X-ray Polarization Structure in the Jet of Markarian 421

We present measurements of the polarization of X-rays in the 2-8 keV energy band of the blazar Markarian 421 (Mrk 421) using data from the Imaging X-ray Polarimetry Explorer (IXPE). Mrk 421 is a prototypical high synchrotron peak (HSP) blazar, with a broad-band SED showing a synchrotron peak at ~ 2 keV, at the low end of the IXPE energy range. This blazar was observed three times with IXPE during the first 6 months of IXPE observations, and these observations are well complemented with optical and radio polarimetry as well as X-ray flux andspectral observations from Swift, XMM-Newton, and NuSTAR. These combined data offer a new and unique window into the magnetic field structure and particle acceleration mechanisms responsible for the X-rays observed from this blazar. We will also discuss these results in the larger context of other blazars already observed with IXPE.