Plasma Physics at West Virginia University
12 Jun

Predicting Aurora Sightings

Amy | June 12th, 2013

By Dr. Amy Keesee, WVU Plasma Physicist.

On April 13, 2013, social media was abuzz with predictions that there would be good viewing conditions for aurora in the northern United States. Many people went outside that night to look up, but unfortunately, there wasn’t much to see. While many may have been disappointed, this was an excellent opportunity for the general public to learn about the Sun, geomagnetic storms, and space weather. In particular, while such activity from the Sun and the resulting geomagnetic storms can have (potentially negative) effects on humans and our technology (see previous posts by Paul and myself), it is important to note that our ability to predict such space weather is in its infancy.

The Sun is monitored by numerous instruments on satellites such as the Solar Dynamics Observatory (SDO), the Solar and Heliospheric Observatory (SOHO), and the dual-satellite mission STEREO. These instruments observed a coronal mass ejection launched from the Sun on April 11th. These images can be seen using the NASA Integrated Space Weather Analysis System.

Using calculations of the speed and direction of the coronal mass ejection (CME) from these images, scientists predict whether the CME will hit Earth, including whether it will be a direct or glancing blow, as well as an approximate time. Scientists also use models to predict these characteristics. One such model is the WSA-ENLIL-CONE model. The model of the Earth-directed CME that launched from the Sun on March 15, 2013 can be seen here. In this image, the Sun is the white circle and Earth is the yellow circle.

AccuWeather created a map of auroral viewing conditions for April 13th.
aurora_viewing

What may have caused confusion with this map is that it was not a prediction of the aurora itself, but an assessment of the cloud cover in the region of possible auroral viewing zones. However, the cutoff used to indicate the “not visible” region is based on a storm with a Kp index of 9. It would require a very intense geomagnetic storm to reach such levels. The Kp index is a scale, similar to the Richter scale for earthquakes, that is based on measurements of the changes in Earth’s magnetic field caused by currents driven in space by the storm. Stronger storms drive stronger currents, and thus larger changes in magnetic field measurements. Aurora are often seen at high latitudes, even during weak storms. As the storms grow stronger, the aurora can be seen at lower latitudes.

It turned out that the CME that hit Earth on April 13th drove a relatively weak event, with a Kp index of 4. One reason for the weak storm was that the CME contained a primarily northward pointing magnetic field. One of the drivers of large storms is magnetic reconnection of the magnetic field in the CME with Earth’s magnetic field. (Learn more about reconnection in Luke’s post.) The magnetic fields must be pointing in opposite directions to occur. Earth’s magnetic field is like a bar magnet, with the magnetic field lines coming out of the South geographic pole and into the North geomagnetic pole, so that they point northward. For reconnection to occur, the magnetic field in the CME must, therefore, point southward. We currently do not have the ability to predict which direction the magnetic field in the CME is pointing. There are satellites with instruments that can measure the magnetic field in the CME, but their location gives about a one-hour notice prior to the CME hitting Earth’s magnetic field.

Plasma physicists at WVU are working to understand many elements of the Sun-Earth system, including coronal mass ejections, magnetic reconnection, and the dynamics of geomagnetic storms. Hopefully in the future, scientists will be able to predict space weather with similar accuracy to the regular weather forecasts.

Find out more about Plasma Physics at West Virginia University at http://ulysses.phys.wvu.edu/~plasma.

2 May

By Bruce Tepke, WVU Plasma Physics graduate student

Introduction

It’s quite common for school children to learn that the space beyond Earth’s atmosphere is a vacuum devoid of substance. While this is approximately true – so little stuff exists in space that aerodynamic drag (the retarding force an object experiences when moving through a fluid) takes little toll on spacecraft and other moving bodies in space (as attested by the multi-billion year lifetime of the Moon’s orbit), there is nonetheless a stream of charged particles streaming from the Sun to fill the void, denominated the solar wind. Since these charged particles can sustain electrical currents and exhibit collective electromagnetic behavior, it’s properly classified as a plasma. This plasma streams past the Earth and interacts with the Earth’s magnetic field, compressing it on the day side and stretching it out on the night side. The plasma region dominated by Earth’s magnetic field, rather than the Sun’s magnetic field, is called the magnetosphere. Those scientists who study the physics of space plasmas have long attempted to model the magnetosphere both to understand what happens in space as well as to better predict variations in that environment (space weather). The topic of this blog post is to describe and compare for the benefit of the nonspecialist some of the ways scientists model the magnetosphere using physics based representations.

Terrellas

The earliest method of modeling the magnetosphere was to simply build a model Earth, complete with magnetic field, place it in a vacuum chamber and shoot a plasma wind past it. Such models are called terrellas. The practice of building terrellas goes back to one of the great pioneers of space physics, Kristian Birkeland, who began his experiments around 1900 and successfully demonstrated the production of the Aurora Borealis and Australis.
terella1
Kristian Birkeland operating his terrella experiment. Source: http://www.astro.umontreal.ca/~paulchar/grps/histoire/newsite/sp/great_moments_e.html

terrella3
Terrella experiment demonstrating Aurora. Source: http://en.wikipedia.org/wiki/File:Aurora_borealis_in_a_lab_dsc04517.jpg

Beginning in the 1950’s, much more sophisticated terrellas were constructed to model the magnetosphere. These experiments have occasionally reemerged, however the difficulty of scaling all the parameters of something as vast as the magnetosphere to the size of a laboratory apparatus is quite a challenging task, thus rendering primarily qualitative rather than quantitative results. In recent years, computer simulations have almost completely replaced terrellas both due to the availability of ever more affordable computing hardware and the possibility of more quantitatively useful output.

