An international team of scientists has developed a model that simulates the evolution of Stealth CMEs, solar storms that puzzled scientists for their lack of typical warning signs as they seem to come from nowhere. Such computer models can help researchers better understand how the Sun affects near-Earth space, and potentially improve our ability to predict space weather.
The team led by the Space Sciences Laboratory at University of California, Berkeley, relied upon NASA missions STEREO and SOHO for this work, fine-tuning their model until the simulations matched the space-based observations, NASA's GSFC explains. Their work shows how a slow, quiet process can unexpectedly create a twisted mass of magnetic fields on the Sun, which then pinches off and speeds out into space, all without any advance warning.
Compared to typical CMEs, which erupt from the Sun as fast as 2900 km/s (1800 miles per second), stealth CMEs move at a rambling gait — between 400 and 700 km/s (250 to 435 miles per second). That’s roughly the speed of the more common solar wind, the constant stream of charged particles that flows from the Sun.
Due to their speed, stealth CMEs aren’t typically powerful enough to drive major space weather events, but they can still cause minor to moderate disturbances to Earth’s magnetic field because of their internal magnetic structure.
To uncover the origins of stealth CMEs, the scientists developed a model of the Sun’s magnetic fields, simulating their strength and movement in the sun’s atmosphere. Central to the model was the Sun’s differential rotation, meaning different points on the sun rotate at different speeds. Unlike Earth, which rotates as a solid body, the Sun rotates faster at the equator than it does at its poles.
The model showed differential rotation causes the Sun’s magnetic fields to stretch and spread at different rates. The scientists demonstrated this constant process generates enough energy to form stealth CMEs over the course of roughly two weeks. The Sun’s rotation increasingly stresses magnetic field lines over time, eventually warping them into a strained coil of energy. When enough tension builds, the coil expands and pinches off into a massive bubble of twisted magnetic fields, and without warning, the stealth CME quietly leaves the Sun.
The evolution of a stealth CME. Differential rotation creates a twisted mass of magnetic fields on the sun, which then pinches off and speeds out into space. The image of the Sun is from NASA’s STEREO. Colored lines depict magnetic field lines, and the different colors indicate in which layers of the Sun’s atmosphere they originate. The white lines become stressed and form a coil, eventually erupting from the Sun. Credits: NASA’s Goddard Space Flight Center/ARMS/Joy Ng, producer
"A model for stealth coronal mass ejections" - B. J. Lynch et al. - Journal of Geophysical Research - November 2016 - DOI: 10.1002/2016JA023432
Stealth coronal mass ejections (CMEs) are events in which there are almost no observable signatures of the CME eruption in the low corona but often a well-resolved slow flux rope CME observed in the coronagraph data. We present results from a three-dimensional numerical magnetohydrodynamics (MHD) simulation of the 1–2 June 2008 slow streamer blowout CME that Robbrecht et al. (2009) called “the CME from nowhere.” We model the global coronal structure using a 1.4 MK isothermal solar wind and a low-order potential field source surface representation of the Carrington Rotation 2070 magnetogram synoptic map. The bipolar streamer belt arcade is energized by simple shearing flows applied in the vicinity of the helmet streamer's polarity inversion line. The flows are large scale and impart a shear typical of that expected from the differential rotation. The slow expansion of the energized helmet streamer arcade results in the formation of a radial current sheet. The subsequent onset of expansion-induced flare reconnection initiates the stealth CME while gradually releasing the stored magnetic energy. We present favorable comparisons between our simulation results and the multiviewpoint SOHO-LASCO (Large Angle and Spectrometric Coronagraph) and STEREO-SECCHI (Sun Earth Connection Coronal and Heliospheric Investigation) coronagraph observations of the preeruption streamer structure and the initiation and evolution of the stealth streamer blowout CME.
Featured image credit: NASA