By synergizing the capabilities of multiple spacecraft strategically positioned throughout our Solar System, researchers have successfully compiled the most comprehensive depiction to date of the demarcation where the Sun’s magnetic pressure ceases to propel the solar wind.
This critical zone is known as the Alfvén surface. Scientists have not only delineated its form but also tracked its transformation during the initial half of Solar Cycle 25, the prevailing period of solar activity characterized by a crescendo and subsequent decline in sunspot activity, solar flares, and coronal mass ejections over an approximately 11-year span.
This marks the inaugural instance where this dynamic structure has been continuously reconstructed through measurements from numerous spacecraft, furnishing indispensable insights into the Sun’s intensely hot atmosphere.
“Data acquired by the Parker Solar Probe from regions beneath the Alfvén surface holds the potential to resolve fundamental inquiries regarding the Sun’s corona, such as the underlying cause of its extreme temperature,” asserts Dr. Sam Badman, an astrophysicist affiliated with the Harvard & Smithsonian Center for Astrophysics (CfA) and the study’s lead author.
“However, to effectively address these questions, a precise understanding of the boundary’s location is paramount.”
An astrophysical boundary is conventionally defined as the threshold where the physical principles governing the behavior of a particular region undergo a significant alteration.
In the context of the Alfvén surface—a boundary recognized by the scientific community for many years—it signifies the point of no return, beyond which the Sun’s magnetic dominion attenuates to such an extent that disturbances within the solar material can no longer propagate backward toward the Sun, and the outward-flowing solar wind loses its direct magnetic connection to our star.
The solar wind is a continuous stream of charged particles emanating from the Sun and propagating outward through the Solar System. While it can escape from below the Alfvén surface, this surface acts as the critical interface where the wind transitions from a magnetically directed flow to an unimpeded outward expansion.
The way this interface undulates, forms protrusions, and expands or contracts has a demonstrable impact on its interactions with Earth and other planets. This plays a pivotal role in the space weather phenomena that can disrupt communication technologies, power grids, and satellite operations on our home world.
Furthermore, the Sun is unique among stars in the cosmos for which we possess the instrumentation to directly measure the Alfvén surface. Specifically, one instrument stands out: the Parker Solar Probe.
Since 2021, the Parker probe has executed recurrent descents below the Alfvén surface, transmitting valuable data that scientists have now definitively identified as representing unambiguous sampling of sub-Alfvénic phenomena.
“This research unequivocally validates that with each orbital pass, the Parker Solar Probe is delving deeply into the very region where the solar wind originates,” explains Dr. Michael Stevens, an astronomer at CfA.
“We are now entering an exhilarating phase where the probe will have direct observational access to how these processes evolve as the Sun progresses into the subsequent stage of its activity cycle.”
The research team scrutinized data gathered by the probe during its perihelion passages—audacious plunges into the Sun’s ethereal atmosphere.
This collected information was then cross-referenced with observations from the Solar Orbiter, which monitors the Sun from a comparatively safer vantage point, in addition to data from three spacecraft positioned at the L1 Lagrange point. This point is a zone of gravitational equilibrium situated between Earth and the Sun, established by the interplay of their mutual gravitational influences coupled with centripetal forces.
These three spacecraft provided critical measurements pertaining to the velocity, density, temperature, and magnetic field characteristics of the outward-bound solar wind.
The analysis revealed that during the majority of its perihelion encounters, the Parker probe traversed regions characterized by bulges within the turbulent Alfvén surface.
It was only during its two most profound descents, undertaken at the zenith of solar maximum—the peak of the Sun’s 11-year activity cycle—that the probe achieved penetration deep below the Alfvén surface.
The aggregated data, spanning a six-year period as solar activity intensified during the initial half of the solar cycle, also indicated a significant expansion of the Alfvén surface by approximately 30 percent of its mean altitude as solar activity escalated.
It is anticipated that this effect would be proportionately greater or lesser for solar cycles exhibiting higher or lower overall activity, respectively.
“As the Sun progresses through its cycles of activity, we are observing a concomitant enlargement and an increase in the ‘spikiness’ of the Alfvén surface surrounding the Sun,” Badman elaborates.
“This is, in fact, consistent with our prior theoretical predictions, which are now directly corroborated by empirical evidence.”
These findings are poised to significantly enhance our comprehension of solar physics, particularly as additional perihelion data is acquired by the Parker probe during the Sun’s quiescent period leading up to solar minimum.
The implications extend to the study of exoplanetary systems as well. Stars exhibiting more robust magnetic fields, for instance, are likely to possess substantially larger Alfvén boundaries, which could profoundly influence planets in close orbits and potentially compromise their habitability.
“Previously, our estimations of the Sun’s boundary relied on distant observations without any means of empirical verification, but we now possess an accurate cartographic representation that can serve as our guide during ongoing investigations,” Badman concludes.
“Crucially, we are also afforded the capability to monitor its dynamic evolution and correlate these changes with in-situ measurements. This provides us with a far more lucid perspective on the intricate processes occurring in the Sun’s immediate vicinity.”
