The solar wind, which is produced by streams of charged particles that
continually blast off the sun's atmosphere and radiate forth at millions of
kilometers per hour, is so large that it constitutes the outer boundary of
our solar system.
Despite this wind's extensive range, its genesis has long baffled
scientists. The solar wind is now said to be propelled by a collection of
irregular, small-scale jetlike eruptions in the sun's corona, or outer
layer. "The idea is similar to how individual clapping sounds in an
auditorium become a steady roar as an audience applauds," said Craig
DeForest, a solar physicist at the Southwest Research Institute in Boulder,
Colorado, and one of the study's co-authors.
tiny jetlets that normally endure for several minutes are found in the
corona, but until recently, scientists had only found a tiny number of them,
mostly near the foot of plumes that emerged from colder, less dense portions
of the corona known as coronal holes.
They are pervasive, according to the recent study. "Once you know how to
find them, you see that they are everywhere in basically every structure in
the corona all the time," said co-author and Southwest Research Institute
solar physicist
Dan Seaton.
The scientists discovered that the jetlets, which are each between 1,000
and 3,000 kilometers broad, are still there even during the solar minimum,
the sun's 11-year cycle's least active period. This finding is in line with
the solar wind's pervasiveness. Lead author of the study and solar physicist
Nour Raouafi
of the Johns Hopkins University Applied Physics Laboratory stated, "You can
randomly pick any day and the jetlets are there, just like the solar
wind."
The team presents proof that the jetlets are triggered by a mechanism known
as magnetic reconnection, which heats and accelerates a plasma of charged
particles, in the study outlining the new results, published last month in
the
Astrophysical Journal. According to the researchers, the jetlets then generate waves that heat
the corona, allowing the plasma to defy gravity and combine into the solar
wind.
"The numbers come out looking promising and show it is really quite
possible that jetlets could supply the mass lost by the sun to the solar
wind," said
Charles Kankelborg, a solar physicist at Montana State University who was not affiliated with
the study.
The Motor
The work of Eugene Parker, a pioneering solar physicist who passed away
last year, gave rise to the hypothesis that a variety of small-scale,
irregular occurrences may act as a single driving force for the solar wind.
He proposed in 1988 that a "swarm of nanoflares" fueled by brief spikes in
magnetic reconnection may heat the corona sufficiently to generate
wind.
Due to the limited precision of magnetic measurements, it has been
difficult to uncover proof of this small-scale reconnection.
The
GOES-R satellites,
better known as weather satellites, the Goode Solar Telescope at the Big
Bear Solar Observatory, and NASA's Solar Dynamics Observatory were among the
sources of high-resolution photos used in the current study. They discovered
that areas of the corona that had previously seemed to be free of magnetic
flux were really occupied by intricate magnetic fields. Several jetlets were
also connected to particular reconnection occurrences by the scientists.
Higher reconnection and jetting rates may be revealed by even finer-scale
magnetic field measurements, according to the researchers.
The research team went on to propose that the jetlets produce a particular
type of wave, known as an Alfvén wave, which heats the corona. Alfvén waves
has been considered as a rival mechanism that may account for the solar
wind. However, there is a growing consensus that these processes can
cooperate. According to
Judith Karpen, a solar physicist at NASA's Goddard Space Flight Center, "the widespread
presence of these reconnection-driven jetlets offers a natural explanation
for both reconnection and Alfvén waves powering the solar wind."
Future research should disclose coronal processes in unprecedented detail,
according to researchers. They place their faith in more recent telescopes,
such the Daniel K.
Inouye Solar Telescope
at the National Solar Observatory and the
Solar Orbiter, a joint NASA-ESA mission that will launch in 2020.
Raouafi predicted that the range of jetting might extend from reasonably
big events to the tiniest sizes, Parker's nanoflares.
Additionally, according to
Jie Zhang, a solar physicist from George Mason University, jetlets could be
connected to the sun's large-scale phenomena like flares and coronal mass
ejections. According to him, "small-scale eruptions may play a role in
transforming magnetic configurations into more coherent large-scale
structures that can store large amounts of energy before erupting."
The legacy of Parker and his colleagues has, for the time being, been
confirmed by the recent jetlet discoveries. According to certain findings
made 30 years later, they were most likely correct, said Kankelborg.
