Astronomy Object of the Month: 2021, July

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Kinematics of coronal mass ejections in the LASCO field of view

An accurate understanding of the propagation of coronal mass ejections (CMEs) is crucial in the prediction of space weather. CMEs generate geomagnetic storms causing catastrophic damages to power grids on Earth and are serious radiation threat to satellites on low-Earth orbit and their crew during spacewalks. Basic parameters such as their velocity and acceleration varying with time and heliospheric distance away from the Sun gives researchers the opportunity to predict their arrival time in the vicinity of the Earth. In this paper, we analyze the trend of this parameter in regards to the solar cycles 23 and 24.

Space weather is mostly controlled by coronal mass ejections (CMEs), which are huge expulsions of magnetized plasma from the solar atmosphere. They have been intensively studied for their significant impact on the Earth’s environment. The first CME was recorded by the coronograph on board the 7th Orbiting Solar Observatory (OSO-7) satellite. Since 1995 CMEs have been intensively studied using the sensitive Large Angle and Spectrometric Coronagraph (LASCO) instrument on board the Solar and Heliospheric Observatory (SOHO) spacecraft. SOHO/LASCO recorded about 30,000 CMEs until December 2017. The basic attributes of CMEs, determined manually from LASCO images, are stored in the SOHO/LASCO catalog. The initial velocity of CMEs, obtained by fitting a straight line to the height-time data points, has been the basic parameter used in prediction of geoeffectiveness of CMEs.

The two basic parameters, velocity and acceleration of CMEs, are obtained by fitting a straight and quadratic line to all the height-time data points measured for a given event. The parameters determined in this way, in some sense, reflect the average values in the field of view of the LASCO coronagraphs. Nevertheless, it is evident that both these parameters are continuously changing with distance and time after CME onset from the Sun. Therefore, the average values of velocity and acceleration, used in many studies, do not give a correct description of CME propagation. In this paper we present a statistical study of the kinematic properties of 28894 CMEs recorded by LASCO from 1996 to mid-2017. This research covers a large number of events observed during the 23 and 24 solar cycles. For the study, we employed SOHO/ LASCO catalog data and a new technique to determine the speed of ejections.

The presented statistical analysis reveals that at the beginning of their expansion, in the vicinity of the Sun, CMEs are subject to several factors (Lorentz Force, CME-CME interaction, speed differences between leading and trailing parts of the CME) that determine their propagation. Although their average values of catalog accelerations are always close to zero, a more detailed study shows that their instantaneous accelerations may be quite different depending on the conditions prevailing in the Sun and the environment in which they propagate. These conditions vary depending on the individual eruption and over time as the solar activity changes. The initial acceleration phase is characterized by a rapid increase in CME velocity just after eruption in the inner corona. This phase is followed by a non-significant residual acceleration (deceleration) characterized by an almost constant speed of CMEs. We demonstrate that the initial acceleration is in the range 0.24–2616 ms⁻² with median (average) value of 57 ms⁻² (ms⁻²) and it takes place up to a distance of about 28 R_SUN with median (average) value of 7.8 R_SUN (6 R_SUN).

We note that the significant driving force of CME, namely Lorentz force, can operate up to a distance of 6 R_SUN from the Sun during the first 2 hours of propagation. We found a significant anti-correlation between the initial acceleration magnitude and the acceleration duration, whereas the residual acceleration covers a range from −1224 to 0 m ms⁻² with a median (average) value of −34 ms⁻² (−17 ms⁻²). One intriguing finding is that the residual acceleration is much smaller during the 24 cycle in comparison to the 23 cycle of solar activity. Our study has also revealed that the considered parameters, initial acceleration (ACC_INI), residual acceleration (ACC_RES), maximum velocity (V_MAX), and time at maximum velocity (Time_MAX) mostly follow solar cycles and the intensities of the individual cycle.




Original publication: Kinematics of coronal mass ejections in the LASCO field of view, Ravishankar, A., Michałek, G., Yashiro, S., A&A, 639, A68, 2020.


The research was conducted at the Department of High Energy Astrophysics of the Jagiellonian University’s Astronomical Observatory (OAUJ). The work was supported by the Polish National Science Centre through the grant UMO-2017/25/B/ ST9/00536 and DSC grant N17/MNS/000038. This work was also supported by NASA LWS project led by Dr. N. Gopalswamy.



Contact:

Grzegorz Michałek Astronomical Observatory Jagiellonian University G.Michalek [at] uj.edu.pl

Anitha Ravishankar Postdoctoral Associate University of Calgary, Canada