Seeking to go to the bottom of reasons for declining activities on sun, Indian scientists are currently engaged in finding out the reasons for a comparatively lower level of explosive activities on sun’s surface in the last 11-years cycle than the previous 1996-2007 cyclic period.

During their investigations, scientists have found that there has been a significant decrease in the mass, size and internal pressure of explosive phenomena of the sun. This conclusion has been arrived at on the basis of a comparative study of the 2008-2019 11-year cycle-24 of the solar activities on the sun with the previous cycle.

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A team of scientists led by Wageesh Mishra of the Indian Institute of Astrophysics (IIA), Bengaluru, an autonomous institute of the Department of Science and Technology, Government of India, have shown that the average radial size of CMEs or coronal mass ejections in the last decades (during solar cycle 24 from 2008-2019) is only two-thirds of its value in the previous cycle.

Coronal Mass Ejections – or CMEs – are powerful eruptions on the sun’s surface. These eruptions are caused by instabilities in the sun’s magnetic field. These eruptions can launch a billion tons of superheated gas into space. Most drift harmlessly across the solar system, but occasionally one is aimed at Earth as well.

“This reduction in average radial size has been baffling to the scientists as, for them, reduced ambient pressure implied that CMEs were expanding into interplanetary space to a significantly larger size, expectedly giving rise to large radial size.  However, they found the opposite happening,” a Ministry of Science and Technology note pointed out.

Explaining this unexpected finding Wageesh Mishra said, “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 of scientists further established that the gas pressure in the interplanetary space in cycle 24 was only 40 per cent of the pressure in cycle 23.  Besides, the rate at which the Sun was losing its mass through these episodic ejections was 15 per cent less in cycle 24 than cycle 23. Besides, the rate of loss of quasi-steady matter by the Sun was also 10 per cent lower in cycle-24.

This work has been published in the Frontiers in Astronomy and Space Sciences journal and is co-authored by Professor Nandita Srivastava from Physical Research Laboratory, Udaipur, and Urmi Doshi from the M S University of Baroda, Vadodara, Gujarat. In their 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 the missions launched by NASA in 1995 and 1997, respectively.

The scientists maintained that 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.

Underlining the importance of keeping an eye on CMEs scientists say that CMEs need to be observed at different distances from the Sun to better understand the evolution of their radial sizes and expansion behaviour.

Understanding such activity of the Sun, particularly the propagation of CMEs, is important since they cause major perturbations in the Earth’s magnetosphere. They affect 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.

“Such a study would be possible using observations from several space missions such as Aditya-L1 to be launched in the coming year by India’s as well as the Parker Solar Probe launched by NASA, Solar Orbiter launched by ESA,” a senior scientist said.