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Research on magnetic-flux eruptions from the sunExploring atmospheric, Earth system, and solar processes, and the variability and change of these processes, are critical components to reaching NCAR's strategic goal #1, “Improve understanding of the atmosphere, the Earth system, and the Sun.” The Earth and Sun System Laboratory (ESSL), with partnerships with the universities and other agencies, has the scientific knowledge to attack the problems associated with attaining this goal. Exploration into these areas will focus on three key activities: simulating the natural Earth system variability, researching the magnetic-flux eruptions from the sun, and investigating the coupling between the upper troposphere and lower stratosphere (including gravity waves). For each of these areas, ESSL has numerous activities underway and they are highlighted below. Note the special laboratory highlights on paleoclimate which offers a remarkably fertile ground for testing climate models and the underlying parameterizations and on the recovery and usage of the HIRDLS satellite as well as the discovery of a way to predict the next solar cycle using the memory term linked to the meridional circulation in the Sun. Furthermore, this priority has two highlights at the level of NCAR, namely the research on Space Weather and that on the building of a coronal magnetometer which allows for the first time a direct measurement of the magnetic field in the solar corona. Predictive flux-transport dynamo developments - DISCOVERY
HAO
Predictive flux-transport dynamo developments
Over the past 50 years, many attempts to predict important properties of the next solar been made, including its amplitude, duration, and aspects of the patterns of solar activity that are produced, such as so-called `active longitudes.' These efforts have met with, at best, modest and uncertain success. There is strong motivation to achieve reliable predictions, both to gain deeper understanding of solar magnetism, and to forecast with skill the impacts of solar activity on the Earth. Communications, power transmission, and many other industries and activities are sensitive to the level and type of solar activity. Progress in solar cycle and magnetic activity simulations and predictions supports two strategic priorities of NCAR and ESSL: (i) working towards a comprehensive understanding of the Sun and the sources and manifestations of solar activity, and (ii) working towards a comprehensive understanding of solar influences on the Earth system. This ongoing project involves the development and exploitation of both axisymmetric and nonaxisymmetric, physics-based, predictive models of solar cycle properties. The models are intialized with and constrained by observations of both solar flow fields (the differetial rotation and meridional circulation) and solar magnetic fields (sunspot areas and `butterfly' diagrams, and synoptic magnetograms). Work on the project began in about 2000 when HAO/NCAR scientist Dr. Mausumi Dikpati concluded that the use of such models to make solar cycle predictions was feasible. Actual predictions of solar cycle timing, including the influence of meridional circulation variations in the Sun, were made in FY2004, and the first predictions of solar cycle amplitudes and the evolution of active longitudes were made in FY2006. In project plans for FY2005, it was decided that the first predictions of both the solar cycle amplitude and the evolution of active longitudes would be completed and announced in FY2006; this goal was achieved, with the publication of results for cycle amplitudes in 2006 (GRL, 33, L05102 and ApJ, 649, 498) and active longitudes in 2005 (ApJ, 635, L193). With respect to the cycle amplitudes, the relative peaks of the past 8 solar cycles were successfully simulated, and it was forecast that the peak of the upcoming solar cycle would be 30 - 50% higher than the current cycle 23. This would make cycle 24 on of the strongest cycles on record, the strongest since the dawn of the `space age' in the 1960s. With respect to active longitudes, it was shown that a simple forward propagation of unstable global MHD modes in the solar tachocline can track the evolving positions of solar active regions and active longitudes for at least a year during the rising phase of the current solar cycle. The solar cycle 24 prediction is being used by both U.S. and EU cycle 24 prediction panels to advise NASA, ESA, and industry on what to expect for solar activity and terrestrial response levels over the next decade. The results of the project were publicized in a joint NASA/NSF/UCAR telecon in March 2006, attended by approximately 35 members of the print and broadcast media. At least 200 news-websites subsequently featured articles on the results. During FY2007 and FY2008, efforts will be directed toward accomplishing the following tasks as part of this project: (i) simulating and predicting solar activity peaks separately for the North and South hemispheres of the Sun; (ii) testing predictive skill of the flux-transport dynamo model for various treatments of the properties of past solar cycles and for