ST1.4

Alfvénic fluctuations represent the dominant contributions to turbulent fluctuations in the solar wind, especially, but not limited to, the fastest streams with velocity of the order of 600-700 km/s. Alfvénic fluctuations can contribute to solar wind heating and acceleration via wave pressure and turbulent heating. Observations show that such fluctuations are characterized by a nearly constant magnetic field amplitude, a condition which remains largely to be understood and that may be an indication of how fluctuations evolve and relax in the expanding solar wind. Interestingly, measurements from Parker Solar Probe have shown the ubiquitous and persistent presence of the so-called switchbacks. These are magnetic field lines which are strongly perturbed to the point that they produce local inversions of the radial magnetic field. 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. While there is not yet a general consensus on what is the origin of switchbacks and their connection to coronal activity, a first necessary step to answer these important questions is to understand how they evolve and how long they can persist in the solar wind. Here we investigate the evolution of switchbacks. We address how their evolution is affected by parametric instabilities and the possible role of expansion, by comparing models with the observed radial evolution of the fluctuations’ amplitude. We finally discuss what are the implications of our results for models of switchback generation and related open questions.

How to cite: Tenerani, A., Velli, M., and Matteini, L.: Theory and observations of switchbacks’ evolution in the solar wind, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13400, https://doi.org/10.5194/egusphere-egu21-13400, 2021.

The observations from the Parker Solar Probe during the first
perihelion revealed large numbers of local reversals in the radial
component of the magnetic field with associated velocity spikes. Since
the spacecraft was magnetically connected to a coronal hole during the
closest approach to the sun, one possible source of these spikes is
magnetic reconnection between the open field lines in the coronal hole
and an adjacent region of closed flux. Reconnection in a low beta
environment characteristic of the corona is expected to be bursty
rather than steady and is therefore capable of producing large numbers
of magnetic flux ropes with local reversals of the radial magnetic
field that can propagate outward large radial distances from the
sun. Flux ropes with a strong guide field produce signatures
consistent with the PSP observations. We have carried out simulations
of "interchange" reconnection in the corona and have explored the
local structure of flux ropes embedded within the expanding solar
wind. We have first established that traditional interchange
reconnection cannot produce the switchbacks since bent field lines
generated in the corona quickly straighten. The simulations have been
extended to the regime dominated by the production of multiple flux
ropes and we have established that flux ropes are injected into the
local solar wind. Local simulations of reconnection are also being
carried out to explore the structure of flux ropes embedded in the
solar wind for comparison with observations. Evidence is presented
that flux rope merging may be ongoing and might lead to the high
aspect ratio of the switchback structures measured in the solar wind.

How to cite: Drake, J., Agapitov, O., Swisdak, M., Badman, S., Bale, S., Horbury, T., Kasper, J., MacDowal, R., Mozer, F., Phan, T., Pulupa, M., Szabo, A., and Velli, M.: Magnetic Reconnection in the Corona as a Source of Switchbacks in the Solar Wind, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2865, https://doi.org/10.5194/egusphere-egu21-2865, 2021.

Magnetic switchbacks are Alfvénic structures characterized as intervals of intermittent reversals in the radial componentof magnetic field. Switchbacks comprise of magnetic spikes preceded/succeeded by quiet, pristine solar wind. Determining switch-back generation and evolution mechanisms will further our understanding of the global circulation and transportation of Sun’s openmagnetic flux. Here, we investigate switchback transition regions using measurements from fields and plasma suites aboard the Parker SolarProbe (PSP). Minimum variance analysis (MVA) is applied on switchback transition region magnetic signatures. Discontinuity analysesare performed on 273 switchback transition regions with robust MVA solutions. Our results indicate that switchbacks are rotational discontinuities (RD) in 214 (or 78%) of the cases. 21% of the switchbacktransition regions are categorized as "either" discontinuity (ED), defined as small relative changes in both magnitude and the normalcomponent of magnetic field. RD-to-ED event count is found to reduce with increasing distance from the Sun. On average, plasmabeta falls by −28% across RD-type switchback transition regions and magnetic shear angle is 60 [deg], therefore making switchbacktransition regions theoretically favorable to local magnetic reconnection. The evolution of switchbacks away from the Sun may involve increasing mass flux across RD-type switchback transition regions. The evolution mechanism(s) remain to be discovered. Our results indicate negligible magnetic curvature across switchback transition regions which may inhibit local magnetic reconnection.

