP tests New moments produced using new alpha factors and densities again compared with LEEA densities in energy overlap region. Possible problem - low count rates result in incorrect ratios due to bad statistics. New method for determining alpha – HEEA sensors