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Graphical overview covering most of the various proposed switchback-generation mechanisms, reprinted from https://www.nasa.gov/feature/goddard/2021/switchbacks-science-explaining-parker-solar-probe-s-magnetic-puzzle. The mechanisms are classified into those that form switchbacks (1) directly through interchange reconnection (e.g., Fisk and Kasper 2020, He et al. 2021b, Sterling and Moore 2020); (2) through ejection of flux ropes by interchange reconnection (Drake et al. 2021; Agapitov et al. 2022); (3) from expanding/growing AWs and/or Alfvénic turbulence (Squire et al. 2020; Mallet et al. 2021; Shoda et al. 2021); (4) due to roll up from nonlinear Kelvin-Helmholtz instabilities (Ruffolo et al. 2020); and (5) through magnetic field lines that stretch between sources of slower and faster wind (Schwadron and McComas 2021; see also Landi et al. 2006)
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Launched on 12 Aug. 2018, NASA’s Parker Solar Probe had completed 13 of its scheduled 24 orbits around the Sun by Nov. 2022. The mission’s primary science goal is to determine the structure and dynamics of the Sun’s coronal magnetic field, understand how the solar corona and wind are heated and accelerated, and determine what processes accelerate e...
Citations
... For these reasons, the study of magnetic reconnection has been a fascinating and challenging subject since it was first formulated in 1953 (Dungey, 1953). Its study will continue to thrive with our increasing capability to observe electromagnetic phenomena in the universe [e.g., Bale et al (2023); ; Raouafi et al (2023a); Müller et al (2020)]. The development of reconnection theories is guided and constrained by a wealth of data from numerical simulations, in-situ and remote space observations, and laboratory experiments. ...
... Here instead we focus on the progress in the past 20 years on collisionless reconnection, where our understanding has been accelerated by kinetic simulations and in-situ spacecraft observations of NASA's ongoing Magnetospheric Multiscale (MMS) mission , THEMIS/ARTEMIS (Angelopoulos, 2008;Sweetser et al, 2011), and Cluster (Escoubet et al, 2001). More exciting results are expected from the Parker Solar Probe (Raouafi et al, 2023a) and Solar Orbiter (Müller et al, 2020) missions, but are not discussed here. It is worth noting that Earth's magnetosphere and the solar wind are the most ideal testing grounds for reconnection physics reachable by human probes with current space technology. ...
Magnetic reconnection is a ubiquitous plasma process that transforms magnetic energy into particle energy during eruptive events throughout the universe. Reconnection not only converts energy during solar flares and geomagnetic substorms that drive space weather near Earth, but it may also play critical roles in the high energy emissions from the magnetospheres of neutron stars and black holes. In this review article, we focus on collisionless plasmas that are most relevant to reconnection in many space and astrophysical plasmas. Guided by first-principles kinetic simulations and spaceborne in-situ observations, we highlight the most recent progress in understanding this fundamental plasma process. We start by discussing the non-ideal electric field in the generalized Ohm's law that breaks the frozen-in flux condition in ideal magnetohydrodynamics and allows magnetic reconnection to occur. We point out that this same reconnection electric field also plays an important role in sustaining the current and pressure in the current sheet and then discuss the determination of its magnitude (i.e., the reconnection rate), based on force balance and energy conservation. This approach to determining the reconnection rate is applied to kinetic current sheets of a wide variety of magnetic geometries, parameters, and background conditions. We also briefly review the key diagnostics and modeling of energy conversion around the reconnection diffusion region, seeking insights from recently developed theories. Finally, future prospects and open questions are discussed.
... The unprecedented in situ measurements close to the sun has ushered in a new era of space plasma studies (see e.g. Chen et al. (2020); Adhikari et al. (2020); Shi et al. (2021); Matthaeus (2021); Sioulas et al. (2022); Zank et al. (2022); Chen (2022); Zhang et al. (2022); Sioulas et al. (2023a,b); Dunn et al. (2023); Larosa et al. (2023); McIntyre et al. (2023), and see Raouafi et al. (2023) for a recent review). Notably, new observations from PSP have shown that the 1/f range in standard solar wind turbulence model (Bruno & Carbone 2013) appears to be absent closer to the sun, and instead shallower (∼ f −0.5 ) spectra are found around the Alfvén surface, the region (that may look more like a thin shell due to its non-smooth behaviour) where the solar wind spead overtakes the Alfvén speed. ...
