SPP Theory Group Telecons

Speaker: Jasper Halekas

Speaker: Joe Giacalone

Speaker: 

Title: The Structure and Origin of Switchbacks: Parker Solar Probe Observations

Speaker: Jia Huang (Berkeley)

Abstract: 

Switchbacks are rapid magnetic field reversals that last from seconds to hours. Current Parker Solar Probe (PSP) observations pose many open questions in regard to the nature of switchbacks. For example, are they stable as they propagate through the inner heliosphere, and how are they formed? In this work, we aim to investigate the structure and origin of switchbacks. In order to study the stability of switchbacks, we suppose the small-scale current sheets therein are generated by magnetic braiding, and they should work to stabilize the switchbacks. With more than one thousand switchbacks identified with PSP observations in seven encounters, we find many more current sheets inside than outside switchbacks, indicating that these microstructures should work to stabilize the S-shaped structures of switchbacks. Additionally, we study the helium variations to trace the switchbacks to their origins. We find both helium-rich and helium-poor populations in switchbacks, implying that the switchbacks could originate from both closed and open magnetic field regions in the Sun. Moreover, we observe that the alpha-proton differential speeds also show complex variations as compared to the local Alfvén speed. The joint distributions of both parameters show that low helium abundance together with low differential speed is the dominant state in switchbacks. The presence of small-scale current sheets in switchbacks along with the helium features are in line with the hypothesis that switchbacks could originate from the Sun via interchange reconnection process. However, other formation mechanisms are not excluded.

Recording: https://sppgway.jhuapl.edu/sites/default/files/webform/TheoryGroup/vide…

EGU meeting. 

No PSP Theory telecon. 

Title: A New Simple Theory on the Formation of Magnetic Switchbacks

Presenter: Gabor Toth (UMICH)

Abstract: 

Magnetic switchbacks are rapid high amplitude reversals of the radial magnetic field in the solar wind that do not involve a heliospheric current sheet crossing. First seen sporadically in the seventies in Mariner and Helios data, switchbacks were later observed by the Ulysses spacecraft beyond 1 au and have been recently discovered to be a typical phenomenon in the inner heliosphere by the Parker Solar Probe. While the nature of switchbacks is now well-understood thanks to Parker Solar Probe observations, their formation has been an intriguing and unsolved puzzle. Here we provide a simple yet predictive theory for the formation of these magnetic reversals: the switchbacks are produced by the shear of circularly polarized Alfven waves by a transversely varying radial wave propagation velocity. We provide an analytic expression for the magnetic field variation, establish the necessary and sufficient conditions and show that the mechanism works in a realistic solar wind scenario. We also show that the predictions of this theory are in excellent agreement with observations.

Recording:

https://sppgway.jhuapl.edu/sites/default/files/webform/TheoryGroup/vide…

Title: Characterizing Earth-directed magnetic clouds through in-situ observations

Presenter: Debesh Bhattacharjee (IISER)

Abstract: 

Coronal mass ejections (CMEs) are occasional expulsions of plasma and magnetic fields from the solar corona. It is well-known that Earth-directed CMEs are the primary drivers of geomagnetic storms that lead to space weather disturbances in the near-Earth space environment like magnetosphere and ionosphere. Such space weather disturbances affect a wide range of technologies we rely on. A good understanding of CME dynamics  and realistic estimates of Sun-Earth CME propagation times are therefore very crucial for space weather forecasting. In-situ measurements of plasma parameters by near-Earth satellites provide detailed information on CME interiors along the line of intercept of the spacecraft. We analyze and interpret such data for a large number of well-observed Earth-directed CMEs in my thesis. We study the amplitude of turbulent fluctuations in the proton density and total magnetic field for a large sample of near-Earth CMEs. We find that the velocity fluctuations inside and in the boundaries of CMEs are subsonic in nature. Our results show that the anomalous resistivity coming from the electrons scattering due to magnetic field turbulence is significantly higher than the Spitzer resistivity in the CME plasma. Such enhanced resistivity might supplement Joule heating within CMEs. The pressure and specific energy of CME plasma are assumed to play a key role in governing the dynamics of CMEs during their propagation through the solar wind. We estimate the total specific energy (comprising kinetic, thermal and magnetic field contributions) inside near-Earth CMEs and compare it with the ambient solar wind background. We also examine if the excess thermal+magnetic specific energy in CMEs might make them resemble rigid objects in the context of CME aerodynamic drag.

