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 ( 



Date Presentation

April 25, 2019


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

Presenter: Nicholleen Viall (NASA GSFC; e-mail:




March 28, 2019

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

Presenter: Gareth Roberg-Clark (University of Maryland; e-mail:


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.

February 28, 2019


Title: TBD

Presenter: Simone Landi (University of Firenze)




January 24, 2019

Title: A New Approach to Fluid Closure for Coronal Plasma

Presenter: Jack Scudder (University of Iowa; e-mail:


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


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.


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; e-mail:


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; e-mail:


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 22, 2016

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

Presenter: Kamen Kosarev (Smithsonian Astrophysical Observatory, USA; e-mail:


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.


Powerpoint File: Kozarev_Slides.pptx

August 18, 2016

Title: What is behind the maltese cross?

Presenter: Andrea Verdini (Observatoire de Paris, France; e-mail:


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.


PDF File: Verdini_Slides.pdf

Telecon Recording: Verdini_Webex.arf


Page Last Modified: February 8, 2019