diff --git a/documentation/khione/user/latex/2_heat_budget.tex b/documentation/khione/user/latex/2_heat_budget.tex
index fb12f85cbf7dc595ea7b1713d82e007ae6fccd5d..2d4a55795298d5b1b76951781606f6d8a6398127 100644
--- a/documentation/khione/user/latex/2_heat_budget.tex
+++ b/documentation/khione/user/latex/2_heat_budget.tex
@@ -350,7 +350,7 @@ in which,
 In summary, the effective back radiation, $\phi_B$, is as follows:
 
 \begin{equation} \label{eq:phiball}
-\phi_B=-0.97\sigma\Big[\ T_{sk}^4-\epsilon_a ( 1+kC^2 ) T_{sk}^4\ \Big]
+\phi_B=-0.97\sigma\Big[\ T_{sk}^4-\epsilon_a ( 1+kC^2 ) T_{ak}^4\ \Big]
 \end{equation}
 
 %===============================================================================