Langmuir Probe observations during eclipses of Cassini with Saturn and the Main Rings: ring optical depths and photoelectrons

The Langmuir Probe (LP) onboard Cassini was one of the three experiments that could measure the cold inner magnetospheric plasma, along with the Radio and Plasma Waves Science (RPWS) and the Cassini Plasma Spectrometer (CAPS). While the century-old LP theory looks quite straight-forward, in reality things are much more complicated.

The operation of the LP is quite simple: by applying positive bias voltages, the probe attracts the electrons and repels the ions of the surrounding plasma. From the resulting current-voltage curve characteristics of the ambient electrons can be estimated, i.e. density and temperature. When negative bias voltages are applied to the probe the characteristics of the ambient ions can be estimated, i.e. density, temperature, and mass.

Though the LP operation and interpretation are quite simple and straightforward, there are assumptions made and therefore the theoretical models may not always reflect the actual plasma conditions in Saturn’s magnetosphere. For this study we are focused on the effect of the photoelectrons, i.e. electrons generated by the incident sunlight on Cassini’s surfaces, that are difficult to calibrate for on the ground and then observe and characterise in the LP data.

We present algorithms for identifying when Cassini is in the shadow of Saturn and its rings, and when the LP is in the shadow of Saturn, its rings or Cassini itself. The LP data inside and outside the eclipses are compared using the algorithms developed. In this presentation we will first discuss the impact of the photoelectron generation from the spacecraft surfaces to the LP current-voltage curves, and understand the variations of the measured plasma density connected with the photoelectrons. Then, using that knowledge, we attempt to define the optical depth of the rings in the wavelengths the LP operates in.