Recording: https://sppgway.jhuapl.edu/sites/default/files/webform/TheoryGroup/vide…

Slides: https://sppgway.jhuapl.edu/sites/default/files/webform/TheoryGroup/Debe…

Title: Novel parallel-kinetic perpendicular-moment model for magnetized plasmas

Presenter: Jimmy Juno (Princeton Plasma Physics Laboratory)

Abstract: 

Many astrophysical plasma systems, from pulsar magnetospheres to the solar wind, are highly magnetized. However, the derivation of large magnetization asymptotic models applicable to this wide variety of plasmas is challenging. Relativistic energies, strong flows, and temperature anisotropies complicate the asymptotics and even if the derivation can be made sufficiently rigorous, the subsequent equations may resist easy discretization via standard numerical methods.  

I will discuss a recent innovation which addresses these challenges by separating the parallel and perpendicular dynamics starting from the kinetic equation while staying agnostic to the inclusion of effects such as relativity or strong flows. The key component of the derivation lies in a spectral expansion of only the perpendicular degrees of freedom, analogous to spectral methods which have grown in popularity in recent years for gyrokinetics, while retaining the complete dynamics parallel to the magnetic field. We thus leverage our intuition that a magnetized plasma's motion is different parallel and perpendicular to the magnetic field, while allowing for the treatment of complex phase space dynamics parallel to the magnetic field. This approach also naturally couples to Maxwell’s equations, allowing easy transitions across energy scales.

Recording: https://sppgway.jhuapl.edu/sites/default/files/webform/TheoryGroup/Jimm…

Slides: https://sppgway.jhuapl.edu/sites/default/files/webform/TheoryGroup/Juno…

Title: The interplay between magnetic switchbacks and solar wind turbulence in the inner heliosphere

Presenter: Andrea Larosa (Queen Mary University, London)

Abstract: 

Since its launch Parker Solar Probe (PSP) has surprised us with many new results. Among these are the ubiquitous large-amplitude deflections of the magnetic field known as switchbacks (SBs). Their origins are still debated. Some authors interpret them as part of the wave/turbulence field, whereas others suggest different origins. In both scenario switchbacks are likely interacting with the background solar wind turbulence in ways not yet understood.
In this work, we investigate the extent to which the SBs can be considered part of the turbulent solar wind background vs arising as a separate population of structures, and how these processes, if separate, interact with one another.We address this problem by studying the distributions of various different measures of the turbulence and SBs and their radial evolution. These results are important for the understanding of switchbacks formation and of how turbulence generate structures in the solar wind.

Recording: https://sppgway.jhuapl.edu/sites/default/files/webform/TheoryGroup/Andr…

Title: Generalized fluid models of the Braginskii-type (Why does the Braginskii viscosity and heat flux “explode” 
when transitioning from the solar chromosphere to corona?)

Presenter: Peter Hunana

Abstract:

Several generalizations of the well-known fluid model of Braginskii (1965) are considered. We use the Landau collisional operator and the moment method of Grad. We focus on the 21-moment model that is analogous to the Braginskii model, and we also consider a 22-moment model. Both models are formulated for general multispecies plasmas with arbitrary masses and temperatures, where all of the fluid moments are described by their evolution equations. The 21-moment model contains two “heat flux vectors” (third- and fifth-order moments) and two “viscosity tensors” (second- and fourth-order moments). The Braginskii model is then obtained as a particular case of a one ion–electron plasma with similar temperatures, with decoupled heat fluxes and viscosity tensors expressed in a quasistatic approximation. We provide all of the numerical values of the Braginskii model in a fully analytic form (together with the fourth- and fifth-order moments). For multispecies plasmas, the model makes the calculation of the transport coefficients straightforward. Formulation in fluid moments (instead of Hermite moments) is also suitable for implementation into existing numerical codes. It is emphasized that it is the quasistatic approximation that makes some Braginskii coefficients divergent in a weakly collisional regime. Importantly, we show that the heat fluxes and viscosity tensors are coupled even in the linear approximation, and that the fully contracted (scalar) perturbations of the fourth-order moment, which are accounted for in the 22-moment model, modify the energy exchange rates.

