Dynamics of CMEs in the LASCO Field of View – Statistical Analysis

A large set of CMEs (2207) has been selected to study their dynamics in the SOHO/LASCO field of view (LFOV). These selected events have at least seven height–time measurements and their acceleration can be determined separately in the C2 and C3 LFOVs. It was demonstrated that the balance between the aerodynamic drag and driving Lorentz forces may change during CME propagation in the LFOV. The drag force dominates the CME propagation close to the Sun (in the C2 LFOV), but farther from the Sun (in the C3 LFOV) the Lorentz force takes over the drag force. We also demonstrated that the acceleration of CMEs depends not only on their velocities but also on their masses and widths. Less massive CMEs (mass?<3×10¹⁴?kg) are generally decelerated and more massive CMEs (mass?>3×10¹⁴?kg) are accelerated in the LFOV. Based on the acceleration behavior we are able to classify the observed events into four different types.     

An Asymmetric Cone Model for Halo Coronal Mass Ejections

Due to projection effects, coronagraphic observations cannot uniquely determine parameters relevant to the geoeffectiveness of CMEs, such as the true propagation speed, width, or source location. The cone model for Coronal Mass Ejections (CMEs) has been studied in this respect and it could be used to obtain these parameters. There are evidences that some CMEs initiate from a flux-rope topology. It seems that these CMEs should be elongated along the flux-rope axis and the cross section of the cone base should be rather elliptical than circular. In the present paper we applied an asymmetric cone model to get the real space parameters of frontsided halo CMEs (HCMEs) recorded by SOHO/LASCO coronagraphs in 2002. The cone model parameters are generated through a fitting procedure to the projected speeds measured at different position angles on the plane of the sky. We consider models with the apex of the cone located at the center and surface of the Sun. The results are compared to the standard symmetric cone model.     

Relation Between Type II Bursts and CMEs Inferred from STEREO Observations

The inner coronagraph (COR1) of the Solar Terrestrial Relations Observatory (STEREO) mission has made it possible to observe CMEs in the spatial domain overlapping with that of the metric type II radio bursts. The type II bursts were associated with generally weak flares (mostly B and C class soft X-ray flares), but the CMEs were quite energetic. Using CME data for a set of type II bursts during the declining phase of solar cycle 23, we determine the CME height when the type II bursts start, thus giving an estimate of the heliocentric distance at which CME-driven shocks form. This distance has been determined to be ～1.5R _s (solar radii), which coincides with the distance at which the Alfvén speed profile has a minimum value. We also use type II radio observations from STEREO/WAVES and Wind/WAVES observations to show that CMEs with moderate speed drive either weak shocks or no shock at all when they attain a height where the Alfvén speed peaks (～3R _s?–?4R _s). Thus the shocks seem to be most efficient in accelerating electrons in the heliocentric distance range of 1.5R _s to 4R _s. By combining the radial variation of the CME speed in the inner corona (CME speed increase) and interplanetary medium (speed decrease) we were able to correctly account for the deviations from the universal drift-rate spectrum of type II bursts, thus confirming the close physical connection between type II bursts and CMEs. The average height (～1.5R _s) of STEREO CMEs at the time of type II bursts is smaller than that (2.2R _s) obtained for SOHO (Solar and Heliospheric Observatory) CMEs. We suggest that this may indicate, at least partly, the density reduction in the corona between the maximum and declining phases, so a given plasma level occurs closer to the Sun in the latter phase. In two cases, there was a diffuse shock-like feature ahead of the main body of the CME, indicating a standoff distance of 1R _s?–?2R _s by the time the CME left the LASCO field of view.     

Statistical Analysis of Periodic Oscillations in LASCO Coronal Mass Ejection Speeds

