Non–equilibrium ionization and the interpretation of Yohkoh/SXT data during solar dynamic events

Yohkoh has been dedicated to study the structure and evolution of the solar corona, especially during dynamic events. During such events, however, the collisional equilibrium ionization – which is invariably assumed when deriving the plasma parameters from the observations – may not apply. We explore the influence of the non–equilibrium ionization effects (NEI) on the values of temperature and emission measure derived from the data collected with the Solar X–ray Telescope (SXT) on board the Yohkoh satellite during solar dynamic events. To this end, we have simulated such dynamic events, assuming that the plasma temperature rises instantaneously from the value T0 to the value T1, and then it remains constant. We have considered two different temperature jumps: the first one from 106.3 K to 107.3 K to simulate a flare and the other, from 105.7 K to 106.5 K, to simulate a microflare. We show that the temperature determination with the SXT wide band filter ratio is, to some extent, affected by non– equilibrium ionization: the maximum fractional error on T1 amounts to ∼ 0.7 for the flare and ∼ 0.5 for the microflare we have simulated. On the other hand, the emission measure determination can be considerably affected by such departures: the maximum fractional error on EM amounts to ∼ 2 for the flare and ∼ 7 for the microflare we have simulated. Therefore NEI effects can be important on the temperature and emission measure determination during fast evolving phenomena.