[Tex/LaTex] math equation with underline has different heights

Question

I struggle with some equations in my research paper. I want to underline some variable, because they are complex, but if I use \underline the line is not at the same level in my example:

\documentclass{article}
\usepackage{amsmath}
\usepackage[normalem]{ulem}
\begin{document}

%underline example
\begin{align}
m = -\dfrac{3}{2} z_\mathrm{p} \operatorname{Im}\{\underline{\psi}_1^{\angle k} \underline{i}_1^{\angle k *} \}
\end{align}

%ulem package example
\begin{align}
m = -\dfrac{3}{2} z_\mathrm{p} \operatorname{Im}\{\uline{\psi}_1^{\angle k}\uline{i}_1^{\angle k *} \}
\end{align}
\end{document}

I hope you can read the example. I also wanted to use the ulem packages or soul package, but it didn't work with the \align environment.
I would be really happy if anybody could help me!

Thx
Max

Edit: I also added the example with ulem but still the same problem

Answer 1

\vphantom{\psi} helps. Then the underlining command for i sees the larger descender of \psi, while \psi is not visible:

\documentclass{article}
\usepackage{amsmath}
\usepackage[normalem]{ulem}
\begin{document}

%underline example
\begin{align}
m = -\dfrac{3}{2} z_\mathrm{p} \operatorname{Im}\{\underline{\psi}_1^{\angle
k} \underline{i\vphantom{\psi}}_1^{\angle k *} \}
\end{align}

%ulem package example
\begin{align}
m = -\dfrac{3}{2} z_\mathrm{p} \operatorname{Im}\{\uline{\psi}_1^{\angle
k}\uline{i\vphantom{\psi}}_1^{\angle k *} \}
\end{align}
\end{document}