Abstracts

  • Observational Constraints on the Origin and Acceleration of Solar Wind from Coronal Holes

    Michael Hahn

    Columbia University

    Daniel W. Savin Co-I
    Stephan Hofmeister Postdoc

    Science Goals and Objectives:

    Understanding the solar wind requires determining the source regions for different types of solar wind and the physical processes that accelerate them. We propose to investigate the solar wind from within and near coronal holes (CHs). These open field regions are the source of fast solar wind. Their boundaries are thought to be a source slow solar wind. Theories propose that at CH boundaries reconnection occurs between open and closed magnetic field, releasing stored plasma onto open field lines to form the slow solar wind. Many characteristics of this interchange reconnection process are unknown.

    We propose to use elemental abundances as a diagnostic to understand interchange reconnection. The abundances of elements with a low first ionization potential (FIP) grow over time on closed loops. This FIP effect does not occur on open field lines. Thus, the FIP effect can be used to determine how recently a closed field line has undergone reconnection with an open field line. We will study the FIP-effect at the boundaries of CHs and use our results to infer the length and time scales over which interchange reconnection occurs at the CH boundary.

    Reconnection is driven by random convective motions and by large scale flows, such as differential rotation. CHs are known to not be sheared by differential rotation of the photosphere. Theories suggest that this is because reconnection at the boundary maintains the CH's shape. We will study differences in the FIP effect gradients on the leading and trailing edges of CHs to determine whether they are consistent with this theory for CH rigid rotation.

    A well-known empirical property of the solar wind is the inverse correlation between the wind speed and the expansion factor describing the divergence of magnetic field lines in the low corona. It has been argued that the expansion factor is just a proxy for the solar wind source distance from the coronal hole boundary (DCHB). These different correlations are related to different physics as the expansion factor is important for wave-turbulence driven solar wind models, whereas the DCHB is important for reconnection-driven models.

    We will measure the solar-wind outflow velocity at low heights in the corona and determine the correlation between velocity, expansion factor, and DCHB. This will show whether the correlation observed in the solar wind is present at low heights and whether expansion factor/wave-driven models or DCHB/turbulence-driven models are more important.

    Methodology:

    We will use spectroscopic data from EIS on Hinode, as well as magnetograms and images from SOHO and SDO, to study low latitude CHs. Many suitable public archival datasets already exist. From the EIS data, we will derive the elemental abundances and Doppler velocities and produce maps of these quantities throughout the field of view. We will determine how the FIP effect varies across the CH boundary. We will also study how the abundances vary as a function of latitude at the leading versus trailing edges of the CH in order to observe the effects of differential rotation.

    From the magnetograms we will extrapolate the coronal magnetic field using magnetic field models. Potential field models are expected to be sufficiently accurate, but we will use other models to quantify systematic uncertainties. These results will allow us to de-project the line-of-sight velocities to measure the flow velocity along the field. From the model, we will also obtain the expansion factors and find the correlation of flow velocity.

    Imaging data will be used to specify the boundary of the CH. In reality, the boundary is gradual and so different conventions and algorithms have been proposed in order to specify a particular location. We will use different conventions in order to quantify systematic uncertainties. Then we will find the correlation between flow velocity and DCHB.

  • Understanding Solar Wind Acceleration from Global Models, Remote Sensing and In-situ Observations

    Enrico Landi

    University of Michigan

    Bart van der Holst Co-I
    Susan Lepri Co-I
    Judit Szente Postdoc

    Science Goals and Objectives:

    In the present investigation we will study solar wind heating and acceleration through Alfven waves by combining existing in-situ measurements obtained with the SWICS instrument on board the ACE and Ulysses spacecraft, high resolution spectra and narrow band images of the solar atmosphere with Hinode, SDO and SoHO, and the 3D MHD AWSoM model of the solar corona and the solar wind. This work will encompass cycles 23 and 24 through the use of multiple in-situ and remote sensing instruments, and can be extended, capitalizing on the results obtained with existing observations, to data from the Parker Solar Probe and the upcoming Solar Orbiter mission.

