MSSL * I. Rozum, A.N. Fazakerley, A.D. Lahiff, H. Bacai and C. Anekallu PEACE Calibration Status 8 th Cross Calibration Workshop, Kinsale, Ireland, 28 – 30 October 2008

Talk Outline Introduction In-flight PEACE calibrations Calibrations v5.1 & v5.2 On-board PEACE calibrations PEACE calibration issues (CL-3,4 LEEA) Cross Calibrations -- PEACE – WHISPER comparison studies -- PEACE – WBD comparison studies Conclusions MSSL

Introduction MSSL

Why Do We Need to Cross-calibrate PEACE Data? MSSL The plasma electron density can be determined with good knowledge from measurements of plasma frequencies (WHISPER and WIDEBAND can provide densities for a subset of the density range that PEACE can measure) The plasma electron density & plasma electron velocities can also be determined from PEACE measurements (if the full electron population is measured, spacecraft potential correction is made, non-plasma electrons are excluded and PEACE calibration is good) We use WHISPER & WIDEBAND densities and CODIF, HIA & EFW/FGM velocities as reference values in order to decide if PEACE plasma electron densities & velocities are reliable

V z Issue MSSL C1 PEACE C2 PEACE C3 PEACE C4 PEACE C4 CODIF Example: V z comparison Uncorrected PEACE V z are quite different from CODIF data NOTE: CODIF V z is not ideal for PEACE – CODIF V z comparison as CODIF have their own calibration issues and also CODIF does not work on all spacecraft. We use (V  ) z (defined as (ExB)/B 2 ) as an additional test.

PEACE moments are calculated by integrating electron phase space densities of a complete spin Example: velocities v = 1/n ∫ f(v) dV V x and V y are in the spin plane => integration involves adding fluxes measured by the same anodes for all spin phases V z is aligned with the spin axes => integration involves adding fluxes measured by different anodes. Equal fluxes measured by opposite anodes cancel each other and give 0 km/s. A calibration error in one anode and not the other can lead to a false contribution to the spin axis velocity sum V z Issue MSSL beam Good inter-anode calibration is essential to get accurate V z

V z Issue MSSL Example: (V  ) z of original PEACE HEEA data and CODIF data for 208 plasma- sheet intervals in HEEA data are based on ground calibrations done under conditions of high MCP gain, does not account for gain variations with time during the mission (V  ) z should centre on 0 km/s PEACECODIF

PEACE In-flight Calibrations MSSL

In general, calibrations involve three main steps: A. Inter-anode calibration => corrected velocities (V z ) B. Density calibration (comparison with WHISPER) => corrected (n) C. Inter-energy calibration => corrected PSD ratios (& n) MSSL Calibration: What is Involved?

MSSL Main parts: Step A: Inter-anode calibration => good V z – is done independently for each sensor – any sc separation can be used – gives correction factors for each energy & each anode & each sensor Step B: Density calibration => good n – is done independently for each sensor – any sc separation can be used – correction factors due to variable MCP gains are important (α-factors) – gives correction factors for each sensor which apply to all energies & all anodes Step C: Inter-energy calibration => good V z, n, PSD – small sc separations only – compare all sensors to a reference sensor – this step should be done only after density calibration has been done – gives correction factors for each energy which apply to all anodes for each sensor (except the reference sensor)

MSSL Calibration procedure: For each energy bin fit a simple function f(PA) to the PEACE differential energy flux (DEF) versus pitch angle (PA). We currently use a parabola centred on 90 o : f(PA) = A0 + A1*(PA 2 – 180*PA) A parabola can cope with isotropic plasmas, bi-directional beams and peaks at 90 o. For each energy bin we multiply the DEF from each anode by a correction factor, determined by fitting to the previously determined function f(PA). The fits for each anode, energy bin, sensor and spacecraft are completely independent. Correction factors are determined for each anode for energy bins which satisfy the following criteria: – the number of counts in every data record during the calibration event is above a minimum threshold (poor statistics can lead to unusual correction factors) – the energy bin is above the sc potential Inter-anode Calibration -each colour represents a different anode -the blue line is f(PA) -firstly fit f(PA) to the original PEACE DEF -then modify the DEF from each anode until all anodes agree well with f(PA)

Modifying VIDFs In the idfs all calibration information (geometric factors, energy efficiencies and energy levels) is stored in VIDFs – Virtual Instrument Definition Files Inter-anode calibration procedure is applied to a number of selected events where the anisotropy does not vary much => this gives a large amount of correction factors for each anode and different energy bins Knowing the MCP threshold, and hence gain and α-factors, for each daily VIDF we generate a set of correction factors for as many energy bins as possible The most frequently occurring correction factors (mode correction factors) are selected and combined using linear interpolation to form a surface This surface is then used to modify energy efficiencies tables in daily VIDFs MSSL