Recent observations of Parker Solar Probe (PSP) from around the Alfv\'en surface have shown that the trace magnetic power spectrum density (PSD) is often characterized by a shallow-inertial double power law, where in the low frequency energy injection range, the power spectrum is shallow (flatter than $1/f$), and in the inertial range the spectrum is steep, with a scaling index of [1.5, 1.67]. Consequently, close to the sun, the majority of the fluctuation energy concentrates in a small frequency range around the low frequency power spectral break. In this work, we conduct a systematic survey of PSP observations for the first 17 encounters to statistically study the energy behaviors of the magnetic fluctuations. Our results show that the center frequency of fluctuation energy systematically drifts to around 3-minute for the most pristine solar wind (smallest solar wind advection time). Moreover, the center frequency rapidly drifts to lower frequency as solar wind advection time increases, as expected for active turbulence. The concentration of fluctuation energy around 3-minutes suggests that Alfv\'enic fluctuations in solar wind might mostly be coming from resonant p-mode oscillations in the photosphere, though other potential sources are discussed.
... This latter category of slow wind is referred to as the Alfvénic slow wind (ASW). Numerous intervals of ASWs have been observed, for example, by the PSP (Raouafi et al. 2023a; see also Bale et al. 2019;Kasper et al. 2019;Bourouaine et al. 2020Bourouaine et al. , 2022Chen et al. 2021). ...
In this work we analyze plasma and magnetic field data provided by the Parker Solar Probe and Solar Orbiter missions to investigate the radial evolution of the heating of Alfvénic slow wind by imbalanced Alfvén-wave (AW) turbulent fluctuations from 0.06 to 1 au. in our analysis we focus on slow solar-wind intervals with highly imbalanced and incompressible turbulence (i.e., magnetic compressibility C B = δ B / B ≤ 0.25, plasma compressibility C n = δ n / n ≤ 0.25, and normalized cross helicity σ c ≥ 0.65). First, we estimate the AW turbulent dissipation rate from the wave energy equation and find that the radial profile trend is similar to the proton heating rate. Second, we find that the scaling of the empirical AW turbulent dissipation rate Q W obtained from the wave energy equation matches the scaling from the phenomenological AW turbulent dissipation rate Q CH09 (with Q CH09 ≃ 1.55 Q W ) derived by Chandran & Hollweg based on the model of reflection-driven turbulence. Our results suggest that, as in the fast solar wind, AW turbulence plays a major role in the ion heating that occurs in incompressible slow-wind streams.
... The mission consists of 24 highly elliptical orbits and seven Venus gravity assist flybys (used to gradually decrease the perihelion every few orbits). The perihelion of orbit 22 will reach 9.86 solar radii on 2024 December 24 (Raouafi et al. 2023). During the mission, PSP will provide unprecedented measurements of the solar wind closer to the Sun. ...
We present a statistical investigation of the radial evolution of 28 interplanetary coronal mass ejections (ICMEs), measured in situ by the Parker Solar Probe spacecraft from 2018 October to 2022 August. First, by analyzing the radial distribution of ICME classification based on magnetic hodograms, we find that coherent configurations are more likely to be observed close to the Sun. By contrast, more complex configurations are observed farther out. We also notice that the post-ICME magnetic field is more impacted following an ICME passage at larger heliocentric distances. Second, with a multilinear robust regression, we derive a slower magnetic ejecta (ME) expansion rate within 1 au compared to previous statistical estimates. Then, investigating the magnetic field fluctuations within ICME sheaths, we see that these fluctuations are strongly coupled to the relative magnetic field strength gradient from the upstream solar wind to the ME. Third, we identify ME expansion as an important factor in the formation of sheaths. Finally, we determine the distortion parameter (DiP), which is a measure of magnetic field asymmetry in an ME. We discover lower overall asymmetries within MEs. We reveal that even for expanding MEs, the time duration over which an ME is sampled does not correlate with DiP values, indicating that the aging effect is not the sole contributor to the observed ME asymmetries.
... Understanding the aging effect becomes more and more important since the recent launch of the Parker Solar Probe (PSP; Fox et al. 2016;Raouafi et al. 2023). We have started to measure CMEs when they are very close to the Sun (<0.1 au; see, e.g., Long et al. 2023;McComas et al. 2023;Romeo et al. 2023), and to this day, it is still unclear how strong the aging effect is closer to the Sun. ...
