FACE OFF WITH THE SUN
The civilizations the world over have many myths and legends depicting powerful and divine individuals making a go at the sun. The end is more often than not quite discouraging. They always burn out near the sun. But, these folklores are symptomatic of humanity’s pathological desire to reach the ‘searingly bright’ spot in the universe. Sun has been studied quite extensively in the last few centuries. Looking out at sun unraveled the sunspot (cooler surfaces on sun) occurrence and the regularity with which they waxed and waned. The sunspot cycle is now tied up with temperature changes on earth, apart from the intensity of the magnetic activity on sun.
Scientists are clearly amazed by the flurry of physicochemical activity at sun and are now in a position to almost fully understand its implications on the life forms inhabiting the earth, and technological innovations that are dangling out there in space. But, first, let’s indulge in some history. In 1931, Sydney Chapman was greatly impressed by how far the corona spread, which according to him even enveloped the earth. He reckoned either the earth moved around the sun in very close proximity or the corona expanded to a considerable length, and continuously. If the corona expanded incessantly then it had to be perpetually renewed and rejuvenated at the surface of the sun. This, he reckoned, would allow continuous outflow of charged particles out of the sun in all directions, which disturbed earth’s magnetic field as they passed close by it. This supposition was reinforced by the work carried out in the 1950’s by Ludovig Franz Biermann, the German astrophysicist. In the first half of the 20th century, it was believed that the cometary tails were formed by the pressure of light from sun. The cometary tails always point away from sun and the length of their tail is seen to increase as they approach the sun. Biermann showed that the light pressure alone was not responsible for producing the cometary tails. There was something else. This ‘something else’ had to be stronger and capable of giving a ‘push’ to the cometary material to turn it into a tail. This ‘something else’ was the charged particles that were emitted out from the sun. The American physicist Eugene Norman Parker went a step further and announced his proclivity for a steady outflow of particles, with additional bursts at the time of solar flares. It was Parker himself who coined the term ‘Solar Wind’ in 1958 to explain this phenomenon. The solar probe is named after this Parker. It goes without saying that he built complex mathematical equations explaining this phenomenon. Parker’s solar wind did not languish long in the theoretical realm. The Soviet Satellites Lunik I and Lunik II demonstrated their presence in 1959 and 1960 respectively. The American planetary probe Mariner II also confirmed the presence of solar wind.
Now let’s get back to present and future. Parker probe is ostensibly launched to study sun, more specifically its corona, at close quarters. The aim is to dive into the scorching hot corona and see with our very own eyes (albeit robotic) what is actually happening out there. Corona is the outermost layer of the solar atmosphere. Here the temperatures are approximately 1 million degrees Celsius. But, at the surface of the sun (which is just 1600 km down) has temperature, just a shade above 500 degrees Celsius. Now this is mystery. Why this difference? We need to understand that.
In the earlier days corona was studied only during the total solar eclipse, when the shadow of moon blocked the sun’s bright face revealing the ‘flamy’ corona. How do we know about the temperature of corona and solar surface? This is the cue for the entry of spectral lines. Different elements emit light at characteristic wavelengths. Rainbow colors are a manifestation of different wavelengths. Spectrometers are used to analyze the light emanating from sun, which is then used to identify its composition. In 1869 when scientists were studying the spectral lines from corona they found it to be green in color. But, back then, in 1869, the green line did not correspond with any known elements on earth. So, the scientists thought they discovered a new element and named it Coronium.
It took almost 70 years, for a Swedish physicist, Bengt Edlén, to understand the spectral lines came from iron. Does iron give out this green spectrum on earth? The answer is no. For an ordinary iron to give out green color will require it to be superheated to a point that it is ionized 13 times. After such extensive heating the ordinary iron will then left with just half the electrons it originally had before heating. Now this is the problem. To reach such high levels of ionization would require coronal temperatures to be more than one million degrees Celsius. This is almost 200 times more than the temperature on the surface of the sun.
As if the temperature conundrum was not enough, nature has put forth to us a couple more riddles associated with corona and surface of the sun. The place from where the solar wind ventures out is fairly understood. This wind encircles the earth because of the roadblock earth’s magnetic field presents to this fast flowing magnetic wind. Though the wind is not visible to naked eyes, it manifests itself in the form of scintillating aurorae at the magnetic poles. What is not understood is how this wind chugs out of the sun and suddenly accelerates, at a certain point, at speeds quite unimaginable to us. This is where the Parker solar probe will come in handy to understand the dynamics of this phenomenon. Theoretical and simulation studies carried out, on the basis of all available data, from different probes have indicated this acceleration takes place somewhere in the coronal region. The probe will be travelling through corona and will make direct measurements within the realm of the corona making it easier for scientists to have an understanding of the actual events that take place in this arena.
The second important phenomenon that will be searched by Parker solar probe is to understand why the corona is thousands of times hotter than the surface of the sun. It is like a bon fire is lit and the heat is felt unbearably miles away from it and not close to it.
The energetic particles that zoom around in space have the potential to harm satellites and create power outages (in extreme cases) in the high latitude regions. It can also put into disarray the commercial and defense planes that traverse the space around the polar regions. The humble compasses can also change their direction by quite an appreciable distance. Parker solar probe will be travelling through the ethereal space from the earth to sun. Through this exciting travel it will be sending back the data, the actual snapshots of the space, revealing for the first time the ‘observed’ happenings in space. For this, it is carrying with it four major instruments to undertake all the stated functions. The first instrument will measure the electric and magnetic field around the spacecraft. This will help understand waves, energy pulses and reconnection phenomena that are capable of generating enormous amounts of energy. There is only one imaging instrument that will be recording all the activities associated with this probe. There is another instrument, a combination of two, to measure and count the charged particles like protons, ions to understand their velocity, density and temperature. This will help understand how and where they originate, how they travel out from sun and corona, and through the space leading towards earth.
