Mystery of solar cycle illuminated
In the convection zone of the star, the plasma currents make a huge
turnover that lasts about 22 years

Date:
June 25, 2020
Source:
Max Planck Institute for Solar System Research
Summary:
The sun's convection zone plays a key role in the generation
and evolution of the Sun's magnetic field. Analyzing data sets
spanning more than 20 years, researchers have obtained the most
comprehensive picture of the north-south flow of plasma in the
convection zone ever. The flow goes around the convection zone in
each hemisphere in about 22 years.



FULL STORY ========================================================================== [Sun's surface (stock | Credit: (c) lukszczepanski / stock.adobe.com]
Sun's surface (stock image; elements furnished by NASA).

Credit: (c) lukszczepanski / stock.adobe.com [Sun's surface (stock |
Credit: (c) lukszczepanski / stock.adobe.com] Sun's surface (stock image; elements furnished by NASA).

Credit: (c) lukszczepanski / stock.adobe.com Close Solar activity
fluctuates in a rhythm of about eleven years, which is reflected among
other things in the frequency of sunspots. A complete magnetic period
lasts 22 years. Scientists have long been puzzling over what causes
this cycle.

It must be related to the conditions beneath the "skin" of our star: A
layer of hot plasma -- electrically-conductive gas -- extends from the
surface to 200,000 kilometers below. The plasma within this convection
zone is constantly in motion.


==========================================================================
A team of scientists from the Max Planck Institute for Solar System
Research, the University of Go"ttingen and New York University Abu
Dhabi has now succeeded in drawing the most comprehensive picture of
the plasma flows in nort-south-direction to date. The researchers have
found a remarkably simple flow geometry: the plasma describes a single
turnover in each solar hemisphere, which lasts for about 22 years. In
addition, the flow in the direction of the equator at the bottom of the convection zone causes spots to form closer and closer to the equator
during the solar cycle.

The number of sunspots on the visible solar surface varies; sometimes
there are more, sometimes fewer. The distance between two sunspot maxima
is about eleven years, after 22 years the sunspots are again magnetically polarized in the same way. During the maximum not only large sunspots
appear, but also active regions. In addition, impressive arcs of hot
plasma reach far into the solar atmosphere, particles and radiation are
hurled into space in violent eruptions.

At the activity minimum, however, the sun calms down noticeably.

"Over the course of a solar cycle, the meridional flow acts as a conveyor
belt that drags the magnetic field along and sets the period of the solar cycle," says Prof. Dr. Laurent Gizon, MPS Director and first author of
the new study.

"Seeing the geometry and the amplitude of motions in the solar interior
is essential to understanding the Sun's magnetic field," he adds. To
this end, Gizon and his team used helioseismology to map the plasma flow
below the Sun's surface.

Helioseismology is to solar physics what seismology is to geophysics.

Helioseismologists use sound waves to probe the Sun's interior, in much
the same way geophysicists use earthquakes to probe the interior of
the Earth.

Solar sound waves have periods near five minutes and are continuously
excited by near surface convection. The motions associated with solar
sound waves can be measured at the Sun's surface by telescopes on
spacecrafts or on the ground.

In this study, Gizon and his team used observations of sound waves at
the surface that propagate in the north-south direction through the
solar interior.

These waves are perturbed by the meridional flow: they travel faster along
the flow than against the flow. These very small travel-time perturbations (less than 1 second) were measured very carefully and were interpreted
to infer the meridional flow using mathematical modeling and computers.

Because it is small, the meridional flow is extremely difficult to see
in the solar interior. "The meridional flow is much slower than other components of motion, such as the Sun's differential rotation," Gizon
explains. The meridional flow throughout the convection zone is no more
than its maximum surface value of 50 kilometers per hour. "To reduce the
noise level in the helioseismic measurements, it is necessary to average
the measurements over very long periods of time," says Dr. Zhi-Chao
Liang of MPS.

The team of scientists analyzed, for the first time, two independent
very long time series of data. One was provided by SOHO, the oldest
solar observatory in space which is operated by ESA and NASA. The data
taken by SOHO's Michelson Doppler Imager (MDI) covers the time from 1996
until 2011. A second independent data set was provided by the Global Oscillation Network Group (GONG), which combines six ground-based solar telescopes in the USA, Australia, India, Spain, and Chile to offer nearly continuous observations of the Sun since 1995.

"The international solar physics community is to be commended for
delivering multiple datasets covering the last two solar cycles," says
Dr. John Leibacher, a former director of the GONG project. "This makes
it possible to average over long periods of time and to compare answers,
which is absolutely essential to validate inferences," he adds.

Gizon and his team find the flow is equatorward at the base of the
convection zone, with a speed of only 15 kilometers per hour (running
speed). The flow at the solar surface is poleward and reaches up to 50 kilometers per hour. The overall picture is that the plasma goes around
in one gigantic loop in each hemisphere. Remarkably, the time taken for
the plasma to complete the loop is approximately 22 years -- and this
provides the physical explanation for the Sun's eleven-year cycle.

Furthermore, sunspots emerge closer to the equator as the solar cycle progresses, as is seen in the butterfly diagram. "All in all, our study supports the basic idea that the equatorward drift of the locations
where sunspots emerge is due to the underlying meridional flows," says
Dr. Robert Cameron of MPS. "It remains to be understood why the solar meridional flow looks like it does, and what role the meridional flow
plays in controlling magnetic activity on other stars" adds Laurent Gizon.


========================================================================== Story Source: Materials provided by Max_Planck_Institute_for_Solar_System_Research. Note: Content may be
edited for style and length.


========================================================================== Journal Reference:
1. Laurent Gizon et al. Meridional flow in the Sun's convection zone
is a
single cell in each hemisphere. Science, 2020 DOI: 10.1126/
science.aaz7119 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200625144835.htm

--- up 22 weeks, 2 days, 2 hours, 34 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1337:3/111)