How do fits of simulated magnetic clouds correspond to their real shapes in 3-D?

Magnetic clouds are important objects for space weather forecasters due to their impact on the Earth's magnetosphere and their consequences during geomagnetic storms. Being considered as cylindrical or toroidal flux ropes, their size, velocity, magnetic field strength, and axis orientation determine its impact on Earth. Above mentioned parameters are usually extracted from model fits using measurements from one-spacecraft crossings of these structures. In order to relate solar events with these spacecraft observations, the parameters are then compared to situation at the Sun around a most probable source region with a goal to correlate them with near-Sun observed quantities for prediction purposes. In the past we performed three-dimensional simulations of magnetic cloud propagation in the inner heliosphere. Simulated spacecraft measurements are fitted by models of magnetic clouds and resulting parameters are compared with real shapes of magnetic clouds which can be directly obtained from our simulations. The comparison shows that cloud parameters are determined quite reliably for spacecraft crossings near the cloud axis

Magnetic field disturbances in the sheath region of a super-sonic interplanetary magnetic cloud

It is well-known that interplanetary magnetic clouds can cause strong geomagnetic storms due to the high magnetic field magnitude in their interior, especially if there is a large negative <I>B<sub>z</sub></I> component present. In addition, the magnetic disturbances around such objects can play an important role in their "geo-effectiveness". On the other hand, the magnetic and flow fields in the CME sheath region in front of the body and in the rear of the cloud are important for understanding both the dynamics and the evolution of the interplanetary cloud. The "eventual" aim of this work is to calculate the magnetic field in this CME sheath region in order to evaluate the possible geo-efficiency of the cloud in terms of the maximum |<I>B<sub>z</sub></I>|-component in this region. In this paper we assess the potential of this approach by introducing a model with a simplified geometry. We describe the magnetic field between the CME shock surface and the cloud's boundary by means of a vector potential. We also apply our model and present the magnetic field distribution in the CME sheath region in front of the body and in the rear of the cloud formed after the event of 20 November 2003

Plasma flows around magnetic obstacles in solar wind

Context. Recent numerical simulations and data analysis have shown that the area in front of magnetic clouds is very important from the point of view of its geo-efficiency. This area has very complicated magnetic and plasma structures. It is necessary to describe the plasma parameter distributions in the vicinity of magnetic clouds and other stable structures in the solar wind. Assuming that the magnetic field around the object is determined or measured, the velocity field is calculated from the frozen-in equation, while the density and pressure are given by explicit formulas expressing P and p as functions of only B and V. An alternative method is to solve the full system of MHD equations numerically, but even in this case the analytical estimates determined here are also useful when formulating initial and boundary conditions. Aims. The aim is to treat the region in front of interplanetary magnetic clouds in terms of analytical functions for a detailed consideration of general phenomena and also for particular phenomena of specific clouds. Methods. First, the velocity and magnetic field distributions satisfying the boundary conditions and the frozen-in condition are determined. Next, the plasma density and pressure are calculated. Results. The three-dimensional plasma parameter distributions are found for the general case of an inclined cylindrical cloud. © ESO 2007.status: publishe

Plasma flows around magnetic obstacles in the solar wind

Context.Recent numerical simulations and data analysis have shown that the area in front of magnetic clouds is very important from the point of view of its geo-efficiency. This area has very complicated magnetic and plasma structures. It is necessary to describe the plasma parameter distributions in the vicinity of magnetic clouds and other stable structures in the solar wind. Assuming that the magnetic field around the object is determined or measured, the velocity field is calculated from the frozen-in equation, while the density and pressure are given by explicit formulas expressing P and ρ as functions of only ${\vec B}$ and ${\vec V}$. An alternative method is to solve the full system of MHD equations numerically, but even in this case the analytical estimates determined here are also useful when formulating initial and boundary conditions. Aims.The aim is to treat the region in front of interplanetary magnetic clouds in terms of analytical functions for a detailed consideration of general phenomena and also for particular phenomena of specific clouds. Methods.First, the velocity and magnetic field distributions satisfying the boundary conditions and the frozen-in condition are determined. Next, the plasma density and pressure are calculated. Results.The three-dimensional plasma parameter distributions are found for the general case of an inclined cylindrical cloud

Comparative study of a constant-alpha force-free field and its approximations in an ideal toroid

Aims. Magnetic clouds in the solar wind are large, loop-like interplanetary flux ropes and may be locally approximated by a toroidal flux rope. We compare approximate constant-alpha force-free fields in an ideal toroid, used in magnetic cloud analysis, with the exact solution, and examine their validity for low aspect ratios, which can be found in magnetic clouds. The approximate toroidal solutions were originally derived under the assumption of large aspect ratios. Methods. Three analytic simple approximate constant-alpha force-free solutions and the exact analytic solution are compared with respect to magnetic field profiles, magnetic field magnitude distributions, and magnetic helicity, with moderate (2–3) and very low (<2) aspect ratios. Results. The Miller & Turne

Magnetic cloud fit by uniform-twist toroidal flux ropes

Context. Detailed studies of magnetic cloud observations in the solar wind in recent years indicate that magnetic clouds are interplanetary flux ropes with a low twist. Commonly, their magnetic fields are fit by the axially symmetric linear force-free field in a cylinder (Lundquist field), which in contrast has a strong and increasing twist toward the boundary of the flux rope. Therefore another field, the axially symmetric uniform-twist force-free field in a cylinder (Gold-Hoyle field) has become employed to analyze magnetic clouds. Aims. Magnetic clouds are bent, and for some observations, a toroidal rather than a cylindrical flux rope is needed for a local approximation of the cloud fields. We therefore try to derive an axially symmetric uniform-twist force-free field in a toroid, either exactly, or approximately, and to compare it with observations. Methods. Equations following from the conditions of solenoidality and force-freeness in toroidally curved cylindrical coordinates were solved analytically. The magnetic field and velocity observations of a magnetic cloud were compared with solutions obtained using a nonlinear least-squares method. Results. Three solutions of (nearly) uniform-twist magnetic fields in a toroid were obtained. All are exactly solenoidal, and in the limit of high aspect ratios, they tend to the Gold-Hoyle field. The first solution has an exactly uniform twist, the other two solutions have a nearly uniform twist and approximate force-free fields. The analysis of a magnetic cloud observation showed that these fields may fit the observed field equally well as the already known approximately linear force-free (Miller-Turner) field, but it also revealed that the geometric parameters of the toroid might not be reliably determined from fits, when (nearly) uniform-twist model fields are used. Sets of parameters largely differing in the size of the toroid and its aspect ratio yield fits of a comparable quality

On plasma flows around magnetic obstacles in the solar wind

status: publishe

A force-free field with constant alpha in an oblate cylinder: A generalization of the Lundquist solution

A force-free magnetic field with constant alpha for a circular cylindrical flux rope (Lundquist solution) is widely used to describe the magnetic field configuration in interplanetary flux ropes. Observations as well as MHD simulations indicate that interplanetary flux ropes are not circular but have an oblate shape. Here we present an analytical solution for a force-free magnetic field with constant alpha in an elliptic flux rope which may be regarded as a direct generalization of the Lundquist solution. An alternative simpler solution for a force-free magnetic field with constant alpha in an oblate flux rope is discussed