The mystery of how the solar corona, the Sun’s outer atmosphere, reaches temperatures of millions of degrees may have an unexpected explanation: cosmic dust traveling along magnetic waves and carrying plasma on the solar wind. The new discovery is reported in The Astrophysical Journal.
“For decades, researchers primarily focused on how electrons, ions, magnetic fields, and plasma waves transport and dissipate energy in the solar atmosphere,” stated lead researcher Syed Ayaz from the University of Alabama in Huntsville. “Our work adds a new element to this picture: dust particles.”
This breakthrough was made possible by NASA’s Parker Solar Probe, which has approached the Sun closer than any other spacecraft, skimming the corona at a distance of 6.1 million kilometers. Anyone who has witnessed a total solar eclipse or even seen a photograph of one is familiar with the corona—ghostly, glowing filaments surrounding the eclipsed Sun. These filaments consist of plasma, or ionized gas, at temperatures exceeding half a million degrees Celsius, compared to the Sun’s visible surface, the photosphere, which shines at around 5,500 degrees Celsius. At such temperatures, the photosphere only outshines the corona because the plasma in the corona is extremely sparse. This is why we can only see the corona during a total solar eclipse, when the photosphere’s light is blocked.
The Parker spacecraft does not carry a cosmic dust detector on board, and this is because dust was not previously considered a significant component of the solar atmosphere. Indeed, it was believed that at the extremely high temperatures of the solar corona, dust could not survive for long and would therefore have no impact.
However, Parker is equipped with numerous antennas and magnetometers, collectively known as the FIELDS experiment, designed to measure electromagnetic fields and radio emissions in the solar corona. These antennas have consistently recorded unexpected voltage spikes, which Ayaz and his team attribute to clouds of charged particles generated when tiny dust particles collide with Parker at high speed.
These dust particles accumulate an electrostatic charge, which can interact with the electromagnetic field carried by the solar wind as it breaks away from the Sun. This, in turn, can influence plasma waves propagating within this electromagnetic field, known as Alfvén waves.
There are two possible, competing ways in which dust can affect Alfvén waves, thereby determining how energy is transferred into the corona and heats it. On one hand, the mass of the dust can provide additional inertia to the plasma moving with the solar wind, allowing plasma energy to be transported over greater distances. On the other hand, the electric charge on the dust particles can enhance the interaction between charged particles in the plasma, the Alfvén waves, and the Sun’s electromagnetic field.
“If dust mass dominates, the energy of Alfvén waves can travel further into the corona,” said Ayaz. “If dust charge effects dominate, energy can be released more locally in the form of particle heating.”
Thus, the balance between these two effects provides a means of controlling where and when energy is released in the corona, concentrating it in certain regions and causing sharp temperature increases in those locations.
According to Ayaz, future solar missions will now need to account for dust, using specialized detectors designed to measure the properties of dust near the Sun.
“A more important question is very intriguing,” said Ayaz. “Is dust merely passing through the near-solar environment, or is it actively helping to shape the conversion of electromagnetic energy into heat and the motion of the solar wind?”
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