Title: Taylor Microscale and Effective Reynolds Number near the Sun from PSP

Presenter: Riddhi Bandyopadhyay (Princeton)

Abstract:

The Taylor microscale is a fundamental length scale in turbulent fluids, representing the end of fluid properties and onset of dissipative processes. It can also be used to evaluate the Reynolds number in classical turbulence theory. Although the solar wind is weakly collisional, it approximately behaves as a magnetohydrodynamic (MHD) fluid at scales larger than the kinetic scale. Therefore, a Taylor microscale can be used to estimate an effective Reynolds number in the solar wind. NASA’s Parker Solar Probe (PSP) has reached progressively closer to the Sun than any other spacecraft before. We use the PSP data to estimate the Taylor microscale and effective Reynolds number near the Sun. We find that the Taylor microscale and Reynolds number are small compared to the corresponding near-Earth values, indicating a solar wind that has been less processed by turbulence, with very small-scale dissipative processes near the Sun.

Recording: https://sppgway.jhuapl.edu/sites/default/files/webform/TheoryGroup/Ridd…;

Title: The role of interplanetary dust in the formation of He+ in the solar wind 

Presenter: Yeimy Rivera (CfA)

Abstract: 

The He+ population in the solar wind is often attributed to interstellar or inner source pick up ions (PUIs). PUIs are not native to the solar wind but instead formed from neutrals of an interstellar or interplanetary dust origin that become ionized.  Their ionization can result from photoionization, charge exchange, or collisions with solar wind ions and electrons, respectively. Once ionized, the newly charged particles become responsive to the interplanetary magnetic field, joining the solar wind particles on their outward trajectory through the heliosphere. However, clues about their formation and origin lie in the particle's velocity distribution function (VDF). Along with typical non-thermal VDF features that indicate the particles have not yet coupled to solar wind conditions, PUI VDFs can also exhibit a highly peaked distribution around the solar wind speed that is thought to be formed through the interaction of interplanetary dust and the solar wind. Using a nonequilibrium ionization model of the solar wind we will explore charge exchange between solar wind alpha particles and neutral dust particles as the leading source of the peaked He+ population in the solar wind.

Title: The Multi-scale Inner Structure of Coronal Streamers Imaged by WISPR/Parker Solar Probe

Presenter: Paulett Liewer (JPL)

Abstract: 

In its eighth orbit, Parker Solar Probe (PSP) reached a perihelion of 16 R_sun. From this vantage point, the inner edge (13.5^o from Sun center) of WISPR’s 105^o FOV reached into 3.7 R_sun. Near perihelion, flying faster than the solar corotation speed, PSP crossed the Heliospheric Current Sheet (HCS). This enabled WISPR to image the coronal streamer belt from within as it flew through it.  Three specific findings to be discussed here are (1) As PSP entered the HCS region, the diffuse coronal streamer belt is resolved into multiple smaller rays; (2) Very small-scale density inhomogeneities are seen in the coronal rays, implying that this solar wind is highly variable deep inside the coronal (R < 5 R_sun); and (3) Magnetic flux ropes and blobs of various size scales were observed.  We used the Tracking and Fitting Technique, developed previously (Liewer et al., Solar Physics, 2020; 2022), to extract the 3D locations of two of the resolved coronal rays and the trajectories of some of the larger blobs in a Heliocentric coordinate system. The technique makes use of the multiple viewpoints of features provided by PSP’s rapid motion to extract their 3D coordinates. 

Parker Two meeting. 

No PSP Theory telecon. 

