A system for assessing one or more temperatures in an opening in a subsurface formation includes an electrical conductor and electrical insulation at least partially surrounding the electrical conductor. The electrical insulation includes magnesium oxide and a ferroelectric material. An electrically conductive sheath at least partially surrounds the electrical insulation. A profile of one or more dielectric properties of the electrical insulation along a length of the electrical insulation is assessed during use to assess a temperature profile with spatial resolution along the length of the electrical insulation.