5 Instrumentation


The CELIAS/STOF (Suprathermal Time Of Flight) mass spectrometer on board SOHO measures the elemental and charge composition of ions with suprathermal energies in the range 35--4000 keV/e. The sensor applies electrostatic deflection, time-of-flight measurement, and the detection of the residual energy in a solid state detector to determine the mass, charge, and energy of an ion. STOF measures the elemental and ionic composition in the energy-per-charge range from 35~keV/e to 660 keV/e and the subsystem HSTOF (H stands for Highly) the elemental composition of particles with energies per charge between 80 keV/e and ≈ 4 MeV/e. The geometrical factor of STOF is 0.05 cm2 sr, and that of HSTOF 0.22 cm2 sr. A detailed description of the CELIAS sensors is given in
Hovestadt et al. [1995].


Figure 6;Schematic drawing of the (H)STOF sensor (from
Hovestadt et al. [1995])

Data Analysis

The data analysis is based on two sets of data: The so-called Pulse Height Analysis (PHA) words indicate the time of flight, the energy measured in the solid-state detector, the position on the microchannel plates creating the start and the stop signal, respectively, the gain flag of the solid-state detector, and the energy per charge of an ion when entering the instrument. PHA words allow us to perform consistency checks and to disregard events in high-background (low signal-to-noise ratio) pixels of the solid-state detectors.
If the particle fluxes are high, as they are for example during the Bastille Day Event (July 14--16, 2000), only a subset of PHA words is transmitted. The rates are accumulated on board after a classification and selection scheme for STOF and HSTOF. The rates have better statistics, whereas the PHA words contain more information about a single event. We have verified that both data sets deliver consistent results.
A detailed description of the analysis is given in Bamert et al. [2002]