Calibration Release v5.1 & v5.2 MSSL

v 5.1 & v 5.2 As it was mentioned at the 7 th Cross-Calibration meeting, we discovered that some correction factors in v5.1 were generated for bins below the spacecraft potential This means that some correction factors were obtained from photo- electrons This error will be corrected in v5.2 v5.2 should give better results at low energies, eg. in the solar wind Our target is to release calibration v5.2 VIDFs in November 2008 MSSL

v 5.1 & v 5.2 Example: C solar wind MSSL PEACE v5.1 PEACE v5.2 HIA V z is significantly improved!

v 5.1 & v 5.2 Example: C solar wind MSSL PEACE v5.1 PEACE v5.2 HIA V z is significantly improved!

v 5.1 & v 5.2 Comparison between (V  ) z calculated using inter-anode calibrations v5.1 & v5.2 MSSL Y axis – a number of events Used selected plasma-sheet events: C1 2001/06 – 2006/01 C2 2001/06 – 2006/12 C3 2001/06 – 2005/07 C4 2001/06 – 2006/11 C3 HEEA data used are up to Jul 2005 as one C3 HEEA anode broke in Aug 2005 If the calibration is successful we expect tall narrow peaks centred at 0 km/s plasma-sheet

v 5.1 & v 5.2 PEACE – EFW (V  ) z comparison: results were calculated using inter-anode calibrations v5.1 & v5.2 MSSL Y axis – a number of events Used selected plasma-sheet events in 2001 – 2004, for which CAA EFW data are available If the calibration is successful we expect tall narrow peaks centred at 0 km/s Could CAA provide ExB V  data for years after 2004 please?

v 5.2 PEACE – CODIF (V  ) z comparison: results were calculated using inter-anode calibrations v5.2 MSSL Used selected plasma-sheet events in 2001 – 2004, for which CODIF data are available Peaks are slightly offset from 0 km/s, but still better than without any inter-anode calibrations Number of events

On-board PEACE Calibrations MSSL

On-board Calibrations MSSL PEACE on-board moments also have not perfect V z due to not perfect on-board calibrations The latest on-board calibrations v4 were up-linked on all spacecraft in October 2007, that resulted in significant improvements in V z, however on-board calibrations are still not perfect Inter-anode correction factors are different at different gains On-board calibrations v4 were created assuming that MCPs were working at the October 2007 operational level. There is not enough space in the DPU for additional calibrations to take into account lowered MCP levels. We performed intensive testing of on-board calibrations v4 by comparing PEACE on-board velocities with HIA PP data and found that when MCP levels are lowered, as in the magnetosheath, calibrations v4 do not work well

On-board Calibrations MSSL CL :20 – 15:20 Magnetosheath: MCPs lowered by 1 level red – real onboard moments green – HIA blue – 3DR, moments calculated on the ground using on-board calibrations Not very good agreement!

On-board Calibrations MSSL CL :40 – 14:00 Mid-altitude cusp: MCPs at operational level red – real onboard moments green – HIA blue – 3DR, moments calculated on the ground using on-board calibrations Good agreement!

On-board Calibrations MSSL CL :00 – 14:30 Magnetosheath: MCPs lowered by 1 level red – real onboard moments green – HIA blue – 3DR, moments calculated on the ground using on-board calibrations Not very good agreement!

PEACE Calibrations Issues: CL-3 & CL-4 LEEA MSSL Work in progress

PEACE Calibration Issues MSSL On some instruments we can no longer determine MCP thresholds, and therefore gains & α-factors, using the usual method because the MCP curve does not turn over CL-3 LEEA CL-4 LEEA The threshold is the MCP voltage at which the MCP is 50% effective. The gain is the estimated number of electrons produced by an incident electron. The α-factor accounts for variable MCP sensitivity. The geometric factor GF relates incoming particle flux to a flux reaching the detector.

