In May 2017, I accepted a job offer at the Wisconsin IceCube Particle Astrophysics Center (WIPAC) in Madison - a job that sent me to the bottom of the Earth. IceCube is a giant Neutrino detector at the South Pole, and it will be my job to keep its computers running. For an entire year (November 2017 to December 2018) I will live and work at the Amundson-Scott South Pole Station in Antarctica. Being an IceCube "Winterover" has been my dream job for years - und now the dream is real. This page is my journal of this once-in-a-lifetime adventure.
Most of the content will be in English, but I might write in German occasionally. The journal entries are sorted by date (latest first).
It's sad that some people have to be reminded, but I own all the contents and photos on this blog (if not denoted otherwise), and by re-posting them without my permission, be it on social media, other websites or in print, you are committing copyright infringement. If you are interested in a particular text or photo, feel free to contact me via email.
Science Facts #9 Blue lightning in the dark: How IceCube detects neutrinos
The IceCube Neutrino Observatory detects ultra-high-energy neutrinos from outer space, hoping to find out more about the properties and origins of those tiny particles. But how exactly does that work? In Science Facts #1 and Science Facts #6 I talked about IceCube and neutrinos a little bit already, but let's look into the process of how neutrinos turn into charged particles, how these particles emanate light, how that light is caught on IceCube's cameras, and what the recordings can tell us about the neutrino.
First of all, neutrinos are usually invisible to detectors. The only chance to see them is when they hit something really hard - like a molecule of ice, with almost the speed of light. IceCube is waiting for exactly that to happen. And South Pole has a lot of ice, which makes it more likely that a neutrino bumps into it in IceCube's field of view. The event of a high-energy neutrino hitting the ice, or to be exact, a nucleon (a proton or neutron) of the ice, is called a "deep inellastic scattering", in which the neutrino transfers part of its huge amounts of energy onto the nucleon. There are several kinds of these scatterings: Some are neutral current interactions, in which the neutrino leaves the scene unharmed (minus some energy) and goes about its way; others are charged current interactions, where the neutrino gets transformed into its corresponding lepton brother - the electron-neutrino into an electron, the muon-neutrino into a muon, and the tau-neutrino into a tau. The important thing is: In all these interactions, the powerful collision generates charged particles, either from the neutrino being transformed into a lepton, or from the particle shower that is immediately set off by the very upset nucleon - or both.
At this point after the collision, the IceCube detector is still completely unaware of what's going on. It's what happens next that makes the magic: The charged particles now travel through the ice, and they do that faster than light. ... Wait a minute, that's not possible! Except that it is. You see, in a perfect vacuum, no particle can move faster than light. Period. But in a medium, e.g. ice, light is slowed down, so that particles in fact can outpace it. No kidding! Anyway, the surrounding ice is now polarized by the fast moving particles, and starts to emanate blue light along their path. This is called the Cherenkov Effect. This process of polarization and light emission has a more famous equivalent in the acoustic domain, which is responsible for the sound cone emanating from a supersonic aircraft.
It is this blue light that sounds IceCube's alarm bells. The detector is equipped with 5160 very sensitive optical sensors (DOMs) that are on the lookout for the blue flashes literally all around the clock. In a high-energy neutrino event, not only one but hundreds of DOMs detect that light, and the intensities and detection times for all those DOMs are recorded. When put together after analysis, the big picture reveals a lot about the neutrino itself. Two of those results are shown in the graphic above: An electron-neutrino leaves a cascade-like signature in the detector, a muon-neutrino's looks more like a track. From these analyses, the IceCube scientists can extract information like the neutrino flavour, it's energy, and sometimes even the direction it came from, and with a little luck that leads them to an insanely powerful cosmic object hundreds of millions of light years away.
Science Facts #8 From the treasure chest of confusing astronomy: The Sidereal Day
I am very much NOT a phone person. Seriously, my life results tripled the day I found out I can order pizza online. I only ever call my family and my closest friends from South Pole - and the only time when it really makes sense to attempt a call is during the DSCS pass when we have about four hours of our fastest internet. My people might have noticed that each time, my calls happen to an earlier hour of the day. But why is that?
The reason for that is the way our planet orbits the Sun, or rather what we make of it. You know, for us, a day has 24 hours - from noon to noon, when the Sun reaches its highest position above the horizon. This is called a solar day, and it's based upon the apparent motion of the Sun in the sky. The problem: This does not, as you might think, equal the time the Earth needs for one full rotation around its axis - because our planet also rotates around the Sun. Look at the picture on the right: The Earth starts out at point (1). After a full circle (2), it has changed it's position relative to the Sun, so it has to rotate just a tiny little bit more to make it "noon" again and to complete the solar day (3). The difference between a full rotation of the Earth and a solar day is about 3 minutes and 56 seconds.
