Sun  ID: 4958

Parker Solar Probe: Crossing the Alfven Surface

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.

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Visualization Credits

Tom Bridgman (Global Science and Technology, Inc.): Lead Visualizer
Justin Kasper (University of Michigan): Lead Scientist
Samuel Badman (University of California at Berkeley): Scientist
Jia Huang (University of Michigan): Scientist
David Stansby (University College London): Scientist
Joy Ng (KBR Wyle Services, LLC): Producer
Mara Johnson-Groh (Wyle Information Systems): Writer
Laurence Schuler (ADNET Systems, Inc.): Technical Support
Ian Jones (ADNET Systems, Inc.): Technical Support
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Parker Solar Probe

Data Used:
SPICE Ephemerides also referred to as: SPICE Ephemerides
Ephemeris - NASA/JPL
Satellite and planetary ephemerides
Parker Solar Probe/FIELDS also referred to as: FIELDS
University of California at Berkeley
Measurements of electric and magnetic fields and waves.
Note: While we identify the data sets used in these visualizations, we do not store any further details nor the data sets themselves on our site.

This item is part of this series:
AGU 2021 - Parker Solar Probe

SVS >> Solar Wind
SVS >> Space Weather
SVS >> Hyperwall
SVS >> Heliophysics
SVS >> Corona
SVS >> Solar Magnetic Field
NASA Science >> Sun
SVS >> Parker Solar Probe