All this is fine. But, will the Parker spacecraft not melt when it enters the punishing temperatures of the solar realm? Can it withstand such high temperatures? Before we arrive at the answer let us first understand what we mean by temperature and heat. High temperatures do not necessarily ‘heat’ up the things. This is especially true in space. In space, the temperature could be thousands of degrees but this ‘high’ temperature is not capable of burning anything. Even if we put our hand out to feel the temperature in such places we will not be able to feel the ‘heat’. In fact, it could be ‘cold’ out there. Why is this so? Because, temperature is the measurement of how ‘fast’ particles move. Heat, on the other hand, is the measure of how much energy is transferred. When the particles move very fast they are perceived to possess very high temperature. In space the particles move very fast but there are pockets where their density or their ‘count’ is very low. At such places the speed (temperature) does not translate into transfer of energy.
The corona where the probe will be traveling through does have very high temperatures, but the density of particles is very low. Hence, the transfer of energy will not be at its optimum. In fact, the temperature at the surface is less, but the surface is ‘dense’ than the corona. So effectively the transfer of energy will be more at the surface than at corona. In practical terms it means that the Parker solar probe will travel through a space having temperatures that touch several million degrees. But, the transfer of heat to the spacecraft will just be to the tune of 1500 degree Celsius. To have an idea of how ‘hot’ this temperature is let us compare it with our volcanoes. The lava that spews out could be anywhere between 700 and 1200 degree Celsius. But, withstanding even 1500 degree Celsius is no mean task. The spacecraft is equipped with a thermal shield that maintains a cosy temperature of 30 degree Celsius behind I,t where all the instruments have been kept, except the one that ‘counts’ charged particles.
The heat shield is called the thermal protection system (TPS) and is 2.4 meter in diameter and 4.5 inches thick built using carbon composite foam sandwiched between two carbon plates. This unit is tested to withstand temperatures to the tune of almost 1650 degree Celsius. The solar probe cup (SPC), also called the Faraday cup, will not be behind the heat shield for obvious reasons. It is designed to count the ion and electron content and their angle of flow from the solar wind. The advancement in technology will actually be tested in this unit for the sheer stress and strain that it would put itself under through the travelling that it will do in space and the coronal region. It will face a gamut of dynamic processes that will be diverse in form and features. The cup is made from Molybdenum alloy sheets of Titanium-Zirconium-Molybdenum having a melting point of 2350 degree Celsius. Tungsten, whose highest melting point is known to be 3422 degree Celsius, is used in the grids that produce electric field for the SPC. To overcome the challenge of wires melting under the heat and energy emanating out from the sun, scientists grew sapphire crystal tubes to suspend the wiring, and made the wires from niobium. The solar panels, which will use the energy from sun, need protection from overheating. This is provided by working out an elaborate cooling system that will not allow the solar panels to heat by using a deionized water as a coolant.
Parker solar probe will be travelling through space to have its rendezvous with the suns corona. The light from sun takes eight minutes to reach earth. If something is amiss on this probe the signal of that mishap will take about eight minutes to reach the control room. The signals that will be sent to rectify the anomaly will take similar time interval to reach the spacecraft. This time lapse is likely to kill the probe. The technologists and scientists have found a solution to this problem. The spacecraft is designed to autonomously keep itself safe and on track to the sun. Several sensors have been attached to heat shield of the spacecraft. These sensors are about half the size of our cell phones. When these sensors detect sunlight they alert the central computer to initiate corrective measures to protect the equipment by changing the course of the spacecraft to avoid anomalous heating of the probe. The computer program will be acting and initiating actions that will keep the probe on its course without need for any human intervention.
The Parker probe is already sun-bound and will be travelling incessantly for the next three months. In the course of seven years of its planned mission it will make 24 orbits around the sun and on its every approach it will sample the solar wind, observe the corona and film the sun from close quarters. It will be an exciting phase of solar adventure that will reveal many new facets of the suns activity to all of humankind. Some new and exciting news about Venus will also be available to all of us.
Indian Institute of Geomagnetism will be greatly benefited from the in situ observations of the sun. Apart from the just launched Parker probe, access to space plasma and its dynamics is very minimal. The satellites that are soaring far away in space have tracked the flow of plasma from sun to earth. However, these observations have been far and few. Parker probe is the technological breakthrough that can track the real time plasma progression from sun to earth. It will now reveal the journey of plasma and the plethora of dynamical processes that the protons and ions go through. The turbulence and the battles that are waged on in space between the charged particles and the waves will now be within the grasp of human intellect. Scientists at IIG, and at many other institutes of India and abroad, are trying to do their best by virtually ‘recreating’ the dark space environment. They know from satellite observations how a wave and particle behaved at one location and how it behaved at other. However, between these two locations no observational data is available. They try to fill in this blank by assuming a set of conditions that gives them an opportunity to recreate the ‘observed’ starting and ending features, with all set of probabilities that may occur between these two end points. They try to cover all the possibilities that may exist in terms of how the waves and particles behave from one location to another. This is simulation and is a powerful tool to understand nature when there is no direct observational data in hand.
The understanding of the mechanism of how plasma waves trigger or govern processes operating in space is not just of academic interest. It has wide applications even in our daily lives. These waves are a handy tool to explore the diagnostic characteristics of space plasma. They are an enormous asset in monitoring micro- and macro-scale events from a distance using ground- and space-based systems.
Parker solar probe will make all these techniques a thing of the past. There is no substitute to actual observations.