Picoflare jets in the coronal holes and their link to the solar wind

(Solar Orbiter Nugget #56 by L. P. Chitta^1*, Z. Huang¹, R. D’Amicis², D. Calchetti¹, A. N. Zhukov^3,4, E. Kraaikamp³, C. Verbeeck³, R. Aznar Cuadrado¹, J. Hirzberger¹, D. Berghmans³, T. S. Horbury⁵, S. K. Solanki¹, C. J. Owen⁶, L. Harra^7,8, H. Peter^1,9, U. Schühle¹, L. Teriaca¹, P. Louarn¹⁰, S. Livi¹¹, A. S. Giunta¹², D. M. Hassler¹³, Y.-M. Wang¹⁴)

1. Abstract

Charged particles continuously stream out from the million Kelvin hot corona as the solar wind. High-resolution extreme ultraviolet observations from the Solar Orbiter are providing new insights into the generation process of the solar wind. In particular, high-speed jets are observed to be widespread in darker interiors of coronal holes where the magnetic field stretches into the heliosphere. These jets could be the intermittent but prevalent seeds of the solar wind. Here we present our latest study from the Solar Orbiter mission and discuss their implications for the magnetic processes governing the solar wind origin.

2. Introduction

Similar to the million Kelvin hot corona, the solar wind is another important characteristic of the Sun. The solar wind is formed by streams of charged particles that escape into interplanetary space, inflating heliosphere to an extent of ~ 120 astronomical units, from the Sun. Coronal holes, formed by the magnetic field that opens into the heliosphere, that appear darker in the extreme ultraviolet (EUV) and X-ray wavelengths, are important source regions of this solar wind. 

Observations of coronal holes over the last two decades have revealed numerous small-scale apparent collimated flows through the solar atmosphere. These include solar chromospheric spicules (De Pontieu et al. 2007; McIntosh et al. 2011), transition region network jets and EUV propagating intensity disturbances, termed jetlets, in plume regions (Raouafi & Stenborg 2014; Kumar et al. 2022; Raouafi et al. 2023). These features are likely driven by magnetic interchange reconnection operating on small spatial scales of a few 100 km in the solar atmosphere (Tian et al. 2014; Tripathi et al. 2021; Kumar et al. 2022; Raouafi et al. 2023; Chitta et al. 2023b), triggered by rapidly changing photospheric magnetic field one timescales less than 5 minutes (Chitta et al. 2023a). The role of these small jet features and their direct link to the solar wind is a topic of active debate.
 

3. New observations from the Solar Orbiter mission

Unprecedented EUV observations from the high-resolution imager of the Extreme Ultraviolet Imager (EUI/HRI_EUV) instrument (Rochus et al. 2020) on the Solar Orbiter spacecraft (Müller et al. 2020), have revealed a small-scale jet activity in the darker interiors of a polar coronal hole (Chitta et al. 2023b). They have high propagating speeds on the order of 100 km s⁻¹, and possess kinetic energy content in the range of picoflares (10²¹–10²⁴ erg). Their widespread existence in the observed coronal hole could imply that the origin of the solar wind is highly intermittent. 

In a follow-up study (Chitta et al. 2025), we combined remote-sensing and in-situ measurements from Solar Orbiter to address the prevalence of these picoflare jets in other coronal holes and their potential link to the solar wind structures. In 2022 October and in 2023 April, Solar Orbiter observed two latitude coronal holes. The Solar Orbiter magnetometer (MAG) (Horbury et al. 2020) and Solar Wind Analyzer (Owen et al. 2020) on board Solar Orbiter sampled highly Alfvénic solar wind streams during the periods considered for analysis. In October 2022, the wind was fast with speed > 500 km s⁻¹ (Fig. 1) and in April 2023, the streams were slower with speeds < 400 km s⁻¹. 
 

Fig. 1. Fast wind streams emerging from a low-latitude coronal hole. Time series of the solar wind radial velocity, proton temperature and the radial magnetic field are shown in panels (A) to (C). A measure of the solar wind Alfvénicity, covering the same time period, is plotted in panel (D). The colored vertical bars identify the timestamps for which the magnetic footpoints of the solar wind at the source region were determined. The sub-spacecraft points (plus symbols) and the magnetic connectivity of the spacecraft (circles) are overlaid on the photospheric magnetic field and the coronal Carrington maps in panels (E) and (F). The white solid rectangle in the lower panels outlines the partial field of view of the low-latitude coronal hole observed by the EUI/HRIEUV. The dashed rectangle is the region where we evaluated the coronal hole magnetic field expansion factor in this particular case. Figure adapted from Chitta et al. (2025).

In Fig. 1A–C we show the in-situ observations of the fast solar wind streams intercepted by the Solar Orbiter spacecraft in October 2022. The correlation between the solar wind magnetic field and velocity components, indicative of the Alfvénicity (CVB), is very high in this case (Fig. 1D). Based on a two-step ballistic back-mapping technique with the aid of potential field source surface magnetic modelling, the source region of these fast wind streams was identified to be a low-latitude coronal hole (Fig. 1E–F).

In the vicinity of the magnetic footpoints of the spacecraft, we identified persistent picoflare jet activity emerging from the dark areas of the observed coronal hole (Fig. 2A displays a section of the coronal hole observed by HRI_EUV). The jets are closely linked to the underlying magnetic network lanes (examples shown in the time-distance maps displayed in Fig. 2B–H). In a similar way, we traced the source region of the Alfvénic slow solar wind streams during April 2023 to a small low-latitude coronal hole, exhibiting picoflare jet activity. 
 

Fig. 2.  Picoflare jets from the interior of a low-latitude coronal hole. (A) Contrast-enhanced HRIEUV image of the coronal hole observed by Solar Orbiter on 2022 October 13. Photospheric magnetic field patches as recorded by the Polarimetric and Helioseismic Imager instrument on the Solar Orbiter spacecraft (Solanki et al. 2020) are outlined in blue (negative polarity) and red (positive polarity) contours. Slanted rectangles S1--S7 are the slits that were used to obtain the respective time-distance maps displayed in panels B to H. Some picoflare jets and their propagating speeds are marked by slanted lines in these maps. Figure adapted from Chitta et al. (2025).

Despite their close connection to the same type of widespread coronal hole picoflare jet activity, the wind streams in October 2022 and April 2023 have distinct speeds. This distinction likely results from the distinction in the magnetic properties of the coronal holes themselves. We quantified this by computing the coronal hole expansion factor, a measure that characterizes the rate of expansion of a magnetic flux tube from the photosphere to the source surface (placed at 2.5 solar radii). The expansion factor of the coronal hole that was identified to be the source region of the October 2022 fast wind streams is generally lower than that of the April 2023 case (Fig. 3). This implies that owing to a larger magnetic expansion factor, the energy dissipation with height steeply falls off over the smaller coronal hole of April 2023. Most of the energy will be dissipated inside the sonic point in this case (Wang 2020). This further implies a reduction in the solar wind speed compared to the fast wind case of October 2022.

Fig. 3.  Coronal hole expansion factor. Histograms of the magnetic field expansion factor for the case of fast wind (grey) and the Alfvénic slow wind (black). Figure adapted from Chitta et al. (2025).

Overall, a comprehensive picture emerges that the small-scale jets from the interiors of coronal holes power the Alfvénic (fast and slow) solar wind streams. We suggest that the expansion factor of the coronal hole ultimately governs the wind speed despite it having been linked to the similar type of coronal hole jet activity at its base.

Affiliations

(1) Max-Planck-Institut für Sonnensystemforschung, 37077 Göttingen, Germany
(2) National Institute for Astrophysics (INAF), Institute for Space Astrophysics and Planetology (IAPS), Via Fosso del Cavaliere, 100, 00133 Rome, Italy
(3) Solar-Terrestrial Centre of Excellence, Solar Influences Data analysis Centre, Royal Observatory of Belgium, 1180 Brussels, Belgium
(4) Skobeltsyn Institute of Nuclear Physics, Moscow State University, 119991 Moscow, Russia 
(5) Department of Physics, Imperial College London, SW7 2AZ London, UK
(6) Mullard Space Science Laboratory, Holmbury St Mary, RH5 6NT, UK
(7) Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, 7260 Davos Dorf, Switzerland
(8) Die Eidgenössische Technische Hochschule Zürich, 8093 Zürich, Switzerland
(9) Institut für Sonnenphysik (KIS), Georges-Köhler-Allee 401a, 79110 Freiburg, Germany
(10) Institut de Recherche en Astrophysique et Planétologie, CNRS, Université de Toulouse, CNES, Toulouse, France
(11) Southwest Research Institute, San Antonio, TX 78238, USA
(12) RAL Space, UKRI STFC Rutherford Appleton Laboratory, Didcot OX11 0QX, UK
(13) Southwest Research Institute,  Boulder, CO 80302, USA
(14) Space Science Division, Naval Research Laboratory, Washington, DC 20375, USA

*To whom correspondence should be addressed; E-mail: chitta@mps.mpg.de.
 

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Acknowledgements

Solar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement Nos. 10103984 – project ORIGIN; 101097844 – project WINSUN). ZH conducted the work in this paper in the framework of the International Max Planck Research School (IMPRS) for Solar System Science at the Technical University of Braunschweig.