Astronomers have investigated a Coronal Mass Ejection (CME) that travelled all the way from the Sun to Earth in March 2023, through a coronal hole, an opening in the Sun’s magnetic field lines, leading to leakage of solar wind streams. The study highlighted how a subtle solar CME could trigger intense geomagnetic storms on earth increasing the challenges of forecasting the effects of space weather. 

Coronal Mass Ejections (CMEs) are powerful expulsions of plasma and magnetic fields from the Sun's atmosphere that can sometimes cause intense geomagnetic storms, disrupting satellites, communication systems, and power grids on Earth.

However, around 10% of intense geomagnetic storms do not seem to arise from any such large-scale eruptions on the solar disk, instead from weak or stealthy eruptions that are typically missed due to current observational limitations. Understanding these ‘Stealth Coronal Mass Ejections’ is crucial to predict space weather effects on Earth even when no visible eruptions are seen on the Sun.

In a recent study, astronomers investigated one such Stealth CME that occurred on 19 March 2023, which led to an intense storm on Earth around 3 days later, utilizing NASA’s Multi spacecraft observations, and found evidence for significant effect on earth of weak, stealth CMEs with southward component of magnetic field and enhanced density.

This CME emerged from the eruption of a longitudinal-filament channel near the Sun’s centre. Unlike typical strong CMEs, which are accompanied by X-ray flares and/or radio bursts, this event occurred without these standard solar warning signs, making it exceptionally elusive.

This stealth CME was investigated by researchers at the Indian Institute of Astrophysics (IIA), an autonomous institution under the Department of Science and Technology, Government of India. “Such weak CMEs leave no detectable signatures on the Sun and hence are extremely difficult to identify with current observational sensitivity,” explained P. Vemareddy, the lead author from IIA. They used data from a number of spacecrafts, including NASA’s Solar Dynamic Observatory (SDO), Solar Orbiter (SolO), STEREO-A, and WIND.

Extreme Ultraviolet images from SDO revealed presence of a coronal hole near the CME source region. When CMEs erupt close to these coronal holes, they are often carried away by high-speed solar wind. “This stealthy eruption was likely aided by the nearby coronal hole, enabling the CME to travel all the way from the Sun to Earth, where otherwise it might have dissipated near the Sun,” Vemareddy added. This finding highlights the role of coronal holes in influencing the propagation of such subtle solar eruptions.

The study also examines the radial evolution of the interplanetary coronal mass ejection (ICME) using in situ observations from spacecraft nearly aligned in radial distance from the Sun: SolO, STEREO-A, and WIND. The ICME, traveling behind a high-speed solar wind, was detected without a clear shock or sheath.

The observations indicated that expansion of the associated magnetic cloud within the ICME, characterized by decreasing velocity, increasing radial size (0.08 AU at SolO to 0.18 AU at STA, where 1 AU is the distance between the Sun and the Earth), and reduced expansion speed. The magnetic field structure showed rotation during propagation, with right-handed helicity consistent with the source region.

Fig: The source region of the CME on the Sun’s disk, showing the coronal hole and the filament (a) Ultraviolet AIA image of the Sun at  at 193 Å, (b) UV image of the Sun at 212 Å, (c ) magnetic field distribution, (d) zoom-in of the source region, (e-f) difference images showing faint bright channels along the filament.

The ICME was observed with enhanced plasma density towards the boundaries of the magnetic cloud. The study also modelled the observed geomagnetic index which is a function of solar wind velocity, density, ICME magnetic field, and electric fields. The modelled storm intensity shows strong agreement with observed geomagnetic indices, especially when accounting for solar wind density and electric field variations.

The study highlights how a subtle CME, inconspicuous near the Sun but with southward magnetic components and enhanced density, can drive intense geomagnetic storms marked by complex solar wind structures and evolving magnetic signatures as it travels through the heliosphere. This dynamic evolution underscores the challenges in forecasting space weather effects from stealthy CMEs.

This study was published in .”The Astrophysical Journal “in a paper entitled “An Intense Geomagnetic Storm Originated from Stealth Coronal Mass Ejection: Remote and In Situ Observations by Near Radially Aligned Spacecraft”, authored by P. Vemareddy of IIA and K. Selva Bharathi of IISER Tirupati ,  an MSc internship student at IIA.

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