  The proton-proton ($pp$) fusion cross-section found at the heart of solar
models is unconstrained experimentally and relies solely on theoretical
calculations. Effective field theory provides an opportunity to constrain the
$pp$ cross-section experimentally, however, this method is complicated by the
appearance of two-nucleon effects in the form of an unknown parameter
$L_{1,A}$. We present a method to constrain $L_{1,A}$ using the Standard Solar
Model and helioseismology. Using this method, we determine a value of $L_{1,A}$
= 7.0 fm$^{3}$ with a range of 1.6 to 12.4 fm$^{3}$. These results are
consistent with theoretical estimates of $L_{1,A} \approx$ 6 fm$^{3}$.