variations in the meridional circulation; (iii) determining the skill of predictions for solar cycles more than one cycle ahead; (iv) developing and exploiting nonlinear hydrodynamic and magnetohydrodynamic models of the solar tachocline for the purpose of building and testing a prediction system for active longitudes; and (v) continuing developmental work on a nonaxisymmetric flux transport dynamo model, to simulate and predict solar cycles and active longitudes together. It is expected that several new findings will be reported in publications, and multiple forecasts for future cycles will be announced. These results are expected to be used by NASA for mission planning, and by solar activity-sensitive industries. Forecasts of future solar cycles will have impacts on both government and industry, as well as on the development of climate modeling scenarios for the 21st century. This project has been supported by the NSF as well as by NASA Living with a Star funds. Space weather
The NCAR program in space weather, led by scientists in the ESSL High Altitude Observatory, seeks to understand and ultimately predict disturbances in near-Earth space and the upper atmosphere that are caused by solar events and variability. This is a complex activity because of the range of time scales involved, from minutes to decades, and the variety of phenomena, including the solar magnetic cycle, eruption of magnetic flux in active regions and sunspots, coronal mass ejections, solar ultraviolet irradiance, flares, solar energetic particles, the interplanetary magnetic field, radiation belt dynamics, geomagnetic storms, ionospheric disruptions, and changes in upper atmosphere density, composition, and chemistry. Some of these are relatively well understood and can be described using numerical models of the space environment; in other areas fundamental advances in understanding the physical processes involved are needed. Understanding space weather is important for human space flight, especially beyond Earth orbit, because of the high-energy radiation hazards. It is also a key factor in satellite design and tracking because of the effects of radiation on computer circuits and power systems, and because changes in solar ultraviolet irradiance and geomagnetic disturbances during the 11-year solar cycle alter the density of the thermosphere and ionosphere, which perturbs satellite orbits. Ionospheric variations during geomagnetic storms disrupt communications and navigation systems, which, together with increased radiation danger, are particularly important for aviation on polar routes, where protection from the magnetosphere is less and the effects of auroral activity are greatest. Acquiring a predictive capability for the most severe events, even if with short lead-time and approximate specificity, is a primary goal of the national space weather program. Recent progress in space weather prediction and analysis has been accomplished at NCAR at the extremes of the applicable time scales. An important new advance in understanding the 11-year solar cycle and predicting its future magnitude was made by Dikpati et al., who developed a flux-transport model of the solar interior dynamo to describe the transport and re-cycling of magnetic flux that causes the seemingly random variation of the cyclical intensity. These predictions are now being used to drive calculations of future thermospheric density, superimposed on the gradual cooling and contraction of the upper atmosphere caused by increasing CO2 levels. At the other end of the time spectrum, detecting the precursors and modeling the processes involved in sudden eruptions of magnetic flux into coronal mass ejections has been advanced by Fan and Gibson using 3D magnetohydrodynamic simulations of twisted flux tubes emerging into the corona. Projecting these events through the solar wind remains a challenging problem, but since the solar wind and the interplanetary magnetic field are monitored by spacecraft upstream of the magnetosphere, it is possible to measure the resulting perturbations 30 to 60 minutes before they arrive at the Earth. The resulting geomagnetic storms and thermosphere/ionosphere disturbances are described using coupled models developed by collaborative efforts between NCAR and university researchers. Future developments, allowing solar observations to drive realistic models of the corona and solar wind that can be used as input to geospace models, will require better understanding and measurement of the behavior of the solar magnetic field in the corona, particularly during coronal mass ejections, as it drives the interplanetary magnetic field and solar wind. To address these problems, solar physicists at NCAR, the University of Hawaii, and the University of Michigan, working with the HAO instrumentation group, are designing a new large coronagraph, the Coronal Solar Magnetism Observatory (COSMO). NCAR is a partner in the Center for Integrated Space weather Modeling
(CISM), a NSF Science and Technology Center led by Prof. W. Jeffrey Hughes
at Boston University, and also supports several strategic partnerships
in coupled model development for the NASA Living-with-a-Star program.
