Solar Physics 259 (1/2), 179−197, 2009
STEREO Science Results at Solar Minimum
E.R. Christian, M.L. Kaiser, T.A. Kucera, O.C. St Cyr (guest eds.)
© Springer Verlag
The impact of geometry on observations of CME brightness and propagation
J.S. Morrill, R.A. Howard and A. Vourlidas
Space Sciences Division, Naval Research Lab., Washington, DC, USA
D.F. Webb
Inst. for Scientific Research, Boston College, Chestnut Hill, MA, USA
Air Force Research Lab., Space Vehicles Directorate, Hanscom AFB, MA, USA
V. Kunkel
George Mason University, Fairfax, VA, USA
Abstract
Coronal mass ejections (CMEs) have a significant impact on space weather and
geomagnetic storms and so have been the subject of numerous studies. Most CME observations
have been made while these events are near the Sun (e.g., SOHO/LASCO). Recent data
from the Coriolis/SMEI and STEREO/SECCHI-HI instruments have imaged CMEs farther
into the heliosphere. Analyses of CME observations near the Sun measure the properties of
these events by assuming that the emission is in the plane of the sky and hence the speed
and mass are lower limits to the true values. However, this assumption cannot be used to analyze
optical observations of CMEs far from the Sun, such as observations from SMEI and
SECCHI-HI, since the CME source is likely to be far from the limb. In this paper we consider
the geometry of observations made by LASCO, SMEI, and SECCHI. We also present
results that estimate both CME speed and trajectory by fitting the CME elongations observed
by these instruments. Using a constant CME speed does not generally produce profiles that
fit observations at both large and small elongation, simultaneously.We include the results of
a simple empirical model that alters the CME speed to an estimated value of the solar wind
speed to simulate the effect of drag on the propagating CME. This change in speed improves
the fit between the model and observations over a broad range of elongations.