How to cite: Akhavan-Tafti, M., Kasper, J., Huang, J., and Bale, S.: Discontinuity analysis and evolution of magnetic switchbacks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7879, https://doi.org/10.5194/egusphere-egu21-7879, 2021.

Parker Solar Probe (PSP) routinely observes magnetic field deflections in the solar wind at distances less than 0.3 au from the Sun. These deflections are related to structures commonly called 'switchbacks' (SBs), whose origins and characteristic properties are currently debated. Here, we use a database of visually selected SB intervals - and regions of solar wind plasma measured just before and after each SB - to examine plasma parameters, turbulent spectra from inertial to dissipation scales, and intermittency effects in these intervals. We find that many features, such as perpendicular stochastic heating rates and turbulence spectral slopes are fairly similar inside and outside of SBs. However, important kinetic properties, such as the characteristic break scale between the inertial to dissipation ranges differ inside and outside these intervals, as does the level of intermittency, which is notably enhanced inside SBs and in their close proximity, most likely due to magnetic field and velocity shears observed at the edges. We conclude that the plasma inside and outside of a SB, in most of the observed cases, belongs to the same stream, and that the evolution of these structures is most likely regulated by kinetic processes, which dominate small scale structures at the SB edges. 

How to cite: Martinović, M., Klein, K., Huang, J., Chandran, B., Kasper, J., Lichko, E., Bowen, T., Chen, C., Matteini, L., Stevens, M., Case, A., and Bale, S.: Multiscale Solar Wind Turbulence Properties inside and near Switchbacks measured by Parker Solar Probe, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6686, https://doi.org/10.5194/egusphere-egu21-6686, 2021.

Switchback boundaries separate two plasmas moving with different velocities, that may have different temperatures and densities and typically manifest sharp magnetic field deflections through the boundary. They may be analyzed similarly to MHD discontinuities. The first step of their characterization consists of analysis in terms of MHD discontinuities. Such an analysis was performed by Larosa et al., (2021) who has found that 32% of them may be attributed to rotational discontinuities, 17% to tangential, about 42% to the group of discontinuities that are difficult to unambiguously define whether they are tangential or rotational, and 9% that do not belong to any of these two groups. We describe and apply hereafter for two events another approach for the characterization of the boundaries based on classification of the general type discontinuity in MHD approximation. It is based on the problem of the decay of the general type of discontinuity. It is well known [Kulikovsky and Lyubimov, 1962, Gogosov, 1959} that general type MHD discontinuity decays on 7 separate discontinuities belonging to different types of MHD waves, namely, entropic wave, two slow mode waves, two Alfvenic waves, and two fast mode waves. Entropic wave is standing in the reference frame of the discontinuity; other wave modes are supposed to run in the opposite directions from the initial discontinuity with their characteristic velocities. Making use of plasma parameters from two sides of the boundary one can evaluate the fraction of each wave mode present in the discontinuity. We apply this method to two boundary crossings. This repartition of the discontinuity allows characterizing the deviation from Alfvenicity quantitatively.

References

Larosa, A., et al., A&A, 2021, (accepted)

Kulikovsky, Lyubimov, Magnetohydrodynamics, (1962)

Gogosov, V.V., Decay of the MHD discontinuity, (1959)

How to cite: Krasnoselskikh, V., Larosa, A., Dudok de Wit, T., Agapitov, O., Froment, C., Kretzschmar, M., Jagarlamudi, V., Velli, M., Bale, S. D., Goetz, K., Harvey, P., Kasper, J., Korreck, K., Larson, D., MacDowall, R., Malaspina, D., Mozer, F., Pulupa, M., Reveillet, C., and Stevens, M.: Decomposition of the switchback boundary on MHD wave modes., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15180, https://doi.org/10.5194/egusphere-egu21-15180, 2021.