Scientists in the HAO Atmosphere-Ionosphere-Magnetosphere section developed
the Coupled Magnetosphere Ionosphere Thermosphere (CMIT) model, beginning
in 2003, working with researchers at Dartmouth College, Boston University,
the University of Maryland, and Rice University. This model couples the
Lyon-Fedder-Mobarry global MHD magnetospheric model with the NCAR Thermosphere-Ionosphere-Electrodynamics
General Circulation Model (TIE-GCM) to provide thermosphere-ionosphere
system with high time resolution particle fluxes and ion drift paths,
and to specify realistic conductivity and neutral-wind-driven current
feedback to the magnetosphere. This model forms one of the core components
of the physics-based numerical modeling chain being developed by the CISM
program. It will be made available to the geoscience community through
a collaboration with the Community Coordinated Modeling Center at NASA
Goddard. Coronal magnetometry (CoMP, and COSMO)
The solar corona is the seat of much of the most important and dramatic activity exhibited by our star. Coronal magnetic fields are sufficiently strong to dominate the force balance, and the free magnetic energy in coronal current systems is believed to be responsible for driving the heating and dynamics of the corona. This heating is both quiescent and explosive in nature. Under flaring conditions, explosive plasma heating leads to large perturbations in solar radiation, most obvious in bursts of radio, EUV, X-ray, and gamma-ray emission. Coronal mass ejections, frequently associated with large flares, involve the sudden release of magnetic energy stored in the corona, with large volumes and masses of coronal plasma ejected into space from previously magnetically closed regions. The corona is also responsible for the acceleration of high- energy particles. Collectively, these phenomena, known as space weather, are responsible for driving the space environment and its evolution at Earth. Space weather poses hazards to astronauts and adversely affects spacecraft, GPS systems, high frequency communications, power grids and other vulnerable technologies on Earth. Driven by society's need to understand the origins of space weather, NCAR scientists at the High Altitude Observatory, along with colleagues at the University of Hawaii and the University of Michigan, plan to build the Coronal Solar Magnetism Observatory (COSMO). By providing unique, synoptic observations of the global coronal magnetic field, COSMO will complement and enhance major projects of importance to the National Space Weather Program and NASA's Living With a Star program, notably ATST, SDO, Solar-B, SOLIS, and STEREO. The dominance of the coronal magnetic field has been known for decades, yet until recently, magnetic fields in the corona have been extremely difficult to measure under typical coronal conditions. The COSMO facility will remedy this situation by obtaining routine synoptic observations of coronal magnetic fields with a 1-meter class coronagraph and associated instrumentation. The concepts underlying COSMO have been developed over the past four years through the support of NCAR's Strategic Initiative on Coronal Magnetism and HAO base funding. This has allowed the parallel development of theoretical and instrumental tools with which to advance the state of observation and interpretation of coronal magnetism. Instrument development efforts have concentrated on the design and construction of a prototype instrument capable of inferring the properties of coronal magnetic fields through the observation of the polarization of forbidden coronal emission lines at infrared wavelengths. This instrument is called the Coronal Multi-channel Polarimeter (CoMP). Measurement of the circular polarization due to the longitudinal Zeeman effect yields information on the strength of the line-of-sight component of the magnetic field while the observation of linear polarization due to resonance scattering constrains the plane-of-sky field direction. Line-of-sight velocity is obtained through measurement of the Doppler effect, and the ratio of emission line intensities provides a diagnostic of plasma density. HAO/NCAR researchers have started taking exciting observations using the CoMP instrument at the 20-cm aperture coronagraph operated by the National Solar Observatory at Sacramento Peak, New Mexico; an example of the observations is shown in the first of the accompanying figures. To prepare for the exploitation of these new observations, HAO/NCAR scientists have begun to develop tools and techniques for comparing forward models of coronal magnetic fields with the anticipated observations. The second figure shows a simulation of an axisymmetric equatorial current sheet embedded in the Sun's corona, as it would appear if observed in the emission line of twelve-times ionized iron (Fe XIII) with wavelength 1074.7 nanometers. The current sheet is derived from the analytical model of developed by Low, Fong and Fan (2003, ApJ, 594, 1060). To obtain these results, the plasma emissivity was integrated along the line of sight, assuming an optically thin, spherically symmetric, hydrostatic atmosphere with temperature 1.6 million degrees Kelvin. The strength of the linear and circular polarization is shown for two cases; the top two panels are for a current sheet with sufficient magnetic free energy to launch a CME, while the bottom two panels show the case of a weaker current sheet with insufficient energy for CME launch. Both cases have an identical value of the radial component of the magnetic field at the solar surface. Note, however, the polarization signatures are extremely different for the two cases. This example illustrates the sensitivity of measurements of the polarization of coronal emission lines to the degree of magnetic free energy. Such differences in linear and circular polarization signals will be measurable by COSMO over scientifically interesting spatial and temporal scales. In FY2006, significant COSMO milestones were 1) the completion of a white paper outlining the scientific motivations for the COSMO facility, 2) a presentation on COSMO to the NSF/ATM on June 30, 3) the formation and organization of a seven-member Scientific Advisory Panel to forge close connections between the project and the solar and heliospheric communities, and 4) the organization of a workshop dedicated to specifying the scientific requirements of the COSMO facility, held in Boulder on August 22-23. The major activity for the COSMO project in FY2007 will be the submission of a proposal to the NSF/ATM Mid-Range Infrastructure account. Engineering studies are currently underway to develop designs for the COSMO coronagraph and associated instrumentation which will form the basis for estimates of cost and schedule. Also, an ongoing evaluation of the sky conditions at two candidate sites in Hawaii will continue. Simulations of magnetic flux emergence/CME initiation
