Parker Solar Probe Theory Group presentations occur by telecon at 12 PM EST on the fourth Thursday of the month. 

To join or contribute a talk please email Marco Velli or Aleida Higginson.





June 27, 2019


Title: Connecting the Properties of Coronal Shock Waves with Those of Solar Energetic Particles

Presenter: Athanasios Kouloumvakos (L'Institut de Recherche en Astrophysique et Planétologie (IRAP))


During COROSHOCK project we have developed and exploited a new catalog of coronal pressure waves modeled in 3D. Using coronagraphic observations and techniques of shock wave 3D reconstructions we model the shock waves expansion in 3D and we derive the waves velocity along the entire front as a function of time. Combining the reconstruction techniques with global models of the solar corona, we derive the 3D distribution of basic physical shock parameters such as Mach numbers, compression ratios, and shock geometry. Our sample comprises of modeled shocks that observed during solar cycle 24 and are associated with major solar energetic particles (SEPs) events, each distributed over a broad range of longitudes. To study the potential role of these waves in accelerating SEPs measured in situ, we model in a time-dependent manner how the shock wave connects magnetically with spacecraft making in situ measurements of SEPs. This allows us to compare modeled shock parameters deduced at the magnetically well-connected regions, with different key parameters of SEPs. Our approach accounts for projection effects associated with remote-sensing observations and constitutes the most extensive study to date of shock waves in the corona and their relation to SEPs. I will discuss the implications of that work for understanding particle acceleration in the corona.

May 23, 2019

Title: Short, Large-amplitude Speed Enhancements in the Near-Sun Fast Solar Wind

Presenter: Timothy Horbury (Imperial College London)


Helios measurements of fast, Alfvénic solar wind streams at 0.3 au reveal the presence of pervasive, intermittent, short discrete enhancements in the bulk velocity. Lasting tens of seconds to minutes in spacecraft measurements at 0.3 au, speeds inside these enhancements can reach 1000 km/s, corresponding to a kinetic energy up to twice that of the bulk high-speed solar wind. These events are Alfvénic in nature with large magnetic field deflections and are the same temperature as the surrounding plasma, in contrast to the bulk fast wind which has a well-established positive speed–temperature correlation. The origin of these speed enhancements is unclear but they may be signatures of discrete jets associated with transient events in the chromosphere or corona (e.g. Roberts et al. 2018). (See Horbury et al., MNRAS, 2018)

PDF File:  Horbury_Slides.pdf

Telecon Recording: Horbury_Webex.arf

April 25, 2019


Title: Helios Observations of Quasi-periodic Density Structures in the Slow Solar Wind at 0.3, 0.4, & 0.6 AU

Presenter: Nicholeen Viall (NASA / GSFC)


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.

PDF File:  Viall_Slides.pdf

Telecon Recording: Viall_Webex.arf

March 28, 2019

Title: Wave Generation and Heat Flux Suppression in Astrophysical Plasma Systems

Presenter: Gareth Roberg-Clark (University of Maryland)


Thermal conduction in weakly collisional, weakly magnetized plasmas such as the intracluster medium of galaxy clusters and the solar wind is not fully understood. One possibility is that plasma turbulence at the spatial and temporal scales of the electron gyro-orbits can scatter electrons and inhibit thermal fluxes. Here we present particle-in-cell (PIC) simulations and analytic analysis demonstrating this behavior. In our numerical model two thermal reservoirs at different temperatures drive an electron heat flux that destabilizes oblique whistler waves. The whistlers grow to large amplitude and resonantly scatter the electrons, strongly suppressing the heat flux. The rate of thermal conduction is controlled by the finite propagation speed of the whistlers, which act as mobile scattering centers that convect the thermal energy of the hot reservoir. Unlike classical (Spitzer) thermal conduction, the resulting steady-state heat flux is largely independent of the thermal gradient. The derived scaling law for thermal conduction has been confirmed in solar wind measurements by Tong et al. (arXiv 2018). We have extended our results to lower beta, which is more relevant for the solar wind and corona. In this regime whistlers gradually become subdominant and the heat flux is mostly regulated by electrostatic double layers, which modify the thermal conduction scaling.

