Following the central region of the Saturnian nightside plasma sheet

Abstract

We use magnetic field data from Cassini’s on board magnetometer (MAG), plasma measurements by Cassini Plasma Spectrometer (CAPS) and electron density measurements from the RPWS Langmuir probe (LP) on board Cassini, for an extended set of orbits from 2004 to 2010 and for distances up to 30 Rs from the planet. The central region of the nightside plasma sheet is identified using the radial component of the magnetic field Br, in contrary to solely geometrical criteria used in previous studies, such as the vertical distance z from the equatorial plane. This parameter reverses its sign around the center of the plasma sheet regardless of the variable equatorial displacement of the Saturnian magnetotail. We give the radial distribution for all data points used in this study to point the magnitude of our data set towards our statistics. We analyze two characteristic examples of Cassini spacecraft nightside plasma sheet pass. An equatorial pass for days 143 to 145 of 2006 and a near equatorial pass for days 222 to 225 of 2010 showing respectively how we can misleadingly conclude that the spacecraft remains continuously in or out of the plasma sheet throughout a nightside pass. We filter our data set using two Br criteria: Br<0.5 nT and Br<0.2 |B|. This way we consider only time periods during which the radial component of the magnetic field is below 0.5 nT and less than 1/5 of the total magnetic field, so that the field vector points within ~11° from the local B_Θ-B_Φ plane and we end up with 1806 10-min data points. We produce an electron number density radial distribution for the nightside plasma sheet of the planet considering only distances greater than 15 Rs and compare our results to the results from Morooka et al. and Persoon et al. Our results follow the described decreasing trend of these studies for the electron number density as we move outwards from the planet but with a decreasing rate of approximately L^-3.59 accompanied by an uncertainty given by the standard error equal to ± 0.42. The derived fit line approaches the upper limit given by Morooka et al. varying as L^-3 while remaining higher than the electron density model fit line by Persoon et al. varying as L^-4.14. The ratio of Br to z filtered electron density (Ne) data is also given, taking values greater than one for more than 90 per cent of the cases. The data distribution is also examined for three different space parameters: The distance from the planet Rs, the local time LT and the vertical equatorial distance z, while the distribution of the ratio of the Br filtered to total data for each parameter is also studied. We further search for seasonal change of the Br filtered data around the central region of the Saturnian plasma sheet before and after decimal year 2008.5 in different spatial distributions (the limit of 2008.5 refers to a 5˚ angle between the solar wind flow direction and the magnetic equatorial plane of Saturn which roughly delimits the beginning of the near Saturnian equinox conditions). Finally we search for seasonal change of the plasma sheet position for three near vertical nightside plasma sheet crossings and attempt an initial estimation of the plasma sheet thickness. We conclude that the Br criteria give overall higher electron density values as we follow closer the plasma sheet center in its daily and seasonal motion and can be used to further improve Morooka et al. study results.

Results

• Final report

Cassini at Saturn. [Image credit: NASA]