Coronal mass ejections (CMEs) are arguably the most important solar drivers of space weather. A CME is a spontaneous eruption of closed magnetic fields previously in equilibrium. It results in an explosive release of the free magnetic energy stored in the pre-eruption twisted magnetic fields. The storage and release of magnetic energy in a CME thus critically depends upon the form of these pre- eruption magnetic fields. HAO/NCAR researchers continue to study the structure and evolution of magnetic configurations capable of driving CMEs, using 3D magnetohydrodynamic (MHD) numerical simulations to model twisted magnetic flux tubes emerging into the corona. As the tube emerges, the coronal field evolves quasistatically through increasingly energized equilibria, until a magnetic twist threshold is crossed, leading to loss of equilibrium and eruption. Numerical simulations of evolving coronal magnetic configurations are performed in an effort to (1) understand how CMEs are triggered, and (2) compare the structures, both in equilibrium and eruption, to observations. Studies of the mechanisms responsible for energetic ejections of plasma and magnetic fields from the Sun support NCAR's efforts to improve understanding of the solar origins of space weather and the impacts of these events on the space environment and upper atmosphere of the Earth. By experimenting with an idealized configuration of a twisted magnetic flux tube emerging into a pre-existing coronal potential arcade field, such simulations have revealed two mechanisms that lead to loss of equlibrium and eruption of the twisted magnetic fields (see highlighted image). In one case, the overlying arcade field declines with height sufficiently slowly such that the emerging flux tube remains confined until a high amount of magnetic twist is built up and the flux tube becomes significantly kinked. The kinking motion causes rotation of the tube to an orientation that makes it easier for it to rupture through the arcade field, leading to an eruption in a localized region, with most of the arcade field remaining closed. In the second case, the overlying field declines more rapidly with height so that the emerging flux tube loses equilibrium via the toroidal instability before it becomes significantly kinked. In this case, the onset of acceleration begins when the tube axis is still nearly planar. The two different mechanisms entail different properties for the motions of the erupting prominences and result in different post-eruption states. The observable properties of prominences and associated coronal structures have been examined and compared with the predictions of the simulations, before, during, and after eruption. It is found that the simulation of a twisted flux tube in equilibrium reproduces the general structure and dynamics of a quiescent (non-eruptive) prominence, along with its relationship to a coronal cavity and soft X-ray sigmoid. Furthermore, the eruption of the kinked tube results in its partial ejection, providing physical explanations for many phenomena observed during CMEs (see image below): partially erupting filaments, X-ray sigmoids which transition to cusps and then back to sigmoids, and the bodily eruption of and reformation of coronal cavities.
Mauna Loa Solar Observatory Studies (MLSO)
of
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| Above is an image of the solar
corona taken from the Mauna Loa Solar Observatory MK4 K-Coronameter
on July 22, 2002. A coronal cavity (circled in red) was observed for
a few days prior to this image and was seen to erupt the following
day. Cavities are often observed as stable structures in the solar
atmosphere but eventually lose equilibrium or become unstable. High resolution figure |
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| The above movie was taken by the
MK4 K-Coronameter on Dec 14, 2004 recording a coronal cavity erupting
directly into a CME (visible in upper left portion of movie). Coronal
cavities are consistent with models of magnetic flux ropes which can
contain sufficient magnetic energy to drive CMEs (see movie
by Yuhong Fan). Cavity observations provide important information
on coronal conditions that lead to the formation of solar activity
such as CMEs and prominence eruptions. High resolution figure |
A major research goal of ESSL is to understand the Sun's continuous and dynamic release of plasma and energy and to ultimately predict its impacts on the interplanetary environment. To support this goal, HAO, a division of ESSL, operates the Mauna Loa Solar Observatory (MLSO) on the island of Hawaii. MLSO instruments routinely record unique images of the solar chromosphere (in the neutral hydrogen alpha and neutral helium 1083 nanometer lines) and low corona every 3 minutes for approximately 9 hours/day (weather permitting). These data are essential for detecting the formation and occurrence of solar activity such as Coronal Mass Ejections (CMEs), prominence eruptions, flares, and related phenomena. MLSO has been in operation since 1965, providing the longest extant record of changes in the density structure of the solar corona over multiple, 11-year solar cycles. MLSO data are available via the internet at: http://mlso.hao.ucar.edu.
Observations of the lower solar atmosphere, such as those acquired at MLSO, are needed to understand the heating of the solar corona, the continual outflow of material that comprises the solar wind, and the causes of solar activity and space weather. Conditions in the solar atmosphere vary dramatically from those that exist in the solar photosphere. The million-degree coronal plasma is highly electrically conducting and `frozen' onto magnetic field lines. Unlike the solar photosphere, the energy of the low corona is dominated by the magnetic field, which organizes the corona into magnetically `closed' and `open' regions that are the sources of the solar wind. The solar wind ultimately determines the conditions in interplanetary space at Earth and throughout the solar system. The Earth's magnetic field and associated current systems are continually reacting to changing conditions in the solar wind, driven by processes occurring at the Sun. During periods of high solar activity, highly energized particles, accelerated by CMEs and flares can stream toward Earth and pose hazards to astronauts and satellites. CMEs can generate severe geomagnetic storms, which can damage power grids, satellites and affect GPS and other important navigation and communication systems. Understanding the causes of such magnetic activity in the Sun's corona, the propagation of the disturbances it produces through interplanetary space, and the occurrence and impacts of associated space weather events on the Earth's magnetosphere and upper atmosphere are among the highest scientific priorities of HAO, ESSL, and NCAR.
