# SPP Theory Group Telecons

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 Parisa Mostafavi or Marco Velli

Date

Presentation

March 25, 2021

Title:

Presenter: Gary Zank (UAH)

Abstract:

February 25, 2021

Title: Inferred Linear Stability of SPANi Observations using One- and Two-Component Proton Velocity Distributions

Presenter: Kris Klein (University of Arizona)

Abstract:

The hot and diffuse nature of the Sun's extended atmosphere allows it to persist in non-equilibrium states for long enough that wave-particle instabilities can arise and modify the evolution of the expanding solar wind. Determining which instabilities arise, and how significant a role they play in governing the dynamics of the solar wind, has been a decades-long process involving in situ observations at a variety of radial distances. With new measurements from Parker Solar Probe (PSP), we can study what wave modes are driven near the Sun, and calculate what instabilities are predicted for different models of the underlying particle populations. We model two hours-long intervals of PSP/SPAN-i measurements of the proton phase-space density during PSP's fourth perihelion with the Sun using two commonly used descriptions for the underlying velocity distribution. The linear stability and growth rates associated with the two models are calculated and compared. We find that both selected intervals are susceptible to resonant instabilities, though the growth rates and kind of modes driven unstable vary depending on if the protons are modeled using one or two components. In some cases, the predicted growth rates are large enough to compete with other dynamic processes, such as the nonlinear turbulent transfer of energy, in contrast with relatively slower instabilities at larger radial distances from the Sun.

Preprint available at https://arxiv.org/abs/2101.10937

January 28, 2021

Title:  Spacetime Multiscale properties of plasma turbulence at sub-ion scales: structures or waves?

Presenter: Emanuele Papini (UniFI)

Abstract:

We present results from a spacetime study of numerical simulations of decaying turbulence. We exploit Fourier analysis and Iterative Filtering (a new technique developed for the analysis of nonstationary nonlinear signals) to calculate the $k$\textendash$\omega$ power spectrum of the magnetic and velocity fluctuations at the maximum of turbulent activity.Results show that the turbulence at sub-ion scales is formed by localized structures and/or perturbations with temporal frequencies much smaller than the ion-cyclotron frequency. Going toward smaller ion-kinetic scales, the contribution of low-medium frequency perturbations to the magnetic spectrum becomes important. Through the analysis of their polarization properties, we show that such perturbations have no kinetic-Alfvén neither Ion-cyclotron origin. At higher frequencies, we clearly identify signatures of both whistler and kinetic Alfvén waves activity, however their energetic contribution to the magnetic power spectrum is negligible. We conclude that the dynamics of turbulence at sub-ion scales is mainly shaped by localized intermittent structures, with no contribution of KAW-like or FW interactions.

December 2020

No meeting.

November 19, 2020

Title:  Fluid theory of coherent magnetic vortices in high-beta space plasmas

Presenter: Dušan Jovanović (Institute of Physics, University of Belgrade, Serbia)

Abstract:
In-situ observations in the Earth's and Saturn's magnetosheaths and in the solar wind reveal the presence of Alfvén vortices as intermittent structures in the range of scales from fluid lengths down to few ion lengths. The density and the magnetic field associated with them appear to be compressible for higher plasma betas. Until now, only incompressible Alfvén vortices have been known. Motivated by space plasma observations we develop a new model of magnetic vortices in high-beta plasmas with anisotropic temperature, possessing compressible density and magnetic field, whose typical size ranges from fluid to ion scales. At magneto-fluid scales we find novel non-propagating field-aligned cylindrical monopoles and inclined propagating dipoles. Their transverse magnetic and velocity fluctuations are aligned, but not identical, and they exhibit density and compressible magnetic field fluctuations δn and δB_ǁ localized inside the vortex core. In the presence of thermal anisotropy and acoustic effects, they may be correlated or anti-correlated δn/δB_ǁ = constant > 0 or < 0; fluctuations whose velocity along the magnetic field is below the ion thermal speed are always correlated. At ion or kinetic scales (with the smallest radii of the order c/ω_pi or ρ_Li) and in the absence of acoustic perturbations, only dipolar Alfvén vortices survive with properties similar as those at fluid scales, except that δn/n_0 reaches the level of δB_ǁ/B_0. We also find pressure balanced kinetic slow magnetosonic dipoles, possessing finite finite component of the electric field parallel to the magnetic field, E_ǁ, and a purely compressional magnetic field perturbation δB_ǁ, whose existence is facilitated by a strong ion temperature anisotropy.

