Parker solar probe discovers fine structure of solar wind

The Parker Solar Probe (PSP) by NASA has ventured close to the sun, enabling scientists to observe the intricate details of the solar wind near its origin at the sun’s surface. This discovery reveals information that is typically lost as the solar wind exits the corona and appears as a uniform burst of charged particles.

The analogy used to describe this phenomenon is akin to perceiving jets of water emanating from a showerhead while being struck by the water in the face.

In a forthcoming paper to be published in the journal Nature, a team of researchers led by Stuart D. Bale, a physics professor at the University of California, Berkeley, and James Drake from the University of Maryland-College Park, report that the PSP has detected streams of high-energy particles that align with the supergranulation flows present within coronal holes. This finding suggests that these regions are the sources of the so-called “fast” solar wind.

Coronal holes are areas where magnetic field lines emerge from the sun’s surface without looping back, leading to the formation of open field lines that expand outward and occupy most of the space around the sun. Typically, these holes are situated near the sun’s poles during periods of solar calm, causing the fast solar wind they generate to miss Earth. However, during the sun’s active phase, which occurs every 11 years during its magnetic field reversal, these holes appear across the sun’s surface, resulting in bursts of solar wind directed toward Earth.

Understanding the origin and characteristics of the solar wind is crucial for predicting solar storms. While these storms create stunning auroras on Earth, they can also cause disruptions to satellites and the electrical grid.

Drake emphasized the importance of comprehending the mechanisms behind the solar wind, stating that it impacts our ability to understand how the sun releases energy and triggers geomagnetic storms, which pose a threat to our communication networks.

The team’s analysis suggests that coronal holes resemble showerheads, with jets emerging evenly spaced from bright spots where magnetic field lines converge and diverge from the sun’s surface. The scientists argue that when oppositely directed magnetic fields interact within these convergence zones, which can span 18,000 miles, the fields often break and reconnect, propelling charged particles away from the sun.

Bale explains that the sun’s photosphere is covered by convection cells similar to those in a boiling pot of water, and the larger-scale convection flow is referred to as supergranulation. When these supergranulation cells intersect and move downward, they drag the magnetic field along with them, creating a downward funnel-like structure. The magnetic field intensifies in these regions due to the constriction, resembling a bundle of magnetic field being drawn into a drain. The researchers have now observed the spatial distribution of these funnels, thanks to the data collected by the Parker Solar Probe.

Based on the detection of extremely high-energy particles by the PSP, traveling 10 to 100 times faster than the average solar wind, the scientists conclude that this process, known as magnetic reconnection, is the only plausible explanation for the creation of the solar wind. The Parker Solar Probe was launched in 2018 with the primary goal of resolving the conflicting explanations surrounding the origin of high-energy particles that compose the solar wind: magnetic reconnection or acceleration via plasma or Alfvén waves.

Bale asserts that the significant finding is that it is magnetic reconnection within these funnel structures that serves as the energy source for the fast solar wind. The fast solar wind does not arise uniformly throughout a coronal hole but is rather substructured within the supergranulation cells of these holes. It originates from these small bundles of magnetic energy associated with the convection flows. The researchers believe that their findings provide strong evidence supporting the role of magnetic reconnection in this process.

The funnel structures likely correspond to the bright jetlets that have been observed from Earth within coronal holes, as recently reported by Nour Raouafi, a co-author of the study and the project scientist for the Parker Solar Probe at the Applied Physics Laboratory at Johns Hopkins University. The Applied Physics Laboratory (APL) was responsible for designing, building, managing, and operating the spacecraft.

Plunging into the sun

When the solar wind reaches Earth, located 93 million miles away from the sun, it has transformed into a turbulent flow consisting of intertwined magnetic fields and charged particles. These particles interact with Earth’s magnetic field and release electrical energy into the upper atmosphere. This energy excites atoms, resulting in the formation of beautiful auroras at the poles. However, the effects of the solar wind extend beyond the auroras and have implications for Earth’s atmosphere. Predicting the intensity of the most powerful solar winds, known as solar storms, and understanding their impacts on Earth is one of the objectives of NASA’s Living With a Star program.

The Parker Solar Probe was specifically designed to investigate the nature of this turbulent solar wind near its origin in the sun’s surface, known as the photosphere. The probe aimed to understand how the charged particles, such as protons, electrons, and helium nuclei, are accelerated to overcome the sun’s gravitational pull and escape into space.

To achieve this, the Parker Solar Probe had to approach the sun at a distance closer than 25 to 30 times the sun’s radius, equivalent to approximately 13 million miles. By reaching altitudes below this threshold, where there is less evolution of the solar wind, the probe could observe the structures and imprints originating from the sun more clearly, providing valuable insights.

In 2021, the instruments onboard the Parker Solar Probe detected magnetic field switchbacks in the Alfvén waves, which appeared to be associated with the regions where the solar wind originates. As the probe approached a distance of about 12 solar radii from the sun’s surface (approximately 5.2 million miles), it became evident from the data that the probe was passing through jets of material rather than mere turbulence. Stuart Bale, James Drake, and their research team traced these jets back to the supergranulation cells in the photosphere, where magnetic fields accumulate and funnel into the sun.

The key question was whether the charged particles within these funnels were accelerated by magnetic reconnection, which slingshots particles outward, or by waves of hot plasma composed of ionized particles and magnetic fields, akin to surfing a wave.

The detection of extremely high-energy particles within these jets, ranging from tens to hundreds of kiloelectron volts (keV), compared to just a few keV for most solar wind particles, led Bale to conclude that magnetic reconnection must be responsible for the particle acceleration and the generation of Alfvén waves, which likely provide an additional boost to the particles.

Bale explains that the team’s interpretation is that these jets of reconnection outflow stimulate the propagation of Alfvén waves. A similar observation is well-known from Earth’s magnetotail, where comparable processes occur. The team does not understand how wave damping alone can produce particles with energies up to hundreds of keV, while such energies naturally arise from the reconnection process. This finding is also supported by their simulations.

Due to the limitations of the Parker Solar Probe’s instruments, the closest it can approach the sun is about 8.8 solar radii above the surface, approximately 4 million miles. Bale expects that data collected at this altitude will further solidify the team’s conclusions. However, during the current solar maximum phase, characterized by increased activity and chaos, the processes they are studying may be more difficult to observe.

Bale highlights the fortunate timing of launching the Parker Solar Probe during the solar minimum, the least active phase of the solar cycle. This period allowed for a clearer understanding of the observed phenomena without the added complexity of heightened solar activity.

Source: University of California – Berkeley

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