Title: Turbulent Properties of the Sub-Alfvénic Solar Wind Measured by the Parker Solar Probe

Presenter: Lingling Zhao (UAH)

Abstract: 

During its 10th orbit around the Sun, Parker Solar Probe (PSP ) sampled two intervals where the local Alfvén speed exceeded the solar wind speed, lasting more than 10 hours in total. We analyze the turbulence and wave properties during these periods. The turbulence is observed to be Alfvénic and unbalanced, dominated by outward propagating modes. The power spectrum of the outward propagating Elsässer z+ mode steepens at high frequencies while that of the inward propagating z− mode flattens. The observed Elsässer spectra can be explained by the nearly incompressible (NI) MHD turbulence model with both 2D and Alfvénic components. The modeling results show that the z+ spectra are dominated by the NI/slab component, and the 2D component mainly affect the z− spectra at low frequencies. An MHD wave decomposition based on an isothermal closure suggests that outward propagating Alfvén and fast magnetosonic wave modes are prevalent in the two sub-Alfvénic intervals, while the slow magnetosonic modes dominate the super-Alfvénic interval in between. The slow modes occur where the wavevector is nearly perpendicular to the local mean magnetic field, corresponding to non-propagating pressure-balanced structures. The alternating forward and backward slow modes may also be features of magnetic reconnection in the near-Sun heliospheric current sheet.

Recording: https://sppgway.jhuapl.edu/sites/default/files/webform/TheoryGroup/Ling…

Title: Stability of current sheet with normal component of magnetic field and plasma flows

Presenter: Chen shi (UCLA)

Abstract: 

The stability problem of current sheets is one of the most important topics in space physics as it is directly related to the onset of various explosive energy-release processes such as the solar flares and geomagnetic substorms. In this study, we calculate the linear growth rate of tearing mode instability in a current sheet with a finite normal component of magnetic field. Two field-aligned ion flows that mimic the non-gyrotropic ion pressure tensor are used to balance the magnetic tension force induced by the normal magnetic field. We show that the current sheet is strongly stabilized by the normal magnetic field, and the most stable case is when the ion flows are exactly counter-streaming with zero net flow. The case where the two ion flows merge into a single flow is the most unstable one, but it cannot overtake the stabilizing role of the normal magnetic field. 

Slides: 

https://sppgway.jhuapl.edu/sites/default/files/webform/TheoryGroup/PSP_…

Recording: 

https://sppgway.jhuapl.edu/sites/default/files/webform/TheoryGroup/Chen…

Presenter: Victor Reville (IRAP, CNRS TOULOUSE)

Abstract: 

The solar wind, in particular the slow component, harbors many dynamical structures. Density perturbations have been observed with coronagraphs and heliospheric imagers for more than 20 years. These so-called « blobs » seem to be released periodically from the low corona, in association with downflows that could be the signature of magnetic reconnection. In situ measurements have been able to associate (at least) part of these density structures to flux ropes, i.e. helical structures connected to the Sun. In this talk, I will review recent efforts to explain the origin of these structures and their relation to helmet streamers and the heliospheric current sheet (HCS). Using 2.5D and 3D simulations, I will show how helmet streamers are naturally unstable and lead, in a two step process, to the release of flux ropes and density perturbations. The periodicity recovered in the simulations are consistent with observations and involve the ideal tearing mode at high Lundquist numbers. Comparing 3D MHD simulations with  data of Parker Solar Probe and Solar Orbiter, I will show that a lot of the observed dynamics is consistent with numerous flux ropes born at the tip of helmet streamers and propagating close to the HCS. Finally, I will discuss the 3D structures of these flux ropes and the possible relation between the onset of the streamers instability and the thermal structure of the corona, controlled by the separatrice and quasi-separatrice network.

Recording: https://sppgway.jhuapl.edu/sites/default/files/webform/TheoryGroup/GMT2…

Title: Characterization of the Sub-Alfvénic Solar Wind Observed by the Parker Solar Probe

Presenter: Riddhi Bandyopadhyay (Princeton University)

Abstract:

In the lower solar coronal regions where the magnetic field is dominant, the Alfvén speed is much higher than the wind speed. In contrast, the near-Earth solar wind is strongly super-Alfvénic, i.e., the wind speed greatly exceeds the Alfvén speed. The transition between these regimes is classically described as the “Alfvén point” but may in fact occur in a distributed Alfvén critical region. NASA’s Parker Solar Probe (PSP) mission has entered this region, as it follows a series of orbits that gradually approach more closely to the Sun. During its 8th and 9th solar encounters, at a distance of about 16 solar radii from the Sun, PSP sampled four extended periods in which the solar wind speed was measured to be smaller than the local Alfvén speed. These are the first in situ detections of sub-Alfvénic solar wind in the inner heliosphere by PSP. We discuss the properties of these recently observed sub-Alfvénic solar wind, which may provide important previews of the physical processes operating at lower altitude.

