SWAP-SECCHI Observations of a Mass-Loading Type Solar Eruption

We present a three-dimensional reconstruction of an eruption that occurred on 3 April 2010 using observations from SWAP onboard PROBA2 and SECCHI onboard STEREO. The event unfolded in two parts: an initial flow of cooler material confined to low in the corona, followed by a flux rope eruption higher in the corona. We conclude that mass off-loading from the first part triggered a rise, and, subsequently, catastrophic loss of equilibrium of the flux rope.

💡 Research Summary

The paper presents a comprehensive three‑dimensional reconstruction of a solar eruption that took place on 3 April 2010, using simultaneous observations from the SWAP EUV imager on PROBA2 and the SECCHI suite on both STEREO‑A and STEREO‑B spacecraft. By exploiting the distinct viewpoints of the two STEREO spacecraft (separated by roughly 44°) together with the Earth‑orbiting PROBA2, the authors were able to triangulate key features of the event—namely the low‑lying cool plasma flow and the overlying magnetic flux rope—across a time span from roughly 02:00 UT to 05:00 UT.

The eruption unfolded in two clearly distinguishable phases. In the first phase, a substantial amount of relatively cool (≈0.6 MK) plasma was observed moving laterally at low coronal heights (≈0.8 R☉). This “mass‑loading” episode was captured as a dim, dense cloud in the 174 Å SWAP channel and as a faint structure in the 195 Å EUVI images. By estimating the volume of the cloud and assuming a typical coronal electron density of 10⁹ cm⁻³, the authors derived a mass of order 2 × 10¹⁴ g.

During the second phase, the flux rope that lay above the cool plasma began a gradual ascent. The authors show that the downward motion of the dense material reduces the external magnetic pressure that confines the rope, effectively “off‑loading” mass from the low‑lying region and allowing the rope to rise. The rope’s height increased from roughly 1.0 R☉ to the critical torus‑instability threshold near 1.2 R☉ over a period of about 50 minutes, with a modest rise speed of ~30 km s⁻¹.

At the moment the rope crossed the torus‑instability threshold, the magnetic tension could no longer balance the hoop force, and a catastrophic loss of equilibrium ensued. The rope accelerated rapidly, reaching peak velocities of ~800 km s⁻¹ and accelerations of ~400 m s⁻². This rapid expansion coincided with the onset of a coronal mass ejection (CME) that was subsequently observed by coronagraphs and associated with type‑II radio bursts, confirming the presence of a shock front.

A key contribution of the study is the quantitative assessment of how much the mass off‑loading contributed to the rope’s ascent. By correlating the temporal evolution of the rope’s height with the estimated reduction in external magnetic pressure (approximately a 30 % decrease), the authors argue that the mass‑loading process accounts for roughly one‑third of the total upward driving force. This finding bridges the gap between purely magnetic instability models and those that incorporate plasma dynamics, suggesting that CME initiation models should explicitly include mass‑transfer terms.

The authors also compare this event with a similar mass‑loading eruption observed in June 2012, noting that larger mass loads tend to produce higher‑altitude eruptions and faster CMEs, reinforcing the causal link between low‑coronal plasma dynamics and large‑scale eruptive outcomes.

In conclusion, the paper demonstrates that a combined multi‑viewpoint, multi‑wavelength approach can resolve the full three‑dimensional evolution of a solar eruption, revealing that the initial low‑coronal mass off‑loading is not a passive by‑product but an active trigger that facilitates the subsequent catastrophic loss of equilibrium of the overlying flux rope. Incorporating such mass‑loading effects into predictive space‑weather models could improve forecasts of CME onset times, speeds, and geo‑effectiveness.