Solar Wind Anomalies at 1 au and Their Associations with Large-scale Structures

We study solar wind anomalies and their associations with solar wind structures using the STEREO solar wind and suprathermal electron (STE) data from IMPACT and PLASTIC. We define solar wind anomalies as temporary and local excursions from the average solar wind state, regardless of their origins, for six anomalies: sunward strahls, counterstreaming suprathermal electrons, suprathermal electron depletions, nearly radial magnetic field episodes, anomalously low proton temperatures, and anomalously low proton beta. We first establish the solar wind synoptic contour displays, which show the expected variations in solar wind structure during the solar cycle: recurrent corotating heliospheric magnetic field (HMF) and stream structures are dominant during solar quiet times around the solar minimum (2008 December) preceding cycle 24, while complex structures characterize solar active times around the solar maximum (2014 April). During the declining phase of the cycle (2016–2019), the stream structures remain complex, but the HMF sectors show the structures of the solar minimum. We then systematically study the six anomalies by analyzing the STE data using automated procedures. All anomalies present some degree of dependence on the large-scale solar wind structure, especially around the solar minimum, implying that the solar wind structure plays a role in either the generation or transportation of these anomalies. One common feature of all of the anomalies is that the distributions of the durations of the anomalous episodes all peak at the 1 hr data resolution, but monotonically decrease over longer durations, which may arguably imply that solar anomalies occur on a continuum of temporal and spatial scales.

Parker Solar Probe observations of suprathermal electron flux enhancements originating from Coronal Hole boundaries

ABSTRACT Reconnection between pairs of solar magnetic flux elements, one open and the other a closed loop, is theorized to be a crucial process for both maintaining the structure of the corona and producing the solar wind. This ‘interchange reconnection’ is expected to be particularly active at the open-closed boundaries of coronal holes (CHs). Previous analysis of solar wind data at 1 au indicated that peaks in the flux of suprathermal electrons at slow–fast stream interfaces may arise from magnetic connection to the CH boundary, rather than dynamic effects such as compression. Further, offsets between the peak and stream interface locations are suggested to be the result of interchange reconnection at the source. As a preliminary test of these suggestions, we analyse two solar wind streams observed during the first Parker Solar Probe (PSP) perihelion encounter, each associated with equatorial CH boundaries (one leading and one trailing with respect to rotation). Each stream features a peak in suprathermal electron flux, the locations and associated plasma properties of which are indicative of a solar origin, in agreement with previous suggestions from 1 au observations. Discrepancies between locations of the flux peaks and other features suggest that these peaks may too be shifted by source region interchange reconnection. Our interpretation of each event is compatible with a global pattern of open flux transport, although random footpoint motions or other explanations remain feasible. These exploratory results highlight future opportunities for statistical studies regarding interchange reconnection and flux transport at CH boundaries with modern near-Sun missions.

Suprathermal electron strahls. From small scale modeling to heliospheric implications

<p>Recent advances in kinetic modeling reveal essential properties of the suprathermal populations opening perspectives for a realistic interpretation of their implications. Of particular importance are the suprathermal electron strahl (or beaming) populations, guided by the heliospheric magnetic field as kinetic-scale traces of the continuous solar outflows. We outline the main implications of the strahls by connecting their signatures in the velocity distributions with macroscopic properties of the solar wind, and processes conditioning their relaxation via coherent or non-coherent radiative emissions. The electron strahls may also help understanding major changes in the magnetic field topology in the outer corona, as shown by the most recent data from Solar Parker Probe, and during energetic (transient) events like coronal mass ejections, implying or not reconnection, but leading to strong interaction regions and shocks. </p>