In-flight calibrati n of Hot Ion Analyser onboard Cluster

Introduction Conclusions References


Introduction
The Hot Ion Analyser (HIA) and the COmposition and DIstribution Function (CODIF) analyzer are the two sensors of the Cluster Ion Spectrometry (CIS) experiment (Rème et al., 2001) onboard Cluster, having the objective to measure the three-dimensional velocity distributions of ions.As a major difference from CODIF, the HIA instrument does not provide mass resolution; however, HIA offers other important advantages, like higher detection efficiency, better angular and energy resolution, faster electronics capable to handle higher count rates etc.
HIA detection system is based on micro-channel plate (MCP) technology.The instrument efficiency has been determined on ground through extensive pre-flight calibrations.However, due to various reasons, like MCP gain fatigue and aging (Prince and Cross, 1971) or because of the penetrating radiation (in the radiation belts or from cosmic ray bombardment), the detector efficiency changes in the course of the mission, requiring periodic in-flight calibration.An in-flight calibration is needed as well whenever the HIA operating point is changed by commands from ground.The multi-point Introduction

Conclusions References
Tables Figures

Back Close
Full character of Cluster and its complex payload made possible to asses the HIA in-flight performance at an unprecedented level of accuracy.
The calibration methodology to be presented in this paper was developed by taking advantage of the large cross-calibration effort carried on in the framework of ESA's Cluster Active Archive (CAA) program.Before the CAA initiative, the resources allocated to this activity were relatively small when compared with the complexity of the work; also, the less accurate Cluster Prime Parameter data set has been used as a reference for the total electron density (Vallat, 2001).
The CIS experiment is prepared by an international consortium, under the principal responsibility of Institut de Recherche en Astrophysique et Planétologie (IRAP) in Toulouse (formerly Centre d'Etude Spatiale des Rayonnements).Since 2009, the Institute for Space Sciences in Bucharest assumed a key role in HIA in-flight calibration, in close collaboration with IRAP.
The CIS data sets available through the CAA interface are described in Dandouras et al. (2010).An updated report on the CIS calibration activities can be found on the CAA web page (current version: 1.3, see Dandouras et al., 2012).
The paper is organized as follows: in Sect. 2 the instrument and its specific parameters are presented.Section 3 discusses the HIA operation modes and data caveats.Section 4 provides details on the calibration tasks and illustrates the calibration methodology by two examples.The next section presents two statistical studies carried out for validating the HIA in-flight calibration.In Sect.6 the results of HIA inflight calibration are summarized and plans to extend this activity are discussed.

Instrument presentation
Figure 1 presents the HIA operational principles.The instrument employs a 360 • angle imaging "top-hat" (Carlson et al., 1982) toroidal electrostatic analyzer (EA) and a fast detection system, based on MCP electron multipliers.The ions moving along different directions in the plane of the instrument entrance aperture (different polar angle in the Introduction

Conclusions References
Tables Figures

Back Close
Full upper part of Fig. 1) are deflected and focused by the EA (middle part) on the exit plane, where are recorded by a system of position encoding discrete anodes (lower part).Ion energies from 5 eV to 32 keV are sequentially measured by rapidly varying (sweeping), in logarithmically spaced steps, the voltage across the hemispherical EA plates.In the detection plane, the MCP plates are arranged in chevron pair configuration in order to achieve higher gain of secondary electrons emission.For a better detection efficiency, the ions are post-accelerated by a ∼ 2300 − 2500 V potential applied between the front of the MCP and a high-transparency grid located ∼ 1 mm above.The MCP gain can be checked by occasionally stepping this high voltage and by adjusting the discrimination level of the collecting charge amplifiers.Coverage in azimuthal angle is achieved by using the satellite spin.
To accommodate the large dynamic range of ion fluxes that occur in different regions sampled by Cluster, the entrance aperture consists of two narrow fans covering each 180 • in polar angle and having sensitivities that differ by a factor of ∼ 25.The high sensitivity (HS or "high G") section (entrance aperture on the right in the upper part of Fig. 1), selects ions with the appropriate energy per charge (E/Q) and concentrates them on 16 anodes, 11.25 • each, located in the exit plane (on the left in the bottom part of the figure).This section is designed for analyzing magnetospheric ions.Similarly, the low sensitivity (LS or "low g") section (entrance aperture on the left in the upper part of of ion species.On the other hand, for the HIA detector the efficiency is much larger, primarily because this sensor has no TOF section.In addition, HIA provides a higher angular resolution (up to 5.6 • × 5.6 • , to be compared with 11.25 • × 22.5 • for CODIF) and faster electronics, capable to handle higher count rates.This makes HIA more suitable for the study of the solar wind environment.Table 1 summarizes the HIA specific parameters.

