[Paper Review] Magnetic Switchback Occurrence Rates in the Inner Heliosphere: Parker Solar Probe and 1 au

Francesco Pecora, W. H. Matthaeus+5 more|arXiv (Cornell University)|Feb 8, 2022

Solar and Space Plasma Dynamics47 references33 citations

TL;DR

This study analyzes magnetic switchback occurrence rates using 8 orbits of Parker Solar Probe (PSP) data, defining switchbacks as magnetic field reversals exceeding 90° relative to local averages. It finds a sharp drop in occurrence rate near 0.2 au (40 R⊙), followed by a gentle increase outward, suggesting switchback generation is linked to dynamics near the Alfvén critical radius, with robust results confirmed across varying data cadences and averaging windows.

ABSTRACT

The subject of switchbacks, defined either as large angular deflections or polarity reversals of the magnetic field, has generated substantial interest in the space physics community since the launch of Parker Solar Probe (PSP) in 2018. Previous studies have characterized switchbacks in several different ways, and have been restricted to data available from the first few orbits. Here, we analyze the frequency of occurrence of switchbacks per unit distance for the first full eight orbits of PSP. In this work, are considered switchback only the events that reverse the sign of magnetic field relative to a regional average. A significant finding is that the rate of occurrence falls off sharply approaching the sun near 0.2 au (40 $R_\odot$), and rises gently from 0.2 au outward. The analysis is varied for different magnetic field cadences and for different local averages of the ambient field, confirming the robustness of the results. We discuss implications for the mechanisms of switchback generation. A publicly available database has been created with the identified reversals.

Motivation & Objective

• To quantify magnetic switchback occurrence rates across the inner heliosphere using PSP's first 8 orbits.
• To determine whether switchback frequency varies systematically with heliocentric distance.
• To assess the robustness of results across different magnetic field cadences and local field averaging methods.
• To explore implications for switchback generation mechanisms, particularly near the Alfvén critical radius.
• To create and release a publicly accessible database of identified switchbacks for community use.

Proposed method

• Data from PSP's FIELDS MAG instrument (10s and 60s cadence) and SWEAP SPC for solar wind speed were used over 6-hour intervals.
• Switchbacks were identified using the normalized deflection parameter z = 1/2(1 - cos(α)), where α is the angle between the instantaneous field and a 6-hour local average.
• Events with z ≥ 0.5 (deflection >90°) were classified as switchbacks.
• Waiting times and durations were computed, with outliers (WT > μ + 3σ) excluded to avoid spurious values.
• Analyses were repeated with 3-hour intervals and 15% missing data threshold to test robustness.
• Results were compared across PSP and Wind data at 1 au, and scaled to correlation length λc for turbulence-based comparison.

Experimental results

Research questions

• RQ1 How does the radial distribution of switchback occurrence rates vary from 0.2 au to 1 au?
• RQ2 Is the observed switchback rate profile robust to changes in magnetic field cadence and local field averaging window?
• RQ3 Does the occurrence rate of switchbacks correlate with turbulence correlation scales in the inner heliosphere?
• RQ4 What does the radial trend in switchback rates imply about their generation mechanism?
• RQ5 How do switchback waiting times and durations relate to solar wind turbulence characteristics?

Key findings

• The switchback occurrence rate drops sharply near 0.2 au (40 R⊙), with a naive extrapolation suggesting zero rate at ~20 R⊙, coinciding with the Alfvén critical radius.
• The rate rises gently from 0.2 au outward, showing a smooth increase that extrapolates reasonably to Wind data at 1 au.
• The radial trend is robust across different magnetic field cadences (10s and 60s) and averaging windows (3h and 6h), confirming consistency.
• The distribution of switchback durations follows a power law with a slope near -2, indicating a broad range of event lengths.
• When scaled to the correlation length λc, the data show systematic ordering, with rates approaching zero near 0.1 au and smoothly increasing outward.
• The results support the hypothesis that switchbacks are generated by dynamics near the Alfvén critical radius, such as MHD mixing layer instabilities, rather than solely by lower-altitude processes.