Magnetic switchbacks are magnetic field reversals in the radial component of magnetic field Br, accompanied by proton bulk flow enhancements. The switchback generation mechanism(s) is still unknown, though their evolution is believed to contribute to solar wind heating. Our exhaustive investigation of Parker Solar Probe (PSP) encounters 1-14 reveals clear absence of Br reversals in the solar corona. Our results indicate that (1) magnetic switchbacks must be formed locally in the solar wind at or above the Alfven critical surface, and that (2) interchange reconnection in the solar corona is likely necessary for creating the conditions for switchbacks to form in the solar wind.
Physical Sciences - Article
No Switchbacks Are Observed in the Solar Corona
https://doi.org/10.21203/rs.3.rs-3405280/v1
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NASA's flagship Parker Solar Probe (PSP) mission discovered magnetic switchbacks in the young solar wind (Bale et al. 2019; Kasper et al. 2019). Magnetic switchbacks are characterized by a radial magnetic field reversals Br. Other signatures include supra-thermal electrons (Kahler et al. 1996), the differential streaming of alpha particles (Yamauchi et al. 2004) and proton beams (Neugebauer and Goldstein 2013), and the directionality of Alfvén waves (Balogh et al. 1999). The observations indicate that switchbacks correspond to local folds in the magnetic field rather than changes in the magnetic connectivity to solar source regions.
Theories concerning the generation of switchbacks are divided into two main categories: (1) “ex situ" transient taking place in the solar corona where proton bulk flow vp < vA, where vA is the local Alfven velocity, including interchange reconnection (Fisk and Kasper 2020; Drake et al, 2021; Zank et al., 2021), and (2) “in situ" processes occurring locally within the solar wind, including those due to flow shear (e.g., Toth et al., 2023) and turbulence (e.g., Ruffolo et al., 2020), as the solar wind propagates radially outward.
Akhavan-Tafti et al. (2021; 2022) concluded that switchbacks may be generated in the solar corona and that their evolution may contribute to solar wind acceleration or heating. Kasper et al. (2021) reported that PSP, for the first time, crossed the Alfven critical surface RA and entered the solar corona. In this study, we utilize our automated algorithm to detect all Br reversals in PSP encounters 1-14. Of more than 4,800 Br reversals and 9 sub-Alfvenic flow encounters, only few are observed in Alfvenic solar wind vp ~ vA, and none in the sub-Alfvenic solar wind –solar corona vp < vA. This is strong evidence that switchbacks are most likely formed via “in-situ" mechanisms, locally in the solar wind, at or beyond the Alfven critical surface.
The data utilized in this work is provided by the FIELDS instrument suite (Bale et al., 2016) and the Solar Wind Electrons, Alphas, and Protons (SWEAP) instrument suite (Kasper et al., 2016) aboard PSP. In this study, the data analysis relies on a sampling frequency of four samples per cycle (0.874 s). The SWEAP suite encompasses a trio of essential components: the Solar Probe Cup (SPC; Case et al., 2020), the Solar Probe Analyzer for Electrons (SPAN-E; Whittlesey et al., 2020), and the Solar Probe Analyzer for Ions (SPAN-I; Livi et al., 2022).
In this study, our well-established algorithm (Akhavan-Taft et al., 2021; Akhavan-Tafti et al., 2022; and Huang et al., 2023) to identify Br reversals, as well as switchbacks, characterized based on: 1) five magnetically-distinguishable regions, chronologically separated as the leading pristine solar wind, leading transition region, spike region, trailing transition region, and trailing solar wind region; (2) enhancement in radial proton velocity, corresponding to a magnetically uni-polar spike region; and (3) uniform electron pitch angle distribution (e-PAD) to rule out heliospheric current sheet (HCS) crossings.
PSP crossed the Alfven critical surface (RA) up to 3 times over the 14-day period near the encounter 12 perihelion, shown in Figure 1a. The panels include: (a) the magnitude B (left, black solid line), and the radial component Br of magnetic field (right, cyan solid line), (b) the radial component of proton bulk flow Vr (left, black solid line), and the local Alfven velocity (right, cyan solid line), (c) proton density, (d) proton beta β, defined as the ratio of thermal to magnetic pressures, (e) Alfven Mach number MA, and (f) normalized electron pitch angle distribution (e-PAD; energy channel=314.45 eV). The RA crossings are marked by orange shadings.
Panel (a) shows that B and Br both increased toward perihelion, consistent with the inverse square dependence on heliocentric distance (Badman et al., 2021). However, near the perihelion, on June 2, 2022, PSP crossed the heliospheric current sheet (HCS) as evidenced by reduced B, reversal in Br polarity, enhanced β, and reversal in the sign of the electron heat flux. The numbers of magnetic switchbacks and nonlinear Alfven waves increases at the perihelion and near the HCS crossing, as evidenced by reversals and deflections in Br with no corresponding change in B. However, such Br reversals disappear in the shaded regions, once PSP dips below RA.
