============================================================================== SWICS 2.0 Level 3 Version 1.11 Data Release Notes Jason A. Gilbert (to whom questions should be asked), George Gloeckler (SWICS lead Co-I), Sue T. Lepri, Jim M. Raines, Paul Shearer, Micah J. Weberg, and Thomas H. Zurbuchen (Co-I) ============================================================================== The SWICS 2.0 dataset consists of time series measurements by ACE/SWICS of the elemental abundance, charge state composition, and kinetic distribution of heavy ions in the solar wind. This data set begins after August 23, 2011, when a radiation and age-induced hardware anomaly altered the instrument's operational state. It should not be confused with SWICS 1.1, the recalibrated data set extending from launch up to the anomaly. SWICS 2.0 continues to make heavy ion measurements which are not available from any other instrument, and new data analysis methods have been developed to address the statistical and calibration issues of the current instrument state.However, SWICS 2.0 is a different instrument than SWICS 1.1, with different measurement and statistical properties. All users of this data should read these release notes carefully. As always, the instrument team is available for consultation prior to publication of any SWICS 2.0 data. Table of Contents ----------------- * Method * Data Products * Missing Data * Systematic and Statistical Error * Data Quality and Usage Caveats * Computational Details * References Method ------ SWICS 2.0 measures ions in double-coincidence using the energy-per-charge (E/q) and measured energy (E_meas) of each ion (Gloeckler et al. 1998). The instrument design is therefore modified, as the August 2011 space-weather induced hardware anomaly caused a high rate of spurious triggering in the TOF start signal, leading to increased background and invalid TOF measurements, changing it from its original design. In the SWICS 2.0 measurement methodology, an accumulation of ion measurements is represented as a two-dimensional histogram with respect to E/q and E_meas The counts are assigned to species by the maximum likelihood algorithm used by Shearer et al. (2014), in this case applied to the two-dimensional (E/q-E_meas) histogram instead of the (E-TOF) histograms which are no longer available. The maximum likelihood algorithm requires a model distribution for each species in both dimensions. The E_meas distribution was taken from the pre-August 2011 model, while the E/q distribution must be determined separately for each histogram due to solar wind kinetic variability. For He2+ the E/q distribution is measured directly, but for heavier ions it usually cannot be measured. Instead, it must be predicted using an empirical formula which describes the statistical relationship between heavy ion E/q distributions and the He2+ E/q distribution. This formula is derived from pre-August 2011 heavy ion data - effectively using SWICS 1.1 as a many-year calibration data-set. This method has been tested in detail by applying it to pre-August 2011 data and comparing the results to the SWICS 1.1 data, which is taken as a reliable ground truth. The tests revealed that the SWICS 2.0 data is generally biased and suffers from occasional extreme values. Therefore, each data product in the SWICS 2.0 data was debiased using the average statistical relationship with SWICS 1.1 data, which is assumed to remain valid after August 2011. After debiasing, the residual variability in the relationship was used as an estimate of total systematic and statistical measurement error in the SWICS 2.0 data. Data Products ------------- Time resolutions: 2 hour * Bulk velocity [km/s]: He2+ * Thermal velocity [km/s]: He2+ * Charge state ratios: C6+/C5+, O7+/O6+, O8+/O6+ * Elemental abundance ratios: Fe[7-17+]/O[6-7+] * Average charge state: Fe[7-17+] Missing Data and Quality Flags ------------------------------ In data downloaded from the ACE Science Center at Caltech, Missing data is marked with a value of -9999.9. Data is generally removed when - a numerical error is encountered (e.g. a zero denominator in a density ratio) - anomalous instrument operation was identified by the operations or analysis team For the data products provided, the relationship between SWICS 2.0 and SWICS 1.1 values is fairly correlated, but only over a limited range of SWICS 2.0 values where signal-to-noise and other measurement effects allow. Outside this range the SWICS 2.0 value is flagged as saturated, and should be considered only as an estimated lower or upper limit. The saturation quality flag values are 0, 1, 2, and signify the following: 0: value within sensitive range of SWICS 2.0 1: value saturated at lower limit 2: value saturated at upper limit Systematic and Statistical Error -------------------------------- For the average Fe charge state, the error provided is simply the 1 sigma error based on a normal distribution derived from our cross-calibrations. For example, if the measured average charge state of Fe is 8+ and the error is 0.5, the actual value is 8 +/- 0.5 with a probability of 68%, as expected for 1 sigma errors. For all other data products, the error is represented as an "error factor" to be multiplied with the observed value. For example, if the measured C6+/C5+ ratio is 0.3 and the error factor is 2, this means that the actual C6+/C5+ ratio is expected to be in the interval [0.15,0.6] with a probability of 68%. The percentage representation of error was chosen to better represent data products with a large dynamic range. Data Quality and Usage Caveats ------------------------------ The errors provided in this dataset are largely systematic and not statistical in nature. They are not independent from point to point. It should not be assumed that the error can be reduced by statistical processing, for example averaging multiple data points. Saturated values are subject to the most systematic uncertainty and should be handled with care. For example, during the previous solar minimum (2008-2010), the actual O7/O6 ratio in fast solar wind (>600 km/s) was often several times lower than the SWICS 2.0 value, even after taking debiasing and error bars into account. Saturation issues are most common in the O7/O6, O8/O6, and C6/C5 data. The Fe/O is less affected, and the average Fe charge state least affected. References ---------- Gloeckler et al. (1998). Investigation of the composition of solar and interstellar matter using solar wind and pickup ion measurements with SWICS and SWIMS on the ACE spacecraft. Space Science Reviews, Volume 86, pp. 497-539. Lepri, S. T., and T. H. Zurbuchen (2010). Direct observational evidence of filament material within interplanetary coronal mass ejections. ApJL, 723.1, L22. Shearer et al. (2014). The solar wind neon abundance observed with ACE/SWICS and Ulysses/SWICS. ApJ, Volume 789, Issue 1, p. 60. von Steiger and Zurbuchen (2011). Polar coronal holes during the past solar cycle: Ulysses observations. JGR Space Physics, Volume 116, Number A1.