Prominence with vertical filament structures over the solar limb (bottom), visible in the red hydrogen alpha spectral line (H-alpha); image taken by Dr. Eberhard Wiehr (Göttingen University) with the vacuum tower telescope (VTT) on Tenerife.
Top view into the structure of a prominence filament in front of the solar disk made visible through absorption of the red H-alpha spectral line; image taken by Prof. Oddbjørn Engvold (University of Oslo) with the Swedish solar telescope (SST) on La Palma.
Prominences over the solar limb visualised in the red H-alpha spectral line with a spectrograph slit (white) and the blue emission of ortho-helium superposed by the spectrum of the sky's stray light; below, the 100-fold attenuated spectrum of the solar disk, from which helium is not emitted; photomontage: Uwe Nolte (GWDG).
Astrophysicists observe the exact structure of plasma clouds for the first time.
(pug) Solar prominences are clouds of plasma, or ionised gas, that float far above the sun’s surface and can extend more than 100,000 kilometres beyond its limb. What we previously knew about the structure of these clouds was that, inside, they are made up of “filaments” with widths of up to 150 kilometres. At temperatures approaching 7,000 °C, they are significantly colder than the surrounding solar corona, which can heat up to 1.5 million °C. An international team of researchers affiliated with Göttingen University has now discovered how these prominences shield themselves against their ultra-hot environment. Apparently, every single filament is surrounded by an isothermal corona with a prominence-corona transition region (PCTR). This PCTR is a type of sheath similar to the insulation layer on a cable where the temperature increases outwardly. The results have been published in the scientific journal Solar Physics.
For their study, astrophysicists used the Gregory–Coudé telescope at the Locarno Solar Observatory. The special construction of this telescope subtracts the superposed aureole spectrum, thereby making it possible to measure the very weak helium emissions in the prominences. They examined three different forms of helium, brought to radiance at various temperatures. Their experiments showed that the temperature from inside the filament increased in the direction of the solar corona from 7,000 °C to 8,750 °C, reaching up to around 25,000 °C. “The outer layer of this transition region may only be a few kilometres in width which would explain why it is no longer visible in any telescope and can only be determined indirectly,” notes Dr. Eberhard Wiehr of the Institute for Astrophysics in Göttingen.
Moreover, the researchers tried to find out whether a PCTR only surrounds each individual thread or the prominence as a whole. “The emitted energy of the three atomic states of helium demonstrated a constant ratio in all varying sizes of prominences investigated. This is a hint that there are many narrow PCTRs around each individual filament,” Dr. Wiehr explains. Further investigations will now aim to model the various plasma layers in the PCTRs and find an explanation for the threadlike structure of the prominences.
Original publication: R. Ramelli et al. Helium Emissions Observed in Ground-Based Spectra of Solar Prominences. Solar Physics (2012) 281: 697-706. Doi: 10.1007/s11207-012-0118-2.