The solar corona, the Sun's outer atmosphere, stretches millions of kilometers, yet it's visible only during a total eclipse or with specialized instruments. Typically, we gauge the corona's density by how it scatters white light, but that method relies on simplifications that can skew the results. Spectroscopy can't help directly here, because the corona is far too tenuous.
Now there's another way: catch the radio waves the corona emits itself. The OVRO-LWA radio telescope in California picks up low-frequency radiation from the Sun and turns it into images. From the brightness of the radio signal at different frequencies, you can calculate how many free electrons lie along the line of sight. It's like gauging how dense the air is by the loudness of an echo in the mountains.
The new measurements matched data from coronagraphs and theoretical models. The researchers also derived a formula describing how electron density falls off with distance from the Sun. It's simple: the farther from the Sun, the fewer particles, and this decline is described by a sum of three terms.
The radio method is fast and doesn't depend on Earth's weather. OVRO-LWA delivers ready-made images of the corona with little delay, so such measurements can be made regularly, like a daily weather report for the solar atmosphere. This will help us better understand how the corona feeds the solar wind and affects space weather.
The electron density in the middle corona is about a billion times lower than in the air at Earth's surface, yet it's these electrons that scatter radio waves and let us 'weigh' the corona.