Magnetic Clouds and their Solar Sources

Much effort - in theoretical modelling, data interpretation, and global MHD simulations - is increasingly being devoted to the understanding of interplanetary coronal mass ejections (CMEs) and magnetic clouds (MCs). These explosive expulsions of mass and energy from the Sun, besides being among the most spectacular manifestations of solar activity, are largely responsible for the strongest disturbances in the Earth`s geophysical environment, since their strong, southward magnetic fields of long duration can apply strong forcing to the magnetosphere. As human society increasingly relies on technologies which are situated outside or at the edge of the Earth`s atmosphere and ionosphere, forecasting such events (so called "Space Weather") has become an urgent need for an increasing amount of institutions, companies and people. The new NASA STEREO mission (launched in October 2006) offers for the first time a stereoscopic view of the entire evolution of CMEs propagating from the Sun to the Earth. This raises the interesting possibility of following these large disturbances from their origin on the Sun down to their consequences as they arrive at the Earth. MCs show a special in situ behaviour, namely a strong magnetic field vector rotating smoothly through a large angle in a low beta plasma, from which they can be quite easily detected by near-Earth satellites and thus, makes them ideal objects to study geomagnetic storms and solar-terrestrial relations in general. In an attempt to advance our understanding of the structure and inner-heliospheric evolution of these transients further, we propose using a sophisticated 2.5 D numerical technique to reconstruct the magnetic and plasma structure in a cross-section of these ejecta intercepted by spacecraft, and applying it to a representative and large sample of ejecta. The method, based on solving the Grad-Shafranov (GS) equation, makes no preconceptions on the geometry and, in particular, does not assume an underlying cylindrically symmetric magnetic flux tube. If it would be possible to identify the parameters on the Sun that indicate the special properties to generate a geoeffective MC, then one could predict the interplanetary consequences of solar eruptions. In order to gain better knowledge on the pre-eruptive structures of CMEs/MCs we will analyse their morphology and underlying magnetic field configuration, and will trace them during their eruption phase and subsequent propagation through the interplanetary space. We will analyse different data sets (photospheric, chromospheric, coronal) using ground- based and space-borne instruments (STEREO, Hinode, SoHO, TRACE) in order to evaluate "key" parameters (orientation, magnetic flux, magnetic twist) which will be compared to in situ MC observations using the new multi-spacecraft GS-reconstruction method. The time of the minimum and rising phase of solar cycle 24 during which the project is planned (late 2007 until late 2010) is a time range which is ideal a) due to the operation of the new STEREO spacecraft which will be fundamental for the heliospheric research for the next years and b) since it will be possible to relate isolated MC signatures back to their solar counterparts.

This project dealt with observations and modeling using new space-based datasets to advance our understanding of Solar Coronal Mass Ejections (CMEs). CMEs are enormous expulsions of plasma and magnetic flux from the hot outer layers of the solar atmosphere which attain speeds of millions of kilometers per hour. Their impact on the Earth`s magnetosphere may result in the strongest geomagnetic storms, which can pose a great threat to our modern technological infrastructure, in particular to satellites in Earth orbit (e.g. GNSS systems) and to power grids on the ground. Thus, forecasting these `hurricanes of space weather` has become an urgent need for an increasing amount of institutions, companies and people. The aim of this project was to increase our basic knowledge on the relationship between measurements of CMEs by spacecraft directly in the solar wind (STEREO, Wind, ACE missions), their solar source regions (SOHO and STEREO missions) and images obtained during their propagation from Sun to earth (STEREO). This is of importance to understand how magnetic clouds (MCs, the `cores` of CMEs which drive the strongest geomagnetic storms) originate in the solar corona, how their magnetic fields are determined and to find better constraints on their global shape and magnetic field topology. These results are useful to enhance schemes for predicting the effects of CMEs at Earth. Most important results from our project are: 1) Through an extension of a modeling method for MCs to multi- spacecraft observations we could confirm the magnetic field topology of MCs as consisting of helical magnetic field lines, found a close to circular MC cross section, and a likely formation of the MC during the eruption (`flare`) on the Sun. 2) Combining images of a CME during interplanetary propagation (STEREO Heliospheric Imager instrument) with in situ plasma and magnetic field parameters confirmed the following hypotheses: a) the intensity `void` inside CMEs seen in coronagraph images may correspond to the MC, and b) a glancing encounter of a CME with Earth can be the cause of special MC-like plasma and field signatures, leading to a long storm growth phase. 3) The 3D orientation of the MC was shown to be consistent with the morphology and orientation of models used on the heliospheric images, opening up the possibility to predict the axis orientation and thus the geo-effects from heliospheric images. 4) A new method was developed to predict the impact direction, speed and arrival time from heliospheric images, and applied to real time events observed by STEREO, contributing to a future space mission for space weather forecasting to be positioned at the Lagrangian L5 and L4 points in the Sun-Earth system.