The discovery of dark jets in the solar atmosphere
Peter Young (George Mason University)
The modern era of Solar Physics has revealed that the Sun's atmosphere is extremely dynamic at all scales, with even so-called "quiet Sun" regions continually twinkling with small flashes. One of the more striking features that the Sun produces are short-lived jets: streaks of hot plasma ejected from small nests of activity. An example is shown in the movie below.
Figure 1: AIA 193 Å images of a small solar region showing the evolution of a jet observed on 9-Feb-2011.
Jets are commonly seen in coronal holes, a recent example of which is shown in the image below. Coronal holes are not actually places where there is no corona, but there is less coronal plasma and it is cooler there (more like 1 million degrees instead of 1.5 million degrees).
Figure 2: An AIA 193 Å image from 2015 February 28, 12:00 UT showing a large coronal hole at the south pole of the Sun.
The reason why there is less corona in coronal holes is that the magnetic field of the Sun is open in these regions and so hot plasma can escape out into the solar wind. Coronal holes are thus the dominant source of solar wind.
Although the large-scale magnetic field is open in coronal holes, there are a lot of small bright points within coronal holes that represent little magnetic bipoles on the Sun's surface (you can see a lot of them in the coronal hole image above). These bipoles are jostled by the Sun's continuously convecting surface layer and can be driven to interact with the larger-scale open magnetic field. It is this process that can lead to jets being created.
New magnetic connections are created between the bipole and the surroundings, liberating energy that causes heating and the ejection of plasma from the bipole. One place for this ejected plasma to go is out along the open magnetic fields lines in the corona, i.e., a jet.
The jet example shown in the movie above is typical, with the jet being narrow and bright compared to the surroundings. Using the EIS instrument a new type of jet has been found for which we do not see the ejected plasma directly. An example is shown in the movie below.
Figure 3: The left panel shows AIA 193 Å images of a coronal hole bright point observed on 8-Feb-2011. The right panel shows how the EIS Dopplergram is built up over time, revealing a blue jet from the bright point. The contours show the intensity of the bright point. The line-of-sight velocities range from -20 to 20 km/s (blue to red).
The left panel shows AIA 193 Å images of a coronal hole bright point observed on 8-Feb-2011. The right panel shows how the EIS Dopplergram is built up over time, revealing a blue jet from the bright point. The contours show the intensity of the bright point. The line-of-sight velocities range from -20 to 20 km/s (blue to red).
EIS views the Sun through a narrow slit, and images are built up by scanning the slit in the X-direction. In the movie, the right panel shows an EIS Dopplergram image being built up over time, and the left panel shows images of the Sun obtained at the same location by the AIA instrument on board the Solar Dynamics Observatory. A Dopplergram is an image that shows the motion of the Sun's corona in the line-of-sight direction to the observer: blue indicates plasma coming towards the observer, and red indicates plasma moving away from the observer. It is obtained by measuring the Doppler shift of the emission feature being observed.
What we see is a small bright point (the bright feature in the left panel) in the south coronal hole of the Sun. The Dopplergram shows a blue jet coming out of the bright point, southwards away from the Sun's surface. However, this jet can not be seen in the left panel. It is a dark jet that is invisible to the imaging instrument, but appears only in the Dopplergram obtained by EIS.
The dark jet was found during an observation in which a coronal hole was scanned repeatedly over a 2 day period. In total I found 24 jets during this period, and 11 of them I classed as dark jets. The presence of so many dark jets shows that there are many more jets than are visible in images alone.
It has been suggested in the past that jets could make a significant contribution to the solar wind, and so the discovery of dark jets helps strengthen this claim. However, because dark jets don't have a strong emission (hence the name) then they don't contain as much hot plasma as normal jets and so contribute relatively less to the solar wind. They do provide a striking example of how some events on the Sun can be hidden from view, and only revealed by the spectroscopic power of EIS!
The work was funded by the National Science Foundation. The data presented were obtained by the Extreme ultraviolet Imaging Spectrometer on board the Hinode spacecraft (JAXA, NASA, STFC, ESA), and the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory (NASA).