Resonance interaction loci along meridional field loops (﹩{\phi }_{* }=0^\circ ﹩) for observer viewing angles ﹩{\theta }_{v}=60^\circ ﹩ (left panel) and ﹩{\theta }_{v}=120^\circ ﹩ (right panel). Contours of constant ﹩{\rm{\Psi }}\equiv {B}_{p}/[2{\varepsilon }_{f}{\gamma }_{e}]﹩ are depicted, with a range ﹩{10}^{-4}\lt {\rm{\Psi }}\lt 4\times {10}^{3}﹩, being color-coded according to the value of the dimensionless final photon energy ﹩{\varepsilon }_{f}﹩, as indicated in the legend on the right; the black loci constitute the value of ﹩{\varepsilon }_{f}={10}^{-0.5}\approx 160﹩ keV (i.e., ﹩{\rm{\Psi }}\approx 0.16﹩). The colatitudes of these loci as functions of the altitude are solutions of Equation (32). The field strength ﹩{B}_{p}=100﹩ and electron Lorentz factor ﹩{\gamma }_{e}={10}^{3}﹩ are kept fixed. As the magnetic field drops with increasing altitude, the interaction points converge toward a radial dotted blue line with the colatitude ﹩{\vartheta }_{0}﹩ that satisfies Equation (34), which is here ﹩{\vartheta }_{0}\approx 0.62\pi =111\buildrel{\circ}\over{.} 6﹩ for the left panel and ﹩{\vartheta }_{0}\approx 0.78\pi =141\buildrel{\circ}\over{.} 6﹩ for the right panel.