Figure 1b shows a switchback event observed on the Alfven critical surface, MA~1. The switchback has five magnetically distinct regions, identified with different shares, including: leading and trailing pristine solar wind (blue), leading and trailing transition regions (orange), and the spike region (red) (Akhavan-Tafti et al., 2021; 2022).
During the first 14 encounters, PSP reported up to 9 RA crossings, defined as intervals wherein MA<1 for more than 15 minutes in duration, continuously. Table 1 summarizes the RA crossings. Next, we applied our automated algorithm on PSP magnetic field data to identify all Br reversals. More than 4,800 Br reversal events were identified, about 10% of which were further characterized as switchbacks.
Table 1: List of PSP RA crossings and Br reversal statistics.
Br reversals were then categorized based on their bulk flow properties relative to the local Alfven speed. Figure 2a shows encounter 13, wherein |B| enhances at the perihelion, coinciding with HCS crossings, as evidenced by the sharp rotations in Br and e-PAD. As shown, the intervals leading up to and moving away from perihelion are marked with enhanced number of Br reversals, and in some cases, switchbacks. Figure 2b further zooms into the interval during which PSP enters the solar corona, marked by MA<1 consistently, for more than 3 hours. It is revealed that, while Br deflections, defined as sharp reduction in Br without a change in |B|, are frequently observed, no Br reversals can be found below RA. Our results are in contrast with Telloni et al. (2022) which reported the existence of switchbacks in the solar corona using remote sensing observations aboard Solar Orbiter.
Switchbacks are believed to be generated by interchange in reconnection-in the solar corona (Fisk and Kasper, 2020; Drake et al., 2021), thus requiring 1) that the number of switchbacks increase with reducing distance from the Sun (e.g., Akhavan-Tafti et al., 2022), and 2) switchbacks to be dominantly rotational-type discontinuities (RD) having relatively-constant |B| and Np across all five regions, as well as enhanced Vr and radial proton temperature in the spike region (Akhavan-Tafti et al., 2021, 2022). While those have been shown near the Sun, our results clearly show that switchbacks are absent in the solar corona, where MA<1. Instead, switchbacks are observed at and above the Alfven critical surface, where solar wind becomes super-Alfvenic. This indicates that magnetic reconnection in the solar corona is necessary, but not sufficient, to generate switchbacks. Additionally, we report that magnetic deflections, DBr/Br~1 and D|B|~0, are observed ubiquitously in the solar corona.
Therefore, we hypothesize, as shown in Figure 3, that magnetic reconnection in the solar corona (1) leads to the generation of waves (2) in the reconnection outflow region, causing the kinking or deflection of open flux tubes. Only under specific conditions, the reconnection-induced waves, and thus the open flux kinks, grow while buoying toward the Alfven critical surface, as evidenced by enhanced magnetic deflections in the solar corona. Reconnection characteristics (rate, etc.) will determine the conditions within which the induced waves evolve. Hence, it is predicted that not all induced waves in the reconnection outflow region will survive the journey toward and past the Alfven critical surface (Squire et al., 2022). However, once at or above the Alfven critical surface (3), plasma instabilities, including flow shear across the reconnection outflow boundaries –referred to as separatrices (characterized by their enhanced field-aligned currents and diamagnetic suppression of |B|), are expected to develop. This, in turn, will likely result in accelerated growth of the induced waves (Toth et al., 2023) and the formation of switchbacks. While propagating in the solar wind (4), RD-type switchbacks will straighten out, as evidenced by increasingly tangential-type discontinuity (TD) switchbacks with lower magnetic pressure with increasing heliocentric distance (Akhavan-Tafti et al., 2022). It is therefore expected that the switchback evolution will result in solar wind acceleration and/or (preferential) heating (Akhavan-Tafti et al., 2019b).
In summary, it is discovered that switchbacks cease to exist below the Alfven critical surface. This evidence confirms that switchbacks are formed in the solar wind at or beyond the Alfven critical surface. Ex-situ mechanisms in the solar corona, such as interchange reconnection, are likely necessary for creating the conditions for the formation of switchbacks in the solar wind.
Acknowledgement:
The authors are grateful for the dedicated efforts of the PSP team. This work was supported by NASA contract Nos. NNN06AA01C, 80NSSC20K1847, 80NSSC20K1014, and 80NSSC21K1662. The authors would like to acknowledge the insightful conversations at the switchback workshop at the International Space Science Institute (ISSI), Bern, Switzerland in Fall 2023, which informed our conclusion(s).
- Akhavan-Tafti, M., Kasper, J., Huang, J., & Thomas, L. (2022). Magnetic switchbacks heat the solar corona. The Astrophysical Journal Letters, 937(2), L39.
- Akhavan-Tafti, M., Kasper, J., Huang, J., & Bale, S. (2021). Discontinuity analysis of the leading switchback transition regions. Astronomy & Astrophysics, 650, A4.
- Badman, S. T., Bale, S. D., Rouillard, A. P., Bowen, T. A., Bonnell, J. W., Goetz, K., ... & Pulupa, M. (2021). Measurement of the open magnetic flux in the inner heliosphere down to 0.13 AU. Astronomy & Astrophysics, 650, A18.
