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We test the X-ray emission predictions of galactic fountain models againstXMM-Newton measurements of the emission from the Milky Way's hot halo. Thesemeasurements are from 110 sight lines, spanning the full range of Galacticlongitudes. We find that a magnetohydrodynamical simulation of asupernova-driven interstellar medium, which features a flow of hot gas from thedisk to the halo, reproduces the temperature but significantly underpredictsthe 0.5-2.0 keV surface brightness of the halo (by two orders of magnitude, ifwe compare the median predicted and observed values). This is true for versionsof the model with and without an interstellar magnetic field. We considerdifferent reasons for the discrepancy between the model predictions and theobservations. We find taking into account overionization in cooled halo plasma,which could in principle boost the predicted X-ray emission, is unlikely inpractice to bring the predictions in line with the observations. We also findthat including thermal conduction, which would tend to increase the surfacebrightnesses of interfaces between hot and cold gas, would not overcome thesurface brightness shortfall. However, charge exchange emission from suchinterfaces, not included in the current model, may be significant. Thefaintness of the model may also be due to the lack of cosmic ray driving,meaning that the model may underestimate the amount of material transportedfrom the disk to halo. In addition, an extended hot halo of accreted materialmay be important, by supplying hot electrons that could boost the emission ofthe material driven out from the disk. Additional model predictions are neededto test the relative importance of these processes in explaining the observedhalo emission.

The Origin of the Hot Gas in the Galactic Halo: Testing Galactic Fountain Models' X-ray Emission