Dynamic spectra of the τ Boo b observation L570725 in Stokes-V (). Panel a: ON-beam. Panel b: OFF-beam 2. Panel c: ON-beam minus OFF-beam 2. Panel d: OFF-beam 1 minus OFF-beam 2. Panel e: ON-beam divided by the OFF-beam 2. Panel f: OFF-beam 1 divided by the OFF-beam 2. Data from the range 21–29 MHz is shown. The ON-beam signal is clearly seen as structured excess emission in panels a, c, and e. A faint signal is seen in the OFF beams (e.g., panel c) likely due to ionospheric refraction of the ON-beam signal (also see Appendix G). However, there is no visible excess difference when the two OFF-beams are subtracted by each other (panel d) suggesting that whatever is causing the faint OFF-beam signal is the same for both beams.