HIA operation mode and data caveats
There are 16 operating modes for the CIS instrument, that can be roughly grouped in 2 classes, i.e. "magnetospheric" and "solar wind" modes.In the "magnetospheric" modes the full energy-angle ranges are covered and the different data products are based on the counts accumulated on the "high G" section.In the "solar wind" modes the plasma moments are based on data accumulated on the "low g" section when this side is facing the solar wind direction.In each mode, HIA and CODIF share the telemetry bit-rate allocated to CIS for transmitting scientific products (on-board computed moments, one-, two-and three-dimensional distributions and pitch-angle distributions) to the ground.
The HIA instrument involves extensive on-board data processing, including the computation of the moments of the velocity distribution functions (density, bulk velocity vector, pressure tensor, and heat flux vector).The moments are transmitted to the ground every spin period, i.e. about 4 s.The computation uses a table of efficiency coefficients (values dependent on the energy and angular sector θ) based on the ground calibration performed before launch.Assuming the same energy dependence and symmetric anode efficiency evolution in time, the values of these moments are periodically adjusted on ground through the so called absolute calibration (see Sect. 4).
The transmission to the ground of the complete 3-D distribution function (i.e. at full angular and energy resolution) is not possible due to the limited telemetry rates allocated to the CIS experiment.For example, the nominal operation of HIA HS section Introduction

Conclusions References
Tables Figures

Back Close
Full would require the transmission, every 4 s, of a matrix having 62 (or 31) energy channels × 16 elevation angles × 32 azimuth angle = 31.744(or 15.872) elements.Therefore a reduced distribution function (particle counts typically binned in 31 energy channels and 88 angular directions) is computed on-board and transmitted to the ground with a time resolution of multiple spin periods.Based on the reduced distribution function, the so called "ground" plasma moments can be computed, where in principle correction for the efficiency energy dependence and asymmetric anode efficiency evolution can be made.Since in the solar wind spectrogram, the He ++ trace is clearly separated, appearing as an ion beam at roughly twice the mean proton energy, HIA is providing the plasma moments for this ion species as well.So far this data product has not been calibrated.
There are a number of data caveats of particular importance for the instrument calibration as well as for regular exploitation of HIA data.These aspects, briefly summarized below, are closely checked when selecting the calibration intervals.For a detailed discussion the reader is referred to the instrument web-page http://cluster.irap.omp.eu/(see also Rème et al., 2001;Dandouras and Barthe, 2012).
-The accuracy of computed moments is affected by the instrument finite energy and angle resolution, and by its finite energy range.Also, a reliable plasma moment computation requires that enough counts (minimum 100) are accumulated over the spin period.
-Inappropriate operational mode adversely affects the data accuracy.For example, when HIA is in solar wind mode while the measurements are taken in the magnetosphere, a large portion of the ion distribution is excluded.Similarly, when HIA is in the magnetospheric mode but measures in the solar wind, detector saturation may occur, leading to underestimated values for the plasma density.
-Due to the penetrating particles from the radiation belts, HIA measures a high background around perigee passes.Similar effect may occur also during some intense solar particle events.Introduction

Conclusions References
Tables Figures

Back Close
Full -The detection of low-energy ions may be affected by the spacecraft charging to a positive floating potential that repels these ions.
-On some occasions, instrument artifacts (wrong time-tagging, sudden density drops, high-voltage discharges, wrong discriminator levels etc) may occur.These events are listed on the CIS Data Caveats list, available on the instrument webpage.
Since the beginning of the mission, the HIA sensors were operational only on Cluster 1 (C1) and C3.Since November 2009, after almost 10 yr of very good performance, the CIS experiment onboard C3 is not operational any more.Also, after June 2011 the HIA operations on C1 are restricted to magnetospheric modes only, with the instrument switched into a safe stand-by mode when in the solar wind.

