CRIS Level 2 Data Documentation
|Sensor||Full Name||Measured Species||Measured Quantities||Energy Range (MeV/nuc.)||Measurement Technique|
|Cosmic Ray Isotope Spectrometer||3 <= Z <= 28||Z, M, E||~ 100 - 500||dE/dx - E|
The CRIS Instrument on ACE
The Cosmic Ray Isotope Spectrometer (CRIS) on the Advanced Composition Explorer (ACE) spacecraft is intended to be a major step in ascertaining the isotopic composition of the Galactic Cosmic Rays and hence a major step in determining their origin. The GCRs (Galactic Cosmic Rays) consist, by number, primarily of hydrogen nuclei (~92%) and He nuclei (~7%). The heavier nuclei (1%) provide most of the information about cosmic-ray origin through their elemental and isotopic composition. The intensities of these heavy cosmic rays are very low and progress in the past has been impeded by limited particle collection power, particularly regarding individual isotopes. CRIS is designed to have far greater collection power (~250 cm2*sr) than previous satellite instruments (< 10 cm2*sr) while still maintaining excellent isotopic resolution through Z=30 (Zinc) and beyond.
For more information about the CRIS instrument, visit the CRIS Home Page, at Caltech.
CRIS Data Description
Jan 23, 2013: Re-release of CRIS Level 2 data
The CRIS Level 2 data product has been re-evaluated following changes made to the data cuts and the calculation of the geometry factors and SOFT hodoscope efficiencies. Bugs in the positioning and thicknesses of the silicon detector stacks and the calculation of the depth of incident particles have been corrected. Please read the Release notes for details. ***CAUTION*** CRIS DATA ARE OUT OF CALIBRATION BETWEEN 28 AUGUST 1997 AND 4 DECEMBER 1997. Early in the mission the camera discriminator in the SOFT hodoscope was adjusted several times. Between 28 Aug and 4 Dec 1997 the discriminator settings were not optimal, resulting in a bias against heavily-ionizing particles in the hodoscope. For species below about Si (Z=14) we do not notice any problems, but the intensities reported in the CRIS level 2 data (both in this new release and in all previous versions) are too low during this period for heavier species, by ~10% for Fe (Z=26) to as much as ~30% for Ni (Z=28). This fact was overlooked in the George et al. paper cited above and will be addressed in an upcoming erratum. We are working to determine the appropriate correction factors and will implement them in a future level 2 release when they are available.
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.
CRIS 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:
- 27 days (1 Bartels rotation)
Currently, flux data are available for 24 elements, in units of particles/(cm2*sr*sec*Mev/nucleon), in seven energy bands. The energy bands are different for each element, and are documented in this text file. The elements for which data are available are:
- B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni
Release notes for CRIS level 2 data - provided by the CRIS instrument team. All users of CRIS data should read these notes.
The following other notes apply:
- Systematic uncertainties in the CRIS data are addressed in detail in the CRIS data release notes.
- A flux value of -999.9 indicates bad or missing data.
- Isotopic data will become available at a later date.
- Time periods during which solar activity is high are not covered by CRIS, since the instrument was not designed to operate during such conditions.
- The data are for a detector acceptance angle of 30 degrees.
The number of counts is presented from which the statistical uncertainties can be calculated. The statistical uncertainties in the flux can be calculated as flux/sqrt(N), where N is the number of counts or events during the averaging period.
Flux Averaging method
The CRIS team provides element flux data in 256-sec or 1024-sec time intervals, depending on the element abundance. These short time-averages are not generally useful, so the ASC only provides the hourly, daily and 27-day averages in the level 2 data. These averages are calculated as follows:
Average flux = Sum(flux)/(Number of samples), for all the good 256-second (or 1024-second) samples in the time period.
CRIS does not operate during periods of high solar activity, and CRIS livetimes are fairly constant otherwise, so the above averaging method should be adequate.
Livetime is the fraction of the time period that the instrument was ready to collect particle data.
Last Updated: 7 November, 2007 Return to ASC Home Page