Recent observations using the instruments comprising the Advance Coronal Observing System (ACOS) at MLSO have yielded valuable insights concerning the coronal conditions that prevail prior to the initiation of a CME, the phenomena related to the occurrence of a CME, and the acceleration of CMEs in the low corona following eruption. For example, observations with the Mark 4 Coronameter have revealed that coronal cavities are ubiquitous, stable features in the low corona that erupt as part of a CME. Such cavities are consistent with the presence of magnetic flux ropes in pre-CME magnetic configurations, and observations of them therefore provide information about the state of corona prior to the occurrence of a CME. MLSO neutral helium observations show that transient coronal holes are also associated with CMEs. These features form during the impulsive phase of solar flares and are co-spatial and co-temporal with transient coronal holes observed in extreme ultraviolet light, suggesting that they are a manifestation of the decrease in coronal density that results from the occurrence of a CME. Similarly, MLSO neutral helium observations of chromospheric waves, also connected with CMEs and flares, demonstrate that they are co-spatial with waves observed in the extreme ultraviolet. Multiple waves are observed during each CME event, an indication that a slow-mode wave compression in the chromosphere is followed by a slow-mode wave rarefaction, and that multiple drivers (e.g. CMEs, and flares) may be present for a given event. MLSO observations have furthermore shown that nearly all (80 to 90%) CMEs are linked with an active or erupting prominence and that CME acceleration peaks in the low corona (below 3 solar radii). Moreover, the CMEs with the largest accelerations (and highest speeds) are found to be strongly associated with solar energetic particle events, while the prominence accelerations appear to be correlated with the speed of the overlying CME. In addition to continued daily monitoring of the physical state of the low corona, future activities will include support of efforts to develop the COronal Solar Mangetism Observatory (COSMO), a community facility for studying the magnetic structure and dynamics of the outer solar atmosphere. This facility will house the next generation of coronal instrumentation, and will make possible coronal and prominence magnetic field measurements that will further understanding of the coronal responses to changes in photospheric magnetic flux on both short and long time scales.
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| This scientific visualization
shows results from the CMIT model depicting the impact of a shock
at the head of a coronal mass ejection on the magnetosphere. The visualization
clearly shows the inward motion of the magnetopause and the compression
of the magnetosphere following the arrival of the shock (modeled from
solar wind observations), as well as effects throughout the coupled
magnetosphere-ionosphere-thermosphere system. Accurate modeling of
events of this kind is essential for making reliable space weather
predictions. High resolution figure |
Scientists in the Atmosphere - Ionosphere - Magnetosphere (AIM) section of HAO/NCAR, working with members of the Center for Integrated Space Weather Modeling (CISM), have developed the Coupled Magnetosphere Ionosphere Thermosphere (CMIT) model, a project that began in 2003. Basically, this model couples the Lyon - Fedder - Mobarry global MHD magnetospheric (LFM) model with the Thermosphere Ionosphere Nested Grid (TING) model to provide the magnetosphere with a more sophisticated conductivity model and the thermosphere-ionosphere systems with high time resolution particles fluxes and ion drift paths.
This model forms one of the core components of the physics-based numerical modeling chain that is being developed by the CISM program. This chain includes models cover the origination of the space weather drivers in the solar corona, their propagation through interplanetary space, and their ultimate impact on geospace. The CMIT model and the work related to it are important components of NCAR efforts to develop a comprehensive model of the solar-terrestrial system. CMIT can be used to investigate the magnetospheric impacts of disturbances in the interplanetary medium, for example, the motion of the magnetopause and enhancements of the radiation belts, as well as their ionospheric impacts, including heating of the neutral atmosphere, changes in NmF2, and other phenomena. All of these effects have impacts on our space and ground-based technology systems and are key components of space weather predictions.
The CMIT team has begun the process of making the model available to the geoscience community through a collaboration with the Community Coordinated Modeling Center (CCMC) at NASA Goddard. This collaboration will allow scientists to complete runs of the model on demand, and to work with NCAR personnel in analyzing and utilizing the results. In addition, numerous improvements to the model are planned. On the ionospheric side, the group's framework technology will be used to replace the extant ionosphere-thermosphere model with the HAO/NCAR Thermosphere-Ionosphere-Electrodynamics General-Circulation Model (TIE-GCM) in order to include the effects of the equatorial electrojet and improve numerical resolution. The magnetospheric enhancements will include utilization of an MPI-based parallel version of the code for improved resolution and the inclusion of an inner magnetosphere model for a better description of the important region 2 current systems.
