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 Photo of An artist rendering of the MMS formation flying through Earth's magnetosphere. (not to scale)
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NASA’s Magnetospheric Multiscale (MMS) Mission Entered its Harvest Season

NASA’s Magnetospheric Multiscale (MMS) mission entered its harvest season (also known as Phase-E) on September 1, 2015. The commissioning and testing process is over, and now MMS is ready to begin gathering data.

Over the next six months, the highest point (about 5 Earth diameters from Earth’s center) in MMS’ highly elliptical daily orbit will sweep from the dusk meridian, through the noon (or sub-solar) meridian and the morning side, finally to dawn. In so doing, the four-spacecraft tetrahedral formation will skim Earth’s magnetopause -- the boundary between Earth-space and interplanetary space -- near every apogee, sampling that boundary and the myriad of plasma dynamical processes that occur there. The primary focus will be on what is arguably the most important and least understood of those processes: magnetic reconnection.  

During magnetic reconnection, a phenomenon that requires a break-down of magnetohydrodynamic physics, magnetic fields from different sources interlink and release energy. It also allows mixing of the electrodynamically active plasmas from the two different sources. Near Earth, this mixing at the magnetospheric boundaries through which MMS travels is of plasmas from Earth space and interplanetary space.

During magnetic reconnection, particles are accelerated to potentially very high energies and previously confined plasmas are allowed, or even driven to expand into newly accessible spatial domains. Sometimes this occurs explosively -- as in the case of solar flares and auroral storms -- when reconnection suddenly releases large quantities of plasma previously magnetically confined under high pressure.

Exactly how magnetic reconnection proceeds is not well understood by the plasma physics community. With MMS, NASA uses near Earth space, where magnetic reconnection is known to occur, as a natural laboratory in which we can observe in great detail and therefore understand the underlying physics of the process throughout the universe—as well as in terrestrial laboratories studying nuclear fusion and plasma processes.  

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Learning from Commercial Analysis to Search Solar Data

08.11.2015   |  Lars Kristen Selberg Daldorff
Mankind has been fascinated by the sun, since the dawn of history. Records can be found of both worship and early forms of scientific study of it. Today we are no different. We still look up, we are still mystified, we are still trying to understand the Sun -- but we have evolved the tools we use to study it. These sophisticated new tools and equipment have introduced new types of challenges, they produce so much data that we are drowning in information without being able to transform it into insight and knowledge.

When physicists use large supercomputers to simulate the Sun, it produces massive amounts of data, but the phenomena of interest is usually located at a specific point in time and space, essentially creating a needle in a haystack situation. The large quantity of data has forced physicist to reduce the amounts, which they do by looking at small portions of the data at the time, making the process long and slow before any true insight can be found. But what if, you could scan the entire haystack at once to find the needle?

The commercial industry has developed many new tools to search, categorize and filter data, giving rise to the data warehouse industry. We combined state-of-the-art techniques from academic and commercial worlds—each side learning from the other and tackle these new challenges.

One of the tools increasingly used to study human behavior by many companies is different analytical methods that combine computational power and statistics turning information into insight. Our team, consisting of a numerical physicist at NASA (Lars K. S. Daldorff) and Data Scientist consultant (Siavoush Mohammadi) in Sweden, once fellow researchers -- combined experience and methods to examine magnetic loops on the Sun.

Each loop visible in Fig 1. is built up of a large number of magnetic field lines moving around and interacting, thereby releasing energy that can heat up the loop. We can make numerical models describing the behavior of the loop to find out what happens on small scales that we can not observe. But we need to find the places where the changes first happen -- the needle in the haystack. Traditionally we have done an interactive circular process, narrowing down the data until we have located the needle (figure 2). Instead, we now applied standardized method widely used in the business community -- like a decision tree to search and group the data and easily identify where the transitions exist. We replace a circular process as in figure 2 with the linear process described in (figure 3).

Not only can this method produce a quicker way to analyze the data, but it provided the increased benefits of showing us things we had not even thought about.

