Why the corona and the photosphere of stars have different compositions is one of the long standing open questions in astrophysics. Researchers from UiO and six other research institutes and universities, and the WaLSA international working group, have come up with some answers in a worldwide cooperation.
The facts
The Sun is the local laboratory where the scientists can test theories and make measurements to further our understanding of the make up of local neighborhoods as well as throughout the Universe. The outer layer of the Sun, the corona which is visible during a solar eclipse, is physically linked to its lower layers, including the surface of the Sun known as the photosphere, from where the magnetized gas or plasma comprising the corona originates.
The questions
As a consequence, one would naturally expect the chemical composition of the corona to be similar to the photosphere but, surprisingly, this is not the case. This fact was already noted some over 50 years ago in the Sun and more recently in other Sun-like stars.
Why the corona and photosphere of stars have different composition is one of the long standing open questions in astrophysics.
The theories
One theory is that magnetic waves may play a role in separating different elements in the layer of the Sun’s atmosphere that lies in between the photosphere and the corona — the chromosphere. However, while theoretical modelling had highlighted the possible role of magnetic waves, no observational confirmation was available so far.
The breakthrough
Today, thanks to a unique combination of high resolution simultaneous ground-based and space observations of the solar atmosphere, it has been possible not only to unambiguously detect magnetic waves in a sunspot atmosphere at chromospheric heights, but also to identify a link between these waves and coronal regions where this chemical composition anomaly is found. These results show, for the first time, a definite link has been identified between chromospheric activity and chemical composition variations observed in the corona.
The tools
This result was made possible by simultaneously exploiting a combination of chromospheric spectropolarimetric observations acquired by IBIS, the high resolution spectropolarimetric imager at the Dunn Solar Telescope (New Mexico, USA), EIS the EUV imaging spectrometer onboard the JAXA Hinode solar mission, and data from the NASA Solar Dynamics Observatory (SDO).
The findings
Interestingly, this chemical composition which is the signature of the atmospheric plasma can be exploited to trace the mass and energy flow from the surface of the Sun into the heliosphere.
Understanding the physical mechanisms underlying this is therefore of paramount importance to link the solar wind to its source in the solar atmosphere.
The findings of this collaborative research provide a foundation for future science with the upcoming modern facilities such as Solar Orbiter and Daniel K. Inouye Solar Telescope (DKIST). These will provide a unique combination of data which will link all layers of the Sun with the Heliosphere shedding new light on the mechanisms and processes through which our star creates and controls its local environment.
The scientists
The research was led by scientists from UCL/MSSL and the Italian Space Agency (ASI) and involved researchers from seven research institutes and universities, including Shahin Jafarzadeh of the Rosseland Centre for Solar Physics (RoCS) of the University of Oslo, and the WaLSA (Waves in the Lower Solar Atmosphere) international working group, bringing together a number of different and complementary expertises necessary to exploit data at different layers in the solar atmosphere.
The results of this study are now published in the Astrophysical Journal and the special issue of the Philosophical Transaction of the Royal Society A.
Interested in knowing more? Take a look at this video:
Links to the papers :
Alfvenic Perturbations in a Sunspot Chromosphere Linked to Fractionated Plasma in the Corona
