Ten years of data from NASA’s Solar Dynamics Observatory combined with
numerical models reveal the deep low musical notes of the Sun.
These motions were measured by analyzing 10 years of observations from
NASA’s Solar Dynamics Observatory (SDO). Using computer models, the
scientists have shown that the newly discovered oscillations are resonant
modes and owe their existence to the Sun’s differential rotation. The
oscillations will help establish novel ways to probe the Sun’s interior and
obtain information about our star’s inner structure and dynamics. The
scientists describe their findings in the journal Astronomy &
Astrophysics.
In the 1960s the Sun’s high musical notes were discovered: The Sun rings
like a bell. 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. These 5-minute oscillations have been
observed continuously by ground-based telescopes and space observatories
since the mid 1990s 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. 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).
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. “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. “Detecting the long-period
oscillations of the Sun requires measurements of the horizontal motions at
the Sun’s surface over many years. The continuous observations from the
Helioseismic and Magnetic Imager (HMI) onboard SDO are perfect for this
purpose.”
The team observed many tens of modes of oscillation, each with its own
oscillation period and spatial dependence. Some modes of oscillation have
maximum velocity at the poles, some at mid-latitudes, and some near the
equator. The modes with maximum velocity near the equator are Rossby modes,
which the team had already identified in 2018. “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. Kiran Jain from NSO, together with B. 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.
To identify the nature of these oscillations, the team compared the
observational data to computer models. “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 obtain the model
oscillations, the team began with a model of the Sun’s structure and
differential rotation inferred from helioseismology. In addition, the
strength of the convective driving in the upper layers, and the amplitude of
turbulent motions are accounted for in the model. The free oscillations of
the model are found by considering small-amplitude perturbations to the
solar model. The corresponding velocities at the surface are a good match to
the observed oscillations and enabled the team to identify the modes.
“All of these new oscillations we observe on the Sun are strongly affected
by the Sun’s differential rotation”, says MPS scientist Damien Fournier. The
dependence of the solar rotation with latitude determines where the modes
have maximum amplitudes. “The oscillations are also sensitive to properties
of the Sun’s interior: in particular to the strength of the turbulent
motions and the related viscosity of the solar medium, as well as to the
strength of the convective driving,” says Robert Cameron from MPS. This
sensitivity is strong at the base of the convection zone, about two hundred
thousand kilometers beneath the solar surface. “Just like we are using
acoustic oscillations to learn about the sound speed in the solar interior
with helioseismology, we can use the long-period oscillations to learn about
the turbulent processes”, he adds.
“The discovery of a new type of solar oscillations is very exciting because
it allows us to infer properties, such as the strength of the convective
driving, which ultimately control the solar dynamo”, says Laurent Gizon. The
diagnostic potential of the long-period modes will be fully realized in the
coming years using a new exascale computer model being developed as part of
the project WHOLESUN, supported by a European Research Council 2018 Synergy
Grant.
Reference:
Solar inertial modes: Observations, identification, and diagnostic promise”
by Laurent Gizon, Robert H. Cameron, Yuto Bekki, Aaron C. Birch, Richard S.
Bogart, Allan Sacha Brun, Cilia Damiani, Damien Fournier, Laura Hyest, Kiran
Jain, B. Lekshmi, Zhi-Chao Liang and Bastian Proxauf, 6 August 2021,
Astronomy & Astrophysics.
Tags:
Space & Astrophysics