A Study of Fluctuations in Magnetic Cloud‐Driven Sheaths.

Interplanetary coronal mass ejections are at the center of the research on geomagnetic activity. Sheaths, highly fluctuating structures, which can be found in front of fast interplanetary coronal mass ejections, are some of the least known geoeffective solar transients. Using Morlet transforms, we analyzed the magnetic fluctuations in a list of 42 well‐identified and isolated magnetic clouds driving a sheath and shock (Masías‐Meza et al., 2016, https://doi.org/10.1051/0004-6361/201628571. We studied the fluctuations inside sheaths by defining two quantities: the power and the anisotropy. With a simple statistical approach we found that sheaths, in particular, those driven by a fast magnetic cloud, encountering a highly turbulent solar wind, and forming a high Alfvén Mach number shock have high levels of turbulent energy (∼10 times compared with the solar wind) as well as a low anisotropy (approximately halved compared with the solar wind) of their fluctuations. On the other hand, the effect of the shock angle and plasma beta in the solar wind are less straightforward: If the shock is quasi‐parallel or the beta in the solar wind is high, both the turbulent energy in the sheaths and the anisotropy of the fluctuations are reduced; but for quasi‐perpendicular shocks or low beta solar wind the turbulent energy and anisotropy can take any value. Plain Language Summary: Solar flares are sometimes linked with the emission of interplanetary structures, which may collide with Earth. When this happens, it can lead to temporary changes in the magnetic field of Earth and possibly affect human technology. These effects are a subset of what is known as geoeffectiveness. We do have an idea of which types of structures may or may not have consequences on Earth, and, for example, magnetic clouds are quite well known for their large impact on the Earth magnetic field; however, we still struggle to understand the consequences of some puzzling interplanetary structures called sheaths. These can often be found preceding a magnetic cloud when the latter is fast enough to generate a shock wave. We think that one of the reasons these sheaths keep having surprising effects on the Earth's magnetic field is because they, themselves, are not yet very well known. The present paper aims at characterizing one of the key properties of sheaths: their magnetic fluctuations, that is, the rapid temporal variation of the magnetic field. We found that those fluctuations are indeed quite particular in sheaths: They have markedly more energy than in the usual solar wind (about 10 times more) and tend to change direction all the time. Conversely, in the solar wind, some directions seem to be privileged and the energy is relatively low. In this paper, we also show that these particular properties of the magnetic fluctuations are all the more pronounced when the magnetic cloud driving the sheath is moving faster and when the solar wind in front of the sheath already has strong fluctuations and magnetic pressure. This work gives us a better insight into the dynamics of the sheath, which may eventually improve our understanding of their geoeffectiveness. Key Points: The fluctuations in sheaths have increased power (∼10 times) and compressibility (∼2 times) compared to the solar wind'sThose characteristics depend on magnetic clouds' speed, preexisting fluctuations in the solar wind, and shock's parameters [ABSTRACT FROM AUTHOR]