Predicting the magnetic vectors within coronal mass ejections arriving at Earth: 1. Initial Architecture

The process by which the Sun affects the terrestrial environment on short timescales is predominately driven by the amount of magnetic reconnection between the solar wind and Earths magnetosphere. Reconnection occurs most efficiently when the solar wind magnetic field has a southward component. The most severe impacts are during the arrival of a coronal mass ejection (CME) when the magnetosphere is both compressed and magnetically connected to the heliospheric environment. Unfortunately, forecasting magnetic vectors within coronal mass ejections remains elusive. Here we report how, by combining a statistically robust helicity rule for a CMEs solar origin with a simplified flux rope topology the magnetic vectors within the Earth-directed segment of a CME can be predicted. In order to test the validity of this proof-of-concept architecture for estimating the magnetic vectors within CMEs, a total of eight CME events (between 2010 and 2014) have been investigated. With a focus on the large false alarm of January 2014, this work highlights the importance of including the early evolutionary effects of a CME for forecasting purposes. The angular rotation in the predicted magnetic field closely follows the broad rotational structure seen within the in situ data. This time-varying field estimate is implemented into a process to quantitatively predict a time-varying Kp index that is described in detail in paper II. Future statistical work, quantifying the uncertainties in this process, may improve the more heuristic approach used by early forecasting systems.

Inner Heliospheric Evolution of a Stealth CME Derived From Multi-view Imaging and Multipoint In--situ observations: I. Propagation to 1 AU

Coronal mass ejections (CMEs) are the main driver of Space Weather. Therefore, a precise forecasting of their likely geo-effectiveness relies on an accurate tracking of their morphological and kinematical evolution throughout the interplanetary medium. However, single view-point observations require many assumptions to model the development of the features of CMEs, the most common hypotheses were those of radial propagation and self-similar expansion. The use of different view-points shows that at least for some cases, those assumptions are no longer valid. From radial propagation, typical attributes that can now been confirmed to exist are; over-expansion, and/or rotation along the propagation axis. Understanding of the 3D development and evolution of the CME features will help to establish the connection between remote and in-situ observations, and hence help forecast Space Weather. We present an analysis of the morphological and kinematical evolution of a STEREO B-directed CME on 2009 August 25-27. By means of a comprehensive analysis of remote imaging observations provided by SOHO, STEREO and SDO missions, and in-situ measurements recorded by Wind, ACE, and MESSENGER, we prove in this paper that the event exhibits signatures of deflection, which are usually associated to changes in the direction of propagation and/or also with rotation. The interaction with other magnetic obstacles could act as a catalyst of deflection or rotation effects. We propose, also, a method to investigate the change of the CME Tilt from the analysis of height-time direct measurements. If this method is validated in further work, it may have important implications for space weather studies because it will allow infer ICME orientation.

A Plasma {beta} Transition Within a Propagating Flux Rope

We present a 2.5D MHD simulation of a magnetic flux rope (FR) propagating in the heliosphere and investigate the cause of the observed sharp plasma beta transition. Specifically, we consider a strong internal magnetic field and an explosive fast start, such that the plasma beta is significantly lower in the FR than the sheath region that is formed ahead. This leads to an unusual FR morphology in the first stage of propagation, while the more traditional view (e.g. from space weather simulations like Enlil) of a `pancake shaped FR is observed as it approaches 1 AU. We investigate how an equipartition line, defined by a magnetic Weber number, surrounding a core region of a propagating FR can demarcate a boundary layer where there is a sharp transition in the plasma beta. The substructure affects the distribution of toroidal flux, with the majority of the flux remaining in a small core region which maintains a quasi-cylindrical structure. Quantitatively, we investigate a locus of points where the kinetic energy density of the relative inflow field is equal to the energy density of the transverse magnetic field (i.e. effective tension force). The simulation provides compelling evidence that at all heliocentric distances the distribution of toroidal magnetic flux away from the FR axis is not linear; with 80% of the toroidal flux occurring within 40% of the distance from the FR axis. Thus our simulation displays evidence that the competing ideas of a pancaking structure observed remotely can coexist with a quasi-cylindrical magnetic structure seen in situ.

A study of the Heliocentric dependence of Shock Standoff Distance and Geometry using 2.5D MHD Simulations of CME-driven shocks

We perform four numerical magnetohydrodynamic simulations in 2.5 dimensions (2.5D) of fast Coronal Mass Ejections (CMEs) and their associated shock fronts between 10Rs and 300Rs. We investigate the relative change in the shock standoff distance, Sd, as a fraction of the CME radial half-width, Dob (i.e. Sd/Dob). Previous hydrodynamic studies have related the shock standoff distance for Earths magnetosphere to the density compression ratio (DR,Ru/Rd) measured across the bow shock (Spreiter, Summers and Alksne 1966). The DR coefficient, kdr, which is the proportionality constant between the relative standoff distance (Sd/Dob) and the compression ratio, was semi-empirically estimated as 1.1. For CMEs, we show that this value varies linearly as a function of heliocentric distance and changes significantly for different radii of curvature of the CMEs leading edge. We find that a value of 0.8+-0.1 is more appropriate for small heliocentric distances (<30Rs) which corresponds to the spherical geometry of a magnetosphere presented by Seiff (1962). As the CME propagates its cross section becomes more oblate and the kdr value increases linearly with heliocentric distance, such that kdr= 1.1 is most appropriate at a heliocentric distance of about 80Rs. For terrestrial distances (215Rs) we estimate kdr= 1.8+-0.3, which also indicates that the CME cross-sectional structure is generally more oblate than that of Earths magnetosphere. These alterations to the proportionality coefficients may serve to improve investigations into the estimates of the magnetic field in the corona upstream of a CME as well as the aspect ratio of CMEs as measured in situ.