VIRTUAL


Thursday, July 1st    2021

3:45 – 4:45 p.m.

WEBEX

 
Speaker:  Prof. William. H. Matthaeus,

Department of Physics and Astronomy, University of Delaware 

 

 

 

“Origin, evolution and fate of turbulence in the solar wind"

 

 

 

Abstract: 

Many observed properties of the supersonic solar wind plasma can be attributed to turbulent fluctuations. Besides its likely involvement in the origin and evolution of the wind itself, plasma turbulence, much of which resides at magnetohydrodynamic (MHD) scales, also regulates transport of energetic particles throughout the heliosphere.  Spacecraft measurements have confirmed the basic properties of solar wind turbulence, reviewed here, with much of the available in situ data concentrated near Earth orbit with a few major exceptions such as Helios and Voyager data. The known or inferred radial evolution of the wind indicates that the turbulence contributes to the observed heating and acceleration.  Where does this turbulence come from? The standard view that it is supplied exclusively at the coronal base through perturbations of the magnetic field lines by photosphere motions appears to be inadequate for several reasons. A different picture is presented here, in which turbulence is resupplied throughout much of the inner heliosphere by shear driving. Recent evidence from STEREO and Parker Solar Probe reinforce this perspective:  First, remote sensing observation of plasma activity at several tens of solar radii indicate a transition from a highly asymmetric state to a more isotropic state, a transition we describe as “striated” to “flocculated.”  Second, in roughly the same region, the Parker Solar Probe (PSP) mission has detected numerous large directional deviations of the magnetic field, called “switchbacks”, often accompanied by “Alfvenic jets.”  Commonly encountered explanations often reply on magnetic activity deeper in the corona, usually some form of interchange reconnection, to explain switchbacks.  We offer a different view in which the transition from striated solar coronal structures to more isotropic, flocculated fluctuations, and the appearance of switchbacks, are explained by a single phenomenon -- shear driven MHD turbulence.  We suggest that these phenomena, first occurring somewhat outside the Alfven critical zone, and in the vicinity of the first plasma beta=1 surface, are powered by the relative velocities of adjacent coronal magnetic flux tubes.   The additional energy deposited in the inertial range by this process should substantially boost the low-frequency plasma turbulence throughout the explored solar system wherever strong shear is present in the solar wind flow. This research is supported partially by NASA through the PSP/ISOIS project and the PUNCH mission.