Coronal heating & solar wind, returns from solar missions shed light on Sun’s mysteries

by Chris Gebhardt

It’s one of the greatest mysteries in solar physics.  Dubbed the coronal heating problem, the issue arises from the fact that the solar corona, or Sun’s atmosphere, is millions of degrees hotter than the layers directly below it.

This goes against modeling that suggests the corona should be far cooler than the layers below it.

Numerous theories have been proposed to explain the problem, but a persistent issue in exploring this solar dynamic has been a lack of technological ability to directly observe the underlying processes that could account for the heat.

The problem of seeing this region of the Sun is dwindling, not just because of new missions such as NASA’s Parker Solar Probe and European Space Agency’s/NASA’s Solar Orbiter

Additionally, advancements in computer technology now allow images and observations from other spacecraft, like NASA’s Solar Dynamics Observatory, to be reprocessed to extract more detailed data.

One of the theories put forward to explain the coronal heating problem came in 1972 from Dr. Eugene Parker, for whom the Parker Solar Probe is named.  He proposed that nanoflares, tiny eruptions on the Sun’s surface that are one-billionth the size of a normal solar flare, cause the corona to heat up to its observed temperatures. 

The issue with nanoflares is that even with advancements in technology, they are incredibly difficult to detect, and just because a bright flash is observed, it is not necessarily a nanoflare. 

Credit: NASA

In order to be classified as a nanoflare, the event has to meet two specific criteria.  First, it must be triggered by magnetic reconnection, a phenomenon previously reported on — as well as their connection to nanoflares — by NASASpaceflight in August 2020. 

Second, and crucially, it must heat the solar corona, which can lie upwards of thousands of kilometers above where the nanoflares occur.

“You have to examine if the energy from a nanoflare can be dissipated in the corona,” said Shah Bahauddin, Research Faculty at the Laboratory of Atmospheric and Space Physics at the University of Colorado, Boulder — lead author of a new study revealing the first look at what is widely accepted to be a nanoflare’s full life-cycle.

“If the energy goes somewhere else, that doesn’t solve the coronal heating problem.”

The discovery stemmed from an observation by Bahauddin of tiny bright loops approximately 100 kilometers apart in the Transition Zone (the layer below the corona) as seen in observations from NASA’s Interface Region Imaging Spectrograph (IRIS) spacecraft.

A deeper investigation revealed the bright spots to be millions of degrees hotter than their surroundings and contained smaller amounts of lighter elements and a larger proportion of heavier elements which also showed as brighter and more energetic.

The elemental discovery was the opposite of expectation; lighter elements should move more quickly than heavier elements.  But Bahauddin observed the lighter elements were not moving while the heavy elements were traveling upwards towards the corona at speeds of 100 km/s.

For comparison, the International Space Station orbits the Earth at 7.6 km/s.

Bahauddin and his team ran simulations to determine what underlying mechanism unseen in the IRIS data could be responsible.  The simulations showed that magnetic reconnection was the only known possible cause.

During these brief, energy-packed magnetic reconnection events, heavier elements are able to keep moving in the electric field created as their momentum is greater than that of their lighter counterparts.

The heavy elements gain more and more energy in the electric field as they are accelerated upward into the corona… where they can then transfer that energy in the form of heat.

With that data in hand, the team realized they could be dealing with a nanoflare and sought subsequent observations of the same region at the same time from other missions.

A loop brightening. Each inset frame zooms in to the selected region in the frame to its left. The frame on the far right is the most zoomed in, showing the potential nanoflare.
(Credit: NASA/SDO/IRIS/Shah Bahauddin)

That led them to the Solar Dynamics Observatory (SDO), data from which was able to confirm that just 20 seconds after the nanoflare was observed, a sudden-heating of the corona by multi millions of degrees occurred directly above.

“SDO gave us this important information: Yes, this is indeed increasing the temperature, transferring energy to the corona,” said Bahauddin.

Based on findings presented in December 2020 in the journal Nature Astronomy, researchers, and their peers, generally agree that this is a nanoflare as well as the first full life-cycle observation of such an event.

