arXiv subjects

M. F. Thomsen

Publications and source records attributed to M. F. Thomsen.

3 recordsLinked to original sources

arXiv2018

Cassini CAPS identification of pickup ion compositions at Rhea

Saturn's largest icy moon, Rhea, hosts a tenuous surface-sputtered exosphere composed primarily of molecular oxygen and carbon dioxide. In this Letter, we examine Cassini Plasma Spectrometer velocity space distributions near Rhea and confirm that Cassini detected nongyrotropic fluxes of outflowing CO$_2^+$ during both the R1 and R1.5 encounters. Accounting for this nongyrotropy, we show that these possess comparable alongtrack densities of $\sim$2$\times$10$^{-3}$ cm$^{-3}$. Negatively charged pickup ions, also detected during R1, are surprisingly shown as consistent with mass 26$\pm$3 u which we suggest are carbon-based compounds, such as CN$^-$, C$_2$H$^-$, C$_2^-$, or HCO$^-$, sputtered from carbonaceous material on the moons surface. These negative ions are calculated to possess alongtrack densities of $\sim$5$\times$10$^{-4}$ cm$^{-3}$ and are suggested to derive from exogenic compounds, a finding consistent with the existence of Rhea's dynamic CO$_2$ exosphere and surprisingly low O$_2$ sputtering yields. These pickup ions provide important context for understanding the exospheric and surface-ice composition of Rhea and of other icy moons which exhibit similar characteristics.

physics.space-ph↗

arXiv2013

Electron acceleration to relativistic energies at a strong quasi-parallel shock wave

Electrons can be accelerated to ultrarelativistic energies at strong (high-Mach number) collisionless shock waves that form when stellar debris rapidly expands after a supernova. Collisionless shock waves also form in the flow of particles from the Sun (the solar wind), and extensive spacecraft observations have established that electron acceleration at these shocks is effectively absent whenever the upstream magnetic field is roughly parallel to the shock surface normal (quasi-parallel conditions). However, it is unclear whether this magnetic dependence of electron acceleration also applies to the far stronger shocks around young supernova remnants, where local magnetic conditions are poorly understood. Here we present Cassini spacecraft observations of an unusually strong solar system shock wave (Saturn's bow shock) where significant local electron acceleration has been confirmed under quasi-parallel magnetic conditions for the first time, contradicting the established magnetic dependence of electron acceleration at solar system shocks. Furthermore, the acceleration led to electrons at relativistic energies (~MeV), comparable to the highest energies ever attributed to shock-acceleration in the solar wind. These observations demonstrate that at high-Mach numbers, like those of young supernova remnant shocks, quasi-parallel shocks become considerably more effective electron accelerators.

astro-ph.HE↗

arXiv2010

Influence of Hot Plasma Pressure on the Global Structure of Saturn's Magnetodisk

Using a model of force balance in Saturn's disk-like magnetosphere, we show that variations in hot plasma pressure can change the magnetic field configuration. This effect changes (i) the location of the magnetopause, even at fixed solar wind dynamic pressure, and (ii) the magnetic mapping between ionosphere and disk. The model uses equatorial observations as a boundary condition-we test its predictions over a wide latitude range by comparison with a Cassini high-inclination orbit of magnetic field and hot plasma pressure data. We find reasonable agreement over time scales larger than the period of Saturn kilometric radiation (also known as the camshaft period).

astro-ph.EP↗