Author name code: viall ADS astronomy entries on 2022-09-14 author:"Viall, Nicholeen M." ------------------------------------------------------------------------ Title: Defining the Middle Corona Authors: West, Matthew J.; Seaton, Daniel B.; Wexler, David B.; Raymond, John C.; Del Zanna, Giulio; Rivera, Yeimy J.; Kobelski, Adam R.; DeForest, Craig; Golub, Leon; Caspi, Amir; Gilly, Chris R.; Kooi, Jason E.; Alterman, Benjamin L.; Alzate, Nathalia; Banerjee, Dipankar; Berghmans, David; Chen, Bin; Chitta, Lakshmi Pradeep; Downs, Cooper; Giordano, Silvio; Higginson, Aleida; Howard, Russel A.; Mason, Emily; Mason, James P.; Meyer, Karen A.; Nykyri, Katariina; Rachmeler, Laurel; Reardon, Kevin P.; Reeves, Katharine K.; Savage, Sabrina; Thompson, Barbara J.; Van Kooten, Samuel J.; Viall, Nicholeen M.; Vourlidas, Angelos Bibcode: 2022arXiv220804485W Altcode: The middle corona, the region roughly spanning heliocentric altitudes from $1.5$ to $6\,R_\odot$, encompasses almost all of the influential physical transitions and processes that govern the behavior of coronal outflow into the heliosphere. Eruptions that could disrupt the near-Earth environment propagate through it. Importantly, it modulates inflow from above that can drive dynamic changes at lower heights in the inner corona. Consequently, this region is essential for comprehensively connecting the corona to the heliosphere and for developing corresponding global models. Nonetheless, because it is challenging to observe, the middle corona has been poorly studied by major solar remote sensing missions and instruments, extending back to the Solar and Heliospheric Observatory (SoHO) era. Thanks to recent advances in instrumentation, observational processing techniques, and a realization of the importance of the region, interest in the middle corona has increased. Although the region cannot be intrinsically separated from other regions of the solar atmosphere, there has emerged a need to define the region in terms of its location and extension in the solar atmosphere, its composition, the physical transitions it covers, and the underlying physics believed to be encapsulated by the region. This paper aims to define the middle corona and give an overview of the processes that occur there. Title: New Insights into the First Two PSP Solar Encounters Enabled by Modeling Analysis with ADAPT-WSA Authors: Wallace, Samantha; Jones, Shaela I.; Arge, C. Nick; Viall, Nicholeen M.; Henney, Carl J. Bibcode: 2022ApJ...935...24W Altcode: 2022arXiv220513010W Parker Solar Probe's (PSP's) unique orbital path allows us to observe the solar wind closer to the Sun than ever before. Essential to advancing our knowledge of solar wind and energetic particle formation is identifying the sources of PSP observations. We report on results for the first two PSP solar encounters derived using the Wang-Sheeley-Arge (WSA) model driven by Air Force Data Assimilative Photospheric Flux Transport (ADAPT) model maps. We derive the coronal magnetic field and the 1 R source regions of the PSP-observed solar wind. We validate our results with the solar wind speed and magnetic polarity observed at PSP. When modeling results are very reliable, we derive time series of model-derived spacecraft separation from the heliospheric current sheet, magnetic expansion factor, coronal hole boundary distance, and photospheric field strength along the field lines estimated to be connected to the spacecraft. We present new results for Encounter 1, which show time evolution of the far-side mid-latitude coronal hole that PSP corotates with. We discuss how this evolution coincides with solar wind speed, density, and temperature observed at the spacecraft. During Encounter 2, a new active region emerges on the solar far side, making it difficult to model. We show that ADAPT-WSA output agrees well with PSP observations once this active region rotates onto the near side, allowing us to reliably estimate the solar wind sources retrospectively for most of the encounter. We close with ways in which coronal modeling enables scientific interpretation of these encounters that would otherwise not have been possible. Title: The Solaris Solar Polar MIDEX-Class Mission Concept: Revealing the Mysteries of the Sun's Poles Authors: Hassler, Donald M.; Harra, Louise K.; Gibson, Sarah; Thompson, Barbara; Gusain, Sanjay; Berghmans, David; Linker, Jon; Basu, Sarbani; Featherstone, Nicholas; Hoeksema, J. Todd; Viall, Nicholeen; Newmark, Jeffrey; Munoz-Jaramillo, Andres; Upton, Lisa A. Bibcode: 2022cosp...44.1528H Altcode: Solaris is an exciting, innovative & bold mission of discovery to reveal the mysteries of the Sun's poles. Solaris was selected for Phase A development as part of NASA's MIDEX program. Solaris builds upon the legacy of Ulysses, which flew over the solar poles, but Solaris provides an entirely new feature remote sensing, or IMAGING. Solaris will be the first mission to image the poles of the Sun from ~75 degrees latitude and provide new insight into the workings of the solar dynamo and the solar cycle, which are at the foundation of our understanding of space weather and space climate. Solaris will also provide enabling observations for improved space weather research, modeling and prediction with time series of polar magnetograms and views of the ecliptic from above, providing a unique view of the corona, coronal dynamics, and CME eruption. To reach the Sun's poles, Solaris will first travel to Jupiter, and use Jupiter's gravity to slingshot out of the ecliptic plane, and fly over the Sun's poles at ~75 degrees latitude. Just as our understanding of Jupiter & Saturn were revolutionized by polar observations from Juno and Cassini, our understanding of the Sun will be revolutionized by Solaris. Title: 4π Heliospheric Observing System - 4π-HeliOS: Exploring the Heliosphere from the Solar Interior to the Solar Wind Authors: Raouafi, Nour E.; Gibson, Sarah; Ho, George; Laming, J. Martin; Georgoulis, Manolis K.; Szabo, Adam; Vourlidas, Angelos; Mason, Glenn M.; Hoeksema, J. Todd; Velli, Marco; Berger, Thomas; Hassler, Donald M.; Kinnison, James; Viall, Nicholeen; Case, Anthony; Newmark, Jeffrey; Lepri, Susan; Krishna Jagarlamudi, Vamsee; Raouafi, Nour; Bourouaine, Sofiane; Vievering, Juliana T.; Englander, Jacob A.; Shannon, Jackson L.; Perez, Rafael M.; Chattopadhyay, Debarati; Mason, James P.; Leary, Meagan L.; Santo, Andy; Casti, Marta; Upton, Lisa A. Bibcode: 2022cosp...44.1530R Altcode: The 4$\pi$ Heliospheric Observing System (4$\pi$-HeliOS) is an innovative mission concept study for the next Solar and Space Physics Decadal Survey to fill long-standing knowledge gaps in Heliophysics. A constellation of spacecraft will provide both remote sensing and in situ observations of the Sun and heliosphere from a full 4$\pi$-steradian field of view. The concept implements a holistic observational philosophy that extends from the Sun's interior, to the photosphere, through the corona, and into the solar wind simultaneously with multiple spacecraft at multiple vantage points optimized for continual global coverage over much of a solar cycle. The mission constellation includes two spacecraft in the ecliptic and two flying as high as $\sim$70$^\circ$ solar latitude. 4$\pi$-HeliOS will provide new insights into the fundamental processes that shape the whole heliosphere. The overarching goals of the 4$\pi$-HeliOS concept are to understand the global structure and dynamics of the Sun's interior, the generation of solar magnetic fields, the origin of the solar cycle, the causes of solar activity, and the structure and dynamics of the corona as it creates the heliosphere. The mission design study is underway at the Johns Hopkins Applied Physics Laboratory Concurrent Engineering Laboratory (ACE Lab), a premier mission design center, fostering rapid and collaborative mission design evolutions. Title: Relating the variability of the middle corona to the structure of the slow solar wind Authors: Higginson, Aleida; DeVore, C. Richard; Antiochos, Spiro; Viall, Nicholeen Bibcode: 2022cosp...44.1320H Altcode: The recent revolution in heliospheric measurements, brought about by NASAʼs Parker Solar Probe and ESAʼs Solar Orbiter, has shown that processes in the middle corona can influence the structure and dynamics of the solar wind across spatial scales. Understanding the formation of the young solar wind structures currently being measured by Parker Solar Probe and Solar Orbiter is now essential. Numerical calculations have shown that magnetic field dynamics at coronal hole boundaries in the middle corona, in particular interchange reconnection driven by photospheric motions, can be responsible for the dynamic release of structured slow solar wind, including along huge separatrix-web (S-Web) arcs formed by pseudostreamers. Quantifying the plasma and magnetic variability along the heliospheric current sheet and these S-Web arcs is crucial to furthering our understanding of how coronal magnetic field dynamics can influence the slow solar wind throughout the heliosphere. Here we present fully dynamic, 3D numerical calculations of a coronal hole boundary driven continuously by realistic photospheric motions at its base. We consider our simulation results within the context of Parker Solar Probe and Solar Orbiter, and make predictions for the structure and variability of the young slow solar wind. Title: Investigating Solar Wind Formation in the Inner Corona Using ADAPT-WSA Authors: Wallace, Samantha; Young, Peter; Arge, Charles; Viall, Nicholeen; Jones, Shaela Bibcode: 2022cosp...44.1321W Altcode: Several fundamental outstanding questions in heliophysics pertain to the genesis and energization of the solar wind - both of which are driven by physical processes that largely occur in the inner solar corona. Recent and upcoming missions enable more direct measurements of the inner corona; however, the use of a model is required to bridge in situ and remote observations to investigate how the solar wind was formed. We present results from aggregate work that support this claim, where we use the Wang-Sheeley-Arge (WSA) model driven by Air Force Data Assimilative Photospheric Flux Transport (ADAPT) time-dependent photospheric field maps to connect in situ solar wind observations from various spacecraft (e.g., PSP, SolO, ACE, Helios) to their source regions at 1 Rs. We show results in which we apply our modeling to test solar wind formation theories (e.g., reconnection/S-web, waves-turbulence, expansion factor), and to characterize the solar wind from specific sources (e.g., active region vs. quiet Sun coronal hole boundaries, deep inside coronal holes). We discuss several current and former collaborations, including connecting PSP in situ measurements to remote composition measurements from Hinode/EIS, and identifying the sources of transient eruptions observed at PSP. We close with how ADAPT-WSA is currently supporting both the PSP and SolO missions. Title: Exploring Structures and Flows with NASA's under-construction PUNCH mission Authors: DeForest, Craig; Gibson, Sarah; Thompson, Barbara; Malanushenko, Anna; Desai, Mihir; Elliott, Heather; Viall, Nicholeen; Cranmer, Steven; de Koning, Curt Bibcode: 2022cosp...44.1077D Altcode: The Polarimeter to UNify the Corona and Heliosphere is a NASA Small Explorer to image the corona and heliosphere as parts of a single system. PUNCH comprises four ~50kg smallsats, each carrying one imaging instrument, that work together to form a single "virtual coronagraph" with a 90° field of view, centered on the Sun. Scheduled for joint launch with NASA's SPHEREx mission, PUNCH starts its two-year prime science phase in 2025. PUNCH will generate full polarized image sequences of Thomson-scattered light from free electrons in the corona and young solar wind, once every four minutes continuously. This enables tracking the young solar wind and turbulent structures within it as they disconnect from the Sun itself, as well as large transients such as CMEs, CIRs, and other shocks within the young solar wind. A student-contributed X-ray spectrometer (STEAM) will address questions of coronal heating and flare physics. We present motivating science, expected advances, mission status, and how to get involved with PUNCH science now. Title: Expected results for the cradle of the Solar Wind with the Polarimeter to UNify the Corona and Heliosphere (PUNCH) Authors: DeForest, Craig; Gibson, Sarah; De Koning, Curt A.; Thompson, Barbara; Malanushenko, Anna; Desai, Mihir; Elliott, Heather; Viall, Nicholeen; Cranmer, Steven Bibcode: 2022cosp...44.1324D Altcode: The Polarimeter to UNify the Corona and Heliosphere is a NASA Small Explorer to image the corona and heliosphere as parts of a single system. Imaging the corona and heliosphere together from a constellation of four synchronized smallsats, PUNCH will — starting in 2025 — provide a unique window on global structure and cross-scale processes in the outer corona and young solar wind. PUNCH science is informed by, and complements, the results of PSP and Solar Orbiter; and will synergize with PROBA3/ASPIICS. We present early prototype results from STEREO/SECCHI and current preparation work to enable PUNCH science when data arrive, discuss anticipated results from the deeper-field, higher time resolution imaging that PUNCH will provide, and describe how to get involved with PUNCH science now. Title: Remote Sensing of Turbulence and Solar Wind Structure with the PUNCH mission Authors: DeForest, Craig; Gibson, Sarah; Matthaeus, William; Viall, Nicholeen Bibcode: 2022cosp...44.1212D Altcode: The Polarimeter to UNify the Corona and Heliosphere is a mission to observe the corona and the inner heliosphere as a unified system. PUNCH will produce continuous images of the solar wind and corona between 1.5° and 45° from the Sun, over a two year prime science mission scheduled to start in early 2025. PUNCH uses visible sunlight scattered by free electrons in the corona, to track density structures in the corona and solar wind. We will describe PUNCH's unique 3D imaging capability, mission structure, and anticipated results measuring the development of large-scale turbulence, and the large- and meso-scale structure of the solar wind itself. Title: Periodic Solar Wind Structures Observed in Measurements of Elemental and Ionic Composition in situ at L1 Authors: Gershkovich, Irena; Lepri, Susan T.; Viall, Nicholeen M.; Matteo, Simone Di; Kepko, Larry Bibcode: 2022ApJ...933..198G Altcode: Mesoscale periodic structures observed in solar wind plasma serve as an important diagnostic tool for constraining the processes that govern the formation of the solar wind. These structures have been observed in situ and in remote data as fluctuations in proton and electron density. However, only two events of this type have been reported regarding the elemental and ionic composition. Composition measurements are especially important in gaining an understanding of the origin of the solar wind as the composition is frozen into the plasma at the Sun and does not evolve as it advects through the heliosphere. Here, we present the analysis of four events containing mesoscale periodic solar wind structure during which the Iron and Magnesium number density data, measured by the Solar Wind Ion Composition Spectrometer (SWICS) on board the Advanced Composition Explorer spacecraft, are validated at statistically significant count levels. We use a spectral analysis method specifically designed to extract periodic signals from astrophysical time series and apply it to the SWICS 12 minute native resolution data set. We find variations in the relative abundance of elements with low first ionization potential, mass dependencies, and charge state during time intervals in which mesoscale periodic structures are observed. These variations are linked to temporal or spatial variations in solar source regions and put constraints on the solar wind formation mechanisms that produce them. Techniques presented here are relevant for future, higher-resolution studies of data from new instruments such as Solar Orbiter's Heavy Ion Sensor. Title: Scattered light in the Hinode/EIS and SDO/AIA instruments measured from the 2012 Venus transit Authors: Young, Peter R.; Viall, Nicholeen M. Bibcode: 2022arXiv220709538Y Altcode: Observations from the 2012 transit of Venus are used to derive empirical formulae for long and short-range scattered light at locations on the solar disk observed by the Hinode Extreme ultraviolet Imaging Spectrometer (EIS) and the Solar Dynamics Observatory Atmospheric Imaging Assembly (AIA) instruments. Long-range scattered light comes from the entire solar disk, while short-range scattered light is considered to come from a region within 50" of the region of interest. The formulae were derived from the Fe XII 195.12 A emission line observed by EIS and the AIA 193 A channel. A study of the weaker Fe XIV 274.20 A line during the transit, and a comparison of scattering in the AIA 193 A and 304 A channels suggests the EIS scattering formula applies to other emission lines in the EIS wavebands. Both formulae should be valid in regions of fairly uniform emission such as coronal holes and quiet Sun, but not faint areas close (around 100") to bright active regions. The formula for EIS is used to estimate the scattered light component of Fe XII 195.12 for seven on-disk coronal holes observed between 2010 and 2018. Scattered light contributions of 56% to 100% are found, suggesting that these features are dominated by scattered light, consistent with earlier work of Wendeln \& Landi. Emission lines from the S X and Si X ions - formed at the same temperature as Fe XII and often used to derive the first ionization potential (FIP) bias from EIS data - are also expected to be dominated by scattered light in coronal holes. Title: On Differentiating Multiple Types of ULF Magnetospheric Waves in Response to Solar Wind Periodic Density Structures Authors: Di Matteo, S.; Villante, U.; Viall, N.; Kepko, L.; Wallace, S. Bibcode: 2022JGRA..12730144D Altcode: Identifying the nature and source of ultra-low frequencies (ULF) waves (f ⪅ 4 mHz) at discrete frequencies in the Earth's magnetosphere is a complex task. The challenge comes from the simultaneous occurrence of externally and internally generated waves, and the ability to robustly identify such perturbations. Using a recently developed robust spectral analysis procedure, we study an interval that exhibited in magnetic field measurements at geosynchronous orbit and in-ground magnetic observatories both internally supported and externally generated ULF waves. The event occurred on 9 November 2002 during the interaction of the magnetosphere with two interplanetary shocks that were followed by a train of 90 min solar wind periodic density structures. Using the Wang-Sheeley-Arge model, we mapped the source of this solar wind stream to an active region and a mid-latitude coronal hole just prior to crossing the Heliospheric current sheet. In both the solar wind density and magnetospheric field fluctuations, we separated broad power increases from enhancements at specific frequencies. For the waves at discrete frequencies, we used the combination of satellite and ground magnetometer observations to identify differences in frequency, polarization, and observed magnetospheric locations. The magnetospheric response was characterized by: (a) forced breathing by periodic solar wind dynamic pressure variations below ≈1 mHz, (b) a combination of directly driven oscillations and wave modes triggered by additional mechanisms (e.g., shock and interplanetary magnetic field discontinuity impact, and substorm activity) between ≈1 and 4 mHz, and (c) largely triggered modes above ≈4 mHz. Title: A Strategy for a Coherent and Comprehensive Basis for Understanding the Middle Corona Authors: West, M. J.; Seaton, D. B.; Alzate, N.; Caspi, A.; DeForest, C. E.; Gilly, C. R.; Golub, L.; Higginson, A. K.; Kooi, J. E.; Mason, J. P.; Rachmeler, L. A.; Reeves, K. K.; Reardon, K.; Rivera, Y. J.; Savage, S.; Viall, N. M.; Wexler, D. B. Bibcode: 2022heli.conf.4060W Altcode: We describe a strategy for coherent and comprehensive observations needed to achieve a fundamental understanding of the middle solar corona. Title: Relating Solar Wind Variability to the Magnetic Topology of its Coronal Source Region Authors: Lynch, Benjamin; Viall, Nicholeen; Higginson, Aleida; Zhao, Liang; Lepri, Susan; Sun, Xudong Bibcode: 2021AGUFMSH32B..07L Altcode: The global magnetic configuration of the solar corona directly determines the structure of the solar wind outflow and decades of in-situ observations have shown that the solar wind properties reflect the coronal conditions and magnetic structure of its origin. Connecting the solar wind observed throughout the heliosphere to its origins in the solar corona is a fundamental science objective of the Parker Solar Probe and Solar Orbiter missions and one of the central aims of heliophysics. The slow solar wind exhibits significant variability on relatively short timescales, from minutes to days. This short-term variability in the magnetic field, bulk plasma, and composition properties of the slow wind likely results from magnetic reconnection processes in the extended solar corona. We present a detailed analysis of the solar wind from 2003 April 15 to May 13, corresponding to Carrington Rotation 2002. We identify regions of enhanced variability and composition signatures and demonstrate their relationship to the large-scale magnetic topology of the solar corona. Specifically, there are four pseudostreamer wind intervals and two heliospheric current sheet crossings (and an ICME) which all exhibit enhanced alpha-to-proton ratios and signatures in the charge states of carbon, oxygen, and iron. We investigate whether other turbulence properties of these intervals, e.g. the Alfvenicity, Hm-PVI structures, etc, can be related to coronal topological features such as the S-Web arcs of the pseudostreamers or the heliospheric current sheet/plasma sheet crossings. We discuss these results in the context of upcoming PSP and SolO joint observational campaigns. Title: Towards a Coherent View of the Sun/Corona/Heliosphere: Combining Remote Sensing Data Products with PSP In Situ Measurements Authors: Alzate, Nathalia; Morgan, Huw; Seaton, Daniel; Thompson, Barbara; Nieves-Chinchilla, Teresa; Di Matteo, Simone; Viall, Nicholeen Bibcode: 2021AGUFMSH24C..02A Altcode: In situ (IS) heliospheric measurements and remote sensing observations contribute crucial information to our understanding of the Sun/Corona/Heliosphere as a single system. In Situ measurements by, e.g., Parker Solar Probe (PSP), are detailed and precise measurements of physical observables (e.g., magnetic field components), which cannot be gained from remote observations. Remote sensing observations, in turn, provide the large-scale context, which is absent from the in situ data taken at one point in space. Therefore, to understand the Sun/Corona/Heliosphere system, we need to effectively establish a link between in situ measurements and remote sensing observations by characterizing structure and plasma properties of the inner corona. Additionally, we need to resolve the line-of-sight limitations of white-light (WL) coronagraph and Extreme Ultraviolet (EUV) observations to properly identify the location of structures and their temporal density changes. Previous studies have identified outward propagating density variations in the solar wind (on timescales of hours up to ~3 days) that have a plasma composition of coronal origin and that can be traced down through the field of view of STEREO/COR2 (~2.5-15 Rs). Our advanced image processing techniques can reveal structures (on various timescales) in both EUV and WL data providing continuous tracking of brightness enhancements from the coronal base out to the radial extended corona. Our most recent work using STEREO/COR1 and GOES-R/SUVI observations has proven crucial in linking the low to high corona and has facilitated the interpretation of PSP data. Further, our time-dependent rotational tomography of coronal data yields empirically derived coronal density distribution directly comparable to PSP measurements at perihelion. We present our current work that combines PSP data with remote sensing EUV/WL observations of the corona, via the use of coronal rotational tomography from SOHO/LASCO and STEREO/COR2 observations, which provides the capabilities to reconstruct features in the solar wind and subsequently study the evolution between EUV/WL and in situ of the plasma flows that give rise to them. Title: Solar Wind Driven Ultra-Low Frequency Waves Properties and Effects on Radiation Belt Electrons Loss Authors: Di Matteo, Simone; Viall, Nicholeen; Kepko, Larry; Breneman, Aaron; Halford, Alexa; Villante, Umberto Bibcode: 2021AGUFMSM44A..05D Altcode: The Ultra-Low Frequency (ULF) waves in the Earth's magnetosphere are created by a variety of external and internal generation mechanisms. Among possible solar wind driving processes, magnetospheric forced breathing due to solar wind periodic density structures (PDSs) with size scales of the order of the magnetosphere cavity are important for fluctuations at frequencies below ~4 mHz. In the rest frame of a spacecraft or Earth, the PDS length scale and the solar wind velocity determine the apparent frequency of the PDSs and the associated dynamic pressure fluctuations. These structures directly drive magnetospheric field oscillations at similar frequencies and may also trigger additional ULF waves. These magnetospheric field fluctuations can cause the loss of radiation belt electrons directly, e.g., via loss cone modulation, or indirectly, e.g., via inward radial transport to a region of larger loss cone or triggering the growth of other magnetospheric processes such as cyclotron waves, which in turn scatter electrons causing an increase in the precipitation loss. In this work, we applied a recently developed spectral analysis procedure, based on the multitaper method, to identify periodicity in solar wind density and magnetospheric field at the geostationary orbit and ground. The analysis is also extended to indirect measurements of radiation belt electron losses in the form of bremsstrahlung X-rays from both the first and second campaigns of the Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) mission. We discuss case events in which we used the combination of satellites and ground magnetometer observations to characterize ULF wave differences in frequency, polarization, and preferential magnetospheric locations. We also discuss preliminary results of electron losses in terms of duration, location, and observed periodicity. We show that both externally driven (by PDSs) and internally supported (e.g., Field Line Resonance) waves occur simultaneously. In addition, while both internally and externally driven ULF waves are important for radiation belt dynamics, we find instances of BARREL X-ray counts that closely follow the solar wind driver. Title: Understanding Solar Eruptions, Solar Wind Formation, and how the Sun Connects to the Heliosphere through a Polar Perspective Authors: Viall, Nicholeen; Gibson, Sarah; Hassler, Don; Newmark, Jeffrey; Seaton, Daniel; Downs, Cooper Bibcode: 2021AGUFMSH34D..01V Altcode: A major limitation to our understanding of how the Sun connects to the heliosphere is due to our ecliptic bias: all remote observations of the Sun and corona have been made from the ecliptic. The ecliptic viewpoint by itself can never capture the global corona and its connection to the heliosphere. The ecliptic view has large uncertainties in measurements of the polar magnetic fields and has limited ability to measure longitudinal coronal structure. A polar perspective can provide new ways to test theories of a host of solar and heliospheric physics problems, from the quiescent processes involved in solar wind formation, up through transient solar eruptions and coronal mass ejections (CMEs). Because the structure and strength of the polar photospheric magnetic fields shape the corona and provide key input to coronal and heliospheric models, measuring and tracking the evolution of the polar magnetic fields provides the bones of the corona-heliosphere connection as well as information on the storage and release of explosive energy. Images of the corona in EUV and white light provide the coronal counterpart to the photospheric magnetic field measurements for connecting the Sun to the heliosphere. They capture global coronal connectivity and interactions, longitudinal expansion and structure, and the effects of co-rotation. Since CMEs tend to deflect toward the equator, a polar view captures essentially all Earth-and planet-directed CMEs from a view perpendicular to their direction of propagation. Overall, the discovery space for a polar imager is enormous. We describe progress on these topics that can be expected with Solar Orbiter, which will get to 30 degrees orbital inclination in the extended mission. We also discuss the unique science that can be done by continuous imaging of the polar magnetic fields and corona from above 70 degrees for at least a solar rotation, such as proposed by the Solaris mission. Title: Understanding the Corona-Heliosphere Connection by Identifying the Origins of In Situ Solar Wind Observations Authors: Wallace, Samantha; Viall, Nicholeen; Arge, Charles Bibcode: 2021AGUFMSH24C..03W Altcode: The corona and heliosphere are fundamentally connected by the solar wind outflow and magnetic field emanating from the Sun. Thus, accurate knowledge of the physical processes responsible for solar wind formation (i.e., origin, release, and acceleration), and evolution as the wind propagates are critical to understanding the corona and heliosphere as a globally connected system. We present evidence from aggregate work that demonstrates the importance of knowing the specific solar wind source regions when investigating solar wind formation. Each study makes use of the Wang-Sheeley-Arge (WSA) model driven by Air Force Data Assimilative Photospheric Flux Transport (ADAPT) time-dependent photospheric field maps to connect in situ solar wind observations from various spacecraft (e.g., ACE, PSP) to their source regions at 1 Rs. We highlight one study which investigates the properties of observed solar wind streams in relationship to their coronal streamer source. Specifically, we test whether the coronal plasma characteristics (e.g., hot/dense active region vs. cooler/more tenuous quiet Sun) or the global magnetic structure (e.g., pseudostreamer vs. helmet streamer) of the coronal streamer are more important in determining the resulting solar wind characteristics. Title: The Multiview Observatory for Solar Terrestrial Science (MOST) Authors: Gopalswamy, Nat; Kucera, Therese; Leake, James; MacDowall, Robert; Wilson, Lynn; Kanekal, Shrikanth; Shih, Albert; Christe, Steven; Gong, Qian; Viall, Nicholeen; Tadikonda, Sivakumar; Fung, Shing; Yashiro, Seiji; Makela, Pertti; Golub, Leon; DeLuca, Edward; Reeves, Katharine; Seaton, Daniel; Savage, Sabrina; Winebarger, Amy; DeForest, Craig; Desai, Mihir; Bastian, Tim; Lazio, Joseph; Jensen, P. E., C. S. P., Elizabeth; Manchester, Ward; Wood, Brian; Kooi, Jason; Wexler, David; Bale, Stuart; Krucker, Sam; Hurlburt, Neal; DeRosa, Marc; Pevtsov, Alexei; Tripathy, Sushanta; Jain, Kiran; Gosain, Sanjay; Petrie, Gordon; Kholikov, Shukirjon; Zhao, Junwei; Scherrer, Philip; Woods, Thomas; Chamberlin, Philip; Kenny, Megan Bibcode: 2021AGUFMSH12A..07G Altcode: The Multiview Observatory for Solar Terrestrial Science (MOST) is a comprehensive mission concept targeting the magnetic coupling between the solar interior and the heliosphere. The wide-ranging imagery and time series data from MOST will help understand the solar drivers and the heliospheric responses as a system, discerning and tracking 3D magnetic field structures, both transient and quiescent in the inner heliosphere. MOST will have seven remote-sensing and three in-situ instruments: (1) Magnetic and Doppler Imager (MaDI) to investigate surface and subsurface magnetism by exploiting the combination of helioseismic and magnetic-field measurements in the photosphere; (2) Inner Coronal Imager in EUV (ICIE) to study large-scale structures such as active regions, coronal holes and eruptive structures by capturing the magnetic connection between the photosphere and the corona to about 3 solar radii; (3) Hard X-ray Imager (HXI) to image the non-thermal flare structure; (4) White-light Coronagraph (WCOR) to seamlessly study transient and quiescent large-scale coronal structures extending from the ICIE field of view (FOV); (5) Faraday Effect Tracker of Coronal and Heliospheric structures (FETCH), a novel radio package to determine the magnetic field structure and plasma column density, and their evolution within 0.5 au; (6) Heliospheric Imager with Polarization (HIP) to track solar features beyond the WCOR FOV, study their impact on Earth, and provide important context for FETCH; (7) Radio and Plasma Wave instrument (M/WAVES) to study electron beams and shocks propagating into the heliosphere via passive radio emission; (8) Solar High-energy Ion Velocity Analyzer (SHIVA) to determine spectra of electrons, and ions from H to Fe at multiple spatial locations and use energetic particles as tracers of magnetic connectivity; (9) Solar Wind Magnetometer (MAG) to characterize magnetic structures at 1 au; (10) Solar Wind Plasma Instrument (SWPI) to characterize plasma structures at 1 au. MOST will have two large spacecraft with identical payloads deployed at L4 and L5 and two smaller spacecraft ahead of L4 and behind L5 to carry additional FETCH elements. MOST will build upon SOHO and STEREO achievements to expand the multiview observational approach into the first half of the 21st Century. Title: Periodic Structures in Solar Wind Composition Observed by the ACE/SWICS Instrument: Event Studies and Superposed Epoch Analysis Authors: Gershkovich, Irena; Lepri, Susan; Viall, Nicholeen; Di Matteo, Simone Bibcode: 2021AGUFMSH25F2157G Altcode: Periodic structures in the solar wind plasma are extremely abundant within the ACE SWICS 12-minute-resolution composition data set. We have demonstrated this statistically in our 2020 AGU Fall Meeting poster [Gershkovich et al., 2020]. These structures, observed in-situ, are indicators of variations in composition at the Corona being imprinted onto the solar wind. Previous work showed that specific periodic structures in proton and alpha density exist at statistically significant levels and cannot be attributed to turbulence [Viall et al., 2009]. Prior to our work, only one periodic event had been identified in the ACE SWICS composition data [Kepko et al., 2016]. We are greatly expanding upon knowledge in this area by analyzing 13 years (1998-2011) of SWICS 12-minute composition data, looking at all ions and charge states of sufficiently high data quality. Composition is excellent for studying processes occurring at the Sun as it is frozen into the solar wind and has no means to evolve as the plasma advects from the solar atmosphere towards the point of observation. Here, we present the results of a superposed epoch analysis consisting of several events and examine trends in charge-state, mass and First Ionization Potential (FIP) in the 24-hour windows before, during and after each event. We compare the composition ratio and charge state data to previously established typical coronal hole and streamer values and discuss the patterns that emerge. Title: An Analysis of Spikes in Atmospheric Imaging Assembly (AIA) Data Authors: Young, Peter R.; Viall, Nicholeen M.; Kirk, Michael S.; Mason, Emily I.; Chitta, Lakshmi Pradeep Bibcode: 2021SoPh..296..181Y Altcode: 2021arXiv210802624Y The Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) returns high-resolution images of the solar atmosphere in seven extreme ultraviolet (EUV) wavelength channels. The images are processed on the ground to remove intensity spikes arising from energetic particles hitting the instrument, and the despiked images are provided to the community. In this article, a three-hour series of images from the 171 Å channel obtained on 28 February 2017 was studied to investigate how often the despiking algorithm gave false positives caused by compact brightenings in the solar atmosphere. The latter were identified through spikes appearing in the same detector pixel for three consecutive frames. 