October 22, 2020

Title:  On the Kinematics of space-time correlations in MHD turbulence and its connection to Taylor Hypothesis and the Solar wind

Presenter: Sofiane Bourouaine (JHU/APL)

Abstract:

Taylor's hypothesis (TH) is commonly used in the interpretation of solar-wind data provided by many spacecraft, including the recently launched PSP spacecraft. Several concerns were raised about the validity of using TH in near-sun regions where PSP is expected to explore and in some other solar wind conditions. In this presentation we will show that the limitations of TH can be well described within the framework of the recently proposed phenomenological model (Bourouaine & Perez 2019) of spacetime correlations in MHD turbulence. The outcomes from this model will allow us not only to study the validity of TH but interpret solar-wind turbulent time signal even when TH is not valid. The proposed model has been tested using different sets of reduced MHD numerical simulations. Results from MHD numerical simulations will be also discussed. A Methodology on how to use the model to interpret solar wind data including the derivation of the reduced energy spectrum in the plasma-frame from the measured spacecraft-frame power spectrum will be shown. This methodology might be used as an alternative to TH.

September 24, 2020

Title:  Exploring Solar Wind Origins and Connecting Plasma Flows from the Parker Solar Probe to 1 au: Encounters 1, 2, 4 and 5

Presenter: Olga Panasenco (Advanced Heliophysics Inc.)

Abstract:
Since the launch of the Parker Solar Probe (PSP) in 2018, a new window has opened into understanding the inner heliosphere. The first Probe encounters, with a perihelion at 35.8 solar radii (Rs) from Sun-center illustrated the complexity of the mapping of the magnetic field at the Sun even into the inner heliosphere. In Encounter 1, Probe connected to a small, overexpanding coronal hole, and the resulting slow solar wind flow was dominated by highly Alfvénic fluctuations. The recent Encounters 4 and 5, with perihelia at a distance of 27.8 Rs, show the importance of the mixing of spatial and intrinsically time-dependent behavior. We describe the general features of the solar wind seen by PSP, with specific emphasis on the polarity of the field, the properties of the fluctuations observed, and their association with the regions of origin of the wind and intrinsically time-dependence effects. We use the Potential Field Source-Surface (PFSS) model of De Rosa and Schrijver, which uses HMI magnetogram data in conjunction with photospheric transport, to extrapolate the field from the solar surface out to an appropriate source surface. We then use coronagraph images from STEREO/COR2 and SOHO/LASCO/C2 and C3 to compare the results with the magnetic field and plasma seen by PSP. In situ measurements are then used to compute plasma and turbulence properties, such as alfvénicity. Only some of the strongly Alfvénic slow wind streams seen by PSP survive and are observed at 1 AU. Paper: https://iopscience.iop.org/article/10.3847/1538-4365/ab61f4/pdf

August 27, 2020

Title:  Stellar Rotation Effects on the Stellar Wind

Presenter: Bhimsen Shivamoggi (University of Central Florida)

Abstract:

We discuss the role of the azimuthal stellar wind flow in the stellar-rotation braking mechanism (Shivamoggi [1]). The stellar rotation is shown to cause the slow magnetosonic critical point to occur lower in the corona, so the stellar wind experiences a stronger afterburner (as in an aircraft jet engine) action in the corona, and hence an enhanced stellar wind acceleration. Stellar rotation effects are shown to produce considerably enhanced stellar wind acceleration even for moderate rotators like Sun. For strong rotators, the stellar wind is shown to experience an immensely enhanced acceleration in a narrow shell adjacent to the star. For strong rotators, the magnetosonic critical point is shown to be determined only by the basic stellar parameters like mass and angular velocity of the star.