    Methodology:

    This methodology will combine the AWSoM model predictions of the 3D distribution of plasma speed, temperature and density with 1) the SPECTRUM module, which calculates line-of-sight images and high resolution spectra of the solar corona from any user-defined line of sight, and 2) the Michigan Ionization Code (MIC), which allows to calculate the evolution of the charge state distribution of the wind plasma with distance. Individual wind source regions, such as polar and equatorial coronal holes, streamers, active regions, will be connected to ACE and Ulysses positions through AWSoM magnetic field calculations; comparison of AWSoM/MIC/SPECTRUM predictions of spectra, images and plasma properties of the wind source regions with remote sensing observations and measurements, and AWSoM/MIC predictions of wind charge state distribution with the measurements magnetically connected to the wind source at the Sun, will allow us to 1) assess AWSoM s ability at predicting plasma heating and acceleration, 2) carry out empirical modeling of the wind evolution, to determine the plasma temperature, density and speed before the freeze-in point, to be compared with AWSoM predictions, and 3) characterize Alfven wave properties through comparison of predicted and observed line widths.

    Proposed Contributions to the Focus Science Team Effort:

    Relevance to FST Scientific objectives: This combination of observation and modeling provides unique contributions to the FST: it will allow the determination of which observables (individual spectral line intensities and profiles, charge states etc) are critical towards understanding the effects of Alfven waves on heating and acceleration; it will allow us to determine how the solar wind charge state composition is set; it will provide vital input towards improving the AWSoM model, enhancing its space weather and solar wind predictive capabilities; it will allow us to understand how the effects of observational and modeling uncertainties on wind diagnostics can be mitigated.

    Potential Contributions to FST Effort:

    This investigation combines observations and numerical modeling to directly address several Types of Investigations such as: minor ions and their role in the origin and the evolution of the solar wind, solar wind source models based on charge state and elemental composition, evolution of solar wind properties through the solar cycle.

    Metrics and Milestones to Success:

    During the first year, remote sensing and in-situ observations will be used to identify the best diagnostics of wind heating and acceleration. In year 2, high resolution spectra and charge states will be used to determine, together with the diagnostic techniques identified during year 1, , the ability of AWSoM to match measurements for a few select Carrington Rotations across the solar cycle. Years 3 and 4 will apply these diagnostics on predictions from the fine-tuned model, and investigate the evolution of the source regions, the Alfven wave properties, and the plasma properties in the solar atmosphere during cycles 23 and 24.

  • The Spatial, Temporal, and Charge-State Variability of the Solar Wind

    Roberto Lionello

    Predictive Science Incorporated

    Cooper Downs Co-I
    Viacheslav Titov Co-I
    Pete Riley Co-I
    Susan Lepri Co-I

    The solar wind expands outward from the solar corona and fills the heliosphere. It is the medium by which solar-driven space weather phenomena transmit their effects to Earth and the surrounding space environment. Fundamental questions remain about the wind's origin and acceleration, its connection to smaller-scale dynamical phenomena close to the Sun, and the partitioning of the wind into fast and slow streams. Specific examples include:

    1. Are topological changes to the magnetic field (e.g. interchange reconnection) in the corona essential for explaining heliospheric solar wind properties?
    2. Are density fluctuations related to such processes, and if so, what determines their observed time period?
    3. What coronal processes are required to explain ionization charge states in the heliosphere?

    To address these questions, we propose to use a magnetohydrodynamic (MHD) model of the solar corona and heliosphere developed at PSI to model specific time periods. The model incorporates the time-dependent evolution of the magnetic field in response to photospheric motions, a wave-turbulence-driven (WTD) description of corona heating, Alfvén wave acceleration of the solar wind, and modeling of ionization charge states. It will be supplemented with comprehensive topological analysis of the magnetic field that identifies different types of magnetic reconnection. Specifically, we will:

    • Model the solar wind, in conjunction with the charge states, as it evolves in response to the magnetic field evolution on the solar surface, and compare our results with remote observations and in-situ measurements.
    • Utilize an advanced technique for identifying topological changes (i.e., interchange reconnection) in the magnetic field and unravel their role in producing plasma properties.
    • Determine whether density fluctuations are also a consequence of the topological evolution of the Sun's magnetic field.
    • Connect in situ measurements of the solar wind with the topology of the coronal magnetic field, and provide our results to the LWS team and the larger scientific community.