1096 examples were found from the 900 image frames. These "three-spikes" were assigned to 126 dynamic solar features, and it is estimated that the three-spike method identifies 19% of the total number of features affected by despiking. For any ten-minute sequence of AIA 171 Å images there are around 37 solar features that have their intensity modified by despiking. The features are found in active regions, quiet Sun, and coronal holes and, in relation to solar surface area, there is a greater proportion within coronal holes. In 96% of the cases, the despiked structure is a compact brightening with a size of two arcsec or less, and the remaining 4% have narrow, elongated structures. By applying an EUV burst detection algorithm, we found that 96% of the events could be classified as EUV bursts. None of the spike events are rendered invisible by the AIA processing pipeline, but the total intensity over an event's lifetime can be reduced by up to 67%. Users are recommended to always restore the original intensities in AIA data when studying short-lived or rapidly evolving features that exhibit fine-scale structure. Title: Connecting the Low to the High Corona: A Method to Isolate Transients in STEREO/COR1 Images Authors: Alzate, Nathalia; Morgan, Huw; Viall, Nicholeen; Vourlidas, Angelos Bibcode: 2021ApJ...919...98A Altcode: 2021arXiv210702644A We present a method that isolates time-varying components from coronagraph and extreme ultraviolet images, allowing substreamer transients propagating within streamers to be tracked from the low to the high corona. The method uses a temporal bandpass filter with a transmission bandwidth of ~2.5-10 hr that suppresses both high- and low-frequency variations in observations made by the STEREO/SECCHI suite. We demonstrate that this method proves crucial in linking features in the low corona, where the magnetic field is highly nonradial, to their counterparts in the high corona, where the magnetic field follows a radial path, through the COR1 instrument. We also apply our method to observations by the COR2 and EUVI instruments on board SECCHI and produce height-time profiles that reveal small density enhancements, associated with helmet streamers propagating from ~1.2 R out to beyond 5 R. Our processing method reveals that these features are common during the period of solar minimum in this study. The features recur on timescales of hours, originate very close to the Sun, and remain coherent out into interplanetary space. We measure the speed of the features and classify them as slow (a few to tens of kilometers per second) or fast (~100 km s-1). Both types of features serve as an observable tracer of a variable component of the slow solar wind to its source regions. Our methodology helps overcome the difficulties in tracking small-scale features through COR1. As a result, it proves successful in measuring the connectivity between the low and high corona and in measuring the velocities of small-scale features. Title: Understanding Heating in Active Region Cores through Machine Learning. II. Classifying Observations Authors: Barnes, W. T.; Bradshaw, S. J.; Viall, N. M. Bibcode: 2021ApJ...919..132B Altcode: 2021arXiv210707612B Constraining the frequency of energy deposition in magnetically closed active region cores requires sophisticated hydrodynamic simulations of the coronal plasma and detailed forward modeling of the optically thin line-of-sight integrated emission. However, understanding which set of model inputs best matches a set of observations is complicated by the need for any proposed heating model to simultaneously satisfy multiple observable constraints. In this paper, we train a random forest classification model on a set of forward-modeled observable quantities, namely the emission measure slope, the peak temperature of the emission measure distribution, and the time lag and maximum cross-correlation between multiple pairs of AIA channels. We then use our trained model to classify the heating frequency in every pixel of active region NOAA 1158 using the observed emission measure slopes, peak temperatures, time lags, and maximum cross-correlations, and are able to map the heating frequency across the entire active region. We find that high-frequency heating dominates in the inner core of the active region while intermediate-frequency dominates closer to the periphery of the active region. Additionally, we assess the importance of each observed quantity in our trained classification model and find that the emission measure slope is the dominant feature in deciding with which heating frequency a given pixel is most consistent. The technique presented here offers a very promising and widely applicable method for assessing observations in terms of detailed forward models given an arbitrary number of observable constraints. Title: Erratum: "On the Relationship between Magnetic Expansion Factor and Observed Speed of the Solar Wind from Coronal Pseudostreamers" (2020, ApJ, 898, 78) Authors: Wallace, Samantha; Arge, C. Nick; Viall, Nicholeen; Pihlström, Ylva Bibcode: 2021ApJ...919...68W Altcode: No abstract at ADS Title: Mesoscale Structure in the Solar Wind Authors: Viall, N. M.; DeForest, C. E.; Kepko, L. Bibcode: 2021FrASS...8..139V Altcode: Structures in the solar wind result from two basic mechanisms: structures injected or imposed directly by the Sun, and structures formed through processing en route as the solar wind advects outward and fills the heliosphere. On the largest scales, solar structures directly impose heliospheric structures, such as coronal holes imposing high speed streams of solar wind. Transient solar processes can inject large-scale structure directly into the heliosphere as well, such as coronal mass ejections. At the smallest, kinetic scales, the solar wind plasma continually evolves, converting energy into heat, and all structure at these scales is formed en route. `Mesoscale' structures, with scales at 1 AU in the approximate spatial range of 5 Mm -10,000 Mm and temporal range of 10 s - 7 hrs, lie in the orders of magnitude gap between the two size-scale extremes. Structures of this size regime are created through both mechanisms. Competition between the imposed and injected structures with turbulent and other evolution leads to complex structuring and dynamics. The goal is to understand this interplay and to determine which type of mesoscale structures dominate the solar wind under which conditions. However, the mesoscale regime is also the region of observation space that is grossly under-sampled. The sparse in situ measurements that currently exist are only able to measure individual instances of discrete structures, and are not capable of following their evolution or spatial extent. Remote imaging has captured global and large scale features and their evolution, but does not yet have the sensitivity to measure most mesoscale structures and their evolution. Similarly, simulations cannot model the global system while simultaneously resolving kinetic effects. It is important to understand the source and evolution of solar wind mesoscale structures because they contain information on how the Sun forms the solar wind, and constrains the physics of turbulent processes. Mesoscale structures also comprise the ground state of space weather, continually buffeting planetary magnetospheres. In this paper we describe the current understanding of the formation and evolution mechanisms of mesoscale structures in the solar wind, their characteristics, implications, and future steps for research progress on this topic. Title: Periodic Solar Wind Density Structures Observed with Parker Solar Probe WISPR Authors: Viall, N. M.; Vourlidas, A.; Howard, R.; Linton, M.; Kepko, L.; Di Matteo, S.; Higginson, A. K. Bibcode: 2021AAS...23812305V Altcode: Periodic trains of mesoscale structures in solar wind density have been observed close to the Sun with in situ data from the Helios spacecraft, as well as remotely in STEREO/COR2 and STEREO/HI1 white light imaging data. While some periodic density structures may be a consequence of the development of dynamics en route, many are remnants of the formation and release of the solar wind, and thus provide important constraints on solar wind models. The instrument suite on Parker Solar Probe offers an unprecedented viewpoint of the ambient solar wind and structure therein, shortly after its formation and release from the solar corona. Here, we report on the first observations of periodic trains of mesoscale structures in solar wind density observed by the Wide-field Imager for Parker Solar PRobe (WISPR). We describe our open-source Fourier analysis and robust spectral background estimation technique used to identify the periodic density structures. The observation of periodic density structures so near to the Sun allows us to begin disentangling how much structure is created during solar wind formation, versus how much is due to evolution as the solar wind advects outward. Title: Evidence For Active Region Coronal Heating By Nanoflares Based On Time-lag Measurements In EUV Light Curves From EIS Authors: Brosius, J.; Viall, N. Bibcode: 2021AAS...23832813B Altcode: The nanoflare model of solar coronal heating is based on the idea that ubiquitous tiny, independent heating events occur on individual sub-resolution strands within coronal loops. Each heating event raises its strand plasma to temperatures (6-10 MK) that are greater than the average active region temperature (about 2 MK). After the impulsive energy release, the loop strand increases in density and cools by conduction and radiation. The strand spends more time at higher density in the radiative cooling phase than it does in any other phase of the heating and cooling cycle. Thus, even when observed on spatial scales larger than the unresolvable individual strands, the solar atmosphere is expected to exhibit an overall cooling trend. Evidence for this has been presented based on correlations among light curves from AIA's six EUV channels. While this supports the nanoflare model of coronal heating, AIA's lack of temperature fidelity means that precise cooling information for small locations or single events are less than conclusive. Here we report results from an investigation of time-lag diagnostics based on EUV light curves derived from stare spectra obtained with a new EIS study designed to investigate time lags in non-flaring active regions. This study observes line emission from ten successive ionization stages of iron (VIII-XVII, 0.45-4 MK), as well as Fe XXIII (seen in flares and microflares) and lines formed at lower temperatures. We find evidence of post-nanoflare cooling in AR 12759 from 2.8 to 0.45 MK, but note that not all locations cool to temperatures this low, possibly indicating a mixture of medium and low frequency nanoflares. The AR periphery is cooler than its core, and exhibits post-nanoflare cooling only from 1.7 to 1.4 MK, suggestive of higher frequency nanoflares. Title: The Heating of the Solar Corona Authors: Viall, Nicholeen M.; De Moortel, Ineke; Downs, Cooper; Klimchuk, James A.; Parenti, Susanna; Reale, Fabio Bibcode: 2021GMS...258...35V Altcode: No abstract at ADS Title: The Solar Wind Authors: Rouillard, Alexis P.; Viall, Nicholeen; Pierrard, Viviane; Vocks, Christian; Matteini, Lorenzo; Alexandrova, Olga; Higginson, Aleida K.; Lavraud, Benoit; Lavarra, Michael; Wu, Yihong; Pinto, Rui; Bemporad, Alessandro; Sanchez-Diaz, Eduardo Bibcode: 2021GMS...258....1R Altcode: No abstract at ADS Title: Understanding Solar Wind Formation by Identifying the Origins of In Situ Observations Authors: Wallace, Samantha; Viall, Nicholeen M.; Arge, Charles N. Bibcode: 2021EGUGA..23.6200W Altcode: Solar wind formation can be separated into three physical steps - source, release, and acceleration - that each leave distinct observational signatures on plasma parcels. The Wang-Sheeley-Arge (WSA) model driven by Air Force Data Assimilative Photospheric Flux Transport (ADAPT) time-dependent photospheric field maps now has the ability to connect in situ observations more rigorously to their precise source at the Sun, allowing us to investigate the physical processes involved in solar wind formation. In this talk, I will highlight my PhD dissertation research in which we use the ADAPT-WSA model to either characterize the solar wind emerging from specific sources, or investigate the formation process of various solar wind populations. In the first study, we test the well-known inverse relationship between expansion factor (fs) and observed solar wind speed (vobs) for solar wind that emerges from a large sampling of pseudostreamers, to investigate if field line expansion plays a physical role in accelerating the solar wind from this source region. We find that there is no correlation between fs and vobs at pseudostreamer cusps. In the second study, we determine the source locations of the first identified quasiperiodic density structures (PDSs) inside 0.6 au. Our modeling provides confirmation of these events forming via magnetic reconnection both near to and far from the heliospheric current sheet (HCS) - a direct test of the Separatrix-web (S-web) theory of slow solar wind formation. In the final study, we use our methodology to identify the source regions of the first observations from the Parker Solar Probe (PSP) mission. Our modeling enabled us to characterize the closest to the Sun observed coronal mass ejection (CME) to date as a streamer blowout. We close with future ways that ADAPT-WSA can be used to test outstanding questions of solar wind formation. Title: Power Spectral Density Background Estimate and Signal Detection via the Multitaper Method Authors: Di Matteo, S.; Viall, N. M.; Kepko, L. Bibcode: 2021JGRA..12628748D Altcode: We present a new spectral analysis method for the identification of periodic signals in geophysical time series. We evaluate the power spectral density with the adaptive multitaper method, a nonparametric spectral analysis technique suitable for time series characterized by colored power spectral density. Our method provides a maximum likelihood estimation of the power spectral density background according to four different models. It includes the option for the models to be fitted on four smoothed versions of the power spectral density when there is a need to reduce the influence of power enhancements due to periodic signals. We use a statistical criterion to select the best background representation among the different smoothing + model pairs. Then, we define the confidence thresholds to identify the power spectral density enhancements related to the occurrence of periodic fluctuations (γ test). We combine the results with those obtained with the multitaper harmonic F test, an additional complex valued regression analysis from which it is possible to estimate the amplitude and phase of the signals. We demonstrate the algorithm on Monte Carlo simulations of synthetic time series and a case study of magnetospheric field fluctuations directly driven by periodic density structures in the solar wind. The method is robust and flexible. Our procedure is freely available as a stand alone IDL code at https://zenodo.org/record/3703168. The modular structure of our methodology allows the introduction of new smoothing methods and models to cover additional types of time series. The flexibility and extensibility of the technique makes it broadly suitable to any discipline. Title: Power spectrum power-law indices as a diagnostic of coronal heating Authors: Ireland, Jack; Bradshaw, Stephen; Kirk, Michael; Viall, Nicholeen Bibcode: 2021cosp...43E1805I Altcode: We investigate the coronal heating of active regions using time-series analysis and hydrodynamic modeling. Viall & Klimchuk 2011, 2012, 2014, 2017 have shown that the timing of active region coronal emission brightenings in multiple channels of Solar Dynamics Observatory Atmospheric Imaging Assembly (SDO/AIA) approximates that derived from simulations of a nanoflare-heated corona. Using Numerical HYDrodynamic RADiative Emission Model for the Solar Atmosphere (HYDRAD)-based simulations of AIA emission for an AR, Bradshaw & Viall 2016 have shown that the timing of coronal emission brightenings is dependent on the properties of the nanoflare energy distribution and occurrence rate. Relatedly, Ireland et al. 2015 show that average power spectra $P(f)$ (where $f$ is frequency) of time series of AIA 171Å and 193Å AR images are dominated by power laws, $P(f)\approx f^{-z}$, $z>0$. This may be explainable by assuming a distribution of exponentially decaying events of emission along the line-of-sight which can also result in power-law power spectra. We present analyses that test the hypothesis that a distribution of nanoflare events causes both the emission power-law power spectrum in AIA time-series as well as the observed brightening time-lags. Firstly, we show that the power-law indices of Fourier power spectra of the same simulated data described in Bradshaw & Viall 2016 depends on the frequency of nanoflares used. Secondly, using the observational AIA time-series data analyzed by Viall & Klimchuk (2013), we obtain and discuss the correlations of the cross-channel time-lags with the power-law indices of Fourier power spectra in each AIA channel. Finally, we discuss the ability of power-law indices and time-lags together to constrain the underlying nanoflare frequency distribution. Title: A Synthesis of First Results from Parker Solar Probe and Solar Orbiter Authors: Viall, Nicholeen Bibcode: 2021cosp...43E.930V Altcode: Parker Solar Probe, which launched on August 12, 2018, has now completed several orbits, traveling closer to the Sun than any spacecraft before. Together with Solar Orbiter, which was launched February 10, 2020, these missions provide a remarkable combination of vantage points and measurements of the connection between the Sun and the heliosphere. The instrument suites measure in situ thermal plasma, energetic particles, magnetic and electric fields and waves, and the remote measurements include measurements of the photospheric magnetic field, extreme ultraviolet and X-ray emissions, and white light emission from the corona and heliosphere. These data are already yielding great advancements in the understanding of coronal heating, solar wind formation and acceleration, and coronal mass ejections and energetic particle acceleration. We describe first results from Parker Solar Probe and Solar Orbiter on the Sun and how it creates the heliosphere. Title: Investigating power law power spectra as a diagnostic of nanoflare coronal heating in active regions Authors: Ireland, J.; Bradshaw, S. J.; Viall, N. M.; Kirk, M. S. Bibcode: 2020AGUFMSH0370006I Altcode: Power spectra of time series of synthetic AIA emission derived from simulations of coronal heating in a realistic active region geometry are analyzed. The synthetic AIA emissions are the same as those described in Bradshaw & Viall 2016 ApJ, 821, 63. In those simulations, low, intermediate and high frequency nanoflare occurrence rates are postulated and the consequent synthetic AIA observations are calculated. Bradshaw & Viall (2016) calculate the time lags between hotter and cooler synthetic AIA channels derived via cross-correlation of time series, and show that these timelags are broadly similar to those derived from observational data. Power spectra of time series of synthetic AIA emission are fit by a model P(f) = Af-n + C, where f is frequency, n>0, A>C>0. For all six synthetic AIA channels, it is shown that the fit power law index n depends on AIA channel, spatial location, and the frequency of nanoflare energy deposition. This suggests that power spectra of time series of observational AIA data contain information on the frequency of nanoflare energy deposition. We discuss the use of power law power spectra as a possible diagnostic of nanoflare heating in observational data. We demonstrated that the analysis of power spectra may provide further constraints on the distribution of heating frequencies in active regions, complementary to existing constraints derived from other observables such as emission measure slopes and time lags. Title: Contemporary Analysis Methods for Coronagraph and Heliospheric Imager Data Authors: Thompson, B. J.; Attie, R.; Chhiber, R.; Cranmer, S. R.; DeForest, C.; Gallardo-Lacourt, B.; Gibson, S. E.; Jones, S. I.; Moraes Filho, V.; Reginald, N. L.; Uritsky, V. M.; Viall, N. M. Bibcode: 2020AGUFMSH031..05T Altcode: Coronagraphs, polarimeters, and heliospheric imagers are providing new insight into how structures in the solar wind form and develop as they flow from the inner corona into the heliosphere. With this comes a whole new frontier of physical observables in 3D, including kinetic (velocity and acceleration), thermodynamic (density, temperature, and shock boundary), and magnetic field properties. These measurements inform and challenge models of global solar wind flow, turbulence, and CME propagation. We will discuss recent advances in quantifying physical properties of the corona and solar wind using coronagraph and heliospheric imager data, and make predictions of what new models and instrumentation (including the in-development PUNCH mission) will bring us in the future. Title: A New Spectral Analysis Procedure for the Identification of ULF Waves. Authors: Di Matteo, S.; Viall, N. M.; Kepko, L. Bibcode: 2020AGUFMSM0060002D Altcode: In the analysis of time series, one of the major diagnostic tools for the identification of periodic fluctuations is the frequency domain characterization via the power spectral density (PSD). Periodic signals manifest as enhancements relative to the continuous PSD. While harmonic analysis to identify the occurrence of periodic variations compared to a flat PSD are well established, there is a lack of standard techniques to assess the significance of a periodicity against colored noise, such as in the identification of ULF waves with respect to the colored PSD typically found in geophysical time series. Here, we present a new procedure based on the adaptive multitaper method (an open source IDL version is available at https://zenodo.org/record/3703168). This sophisticated non-parametric spectral analysis approach, suitable for colored PSD, provide an additional complex-valued regression model from which it is possible to estimate the amplitude and phase of the signals: the harmonic F test. A priori knowledge of the statistical properties of the multitaper PSD estimates allows a robust maximum likelihood fitting of different continuous PSD background models. Then, the best representation is selected via objective statistical criteria. Confidence thresholds are used to determine statistically significant PSD enhancements, that, when combined with the harmonic F test, provide robust estimates of the frequency of periodic oscillations occurring in the time series. After testing the performance of this method on Monte Carlo simulation of synthetic time series, we identify ULF fluctuations in magnetospheric field observations at geostationary orbit and in ground observatories. We present a brief tutorial of our procedure, and we discuss our analysis of the ULF fluctuations identified with our method. Title: Changes in Alpha-to-Proton Ratios During Periodic Solar Wind Density Structures Authors: Kepko, L.; Viall, N. M. Bibcode: 2020AGUFMSH0440030K Altcode: Trains of mesoscale density structures in the solar wind are often periodic, with length scales between ~100-1000 Mm, and frequencies corresponding to their advected timescales of ~0.1-5 mHz. Previous work has shown that periodic density structure (PDSs) occur at recurrent scale sizes, and that those scale sizes are correlated with the terminator signaling end of the solar cycle. However, only a few event studies have examined the compositional changes associated with PDSs. Compositional changes, such as the alpha-to-proton ratio, are frozen into the plasma low in the corona, and so do not evolve as the solar wind advects and fills the Heliosphere. Therefore, alpha-to-proton ratio changes within PDSs provide critical insights into and constraints on their generation - i.e., whether they are generated in situ during transit to 1 AU, or created as part of solar wind formation. Using 25 years of Wind proton and alpha solar wind data measured at 90-s cadence, we examine the occurrence distributions of periodic density structures. Our analysis utilizes multiple temporal (between 6 and 48 hours) and length-scale (9-72 Gm) spectral analysis windows to fully explore the distribution of alpha-to-proton ratio changes during PDSs placing important constraints on their generation mechanism and the formation of the solar wind. Title: Evidence of Solar Coronal Heating by Nanoflares Based on Time-Lag Measurements in EUV Light Curves from EIS Authors: Brosius, J. W.; Viall, N. M. Bibcode: 2020AGUFMSH0370004B Altcode: The nanoflare model is a major contender to explain solar coronal heating. The model is based on the idea that ubiquitous tiny, independent heating events occur on individual sub-resolution strands within coronal loops. Each heating event raises its strand plasma to temperatures (6 - 10 MK) that are greater than the average active region temperature (~2 MK). Compelling evidence for this mechanism is pervasive faint emission at flare-like temperatures, such as that detected in an active region by the EUNIS sounding rocket. After the impulsive energy release, the loop strand cools by conduction and radiation, during which it spends more time at higher density and at colder temperatures than it does at hotter temperatures. Thus, even when observed on spatial scales larger than the unresolvable individual strands, the solar atmosphere is expected to exhibit an overall cooling trend. Evidence for this cooling trend has been sought and found based on correlations among light curves from AIA's six EUV channels. While this provides further support for the nanoflare model of coronal heating, AIA's lack of temperature fidelity means that precise cooling information for small locations or single events are less than conclusive. Here we report preliminary results from an investigation of time-lag diagnostics based on EUV light curves from Hinode/EIS spectra. We present results for non-flaring active regions and quiet-sun areas derived from stare spectra obtained with several different EIS studies that observe unblended emission lines formed at temperatures that range from 0.14 to 14 MK. We performed time-lag diagnostics on light curves of Fe XXIII, Fe XVII, Fe XVI, Fe XIV, S X, Si VII, Mg VI, O IV, and other lines. For example, for a 2014 March 11 observing run on AR 12002, EIS observed fan loops that cooled slowly between 1.4 and 0.6 MK on timescales of ~3000s; microflares that cooled from 14 to 2 MK on timescales of ~1000 s; and core loops that cooled from about 4 to 0.1 MK on timescales ~1500 s. Properties such as peak temperature, the timescales of the cooling, and a determination of whether the cooling is full or partial all provide valuable constraints on nanoflares as a source of coronal heating. Title: On the Relationship between Magnetic Expansion Factor and Observed Speed of the Solar Wind from Coronal Pseudostreamers Authors: Wallace, S.; Arge, C. N.; Viall, N. M.; Pihlstrom, Y. Bibcode: 2020AGUFMSH041..06W Altcode: For the past 30+ yr, the magnetic expansion factor ( f s ) has been used in empirical relationships to predict solar wind speed ( v obs) at 1 AU based on an inverse relationship between these two quantities. Coronal unipolar streamers (i.e., pseudostreamers) undergo limited field line expansion, resulting in f s -dependent relationships to predict the fast wind associated with these structures. However, case studies have shown that the in situ observed pseudostreamer solar wind was much slower than that derived with f s . To investigate this further, we conduct a statistical analysis to determine if f s and v obs are inversely correlated for a large sample of periods when pseudostreamer wind was observed at multiple 1 au spacecraft (i.e., ACE, STEREO-A/B). We use the Wang-Sheeley-Arge model driven by Air Force Data Assimilative Photospheric Flux Transport (ADAPT) photospheric field maps to identify 38 periods when spacecraft observe pseudostreamer wind. We compare the expansion factor of the last open field lines on either side of a pseudostreamer cusp with the corresponding in situ measured solar wind speed. We find that only slow wind ( v obs < 500 km s −1) is associated with pseudostreamers and that there is not a significant correlation between f s and v obs for these field lines. This suggests that field lines near the open-closed boundary of pseudostreamers are not subject to the steady-state acceleration along continuously open flux tubes assumed in the f s - v obs relationship. In general, dynamics at the boundary between open and closed field lines such as interchange reconnection will invalidate the steady-state assumptions of this relationship. Title: Signatures of Type III Solar Radio Bursts from Nanoflares: Final Results Authors: Chhabra, S.; Klimchuk, J. A.; Gary, D. E.; Viall, N. M. Bibcode: 2020AGUFMSH0430016C Altcode: The heating mechanisms responsible for the million degree solar corona remain one of the most intriguing problems in space science. It is widely agreed, that the ubiquitous presence o f reconnection events and the associated impulsive heating (nanoflares) are a strong candidate in solving this problem [Klimchuk J.A., 2015 and references therein].

