Data set Overview: On March, 13/14 1986, the Giotto spacecraft flew within 600 km from comet Halley through the coma. During this flyby the Neutral Mass Spectrometer NMS measured the neutral gas and the ionized plasma of the comet as a function of distance. The present data represent the curren- tly available dataset of the innermost coma. Not all masses are calib- rated and evaluated to the same degree of accuracy. Therefore, care has to be taken when using these data. The paragraph summarizing the correlated errors should be carefully read before working with these data. There are no data available for other periods of the mission. This data set supersedes GIO-C-NMS-4-86P-V1.0.Version History:The first version of the this data set was archived as GIO-C-NMS-4-86P-V1.0.After an internal and external review by the Planetary Data System (PDS)Small Bodies Node (SBN), several technical issues were discovered. First,this data set was renamed to GIO-C-NMS-4-HALLEY-V1.0 since this is a data setfor comet 1P/Halley, not 86P/Wild. All catalog files have been updated.Support files have been populated, updated, and/or corrected to comply withPDS3 standards. The data files have remained unchanged, though the formatfiles that describe them have been corrected as needed.Content of archive: - Neutral density data from the mass analyzer - Neutral density data from the energy-analyzer - Ion density data from the mass analyzer - Neutral gas velocity data - Ion temperature data - Yields for different ions for the NMS detectors - Data for mass 18, 28 and 31 for the mass analyzer which have been published in the literature - Data for mass 18 and 44 for the energy analyzer which have been published in the literature - List of references - PhD work of R. Meier and M. Reber, University of BernData structure--------------Product Column 1 Column 2 Column 3 Column 4------- -------- -------- -------- --------Neutral mass density Distance Density(cm- Relative N/Aprofile M-Analyzer from comet 3) error percent (km)Neutral mass density Distance Density(cm- Relative N/Aprofile E-Analyzer from comet 3) error percentIon density profile M- Distance Density(cm- Relative N/AAnalyzer from comet 3) error percentNeutral mass density Distance Density(cm- Relative N/Aprofile M-Analyzer, from comet 3) error percentpublishedNeutral mass density Distance Density(cm- Relative N/Aprofile E-Analyzer, from comet 3) error percentpublishedNeutral gas Velocity Distance Velocity N/A N/A from comet (km/s)Ion temperature Distance Temperature N/A N/A from comet (K)Detector Yield Mass Molecule Yield(neu Yield tral) (ion)Calibration:(Meier, 1992) [MEIER1992]The calibration of the flight unit was performed with the toggle 0mode with a N2+-ion beam at 700 eV beam energy and a N2-neutral beamat 700 eV. This calibration leads to a mean detector gain for thecomplete detector. In order to expand this calibration to allspecies the following correction factors have to be applied: _ A relative correction factor for the individual pixels is given by the deviation of the local gain to the mean gain. _ Because the gain decreases at high count rates a correction factor for the non-linearity was introduced. _ For other ions than N2 species dependent correction factors have to be applied: o For neutrals the ionization cross section of the electron bombardment in the ion source is species dependent. o The instrument transmission is a function of the ion mass and the toggle mode. A transmission correction factor des- cribes the difference of transmission for a certain mass to the transmission for N2 in toggle mode 0 (standard). o Not only the transmission but also the detector gain depends on the ion species. The detector gain has to be corrected for the species dependent yield of the detector.The different factors are discussed below in detail (from Meier,1992)Mean detector gain Guni(GS):---------------------------The gain of the detector depends on the high voltage between frontand back of the microchannel plate. This voltage assumed 15 discretevalues (gain steps, GS) between hex 1 (gain ~10^-2) and hex F (gain~10^6). For each of these values a mean gain factor (unified gain:Guni(GS)) was determined for the bare detector.Relative correction factor for the individual pixels----------------------------------------------------Each