10 th CAA Cross-calibration meeting, 2nd – 4th November 2009, Paris N. Doss, A. Fazakerley, B. Mihaljčić and I. Rozum UCL Department of Space and Climate Physics Mullard Space Science Laboratory Many thanks to WHISPER, WIDEBAND and EFW teams for their support! PEACE Cross-calibration comparison studies

Outline PEACE – WHISPER comparison studies PEACE – WBD comparison studies PEACE – EFW comparison studies

PEACE – WHISPER

At the previous X-Cal meeting (9) we presented PEACE – WHISPER density comparisons for a set of specific events in 2006 and 2007 for which the WHISPER team provided us with data Summary of PEACE-WHISPER comparisons C1: Generally good agreement C2: Generally good agreement C3: After Feb 2006, we found surprisingly poor agreement in the period prior to the expected calibration validity cut-off (April 2007). C4: Generally bad agreement, just as expected, since the events studied are after the expected calibration validity cut-off (May 2006)

2006 Nov UT C3 Event 9 Disagreement shows degrading C3- LEEA calibration Before our validity cutoff, 10 Apr 2007, showing a problem we didn’t know about Extract from Xcal-9 PEACE presentation

2006 Dec UT C3 Event 10 Disagreement shows degrading C3- LEEA calibration Before our validity cutoff, 10 Apr 2007, showing a problem we didn’t know about Extract from Xcal-9 PEACE presentation

2006 Dec UT C3 Disagreement shows degrading C3- LEEA calibration Before our validity cutoff, 10 Apr 2007, showing a problem we didn’t know about Event 11 Extract from Xcal-9 PEACE presentation

2006 Nov UT C4 Event 8 Disagreement as expected due to poor C4-LEEA calibration Extract from Xcal-9 PEACE presentation

2006 Dec UT C4 Disagreement as expected due to poor C4-LEEA calibration Event 10 Extract from Xcal-9 PEACE presentation

2007 Nov UT C4 Disagreement as expected due to poor C4-LEEA calibration Event 12 Extract from Xcal-9 PEACE presentation

The reason we require density data after 2005 is to help us extend our time history of MCP sensitivity on all sensors, especially C3-LEEA and C4-LEEA. C3-LEEA Poor curve C1-HEEA Good curve Sensitivity history - Problems here Sensitivity-related curves for C1,2,3,4 LEEA

Since the last meeting... We have compared CAA PEACE Moments densities with CAA WHISPER ACTIVE densities for all magnetosheath intervals (where there is data from both instruments) for: - 01 December 2005 – 31 May December 2006 – 31 May 2007 We expect to see worsening agreement after (and possibly slightly before as suggested by previous comparison studies) the expected calibration validity cut-off dates for Cluster-3 and Cluster-4.

2006/01/30 (09:15) – 2006/01/31 (16:00) On all spacecraft PEACE misses part of the plasma above the spacecraft potential, resulting in underestimated densities Hence we do expect some PEACE/WHISPER ratios < 1 In future we will be able to filter out these events when we make comparisons. (Status Partial Coverage)

Cluster 1

Magnetosheath intervals 2005/12/01 – 2006/06/01 Pretty good agreement, but we are beginning to see a slight decrease in the ratio with time. Cluster-1 expected calibration validity cut-off: July 2008

Magnetosheath intervals 2006/12/01 – 2007/06/01 Pretty good agreement. Ratio slightly decreasing with time. Cluster-1 expected calibration validity cut-off: July 2008

Cluster 2

Magnetosheath intervals 2005/12/01 – 2006/06/01 Generally good agreement Cluster-2 expected calibration validity cut-off: July 2008

Magnetosheath intervals 2006/12/01 – 2007/06/01 Generally good agreement Cluster-2 expected calibration validity cut-off: July 2008

Cluster 3

Magnetosheath intervals 2005/12/01 – 2006/06/01 Generally good agreement Cluster-3 expected calibration validity cut-off: April 2007 (LEEA)

Magnetosheath intervals 2006/12/01 – 2007/06/01 As expected, we see a poor agreement after the expected calibration validity cut- off date In fact the PEACE/WHISPER density ratio begins to decrease before the cut-off Cluster-3 expected calibration validity cut- off: April 2007 (LEEA)

Cluster 4

Magnetosheath intervals 2005/12/01 – 2006/06/01 This period is before the validity cut-off date but we are already seeing poor agreement Cluster-4 expected calibration validity cut-off: May 2006 (LEEA)

Magnetosheath intervals 2006/12/01 – 2007/06/01 Very poor agreement for this period. We are 6 months+ past the validity cut-off date Cluster-4 expected calibration validity cut-off: May 2006 (LEEA)

