Parker Solar Probe: Crossing the Alfven Surface

  • Released Tuesday, December 14th, 2021
  • Updated Wednesday, November 15th, 2023 at 12:17AM
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Split window view illustrating the orbit of Parker with the orbit trail colored based on the Mach number of the solar wind and the magnetic field lines (represented as gold) connecting back to the Sun. The Mach number drops below unity (one) when a field line transitions between two different coronal hole regions (the blue and red regions marked on the Sun).

The Sun's corona extends far beyond the solar surface, or photosphere and is considered the outer boundary of the Sun. It marks the transition to the solar wind which moves through the solar system. This limit is defined by the distance at which disturbances in the solar wind cannot propagate back to the solar surface. Those disturbances cannot propagate back towards the Sun if the outbound solar wind speed exceeds Mach one, the speed of 'sound' as defined for the solar wind. This distance forms an irregular 'surface' around the Sun called the Alfvén surface.

Parker Solar Probe has now reached close enough to the Sun that it has begun to penetrate this Alfvén surface. From measurements of the solar wind plasma environment by the Parker's FIELDS and SWEAP instruments, scientists can compute the 'speed of sound' for the plasma, which exhibits brief periods when the Mach number drops below unity (one).

At Parker's distance during encounter 8, the Mach number dropped below unity on several occasions. Propagating the magnetic field vector at Parker back to the solar photosphere revealed that these regions corresponded to significant changes in the magnetic field on the photosphere, particularly that fields lines of 'open' magnetic flux were transitioning from one location to another.

In the visualizations below, we see the measured Mach number at Parker propagated along the orbit, with green representing Mach number greater than one, grey represents Mach number approximately one, and red represents Mach number less than one. When we trace the field lines at these moments back to the Sun, we see the field line jumping between isolated regions of 'open' magnetic flux - blue for inward magnetic flux and red for outbound magnetic flux.

A visualization illustrating the orbit of Parker, orbit trail colored based on the Mach number of the solar wind, with the magnetic field lines (represented in gold) connecting back to the Sun. The Mach number drops below unity (one) when a field line transitions between two different coronal hole regions (the blue and red regions marked on the Sun).

A top-down view from the ecliptic pole of the orbit of Parker Solar Probe for Encounter 8. FIELDS instrument magnetic vector data are projected from the spacecraft position as arrows. The arrows are deep blue for sunward vectors, deep red for anti-sunward, and in between for directions off from this line. The heliospheric magnetic field lines are the gold lines, representing the propagation of the average field measured at Parker, propagated back to the solar photosphere.

View of FIELDS instrument data measured at Parker Solar Probe during encounter 8. FIELDS magnetic vector data are projected from the spacecraft position as arrows. The arrows are colored deep blue for sunward vectors, deep red for anti-sunward, and in between for directions off from this line. The heliospheric magnetic field lines are the gold lines, representing the propagation of the average field measured at Parker, propagated back to the solar photosphere.



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