As far as can be told, the 'solar mirror' phenomenon is a) normal within stars in this fictional universe and b) a purely fictional construct of the author.
There are literally pages of borderline pseudo-science about this, but the most obvious passage to describe the phenomenon of solar mirror reflectivity is below.
But now, Ye made the first step in confirming her guess about the gain reflectivity of solar energy mirrors: The energy mirrors not only reflected radiation coming from the lower-frequency side, but amplified it. All the mysterious sudden fluctuations within narrow frequency bands that she had observed were in fact the result of other radiation coming from space being amplified after reflecting off an energy mirror in the sun. That was why there were no observable disturbances on the surface of the sun.
This time, after the Jovian radio outbursts reached the sun, they were re-emitted, as if by a mirror, after being amplified about a hundred million times. The Earth received both sets of emissions, before and after the amplification, separated by sixteen minutes and forty-two seconds.
The sun was an amplifier for radio waves.
However, there was a question: The sun must be receiving electromagnetic radiation from space every second, including radio waves emitted by the Earth. Why were only some of the waves amplified? The answer was simple: In addition to the selectivity of the energy mirrors for frequencies they would reflect, the main reason was the shielding effect of the solar convection zone. The endlessly boiling convection zone situated outside the radiation zone was the outermost liquid layer of the sun. The radio waves coming from space must first penetrate the convection zone to reach the energy mirrors in the radiation zone, where they would be amplified and reflected back out. This meant that in order to reach the energy mirrors, the waves would have to be more powerful than a threshold value. The vast majority of Earth-based radio sources could not cross this threshold, but the Jovian radio outburst did—
And Red Coast’s maximum transmission power also exceeded the threshold.
The problem with solar outages was not resolved, but another exciting possibility presented itself: Humans could use the sun as a superantenna, and, through it, broadcast radio waves to the universe. The radio waves would be sent with the power of the sun, hundreds of millions of times greater than the total usable transmission power on Earth.
Earth civilization had a way to transmit at the level of a Kardashev Type II civilization.