    Our proposed investigation, which aims at furthering our understanding of the solar wind by connecting topological properties with in-situ measurements, density fluctuations, charge states, and the formation of the wind itself, is relevant to FST #2: Origins, Acceleration and Evolution of the Solar Wind. It contains modern theoretical (topological analysis), numerical modeling (MHD), and observational analysis (charge states) elements. We will quantitatively compare our results with remote observations from NASA missions such as SDO, SOHO, and STEREO, and in situ measurements from ACE, PSP, and STEREO. Our project will provide tools for linking these in situ measurements with the remote observations.

  • Improving Localization of the Source Regions of the Solar Wind

    Nariaki Nitta

    Lockheed Martin Solar and Astrophysics Lab

    Rebecca Centeno Co-I
    Jon Linker Co-I
    Yang Liu Collaborator

    This proposal aims at better localization of the source regions of the solar wind observed in near-Earth space, by improving full-surface (synoptic) magnetic maps commonly used to compute macroscopic properties of the solar wind. We concentrate on updating the magnetic field in the polar regions. The solar wind is generally divided into two categories, fast and slow. We often associate the fast solar wind with coronal holes, dark regions in EUV and soft X-ray images. In comparison, there is hardly any consensus as to the origin of the slow solar wind. It is of vital importance to reliably locate the source regions in order to test the models of the solar wind. Knowing the source regions will also help us understand various properties of the solar wind, in terms of the properties of the source region. However, without the actual measurement of the solar wind very close to the Sun, we cannot directly locate the source regions. Instead, we have to depend on models that show where on the Sun we are magnetically connected to and thus exposed to the solar wind. These models generally require the radial component of the magnetic field of the full solar surface, i.e. a synoptic magnetic map, as boundary conditions. This is not observationally available.

    Presently, the solar magnetic field is measured exclusively from the Earth. Only less than one half of the surface is adequately sampled by these measurements. We may use helioseismic farside imaging combined with STEREO EUV images to detect strong magnetic field regions on the backside. Magnetograms taken several days before or after may tell us whether there were indeed such regions on the backside, assuming that they would survive without drastic changes for several days. However, the polar regions are inherently very hard to observe from the ecliptic, and the historical measurements of magnetic field close to the poles may have large uncertainties. It is possible that these uncertainties may seriously impact our understanding of the magnetic connection between the Sun and Earth. In polar regions, the radial component of the field becomes almost perpendicular to the line of sight, and high-resolution and high-sensitivity vector measurements become essential.

    We propose to improve synoptic maps that serve as the lower boundary conditions for models, simple and complex alike, by incorporating measurements of the vector field in the polar regions by the Hinode Spectro-polarimeter (SP) and SDO Helioseismic and Magnetic Imager (HMI). To date, SP is the best resource for the polar field due to its high spatial and spectral resolution. Inversions not included in the pipeline will be explored on SP data to ensure the best results for the polar regions. HMI is needed not only to correct for the SP pointing but also to address the time variability of the polar field. We will calibrate HMI data with SP, and use their radial field for updating the polar regions in the synoptic maps from HMI line-of-sight magneotograms. Using these maps, we run both the potential field source surface (PFSS) model and magnetohydrodynamic (MHD) model to locate the magnetic footpoint of the observer at L1 and estimate the uncertainties of the location. We will systematically conduct this study for periods selected on the basis of the difficulty of locating the source region of the solar wind. The proposed research should be an important part of FST#2, and will form the basis of other projects in the team whose emphasis may be the mechanisms of heating and acceleration of the solar wind.

  • Investigating the Interaction between Solar Wind Ions and Electromagnetic Waves Using New Observations and Hybrid Simulations

    Lan Jian

    NASA Goddard Space Flight Center

    Leon Ofman Co-I
    Michael Stevens Co-I
    Hanying Wei Co-I
    Daniel Gershman Co-I

    Electromagnetic cyclotron waves (ECWs) are extensively observed in the solar wind from 0.3 to 1 AU. They appear to be left-hand or right-hand polarized in the spacecraft frame, and propagate in directions close to the background magnetic field. On the other hand, the solar wind is often not in equilibrium, featured with the temperature anisotropy of particles with respect to the background magnetic field and relative drifts among ion components (proton core, proton beam, and alpha particles). The ECWs near the proton cyclotron frequency are most likely to have Landau and/or cyclotron resonances with solar wind ions. Using well-calibrated magnetic field and plasma data from Wind (continuous solar wind monitoring) and MMS mission (providing about 3 hours of high-cadence solar wind data per 3-day orbit), we will investigate the interaction between ECWs and solar wind ions.

