The effect of hydrostatic weighting on the vertical temperature structure of the solar corona

We investigate the effect of hydrostatic scale heights lambda(T) in coronal loops on the determination of the vertical temperature structure T(h) of t he solar corona. Every method that determines an average temperature at a p articular line of sight from optically thin emission (e.g., in EUV or soft X-ray wavelengths) of a mutlitemperature plasma is subject to the emission measure-weighted contributions dEM(T)/dT from different temperatures. Becau se most of the coronal structures (along open or closed field lines) are cl ose to hydrostatic equilibrium, the hydrostatic temperature scale height in troduces a height-dependent weighting function that causes a systematic bia s in the determination of the temperature structure T(h) as function of alt itude h. The net effect is that the averaged temperature seems to increase with altitude, dT(h)/dh > 0, even if every coronal loop (of a multitemperat ure ensemble) is isothermal in itself. We simulate this effect with differe ntial emission measure distributions observed by SERTS for an instrument wi th a broadband temperature filter such as Yohkoh/Soft X-Ray Telescope and f ind that the apparent temperature increase due to hydrostatic weighting is of order Delta T approximate to T(0)h/r(circle dot). We suggest that this e ffect largely explains the systematic temperature increase in the upper cor ona reported in recent studies (e.g., by Sturrock et al., Wheatland et al., or Priest et al.), rather than being an intrinsic signature of a coronal h eating mechanism.