Recording: https://sppgway.jhuapl.edu/sites/default/files/webform/TheoryGroup/Ridd…

Title: Modeling Proton and Alpha Particle Beams Observed by the Parker Solar Probe Near Perihelia

Presenter: Leon Ofman (NASA-Goddard)

Abstract:

Recent in-situ observations from the Parker Solar Probe (PSP) mission in the inner heliosphere near perihelia show evidence of ion beams, temperature anisotropies, and kinetic wave activity. The proton and alpha particle beams were detected by PSP/SPAN-I with related kinetic ion-scale waves were observed by the FIELDS instrument. The observations show that beam-core relative speed often exceed the local Alfven speed in E4 with even faster super-Alfvenic beams in subsequent encounters, and that the ions temperatures are anisotropic with T_perp/T_||>1. Motivated by these observations we develop 2.5D and 3D hybrid-particle-in-cell (hybrid-PIC) models of proton and alpha particle super-Alfvenic beams in the solar wind plasma. These beams drive ion kinetic instabilities with associated ion-scale wave spectra in the inner-heliospheric solar wind. We model the evolution of the anisotropic core-beam ion VDFs with ion relative drifts speeds, and ion temperature anisotropies for solar wind conditions. The solar wind heating is evident as the beam kinetic energy is converted to perpendicular and parallel thermal energies of the ions. We model the partition of energies between the protons, alphas, and the magnetic field, as well as the evolution of magnetic energy at the various nonlinear stages of the beam instability, and compare to observations. We conclude that the unstable ion beams are important components of solar wind plasma heating.

Recording: https://sppgway.jhuapl.edu/sites/default/files/webform/TheoryGroup/Leon…

Title: HelioDISC: A mission to untangle the dynamic mesoscale Sun-Earth connection

Presenter: Robert Allen (JHU/APL)

Abstract:

Cross-scale and mesoscale dynamics are a fundamental science priority in space physics but fall within an observational gap of past, current, and planned missions. Particularly in the solar wind, measurements at the mesoscale (100’s R_E to a few degrees heliographic longitude at 1 au) are crucial for understanding the connection between the corona and an observer anywhere within the heliosphere, as well as for revealing the currently unresolved physics regulating particle acceleration and transport, magnetic field topology, and the causes of variability in the composition and acceleration of solar wind plasma. Multi-point measurements with mesoscale separations are required to address this fundamental gap in our understanding, as studies using single-point observations do not allow for investigations into cross-scale and mesoscale solar wind dynamics and plasma variability, nor do they allow for the exploration of sub-structuring of large-scale solar wind structures like coronal mass ejections (CMEs), co-rotating/stream interaction regions (CIR/SIRs), and the heliospheric plasma sheet. The Heliospheric Distributed In-Situ Constellation (HelioDISC) mission concept would employ the novel use of four identical spacecraft in Earth-trailing orbits near 1 au with varying drift speeds to span a range of mesoscale separations in the solar wind to achieve significant and innovative science return. Simultaneous, longitudinally-separated measurements of structures co-rotating over the spacecraft would allow for disambiguation of spatio-temporal variability within the solar wind and mesoscale dynamics related to large-scale structures, tracking of the evolution of solar wind structures, and determination of how energetic particle acceleration and transport to 1 au is impacted by these variabilities.

Slides: https://sppgway.jhuapl.edu/sites/default/files/webform/TheoryGroup/Heli…

Recording: https://sppgway.jhuapl.edu/sites/default/files/webform/TheoryGroup/Robe…

Title:Geometry of Magnetic Fluctuations near the Sun from PSP

Presenter: Riddhi Bandyopadhyay (Princeton University)

Abstract:

Solar wind magnetic fluctuations exhibit anisotropy due to the presence of a mean magnetic field in the form of the Parker spiral. Close to the Sun, direct measurements were not available until the recently launched Parker Solar Probe (PSP) mission. The nature of anisotropy and geometry of the magnetic fluctuations play a fundamental role in dissipation processes and in the transport of energetic particles in space. Using PSP data, we present measurements of geometry and anisotropy of the inner heliosphere magnetic fluctuations, from fluid to kinetic scales. The results are surprising and different from 1 au observations. We propose that this behavior is due to the nature of large-scale forcing outside the solar corona.