    Since the ion kinetic scale marks the transition from the inertial range to the dissipation range, our investigation directly addresses the acceleration and evolution of solar wind, which is the second Focused Science Topic of this LWS solicitation. In particular, we aim to answer the following three science questions.

    1. How often are non-equilibrium ion velocity distributions and ECWs observed together in the solar wind?
    2. In the case of multiple unstable modes of ion kinetic instabilities, how do the different modes interact?
    3. Do all the instabilities propagate along with the solar wind?

    In the solar wind, three types of ion-driven instabilities can generate such parallel-propagating ECWs: ion cyclotron, parallel firehose, and ion beam instabilities. There is often multiple sources of free energy for waves to grow. To fully investigate the nonlinear interaction between the different instabilities, we will conduct the hybrid simulation guided by solar wind observations. Our methodology includes the following three aspects.

    1. We identify ECWs using Wind data and compare their existence with the growth rates of multiple ion kinetic instabilities to explain the discrepancies of their occurrence rates.
    2. We search 20 long-lasting ECW events of multiple coexistent unstable instabilities using 2005-2007 Wind data, and examine how the different modes interact using hybrid simulation.
    3. We search ECWs (especially the cases of close appearance of opposite polarizations) in the solar wind using MMS data and determine the wavevector and intrinsic polarization. By conducting hybrid simulations for selected events, we determine whether the unstable instabilities propagate along or against the solar wind.

    The coherent electromagnetic waveforms and detailed particle velocity distributions provide unique opportunities to study the wave-particle interaction at ion kinetic scales. The improved understanding of ion kinetic physics through this coordinated project will be applicable to plasma in planetary magnetospheres and ionospheres, astrophysical systems, and laboratory plasma experiments. As more solar wind models and CME models start to incorporate the kinetic effects, the better understanding of wave-particle interaction gained from this project will ultimately help improving the prediction of solar wind and CME properties.

  • Coupling Electron and Proton Kinetic Physics in the Solar Wind

    Michael Shay

    University of Delaware

    William Matthaeus Co-I
    Chadi Salem Co-I
    Bennett Maruca Co-I
    Tulasi Parashar Collaborator

    Science Goals:

    The mean free path of particles in solar wind is ~1AU or larger, making it larger than most scales of interest. This implies that the solar wind is extremely weakly collisional. Lack of direct inter-species collisions drives the system into a highly non equilibrium state where protons and electrons are not in a thermal equilibrium. However a lack of collisions and thermal equilibrium between different plasma species does not imply that the species do not affect each other. Both species affect the evolution and kinetic behavior of the other via electromagnetic interactions. However, which processes and channels enable this interaction, and the resultant effects have not gained much attention at all. It is our aim to develop a research program that addresses this problem using a combination of spacecraft data and kinetic simulations. The objectives of the proposal are:

    1. Identify the channels of interaction between protons and electrons, and their relation to reconnection and turbulence in the solar wind
    2. Identify the effects of this interaction on kinetic physics of both protons and electrons
    3. Create statistical surveys of correlations between various physical quantities of interest such as relative species heating, turbulence amplitudes, individual species temperatures, temperature ratio of both species, solar wind speed etc.

    The fundamental goal is to create a comprehensive knowledge base that can guide global simulations of solar wind to improve the thermodynamic representation of these species.

    Methodology:

    The project will employ both, analysis of spacecraft data as well as kinetic simulations to address this problem. The strategy will be two pronged:

    1. Use kinetic simulations to identify the interaction channels between the two species, and quantify their roles.
    2. Use spacecraft data to perform surveys of statistical correlations between various quantities of interest, and to identify specific intervals of interest to simulate using kinetic simulation models.

    For kinetic simulations, primarily fully parallel electromagnetic kinetic code P3D will be used. Our team has used this code extensively for studying turbulence and reconnection. For spacecraft observations, solar wind data from Wind, ACE, Cluster, Helios, and MMS spacecraft will be used. Single-spacecraft and multi-spacecraft techniques will be used where appropriate. Our team has expertise in analyzing data from all these spacecraft for various turbulence studies.

