SIS: The Solar Isotope Spectrometer

Designed and developed by:

California Institute of Technology
Goddard Space Flight Center, NASA
Jet Propulsion Laboratory

Introduction

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 will measure the isotopic composition of the solar corona, while during solar quiet times SIS will measure 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.

SIS is also part of the Real Time Solar Wind (RTSW) set of instruments flying aboard ACE. Four of ACE's nine instruments will be constantly monitored by the National Oceanic and Atmospheric Administration (NOAA) operated ground stations. The data from these instruments will be used by NOAA to evaluate the risk of geomagnetic storms from solar events and to make predictions of these storms rapidly available.

Solar Energetic Particles and Anomalous Cosmic Rays

The above figure illustrates the major particle populations that will be observed by SIS, along with data collected by other missions such as IMP-8. Galactic cosmic rays are discussed on the CRIS page, while SEPs and ACRs are discussed below.

Solar Energetic Particles

Solar energetic particles represent a sample of solar material that can be used to make direct measurements of the Sun's elemental and isotopic makeup, and can be used to study the most energetic acceleration processes that occur in our solar system. Although the sun contains the vast majority of solar system material, we have only limited direct knowledge of its elemental and isotopic composition. Spectroscopic observations of solar isotopes are very difficult; there are isotopic observations for only a few elements and the uncertainties are large. With its greatly improved collecting power over other instruments, it is hoped that SIS can make a major advance in our knowledge of SEP isotopic composition. The figure below shows an estimate of the number of events that SIS would have observed in the SEP event of 10/30/92 measured by the SAMPEX spacecraft (Selesnick, R.A., et al., 1993). Arrows indicate upper limits.

[SEP Event Fluxes from
10/30/92, as would be seen by SIS]

Anomalous Cosmic Rays

During solar minimum conditions there are seven elements (H, He, C, N, O, Ne, and Ar) whose energy spectra have shown anomalous increases in flux above the quiet time galactic cosmic ray spectrum. This so-called "anomalous cosmic ray" (ACR) component is now thought to represent neutral interstellar particles that have drifted into the heliosphere, become ionized by the solar wind or UV radiation, and then been accelerated to energies >10 MeV/nucleon, most likely at the solar wind termination shock.

Anomalous cosmic ray observations offer a unique opportunity to study a sample of matter from local interstellar space. Because the ACR component apparently represents a direct sample of the local interstellar medium, it carries important information about galactic evolution in the solar neighborhood since the formation of the solar nebula - information that can be obtained by comparing the isotopic composition of ACR nuclei with that of solar system abundances, including those measured by SIS in solar energetic particles. The figure below illustrates what is currently known about the 22Ne/20Ne isotopic ratio over a wide energy interval, including ACR neon. Note that the ACR (lower energy) Ne isotopic ratio is lower than it is in galactic cosmic rays, implying that the GCRs contain a component rich in 22Ne.

[Ne isotopes over wide energy
range]

SIS Physical Description

[SIS
Element/Isotopic Identification Regions]

If SIS is to achieve the objectives of excellent mass resolution and large collecting power beyond that of previous instruments that have measured ACRs and SEPs, it must satisfy several design requirements. It must have a mass resolution of ~0.25 amu or better. The contributions to this are similar to those in the CRIS physical description. In addition to these issues, measurements of solar energetic particles are usually made in a very hostile environment in which the flux of protons >1 MeV may exceed 10^5/cm²-sr-sec. Chance coincidences between these low energy protons and the heavier nuclei that are of primary interest to SIS can lead to ambiguous trajectories or distorted energy loss measurements. It is therefore necessary that SIS be capable of returning accurate composition measurements in the presence of high fluxes of low energy protons and helium nuclei. To ensure that as few as possible of the nuclei with Z>=10 are missed requires that the instrument be capable of selecting the most interesting nuclei for analysis and that the bit rate be sufficient to transmit several events per second. The above figure shows the energy and charge intervals for which isotopic analysis is possible (in gray). Particle elemental identification can be continued to higher energies, while integral fluxes can be calculated from events fully penetrating the telescope.

Measurements of anomalous cosmic rays, free from contamination of solar and interplanetary particles at lower energy and free from GCR contamination at higher energies are best made in the energy interval from ~5 to 25 MeV/nucleon, where the flux is a decreasing function of energy. Similarly, SEP spectra typically decrease rather steeply with increasing energy. It follows that to maximize the number of detected particles for both of these species requires the use of thin detectors with as low a threshold for penetration as possible, combined with a large geometry factor. For this reason SIS has two telescopes composed of the largest area devices available (~65 cm² each). It is also of interest to extend measurements of SEPs to as high an energy as possible to understand the acceleration process in these events. The SIS detector stack is composed of devices of graduated thicknesses in order to cover a broad energy range. There are two identical telescopes in SIS (one pictured here), each composed of 17 high-purity silicon detectors.

[SIS Matrix photo] The first two detectors, M1 and M2, are position-sensitive "matrix" devices (pictured at left) that form the hodoscope measuring the trajectory and energy loss of incident nuclei. The matrix detectors are octagonal in shape, 70 to 80 µm in thickness, and have 34 cm² active areas that are divided into 64 strips. Each of the strips on M1 and M2 is individually pulse-height analyzed with its own 12-bit ADC when an event occurs so that the trajectory of heavy ions traversing the system can be separated from the tracks of low energy H or He that might happen to hit one of these detectors at the same time. Detectors M1 and M2 are separated by 6 cm; the resulting rms angular resolution of the system is ~0.25 degrees, averaged over all angles.

Back to CRIS/SIS homepage.
Back to Caltech SRL ACE homepage.
last modified 28 November 2007
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