Whether nanoflares accelerate energetic particles like full sized flares is unknown. The lack of strong emission in hard X rays suggests that the quantity of highly energetic particles is small. There could, however, be large numbers of mildly energetic particles (~ 10 keV). We investigate such particles by searching for the type III radio bursts that they may produce. If energetic electron beams propagating along magnetic field lines generate a bump on tail instability, they will produce Langmuir waves, which can then interact with other particles and waves to give rise to emission at the local plasma frequency and its first harmonic. Type III radio bursts bursts are characteristically known to exhibit high frequency drifts as the beam propagates through a density gradient. The time lag technique that was developed to study subtle delays in light curves from different EUV channels [Viall & Klimchuk 2012] can also be used to detect subtle delays at different radio frequencies. We have modeled the expected radio emission from nanoflares, which we used to test and calibrate the technique. We will present the final results of our modeling efforts along with results from application of the technique to actual radio observations from VLA (Very Large Array), MWA (Murchison Widefield Array) and seeking data from LOFAR (Low Frequency Array) as well.We are also using data from the PSP (Parker Solar Probe) to look for similar reconnection signatures in the Solar Wind. Our goal is to determine whether nanoflares accelerate energetic particles and to determine their properties. The results will have important implications for both the particle acceleration and reconnection physics. Title: The Coronal Diagnostic Experiment (CODEX) Authors: Newmark, J. S.; Gopalswamy, N.; Kim, Y. H.; Viall, N. M.; Cho, K. S. F.; Reginald, N. L.; Bong, S. C.; Gong, Q.; Choi, S.; Strachan, L.; Yashiro, S. Bibcode: 2020AGUFMSH0280011N Altcode: Understanding solar wind sources and acceleration mechanisms is an overarching solar physics goal. Current models are highly under-constrained due to the limitations of the existing data, particularly in the ~3-10 Rs range. COronal Diagnostic EXperiment (CODEX) is designed to deliver the first global, comprehensive data sets that will impose crucial constraints and answer targeted essential questions, including: Are there signatures of hot plasma released into the solar wind from previously closed fields? What are the velocities and temperatures of the density structures that are observed so ubiquitously within streamers and coronal holes?

To provide these crucial measurements, NASA's Goddard Space Flight Center, in collaboration with the Korea Astronomy and Space Science Institute, will develop a next-generation coronagraph for the International Space Station. This imaging coronagraph uses multiple filters to obtain simultaneous measurements of electron density, temperature, and velocity within a single instrument. This will be the first time all three have been measured simultaneously for this critical field-of-view, and CODEX achieves these measurements multiple times a day. Title: Connecting the Low to High Corona: Tracking Outward Propagating Small-Scale Structures Using EUV and Coronagraph Observations Authors: Alzate, N.; Seaton, D. B.; Morgan, H.; Viall, N. M. Bibcode: 2020AGUFMSH0300010A Altcode: Previous studies have identified density structures in the slow solar wind that can be traced down through the field of view (FOV) of STEREO A/COR2. Advanced image processing techniques can reveal small, faint, formerly hidden structures in extreme ultraviolet (EUV) and white light (WL) data providing continuous tracking of brightness enhancements from the coronal base to the radial extended corona. Our most recent work, carried out using STEREO/COR1 observations, proved crucial in linking the low to the high corona. Our method for processing COR1, which facilitates the tracking of small-scale outward propagating structures, was applied to several time periods of observations during Solar cycle 24 for the study of the sources of the slow solar wind. We identified the outflows in ancillary data from multiple WL coronagraphs as well as EUV instruments, including STEREO/EUVI and GOES-R/SUVI when available. The outflows were tracked across the different FOVs from carefully chosen datasets based on spacecraft position. The larger FOV of SUVI enabled an uninterrupted view of outflows from the Sun through images from corresponding coronagraph observations. In doing so, we were able to link structures near the Sun where the magnetic field structure is highly non-radial to their counterparts higher up where the magnetic field follows a radial path. Our work opens the door for studies of the origin of small-scale structures in the heliosphere and their implications for the sources of the slow solar wind. Title: Inherent Spatial Scales of Solar Wind Periodic Structures Found in ACE/SWICS Data Authors: Gershkovich, I.; Lepri, S. T.; Viall, N. M.; Di Matteo, S. Bibcode: 2020AGUFMSH0440028G Altcode: The existence of prevalent, statistically significant, periodic structures in the in-situ observations of number density and solar wind composition at 1 AU can meaningfully address outstanding questions about solar wind formation and evolution. Past work [ Viall et al., 2009] has revealed that specific periodicities in solar wind proton and alpha density, as well as dayside magnetospheric oscillations, exist and suggest that mechanisms beyond what can be attributed to turbulence are responsible. Composition is frozen into the solar wind and does not evolve as the plasma advects from the upper atmosphere of the Sun, making it invaluable to studying the processes that form the heliosphere. Although a periodic event has been identified in the ACE SWICS data previously [ Kepko et al., 2016], we greatly expand upon the scope of all prior work by analyzing 13 years (1998-2011) of SWICS 12-minute solar wind composition data, considering all ion species and charge states with sufficient data quality. We now have a database of validated windows, for a variety of ion species and charge states, which have sufficient continuity, counts and low enough errors to be suitable for any time series analysis. Spectral peaks identified by our algorithm are required to pass both an amplitude and a harmonic F-test at a 90% or greater confidence level. This ensures that rigorous statistical significance criteria have been met. We are conducting an unprecedented study of the relationships between the periodicities of different ion densities and their dependence on First Ionization Potential (FIP) bias, charge state, and mass. Here, we present statistical results that identify generally prevalent periodic enhancements as well as event studies that explore the spectral behaviors associated with heliospheric current sheet crossings, ICME sheaths and stream interaction regions. This analysis of composition variations informs our understanding of solar wind formation and evolution. Title: Using SDO/AIA to Understand the Thermal Evolution of Solar Prominence Formation Authors: Viall, Nicholeen M.; Kucera, Therese A.; Karpen, Judith T. Bibcode: 2020ApJ...905...15V Altcode: We investigated the thermal properties of prominence formation using time series analysis of Solar Dynamics Observatory's Atmospheric Imaging Assembly (SDO/AIA) data. Here, we report the first time-lag measurements derived from SDO/AIA observations of a prominence and its cavity on the solar limb, made possible by AIA's different wave bands and high time resolution. With our time-lag analysis, which tracks the thermal evolution using emission formed at different temperatures, we find that the prominence cavity exhibited a mixture of heating and cooling signatures. This is in contrast to prior time-lag studies of multiple active regions that chiefly identified cooling signatures and very few heating signatures, which is consistent with nanoflare heating. We also computed time lags for the same pairs of SDO/AIA channels using output from a one-dimensional hydrodynamic model of prominence material forming through thermal nonequilibrium (TNE). We demonstrate that the SDO/AIA time lags for flux tubes undergoing TNE are predicted to be highly complex, changing with time and location along the flux tube, and are consistent with the observed time-lag signatures in the cavity surrounding the prominence. Therefore, the time-lag analysis is a sensitive indicator of the heating and cooling processes in different coronal regions. The time lags calculated for the simulated prominence flux tube are consistent with the behavior deduced from the AIA data, thus supporting the TNE model of prominence formation. Future investigations of time lags predicted by other models for the prominence mass could be a valuable method for discriminating among competing physical mechanisms. Title: Using Coronagraphs and Heliospheric Imagers to Answer the Outstanding Questions of Solar Wind Physics Authors: Viall, N. M.; Borovsky, J. Bibcode: 2020AGUFMSH0280003V Altcode: As a part of the American Geophysical Union's Centennial celebration, the Journal of Geophysical Research commissioned papers on the Grand Challenges in the Earth and Space Sciences. We present our Grand Challenge paper on nine outstanding questions of solar wind physics that synthesizes input from the heliophysics community. These involve questions about the formation of the solar wind, about the inherent properties of the solar wind (and what the properties say about its formation), and about the evolution of the solar wind. The nine questions focus on (1) origin locations on the Sun, (2) plasma release, (3) acceleration, (4) heavy-ion abundances and charge states, (5) magnetic structure, (6) Alfven waves, (7) turbulence, (8) distribution-function evolution, and (9) energetic-particle transport. We address the aspects of these questions where progress is being made with the coronagraphs and heliospheric imagers on current missions such as Solar TErrestrial RElations Observatory (STEREO), Parker Solar Probe, and Solar Orbiter. We conclude with the aspects that require observations from the coronagraphs and heliospheric imagers on the upcoming missions Polarimeter to Unify the Corona and Heliosphere (PUNCH), COronal Diagnostic EXperiment (CODEX), as well as a mission with a polar view, such as Solaris . Title: SPD_MTM: a spectral analysis tool for the SPEDAS framework Authors: Di Matteo, Simone; Viall, Nicholeen; Kepko, Larry Bibcode: 2020zndo...3703168D Altcode: This is a new spectral analysis method for the identification of periodic signals in geophysical time series. We evaluate the power spectral density with the adaptive multitaper method, a sophisticated non-parametric spectral analysis technique suitable for time series characterized by colored power spectral density. Our method provides a maximum likelihood estimation of the power spectral density background according to four different models. It includes the option for the models to be fitted on four smoothed versions of the power spectral density when there is a need to reduce the influence of power enhancements due to periodic signals. We use a statistical criterion to select the best background representation among the different smoothing+model pairs. Then, we define the confidence thresholds to identify the power spectral density enhancements related to the occurrence of periodic fluctuations. We combine the results with those obtained with the multitaper harmonic F test, an additional complex-valued regression analysis from which it is possible to estimate the amplitude and phase of the signals. A complete description of the procedure is available at https://doi.org/10.1002/essoar.10502619.2. This work is under review for publication in JGR: Space Physics - Technical Reports: Methods. Title: Inherent Length Scales of Periodic Mesoscale Density Structures in the Solar Wind Over Two Solar Cycles Authors: Kepko, L.; Viall, N. M.; Wolfinger, K. Bibcode: 2020JGRA..12528037K Altcode: It is now well established through multiple event and statistical studies that the solar wind at 1 AU contains contains periodic, mesoscale (L ∼ 100-1,000 Mm) structures in the proton density. Composition variations observed within these structures and remote sensing observations of similar structures in the young solar wind indicate that at least some of these periodic structures originate in the solar atmosphere as a part of solar wind formation. Viall et al. (2008, https://doi.org/10.1029/2007JA012881) analyzed 11 years of data from the Wind spacecraft near L1 and demonstrated a recurrence to the observed length scales of periodic structures in the solar wind proton density. In the time since that study, Wind has collected 14 additional years of solar wind data, new moment analysis of the Wind SWE data is available, and new methods for spectral background approximation have been developed. In this study, we analyze 25 years of Wind data collected near L1 and produce occurrence distributions of statistically significant periodic length scales in proton density. The results significantly expand upon the Viall et al. (2008, https://doi.org/10.1029/2007JA012881) study and further show a possible relation of the length scales to solar "termination" events. Title: On the Relationship between Magnetic Expansion Factor and Observed Speed of the Solar Wind from Coronal Pseudostreamers Authors: Wallace, Samantha; Arge, C. Nick; Viall, Nicholeen; Pihlström, Ylva Bibcode: 2020ApJ...898...78W Altcode: 2020arXiv200716168W For the past 30+ yr, the magnetic expansion factor (fs) has been used in empirical relationships to predict solar wind speed (vobs) at 1 au based on an inverse relationship between these two quantities. Coronal unipolar streamers (i.e., pseudostreamers) undergo limited field line expansion, resulting in fs-dependent relationships to predict the fast wind associated with these structures. However, case studies have shown that the in situ observed pseudostreamer solar wind was much slower than that derived with fs. To investigate this further, we conduct a statistical analysis to determine if fs and vobs are inversely correlated for a large sample of periods when pseudostreamer wind was observed at multiple 1 au spacecraft (i.e., ACE, STEREO-A/B). We use the Wang-Sheeley-Arge model driven by Air Force Data Assimilative Photospheric Flux Transport (ADAPT) photospheric field maps to identify 38 periods when spacecraft observe pseudostreamer wind. We compare the expansion factor of the last open field lines on either side of a pseudostreamer cusp with the corresponding in situ measured solar wind speed. We find that only slow wind (vobs < 500 km s-1) is associated with pseudostreamers and that there is not a significant correlation between fs and vobs for these field lines. This suggests that field lines near the open-closed boundary of pseudostreamers are not subject to the steady-state acceleration along continuously open flux tubes assumed in the fs-vobs relationship. In general, dynamics at the boundary between open and closed field lines such as interchange reconnection will invalidate the steady-state assumptions of this relationship. Title: Nine Outstanding Questions of Solar Wind Physics Authors: Viall, Nicholeen M.; Borovsky, Joseph E. Bibcode: 2020JGRA..12526005V Altcode: In situ measurements of the solar wind have been available for almost 60 years, and in that time plasma physics simulation capabilities have commenced and ground-based solar observations have expanded into space-based solar observations. These observations and simulations have yielded an increasingly improved knowledge of fundamental physics and have delivered a remarkable understanding of the solar wind and its complexity. Yet there are longstanding major unsolved questions. Synthesizing inputs from the solar wind research community, nine outstanding questions of solar wind physics are developed and discussed in this commentary. These involve questions about the formation of the solar wind, about the inherent properties of the solar wind (and what the properties say about its formation), and about the evolution of the solar wind. The questions focus on (1) origin locations on the Sun, (2) plasma release, (3) acceleration, (4) heavy-ion abundances and charge states, (5) magnetic structure, (6) Alfven waves, (7) turbulence, (8) distribution-function evolution, and (9) energetic-particle transport. On these nine questions we offer suggestions for future progress, forward looking on what is likely to be accomplished in near future with data from Parker Solar Probe, from Solar Orbiter, from the Daniel K. Inouye Solar Telescope (DKIST), and from Polarimeter to Unify the Corona and Heliosphere (PUNCH). Calls are made for improved measurements, for higher-resolution simulations, and for advances in plasma physics theory. Title: Magnetic Origins of Cool Plasma in the Sun's Corona Authors: Mason, E.; Antiochos, S.; Viall, N. Bibcode: 2020AAS...23610606M Altcode: Much of solar physics research focuses on two questions: how the corona's temperature becomes hundreds of times hotter than the surface, and how the slow solar wind forms. Among the most fascinating phenomena produced by coronal heating is coronal rain, in which plasma undergoes rapid cooling (from roughly 106 to 103 K), condenses, and falls to the surface. One proposed rain origin theory, thermal nonequilibrium (TNE), posits a height restriction in coronal heating. By studying condensations, physicists hope to better understand coronal heating. Solar wind is often subdivided into fast and slow wind. The former originates in coronal hole regions; slow wind's source, however, is still under debate. One leading theory postulates that it comes from coronal hole boundaries, where magnetic field lines frequently reconnect. This research investigates the origins and dynamics of coronal rain via study of recently-discovered structures called raining null-point topologies, or RNPTs. RNPTs — the first identification and characterization of which comprise part of this work — are decaying active regions situated near coronal hole boundaries, between 50-150 Mm in height. They are host to long periods of continuous coronal rain formation, and provide insight into coronal heating, slow solar wind origins, and coronal dynamics. We focus on identifying and analyzing RNPTs' observational characteristics. We process and analyze RNPT data using both the Solar Dynamics Observatory Atmospheric Imaging Assembly and the Helioseismic and Magnetic Imager. Potential field source-surface extrapolations that model the magnetic field in the corona aid in the interpretation of the structures' topology. Results indicate that RNPTs experience two rain-forming mechanisms, TNE and interchange reconnection. The interchange reconnection is posited to power much of the early bursts of coronal rain, which constitutes a new rain-formation mechanism and allows for plasma from closed loops to escape into the slow solar wind. Observations also show evidence of partial condensations, which condense but do not fully cool. Title: The Solaris Solar Polar Mission Authors: Hassler, Donald M.; Newmark, Jeff; Gibson, Sarah; Harra, Louise; Appourchaux, Thierry; Auchere, Frederic; Berghmans, David; Colaninno, Robin; Fineschi, Silvano; Gizon, Laurent; Gosain, Sanjay; Hoeksema, Todd; Kintziger, Christian; Linker, John; Rochus, Pierre; Schou, Jesper; Viall, Nicholeen; West, Matt; Woods, Tom; Wuelser, Jean-Pierre Bibcode: 2020EGUGA..2217703H Altcode: The solar poles are one of the last unexplored regions of the solar system. Although Ulysses flew over the poles in the 1990s, it did not have remote sensing instruments onboard to probe the Sun's polar magnetic field or surface/sub-surface flows.We will discuss Solaris, a proposed Solar Polar MIDEX mission to revolutionize our understanding of the Sun by addressing fundamental questions that can only be answered from a polar vantage point. Solaris uses a Jupiter gravity assist to escape the ecliptic plane and fly over both poles of the Sun to >75 deg. inclination, obtaining the first high-latitude, multi-month-long, continuous remote-sensing solar observations. Solaris will address key outstanding, breakthrough problems in solar physics and fill holes in our scientific understanding that will not be addressed by current missions.With focused science and a simple, elegant mission design, Solaris will also provide enabling observations for space weather research (e.g. polar view of CMEs), and stimulate future research through new unanticipated discoveries. Title: The Heliospheric Current Sheet and Plasma Sheet during Parker Solar Probe's First Orbit Authors: Lavraud, B.; Fargette, N.; Réville, V.; Szabo, A.; Huang, J.; Rouillard, A. P.; Viall, N.; Phan, T. D.; Kasper, J. C.; Bale, S. D.; Berthomier, M.; Bonnell, J. W.; Case, A. W.; Dudok de Wit, T.; Eastwood, J. P.; Génot, V.; Goetz, K.; Griton, L. S.; Halekas, J. S.; Harvey, P.; Kieokaew, R.; Klein, K. G.; Korreck, K. E.; Kouloumvakos, A.; Larson, D. E.; Lavarra, M.; Livi, R.; Louarn, P.; MacDowall, R. J.; Maksimovic, M.; Malaspina, D.; Nieves-Chinchilla, T.; Pinto, R. F.; Poirier, N.; Pulupa, M.; Raouafi, N. E.; Stevens, M. L.; Toledo-Redondo, S.; Whittlesey, P. L. Bibcode: 2020ApJ...894L..19L Altcode: We present heliospheric current sheet (HCS) and plasma sheet (HPS) observations during Parker Solar Probe's (PSP) first orbit around the Sun. We focus on the eight intervals that display a true sector boundary (TSB; based on suprathermal electron pitch angle distributions) with one or several associated current sheets. The analysis shows that (1) the main density enhancements in the vicinity of the TSB and HCS are typically associated with electron strahl dropouts, implying magnetic disconnection from the Sun, (2) the density enhancements are just about twice that in the surrounding regions, suggesting mixing of plasmas from each side of the HCS, (3) the velocity changes at the main boundaries are either correlated or anticorrelated with magnetic field changes, consistent with magnetic reconnection, (4) there often exists a layer of disconnected magnetic field just outside the high-density regions, in agreement with a reconnected topology, (5) while a few cases consist of short-lived density and velocity changes, compatible with short-duration reconnection exhausts, most events are much longer and show the presence of flux ropes interleaved with higher-β regions. These findings are consistent with the transient release of density blobs and flux ropes through sequential magnetic reconnection at the tip of the helmet streamer. The data also demonstrate that, at least during PSP's first orbit, the only structure that may be defined as the HPS is the density structure that results from magnetic reconnection, and its byproducts, likely released near the tip of the helmet streamer. Title: Relating Streamer Flows to Density and Magnetic Structures at the Parker Solar Probe Authors: Rouillard, Alexis P.; Kouloumvakos, Athanasios; Vourlidas, Angelos; Kasper, Justin; Bale, Stuart; Raouafi, Nour-Edine; Lavraud, Benoit; Howard, Russell A.; Stenborg, Guillermo; Stevens, Michael; Poirier, Nicolas; Davies, Jackie A.; Hess, Phillip; Higginson, Aleida K.; Lavarra, Michael; Viall, Nicholeen M.; Korreck, Kelly; Pinto, Rui F.; Griton, Léa; Réville, Victor; Louarn, Philippe; Wu, Yihong; Dalmasse, Kévin; Génot, Vincent; Case, Anthony W.; Whittlesey, Phyllis; Larson, Davin; Halekas, Jasper S.; Livi, Roberto; Goetz, Keith; Harvey, Peter R.; MacDowall, Robert J.; Malaspina, D.; Pulupa, M.; Bonnell, J.; de Witt, T. Dudok; Penou, Emmanuel Bibcode: 2020ApJS..246...37R Altcode: 2020arXiv200101993R The physical mechanisms that produce the slow solar wind are still highly debated. Parker Solar Probe's (PSP's) second solar encounter provided a new opportunity to relate in situ measurements of the nascent slow solar wind with white-light images of streamer flows. We exploit data taken by the Solar and Heliospheric Observatory, the Solar TErrestrial RElations Observatory (STEREO), and the Wide Imager on Solar Probe to reveal for the first time a close link between imaged streamer flows and the high-density plasma measured by the Solar Wind Electrons Alphas and Protons (SWEAP) experiment. We identify different types of slow winds measured by PSP that we relate to the spacecraft's magnetic connectivity (or not) to streamer flows. SWEAP measured high-density and highly variable plasma when PSP was well connected to streamers but more tenuous wind with much weaker density variations when it exited streamer flows. STEREO imaging of the release and propagation of small transients from the Sun to PSP reveals that the spacecraft was continually impacted by the southern edge of streamer transients. The impact of specific density structures is marked by a higher occurrence of magnetic field reversals measured by the FIELDS magnetometers. Magnetic reversals are associated with much stronger density variations inside than outside streamer flows. We tentatively interpret these findings in terms of magnetic reconnection between open magnetic fields and coronal loops with different properties, providing support for the formation of a subset of the slow wind by magnetic reconnection. Title: Imaging the Solar Corona From Within Authors: Hess, P.; Howard, R.; Vourlidas, A.; Bothmer, V.; Colaninno, R.; DeForest, C.; Gallagher, B.; Hall, J. R.; Higginson, A.; Korendyke, C.; Kouloumvakos, A.; Lamy, P.; Liewer, P.; Linker, J.; Linton, M.; Penteado, P.; Plunkett, S.; Poirer, N.; Raouafi, N.; Rich, N.; Rochus, P.; Rouillard, A.; Socker, D.; Stenborg, G.; Thernisien, A.; Viall, N. Bibcode: 2020AAS...23514907H Altcode: Parker Solar Probe (PSP), launched, in August 2018 is humanity's first probe of a stellar atmosphere. It will make measurements of the near-Sun plasma from 'within' the outer corona with gradually reduced perihelia from its first perihelia of 35 Rs in 2018-19 to 9.8 Rs in 2025. Here we report the results from the imaging observations of the electron and dust corona, whe PSP was 35-54 Rs from the solar surface, taken by the Wide-field Imager for Solar Probe (WISPR). The spacecraft was near-corotating with the solar corona throughout the observing window, which is an unprecedented situation for any type of coronal imaging. Our initial analysis uncovers a long-hypothesized depletion of the primordial dust orbiting near the Sun, reveals the plasma structure of small-scale ejections, and provides a strict test for validating model predictions of the large-scale configuration of the coronal plasma. Thus, WISPR imaging allows the study of near-Sun dust dynamics as the mission progresses. The high-resolution images of small transients, largely unresolved from 1 AU orbits, unravel the sub-structures of small magnetic flux ropes and show that the Sun continually releases helical magnetic fields in the background wind. Finally, WISPR's observations of the coronal streamer evolution confirm the large-scale topology of the solar corona but they also reveal that, as recently predicted, streamers are composed of yet smaller