Parker Solar Probe Provides Hitherto Unknown Insights Into The Corona

Artist's conception of the Parker Solar Probe

The Parker Solar Probe  is in the news again- a welcome break from Trump -  with further discoveries providing novel insights into the physics of our Sun.   Important,  because the Sun is the only star we can study up close, using actual sacecraft and instruments..  To many of us who've invested  significant segments of our lives in solar research it is fitting the probe is named after Eugene Parker, seen as the father of solar physics. It was while Dr. Parker was still a budding young astrophysicist at the University of Chicago that he wrote a seminal paper in 1958 about the solar wind and its association with the interplanetary magnetic field. (Parker, E.N. : Dynamics of the interplanetary gas and magnetic fields,” 128, 664, Astrophys. J., 1958.) The paper can be accessed at the link below for those interested:

http://adsabs.harvard.edu/full/1958ApJ...128..664P

Fast forward some 21 years, to ca. 1979. Measurements over decades of the so -called Evershed effect showed the plasma motions to be radial and inwards. There did not appear to be any 'escape hatch' for the rising gas columns represented by the umbral dots. This being the case sunspots ought to heat up and reach equilibrium with the surrounding photosphere after a few days, and yet spots with umbral dots were observed to last weeks.

And so the "multiple flux tube" model of Eugene Parker was born (cf. Astrophys. J., 230, 905-13). In the diagram shown below note the geometry of the field lines extending from beneath the photosphere (in the convective zone) to far above it. The 'flaring field' on top is buoyant for reasons that have to do with the stratification of the solar atmosphere. 

Parker in his paper (ibid.) showed that the downdraft velocity needed to remove heat from beneath a sunspot  (at a depth of 2500- 5000 km) is on the order of the Alfven velocity  e.g.

v  _A   = B_o  / [m ₀  r _o]  ^{1/ 2}

for this region, where   B_o  is the equilibrium magnetic field,  m ₀  is the magnetic permeability of free space,  and r _o   is the plasma density. This leads to v  _A   =  about 2 kilometers per second. This then is adequate to provide the observed umbral energy flux of 0.2 F _o  where F _o  denotes the normal photospheric flux.

The full paper can be accessed here:

http://articles.adsabs.harvard.edu//full/1979ApJ...230..905P/0000905.000.html

We now know that NASA's  probe  has flown closer to the Sun than any spacecraft before. In so doing it has beamed back its first observations from the edge of the Sun’s atmosphere, called the corona.   The first tranche of data offers clues to its long-standing mysteries, including why the the corona, is hundreds of times hotter than its surface, as well as the precise origins of the solar wind.  According to Prof Stuart Bale, a physicist at the University of California, in Berkeley, who led the analysis from one of the craft’s instruments:

“The first three encounters of the solar probe that we have had so far have been spectacular.  We can see the magnetic structure of the corona, which tells us that the solar wind is emerging from small coronal holes; we see impulsive activity, large jets or switchbacks, which we think are related to the origin of the solar wind. And we are also surprised by the ferocity of the dust environment.”

Here is the paradox we face  in solar physics: the corona is at an estimated  2 million degrees Kelvin, but the actual solar surface (photosphere) is only in the "thousands” of degrees.   This pointed out by  Prof Tim Horbury, a co-investigator on the Parker Solar Probe Fields instrument (based at Imperial College London.)   According to Prof. Horbury:

 “It’s as if the Earth’s surface temperature were the same, but its atmosphere was many thousands of degrees. How can that work? You’d expect to get colder as you moved away.”

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.   

“It’s bang, bang, bang,” said Horbury, in a recent interview with the UK Guardian. Fine, but what about the "bang"? Exactly what are these 'switchbacks'?

They appear to be, as the name implies, rapid "flips" in the direction of the magnetic field which flows out from the Sun, embedded in the solar wind. 

These switchbacks into the corona could help explain the temperature anomaly by way of their rapid reversals of  the localized magnetic fields and the rapid release  of stored magnetic energy into the ambient plasma..  This then affecting the "Strahl" electrons of the solar wind stream.  (See e.g. the linked paper at the end :'A Step Closer To The Sun's Secrets')   Thus, localized field reversals - which can last anywhere from a few seconds to several minutes -  are the drivers for the localized enhancements in the radial component of the plasma velocity .(The component directed away from the Sun's center)  The energy released into the plasma can  be enormous given the stored magnetic energy:

E _m   =      B² / 2 m

Where B is the magnetic induction (B =  m H )  and  m is the magnetic permeability. Given a magnitude of field intensity (H)  of 0.1 T (Tesla) and an order of magnitude for  m  of  10^-7  H/m  it can easily be seen the magnetic energy would be sufficient to trigger explosive release, say if the energy change occurred in seconds.  According to one solar physicist (Justin Kasper)  from the Univ. of Michigan:

"We are detecting remnants of structures from the Sun being hurled into space and violently changing the organization of the flows and magnetic field. This will dramatically change our theories for how the corona and solar wind are being heated."

Yet another  Parker probe surprise was the dustiness of the region close to the Sun. During the nearest approach of its orbit, the probe was peppered with a fine dust, chipping tiny pieces off its heat shield.  These detached pieces showed up as white streaks in images captured by the high-resolution camera. The dust is thought to be the remains of asteroids and comets that came close to the sun, causing them to evaporate, leaving behind just a dusty haze.

The new observations were made when Parker was about 15m miles (24m km) from the Sun. But this is just a tease for what will follow, given 21 more progressively closer passes.  The final one will  fly to about 6m km of the solar surface — more than seven times closer than the previous closest mission, the Helios 2 spacecraft in 1976.

The extreme conditions faced by Parker have required the use of unconventional materials and spacecraft design. The craft’s white ceramic heat shields will reach a temperature of nearly 1,400C (2,552F) during the mission’s closest approach. As it passes close to the sun, its solar panels are retracted into the shadow of the heat shield, with just a tiny area remaining exposed to generate power. The craft has also broken the record for the fastest moving spacecraft, relative to the sun. It will reach speeds of nearly 435,000 mph (700,000 km/h) in 2024.

“It’s a very bold mission, it’s really extreme and it’s an enormously impressive engineering effort,” said Prof.  Horbury.

Interested readers can access the primary findings  in three papers in the journal Nature.  (Each paper opened has links to the others, including pdfs.)