In situ measurements of coronal mass ejections (CMEs) when they pass over an interplanetary probe are one of the main ways we directly measure their properties. However, such in situ profiles are subject to several observational constraints that are still poorly understood. This work aims at quantifying one of them, namely, the aging effect, using a CME simulated with a three-dimensional magnetohydrodynamical code. The synthetic in situ profile and the instantaneous profile of the magnetic field strength differ more from each other when taken close to the Sun than far from it. Moreover, out of three properties we compute in this study (i.e., size, distortion parameter, and expansion speed), only the expansion speed shows a dependence of the aging as a function of distance. It is also the property that is the most impacted by the aging effect as it can amount to more than 100 km s ⁻¹ for CMEs observed closer than 0.15 au. This work calls for caution when deducing the expansion speed from CME profiles when they still are that close to the Sun since the aging effect can significantly impact the derived properties.
... The angular velocity of the orbital motion of the PSP (Raouafi et al. 2023b) spacecraft exceeds the angular rotation velocity of the surface of the Sun at perihelion, while farther out in PSP's orbit the Sun's surface angular velocity exceeds that of PSP. For about 2 days during both the inbound and outbound legs of the orbit, PSP's angular velocity roughly matches that of the Sun's surface. ...
Parker Solar Probe (PSP) observations of a small dispersive event on 2022 February 27 and 28 indicate scatter-free propagation as the dominant transport mechanism between the low corona and greater than 35 solar radii. The event occurred during unique orbital conditions that prevailed along specific flux tubes that PSP encountered repeatedly between 25 and 35 R s during outbound orbit 11. This segment of the PSP orbit exhibits almost stationary angular motion relative to the rotating solar surface, such that in the rotating frame, PSP’s motion is essentially radial. The time dispersion often observed in impulsive solar energetic particle (SEP) events continues in this case down to velocities including the core solar-wind ion velocities. Especially at the onset of this event, the ³ He content is much larger than the usual SEP abundances seen in the energy range from ∼100 keV to several MeV for helium. Later in the event, iron is enhanced. The compositional signatures suggest this to be an example of an acceleration mechanism for generating the seed energetic particles required by shock (or compression) acceleration models in SEP events to account for the enrichment of various species above solar abundances in such events. A preliminary search of similar orbital conditions over the PSP mission has not revealed additional such events, although favorable conditions (isolated impulsive acceleration and well-ordered magnetic field connection with minimal magnetic field fluctuation) that would be required are infrequently realized, given the small fraction of the PSP trajectory that meets these observation conditions.
... Parker measurements have been used to constrain photospheric flux transport models (e.g., Wallace et al. 2022) and coronal heating rates in the solar wind using simultaneous remote sensing and in situ measurements from SolO (e.g., Telloni et al. 2023). For a recent review of Parker's discoveries, see Raouafi et al. (2023). ...
The launches of Parker Solar Probe (Parker) and Solar Orbiter (SolO) are enabling a new era of solar wind studies that track the solar wind from its origin at the photosphere, through the corona, to multiple vantage points in the inner heliosphere. A key ingredient for these models is the input photospheric magnetic field map that provides the boundary condition for the coronal portion of many heliospheric models. In this paper, we perform steady-state, data-driven magnetohydrodynamic (MHD) simulations of the solar wind during Carrington rotation 2258 with the Grid GAMERA model. We use the ADAPT and AFT flux transport models and quantitatively assess how well each model matches in situ measurements from Parker, SolO, and Earth. We find that both models reproduce the magnetic field components at Parker quantitatively well. At SolO and Earth, the magnetic field is reproduced relatively well, though not as well as at Parker, and the density is reproduced extremely poorly. The velocity is overpredicted at Parker, but not at SolO or Earth, hinting that the Wang–Sheeley–Arge (WSA) relation, fine-tuned for Earth, misses the deceleration of the solar wind near the Sun. We conclude that AFT performs quantitatively similarly to ADAPT in all cases, and that both models are comparable to a purely WSA heliospheric treatment with no MHD component. Finally, we trace field lines from SolO back to an active region outflow that was observed by Hinode/EIS, and which shows evidence of elevated charge state ratios.
... A series of studies has reported widespread observations of enhanced magnetic fluctuations near proton/electron cyclotron frequencies in the inner heliosphere (Jian et al. 2009;Breneman et al. 2010;Lacombe et al. 2014;Boardsen et al. 2015;Gary et al. 2016;Stansby et al. 2016;Zhao et al. 2019;Tong et al. 2019;Agapitov et al. 2020;Bowen et al. 2020;Cattell et al. 2020;Verniero et al. 2020;Liu et al. 2023). These electromagnetic waves at the ion/electron scale include Alfven ion cyclotron, fast magnetosonic, and whistler waves, and they are believed to play a crucial role in heating, accelerating, scattering coronal and solar wind plasma particles and regulating heat flux in solar wind through wave-particle resonant interactions (Pagel et al. 2007;Cattell & Vo 2021;Bowen et al. 2022;Squire et al. 2022;Raouafi et al. 2023). While many issues concerning the origin of such kinetic-scale waves are still debated, recent observations suggest that these waves could be locally generated in an interplanetary medium through plasma kinetic instabilities (Gary et al. 2016;Liu et al. 2023). ...
