Charge State Measurements of Solar Energetic Particles Observed with SAMPEX

Abstract

We have measured the ionization states of major elements accelerated in two large solar energetic particle events by use of the geomagnetic cutoff technique. The observations were made in 1992 October/November using instrumentation on the SAMPEX satellite, which is in an 82° inclination low Earth orbit. We calibrate the geomagnetic cutoff rigidity by assuming that protons and He observed on SAMPEX are fully stripped. The mean charge states of the heavier elements are then determined from their measured cutoffs, along with this calibrated scale. Our results cover the energy range ~0.5-5 MeV nucleon^(-1) and are in good agreement with earlier studies by Luhn et al. (1985) near -1 MeV nucleon^(-1) in all cases except for Fe. We find a mean charge state of 11.04±0.22 for Fe, while the average from Luhn is 14.09±0.09. The lower ionization state of Fe reported here appears to be consistent with the general range of values recently measured in solar wind Fe on the Ulysses deep space probe. Although the observational picture is still incomplete, overall the present results confirm and extend previous evidence that the solar energetic particles in large events are accelerated from the coronal or solar wind material.