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HD 219134 hosts several planets, with seven candidates reported, and the twoshortest period planets are rocky (4-5 $M_{\oplus}$) and transit the star. Herewe present contemporaneous multi-wavelength observations of the star HD 219134.We observed HD 219134 with the Narval spectropolarimeter at the Observatoire duPic du Midi, and used Zeeman Doppler Imaging to characterise its large-scalestellar magnetic field. We found a weak poloidal magnetic field with an averageunsigned strength of 2.5 G. From these data we confidently confirm the rotationperiod of 42 days, measure a stellar inclination of 77$\pm$8 degrees, and findevidence for differential rotation. The projected obliquity of the twotransiting super-Earths is therefore between 0 and 20 degrees. We employed HSTSTIS observations of the Ly$\alpha$ line to derive a stellar wind mass-lossrate of half the solar value ($10^{-14} M_{\odot} {\rm yr}^{-1}$). We furthercollected photometric transit observations of the closest planet at near-UVwavelengths centred on the Mg II h&k lines with AstroSat. We found nodetectable absorption, setting an upper limit on the transit depth of about 3%,which rules out the presence of a giant magnesium cloud larger than 9 planetradii. Finally, we estimated the high-energy flux distribution of HD 219134 asseen by planets b and c. These results present a detailed contemporaneouscharacterisation of HD 219134, and provide the ingredients necessary foraccurately modelling the high-energy stellar flux, the stellar wind, and theirimpact on the two shortest-period planets, which will be presented in thesecond paper of this series.

Characterisation of the HD 219134 multi-planet system I. Observations of stellar magnetism, wind, and high-energy flux