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.)

The Parker Solar Probe - Why It's A Monumental Engineering Feat - And The First Image Delivered

Artist's conception of the Parker Solar Probe which will approach within 4 million miles of the Sun.   

The level and rate of successes - including newly processed imagery- of the latest solar space craft are almost too numerous to tally. One can then understand-  even if one is not a solar physicist - why the Parker solar probe had been renamed from the original "Solar Probe Plus".   In this post I want to examine why the Parker solar probe has been such a success, especially in terms of the engineering components and the associated experiments to be carried out over its 7-year mission. (Okay, actually 6 years and 321 days)

In terms of solar physics there is perhaps no more towering personage than Eugene Parker. Indeed, it was reading his original research papers on the magnetic structure of sunspots - portrayed as assemblies of multiple flux tubes - that enticed me to go into the field of solar physics 40 years ago.

Why is Eugene Parker so special ?  Well, to many of us who've invested segments of our lives into solar research he is 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. The Wilson depression is shown as the indentations at the umbral surface on either side.

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 _o  r _o]  ^{1/ 2}

for this region, where   B_o  is the equilibrium magnetic field, m _o   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

A key fact relevant here is that heat flux and magnetic field strength is independent of sunspot area. The parameter that best helps to explain this is the vertical distance 'x'  which the model predicts is characteristic of all sunspots whether they be 4,000 km or 40,000 km across. Calculations by Parker show x = 1150 km approximately. It is the limiting distance below which an instability would occur in a single flux tube.

From all these points of view, the adoption of Eugene Parker's name for this one of a kind  solar craft - the first ever long term mission to our nearest star- is welcome and quite understandable. Indeed, the mission is a culmination of Prof. Parker's research in the fields of solar physics and heliophysics (the latter distinguished from the former on the basis of the extent of the solar wind, e.g. into the heliosphere - or the magnetically affected region that extends beyond Pluto's orbit).

Now what makes this solar mission so extraordinary?  First, back in November last year the craft made the first of 24 "oscillating" sweeps into the solar atmosphere. By that I mean the flight path carried it in then back out. In the November approach it came to within 15 million miles of the Sun's surface or photosphere. This is far closer than any earlier craft has ever gone. The previous record was set by Helios B in 1976 and broken by Parker on Oct. 29 — and this maneuver has exposed the spacecraft to intense heat and solar radiation in a complex solar wind environment. 

 Second, Parker features marvelous engineering designs which enable it to make such close passes.  (And bear in mind this was the first of 24 such oscillating in and out orbits, where the sunlit sie can reach as high as 1370 C.)   The main component here is a 2.5 meter wide, 72.5 kilogram heat shield, made of carbon foam sandwiched between two carbon sheets.  The whole shield is only 11. 5 centimeters thick (about 4. 6 inches).  It's coated on the Sun facing side with white ceramic paint to reflect as much sunlight as possible. Even then that side is expected  heat up to more than 1300 degrees Celsius.   Behind it, the bulk of the craft will coll to just 30 C on average, or about 86 F - about like a balmy spring day in Barbados.  

Other features of the Parker solar craft worth noting:

1) A component experiment for 'Integrated Science Investigation of the Sun' which is designed to detect solar particles across a wide range of energies.  This will allow solar physicists to decipher how the Sun accelerates the solar wind.  One detector will search for low energy particles, while a different one will search for high energy ones.  

  2) A component designated WISPR (Wide -field Imager for Solar Probe) will take images of the solar corona, solar wind, shocks and solar flares.  These images will help scientists properly interpret data from the other instruments.   

3) The craft features two solar panels mounted on a movable joint to control how much sun light (radiant energy) the panels absorb.  In close passes to the Sun - such as last November-  the panels will fold behind the heat shield, leaving only a last row of solar cells to absorb energy.   

4)The craft will be able to expel heat through a set of radiators composed of black material, which will be worn like a collar between the heat shield and the bulk of the space craft. Tubes of water will carry heat from the solar panels to the radiators where the heat can then escape into space.  At all temperatures the probe's solar panels will need to stay cool and the design of this system ensures it - especially the fact the panels are threaded with 'veins' to carry cooling water. 

4) Another experimental component, designated FIELDS, is comprised of five antennas to measure electric and magnetic field strengths in the solar neighborhood.  These measurements ought to help us to understand what makes the solar corona so hot (at nearly 2 million K).  To this end, four of the antennas are made of special material to protect them from direct sunlight..  