- Bale, S. D., Badman, S. T., Bonnell, J. W., Bowen, T. A., Burgess, D., Case, A. W., ... & Wygant, J. R. (2019). Highly structured slow solar wind emerging from an equatorial coronal hole. Nature, 576(7786), 237-242.
- Bale, S. D., Goetz, K., Harvey, P. R., Turin, P., Bonnell, J. W., Dudok de Wit, T., ... & Wygant, J. R. (2016). The FIELDS instrument suite for Solar Probe Plus: measuring the coronal plasma and magnetic field, plasma waves and turbulence, and radio signatures of solar transients. Space science reviews, 204, 49-82.
- Balogh, A., Forsyth, R. J., Lucek, E. A., Horbury, T. S., & Smith, E. J. (1999). Heliospheric magnetic field polarity inversions at high heliographic latitudes. Geophysical research letters, 26(6), 631-634.
- Case, A. W., Kasper, J. C., Stevens, M. L., Korreck, K. E., Paulson, K., Daigneau, P., ... & Martinović, M. M. (2020). The solar probe cup on the Parker Solar Probe. The Astrophysical Journal Supplement Series, 246(2), 43.
- Drake, J. F., Agapitov, O., Swisdak, M., Badman, S. T., Bale, S. D., Horbury, T. S., ... & Velli, M. (2021). Switchbacks as signatures of magnetic flux ropes generated by interchange reconnection in the corona. Astronomy & Astrophysics, 650, A2.
- Fisk, L. A., & Kasper, J. C. (2020). Global circulation of the open magnetic flux of the Sun. The Astrophysical Journal Letters, 894(1), L4.
- Huang, J., Kasper, J. C., Fisk, L. A., Larson, D. E., McManus, M. D., Chen, C. H. K., ... & Halekas, J. S. (2023). The Structure and Origin of Switchbacks: Parker Solar Probe Observations. arXiv preprint arXiv:2301.10374.
- Kahler, S. W., Crocker, N. U., & Gosling, J. T. (1996). The topology of intrasector reversals of the interplanetary magnetic field. Journal of Geophysical Research: Space Physics, 101(A11), 24373-24382.
- Kasper, J. C., Klein, K. G., Lichko, E., Huang, J., Chen, C. H. K., Badman, S. T., ... & Zank, G. P. (2021). Parker solar probe enters the magnetically dominated solar corona. Physical review letters, 127(25), 255101.
- Kasper, J. C., Abiad, R., Austin, G., Balat-Pichelin, M., Bale, S. D., Belcher, J. W., ... & Zank, G. (2016). Solar wind electrons alphas and protons (SWEAP) investigation: Design of the solar wind and coronal plasma instrument suite for solar probe plus. Space Science Reviews, 204, 131-186.
- Livi, R., Larson, D. E., Kasper, J. C., Abiad, R., Case, A. W., Klein, K. G., ... & McManus, M. D. (2022). The Solar Probe ANalyzer—Ions on the Parker Solar Probe. The Astrophysical Journal, 938(2), 138.
- Neugebauer M. and Goldstein B. E. 2013 AIP Conf. Proc. 1539, Proc. of the Thirteenth International Solar Wind Conf. (Melville, NY: AIP) 46
- Ruffolo, D., Matthaeus, W. H., Chhiber, R., Usmanov, A. V., Yang, Y., Bandyopadhyay, R., ... & Kasper, J. C. (2020). Shear-driven transition to isotropically turbulent solar wind outside the Alfvén critical zone. The Astrophysical Journal, 902(2), 94.
- Squire, J., Meyrand, R., Kunz, M. W., Arzamasskiy, L., Schekochihin, A. A., & Quataert, E. (2022). High-frequency heating of the solar wind triggered by low-frequency turbulence. Nature Astronomy, 6(6), 715-723.
- Telloni, D., Zank, G. P., Stangalini, M., Downs, C., Liang, H., Nakanotani, M., ... & Zhukov, A. N. (2022). Observation of a magnetic switchback in the solar corona. The Astrophysical journal letters, 936(2), L25.
- Toth, G., Velli, M., & van der Holst, B. (2023). Formation of Magnetic Switchbacks Observed by Parker Solar Probe. arXiv preprint arXiv:2301.02572.
- Whittlesey, P. L., Larson, D. E., Kasper, J. C., Halekas, J., Abatcha, M., Abiad, R., ... & Verniero, J. L. (2020). The Solar Probe ANalyzers—electrons on the parker solar probe. The Astrophysical Journal Supplement Series, 246(2), 74.
- Yamauchi, Y., Suess, S. T., Steinberg, J. T., & Sakurai, T. (2004). Differential velocity between solar wind protons and alpha particles in pressure balance structures. Journal of Geophysical Research: Space Physics, 109(A3).
- Zank, G. P., Nakanotani, M., Zhao, L. L., Adhikari, L., & Kasper, J. (2020). The origin of switchbacks in the solar corona: Linear theory. The Astrophysical Journal, 903(1), 1.
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