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Cluster Data Leads to First Ever Models of Generation of Equatorial Magnetosonic Waves

07.21.2015   |  Michael Balikhin
Earth’s magnetic field shields the planet from the solar wind -- the flow of electrons and ions that are constantly emitted from the Sun. The solar wind's interaction with the terrestrial magnetic field forms the magnetosphere around Earth. In this magnetic environment, plasma waves play a very important role: Advanced knowledge of the wave dynamics in the magnetosphere is central to understanding the dynamics of high-energy electrons in the environment and mitigating hazardous effects on spacecraft operation.

We used the ESA/NASA Cluster mission to study a kind of magnetospheric wave called equatorial magnetosonic waves (EMW). A remarkable data set of Cluster plasma measurements enabled us to understand and accurately model the generation mechanism of EMW for the first time.  The results were published in Nature Communications on July 14, 2015.

Matter in the magnetosphere is so rarefied that collisions between particles almost never happen. Instead, waves are the key to transferring information from one particle to the other. Without waves, no particle in the magnetosphere would ever experience the influence of the other particles present. The physics is constantly dynamic: Waves consume energy from some of the particles, grow in magnitude, and then transfer the energy to other particles.

EMW are one example of these crucial waves. They were discovered in the 1960s by NASA’s Orbiting Geophysical Observatories (OGO-3) spacecraft. The noisy structure of waves and their location near the geomagnetic equator gave them the name "equatorial noise." They are one of the most frequently observed emissions in the near-Earth space, being observed on approximately 60% of satellite passes through the geomagnetic equator.

Cluster was able to observe these waves directly in their generation region on July 6, 2013. Unlike the typical observations that show noise near the equator, these observations showed 13 clear lines on the spectrogram. This clear data allowed us to answer a long-standing question of how these waves are generated. 

The Cluster plasma measurements allowed us to observed the source particles for these waves and accurately model the generation of these waves. The model results were remarkably similar to the observations.  We have shown that these wave are generated by what's known as "proton ring distributions." These are distribution of protons in which the number of protons that possess higher velocity exceeds the number of those with lower velocities.

The solution to how EMW are generated advances the goal to develop a comprehensive model of energetic electrons dynamics in the magnetosphere and to eventually forecast their fluxes. Such a predictive model would provide spacecraft operators ample time to mitigate the effects of potentially-damaging space weather on spacecraft.

These results are part of a larger Cluster Inner Magnetopshere Campaign that seeks observations of key magnetopsheric waves -- EMW, Chorus and EMIC -- in their generation regions. The flying formation of the four Cluster spacecraft has been optimized to distinguish between the spatial and temporal variations in the wave field. For the period of the campaign, the interspacecraft separation for a pair of spacecraft was at times only a few kilometers apart -- the closest distance not only for previous 12 years of Cluster but for all previous magnetopsheric missions. 

This study clearly shows the importance of having multi-point measurements in the magnetosphere. NASA’s Heliophysics System Observatory, which includes in it’s fleet a a number of inner magnetospheric missions including Van Allen Probes, THEMIS, MMS, Cluster missions plays a crucial role in determining the physics behind magnetospheric waves – a feat that can only be accomplished with observations from numerous spots simultaneously.

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 Photo of Animated gif of a gigantic filament eruption.
 Photo of SDO image with overlaid data from RHESSI
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RHESSI Spots an Unexpected X-ray Flare In a Quiet Region of the Sun

06.03.2015   |  Adi Foord
On Sept. 29, 2013, a giant filament on the sun, stretching some 200,000 miles long across the northwest quadrant of the sun, erupted in a spectacular fashion, earning the nickname "Canyon of Fire." This eruption was also associated with a fast coronal mass ejection, or CME, which impacted Earth -- but only a fairly weak, C-level solar flare. We pinpointed the source of the X-rays to a smaller region using NASA's Reuven Ramaty High Energy Solar Spectroscope Imager, or RHESSI. NOAA's GOES soft X-ray imager confirmed the origin of the X-ray burst to be, indeed, in the quiet sun at the location of the erupting filament.

