# The Sun and Space Weather
## 2 Basic Properties of CMEs
### Kinematic Properties  
Figure 2 shows the speed and width distributions of CMEs detected in the SOHO/LASCO FOV (2.5 to 32 Rs) covered by the C2 and C3 telescopes. The speed of CMEs measured in  
Figure 1: SOHO/LASCO/C2 images of the 15 January 2005 CME showing the basic morphology. The 06:24 UT image (**a**) shows the pre-eruption corona featuring several streamers. The white circle on the image of the coronagraph occulting disk denotes the optical Sun. Six minutes later, a CME appears in the north–northwest part of the corona with a bright front (BF) and bright core (**b**). The void can be seen separating BF and the core. “S” denotes a kink that appears outside of the BF. The difference between the frames (**a,b**) shows the coronal changes occurring over a period of 6 min (**c**). The kink S is at the outer boundary of the diffuse structure that envelopes BF. An EUV difference image, superposed on the LASCO difference image, shows disturbances on the solar disk indicating the eruption (adapted from [17]). the sky plane varies by over two orders of magnitude from ~50 to > 3000 km s\({}^{-1}\), while the width ranges from <20\({}^{\circ}\) to >120\({}^{\circ}\). The typical speed and width are ~400 km/s and ~40\({}^{\circ}\), respectively. The speed is lognormally distributed, which has been attributed to the complexities of the elementary reconnection processes during an eruption [42]. The CME widths > 120\({}^{\circ}\) are mainly due to projection effects. The last width bin corresponds to full halo CMEs, which constitute only ~2.5% of all CMEs. For a given coronagraph, halo CMEs represent an energetic population with the inherent width and speed larger than the average values shown in Figure 2[43]. The accelerations have a large scatter, but there is a clear tendency for faster CMEs to decelerate on average. On the other hand, very slow CMEs (speed < 480 km/s) have a positive acceleration. However, there are many fast CMEs that do have positive acceleration within the LASCO FOV. All CMEs have to accelerate from zero speed, so the initial acceleration is always positive. What is shown in Figure 2 is the residual acceleration after the CMEs have attained their peak speeds, and hence the initial acceleration is often missed. Observations from SOHO's Extreme-ultraviolet Imaging Telescope (EIT) and LASCO's C1 telescope reveal the extent of the initial acceleration. Case studies that include CME motion below the LASCO/C2 occulting disk reveal the CME initial acceleration, which is much higher than the residual acceleration [44; 45; 46; 47]. Figure 3 shows the 11 June 1998 CME observed close to the Sun by SOHO/EIT and LASCO. The height-time plot of the CME has an S-shape because of the initial positive acceleration and later deceleration. If we use LASCO/C2 and C3 data alone, we see only the decelerating part ([[https://cdaw.gsfc.nasa.gov/CME_list/UNIVERSAL/19](https://cdaw.gsfc.nasa.gov/CME_list/UNIVERSAL/19)]([https://cdaw.gsfc.nasa.gov/CME_list/UNIVERSAL/19](https://cdaw.gsfc.nasa.gov/CME_list/UNIVERSAL/19)) 98_06/htpng/19980611.102838.p097g.ht.html, accessed on 1 October 2022). The residual acceleration is ~36 m s\({}^{-2}\). The two EIT and two LASCO/C1 data points are able to capture the initial positive acceleration, which is an order of magnitude higher than the residual acceleration. Unfortunately, SOHO/LASCO/C1 ceased operations in June 1998. The COR1 coronagraph on board the Solar Terrestrial Relations Observatory (STEREO) [48] has observed CMEs closer to the Sun since 2006, and the acceleration has been confirmed to be in the range 0.02 to 6.8 km s\({}^{-2}\) using a larger number of events [49]. Studies of initial acceleration have shown that CMEs attain peak acceleration within ~1.5 Rs; the peak acceleration is inversely proportional to the duration of acceleration [47; 49]. The source regions that produce high impulsive acceleration are compact compared to those that produce small gradual acceleration.