The HIA in-flight calibration
The HIA detection efficiency as a function of position (polar angle θ) and particle energy E is given by the formula (see Bosqued, 2000, reporting on the HIA ground calibration) The first part of the RHS describes the position (anode) dependent efficiency, with Norm_θ designating the anode normalization coefficients (one for each sensitivity side) and Cheff(θ) the relative anode dependent efficiency coefficients.The second part of RHS describes the efficiency energy dependence, with A, B, T 0,1,2 being the calibration coefficients and E t = E + E g the total energy (sum of the particle energy E and the MCP-grid acceleration energy E g ) employed for describing the MCP energy-dependent efficiency.In total there are 2 + 2 • 16 + 2 • 2 + 3 = 41 (39 independent) efficiency calibration coefficients for each validity period and spacecraft.Their values are specified in the calibration files that are constantly provided as the mission progresses.Introduction

Conclusions References
Tables Figures

Back Close
Full The efficiency coefficients of the HIA instruments have been determined on ground through extensive pre-flight calibrations at IRAP vacuum test facilities in Toulouse.Using ion beams of energies from a few tens of eV up to 30 keV, detailed studies of MCPs gain levels, MCP matching, and angular-energy resolution for each sector (each θ) were performed.Based on these tests, a table of efficiency coefficients is stored onboard in the non-volatile memory and used by the processing software to compute the on-board moments from the full angular and energy resolution 3-D ion distribution function.
However, the detector efficiencies change with time due to various reasons presented in Sect. 1.Therefore, as the missions progresses, the on-board calculation are based on out-of-date/in-accurate efficiencies.These unavoidable change with time of the channel-plate detectors require continuous in-flight calibration.Also, the MCP high voltage is periodically increased by ground commands, to compensate for the MCP gain fatigue.Since the procedure has a direct impact on detector efficiency, an in-flight calibration is subsequently required.So far, this operation has been performed five times (see Fig. 8).
The standard procedure to calibrate in-flight the HIA instrument, called the absolute calibration, relies on comparing HIA ion number density with the electron number density provided by the WHISPER experiment onboard Cluster (Décréau et al., 2001).While HIA detects individual particles to measure the ion distribution function, WHIS-PER is based on a different method to determine the plasma density, i.e. by analyzing, both actively and passively, the electric signals in the neighboring plasma.In active mode, WHISPER measures the total electron density, while in passive mode it provides a survey of natural emissions from about 2 to 80 kHz, that covers the electron plasma resonance frequency.
There are a number of assumptions involved in the absolute calibration procedure, as follows: Introduction