This is the first time RHESSI has identified an X-ray flare outside of an active region. Such observations lay the groundwork for better understanding of what events cause what kinds of eruptions – and better diagnosis of the power of the quiet sun.

Filaments lying outside solar active regions – termed "quiescent filaments" – are well known and are often much longer than the diameter of a typical active region. Despite their name, quiescent filaments do erupt and can be associated with a CME that can excite major geomagnetic storms. However, they never produce strong solar flares. Understanding the origin of solar magnetic activity and space weather requires understanding what types of sources on the sun trigger what kinds of events – that is, what is the connection between quiescent filament eruptions, filament eruptions from active regions, flares, and CMEs?

To explore the connections further, we examined imagery during the event from the Atmospheric Imaging Assembly on NASA's Solar Dynamics Observatory, or SDO. The AIA image shows two bright ribbons that formed below the original location of the filament and gradually spread apart as the filament erupted, a pattern of evolution typical of two-ribbon flares. The RHESSI X-ray source is located along part of the western ribbon. It followed the westward movement of the ribbon and also moved somewhat along its length, toward the north. There is an active region to the west of the southern end of the ribbons, but the RHESSI source is clearly associated with the ribbon of the erupted quiescent filament, not the active region.

Why is the flare located along only a small fraction of the extended filament? The answer appears to be a small dipolar region that emerged earlier in the day before the start of the eruption. It emerged below the filament about where the RHESSI source is later observed. The dipole field builds to a strength exceeding 1,000 Gauss. This field strength decays slowly during the filament eruption and decays more rapidly during the flare. Magnetic reconnection between this dipolar field and the filament arcade was likely the driver of enhanced local plasma heating responsible for the thermal bremsstrahlung emission from the hot plasma observed by RHESSI. Emerging flux regions such as this small dipole are frequently associated with quiescent filament eruptions and may be involved in triggering the eruption.

The event reveals the presence of intense energy release even in a quiet Sun, in the form of an otherwise innocuous-looking quiescent filament. The power of the CME that resulted is not associated with a large solar flare as often happens in an active-region event. Nevertheless, the eruptions produced high temperatures and X-rays. Additional associations of RHESSI X-ray emission with quiescent filament eruptions have been identified. Analysis of these events will provide valuable insights into the origin and evolution of quiescent filament eruptions and solar eruptive events in general.

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Catastrophes For Life From the Sun: Distant Past and Near Future

02.02.2015   |  Vladimir Airapetian
Understanding the evolution of Earth requires understanding its relationship to the sun. The early sun was an extremely magnetically active star and its space weather effects may well have had crucial effects for the origin and development of life on Earth. In this research, we explore how the magnetic activity of the sun -- in terms of its frequent and energetic coronal mass ejections, or CMEs – could have affected the atmosphere of early Earth and life on it.

It took over half a billion years for early life to develop on Earth after the first appearance of water on the planet about 4.3 billion years ago. We researched what could have affected this timing by assessing the role of the magnetic processes on the early sun. Specifically, we modeled the interaction of frequent and energetic CMEs with Earth's magnetosphere. Our research suggests that CMEs from the early sun continuously destroyed the sub-solar parts of Earth's magnetosphere at heights less than 1 Earth radius.

We used a sophisticated 3-dimensional magnetohydrodynamic model used for space weather forecasting, which is currently available at the Community Coordinated Modeling Center at NASA's Goddard Space Flight Center. We applied the model to the harshest conditions in early Earth. The model showed that the dipole magnetic field around Earth was severely affected by magnetic reconnection from the CMEs, forming "perfect storms" every day.

This level of solar storm would have caused erosion of Earth's atmosphere, the ozone layer, leading to lethal UV radiation passing from the sun to Earth's surface and to a greenhouse effect due to escape of carbon dioxide from the atmosphere. Our research provides a potential answer to what's known as the "faint young sun” paradox, in which the stellar evolution theory that the Sun was only 75% as bright 4 billion years ago as it is today doesn't jibe with the fact that the planet was warm enough to support liquid oceans. Specifically, our model shows that the direct heating from CME magnetic interaction as well as from the energetic protons produced in a CME associated shocks may have been enough to account for the warm conditions on early Earth. The model may also be consistent with recent findings -- from the sizes of tiny craters formed by water droplets over 2.7 billion years ago -- that early Earth's atmospheric pressure was around half the present value.