sub-streamers channeling continual density fluctuations at all visible scales. Title: The Importance of Periodic Density Structures Within Stream Interaction Regions on Outer Zone Electrons Authors: Kepko, L.; Viall, N. M. Bibcode: 2019AGUFMSM52A..04K Altcode: While ULF waves are known to be important for radial transport of radiation belt electrons, particularly in the outer zone, the current state of the art relies on empirical relationships of ULF wave power to geomagnetic indices (e.g., Kp) to model this transport. However, ULF power departs significantly from these empirical representations on short timescales, and empirical representations are often unable to accurately describe the diffusion rates. One such departure is the existence of periodic number density structures (PDSs) in the solar wind, which are discrete (not broadband) oscillations, and that are not included in parameterized models. Here we present six PDS events that were embedded within the leading edge of stream interaction regions (SIRs). The events ranged from strong SIRs, with a large jump in velocity across the region and a forward shock, to weak SIRs, where the velocity change was minor and no shock had yet developed. Despite the range of velocity change, each event contained similar periodic density structures that drove global, compressional (poloidal) ULF waves with periods of 5-20 minutes, and had noticeable impacts on MeV electrons in the outer zone. Similar periodic density structures have been observed previously, but never in the context of SIRs, where their existence may play an important role in magnetospheric particle acceleration, loss, and transport, particularly for outer zone electrons that are highly responsive to ULF wave activity. Since these directly driven pulsations extend beyond the Pc5 band, their magnetospheric impacts are underexplored, and we argue that studies examining magnetospheric impacts of low frequency ULF pulsations should therefore not focus exclusively on the Pc5 bandwidth. The association of stream interaction regions (SIRs), which are known to have substantial impacts upon the radiation belts, with periodic density structures likely enhances their effectiveness. While our results demonstrate that there is driving of energetic particles by periodic solar wind density structures, the effects are not simple. There are global and local particle responses to the interaction, and the response has time-dependent and quasi-static aspects that are energy dependent, and should be taken into account particularly when modeling such interactions. Title: Imaging the Solar Corona from Within: First Results from the Parker Solar Probe Telescope Authors: Howard, R. A.; Vourlidas, A.; Bothmer, V.; Colaninno, R. C.; DeForest, C.; Gallagher, B.; Hall, J. R.; Hess, P.; Higginson, A. K.; Korendyke, C.; Kouloumvakos, A.; Lamy, P.; Liewer, P. C.; Linker, J.; Linton, M.; Penteado, P. F.; Plunkett, S. P.; Poirier, N.; Raouafi, N.; Rich, N.; Rochus, P. L.; Rouillard, A. P.; Socker, D. G.; Stenborg, G.; Thernisien, A.; Viall, N. M. Bibcode: 2019AGUFMSH11A..04H Altcode: Parker Solar Probe (PSP) launched in August 2018 is humanity's first probe of a stellar atmosphere. It will make measurements of the near-Sun plasma from 'within' the outer corona with gradually reduced perihelia from its first perihelia of 35 Rs in 2018-19 to 9.8 Rs in 2025. Here we report the results from the imaging observations of the electron and dust corona, whe PSP was 35-54 Rs from the solar surface, taken by the Wide-field Imager for Solar Probe (WISPR). The spacecraft was near-corotating with the solar corona throughout the observing window, which is an unprecedented situation for any type of coronal imaging. Our initial analysis uncovers a long-hypothesized depletion of the primordial dust orbiting near the Sun, reveals the plasma structure of small-scale ejections, and provides a strict test for validating model predictions of the large-scale configuration of the coronal plasma. Thus, WISPR imaging allows the study of near-Sun dust dynamics as the mission progresses. The high-resolution images of small transients, largely unresolved from 1 AU orbits, unravel the sub-structures of small magnetic flux ropes and show that the Sun continually releases helical magnetic fields in the background wind. Finally, WISPR's observations of the coronal streamer evolution confirm the large-scale topology of the solar corona but they also reveal that, as recently predicted, streamers are composed of yet smaller sub-streamers channeling continual density fluctuations at all visible scales. Title: The Properties of Periodic Mesoscale Density Structures in the Solar Wind Authors: Kepko, L.; Wolfinger, K.; Viall, N. M. Bibcode: 2019AGUFMSH43C3380K Altcode: It is now well-established that the solar wind at 1 AU contains periodic structures in the proton densities that are recurrent at specific radial length-scales. Event studies using in situ composition data and remote sensing images shows that many of these structures originate deep within the solar corona, and advect with the solar wind, as opposed to being generated by turbulence within the solar wind during transit. Periodic density structures directly drive global waves in Earth's magnetosphere, which can affect the energetic particle populations in the radiation belts, making them a potentially an important driver of space weather. Viall, Kepko, and Spence (2008) previously analyzed 11 years of data from the Wind satellite and found hints of a solar cycle dependence in the sets of preferred length-scales, but they had insufficient data to examine long term trends. Here, we utilize the longevity of the Wind spacecraft, and new background estimation techniques, to extend the statistical analysis of periodic solar wind density structures to cover more than a full Hale cycle: a total of 24.5 years, from 1995 through May of 2019, in order to probe which radial wavelengths preferentially occur during the solar cycle. We required spectral peaks from the density data to simultaneously pass a stringent combination of an amplitude test and a harmonic F-test. We test three different noise backgrounds: first-order auto-regressive, bending power law, and a spectral running-average. We produce occurrence distributions from the statistically significant length-scales. They display enhancements above the background at persistent length-scales across the full span of years covered, confirming the earlier results of Viall et al. 2008, but with some intriguing differences that we will discuss. The solar cycle dependence of preferred periodicities provides important constraints on the formation of these periodic density structures. Furthermore, the solar cycle-dependent statistics of these periodic density structures, which can impact magnetospheric particle acceleration and transport, can be used as inputs to space weather models. Title: Combining Remote and in situ Parker Solar Probe and STEREO Data to Understand Solar Wind Density Structures Authors: Viall, N. M.; Howard, R. A.; Vourlidas, A.; DeForest, C.; Kasper, J. C.; Korreck, K. E.; Case, A. W.; Stevens, M. L.; Whittlesey, P. L.; Larson, D. E.; Livi, R.; Szabo, A.; Kepko, L.; Lavraud, B.; Rouillard, A. P.; Velli, M. Bibcode: 2019AGUFMSH13C3432V Altcode: The instrument suite on Parker Solar Probe offers an unprecedented viewpoint of the ambient solar wind and structure therein, shortly after its formation and release from the solar corona. We take advantage of the synergistic observations of the first Parker Solar Probe encounters and the STEREO COR2 deep field campaigns covering the same time periods to study mesoscale solar wind density structures. They often occur in a quasi-periodic train, especially near the heliospheric current sheet. Some may be a consequence of the development of dynamics en route; many are remnants of the formation and release of the solar wind, and provide important constraints on solar wind models. The opportunity to combine the different observing angles and fields of view of the white light WISPR observations and white light STEREO COR2 observations with in situ density and plasma measurements from SWEAP allows better understanding of the characteristics and properties of mesoscale density structures. The in situ data measure precise size scales, plasma boundaries, and relationships between density and other parameters. They help in the interpretation of the structures seen in white light images and in unraveling projection effects. The white light images enhance the in situ data by providing global heliospheric context, as well as the occurrence rate and 2-D size scales of structures as a function of latitude and distance from the Sun. Together, these observations provide crucial constraints on the formation of structures in the solar wind. Title: Simulations of Thermal Nonequilibrium in Raining Null-Point Topologies Authors: Antiochos, S. K.; Mason, E. I.; Viall, N. M. Bibcode: 2019AGUFMSH53B3381A Altcode: Coronal heating and the origins of slow solar wind remain central open questions of solar physics. The recent discovery of raining null-point topologies allows study of regions that hold implications for both questions. We present observations from SDO AIA that show persistent coronal condensations in null-point topologies formed by decaying active regions located near coronal hole boundaries. Coronal rain - catastrophically-cooled plasma precipitating along flux tubes - can be used as a tracer of several physical processes to provide insight into local heating and cooling dynamics. The rain forms in two observationally-distinct ways: along the lower spine and null, and within the closed loops under the fan surface. The former is attributed to interchange reconnection, while the latter is due to thermal nonequilibrium (TNE). TNE is caused by height-dependent footpoint heating, which creates a runaway cooling affect far from the loop base and triggers condensation. Using the one-dimensional HYDrodynamic and RADiation solver code (HYDRAD, Bradshaw & Mason 2003), we model asymmetric flux tubes with a large expansion factor where the loop apex occurs near the null point. A broad parameter study of heating scale heights and heating rates show the ranges within which rain could occur, and point to highly restricted coronal heating scale heights in the decaying active regions. This study provides predictions for Parker Solar Probe and Solar Orbiter observations. Title: Near-Sun observations of an F-corona decrease and K-corona fine structure Authors: Howard, R. A.; Vourlidas, A.; Bothmer, V.; Colaninno, R. C.; DeForest, C. E.; Gallagher, B.; Hall, J. R.; Hess, P.; Higginson, A. K.; Korendyke, C. M.; Kouloumvakos, A.; Lamy, P. L.; Liewer, P. C.; Linker, J.; Linton, M.; Penteado, P.; Plunkett, S. P.; Poirier, N.; Raouafi, N. E.; Rich, N.; Rochus, P.; Rouillard, A. P.; Socker, D. G.; Stenborg, G.; Thernisien, A. F.; Viall, N. M. Bibcode: 2019Natur.576..232H Altcode: Remote observations of the solar photospheric light scattered by electrons (the K-corona) and dust (the F-corona or zodiacal light) have been made from the ground during eclipses1 and from space at distances as small as 0.3 astronomical units2-5 to the Sun. Previous observations6-8 of dust scattering have not confirmed the existence of the theoretically predicted dust-free zone near the Sun9-11. The transient nature of the corona has been well characterized for large events, but questions still remain (for example, about the initiation of the corona12 and the production of solar energetic particles13) and for small events even its structure is uncertain14. Here we report imaging of the solar corona15 during the first two perihelion passes (0.16-0.25 astronomical units) of the Parker Solar Probe spacecraft13, each lasting ten days. The view from these distances is qualitatively similar to the historical views from ground and space, but there are some notable differences. At short elongations, we observe a decrease in the intensity of the F-coronal intensity, which is suggestive of the long-sought dust free zone9-11. We also resolve the fine-scale plasma structure of very small eruptions, which are frequently ejected from the Sun. These take two forms: the frequently observed magnetic flux ropes12,16 and the predicted, but not yet observed, magnetic islands17,18 arising from the tearing-mode instability in the current sheet. Our observations of the coronal streamer evolution confirm the large-scale topology of the solar corona, but also reveal that, as recently predicted19, streamers are composed of yet smaller substreamers channelling continual density fluctuations at all visible scales. Title: Study of Type III Solar Radio Bursts in Nanoflares Authors: Chhabra, S.; Klimchuk, J. A.; Gary, D. E.; Viall, N. M. Bibcode: 2019AGUFMSH23C3337C Altcode: The heating mechanisms responsible for the million-degree solar corona remain one of the most intriguing problems in space science. It is widely agreed, that the ubiquitous presence of reconnection events and the associated impulsive heating (nanoflares) are a strong candidate in solving this problem [Klimchuk J.A., 2015 and references therein].

Whether nanoflares accelerate energetic particles like full-sized flares is unknown. The lack of strong emission in hard X-rays suggests that the quantity of highly energetic particles is small. There could, however, be large numbers of mildly energetic particles (~ 10 keV). We investigate such particles by searching for the type III radio bursts that they may produce. If energetic electron beams propagating along magnetic field lines generate a bump-on-tail instability, they will produce Langmuir waves, which can then interact with other particles and waves to give rise to emission at the local plasma frequency and its first harmonic. Type III bursts are characteristically known to exhibit high frequency drifts as the beam propagates through a density gradient. The time-lag technique that was developed to study subtle delays in light curves from different EUV channels [Viall & Klimchuk 2012] can also be used to detect subtle delays at different radio frequencies. We have modeled the expected radio emission from nanoflares, which we used to test and calibrate the technique. We are applying the technique to actual radio observations from VLA (Very Large Array), MWA (Murchison Widefield Array) and seeking data from LOFAR (Low-Frequency Array) as well. We also plan to use data from the PSP (Parker Solar Probe) to look for similar reconnection signatures in the Solar Wind. Our goal is to determine whether nanoflares accelerate energetic particles and to determine their properties. The results will have important implications for both the particle acceleration and reconnection physics. Title: Simultaneous occurrence of internally and externally driven ULF waves in the magnetosphere. Authors: Di Matteo, S.; Villante, U.; Viall, N. M.; Kepko, L. Bibcode: 2019AGUFMSM23F3277D Altcode: Earth's magnetosphere is capable of supporting both internal and externally driven ULF oscillations. In such a highly coupled system, the identification of the processes that trigger ULF waves is a complex task. Here, we discuss a unique interval of magnetospheric ULF pulsations that occurred on November 9th, 2002 during the interaction with a complex interplanetary structure (two interplanetary shocks followed by the heliospheric current sheet). We use a joint analysis of interplanetary (WIND and GEOTAIL), magnetospheric (GOES8 and GOES10) and ground observations, that enables us to definitively identify both internal and external sources of ULF waves. Following the second interplanetary shock, solar wind monitors observed three consecutive periodic density structures (PDSs) with a period of ≈90 minutes (0.2 mHz). These periodic structures then directly drove globally coherent compressional magnetospheric field oscillations observed at geostationary orbit and at middle and low latitude ground observatories. In addition, immediately after the impact of the shock onto the magnetosphere, a global oscillation at ≈1.5 mHz was observed in the magnetosphere on the ground. The concomitant occurrence of a ≈1.9 mHz wave at the same time in the solar wind density suggests the stimulation of a global cavity mode at the observed frequency. Additional magnetospheric field and solar wind density oscillations were observed at ≈2.4-2.5 and ≈3.5 mHz, concurrent with the 0.2 mHz oscillations. A signal at ≈5-6 mHz was observed at geostationary orbit, primarily along the poloidal component, with no clear counterpart in the solar wind. This higher frequency oscillations suggests localized compressive oscillations, possibly driven by magnetopause surface eigenmode or Kelvin-Helmholtz instability. We conclude that internally and externally driven ULF waves in the magnetosphere can occur simultaneously. In particular, this event shows three regimes of interaction: at lower frequencies (≈0.2 mHz) the ULF fluctuations are clearly externally driven; the higher frequency oscillations (≈5-6 mHz) are of internal origin; and the ULF waves at frequencies in between ≈1 and 5 mHz can be triggered by both external and internal processes. Therefore, many sources need to be considered before drawing conclusion on their origin. Title: Impacts of small coronal transients at Parker Solar Probe at times of density increases and burst of magnetic switchbacks Authors: Rouillard, A. P.; Kouloumvakos, A.; Vourlidas, A.; Raouafi, N. E.; Lavraud, B.; Stenborg, G.; Kasper, J. C.; Bale, S.; Poirier, N.; Howard, R. A.; Viall, N. M.; Lavarra, M.; Stevens, M. L.; Korreck, K. E.; Case, A. W.; Whittlesey, P. L.; Larson, D. E.; Halekas, J. S.; Livi, R.; Goetz, K.; Harvey, P.; MacDowall, R. J.; Malaspina, D.; Pulupa, M.; Bonnell, J. W.; Dudok de Wit, T. Bibcode: 2019AGUFMSH12A..04R Altcode: A subset at least of the slow solar wind is released in the form of transients ejected continually along streamer rays. The physical mechanisms responsible for these transient releases of dense material are not yet fully understood. We exploit a period when the NASA Solar-TErrestrial RElations Observatory-A (STEREO-A) was in orbital quadrature with Parker Solar Probe (PSP) to track the release and propagation of dense material from the corona to PSP. At the time PSP had passed its second perihelion and was located near the Thomson sphere of the inner Heliospheric Imager (HI-1) onboard STEREO-A. This provided optimal observing conditions to track dense and therefore bright structures from the corona to the Sun-approaching spacecraft. We show that the streamers were continually ejecting bursts of dense structures (so-called 'blobs') many of which exhibiting V-shapes that are reminiscent of either magnetic kinks and/or the well-known back ends of small magnetic flux ropes. The wide-angle imager on Parker Solar Probe imaged similar structures at other locations of the streamers during this second encounter suggesting a global nature of this transient activity. We find evidence in STEREO ultraviolet images for slow reconfigurations of the corona near the estimated source regions of these structures but no one-to-one association is yet clearly established between the lower and upper corona. The exploitation of height-time maps ('J-maps') built from COR-2 and HI-1 images of the solar wind allow us to track the dense features all the way to PSP. We show that the spacecraft was repeatedly impacted by the southern edge of these structures. The passage of the bright coronal material at PSP is associated with clear density increases measured by the plasma instrument as expected. We also find evidence that the impact of the specific dense structures are correlated with a higher occurrence of magnetic field reversals. Part of this work was funded by the European Research Council through the project SLOW_SOURCE - DLV-819189 Title: Tracking Outward Propagating Small-Scale Structures from EUVI through COR1 and COR2 Authors: Viall, N. M.; Alzate, N.; Morgan, H.; Vourlidas, A. Bibcode: 2019AGUFMSH13A..07V Altcode: The challenge of connecting slow solar wind variability to its source at the Sun is primarily due to the limitations on observational data of the low corona. Instrumental light scattering is higher in coronagraph observations very close to the solar surface, resulting in poor signal-to-noise ratios. Additionally, the non-radial nature of the coronal structures in this region hampers the tracking of small-scale structures in observations, since they produce weaker signals. Our work focuses on developing and applying advanced image processing techniques to solar imaging data in an effort to connect the low corona to the high corona with a focus on the identification of the sources of small-scale structures in the slow solar wind. The inner coronagraph, COR1, onboard the STEREO spacecraft, observes the low corona from ~1.1 to 4 Rs, imaging the important connection between the solar wind, its source, and the formation of the solar wind structures. We have developed an approach for processing COR1 that allows the tracking of small-scale structures. The core process is a bandpass filter of the data over time, where the signal from high frequency noise is suppressed, as well as slowly evolving structures. We applied this method to a 10-day period of observations during solar minimum and compared them to observations from the imager EUVI and outer coronagraph COR2. Height-time profiles reveal structures propagating through the different fields of view, establishing a connection between the low and high corona. Further analysis will allow us to characterize these structures and determine occurrence frequencies, size scales, formation height and mechanism. Our method for processing COR1 opens the door to ~13 years of STEREO/COR1 data for studies in general of the connection between the low and high corona, and specifically of small-scale coronal structures. Title: Parker Solar Probe Observations of the Release of Density Blobs and Flux Ropes at the Heliospheric Current Sheet Authors: Lavraud, B.; Fargette, N.; Bale, S. D.; Bonnell, J. W.; Case, A. W.; Dudok de Wit, T.; Eastwood, J. P.; Genot, V. N.; Goetz, K.; Griton, L. S.; Halekas, J. S.; Harvey, P.; Huang, J.; Kasper, J. C.; Klein, K. G.; Korreck, K. E.; Kouloumvakos, A.; Larson, D. E.; Lavarra, M.; Livi, R.; Louarn, P.; MacDowall, R. J.; Maksimovic, M.; Malaspina, D.; Nieves-Chinchilla, T.; Phan, T. D.; Pinto, R.; Poirier, N.; Pulupa, M.; Raouafi, N. E.; Rouillard, A. P.; Stevens, M. L.; Szabo, A.; Toledo-Redondo, S.; Viall, N. M.; Whittlesey, P. L. Bibcode: 2019AGUFMSH13C3442L Altcode: We present Parker Solar Probe observations of several heliospheric current sheet crossings during the first two encounters of the mission. The data show a large variability in the internal structure of the HCS. We focus in particular on the recurrent observation of density enhancements, interleaved with magnetic field enhancements, and which are deemed the signature of the sequential release of flux ropes and blobs from the tip of the helmet streamers. Title: The Source, Significance, and Magnetospheric Impact of Periodic Density Structures Within Stream Interaction Regions Authors: Kepko, L.; Viall, N. M. Bibcode: 2019JGRA..124.7722K Altcode: We present several examples of magnetospheric ultralow frequency pulsations associated with stream interaction regions and demonstrate that the observed magnetospheric pulsations were also present in the solar wind number density. The distance of the solar wind monitor ranged from just upstream of Earth's bow shock to 261 RE, with a propagation time delay of up to 90 min. The number density oscillations far upstream of Earth are offset from similar oscillations observed within the magnetosphere by the advection timescale, suggesting that the periodic dynamic pressure enhancements were time stationary structures, passively advecting with the ambient solar wind. The density structures are larger than Earth's magnetosphere and slowly altered the dynamic pressure enveloping Earth, leading to a quasi-static and globally coherent "forced-breathing" of Earth's dayside magnetospheric cavity. The impact of these periodic solar wind density structures was observed in both magnetospheric magnetic field and energetic particle data. We further show that the structures were initially smaller-amplitude, spatially larger, structures in the upstream slow solar wind and that the higher-speed wind compressed and amplified these preexisting structures leading to a series of quasiperiodic density structures with periods typically near 20 min. Similar periodic density structures have been observed previously at L1 and in remote images, but never in the context of solar wind shocks and discontinuities. The existence of periodic density structures within stream interaction regions may play an important role in magnetospheric particle acceleration, loss, and transport, particularly for outer zone electrons that are highly responsive to ultralow frequency wave activity. Title: Understanding Heating in Active Region Cores through Machine Learning. I. Numerical Modeling and Predicted Observables Authors: Barnes, W. T.; Bradshaw, S. J.; Viall, N. M. Bibcode: 2019ApJ...880...56B Altcode: 2019arXiv190603350B To adequately constrain the frequency of energy deposition in active region cores in the solar corona, systematic comparisons between detailed models and observational data are needed. In this paper, we describe a pipeline for forward modeling active region emission using magnetic field extrapolations and field-aligned hydrodynamic models. We use this pipeline to predict time-dependent emission from active region NOAA 1158 for low-, intermediate-, and high-frequency nanoflares. In each pixel of our predicted multi-wavelength, time-dependent images, we compute two commonly used diagnostics: the emission measure slope and the time lag. We find that signatures of the heating frequency persist in both of these diagnostics. In particular, our results show that the distribution of emission measure slopes narrows and the mean decreases with decreasing heating frequency and that the range of emission measure slopes is consistent with past observational and modeling work. Furthermore, we find that the time lag becomes increasingly spatially coherent with decreasing heating frequency while the distribution of time lags across the whole active region becomes more broad with increasing heating frequency. In a follow-up paper, we train a random forest classifier on these predicted diagnostics and use this model to classify real observations of NOAA 1158 in terms of the underlying heating frequency. Title: Study of Type III Radio Bursts in Nanoflares Authors: Chhabra, Sherry; Klimchuk, James A.; Viall, Nicholeen M.; Gary, Dale E. Bibcode: 2019shin.confE..12C Altcode: The heating mechanisms responsible for the million-degree solar corona remain one of the most intriguing problems in space science. It is widely agreed, that the ubiquitous presence of reconnection events and the associated impulsive heating (nanoflares) are a strong candidate in solving this problem [Klimchuk J.A., 2015 and references therein].