The solar wind is frequently perturbed by transient structures such as magnetic folds, jets, waves and flux-ropes that propagate rapidly away from the Sun over a large range of heliocentric distances. Parker Solar Probe has revealed that rotations of the magnetic field vector occur repeatedly at small heliocentric distances, on regions that also display surprisingly large solar wind rotation rates. Sun-to-spacecraft connectivity analysis shows that a large fraction of the solar wind flows probed so far by Parker Solar Probe were formed and accelerated in the vicinity of coronal hole boundaries.
We show by means of of global MHD simulations that coronal rotation is highly structured in proximity to those boundary regions (in agreement with preceding SoHO/UVCS observations), and that enhanced poloidal and toroidal flow shear and magnetic field gradients also develop there. We identified regions of the solar corona for which solar wind speed and rotational shear are significant, that can be associated with field-aligned and/or transverse vorticity, and that can be favourable to the development of magnetic deflections. Some of these wind flow shears are driven through large radial extensions, being noticeable tens of solar radii away from the surface, and therefore have a potential impact on the propagation of such magnetic perturbations across extended heights in the solar wind. We conclude that these regions of persistent shears are undoubtedly sources of complex solar wind structures, and suggest that they can trigger instabilities capable of creating magnetic field reversals detected in-situ in the heliosphere.
Our simulations furthermore indicate that the spatial structure of the solar wind shear will become more complex as the solar cycle progresses, with strong and extended shears appearing at heliographic latitudes that will be probed by Solar Orbiter in the near future.

How to cite: Pinto, R., Poirier, N., Kouloumvakos, A., Rouillard, A., Griton, L., Fargette, N., Kieokaew, R., Lavraud, B., and Brun, A. S.: Solar wind speed and rotational shear at coronal hole boundaries, impacts on magnetic field inversions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13552, https://doi.org/10.5194/egusphere-egu21-13552, 2021.

Recent studies of the solar wind sunward of 0.25 AU using the Parker Solar Probe spacecraft reveal that that solar wind can be bimodal, alternating between near quiescent regions with low turbulent fluctuation amplitudes and Parker-like magnetic field direction and regions of highly turbulent plasma and magnetic field fluctuations associated with ‘switchbacks’ of the radial magnetic field.  

The quiescent solar wind regions are highly unstable to the formation of plasma waves near the electron cyclotron frequency (fce), possibly driven by strahl electrons, which carry the solar wind heat flux, and may provide one of the most direct particle diagnostics of the solar corona at the source of the solar wind.  These waves are most intense near ~0.7 fce and ~fce. The near-fce waves are found to become more intense and more frequent closer to the Sun, and statistical evidence indicates that their occurrence rate is related to the sunward drift of the core electron population.  

In this study, we examine high time resolution burst captures of these waves, demonstrating that each wave burst contains several distinct wave types, including electron Bernstein waves and extremely narrow band waves that are highly sensitive to the magnetic field orientation. Using properties of these waves we provide evidence to support the identification of their likely plasma wave modes and the instabilities responsible for generating these waves.  By understanding the driving instabilities responsible for these waves, we infer their ability to modify electron distribution functions in the quiescent near-Sun solar wind.  

How to cite: Malaspina, D., Wilson, L., Ergun, R., Bale, S., Bonnell, J., Dudok de Wit, T., Goetz, K., Goodrich, K., Harvey, P., MacDowall, R., Pulupa, M., Halekas, J., Case, A., Larson, D., Stevens, M., and Whittlesey, P.: Plasma Waves Near the Electron Cyclotron Frequency in the Near Sun Solar Wind: Wave Mode Identification and Driving Instabilities, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-955, https://doi.org/10.5194/egusphere-egu21-955, 2021.

How to cite: Chhiber, R., Usmanov, A., Matthaeus, W., Goldstein, M., and Bandyopadhyay, R.: Large-scale Structure and Turbulence Transport in the Young Solar Wind – Comparison of Parker Solar Probe Observations with a Global 3D Reynolds-averaged MHD Model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12726, https://doi.org/10.5194/egusphere-egu21-12726, 2021.