The seeming direct connection between the observed nanoflare and the superheating event of the corona provides support for Dr. Parker’s nanoflare theory to solve the coronal heating problem… but doesn’t prove it.

While Bahauddin’s team has documented 10 further instances of bright loops and coronal super-heating events directly above them since that first observation, the team cautioned a great deal of work remains in determining if these:

  1. are indeed nanoflares, and 
  2. (assuming ‘1’ to be true), if they occur with enough regularity and in sufficiently large quantities to produce the type of extreme heating necessary to heat the overall corona to the temperatures observed today.

Meanwhile, solar physicists focusing on other dynamics of the corona have also used NASA’s Solar Dynamics Observatory to more deeply explore the structures that create the ever-present flow of charged particles — called the solar wind — outward from the Sun to distances well beyond 100 astronomical units (AU).

Changes to the solar wind can have profound impacts on Earth and our technology.  The 1859 Carrington Event, in which the most severe Coronal Mass Ejection ever recorded directly struck Earth just 17.6 hours after erupting from the corona, is an example of how a lack of solar wind can accelerate the impact timing of potentially disastrous solar storms.

In the case of the Carrington Event, a smaller Coronal Mass Ejection 4-5 days prior to the large eruption cleared the region between the Sun and the Earth of most of the solar wind, creating a near resistance-free path for the major Coronal Mass Ejection that followed.

Conversely, the presence of the solar wind can delay a Coronal Mass Ejection’s arrival at various planets by hours or even days, providing critical time to prepare for such an event.

Like Coronal Mass Ejections, it is from the corona which the solar wind is expelled at velocities that allow it to escape the gravitational pull of the star and stream outward in all directions.

Using new technology to further refine and extract information from collected data, a team of researchers led by Vadim Uritsky, solar scientist at the Catholic University of America and NASA’s Goddard Space Flight Center, took a closer look at the structures within the corona that create the solar wind. 

The solar wind itself is ejected from the Sun along open magnetic field lines that stretch thousands of kilometers outward from the corona.  The field lines also create regions where streams of solar material, called plumes, can form.

Plumes are incredibly bright regions, making observations of their detailed structure complicated.

Image processing on high-resolution images of the Sun reveals distinct plumelets within structures of solar plumes. (Credit: NASA/SDO/Uritsky, et al)

However, the new data extraction techniques allowed Uritsky and his team to use NASA’s SDO spacecraft to examine solar flares and then pull incredibly detailed information not previously possible from those observations.

The examination revealed that the plumes are made up of smaller ropes of material, called plumelets. 

“People have seen structure in and at the base of plumes for a while,” added Judy Karpen, a co-author of the paper and chief of the Space Weather Laboratory in the Heliophysics Science Division at NASA Goddard.  

“But we’ve found that the plume itself is a bundle of these denser, flowing plumelets, which is very different from the picture of plumes we had before.”

Critically, the plumelets were found to move individually and not as groups, oscillating on their own with small-scale behavioral changes based on their interaction.

Those interactions could be key to understanding one of the major driving forces behind disruptions in the solar wind and could potentially connect to one of the first observations made by NASA’s Parker Solar Probe in November 2018 when it found sudden reversals in the magnetic field direction of the solar wind. 

Work to identify the cause as well as the nature of those magnetic field reversals is still ongoing, but the new evidence found by Uritsky’s team has led to a hypothesis that the two elements could be connected or that these plumelet  structures could produce similar signatures to those found by the Parker Solar Probe. 

Understanding the solar corona and its complex dynamics is, in part, key to understanding how to better predict the fluctuations and electromagnetic tantrums our host star can sometimes experience.

Better predictive models for solar weather and solar activity can help in fortifying not just satellite and human space-based exploration missions, but could provide key warnings and lead time to secure our technology and precariously fragile electrical grids from the potentially devastating effects of coronal storms — a warning imparted to us in 1859 that has largely gone unheeded.

Lead image: NASA’s Goddard Space Flight Center/Conceptual Image Lab/Adriana Manrique Gutierrez

Related Articles