| dc.description.abstracteng | Coronal Mass Ejections (CMEs) are explosive large-scale outbursts of the Sun’s coronal
plasma and magnetic field. They can induce strong geomagnetic storms at Earth, which
pose serious threats to space systems, communications and navigation. Hence, arrival predictions
of CMEs are of special interest to the humane society. Such predictions require a
meticulous analysis of CME properties in the earliest possible stage. Coronagraph observations
can provide important insights into the CME kinematics, morphology and mass at
CME distances of only a few solar radii away from the Sun. However, the 3-dimensional
structure of CMEs can only by analysed, based on their 2-dimensional projection in coronagraph
images, which means that they are affected by projection effects.
This thesis has the goal to present the state-of-the-art methods of CME parameterisation
derived from coronagraph observations and to discuss arising issues resulting from projection
effects. A focus is laid on the measurements of the CME mass and morphology as well
as the question under which conditions they can be determined with highest accuracy. Further,
the solar mass loss caused by CMEs is investigated. Also, CME mass determination
is currently not feasible in real-time and therefore not applicable in actual terrestrial CME
arrival predictions. Thus, it is discussed how the CME mass and the CME morphology
can be empirically estimated from the CME speed.
The thesis presents a new combined method which enables the measurement of relevant
CME kinematics, morphology and mass in a consistent and comparable manner. The two
vantage points of the COR2 coronagraphs onboard of the twin NASA STEREO spacecraft
are used to apply the method to a set of 122 events with intense brightness. The modelling
results are analysed to derive empirical correlations with the CME speed. Further, a CME
propagation model – the Drag-Based Model (DBM) – is combined with the GCS model to
predict the CME arrival of a sample event at Earth.
It is shown that the largest CME parameterisation uncertainties arise for events emerging
from close to the disk centre towards or away from the observer. For these events the term
”disk events“ is adopted. If an event is seen as disk event in both coronagraphs, the CME
morphology can be overestimated by up to a factor of two from stereoscopical modelling.
Equally the CME mass of disk events can be overestimated by a factor of 10 and more in
the case of overlapping coronal streamers. Therefore, stereoscopical measurements of disk
events are not always reliable, at least under a very active background corona. Though, the
CME mass M can be estimated from the initial apex velocity vapex with the empirically
derived equation
log10(M) = 3.4* 10^(-4) v_apex + 15.479.
This result is used to predict the terrestrial CME arrival of a CME with an Earth-directed
initial speed of 1172 km/s with the GCS plus DBM model. The CME arrival time and the
arrival speed are both strongly affected by the solar wind density and CME mass. For the
presented case the arrival prediction limits spread to DeltaT = 59 h and Deltav = 748 km/s for
typical CME mass and solar wind values. It is demonstrated that the derived empirical
equation can be very valuable to improve the arrival prediction accuracy. | de |