Conclusions References
Tables Figures

Back Close
Full there is a symmetric anode-dependent efficiency evolution with time, and therefore the relative anode-dependent efficiency coefficients Cheff(θ), determined in the pre-flight tests, have not changed.
the coefficients A, B, T 0,1,2 , describing the efficiency energy dependence do not change as well and assume the values determined in the pre-flight tests.
the WHISPER data are well calibrated and free of errors (at least in the statistical sense).The traces in the WHISPER spectrograms are correctly assigned to some characteristic frequencies in plasma, from where electron density can be inferred.In these circumstances, the WHISPER data can be taken as reference.
The first two assumptions greatly simplify the calibration task, basically implying that only two coefficients (one for each sensitivity side) are needed to correct the HIA efficiency.It also means that the on-board moments are accurate up to a multiplication factor determined through calibration, allowing thus to take advantage of the HIA highest temporal, directional and energy resolution.Indeed, e.g. in magnetospheric modes the on-board moments are computed every spin period based on uncompressed data accumulated in 32 energy channels and 16 elevation × 32 azimuth solid angles (Di Lelis and Formisano, 2000), whereas the ground-computed moments are based on the reduced distribution function transmitted to the ground, having typically 31 energy × 88 solid angle bins and poorer time resolution.duced by the on-board processing software in order to comply with the limited capacity of the telemetry; in that process the counts registered by individual anodes are binned in 8 angular sectors.Regarding the second assumption, i.e. constancy of the coefficients describing the efficiency energy dependence, there are some indications that this might not be completely valid (see e.g. the discussion about the HIA measurements in the plasma sheet environment in Sect.6) but so far no careful study addressing this problem has been carried out.However, it seems that a change in the efficiency energy dependence is a second order effect, at least in the plasma environments where the two HIA sides are calibrated (see the statistical studies presented in Sect.5).
One particular aspect that might be of concern is the role of ion composition in the HIA-WHISPER data comparison.The prevalent minor ions in solar wind and magnetosheath plasma (the environments where the two HIA sides are calibrated; see next sections) are the α particles.If one considers a mixture of protons (number density N p , mass m p , and electric charge q p ) and α particles (with corresponding parameters N α , m α , and q α ), then for a detector like HIA, unable to discriminate between the ion species, the number density reported by the instrument will be (see e.g.Paschmann et al., 1998): N HIA = N p + m p /m α N α = N p + N α /2 .On the other hand, the WHISPER instrument will report a number density N WHI = N p + (q α /q p ) N α = N p + 2 N α .Typically, the α particles abundance in the solar wind and magnetosheath plasma is around few percents of the proton number density.Therefore the discrepancy between the readings of the two instruments is of the same magnitude and consequently can be neglected in the first approximation.In addition, the procedure used to select the final set of calibration intervals (see Sect. 4.1) tends to exclude intervals with lower (than expected) value of N HIA /N WHI ratio.
In the case of HIA operating in "solar wind" mode, where the protons and α particles are clearly separated in the energy/charge channels, the on-board software automatically computes the plasma moments separately for the two ion species.Therefore here one compares N HIA = N p with N WHI = N p + 2 N α .However, the clear separation of the 416 Introduction

Conclusions References
Tables Figures

Back Close
Full He ++ trace in the energy spectrogram allows us to select from the beginning intervals with low presence of α, as will be described in Sect.4.2.

Calibration of HIA high sensitivity section
For calibrating the HIA high sensitivity section, magnetosheath (MS) intervals are used since the characteristic values of the plasma parameters in that environment (like density, temperature, energy spectrum within the energy domain covered by HIA) allow for an optimum instrument performance.Each part of the year when Cluster samples the MS environment, i.e. roughly between November and June the following year, is analyzed to obtain one set of calibration coefficients.The number of values in the set depends on the efficiency evolution.
Typically two values, each obtained by combining data from several intervals, are inferred.Nevertheless, when an increase in the MCP HV is commanded from ground, a sudden increase followed by a relatively rapid adjustment in efficiency is expected, which requires the determination of additional values.The last value in the set is assumed to be valid until the beginning of the next MS season.
Intervals for calibration are carefully selected to meet several criteria listed below: the HIA energy spectrogram suggests that, presumably, the vast majority of ions are detected, i.e. no indication of a significant ion population below or above the detector energy range exists.
the evolution of HIA density is regular, useful for revealing potential instrument artifacts.This condition is also important because HIA and WHISPER could provide different values when a steep boundary is encountered (due to Larmor radius effects).It is desirable as well to select intervals where the density values span over a wide range, for a better comparison.
preferably the same intervals for both C1 and C3 are used, to allow for an interspacecraft calibration and to detect potential instrument artifacts.Introduction