    Proposed Contributions to Focused Science Team Effort:

    The interplay of protons and electrons in a collisionless system is of interest on its own at a fundamental level. However, the statistical correlations found in this study will be important to empirically represent the thermodynamics of protons and electrons in global models. A preliminary attempt in this regard has already been made by our collaborator Arcadi Usmanov by incorporating results from one of recent papers into his global heliospheric model. Moreover, the insights gathered from this study will help interpret and analyze data from NASA/ESA missions such as Parker Solar Probe, and Solar Orbiter.

    Relevance:

    The proposed study is directly relevant to LWS program objective 1: "Understand how the sun varies and what drives solar variability" and Focused Science Topic 2: "Origins, Acceleration and Evolution of the Solar Wind". Understanding the interplay of proton and electron kinetic physics is central to a better understanding of the solar wind and its evolution. This study is also relevant to the first science objective defined in the 2014 Heliophysics Roadmap for NASA: "Solve the Fundamental Mysteries of Heliosphere" by understanding not only the fundamental processes that energize particles, but also by enhancing our understanding of the role that magnetic reconnection and turbulence play in the evolution of the solar wind.

  • Heating of Ions in the Low-beta Compressible Solar Wind

    Xiangrong Fu

    New Mexico Consortium

    Fan Guo Co-I
    Hui Li Co-I
    William Matthaeus Co-I
    John Steinberg Collaborator
    Zhaoming Gan Postdoc

    The solar wind is the high speed plasma flow originated from the Sun, carrying magnetic field and energetic particles and propagating throughout the heliosphere. In-situ measurements have shown that solar wind is turbulent and ions are heated, though the heating mechanisms for solar wind ions are still under debate and a subject of active research.

    We propose to study the solar wind ion heating in the regime when the turbulent Mach number is high (between 0.1 and 1) and the plasma beta is low (< 0.1), i.e. the low-beta compressible turbulence (LBCT) regime. This regime is particularly relevant in the near-Sun region where the solar wind originates and the magnetic energy density is large, though such conditions can exist throughout the heliosphere. In particular, we will study the critical role of the parametric decay instability (PDI), which converts a large-amplitude forward Alfven wave into a backward Alfven wave and a slow mode. The backward Alfven waves can further interact with forward Alfven waves and produce a plasma turbulence mixed with compressible and incompressible components. Our recent MHD and kinetic simulations show that, in this regime minor ion species undergo efficient heating, with distinctive signatures in both parallel and perpendicular directions. Specifically, we will address three key science questions (SQ):

    1. Can PDI explain enhanced density fluctuations at the center of the preferential ion heating zone (10-20 solar radii)?
    2. What are the heating rates of compressible and incompressible turbulences on protons and minor ions in the low-beta compressible solar wind?
    3. How does the inclusion of ion heating from compressible turbulence improve global modeling of the solar wind?

    Because LBCT have been observed in the heliosphere from tens of solar radii to a few AUs, with an increasing occurrence when approaching the Sun, our proposed investigation is very timely to address the long-standing ion heating problem in the solar wind because in-situ measurements in the close-to-Sun region will soon be made available by the Parker Solar Probe (PSP).

    We propose to perform local 3D MHD and hybrid simulations to address the problem of solar wind ion heating in the low-beta compressible regime, using turbulence and plasma quantities provided by global MHD simulations. The local MHD simulations will enable us to examine the similarities and differences of properties between compressible turbulence and the well-studied nearly incompressible turbulence. Hybrid simulations will allow us to directly examine the detailed ion heating processes in the low-beta compressible regime. Specifically, we will use the background plasma conditions including magnetic field, plasma density, ion temperature and the solar wind speed in several typical regions: 1 AU, 0.3 AU and 10s R_s (solar radius) and < 2R_s, from observations and global MHD simulations. We anticipate to carry out a large set of hybrid simulations to study PDI and its contribution to density fluctuations. With kinetic ions properly modeled, their heating rates by compressible waves and turbulence can be quantified, and empirical models of heat functions will be derived. These studies will enable us to test competing solar wind heating mechanisms which could have different ion heating signatures and provide critical microphysics inputs for the global solar wind evolution models.