I was reading various reviews of neutrino physics but I couldn't understand the following to complete satisfaction.

How is $\theta_{12}$ identified the Solar mixing angle and $\Delta m^2_{21}$ the Solar mass-squared difference?

The formulae for three-flavor oscillation probabilities, in general, depends on all three mixing angles $\theta_{12},\theta_{23}$ and $\theta_{13}$. It's not clear to me why only one mixing angle be associated with solar neutrino oscillation.

I was reading various reviews on neutrino physics but I couldn't understand the following to complete satisfaction.

How is $\theta_{12}$ identified with the Solar mixing angle and $\Delta m^2_{21}$ the Solar mass-squared difference?

The formulae for three-flavor oscillation probabilities, in general, depends on all three mixing angles $\theta_{12},\theta_{23}$ and $\theta_{13}$:

$$P_{\alpha\rightarrow\beta}=\delta_{\alpha\beta} - 4 \sum_{i>j} {\rm Re}(U_{\alpha i}^{*} U_{\beta i} U_{\alpha j} U_{\beta j}^{*}) \sin^2 \left(\frac{\Delta m_{ij}^2 L}{4E}\right) \\ + 2\sum_{i>j}{\rm Im}(U_{\alpha i}^{*}U_{\beta i}U_{\alpha j} U_{\beta j}^{*}) \sin\left(\frac{\Delta m_{ij}^2 L}{2E}\right).$$

It's not clear to me why only one mixing angle be associated with solar neutrino oscillation. An answer starting from the three-flavour oscillation formulae will be helpful for my understanding.