PDF File: RobergClark_Slides.pdf

Telecon Recording: RobergClark_Webex.arf

January 24, 2019

Title: A New Approach to Fluid Closure for Coronal Plasma

Presenter: Jack Scudder (University of Iowa)


A new approach for fluid closure of plasmas with a finite Knudsen expected along Parker Solar Probe trajectory is discussed. Empirical evidence using Helios I & II and Wind observations is presented that establishes (i) a lower bound for (E defined as E_para / E_D) is order unity and monotonically increasing with wind speed in the accessible solar wind, where Dreicer’s electric field, E_D , is the yardstick; (ii) the size and variability of E and runaway theory accurately reproduces the electron supra thermal density fraction in the solar wind over a range of three orders of magnitude; (iii) the long neglected thermal force is an important additional contributor to E even in formally “collisionless” regimes; (iv) the observed E is responsible for the velocity space bifurcation seen in the non-thermal electron eVDF’s; (v) and explains the over damped preferential shift of the thermals towards the sun and thus the heat flux; and (vi) the break point energy of the eVDF routinely scales with temperature like the low energy threshold for runaway production in Dreicer’s runaway model. These observations have been forged into a model that given the density, temperature and E_para the lowest order observed energy dependence of the eVDF can reproduce nearly all that is known about the solar wind electron distribution function. As E_para goes to zero the system approaches the gradient free initial state for Spitzer’s theory. This new model predicts the return to Maxwellian eVDF as E approaches null. The observed significant size of E_para reflects the impact in astrophysics of gravity, rotation and pressure gradients not considered in usual closure formulations; thus E_para is hardly ever zero and the dimensionless electric field E_para = E_para / E_D can only be small in very high densities or very low temperatures not under consideration in the corona solar wind. Thus the transport problem is NOT about perturbation about Maxwellians but descriptions of the asymmetry of kurtotic distributions. A coupled three fluid program (2 separate electron and one ion) for establishing closure will be discussed and demonstrate that the needed steady heat flux for this closure will be determined by a system of differential equations that solves for the necessary parameters of the skewed lepto-kurtic eVDF to enforce the summational invariants that reflect the conserved quantities in the theory. This system retains all collisional couple between and among the three fluids, including the migration of electrons between their “home” fluid. This system can be used to study the implications of postulated new effects, since the heat flow closure formulation responds to the electrodynamic forces of the new processes.

PDF File: Scudder_Slides.pdf

Telecon Recording: Scudder_Webex.arf

December 20, 2018


Title: AGU Debrief and Discussion

Presenter: Marco Velli and participants


Discussion topics included: the merits and deficiencies of MHD and kinetic models, turbulence in the young solar wind, shocks in the upper corona, electron distributions, as well as new Voyager results and the role of filaments in CMEs and jets.

PDF File: n/a

Telecon Recording: AGU2018_Webex.arf

November 15, 2018

Title: A New Algorithm for Developing an Onboard Wave-Particle Correlator

Presenter: Jennifer Verniero (University of Iowa)


One of the greatest conundrums we have in modern spacecraft missions is the fact that instruments are capable of taking high-resolution full burst-mode data 24 hours a day, but due to telemetry limitations between the spacecraft and Earth, we can only afford to downlink a few minutes of it. If we know a priori what kind of measurements we seek, data compression can be done by performing data analysis onboard the spacecraft and returning the post-processed data back to Earth. In particular for this talk, we seek measurements of collisionless energy transfer between fields and particles by using the novel Field-Particle Correlation technique. This method determines how turbulent energy dissipates into plasma heat by identifying which particles in velocity-space experience a net gain of energy. By utilizing knowledge of discrete particle arrival times, we devise an algorithm for implementing a field-particle correlator onboard spacecraft. Using a gyrokinetic simulation, we create synthetic spacecraft data mapped to realistic phase-space resolutions of modern spacecraft instruments to determine the limitations of this algorithm on resolving the velocity-space signature of Landau damping for ions.

PDF File: Please contact author. 

Telecon Recording: Withheld until publication. 

