Using data from NASA’s SDO (Solar Dynamics Observatory) and STEREO (Solar TErrestrial RElations Observatory) missions, astronomers have detected so-called Rossby waves — a type of global-scale wave that develops in planetary atmospheres – in the solar atmosphere. The discovery is reported in the journal Nature Astronomy.
SDO image of the solar corona in UV light (yellow lines represent the magnetic field on the surface of the Sun). Image credit: NASA / SDO / AIA / LMSAL.
On Earth, Rossby waves are associated with the path of the jet stream and the formation of low- and high-pressure systems, which in turn influence local weather events.
“The waves form in rotating fluids — in the atmosphere and in the oceans,” explained lead author Dr. Scott McIntosh, director of the High Altitude Observatory at the National Center for Atmospheric Research.
“Because the Sun is also rotating, and because it’s made largely of plasma that acts, in some ways, like a vast magnetized ocean, the existence of Rossby-like waves should not come as a surprise.”
And yet solar astronomers have lacked the tools to distinguish this wave pattern until recently.
Unlike Earth, which is scrutinized at numerous angles by satellites in space, scientists historically have been able to study the Sun from only one viewpoint: as seen from the direction of Earth. But for a brief period, from 2011 to 2014, they had the unprecedented opportunity to see the Sun’s entire atmosphere at once.
During that time, observations from NASA’s SDO spacecraft, which sits between the Sun and the Earth, were supplemented by measurements from NASA’s two STEREO satellites. Collectively, the three observatories provided a 360-degree view of the Sun.
“We observed long-lived, slow-moving, westerly features in the combined observations of STEREO and SDO,” the researchers said.
“Those wave-like patterns closely resemble the diagnostics of Rossby wave trains in the Earth’s atmosphere.”
Phase and group velocities of solar Rossby waves. Hovmöller diagrams (longitude versus time at fixed latitude) are shown for the brightpoint density distributions derived from 360 degrees solar observations. (a) the Hovmöller diagram for a band of latitudes 2 degree wide that is centered on 15 degrees in the northern hemisphere; (b) the corresponding Hovmöller diagram for 22 degrees in the southern hemisphere. In both cases, we see that the clusters of enhanced brightpoint density exhibit a westward (left-to-right) phase velocity over their lifetime, while there is an apparent eastward (right-to-left) group velocity. Analysis of the cluster shape yields a westward phase velocity of approximately 3 m/s (red dashed reference line), whereas the analysis of their longitudinal separation yields an eastward group velocity of approximately 24 m/s (white dashed reference line) in each hemisphere. Image credit: Scott W. McIntosh et al, doi: 10.1038/s41550-017-0086.
Dr. McIntosh and his colleagues from Yale University, Texas Tech University, and the University of Maryland, College Park, used images taken by instruments on SDO and STEREO to identify and track coronal bright points.
These small bright features dot the entire face of the Sun and have been used to track motions deeper in the solar atmosphere.
The team plotted the combined data on Hovmöller diagrams, a diagnostic tool developed by meteorologists to highlight the role of waves in Earth’s atmosphere.
What emerged from the analysis were bands of magnetized activity that propagate slowly across the Sun — just like the Rossby waves found on Earth.
The discovery could link a range of solar phenomena that are also related to the Sun’s magnetic field, including the formation of sunspots, their lifetimes, and the origin of the Sun’s 11-year solar cycle.
“It’s possible that it’s all tied together, but we needed to have a global perspective to see that,” Dr. McIntosh said.
“We believe that people have been observing the impacts of these Rossby-like waves for decades, but haven’t been able to put the whole picture together.”
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Scott W. McIntosh et al. 2017. The detection of Rossby-like waves on the Sun. Nature Astronomy 1, article number: 0086; doi: 10.1038/s41550-017-0086
