An international team of researchers for the first time compiled and published a detailed map of the Sun’s boundary, known as the Alfvén surface. This is a dynamic region where our star’s magnetic influence weakens enough that the solar wind—a stream of charged particles—finally detaches from the luminary and begins its unimpeded journey through interplanetary space. The investigation became achievable thanks to the synchronization of data gathered by a whole fleet of spacecraft and covers the first half of the current 25th solar cycle. A pivotal role in this breakthrough was played by the Parker Solar Probe (Parker), which since 2021 has repeatedly undertaken perilous descents into the scorching solar atmosphere, crossing the sought-after boundary. Its measurements were supplemented by observations from the Solar Orbiter craft, operating at a safe distance, as well as three space stations situated at the gravitationally stable Lagrange point L1 between Earth and the Sun. The joint analysis of this information allowed not only the reconstruction of the Alfvén surface’s shape but also, for the first time, the continuous tracking of its evolution during the buildup of solar activity. The findings demonstrated that during most passes, Parker merely grazed the bulges on the turbulent surface. Only twice, during the deepest dives at the peak of solar activity, did the vehicle fully descend beneath this frontier. Data collected over six years also confirmed that as the Sun’s activity grows, the Alfvén surface expands, increasing in altitude by approximately 30% from its average value, and its configuration becomes more “spiky” and irregular. This is direct substantiation of theoretical predictions made previously. As astrophysicists explain, mapping this boundary holds fundamental importance for grasping the physics of the solar corona, particularly the reasons for its abnormally high temperature. Furthermore, the dynamics of the Alfvén surface directly influence the formation of space weather, which impacts Earth, causing geomagnetic storms capable of affecting the operation of satellites, power grids, and communication systems. Since the Sun is the only star whose Alfvén boundary can be measured directly, this study also establishes a basis for studying magnetic environments surrounding other stars in the Universe.
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