NSO: NSF’s GONG Observations of Long-period Oscillations of the Sun
High-latitude inertial mode of oscillation as seen in the east-west velocity using 10 years of observations from GONG. Credit: MPS/ Z.-C. Liang
The Sun is a ball of hot gas, allowing its interior to transmit sound. These sound waves bounce around under the surface, much like a bell. The waves can be characterized by their modes, which are akin of tones of a musical instrument with high and low pitch sound. The properties of these waves (or “solar oscillations”) reveal the invisible dynamics happening in the Sun’s interior.
A new set of solar oscillations with specific long-duration properties have recently been discovered by a group of scientists led by Laurent Gizon of the Max Planck Institute for Solar System Research (Germany) with a contribution from NSO scientists and observations. The newly discovered modes will provide new insight into the internal properties of the Sun, such as the strength of the turbulent motions and the strength of the convective driving at the base of the convection zone. These properties ultimately govern the solar dynamo – the physical process that generates the Sun’s magnetic field.
These long-period oscillations manifest themselves as very slow swirling motions at the surface of the Sun with speeds of about 3 miles per hour. The team used 10 years of observations from both ground-based Global Oscillations Network Group (GONG) and space-borne Solar Dynamics Observatory (SDO) to discover the new waves. Their results are described in a letter published in the journal Astronomy & Astrophysics.
Millions of acoustic modes of oscillation with periods near five minutes are routinely observed by GONG and SDO. These were discovered in the 1960s and inform us primarily about the speed of sound and the rotation in the solar interior.
Other modes with much longer periods, known as inertial modes, were predicted by models several decades ago but until now, have not been confirmed by observations. NSO’s six identical instruments of the GONG network located around the globe have been collecting the velocity observations needed to confirm this theory since mid-1995.
In this new study, the team observed more than 40 inertial modes of oscillation, each with its own unique characteristics, such as oscillation period and spatial dependence. Some modes swirl fastest at the north and south poles, while others at mid-latitudes between poles and the equator. This is the first time oscillations have been clearly identified as resonant modes of oscillation. In addition, long-period modes have been observed near the equator, and these are known as Rossby modes (or planetary waves which also exist in Earth oceans and atmospheres of Earth and other planets)
GONG instruments are located at Learmonth (Australia), Udaipur (India), Tenerife (Canary Islands, Spain), Cerro Tololo (Chile), Big Bear (California, USA) and Mauna Loa (Hawaii, USA). GONG operations are funded by NSF and NOAA.