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Picoflares in the Quiet Solar Corona: Solar Orbiter Observations Halfway to the Sun

O. Podladchikova, A. Warmuth, L. Harra, L. Dolla, C. Verbeeck, M. Mierla, L. Rodriguez, S. Parenti, M. K. Georgoulis, S. J. Hofmeister, N. Engler, M. J. West, A. M. Veronig, P. Antolin, S. Purkhart, D. M. Long, É. Buchlin, M. Haberreiter, A. N. Zhukov, H. Safari, A. F. Battaglia, E. Soubrié, V. Büchel, S. Gissot, A. De Groof, M. Gyo, J. P. Halain, B. Inhester, E. Kraaikamp, D. Müller, D. Pfiffner, P. Rochus, F. Schuller, P. J. Smith, W. Schmutz, K. Stegen

TL;DR

This study uses Solar Orbiter's HRIEUV observations at 0.556 AU to characterize quiet-Sun campfires as the picoflare class, with thermal energies spanning $10^{20}$–$10^{24}$ erg and occurrence rates that imply a substantial, previously unseen energy input to coronal heating. By applying multiple volume models to account for optically thin plasma geometry, the authors show robust power-law energy distributions with indices $oldsymbol{\alpha \,=\,2.32\pm0.35}$ (≥5σ) and $oldsymbol{\\alpha \,=\,2.74\pm0.23}$ (≥3σ), and demonstrate that picoflares occur ~60× more often than nanoflares observed near Earth, contributing about 1% of the quiet-Sun heating requirement. Stereoscopic height measurements place most events at 1–5 Mm above the photosphere, suggesting energy release via field-aligned current dissipation in low-β environments, complemented by rapid reconnection in some structures. The findings extend the flare energy continuum to smaller scales, support the nanoflare heating paradigm, and indicate that closer Solar Orbiter observations could reveal even more fundamental heating events near the 10^18 erg scale. The work highlights the importance of combining near-Sun observations, DEM-based thermodynamics, and volume-uncertainty analysis to constrain coronal heating models and current-sheet dynamics across multiple scales.

Abstract

X-ray observations of the Sun led Eugene Parker to propose nanoflares as the basic energy-release units that heat the solar corona. Decades later, Solar Orbiter's Extreme Ultraviolet Imager (HRIEUV), operating halfway between Earth and the Sun, revealed thousands of even smaller brightenings in the quiet corona - tiny "campfires" that are smaller and far more frequent than the fundamental nanoflares observed from 1 AU. We analyze over 12,000 of these events, deriving their thermal energies using multiple geometric models to account for volume uncertainties. Although absolute values vary, all models yield consistent power-law energy distributions and ranges, confirming their flare-like behavior. These picoflares, spanning $10^{20}$--$10^{24}$ erg, were detected by the Solar Orbiter EUI Imager while the spacecraft was at 0.56 AU from the Sun. They occur up to sixty times more often than nanoflares seen from Earth orbit and supply about 1% of the quiet-Sun coronal heating power. This previously unseen energy source may be a missing component in the solar energy balance. Their discovery extends the flare energy spectrum to smaller scales, and future Solar Orbiter observations at 0.28 AU may reveal the most fundamental flare events that sustain the million-degree solar corona.

Picoflares in the Quiet Solar Corona: Solar Orbiter Observations Halfway to the Sun

TL;DR

This study uses Solar Orbiter's HRIEUV observations at 0.556 AU to characterize quiet-Sun campfires as the picoflare class, with thermal energies spanning – erg and occurrence rates that imply a substantial, previously unseen energy input to coronal heating. By applying multiple volume models to account for optically thin plasma geometry, the authors show robust power-law energy distributions with indices (≥5σ) and (≥3σ), and demonstrate that picoflares occur ~60× more often than nanoflares observed near Earth, contributing about 1% of the quiet-Sun heating requirement. Stereoscopic height measurements place most events at 1–5 Mm above the photosphere, suggesting energy release via field-aligned current dissipation in low-β environments, complemented by rapid reconnection in some structures. The findings extend the flare energy continuum to smaller scales, support the nanoflare heating paradigm, and indicate that closer Solar Orbiter observations could reveal even more fundamental heating events near the 10^18 erg scale. The work highlights the importance of combining near-Sun observations, DEM-based thermodynamics, and volume-uncertainty analysis to constrain coronal heating models and current-sheet dynamics across multiple scales.

Abstract

X-ray observations of the Sun led Eugene Parker to propose nanoflares as the basic energy-release units that heat the solar corona. Decades later, Solar Orbiter's Extreme Ultraviolet Imager (HRIEUV), operating halfway between Earth and the Sun, revealed thousands of even smaller brightenings in the quiet corona - tiny "campfires" that are smaller and far more frequent than the fundamental nanoflares observed from 1 AU. We analyze over 12,000 of these events, deriving their thermal energies using multiple geometric models to account for volume uncertainties. Although absolute values vary, all models yield consistent power-law energy distributions and ranges, confirming their flare-like behavior. These picoflares, spanning -- erg, were detected by the Solar Orbiter EUI Imager while the spacecraft was at 0.56 AU from the Sun. They occur up to sixty times more often than nanoflares seen from Earth orbit and supply about 1% of the quiet-Sun coronal heating power. This previously unseen energy source may be a missing component in the solar energy balance. Their discovery extends the flare energy spectrum to smaller scales, and future Solar Orbiter observations at 0.28 AU may reveal the most fundamental flare events that sustain the million-degree solar corona.