The fourth orbit of Parker Solar Probe (PSP) reached heliocentric distances down to 27.9 Rs, allowing solar wind turbulence and acceleration mechanisms to be studied in situ closer to the Sun than previously possible. The turbulence properties were found to be significantly different in the inbound and outbound portions of PSP's fourth solar encounter, likely due to the proximity to the heliospheric current sheet (HCS) in the outbound period. Near the HCS, in the streamer belt wind, the turbulence was found to have lower amplitudes, higher magnetic compressibility, a steeper magnetic field spectrum (with spectral index close to -5/3 rather than -3/2), a lower Alfvenicity, and a "1/f" break at much lower frequencies. These are also features of slow wind at 1 au, suggesting the near-Sun streamer belt wind to be the prototypical slow solar wind. The transition in properties occurs at a predicted angular distance of ~4 degrees from the HCS, suggesting ~8 degrees as the full-width of the streamer belt wind at these distances. While the majority of the Alfvenic turbulence energy fluxes measured by PSP are consistent with those required for reflection-driven turbulence models of solar wind acceleration, the fluxes in the streamer belt are significantly lower than the model predictions, suggesting that additional mechanisms are necessary to explain the acceleration of the streamer belt solar wind.

How to cite: Chen, C., Chandran, B., Woodham, L., Jones, S., Perez, J., Bourouaine, S., Bowen, T., Klein, K., Moncuquet, M., Kasper, J., and Bale, S.: The Near-Sun Streamer Belt Solar Wind: Turbulence and Solar Wind Acceleration, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3426, https://doi.org/10.5194/egusphere-egu21-3426, 2021.

How to cite: Duan, D., He, J., Zhu, X., Verscharen, D., Bowen, T., Badman, S., and Bale, S.: Radial Evolution of the Solar Wind and Associating Turbulence Based on the Synergetic Measurement from Parker Solar Probe and 1 au Observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15718, https://doi.org/10.5194/egusphere-egu21-15718, 2021.

Magnetic flux rope, formed by the helical magnetic field lines, can sometimes remain its shape while carrying significant plasma flow that is aligned with the local magnetic field. We report the existence of such structures and static flux ropes by applying the Grad-Shafranov-based algorithm to the Parker Solar Probe (PSP) in-situ measurements in the first five encounters. These structures are detected at heliocentric distances, ranging from 0.13 to 0.66 au, in a total of 4-month time period. We find that flux ropes with field-aligned flows have certain properties similar to those of static flux ropes, such as the decaying relations of the magnetic fields within structures with respect to heliocentric distances. Moreover, these events are more likely with magnetic pressure dominating over the thermal pressure and occurring more frequently in the relatively fast-speed solar wind. Taking into account the high Alfvenicity, we also compare these events with switchbacks and present the cross-section maps via the new Grad-Shafranov type reconstruction. Finally, the possible evolution and relaxation of the magnetic flux rope structures are discussed.

How to cite: Chen, Y., Hu, Q., and Zhao, L.: Small-scale Magnetic Flux Ropes with Field-aligned Flows via the PSP In-situ Observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8043, https://doi.org/10.5194/egusphere-egu21-8043, 2021.

Magnetic flux ropes can play important roles in transferring the mass, momentum, and energy in the interplanetary environment and in affecting space weather. Small-scale flux ropes (SFRs) are common in the interplanetary environment. However, SFRs with medium and high Alfvénicity are generally discarded in previous identification procedures. Using Parker Solar Probe measurements, we identify an SFR event with medium Alfvénicity in the inner heliosphere (at ~ 0.2 au). Based on high correlations between the magnetic field and velocity fluctuations, we show Alfvénic waves arising inside such SFR. We also show occurrence of quasi-monochromatic electromagnetic waves at the leading and trailing edges of this SFR. These waves are well explained by the outward-propagating ion-cyclotron waves, which have wave frequencies ~ 0.03 - 0.3 Hz and wavelengths ~ 60 - 2000 km in the plasma frame. Furthermore, we show that the power spectral density of the magnetic field in SFR middle region follows the power-law distribution, where the spectral index changes from -1.5 (f <~ 1 Hz) to -3.3 (f >~ 1 Hz). These findings would motivate developing an automated program to identify SFRs with medium and high Alfvénicity from Alfvénic waves structures.

How to cite: Shi, C., Zhao, J., Huang, J., Wang, T., Wu, D., Chen, Y., Hu, Q., Kasper, J. C., and Bale, S. D.: Parker Solar Probe Observations of Alfvénic Waves and Ion-cyclotron Waves in a Small-scale Flux Rope, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14934, https://doi.org/10.5194/egusphere-egu21-14934, 2021.

