Monday, May 7, 2018

Rossby Waves Influencing Weather on the Sun and on Earth

In international team of scientists, led by Laurent Gizon, co-principal investigator of the Center for Space Science at NYU Abu Dhabi (NYUAD), have discovered planetary waves of vorticity on and inside the Sun similar to those that significantly influence weather on Earth.

Rossby waves are a natural phenomenon in the atmospheres and oceans of planets that form in response to the rotation of the planet. Like Earth, the Sun also rotates and should support Rossby waves, but their existence on the Sun has been debated, until now.

"There's no doubt what we're seeing are Rossby waves due to the measured, textbook relationship between frequency and wavelength, said Gizon.

Solar Rossby waves are gigantic in size, Gizon explained, with wavelengths comparable to the solar radius. They are an essential component of the Sun's internal dynamics because they contribute half of the Sun's large-scale kinetic energy.'

"That these waves are so big and are only seen in the equatorial regions of the Sun is completely unexpected," he said.

Figure: Rossby waves on the Sun are waves of vorticity that move in the direction opposite to rotation. Associated flows have amplitudes of about one meter per second that peak in the Sun's equatorial regions. 
Credit: Max Planck Institute for Solar System Research.

Astrophysicists from NYUAD, the Max Planck Institute for Solar System Research, and Stanford University studied six years of space data, which revealed the Rossby waves moving in the direction opposite to the Sun's rotation.

Rossby waves on the Sun are close relatives to those known to occur in the Earth's atmosphere and oceans, Gizon said, but are extremely difficult to detect on the Sun because they have very small flow amplitudes, around one meter per second.

Solar Rossby Waves Characteristics

- waves of vorticity

- move in the direction opposite to rotation

- well-defined relationship between frequency and wavelength

- found only near the equator

- small amplitude, difficult to detect

- live for several months

- contribute half of the Sun's kinetic energy at large scales

Earth's Rossby Waves Characteristics

- found in at mid-latitudes in the atmosphere and ocean

- significant role in shaping weather

Analysis and confirmation

Scientists analyzed data collected from 2010-2016 by the Heliospheric and Magnetic Imager (HMI) instrument on board NASA's Solar Dynamics Observatory. The study required high-precision observations of the Sun over many months.

In this north pole view of the sun, the brightpoints can be seen circling counter-clockwise, revealing the magnetized Rossby waves flowing beneath the surface.
Credit: NASA

Granules were used as passive tracers to uncover the underlying, much larger vortex flows associated with Rossby waves. "The HMI images have sufficiently high spatial resolution to allow us to follow the movement of photospheric granules on the Sun's visible surface," said Bjoern Loeptien, scientist at the Max Planck Institute and first author of the paper. These granules are small convective cells roughly 1,500 kilometers in size on the solar surface.

Helioseismology, the study of the solar interior using solar internal acoustic waves, was used to verify the findings and observe the Sun's Rossby waves at depths up to 20,000 kilometers. "The results from helioseismology and granulation tracking are in excellent agreement," asserted Gizon.

"We don't yet know what role Rossby waves play in the Sun, but know that they can't be ignored in future studies," added Katepalli R. Sreenivasan, NYUAD Center for Space Science principal investigator, "their presence may help us understand solar convection at the largest spatial scales, which remains poorly understood. They are very hard to find because of low signal levels but this research team has used ingenious data processing techniques to discover their existence."

Their findings are reported in the journal Nature Astronomy.

NYUAD Provost Fabio Piano said, "We congratulate the researchers, including NYUAD Research Professor Laurent Gizon, for their work on this important discovery confirming the presence of Rossby waves on the Sun. The Center for Space Science is running a world-class research and outreach program in solar, stellar, and exoplanet science. In addition to being a hub of intellectual activity within NYUAD, the Center is quickly becoming a significant resource in supporting the priority space sector within the UAE."

Contacts and sources:
Adam Pockriss
Rubenstein Associates Inc.

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