Summary of PEACE-WHISPER comparisons C1: Generally good agreement for 2006 and 2007 but we are seeing the PEACE/WHISPER density ratio slightly decrease with time C2: Generally good agreement for 2006 and 2007 C3: Generally good agreement for Poor agreement after the expected calibration validity cut-off date, as expected. However surprisingly we begin to see a decline in the PEACE/WHISPER density ratio from early 2007, a few months prior to the cut-off (April 2007). C4: Bad agreement in for 2007, just as expected, since this is after the expected calibration validity cut-off (May 2006). However we surprisingly see poor agreement also in the 6 months prior to the cut- off

What next? - We will soon start working on a correction method for C3 and C4 - We will need to filter out cases where PEACE moments are compromised and assume that the WHISPER densities are reliable - We would like to see similar WHISPER data for 2008 and 2009 soon if possible

WHISPER density from CAA vs. WHISPER density from examples circulated by Gabor by The intervals are from the years 2006 and Data provided by Gabor typically has more data points than the CAA files (it is said in CAA doc’s that any unreliable values have been filtered out) so only those data points in Gabor’s files for which there is a data point at the same time in the CAA files have been taken. The CAA quality parameter is shown in our plots. It ranges from 0 for bad to 1 for best quality. However, this refers to the strength of the signature – unreliable data is not in the CAA files according to the User Guide documentation. Quality -1 means EFW potential data together with active mode sounding is used. We also plotted the ratio of (density in received data from Gabor) to (density from CAA files) the difference of (density in received data from Gabor) to (density from CAA files) the “uncertainty” parameter from the CAA files (which is in density units)‏ the “quality” parameter from the CAA files Full set of investigated intervals sent to WHISPER team on June 26 and July 31

Conclusions: In most cases the active mode densities delivered by Gabor are slightly larger than the active mode densities in the CAA, but usually agree to within 4% or less. Ratios vary from up to 5% for C1/C3 on 2007-Jan-08, to as small as 1% on C Dec-31). For natural mode with a quality of 0-1, in most cases the data in the CAA is identical to that provided by Gabor. For natural mode with quality of -1, in 11 cases the data does not agree well. In 4 cases, C Mar-17, C Dec-05, C1 & C Dec-27 and C Dec-21, agreement is good. The CAA User Guide tells us that quality = -1 means that density was estimated using EFW potential data rescaled using density from Active mode WHISPER results. Future work We may plot active and natural together for files from the two sources. We may add densities from PEACE/CIS

Quality 0 to 1

CL CL3

CL CL3

Quality -1 – Good agreement

CL CL3

Quality -1 – Natural data comparisons show scatter

Ratio +/- ~5% CL CL3

Ratio +/- ~10% CL CL3

PEACE – WBD

WHISPER and PEACE density measurements have provided comparisons for calibration in the 10 eV to 1 KeV energy range, covering the magnetosheath and solar wind regions. We are interested in WBD – PEACE comparisons, as WBD has a larger frequency range than WHISPER and so can resolve lower electron densities. This is very useful in the magnetotail plasma sheet where typical electron energies are in the 1 keV to 10 keV energy range. For useful comparisons we require: - intervals where PEACE covers the entire energy spectrum - that energy spectrum does not lie above the max. energy measured by PEACE Where possible we have also compared with CIS-HIA and CIS-CODIF densities downloaded from the UK Cluster Data Centre for illustration purposes (we note that CIS is not always expected to provide accurate plasma density in these conditions).

We receive wave frequency from the WBD team for a set of intervals. We convert these into densities and compare with PEACE densities. How? WBD measures low cut-off frequencies f L The plasma frequency is given by where f c is the cyclotron frequency determined using the magnetic field vector B from FGM The density is then given by cm -3 WBD can provide high accuracy, high time resolution measurements of the electron density. WBD densities are not reliable if the low cut-off frequency is not well defined

