The evolution of inverted magnetic fields through the inner heliosphere

Abstract

Local inversions are often observed in the heliospheric magnetic field (HMF), but their origins and evolution are not yet fully understood. Parker Solar Probe has recently observed rapid, Alfvénic, HMF inversions in the inner heliosphere, known as ‘switchbacks’, which have been interpreted as the possible remnants of coronal jets. It has also been suggested that inverted HMF may be produced by near-Sun interchange reconnection; a key process in mechanisms proposed for slow solar wind release. These cases suggest that the source of inverted HMF is near the Sun, and it follows that these inversions would gradually decay and straighten as they propagate out through the heliosphere. Alternatively, HMF inversions could form during solar wind transit, through phenomena such velocity shears, draping over ejecta, or waves and turbulence. Such processes are expected to lead to a qualitatively radial evolution of inverted HMF structures. Using Helios measurements spanning 0.3–1 au, we examine the occurrence rate of inverted HMF, as well as other magnetic field morphologies, as a function of radial distance r, and find that it continually increases. This trend may be explained by inverted HMF observed between 0.3 and 1 au being primarily driven by one or more of the above in-transit processes, rather than created at the Sun. We make suggestions as to the relative importance of these different processes based on the evolution of the magnetic field properties associated with inverted HMF. We also explore alternative explanations outside of our suggested driving processes which may lead to the observed trend.

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

“…A range of other viable mechanisms to produce HMF reversals exist, including coronal jets (also related to reconnection, Raouafi et al 2016;Horbury et al 2018;Sterling & Moore 2020), stream shears (Landi et al 2005(Landi et al , 2006Owens et al 2018;Lockwood et al 2019), and solar wind turbulence (e.g., Squire et al 2020). In situ processes likely make a sizeable contribution to HMF reversals observed at 0.3 to 1 AU, since the occurrence of inverted HMF increases with solar distance over this range (Macneil et al 2020). A solar origin, such as jets or interchange, remains a viable explanation for the switchbacks observed by PSP far closer to the Sun.…”

Evolving solar wind flow properties of magnetic inversions observed by Helios

“…Alfvénic inversions were shown to exist in fast solar wind in the inner portion of the Helios orbit (around 0.3 AU) by Horbury et al (2018), and in high-latitude fast wind at distances >1 AU by Matteini et al (2013), but further insight on inversion properties over a continuous range of distances and in other solar wind conditions can still be extracted from the Helios data set. In this paper we extend this work and the work of Macneil et al (2020b); performing analysis of changes in proton velocity, density, and magnetic field magnitude as a function of magnetic field azimuthal deflection. We do so over the full time range of the Helios 1 data set, enabled by the magnetic sector information provided by the strahl.…”

“…Inverted HMF which is positive (negative) along the expected Parker spiral direction will have anti-parallel (parallel) strahl. This information, summarised in Figure 1 of Macneil et al (2020b), is used to tentatively classify all HMF samples which have a detectable mono-directional strahl as either uninverted or inverted.…”

“…A further result of Macneil et al (2020b) was that inversions did not exhibit a particular bias in the direction of deflection of the azimuthal magnetic field. For this reason, a driving process such as turbulence was favoured as an explanation for the increase in inverted HMF.…”

“…Stream shear will produce a structured velocity field at the inversion, while draping will involve compression. Macneil et al (2020b) performed a statistical study of inverted HMF occurrence using data from the Helios 1 spacecraft. They used the direction of the suprathermal beam of electrons known as the 'strahl', which predominantly travels away from the Sun along the HMF (Feldman et al 1975;Hammond et al 1996;Pierrard et al 2001) to discriminate between locally inverted HMF and true sector reversals.…”