Magnetic Structure and Propagation of Two Interacting CMEs From the Sun to Saturn

One of the grand challenges in heliophysics is the characterization of coronal mass ejection (CME) magnetic structure and evolution from eruption at the Sun through heliospheric propagation. At present, the main difficulties are related to the lack of direct measurements of the coronal magnetic fields and the lack of 3D in-situ measurements of the CME body in interplanetary space. Nevertheless, the evolution of a CME magnetic structure can be followed using a combination of multi-point remote-sensing observations and multi-spacecraft in-situ measurements as well as modeling. Accordingly, we present in this work the analysis of two CMEs that erupted from the Sun on April 28, 2012. We follow their eruption and early evolution using remote-sensing data, finding indications of CME-CME interaction, and then analyze their interplanetary counterpart(s) using in-situ measurements at Venus, Earth, and Saturn. We observe a seemingly single flux rope at all locations, but find possible signatures of interaction at Earth, where high-cadence plasma data are available. Reconstructions of the in-situ flux ropes provide almost identical results at Venus and Earth but show greater discrepancies at Saturn, suggesting that the CME was highly distorted and/or that further interaction with nearby solar wind structures took place before 10 AU. This work highlights the difficulties in connecting structures from the Sun to the outer heliosphere and demonstrates the importance of multi-spacecraft studies to achieve a deeper understanding of the magnetic configuration of CMEs.

An interplanetary shock traced by planetary auroral storms from the Sun to Saturn

A relationship between solar activity and aurorae on Earth was postulated long before space probes directly detected plasma propagating outwards from the Sun. Violent solar eruption events trigger interplanetary shocks that compress Earth's magnetosphere, leading to increased energetic particle precipitation into the ionosphere and subsequent auroral storms. Monitoring shocks is now part of the 'Space Weather' forecast programme aimed at predicting solar-activity-related environmental hazards. The outer planets also experience aurorae, and here we report the discovery of a strong transient polar emission on Saturn, tentatively attributed to the passage of an interplanetary shock--and ultimately to a series of solar coronal mass ejection (CME) events. We could trace the shock-triggered events from Earth, where auroral storms were recorded, to Jupiter, where the auroral activity was strongly enhanced, and to Saturn, where it activated the unusual polar source. This establishes that shocks retain their properties and their ability to trigger planetary auroral activity throughout the Solar System. Our results also reveal differences in the planetary auroral responses on the passing shock, especially in their latitudinal and local time dependences.