Solar Energetic Particle Ionic Charge Analyzer at UNH

SEPICA In Brief:

SEPICA is one of nine instruments aboard the ACE spacecraft. Its mission is to collect information about particles emanating from the Sun. SEPICA detects the ionic charge state, kinetic energy, and nuclear charge of ions coming from the Sun, and with that information, one can determine not only the type of ions present, but also the history of those ions within the Sun. This aids scientists in their understanding of the Sun and the processes that take place within it. For more information, read the full Instrument Description.


  1. Picture of the SEPICA Instrument without its housing A Brief History of SEPICA
  2. Scientific Objectives of the SEPICA Project
  3. Brief Description of the SEPICA Instrument
  4. Acknowledgments

Note: This page is a distillation of the instrument paper, which has been published as: Möbius, E., et al., The Solar Energetic Particle Ionic Charge Analyzer (SEPICA) and the Data Processing Unit (S3DPU) for SWICS, SWIMS and SEPICA,Space Science Reviews, 86, 447, 1998. It is recommended that upper level students and educators read the paper as well as this page, for a more thorough understanding of the instrument and its purpose aboard ACE.

A Brief History of SEPICA

Prior to the development of ACE and SEPICA, the measurement of solar activity was accomplished by the International Sun-Earth Explorer (ISEE). ISEE took the same kind of data that SEPICA does, using similar instruments, but it could only take a small fraction of the data that SEPICA can in a given amount of time. Because of this limitation, ISEE could not study individual solar events, as they occur in a much smaller time span than ISEE's resolution. Therefore, ISEE missed many of the unique features found only during solar events, such as large and small scale flares and the like, and in doing so, limited the analysis that could be done.

SEPICA was built as a collaboration between the UNH Space Science Group and the National Aeronautics and Space Administration (NASA). SEPICA's mission was proposed in 1986. The project was kicked off into Phase B in May of 1991, with the development and implementation of Phase C started in January, 1994. Satellite integration and testing (Phase D) was carried out through 1996 and 1997. ACE itself was launched on 25 August, 1997, on a Delta II launch vehicle, from the Kennedy Space Center in Florida. Since then, SEPICA has been returning data on the solar wind, some of which can be found here online.

Scientific Objectives of the SEPICA Instrument

NASA's Advanced Composition Explorer (ACE) mission will probe material from within as well as outside the solar system for its composition. With state-of-the-art instrumentation, material that is accessible in the near-Earth interplanetary space will be investigated with unprecedented precision. Examples of accessible particle populations are:

The Solar Energetic Particle Ionic Charge Analyzer (SEPICA) is the key instrument on ACE to determine the ionic charge state of solar energetic particles and particles accelerated in interplanetary space. The charge state of energetic particles contains information about the temperature at the source of the particles, as well as related acceleration transport processes.

Brief Description of the SEPICA Instrument

The SEPICA instrument is the prime sensor on ACE, which is used to determine the charge state distribution of energetic particle distributions. SEPICA is designed to measure the ionic charge state, Q, the kinetic energy, E, and the nuclear charge, Z, of energetic ions above 0.2 MeV/Nuc (Stone et al., 1990). This includes ions accelerated in solar flares as well as in interplanetary space during energetic storm particle (ESP) and co-rotating interaction region (CIR) events. For low mass numbers SEPICA will also separate isotopes -- for example, 3He and 4He. During solar quiet times, SEPICA should also be able to directly measure the charge states of anomalous cosmic ray nuclei, including H, N, O, and Ne, which are presumed to be singly-charged. With the capability to differentiate the charge states of ions, the instrument will also be able to separate neutral atoms (Q = 0) from ions. Thus it may be able to identify energetic neutrals created through charge exchange.

The instrument is based on the design of the ULEZEQ (Ultra Low Energy Z E Q Analyzer) sensor flown on the ISEE spacecraft (Hovestadt et al., 1978). The sensor combines the determination of the electrostatic deflection of incoming ions in a collimator-analyzer assembly by the measurement of the impact position in the detector plane and a dE/dx - E telescope with a proportional counter solid state detector combination. The background from penetrating radiation is suppressed by the use of an anti-coincidence detector. The scientific objectives of the ACE mission call for significant improvements over the ULEZEQ sensor in the following parameters of the instrument:

  1. Increase of the geometrical factor by at least a factor of 10 (to improve the measurement statistics significantly)
  2. Improvement of the charge resolution to deltaQ/Q 0.1 below 0.7 MeV/nucleon (to allow resolution of individual charge states for elements up to oxygen.

For more in-depth information on the instrument, read the full Instrument Description.


SEPICA is being developed by The Space Science Center of UNH in collaboration with the Max-Planck-Institut fur extraterrestrische Physik (MPE), Garching, Germany.