PEACE Calibration Issues MSSL We have three methods for determining α-factors: Method 1. (“usual” method). MCP thresholds for both sensors can be determined from weekly MCP tests; α-factors are calculated using weekly MCP test data. This significantly improves densities for low gains. This method was used in calibrations v5.0 – v5.2 We have been experimenting with two methods for cases when MCP thresholds, and therefore gains, cannot be determined: Method 2. MCP threshold for one sensor cannot be determined, the monitor sensor is good Method 3. MCP thresholds for both sensors cannot be determined

PEACE Calibration Issues MSSL Assumption: MCP test counts ratio curve is proportional to the α-factor For the LEEA sensor: α LEEA = C R α HEEA N F C R = 3.75 * counts(LEEA) / counts(HEEA), 3.75 is GF, α HEEA is the HEEA α-factor corresponding to the MCP gain of the HEEA sensor during the MCP test, N F is a normalization factor. The counts ratio from all MCP tests are normalized such that at high MCP gains the curves agree with mathematically determined α-factor The curve is fitted to this data and used as the new α-factor Method 1: good MCP test data from both sensors The counts ratios for each sensor have been normalized to the alpha factor

PEACE Calibration Issues MSSL Assumption: MCP test counts ratio curve is proportional to the α-factor For the LEEA sensor (as in method 1, but α LEEA is unknown): α LEEA = C R α HEEA N F C R = 3.75 * counts(LEEA) / counts(HEEA), α HEEA is the HEEA α-factor corresponding to the MCP gain of the HEEA sensor during the test, N F is a normalization factor, it is a free parameter and can be varied to achieve good normalization. It is obtained as: 1. the counts ratio from every MCP test is multiplied by a free parameter so that it agrees with the previously determined α-factor for good gain 2. N F is the most frequently-occurring value Knowing N F we can determine the α-factor for a particular sensor from any MCP test from the counts ratio for the required MCP level Method 2: one sensor gives bad MCP test data

PEACE Calibration Issues MSSL C omparison between Red points – the “usual” method 1 Black points – method 2 using counts ratios directly from MCP tests (fit used) Method 2: one sensor gives bad MCP test data Method 2 gives different results that can be better than method 1 results at low MCP gains

PEACE Calibration Issues MSSL Uses data from a single sensor We can determine empirical relations between the inter-anode correction factors for each anode and each sensor and the MCP gain for situations when the MCP threshold is known, then use it to predict the MCP gain Procedure: 1. determine the relation: the lower the MCP gain, the further the correction factors are away from 1 2. use a least-square minimization to determine the MCP gain using (i) a set of mode correction factors for the time we are interested in and (ii) curves determined in step 1 Method 3: bad MCP test results from both sensors NOTE: Here “mode” is the most frequently-occurring correction factor value

PEACE Calibration Issues MSSL The relation: the lower the MCP gain, the further the correction factors are away from 1 Method 3: bad MCP test results from both sensors gain > 0.9 (“good”) 0.6 < gain < 0.9 gain < 0.6 (“poor”) gain in 10 6 electrons This example, for a particular energy (~1 keV) clearly shows that the inter-anode calibration changes as the MCP gain is reduced. The correction factors needed for “good” gain are of order only 5%, showing that the original ground calibration was quite good. The correction factors needed for “poor” gain are up to 20% in this example.

PEACE Calibration Issues MSSL Example: C4 LEEA MCP gain in cal v5.1 : 0.24 MCP gain determined using method 3 : 0.14 Agreement with WHISPER better but not perfect Method 3 Black - WHISPER CAA Red - v5.1 PEACE Green – method 3

PEACE Calibration Issues MSSL Manual tests on several events gave the following gains: C4 LEEA : actual gain = 0.42; predicted gain = 0.44 C4 LEEA : actual gain = 0.95; predicted gain = 1.02 C4 LEEA : actual gain = 1.28; predicted gain = 1.27 C4 LEEA : actual gain = 1.78; predicted gain = 1.66 C4 LEEA : actual gain = 0.66; predicted gain = 0.68 C4 LEEA : actual gain = 0.90; predicted gain = 0.93 Advantages: 1. Can use only one sensor 2. Does not use MCP test data Disadvantages: 1. Need to have calibration events for a wide variety of known MCP gains, including very low MCP gains 2. Minimization needs to be carried out carefully as one can get multiple minima 3. Tests showed that in many cases this method gives unreliable gains Method 3

PEACE Calibration Issues MSSL Summary In calibrations v5.2 and below we have not tried to solve the problem with C3 and C4 LEEA sensors, but we hope to do so in a further release We are investigating possibilities to use methods 2 & 3 for cases when MCP test data from one or both sensors are not good In many cases method 3 gives unreliable gains Cannot calibrate broken C3 HEEA anode without extra work, e.g. interpolation to fill in the data from the bad anode, before starting inter- anode calibration