July 30, 2020

Title:  Effect of Minimum Variance Criteria on Foreshock Wave Properties for 150,000+ Venus Express Intervals

Presenter: Alexandra Brosius (Penn State)

Abstract:
Unlike Earth, Venus lacks a magnetic dynamo. The motion of planetary ions and electrons generates an induced magnetosphere and a planetary bow shock. The foreshock is a dynamic region upstream of the shock populated by reflected ions. Foreshock waves and magnetic structures affect ionospheric dynamics yet are not well-understood. The purpose of this talk is two-fold: to demonstrate the limitations of minimum variance analysis, and to show Venus Express wave observations in the Venusian foreshock. Characterization of foreshock phenomena typically involves minimum variance analysis (MVA), but without careful consideration MVA-derived results may reflect the choice of analysis parameters rather than the electromagnetic fluctuations. As one of the most sensitive environments to solar wind-magnetosphere coupling, the Venusian foreshock is an ideal laboratory for MVA method comparisons. We analyze upstream regions of 42 Venus Express orbits at 32 samples/second cadence using three distinct MVA cases: strict, intermediate, and lenient. Each of the three cases result in >150,000 wave intervals spanning 0.5-4.3 Hz. The comparison of the three MVA cases results in seemingly distinct distributions of wave propagation angles. Example orbits for the stringent MVA case highlight the diversity of upstream wave fields. Our analyses suggest wave propagation angle variability on a faster time scale than may be captured by software designed for frequency-space analysis. The foreshock characterization in this work is relevant to Parker Solar Probe and Solar Orbiter observations near Venus, and the method comparisons are applicable to wave analyses for missions in diverse plasma environments.

July 23, 2020

Title:  On the stability of alfvénic fluctuations and switchbacks in the solar wind

Presenter: Anna Tenerani (UT Austin)

Abstract:

Large amplitude, turbulent Alfvénic fluctuations have been commonly observed in the solar wind since the first in-situ measurements, and they are thought to provide a possible mechanism to heat the solar corona and accelerate the solar wind. An important property that remains unexplained is that, despite the large excursion of such fluctuations, the magnitude of the total magnetic field remains nearly constant. This condition corresponds to spherical polarization and it implies an intrinsic degree of phase coherence in the fluctuating fields, which is necessary in order to maintain such a nonlinear state. How is this Alfvénic turbulent state achieved and maintained in the solar wind remains a fundamental open question in space physics. The mystery only deepens with Parker Solar Probe, whose observations during the first perihelion have shown the ubiquitous and persistent presence of magnetic field lines which are strongly perturbed to the point that they produce local inversions of the radial magnetic field, known as switchbacks. The corresponding signature of switchbacks in the velocity field is that of local enhancements in the radial speed (or jets) that display the typical velocity-magnetic field correlation that characterizes Alfvén waves propagating away from the Sun. After reviewing the main properties of Alfvénic fluctuations and switchbacks in the solar wind, we will address how their stability and evolution is affected by nonlinearities and kinetic effects in different plasma beta regimes. Emphasis will be given to the onset and evolution of parametric instabilities and to the impact of kinetic effects on the saturated nonlinear state. The implications of our results for models of switchback generation will be discussed, and we will conclude by outlining remaining open issues.