The Korean heliospheric community, led by the Korea Astronomy and Space Science Institute (KASI), is currently assessing the viability of deploying a spacecraft at the Sun-Earth Lagrange Point L4 in collaboration with National Aeronautics and Space Administration (NASA). The aim of this mission is to utilize a combination of remote sensing and in situ instruments for comprehensive observations, complementing the capabilities of the L1 and L5 observatories. The paper outlines longterm scientific objectives, underscoring the significance of multi-point in-situ observations to better understand critical heliospheric phenomena. These include coronal mass ejections, magnetic flux ropes, heliospheric current sheets, kinetic waves and instabilities, suprathermal electrons and solar energetic particle events, as well as remote detection of solar radiation phenomena. Furthermore, the mission’s significance in advancing space weather prediction and space radiation exposure assessment models through the integration of L4 observations is discussed. This article is concluded with an emphasis on the potential of L4 observations to propel advancements in heliospheric science.
... Switchbacks are local polarity reversals of the radial magnetic field and are typically Alfvénic structures, with highly correlated velocity and magnetic field fluctuations and nearly constant magnetic field strength (McManus et al. 2020;Woolley et al. 2020;Larosa et al. 2021). For a comprehensive description of switchbacks and their properties, please refer to the review paper (Raouafi et al. 2023a) and references therein. ...
Parker Solar Probe observations reveal that the near-Sun space is almost filled with magnetic switchbacks (“switchbacks” hereinafter), which may be a major contributor to the heating and acceleration of solar wind. Here, for the first time, we develop an analytic model of an axisymmetric switchback with uniform magnetic field strength. In this model, three parameters control the geometry of the switchback: height (length along the background magnetic field), width (thickness along radial direction perpendicular to the background field), and the radial distance from the center of switchback to the central axis, which is a proxy of the size of the switchback along the third dimension. We carry out 3D magnetohydrodynamic simulations to investigate the dynamic evolution of the switchback. Comparing simulations conducted with compressible and incompressible codes, we verify that compressibility, i.e., parametric decay instability, is necessary for destabilizing the switchback. Our simulations also reveal that the geometry of the switchback significantly affects how fast the switchback destabilizes. The most stable switchbacks are 2D-like (planar) structures with large aspect ratios (length to width), consistent with the observations. We show that when plasma beta ( β ) is smaller than one, the switchback is more stable as β increases. However, when β is greater than 1, the switchback becomes very unstable as the pattern of the growing compressive fluctuations changes. Our results may explain some of the observational features of switchbacks, including the large aspect ratios and nearly constant occurrence rates in the inner heliosphere.
... The Parker Solar Probe mission (PSP; Fox et al. 2016;Raouafi et al. 2023) created a unique opportunity to observe the environment around the Sun from an exceptionally close distance with both in situ and remote-sensing instruments. The remote-sensing instrument onboard PSP is the Wide-field Imager (WISPR; Vourlidas et al. 2016). ...
We present a comprehensive analysis aimed at proving the hypothesis that a train of small-scale features observed by the Wide-field Imager (WISPR) onboard the Parker Solar Probe (PSP) are the signature of a Kelvin–Helmholtz instability (KHI). These features were seen near the flank of a Coronal Mass Ejection (CME) wake between 7.5 R ⊙ and 9.5 R ⊙ , lasting for about 30 minutes. The CME was a slow event, associated with a streamer blowout. We analyzed the size of the eddies and found growth during their evolution while maintaining separation distances and alignment typical of Kelvin–Helmholtz vortexes. We then assessed the magnetic field conditions that would make the observation of such an instability plausible. Two methods were used to cross-check our findings. The measured thickness of the boundary layer supports KHI candidacy, and the estimated linear growth rate suggests nonlinear saturation within the expected timescale. We conclude that a KHI is a plausible explanation for the observed features, and therefore that such instabilities might exist in the low and middle solar corona (within ∼15 R ⊙ ) and can be detected in white light observations. Their observation, however, might be rare due to stringent conditions like the observer’s proximity, suitable viewing circumstances, magnetic field topology, and flow properties. This study highlights the unique capability of PSP/WISPR in observing such phenomena, especially as PSP perihelia reach closer distances to the Sun.
