SIS Level 2 Data Documentation

Sensor		Full Name		Measured Species		Measured Quantities		Energy Range (MeV/nuc.)		Measurement Technique
SIS		Solar Isotope Spectrometer		2 <= Z <= 28		Z, M, E		5 - 150		dE/dx - E

The SIS Instrument on ACE

The Solar Isotope Spectrometer (SIS) is designed to provide high resolution measurements of the isotopic composition of energetic nuclei from He to Ni (Z=2 to 28) over the energy range from ~10 to ~100 MeV/nucleon. During large solar events, when particle fluxes can increase over quiet-time values by factors of up to 10000, SIS measures the isotopic composition of the solar corona, while during solar quiet times SIS measures the isotopes of low-energy Galactic cosmic rays and the composition of the anomalous cosmic rays which are thought to originate in the nearby interstellar medium. The solar energetic particle measurements are useful to further our understanding of the Sun, while also providing a baseline for comparison with the Galactic cosmic ray measurements carried out by CRIS.

SIS has a geometry factor of ~40 cm²--sr, which is significantly larger than previous satellite solar particle isotope spectrometers. It is also designed to provide excellent mass resolution during the extremely high particle flux conditions which occur during large solar particle events.

For more information about the SIS instrument, visit the SIS Home Page, at Caltech.

July 22 2014: The SIS team has released a new version of the SIS Level 2 data. The new analysis includes a better calculation of the time-dependent geometry factor and a data cut to eliminate chance-coincidence events resulting from disabled strips. See the Release Notes for details.

June 23 2008: The SIS team has released a new version of the SIS Level 2 data. Intensities for Na, Al, Ar, Ca, and Ni have been added. Also, The range 7 He data have been reinstated. A few other minor changes in the analysis code have been made. See the Release Notes for details.

Nov 08 2007: Due to suspected inefficiencies in the detection of range 7 (29.3 - 41.2 MeV/n) He by the SIS B telescope, all the range 7 He intensities have been set to fill data from 1/1/2005 onwards. The source of these inefficiencies and possible corrections are currently being investigated by the SIS team. Once this examination is complete, the range 7 He intensities will be recalculated and posted.

April 26 2007: The SIS team has released a new version of the SIS Level 2 data. See the Release Notes for details.

Access SIS Level 2 data

SIS Data Description

Time Data
All level 2 data records for all ACE instruments contain timing information in the same format. The format of the timing information is described here.

Element Fluxes
SIS level 2 data is organized into 27-day time periods (Bartels Rotations - roughly one solar rotation period). For each Bartels Rotation, the level 2 data contains time averages of energetic charged particle fluxes over the following time periods:

256 seconds
hourly
daily
27 days (1 Bartels rotation)

Species and Energy Ranges
Currently, flux data are available for 14 elements, in units of particles/(cm² Sr sec MeV/nucleon), in eight energy ranges. The energy ranges are different for each element - download this text file of energy ranges.

The elements for which data are available are:

He, C, N, O, Ne, Na, Mg, Al, Si, S, Ar, Ca, Fe, and Ni.

Release notes for SIS level 2 data - provided by the SIS instrument team. All users of SIS data should read these notes.

The following notes also apply:

The SIS team currently recommends that a systematic uncertainty of 5% be added to the statistical uncertainties derived from the counts data. Systematic uncertainties in the SIS data are addressed in more detail in the SIS data release notes.

A flux value of -999.9 indicates bad or missing data.

For the 27-day Bartels Rotation averages, two kinds of averages are provided:

Quiet-time flux averages, where data from Solar Energetic Particle Event time periods have been excluded.
Flux averages computed using all the data.

Element Counts
These are the actual event counts used to calculate the flux data. The statistical uncertainty of the flux data can be derived from these counts (N). The flux uncertainties can be calculated as flux/sqrt(N), where N is the number of events in the averaging period.
Caution: Counts for He may be low during periods of high solar activity. SIS livetimes for He can be low during these periods.

Flux Averaging method
Average flux = Sum(flux)/(Number of samples), for all the good 256-second samples in the time period.

Livetime
uptime_fraction is based on the number of 256-second averaging periods contained in the given averaging period. This may later be upgraded to a true livetime.

Solar Activity Flag
This variable is set to 1 if either the SIS helium or oxygen flux is higher than a certain threshold, for a given averaging period. The variable is intended to indicate periods of high solar activity. If this variable is zero, low solar activity for the period is indicated.
Note: the flag is set to 1 (non-quiet) if there is no Helium data available for a given time period. The thresholds are defined as follows:

threshold_He: (flux_He[0]+flux_He[1]+flux_He[2])/3. > 1.e-4

(Note: Prior to Jan. 28, 1998, the second term for He is not used, i.e.
threshold_He = (flux_He[0]+flux_He[2])/2. > 1.e-4)   

threshold_O: (flux_O[0]+flux_O[1]+flux_O[2])/3. > 2.e-5

For the Bartels rotation data, there is no solar_activity_flag. However, there are two sums. The first sum is over all the data during the period regardless of solar_activity. The second sum is over only the quiet days, i.e. those days where the solar_activity_flag is zero.

Download SIS level 2 data

Last Updated: 7 November, 2007

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