PEACE – WHISPER Comparison Studies MSSL

PEACE – WHISPER Comparison MSSL These studies aim to show if our curve that shows sensitivity changes from lower to higher energies gives correct densities at high energies in the tail where E > 1000 eV; we know that our LEEA GF calibrations at sheath energies E < 1000 eV are good. Last year we received WHISPER data for 14 tail intervals in 2002 – 2003 (most of these events have only a few data points) This year we received dayside data for 2006 – 2007 from the WHISPER team (thank you WHISPER team for providing these data so quickly!) We found that in the majority of cases PEACE densities agree with WHISPER densities

PEACE – WHISPER Comparison MSSL Example: tail, good gain, low energy event 10 eV < E < 1300 eV Good agreement! dots – WHISPER dashed line – PEACE black – C1 red – C2 green – C3 blue – C4

PEACE – WHISPER Comparison MSSL Example: tail, good gain, low energy event 100 eV < E < 10 keV Good agreement! dots – WHISPER dashed line – PEACE black – C1 red – C2 green – C3 blue – C4

PEACE – WHISPER Comparison MSSL Example: tail, good gain, high energy event 70 eV < E < 2000 eV Good agreement! dots – WHISPER dashed line – PEACE black – C1 red – C2 green – C3 blue – C4

PEACE – WHISPER Comparison MSSL Example: tail, good gain, high energy event 30 eV < E < 1300 eV Good agreement! dots – WHISPER dashed line – PEACE black – C1 red – C2 green – C3 blue – C4

PEACE – WHISPER Comparison MSSL Example: CL dayside, solar wind, low energy event 20 eV < E < 110 eV Red – WHISPER Blue – 3DR LEEA What is wrong with WHISPER density? LEEA covers the full distribution – we expect to get good densities

PEACE – WHISPER Comparison MSSL Example: CL dayside, solar wind 20 eV < E < 100 eV Red – WHISPER Blue – 3DR LEEA LEEA covers the full distribution – we expect to get good densities Good agreement!

PEACE – WHISPER Comparison MSSL Example: CL dayside, solar wind 20 eV < E < 110 eV Red – WHISPER Blue – 3DR LEEA LEEA covers the full distribution – we expect to get good densities WHISPER densities fluctuate too much, but the agreement is reasonably good

PEACE – WHISPER Comparison MSSL Example: CL dayside, solar wind 20 eV < E < 110 eV Red – WHISPER Blue – 3DR LEEA LEEA covers the full distribution – we expect to get good densities WHISPER density fluctuates too much, but the agreement is reasonably good

PEACE – WHISPER Comparison MSSL Example: CL solar wind Red – WHISPER Blue – 3DR LEEA WHISPER density disagrees with PEACE density Calibration validity end PEACE calibration used on eV < E < 100 eV

PEACE – WHISPER Comparison MSSL Example: CL dayside, sheath Red – WHISPER Blue – 3DR LEEA WHISPER density disagrees with PEACE density Calibration validity end PEACE calibration used on eV < E < 100 eV

PEACE – WHISPER Comparison MSSL Summary PEACE densities are generally agree with WHISPER There are a few intervals where the agreement is poor We would like to have more -- WHISPER sheath density data for 2006, 2007 & 2008 as soon as possible please! We prefer not to deliver PEACE 2006 data until we have been able to use the WHI 2006 data to check that we have got our calibrations right

PEACE – WBD Comparison Studies MSSL

PEACE – WBD density comparison MSSL 7 th XCal meeting AI-3: -- We received wave frequency vs time data from the WBD team for a set of intervals, we calculated densities from these data and compared with PEACE densities -- We found that generally WBD densities agree well with PEACE densities in both magnitude and trend, but found 3 events where they disagree Ondrej Santolik (WBD team) visited MSSL; together we analyzed these intervals Why WBD? WBD does measurements from 0.1 kHz to 9.5 kHz => can determine densities lower than WHISPER – good for density comparison in the lobes and solar wind How? WBD measures low cut-off frequencies: f L ≈ f p – ½ f cl [kHz] WBD densities are calculated then as: n = (f p / q) 2 = (f p / 8.98) 2 [cm -2 ] where f p is a plasma frequency, f cl is a cyclotron frequency, q is an electric charge. Accuracy WBD densities are not reliable if the low cut-off frequency is not well defined

MSSL ● SPINPAD densities agree with WBD densities ● 3DR densities are consistent with the WBD densities (poor 3DR resolution removes most of the features) Good agreement! PEACE – WBD density comparison Clear cut-off – reliable WBD density CL-4 Lobes :25 – 11:34 E < 100 eV