Magnetohydrodynamics and computer simulation

To use a computer to model the physical processes, one must choose the correct details to represent. The number of particles trapped in the magnetosphere is too vast to expect either the computers of today of those of the foreseeable future to model. Therefore, one must choose a representation which captures enough physical content to be useful but not so much as to make computer simulations impractical. That compromise which most space physicists choose is called magnetohydrodynamics. Magneto refers to the the fact that we assume the plasma to be electrically conductive (often perfectly conductive) enough to interact with magnetic fields. Hydrodynamics is the study of how fluids flow, thus indicating that we treat the plasma as a fluid. To be sure, there are other physical effects which are ignored by treating the plasma as a conductive fluid, but this approach has the benefit of being good enough to frequently be useful, often surprisingly so, and simple enough to be the basis of a global model of the magnetosphere on the computers of today.

Digital simulation – an analogy

While the nitty gritty of computer simulations are beyond the scope of this post, it’s useful to remember that computers can’t perfectly represent continuous objects or continuously varying processes. Rather, they by chopping a continuously varying quantity into discreet amounts. This is analogous to how a video camera breaks up a movie. The camera typically takes 30 frames in a second to create the illusion that it captures continuous motion. The individual frames aren’t continuous to the finest details and textures, but broken into discreet building blocks called pixels. Further, the colors and tones represented aren’t continuous, but themselves rounded to some value in a finite set of numbers in the computer memory. The same is true for computer simulations of the magnetosphere. The fluid must be represented by numeric values assigned to given locations and times within the realm that is being modeled. However, unlike a movie camera, there are several ways to break the model up into discreet parts. The various magnetohydrodynamic models of the magnetosphere in use today all make different trade offs in how they do this. I will conclude with a brief overview of the most popular models in use.

Ogino Code – Ogino’s code from Nagoya, Japan is seldom used for Earth’s magnetosphere these days. However is is worth mentioning because a version of it is freely available for download and study. It also sees continued use for modeling the magnetospheres of other planets.

The Ogino code breaks up the magnetosphere into a uniform grid of cubic blocks. This is relatively simple to program, but with the disadvantage that to get enough resolution where it is needed require a lot of wasted computations in regions where high resolution is not needed.

Open GGCM – This code is the result of Jimmy Raeder’s group at the University of New Hampshire. It works to place resolution where its needed by taking the cubic grid mentioned above and stretching it.

LFM – The Lyon-Fedder-Mobarry (LFM) model uses the unique, but difficult to program, approach of chopping the simulated region into a spherical geometry (think of map coordinates on a globe to visualize this) which is then stretched and contorted to provide resolution where it is needed.

BATSRUS – This model from the University of Michigan uses regular cubic blocks, similar to the Ogino code. However, the code can automatically detect which regions need more resolution and subdivide those parts into smaller blocks adaptively.
idl_7199_20000101_023000
Magnetosphere density modeled using BATSRUS. Plotted at NASA CCMC.

GUMICSGUMICS is an interesting case. It adaptively subdivides its grid, similar to BATSRUS. Unlike all the other codes, it can also adaptively vary the timestep used to break up time so high resolution regions have finer timesteps. This has the advantage of producing a code efficient enough to run on a desktop computer. The disadvantage is that it’s so complicated that it has not yet been made available for parallel computers with all the speed advantages one would expect from such hardware advances.

OpenGGCM, LFM, BATSRUS, and GUMICS are available to researchers through NASA’s excellent Community Coordinated Modeling Center (CCMC).

Further Online Reading:

http://www.mhs.ox.ac.uk/sphaera/index.htm?issue7/articl6

http://www-spof.gsfc.nasa.gov/Education/wterrell.html

http://ccmc.gsfc.nasa.gov/models/models_at_glance.php

http://center.stelab.nagoya-u.ac.jp/web1/simulation/index.html

You can find out more about Plasma Physics at West Virginia University at http://ulysses.phys.wvu.edu/~plasma.

12 Mar

By Dustin McCarren, WVU Plasma Physics graduate student

Hall thrusters are a type of plasma propulsion device that may be the key to unlocking the cosmos. Hall thrusters accelerate ions, positively charged particles, with an electric field to generate thrust. To better understand how this process transpires we have to take a closer look. Well, maybe a much closer look.

Imagine, if you will, that you’re an electron. You have been transformed into a negatively charged particle so small that a fine grain of sand would now appear to be as vast as all of Earth’s oceans combined. You reside on the surface of a device called a cathode and things are starting to heat up for you, literally. The cathode has started creating a massive amount of heat on its surface and some of the thermal energy is being transferred to you. Suddenly you have enough energy to break away and you go shooting away from the surface of the cathode.

You are a free agent now; the world is your oyster! As you zoom gleefully away from your former place of residence you start to feel a tug. You immediately recognize this particular feeling as the sensation associated with the pull from a strong electric field. Being a negatively charged electron, you feel compelled to seek out the source of the electric field. You adjust your current trajectory and speed towards your new objective.

