A fortuitous alignment of two spacecraft studying the sun has potentially unraveled a long-standing solar mystery. Recent data gathered by NASA’s Parker Solar Probe and the European Space Agency’s Solar Orbiter suggests that plasma waves known as Alfvén waves may play a crucial role in accelerating and heating the solar wind as it emanates from the sun’s outer atmosphere. These findings, published in Science on August 29, provide new insights into why the solar wind is much faster and hotter than previously thought.
### Probing the Solar Wind
Ever since the early days of space exploration, scientists have been puzzled by the behavior of the solar wind, a stream of charged particles released by the sun. The solar wind accelerates as it travels through the solar system, a phenomenon that has been well-documented by robotic probes sent beyond the Earth’s atmosphere. Theoretical calculations suggest that the temperature of the solar wind should decrease as it expands into space, but measurements have shown that this drop occurs at a slower rate than expected.
Previous observations from Earth have detected the presence of Alfvén waves near the sun, which are oscillations in the magnetic fields of the plasma discharged by the sun. These waves, sometimes forming “switchbacks” due to their large size, appeared to hold the key to understanding the solar wind’s speed and temperature anomalies. However, direct evidence linking these waves to the acceleration and heating of the solar wind was lacking until now.
### The Perfect Alignment
In a stroke of luck, the Parker Solar Probe and Solar Orbiter happened to be in the right place at the right time to capture crucial data on the solar wind. In late February 2022, Parker Solar Probe was traversing a region about one-fifth of the distance between the sun and Mercury, precisely where the Alfvén waves were observed to be active. Just under two days later, Solar Orbiter passed through the same plasma stream near the orbit of Venus.
By measuring the properties of the solar wind at these two locations, researchers were able to quantify the energy of the Alfvén waves and their impact on the solar wind. Parker Solar Probe recorded the plasma stream moving at approximately 1.4 million kilometers per hour, while Solar Orbiter observed it speeding along at 1.8 million km/h. The plasma temperature at Solar Orbiter’s location was a scorching 200,000 degrees Celsius, three times hotter than expected based on theoretical models.
### Unraveling the Mystery
The dissipation of the Alfvén waves between the two spacecraft’s measurements provided a crucial piece of the puzzle. This dissipation injected just the right amount of energy into the solar wind to explain the increased speed and temperature observed by Solar Orbiter. The phenomenon is likened to creating waves in a wind tunnel that mix energy into the surrounding air, ultimately affecting its properties.
While the findings offer a compelling explanation for the solar wind’s behavior, some experts remain cautious. There are concerns that the complexity of the solar wind may not have been fully accounted for in the analysis, leading to uncertainties about whether the two spacecraft intercepted the same plasma stream. However, the researchers behind the study maintain that they conducted multiple checks to ensure the validity of their observations, including confirming the presence of the same amount of helium in the plasma streams.
Moving forward, the team hopes to delve deeper into the intricate physics underlying the interaction between Alfvén waves and the solar wind to further validate their findings. By exploring these mechanisms in more detail, they aim to solidify their conclusions and shed more light on the mysteries of our closest star.
### Conclusion
The alignment of the Parker Solar Probe and Solar Orbiter has provided a unique opportunity to uncover the secrets of the solar wind acceleration and heating. The presence of Alfvén waves appears to play a significant role in energizing the solar wind as it flows from the sun, potentially explaining why it is hotter and faster than anticipated. While further research is needed to confirm these findings, the study marks a significant step forward in our understanding of the dynamics of the sun and its influence on the space environment.