We have been supplied with WBD data for the following dates: (Courtesy of Ondřej Santolík) 29 C4_2007_06_28_0748_ C4_2007_06_28_0809_ C4_2007_06_28_0831_ C4_2007_06_28_0833_ C4_2007_08_12_0941_ C4_2007_08_12_0946_ C4_2007_08_12_0948_ C4_2007_08_12_1124_ C4_2007_08_12_1125_ C4_2007_08_12_1132_ C4_2007_10_29_1856_ C4_2007_10_29_1857_ C4_2007_10_29_1858_ C4_2007_10_29_1901_ C4_2007_10_29_1902_ C4_2007_10_29_1906_ C4_2007_10_29_2008_ C4_2007_10_29_2117_ C4_2007_10_29_2155_ C4_2007_10_29_2157_ C4_2007_11_22_1939_ C3_2007_09_05_0726_ C3_2007_09_05_0728_ C3_2007_09_05_0729_ C2_2007_09_05_0728_ C2_2007_09_05_0729_ C2_2007_09_05_0730_ C2_2007_09_05_0737_ C2_2007_11_27_1306_ C2_2007_11_27_1310_ C1_2007_08_12_0956_ C1_2007_08_12_1002_ C1_2007_08_12_1003_ C1_2007_08_12_1014_ C1_2007_08_12_1121_ C1_2007_08_12_1123_ C1_2007_08_12_1125_ C1_2007_08_12_1126_ C1_2007_09_05_0724_ C1_2007_09_05_0725_ C1_2007_09_05_0726_ C1_2007_09_05_0727_ C1_2007_09_05_0728_ C1_2007_09_05_0729_ C1_2007_09_05_0730_ C1_2007_09_05_0734_ C1_2007_09_05_0737_ C1_2007_09_05_0740_ C1_2007_11_27_1259_1300 Events supplied by Ondrej Santolik (16-Mar-2009) The colouring in each column group together particular spacecraft events which happened during the same date and similar times.

Cluster-1 Cluster-4

CL1: :21 – 11:23 88m28-66h36-cplr Excellent agreement between WBD and 3DR HEEA HEEA sees all the plasma and should give correct total density

CL1: :25 – 11:26 88m28-66h36-cplr Very good agreement between WBD and 3DR HEEA HEEA sees all the plasma and should give correct total density

CL4: :25 – 11:26 88m28-66h36-cplr Excellent agreement between WBD and 3DR HEEA HEEA sees all the plasma and should give correct total density

CL4: :32 – 11:33 88m28-66h36-cplr Good agreement between WBD and 3DR HEEA HEEA sees all the plasma and should give correct total density

CL1: :02 – 10:03 88m28-66h36-cplr Poor agreement between WBD and 3DR HEEA (>30% error) HEEA sees all the plasma and should give correct total density

CL1: :03 – 10:05 88m28-66h36-cplr Poor agreement between WBD and 3DR HEEA (>30% error) HEEA sees all the plasma and should give correct total density

CL4: :46 – 09:48 88m28-66h36-cplr Poor agreement between WBD and 3DR HEEA (20% error) HEEA sees all the plasma and should give correct total density

Cluster-1 Cluster-2 Cluster-3

CL1: :24 – 07:25 88m28-66h36-cplr Moderate agreement between WBD and 3DR HEEA (15% error) HEEA sees all the plasma and should give correct total density

CL1: :27 – 07:28 88m28-66h36-cplr Moderate agreement between WBD and 3DR HEEA (15% error) HEEA sees all the plasma and should give correct total density

CL1: :37 – 07:38 88m28-66h36-cplr Good agreement with 3DR HEEA HEEA sees all the plasma and should give correct total density

CL2: :30 – 07:31 88m28-66h36-cplr Moderate agreement between WBD and 3DR HEEA (15% error) HEEA sees all the plasma and should give correct total density

CL2: :37 – 07:38 88m28-66h36-cplr Moderate agreement between WBD and 3DR HEEA (10% error) HEEA sees all the plasma and should give correct total density

CL3: :26 – 07:27 66h36-88m28-cplr Good agreement: between WBD and 3DR TH1H2B (10% error) Here the LEEA sensor is measuring the high energy part of the spectrum. We may be underestimating the density as this event is past the expected calibration validity cut-off date for the LEEA sensor - April 2007 (HEEA – June 2008) We use the data from HEEA in the overlap region of the energy spectrum.

CL3: :29 – 07:30 66h36-88m28-cplr Moderate agreement between WBD and 3DR TL1L2B (15% error) Here the LEEA sensor is measuring the high energy part of the spectrum. We may be underestimating the density as this event is past the expected calibration validity cut-off date for the LEEA sensor - April 2007 (HEEA – June 2008) We use the data from HEEA in the overlap region of the energy spectrum.