Your journey takes you into the bowels of a Hall thruster. As you enter the end of the Hall thruster you experience a new sensation, even more potent than the one you had previously encountered with the electric field. Being the worldly electron that you are, you realize that you are now in the presence of a powerful magnetic field. In the presence of any magnetic field you would be obligated to curve your trajectory, but the magnitude of this magnetic field is so great it effectively inhibits you from moving beyond its region of influence.

Despite being trapped by this radially oriented magnetic field (green →), you still feel the incessant pull from the electric field down the axis (yellow →) of the thruster. The two resulting forces cause you and the other electrons to spiral azimuthally (blue →) about the inside of the Hall thruster. This circling motion effectively creates an electric current, or Hall Current, the namesake of this type of thruster. You and your fellow electrons are crowded together, constantly colliding within the confines of the magnetic field in an attempt to reach the source of the electric field. Through the constant jostling, some of your fellow electrons liberate themselves, but their numbers are relatively few.

At the end of the Hall thruster opposite of you a new element has been introduced to the system. Massive particles of neutral gas, each one over a thousand times your size begin drifting down the axis of the Hall thruster. Being neutral, they are blind to the effects of the electric and magnetic fields. There is nothing to prevent these behemoths from barreling into the crowd of electrons in which you reside. Constrained by the magnetic field, you can only watch as the stampede of atoms and molecules smash into the foremost electrons.

In one of the initial electron-neutral collisions, you see that the colliding electron knocks an electron off of the neutral gas particle. The formerly neutral gas particle now has more protons than electrons, making the particle into a positively charged ion! You watch in amazement as the ion’s substantial mass mitigates the influence of the magnetic field on its trajectory. Despite its mass, there are some things that the ion cannot ignore.

An electric field has been erected between you and the ion. In the past when you have come across ions, the effect has been much like two people playing tug of war on a sheet of ice. You pull the ion towards you, while the ion does the same, resulting in you and the ion moving towards one another. However, due to the ion being so heavy, it is usually you that appears to go flying towards the ion. Imagine your surprise when it is the ion that comes flying towards you!

You are still trapped by the radial magnetic field and unable to move axially down the Hall thruster towards the newly created ion. The magnetic field lines are “anchored” to the thruster, meaning that when you get tugged on, the entire thruster has to move towards the ion. As you and the other electrons that constitute the Hall Current drag the ion axially (orange →) towards the end of the thruster, the whole thruster moves almost imperceptibly towards the ion. You and your fellow electrons have just dragged the impossibly large thruster through space! The ion, propelled by the collective pull of you and the other electrons, flies by and shoots out of the Hall thruster.

As the ion emerges from the magnetic field, it encounters a swarm of electrons freshly liberated from the surface of the cathode. A number of electrons break off from their endeavors and surround the positively charged particle of gas. To you, at this distance, the ions and electrons move as a unit that has no charge associated with it. This phenomenon, Debye shielding, effectively severs the connection between you and the ion. Don’t despair though, as you turn your attention away from the ion, you notice that there are now several more ions that require your immediate attention…

What an adventure we just had. You now know, first hand, the inner workings of a Hall thruster. We saw the electrons from the cathode travel into the Hall thruster where they experienced the Hall Effect from the induced magnetic and electric fields. The Hall Effect causes the trapped electrons to form a ring of current, the Hall Current. After that we witnessed the ionization of the gas propellant occur from electron-neutral collisions and then the acceleration of the resulting ions which ultimately created the thrust for the Hall thruster. By repeating this process billions upon billions of times, Hall thrusters can generate enough thrust to be a viable engine for long distance space travel.

hallthruster
Hall Thruster Image: http://www.al.t.u-tokyo.ac.jp/hall/en/projects.html

You can find out more about Plasma Physics at West Virginia University at http://ulysses.phys.wvu.edu/~plasma.

14 Feb

By Matt Beidler, WVU Plasma Physics Graduate Student

It seems the center of an atom has gotten a bad rap in the last century, just saying the word nuclear in public will get you suspicious or fearful eyes staring back. The nucleus of an atom carries a sizable amount of harvestable energy, but because this energy source was weaponized before we could fully control it, anything including the word nuclear evokes images of destruction rather than growth in most peoples’ minds. Despite the difficulties with its perception, nuclear energy has proven to be as effective as first imagined. Fission energy reactors have been available since 1951, with numerous reactors supplying a critical percentage of energy to nations worldwide. For example, France supplies ~75% of their energy utilizing fission. The other process used to extract nuclear energy, nuclear fusion, has been less successful as an energy source. However, with the world agreeing that fusion is a worthwhile endeavor, tremendous progress has been made toward a stable energy reactor.

It’s pretty straightforward to understand where the energy wrapped up in the nucleus is stored. We know that the nucleus of an atom is made up of protons and neutrons (together known as nucleons), which can exist isolated from each other but prefer to enter their lowest energy state by binding together. A simple example of a lowest energy bound state is sitting in a La-Z-Boy; when you relax fully, entering your lowest energy state, you find yourself sinking into the chair, binding to it. You also know you need to (begrudgingly) exert energy to get out of the chair, illustrating that the unbound state has higher energy. However, the interactions between nucleons involve the strong force, which is much more complex than sinking down into a La-Z-Boy. Summarizing these complex interactions, a chart showing the binding energy per nucleon as a function of number of nucleons in different atoms can seen in Figure 1. As you can see, iron (Fe) has the largest binding energy per nucleon, and in the context of our example would be the most comfortable in the La-Z-Boy. All the other elements have less binding energy per nucleon and are therefore in a higher energy state than iron; by breaking apart (fissing) big atoms or sticking together (fusing) small atoms, we can harvest some of the energy stored in the nuclear bindings.