Whether nanoflares accelerate energetic particles like full-sized flares is unknown. The lack of strong emission in hard X-rays suggests that the quantity of highly energetic particles is small. There could, however, be large numbers of mildly energetic particles ( 10 keV). We investigate such particles by searching for the type III radio bursts that they may produce. If energetic electron beams propagating along magnetic field lines generate a bump-on-tail instability, they will produce Langmuir waves, which can then interact with other particles and waves to give rise to emission at the local plasma frequency and its first harmonic. Type III bursts are characteristically known to exhibit high frequency drifts as the beam propagates through a density gradient. The time-lag technique that was developed to study subtle delays in light curves from different EUV channels [Viall & Klimchuk 2012] can also be used to detect subtle delays at different radio frequencies. We have modeled the expected radio emission from nanoflares, which we used to test and calibrate the technique. We have begun applying the technique to actual radio observations from VLA (Very Large Array) and seeking data from MWA (Murchison Widefield Array) as well. We also plan to use data from the PSP(Parker Solar Probe) to look for similar reconnection signatures in the Solar Wind. Our goal is to determine whether nanoflares accelerate energetic particles and to determine their properties. The results will have important implications for both the particle acceleration and reconnection physics." Title: Observations and Modelling of Condensation Formation at Coronal Hole Boundaries Authors: Mason, Emily; Antiochos, Spiro; Viall, Nicholeen; Macneice, Peter; Bradshaw, Stephen Bibcode: 2019shin.confE..40M Altcode: One of the primary mechanisms suggested for slow solar wind formation is interchange reconnection. This tool for leveraging closed-loop plasma into the heliosphere is believed to occur ubiquitously in the corona, but has few definitive observational characteristics. We present recent observations from SDO AIA and STEREO-A of frequent condensations in small null-point topologies. These structures, termed raining null-point topologies, result from decayed active regions bordering on or entirely within coronal holes. These structures may have unique S-Web fingerprints, aiding slow wind detection and prediction capability. Our observations clearly show condensation formation at the open-closed boundary, where interchange reconnection is widely believed to occur. The condensations take the form of coronal rain, catastrophically cooled plasma that is easy to track using remote observations; their formation on apparently newly-opened coronal flux tubes gives interchange reconnection a hallmark signature. We will also present 1D hydrodynamic models for how these condensations can form via interchange reconnection and thermal nonequilibrium. Due to the nature of these null-point topologies and their proximity to coronal holes, they share characteristics common to open-closed boundaries, pseudostreamers, and active regions. Coordination between Parker Solar Probe, DKIST, Solar Orbiter, etc. would provide a deeper understanding of these ideal targets, which encompass such useful signatures for slow wind investigation. Title: COHERENT: Studying the corona as a holistic environment Authors: Caspi, Amir; Seaton, Daniel B.; Case, Traci; Cheung, Mark; Cranmer, Steven; DeForest, Craig E.; de Toma, Giuliana; Downs, Cooper; Elliott, Heather; Gold, Anne U.; Longcope, Dana; Savage, Sabrina L.; Sullivan, Susan; Viall, Nicholeen; Vourlidas, Angelos; West, Matthew J. Bibcode: 2019shin.confE.241C Altcode: The solar corona and the heliosphere must be part of a single physical system, but because the dominant physical processes change dramatically from the magnetically-dominated low corona, through the sparsely-observed middle corona, and into the plasma flow-dominated outer corona and heliospheric interface, unified frameworks to study the corona as a whole are essentially nonexistent. Understanding how physical processes shape and drive the dynamics of the corona as a global system, on all spatiotemporal scales, is critical for solving many fundamental problems in solar and heliospheric physics. However, the lack of unifying observations and models has led to a fragmentation of the community into distinct regimes of plasma parameter space, largely clustering around regions where existing instrumentation has made observations widely available and where models can be sufficiently self-contained to be tractable. We describe COHERENT, the 'Corona as a Holistic Environment' Research Network, a focused effort to facilitate interdisciplinary collaborative research to develop frameworks for unifying existing and upcoming observations, theory, models, and analytical tools to study the corona as a holistic system. Title: Connecting the Low Corona to the High Corona: Outward Propagating Small-Scale Transients Tracked from EUVI Through COR1 and COR2 Authors: Alzate, Nathalia; Viall, Nicholeen; Morgan, Huw; Vourlidas, Angelos Bibcode: 2019shin.confE..59A Altcode: Identification of the source of the slow solar wind has been hampered by the complexity of plasma structures in the very low corona and data limitations in terms of noise reduction. Application of state-of-the-art image processing techniques has revealed very faint structures in the low corona that have an impact on the structure of the extended corona. These techniques, which overcome faint signals and noise in the data, allow us to identify and characterize the sources of the slow solar wind as we explore the variability of coronal density structures over a distance range starting from the solar surface out to tens of radii and beyond using data from the STEREO spacecraft. Having successfully conquered the noise issues in the COR1 instrument, as a proof of concept we revisited a 10-day period in January 2008 in which density enhancements were previously identified as sources of the slow solar wind in in situ, HI2 and COR2 data. Our preliminary results show transients present in the EUVI, COR1 and COR2 FOVs, with several transients propagating through at least two FOVs. Further analysis will enable us to properly determine the exact formation heights of ambient solar wind structures, formation mechanisms, and the features in the low corona they connect to. We will include data from other instruments to study different coronal configurations in order to characterize solar wind structures low in the corona as a function of coronal magnetic complexity, which will be an essential test for theories of solar wind formation. Our method for processing COR1 data opens the door to 12 years of data for studies of small-scale coronal structures and their geoeffectiveness. This work is part of the broad effort in Heliospheric physics to understand the different types of transient structures created in the solar wind as it is formed. Title: Observations of Solar Coronal Rain in Null Point Topologies Authors: Mason, E. I.; Antiochos, Spiro K.; Viall, Nicholeen M. Bibcode: 2019ApJ...874L..33M Altcode: 2019arXiv190408982M Coronal rain is the well-known phenomenon in which hot plasma high in the Sun’s corona undergoes rapid cooling (from ∼106 to <104 K), condenses, and falls to the surface. Coronal rain appears frequently in active region coronal loops and is very common in post-flare loops. This Letter presents discovery observations, which show that coronal rain is ubiquitous in the commonly occurring coronal magnetic topology of a large (∼100 Mm scale) embedded bipole very near a coronal hole boundary. Our observed structures formed when the photospheric decay of active-region-leading-sunspots resulted in a large parasitic polarity embedded in a background unipolar region. We observe coronal rain to appear within the legs of closed loops well under the fan surface, as well as preferentially near separatrices of the resulting coronal topology: the spine lines, null point, and fan surface. We analyze three events using SDO Atmospheric Imaging Assembly observations in the 304, 171, and 211 Å channels, as well as SDO Helioseismic and Magnetic Imager magnetograms. The frequency of rain formation and the ease with which it is observed strongly suggests that this phenomenon is generally present in null point topologies of this size scale. We argue that these rain events could be explained by the classic process of thermal nonequilibrium or via interchange reconnection at the null; it is also possible that both mechanisms are present. Further studies with higher spatial resolution data and MHD simulations will be required to determine the exact mechanism(s). Title: Helios Observations of Quasiperiodic Density Structures in the Slow Solar Wind at 0.3, 0.4, and 0.6 AU Authors: Di Matteo, S.; Viall, N. M.; Kepko, L.; Wallace, S.; Arge, C. N.; MacNeice, P. Bibcode: 2019JGRA..124..837D Altcode: Following previous investigations of quasiperiodic plasma density structures in the solar wind at 1 AU, we show using the Helios1 and Helios2 data their first identification in situ in the inner heliosphere at 0.3, 0.4, and 0.6 AU. We present five events of quasiperiodic density structures with time scales ranging from a few minutes to a couple of hours in slow solar wind streams. Where possible, we locate the solar source region of these events using photospheric field maps from the Mount Wilson Observatory as input for the Wang-Sheeley-Arge model. The detailed study of the plasma properties of these structures is fundamental to understanding the physical processes occurring at the origin of the release of solar wind plasma. Temperature changes associated with the density structures are consistent with these periodic structures developing in the solar atmosphere as the solar wind is formed. One event contains a flux rope, suggesting that the solar wind was formed as magnetic reconnection opened up a previously closed flux tube at the Sun. This study highlights the types of structures that Parker Solar Probe and the upcoming Solar Orbiter mission will observe, and the types of data analyses these missions will enable. The data from these spacecrafts will provide additional in situ measurements of the solar wind properties in the inner heliosphere allowing, together with the information of the other interplanetary probes, a more comprehensive study of solar wind formation. Title: Timescales and radial lengthscales of quasi-periodic density structures observed by the Helios probes Authors: Di Matteo, S.; Viall, N. M.; Kepko, L.; Villante, U. Bibcode: 2019NCimC..42...20D Altcode: Quasi-periodic density structures are important mesoscale structures that constitute the slow solar wind. The associated periodicities play a fundamental role in the understanding of the release processes of solar coronal plasma and provide important constraints on models trying to understand the origin of the slow solar wind. However, observational restrictions have limited their study to coronagraph images near the Sun and in situ measurements at 1 AU, preventing a clear connection between in situ structures to their coronal sources. A first step toward a better understanding of this problem is the analysis of Helios measurements to probe the solar wind between 0.3 and 0.6 AU. Three instances of quasi-periodic density structures have been identified showing characteristic timescales of ≈ 31-33 minutes and ≈ 112-120 minutes. Using the mean radial bulk speed corresponding to the time intervals of the structures, the related radial lengthscales are derived showing a characteristic value of ≈ 120-150 R_E. Title: Power spectrum power-law indices as a diagnostic of coronal heating Authors: Ireland, Jack; Viall, Nicholeen; Bradshaw, Stephen; Kirk, Michael Bibcode: 2018csc..confE.119I Altcode: We investigate the coronal heating of active regions by bringing together novel data analysis techniques with hydrodynamic modeling in a new and unique way. Viall & Klimchuk 2011, 2012, 2014, 2017 have shown that the timing of active region coronal emission brightenings in multiple channels of Solar Dynamics Observatory Atmospheric Imaging Assembly (SDO/AIA) follows that expected from simulations of a nanoflare-heated corona. Using Numerical HYDrodynamic RADiative Emission Model for the Solar Atmosphere (HYDRAD)-based simulations of AIA emission for an AR, Bradshaw & Viall 2016 have shown that the timing of coronal emission brightenings is dependent on the properties of the nanoflare energy distribution and occurrence rate. Relatedly, Ireland et al. 2015 show that average power spectra P(f) (where f is frequency) of time series of AIA 171Å and 193Å AR images are dominated by power laws, P(f) f^{-z}, z>0. Ireland et al. 2015 show that a distribution of exponentially decaying events of emission E along the line-of-sight, where N(E) E^{-m} and the size of the emission depends on its duration T such that E T^{k} creates a power law power spectrum P(f) f^{-k(2-m)}. We present analyses that test the hypothesis that a distribution of nanoflare events causes both the emission power-law power spectrum in AIA time-series as well as the observed brightening time-lags. Firstly, we show that the power-law indices of Fourier power spectra of the same simulated data described in Bradshaw & Viall 2016 depends on the frequency of nanoflares used. Secondly, using the same observational AIA time-series data analyzed by Viall & Klimchuk (2012), we obtain correlations of the cross-channel time-lags with the power-law indices of Fourier power spectra in each AIA channel. Finally, the ability of power-law indices and time-lags together to constrain the underlying nanoflare frequency distribution is discussed. Title: Using SDO/AIA to Understand the Thermal Evolution of Solar Prominence Formation Authors: Viall, Nicholeen; Kucera, Therese; Karpen, Judith Bibcode: 2018csc..confE.124V Altcode: We investigate prominence formation using time series analysis of Solar Dynamics Observatory's Atmospheric Imaging Assembly (SDO/AIA) data. We examine the thermal properties of forming prominences by analyzing observed light curves using the same technique that we have already successfully applied to active regions to diagnose heating and cooling cycles. This technique tracks the thermal evolution using emission formed at different temperatures, made possible by AIA's different wavebands and high time resolution. We also compute the predicted light curves in the same SDO/AIA channels of a hydrodynamic model of thermal nonequilibrium formation of prominence material, an evaporation-condensation model. In these models of prominence formation, heating at the foot-points of sheared coronal flux-tubes results in evaporation of material of a few MK into the corona followed by catastrophic cooling of the hot material to form cool ( 10,000 K) prominence material. We investigate prominences from different viewing angles to evaluate possible line of sight effects. We demonstrate that the SDO/AIA light curves for flux tubes undergoing thermal nonequilibrium vary at different locations along the flux tube, especially in the region where the condensate forms, and we compare the predicted light curves with those observed. Title: Source and Effect of Mesoscale Solar Wind Structures in the Inner Heliosphere Authors: Viall, Nicholeen Mary Bibcode: 2018shin.confE..45V Altcode: We discuss the mounting evidence from remote white light imaging data and from in situ Helios data that a large fraction of the solar wind in the inner heliosphere is comprised of mesoscale structures. Mesoscale structures have spatial scales of 100-10,000 Mm and timescales of tens of minutes to many hours, and they exhibit changes in the plasma density, temperature and/or composition that are concurrent with flux tube boundaries. Remote imaging shows that mesoscale structures are already present as low as 2.5 Rs from the Sun - and possibly lower. We will discuss the numerous possible sources and physical processes involved, including magnetic reconnection, coronal jets, waves, and filamentary structure from the low corona. It is likely that multiple physical processes are involved. Regardless of their source, the magnetic field changes in the mesoscale structures can affect SEP propagation, and the dynamic pressure changes associated with the density can drive global dynamics in the magnetospheres of the terrestrial planets. Title: Observational Evidence of Coronal Magnetic Reconnection during Quiescent Conditions Authors: Viall, Nicholeen Mary Bibcode: 2018shin.confE..67V Altcode: The slow solar wind is often released through magnetic reconnection in the high corona. White light images of the corona show evidence of the associated dipolarization flows collapsing back towards the solar surface. In situ data in the inner heliosphere show composition and temperature changes in solar wind structures with concurrent magnetic field signatures - including flux ropes - indicating that closed field plasma was released into the solar wind through magnetic field reconnection. Magnetic reconnection is also likely important for heating the solar corona, though direct evidence is difficult to find due to instrumental constraints. Current observations suggest that Parker Solar Probe will most often fly above the reconnection sites. Given the existing methods used to identify magnetic reconnection, we will discuss the types of signatures that Parker Solar Probe may see. This includes (but not limited to) temperature changes, alpha/proton abundance changes, flux ropes, magnetic connectivity changes, and Type III radio bursts. Title: Tracing the Origins of the Solar Wind by Tracking Flows and Disturbances in Coronagraph Data Authors: Thompson, Barbara J.; Attie, Raphael; DeForest, Craig E.; Gibson, Sarah E.; Hess Webber, Shea A.; Ireland, Jack; Kirk, Michael S. F.; Kwon, Ryun Young; McGranaghan, Ryan; Viall, Nicholeen M. Bibcode: 2018shin.confE..47T Altcode: The challenge of identifying transient motions in solar imagery has been addressed in a number of ways. A variety of methods have been developed to detect and characterize the motion and extent of coronal mass ejections, for example. We discuss the adaptation of CME and solar transient detection methods to trace smaller-scale perturbations consistent with solar wind motions in the inner heliosphere (out to 10 RSun). We evaluate several methods, and compare the speed and structure results to model predictions. In particular, we discuss how high-cadence heliospheric imagery can be used to track small scale solar density variations throughout the solar wind, serving as a proxy for in situ velocity detection, but with global and continuous coverage. Title: Study of Type III Radio Bursts in Nanoflares Authors: Chhabra, Sherry; Klimchuk, James A.; Viall, Nicholeen M. Bibcode: 2018shin.confE..18C Altcode: The heating mechanisms responsible for the million-degree solar corona remain one of the most intriguing problems in space science. It is widely agreed, that the ubiquitous presence of reconnection events and the associated impulsive heating (nanoflares) are a strong candidate in solving this problem [Klimchuk J.A., 2015 and references therein]. Title: The Highly Structured Outer Solar Corona Authors: DeForest, C. E.; Howard, R. A.; Velli, M.; Viall, N.; Vourlidas, A. Bibcode: 2018ApJ...862...18D Altcode: We report on the observation of fine-scale structure in the outer corona at solar maximum, using deep-exposure campaign data from the Solar Terrestrial Relations Observatory-A (STEREO-A)/COR2 coronagraph coupled with postprocessing to further reduce noise and thereby improve effective spatial resolution. The processed images reveal radial structure with high density contrast at all observable scales down to the optical limit of the instrument, giving the corona a “woodgrain” appearance. Inferred density varies by an order of magnitude on spatial scales of 50 Mm and follows an f -1 spatial spectrum. The variations belie the notion of a smooth outer corona. They are inconsistent with a well-defined “Alfvén surface,” indicating instead a more nuanced “Alfvén zone”—a broad trans-Alfvénic region rather than a simple boundary. Intermittent compact structures are also present at all observable scales, forming a size spectrum with the familiar “Sheeley blobs” at the large-scale end. We use these structures to track overall flow and acceleration, finding that it is highly inhomogeneous and accelerates gradually out to the limit of the COR2 field of view. Lagged autocorrelation of the corona has an enigmatic dip around 10 R , perhaps pointing to new phenomena near this altitude. These results point toward a highly complex outer corona with far more structure and local dynamics than has been apparent. We discuss the impact of these results on solar and solar-wind physics and what future studies and measurements are necessary to build upon them. Title: Timelag Analysis of Simulated Active Region Cores Heated by Nanoflares Authors: Barnes, Will; Bradshaw, Stephen J.; Viall, Nicholeen M. Bibcode: 2018tess.conf22403B Altcode: The interpretation of remote sensing data in the context of the underlying coronal heating mechansim is complicated by several factors, including limited instrument sensitivity, nonequilibrium ionization and multiple emitting structures along the line of sight. In this presentation, we investigate observable signatures of impulsive heating in active region NOAA 1158 through efficient hydrodynamic loop models, magnetic field extrapolations, and advanced forward modeling techniques. To compute synthetic observations for all EUV channels of the Atmospheric Imaging Assembly (AIA), we calculate the emissivity for the relevant ions using CHIANTI, fold this information through the appropriate instrument response functions, and then map it to the extrapolated field geometry. The timelag, the temporal offset which gives the maximum cross-correlation between two channels, is calculated in each pixel of our synthesized AIA observations. We investigate the impact of three different parameters on the simulated timelags: the frequency at which the loops are reheated, the assumption that the ion populations are in equilibrium with the surrounding electrons, and the orientation of the active region relative to the line of sight. We make detailed comparisons to observed timelag maps of the same active region. Additionally, our forward-modeling software has been developed to be both modular and generally applicable. We briefly discuss how this framework for synthesizing observations might be useful to the larger solar physics community. Title: Tracking Flows and Disturbances in Coronagraph Data Authors: Thompson, Barbara J.; Attie, Raphael; DeForest, Craig E.; Gibson, Sarah E.; Hess Webber, Shea A.; Inglis, Anfew R.; Ireland, Jack; Kirk, Michael S.; Kwon, RyunYoung; Viall, Nicholeen M. Bibcode: 2018tess.conf30922T Altcode: The challenge of identifying transient motions in solar imagery has been addressed in a number of ways. A variety of methods have been developed to detect and characterize the motion and extent of coronal mass ejections, for example. We discuss the adaptation of CME and solar transient detection methods to trace smaller-scale perturbations consistent with solar wind motions in the inner heliosphere (over 10 RSun). We evaluate several methods, and compare the speed and structure results to model predictions. In particular, we discuss how high-cadence heliospheric imagery can be used to track small scale solar density variations throughout the solar wind, serving as a proxy for in situ velocity detection, but with global and continuous coverage. Title: Turtles All The Way Down: The finely structured outer corona, and its implications for PSP Authors: DeForest, Craig E.; Howard, Russell A.; Velli, Marco C. M.; Viall, Nicholeen M.; Vourlidas, Angelos Bibcode: 2018tess.conf30928D Altcode: Based on optical resolution of the starfield with SOHO/LASCO, STEREO/COR, and other coronagraphs, there is widespread intuition that the solar corona becomes more smooth with altitude. This is an optical illusion, caused by the interplay between signal-to-noise ratio (SNR) and feature size in typical coronal images. Processed, low-noise, deep-field COR2 images of the outer corona reveal rich structure at all observable scales, with surprising time variability and very short spatial correlation scales under 50 Mm, at altitudes near 10 Rs. This has deep implications not only for the solar wind and outer coronal physics, but also for the types of structure that Parker Solar Probe will encounter. We will present and discuss the fundamental result, and explore its implications for in-situ science and required context imaging from PSP. We will also make specific predictions about the environment PSP will encounter at solar altitudes of 10-15 Rs. Title: Using Solar Wind Structures as a Rosetta Stone for Understanding Solar Wind Formation Authors: Viall, Nicholeen M.; Kepko, Larry; Antiochos, Spiro K.; Higginson, Aleida Katherine; Vourlidas, Angelos; Lepri, Susan T. Bibcode: 2018tess.conf31702V Altcode: In the inner heliosphere, the slow solar wind is often comprised of mesoscale structures: structures with timescales of hours and length scales of hundreds of mega meters. White light coronagraph data suggest that these mesoscale structures are formed and embedded in the solar wind within the first several solar radii above the solar surface, which is still below even the closest approach of Parker Solar Probe at nine solar radii. We argue that these mesoscale structures represent a 'Rosetta Stone' for using the embedded solar wind plasma signatures to understand the fundamental release and acceleration of solar wind plasma. We study events identified in data from current missions to demonstrate how mesoscale structures can link dynamics observed remotely in the lower corona with in situ observations. We discuss the observations that Parker Solar Probe will make and how to capitalize on this remote-to-in situ data connection. Title: From the Magnetosphere to the Sun: How we Used Waves in Earth's Magnetosphere to Understand the Dynamic Nature of the Formation of the Slow Solar Wind Authors: Viall, Nicholeen M. Bibcode: 2018tess.conf31001V Altcode: Constant buffeting of Earth's magnetosphere by the highly dynamic solar wind produces a broad range of ULF oscillations in the mHz and sub mHz range (timescales of minutes to hours). Such oscillations have been observed for decades by in situ and ground-based observations. The magnetospheric boundaries are sharp and capable of reflecting oscillations, leading to the intriguing idea of cavity mode oscillations (standing mode waves), in which the magnetospheric cavity acts like a ringing bell, struck by the hammer that is the solar wind. As L1 solar wind measurements became more prevalent, it became clear that there were times when the magnetosphere was not ringing like a bell, but was directly driven by quasi-periodic dynamic pressure structures in solar wind that appeared to be ubiquitous - an unexpected finding. Yet, while this discovery answered one question, a major new question immediately arose: why are there quasi-periodic structures in the solar wind in the first place? And with that question, my career in heliophysics took a turn for the Sun to look for their source. In this talk, I discuss my research connecting the waves in the magnetosphere to quasi-periodic structures in the solar wind observed at L1, and further connecting those to the dynamic nature of the corona and of solar wind formation. We show that the slow solar wind in particular is released through magnetic reconnection with characteristic time scales. The result is quasi-periodic mesoscale structures in the slow solar wind that retain the magnetic and plasma signatures of this process, eventually driving magnetospheric pulsations several days later. Title: Dressing the Coronal Magnetic Extrapolations of Active Regions with a Parameterized Thermal Structure Authors: Nita, Gelu M.; Viall, Nicholeen M.; Klimchuk, James A.; Loukitcheva, Maria A.; Gary, Dale E.; Kuznetsov, Alexey A.; Fleishman, Gregory D. Bibcode: 2018ApJ...853...66N Altcode: The study of time-dependent solar active region (AR) morphology and its relation to eruptive events requires analysis of imaging data obtained in multiple wavelength domains with differing spatial and time resolution, ideally in combination with 3D physical models. To facilitate this goal, we have undertaken a major enhancement of our IDL-based simulation tool, GX_Simulator, previously developed for modeling microwave and X-ray emission from flaring loops, to allow it to simulate quiescent emission from solar ARs. The framework includes new tools for building the atmospheric model and enhanced routines for calculating emission that include new wavelengths. In this paper, we use our upgraded tool to model and analyze an AR and compare the synthetic emission maps with observations. We conclude that the modeled magneto-thermal structure is a reasonably good approximation of the real one. Title: Understanding Coronal Heating through Time-Series Analysis and Nanoflare Modeling Authors: Romich, Kristine; Viall, Nicholeen Bibcode: 2018AAS...23135911R Altcode: Periodic intensity fluctuations in coronal loops, a signature of temperature evolution, have been observed using the Atmospheric Imaging Assembly (AIA) aboard NASA’s Solar Dynamics Observatory (SDO) spacecraft. We examine the proposal that nanoflares, or impulsive bursts of energy release in the solar atmosphere, are responsible for the intensity fluctuations as well as the megakelvin-scale temperatures observed in the corona. Drawing on the work of Cargill (2014) and Bradshaw & Viall (2016), we develop a computer model of the energy released by a sequence of nanoflare events in a single magnetic flux tube. We then use EBTEL (Enthalpy-Based Thermal Evolution of Loops), a hydrodynamic model of plasma response to energy input, to simulate intensity as a function of time across the coronal AIA channels. We test the EBTEL output for periodicities using a spectral code based on Mann and Lees’ (1996) multitaper method and present preliminary results here. Our ultimate goal is to establish whether quasi-continuous or impulsive energy bursts better approximate the original SDO data. Title: Combining Remote and In Situ Observations with MHD models to Understand the Formation of the Slow Solar Wind Authors: Viall, N. M.; Kepko, L.; Antiochos, S. K.; Lepri, S. T.; Vourlidas, A.; Linker, J. Bibcode: 2017AGUFMSH21C..05V Altcode: Connecting the structure and variability in the solar corona to the Heliosphere and solar wind is one of the main goals of Heliophysics and space weather research. The instrumentation and viewpoints of the Parker Solar Probe and Solar Orbiter missions will provide an unprecedented opportunity to combine remote sensing with in situ data to determine how the corona drives the Heliosphere, especially as it relates to the origin of the slow solar wind. We present analysis of STEREO coronagraph and heliospheric imager observations and of in situ ACE and Wind measurements that reveal an important connection between the dynamics of the corona and of the solar wind. We show observations of quasi-periodic release of plasma into the slow solar wind occurring throughout the corona - including regions away from the helmet streamer and heliospheric current sheet - and demonstrate that these observations place severe constraints on the origin of the slow solar wind. We build a comprehensive picture of the dynamic evolution by combining remote imaging data, in situ composition and magnetic connectivity information, and MHD models of the solar wind. Our results have critical