Model of how the emission measure scales with height for the flux tubes in Figure 19. The functions plotted are n ² (dotted line), n ² B ^−1/2 (dashed line), and n ² B ⁻¹ (solid line). We assume a hydrostatic atmosphere n∝exp (− z/ z ₀) with z ₀ = 50 Mm for all curves and the field strength dropping like that of a point dipole, B( z) = z ₀/ z ³ (the constant for B is chosen such that all three curves intersect at one density scale height).