Recording: https://sppgway.jhuapl.edu/sites/default/files/webform/TheoryGroup/Ridd…

September 22, 2021

(Wednesday at 2 pm ET)

Joint PSP Theory and HERMES DRIVE Science Center telecon

Title: Using PSP Data and Solar Wind Models to Support Venusian Aurora Observations

Presenter: Sarah Kovac (New Mexico State University)

Abstract:

Formation and evolution are key topics in planetary science, and we cannot fully understand planetary atmospheres without accounting for their interactions with the solar wind. The presence of aurora is an important manifestation and tracer of the interaction between the solar wind and planetary ionospheres. The OI (1S-1D) 557.7 nm (oxygen green line) is a bright auroral line in the terrestrial atmosphere and is detected on the Venusian nightside after major solar storms. Currently, the processes responsible for producing the green line emission on Venus are poorly understood, yet the observed variability of this feature is clearly linked to the solar wind environment. Here, we use the Wang – Sheeley – Arge (WSA) model and in situ data from Parker Solar Probe (PSP) to look at the solar wind conditions during a detection of the Venusian green line on 11 July 2020, when PSP was making its closest approach to Venus.

NOTE: we will be using a different Zoom for this meeting.

Recording: https://sppgway.jhuapl.edu/sites/default/files/webform/TheoryGroup/Kova…

Title: Evolution of MHD turbulence in the solar wind: Parker Solar Probe observations and numerical simulations

Presenter: Chen Shi (UCLA)

Abstract:

Parker Solar Probe (PSP) measures the magnetic field and plasma parameters of the solar wind at unprecedentedly close distances to the Sun, providing a great opportunity to study the early-stage evolution of magnetohydrodynamic (MHD) turbulence in the solar wind. Here we use PSP data to explore the nature of solar wind turbulence focusing on the Alfvénic character and power spectra of the fluctuations and their dependence on heliocentric distance and context (i.e., large-scale solar wind properties), aiming to understand the role that different effects such as source properties, solar wind expansion, and stream interaction might play in determining the turbulent state. We carried out a statistical survey of the data from the first five orbits of PSP with a focus on how the fluctuation properties at the MHD scales vary with different solar wind streams and the distance from the Sun. A more in-depth analysis from several selected periods is also presented. By tracing the magnetic footpoints of the wind streams measured by PSP, we show that the source region of the slow solar wind may have a great impact on its Alfvénicity. We compare these observational results with a series of 2D expanding-box-model (EBM) MHD simulations. The simulation results show that the large-scale solar wind structures, e.g. the stream shears and heliospheric current sheets, may explain the observed evolution of certain properties of the turbulence such as the normalized residual energy and normalized cross helicity.

Recording: https://drive.google.com/file/d/10BrEfPx1fV46pke306BNjKCfepod1Pji/view?…

July 22, 2021

Title: Suprathermal Ion Intensity Enhancements in the vicinity of the Heliospheric Current Sheet Crossings near Parker Solar Probe’s Perihelion 7

Presenter:  Mihir Desai (SwRI)

Abstract: 

On January 17, 2021 when Parker Solar Probe (PSP) was close to perihelion during orbit 7, the Integrated Science Investigation of the Sun-Energetic Particle-Lo instrument (ISOIS/EPI-Lo) observed significant increases in the intensities of <100 keV/nucleon suprathermal protons and heavy ions, such as He, O, and Fe, in the vicinity of two distinct crossings of the heliospheric current sheet (HCS) as detected by the FIELDS and SWEAP instruments (see Phan et al., Bale et al.). These enhancements occurred over a ~14 hr interval centered on the HCS crossing, and are the first such measurements observed at a radial distance of <0.3 au from the Sun. Our key observations are: 1) the ion intensities exhibit no clear velocity dispersion, drop-off during the first full HCS crossing, and peak just after an IMF polarity reversal associated with the second partial HCS crossing; 2) the He spectral slope softens either side of the HCS crossing; and 3) the He pitch-angle distributions (PADs) near the HCS crossings show anti-sunward field-aligned beams or flows, with the PADs closely tracking the magnetic field polarity reversals. We examine the temporal evolution of the PADs in the PSP frame and in the local plasma frame and of the heavy ion energy spectra. We discuss the implications of these new observations in terms of potential sources, expectations of remote vs. local acceleration mechanisms, and existing propagation models of suprathermal ions in the inner heliosphere.

