To obtain the model oscillations, the team began with a model of the Sun’s structure and differential rotation inferred from helioseismology.
SPEED OF SOUND MPS FULL
“The models allow us to look inside the Sun’s interior and determine the full three-dimensional structure of the oscillations”, explains MPS graduate student Yuto Bekki. To identify the nature of these oscillations, the team compared the observational data to computer models. Lekshmi and Bastian Proxauf from MPS, confirmed the results with data from the Global Oscillation Network Group (GONG), a network of six solar observatories in the USA, Australia, India, Spain, and Chile. “The long-period oscillations manifest themselves as very slow swirling motions at the surface of the Sun with speeds of about 5 kilometers per hour – about how fast a person walks”, says Zhi-Chao Liang from MPS. The modes with maximum velocity near the equator are Rossby modes, which the team had already identified in 2018. Some modes of oscillation have maximum velocity at the poles (movie 1), some at mid-latitudes (movie 2), and some near the equator (movie 3). The team observed many tens of modes of oscillation, each with its own oscillation period and spatial dependence.
Sound: filtered data (73 ± 10 nHz) shifted to the audible spectrum the sound variations inform us about the excitation and damping of the mode. Left: observations using the SDO/HMI instrument. The east-west velocity associated with the retrograde propagating mode of oscillation. The continuous observations from the Helioseismic and Magnetic Imager (HMI) onboard SDO are perfect for this purpose.” “Detecting the long-period oscillations of the Sun requires measurements of the horizontal motions at the Sun’s surface over many years. “The long-period oscillations depend on the Sun’s rotation they are not acoustic in nature”, says Laurent Gizon, lead author of the new study and director at the MPS.
In addition to the 5-minute oscillations, much longer-period oscillations were predicted to exist in stars more than 40 years ago, but had not been identified on the Sun until now. One of the triumphs of helioseismology is to have mapped the Sun’s rotation as a function of depth and latitude (the solar differential rotation). These 5-minute oscillations have been observed continuously by ground-based telescopes and space observatories since the mid 1990’s and have been used very successfully by helioseismologists to learn about the internal structure and dynamics of our star – just like seismologists learn about the interior of the Earth by studying earthquakes. Millions of modes of acoustic oscillations with short periods, near 5 minutes, are excited by convective turbulence near the solar surface and are trapped in the solar interior. In the 1960’s the Sun’s high musical notes were discovered: The Sun rings like a bell. Sound: filtered data (86 ± 10 nHz) shifted to the audible spectrum the sound variations inform us about the excitation and damping of the mode.
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