binding energy
Figure Credit: http://www.schoolphysics.co.uk/age16-19/Nuclear%20physics/Nuclear%20structure/text/Binding_energy_per_nucleon/index.html

To harvest the energy stored in the nucleus of atoms, nuclear reactors are built to sustain an environment in which fission or fusion can take place in an energy profitable way. Nuclear fission, the breaking apart of large atoms, relies on a chain reaction of energetic neutrons to be sustained. You can think of energetic neutrons as bullets that slam into big uranium atoms to shatter them apart, into smaller atoms and additional energetic neutrons. When, on average, one energetic neutron from a fission reaction starts another fission reaction, a chain reaction is said to be taking place. You can imagine chain reactions could easily get out of hand; on average, if multiple energetic neutrons from a single fission reaction each start a reaction of their own, energetic neutron bullets would be flying everywhere, exciting more and more reactions. This is how meltdowns happen. By continuing to study and understand the fission process, safety measures and protocols at reactors have improved along with the fast pace of technology. Current reactors have an extremely low risk of meltdowns, however that chance is still finite.

In the fusion process, on the other hand, we attempt to slam together light atoms in such a way that two atoms collide and become one heavier atom (that is still lighter than iron.) As commented on in an earlier post, there are two distinct fusion processes currently employed, inertial confinement and magnetic confinement. While the two processes are surprisingly different, the main concept is the same in each: confining many light particles in the form of a plasma together for a sufficient amount of time and heating them up to a sufficient temperature so they collide and fuse together. This task is far from simple though because as the temperature of the plasma rises, energy is funneled into random motions of the plasma that work to break the confinement through many processes. To maintain confinement, inertial and magnetic systems must be able to mitigate these disruptions or else the fusion reaction cannot be sustained. This is by far the main advantage of fusion energy as opposed to fission; there is no chance of the reaction getting out of hand because fusion is unstable in the conditions in which we live. Not only is fusion safer than fission, but it’s also a more efficient process in terms of energy production in addition to its fuel being more abundant.

The techniques used in the magnetically confined fusion (MCF) process to heat the plasma and increase the time of plasma confinement are becoming as complex as the fusion process itself. On December 2-6, I (along with Rich) traveled to India for the International Iter School focusing on current drive and heating techniques used in tokamaks (the vessel for MCF). Iter is the name of the machine currently being built by a coalition of 7 countries; it used to be an acronym for the International Thermonuclear Experimental Reactor. The acronym had to be dropped due to the stigma surrounding the word nuclear, exactly like the MRI we use as a medical imaging tool used to be known as NMRI (and I bet you can guess what the ‘N’ stood for.) The Iter School brought in experts from each nation in the coalition to cover the basics of the current drive process, which is a way to funnel energy into the plasma in a precise way to heat and contain it more efficiently. Current drive techniques, in essence, transfer externally generated energy to specific populations of particles in the plasma. (Those interested in the topics covered can see the presentations here.)

With such an available source of energy locked up in the nucleus of atoms, processes to extract this energy are going to be essential as our species continues to grow. Many young nations, China and India for example, have large populations of people ready to consume many resources, energy included. While we continue to research alternative sources of energy to sate this hunger, nuclear energy has the ability to be a safe and abundant source whether obtained through the fission process or the superior fusion process.

You can find out more about Plasma Physics at West Virginia University at http://ulysses.phys.wvu.edu/~plasma.

15 Jan

Predicting the Weather - in Space!

Amy | January 15th, 2013

By Dr. Paul Cassak, WVU Plasma Physics Assistant Professor

In early March of 2012, the world awoke to news that a huge solar flare occurred in the solar corona (the Sun’s atmosphere). Associated with the flare was a coronal mass ejection, in which a blob of hot gas – known as plasma – is launched into interplanetary space. Large flares can release the same amount of energy as 40,000,000,000 atom bombs in a matter of minutes, so they are a force to be reckoned with! Flares and coronal mass ejections are common, but usually the material flies out into space with no effect on the Earth. The flare in March was different – the material from the Sun was on a collision course with the Earth. There were a tense couple of days when it was unknown how the impact of this material would play out on Earth.

Why does it matter if material from the Sun hits the Earth? First, the energetic charged particles coming from these “solar storms” can damage satellites. In our modern world, this is a major issue. Satellites are used for communication, cell phone service, GPS, and so on. For example, AT&T lost a satellite to a solar storm in 1997! Another way satellites can be affected is that they are carefully placed in particular orbits, but the material from the Sun can increase friction (drag) and cause them to go out of these designed orbits. A third dramatic result of solar storms is power outages – the charged particles moving above the Earth create a magnetic field, which induces a current in the electrical power grid. If the current gets too large, it can damage transformers in the power grid (see the picture below), which famously occurred in Quebec in 1989, causing a power outage over a large portion of the US and Canada.

transformer
Photo credit – http://www.windows2universe.org/space_weather/sw_in_depth/sw_voltage_transformer_damage.html

In the solar storm of March 2012, we got lucky – it turned out that the magnetic field in the plasma blob from the Sun was pointed in the wrong direction, so the blob bounced off the Earth’s magnetic field instead of penetrating it. At the present time, there is no way to definitively tell if a given solar storm will cause damage when it strikes the Earth. We can see if there is a solar storm and predict if it will hit us, but not if it will cause damage. One goal of scientists working in the field of space plasma physics is to learn about what causes solar storms, how the plasma blobs they create travel toward Earth, and what happens when the blobs hit. This will help to figure out how to develop an ability to predict solar storms and their effects the same way we now predict the weather. For this reason, this field is called “space weather.”

