Evolution Of Magnetic Clouds Research Articles

Main Cause of the Poloidal Plasma Motion Inside a Magnetic Cloud Inferred from Multiple-Spacecraft Observations

Although the dynamical evolution of magnetic clouds (MCs) has been one of the foci of interplanetary physics for decades, only few studies focus on the internal properties of large-scale MCs. Recent work by Wang et al. (J. Geophys. Res.120, 1543, 2015) suggested the existence of the poloidal plasma motion in MCs. However, the main cause of this motion is not clear. In order to find it, we identify and reconstruct the MC observed by the Solar Terrestrial Relations Observatory (STEREO)-A, Wind, and STEREO-B spacecraft during 19 – 20 November 2007 with the aid of the velocity-modified cylindrical force-free flux-rope model. We analyze the plasma velocity in the plane perpendicular to the MC axis. It is found that there was evident poloidal motion at Wind and STEREO-B, but this was not clear at STEREO-A, which suggests a local cause rather than a global cause for the poloidal plasma motion inside the MC. The rotational directions of the solar wind and MC plasma at the two sides of the MC boundary are found to be consistent, and the values of the rotational speeds of the solar wind and MC plasma at the three spacecraft show a rough correlation. All of these results illustrate that the interaction with ambient solar wind through viscosity might be one of the local causes of the poloidal motion. Additionally, we propose another possible local cause: the existence of a pressure gradient in the MC. The significant difference in the total pressure at the three spacecraft suggests that this speculation is perhaps correct.

Propagation and evolution of a magnetic cloud from ACE to Ulysses

The chance of a magnetic cloud (MC) being observed by widely separated spacecraft is rare. However, such an event provides us a good opportunity to study the propagation and evolution of magnetic cloud in the interplanetary space. A magnetic cloud event observed by ACE at Earth on 4–6 March 1998 is tracked from the location of ACE to Ulysses. Using a one‐dimensional (1‐D) MHD solar wind model, we propagate the ACE data to the location of Ulysses and confirm that the two magnetic clouds observed by both spacecraft have the same solar origin. The Grad‐Shafranov (GS) reconstruction technique is then employed to recover the 2‐D cross sections of the magnetic clouds at locations of ACE and Ulysses, respectively. The magnetic clouds observed at ACE and Ulysses both show magnetic flux rope configurations of the same chirality and unidirectional axial magnetic field along approximately the same direction. It is found that the magnetic cloud is expanding while propagating outward. Their relevant toroidal (axial) and poloidal magnetic flux contained within each flux rope are different but approximately of the same order of magnitude. However, the relative magnetic helicity contained in each flux rope differs significantly. We discuss the causes of such differences, taking into account the underestimate of the area of the flux rope at Ulysses and the effect of magnetic cloud evolution that may not be fully addressed by 2‐D models.

Magnetic cloud evolution in a two‐speed solar wind

The Ulysses spacecraft has observed interplanetary coronal mass ejections in pure high‐speed solar wind, in the shear layer between high‐speed and low‐speed, and, in conjunction with other satellites, the same coronal mass ejection (CME) has been seen in both high‐ and low‐speed wind. If CMEs originate in the equatorial streamer belt, they must be able to somehow penetrate into high‐speed solar wind flows. This paper presents numerical simulations of the interaction of the subset of CMEs known as magnetic clouds (i.e., a flux rope geometry) with a velocity shear layer. A spherical, two‐speed solar wind model represents the stream interface. It is shown that provided the CME has a large enough poleward velocity at the Sun, it can readily penetrate the stream interface and enter the high‐speed wind. It is also shown that under some initial conditions, internal magnetic reconnection can take place within the flux rope. As the flux rope field lines are sheared, oppositely directed fields are generated, which press together and reconnect.

Magnetic cloud evolution: A comparison of analytical and numerical solutions

The propagation of magnetic clouds in the interplanetary medium between the Sun and Earth is studied using two different numerical methods. No numerical method can account completely for the complexity of the physical processes that occur in the solar wind because approximations need to be made. In solving the time‐dependent MHD equations, a number of simplifications and assumptions are made. These may have significant influence on the results obtained. We compare the results of two‐dimensional simulations obtained from two different MHD codes based on different numerical methods, namely, the two‐step Lax‐Wendroff and the total variation diminishing Lax‐Friedrich numerical schemes. The two codes give similar results, which are in general agreement with observational data. The numerical results are then compared with analytical solutions of an evolution of a flux rope published by Osherovich et al. [1995] and applied to magnetic clouds. It follows from this solution that the polytropic index γ must be less than 1 in order for a cloud to expand, i.e., as is observed. Polytropic index γ > 1 allows only for oscillations of the cloud. In the present paper it is shown that under the same conditions, the analytical solution and the numerical code give the same results, but when more realistic initial conditions are used, the modeled magnetic clouds behave differently.