pixel has an individual gain which is taken into account by therelative gain REL(GS, i) (GS: gain step, i: number of pixel).Correction factor for the non-linearity (NL(GS,i,AN,T)):-------------------------------------------------------The detector gain depends on the count rate AN(i) and decreases withincreasing count rate. The correction factor NL(GS,i,AN,T) depends onthe count rate, the gain step GS, the pixel number i and the detectortemperature T.Effective Gain--------------The effective gain Geff (GS,i,AN(i),T) of each pixel is calculatedby:Geff(GS,i,AN(i),T):REL(GS,i)*Guni(GS)*NL(GS,I,An,T) [1]Detector background-------------------The background is the result of residual gas in the instrument. Thethermal noise is already filtered by the DPU. The background ismeasured and transmitted separately and is deducted from thespectra.Mapping of the mass lines-------------------------The counts are added over five pixels around the center of the massline. The overlap of adjacent peaks is taken into account. Thecorresponding uncertainty is ideally less than 1 percent, in general lessthan 5 percent.Toggle correction krel(M,Toggle)--------------------------------Periodically the mass spectra were shifted on the detector (togglemode). The main reason was the fact, that due to the four MCP's whichare aligned side by side, some masses fall onto the gap in betweenand could therefore not be evaluated. Due to the grids in front ofthe detector, a systematic difference in the peak height betweenshifted and unshifted spectrum can be observed. As a function of massand instrumental mode (neutral or ion mode) the peak height of thetwo types of spectra were adjusted.The toggle correction can be expressed by a 3rd degree polynomial:Krel(M,Toggle0):-2.873*10^-5*M^3+2.532*10^-3*M^2-8.109*10^-2*M+1.9166Krel(M,Toggle1):-3.368*10^-5*M^3+3.012*10^-3*M^2-9.616*10^-2*M+1.9293 [2]GS/HG-effect------------To enlarge minor peaks the NMS records every second spectrum at ahigher MCP voltage. The difference corresponds to three gain steps.Again a systematic offset could be observed. A scaling factor wasderived to determine the true particle counting rate of the regularand of the enlarged spectrum. The GS/HG effect results from a gainshift during the time between the laboratory calibration phase andthe flyby at comet P/Halley. Except for the detector number 3 thecorresponding factor is compatible with 1.Relative detector sensitivity-----------------------------The detector sensitivity does not only depend on the MCP voltageapplied but also on the incident velocity and chemical structure ofthe projectile. This can be described by a factor Y(X+, E, MHV)whereby X+ is the ion species, E is the initial energy plus theacceleration in the sensor and MHV is the detector high voltage (orgain step). A list for different species can be found in the archive(specific detector yield data) normalized to the flyby velocity of68.37 km/s and to the gain step hex F(used for neutrals) and hexC(used for ions). This relative detector sensitivity is 1 for N2+ at700 eV.Absolute sensitivity--------------------P(M) is the experimentally determined count rate which was normalizedto the uppermost gain step (hex F) and which is corrected withrespect to the gain and the non-linearity.P(M):Sum (An(i)*Guni(hexF)/ Geff(GS,I,An(i),T)) where the sum is over 5 pixels i which contribute to Mass M [3]AN(i) is the effective count rate after separation of mass peaks anddeduction of background.Absolute ion sensitivity------------------------The absolute sensitivity for ions Sion gives the relation between thecount rate P(M) and the effective ion density n(X+) of a certain ionspecies X+.n(X+):P(M)/(krfel(M,Toggle)*Y(X+,E,MHV))*Sion [4]Sion is given by the following expression:Sion : 1/(t*Guni(hexF)*Q*e*A*tau*v : 8.05*10^-5 cm^-3 [5]with:t : 0.161 Transmission of N2+, 700 eV, Toggle 0 (standard)Guni (hex F) : 2.104 106 Unified gain at GS : hex FQ : 2.95 1013 C-1 Absolute sensitivitye : 1.6022 10-19 C elemental chargeA : 0.36 x 5.0 mm2 Area of the entrance slittau : 62.96 ms Integration timev : 68.373 km/s Ion velocity relative to the spacecraftAbsolute neutral sensitivity----------------------------Sneutr connects the count rate to the density of the neutral