June 25, 2020

Title:  Radial and Temporal Evolution of Stream Interaction Regions and Co-rotating Interaction Regions

Presenter: Robert Allen (JHU/APL)

Abstract:
Parker Solar Probe, as part of the Heliophysics System Observatory, provides a unique opportunity to better understand the radial and temporal evolution of stream interaction regions (SIRs) and co-rotating interaction regions (CIRs). SIRs and CIRs are known to evolve both with radial distance as the structures propagate from the Sun, as well as in time, as the structures co-rotate. While earlier observations were able to investigate the radial evolution of SIRs in a statistical sense, simultaneous, coordinated measurements at different radial distances were unavailable to probe the spatial variation and evolution of SIRs/CIRs unambiguously. Studies involving STEREO, ACE, and Wind have been able to investigate temporal variations of SIRs/CIRs; however, these measurements are all at the same radial distance. With PSP measurements, we have identified SIRs/CIRs for the first four orbits at various radial distances along the PSP orbit, as well as SIR/CIR events with favorable conjunctions between PSP and STEREO-A, Wind, and ACE. Here we focus on two such events, and examine the temporal and radial evolution of the SIRs/CIRs, particularly focusing on the energetic particle population at the SIR/CIR interface. The PSP observations, in concert with observations at 1 au, enable us to dive further into the different acceleration and transport mechanisms of suprathermal to energetic particle populations associated with SIRs/CIRs.

PDF File:  Allen_Presentation.pdf

May 28, 2020

Title:  The Heliospheric Current Sheet and Plasma Sheet During Parker Solar Probe’s First Orbit

Presenter: Benoit Lavraud (IRAP)

Abstract:

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 by-products, likely released near the tip of the helmet streamer.

April 23, 2020

Title:  Electron Energy Partition Across Interplanetary Shocks

Presenter: Lynn Wilson (UNASA/GSFC)

Abstract:
Analyses of 15,314 electron velocity distribution functions (VDFs) within ±2 hr of 52 interplanetary (IP) shocks observed by the Wind spacecraft near 1 au are introduced. The electron VDFs are fit to the sum of three model functions for the cold dense core, hot tenuous halo, and field-aligned beam/strahl component. The best results were found by modeling the core as either a bi-kappa or a symmetric (or asymmetric) bi-self-similar VDF, while both the halo and beam/strahl components were best fit to bi-kappa VDF. This is the first statistical study to show that the core electron distribution is better fit to a self-similar VDF than a bi-Maxwellian under all conditions. The self-similar distribution deviation from a Maxwellian is a measure of inelasticity in particle scattering from waves and/or turbulence. The ranges of values defined by the lower and upper quartiles for the kappa exponents are κ ec ∼ 5.40-10.2 for the core, κ eh ∼ 3.58-5.34 for the halo, and κ eb ∼ 3.40-5.16 for the beam/strahl. The lower-to-upper quartile range of symmetric bi-self-similar core exponents is s ec ∼ 2.00-2.04, and those of asymmetric bi-self-similar core exponents are p ec ∼ 2.20-4.00 for the parallel exponent and q ec ∼ 2.00-2.46 for the perpendicular exponent. The nuanced details of the fit procedure and description of resulting data product are also presented. The statistics and detailed analysis of the results are presented in Paper II and Paper III of this three-part study. https://ui.adsabs.harvard.edu/abs/2019ApJS..243....8W/abstract https://ui.adsabs.harvard.edu/abs/2019ApJS..245...24W/abstract https://ui.adsabs.harvard.edu/abs/2020ApJ...893...22W/abstract

March 26, 2020

Title:  Coronal Electron Temperature inferred from the Strahl Electrons in the Inner Heliosphere

Presenter: Laura Bercic (University of Florence, Italy)

Abstract:

The shape of the electron velocity distribution function plays an important role in the dynamics of the solar wind acceleration. Electrons are normally modelled with three components, the core, the halo, and the strahl. We investigate how well the fast strahl electrons in the inner heliosphere preserve the information about the coronal electron temperature at their origin. We analysed the data obtained by two missions, Helios spanning the distances between 65 and 215 Rs, and Parker Solar Probe (PSP) reaching down to 35 Rs during its first two orbits around the Sun. The electron strahl was characterised with two parameters, pitch-angle width (PAW), and the strahl parallel temperature (T_s). Most of the strahl measured by PSP appear narrow with PAW reaching down to 30 deg. The portion of the strahl velocity distribution function aligned with the magnetic field is for the measured energy range well described by a Maxwellian distribution function. T_s was found to be anti-correlated with the solar wind velocity, and independent of radial distance. These observations imply that T_s carries the information about the coronal electron temperature. The obtained values are in agreement with coronal temperatures measured using spectroscopy (David et al. 1998), and the inferred solar wind source regions during the first orbit of PSP agree with the predictions using a PFSS model (Bale et al. 2019; Badman et al. 2019).