Plasma turbulence can be viewed as a magnetic landscape populated by large- and small-scale coherent structures, consisting notionally of magnetic flux tubes and their boundaries. Such structures exist over a wide range of scales and exhibit diverse morphology and plasma properties.  Moreover, interactions of particles with turbulence may involve temporary trapping in, as well as exclusion from, certain regions of space, generally controlled by the topology and connectivity of the magnetic field.  In some cases, such as SEP "dropouts'' the influence of the magnetic structure is dramatic; in other cases, it is more subtle, as in edge effects in SEP confinement. With Parker Solar Probe now closer to the sun than any previous mission, novel opportunities are available for examination of the relationship between magnetic flux structures and energetic particle populations. 

We present a method that is able to characterize both the large- and small-scale structures of the turbulent solar wind, based on the combined use of a filtered magnetic helicity (H_m) and the partial variance of increments (PVI). The synergistic combination with energetic particle measurements suggests whether these populations are either trapped within or excluded from the helical structure.

This simple, single-spacecraft technique exploits the natural tendency of flux tubes to assume a cylindrical symmetry of the magnetic field about a central axis. Moreover, large helical magnetic tubes might be separated by small-scale magnetic reconnection events (current sheets) and present magnetic discontinuity with the ambient solar wind. The method was first validated via direct numerical simulations of plasma turbulence and then applied to data from the Parker Solar Probe (PSP) mission. In particular, ISOIS energetic particle (EP) measurements along with FIELDS magnetic field measurements and SWEAP plasma moments, are enabling characterization of observations of EPs closer to their sources than ever before.
 
This novel analysis, combining H_m and PVI methods, reveals that a large number of flux tubes populate the solar wind and continuously merge in contact regions where magnetic reconnection and particle acceleration may occur. Moreover, the detection of boundaries, correlated with high-energy particle measurements, gives more insights into the nature of such helical structures as "excluding barriers'' suggesting a strong link between particle properties and fields topology. This research is partially supported by the Parker Solar Probe project. 

How to cite: Pecora, F., Servidio, S., Greco, A., Bale, S. D., McComas, D. J., Joyce, C. J., and Matthaeus, W. H.: Flux tubes and energetic particles in Parker Solar Probe orbit 5: magnetic helicity - PVI method and ISOIS observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9911, https://doi.org/10.5194/egusphere-egu21-9911, 2021.

In 2020 May-June, six solar energetic ion events were observed by the Parker Solar Probe/ISoIS instrument suite at ~0.35 AU from the Sun.  From standard velocity-dispersion analysis, the apparent ion path length is ~0.625 AU at the onset of each event. We develop a formalism for estimating the path length of random-walking magnetic field lines, to explain why the apparent ion path length at event onset greatly exceeds the radial distance from the Sun for these events. We developed analytical estimates of the average increase in path length of random-walking magnetic field lines, relative to the unperturbed mean field. Both a simple estimate and a rigorous theoretical formulation are obtained for field-lines' path length increase as a function of path length along the large-scale field. Monte Carlo simulations of field line and particle trajectories in a model of solar wind turbulence are used to validate the formalism and study the path lengths of particle guiding-center and full-orbital trajectories. From these simulated trajectories, we find that particle guiding centers can have path lengths somewhat shorter than the average field line path length, while particle orbits can have substantially larger path lengths due to their gyromotion with a nonzero effective pitch angle. The formalism is also implemented in a global solar wind model, and results are compared with ion path lengths inferred from ISoIS observations. The long apparent pathlength during these solar energetic ion events can be explained by 1) a magnetic field line path length increase due to the field line random walk, and 2) particle transport about the guiding center with nonzero effective pitch angle due to pitch angle scattering. This research partially supported by the PSP /ISOIS project.

How to cite: Matthaeus, W., Chhiber, R., Cohen, C. M. S., Ruffolo, D., Sonsrettee, W., Tooprakai, P., Seripienlert, A., Chuychai, P., Usmanov, A. V., Goldstein, M. L., and McComas, D. J. and the PSP/ISOIS Team: Magnetic Field Line Random Walk and Solar Energetic Particle Path Lengths, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13959, https://doi.org/10.5194/egusphere-egu21-13959, 2021.