Enhanced proton parallel temperature inside patches of switchbacks in the inner heliosphere

Woodham, L. D.; Horbury, T. S.; Matteini, Lorenzo; Woolley, T.; Laker, R.; Bale, S. D.; Nicolaou, Georgios; Stawarz, J. E.; Stansby, David; Hietala, Heli; Larson, D. E.; Livi, R.; Verniero, J. L.; McManus, Michael D.; Kasper, J. C.; Korreck, K. E.; Raouafi, N. E.; Moncuquet, M.; Pulupa, M.

doi:10.1051/0004-6361/202039415

Cited by 55 publications

(45 citation statements)

References 59 publications

(20 reference statements)

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“…Surprisingly, Phan et al (2020) have reported that no reconnection signatures were observed inside the switchback transition regions, suggesting that switchbacks are isolated RD-type current sheets that do not undergo reconnection. On the other hand, Froment et al (2021) have recently provided direct evidence for magnetic reconnection occurring within the switchback transition regions, and (2) if magnetic reconnection does not commonly occur, what is the source (generation and/or evolution mechanisms) of the observed proton temperature increase (Woodham et al 2021) at the leading switchback transition regions? Recently, Woolley et al (2020) have shown that the parallel proton temperature does not vary across the switchback transition regions, further highlighting the role of local mechanisms in switchback evolution.…”

Section: Discussionmentioning

confidence: 99%

Discontinuity analysis of the leading switchback transition regions

Akhavan‐Tafti

Kasper

Huang

et al. 2021

A&A

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Context. Magnetic switchbacks are magnetic structures characterized as intervals of sudden reversal in the radial component of the pristine solar wind’s magnetic field. Switchbacks comprise of magnetic spikes that are preceded and succeeded by switchback transition regions within which the radial magnetic field reverses. Determining switchback generation and evolution mechanisms will further our understanding of the global circulation and transportation of the Sun’s open magnetic flux. Aims. The present study juxtaposes near-Sun switchback transition regions’ characteristics with similar magnetic discontinuities observed at greater radial distances with the goal of determining local mechanism(s) through which switchback transition regions may evolve. Methods. Measurements from fields and plasma suites aboard the Parker Solar Probe were utilized to characterize switchback transition regions. Minimum variance analysis (MVA) was applied on the magnetic signatures of the leading switchback transition regions. The leading switchback transition regions with robust MVA solutions were identified and categorized based on their magnetic discontinuity characteristics. Results. It is found that 78% of the leading switchback transition regions are rotational discontinuities (RD). Another 21% of the leading switchback transition regions are categorized as “either” discontinuity (ED), defined as small relative changes in both magnitude and the normal component of the magnetic field. The RD-to-ED event count ratio is found to reduce with increasing distance from the Sun. The proton radial temperature sharply increases (+ 29.31%) at the leading RD-type switchback transition regions, resulting in an enhanced thermal pressure gradient. Magnetic curvature at the leading RD-type switchback transition regions is often negligible. Magnetic curvature and the thermal pressure gradient are parallel (i.e., “bad” curvature) in 74% of the leading RD-type switchback transition regions. Conclusions. The leading switchback transition regions may evolve from RD-type into ED-type magnetic discontinuities while propagating away from the Sun. Local magnetic reconnection is likely not the main driver of this evolution. Other drivers, such as plasma instabilities, need to be investigated to explain the observed significant jump in proton temperature and the prevalence of bad curvature at the leading RD-type switchback transition regions.

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Section: Discussionmentioning

confidence: 99%

Discontinuity analysis of the leading switchback transition regions

Akhavan‐Tafti

Kasper

Huang

et al. 2021

A&A

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“…The ion (proton) velocity, v i , and temperature, T i were primarily obtained from the SPAN-I (Livi et al 2020), but also the SPC (Case et al 2020), instruments of the SWEAP suite (Kasper et al 2016). The SPAN-I data consist of bi-Maxwellian fits to the proton core population, described in Woodham et al (2020), with the same selection criteria used for excluding bad fits from the dataset, and these data are used for v i and T i unless stated otherwise. Since the fluctuations investigated in this letter are at MHD scales, the solar wind velocity v is considered to be equal to v i .…”

Section: Datamentioning

confidence: 99%

The near-Sun streamer belt solar wind: turbulence and solar wind acceleration

Chen

Chandran

Woodham

et al. 2021

A&A

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The fourth orbit of Parker Solar Probe (PSP) reached heliocentric distances down to 27.9 R , 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, which was 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 a spectral index close to -5/3 rather than -3/2), a lower Alfvénicity, 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 • from the HCS, suggesting ≈ 8 • as the full-width of the streamer belt wind at these distances. While the majority of the Alfvénic 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.

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“…Our main reason for using the data from four orbits of PSP, out of eight in total to date, is the availability of the SPAN-I data, which improve significantly after encounter 3. A Faraday cup instrument (SPC; Case et al 2019) also measures solar wind proton velocity on PSP but discrepancies between the two instruments exist (Woodham et al 2021). We choose to use data from SPAN-I, which provides more accurate data closer to the Sun, where the aberration allows the solar wind protons to fly into the instrument protected by the heat shield.…”

Section: Data Setmentioning

confidence: 99%

Ambipolar Electric Field and Potential in the Solar Wind Estimated from Electron Velocity Distribution Functions

Berčič

Maksimović

Halekas

et al. 2021

ApJ

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The solar wind escapes from the solar corona and is accelerated, over a short distance, to its terminal velocity. The energy balance associated with this acceleration remains poorly understood. To quantify the global electrostatic contribution to the solar wind dynamics, we empirically estimate the ambipolar electric field (E ∥) and potential (Φr,∞). We analyze electron velocity distribution functions (VDFs) measured in the near-Sun solar wind between 20.3 R S and 85.3 R S by the Parker Solar Probe. We test the predictions of two different solar wind models. Close to the Sun, the VDFs exhibit a suprathermal electron deficit in the sunward, magnetic-field-aligned part of phase space. We argue that the sunward deficit is a remnant of the electron cutoff predicted by collisionless exospheric models. This cutoff energy is directly linked to Φr,∞. Competing effects of E ∥ and Coulomb collisions in the solar wind are addressed by the Steady Electron Runaway Model (SERM). In this model, electron phase space is separated into collisionally overdamped and underdamped regions. We assume that this boundary velocity at small pitch angles coincides with the strahl break-point energy, which allows us to calculate E ∥. The obtained Φr,∞ and E ∥ agree well with theoretical expectations. They decrease with radial distance as power-law functions with indices α Φ = −0.66 and α E = −1.69. We finally estimate the velocity gained by protons from electrostatic acceleration, which equals 77% calculated from the exospheric models, and 44% from the SERM model.

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Enhanced proton parallel temperature inside patches of switchbacks in the inner heliosphere

Cited by 55 publications

References 59 publications

Discontinuity analysis of the leading switchback transition regions

Discontinuity analysis of the leading switchback transition regions

The near-Sun streamer belt solar wind: turbulence and solar wind acceleration

Ambipolar Electric Field and Potential in the Solar Wind Estimated from Electron Velocity Distribution Functions

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