January 7, 2009 | On November 8, 2007, scientists Giuliana de Toma and Roberto Casini noted something very unusual in observations of a solar prominence collected at NCAR's Mauna Loa Solar Observatory (MLSO). Prominences are magnetic structures that appear as clouds of cool, very dense material suspended for days or weeks in the much hotter, less dense solar corona. Most prominences end their lives in eruptions, often associated with flares and coronal mass ejections that can send plasma toward Earth and cause stormy space weather. In this case, however, a large, dark bubble with a bright core rose through a very stable, or quiescent, prominence in less than an hour without causing it to erupt.
"We observe prominences every day at MLSO, but we had never seen such a large, organized, and rapidly moving disturbance inside a quiescent prominence,” says instrument scientist Joan Burkepile. Although the bubble spanned about a third of the entire prominence, the shape and size of the prominence were mostly unaffected by the bubble’s passage. The incident is described in the November 10 issue of Astrophysical Journal.
This ground-based discovery shows that a prominence can have a remarkably dynamic interior without losing its overall stability. The Japanese space mission Hinode has observed many dynamic prominences at high resolution since 2006, but the MLSO observations were unprecedented, says de Toma. “This bubble and its core moved like a bullet through the prominence,” she notes. The speedy motion produced a front of compressed plasma (the bright arc seen just below the dark bubble). Other plasma was trapped at the heart of the bubble itself (the bright core).
HAO’s Boon Chye Low points out that these observations from MLSO and Hinode challenge the current magnetohydrodynamic understanding of prominences and space weather. “This reinforces the need to directly measure the magnetic fields of prominences,” he says. With Casini as lead, HAO is now completing the Prominence Magnetometer (ProMag), which will collect these much-needed observations from Sacramento Peak Observatory.