function get_mf_from_slope_freq, f, pol,spi
;magnetic field from curcular polarization and local slope
;pol - polarization in % at f2
;f2 in Hz

;sub=where(spi eq 0 or abs(pol) lt 0.01)
sub=where(abs(spi) le 1e-6,count)
print,f, 100*count/504./504.
;sub=where(spi le 0)
if count ge 1 then spi(sub)=1e-6
mf=pol*f/2.8e6/1e2/spi ;else mf=0.
if count ge 1 then mf(sub)=0.0
return,mf
end