November 1, 2018


Title: Solar-Wind Observations of Collisional Thermalization among Multiple Ion-Species

Presenter: Bennet Maruca (University of Delaware)


The rate of Coulomb collisions among ions in the solar wind is low enough that significant departures from thermal equilibrium (e.g., different ion species having different temperatures) are frequently observed.  Nevertheless, collisions have been found to play an important role in the plasma's large-scale evolution as it expands from the corona and through the heliosphere.  Many statistical analyses have found that the temperature ratio of the two most abundant ions, protons (ionized hydrogen) and alpha-particles (fully ionized helium), is heavily influenced by collisional thermalization.  This ongoing study expands on this work by including oxygen +6, which, during select periods (of cold, slow, dense plasma), the Wind spacecraft's Faraday Cups can measure at high cadences.  Using well-established models of collisional relaxation, the in-situ measurements at 1 AU can be used to estimate ion conditions earlier in the plasma's expansion history.  Assessing the physicality of these predictions can indicate to what degree preferential heating and/or heating beyond the corona affected the plasma's evolution.

PDF File: Maruca_Slides.pdf

Telecon Recording: Maruca_Webex.arf

September 27, 2018

Title: Radial Evolution of Pure High-Speed Streams in the Inner Heliosphere

Presenter: Denise Perrone (Imperial College London, UK)


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Telecon Recording: Perrone_Webex.arf

June 28, 2018


Title: Coronal Heating and Solar Wind Accleration by MHD Turbulence

Presenter: William Matthaeus (University of Delaware) 


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Telecon Recording: Matthaeus_Webex.arf

June 14, 2018 


Title: Collisional Dissipation in Turbulent Weakly Collisional Plasmas

Presenter: Oreste Pezzi (Universita Della Calabria, Italy) 


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Telecon Recording: Pezzi_Webex.arf

April 26, 2018

Title: Marginal Stability of Sweet-Parker Type Current Sheets at Low Lundquist Numbers

Presenter: Chen Shi (UCLA)


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Telecon Recording: Shi_Webex.arf

March 22, 2018


Title: Turbulence in Solar Wind: Why Comobile Coordinates?

Presenter: Roland Grappin (LPP Ecole polytechnique)


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Telecon Recording: Grappin_Webex.arf

February 22, 2018

Title: CME-Turbulence Interaction with 3-Temperature Plasma Instabilities

Presenter: Bart van der Holst (University of Michigan)


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Telecon Recording: vanderHolst_Webex.arf

November 30, 2017


Title: Global Energetics, Particle Acceleration and Turbulence in Standard Solar Flare Model

Presenter: Eduard Kontar (University of Glasgow, UK) 


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Telecon Recording: Kontar_Webex.arf

November 2, 2017

Title: The Solar Angular Momentum: Perspectives with Parker Solar Probe

Presenter: Victor Reville (UCLA EPSS) 


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Telecon Recording: Reville_Webex.arf

August 24, 2017


Title: The Polarization of Compressive Fluctuations in the Solar Wind

Presenter: Daniel Verscharen (Mullard Space Science Laboratory, University College London)


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Telecon Recording: Verscharen_Webex.arf

July 27, 2017

Title: Hybrid Simulations of Turbulence and Kinetic Instabilities at Ion Scales in the Expanding Solar Wind

Presenter: Simone Landi (University of Florence, Italy)


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Telecon Recording: Landi_Webex.arf

June 22, 2017


Title: Solar Wind Turbulence at Plasma Kinetic Scales: Observational Point of View

Presenter: Olga Alexandrova (Astronome-adjointe, LESIA/Observatoire de Paris)


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Telecon Recording: Alexandrova_Webex.arf

May 25, 2017

Title: How to simulate turbulence kinetically, accurately, at low computational costs?

Presenter: Maria Elena Innocenti 


Fully kinetic simulations are a precious tool for the study of turbulence because the interaction of ions and electrons with electric and magnetic fields is reproduced virtually without approximation, contrarily to many other simulation approaches. The drawback is a very high computational cost, since these simulations must incorporate 1) the large scales of energy injection and 2) the small scales at which field/ particle interaction takes place. The Multi-Level Multi-Domain (MLMD) method [Innocenti 2013] is the solution we propose to this challenge. MLMD simulations are fully kinetic, semi-implicit, adaptive: simulations of turbulence at realistic mass ratio between ions and electrons are feasible at low computational costs. MLMD simulations have been measured to be 70 times faster than corresponding single-level, semi-implicit simulations. We demonstrate the method with simulations of turbulence generated by the Lower Hybrid Drift Instability in the terrestrial magnetotail.

