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SWICS Level 2 Version 1.1 Proton Data Release Notes

Jason A. Gilbert (to whom questions should be directed), George Gloeckler
(SWICS lead, Co-I), Manan Kocher, Sue T. Lepri, Jim M. Raines, Paul Shearer,
Micah J. Weberg, and Thomas H. Zurbuchen (Co-I)
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Update for Version 1.2
-----------
In Version 1.2, an issue has been corrected in which, for proton speeds lower
than 300 km/s, the calculation of proton densities was sometimes off by an
aspect-angle dependent scaling factor.

Update for Version 1.1
-----------
Version 1.1 contains optimizations to the peak selection model as well as to
the recovery of velocity distribution functions. The accuracy of the peak
selection model has been increased to ensure that alternate peaks are not
selected at the expense of the proton peak during any given time step. This
improves the calculation of both speed and density during time periods of
unusual solar wind, and brings those values into better agreement with those
measured by ACE/SWEPAM and Wind/SWE. The level of background has also been
more effectively subtracted and a new method of cross-calibration with the
SWICS main channel has been used to remove excess helium counts from the tail
of the proton count distribution. This process reveals more of the proton peak
and allows the calculation of extended velocity distribution functions that
should improve the accuracy of thermal speed calculations.

Overview
--------
The SWICS proton data (1998-present) is a time series of proton speeds, thermal speeds, and 
densities that are cross-calibrated to ACE/SWEPAM (for the years 1998-2004) and WIND/SWE 
(for 2005-present). The proton data comes from an auxiliary channel on the SWICS instrument, 
separate and distinct from the main channel that provides heavy ion composition data. The 
proton data is gathered by stepping through a range of incident energies, and delivered in 12-
minute accumulations - the accumulation time required for a single, complete energy scan. The 
hardware anomaly experienced by SWICS in 2011 affected only the time-of-flight accuracy in 
the main channel. Since the proton data is derived from the separate auxiliary channel, it has 
not been affected by the anomaly and is provided here as a continuous data set for the entire 
ACE mission. A gap from 26 Aug - 15 Sep 2011 in the proton data represents a time when 
instrument settings were being adjusted to diagnose the anomaly; normal instrument settings 
resumed in the proton channel afterward. To provide accurate reconstructions of the proton 
densities near Earth, the proton densities measured by SWICS are cross-calibrated with those 
reported by the higher resolution proton sensors ACE/SWEPAM and WIND/SWE. The 
robustness of this cross-calibration is improved by long accumulations of the higher resolution 
data. Time steps for which telemetry data was unavailable, or for which the proton peak was 
not clearly identifiable, have been omitted.  To ensure the quality of the SWICS densities, new 
proton data will be delivered at a regular schedule that allows confidence in the cross-
calibration. 

Method
--------------------
For each year, a unique set of empirical corrections has been calculated by taking the average 
of proton density ratios from SWICS relative to either SWEPAM or SWE. To account for changes 
in the density due to the speed-dependent detection efficiency and the solar wind aspect angle, 
the densities are first cross-calibrated according to their aspect angle (in 1-degree bins) and 
then according to their speed (in bins of 50 km/s). Typical values of the density correction ratio 
as a function of aspect angle range between [0.2-3.8], with the annual averages ranging from 
[0.5-0.9] and standard deviations from the annual mean in the range of [0.2-0.5]. The density 
correction ratio as a function of speed ranges between [0.4-1.4], with the annual averages 
ranging from [0.8-1.1] and standard deviations from the annual mean in the range of [0.1-0.2].

Data Products
-------------------------------
The proton data maintains the SWICS sensor's native 12-minute measurement cycle, and 
contains the following quantities:

1) Proton density (cm^-3) with uncertainties
2) Proton bulk speed (km/s)
3) Proton thermal speed (km/s)

Data Quality and Usage Caveats
--------------------------
The density and their uncertainties are calculated from accumulations of ion counts as 
described in the release notes for the SWICS 1.1 heavy ion data. Bulk and thermal velocities are 
calculated from the first and second moments of the distribution and this simple moment-
based method may be biased when the distribution is significantly non-Gaussian (e.g., when the 
bulk speed is highly variable on a timescale below the data time resolution, or when the 
distribution exhibits beaming or suprathermal tails).
The thermal velocity accounts for the solar wind's thermal distribution along the direction of 
flow, as this is the only direction where speed distribution is observable by SWICS. Formally, the 
thermal velocity is computed from the 1-D reduced velocity distribution function along the 
SWICS look direction, which is formed (to a good approximation) by marginalizing out the two 
perpendicular dimensions from the full 3-D velocity distribution. The thermal velocity we report 
is the root-mean-square deviation of the 1-D reduced distribution from the bulk speed.