Referee report



The paper by Prabhu et al. describes a novel way to compute helicity proxies using the observed linear polarization Stokes profiles or the vector magnetic fields inferred from inverting them. The paper finds a significant correlation of the helicity proxies and the solar hemisphere where the active regions are observed. This result is very relevant given the difficulties encountered when measuring helicity using older methods.  I only have minor comments:

1.- Section 2 first paragraph. It says “S is the source-function vector, which, under the assumption of local thermodynamic equilibrium (LTE), can be approximated as S≡(Sν(T),0,0,0), where Sν(T ) is the source function.” I think what the authors really want to say is “can be approximated as S≡(Bν(T),0,0,0), where Bν(T ) is the Planck function.”

2.- Equation 5 uses a factor epsilon that is called emissivity that is poorly defined. The relation between Stokes Q and U and the transverse components b_theta and b_phi is very complex (even in LTE) if one goes to the exact formulation of the radiative transfer equation. Equation 5 is an approximation. Understanding the assumptions behind equation 5 requires a better definition of the emissivity (even if later is set equal to 1).   Note that the radiative transfer equation for polarized light uses an emissivity too, but it is unclear as the authors refer to it. 

I went to Brandenburg et al (2019) and the emissivity is equally poorly defined there. Please provide a proper definition of epsilon.

3.- Section 3.1 and discussion on the wavelengths of SDO. The paper says several times that the wavelengths closer to line center are lambda_2 and lambda_3. This is true when there are no Doppler shifts of the lines cause by solar conditions (solar rotation, Evershed flow) or by the satellite orbital motions. This is recognized in Sections 3.6 (when explaining the change in sing of E for AR 11542) and in Section 3.7 where the orbital velocity of SDO is explicitly mentioned. This is correct, but the SDO orbital velocity and the combination of Solar rotation can contribute to a Doppler shift of 6 km/s. This shift is as large as 120 miliangstroms, meaning that line center in extreme cases will be centered on lambda_1 or lambda_4. Thus, these wavelengths are also potentially impacted by Faraday rotation, admittedly less than the two central ones. 

4. The paper ends with this statement “Stokes V carries with it additional information about the directionality of the line-of-sight magnetic field, which has not been used in the present study.” This is not entirely correct. The authors have used inversion codes in the paper (VFISV and HeLIx+) that do use Stokes V. Please rephrase admitting that it is only in using equation 5 that Stokes V is not used.