Many aspects of space weather research are being carried out at WVU, as has been discussed in previous posts. For example Rich’s post on plasma self-organization of sunspots, Luke’s post on energy release in solar flares, Dimitris’ post on measuring magnetic fields, Earl’s post how plasmas get heated, Stephanie’s post on coronal heating, Mattias’ post on Earth’s radiation belts, and Amy’s post on space weather.

A colleague and I are doing research on a particular aspect of solar flares. It was proposed a long time ago that the energy given off in a solar flare comes from the energy stored in the stretched magnetic field in the solar corona. The way it gets released is through a process called “magnetic reconnection,” where magnetic field lines break and snap apart like rubber bands, as described in the post by Luke. The standard model of a flare has a reconnection site high above the solar surface, and it shoots plasma towards and away from the Sun. This process is not directly observable in the Sun because it takes place in such a small region and it does not give off much light, but the after effects of reconnection are routinely observed.

An interesting development occurred in 1999. Material from a flare site was observed traveling down toward the surface of the Sun. Interestingly, the observed flows moving downward from the flare site appeared dark against a light background, and people have since learned that it is because these regions have less plasma than the bright regions. They have been called “supra-arcade downflows” as a way to describe their properties, but they have also been called “tadpoles” because of their shape, as can be seen near the arrows in the picture below.

Trace
Photo credit – Reeves et al., Journal of Geophysical Research, 113, A00B02, 2008.

Since magnetic reconnection cannot be directly observed, the tadpoles were exciting because their properties provide a potential window into inferring information about the way reconnection occurs during the flare. The tadpoles appear in a bursty manner, which implies that the reconnection itself is bursty. It has been suggested that the reconnection is bursty in time, meaning that reconnection starts and stops in a short time.

In work done at WVU, we have argued that the tadpoles are more representative of reconnection being bursty in space rather than in time. In other words, reconnection occurs in localized regions in space, like fireworks going off at different parts of the sky. We think a key aspect of their appearance is that the solar atmosphere is more dense near the surface and gets less dense as you go away from the surface. We performed large computer simulations on supercomputers to test this idea. (Supercomputers are discussed in the post by Luke.) The computer simulations confirmed that the tadpole-like structures only appear when there is a region of higher density. This work may help researchers understand where tadpoles come from and how the reconnection process works in flares, which will be of use in space weather modeling.

You can find out more about Plasma Physics at West Virginia University at http://ulysses.phys.wvu.edu/~plasma.

24 Sep

Plasma self-organization

Amy | September 24th, 2012

By Dr. Rich Magee, WVU Plasma Physics postdoctoral researcher

Examples abound in the natural world of order seemingly arising from disorder: the tumultuous roiling atmosphere of Jupiter gives rise to a surprisingly regular banded structure; the random crystallization of water vapor in a cloud creates snowflakes with striking fractal beauty; and highly structured organisms subsist on the destruction of complex carbohydrates. Phenomena such as these are generally called self-organization, and they share a few common features. There is a source of energy, for example, and that energy is in some way dissipated. More importantly, the systems are complex. That is, they are composed of many parts interacting with each other in feedback loops across a wide range of length scales. Plasmas are the epitome of a complex system – the dynamics of the charged particles are influenced by electric and magnetic fields which are in turn modified by the moving charges themselves. They are therefore an ideal medium in which to observe self-organization, and we observe it in several manifestations in our research in the WVU plasma physics group.

The fusing plasma at the center of the solar system – the sun – is a prime example. Whereas the magnetic field of the Earth is relatively ordered, with field lines pointing from the geographic south pole to the north, the magnetic field of the sun is a tangled, turbulent mess. The flowing plasma constantly moves field lines around, which leads to the generation of magnetic waves (Stephanie’s post), ion heating (Earl’s post), and solar storms that can affect Earth (posts by Amy and Luke). This chaotic interaction between flowing plasma and a jumbled magnetic field is also responsible for the near clockwork-like solar cycle. Every 11 years (on average) the sun oscillates between times of intense activity and relative calm, illustrated in Figure 1. The “butterfly” diagram shows the size and latitude of sunspots as a function of time. The regularity of this oscillation stands in stark contrast with the processes responsible for it and is the hallmark of self-organization.

Sunspot-bfly
Figure 1 The regularity of the solar cycle can be seen in the size and latitude of sunspots as a function of time. (Taken from reference 1.)

The sun, in addition to exemplifying plasma self-organization, is the ultimate energy source of the solar system, providing 10 billion years-worth of energy via the fusion reactions in its core. Containing that reaction here on Earth would provide an almost ideal energy source: cheap and abundant fuel, no greenhouse gases, no radioactive waste. But this is a daunting task. The fusion reaction requires such high temperatures (1,000,000 deg. F) that it cannot be contained with material walls. The sun contains its plasma with gravity, here on Earth we are trying to do the same with magnetic fields. Scientists have made tremendous strides advancing the concept of magnetic confinement fusion energy over the last 50 years, but perhaps the greatest single advancement was made by the plasma itself.

