14 th CAA Cross-Calibration Workshop, 5th – 7th October 2011, York, UK 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 Nov 2004-Dec 2009 corrected for thruster firings, year boundaries and other issues 2. Alpha factors for Jan – Jun 2010 determined for all LEEA sensors 3. Alpha factors for Jan – Jun 2010 determined for SC1 & SC2 HEEA sensors 4. Currently working on alpha factors for Jan – Jun 2010 for SC3 & 4 HEEA

1. Corrections to Nov 2004 – Dec 2009 alpha factors

Corrections to Nov 2004 – Dec 2009 Final corrections made to alpha factors for all sensors for - thruster firings - year boundary discontinuities - smoothing issues - ‘bumps’ In depth study of what the PEACE/WHISPER & PEACE LEEA/PEACE HEEA ratios are doing in problem regions instead of using weekly averages.

Calibration v6.0 alpha factorsLEEA Key: Corrected v6.0 alpha factors Previous v6.0 alpha factors MCP operational level changes Thruster firings Cluster-1LEEA Cluster-2LEEA Cluster-3LEEA Cluster-4LEEA Plots show corrections to v6.0 α-factors for LEEA sensors for Nov 2004 to Dec 2009

Calibration v6.0 alpha factorsHEEA Cluster-1HEEA Cluster-2HEEA Cluster-3HEEA Cluster-4HEEA Plots show corrections to v6.0 α-factors for HEEA sensors for Nov 2004 to Dec 2009 Key: Corrected v6.0 alpha factors Previous v6.0 alpha factors MCP operational level changes Thruster firings

Jan –Jun Alpha Factors LEEA sensors – All SC

Following 4 slides show the degradation for each LEEA sensor between Jan-Jun 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. The effect of raising the MCP operational levels is clearly seen. The lowered MCP levels was increased one week later. The effect of some of the thruster firings are also visible in these plots. LEEA degradation 2010

2010 LEEA (ground g.f. only) vs WHI density CLUSTER-1 CLUSTER-2 Density ratio MCP - lowered MCP - operational

2010 LEEA (ground g.f. only) vs WHI density CLUSTER-3 CLUSTER-4 Density ratio MCP - operational 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.

2010 Jan – Jun LEEA alpha factors v6.0 v6.0 alpha factorsThruster firingsMCP level raise C1C2 C3C4

2010 LEEA (v6.0 cal.) vs WHI density CLUSTER-1 CLUSTER-2 Density ratio MCP - lowered MCP - operational

2010 LEEA (v6.0 cal.) vs WHI density CLUSTER-3 CLUSTER-4 Density ratio MCP - operational

First iteration of alpha factors for the LEEA sensors looks very good Small corrections required for some sensors 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.

Jan –Jun Alpha Factors HEEA sensors – SC 1 & 2

Following 4 slides show the degradation for the HEEA sensors on SC1 & 2 in Jan- Jun 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 on 20 th Feb. The effect of raising the MCP operational level is clearly seen. The lowered MCP level was increased one week later. There was no MCP level raise for SC2 HEEA. The effect of some of the thruster firings are also visible in these plots. HEEA degradation 2010

2010 HEEA (ground g.f. only) vs LEEA (v6.0) CLUSTER-1 CLUSTER-2 Density ratio MCP - lowered MCP - operational

2010 HEEA (ground g.f. only) vs LEEA (v6.0) 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).

2010 Jan – Jun HEEA alpha factors v6.0 v6.0 alpha factorsThruster firingsMCP level raise C1C2

CLUSTER-1 CLUSTER-2 Density ratio MCP - lowered MCP - operational 2010 HEEA (v6.0) vs LEEA (v6.0)

First iteration of alpha factors for the SC1 & 2 HEEA sensors looks very good. Small corrections may be required.

Jan –Jun Alpha Factors HEEA sensors – SC 3 & 4

CLUSTER-3 Density ratio MCP - operational 2010 HEEA (ground g.f. only) vs LEEA (v6.0) 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 is instrument mode dependent.

CLUSTER-3 Density ratio MCP - operational 2010 HEEA (ground g.f. only) vs LEEA (v6.0) CLUSTER-4 Data from intervals where the mode is 88h58-76m16 (sheath) have been removed. In this mode the HEEA sensor is looking at high energies above the sheath, so there are very few counts in the overlap region as shown in the example below (CAA flag Status_Counts_Stats as produced by Yulia Bogdanova) resulting in bad statistics

This 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. Further detailed investigation of all instrument modes used is required. One possibility is to use the now available Status_Counts_Stats flag to filter out bad intervals with saturation and low counts.

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 March 2011 However in order to make PEACE moments we require CSDS AUX data. This is only available until the end of October 2010 (some days in Nov 2010) Therefore we are only able to produce alphas until end Jun (NB Jul-Oct alphas are an interpolation between two dayside seasons). We look forward to receiving AUX data for the rest of 2010 (and some 2011) to continue alpha factor work. It would also be valuable for operational reasons (current and future operations) to have the most up to date alpha factors We plan to do some special commanding with X-cal in mind in the upcoming dayside

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