MSSL ● 3DR densities are consistent with WBD densities Good agreement ! PEACE – WBD density comparison Clear cut-off – reliable WBD density CL-4 Plasma-sheet :25 – 09: eV < E < 2000 eV

PEACE – WBD density comparison MSSL CL-2 Lobes :06 – 10:20 E < 100 eV Vertical red lines - magnetic field from a search coil (STAFF) Not a good interval because: 1. cannot estimate cut-off f during red lines 2. intense low f emission 3. the cut-off is quite fuzzy in places WBD density is unreliable in this interval

MSSL CL-2 Plasma-sheet :25 – 09: eV – 2000 eV Not a good interval because: 1. cut-off is not sharp in some places 2. estimated density can be non-local 3. agree with 3DR HEEA 09:28 – 09:31 WBD density is unreliable outside of the above time interval PEACE – WBD density comparison

MSSL Not a good interval because: 1. cut-off is fuzzy WBD density cannot be estimated correctly CL-2 Plasma-sheet :33 – 07: eV < E < 2000 eV

PEACE – WBD density comparison MSSL Summary PEACE and WBD densities agree in the majority of cases (for low and high energies) AI-3: The intervals where PEACE and WBD densities disagree have no well- defined cut-off => WBD density cannot be estimated reliably for such events and should not be used for PEACE – WBD density comparison studies Ondrej Santolik agreed to deliver more WBD data for our comparison studies during his visit to MSSL, however, we did not set any delivery deadline We need more tail WBD data please! PEACE densities agree with WBD densities. We do not need to do any PEACE density correction at present.

PadSelection Quality Flags MSSL

PadSelection Quality Flags MSSL One has to be careful about interpreting pitch angle distributions if the magnetic field direction changes rapidly. PEACE pitch angle data products for the CAA will be "rebinned" pitch angle data, calculated at the time resolution of each individual spin sample. PadSelection quality flags code checks if the energy sweeps/polar zones that we transmit in PAD include the pitch angles nearest to the magnetic field direction or not (as it happens when the magnetic field rotates during the spin) PadSelection quality flag values (one flag every half of spin): 0 – PAD selection is correct 1 – PAD selection is incorrect

PadSelection Quality Flags MSSL top HEEA overlap LEEA overlap bottom CL :40 – 09:50 rapidly changing B CL :10 – 09:20 steady B

PadSelection Quality Flags MSSL CL :15 – 06:25CL :10 – 13:20 Harri’s events

PadSelection Quality Flags MSSL CL :10 – 06:20CL :40 – 18:50 Harri’s events

PadSelection Quality Flags MSSL CL :05 – 20:15CL :40 – 01:50 Harri’s events

Conclusions v5.1 codes have been adjusted to exclude counts from below the spacecraft potential We aim to release v5.2 in November 2008 Currently we are experimenting with 2 methods to determine α-factors when MCP gains for one or both sensors cannot be determined by usual techniques PEACE-WBD comparison events where WBD densities disagree with PEACE were analyzed; we found that all these events have unreliable WBD density (AI-3 is closed) WBD and WHISPER densities generally agree with PEACE densities: density corrections are not needed We need more WBD and WHISPER data for our calibration studies MSSL

65 Introduction to PEACE Each Cluster spacecraft carries two PEACE sensors, HEEA and LEEA Each sensor is a Top Hat electrostatic analyser with an MCP detector The anode beneath the MCP is divided into 12 equal parts which give 15º polar angle resolution Calibration Parameters Geometric Factor: –relates incoming particle flux to number of particles reaching the detector –function of analyser design and can be confirmed in ground calibration Detector Efficiency: –relates the number of particles reaching the detector that generate a countable signal –the response of the MCP to arriving electrons has an energy independent component and an energy dependent component –the response of the MCP can vary according to where on the MCP an incoming electron arrives –the response of the MCP can vary over time

PadSelection Quality Flags MSSL One has to be careful abut interpreting pitch angle distributions if the magnetic field direction changes rapidly. PEACE pitch angle data products for the CAA will be "rebinned" pitch angle data, calculated at the time resolution of each individual spin sample. PadSelection quality flags code checks if the energy sweeps/polar zones that we transmit in PAD include the pitch angles nearest to the magnetic field direction or not (as it happens when the magnetic field rotates during the spin) PadSelection quality flag values: 0 – PAD selection is correct 1 – PAD selection is incorrect These Flags are generated by taking the dot product of the four corners of the bin, in which magnetic field was, and the CFUNIT vectors corresponding to the time of PAD collection.

PadSelection Quality Flags MSSL CL :15 – 06:25CL :10 – 13:20 Harri’s events