gas:P(M)/(krel(M,Toggle)*Y(X(M)+,E,MHV))*Sneutr(X,M,IE):n(X) [6]withSneutr(X,M,IE):S*pi*a0^2/(sigmatot(X,IE)*delta(X,M,IE)) [7]X is the neutral species.X(M)+ is the ion with the mass M, which is created by the electron bombard- ment from X.Sneutr(X,M,IE) is the neutral sensitivity, if ions of the neutral species X created with the electron energy IE generate the mass line M.sigmatot(X,IE) is the ionization cross section for a species X with the electron energy IE.The cross section is given in units of pi*a0^2 (: 8..80 10^17 cm^2, a0 is the Bohr radius).The factor delta(X;M;IE) is the fraction of the total ions created from the species X which fall onto mass M.S has been determined for the two electron energies 18 eV (LeV) and 90 eV (HeV)as: SHeV : 230 cm^-3 SLeV : 362 cm^-3All archived data were taken in the HeV modeThe following data analysis has been performed on the archived data:-------------------------------------------------------------------- - The mass peaks have been separated - The background has been subtracted - The count rates have been normalized to the gain step hex F according to [3] - The ion density has been calculated according to [4], but with a specific yield for N+2. - The neutral densities have been calculated according to [6] and [7] but with a specific yield for N2.It is the task of the user of the archive to calculate species dependentyields according to the assumptions made for the composition of the cometarycoma, that is to choose the correct relative detector sensitivity (for ionsand neutrals) and the ionization cross section and fraction for a specificion in the case of neutrals. Coordinate Systems : No coordinate is specified for the data. Software : No software is provided. Media/Format : The standard distribution format for the data is an electronic volume.

Correlated and uncorrelated uncertainties of the data: Uncorrelated uncertainties due to counting statistics: ------------------------------------------------------ P*C : n with P:Number of counts, C Calibration factor, n density in cm-3 (see formula [4]) It follows: delta(n)/n : sqrt(p)/p : 1/sqrt(p) : 1/sqrt(n/C) : sqrt(C)*sqrt(n)/n In the case of neutrals C~100. It then follows: delta(n)/n : sqrt(n)/n*10 , For ion densities C~10-4, that means delta(n)/n : sqrt(n)/n*0.01 Correlated uncertainties ------------------------Different contributions: - Due to an unexplained difference between mass analyzer and energy analyzer absolute ion densities have an uncertainty of a factor 2.25. - The detector yield has been determined as a function of the ionized species with an uncertainty of at least 2 percent (1 digit) - The overlap of neighboring peaks gives an additional mean uncertainty of < 3 percent (Meier, p15) for large enough peaks, may be larger for small peaks - Other uncertainties which are independent of the mass give an addi- tionnal contribution of 20 percent. (Toggle Effect, Unified Gain, background correction, etc.). - Because after 20 years the only data usable were plots on paper the data had to be digitalized which causes an uncertainty in density as well as in distance. This amounts to 50 km for the distances and to less than 2percent for densitiesFor absolute densities the correlated uncertainty amounts to at least 20percent for neutrals and >50 percent for ions. For relative densities (e.g.mass18/mass 19) the correlated uncertainty is at least 20 percent forneutrals and for ions. The errors may be considerably larger for smalldensities and care has to be taken when looking at minor masses. Due to thedegrees ion densities outside the contact surface (4600 km) have to benarrow field of view of 4 corrected for their incoming velocitiy vector andtheir temperature. They fall of much more rapidly than e.g. the data from theIMS (field of view 15 degrees). Data which have been published have been verycarefully evaluated (mainly peak separation and background subtraction) andtheir uncertainties lie generally below the above values. However, in thefirst publication, Krankowsky et al.,Nature, 1986 [KRANKOWSKYETAL1986B], thedistances are wrong due to a insufficiently known time for closest encounter.In addition, later calibration and data evaluation efforts lead to a slightlyaltered water density (compare Krankow-sky et al., Nature. 1986[KRANKOWSKYETAL1986B] and Meier, PhD thesis, 1993)