A Tale of Two Shocks

It was the best of times, it was the worst of times – for two traveling interplanetary shocks observed by Solar Orbiter on November 29 and 30 in 2023. We investigate these two very dissimilar shocks which were observed within less than 27 hours to elucidate obviously present non-equilibrium features and test the assumption of gyrotropy. We find very different behavior of particles at the two shocks which are – of course – due to differences in the two shocks.

The first of the two shocks was observed at 07:51:17 on Nov. 29, 2023, was quasi-parallel (θ_Bn ≈ 33°) and the weaker of the two shocks (fast magnetosonic (Alfvénic) Mach number of 2.4 (2.6)). It had no or only a minimal effect on the suprathermal (E less than ~1 MeV) particle population which was anisotropic and streaming away from the Sun. It was likely running into a small ICME and exhibited upstream wave activity with a dominant period just below one second.

The following shock on Nov 30, 2023 (10:47:26) was stronger (fast magnetosonic (Alfvénic) Mach number of 3.8 (4.5)) and quasi-perpendicular (θ_Bn ≈ 81°). Wave activity upstream of this shock was weaker than at the first and limited to the shock ramp, as expected for a quasi-perpendicular shock. Upstream protons and He2+ particles show clear core-beam velocity distribution functions. Suprathermal ions upstream of the second shock are more isotropic than around the first shock but nevertheless show a clear bump-on-tail distribution which lasts for approximately two gyroperiods. The level of fluctuations of the interplanetary magnetic field (IMF) is low which probably allows this “beam” to survive. The region downstream of this shock is rich in further unusual properties of the suprathermal ions. These exhibit strong non-equilibrium features in their differential intensities and anisotropic features which suggest non-gyrotropic behavior.