implications for the magnetic topology involved in slow solar wind formation and magnetic reconnection dynamics. Crucially, this analysis pushes the limits of current instrument resolution and sensitivity, showing the enormous potential science to be accomplished with the Parker Solar Probe and Solar Orbiter missions. Title: Thermal Time Evolution of Non-Flaring Active Regions Determined by SDO/AIA Authors: Wright, Paul James; Hannah, Iain; Viall, Nicholeen; MacKinnon, Alexander; Ireland, Jack; Bradshaw, Stephen Bibcode: 2017SPD....4840203W Altcode: We present the pixel-level time evolution of DEM maps from SDO/AIA data using two different methods (Hannah et al. 2012; Cheung et al. 2015). These sets of Differential Emission Measure (DEM) maps allow us to determine the slopes of the DEM throughout non-flaring structures, and investigate how this changes with time, a crucial parameter in terms of how these flux tubes are being heated. We present this analysis on both real and synthetic data allowing us to understand how robustly we can recover the thermal time evolution. As this analysis also produces the time series in different temperature bands we can further investigate the underlying heating mechanisms by applying a variety of techniques to probe the frequency and nature of the heating, such as time-lag analysis (Viall & Klimchuck 2012; 2016), power spectrum analysis (Ireland et al. 2015), and Local Intermittency Measure (Dinkelaker & MacKinnon 2013a,b). Title: Diagnosing Coronal Heating in a Survey of Active Regions using the Time Lag Method Authors: Viall, Nicholeen; Klimchuk, James A. Bibcode: 2017SPD....4840202V Altcode: In this paper we examine 15 different active regions observed with the Solar Dynamics Observatory and analyze their nanoflare properties using the time lag method. The time lag method is a diagnostic of whether the plasma is maintained at a steady temperature, or if it is dynamic, undergoing heating and cooling cycles. An important aspect of our technique is that it analyses both observationally distinct coronal loops as well as the much more prevalent diffuse emission surrounding them. Warren et al. (2012) first studied these same 15 active regions, which are all quiescent and exhibit a broad range of characteristics, including age, total unsigned magnetic flux, area, hot emission, and emission measure distribution. We find that widespread cooling is a generic property of both loop and diffuse emission from all 15 active regions. However, the range of temperatures through which the plasma cools varies between active regions and within each active region, and only occasionally is there full cooling from above 7 MK to well below 1 MK. We find that the degree of cooling is not well correlated with slopes of the emission measure distribution measured by Warren et al. (2012). We show that these apparently contradictory observations can be reconciled with the presence of a distribution of nanoflare energies and frequencies along the line of sight, with the average delay between successive nanoflare events on a single flux tube being comparable to the plasma cooling timescale. Warren, H. P., Winebarger, A. R., & Brooks, D. H. 2012, ApJ, 759, 141 Title: Simulations and Observations of the Structured Variability in the Slow Solar Wind Authors: Lynch, Benjamin J.; Higginson, Aleida K.; Zhao, Liang; Viall, Nicholeen; Lepri, Susan T. Bibcode: 2017SPD....4840401L Altcode: In addition to the long-term heliospheric evolution on timescales of months to years, the slow solar wind exhibits significant variability on much shorter timescales—from minutes to days. This short-term variability in the magnetic field, bulk plasma, and composition properties of the slow solar wind likely results from magnetic reconnection processes in the extended solar corona. Here, we continue our analysis of the Higginson et al. (2017, ApJ 840, L10) numerical MHD simulation to investigate the following sources of structured slow solar wind variability. First, we examine the formation and evolution of 3D “streamer blob” magnetic flux ropes from the cusp of the helmet streamer belt by reconnection in the heliospheric current sheet (HCS). Second, we examine the large-scale torsional Alfven wave that propagates to high latitudes along the Separatrix-Web (S-Web) arc. We argue that the in-situ Alfven wave signatures in our simulation should be representative of the field and plasma signatures associated with interchange reconnection process in the corona. Therefore, we predict that streamer blob magnetic island flux ropes should be found primarily near the HCS but the torsional Alfven wave signatures should be present in both the streamer belt/HCS slow wind and in the slow wind in the S-Web arcs of pseudostreamers. We present preliminary results of our analysis of the field, plasma, and composition variability in select intervals of slow solar wind in Carrington Rotation 2002 and show these are in excellent agreement with the numerical simulation predictions. Title: A Survey of Nanoflare Properties in Active Regions Observed with the Solar Dynamics Observatory Authors: Viall, Nicholeen M.; Klimchuk, James A. Bibcode: 2017ApJ...842..108V Altcode: In this paper, we examine 15 different active regions (ARs) observed with the Solar Dynamics Observatory and analyze their nanoflare properties. We have recently developed a technique that systematically identifies and measures plasma temperature dynamics by computing time lags between light curves. The time lag method tests whether the plasma is maintained at a steady temperature, or if it is dynamic, undergoing heating and cooling cycles. An important aspect of our technique is that it analyzes both observationally distinct coronal loops as well as the much more prevalent diffuse emission between them. We find that the widespread cooling reported previously for NOAA AR 11082 is a generic property of all ARs. The results are consistent with impulsive nanoflare heating followed by slower cooling. Only occasionally, however, is there full cooling from above 7 MK to well below 1 MK. More often, the plasma cools to approximately 1-2 MK before being reheated by another nanoflare. These same 15 ARs were first studied by Warren et al. We find that the degree of cooling is not well correlated with the reported slopes of the emission measure distribution. We also conclude that the Fe xviii emitting plasma that they measured is mostly in a state of cooling. These results support the idea that nanoflares have a distribution of energies and frequencies, with the average delay between successive events on an individual flux tube being comparable to the plasma cooling timescale. Title: Methods on Efficiently Relating Data from the Sun to In-situ Data at L1: An Application to the Slow Solar Wind Authors: McQuillan, Maria; Viall, Nicholeen Bibcode: 2017AAS...22933908M Altcode: Understanding space weather has become increasingly important as scientists and spacecraft extend their reach further into the universe. The solar wind is highly ionized plasma that constantly bombards the earth. It causes compression and relaxation in our magnetosphere, and affects spacecraft and astronauts in outer space. There are two types of solar wind, fast wind and slow wind. The fast wind is considered to be steady in composition and speed, and travels at speeds greater than 500 km/s. The slow solar wind is known for being highly variable in composition and speed, and travels at speeds less than 500 km/s. Fast solar wind originates from coronal hole regions on the sun, while the slow solar wind’s origin is very controversial. There are currently two types of theories for slow solar wind. One theory involves wave heating dynamics, while the other contends that slow solar wind originates from magnetic reconnection that continually opens magnetic field lines. These models are currently under-constrained with both types able to reproduce the long-term, average behavior of the wind. To further constrain these models it was necessary to research small scale structure in the solar wind, however analyzing these structures pushes the limits of the current instrument capabilities. We developed techniques that provide an automated process to quickly generate results from multiple different analysis techniques, allowing the user to compare data from STEREO’s Heliospheric Imager (HI) and from data taken at L1. This increases the efficiency and ability to relate data from the sun in HI and data at Earth at L1. These techniques were applied to a study on the slow solar wind which lead to possible evidence for the S-Web model. Title: On the Origin of the Slow Solar Wind: Periodic Plasma Release from Pseudostreamers Authors: Viall, N. M.; Kepko, L.; Antiochos, S. K. Bibcode: 2016AGUFMSH54A..05V Altcode: We present observations of quasi-periodic release of plasma from pseudostreamers, and demonstrate that these observations place severe constraints on the origin of both the slow solar wind and pseudostreamer dynamics. Though quasi-periodic release of slow solar wind plasma is routinely observed in remote white light images, such plasma release is often associated with the tips of helmet streamers and the heliospheric current sheet. Helmet streamers and the heliospheric current sheet are natural locations for magnetic reconnection to occur, both in the form of complete disconnections and interchange reconnection. However, pseudostreamers are not associated with the heliospheric current sheet, and are predicted by some models to have steady solar wind release. In contrast, in the S-web model of solar wind formation, pseudostreamers and their magnetic extensions into the heliosphere are also locations where slow solar wind is released sporadically through magnetic reconnection. We present the first observations demonstrating that quasi-periodic plasma release occurs in pseudostreamers as well. We build a comprehensive picture of the dynamics by combining remote-sensing data with in situ composition and magnetic connectivity information. Our results have critical implications for the magnetic topology of pseudostreamers and for their reconnection dynamics. This analysis pushes the limits of current instrument resolution and sensitivity, showing the enormous potential science to be accomplished with Solar Probe Plus and Solar Orbiter. Title: Imaging the Top of the Solar Corona and the Young Solar Wind Authors: DeForest, C. E.; Matthaeus, W. H.; Viall, N. M.; Cranmer, S. R. Bibcode: 2016AGUFMSH53A..05D Altcode: We present the first direct visual evidence of the quasi-stationary breakup of solar coronal structure and the rise of turbulence in the young solar wind, directly in the future flight path of Solar Probe. Although the corona and, more recently, the solar wind have both been observed directly with Thomson scattered light, the transition from the corona to the solar wind has remained a mystery. The corona itself is highly structured by the magnetic field and the outflowing solar wind, giving rise to radial "striae" - which comprise the familiar streamers, pseudostreamers, and rays. These striae are not visible in wide-field heliospheric images, nor are they clearly delineated with in-situ measurements of the solar wind. Using careful photometric analysis of the images from STEREO/HI-1, we have, for the first time, directly observed the breakup of radial coronal structure and the rise of nearly-isotropic turbulent structure in the outflowing slow solar wind plasma between 10° (40 Rs) and 20° (80 Rs) from the Sun. These observations are important not only for their direct science value, but for predicting and understanding the conditions expected near SPP as it flies through - and beyond - this final frontier of the heliosphere, the outer limits of the solar corona. Title: Probing Prominence Formation with Time Series Analysis of Models and AIA Data Authors: Kucera, T. A.; Viall, N. M.; Karpen, J. T. Bibcode: 2016AGUFMSH43C2583K Altcode: We present a observational and modeling study of the formation and dynamics of prominence plasma, using a time series analysis of data from the Solar Dynamic Observatory's Atmospheric Imaging Assembly (SDO/AIA). The analysis consists of a diagnosis of heating and cooling events by comparing the time profiles of emission formed at different temperatures and observed by different AIA bands. We apply this analysis both to prominences observed by AIA and to model runs from the thermal non-equilibrium model in which heating at the foot-points of sheared coronal flux-tubes results in evaporation of hot (a few MK) material into the corona and subsequent catastrophic cooling of the hot material to form the cool ( 10,000 K) prominence material. We find that both the data and model show characteristic heating and cooling signatures that are significantly different from those seen in active regions. Supported by NASA's Living with a Star program. Title: The Dynamics of Open-Field Corridors Authors: Viall, N. M.; Antiochos, S. K.; Higginson, A. K.; DeVore, C. R. Bibcode: 2016AGUFMSH54A..06V Altcode: The source of the slow solar wind and the origins of its dynamics have long been major problems in solar/heliospheric physics. Due to its observed location in the heliosphere, its plasma composition, and its variability, the slow wind is widely believed to be due to the release of closed-field plasma onto open field lines. In the S-Web model the slow wind is postulated to result from the driving of the open-closed boundary in the corona by the quasi-random photospheric convective motions. A key feature of the model is the topological complexity of the open field regions at the Sun, in other words, the distribution and geometry of coronal holes. In particular, narrow corridors of open field and even singular topologies are required in order to account for the observed angular extent of the slow wind in the heliosphere. We present the first calculations of the dynamics of an open-field corridor driven by photospheric flows. The calculations use our high-resolution MHD code and an isothermal approximation for the coronal and solar wind plasma. We show that the corridor dynamics do, in fact, result in the release of closed field plasma far from the heliospheric current sheet, in agreement with observations and as predicted by the S-Web model. The implications of our results for understanding the corona-heliosphere connection and especially for interpreting observations from the upcoming Solar Orbiter and Solar Probe Plus missions will be discussed. This research was supported by the NASA LWS programs. Title: Using SDO/AIA to Understand the Thermal Evolution of Solar Prominence Formation Authors: Viall, Nicholeen; M.; Kucera, Therese T.; Karpen, Judith Bibcode: 2016usc..confE..49V Altcode: In this study, we investigate prominence formation using time series analysis of Solar Dynamics Observatory's Atmospheric Imaging Assembly (SDO/AIA) data. We investigate the thermal properties of forming prominences by analyzing observed light curves using the same technique that we have already successfully applied to active regions to diagnose heating and cooling cycles. This technique tracks the thermal evolution using emission formed at different temperatures, made possible by AIA's different wavebands and high time resolution. We also compute the predicted light curves in the same SDO/AIA channels of a hydrodynamic model of thermal nonequilibrium formation of prominence material, an evaporation-condensation model. In these models of prominence formation, heating at the foot-points of sheared coronal flux-tubes results in evaporation of material of a few MK into the corona followed by catastrophic cooling of the hot material to form cool ( 10,000 K) prominence material. We demonstrate that the SDO/AIA light curves for flux tubes undergoing thermal nonequilibrium vary at different locations along the flux tube, especially in the region where the condensate forms, and we compare the predicted light curves with those observed. Supported by NASA's Living with a Star program. Title: Signatures of Steady Heating in Time Lag Analysis of Coronal Emission Authors: Viall, Nicholeen M.; Klimchuk, James A. Bibcode: 2016ApJ...828...76V Altcode: 2016arXiv160702008V Among the multitude of methods used to investigate coronal heating, the time lag method of Viall & Klimchuk is becoming increasingly prevalent as an analysis technique that is complementary to those that are traditionally used. The time lag method cross correlates light curves at a given spatial location obtained in spectral bands that sample different temperature plasmas. It has been used most extensively with data from the Atmospheric Imaging Assembly on the Solar Dynamics Observatory. We have previously applied the time lag method to entire active regions and surrounding the quiet Sun and created maps of the results. We find that the majority of time lags are consistent with the cooling of coronal plasma that has been impulsively heated. Additionally, a significant fraction of the map area has a time lag of zero. This does not indicate a lack of variability. Rather, strong variability must be present, and it must occur in phase between the different channels. We have previously shown that these zero time lags are consistent with the transition region response to coronal nanoflares, although other explanations are possible. A common misconception is that the zero time lag indicates steady emission resulting from steady heating. Using simulated and observed light curves, we demonstrate here that highly correlated light curves at zero time lag are not compatible with equilibrium solutions. Such light curves can only be created by evolution. Title: Fading Coronal Structure and the Onset of Turbulence in the Young Solar Wind Authors: DeForest, C. E.; Matthaeus, W. H.; Viall, N. M.; Cranmer, S. R. Bibcode: 2016ApJ...828...66D Altcode: 2016arXiv160607718D Above the top of the solar corona, the young, slow solar wind transitions from low-β, magnetically structured flow dominated by radial structures to high-β, less structured flow dominated by hydrodynamics. This transition, long inferred via theory, is readily apparent in the sky region close to 10° from the Sun in processed, background-subtracted solar wind images. We present image sequences collected by the inner Heliospheric Imager instrument on board the Solar-Terrestrial Relations Observatory (STEREO/HI1) in 2008 December, covering apparent distances from approximately 4° to 24° from the center of the Sun and spanning this transition in the large-scale morphology of the wind. We describe the observation and novel techniques to extract evolving image structure from the images, and we use those data and techniques to present and quantify the clear textural shift in the apparent structure of the corona and solar wind in this altitude range. We demonstrate that the change in apparent texture is due both to anomalous fading of the radial striae that characterize the corona and to anomalous relative brightening of locally dense puffs of solar wind that we term “flocculae.” We show that these phenomena are inconsistent with smooth radial flow, but consistent with the onset of hydrodynamic or magnetohydrodynamic instabilities leading to a turbulent cascade in the young solar wind. Title: Using Periodic Density Structures to Understand the Origin of the Slow Solar Wind Authors: Viall, Nicholeen Bibcode: 2016shin.confE..81V Altcode: Variability in the slow solar wind has many sources. One source of variability in the slow solar wind involves its formation and/or its origin. Understanding the origin of the slow solar wind has challenged scientists for many years. The challenge understanding slow solar wind formation and how that creates variability is that the data are very disparate, with global-scale, lower resolution remote sensing of the corona and high resolution, in situ point measurements near Earth. In this presentation, we show how a particular kind of solar wind density variations can be used to understand slow solar wind formation. We present new analysis of STEREO coronagraph and heliospheric imager observations and of in situ ACE measurements that reveal an important connection between the dynamics of the corona and of the solar wind. In particular, we have discovered quasi-periodic density variations in the slow wind that propagates along the so-called streamers that connect back to the large-scale closed field regions at the Sun. Furthermore, we have discovered similar quasi-periodic variations in the plasma properties of the slow wind near Earth. We discuss the extension of this study to the upcoming Solar Orbiter and Solar Probe Plus missions. Title: The Transition Region Response to a Coronal Nanoflare: Forward Modeling and Observations in SDO/AIA Authors: Viall, Nicholeen; Klimchuk, James A. Bibcode: 2016SPD....4720202V Altcode: The corona and transition region (TR) are fundamentally coupled through the processes of thermal conduction and mass exchange. Yet the temperature-dependent emissions from the two locations behave quite differently in the aftermath of an impulsive heating event such as a coronal nanoflare. In this presentation, we use results from the EBTEL hydrodynamics code to demonstrate that after a coronal nanoflare, the TR is multithermal and the emission at all temperatures responds in unison. This is in contrast to the coronal plasma, which cools sequentially, emitting first at higher temperatures and then at lower temperatures. We apply the time lag technique of Viall & Klimchuk (2012) to the simulated Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory emission and show that coronal plasma light curves exhibit post-nanoflare cooling time lags, while TR light curves show time lags of zero, as observed. We further demonstrate that time lags of zero, regardless of physical cause, do not indicate a lack of variability. Rather, strong variability must be present, and it must occur in unison in the different channels. Lastly, we show that the 'coronal' channels in AIA can be dominated by bright TR emission. When defined in a physically meaningful way, the TR reaches a temperature of roughly 60% the peak temperature in a flux tube. The TR resulting from impulsive heating can extend to 3 MK and higher, well within the range of the 'coronal' AIA channels. Title: Implications of L1 observations for slow solar wind formation by solar reconnection Authors: Kepko, L.; Viall, N. M.; Antiochos, S. K.; Lepri, S. T.; Kasper, J. C.; Weberg, M. Bibcode: 2016GeoRL..43.4089K Altcode: While the source of the fast solar wind is known to be coronal holes, the source of the slow solar wind has remained a mystery. Long time scale trends in the composition and charge states show strong correlations between solar wind velocity and plasma parameters, yet these correlations have proved ineffective in determining the slow wind source. We take advantage of new high time resolution (12 min) measurements of solar wind composition and charge state abundances at L1 and previously identified 90 min quasiperiodic structures to probe the fundamental timescales of slow wind variability. The combination of new high temporal resolution composition measurements and the clearly identified boundaries of the periodic structures allows us to utilize these distinct solar wind parcels as tracers of slow wind origin and acceleration. We find that each 90 min (2000 Mm) parcel of slow wind has near-constant speed yet exhibits repeatable, systematic charge state and composition variations that span the entire range of statistically determined slow solar wind values. The classic composition-velocity correlations do not hold on short, approximately hourlong, time scales. Furthermore, the data demonstrate that these structures were created by magnetic reconnection. Our results impose severe new constraints on slow solar wind origin and provide new, compelling evidence that the slow wind results from the sporadic release of closed field plasma via magnetic reconnection at the boundary between open and closed flux in the Sun's atmosphere. Title: Patterns of Activity Revealed by a Time Lag Analysis of a Model Active Region Authors: Bradshaw, Stephen; Viall, Nicholeen Bibcode: 2016SPD....4720201B Altcode: We investigate the global activity patterns predicted from a model active region heated by distributions of nanoflares that have a range of average frequencies. The activity patterns are manifested in time lag maps of narrow-band instrument channel pairs. We combine an extrapolated magnetic skeleton with hydrodynamic and forward modeling codes to create a model active region, and apply the time lag method to synthetic observations. Our aim is to recover some typical properties and patterns of activity observed in active regions. Our key findings are: 1. Cooling dominates the time lag signature and the time lags between the channel pairs are generally consistent with observed values. 2. Shorter coronal loops in the core cool more quickly than longer loops at the periphery. 3. All channel pairs show zero time lag when the line-of-sight passes through coronal loop foot-points. 4. There is strong evidence that plasma must be re-energized on a time scale comparable to the cooling timescale to reproduce the observed coronal activity, but it is likely that a relatively broad spectrum of heating frequencies operates across active regions. 5. Due to their highly dynamic nature, we find nanoflare trains produce zero time lags along entire flux tubes in our model active region that are seen between the same channel pairs in observed active regions. Title: Patterns of Activity in a Global Model of a Solar Active Region Authors: Bradshaw, S. J.; Viall, N. M. Bibcode: 2016ApJ...821...63B Altcode: 2016arXiv160306670B In this work we investigate the global activity patterns predicted from a model active region heated by distributions of nanoflares that have a range of frequencies. What differs is the average frequency of the distributions. The activity patterns are manifested in time lag maps of narrow-band instrument channel pairs. We combine hydrodynamic and forward modeling codes with a magnetic field extrapolation to create a model active region and apply the time lag method to synthetic observations. Our aim is not to reproduce a particular set of observations in detail, but to recover some typical properties and patterns observed in active regions. Our key findings are the following. (1) Cooling dominates the time lag signature and the time lags between the channel pairs are generally consistent with observed values. (2) Shorter coronal loops in the core cool more quickly than longer loops at the periphery. (3) All channel pairs show zero time lag when the line of sight passes through coronal loop footpoints. (4) There is strong evidence that plasma must be re-energized on a timescale comparable to the cooling timescale to reproduce the observed coronal activity, but it is likely that a relatively broad spectrum of heating frequencies are operating across active regions. (5) Due to their highly dynamic nature, we find nanoflare trains produce zero time lags along entire flux tubes in our model active region that are seen between the same channel pairs in observed active regions. Title: Nanoflare Heating of the Quiet Sun Authors: Viall, N. M.; Klimchuk, J. A. Bibcode: 2015AGUFMSH31D..05V Altcode: How the solar corona is heated to temperatures of over 1 MK, while the photosphere below is only ~ 6000 K remains one of the outstanding problems in all of space science. Solving this problem is crucial for understanding Sun-Earth connections, and will provide new insight into universal processes such as magnetic reconnection and wave-particle interactions. We use a systematic technique to analyze the properties of coronal heating throughout the solar corona using data taken with the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory. Our technique computes cooling times of the coronal plasma on a pixel-by-pixel basis and has the advantage that it analyzes all of the coronal emission, including the diffuse emission surrounding distinguishable coronal features. We have already applied this technique to 15 different active regions, and find clear evidence for dynamic heating and cooling cycles that are consistent with the 'impulsive nanoflare' scenario. What about the rest of the Solar corona? Whether the quiet Sun is heated in a similar or distinct manner from active regions is a matter of great debate. Here we apply our coronal heating analysis technique to quiet Sun locations. We find areas of quiet Sun locations that also undergo dynamic heating and cooling cycles, consistent with impulsive nanoflares. However, there are important characteristics that are distinct from those of active regions. Title: Periodic Density Structures and the Origin of the Slow Solar Wind Authors: Viall, Nicholeen M.; Vourlidas, Angelos Bibcode: 2015ApJ...807..176V Altcode: The source of the slow solar wind has challenged scientists for years. Periodic density structures (PDSs), observed regularly in the solar wind at 1 AU, can be used to address this challenge. These structures have length scales of hundreds to several thousands of megameters and frequencies of tens to hundreds of minutes. Two lines of evidence indicate that PDSs are formed in the solar corona as part of the slow solar wind release and/or acceleration processes. The first is corresponding changes in compositional data in situ, and the second is PDSs observed in the inner Heliospheric Imaging data on board the Solar Terrestrial Relations Observatory (STEREO)/Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) suite. The periodic nature of these density structures is both a useful identifier as well as an important physical constraint on their origin. In this paper, we present the results of tracking periodic structures identified in the inner Heliospheric Imager in SECCHI back in time through the corresponding outer coronagraph (COR2) images. We demonstrate that the PDSs are formed around or below 2.5 solar radii—the inner edge of the COR2 field of view. We compute the occurrence rates of PDSs in 10 days of COR2 images both as a function of their periodicity and location in the solar corona, and we find that this set of PDSs occurs preferentially with a periodicity of ∼90 minutes and occurs near streamers. Lastly, we show that their acceleration and expansion through COR2 is self-similar, thus their frequency is constant at distances beyond 2.5 solar radii. Title: Using the fingerprints of solar magnetic reconnection to identify the elemental building blocks of the slow solar wind Authors: Kepko, Larry; Viall, Nicholeen M.; Kasper, Justin; Lepri, Sue Bibcode: 2015TESS....110802K Altcode: While the source of the fast solar wind is well understood to be linked to coronal holes, the source of the slow solar wind has remained elusive. Many previous studies of the slow solar wind have examined trends in the composition and charge states over long time scales and found strong relationships between the solar wind velocity and these plasma parameters. These relationships have been used to constrain models of solar wind source and acceleration. In this study, we take advantage of high time resolution (12 min) measurements of solar wind composition and charge-state abundances recently reprocessed by the ACE Solar Wind Ion Composition Spectrometer (SWICS) science team to probe the timescales of solar wind variability at relatively small scales. We study an interval of slow solar wind containing quasi-periodic 90 minute structures and show that they are remnants of solar magnetic reconnection. Each 90-minute parcel of slow solar wind, though the speed remains steady, exhibits the complete range of charge state and composition variations expected for the entire