Recording: July_2021_Mihir_Desai.mp4

June 24, 2021

No PSP Theory telecon this month!

Parker one conference: June 14-18 2021.

May 27, 2021

Title: Evolution of large-amplitude Alfven waves and generation of switchbacks in the expanding solar wind

Presenter: Alfred Mallet (UC Berkeley)

Abstract: 

Motivated by recent Parker Solar Probe observations of “switchbacks” (abrupt, large- amplitude reversals in the radial magnetic field, which exhibit Alfvenic correlations) we examine the dynamics of large-amplitude Alfven waves in the expanding solar wind. We develop an analytic model which makes several predictions: switchbacks should preferentially occur in regions where the solar wind plasma has undergone a greater expansion, the switchback fraction at radii comparable to PSP should be an increasing function of radius, and switchbacks should have their gradients preferentially perpendicular to the mean magnetic field direction. The expansion of the plasma generates small compressive components as part of the wave’s nonlinear evolution: these are maximized when the normalized fluctuation amplitude is comparable to sin θ, where θ is the angle between the propagation direction and the mean magnetic field. These compressive components steepen the primary Alfvenic waveform, keeping the solution in a state of nearly constant magnetic field strength as its normalized amplitude δB/B grows due to expansion. The small fluctuations in the magnetic-field-strength are minimized at a particular θ-dependent value of β, usually of order unity, and the density and magnetic- field-strength fluctuations can be correlated or anticorrelated depending on β and θ. Example solutions of our dynamical equation are presented; some do indeed form magnetic-field reversals. Our predictions appear to match some previously unexplained phenomena in observations and numerical simulations, providing evidence that the observed switchbacks result from the nonlinear evolution of the initially small-amplitude Alfven waves already known to be present at the coronal base.

Recording: May_2021_Alfred_Mallet.mp4

Title: Energetic Particles Associated with a CME-Driven Shock as it Interacts with an Isolated Magnetic Structure

Presenter: Joe Giacalone (University of Arizona)

Abstract: 

The physics of particle acceleration at a shock which interacts with an isolated flux-rope-like magnetic structure (FRLS) will be discussed. This topic is motivated by recent PSP observations. On Nov 30, 2020, PSP was crossed by a CME-driven shock which was also crossing a FRLS at about the same time: its leading edge coincided with the crossing of the shock, while the trailing edge crossed PSP about 7 minutes later. Prior to the arrival of the shock, the intensity of ~30keV-3MeV ions increased gradually, peaking at the time of the shock passage, in a manner that is generally consistent with the theory of diffusive shock acceleration. However, during the crossing of the FRLS, in the CME-shock sheath region, the energetic-ion intensity dropped dramatically, before recovering at about the time of the crossing of the trailing edge of the FRLS. We present an interpretation of this invoking theoretical aspects of particle acceleration at shocks, and results from a numerical simulation model which qualitatively agrees with these observations.

Recording: April_2021_Joe_Giacalone.mp4

March 25, 2021

Title: The Origin of Switchbacks in the Solar Corona

Presenter: Gary Zank and Haoming Liang (Center for Space Plasma and Aeronomic Research (CSPAR) and Department of Space Science, UAH)

Abstract: 