We are working with atmospheric scientists Domagal-Goldman and Alex Pavlov from NASA Goddard to model these conditions for early Earth. We also find that abundant energetic protons are capable of penetrating Earth's atmosphere and breaking atmospheric nitrogen -- the major ingredient of the early Earth’s atmosphere -- into atomic nitrogen. This process produces hydrogen cyanide, which is an essential molecule for producing prebiotic life chemistry including RNA molecules, the precursor of life. This raises an intriguing possibility that the frequent super-CMEs could also have been catalysts of the first life forms on Earth.

Not only does this research address important questions about the evolution of the early sun and our planet, but it may provide clues on the effects of future severe space weather on our Earth. The next step is to assess the consequences of such an event if it occurred today. Such information also helps us better understand the space weather system around planets like Earth elsewhere in the solar system.

Presented at the International Meeting "Toward Other Earths II", Sept 13-18, 2014, Porto, Portugal in collaboration with Alex Glocer (673).
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Mapping Earth's Magnetotail

01.26.2015   |  David Gary Sibeck
Like a windsock near a breezy airport runway, Earth’s magnetotail flaps back and forth in the gusty solar wind. Results from recent simulations help us understand how, and how quickly, the magnetotail responds to these solar wind variations, thereby resolving longstanding controversies and providing important clues to the nature of the solar wind-magnetosphere interaction. New models show that the newly reconnected field lines are laid down in place in the tail, with no subsequent realignment. This leads to a much quicker event than previously thought.

Earth’s magnetic field carves out a cavity, known as the magnetosphere, in the oncoming supersonic solar wind plasma. The solar wind-magnetosphere interaction stretches Earth’s magnetic field lines within this cavity into a long magnetotail extending many thousands of miles away from Earth in the anti-sunward direction. Theory indicates that the size, shape, structure and length of the magnetotail should depend upon solar wind conditions. Such conditions include both the solar wind plasma pressure and the geometry of its magnetic fields, namely the direction of the interplanetary magnetic field, or IMF. In particular, theory predicts that the forces applied by the IMF to the magnetotail should generally elongate the cross-section of the magnetotail in the east/west direction and flatten it in the north/south direction. However, models based on observations have yielded contradictory results: Previous studies using extensive but isolated ISEE-3 and Geotail spacecraft data included circular north/south, and east/west elongated cross-sections.

To resolve disputes concerning the nature of Earth’s distant magnetotail, we ran the University of Michigan’s BATS-R-US global magnetohydrodynamic model for the interaction of the solar wind with Earth’s magnetosphere at Goddard Space Flight Center’s Community Coordinated Modeling Center. We ran the simulation with enhanced spatial and temporal resolutions at lunar distances and examined the magnetotail's response to abrupt variations in the IMF orientation. Within 15-20 minutes of a change in the IMF orientation, the magnetotail flattened in precisely the manner predicted by theory.

If so, why did previous research come to such different conclusions? The answer may lie in the different magnetotail identification criteria employed. The model predicts very gradual transitions in plasma and magnetic field parameters at the eastern and western edges of the magnetotail, but sharp transitions at the northern and southern edges. Consequently, the east/west dimensions depend sensitively upon the criteria used for magnetotail identification: more liberal criteria result in greater extents (and east/west elongated extents), stricter criteria in lesser extents (and either circular or north/south elongate magnetotails).

In addition to resolving a longstanding question concerning the shape of the distant magnetotail, the study demonstrates that its response to solar wind variations is very rapid. Since squeezing the magnetotail into new configurations would take one or more hours, the 10-15 minute response must involve some other, faster, process. The results indicate that this process is magnetic reconnection at the dayside magnetopause. The solar wind flow sweeps newly reconnected interplanetary and magnetospheric magnetic field lines antisunward, and simply deposits them immediately in magnetotail locations that depend upon the IMF orientation. Consequently, future work on the size, shape, and structure of the magnetotail may also provide important clues as to the nature of reconnection far away, on the dayside magnetopause.