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| The left panel shows time evolution
of magnetic field (toroidal field as color shades with overlying poloidal
field lines), while the right panel shows the time varying component
of the differential rotation (torsional oscillations). This movie
shows the non-linear feedback of the dynamo on the differential differential
rotation, leading to a poleward propagating pattern of oscillations
at high lattitudes (driven by Lorentz force), and an equatorward propagating
pattern at low latitudes (thermally driven). Torsional oscillations
contain valuable information about processes in the solar interior
that help to understand the solar dynamo. High resolution figure |
Understanding solar magnetic activity is one of the most intriguing problems in solar physics. The most obvious manifestation of solar activity on the large scale is the 11-year sunspot cycle, which is a consequence of a dynamo operating in the solar convection zone. Recently, significant progress has been made in understanding the process of magnetic field generation in the Sun through the development flux-transport dynamo models. Helioseismology has revealed significant information about the solar-cycle variations of the large-scale flows in the solar convection zone (the differential rotation and meridional circulation) that play important roles in the operation a flux-transport dynamo. The 11-year periodicity of these flow variations makes evident their close connection to the Sun's magnetic cycle. A comprehensive understanding of the dynamo process and the dynamics of the large-scale flows requires non-kinematic flux-transport dynamo models that incorporate the non-linear interaction between magnetic fields and flow fields in the solar convection zone.
Understanding the origins of the Sun's magnetism and its variability is one of the four research cornerstones of ESSL's HAO. It is also the key for understanding and predicting the activity- related magnetic and particulate influences on the Earth system, an interdisciplinary research priority within ESSL and NCAR.
To date, most flux-transport dynamo models have been kinematic, using the observed differential rotation and meridional flow to understand the evolution of the large-scale solar magnetic field. A non-kinematic model allows for the non-linear feedback of the magnetic field (via Lorentz force and indirect thermal effects) on these flow fields, leading to a comprehensive understanding of the operation of the dynamo in the non-linear regime, as well to as a theoretical explanation of the observed variations of the differential rotation (torsional oscillations) and meridional flow. To this end, a non-kinematic flux transport dynamo model has been developed (Rempel 2006, ApJ, 647, 622), incorporating mean-field descriptions of the effects of small-scale turbulence on both the dynamics of the large-scale flows and the generation of the large-scale magnetic field. Results from model runs show that a solar-like flux-transport dynamo saturates through a reduction of the mean differential rotation when the toroidal field at the base of the convection zone attains a strength of about 15 kilogauss. The feedback on the meridional flow turns out to be small for a field of this strength, a favorable result since the dynamo relies on the advection of the magnetic field by this flow in order to operate. The meridional flow is a weak but strongly driven flow, meaning that its energy content is replenished on a short time scale, of the order of one day. The cycle variations of the differential rotation (torsional oscillations) produced by the macroscopic Lorentz force feedback show a pattern of slower and faster rotation propagating poleward that agrees well with the pattern observed at high latitudes. However, the observed low latitude branch is missing in the model. It is possible to produce a low-latitude branch by incorporating surface radiative effects, as has been previously proposed by H. C. Spruit (2003, Solar Phys., 213, 1). Cycle variations of the meridional surface flow produced by this effect are also in agreement with observations. Planned future work includes further refinement and use of the model, with particular emphasis on understanding the low- latitude branch of torsional oscillations (in close interplay with meridional flow observations by local helioseismology), as well as studying the influence of the rotation rate on the dynamo, a dependence of great interest for interpreting observations of solar-like stars.
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| >Results from two simulations
of the rise of a 100 kilogauss buoyant flux tube from the base of
the convective zone, each with a different initial twist rate: q =
-0.054 rad/Mm (top row) and q= -0.027 rad/Mm (bottom row), respectively.
In both cases, the two left images show a volume rendering of the
absolute magnetic field strength of the flux tube as it reaches the
top boundary, and the right panels show the distribution of the vertical
magnetic flux in a horizontal cross-section near the top boundary.
In the higher twist case, the flux tube rises more cohesively, but
the twist induced tilting of the emerging loop causes the leading
polarity to be farther away from the equator than the following; in
the lower twist case, there is severe flux loss during the rise, but
the emerging flux shows shows an equatorward tilt that is consistent
with observations. High resolution figure |
Active regions and sunspots on the visible solar photosphere are believed to form as a result of the buoyant rise of magnetic flux tubes from the base of the solar convection zone, where strong, predominantly toroidal magnetic fields are being generated by the solar dynamo mechanism. Understanding the dynamics of rising magnetic flux tubes in the solar convective envelope (the outer 30% of the solar interior, about 200 megameters in extent) is therefore crucial for understanding the origin of solar active regions and their link to the dynamo-generated magnetic fields in the interior.