Cluster-1 Cluster-2

CL1: :59 – 13:00 88h58-76m16-cplr Poor agreement between WBD and 3DR TH1H2B Care was taken to eliminate spacecraft photo-electrons from the moments determination

CL2: :06 – 13:07 88h58-76m16-cprl Good agreement between WBD and 3DR TH1H2B Care was taken to eliminate spacecraft photo-electrons from the moments determination

CL2: :10 – 13:11 88h58-76m16-cprl Good agreement between WBD and 3DR TH1H2B Care was taken to eliminate spacecraft photo-electrons from the moments determination

Cluster-4

CL4: :09 – 08:10 88h58-76m16-cplr Poor agreement between WBD and 3DR TH1H2B (40% error) We may be underestimating the density as this event is past the expected calibration validity cut-off for the LEEA sensor – May 2006 (HEEA – June 2008) In the overlap region we use data from the HEEA sensor Care was taken to eliminate spacecraft photo-electrons from the moments determination

CL4: :33 – 08:34 88h58-76m16-cplr Poor agreement between WBD and 3DR TH1H2B (25% error) We may be underestimating the density as this event is past the expected calibration validity cut-off for the LEEA sensor – May 2006 (HEEA – June 2008) In the overlap region we use data from the HEEA sensor Care was taken to eliminate spacecraft photo-electrons from the moments determination

Cluster-4

CL4: :58 – 19:00 88m28-66h36-cplr Poor agreement between WBD and 3DR HEEA (30% error) HEEA covers a larger energy range than the LEEA sensor so we use 3DR HEEA only (HEEA expected calibration validity cut-off – June 2008) Saturation of HEEA sensor? Is the low cut-off frequency well defined by WBD? Can low frequency emissions be affecting the WBD density?

CL4: :02 – 19:03 88m28-66h36-cplr Poor agreement between WBD and 3DR HEEA (30% error) HEEA covers a larger energy range than the LEEA sensor so we use 3DR HEEA only (HEEA expected calibration validity cut-off – June 2008) Saturation of HEEA sensor? Is the low cut-off frequency well defined by WBD?

CL4: :08 – 20:09 88m28-66h36-cplr Good agreement between WBD and 3DR HEEA

Summary of PEACE-WBD comparisons - We grouped together different spacecraft on the same date and time - Even for the same spacecraft and same day, there is a large variation in the agreement -Few events have excellent agreement -The majority has moderate agreement with errors in the range %

Summary - Good cases - excellent agreement for C1 and C4 on around 11:20-11:33 - very good agreement for C Bad cases (problem with PEACE?) - On C4 the estimated calibration validity cut-off for the LEEA sensor is May Therefore in some cases we may be underestimating the density e.g On C3 the estimated calibration validity cut-off for the LEEA sensor is April Again in some cases we may be underestimating the density e.g Bad cases (problem with WBD?) - In some of the WBD spectrograms it appears that the low cut-off frequency may not be well defined or inaccurate due to low frequency emissions e.g ,

PEACE – EFW

Spacecraft Potential Estimation from PEACE spectra A diagram of the secondary e - populations which might be encountered at various times by the instruments. Energy bins CR 0 / CR 1 < 1 CR 1 / CR 2 ~ 1 CR 7 / CR 8 ~ 1 CR 6 / CR 7 << 1 … CR 4 / CR 5 > 1 … E1E1 E2E2 E3E3 E4E4 E5E5 E6E6 E7E7 E8E8 E0E0 … photo-electrons The spacecraft potential will be a minimum in the energy spectrum Based on the min of ratios of electron count rates for consecutive energy bins Accuracy determined by energy resolution of measurements => EFW results will always be better!

This method gives sensible looking spacecraft potential estimates in most cases The EFW probe potential is 1-2 eV lower than the spacecraft potential from PEACE spectra; the spacecraft potential calculated from the EFW probe potential should match with the one from PEACE EFW (thin line), from PEACE (thick line)‏ Energy (eV)‏ CR ratio Example: CL Lowest lying minimum should give the spacecraft potential

S/C potentials for 2004 Cluster Spacecraft Potential Potential inferred from PEACE electron spectra vs CSDS EFW potential Produced by Andrew Lahiff in 2005 (tbc) Extract from Xcal-9 PEACE presentation

S/C potentials for 2005 Cluster Spacecraft Potential Potential inferred from PEACE electron spectra vs CSDS EFW potential Produced by Andrew Lahiff in 2005 (tbc) Extract from Xcal-9 PEACE presentation

Spacecraft Potential These results puzzled us at the time as they conflicted with EFW team advice that the spacecraft potential was given by : Vsc = “probe-spacecraft potential” + 1V Recent paper by Cully et al., leads to a new expression: Vsc = 1.25 x “probe-spacecraft potential” + 0.9V Extract from Xcal-9 PEACE presentation

We have compared PEACE spacecraft potential (inferred from PEACE electron spectra) with EFW potential for for all spacecraft. We present scatter plots of PEACE vs EFW potential to see if it follows the new expression given by Cully et al.

2001

2002

2003

2004

2005

Summary of PEACE – EFW comparisons New expression: Vsc = 1.25 x “probe-spacecraft potential” + 0.9V is much more consistent with our findings.

Thanks to the WHISPER, WBD and EFW teams.