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One After Another: The Effect of Repeated Coronal Mass Ejections

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white-light coronagraph image of a coronal mass ejection
In this image from 23 January 2013, a coronal mass ejection extends into space above the Sun's disk, which is obscured by a coronagraph. [ESA & NASA/SOHO]

The Sun launches tangled masses of plasma and magnetic fields in the form of coronal mass ejections. But these explosions don’t occur in a (literal or metaphorical) vacuum — how is the passage of a coronal mass ejection affected by the eruptions that preceded it?

Capturing Coronal Mass Ejections

Illustration of the sun setting off a coronal mass ejection headed to Earth and it hitting Earth's magnetosphere

Illustration of a coronal mass ejection headed toward Earth. [SOHO/LASCO/EIT (ESA & NASA)]

When the Sun’s activity cycle ramps up to its maximum, as it will in 2025, the Sun will unleash two or three coronal mass ejections every day. These explosions blast out into the solar system, speeding toward Earth and the other planets at hundreds of kilometers a second. When a coronal mass ejection collides with Earth’s protective magnetic field, the ensuing magnetic tussle can fling high-energy particles into Earth’s atmosphere, creating the aurora — and potentially damaging spacecraft electronics.

Given the risk of damaging effects on Earth-orbiting spacecraft, researchers have developed models to predict the path a coronal mass ejection will take after detaching from the Sun. These models estimate how a loop of magnetized plasma twists, expands, and is deflected as it travels through the tenuous swirls of the solar wind. However, many such models trade accuracy for speed in order to quickly assess the danger to Earth, failing to capture the variable and turbulent nature of the space between Sun and Earth, which may affect how coronal mass ejections travel through that space.

A Series of Solar Events

A team led by Chin-Chun Wu (Naval Research Laboratory) used magnetohydrodynamics simulations to explore how a coronal mass ejection moves through the wake left by previous solar eruptions. Wu and collaborators opted to model a series of five coronal mass ejections that occurred in two and a half weeks in July 2012, for which we have extensive data.

Modeled radial solar wind speed for four time periods

Modeled radial solar wind speed for four time periods. The red areas indicate coronal mass ejections. Click to enlarge. [Adapted from Wu et al. 2022; full time series available here]

The team’s model uses observations of these events to determine each coronal mass ejection’s initial speed, trajectory, and the time it departs the Sun. By solving fluid dynamics equations to understand how each event evolves over time, the model outputs key parameters like the plasma density and temperature, the speed of the background solar wind, and the magnetic field strength, all of which can be compared against measurements made by satellites.

Creating a Path to Follow

comparison of observed and modeled plasma parameters

Observed (black points) and modeled (red and purple lines) plasma parameters. The model results for a series of three coronal mass ejections are shown as red dotted lines, and the model results for a single coronal mass ejection are shown in purple lines. From top to bottom, the parameters shown are the radial velocity (vr), the plasma density (np), the plasma temperature (Tp), and the magnetic field strength (B). Click to enlarge. [Wu et al. 2022]

Previous work has suggested that when one coronal mass ejection closely follows another, the second event moves faster than it would otherwise. To test this theory, Wu and coauthors compared a model of a series of coronal mass ejections to another of just the final event in the series. These simulations showed that a coronal mass ejection following in the wake of other explosions travels faster than one forging ahead solo — the passage of a previous shock wave reduces the density and increases the speed of the solar wind, allowing the final coronal mass ejection to surf its way to Earth’s orbit 30 minutes faster.

Ultimately, the authors concluded that their model was able to match the observed parameters of the five coronal mass ejections fairly well. Their simulations allowed them to show that coronal mass ejections are affected by those that came before, suggesting that multiple events should be accounted for in modeling these eruptions.

Citation

“Magnetohydrodynamic Simulation of Multiple Coronal Mass Ejections: An Effect of Pre-events,'” Chin-Chun Wu et al 2022 ApJ 935 67. doi:10.3847/1538-4357/ac7f2a