================================================================================= ACE/SWICS Level 3 Proton and Alpha Energy Spectra Data Release Notes (v1.0) ================================================================================= Micah J. Weberg, Susan T. Lepri, Jim M. Raines, Jason A. Gilbert, Patrick Tracy, and Thomas H. Zurbuchen Last Revision: Jan 29th, 2016 Please direct inquires to Sue Lepri at slepri@umich.edu Overview -------- This document describes the processing, calibration, and validation of proton (H+)and alpha (He++) energy spectra in the core solar wind as measured by the Solar Wind Ion Composition Spectrometer (SWICS) aboard the Advanced Composition Explorer (ACE). This new data product complements the existing ACE/SWICS datasets and extends the scientific utility of SWICS measurement to include new topics of research. Table of Contents ----------------- * The SWICS Instrument * Analysis Method * Data Products and Time Coverage * Calibration and Statistical Errors * Quality Flags * Validation Plots * References The SWICS Instrument -------------------- SWICS is an advanced time-of-flight, mass spectrometer capable of measuring the densities, bulk speeds, and thermal velocities of over 40 different charge states between the elements of H, He, C, N, O, Ne, Mg, Si, S, & Fe. The SWICS instrument has two measurement channels. The main channel (MAIN) uses a triple coincidence technique to identify the energy-per-charge (E/q), velocity, and total energy of incident ions E/q values in the range of 0.657 – 86.61 keV/e (~251 - 2890 km/s for He++). The benefits of the triple coincidence method are very low signal-to-noise ratios and a robust identification and separation of different ions. However, the E/q range and energy threshold of the main channel are unsuitable for measuring protons (H+) which compose the majority of solar wind plasmas. The auxiliary channel (AUX) accounts of this limitation by using a simpler, single coincidence method and is designed to measure solar wind protons and alphas with E/q values of 0.16 – 20.8694 keV/e (~174 – 2000 km/s for H+; ~123 – 1419 km/s for He++). More information about the design and nominal operations of SWICS can be found in Gloeckler et al., 1998. Analysis Method --------------- This work builds upon existing SWICS data processing pipelines, most notably the recent version 4 dataset which significantly improves the ion identification scheme for the MAIN channel (see Shearer et al. 2014 as well as the SWICS 1.1 data release notes found on the ACE Science Center). Below we briefly describe our method for obtaining energy spectra (i.e. velocity distribution functions) for protons and alphas over a large range of solar wind speeds. We begin by taking the 12 minute count rates in each E/q bin for the AUX channel. Background noise is removed from the count distribution by sampling data in E/Q bins far from the ion peaks and then applying a timestep-dependent threshold. It is important to note here that the base rates in the AUX channel contain two peaks: the larger one corresponds to protons and the second, smaller peak corresponds to alphas. Under normal conditions, this does not pose a problem for calculating bulk solar wind properties, since the peaks are separated by a factor of 4 (due to different q/m ratios). However, separating the high energy tail of the proton distribution from the lower energy tail of the alpha distribution provides some challenges. To circumvent this problem we utilize the capability of the MAIN channel to unambiguously identify alpha particles. We next take the 12 minute MAIN channel alpha (He++) count rates as determined by the SWICS v4.09 data processor and interpolate the distribution to the E/q bins locations of the AUX channel using a standard cubic spline. This step is necessary since the bin edges between the two channels are not perfectly aligned. Additionally, there is often a single bin offset between the locations of the AUX and MAIN alpha peaks. This occurs most often at low solar wind speeds and high aspect angles. When there is a bin offset, we compensate appropriately to ensure the most precise cross-channel alignment. We then scale the interpolated MAIN He distribution to the maximum value of the AUX alpha peak and subtract this scaled count distribution from the entire AUX distribution. This “cross-channel He subtraction method” neatly deconvolves AUX proton and alpha counts and results in the most complete proton distribution observable by SWICS. The last and final step is to convert counts to differential flux and correct for instrumental efficiencies and pointing effects using the same process as the level 2 SWICS data. The reported alpha values are from the unmodified MAIN channel count distributions. The above method cannot be used during time periods when MAIN channel alpha data is unavailable, such as after August of 2011 when SWICS experienced an age-induced hardware anomaly that necessitated switching to a different operational mode. At these times we report a truncated proton distribution including only 5 E/q bins on either side of the proton peak. This secondary method is equivalent to the method used for calculating current level 2 SWICS / AUX bulk proton parameters. Data Products and Time Coverage ------------------------------- This preliminary data release includes 12 minute proton and alpha differential flux measurements in units of [particles]/([cm^2]*[Sr]*[sec]*[MeV/nucleon]) for the time period from July 2009 – July 2011 as well as proton only measurements from August 2011 – July 2012. Separate files are provided for protons and alphas. Each file has a total of 120 columns. Timestamps correspond to the START TIMES of each 12 minute scan and invalid data is filled with a value of -999.9. The format for the files is as follows: COLUMN NUMBER, VARIABLE, FORMAT 1 Year I4 2 Fractional Year F13.8 3 Fractional DOY F9.5 4 Data Quality Flag I2 5 – 119 (odds) Differential Flux E12.3 6 – 120 (evens) Error in Diff. Flux E11.4 The last 116 columns contain paired differential flux (odd numbered columns) and error values (even numbered columns) in each of the 58 E/q bins measured by SWICS. The central MeV/nucleon value for each bin is given in the file header. The energy ranges for each ion species are listed below. Please note that the bins are not evenly spaced. Protons --> 2.536e-23 – 3.344e-21 MeV/nuc. Alphas --> 1.053e-22 – 1.388e-20 MeV/nuc. Calibration and Statistical Errors ---------------------------------- The reported differential flux values include adjustments for all instrumental effects such as pointing direction, geometric factor, bandpass coverage, and spacecraft spin. Additionally, a speed-dependent cross-calibration factor has been applied. This factor is based on comparisons with solar wind data from Wind/SWE and was determined by the ACE/SWICS science team as part of the validation process for the level 2 version 4 dataset. Error values were calculated using the statistical uncertainty of sqrt(N) (i.e. basic counting errors) and then converted to units of differential flux. The reported error values also include all the calibration factors applied to the differential flux. Quality Flags ------------- A set of quality flags have been provided to indicate the operations and adjustments made by our processor VALUE , DESCRIPTION 0 Good quality data 1 Cross-channel peak alignment used 2 No valid MAIN channel alpha data available. Limited high energy tails retrieved for AUX protons -1 Missing or invalid data (data filled with a value of -999.9) Validation Plots ---------------- A set of 30-day overview plots have been provided as an aid for analysis and event selection. These plots show (from top to bottom) solar wind speed, data quality flag, AUX nH, AUX proton counts, AUX proton differential flux, MAIN nHe, MAIN alpha counts, and MAIN alpha differential flux. Interplanetary Coronal Mass Ejections (ICMEs) from the list produced by Richardson and Cane (2010) are indicated by vertical solid red (start) and dashed orange (end) lines. These plots are intended to give a quick overview of the data and should not be used in publications. 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. Shearer et al. (2014), The solar wind neon abundance observed with ACE/SWICS and Ulysses/SWICS, ApJ, Volume 789, Issue 1, p. 60. Richardson, I. G., & Cane, H. V. (2010), Solar Phys, Volume 264, p. 189