The 23 hour 56 minutes and 4 second long period of a full rotation is also called the sidereal day (spoken /saɪˈdɪəriəl/) and matches the apparent motion of the stars in the night sky - they are so far away that our relative movement is neglectable.
The satellites that provide internet to South Pole live in sidereal time - while we go after the solar clock. That means the satellites come online about four minutes earlier everyday, and have shifted about two hours after a month. I think it would be a nice experiment to organize life at Pole along the sidereal day, and shift the meal times an hour forward every 15 days to account for the satellite shifts - so that you don't have to get up at 3 in the fuckin morning to catch a tiny little bit of fast internet... If you are curious about our exact satellite times, you can find a schedule on the South Pole intranet - only when our internet is up though.
Science Facts #7 A Groundbreaking Discovery
IceCube is looking for high-energy neutrinos coming from extremely powerful objects outside our galaxy. In the past years of data taking, the collaboration was able to identify a handful of particle signatures in the detector as such neutrinos - which is proof for their existence and was a huge success for the experiment. However, so far none of these neutrinos could be traced back to it's source, i.e. nothing could be found at the locations in the sky where the experts thought those neutrinos were coming from.
On September 22nd 2017, IceCube detected a promising neutrino energy signature and alerted the international community of observatories around the globe and in space to the location where the neutrino originated. Working closely together, the collaboration was able to identify the blazar TXS 0506+056 as the source of that neutrino. The object is located just off Orion's left shoulder and about 4 billion light years away.
A blazar is an object that can be described as a super-massive black hole in the middle of a galaxy. The incredible forces that distort space to its breaking point in the immediate perimeter of a blazar generate gigantic flares that violently blast light and matter into space. These flares - called jets - shoot out perpendicular to the black hole's accretion disk and are subject to rapid fluctuations. Only objects like these are capable of producing cosmic radiation of such energies. The neutrino detected by IceCube had an energy of 300 TeV - that is more than 45 times the energy of the particles generated in the Large Hadron Collider at CERN, the most powerful particle accelerator on Earth.
Why is that a big deal?
First of all: We basically found a space railgun firing alien particles towards Earth. How is that not cool. And here's the real reason: We knew about high-energy neutrinos before, as we did about blazars (there are about 2000 currently observed). Now we know those two are connected, which holds valuable information about the processes that happen inside blazars and blazar-like objects. The only way neutrinos of such unbelievable energies are generated is through proton acceleration. The accerelerated protons decay into pions which then decay into neutrinos - all that happens within the giant jets, which in this case luckily pointed directly towards Earth. That means IceCube and the collaborating telescopes just found a smoking gun of proton acceleration - and busted TXS 0506+056 in the very act.
Knowing that blazars are gigantic hadron accelerators will give scientists many insights in what might be happening in and around super-massive black holes. This groundbreaking discovery is another step towards unravelling the mysteries of this universe.
It's finally boring enough down here so that I can start writing about science. ... Nah just kidding, this is gonna be awesome! Because science is awesome! Science yeah! SCIENCE!
As you might know, I was hired as an astroparticle physicist to watch over the IceCube Neutrino Telescope here at South Pole. The name itself poses a bunch of questions already: What are neutrinos? How is IceCube a telescope? And why at the South Pole?!
Let's tackle the first question today and learn about neutrinos. They are very fascinating little guys, that live in the lepton family of the Standard Model of Particle Physics which is visualized on the right. The model holds all elementary (that means indivisible, unlike neutrons and protons or whole atoms) particles that we know of today: The quarks, which are the building blocks for protons and neutrons and therefore all matter we see around us. The gauge bosons, which are the carriers of the four fundamental forces - gravity, electro-magnetism, strong force and weak force. The recently discovered Higgs boson which "gives" mass to other elementary particles in the model. And the leptons, which are, among other things, defined over their 1/2-integer spin. For each quark and lepton, there exists an antiparticle with identical mass but opposite electrical charge.