range of slow solar wind, which is repeated again in the next 90-minute interval. These observations show that previous statistical results break down on these shorter timescales, and impose new and important constraints on models of slow solar wind creation. We conclude by suggesting these structures were created through interchange magnetic reconnection and form elemental building blocks of the slow solar wind. We also discuss the necessity of decoupling separately the process(es) responsible for the release and acceleration. Title: Nanoflare Heating of the Quiet Sun Authors: Viall, Nicholeen M.; Klimchuk, James A. Bibcode: 2015TESS....121303V Altcode: How the solar corona is heated to temperatures of over 1 MK, while the photosphere below is only ~ 6000 K remains one of the outstanding problems in all of space science. Solving this problem is crucial for understanding Sun-Earth connections, and will provide new insight into universal processes such as magnetic reconnection and wave-particle interactions. We use a new systematic technique to analyze the properties of coronal heating throughout the solar corona using data taken with the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory. Our technique computes cooling times of the coronal plasma on a pixel-by-pixel basis and has the advantage that it analyzes all of the coronal emission, including the diffuse emission surrounding distinguishable coronal features. We have already applied this technique to 15 different active regions, and find clear evidence for dynamic heating and cooling cycles that are consistent with the 'impulsive nanoflare' scenario. What about the rest of the Solar corona? Whether the quiet Sun is heated in a similar or distinct manner from active regions is a matter of great debate. In this paper, we apply our coronal heating analysis technique to quiet Sun locations. We find that the majority of the analyzed quiet Sun locations do undergo dynamic heating and cooling cycles, consistent with impulsive nanoflares. However, there are important characteristics that are distinct from those of active regions.This research was supported by a NASA Guest Investigator grant. Title: Synthetic 3D modeling of active regions and simulation of their multi-wavelength emission Authors: Nita, Gelu M.; Fleishman, Gregory; Kuznetsov, Alexey A.; Loukitcheva, Maria A.; Viall, Nicholeen M.; Klimchuk, James A.; Gary, Dale E. Bibcode: 2015TESS....131204N Altcode: To facilitate the study of solar active regions, we have created a synthetic modeling framework that combines 3D magnetic structures obtained from magnetic extrapolations with simplified 1D thermal models of the chromosphere, transition region, and corona. To handle, visualize, and use such synthetic data cubes to compute multi-wavelength emission maps and compare them with observations, we have undertaken a major enhancement of our simulation tools, GX_Simulator (ftp://sohoftp.nascom.nasa.gov/solarsoft/packages/gx_simulator/), developed earlier for modeling emission from flaring loops. The greatly enhanced, object-based architecture, which now runs on Windows, Mac, and UNIX platform, offers important new capabilities that include the ability to either import 3D density and temperature distribution models, or to assign to each individual voxel numerically defined coronal or chromospheric temperature and densities, or coronal Differential Emission Measure distributions. Due to these new capabilities, the GX_Simulator can now apply parametric heating models involving average properties of the magnetic field lines crossing a given voxel volume, as well as compute and investigate the spatial and spectral properties of radio (to be compared with VLA or EOVSA data), (sub-)millimeter (ALMA), EUV (AIA/SDO), and X-ray (RHESSI) emission calculated from the model. The application integrates shared-object libraries containing fast free-free, gyrosynchrotron, and gyroresonance emission codes developed in FORTRAN and C++, and soft and hard X-ray and EUV codes developed in IDL. We use this tool to model and analyze an active region and compare the synthetic emission maps obtained in different wavelengths with observations.This work was partially supported by NSF grants AGS-1250374, AGS-1262772, NASA grant NNX14AC87G, the Marie Curie International Research Staff Exchange Scheme "Radiosun" (PEOPLE-2011-IRSES-295272), RFBR grants 14-02-91157, 15-02-01089, 15-02-03717, 15-02-03835, 15-02-08028. Title: Investigating the Thermal Evolution of Solar Prominence Formation Authors: Kucera, Therese A.; Viall, Nicholeen M.; Karpen, Judith T. Bibcode: 2015TESS....120315K Altcode: We present a study of prominence formation using time series analysis of Solar Dynamics Observatory’s Atmospheric Imaging Assembly (SDO/AIA) data. In evaporation-condensation models of prominence formation, heating at the foot-points of sheared coronal flux-tubes results in evaporation of hot (a few MK) material into the corona and subsequent catastrophic cooling of the hot material to form the cool (~10,000 K) prominence material. We present the results of a time-lag analysis that tracks the thermal evolution using emission formed at different temperatures. This analysis is made possible by AIA's many wavebands and high time resolution, and it allows us to look for signs of the evaporation-condensation process and to study the heating time scales involved. Supported by NASA’s Living with a Star program. Title: The Transition Region Response to a Coronal Nanoflare: Forward Modeling and Observations in SDO/AIA Authors: Viall, Nicholeen M.; Klimchuk, James A. Bibcode: 2015ApJ...799...58V Altcode: The corona and transition region (TR) are fundamentally coupled through the processes of thermal conduction and mass exchange. It is not possible to understand one without the other. Yet the temperature-dependent emissions from the two locations behave quite differently in the aftermath of an impulsive heating event such as a coronal nanoflare. Whereas the corona cools sequentially, emitting first at higher temperatures and then at lower temperatures, the TR is multithermal and the emission at all temperatures responds in unison. We have previously applied the automated time lag technique of Viall & Klimchuk to disk observations of an active region (AR) made by the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory. Lines of sight passing through coronal plasma show clear evidence for post-nanoflare cooling, while lines of sight intersecting the TR footpoints of coronal strands show zero time lag. In this paper, we use the EBTEL hydrodynamics code to demonstrate that this is precisely the expected behavior when the corona is heated by nanoflares. We also apply the time lag technique for the first time to off-limb observations of an AR. Since TR emission is not present above the limb, the occurrence of zero time lags is greatly diminished, supporting the conclusion that zero time lags measured on the disk are due to TR plasma. Lastly, we show that the ''coronal'' channels in AIA can be dominated by bright TR emission. When defined in a physically meaningful way, the TR reaches a temperature of roughly 60% the peak temperature in a flux tube. The TR resulting from impulsive heating can extend to 3 MK and higher, well within the range of the ''coronal'' AIA channels. Title: Elemental building blocks of the slow solar wind Authors: Kepko, L.; Viall, N. M.; Lepri, S. T. Bibcode: 2014AGUFMSH33A4126K Altcode: While the source of the fast solar wind is well understood to be linked to coronal holes, the source of the slow solar wind has remained elusive. A distinguishing characteristic of the slow solar wind is the high variability of the plasma parameters, such as magnetic field, velocity, density, composition, and charge state. Many previous studies of the slow solar wind have examined trends in the composition and charge states over long time scales and using data with comparatively low temporal resolution. In this study, we take advantage of high time resolution (12 min) measurements of the charge-state abundances recently reprocessed by the ACE SWICS science team to probe the timescales of solar wind variability of coherent structures at relatively small scales (<2000 Mm, or ~ 90 minutes at slow wind speeds). We use an interval of slow solar wind containing quasi pressure-balanced, periodic number density structures previously studied by Kepko et al and shown to be important in solar wind-magnetospheric coupling. The combination of high temporal resolution composition measurements and the clearly identified boundaries of the periodic structures allows us to probe the elemental slow solar wind flux tubes/structures. We use this train of 2000Mm periodic density structures as tracers of solar wind origin and/or acceleration. We find that each 2000 Mm parcel of slow solar wind, though its speed is steady, exhibits the complete range of charge state and composition variations expected for the entire range of slow solar wind, in a repeated sequence. Each parcel cycles through three states: 1) 'normal' slow wind, 2) compositionally slow wind with very high density, and 3) compositionally fast but typical slow solar wind density. We conclude by suggesting these structures form elemental building blocks of the slow solar wind, and discuss whether it is necessary to decouple separately the process(es) responsible for the release and acceleration. Title: Periodic Density Structures and the Origin of the Slow Solar Wind Authors: Viall, N. M.; Vourlidas, A. Bibcode: 2014AGUFMSH21B4114V Altcode: Periodic density structures with length-scales of hundreds to several thousands of Mm and frequencies of tens to hundreds of minutes are observed regularly in the solar wind at 1 AU. These structures coexist with, but are not due to, fluctuations in the plasma resulting from the turbulent cascade. Two lines of evidence - one identifying corresponding changes in compositional data in situ, and another identifying periodic density structures in the inner Heliospheric Imaging data onboard the Solar Terrestrial Relations Observatory (STEREO)/ Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) suite - indicate that periodic density structures are formed in the solar corona as part of the slow solar wind release and/or acceleration processes. The periodic nature of these density structures is an important physical constraint on their origin. In this presentation, we present the results of tracking periodic structures identified in the SECCHI/HI1 images down through the corresponding SECCHI/COR2 images. We demonstrate that the periodic density structures are formed around or below 2.5 solar radii - the inner edge of the COR2 field of view. Further, we compute the occurrence rate of periodic density structures in 10 days of COR2 images as a function of location in the solar corona. We find that this set of periodic density structures occurs preferentially in relation to coronal streamers. Periodic density structures are tracers of solar wind origin and/or acceleration; this study is a pilot for the kinds of investigations that we can carry out with the better temporal and spatial resolution of the heliospheric imagers on Solar Orbiter and Solar Probe Plus. Title: The effect of magnetopause motion on fast mode resonance Authors: Hartinger, M. D.; Welling, D.; Viall, N. M.; Moldwin, M. B.; Ridley, A. Bibcode: 2014JGRA..119.8212H Altcode: The Earth's magnetosphere supports several types of ultralow frequency (ULF) waves. These include fast mode resonance (FMR): cavity modes, waveguide modes, and tunneling modes/virtual resonance. The magnetopause, often treated as the outer boundary for cavity/waveguide modes in the dayside magnetosphere, is not stationary. A rapidly changing outer boundary condition—e.g., due to rapid magnetopause motion—is not favorable for FMR generation and may explain the sparseness of FMR observations in the outer magnetosphere. We examine how magnetopause motion affects the dayside magnetosphere's ability to sustain FMR with idealized Space Weather Modeling Framework (SWMF) simulations using the BATS-R-US global magnetohydrodynamic (MHD) code coupled with the Ridley Ionosphere Model (RIM). We present observations of FMR in BATS-R-US, reproducing results from other global MHD codes. We further show that FMR is present for a wide range of solar wind conditions, even during periods with large and rapid magnetopause displacements. We compare our simulation results to FMR observations in the dayside magnetosphere, finding that FMR occurrence does not depend on solar wind dynamic pressure, which can be used as a proxy for dynamic pressure fluctuations and magnetopause perturbations. Our results demonstrate that other explanations besides a nonstationary magnetopause—such as the inability to detect FMR in the presence of other ULF wave modes with large amplitudes—are required to explain the rarity of FMR observations in the outer magnetosphere. Title: Periodic Density Structures and the Source of the Slow Solar Wind Authors: Viall, Nicholeen; Vourlidas, Angelos Bibcode: 2014AAS...22440202V Altcode: Periodic density structures with length-scales of hundreds to several thousands of megameters, and frequencies of tens to hundreds of minutes, are observed regularly in the solar wind at 1 AU. These structures coexist with, but are not due to, fluctuations in the plasma resulting from the turbulent cascade. Two lines of evidence suggest that periodic density structures are formed in the solar corona as part of the slow solar wind release and/or acceleration processes. The first is the identification of corresponding changes in compositional data in situ, and the other is the identification of periodic density structures in the inner Heliospheric Imaging data onboard the STEREO/SECCHI suite. In this presentation, we show the results of tracking periodic structures identified in the SECCHI/Hi1 images down through the corresponding SECCHI/Cor2 images. We demonstrate that the periodic density structures are formed around or below 2.5 Rs - the inner edge of the Cor2 field of view. Further, we compute the occurrence rate of periodic density structures in 10 days of Cor2 images as a function of location in the solar corona. We find that periodic density structures do not occur throughout the entire space-filling volume of the solar wind; rather, there are particular places where they occur preferentially, suggesting source locations for periodic density structures in the slow solar wind. Title: A Survey of Coronal Heating Properties in Solar Active Regions Authors: Viall, Nicholeen; Klimchuk, James A. Bibcode: 2014AAS...22432315V Altcode: We investigate the properties of coronal heating in solar active regions (AR) by systematically analyzing coronal light curves observed by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory. Our automated technique computes time-lags (cooling times) on a pixel-by-pixel basis, and has the advantage that it allows us to analyze all of the coronal AR emission, including the so-called diffuse emission between coronal loops. We recently presented results using this time-lag analysis on NOAA AR 11082 (Viall & Klimchuk 2012) and found that the majority of the pixels contained cooling plasma along their line of sight. This result is consistent with impulsive coronal nanoflare heating of both coronal loops and the surrounding diffuse emission in the AR. Here we present the results of our time-lag technique applied to a survey of 15 AR of different magnetic complexity, total unsigned magnetic flux, size and age. We show that the post-nanoflare cooling patterns identified in NOAA AR 11082 are identified throughout all of the active regions in this survey, indicating that nanoflare heating is ubiquitous in solar active regions. However, some details of the nanoflare properties, such as the nanoflare energy, are different across these different active regions.We thank the SDO/AIA team for the use of these data, and the Coronal Heating ISSI team for helpful discussion of these topics. This research was supported by a NASA Heliophysics GI. Title: Measuring Temperature-dependent Propagating Disturbances in Coronal Fan Loops Using Multiple SDO/AIA Channels and the Surfing Transform Technique Authors: Uritsky, Vadim M.; Davila, Joseph M.; Viall, Nicholeen M.; Ofman, Leon Bibcode: 2013ApJ...778...26U Altcode: 2013arXiv1308.6195U A set of co-aligned high-resolution images from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory is used to investigate propagating disturbances (PDs) in warm fan loops at the periphery of a non-flaring active region NOAA AR 11082. To measure PD speeds at multiple coronal temperatures, a new data analysis methodology is proposed enabling a quantitative description of subvisual coronal motions with low signal-to-noise ratios of the order of 0.1%. The technique operates with a set of one-dimensional "surfing" signals extracted from position-time plots of several AIA channels through a modified version of Radon transform. The signals are used to evaluate a two-dimensional power spectral density distribution in the frequency-velocity space that exhibits a resonance in the presence of quasi-periodic PDs. By applying this analysis to the same fan loop structures observed in several AIA channels, we found that the traveling velocity of PDs increases with the temperature of the coronal plasma following the square-root dependence predicted for slow mode magneto-acoustic waves which seem to be the dominating wave mode in the loop structures studied. This result extends recent observations by Kiddie et al. to a more general class of fan loop system not associated with sunspots and demonstrating consistent slow mode activity in up to four AIA channels. Title: Modeling the Line-of-sight Integrated Emission in the Corona: Implications for Coronal Heating Authors: Viall, Nicholeen M.; Klimchuk, James A. Bibcode: 2013ApJ...771..115V Altcode: 2013arXiv1304.5439V One of the outstanding problems in all of space science is uncovering how the solar corona is heated to temperatures greater than 1 MK. Though studied for decades, one of the major difficulties in solving this problem has been unraveling the line-of-sight (LOS) effects in the observations. The corona is optically thin, so a single pixel measures counts from an indeterminate number (perhaps tens of thousands) of independently heated flux tubes, all along that pixel's LOS. In this paper we model the emission in individual pixels imaging the active region corona in the extreme ultraviolet. If LOS effects are not properly taken into account, erroneous conclusions regarding both coronal heating and coronal dynamics may be reached. We model the corona as an LOS integration of many thousands of completely independently heated flux tubes. We demonstrate that despite the superposition of randomly heated flux tubes, nanoflares leave distinct signatures in light curves observed with multi-wavelength and high time cadence data, such as those data taken with the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. These signatures are readily detected with the time-lag analysis technique of Viall & Klimchuk in 2012. Steady coronal heating leaves a different and equally distinct signature that is also revealed by the technique. Title: A Survey of Nanoflare Properties in Solar Active Regions Authors: Viall, Nicholeen; Klimchuk, J. A. Bibcode: 2013SPD....44...16V Altcode: We investigate the characteristics of coronal heating using a systematic technique that analyzes the properties of nanoflares in active regions (AR). Our technique computes cooling times, or time lags, using SDO/AIA light curves of all of the coronal AR emission, including the so-called diffuse emission. We recently presented results using this time-lag analysis on NOAA AR 11082 (Viall & Klimchuk 2012). We found that the majority of the pixels had cooling plasma along their line of sight, consistent with impulsive coronal nanoflare heating. Additionally, our results using the AIA 94 channel data showed that the nanoflare energy is stronger in the AR core and weaker in the AR periphery. Are these results representative of the nanoflare characteristics exhibited in the majority of active regions, or is AR 11082 unique? Here we present the time-lag results for a survey of active regions and determine whether these nanoflare patterns are born out in other active regions as well. This research was supported by the NASA Heliophysics Guest Investigator program. Title: Slow mode waves and quasi-periodic upflows in the multi-temperature solar corona as seen by the SDO Authors: Uritsky, Vadim; Davila, J. M.; Viall, N.; Ofman, L. Bibcode: 2013SPD....4410405U Altcode: We report results the analysis of coronal fan loops in a non-flaring solar active region exhibiting temperature-dependent propagating optical disturbances. A 6-hour set of high resolution coronal observations provided by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) has been used for characterizing apparent propagating patterns at multiple coronal temperatures (131A, 171A, 193A and 211A). A new data analysis methodology has been developed enabling an identification of subvisual motions with low signal-to-noise ratios not previously examined in this context. The technique involves spatiotemporal tracking of fan loop filaments containing propagating disturbances, construction of position - time plots for different temperature channels, obtaining the waveforms of the propagating optical features, and evaluation of Fourier spectral power of the waveforms as a function of phase speed and frequency. Using this methodology, we identified the parameters of propagating optical disturbances in different magnetic geometries, and classified these events as waves and/or plasma jets. We explored coronal conditions favoring wave-like and jet-like traveling plasma density enhancements in fan loops and the mechanisms of their generation, damping and interaction. The results obtained are compared with the behavior of a resistive MHD model exhibiting both types of propagating disturbances. Title: Understanding Coronal Heating by Comparing SDO/AIA Observations with Modeled Light Curves Authors: Viall, Nicholeen M.; Klimchuk, James A. Bibcode: 2013enss.confE..18V Altcode: An important signature of nanoflare heated coronal plasma is the sudden appearance of the plasma at hot temperatures, followed by a comparatively slow cooling and draining phase. This is due to the impulsive nature of nanoflare heating and the heat conduction and mass exchange between the corona and chromosphere. Identifying such nanoflare signatures is complicated by the fact that the solar corona is optically thin: many thousands of flux tubes which are heated completely independently are contributing to the total emission along a given line of sight. One approach has been to analyze isolated features such as coronal loops; however the diffuse emission between and around isolated features contribute as much, if not more to the EUV coronal emission, and therefore is crucial to the understanding of coronal heating. In this study we move beyond isolated features and analyze all of the emission in an entire active region and quiet Sun area. We investigate SDO/AIA light curves, systematically identifying nanoflare signatures. We compare the observations with a model of the corona as a line-of-sight integration of many thousands of completely independently heated flux tubes. We consider that the emission from these flux tubes may be due exclusively to impulsive nanoflare bursts, quasi-steady heating, or a mix of both, depending on the cadence of heat release. We demonstrate that despite the superposition of randomly heated flux tubes, different distributions of nanoflare cadences produce distinct signatures in light curves observed with multi-wavelength and high time cadence data, such as those from SDO/AIA. We find that much of the solar corona is heated through impulsive nanoflares. Title: Nanoflare Heating of the Solar Corona: Comparing SDO/AIA Observations with Modeled Light Curves Authors: Viall, N. M.; Klimchuk, J. A. Bibcode: 2012AGUFMSH42A..03V Altcode: A significant outstanding issue in current solar and astrophysical research is that of the heating of the solar corona. Coronal plasma is typically measured to be at temperatures near ~1-3 MK. Is the majority of the coronal plasma maintained at these temperatures through a form of quasi-steady heating, or is this simply a measure of the average temperature of widely varying, impulsively heated coronal plasma? Addressing even this basic question is complicated by the fact that the corona is optically thin: many thousands of flux tubes which are heated completely independently are contributing to the total emission along a given line of sight. There is a large body of work focused on the heating of isolated features - coronal loops in active regions- which are impulsively heated, however understanding of the diffuse emission between loops and the emission from the quiet Sun are also crucial. Therefore in this study we move beyond isolated features and analyze all of the emission in an entire active region and quiet Sun area from all contributing flux tubes. We investigate light curves systematically using SDO/AIA observations. We also model the corona as a line-of-sight integration of many thousands of completely independently heated flux tubes. The emission from these flux tubes may be time dependent, quasi-steady, or a mix of both, depending on the cadence of heat release. We demonstrate that despite the superposition of randomly heated flux tubes, different distributions of nanoflare cadences produce distinct signatures in light curves observed with multi-wavelength and high time cadence data, such as those from SDO/AIA. We discuss the quiet Sun and active region emission in the context of these predicted nanoflare signatures. Title: Evidence for Widespread Cooling in an Active Region Observed with the SDO Atmospheric Imaging Assembly Authors: Viall, Nicholeen M.; Klimchuk, James A. Bibcode: 2012ApJ...753...35V Altcode: 2012arXiv1202.4001V A well-known behavior of EUV light curves of discrete coronal loops is that the peak intensities of cooler channels or spectral lines are reached at progressively later times than hotter channels. This time lag is understood to be the result of hot coronal loop plasma cooling through these lower respective temperatures. However, loops typically comprise only a minority of the total emission in active regions (ARs). Is this cooling pattern a common property of AR coronal plasma, or does it only occur in unique circumstances, locations, and times? The new Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) data provide a wonderful opportunity to answer this question systematically for an entire AR. We measure the time lag between pairs of SDO/AIA EUV channels using 24 hr of images of AR 11082 observed on 2010 June 19. We find that there is a time-lag signal consistent with cooling plasma, just as is usually found for loops, throughout the AR including the diffuse emission between loops for the entire 24 hr duration. The pattern persists consistently for all channel pairs and choice of window length within the 24 hr time period, giving us confidence that the plasma is cooling from temperatures of greater than 3 MK, and sometimes exceeding 7 MK, down to temperatures lower than ~0.8 MK. This suggests that the bulk of the emitting coronal plasma in this AR is not steady; rather, it is dynamic and constantly evolving. These measurements provide crucial constraints on any model which seeks to describe coronal heating. Title: SDO / AIA Observations of Slow Mode Waves in Coronal Fan Loops Authors: Uritsky, Vadim; Davila, J. M.; Viall, N. M. Bibcode: 2012AAS...22032205U Altcode: We investigate slow mode waves in fan coronal loops observed by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory spacecraft ( 12 sec cadence, 0.6 arcsec pixels). The warm fan structure studied here was located at the periphery of quiescent active region NOAA AR 11082. A specialized software package has been developed for extracting subvisual modulations of SDO AIA intensity propagating along dynamic loop segments changing their shape and position during the observation. The processing steps include manual tracking of the segment location in time sequences of co-aligned multispectral AIA images, extracting wave-carrying filamentary segments from the surrounding background, constructing position - time diagrams representing temporal evolution of optical brightness along the filaments, and analysis of the obtained wave signatures. The results reveal a persistent wave activity in many fan loop segments characterized by a well-defined frequency and phase speed, with the best signal-to-noise ratio in 171Å and 193Å channels. For some filamentary segments, the wave parameters remained almost constant over the entire observing interval ( 6 hours). The wave parameters varied across the studied structures. The fastest wave fronts exhibited strictly outward propagation while the slower waves could travel both inward and outward. The estimated phase velocity (80-100 km/s) and period (3-4 min) of the most stable outward wave mode are in a good agreement with earlier SOHO EIT and TRACE observations of slow magnetosonic waves in fan loops. The newly observed features include (1) the remarkable coherency of the wave pattern over a course of several hours, and (2) the detailed wave form of the process enabling quantitative analysis of nonlinear propagation and damping effects. No consistent dependence of the wave speed on the distance from the hot core region of AR 11082 was identified, which challenges the traditional picture of traveling magnetoacoustic oscillations in fan loops. Title: Nanoflare Properties throughout Active Regions: Comparing SDO/AIA Observations with Modeled Active Region Light Curves Authors: Viall, Nicholeen; Klimchuk, J. Bibcode: 2012AAS...22030904V Altcode: Coronal plasma in active regions is typically measured to be at temperatures near 1-3 MK. Is the majority of the coronal plasma in hydrostatic equilibrium, maintained at these temperatures through a form of quasi-steady heating, or is this simply a measure of the average temperature of widely varying, impulsively heated coronal plasma? Addressing this question is complicated by the fact that the corona is optically thin: many thousands of flux tubes which are heated completely independently are contributing to the total emission along a given line of sight. There is a large body of work focused on the heating of isolated features - coronal loops - which are impulsively heated, however it is the diffuse emission between loops which often comprises the majority of active region emission. Therefore in this study we move beyond isolated features and analyze all of the emission in an entire active region from all contributing flux tubes. We investigate light curves systematically using SDO/AIA observations. We also model the active region corona as a line-of-sight integration of many thousands of completely independently heated flux tubes. The emission from these flux tubes may be time dependent, quasi-steady, or a mix of both, depending on the cadence of heat release. We demonstrate that despite the superposition of randomly heated flux tubes, different distributions of nanoflare cadences produce distinct signatures in light curves observed with multi-wavelength and high time cadence data, such as those from SDO/AIA. We conclude that the majority of the active region plasma is not maintained in hydrostatic equilibrium, rather it is undergoing dynamic heating and cooling cycles. The observed emission is consistent with heating through impulsive nanoflares, whose energy is a function of location within the active region.

