Sub-Gauss magnetic fields are crucial for many physical processes but hard to measure. We report the first observational results that measure 3D topology as well as the strength of a sub-milliGauss magnetic field simultaneously beyond our solar system (arXiv:1903.08675). This is the first observational proof in line with the magnetic tracer ground state alignment (GSA) proposed a decade ago. Our analysis reveals the first sub-AU scale magnetic field on 89 Her, that is 1.3kpc away, has a 3D orientation aligned to the stellar outflow axis and a strength 70 ~ 150 microGauss, thus substantially improving the accuracy by five orders of magnitude compared to the previous 10G upper limit set by non-detection of Zeeman effect. GSA has shown great potentials, including abundant observables, the possibility of multi-waveband observation from UV to submillimeter fine-structure lines, as well as a large field strength coverage (from sub-Gauss at the atmosphere of evolving star all the way down to weaker than nanoGauss InterGalactic medium). We anticipate our observation to start a new era of magnetic field mapping in the universe, where much more information and higher precision can be achieved with current observational facility from our Galaxy to the early universe if accounting for GSA.
