The abundance of chlorine is determined from X-ray spectra obtained with the RESIK instrument on CORONAS-F during solar flares between 2002 and 2003 .
Using weak lines of He-like Cl , Cl xvi , between 4.44 and 4.50 Å , and with temperatures and emission measures from GOES on an isothermal assumption , we obtained A ( { Cl } ) = 5.75 \pm 0.26 on a scale A ( { H } ) = 12 .
The uncertainty reflects an approximately factor 2 scatter in measured line fluxes .
Nevertheless our value represents what is probably the best solar determination yet obtained .
It is higher by factors of 1.8 and 2.7 than Cl abundance estimates from an infrared sunspot spectrum and nearby H ii regions .
The constancy of the RESIK abundance values over a large range of flares ( GOES class from below C1 to X1 ) argues for any fractionation that may be present in the low solar atmosphere to be independent of the degree of solar activity .