Citation: Sibeck, D. G. and R.-Q. Lin, Size and shape of the distant magnetotail, J. Geophys. Res., 119, 1028-1043, 2014
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 Photo of F/A-18 approaching the sound barrier.
 Photo of Earth's Magnetic Bubble
 Photo of Artist's concept of THEMIS in orbit
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Energy Dissipation in Collisionless Shocks

01.19.2015   |  Lynn B Wilson III
In the 1960s, scientists discovered a new kind of shock wave that traveled through space plasmas that did not rely upon collisions. Thus, they are known as a collisionless shock waves. These shocks are of great interest in multiple fields of research: they can produce radiation that can negatively impact commercial and military spacecraft operation, as well as the safety of humans in space.

But the mechanisms allowing these shocks to form has been a topic of great debate. Our recent work helps resolve some of these issues by confirming theories that predict how small-scale phenomena can control large-scale dynamics. These small-scale processes, on the order of tens of meters, can regulate structures at scales more than 1 million meters across.

Common shock waves – such as those at the front of a supersonic jet -- occur when an obstacle moves faster than the speed of sound, that is, the speed of a compression wave through a fluid. Such shocks transform the bulk flow from supersonic to subsonic in a thin transition region called the shock ramp, where the lost kinetic energy is converted into heat. With common shock waves, this conversion occurs mostly through binary particle collisions, much like billiard balls colliding and recoiling. These collisions occur over a short distance called the particle mean free path, which, in Earth's atmosphere, is around a micrometer.

In the magnetized bow shock in front of Earth, however, the particle mean free path in the solar wind can be as large as one astronomical unit, that is, the distance between the Earth and the sun or around 150 billion meters. On the other hand, the magnetized shock ramps were found to be less than one-millionth that size. There was no way these shocks could rely upon particle collisions taking place over billions of meters -- thus the name collisionless shock wave. But how could such shocks transform the incident bulk flow into heat in such short distances without collisions?

Four primary mechanisms were proposed. Three of the theories, however, were proposed as steps that, in fact, created a fourth process named anomalous resistivity. Our study focuses on the fourth mechanism, which occurs so quickly that it could only recently be tested using the high cadence field and particle measurements of the THEMIS spacecraft.

The theory for the fourth mechanism arose by recognizing that electromagnetic waves could impede the relative velocities between electrons and ions (i.e., electric currents) that give rise to magnetic shock ramps. These waves can transfer momentum to charged particles by scattering them off their original trajectories and/or acting as an effective drag force, slowing the bulk flows that produce electric currents. This extra resistance gave rise to the name anomalous resistivity. More recently, however, the more generic term wave-particle interactions has been used, as not all of these interactions affect the electric currents.

Since wave-particle interactions occur much faster than particle instruments can currently measure, testing their relative importance poses a problem. We found a way around this issue by estimating the electric currents in collisionless shock ramps by using a combination of magnetic field measurements and the laws governing electromagnetism --Maxwell’s equations. We could then estimate the energy dissipation rate due to wave-particle interactions. The final step in the analysis involved estimating the total energy dissipation rate needed to maintain a stable shock on large-scales.

The results were surprising. First, we discovered that not only are small-scale waves ubiquitous in collisionless shock waves -- but they can be huge. In some cases, the wave amplitudes were so large, they contained as much energy density as is necessary to produce an aurora. Second, we found that the energy dissipation rates due to wave-particle interactions were also very large. So large, in fact, that they could exceed the large-scale dissipation rates by over 10,000 times. In other words, the wave-particle interactions need only be ~0.01% efficient and they could still regulate the large-scale structure of the shock.

This information about waves near Earth can also be extrapolated to inaccessible regions of space. We found that similar processes could provide enough energy to explain the heating of the solar corona, magnetic reconnection rates, and have implications for particle heating and acceleration around stars elsewhere in the universe. These results quantitatively show, for the first time, that small-scale phenomena can control the large-scale dynamics in collisionless plasmas.

