Here's how it works: the sun is surrounded by an atmosphere just like Earth – except, because the sun is outrageously hot, so is its atmosphere. So hot, in fact, that its gasses ionized and became superhot plasma. The sun's plasma atmosphere is called its "corona."
The movement of plasma within the corona creates a magnetic field, and spots where the magnetic field is particularly intense can cause instability. That instability then sends a geyser of plasma into space – thus, a CME.
More than just astronomical phenomena, CMEs have a real bearing on us Earthlings. They are the cause of solar storms, which can stress or even deactivate electrical devices. We've been fortunate to have not been hit with a major storm since the 19th century, but one today could cause billions or even trillions of dollars worth of damage. Fortunately, stealth CMEs move too slowly to cause that sort of damage, but they can still affect Earth's magnetic field.
The damage could be mitigated if we saw the CME coming – except some CMEs happen without warning, in spots without active magnetic fields. CMEs are typically heralded by solar flares, but so-called "stealth CMEs" are not. They just seem to happen without cause.
NASA and the ESA launched a combined effort to uncover the meaning behind these stealth CMEs. "Using data from NASA's STEREO spacecraft and the NASA/European Space Agency Solar and Heliospheric Observatory (SOHO), an international team of researchers has developed a model that simulates the evolution of these mysterious eruptions," NASA wrote in a statement
The sun's equator moves faster than its poles: since the entire star completes a rotation at the same time, but the equator is the longest part of the sphere, it has to move quicker (Earth's equator also rotates faster than its poles). This phenomenon, known as "differential rotation," also affects the sun's magnetic field.
The researchers found that the differential rotation caused "knots" to form in the magnetic fields, as different bands moved at different speeds intersected. Add in a tremendous amount of energy from the corona's plasma, and the magnetic energy is converted to kinetic and thermal energy. Voila: a stealth CME.
Since magnetic fields are much harder to detect than plasma eruptions or solar flares, the cause of stealth CMEs hasn't been known until now.