HAO/NCAR scientists are actively engaged in the development of realistic numerical models for use in simulating the rise of magnetic flux concentrations in the solar convection zone. Such studies provide insights concerning the emergence and subsequent evolution of the magnetic structures responsible for eruptions of plasma and fields that can produce disturbed conditions in interplanetary space. Over the past year, a new, 3D spherical shell, anelastic MHD code was used to perform simulations of the buoyant rise of such magnetic flux ropes, in order to study how the flux tube twist and the Coriolis force affect their trajectory, cohesion, and structure during the ascent to the surface. The simulations started with an initially toroidal magnetic flux tube with field strength in the range 30-100 kilogauss, located at a latitude of 15 degrees at the base of the convective envelope. A sinusoidal entropy variation was imposed, such that the central portion of the tube was in thermal equilibrium with the surroundings and thus maximally buoyant, while the two ends were neutrally buoyant. From these initial conditions, the toroidal flux tube subsequently evolved into an Omega-shaped rising flux tube in the solar convection zone. The simulations assume an adiabatically stratified convective envelope (i.e. marginally stable to convection), so that the flux tube rises through a ``quiescent'' convection zone; buffeting of the tube by convective flows is ignored in order to focus on the dynamical effects of the Coriolis force and field line twist.
These simulations provided the following major new results. First, it is found that in order for a buoyant flux tube to rise cohesively, with most of its flux reaching the top in a coherent fashion, the initial twist rate needs to be at least |q|=0.3/a = 0.054 rad/Mm, where q denotes the angular rate of field line rotation about the axis per unit length of the tube; a is the initial radius of the tube, chosen to be 0.1 times the pressure scale height at the base of the convection zone. Secondly, it is found that a twisted, arched flux tube will develop a writhe of the tube axis, thus producing a tilt of the tube at the apex. This twist-induced tilt is counter-clock-wise (clock-wise) as viewed from the top for a flux tube with a left-handed (right-handed) twist. On the other hand, the Coriolis force acting on the diverging, expanding motion at the apex of a rising flux tube will drive a clock-wise (counter-clock-wise) tilt at the tube apex in the northern (southern) hemisphere. This effect of the Coriolis force has been demonstrated in previous thin flux tube simulations, in which the effect of the twist was ignored. The present 3D MHD simulations show that for tubes having sufficient twist to rise cohesively, the twist-induced tilt dominates that caused by the Coriolis force, and is furthermore, in the wrong direction (opposite to the observed Joy's law) if the twist is left-handed (right-handed) in the northern (southern) hemisphere, following the observed hemispheric preference of the sign of active region twist. In order for the emerging flux tube to show a tilt direction that is consistent with observations, the initial twist rate of the flux tube needs to be smaller than |q| = 0.15/a = 0.027 rad/Mm. A third major result is that a field strength asymmetry develops in an emerging, omega-shaped tube, with the field in the leading (i.e., leading in the direction of rotation) leg being stronger than in the following, producing a more compact flux distribution and a higher peak field strength in the leading polarity of the emerging active region. This behavior is consistent with the observed morphological asymmetry of solar active regions. During the next year, simulations utilizing a more realistic initial condition are planned; rather than initiating the rise from a prescribed thermal equilibrium state, a mechanical equilibrium state will be enforced at the outset. This project is supported by a FY06 NCAR Director's Opportunity Fund Award.
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| Synthetic profiles of the fractional
linear polarization, Q/I, of the D1 and D2 lines of neutral sodium
observed 1 arcsec inside the solar limb, in the presence of a field
of 45 G, inclined by 90-degrees from the local vertical, and uniformly
distributed in azimuth (a "canopy" field); a non-normalized plot of
the absorption intensity profiles (dotted line) is superimposed for
reference. This modeling shows that the appearance of a central peak
in the D1 line, which has been regarded as "enigmatic", is instead
compatible with the presence of moderately weak, canopy magnetic field
in the line forming region. The modeling of these spectral lines can
therefore provide important clues about the complex magnetism of the
quiet-Sun lower chromosphere. High resoltuion figure |
The study of the scattering polarization of atomic and molecular spectral lines, and its modification in the presence of magnetic fields, has in recent years become an essential tool for understanding the magnetism of the solar atmosphere. One of its advantages is that it makes possible the detection of the very weak magnetic fields (below 100 Gauss) that are ubiquitous in quiet solar regions. At the present time, these fields are believed to represent a significant part of the total magnetic energy stored in the surface layers of the Sun, but are almost completely inaccessible to other diagnostics. To understand the signature of these magnetic fields in the scattering polarization of spectral lines, it is necessary to model the complex quantum-mechanical processes that occur when atomic/molecular radiation processes are perturbed by the presence of external fields; among others, these processes include the Hanle effect, the alignment-to-orientation conversion mechanism, and the Paschen-Back effect.