Citation: Wilson et al., in the Journal of Geophysical Research Vol. 119, with titles “Quantified energy dissipation rates in the terrestrial bow shock: 1. Analysis techniques and methodology” and “Quantified energy dissipation rates in the terrestrial bow shock: 2. Waves and dissipation.”
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 Photo of Illustration of Venus Hot Flow Anomalies
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Effects of Space Weather Explosions at Venus

01.12.2015   |  Glyn Alexander Collinson
When the solar wind blows hard enough, the space in front of a planetary bow shock essentially, well, explodes. These are called “hot flow anomalies," and they happen commonly on the outside of Earth’s magnetic bubble, called the magnetosphere. A couple of years ago, I went looking for, and found them for the first time at Venus with data from the ESA Venus Express. Since then I’ve found more, and discovered that not only do they happen multiple times every (Earth) day, but they are so big that they could swallow the planet whole.

One of the biggest mysteries about Venus is why is it so barren and inhospitable. By rights, the surface temperature should only be about 60 degrees C (140 F), but due to its thick carbon dioxide atmosphere and runaway green house effect, it’s actually closer to 500 degrees C (900 F). The atmosphere is in-fact so thick that the lifetime of landers is measured in hours before they are crushed. Another important difference between the two planets is that Earth has a magnetic field, and Venus does not.

Venus is, therefore, a natural laboratory for asking the big “what if” questions about Earth. What happened differently at Earth to make it into the life-supporting planet it is today? What would Earth be like without its magnetic field?

My work answers the question of what effect a kind of space weather called a hot flow anomaly can have on a planet when that protective magnetic field is missing. Hot flow anomalies are hard to detect, but they can nonetheless cause dramatic, planet-scale disruptions. Earth’s magnetosphere projects a magnetic bubble, about ten times larger than our planet, which deflects the solar wind and provides a buffer against hot flow anomalies. At Earth, the anomalies form and rage, releasing so much energy that the solar wind is deflected, but always our magnetic field holds them at arms length.

Without a magnetosphere, what happens at Venus is very different. The only protection Venus has against the solar wind is the charged outer layer of its atmosphere called the ionosphere. A sensitive pressure balance exists between the ionosphere and the solar wind, a balance easily disrupted by the giant energy rush of a hot flow anomaly.

At Earth, hot flow anomalies make our entire magnetic field shudder. At Venus, where they happen right on top of the ionosphere -- and are as large as the planet itself -- just imagine what kind of trouble they could cause. While it is going to take future studies to pin this down, hot flow anomalies have the potential to create dramatic, planet-scale disruptions, possibly sucking the ionosphere up and away from the surface of the planet.
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 Photo of A massive coronal hole observed by Skylab
 Photo of A view of a southern polar coronal hole from Jan. 3, 2015.
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A Casual Introduction to Coronal Holes

01.07.2015   |  Michael S Kirk

Coronal holes are large, ubiquitous, dark features observed in the solar corona, but why should we study them?

Coronal holes are the primary source of the fast component of the solar wind that emanates from the sun and eventually envelopes Earth as well as the rest of the solar system. Understanding the origin of the solar wind is critical in producing accurate space weather forecasts for our satellites, our astronauts, and our planet.

In addition, current models of the solar wind and space environment are highly sensitive to the size and location of the holes on the disk – small changes in the measured physical parameters propagate to large differences in the heliosphere.

The Problems With Defining and Measuring Coronal Holes:

With no data to definitively link the theoretical model of a coronal hole to solar observations, there is no model confirmation, and therefore no consensus in the scientific community as to how to precisely define where coronal holes begin and end.

