Scientists have shown that the average radial size of CMEs in the last decades is only two-thirds of its value in the previous cycle.
Figure: A CME having loop-like structure observed on 03 April 2010 by coronagraphs onbaord NASA’s STEREO-B (left) and STEREO-A (right) is shown.
The Sun, an explosive celestial object, has been much quieter between 2008 and 2019 than it was between 1996 and 2007, and scientists have quantified that radial size of its Coronal Mass Ejections (CMEs) are two-thirds the radial size of CMEs in the last decade. There has been a significant decrease in the mass, size as well as internal pressure of explosive phenomena. Surprisingly, this was also accompanied with decrease in the average radial size of Coronal Mass Ejections (CMEs) – contrary to the expectation that decrease of pressure in the interplanetary medium will be accompanied with increase in radial size of CMEs.
The Sun is known to be very active with sunspots, solar flares, and CMEs-- episodic expulsion of huge magnetized plasma from the Sun out into space. Understanding such activity of the Sun, particularly the propagation of CMEs, is important since they cause major perturbations in the Earth’s magnetosphere. They effect the near-Earth space environment disturbing the orbit of satellites in low-earth orbits, Global Positioning Signals (GPS), long-distance radio communications, and power grids. Intensity of such solar activity is known to vary in 11-year long periodic cycles. It had earlier been traced that Cycle 24 (2008-2019) was weaker than Cycle 23 (1996-2007), and the Sun was weakest in 2019 during the last 100 years.
Since CMEs and other events propagate in this interplanetary space, astronomers expected that weakening of the solar cycle 24 will be reflected in the properties of these CMEs as well. They looked at the radial extent of the CMEs when they reach the Earth, over both the cycles 23 and 24, to investigate their differences.
Scientists led by Dr. Wageesh Mishra of the Indian Institute of Astrophysics (IIA), Bengaluru, an autonomous institute of the Department of Science & Technology, Government of India, have shown that the average radial size of CMEs in the last decades (during solar cycle 24 from 2008-2019) is only two-thirds of its value in the previous cycle. This was baffling to the scientists as, for them, reduced ambient pressure implied that CMEs were expanding into an interplanetary space to a significantly larger size, expectedly giving rise to large radial size. However, they found the opposite happening.
Explaining this unexpected finding, Dr. Wageesh Mishra suggested, “The reduced pressure in the interplanetary space in cycle 24 is compensated by a reduced magnetic content inside CMEs, which did not allow the CMEs to expand enough in the later phase of their propagation”. The scientist’s explanation is further strengthened by the fact that the lack of stronger and bigger CMEs arriving at the Earth during solar cycle 24 had caused reduced geomagnetic perturbations compared to cycle 23.
The team also established that the gas pressure in the interplanetary space in cycle 24 was only 40% of the pressure in cycle 23. Besides, the rate at which the Sun was losing its mass through these episodic ejections was 15% less in cycle 24 than cycle 23. Additionally, the rate of loss of quasi-steady matter by the Sun was also 10% lower in cycle 24.
This work has been published in the Frontiers in Astronomy and Space Sciences journal and is co-authored by Prof. Nandita Srivastava from Physical Research Laboratory, Udaipur, and Urmi Doshi from the M. S. University of Baroda, Vadodara, India. In this research, the team studied the Earth-directed CMEs and interplanetary counterparts of CMEs (ICMEs) using publicly available observations from Solar and Heliospheric Observatory (SOHO) and Advanced Composition Explorer (ACE), both of them missions launched by NASA in 1995 and 1997, respectively.
The expansion history of CMEs which is governed primarily by the difference in the total pressure between the CMEs and the ambient space around them, are difficult to understand. The study used expansion speeds of CMEs close to the Sun and at Earth.
The scientists said that CMEs need to be observed at different distances from the Sun to better understand the evolution of their radial sizes and expansion behaviour. Such a study would be possible using observations from several space missions such as Aditya-L1 to be launched in the coming year by ISRO, India, as well as the Parker Solar Probe launched by NASA, Solar Orbiter launched by ESA.