On February 4th, 1982 scientists at the ASDEX tokamak in Germany observed a plasma spontaneously self-organize in such a way that the confinement time (a measure of how well the magnetic field holds the plasma) increased by a factor of 2. Researchers have since learned how to induce the transition from low-confinement mode (L-mode) to high-confinement mode (H-mode), and, although a complete understanding remains elusive, it appears that small changes in conditions in the very edge of the plasma are involved. Shown in Figure 2 are the density profiles in L-mode (green curve), H-mode (red curve), and an intermediate mode (blue curve). It can be seen that in H-mode the density is much larger, and there is a steep “cliff” at the edge of the plasma (the plasma edge is on the right side of the graph, and the arrow shows the location of the cliff; the center of the plasma is on the left side of the graph). This cliff is referred to as the pedestal, and its formation, although not in any way prescribed by the experimenter, is critical to achieving H-mode. To further our understanding of pedestal formation and the L- to H-mode transition, we have a developed a plasma diagnostic capable of measuring the neutral density in the edge of a magnetic confinement device (described in Matt’s post). These measurements may one day shed light onto how and why the tokamak plasma self-organizes in this manner, and maybe even open up new ways to achieve fusion energy on Earth.

denspro_arrow
Figure 2 The density profiles in a tokamak plasma. In H-mode (red curve) the density is higher than in L-mode (green curve). The red arrow shows the location of the pedestal. (Taken from reference 3.)

References:
1. NASA Solar physics webpage: http://solarscience.msfc.nasa.gov/
2. The story of the discovery of H-mode: http://www.iter.org/newsline/86/659
3. Y. Ma, et. al. Nuclear Fusion 52 (2012) 0230101.

You can find out more about Plasma Physics at West Virginia University at http://ulysses.phys.wvu.edu/~plasma.

11 Sep

Explosive Events on the Sun

Amy | September 11th, 2012

By Luke Shepherd, WVU Plasma Physics graduate student

The sun is home to the most powerful explosions that occur in our solar system. These explosions are commonly known as solar flares. The energy released in a typical solar flare is equivalent to about 100 million Mount St. Helens eruptions from 1980! The energy released in a solar flare can occur in a matter of minutes. It is believed that magnetic reconnection, a fundamental process in hot ionized gases called plasmas, is the mechanism that mediates this energy release. Solar flares can dramatically impact near Earth space weather, the measure of plasma properties in space between the sun and Earth. It is important to understand the processes behind solar flares for space weather applications and predictions, to better protect satellites and astronauts from energetic charged particles.

Magnetic fields play an integral part in magnetic reconnection and plasma processes. In a simplistic version of the sun, we can imagine that the sun is covered in a number of bar magnets that vary in orientation (a bar magnet is pictured below). Bar magnets have a magnetic field that emanates from the north pole and connects with the south pole. In the places where the magnetic fields between magnets become oppositely directed is where magnetic reconnection takes place.
bar magnet
Photo credit: Georgia State University

The figure below is a cartoon of magnetic reconnection. The red and blue lines at the top and bottom indicate oppositely directed magnetic field lines. As these magnetic field lines come together (first picture) in the center of the figure they break and reconnect (second picture) with each other (lines on right and left of figure) and the magnetic configuration changes. After the field lines reconnect, they then “sling” out away from the reconnection site (third picture), this is similar to letting a stretched rubber band go. This process converts stored magnetic energy (bent field lines) into thermal (heat) and kinetic (motion) energies of the plasma.
reconnection

To study magnetic reconnection, I use computer simulations with the goal of applying my results to better understand what is observed on the surface of the sun. In order to capture the physics we are interested in we must use very large simulations. To perform these simulations in a reasonable time frame, we must use a supercomputer. If we were to use a normal desktop computer our simulations would take 30,000+ hours each! Each simulation also produces about 100 GB of data, that is enough to fill up the entire hard drive on the average household computer.

My research over the past few years has dealt with the spreading of magnetic reconnection sites. Naturally occurring magnetic reconnection often begins in a localized region of space and then spreads to nearby regions of space. Reconnection spreading has been observed on the sun, and an example can be seen here in this two minute video clip of the Bastille Day Flare. Reconnection spreading has been observed to have two different physical mechanisms. My research involved determining the conditions for when each kind of spreading will occur. This result is important for helping us to understand observations made of reconnection events, whether they occur in space or in laboratory experiments.

My current research focuses on how the energy gets stored prior to the energy release in solar flares. This is a challenging problem to answer. We can use an analogy to explain why this is a difficult problem. If we think of TNT, we know the source of the energy is the chemical bonds between atoms. We can store the energy in a box without the TNT exploding. We can then release the energy stored in the box by igniting the TNT. If we change this to the reconnection storage problem, we know the source of the energy is twisted and bent magnetic field lines. We know that the stored energy is released by magnetic reconnection. We do not know how the energy gets stored without magnetic reconnection starting-what “ignites” the reconnection? This is an important question to answer as it will contribute to better understanding of solar flare events, which will lead to better predictions for space weather.

You can find out more about Plasma Physics at West Virginia University at http://ulysses.phys.wvu.edu/~plasma.

