Friday, January 13, 2023

'Magnetic Switchbacks' Now The Focus Of Parker Solar Probe's Plumbing of Solar Corona


By October 24, 2022, the Parker Solar Probe, e.g.

Parker Solar Probe Provides Hitherto Unknown Insights Into The Corona 

had completed a full 13 solar orbits of the twenty four scheduled for its seven-year mission. On October 16, 2021, the spacecraft flew by Venus for the fifth time. One month later, it achieved the closest approach yet: 13.28 solar radii from the center of the Sun. Plans are also in place to use Venus for at least tow more gravity assists for the Parker Probe to reach its final perihelion position by December 24, 2024.  This point will be at about 9.86 solar radii or 9.86 x (6.95 x 10 8  km) = 6.85 x 10 9   km, or 4.5% of Earth's distance to the Sun. 

The link above noted the temperature inversion paradox pertaining to the solar corona and surface (photosphere) with the latter in the thousands of degrees Kelvin compared to the corona at 2 million K. The critical clue for this disparity is that there is a rapid release of energy from the solar interior into its corona and this could help explain why the latter is so staggeringly hot compared to the solar surface.   The Parker Fields instrument  observations have indeed  revealed that the particles in the solar wind appear to be released in explosive jets called switchbacks, rather than being radiated out in a steady stream. In this post, I examine how the magnetic switchback phenomenon is being pursued using the Parker Solar probe's instrument suites. 

 But first things first.   Fig. 1 below from a recent (November) Physics Today issue, shows a sequence of images from the 'Wide Field Imager for Solar Probe' suite (WISPR) captured around the perihelion of the 9th solar encounter, when the spacecraft actually flew through the solar corona. 

Careful inspection of the sequence here shows coronal structures moving upward in the upper view panels and downward in the lower  - though to be sure the motion is apparent only.  Smaller structures are also visible in the images which could be seen from a distance of 1 A.U.  Both sets of features reflect the dynamic nature of the solar wind in proximity to the solar surface.  

The Parker Probe's examination of magnetic switchbacks is seen in the signature captured in Fig 2a. below:  What this graphic depicts are sudden reversals of the radial component of the magnetic field vector (in nano Tesla, nT) which behavior precisely defines what a magnetic switchback is.  Basically, the Parker Probe's FIELDS suite of instruments is measuring rapid, large amplitude field reversals associated with jets of plasma.  (More on the physics of the phenomenon below).

Parker's measurement of suprathermal electrons indicates the field lines physically fold over to form an 'S' shape.  This as opposed to disclosing changes magnetic field polarity.  A visual image is also captured in Fig. 2b below:

 An important point is that the switchbacks are Alfvenic in nature, i.e. characterized by the Alfven velocity associated with Alfven waves e.g.   

  • So that the solar wind velocity is highly correlated with the magnetic field, and the directionality conforms to that of the associated Alfven waves. Again, I have to emphasize the crucial importance of these magnetic switchbacks only rose to 'center stage' thanks to the proximity of the Parker Probe to the Sun. Earlier efforts, such as Helios 1, Helios 2 only observed the SBs sporadically.  
  • Thanks to Parker's 'up close and personal' access to the solar wind and corona we now have a much better grasp of the SBs morphology, rate of occurrence and amplitude. Also that they are observed ubiquitously in Alfvenic velocity solar wind.  It is also important to note that we observe a drop off in excess (free) energy with distance from the Sun.  This was observed by the European Space Agency's Solar Orbiter. It follows that the switchbacks must dissipate and release their energy to the ambient plasma, likely in the form of heat which reduces velocity.  One recent model  (Fisk & Kasper, ApJ. Lett., 2020) depicting magnetic switchbacks is shown below.   
  • SB model from Fisk & Kasper, Astrophysical Journal Letters, May 1, 2o20
  • This illustrates the nature of the global field circulation enabled by the interchange reconnection associated with magnetic switchbacks.  In this case we see an open magnetic field line in (A) dragged against a large coronal loop (in light blue) by global circulation in the corona. At (B) we have interchange reconnection and at (C) the escaping switchback - effectively 'jumping' the width or span of the original closed loop - thereby launching an S-shaped switchback into the corona.  
  • While the model of Fisk and Kasper explains a lot in terms of the observed magnetic aspect, S-shaped morphology etc., much work remains to be done to tie the model to the disparity in the temperatures (corona vs. photosphere).  For one thing, one needs to see a rigorous quantification tied to the explicit thermodynamics of the solar wind plasma.  We also need to ascertain the precise origin, which the above graphic depicts in 'cartoon' form but which needs significant elaboration. (We could more accurately call the model an "ansatz" or an initial crude estimate of what's really going on.)  Many questions also need to be answered, including: Is there more than one tupe of switchback?  And, can SBs develop locally in the solar wind?   Also, can SBs form lower in the solar atmosphere and be conveyed upward by the solar wind to Parker Probe altitudes and even beyond?  
  • Once we have answers to these and other related questions solar physicists may finally be on track to understanding the actual physical processes of switchback formation which may then lead to discriminating between the two most prominent theories of the solar wind, to explain its plasma heating and acceleration. Currently these theories are magnetic field reconnection and turbulence.   The most recent Parker Solar Probe observations, in that regard, appear to show a connection between solar wind switchbacks magnetic field structures such as supergranules on the solar surface.  
  • Once the puzzle of magnetic swtichbacks is fully resolved we may then be on our way to understanding the solar wind. 

  • See Also:
  • New Research Into The 'Slow' Solar Wind Sheds Light On Its Source.


Why Jason Lisle is wrong in his Solar Super-granulation Polarity finding

No comments: