Self-similar Outflows at the Source of the Fast Solar Wind: A Smoking Gun of Multiscale Impulsive Reconnection?

Abstract

We present results of a quantitative analysis of structured plasma outflows above a polar coronal hole observed by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) spacecraft. In a 6 hr interval of continuous high-cadence SDO/AIA images, we identified more than 2300 episodes of small-scale plasma flows in the polar corona. The mean upward flow speed measured by the surfing transform technique is estimated to be 122 ± 34 km s−1, which is comparable to the local sound speed. The typical recurrence period of the flow episodes is 10–30 minutes, and the mean duration and transverse size of each episode are about 3–5 minutes and 3–4 Mm, respectively. The largest identifiable episodes last for tens of minutes and reach widths up to 40 Mm. For the first time, we demonstrate that the polar coronal-hole outflows obey a family of power-law probability distributions characteristic of impulsive interchange magnetic reconnection. Turbulent photospheric driving may play a crucial role in releasing magnetically confined plasma onto open field. The estimated occurrence rate of the detected self-similar coronal outflows is sufficient for them to make a dominant contribution to the fast-wind mass and energy fluxes and to account for the wind’s small-scale structure.

How Small-scale Jetlike Solar Events from Miniature Flux Rope Eruptions Might Produce the Solar Wind

“…Recently, Uritsky et al (2023) have published results of a quantitative investigation of the occurrence rate of coronal outflows involving 2300 events. They look at the size scale of the features leaving the Sun above a polar coronal hole in AIA 171 Å images over a 6 hr period.…”

“…We can, however, crudely compare the Uritsky et al (2023) event numbers with ours, by estimating the instantaneous number of their events that would be present over the entire Sun based on what they observe in their limited FOV. From Figure 5(b) of Uritsky et al (2023), most of their 2300 events have a transverse size of between about 500 and 3000 km, based on the half-width of the transverse-size distribution plotted in that figure; we can use these values as range-bar lower and upper limits, and we take the value of 1750 km for our Figure 2 abscissa. For our ordinate, we have to estimate/ guesstimate the number of Uritsky et al (2023) events that would occur over the entire Sun at a given time.…”

“…From Figure 5(b) of Uritsky et al (2023), most of their 2300 events have a transverse size of between about 500 and 3000 km, based on the half-width of the transverse-size distribution plotted in that figure; we can use these values as range-bar lower and upper limits, and we take the value of 1750 km for our Figure 2 abscissa. For our ordinate, we have to estimate/ guesstimate the number of Uritsky et al (2023) events that would occur over the entire Sun at a given time. They observed 2300 events in 6 hr, from a portion of the solar limb that extends for 1/8 of the solar circumference.…”

“…They observed 2300 events in 6 hr, from a portion of the solar limb that extends for 1/8 of the solar circumference. A substantial unknown, however, is from how far along the Earth-Sun line of sight the events observed above the limb by Uritsky et al (2023) originate. That is, among the 2300 events observed in the plane of sky at the limb, some will originate from exactly at the limb, while others will originate from somewhat inside the The horizontal axis shows the size (horizontal extent) of the erupting feature, and the vertical axis shows the number of events that are erupting on the Sun at any given instant in time.…”

Are Coronal Loops Projection Effects?

“…We found that this method is efficient for the filling factors of up to ∼0.5, after which the computational cost of the nonoverlapping constraint shows an explosive growth. Many previous observations suggest that the morphology of the thin solar corona involves light-emitting structures organized across wide ranges of spatial scales (see, e.g., Gomez et al 1993;Uritsky et al 2013Uritsky et al , 2023Uritsky & Davila 2014;Aschwanden et al 2016;Mason & Uritsky 2022 and references therein). In closed-field coronal regions, this tendency manifests itself in the spatial clustering of smallscale loops and loop strands into multiscale loop bundles, possibly due to transient localized magnetic reconnection events resulting in clusters of hot flux tubes of different sizes (Knizhnik et al 2018(Knizhnik et al , 2020Klimchuk et al 2023).…”