HINODE captures record breaking solar magnetic field

Snapshot of a sunspot observed by the Hinode spacecraft.
(top) Visible light continuum image. (bottom) Magnetic
field strength map. The color shows the field strength,
from weak (cool colors) to strong (warm colors). Red
indicates a location with a strength of more than 6,000
gauss (600 mT). Credit: NAOJ/JAXA

Astronomers at the National Astronomical Observatory of
Japan (NAOJ) using the HINODE spacecraft observed the
strongest magnetic field ever directly measured on the
surface of the Sun. Analyzing data for 5 days around the
appearance of this record breaking magnetic field, the
astronomers determined that it was generated as a result of
gas outflow from one sunspot pushing against another
sunspot.


Magnetism plays a critical role in various solar phenomena
such as flares, mass ejections, flux ropes, and coronal
heating. Sunspots are areas of concentrated magnetic fields.
A sunspot usually consists of a circular dark core (the
umbra) with a vertical magnetic field and radially-elongated
fine threads (the penumbra) with a horizontal field. The
penumbra harbors an outward flow of gas along the horizontal
threads. The darkness of the umbrae is generally correlated
with the magnetic field strength. Hence, the strongest
magnetic field in each sunspot is located in the umbra in
most cases.

Joten Okamoto (NAOJ Fellow) and Takashi Sakurai (Professor
Emeritus of NAOJ) were analyzing data taken by the Solar
Optical Telescope onboard HINODE, when they noticed the
signature of strongly magnetized iron atoms in a sunspot.
Surprisingly the data indicated a of 6,250 gauss.
This is more than double the 3,000 gauss field found around
most sunspots. Previously, magnetic fields this strong on the
Sun had only been inferred indirectly. More surprisingly, the
strongest field was not in the dark part of the umbra, as
would be expected, but was actually located at a bright
region between two umbrae.

HINODE continuously tracked the same with high spatial resolution for
several days. This is impossible for ground-based telescopes
because the Earth’s rotation causes the Sun to set and night
to fall on the observatories. These continuous data showed
that the strong field was always located at the boundary
between the bright region and the umbra, and that the
horizontal gas flows along the direction of the magnetic
fields over the bright region turned down into the Sun when
they reached the strong-field area. This indicates that the
bright region with the strong field is a penumbra belonging
to the southern umbra (S-pole). The horizontal gas flows from
the southern umbra compressed the fields near the other umbra
(N-pole) and enhanced the field strength to more than 6,000
gauss.

Okamoto explains, “HINODE’s continuous high-resolution data
allowed us to analyze the sunspots in detail to investigate
the distribution and time evolution of the strong and also the surrounding
environment. Finally, the longtime mystery of the formation
mechanism of a stronger outside an umbra than in the umbra, has
been solved.”

Explore further:

Image: NASA’s Solar Dynamics Observatory watches a
sunspot

More information: Takenori J. Okamoto et al.
Super-strong Magnetic Field in Sunspots, The Astrophysical
Journal
(2018). DOI: 10.3847/2041-8213/aaa3d8