Several observational projects at HAO/NCAR are fundamentally based on the diagnostic potential of scattering polarization: the investigation of coronal magnetic fields with the Coronal Multi-channel Polarimeter (CoMP), the study of the magnetism of solar prominences and filaments with the Prominence Magnetometer (ProMag), and, on a somewhat more fundamental level, the testing of the current quantum theory of polarized line formation with the HAO laboratory scattering experiment. CoMP and ProMag (and the future COSMO facility that will combine and enhance their capabilities) hold great promise for soon enabling routine measurements of the strength and topology of the magnetic field in the outer, coronal layers of the solar atmosphere. This region represents the interface between the Sun and the Earth where the activity that causes space weather events originates. The possibility of monitoring coronal and prominence/filament magnetic fields will contribute in a fundamental way to identifying the conditions that lead to coronal mass ejections, an important objective in NCAR's mission to understand and predict the the solar drivers of space weather events at Earth.
HAO/NCAR researchers have been very active in the theoretical development of the subject of scattering polarization. Recently, they have developed tools that have made it possible to tackle some of the longstanding, unsolved problems of solar polarization diagnostics, including the enigmatic linear polarization profile of the D1 line of sodium and the anomalous net circular polarization (NCP) of the hydrogen-alpha line that has been detected in several quiescent prominences. The latter, in particular, indicates that micro-turbulent electric fields of statistical origin (so-called Holtsmark fields) significantly affect the processes that produce scattering polarization in a magnetized plasma. As a result, the NCP of the hydrogen lines is dramatically increased. This suggests that the traditional magnetic diagnostics based on these lines must be seriously revised, work that will be the focus of future efforts in this area.
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| Two magnetic fields with identical
normal-flux contours on a spherical boundary, illustrated by their
respective field lines drawn thin in one set and thick in the other.
Despite the topological similarity between these two sets, the field
with the thin lines can achieve a quiescent state of equilibrium whereas
the other with the thick lines is capable of explosive plasma heating.
This model is a direct mathematical demonstration of solar coronal
heating by electric current sheets. High resoltuion figure |
Scientists from the HAO and MMM divisions of ESSL, from the SCD component of CISL, the University of Colorado, and their collaborators are investigating the fundamental MHD processes of magnetic helicity transport and electric current sheet formation in the solar atmosphere. These two processes are the basic causes of extreme heating in the corona, such as that occuring in solar flares, and, the creation of long-lived magnetic structures capable of erupting into coronal mass ejections (CMEs). In this inter-divisional collaboration, observations, analytical mathematics, and high-precision numerical methods have contributed to a productive year marked by advancements in fundamental MHD theory, theoretical implications for CME phenomenology, and, new 3D numerical models.
An ongoing study of energy storage in force-free magnetic fields in axisymmetric geometry was extended to include pressure and gravitational forces (Flyer, Fornberg, Thomas, & Low 2005, ApJ, 631, 1235). The numerical methods utilized in this work are being extended to 3D fields. A theoretical MHD conjecture, namely, that the helicity of a force-free magnetic field in an open atmosphere, removed of its sign, has an upper bound defined entirely by the flux distribution at the atmospheric base, was proposed and demonstrated (Zhang, Flyer, & Low 2006, ApJ, 644, 575). On this basis, a CME can be understood as the inevitable expulsion of a structure that has accumulated helicity in excess of the conjectured bound. A primitive form of magnetic helicity, more basic than the classical Woltjer and Berger-Field helicities, has also been theoretically discovered (Low 2006, ApJ, 646, 1288). This development extends existing MHD concepts and suggests a novel approach to the numerical description of a time-dependent magnetic field. Moreover, a general and rigorous mathematical demonstration of the spontaneous formation of electric current sheets in an ideal fluid, first proposed by E. N. Parker of University of Chicago in 1972, has been given (Low 2006, ApJ, 649, 1064). These developments have led to new ideas for both analytical and numerical investigations being carried NCAR researchers. All of these studies are helping to understand the evolution of coronal magnetic fields and the activity it engenders, processes of great relevance to research on space weather and its impacts at NCAR. On the observational side, HAO/NCAR investigators have carried out forward radiative transfer calculations to determine the magnetic signatures present in coronal radiation from around a current-sheet prominence. Their results, suggesting that the topology of a prominence-related magnetic field should be detectable by polarimetric means, support the CoMP and COSMO coronal magnetometry projects.
Research during the next year will include the following components. Work will continue on the numerical construction of nonlinear force-free fields from their boundary values at the atmospheric base. This is a central mathematical problem in the interpretation of vector magnetic field observations. Memebers of the group will develop two sets of 3D time-dependent MHD codes, treating dynamics and, separately, equilibrium states via frictional relaxation. This work, to be supported by a grant from the NCAR Director's Opportunity Fund, aims at developing a new way of describing magnetic fields and using it to demonstrate the Parker current sheets. The group, will further develop the theoretical advancements made, and will investigate connections between theory and observations. Some support is provided through the NSF Small Grants for Exploratory Research.
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