Twisted magnetic flux ropes in the solar wind

Magnetic fields in interplanetary space exhibit various kinds of structures and wave activities. Above all, recent discoveries of the magnetic field twisting (or twisted flux tubes) are considered as a breakthrough to answer many fundamental questions in space plasma dynamics in the solar system, e.g., when and where the twisting appears, how the solar plasma is transported to near-Earth space, how the magnetic fields in the solar corona become eroded or destabilized, and how the solar plasma evolves into turbulence as it streams into interplanetary space. All these questions are important to substantially improve our knowledge not only on the near-Earth plasma environment but also on physical processes in stellar and interstellar plasmas in astrophysics. The project performs a first-time, systematic study of the magnetic field twisting in interplanetary space and makes use of the full potential of the magnetic field data ever recorded in situ by spacecraft at orbits of the Earth (1 astronomical unit) and close to the Sun down to Mercury`s orbit (0.3 astronomical units). The interplanetary space is the only place where one can perform in situ measurements of collisionless plasmas using spacecraft. In the first step of the study, the magnetic field and plasma data are analyzed to determine the properties of field twisting statistically at the Earth orbit. The spacecraft data from the ACE, WIND, and STEREO missions are used, and the properties of the twisted field such as the occurrence rate, the length and diameter scales, the degree of field twisting, and the plasma density and temperature are evaluated. In the second step, the properties of the twisted fields are tracked as a function of the radial distances from the Sun by analyzing the Helios spacecraft data in conjunction with IMP8 and ISEE3 measurements, covering a wide range of distances from the Earth orbit down to Mercury`s one. The research is based on the hypotheses that the magnetic field twisting is the key process in the plasma transport problem in the solar system and that the time evolution and interactions of twist carrying magnetic structures can be tracked at different radial distances from the Sun. The expected impact is an improved understanding of space plasma processes which can be connected with generation of turbulence, erosion of structures, heating of plasma and finally with solar wind magnetosphere interactions and space weather. The project results connect the near-Earth plasma environment with the solar and astrophysical plasmas. The new knowledge obtained from the project is immediately integrated in the planning of the upcoming spacecraft missions such as Solar Orbiter and THOR missions by ESA and Solar Probe Plus by NASA.

The magnetic field is not homogeneously distributed in space plasmas, rather it is organized into non-space-filling filamentary structures, called flux tubes. The internal magnetic field in these tubes is often helical with a spatial twist, representing a flux rope structure rather than a simple tube. Coronal mass ejections originating in eruptive processes on the Sun and propagating through the interplanetary space, usually have a flux rope core region, also called a magnetic cloud (MC). In the project, the emphasis was placed on turbulence generated in front of propagating MCs and on magnetic reconnection (MR) occurring within the boundary layer of MCs. MR converts magnetic to other forms of energy when within narrow spatial regions nearly oppositely oriented magnetic field configurations (current sheets) are present. MR can occur within the boundary of magnetic clouds or in turbulence due to stochastic motions generating both current sheets and vortical structures. Although interactions of helical MCs with the interplanetary magnetic field and ambient plasma represent one of the most important problems, the understanding of the related physics is very limited due to insufficient time resolution of measurements and due to single-point measurements from the existing solar wind missions. Despite the limitations we reached substantial progress in understanding these interactions in three steps: (a) MR and turbulence have been studied using data from missions where multi-point measurements with high time resolution were available. Here MR and the associated wave activity have been observed in the turbulent terrestrial magnetosheath, where fluctuations and other coherent structures co-exist, such as flux ropes, jets, vortices, etc. (b) On this basis single-point proxies for describing turbulent structures, such as current sheets and vortices have been introduced and tested against their four-point versions in the magnetosheath. It was found that enhanced correlations between the proxies are associated with plasma heating in spatial blobs in front of the propagating MCs in the solar wind. Such extended spatial regions of energy conversions and dissipation associated with MCs were observed for the first time in the solar wind. (c) An MR event occurring within the boundary of an MC was studied in detail. Due to the symmetries and slow motion of the reconnection structure the multi-scale features of MC - solar wind interaction could be studied despite the pure time resolution of measurements. The new results indicate that MR occurs because of the oppositely directed interplanetary magnetic field and the twisted magnetic field of the MC and it is associated with rather dynamical processes such as flow shears, generation of flow-related discontinuities, plasma instabilities, particle acceleration, and wave activity. These inter-related processes occur at the same time, leading to massive energy conversions at particle scales and reorganizations of the large-scale magnetic field.