Paper Structure

This paper contains 29 sections, 14 equations, 8 figures, 1 table.

Table of Contents

  1. Introduction
  2. First Observations and Measurements of EUV Campfires
  3. Observational Setup and Data Acquisition
  4. Event Detection and Classification Methodology
  5. Morphological Classification
  6. Thermal Energy in Optically Thin Coronal Plasma
  7. Thermal Energy Framework
  8. Geometrical Parameters and Volume Models
  9. Temperature Determination via DEM Analysis
  10. Electron Density Determination
  11. Method Validation and Parameter Consistency
  12. Uncertainty Quantification and Robustness Analysis
  13. From Campfires to Picoflares: Energy Classification and Heating Contribution
  14. Geometrical Parameters of Campfires
  15. Stereoscopic Determination of Height above the Photosphere
  16. ...and 14 more sections

Figures (8)

  • Figure 1: Previously unresolved solar structures observed by Solar Orbiter HRIEUV 174 Å campfires on May 30, 2020 at the quiet sun (QS) while Solar Orbiter was located half-way to the Sun, above the chromospheric network (bright patterns in panel b). The main frame ($120 \times 120$ Mm) shows the context in Lyman-$\alpha$. The bottom panels ($20 \times 20$ Mm each) highlight specific event types: (1,5) intermittent dissipation along quiet sun QS miniloops; (2) multiple-loop reconnection; (3) single-loop burst initiating at the top; (4) tiny reconnection with plasma ejection; (6) picoflare at loop base. Dynamic views are available online: https://drive.google.com/file/d/1Kmn8mrvktAV210Adx7phBkb5TNJ98TMY/view?usp=sharing, https://drive.google.com/file/d/1vep3o8pf15TRoqfEHz9CMVwMkLwV-1i-/view?usp=sharing, https://drive.google.com/file/d/1NiL9kKfQv_KX6JhlKT0EJXAb2WCJ6wMc/view?usp=sharing, https://drive.google.com/file/d/1g8lA5GmxMVY-vK1cRxyJwWFb86W5m9cb/view?usp=sharing, https://drive.google.com/file/d/1f9sU5JUjBubawXHbnpR3pCITFFmruOr7/view?usp=sharing, https://drive.google.com/file/d/1AhgcDMYg_0a57mz1qMVuZH4Us4EmplfI/view?usp=sharing, https://drive.google.com/file/d/1TZ9WFrTqaMCNRCHoMpr4WtJB902lHlZ4/view?usp=sharing, https://drive.google.com/file/d/1NAMaxNvxBawTGt8mCNMUY8RkARi2y7a-/view?usp=sharing.
  • Figure 2: Aschwanden2000 EUV brightening volume modelling as an elliptical loop model. HRIEUV flaring loops are observed in the low corona, chromospheric segments are truncated $h_{ch}=500$ km from the brightening of the coronal EUV volume. (a) The ellipsoidal axes $a$ and $b$ confine the geometry of the projected loop, from which the projected loop length $L$ and width $w$ can be determined in (b) and (c).
  • Figure 3: Distributions of HRIEUV campfire parameters derived through elliptical fitting: length $L$, width $w$, area $A$, elliptical loop volume $V$, duration $t$, and stereoscopic heights $H$ above the photosphere. Magenta curves show relative measurement errors, highlighting the increasing uncertainties for the smallest events near the detection threshold (1% to 8.3% relative error). The area distribution spans 0.031 to 7.9 Mm$^2$ (elliptical areas). Comparison with TRACE EUV nanoflare parameters reveals campfire volumes are approximately three orders of magnitude smaller while maintaining consistent physical scaling relationships.
  • Figure 4: Power-law distributions of HRIEUV campfire parameters: length $L$, width $w$, area $A$, elliptical loop volume $V$, duration $t$, and heights $H$ above the photosphere. Magenta curves show relative errors in length and width estimation, ranging from 1% for largest dimensions (2.65-3.8 Mm) to 8.3% for smallest dimensions (0.198 Mm). Comparison with TRACE EUV nanoflare parameters (Fig. 4 in Aschwanden2000, Fig. 11 in Aschwanden2016) reveals campfire volumes are approximately three orders of magnitude smaller.
  • Figure 5: Emission measure-temperature relationship for the 1,467 HRIEUV campfire events ($> 5\sigma$) analyzed in this work, using the thermodynamic parameters (emission measure and temperature) established by Berghmans2021. We compare HRIEUV emission measures and temperatures with those of X-ray microflares and flares. The relation for HRIEUV and X-Ray covering the temperature range $0.97-3.5$ MK follows $EM = 10^{36.50} \times T^{1.90 \pm 0.35}$. Campfires would correspond to GOES class flares at least three orders of magnitude lower than those observed in X-rays.
  • ...and 3 more figures