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Telecon Recording: Innocenti_Webex.arf

May 18, 2017


Title: The Solar Wind from Pseudostreamers & their Environs: Opportunities for Observations w/ PSP & SO

Presenter: Olga Panasenco (Advanced Heliophysics)


The solar dynamo and photospheric convection lead to three main types of structures extending from the solar surface into the corona – active regions, solar filaments (prominences when observed at the limb) and coronal holes. These structures exist over a wide range of scales, and are interlinked with each other in evolution and dynamics.  Active regions can form clusters of magnetic activity and the strongest overlie sunspots. In the decay of active regions, the boundaries separating opposite magnetic polarities (neutral lines) develop specific structures called filament channels above which filaments form. In the presence of flux imbalance decaying active regions can also give birth to lower latitude coronal holes. The accumulation of magnetic flux at coronal hole boundaries also creates conditions for filament formation: polar crown filaments are permanently present at the boundaries of the polar coronal holes. Mid-latitude and equatorial coronal holes - the result of active region evolution - can create pseudostreamers (PSs) if other coronal holes of the same polarity are present. While helmet streamers form between open fields of opposite polarities, the pseudostreamer, characterized by a smaller coronal imprint, typically shows a more prominent straight ray or stalk extending from the corona. The pseudostreamer base at photospheric heights is multipolar; often one observes tripolar magnetic configurations with two neutral lines - where filaments can form - separating the coronal holes.

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Telecon Recording: Panasenco_Webex.arf

April 27, 2017

Title: Testing the Shock Origin of Protons Responsible for Solar Long-Duration Gamma-Ray Events 

Presenter: Alexandr Afanasiev (Univ. Turku, Finland)


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Telecon Recording: Afanasiev_Webex.arf

April 13, 2017


Title: Electron Energization in Reconnection: Implications for Solar Probe Plus

Presenter: Joel Dahlin (CPAESS; NASA / GSFC) 


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Telecon Recording: Dahlin_Webex.arf

Feburary 23, 2017

Title: Simulations of Lateral Transport & Dropout Structure of Energetic Particles from Impulsive Solar Flares

Presenter: David Ruffolo (Mahidol Univ., Thailand)


We simulate trajectories of energetic particles from impulsive solar flares for 2D+slab models of magnetic turbulence in spherical geometry to study dropout features (i.e., sharp, repeated changes in the particle density, which have been observed) and the particles' lateral transport.  For the 2D component, we use the output of a 2D MHD simulation, which dynamically generates realistic features of turbulence such as coherent structures.  The magnetic field lines and particles spread non-diffusively (ballistically) to a patchy distribution reaching up to 25 degrees from the injection longitude and latitude at 1 AU.  The initial dropout pattern in particle trajectories is relatively insensitive to particle energy, though the energy affects the pattern's evolution with time.  We make predictions for upcoming observations by Solar Probe Plus and Solar Orbiter: we expect a sharp pulse of outgoing particles along the dropout pattern, followed by backscattering that first remains close to the dropout pattern and later exhibits cross-field transport to a distribution that is more diffusive, yet mostly contained within the dropout pattern found at greater distances.

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Telecon Recording: Ruffolo_Webex.arf

January 26, 2017


Title: Observational Signatures of the Slow Solar Wind Close to the Sun

Presenter: Leon Ofman (Catholic Univ. America)


The SPP orbit in the ecliptic plane close to the Sun would likely result in long periods of slow solar wind (SSW) measurements. Co-rotation with the Sun will facilitate the identification of the sources of the SSW. I will review briefly the past spectroscopic and in-situ observations of the SSW, as well as the related computational models of SSW. I will discuss the possible sources of the SSW in the corona, the compositional signatures, and the likely acceleration and heating mechanisms of SSW. I will review the open questions of SSW formation and origin, and discuss how the SPP could help resolving these questions.

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November 17, 2016

Title: Can Field-Particle Correlations Be Used to Discern the Nature of Solar Wind Heating?