The talk will be split into two parts. In the first, G.P. Zank will discuss the theory that addresses the origin, structure, and propagation characteristics of a switchback, compelling questions posed by Parker Solar Probe (PSP) observations of velocity spikes and magnetic field reversals. By assuming interchange reconnection between coronal loop and open magnetic field, we show that this results in the generation of upward (into the heliosphere) and downward complex structures propagating at the fast magnetosonic speed (i.e., the Alfven speed in the low plasma beta corona) that can have an arbitrary radial magnetic field deflection, including ``S-shaped.'' We derive the evolution equation for the switchback radial magnetic field as it propagates through the inhomogeneous supersonic solar corona. An analytic solution for arbitrary initial conditions is used to investigate the properties of a switchback propagating from launch to ~35 Rs where PSP observed switchbacks during its first encounter. We describe how the model can describe simple single switchbacks, or double-humped structures, and suggest that the clustering of switchbacks and their sometimes complicated structure may be due to the formation of multiple closely spaced switchbacks created by interchange reconnection with numerous open and loop magnetic field lines over a short period. We show that their evolution yields a complex, aggregated group of switchbacks that includes “sheaths” with large-amplitude radial magnetic field and velocity fluctuations. In the second part of the talk, H. Liang will apply the linear theory of Zank et al. (2020) to interpret the switchback observations. We select 96 simple one-humped switchback events during the first encounter of PSP and use a Markov Chain Monte Carlo technique to fit the observed magnetic field and plasma variables with the model predictions for each event. The chi-squared goodness of fit test is used to evaluate the fittings. We find that about 47.9% and 42.7% of the events are classified as good fits below the 95% and 90% critical values respectively. The statistical study validates the reliability of the linear theory of Zank et al. (2020) for a significant number of switchback events. The statistical analysis provides most probable initial conditions for switchbacks generated by interchange reconnection, thereby providing insight into the environment in which interchange reconnection occurs.

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

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)

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)

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.)

August 27, 2020

Title:  Stellar Rotation Effects on the Stellar Wind

Presenter: Bhimsen Shivamoggi (University of Central Florida)

July 30, 2020

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

Presenter: Alexandra Brosius (Penn State)

July 23, 2020

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

Presenter: Anna Tenerani (UT Austin)

June 25, 2020

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

Presenter: Robert Allen (JHU/APL)

May 28, 2020

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

Presenter: Benoit Lavraud (IRAP)

April 23, 2020

Title:  Electron Energy Partition Across Interplanetary Shocks

Presenter: Lynn Wilson (UNASA/GSFC)

March 26, 2020

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

Presenter: Laura Bercic (University of Florence, Italy)

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)

January 23, 2020

Title:  Electrons in the Young Solar Wind

Presenter: Jasper Halekas (University of Iowa)

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)

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)

September 26, 2019

Title: Outstanding Questions of Solar Wind Physics

Presenter: Nicki Viall (NASA Goddard Space Flight Center)

August 22, 2019

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

Presenter: Ben Alterman (University of Michigan)

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))

May 23, 2019

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

Presenter: Timothy Horbury (Imperial College London)

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)

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)

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)

PDF File: Scudder_Slides.pdf

Telecon Recording: Scudder_Webex.arf

December 20, 2018

Title: AGU Debrief and Discussion

Presenter: Marco Velli and participants

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)

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)

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:

PDF File: 

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:

PDF File: 

Telecon Recording: Matthaeus_Webex.arf

June 14, 2018 

Title: Collisional Dissipation in Turbulent Weakly Collisional Plasmas

Presenter: Oreste Pezzi (Universita Della Calabria, Italy) 

Abstract:

PDF File: 

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:

PDF File: 

Telecon Recording: Shi_Webex.arf

March 22, 2018

Title: Turbulence in Solar Wind: Why Comobile Coordinates?

Presenter: Roland Grappin (LPP Ecole polytechnique)

Abstract:

PDF File: 

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:

PDF File: 

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:

PDF File: 

Telecon Recording: Kontar_Webex.arf

November 2, 2017

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

Presenter: Victor Reville (UCLA EPSS) 

Abstract:

PDF File: 

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:

PDF File: 

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:

PDF File: 

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:

PDF File: 

Telecon Recording: Alexandrova_Webex.arf

May 25, 2017

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

Presenter: Maria Elena Innocenti 

PDF File: 

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)

PDF File: 

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: 

PDF File: 

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:

PDF File: 

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)

PDF File: 

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)

PDF File: 

Telecon Recording: 

November 17, 2016

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

Presenter: Kristopher Klein (University of Michigan)

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)

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)

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)

Telecon Recording: Verdini_Webex.arf