31 Jul

By Stephanie Sears, WVU Plasma Physics graduate student

In earlier posts, Jerry has described the basic physics of plasma thrusters, Earl has talked about how plasmas get hot, Matt has discussed a new plasma diagnostic, and I have written about studying solar plasmas. In all of these cases, knowing the plasma flow and temperature is an important aspect of fully understanding the dynamics at work, but how exactly do we make these measurements? In this post, I’ll explain one of the ways in which we measure plasma flow and temperature using a very specialized laser system.

dyelaser
Stephanie working with the dye laser system

The trickiest thing about taking reliable measurements is that putting physical probes inside a laboratory plasma alters the surrounding plasma. Sometimes the change is small enough that we neglect it, but other times, it is large enough to fundamentally and significantly change the plasma. In these latter cases, what we end up measuring is due to the changes we’ve introduced with the probe rather than the actual plasma we are trying to study. One way around this problem is to ‘probe’ the plasma with our specialized laser beam rather than a physical probe.

How we do this is based on fluorescence, like those glow-in-the-dark star-shaped stickers and modern watch faces. When these objects are exposed to light, they absorb some of the light’s energy and then reemit it in a slightly different form. A great example of this absorption and reemission is when my watch face glows a bright aqua-green in the dark, even though daylight is not aqua-green.

Applying this to our plasma, we shine a laser into it at a very specific, single color (rather than all the colors of visible light in daylight, illustrated by Pink Floyd’s The Dark Side of the Moon cover) and then we record the reemitted light or fluorescence we get back, which is also a single, though different, color. The amount or intensity of fluorescence we measure tells us how many ions in the plasma have absorbed our laser light and reemitted it. (If you have questions about ions in plasmas, a good background is here.)

We know exactly which single color our plasma ions absorb when they are at rest, and we know that they absorb a slightly different single color when they are in motion. This slightly different color gives us measurements for plasma flow and temperature. If an ion is moving toward us, we need to make the laser light slightly more red in order to match the absorption color. If an ion is moving away from us, we need to make the laser light slightly more blue. If an ion is moving toward us really quickly, we need to make the light much more red than if it were moving slowly. Similarly, if an ion is moving away from us really quickly, we need to make the light much more blue than if it were moving away slowly. (This motion-dependent absorption is due to the Doppler effect, which you can learn more about here.)

By stepping our laser through a range of ‘more blue’ to ‘more red’ colors and measuring the amount of fluorescence we get, we can tell how all of the ions in the plasma are moving. We can see what speed most of the ions have, how much the slowest speed differs from the fastest, and how many ions we have all together since we’ve collected light from all of them. Obviously, the speed most of the ions have tells us the overall motion of the plasma. The difference between the slowest ions and the fastest ions tells us the temperature, and knowing the total number of ions tells us the density of the plasma. With our one, special laser beam, we are able to measure plasma flow, temperature, and density without ever altering the plasma itself.

You can find out more about Plasma Physics at West Virginia University at http://ulysses.phys.wvu.edu/~plasma.

13 Jul

Plasma Thrusters

Amy | July 13th, 2012

By Jerry Carr, Jr., WVU Plasma Physics graduate student

Like many young children, I wanted to be an astronaut when I grew up. However, my fear of heights became very real to me after my first plane ride as a child. Fortunately, science has given me a chance to help the mission of space travel by studying the physics of a plasma thruster that might enable humans to go deeper into space.

A thruster works by using Newton’s third law; for every action there is an opposite and equal reaction. Picture yourself standing still on a skateboard while holding a bag of hammers, each identical. Also imagine that the ground and wheels are frictionless. As you throw a hammer directly in front of you, you will proceed to move backward. The force pushing you backwards is referred to as thrust. If you throw two hammers at the same speed, you would move twice as fast.

Newton’s second law allows you to substitute smaller particles such as ions for the hammers by providing a mathematical relationship between thrust and how far something will travel because of it. If you continue to “throw” the ions at the same speed as the hammers, the total change of momentum (or impulse) delivered would be dependent on the total mass of all the ions you could launch. If the total mass of the ions is the same as the hammers, you’ll have the same impulse in both cases. Since ions move much faster than the mass ejected in chemical rockets, they don’t require as much ejected mass to deliver the same impulse. Currently, the weight of fuel is a limiting factor for long distance space travel. For this reason, many believe plasma thrusters are the key to making deep space missions to places like Mars a reality.

A plasma thruster can be generated using electrodes that help govern the flow of charged particles such as positively charged ions and negatively charged electrons. However, over time energetic plasma particles can erode these electrodes within the exhaust chambers. One way plasma researchers have attempted to solve this problem is by studying plasma thruster schemes that don’t require electrodes. An example of this kind of system can be found in nature in the form of the aurora borealis (or the northern lights).

By varying the magnetic geometry in our plasma source, I have the ability to study the science behind the aurora and this particular type of electrode-less plasma thruster. I use the same plasma chamber described by Stephanie and Earl – in earlier posts. I also use the laser as a radar gun to get speed and temperature measurements of some of the ions present with my plasma. The speeds obtained from these measurements (~20,000 mph) are comparable to what’s seen in plasma thrusters. I am studying ways to control these fast moving ions. By doing this, I am able to determine additional strengths and inherent limitations for this type of plasma thruster.
Helix
Plasma Source Used to Study the Plasma Thruster

You can find out more about Plasma Physics at West Virginia University at http://ulysses.phys.wvu.edu/~plasma.

14 Jun

By Dr. Dimitris Vassiliadis, WVU Plasma Physics Research Professor

Plasmas are found in many corners of the cosmos, but it is not always practical or even possible to measure them in situ (Latin for “on location”, i.e. using an instrument in direct contact the plasma). They may be simply too hot or too far away. Instead we often detect their presence and measure their properties indirectly, by using magnetic, radio, or optical methods. Today’s post will discuss the magnetic methods.