The best known lepton is probably the electron, which per definition has an electric charge of -1 and can form atoms together with protons and neutrons. The muon and the tau are pretty much just heavier electrons. For each of these three there exists an uncharged counterpart, the electron-, muon- and tau-neutrino, represented by the greek letter υ. We know about them only since they were postulated by Wolfgang Pauli in 1930. Why? Because they're incredibly hard to find. Elementary particles are so tiny that we can only "see" them when they interact with things, like an electron being pushed around in a magnetic field. Neutrinos however do not take part in electro-magnetism, because they do not carry electric charge, unlike their lepton brothers. They only reveal themselves in interactions of the weak force, which - you could say - is the "rarest" of the forces (also pretty complicated, so let's cover that in a different session). "What about gravity?" you might ask. Well, technically you're right, but the neutrino masses are so incredibly small that gravity basically has no impact at all, at least not on a scale we could measure. Fun fact: According to the Standard Model, the neutrinos should be completely massless. Nowadays we know they actually do have tiny masses, although the exact values are still a mistery. Their property of being a pain in the butt to find, and the fact that we still do not know a lot about them, awarded the neutrinos the nickname Ghost Particles.
So why do we care about neutrinos? First of all, because they're there (which, for a physicst, is an absolutely sufficient but not necessarily required condition :D)! Second of all, they are considered ideal cosmic messengers. What does that mean? Well, neutrinos are not influenced by cosmic magnetic fields, neither are they likely to interact with anything like dust clouds or radiation - so they basically travel through space in a straight line unimpressed by whatever may try to deflect them from their path. The highest-energy neutrinos originate in powerful cosmic objects like supermassive black holes or active galactic cores, so they carry precious information from the very inside of these objects with them. No other particle can do that. Neutrinos are the unicorns of space!
Science Facts #5 Solar shenanigans
To understand what's going on in this week's science facts, you need the following vocabulary:
Solstices happen twice a year; they are defined as the days the sun reaches it's respectively highest (in the summer) or lowest (in the winter) altitudes in the sky.
Equinoxes are the two days of the year when day and night are of exactly the same length. At the poles, the equinoxes are the moments of sunrise and sunset.
Earth's rotation axis is tilted about 23.5° relative to the perpendicular to its plane of movement around the sun. That's the reason for seasons on our planet! It's also the reason for night and day at South Pole - if the axis wasn't tilted, Amundsen-Scott South Pole Station would be at the edge of dawn year round. But because things are how they are, we get one long day and one long night. The ostensible movement of the sun across the South Pole sky is illustrated in the picture below: It basically draws big circles seemingly without change in altitude every "day", with those circles slowly moving towards or away from the horizon. The sun's highest point in the sky is at summer solstice and measures 23.5° from the horizon, which equals the tilt of Earth's rotation axis - makes sense, doesn't it? It's hard to wrap your head around it at first, but maybe it helps to look at how the sun "moves" right at the equator: It rises and sets exactly perpendicular to the horizon every day of the year, with its highest altitude at the zenith straight above your head on the equinoxes, and its lowest altitude about 66.5° (= 90° - 23.5°) either from the north or south horizon on the solstices.
The polar day therefore is equivalent to the polar summer, as the polar night is to the polar winter. The nicest consequence of all this: Sunrise and sunset both seem to take about a week! If the weather is not shitty (which it might be according to our meteorologist Janelle) I'll show you some awesome photos next week.
Fun fact: The average time of daylight per 24 hours is exactly 12 hours for EVERY place on Earth, even the Poles. It makes sense if you think about it! :)
Science Facts #4 The IceCube Laboratory
I guess it's about time to talk about my actual job a little bit again. But before I tell you what exactly it is that I do (because I like messing with people who keep asking me that exact question ;)), let me show you my workspace! The IceCube Laboratory, or short ICL, is located in the Dark Sector (I know, right?!) about a kilometer away from Amundsen-Scott South Pole Station. It marks the center point of the IceCube detector which is buried under 1.5 km of ice. All the hundreds of miles of cables that IceCube consists of come together in this little building. Those arm-thick cables enter the ICL through the two large cable towers you can see in the picture above, and are split up inside into smaller red quad cables which each are connected to four DOMs deep down in the ice; their other ends are connected to the DOMHubs, custom-made computers that feed high voltage to the sensors and read their data in return. We've got 97 of those! All the other machines (we've got a total of about 200!) are data processing or infrastructure machines, that means they filter and pre-analyse the data, they host important resources like repositories, mail accounts, or the detector monitoring system, or are responsible for the Iridium connection that lets the winterovers communicate with the North during satellite outages.
So yes, the essence of IceCube is housed in the little blue building in the middle of nowhere - it's cozy, and the noise of all the machines has a somewhat soothing effect on me by now. The dog house, the little blue cube on the roof, is a perfect get-away when you need a break from life on station. I'd spend way more time out there, if the ICL would feature a bathroom with running water...
Fun fact: The ICL is the only building at South Pole that has to be actively cooled. For that purpose the outside air is sucked in, heated up (yes, we're at the South Pole), and blown into the server room. If the air conditioning shuts down for only 20 minutes, the exhaust heat of our computers heats the server room up to almost 60° C - which can be fatal for all kinds of expensive equipment in there! That's why we monitor the temperature very closely with dozens of sensors.