Conclusions References
Tables Figures

Back Close
Full For the selected intervals, the WHISPER density data are requested from the instrument team, that either decides to re-generate it for the purpose of HIA calibration or to re-validate the already available CAA data set.Figure 3 illustrates one example of interval selected for HIA-WHISPER density comparison.The four panels at the top represent the type of plot routinely produced for the identification of calibration intervals.First, the HIA energy spectrograms in three ranges, i.e. above 2 keV, the entire energy range, and below 100 eV, are shown in order to identify intervals that better comply with the requirement that virtually all particles are detected by the instrument.In the fourth panel the HIA (red) and WHISPER (black) raw density data are shown.Here a few sudden HIA density drops can be seen; these signatures are not present in the WHISPER data and are interpreted as instrument artifacts.The next panel compares the two densities after the data processing has been performed, which includes instrument artifacts removal, discarding of short intervals with rapid variation in density, data filtering, averaging and interpolation etc.The sixth panel checks the plasma gyrotropy as measured by the quantity (p ⊥2 − p ⊥1 )/[(p ⊥2 + p ⊥1 )/2], with p ⊥1 and p ⊥2 being the plasma thermal pressure along two orthogonal directions in the plane perpendicular to the magnetic field.The relatively small deviations from gyrotropy, e.g.around or below 5 %, provides supporting evidence of HIA symmetric anode response for this interval.The bottom panel shows the N HIA /N WHI density ratio (blue) and its average value for this interval (magenta straight line).
The result of comparison can also be shown in the form of HIA vs. WHISPER density plot, presented in Fig. 4, where the left part refers to the event described above.The points are clearly scattered along the regression line (in red) forced to cross the origin; its slope is used to estimate of the calibration factor inferred from this interval.The average value of several calibration factors, obtained from different intervals, is then employed to update the calibration files used in processing the HIA data.Introduction

Conclusions References
Tables Figures

Back Close
Full In spite of the careful selection, it can happen that the HIA-WHISPER data comparison brings inconsistent results on some intervals.Typically a lower (than expected) value of the N HIA /N WHI ratio is attributed to the presence of plasma population outside the HIA detection range, to the spacecraft charging or to events with relatively high abundance of α particles.Therefore, the final set of intervals to be used in the calibration is established after an additional inter-spacecraft comparison and/or checking the data provided by other instruments like CODIF, ASPOC, EFW (spacecraft potential).

Calibration of HIA LS section
With some specific differences, the calibration of the HIA low sensitivity section follows a similar procedure.One uses solar wind (SW) data and compares the HIA and WHISPER density on carefully selected intervals.
The selection is based on the type of plot presented in Fig. 5.The top panel shows HIA (red) and WHISPER (black) raw density data, A number of sudden HIA density drops (instrument artifacts), can be seen here as well.In the second panel the energy spectrogram based on data accumulated on the LS and is shown.For the purpose of calibration, it is desirable to select events with low presence of α particles, (seen in the second panel as the faint green line, at around twice the peak proton energy).Since an accurate calibration of the LS section requires no significant counts accumulated on the HS section (blocked when facing the SW direction, see Sect. 2), the third and the fourth panel present the plasma density and the energy spectrogram based on data accumulated on this side.The bottom panel compares the LS (red) and HS (black) count rates.
After the selection of SW intervals suitable for calibration, the raw HIA and WHISPER data are processed in a manner similar to that described in the previous section.For the event presented above, the result of the comparison is showed in the right part of inferred from different intervals, a value is obtained that will be used in the calibration files update.

Statistic comparison between HIA and WHISPER data
To validate the results of calibration methodology presented in Sect.4, two statistical studies have been carried out, each based on data provided by one HIA sensitivity side.
All MS and SW intervals observed on Cluster 1 data between 15 December 2006 and 15 February 2007 have been analyzed in order to compare calibrated HIA and WHISPER density data.Stable detector efficiency is expected in that period since the previous MCP high voltage increase occurred in January2006, the next one being performed on 16 February 2007, i.e. just after the chosen interval.Using selection criteria similar to those presented in Sect.4, a total of 54 MS intervals sampled by HIA HS section have been identified, comprising around 55 h of data.For the LS section, 64 SW intervals have been identified, covering around 96 hours of data.
The results of the analysis are presented in Fig. 6 for the HS section and in Fig. 7 for the LS section.In the former case, the regression line (in red) forced to cross the origin corresponds to a proportionality factor of ∼ 1.01; the relative standard deviation (RSD) of calibrated HIA data with respect to WHISPER date is 8.3 %.For the LS side, the regression line corresponds to a proportionality factor of ∼ 1.02, with the RSD of points of 5.7 %.
There are a number of conclusion that can be drawn from these comparisons: the dependence between N HIA and N WHI is linear.The proportionality factor is close to the ideal value of 1, a result that validates the calibration methodology outlined in Sect. 4.
at least in the two plasma environments used for HIA in-flight calibration, the assumptions of symmetric anode-dependent efficiency evolution and constancy of Introduction