Plasmas are collections of electric charges of various kinds (typically electrons and protons) which are constantly moving with each charge effectively producing a mini-current. Now all currents, whether localized or extended, produce magnetic fields. So if there is a net current, the sum total of all mini-currents in the plasma, we can detect its magnetic field inside the plasma volume but also, provided the current is intense enough, at distances far from the plasma. That way we can “remote-sense” the field and consequently the dynamics of the plasma producing it. The strength of the magnetic field decreases the further away from the plasma we take the field measurements.
Magnetic fields are routinely measured for a number of cosmic plasmas. The nearest such plasmas are those surrounding our planet: its ionosphere and magnetosphere, the top two layers of Earth’s upper atmosphere. We can measure their fields by flying spacecraft through them (in situ) or by using sensitive instruments here on the ground.

A magnetometer in principle can be as simple as the magnetized needle of a compass. The needle adjusts itself so as to be aligned with the magnetic field. If the magnetic field changes direction due to a nearby or distant current, the needle will track that change. This simple magnetic-field sensor originated in China during the 9th century and was useful for navigating across oceans and along unfamiliar coasts. However a compass provides us with only directional information or “bearing”, and does not show the intensity of the field.

Magnetometer technology evolved drastically in the 19th and 20th centuries. A number of techniques were developed such as the Hall-effect magnetometer (end of the 19th century), the fluxgate magnetometer (1930) and the formerly widely used, but now obsolete coil magnetometer. Even those devices however appear bulky in comparison to today’s miniaturized sensors. These rely on nuclear and atomic effects such as the (warning: tongue-twisters ahead) Overhauser effect (1953), the superconducting quantum interference device (SQUID; 1964), the giant magnetoresistive effect (GMR; 1988), and the spin-exchange relaxation-free, or SERF, effect (early 2000s). As a result of this mag glut there is a sensor installed on your average modern smartphone where it is used for apps ranging from navigation to entertainment.

If magnetometers are luxury items on phones, they are a must-have for any spacecraft (and even certain aircraft) where they play an indispensable role in navigation. Thanks to on-board magnetometers we know that the most distant spacecraft, Voyager 1 and 2 have reached the outer edge of the Sun’s sphere of influence, or heliosphere, we know the structure and orientation of that region, and that they are now entering a magnetically turbulent interstellar boundary. Magnetometer wizards such as NASA’s late Mario Acuna, worked hard for decades to develop supersensitive, reliable magnetic sensors that would withstand the temperature fluctuations and radiation doses of outer space. Argentinian-born Acuna who passed away in 2009, is said to have the record of installing high-accuracy, space-rated mags on missions to all solar system planets!

The study of space physics can be defined to commence with the development and use of ground magnetometers. These magnetometers were originally installed (in the early 19th century) to precisely measure the Earth’s crustal field for mapping and mineral exploration. However it was soon found that the magnetic field they displayed was not fixed in time, but kept oscillating, usually unpredictably. This variation had to depend on activity in near-Earth space during the so-called “magnetic storms”. The great German scientist and explorer Alexander von Humboldt noticed the relation between magnetic storms and the presence of auroral lights above Berlin during a magnetic storm in 1806. It was surmised that intense currents flowing in the ionized upper atmosphere, or ionosphere, would produce these variations. More than a century later, the English mathematician Sydney Chapman linked the brightening of auroral lights and magnetic activity to solar events in 1931. We now know that expulsions of solar atmospheric material reach Earth and intensify both ionospheric and magnetospheric currents whose magnetic effects we can monitor from the ground.

The plasma physics group at WVU’s department of physics maintains a ground magnetometer in collaboration with the National Radio Astronomy Observatory at the latter’s grounds in Green Bank, WV. The mountainous plateau is ideally quiet for radio observations, but the immediate surroundings of the instrument are also free of unwanted magnetic effects. The magnetometer, in operation since June 2009, has been used to measure geomagnetic activity and thereby the intensity of ionospheric currents that may interfere with the operation of the radio observatory. As the 11-year solar cycle reached a prolonged nadir in 2009 and has since hummed up to a maximum expected later this year, the geomagnetic activity followed suit. The WVU magnetometer tracked the rise of this activity including all major magnetic storms during these three years.

The WVU instrument is in fact part of a magnetometer array called MEASURE, short for Magnetometers on the Eastern Atlantic Seaboard for University Research and Education. The program is managed by University of Michigan and UCLA. Using entire arrays of sensors, rather than isolated magnetometers allows scientists to measure the footprint of geomagnetic disturbances in addition to their changes in time and obtain a more realistic view of their impact.

An animation of such a footprint of geomagnetic activity during a storm in the spring of 2010 is shown at the website of UCLA researcher James Weygand
http://www.igpp.ucla.edu/jweygand/images/Movie_asi_eic_20100405_0830-0930.wmv
where a total of more than 40 magnetometers from seven arrays are used to measure the magnetic field. The intensity of the ionospheric electric currents is calculated from the field measurements, and is indicated by the length of the arrows. Simultaneous video feed of the aurora borealis, captured on several all-sky cameras, is superposed on the map as well. As the storm evolves the auroral plasma brightens up during the bursts of magnetic activity.

You can find out more about Plasma Physics at West Virginia University at http://ulysses.phys.wvu.edu/~plasma.

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