March 26, 2020

Parker One science conference has been postponed.

February 27, 2020

Title:  Ion Heating in Low-Beta Kinetic Turbulence

Presenter: Silvio Cerri (Princeton University)

Abstract:

We present recent results from 3D hybrid particle-in-cell simulations of continuously driven Alfvenic turbulence at beta ~ 0.1. The interplay between turbulent fluctuations, their intermittency, and different ion-heating mechanisms is discussed. In particular, we find that ion heating is almost entirely perpendicular to B, and is mainly due to stochastic and ion-cyclotron heating mechanisms. This is also associated with pronounced magnetic reconnection activity and a kinetic-range magnetic-field spectrum which is steeper than the one typically found at beta ~ 1. These results have direct implications for the heating mechanisms and turbulence observed by Parker Solar Probe in the inner heliosphere.

January 23, 2020

Title:  Electrons in the Young Solar Wind

Presenter: Jasper Halekas (University of Iowa)

Abstract:

We present electron observations made by the Solar Wind Electrons Alphas and Protons experiment on the first two orbits of the Parker Solar Probe mission, which reached heliocentric distances as small as ~0.17 AU. The SWEAP instrument suite contains two electron sensors - the Solar Probe Analyzers (SPAN-A-E and SPAN-B-E on the ahead and behind faces of the spacecraft) - which together measure the majority of the electron distribution function. We derive the parameters of the electron core, halo, and strahl populations by utilizing a combination of fitting to model distributions and numerical integration of the measurements from the two SPAN sensors.

As expected, the electron core density and temperature increase with decreasing heliocentric distance, while the ratio of electron thermal pressure to magnetic pressure (beta) decreases. The density in the strahl also increases; however, the density of the halo plateaus and even decreases at perihelion, leading to a large strahl/halo ratio at closest approach. The core is anisotropic, and has a sunward drift relative to the proton flow velocity. This core drift is approximately balanced by the strahl, satisfying the zero-current condition necessary to maintain quasi-neutrality globally.

The characteristics of the electron distributions near perihelion are clearly organized by solar wind flow speed, electron beta, and collisional age, including some trends which are no longer apparent at 1 AU. These trends help us understand the mechanisms that shape the solar wind electron distributions at an early stage of their evolution.

December 19, 2019

Title: Post-AGU Discussion

December 5, 2019

Title:  How the Area of Coronal Holes Affects the Propagational Evolution of High-speed Streams in the Inner Heliosphere and Their Properties Near Earth

Presenter: Stefan Hofmeister (University of Graz)

Abstract:
The empirical relationship between the area of coronal holes and the peak velocities of high-speed streams near Earth is well known since the 1970s, but not well understood. By using MHD simulations, we show that this empirical relationship can be explained by the propagational evolution of high-speed streams in the inner heliosphere. At small coronal hole areas, fast high-speed streams near the Sun will be deccelarated resulting in lower velocities near Earth, whereas for large coronal hole areas, the streams stay fast. Further, also the empirical relationships between the high-speed stream velocities, temperatures, and densities are obtained, showing that all of these empirical relationships could be a result of the propagational evolution of high-speed streams in the inner heliosphere. Therefore, we should not simply relate the properties of high-speed streams near Earth to the properties of their solar source regions, but de-couple the main acceleration phase close to the Sun from the propagation phase in the inner heliosphere. This will be possible using Parker Solar Probe in radial alignments with other satellites at farther distances to the Sun.