Conclusions References
Tables Figures

Back Close
Full the coefficients describing the efficiency energy dependence are verified in the statistical sense, any deviation being of a second order importance.
since the instrument artifacts have not been removed before comparison, the results indicate that they have no significant influence on the data, in a statistical sense.

Summary and future work
The first two panels in Fig. 8, showing the evolution of HS and LS detection efficiency in the course of the mission, summarize the results of HIA in-flight calibration activity.
Our comments below will refer mainly to the period starting from October 2005, subject to the calibration methodology presented in this paper.
The top and middle panels in Fig. 8 refer to the HS and LS section, respectively.The shown detector efficiency, obtained by comparing the HIA and WHISPER data densities, are relative to the beginning of the mission.Black lines present the C1 evolution while the red lines the evolution for C3.Each value of the efficiency is shown by a horizontal segment, with the length indicating its validity period.There are no HIA data on C3 after November 2009 and on C1 LS side after June 2011 (see the end paragraph of Sect.3).
It is worth noting that the HIA detection efficiency stayed at a reasonable level in the course of the mission.Taking for example the values corresponding to 2009, the relative efficiency on C1 was around 1 for the HS side and 0.93 for the LS side, whereas in case of C3, the relative efficiency was around 1.25 for the HS side and 1.38 for the LS side.an increase has been noticed.Note also that sometimes the efficiency has slightly increased without raising the MCP HV, like at the end of 2007.This unexpected behavior has been observed on both spacecraft and on both sensitivities, i.e. both on MS and SW intervals, which argues for a real effect.An MCP efficiency recovery has also been reported by the PEACE team in that period.
The HIA in-flight calibration is a complex task that requires considerable effort.This activity will continue and expand in order to ensure the highest data accuracy.Below we present a list of topics to be addressed in the future: to apply the calibration methodology described in this paper to data provided by HIA in the first years of the Cluster mission, i.e. before October 2005.
to investigate more closely whether or not the anode-dependent efficiency factors evolved symmetrically in the course of the mission.Although Fig. 2 indicates a relatively homogeneous response from all HIA angular sectors, no quantitative assessment of this assumption was made so far and only few data intervals have been qualitatively evaluated.In the case of CODIF such an investigation is regularly performed, bringing significant improvements to data quality.
to investigate possible changes in the efficiency energy dependence in the course of the mission.
to calibrate the He ++ data provided by HIA in the SW.
Related to the third item above, there is evidence suggesting the need for correction of the efficiency energy dependence.The situation presented in Fig. 9 is typical for the plasmasheet.The HIA-CODIF data discrepancy in this environment could be explained by changes in the efficiency energy dependence, since the average plasma energy in the plasmasheet is higher than in the magnetosheath, where the instrument was calibrated.It means that a correction that results in lower efficiency for the high energy channels would provide a plasma density correction in the right direction (i.e. an increase in density and, mostly, in pressure).The same type of correction has been suggested by the instrument response to the penetrating radiation during perigee passes (see Ganushkina et al., 2011).Introduction