This research was supported by an appointment to the NASA Postdoctoral Program at GSFC/NASA. Title: Determining the Typical Nanoflare Cadence in Active Regions: Comparing SDO/AIA Observations with Modeled Active Region Light Curves Authors: Viall, Nicholeen M.; Klimchuk, James A. Bibcode: 2012decs.confE..40V Altcode: Coronal plasma in active regions is typically measured to be at temperatures near 1-3 MK. Is the majority of the coronal plasma in hydrostatic equilibrium, maintained at these temperatures through a form of quasi-steady heating, or is this simply a measure of the average temperature of widely varying, impulsively heated coronal plasma which is continually undergoing heating and cooling cycles? Addressing this question is complicated by the fact that the corona is optically thin: many thousands of strands which are heated completely independently are contributing to the total emission along a given line of sight. There is a large body of work focused on the heating of coronal loops, which are impulsively heated, however it is the diffuse emission between loops which often comprises the majority of active region emission. Therefore, a different and necessary approach to analyzing active region heating is to analyze all of the emission in an active region, and account for emission along the line of sight from all of the contributing strands. We investigate light curves systematically in an entire active region using SDO/AIA observations. We also model the active region corona as a line-of-sight integration of many thousands of completely independently heated strands. The emission from these flux tubes may be time dependent, quasi-steady, or a mix of both, depending on the cadence of heat release on each strand. We examine a full range of heat cadences from effectively steady (heat pulse repeat time << plasma cooling time) to fully impulsive (heat pulse repeat time >> plasma cooling time) and model the resulting emission when superposing strands undergoing these differing heat cycles. We demonstrate that despite the superposition of randomly heated strands, different distributions of heat cadences produce distinct signatures in light curves observed with multi-wavelength and high time cadence data, such as those from the AIA telescopes on SDO. Using these model predictions in conjunction with SDO/AIA observations, we evaluate the typical cadence of heat release in different active regions and patterns therein, which is a crucial constraint on coronal heating mechanisms. Title: Determining the Typical Nanoflare Cadence in Active Regions: Modeling Light Curves of Active Regions Authors: Viall, N. M.; Klimchuk, J. A. Bibcode: 2011AGUFMSH33B2057V Altcode: Active region coronal loops visible at 1MK are likely composed of many unresolved strands, heated by storms of impulsive nanoflares. Though well-studied, these loops often contribute only a fraction of the total emission in an active region; the degree to which the entire active region is heated in the same manner as loops are is highly debated. Is the majority of coronal active region plasma heated impulsively, or is the majority of the heating quasi-steady? Addressing this question is complicated by the fact that the corona is optically thin: many thousands of strands which are heated completely independently are contributing to the total emission along a given line of sight. Furthermore, certain geometries preclude even the best background subtraction methods from fully isolating the emission from even a single coronal loop. Therefore, a different and necessary approach to analyzing active region heating is to account for emission along the line of sight from all of the contributing strands. We model the active region corona as a line-of-sight integration of many thousands of completely independently heated strands. The emission from these flux tubes may be time dependent, quasi-steady, or a mix of both, depending on the cadence of heat release on each strand. We examine a full range of heat cadences from effectively steady (heat pulse repeat time << plasma cooling time) to fully impulsive (heat pulse repeat time >> plasma cooling time) and model the resulting emission when superposing strands undergoing these differing heat cycles. We demonstrate that despite the superposition of randomly heated strands, different distributions of heat cadences produce distinct signatures in light curves observed with multi-wavelength and high time cadence data, such as those from the AIA telescopes on SDO. For this reason, high time cadence spectral information for lines sensitive to the 1-10 MK range will be especially useful in future missions. Using these model predictions, we evaluate the typical cadence of heat release in different active regions and patterns therein, which is a crucial constraint on coronal heating mechanisms. Title: Patterns of Nanoflare Storm Heating Exhibited by an Active Region Observed with Solar Dynamics Observatory/Atmospheric Imaging Assembly Authors: Viall, Nicholeen M.; Klimchuk, James A. Bibcode: 2011ApJ...738...24V Altcode: 2011arXiv1106.4196V It is largely agreed that many coronal loops—those observed at a temperature of about 1 MK—are bundles of unresolved strands that are heated by storms of impulsive nanoflares. The nature of coronal heating in hotter loops and in the very important but largely ignored diffuse component of active regions is much less clear. Are these regions also heated impulsively, or is the heating quasi-steady? The spectacular new data from the Atmospheric Imaging Assembly (AIA) telescopes on the Solar Dynamics Observatory offer an excellent opportunity to address this question. We analyze the light curves of coronal loops and the diffuse corona in six different AIA channels and compare them with the predicted light curves from theoretical models. Light curves in the different AIA channels reach their peak intensities with predictable orderings as a function the nanoflare storm properties. We show that while some sets of light curves exhibit clear evidence of cooling after nanoflare storms, other cases are less straightforward to interpret. Complications arise because of line-of-sight integration through many different structures, the broadband nature of the AIA channels, and because physical properties can change substantially depending on the magnitude of the energy release. Nevertheless, the light curves exhibit predictable and understandable patterns consistent with impulsive nanoflare heating. Title: Heating of Active Regions by Impulsive Nanoflares Authors: Viall, Nicholeen Mary; Klimchuk, James A. Bibcode: 2011shin.confE..57V Altcode: It has been proposed that plasma on active regions may be contributing plasma to the slow solar wind. If this is the case, then understanding the heating and dynamics of active regions adds vital knowledge to our understanding of the heating and acceleration of the slow solar wind. It seems largely agreed that many coronal loops--those observed at a temperature of about 1 MK--are bundles of unresolved strands that are heated by storms of impulsive nanoflares. The nature of coronal heating in hotter loops and in the very important but largely ignored diffuse component of active regions is much less clear. Are these regions also heated impulsively, or is the heating quasi steady? The spectacular new data from the Atmospheric Imaging Assembly (AIA) telescopes on the Solar Dynamics Observatory (SDO) offer an excellent opportunity to address this question. We analyze the light curves of coronal loops and the diffuse corona in 6 different AIA channels and compare them with the predicted light curves from theoretical models. Light curves in the different AIA channels reach their peak intensities with predictable orderings as a function of the nanoflare storm properties. These orderings, or time lags, are clearly exhibited in loop observations in all channels. What is especially exciting is that we identify these time lag patterns in observations of the seemingly steady diffuse corona as well. We model the diffuse corona as a line-of-sight integration of many thousands of completely independent, impulsively heated strands. The time lags of the simulated and actual observations are in excellent agreement. Our results suggest that impulsive nanoflare heating is ubiquitous within active regions. Title: Pulsed Flows Along a Cusp Structure Observed with SDO/AIA Authors: Thompson, Barbara; Démoulin, P.; Mandrini, C.; Mays, M.; Ofman, L.; Van Driel-Gesztelyi, L.; Viall, N. Bibcode: 2011SPD....42.2117T Altcode: 2011BAAS..43S.2117T We present observations of a cusp-shaped structure that formed after a flare and coronal mass ejection on 14 February 2011. Throughout the evolution of the cusp structure, blob features up to a few Mm in size were observed flowing along the legs and stalk of the cusp at projected speeds ranging from 50 to 150 km/sec. Around two dozen blob features, on order of 1 - 3 minutes apart, were tracked in multiple AIA EUV wavelengths. The blobs flowed outward (away from the Sun) along the cusp stalk, and most of the observed speeds were either constant or decelerating. We attempt to reconstruct the 3-D magnetic field of the evolving structure, discuss the possible drivers of the flows (including pulsed reconnection and tearing mode instability), and compare the observations to studies of pulsed reconnection and blob flows in the solar wind and the Earth's magnetosphere. Title: Patterns of Nanoflare Heating Exhibited by Active Regions Observed with SDO/AIA Authors: Viall, Nicholeen; Klimchuk, J. Bibcode: 2011SPD....42.2103V Altcode: 2011BAAS..43S.2103V It seems largely agreed that many coronal loops---those observed at a temperature of about 1 MK---are bundles of unresolved strands that are heated by storms of impulsive nanoflares. The nature of coronal heating in hotter loops and in the very important but largely ignored diffuse component of active regions is much less clear. Are these regions also heated impulsively, or is the heating quasi steady? The spectacular new data from the Atmospheric Imaging Assembly (AIA) telescopes on the Solar Dynamics Observatory (SDO) offer an excellent opportunity to address this question. We analyze the light curves of coronal loops and the diffuse corona in 6 different AIA channels and compare them with the predicted light curves from theoretical models. Light curves in the different AIA channels reach their peak intensities with predictable orderings as a function of the nanoflare storm properties. These orderings, or time lags, are clearly exhibited in loop observations in all channels. What is especially exciting is that we identify these time lag patterns in observations of the seemingly steady diffuse corona as well. We model the diffuse corona as a line-of-sight integration of many thousands of completely independent, impulsively heated strands. The time lags of the simulated and actual observations are in excellent agreement. Our results suggest that impulsive nanoflare heating is ubiquitous within active regions.

This research was supported through an appointment to the NASA Postdoctoral Program at the Goddard Space Flight Center, administered by Oak Ridge Associated Universities through a contract with NASA. Title: SDO/AIA Light Curves and Implications for Coronal Heating: Observations Authors: Viall, N. M.; Klimchuk, J. A. Bibcode: 2010AGUFMSH41E..02V Altcode: It seems largely agreed that many coronal loops---those observed at a temperature of about 1 MK---are bundles of unresolved strands that are heated by storms of impulsive nanoflares. The nature of coronal heating in hotter loops and in the very important but largely ignored diffuse component of active regions is much less clear. Is it also impulsive or is it quasi steady? The spectacular new data from the Atmospheric Imaging Assembly (AIA) telescopes on the Solar Dynamics Observatory (SDO) offer an excellent opportunity to address this question. We analyze the light curves of coronal loops and the diffuse corona in 6 different AIA channels and compare them with the predicted light curves from theoretical models. Light curves in the different AIA channels reach their peak intensities with predictable orderings as a function the nanoflare storm properties. We show that while some sets of light curves exhibit clear evidence of cooling after nanoflare storms, other cases are less straightforward to interpret. Complications arise because of line-of-sight integration through many different structures, the broadband nature of the AIA channels, and because physical properties can change substantially depending on the magnitude of the energy release. Nevertheless, the light curves exhibit predictable and understandable patterns. This presentation emphasizes the observational aspects of our study. A companion presentation emphasizes the models. Title: SDO/AIA Light Curves and Implications for Coronal Heating: Model Predictions Authors: Klimchuk, J. A.; Viall, N. M. Bibcode: 2010AGUFMSH41E..03K Altcode: It seems largely agreed that many coronal loops---those observed at a temperature of about 1 MK---are bundles of unresolved strands that are heated by storms of impulsive nanoflares. The nature of coronal heating in hotter loops and in the very important but largely ignored diffuse component of active regions is much less clear. Is it also impulsive or is it quasi steady? The spectacular new data from the Atmospheric Imaging Assembly (AIA) telescopes on the Solar Dynamics Observatory (SDO) offer an excellent opportunity to address this question. We analyze the light curves of coronal loops and the diffuse corona in 6 different AIA channels and compare them with the predicted light curves from theoretical models. Light curves in the different AIA channels reach their peak intensities with predictable orderings as a function the nanoflare storm properties. We show that while some sets of light curves exhibit clear evidence of cooling after nanoflare storms, other cases are less straightforward to interpret. Complications arise because of line-of-sight integration through many different structures, the broadband nature of the AIA channels, and because physical properties can change substantially depending on the magnitude of the energy release. Nevertheless, the light curves exhibit predictable and understandable patterns. This presentation emphasizes the modeling aspects of our study. A companion presentation emphasizes the observations. Title: Examining Periodic Solar-Wind Density Structures Observed in the SECCHI Heliospheric Imagers Authors: Viall, Nicholeen M.; Spence, Harlan E.; Vourlidas, Angelos; Howard, Russell Bibcode: 2010SoPh..267..175V Altcode: 2010arXiv1009.5885V; 2010SoPh..tmp..174V We present an analysis of small-scale, periodic, solar-wind density enhancements (length scales as small as ≈ 1000 Mm) observed in images from the Heliospheric Imager (HI) aboard STEREO-A. We discuss their possible relationship to periodic fluctuations of the proton density that have been identified at 1 AU using in-situ plasma measurements. Specifically, Viall, Kepko, and Spence (J. Geophys. Res.113, A07101, 2008) examined 11 years of in-situ solar-wind density measurements at 1 AU and demonstrated that not only turbulent structures, but also nonturbulent, periodic density structures exist in the solar wind with scale sizes of hundreds to one thousand Mm. In a subsequent paper, Viall, Spence, and Kasper (Geophys. Res. Lett.36, L23102, 2009) analyzed the α-to-proton solar-wind abundance ratio measured during one such event of periodic density structures, demonstrating that the plasma behavior was highly suggestive that either temporally or spatially varying coronal source plasma created those density structures. Large periodic density structures observed at 1 AU, which were generated in the corona, can be observable in coronal and heliospheric white-light images if they possess sufficiently high density contrast. Indeed, we identify such periodic density structures as they enter the HI field of view and follow them as they advect with the solar wind through the images. The smaller, periodic density structures that we identify in the images are comparable in size to the larger structures analyzed in-situ at 1 AU, yielding further evidence that periodic density enhancements are a consequence of coronal activity as the solar wind is formed. Title: Examining Periodic Solar Wind Density Structures in SECCHI HI1A Authors: Viall, Nicholeen; Vourlidas, A.; Spence, H.; Howard, R. Bibcode: 2010AAS...21630303V Altcode: We present an analysis of small-scale periodic solar wind density enhancements observed in SECCHI HI1. We discuss their possible relationship to periodic fluctuations of the proton density observed in-situ with the Wind SWE data. Viall et al. [2008] used 11 years of solar wind density measurements at 1 AU and demonstrated that in addition to turbulent fluctuations, non-turbulent periodic density structures with length scales of tens to hundreds of megameters exist in the solar wind. Event studies of the periodic density structures reveal instances in which the density structures have alpha/proton abundance ratio changes associated with the density structures. Specifically, the alpha density varies with the same periodicity as the protons, but in antiphase. For those events, this strongly suggests either time varying or spatially varying coronal source plasma that created the density structures. If such periodic density structures observed at 1 AU are generated in the corona, then they may be observable in SECCHI HI1 data. We identify periodic density structures as they convect with the solar wind into the field of view of SECCHI HI and follow the train of structures as a function of time. The periodic density structures we analyze are comparable in size to the larger structures identified in-situ at 1 AU.

This research was supported through NASA Grant No. NNG05GK65G and an appointment to the NASA Postdoctoral Program at the Goddard Space Flight Center, administered by Oak Ridge Associated Universities through a contract with NASA. Title: Periodic solar wind density structures Authors: Viall, Nicholeen Mary Bibcode: 2010PhDT.........1V Altcode: This dissertation addresses a specific aspect of the Sun-Earth connection: we show that coronal activity creates periodic density structures in the solar wind which convect radially outward and interact with Earth's magnetosphere. First, we analyze 11 years (1995-2005) of in situ solar wind density observations from the Wind spacecraft and find that periodic density structures occur at particular sets of radial length-scales more often than others. This indicates that these density fluctuations, which have radial length-scales of hundreds of megameters, cannot be attributed entirely to turbulence. Next, we analyze their effect on Earth's magnetosphere. Though these structures are not waves in the solar wind rest frame, they appear at discrete frequencies in Earth's reference frame. They compress the magnetosphere as they convect past, driving global magnetospheric oscillations at the same discrete frequencies as the periodic density structures. Last, we investigate source regions and mechanisms of the periodic solar wind density structures. We analyze the alpha particle to proton abundance ratio during events of periodic density structures. In many events, the proton and alpha density fluctuations are anti- correlated, which strongly argues for either temporally or spatially varying coronal source plasma. We examine white light images of the solar wind taken with SECCHI HI1 on the STEREO spacecraft and find periodic density structures as near to the Sun as 15 solar radii. The smallest resolvable periodic structures that we identify are of comparable length to those found at 1 AU, providing further evidence that at least some periodic density structures are generated in the solar corona as the solar wind is formed. Guided by the properties observed during previous studies and the characteristics established through the work presented here, we examine possible candidate mechanisms in the solar corona that can form periodic density structures. We conclude that: coronal activity creates coherent structures in the solar wind at smaller size scales than previously thought; corona-formed coherent structures persist to 1 AU largely intact; finally, a significant amount of discrete frequency wave power in Earth's magnetosphere is directly driven by these structures once they reach Earth. Title: Are periodic solar wind number density structures formed in the solar corona? Authors: Viall, Nicholeen M.; Spence, Harlan E.; Kasper, Justin Bibcode: 2009GeoRL..3623102V Altcode: We present an analysis of the alpha to proton solar wind abundance ratio (AHe) during a period characterized by significant large size scale density fluctuations, focusing on an event in which the proton and alpha enhancements are anti-correlated. In a recent study using 11 years (1995-2005) of solar wind observations from the Wind spacecraft, N. M. Viall et al. [2008] showed that periodic proton density structures occurred at particular radial length-scales more often than others. The source of these periodic density structures is a significant and outstanding question. Are they generated in the interplanetary medium, or are they a relic of coronal activity as the solar wind was formed? We use AHe to answer this question, as solar wind elemental abundance ratios are not expected to change during transit. For this event, the anti-phase nature of the AHe variations strongly suggests that periodic solar wind density structures originate in the solar corona. Title: Examining Solar Wind Number Density Structures Observed in SECCHI HI 1 Authors: Viall, N. M.; Spence, H. E.; Vourlidas, A.; Howard, R. A. Bibcode: 2009AGUFMSH13B1516V Altcode: We present an analysis of small-scale periodic solar wind density enhancements observed in SECCHI HI 1. We discuss their possible relationship to periodic fluctuations of the proton density observed in-situ with the Wind SWE data. Viall et al. [2008] used 11 years of solar wind density measurements at 1 AU and demonstrated that in addition to turbulent fluctuations, non-turbulent, periodic density structures exist in the solar wind. In the slow wind, periodic density structures occurred most often with radial length-scales of approximately 73, 120, 136 and 180 Mm. In the fast wind, periodic density structures occurred most often with radial length-scales of approximately 187, 270 and 400 Mm. Event studies of the periodic density structures reveal instances in which the density structures have alpha/proton abundance ratio changes associated with the density structures. Specifically, the alpha density varies with the same periodicity as the protons, but in antiphase. For those events, this strongly suggests either time varying or spatially varying coronal source plasma that created the density structures. If such periodic density structures observed at 1 AU are generated in the corona, then they may be observable in SECCHI HI1 data. For instance, larger scale plasmoids have been observed in the corona [e.g. Sheeley et al., 2009] and it is plausible that smaller, periodic structures may exist as well. We identify periodic density structures as they convect with the solar wind into the field of view of SECCHI HI and follow the train of structures as a function of time. The periodic density structures we analyze are comparable in size to the larger structures identified in-situ at 1 AU. Title: Examining Solar Wind Number Density Structures Observed in SECCHI HI 1 Authors: Viall, Nicholeen Mary; Spence, Harlan E.; Vourlidas, Angelos; Howard, Russ Bibcode: 2009shin.confE.133V Altcode: We explore small-scale quasi-periodic solar wind density fluctuations observed in SECCHI HI 1. We discuss their possible relationship to periodic fluctuations of the proton density observed in-situ with the Wind SWE data. Viall et al. [2008] used 11 years of solar wind density measurements at 1 AU and demonstrated that in addition to turbulent fluctuations, non-turbulent, periodic density structures exist in the solar wind. In the slow wind, periodic density structures occurred most often with radial length-scales of approximately 73, 120, 136 and 180 Mm. In the fast wind, periodic density structures occurred most often with radial length-scales of approximately 187, 270 and 400 Mm. Event studies of the periodic density structures reveal instances in which the density structure has alpha/proton abundance ratio changes associated with the density structures. Specifically, the alpha density varies with the same periodicity as the protons, but in antiphase. This strongly suggests either time varying or spatially varying coronal source plasma that created the density structures. If such periodic density structures observed at 1AU are generated in the corona, then they may be observable in SECCHI HI1 data. For instance, larger scale plasmoids have been observed in the corona [e.g. Sheeley et al., 2009] and it is plausible that smaller, quasi-periodic structures may exist as well. We identify quasi-periodic density structures in the SECCHI HI1 images that are comparable in size to those identified in-situ at 1AU. Title: Multipoint Analysis of Meso-scale Structures in the Ambient Solar Wind: STEREO-A, -B, and L1 Observations Authors: Spence, H. E.; Viall, N. M.; Vourlidas, A.; Howard, R. A.; Simunac, K.; Kistler, L. M.; Galvin, A. B.; Kasper, J. C.; Lazarus, A. J. Bibcode: 2008AGUFMSH12A..06S Altcode: We explore sources of apparent time-dependence of meso-scale structures (those lasting two to three days and less) in the ambient solar wind through analysis of measurements from STEREO-A, -B, and L1 spacecraft (WIND, ACE, and SOHO). In early 2008, stable corotating interaction regions and high-speed streams provided excellent boundaries and features for co-registering the large-scale, corotating solar wind observed by several heliospheric spacecraft separated in solar orbital phase near 1 AU. During this period, STEREO-B (located 23 degrees behind the Earth in heliographic longitude) first observed the large-scale corotating stream structures, followed by the WIND, ACE, and SOHO spacecraft at Earth, then finally by STEREO-A (located 22 degrees ahead of the Earth in heliographic longitude). Conspicuous similarities in the macro-scale solar wind flow dominate the comparison between spacecraft observations and permit us to time-adjust the observed flow features reasonably well by assuming a simple corotating solar wind source. While the co-registered, large-scale solar wind structure agrees well, mesoscale flow features can exhibit large measured differences at the various spacecraft. We focus on one such interesting feature which exhibits apparent time dependence. Though this few-day-long, significant flow speed event is observed by the PLASTIC experiments on both STEREO-A and STEREO-B, it is not seen at the L1 spacecraft which the STEREO spacecraft bracket in space and time. We explore potential sources of the apparent time dependence of this meso-scale feature. Latitudinal differences in the multipoint measurements is one source that could account for the apparent mesoscale flow structure variability. We also explore explicit time variation of the solar wind's source, by analyzing relevant coronal holes observed simultaneously by the STEREO spacecraft imagers. This event and analysis underscores that multipoint heliospheric observations and analysis reveals the existence of mesoscale structure in the solar wind and can be used to constrain its possible source(s). Title: On the Source of Periodic Solar Wind Number Density Structures Using the Alpha to Proton Abundance Ratio Authors: Viall, N. M.; Spence, H. E.; Kasper, J. Bibcode: 2008AGUFMSH21A1573V Altcode: Solar wind elemental abundance ratios provide information regarding the source region of the solar wind plasma because they are not modified by propagation. We present an analysis of the alpha to proton abundance ratio during periods in the solar wind with significant long wavelength number density fluctuations. We discuss the implications of the abundance ratio variations on possible source regions of those periodic solar wind number density structures. In a recent study using 11 years (1995-2005) of solar wind number density observations taken on the Wind spacecraft, we analyzed the radial length-scales of periodic number density structures. We performed Fourier analysis on short (approximately equal to six hours) data windows and determined the wavelength (length-scale) of the number density structures. We found that particular length-scales occurred more often than others in the solar wind number density during those eleven years. For the slow wind, the most significant length-scales are approximately 73, 120, 136 and 180 Mm. For the fast wind, the most significant length-scales are approximately 187, 270 and 400 Mm. The outstanding question is the source of these periodic density structures. Using the alpha to proton abundance ratio during periodic solar wind number density events, we address whether periodic density structures are generated in the local interplanetary medium, or if they are more probably generated somewhere near the solar surface or solar corona, frozen into the solar wind, and convected out to Earth. Title: Inherent length-scales of periodic solar wind number density structures Authors: Viall, N. M.; Kepko, L.; Spence, H. E. Bibcode: 2008JGRA..113.7101V Altcode: We present an analysis of the radial length-scales of periodic solar wind number density structures. We converted 11 years (1995-2005) of solar wind number density data into radial length series segments and Fourier analyzed them to identify all spectral peaks with radial wavelengths between 72 (116) and 900 (900) Mm for slow (fast) wind intervals. Our window length for the spectral analysis was 9072 Mm, approximately equivalent to 7 (4) h of data for the slow (fast) solar wind. We required that spectral peaks pass both an amplitude test and a harmonic F-test at the 95% confidence level simultaneously. From the occurrence distributions of these spectral peaks for slow and fast wind, we find that periodic number density structures occur more often at certain radial length-scales than at others, and are consistently observed within each speed range over most of the 11-year interval. For the slow wind, those length-scales are L ∼ 73, 120, 136, and 180 Mm. For the fast wind, those length-scales are L ∼ 187, 270 and 400 Mm. The results argue for the existence of inherent radial length-scales in the solar wind number density. Title: Shock and discontinuity associated periodicities in the solar wind and magnetosphere Authors: Kepko, L.; Viall, N. M.; Spence, H. E. Bibcode: 2006AGUFMSH31A0378K Altcode: Solar wind shocks and discontinuities are often cited as sources of broadband compressional power that couple to magnetospheric eigenmodes. Some recent event studies have indicated that the post-shock solar wind sometimes contains discrete periodicities in the number density. These periodicities lasted 10s of minutes to hours and matched oscillations observed in the magnetosphere that otherwise may have been attributed to global cavity modes. In addition, it has been shown recently that the ambient solar wind contains intervals with significant power at multiple discrete frequencies, and that these density variations drive multiple, discrete magnetospheric oscillations. We present the results of a statistical analysis of solar wind shocks and discontinuities and seek answers to the following questions: 1) How often and under what conditions are number density periodicities observed following shocks and discontinuities? 2) To what degree are the periodicities 'discrete' vs random? 3) Are the shock-associated periodicities related somehow to the multiple, discrete periodicities observed in the ambient solar wind? Title: Magnetospheric Oscillations and Their Relation to Periodic Multiple Discrete Solar Wind Number Density Variations Authors: Viall, N. M.; Kepko, L.; Spence, H. Bibcode: 2006AGUFMSH31A0384V Altcode: Sets of discrete magnetospheric oscillations in the mHz range have been observed in satellite measurements, HF radar measurements, and ground magnetometer data for the last decade. Kepko et al. [2002] and Kepko and Spence [2003] presented case studies showing that, in at least some instances, the apparent frequencies of periodic solar wind number density structures in Earth's reference frame are correlated with observations of magnetospheric oscillations. This strongly suggests that the solar wind is sometimes a driver of multiple discrete magnetospheric oscillations. We recently completed a statistical analysis of 11 years of solar wind data (1995-2005) investigating the occurrence of periodic solar wind number density structures and showed that they occur at preferred frequencies. We defined multiple discrete periodicity events in the solar wind by imposing a criterion that an event contained at least three discrete frequencies, and found stronger evidence for preferred multiple discrete apparent frequencies. In this paper, we extend this analysis to the magnetosphere. We present the results of an analysis of statistically robust magnetospheric peaks in power spectra observed at geosynchronous satellites and ground magnetometers. We compare the occurrence rate of statistically significant spectral peaks in the magnetosphere to our results from the upstream solar wind number density. Title: Discrete Frequency Magnetospheric Oscillations and Their Relation to Periodic Solar Wind Number Density Structures Authors: Viall, N. M.; Kepko, L.; Spence, H. Bibcode: 2006AGUSMSM42A..02V Altcode: Discrete magnetospheric oscillations near f = 1.3, 1.9, 2.6 and 3.4 mHz have been observed in satellite measurements, HF radar measurements, and ground magnetometer data for the last decade. Case studies have shown instances in which the apparent frequencies of periodic solar wind number density structures in Earth's reference frame correlate well with magnetospheric oscillations. This strongly suggests that the solar wind is sometimes a direct driver of discrete magnetospheric oscillations. We have recently completed a statistical analysis of periodic solar wind number density structures that indicated a preference for the solar wind to contain periodicities near the f = 1.3, 1.9, 2.6 and 3.4 mHz oscillations. In this paper we extend this analysis to magnetospheric data and determine the occurrence rate of statistically robust magnetospheric peaks in power spectra using data from GOES, ground magnetometers, Geotail, and LANL EP. We compare the occurrence rate of statistically significant spectral peaks in the magnetosphere to those observed upstream in the solar wind number density. Title: The Occurrence Rate of Solar Wind Periodic Number Density Structures Authors: Viall, N. M.; Kepko, L.; Spence, H. Bibcode: 2005AGUFMSH11B0264V Altcode: The solar wind is generally described as a fully turbulent plasma, with a featureless power-law spectrum that does not contain excess power at discrete frequencies. However, recent observations have indicated that the solar wind sometimes contains highly periodic number density variations in the mHz range in the Earth's reference frame. These periodic structures remained coherent over several hours and were highly geoeffective, driving global magnetospheric oscillations at the same discrete frequencies. These previous studies examined only a few events, and did not determine the occurrence rate of these structures. In this paper we determine the occurrence rate of discrete oscillations in the solar wind number density. We perform a multi-tapered analysis for spectral estimation and robust significance testing using several years of solar wind data. In addition we attempt to reconcile the turbulent picture of the solar wind with the new observations.