Presenter: Kristopher Klein (University of Michigan)


Several classes of dissipation mechanisms have been proposed to heat the solar corona and the solar wind, with no satisfactory observational discrimination between the competing theories. Determination of the nature of this heating is a key science goal for a number of upcoming missions including Solar Probe Plus. We argue that the use of field-particle correlations to examine the velocity dependent energization of the plasma may serve as a useful observational diagnostic of the nature of solar wind heating. This talk details the basic kinetic plasma physics behind the application of field-particle interaction term in the Vlasov equation and the transfer of energy as a function of velocity. Different heating mechanisms will transfer energy in different regions of velocity phase space, allowing a properly constructed correlation to distinguish between heating mechanisms. We apply such correlations to a series of increasingly complex kinetic simulations, ranging from electrostatic waves to strongly turbulent gyrokinetic and hybrid Vlasov models, showing that such correlations can distinguish between the dissipation mechanisms accessible to the system. Lastly, we discuss the application of this technique to measurements that will be made by the FIELDS (electric and magnetic fields) and SWEAP (particle velocity distribution) instruments. 

PDF File: Klein_Slides.pdf

Telecon Recording: Klein_Webex.arf

October 20, 2016


Title: A Stringent Limit on the Amplitude of Alfvénic Perturbations in High-Beta Low-Collisionality Plasmas

Presenter: Jonathan Squire (Caltech)


I will discuss and explore a stringent nonlinear limit on the amplitude of shear-Alfvén waves in low-collisionality plasmas. In particular, the result states that collisionless plasmas cannot support linearly polarized shear-Alfvén fluctuations above the critical amplitude dB_perp / B_0 ~ beta^(-1/2), where beta is the ratio of thermal to magnetic pressure. Above this cutoff, a developing fluctuation will generate a pressure anisotropy that is sufficient to destabilize itself thPlasma Seminar Friday 10/14: A stringent lirough the parallel firehose instability. This causes the wave frequency to approach zero, interrupting the fluctuation before any oscillation, and the magnetic field lines relax into a sequence of angular zig-zag structures. I will conclude by discussing a variety of interesting implications that stem from this restrictive amplitude maximum.

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Telecon Recording: Squire_Webex.arf

September 22, 2016

Title: A Data-Driven Analytic Model for Proton Acceleration by Large-Scale Solar Coronal Shocks 

Presenter: Kamen Kozarev (Smithsonian Astrophysical Observatory)


We have recently studied the development of an eruptive filament-driven, large-scale off-limb coronal bright front (OCBF) in the low solar corona, using remote observations from the Solar Dynamics Observatory’s Advanced Imaging Assembly EUV telescopes. In that study, we obtained high-temporal resolution estimates of the OCBF parameters regulating the efficiency of charged particle acceleration within the theoretical framework of diffusive shock acceleration (DSA). These parameters include the time-dependent front size, speed, and strength, as well as the upstream coronal magnetic field orientations with respect to the front’s surface normal direction. Here we present an analytical particle acceleration model, specifically developed to incorporate the coronal shock/compressive front properties described above, derived from remote observations. We verify the model’s performance through a grid of idealized case runs using input parameters typical for large-scale coronal shocks, and demonstrate that the results approach the expected DSA steady-state behavior. We then apply the model to the event of 2011 May 11 using the OCBF time-dependent parameters derived by Kozarev et al. We find that the compressive front likely produced energetic particles as low as 1.3 solar radii in the corona. Comparing the modeled and observed fluences near Earth, we also find that the bulk of the acceleration during this event must have occurred above 1.5 solar radii. With this study we have taken a first step in using direct observations of shocks and compressions in the innermost corona to predict the onsets and intensities of solar energetic particle events.

PDF File: Kozarev_Slides.ppt

Telecon Recording: Kozarev_Webex.arf

August 18, 2016


Title: What is behind the maltese cross?

Presenter: Andrea Verdini (Observatoire de Paris, France)


The spectral anisotropy of turbulent structures has been measured in the solar wind since 1990, relying on a gyrotropy assumption around the mean magnetic field axis. However, early and recent works (Dong et al. 2014) indicate that this hypothesis might be partially wrong. In this seminar we discuss the questions  (i) Are the usual interpretation of the measurements at 1 AU (the so-called maltese cross) in term of a sum of slab and 2D turbulence correct? (ii) what information is really contained in the maltese cross? To this end,  direct numerical simulations of the MHD equations including the transverse stretching exerted by the mean solar wind flow (EBM equations) are carried out and the genuine 3D anisotropy of turbulence as well as that one resulting from the assumption of axisymmetry around the mean field, B0 are studied and compared. The relevance to observations by SPP will also be discussed.

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Telecon Recording: Verdini_Webex.arf






Page Last Modified: June 4, 2019