5) At the back of the probe will be the SWEAP experiment, for "Solar Wind Electrons Alphas and Protons".   This is designed to catch charged particles including electrons, and alpha particles ("alphas") from the solar wind and also determine their temperature, density and speed.  It will be exposed to sunlight 475 times as intense as felt on Earth.  

We can expect much data as well as imagery to be forthcoming in the coming months and years, as this monumental exploration of our Sun proceeds. Already, Parker has captured the image below of the Sun's corona, taken on December 8th.

The Parker solar adventure is just beginning, stay tuned!


See also:

Parker Solar Probe Heat-Resistant Technology - Spacecraft ...

Alfven Waves Revisited (2)

We left off trying to obtain the x, y-components of velocity associated with the wave, by use of Fourier transforms. The x-component is:

w²  v _x   –  c _s  ²    k ²    v _x  + B  _z    k ²  /  m _o  r _o   [v  _z   B  _x  –  v  _x  B  _z] = 0

The y-component is decoupled from the others (x, z) and can be written:

 w²  v _y   -  B  _x ²   k ²  v _y / m _o   r _o = 0

or simply:

w²   = [B  _x ²   / m _o  r _o] k ²

where the quantity in brackets is the Alfven velocity or alternatively written:

v  _A  = [ w/ k] =  B _x  / [m _o  r _o]  ^{1/ 2}

or

v  _A   = B_o  / [m _o  r _o]  ^{1/ 2}

since B_o    is in the x –direction


For completeness, we should be able to show the z-component equation is:

w²  v _z  -  B _x  k ²  / m _o  r _o [v  _z   B  _x –  v  _x  B  _z] = 0

Here, let me back up and refer readers again to the basic wave equation one can obtain by getting the 2nd derivative of:

¶ v _l / ¶t  = -(c _s  ² ) grad  p  _l  / r  - 1/ m _o  r  [B _o X Curl B ₁]

One can then find the solution in terms of plane waves by assuming:

v  ₁ = v  ₁ *[exp ik.x – iwt]


for which taking the second derivative, of  v  ₁ with respect to t yields the original equation in w we found earlier. All of this the energetic reader should be able to work out, but most of it (after taking derivatives) reduces to brute force algebra!

For completeness, I need to note what happens when you solve the preceding (simultaneous) equations in x, and z.

w ⁴ +  w² [- c _s  ²  k  ²  - B _x ²  k  ²  / m _o  r _o ] + c _s  ²  k  ⁴    [B _x ²  / m _o  r _o ] = 0

or:

w ⁴ -   w²  (c _s  ²   + v  _A ² )k ²  +  c _s  ² v  _A ²  cos  ² (Θ) k ⁴  = 0

and finally,

w²   = ½[( c _s  ² v  _A ²  k ²  +   [(c _s  ² v  _A ²  k  ⁴  –  4 c _s  ² v  _A ²    cos ² (Θ) k⁴ ]^1/2 

Now, if one plots the preceding using for the vertical axis (c _s  ²   + v  _A ² ) and for the horizontal B _o (e.g. x) one will get what is called “Friedrich’s diagram” (see sketch image). It consists of


1)A smaller “dumb bell” or figure-8 shaped graph centered at the origin. This will be for what we call “slow mode” waves

2)A single larger lobe that envelopes the smaller right lobe of the dumb bell. This will be for Alfven waves proper.

3) A circle shaped graph surrounding both 1, 2 above. This will be for what we call the “fast MHD” mode.


The critical thing to note here is that the fast mode is the only MHD wave able to carry energy perpendicular to the magnetic field. This has important ramifications for solar flares, as well as magnetospheric effects (such as the aurora). Meanwhile, the phase velocity (w /k) of the slow mode wave perpendicular to the magnetic field is always zero. In the limit where the sound speed c _s  ²  < < v  _A ², and the Alfven speed v  _A ²<< c _s  ², the slow wave disappears. (Which one can easily validate and confirm for the equation in w²  )

Other properties, points to note:

-the velocity perturbation v ₁ is orthogonal to B _o

-the wave is incompressible since DIV v ₁ = ik.v ₁ = 0

-the magnetic field perturbation (B ₁ ) is aligned with the velocity perturbation. Since both are perpendicular to k and B _o

-the current density perturbation (J ₁) exists as a current perturbation perpendicular to k and B _o  e.g.

J ₁ = k X  B _o

- When c _s  ² <<   v  _A ² the fast mode wave becomes a compressional Alfven wave. This has a group velocity equal to its phase velocity w /k