PDF File:  Hofmeister_Slides.pdf

Telecon Recording: Hofmeister_Webex.arf

November 28, 2019

AMERICAN THANKSGIVING

November 7, 2019

Title: The Role of Alfvén Wave Dynamics on the Large Scale Properties of the Solar Wind: Comparing an MHD Simulation with Parker Solar Probe Encounter 1 Data

Presenter: Victor Reville (IRAP)

Abstract:

In its first orbit, Parker Solar Probe has already observed the solar wind in situ closer than ever before. Measurements at perihelion on November 6th, 2018 revealed a flow that is constantly permeated by large amplitude Alfvénic fluctuations. These include radial magnetic field reversals, or switchbacks, that seem to be a persistent feature of the young solar wind. The measurements also reveal a very strong, unexpected, azimuthal velocity component. In this work, we numerically model the solar corona during this first encounter, solving the MHD equations and account for Alfvén wave transport and dissipation. We find that the large scale plasma parameters are well reproduced, allowing the computation of the solar wind sources at Parker with confidence. We try to understand the dynamical nature of the solar wind to explain both the amplitude of the observed radial magnetic field and of the azimuthal velocities.

PDF File:  Reville_Slides.pdf

Telecon Recording: Reville_Webex.arf

September 26, 2019

Title: Outstanding Questions of Solar Wind Physics

Presenter: Nicki Viall (NASA Goddard Space Flight Center)

Abstract:

In situ measurements of the solar wind have been available for almost 60 years, and the measurements continue to improve. In those 60 years, computer simulation capabilities have commenced and simulation techniques continue to advance. These observations and simulations have yielded an increasingly improved knowledge of fundamental physics and have delivered a remarkable understanding of the solar wind. The wisdom continues to increase. Yet there are longstanding major unsolved issues. Solicited from the solar wind research community, three outstanding questions of solar-wind physics are discussed. On these questions we offer suggestions for future progress, forward looking on what is likely to be accomplished in near future, especially with data from Parker Solar Probe.

PPT File:  Viall_Slides.pptx

Telecon Recording: Viall_Webex.arf

August 22, 2019

Title: Helium Variation Across Two Solar Cycles Reveals a Speed-Dependent Phase Lag

Presenter: Ben Alterman (University of Michigan)

Abstract:

We study the relationship between the solar wind helium-to-hydrogen abundance ratio (A_He), solar wind speed (v_sw), and sunspot number (SSN) over solar cycles 23 and 24. This is the first full 22 year Hale cycle measured with the Wind spacecraft covering a full cycle of the solar dynamo with two polarity reversals. WHile previous studies have established a strong correlation between A_He and SSN, we show that the phase delay between A_He and SSN is a monotonic increasing function of v_sw. Correcting for this lag, A_He returns to the same value at a given SSN over all rising and falling phases and across solar wind speeds. We infer that this speed-dependent lag is a consequence of the mechanism that depletes solar wind A_He from its fast wind value during solar wind formation.

PDF File:  Alterman_Slides.pptx

Telecon Recording: Alterman_Webex.arf

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))

Abstract:

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.

PDF File:  Kouloumvakos_Slides.pdf

Telecon Recording: Kouloumvakos_Webex.arf

May 23, 2019

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

Presenter: Timothy Horbury (Imperial College London)

Abstract:

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)

Abstract:

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)

Abstract:

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)

Abstract:

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

Abstract:

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)

Abstract:

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.

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)

Abstract:

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)

Abstract:

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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)

Abstract:

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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)

Abstract:

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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)

Abstract:

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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)

Abstract:

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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)

Abstract:

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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)

Abstract:

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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)

Abstract:

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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)

Abstract:

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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)

Abstract:

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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)

Abstract:

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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

Abstract:

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

Abstract:

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)

Abstract:

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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)

Abstract:

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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)

Abstract:

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)

Abstract:

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)

Abstract:

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)

Abstract:

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)

Abstract:

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)

Abstract:

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