Specifically, the term “coronal hole” simultaneously refers to three distinct phenomena. These perceptions of coronal holes are causally connected, yet a one-to-one mapping of features between them has never been accomplished. First, the dark patches that appear in coronal x-ray and ultraviolet images are called coronal holes (just to confuse things, these same regions appear bright in the He I 10830 Å triplet). Second, the lowest intensity regions observed outside the limb of the sun are called coronal holes; these regions were first observed during natural (i.e., the sun obscured by the Moon) and artificial (i.e, coronagraphs) eclipses. Third, the term also has a theoretical definition, in which coronal holes are described by open magnetic field lines extending from the solar surface outward into space (the precise length needed for a field line to be considered “open” is up for debate). All three definitions describe regions of low plasma density occurring in the corona where ionized atoms and electrons are free to flow from the sun's surface outward into space along magnetic field lines.

Coronal holes are hard to define on the solar surface as well. Mapping the shape of coronal holes is kind of like trying to measure the edge of a cloud – it is easy to tell what is definitely a hole and what definitely isn’t, but the boundary is fuzzy and different observers will mark the edge differently. To complicate things further, coronal holes appear differently in different wavelengths. The size, shape, and darkness of any given hole are not consistent between separate observing filters. The theoretical definition doesn’t help much either, since it is dependent on coronal magnetic fields and we currently have no way to independently measure the intensity and location of coronal magnetic fields.

Observations of Holes:

Both qualitative and quantitative descriptions of coronal holes have been documented as far back as the early 1900s during solar eclipses. They were first noticed as rays of light emanating from the solar poles, which appeared similar to the magnetic field lines of a bar magnet. Imaging coronal holes on the disk of the sun began early in the space age: In the late 1960s and early 1970s, they could be seen as isolated dark patches in UV and x-ray measurements. The definition of these dark patches was refined to be what we know now as coronal holes by solar instruments on Skylab in 1973 and 1974. The Skylab data and subsequent missions helped to define characteristics of these holes. They appear as dark caps in both polar regions of the sun and as large, often elongated, expanses at mid-latitude and equatorial regions.

Coronal holes are some of the most persistent features on the sun. The mid-latitude and equatorial holes typically last between a couple of months to nearly a year. Polar coronal holes endure for several years at a time, only disappearing completely at solar maximum for a year or so.

Improving Our Current Understanding:

Several compelling local and global models simulating coronal holes and their evolution currently exist using both pure theoretical as well as data driven approaches. However, the best models can reproduce the general size and shape of the holes but lack some of the hallmark features consistently observed. Specifically, the models lack the ambiguity in the boundaries observed between the quiet sun and coronal hole.

Models also currently do not describe small-scale bright loops that frequently occur in the middle of an otherwise unambiguous hole. We are working to incorporate these loops into physical models with a more nuanced treatment of the magneto-hydrodynamics at small scales. Conversely, to understand the boundary that separates a coronal hole from the rest of the sun, there is a fundamental outstanding question of the magnetic stability of a hole. Clearly the holes are magnetically stable enough to persist in a chaotic solar environment for several rotations, however the edge of a hole, unlike a sunspot, is poorly defined and could be interpreted as an evolving boundary layer rather than a hard edge.

How will we resolve these outstanding problems?

Like almost every other outstanding issue in observational science, more data is needed to resolve the problems with distinguishing and simulating coronal holes. Ultimately, we need coronal and chromospheric vector magnetic field measurements to link the off-limb regions to dark patches on the disk, and to models of coronal holes.

Two ground based observatories will get us one step closer to these measurements: ChroMag, and DKIST. The Chromosphere and Prominence Magnetometer (ChroMag) from NCAR is just beginning to take consistent data with the promise of “providing the community with synoptic global observations of the chromospheric vector field.” The Daniel K. Inouye Solar Telescope (DKIST) is expected to image the sun in unprecedented detail beginning in 2019. Among the expected capabilities of DKIST’s instrument suite is the ability to image the off-limb corona with enough signal to measure the emerging coronal magnetic fields. The combination of both of these observatories with our current EUV imaging satellites will hopefully resolve the ambiguities in coronal holes and ultimately give us the ability to forecast the effects of coronal holes on our planet.

For a less casual introduction to coronal holes, I would recommend: Steven R. Cranmer, "Coronal Holes", Living Rev. Solar Phys. 6, (2009), http://www.livingreviews.org/lrsp-2009-3

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