A large set of coronal mass ejections (CMEs, 3463) has been selected to study their periodic oscillations in speed in the Solar and Heliospheric Observatory (SOHO) mission’s Large Angle and Spectrometric Coronagraph (LASCO) field of view. These events, reported in the SOHO/LASCO catalog in the period of time 1996?–?2004, were selected based on having at least 11 height–time measurements. This selection criterion allows us to construct at least ten-point speed–distance profiles and evaluate kinematic properties of CMEs with a reasonable accuracy. To identify quasi-periodic oscillations in the speed of the CMEs a sinusoidal function was fitted to speed–distance profiles and the speed–time profiles. Of the considered events 22 % revealed periodic velocity fluctuations. These speed oscillations have on average amplitude equal to \(87~\mbox{km}\,\mbox{s}^{-1}\) and period \(7.8 R _{\odot}/241~\mbox{min}\) (in distance/time). The study shows that speed oscillations are a common phenomenon associated with CME propagation implying that all the CMEs have a similar magnetic flux-rope structure. The nature of oscillations can be explained in terms of magnetohydrodynamic (MHD) waves excited during the eruption process. More accurate detection of these modes could, in the future, enable us to characterize magnetic structures in space (space seismology).     

Is the Asymmetric Cone Model for Halo Coronal Mass Ejections Correct?

A set of 106 limb CMEs which are wide and could be possible halo events, when directed towards Earth, are used to check the accuracy of the asymmetric cone model. For this purpose characteristics of CMEs (widths and radial speeds) measured for the possible halo CMEs are compared with these obtained for halo CMEs using the asymmetric cone model (Michalek, Solar Phys. 237, 101, 2006). It was shown that the width and speed distributions for both datasets are very similar and with a probability of p>0.93 (using the Kolmogorov – Smirnov test) were drawn from the same distribution of events. We also determined the accurate relationship between radial (V _rad) and expansion (V _exp) speeds of halo CMEs. This relation for the halo CMEs is simply V _rad=V _exp and could be very useful for space weather application.     

Statistical Analysis of Decimetric Radio Bursts,Flares, and Coronal Mass Ejections

A statistical analysis of decimetric radio bursts (RBs), X-ray flares and coronal mass ejections (CMEs) is carried out. We consider all radio bursts recorded by the Cracow Solar Radio Telescope from the beginning of 1996 until the end of 2004. It is found that the decimetric radio bursts are associated and strongly correlated with X-ray flares. Correlation coefficients between RBs durations and the maximal fluxes of the radio bursts and flares are found to be 0.60 and 0.87, respectively. We also demonstrated that a significant population of the decimetric radio bursts are associated with CMEs. The correlation coefficient between the maximal radio flux density multiplied by the duration of the RBs versus velocity multiplied by width of CMEs is found to be 0.55.     

Satellite Measurements of Albedo and Radiant Temperature from Semi-Desert Grassland along the Arizona/Sonora Border

Along the international border separatingthe U.S. (Arizona) and Mexico (Sonora), differencesin the grazing intensity of domestic livestock arecommonly presumed to have created a large differencein vegetation cover between the two countries. Thisvegetation difference is reportedly responsible for anextensive albedo and temperature discontinuity thatmay be affecting regional climate. In this study, weused Landsat Thematic Mapper data to examinetrans-border differences in these two biophysicalparameters. Albedo and radiant temperature estimateswere computed for 25 km-long (east-west) transectsthrough semi-desert grassland on each side of theborder at two different times of year. Only smallaverage trans-border differences in these parameterswere found, and in some cases average albedo andtemperature data were essentially equal on each sideof the border. In addition, we found significantspatial heterogeneity in conditions on both sides ofthe border. These results suggest that, based on asmall sample, it may be difficult to assess whetherthere are significant differences in biophysicalproperties of semi-arid grassland between Arizona andMexico in the vicinity of the border. We concludethat more extensive spatial and temporal sampling iscritical in assessing any possible trans-borderdifferences in average terrain conditions that mightaffect climate, and that this data must be coupledwith more extensive meteorological data to assesswhether a difference in climate also exists.     

Space Weather Application Using Projected Velocity Asymmetry of Halo CMEs

Halo coronal mass ejections (HCMEs) originating from regions close to the center of the Sun are likely to be responsible for severe geomagnetic storms. It is important to predict geoeffectiveness of HCMEs by using observations when they are still near the Sun. Unfortunately, coronagraphic observations do not provide true speeds of CMEs because of projection effects. In the present paper, we present a new technique to allow estimates of the space speed and approximate source location using projected speeds measured at different position angles for a given HCME (velocity asymmetry). We apply this technique to HCMEs observed during 2001 – 2002 and find that the improved speeds are better correlated with the travel times of HCMEs to Earth and with the magnitudes of ensuing geomagnetic storms.