## 6. Plasma filling factor at the top of the PFL systemThe results obtained from CDS electron density and emission measure analysis were used to estimate the geometrical filling factor of the examined PFL system in the Fe XIV line. If we assume that the plasma is isothermal and concentrated in filaments with a typical electron density , we can rewrite the formula for emission measure in this way: where In this integral we calculate the total thickness of the radiating elements along the line of sight. Using these quantities we can define the geometrical filling factor as The density in Eq. 8 has
been derived from the density sensitive line ratio Fe XIV
334.2/353.8. This ratio measures the real electron density. The value
of has been derived by dividing the
emission measure The quantities and are identical only in the case of homogeneous distribution of plasma along the line of sight. In this case the filling factor would be equal to one. If plasma is not distributed uniformly, the electron density obtained from line ratio is greater than the density obtained from the emission measure and the filling factor is less than one. Using this method we determined the filling factor in pixels
26 - 30 (
The value of the geometrical filling factor in flare loops has an important consequence. The majority of flare loops observations have been made by Yohkoh/SXT. However, SXT can measure only the emission measure and not the electron density in observed regions. An approximation which assumes a homogeneous and isothermal plasma is often used to derive the electron density. This approximation does not take into account the possibility that the spatial plasma distribution can be filamentary. If we assume that the typical electron density in such filaments is approximately the same for all of them and the electron density outside the filaments is much smaller than inside, knowing the filling factor we can estimate the real electron density of emitting elements with a much better accuracy. © European Southern Observatory (ESO) 2000 Online publication: March 9, 2000 |