Wednesday, January 16, 2019
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:
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 = Bo / [m o r o] 1/ 2
for this region, where Bo 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:
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!