Science Facts #3 Askaryan Radio Array (ARA)
The Askaryan Radio Arrary ... pardon me, the Askaryan Radio ARRAY is a sister project of IceCube. Like the Cube, it is also looking for high energy Neutrinos, but instead of optical sensors it utilizes radio antennas to detect our favorite particles. The measurement principle of ARA is based upon the Askaryan effect, which describes the generation of charge anisotropies in bulk media (such as ice) caused by high-energy neutrino induced particle cascades. The anisotropy emits coherent radio waves which can be detected by the ARA antennas.
At the end of this summer season, the experiment will consist of six stations with four holes each, where every hole is holding 4 antennas. Once completed, ARA will cover an area far bigger than IceCube, although with a far smaller detector density. It's neutrino detection sweetspot is at energies even higher than IceCube's, which makes it an important addition to the South Pole Neutrino Club.
Science Facts #2 IceTop Snow Measurements
If you paid attention in my IceCube facts #1, you might have noticed that IceCube does not only have in-ice optical sensors to measure the neutrinos, but also features some modules right beneath the surface - these are called IceTop stations. Each of the 86 strings that are deployed in the ice has one of them on top. All together, the IceTop stations are used to measure lower-energy neutrinos, and they also serve as a veto-mechanism for the in-ice DOMs. The problem with stuff that is set up at the surface of South Pole ice plateau: It does not stay at the surface for very long. Things are being burried in snow drift faster than you can say "penguin". Since the amount of snow that covers IceTop has an affect on the measurements, every once in a while the IceCube winterovers have to go out and estimate the snow level on every single IceTop station. This can be a long and cold adventure, depending on the windchill and how many people can be motivated to help. Fortunately, the old winterovers Martin and James were still here (they belong to the handfull of toasty people who are still waiting for a plane to take them back to the real world) to help Johannes and me, so it took us only two afternoons.
Science Facts #1 The IceCube detector
The IceCube South Pole Neutrino Observatory is a huge particle detector buried in the about 2500 m thick Antarctic ice sheet at the geographic South Pole. It has an instrumented detector volume of 1 km3 and weighs over a 100.000.000 tons. 5160 optical sensors, called "DOMs", attached to 86 long cables take data 24 every single day.
IceCube is looking for ultra-high-energy neutrinos from outer space. Upon colliding with the molecules of the ice, the neutrinos produce secondary charged particles which then again generate a little flash of blue light. This is called the "Cherenkov Effect". The light can be seen by the optical sensors. The neutrinos leave a signature in the detector from which the IceCube scientists can extract the particle's energy, and sometimes the direction it was coming from.
The strings and DOMs have been deployed in the ice by melting deep holes with a hot-water drill head that was specifically designed for IceCube. Soon after each string was in its place, the holes froze shut - as long as the Antarctic plateau exists, the IceCube sensors will never see the sunlight again. The data collected by the DOMs is sent to the IceCube Laboratory at the surface, where it is recorded, filtered, processed and forwarded to the Northern hemisphere for analysis.
Why is IceCube so big?
Because of their nature, neutrinos hardly ever interact with matter. And on top of that, the flux of neutrinos with very high energies is pretty low - so the bigger the detector, the better the chance of catching at least a few of them every year.
Why at the South Pole?
In order for the optical sensors to see the Cherenkov light, the surrounding medium must be transparent to the visible and UV spectrum - ice is just perfect for that! The South Pole is the only place on Earth with a sufficiently deep and clear ice sheet. The 1 km long IceCube strings are covered by 1.5 km of ice, to shield the sensitive DOMs from unwanted atmospheric charged particles, like muons, that might leave a signature in the detector that can not be distinguished from a cosmic neutrino. But even despite the big shield, a lot of those particles reach the detector anyway - that's why all downgoing particle events (i.e. that enter IceCube from above) are ditched as false events in the high-energy analysis, because nobody can say for sure whether the signal was created by cosmic neutrino or something else. Upgoing signals, on the other hand, must have been created by particles that have traveled through the whole Earth, and only neutrinos can do that!
Besides the main high-energy array, IceCube features two sub-experiments: "IceTop" at the surface consists of two DOMs on top of every string, and is used mainly as a veto mechanism against downgoing muons. "DeepCore" in the center of IceCube features strings with a much higher DOM density to detect lower-energy neutrinos, e.g. for studying neutrino oscillation.
To get a better idea of IceCube, you can have a look at the picture above (courtesy of the IceCube collaboration) or visit icecube.wisc.edu.