Dear Professor Draine,

I am using DDSCAT 7.3 to calculate the orientationally-averaged values of *Q*_{abs}, *Q*_{sca} and <cos*θ*> of randomly-oriented soot aggregates interacting with monochromatic plane waves with specified polarization states. According to * User Guide for the Discrete Dipole Approximation Code DDSCAT 7.3*, DDSCAT allows the user to specify a general elliptical polarization state for the incident radiation, by specifying the (complex) polarization vector ${\hat e_{01}}$ (see §24). Moreover, if

*ddscat.par*specifies IORTH = 2, the orthonormal polarization state ${\hat e_{02}} = {\hat x_{LF}} \times \hat e_{01}^*$is generated automatically. However, there are some confusing questions on how to specify an arbitrary polarization state for the incident plane wave in

*ddscat.par*as follows:

(1) Assume that the incident plane wave has a specified polarization state represented by the following Jones vector:

(1)where *E*_{0y} and *E*_{0z} are the amplitude of the two components of the electric field vector, while *δ*_{y} and *δ*_{z} are the phase of the two components of the electric field vector, **should we simply set ${\hat e_{01}}$ by specifying (0, 0) (cos β, 0) (sinβcosδ, - sinβsinδ) in ddscat.par ?**

(2) Assume that we simply set ${\hat e_{01}} = {\hat y_{LF}}$ and ${\hat e_{02}} = {\hat z_{LF}}$, and calculate the orientationally-averaged values of *Q*_{abs}, *Q*_{sca} and <cos*θ*> in the two orthonormal polarization states, **can we calculate the orientationally-averaged values of Q_{abs}, Q_{sca} and <cosθ> in an arbitrary incident polarization state, just based on the orientationally-averaged values of Q_{abs}, Q_{sca} and <cosθ> in the two orthonormal polarization states ?**

*wxxxryyy.avg*, there are two lines like this:

Qext | Qabs | Qsca | g(1)=<cos> | <cos^2> | Qbk | Qpha | |

JO=1: | 9.2037E-01 | 8.1852E-01 | 1.0185E-01 | 7.2936E-02 | 4.0114E-01 | 1.0141E-02 | 5.3381E-01 |

JO=2: | 9.2037E-01 | 8.1852E-01 | 1.0185E-01 | 7.2936E-02 | 4.0114E-01 | 1.0141E-02 | 5.3380E-01 |

I am wondering **if the data in the lines of JO=1 and JO=2 in the red box are the calculation results of Q_{abs}, Q_{sca} and <cosθ> in the incident polarization states ${\hat e_{01}}$ and ${\hat e_{02}}$, respectively ?**

It is highly appreciated if you would instruct me on these questions, and I am looking forward to your reply.

Sincerely yours

Ya-fei Wang