CRIS Level 2 Data Documentation
| Sensor | Full Name | Measured Species | Measured Quantities | Energy Range (MeV/nuc.) | Measurement Technique | ||||||
CRIS |
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
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
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:
- hourly
- daily
- 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 ranges. The energy ranges are different for each element - download this text file of energy ranges. 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:
- The CRIS team currently recommends adding an additional 10% to the statistical uncertainties of higher Z elements and 15% for lower Z elements to reflect the geometry factor and trajectory detector efficiency uncertainties. These systematics will be improved in future versions. Systematic uncertainties in the CRIS data are addressed in more 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.
Elemental Counts
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
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