Discovery of long period oscillations of the Sun

A team of solar physicists led by Laurent Gizon from the Max Planck Institute for Solar System Research (MPS) and the University of Göttingen in Germany reported the discovery of global oscillations of the Sun with very long periods, comparable to those of the 27-day Sun. rotation period. The oscillations appear on the solar surface in the form of whirling movements with speeds of the order of 5 kilometers per hour. These movements were measured by analyzing 10 years of observations from NASA’s Solar Dynamics Observatory (SDO). Using computer models, scientists have shown that the recently discovered oscillations are modes of resonance and owe their existence to the differential rotation of the Sun. The oscillations will help establish new ways of probing the Sun’s interior and obtaining information about the internal structure and dynamics of our star. Scientists describe their findings in letter to appear today in the journal Astronomy & Astrophysics.

In the 1960s, the high-pitched musical notes of the Sun were discovered: The Sun rings like a bell. Millions of modes of acoustic oscillations with short periods, nearly 5 minutes, are excited by convective turbulence near the solar surface and are trapped inside the sun. These 5-minute oscillations have been observed continuously by ground telescopes and space observatories since the mid-1990s and have been used with great success by helioseismologists to learn more about the internal structure and dynamics of our star. , just as seismologists discover the interior of the Earth by studying earthquakes. One of the triumphs of helioseismology is to have mapped the rotation of the Sun as a function of depth and latitude (the solar differential rotation).

In addition to the 5-minute oscillations, much longer period oscillations were predicted in stars over 40 years ago, but had not been identified on the Sun until now. “Long-period oscillations depend on the rotation of the Sun; they are not of an acoustic nature, ”explains Laurent Gizon, principal author of the new study and director of the MPS. “Detecting the long-period oscillations of the Sun requires measurements of the horizontal motions on the Sun’s surface over many years. The continuous observations of the Helioseismic and Magnetic Imager (HMI) on board the SDO are perfect for this purpose.”

The team observed dozens of oscillation modes, each with its own period of oscillation and its own spatial dependence. Some modes of oscillation have maximum speed at the poles, some at mid-latitudes, and others near the equator. The modes with a maximum speed near the equator are the Rossby modes, which the team had already identified in 2018. “Long period oscillations manifest themselves as very slow swirling motions on the surface of the Sun with speeds of d ‘about 5 kilometers per hour — about how fast a person walks, “says Zhi-Chao Liang of MPS. Kiran Jain of NSO, along with B. Lekshmi and Bastian Proxauf of MPS, confirmed the results with data from the Global Oscillation Network Group (GONG), a network of six solar observatories in the United States, Australia, India, Spain and Chile.

To identify the nature of these oscillations, the team compared observational data to computer models. “The models allow us to look inside the Sun and determine the full three-dimensional structure of the oscillations,” explains MPS graduate student Yuto Bekki. To obtain the oscillations of the model, the team started with a model of the structure of the Sun and the differential rotation deduced from helioseismology. In addition, the force of the convective conduct in the upper layers and the amplitude of the turbulent movements are taken into account in the model. The free oscillations of the model are found by considering small amplitude disturbances of the solar model. The corresponding surface speeds correspond well to the observed oscillations and allowed the team to identify the modes.

“All these new oscillations that we observe on the Sun are strongly affected by the differential rotation of the Sun”, explains Damien Fournier, scientist at MPS. The dependence of solar rotation on latitude determines where the modes have maximum amplitudes. “The oscillations are also sensitive to the properties of the interior of the Sun: in particular to the force of the turbulent movements and the associated viscosity of the solar medium, as well as to the force of the convective conduct,” explains Robert Cameron of MPS. This sensitivity is strong at the base of the convection zone, about two hundred thousand kilometers below the solar surface. “Just as we use acoustic oscillations to learn more about the speed of sound inside the sun with helioseismology, we can use long-period oscillations to learn more about turbulent processes,” he adds. he.

“The discovery of a new type of solar oscillations is very exciting because it allows us to deduce properties, such as the force of the convective drive, which ultimately control the solar dynamo,” explains Laurent Gizon. The diagnostic potential of long-period modes will be fully exploited in the coming years using a new exascale computer model developed within the framework of the WHOLESUN project, supported by a Synergy 2018 grant from the European Research Council.