9.3  The emission measure loci approach

One direct approach is to plot the ratio I_ob / G(T) for each line as a function of temperature and consider the loci of these curves to constrain the shape of the emission measure distribution. In fact, for each line and temperature T_i the value I_ob / G(T_i) represents an upper limit to the value of the line emission measure EM_L (Equation 23, below) at that temperature, assuming that all the observed emission I_ob is produced by plasma at temperature T_i.

Obtain the G(T) functions from the CHIANTI software, e.g.

gofnt, 'o_5', 580, 635, t_o_5, g_o_5,  d_o_5, $
press=1e15, abund_name=!xuvtop+'/abundance/sun_coronal.abund', $
ioneq_name=!xuvtop+'/ioneq/mazzotta_etal.ioneq'


; [default units are erg cm^+3 s^-1 sr-1 ]

; Suppose you have an O V 630 A intensity of 1812 [erg cm^-23 s^-1 sr-1]:


F_o_5= 1812./g_o_5 

plot, alog10(t_o_5), alog10(F_o_5),$
 xr=[4.5, 7.], yr=[25, 28], /yst, /xst, $
 xtit='Log T [K]',  ytit='Log F(T) [cm-5]', chars=1.3