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  1. Abstract The origin of switchbacks in the solar wind is discussed in two classes of theory that differ in the location of the source being either near the transition region near the Sun or in the solar wind itself. The two classes of theory differ in their predictions of the switchback rate (the number of switchbacks observed per hour) as a function of distance from the Sun. To distinguish between these theories, one-hour averages of Parker Solar Probe data were averaged over five orbits to find the following: (1) The hourly averaged switchback rate was independent of distance from themore »Sun. (2) The average switchback rate increased with solar wind speed. (3) The switchback size perpendicular to the flow increased as R , the distance from the Sun, while the radial size increased as R 2 , resulting in an increasing switchback aspect ratio with distance from the Sun. (4) The hourly averaged and maximum switchback rotation angles did not depend on the solar wind speed or distance from the Sun. These results are consistent with switchback formation in the transition region because their increase of tangential size with radius compensates for the radial falloff of their equatorial density to produce switchback rates that are independent of radial distance. This constant switchback rate is inconsistent with an in situ source. The switchback size and aspect ratio, but not their hourly average or maximum rotation angle, increased with radial distance to 100 solar radii. Additionally, quiet intervals between switchback patches occurred at the lowest solar wind speeds.« less
    Free, publicly-accessible full text available September 1, 2022
  2. Abstract The SWEAP instrument suite on Parker Solar Probe (PSP) has detected numerous proton beams associated with coherent, circularly polarized, ion-scale waves observed by PSP’s FIELDS instrument suite. Measurements during PSP Encounters 4−8 revealed pronounced complex shapes in the proton velocity distribution functions (VDFs), in which the tip of the beam undergoes strong perpendicular diffusion, resulting in VDF level contours that resemble a “hammerhead.” We refer to these proton beams, with their attendant “hammerhead” features, as the ion strahl. We present an example of these observations occurring simultaneously with a 7 hr ion-scale wave storm and show results from amore »preliminary attempt at quantifying the occurrence of ion-strahl broadening through three-component ion VDF fitting. We also provide a possible explanation of the ion perpendicular scattering based on quasilinear theory and the resonant scattering of beam ions by parallel-propagating, right circularly polarized, fast magnetosonic/whistler waves.« less
    Free, publicly-accessible full text available January 1, 2023
  3. Context. Aims. We systematically search for magnetic flux rope structures in the solar wind to within the closest distance to the Sun of ~0.13 AU, using data from the third and fourth orbits of the Parker Solar Probe. Methods. We extended our previous magnetic helicity-based technique of identifying magnetic flux rope structures. The method was improved upon to incorporate the azimuthal flow, which becomes larger as the spacecraft approaches the Sun. Results. A total of 21 and 34 magnetic flux ropes are identified during the third (21-day period) and fourth (17-day period) orbits of the Parker Solar Probe, respectively. Wemore »provide a statistical analysis of the identified structures, including their relation to the streamer belt and heliospheric current sheet crossing.« less
  4. During three of its first five orbits around the Sun, Parker Solar Probe (PSP) crossed the large-scale heliospheric current sheet (HCS) multiple times and provided unprecedented detailed plasma and field observations of the near-Sun HCS. We report the common detections by PSP of reconnection exhaust signatures in the HCS at heliocentric distances of 29.5–107 solar radii during encounters 1, 4, and 5. Both sunward and antisunward-directed reconnection exhausts were observed. In the sunward reconnection exhausts, PSP detected counterstreaming strahl electrons, indicating that HCS reconnection resulted in the formation of closed magnetic field lines with both ends connected to the Sun.more »In the antisunward exhausts, PSP observed dropouts of strahl electrons, consistent with the reconnected HCS field lines being disconnected from the Sun. The common detection of reconnection in the HCS suggests that reconnection is almost always active in the HCS near the Sun. Furthermore, the occurrence of multiple long-duration partial crossings of the HCS suggests that HCS reconnection could produce chains of large bulges with spatial dimensions of up to several solar radii. The finding of the prevalence of reconnection in the HCS is somewhat surprising since PSP has revealed that the HCS is much thicker than the kinetic scales required for reconnection onset. The observations are also in stark contrast with the apparent absence of reconnection in most of the small-scale and much more intense current sheets encountered near perihelia, many of which are associated with “switchbacks”. Thus, the PSP findings suggest that large-scale dynamics, either locally in the solar wind or within the coronal source of the HCS (at the tip of helmet streamers), plays a critical role in triggering reconnection onset.« less