Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userCurrent sheets, plasmoids and flux ropes in the heliosphere. Part II: Theoretical aspects
392
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Our understanding of processes occurring in the heliosphere historically began with reduced dimensionality - one-dimensional (1D) and two-dimensional (2D) sketches and models, which aimed to illustrate views on large-scale structures in the solar wind. However, any reduced dimensionality vision of the heliosphere limits the possible interpretations of in-situ observations. Accounting for non-planar structures, e.g. current sheets, magnetic islands, flux ropes as well as plasma bubbles, is decisive to shed the light on a variety of phenomena, such as particle acceleration and energy dissipation. In part I of this review, we have described in detail the ubiquitous and multi-scale observations of these magnetic structures in the solar wind and their significance for the acceleration of charged particles. Here, in part II, we elucidate existing theoretical paradigms of the structure of the solar wind and the interplanetary magnetic field, with particular attention to the fine structure and stability of current sheets. Differences in 2D and 3D views of processes associated with current sheets, magnetic islands, and flux ropes are discussed. We finally review the results of numerical simulations and in-situ observations, pointing out the complex nature of magnetic reconnection and particle acceleration in a strongly turbulent environment.
rate research
Read More
Magnetic flux ropes (MFRs) are one kind of fundamental structures in the solar physics, and involved in various eruption phenomena. Twist, characterizing how the magnetic field lines wind around a main axis, is an intrinsic property of MFRs, closely related to the magnetic free energy and stableness. So far it is unclear how much amount of twist is carried by MFRs in the solar atmosphere and in heliosphere and what role the twist played in the eruptions of MFRs. Contrasting to the solar MFRs, there are lots of in-situ measurements of magnetic clouds (MCs), the large-scale MFRs in interplanetary space, providing some important information of the twist of MFRs. Thus, starting from MCs, we investigate the twist of interplanetary MFRs with the aid of a velocity-modified uniform-twist force-free flux rope model. It is found that most of MCs can be roughly fitted by the model and nearly half of them can be fitted fairly well though the derived twist is probably over-estimated by a factor of 2.5. By applying the model to 115 MCs observed at 1 AU, we find that (1) the twist angles of interplanetary MFRs generally follow a trend of about $0.6frac{l}{R}$ radians, where $frac{l}{R}$ is the aspect ratio of a MFR, with a cutoff at about $12pi$ radians AU$^{-1}$, (2) most of them are significantly larger than $2.5pi$ radians but well bounded by $2frac{l}{R}$ radians, (3) strongly twisted magnetic field lines probably limit the expansion and size of MFRs, and (4) the magnetic field lines in the legs wind more tightly than those in the leading part of MFRs. These results not only advance our understanding of the properties and behavior of interplanetary MFRs, but also shed light on the formation and eruption of MFRs in the solar atmosphere. A discussion about the twist and stableness of solar MFRs are therefore given.
The magnetic topology and field line random walk properties of a nanoflare-heated and magnetically confined corona are investigated in the reduced magnetohydrodynamic regime. Field lines originating from current sheets form coherent structures, called Current Sheet Connected (CSC) regions, extended around them. CSC field line random walk is strongly anisotropic, with preferential diffusion along the current sheets in-plane length. CSC field line random walk properties remain similar to those of the entire ensemble but exhibit enhanced mean square displacements and separations due to the stronger magnetic field intensities in CSC regions. The implications for particle acceleration and heat transport in the solar corona and wind, and for solar moss formation are discussed.
Magnetic flux rope (MFR) is the core structure of the greatest eruptions, i.e., the coronal mass ejections (CMEs), on the Sun, and magnetic clouds are post-eruption MFRs in interplanetary space. There is a strong debate about whether or not a MFR exists prior to a CME and how the MFR forms/grows through magnetic reconnection during the eruption. Here we report a rare event, in which a magnetic cloud was observed sequentially by four spacecraft near Mercury, Venus, Earth and Mars, respectively. With the aids of a uniform-twist flux rope model and a newly developed method that can recover a shock-compressed structure, we find that the axial magnetic flux and helicity of the magnetic cloud decreased when it propagated outward but the twist increased. Our analysis suggests that the `pancaking effect and `erosion effect may jointly cause such variations. The significance of the `pancaking effect is difficult to be estimated, but the signature of the erosion can be found as the imbalance of the azimuthal flux of the cloud. The latter implies that the magnetic cloud was eroded significantly leaving its inner core exposed to the solar wind at far distance. The increase of the twist together with the presence of the erosion effect suggests that the post-eruption MFR may have a high-twist core enveloped by a less-twisted outer shell. These results pose a great challenge to the current understanding on the solar eruptions as well as the formation and instability of MFRs.
Kinetic aspects of the ion current layer at the center of a reconnection outflow exhaust near the X-type region are investigated by a two-dimensional particle-in-cell (PIC) simulation. The layer consists of magnetized electrons and unmagnetized ions that carry a perpendicular electric current. The ion fluid appears to be nonideal, sub-Alfvenic, and nondissipative. The ion velocity distribution functions contain multiple populations such as global Speiser ions, local Speiser ions, and trapped ions. The particle motion of the local Speiser ions in an appropriately rotated coordinate system explains the ion fluid properties very well. The trapped ions are the first demonstration of the regular orbits in the chaotic particle dynamics [Chen and Palmadesso, J. Geophys. Res., 91, 1499 (1986)] in self-consistent PIC simulations. They would be observational signatures in the ion current layer near reconnection sites.
Studying the evolution of magnetic clouds entrained in coronal mass ejections using in-situ data is a difficult task since only a limited number of observational points is available at large heliocentric distances. Remote sensing observations can, however, provide important information for events close to the Sun. In this work we estimate the flux rope orientation first in the close vicinity of the Sun (2-20 Rs) using forward modeling of STEREO/SECCHI and SOHO/LASCO coronagraph images of coronal mass ejections and then in-situ using Grad-Shafranov reconstruction of the magnetic cloud. Thus, we are able to measure changes in the orientation of the erupted flux ropes as they propagate from the Sun to 1 AU. We present both techniques and use them to study 15 magnetic clouds observed during the minimum following Solar Cycle 23 and the rise of Solar Cycle 24. This is the first multievent study to compare the three-dimensional parameters of CMEs from imaging and in-situ reconstructions. The results of our analysis confirm earlier studies showing that the flux ropes tend to deflect towards the solar equatorial plane. We also find evidence of rotation on their travel from the Sun to 1 AU. In contrast to past studies, our method allows one to deduce the evolution of the three-dimensional orientation of individual flux ropes rather than on a statistical basis.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
