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The Gaia mission has provided the largest ever astrometric chart of the MilkyWay. Using it to map the Galactic halo is helpful for disentangling its mergerhistory. The identification of halo stars in Gaia DR2 with reliable distanceestimates requires special methods because such stars are typically fartheraway and scarce. We apply the reduced proper motion (RPM) method to identifyhalo main sequence stars on the basis of Gaia photometry and proper motions.Using the colour-absolute-magnitude relation for this type of stars, wecalculate photometric distances. Our selection results in a set of $\sim10^7$tentative main sequence halo stars with typical distance uncertainties of $7\%$and with median velocity errors of 20 km/s. The median distance of our sampleis $\sim 4.4$ kpc, with the faintest stars located at $\sim 16$ kpc. Thespatial distribution of the stars in our sample is centrally concentrated.Visual inspection of the mean velocities of stars on the sky revealslarge-scale patterns as well as clear imprints of the GD-1 stream and tentativehints of the Jhelum and Leiptr streams. Incompleteness and selection effectslimit our ability to interpret the patterns reliably as well as to identify newsubstructures. We define a pseudo-velocity space by setting to zero theline-of-sight velocities of our sample stars. In this space, we recover severalknown structures such as the footprint of Gaia-Enceladus (i.e. theGaia-Sausage) as well as the Helmi streams and some other retrogradesubstructures (Sequoia, Thamnos). We show that the two-point velocitycorrelation function reveals significant clustering on scales smaller than 100km/s, of similar amplitude as found for the 6D Gaia halo sample. Thisclustering indicates the presence of nearby streams that are predominantlyphase-mixed.

The Reduced Proper Motion selected halo: methods and description of the catalogue