Conclusions References
Tables Figures

Back Close
Full  Full Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

Fig. 1 )
is tuned for the detection of solar wind ions, i.e. for high ion fluxes with narrow energy and angular range.The required high angular resolution is achieved through the use of 8 × 5.625 • central anodes in the exit plane, the remaining 8 sectors having in principle 11.25 • resolution.In the solar wind mode, the HIA voltage sweep is truncated when the "high G" section is facing the Sun, in order to avoid the solar wind detection and to protect the MCP lifetime.The two sections of the instrument can supply data simultaneously in the solar wind mode.The HIA and CODIF sensors complement each other in terms of sensitivity, mass resolution, and detection efficiency.For CODIF, an additional time-of-flight (TOF) section is present, following the E/Q selection by the EA, allowing thus the separation Introduction Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

Figure 2 ,
based on C1 data from 17 October 2007, presents the individual anode response of HIA high sensitivity section in the plasma sheet environment, where the plasma distribution function is expected to be highly isotropic.Each of the panels corresponds to one sector in elevation angle (θ angle) for the arriving particles.The relatively homogeneous response from all 8 angular sectors supports qualitatively the assumption of symmetric anode-dependent efficiency evolution with time.The same situation is observed for HIA instrument on C3 as well.Note that there are only 8 panels, although according to bottom part of Fig. 1 there should be 16 angular sectors for the HS side of HIA.This is because the distribution function sent to the ground has been re-Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | intervals are not on the Data Caveat list (see Sect. 3).
Discussion Paper | Discussion Paper | Discussion Paper |

Fig. 4
Fig. 4 in the form of HIA vs. WHISPER density plot.The points are aligned along the regression line (in red) forced to cross the origin; its slope is taken as the value of the calibration factor corresponding to this interval.By averaging several calibration factors, 419 Discussion Paper | Discussion Paper | Discussion Paper | The high voltage applied to the MCPs is presented in the bottom panel; the vertical dashed lines indicate the dates when this HIA operating parameter has been raised by ground commands, for the purpose of increasing the detection efficiency.With some exceptions (e.g.see the LS side after the last change, on 17 February 2007) such Introduction Discussion Paper | Discussion Paper | Discussion Paper | Figure 9 shows a comparison between HIA and CODIF data in the plasmasheet environment on 7 September 2001.The energy spectrograms (CODIF first panel and HIA the second panel) support the idea that most of the particles are detected by the two instruments.However, the plasma pressure and density (next panels) are different, with HIA (red lines) providing values around Introduction Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | (CAA), in: The Cluster Active Archive, Studying the Earth's Space Plasma Environment, edited by: Laakso, H., Taylor, M., and Escoubet, C. P., Springer, Berlin, 51-72, 2010.409 Dandouras, I., Barthe, A., Kistler, L. M., and Blagau, A.: Calibration Report of the CIS measurements in the Cluster Active Archive (CAA), http://caa.estec.esa.int/caa/ug_cr_icd.xml(last access: 4 July 2013), 2012.Discussion Paper | Discussion Paper | Discussion Paper | cucci, M. F., Pallocchia, G., Korth, A., Daly, P. W., Graeve, B., Rosenbauer, H., Vasyliunas, V., McCarthy, M., Wilber, M., Eliasson, L., Lundin, R., Olsen, S., Shelley, E. G., Fuselier, S., Ghielmetti, A. G., Lennartsson, W., Escoubet, C. P., Balsiger, H., Friedel, R., Cao, J.-B., Kovrazhkin, R. A., Papamastorakis, I., Pellat, R., Scudder, J., and Sonnerup, B.: First multispacecraft ion measurements in and near the Earth's magnetosphere with the identical Clus-Discussion Paper | Discussion Paper | Discussion Paper |

Fig. 1 .Fig. 2 .Fig. 3 .Fig. 4 .Fig. 5 .Fig. 6 .Fig. 7 .Fig. 8 .Fig. 9 .
Fig. 1.The first two parts present the top view and cross-sectional view of the HIA instrument.At the bottom, the principles of HIA anode sectoring are shown.In the upper and bottom parts of the figure, the spin axis is in the plane of the paper, along the vertical direction, while in the middle part it points into the paper.See text for more details.Figure adapted from Klumpar et al. (2001) and Rème et al. (2001).