An
ultraviolet picture of the sun's chromosphere, the thin layer of solar
atmosphere sandwiched between the visible surface, the photosphere, and
the corona. Astronomers have developed a simulation to address magnetic
reconnection in the chromosphere. The image was taken by the Hinode
spacecraft. JAXA/NASA
The
Sun glows with a surface temperature of about 5500 degrees Celsius. On
the other hand its hot outer layer, the corona, has a temperature of
over a million degrees and ejects a wind of charged particles at a rate
equivalent to about one-millionth of the moon's mass each year. Some of
these particles bombard the Earth, producing auroral glows and
occasionally disrupting global communications. In between these two
regions of the Sun is the chromosphere. Within this complex interface
zone, only a few thousand kilometers deep, the density of the gas drops
with height by a factor of about one million and the temperature
increases. Almost all of the mechanical energy that drives solar
activity is converted into heat and radiation within this interface
zone.
Charged particles are produced by the high temperatures of the gas,
and their motions produce powerful, dynamic magnetic fields. Those field
lines can sometimes break apart forcefully, but movement of the
underlying charged particles often leads them to reconnect. There are
two important, longstanding, and related questions about the hot solar
wind: how is it heated, and how does the corona produce the wind?
Astronomers suspect that magnetic reconnection in the chromosphere plays
a key role.
CfA astronomer Nicholas Murphy and his three colleagues have
completed complex new simulations of magnetic reconnection in hot
ionized gas like that present in the solar chromosphere. (The lead
author on the study, Lei Ni, was a visitor to the CfA.) The scientists
include for the first time the effects of incompletely ionized gas in
lower temperature regions, certain particle-particle effects, and other
details of the neutral and ionized gas interactions. They find that the
neutral and ionized gas is well-coupled throughout the reconnection
region, and conclude that reconnection can often occur in the cooler
portions of the zone. They also note that new, high-resolution solar
telescopes are capable of studying smaller and smaller regions of low
ionization for which their results are particularly applicable.
Reference(s):
"Magnetic
Reconnection in Strongly Magnetized Regions of the Low Solar
Chromosphere," Lei Ni, Vyacheslav S. Lukin, Nicholas A. Murphy, and Jun
Lin, ApJ 852, 95, 2018.