A Spherical Model for Starless Cores of Magnetic Molecular Clouds and Dynamical Effects of Dust Grains

In the current paradigm, isolated low-mass stars form out of magnetic molecular clouds as the clouds evolve because of ambipolar diffusion. A quantitative understanding of this process remains incomplete, because of both physical and technical complexities. As a step toward a quantitative theory for star formation, I explore further the evolution of magnetic clouds with a simplifying spherical geometry, studied first by Safier, McKee, & Stahler. The spherical model has several desirable features as well as some potentially serious difficulties. It highlights the pressing need for two-dimensional, fully dynamic models that treat the coupling between the magnetic field and the cloud matter properly. The model clouds exhibit substantial inward motion during the late stage of core formation, with velocities of order one-half the isothermal sound speed or more over most of the cloud, after a central density enhancement of about 102 (say, from 103 to 105 cm-3). Such precollapse motion may have been detected recently by Tafalla et al. and Williams et al. in the starless core L1544. The motion may also explain why starless dense cores typically last for only a few dynamic times and why they have small but significant nonthermal line widths. The clouds that I study have relatively low mass (of order 10 M☉) and initial magnetic pressure comparable to thermal pressure. They evolve into a density profile of a flat central plateau surrounded by an envelope whose density decreases with radius roughly as a power law, as found previously. The density decline in the envelope is significantly steeper than those obtained by Mouschovias and collaborators, who considered disklike magnetic clouds whose initial magnetic pressure is much larger than the thermal pressure. As the cloud density increases above approximately 105 cm-3, dust grains become dynamically important. Depending on their size distribution, dust grains can enhance the coupling coefficient between the magnetic field and the neutral cloud matter by as much as an order of magnitude or even more. The enhanced coupling makes it difficult for magnetic flux to escape during the advanced, more dynamic phase of the core formation. In spherical geometry, this trapping of magnetic flux leads to an almost homologous collapse of a central region with substantial mass. The implied extremely rapid assemblage of protostellar mass, with more than 0.5 M☉ of material reaching the origin in about 103 yr or less for a typical set of cloud parameters, poses a serious accretion luminosity Magnetic tension in two-dimensional models may help alleviate the problem.

Interplanetary magnetic clouds, helicity conservation, and current‐core flux‐ropes

A current‐core flux‐rope model for interplanetary magnetic clouds is presented which explains their average thermodynamic and magnetic properties. It is assumed that during a magnetic cloud's evolution, its total magnetic helicity, flux and mass are conserved and that the dynamics of a cloud is governed by the Lorentz self‐force acting on its curved portions. Total magnetic energy and current in a magnetic cloud decrease monotonically as it elongates. Part of this magnetic energy is lost in overcoming solar gravity, part goes into the bulk kinetic energy, and the rest can be assumed to go into heating the plasma inside the cloud. Due to this dissipation of magnetic energy as heat, the temperature of an expanding cloud goes through a maximum before the cloud leaves the corona. The temperature may reach 1.7 × 106K. As a cloud expands into interplanetary space, the total plasma beta asymptotically approaches a constant value between 0.39 and 0.52, irrespective of its initial value. Apart from explaining the heating and expansion of magnetic clouds, this model also provides expressions (scaling laws) for the magnetic field strength, temperature, radius, density, asymmetry of the magnetic field strength profile, slope of the plasma velocity profile inside clouds, and plasma beta, as functions of distance from the Sun. These theoretical results are compared with cloud data obtained between 0.3 and 4 AU from the Sun. The comparisons show a good agreement between observation and theory.

Simulation of magnetic cloud propagation in the inner heliosphere in two‐dimensions: 1. A loop perpendicular to the ecliptic plane

We present results of simulations of a magnetic cloud's evolution during its passage from the solar vicinity (18 solar radii) to approximately 1 AU using a two‐dimensional MHD code. The cloud is a cylinder perpendicular to the ecliptic plane. The external flow is explicitly considered self‐consistently. Our results show that the magnetic cloud retains its basic topology up to 1 AU, although it is distorted due to radially expanding solar wind and magnetic field lines bending. The magnetic cloud expands, faster near the Sun, and faster in the azimuthal direction than in the radial one; its extent is approximately 1.5–2× larger in the azimuthal direction. Magnetic clouds reach approximately the same asymptotic propagation velocity (higher than the background solar wind velocity) despite our assumptions of various initial conditions for their release. Recorded time profiles of the magnetic field magnitude, velocity, and temperature at one point, which would be measured by a hypothetical spacecraft, are qualitatively in agreement with observed profiles. The simulations qualitatively confirm the behavior of magnetic clouds derived from some observations, so they support the interpretations of some magnetic cloud phenomena as magnetically closed regions in the solar wind.