Picoflares: Small-Scale Solar Energy Events

Updated 29 October 2025

Picoflares are defined as impulsive energy-release events in the solar atmosphere with energies ranging from 10^20 to 10^24 erg, observable in high-resolution EUV imagery.
They exhibit two distinct mechanisms—looptop tearing-mode reconnection and footpoint anomalous resistivity—each with unique durations and energy signatures.
High-frequency picoflare jets, especially in coronal holes, can contribute up to 20% of the solar wind mass flux, offering new insights into coronal heating and energy transport.

Picoflares are a recently characterized class of small-scale, impulsive energy-release events in the solar atmosphere, distinguished by their thermal or kinetic energies in the range $10^{20}$ – $10^{24}$ erg. They are identified as flare-like brightenings at spatial and energetic scales several orders of magnitude below traditional nanoflares. Solar Orbiter’s high-cadence, high-resolution Extreme Ultraviolet (EUV) observations have revealed that picoflares are abundant in both quiet-Sun coronal regions and coronal holes, with profound implications for the solar wind and the coronal heating problem. Their physical mechanisms, statistical properties, and roles in mass and energy transport constitute an area of active research and debate.

1. Observational Properties and Detection

Picoflares are detected as localized EUV intensity enhancements above 3 $\sigma$ –5 $\sigma$ of background fluctuation levels, typically by wavelet-based algorithms using Solar Orbiter’s HRIEUV imaging at 174 Å (Podladchikova et al., 11 Oct 2025). The minimum event sizes resolved are down to 200 km (instrumental limits $\sim$ 198–237 km/pixel), with durations ranging from 1 s up to 260 s, depending on region and mechanism. Thermal energies are robustly derived from DEM (Differential Emission Measure) analyses, combining EUV channels to estimate event temperature and emission measure, and geometric modeling for volumes—most consistently via elliptical loop assumptions, validated by stereoscopy (Podladchikova et al., 11 Oct 2025).

Typical event properties:

Property	Quiet Sun Picoflares	Coronal Hole Picoflare Jets
Width/Linear Size	0.2–3.8 Mm	200–600 km
Duration	1–260 s	20–100 s (some bases 300–600 s)
Energy	$3 \times 10^{20}$ – $10^{24}$ erg	≥ $10^{23}$ erg
Speed	—	70–440 km/s (typically supersonic)
Occurrence Rate	518 events/s (full Sun, $3\sigma$ )	≥120 jets/9000 Mm² region

The phenomenon occurs at multiplicities as high as 12,000 events detected in a 90 min HRIEUV sequence (Podladchikova et al., 11 Oct 2025), and at least $\sim$ 120 picoflare jets simultaneously within a single coronal hole segment (Chitta et al., 2023). Detections in X-rays prove challenging at these energies; prior X-ray surveys establish a lower physical threshold for microflares ( $10^{25}$ – $10^{26}$ erg) (Mirzoeva, 2015), with picoflares populating the continuum below.

2. Physical Mechanisms and Classification

Recent analyses of Solar Orbiter data resolve two distinct physical mechanisms for picoflare energy release, depending on region and event properties (Podladchikova, 26 Oct 2025):

A. Looptop Tearing-Mode Reconnection ("Population A")

Occurs at loop tops (2.5–5 Mm above the photosphere)
Impulsive, rapid reconnection of perpendicular currents via tearing-mode instability ( $\beta<1$ regime)
Durations: 1–10 s; energies: $10^{22}$ – $10^{24}$ erg
Characterized by rapid, bright, short-lived signatures

B. Footpoint Anomalous Resistivity ("Population B")

Occurs near loop footpoints (1–2.5 Mm, transition region)
Gradual dissipation of parallel currents via anomalous resistivity (electron-ion micro-instabilities), activated above a critical current density $j_\text{crit} = n_e e c_s$ (ion sound speed $c_s\sim50$ km/s)
Durations: 10–100 s; energies: $10^{20}$ – $10^{22}$ erg
Characterized by slower, sustained brightenings

Picoflare jets in coronal holes are morphologically distinguished by their Y-shaped bases, indicative of transient interchange reconnection between small-scale and open magnetic field lines (Chitta et al., 2023). These jets are both frequent and widespread, suggesting a granular-scale reconnection origin.

3. Event Statistics, Energy Distributions, and Scaling Laws

The measured picoflare population displays robust power-law energy distributions (Podladchikova et al., 11 Oct 2025, Podladchikova, 26 Oct 2025). Using the formula:

$N(E_{\mathrm{th}}) \propto E_{\mathrm{th}}^{-\alpha}$

the following slopes are reported:

$\alpha = 2.32 \pm 0.36$ for robust ( $\geq 5\sigma$ ) events
$\alpha = 2.82 \pm 0.11$ for more inclusive ( $\geq 3\sigma$ ) thresholds

Total heating input from resolved picoflares is $P_{\mathrm{in}} \sim 3.0 \times 10^3$ erg s $^{-1}$ cm $^{-2}$ , compared to the required quiet-Sun coronal heating loss of $P_{\mathrm{out}} \sim 3 \times 10^5$ erg s $^{-1}$ cm $^{-2}$ (Podladchikova et al., 11 Oct 2025, Podladchikova, 26 Oct 2025).

A key scaling relation for picoflare emission measure and temperature:

$EM = 10^{36.50} \times T^{1.90\pm0.35}$

with $T$ in MK units, continuous with similar relations for X-ray flares but at lower temperatures.

In coronal holes, picoflare jets have an estimated filling factor $f_j \sim 0.01$ (conservative), accounting for $\geq 20\%$ of the solar wind’s required mass flux; extrapolation to fainter jets suggests a potential dominant contribution (Chitta et al., 2023).

4. Role in Coronal Heating and Solar Wind Generation

Picoflares represent a previously missing low-energy contribution to coronal heating, with direct implications for the quiet Sun and coronal hole environments. Their high occurrence rates and steep power-law slopes ( $\alpha > 2$ ) suggest small-scale events are energetically dominant if the pattern extends down to unresolved regimes (Podladchikova et al., 11 Oct 2025, Podladchikova, 26 Oct 2025).

In quiet regions, the summed observed picoflare input constitutes $\sim 1\%$ of the coronal radiative/conductive losses (Podladchikova et al., 11 Oct 2025). A plausible implication is that the majority of heating could arise from yet-smaller, currently undetectable events if the energy distribution remains unbroken.

In coronal holes, picoflare jets—driven by magnetic reconnection—supply $\sim20\%$ (minimum) of the observed solar wind mass and energy flux, suggesting a fundamentally intermittent source structure for the wind. The granular and supergranular clustering of these jets may correspond to the multi-scale modulations observed in the heliospheric wind (Chitta et al., 2023).

5. Distinction from Microflares and Nanoflares

Microflare observations in soft X-rays ( $10^{25}$ – $10^{26}$ erg) demonstrate a physical lower limit; the flare event distribution turns over below this, and only events generating sufficient nonthermal or thermal electron populations are detected as flares (Mirzoeva, 2015). Picoflares populate the energy-transfer continuum below this threshold, primarily visible in EUV/coronal imaging. The distinction is physical, not merely instrumental: at low energies, events may fail to generate detectable X-ray emission, signifying plasma heating episodes rather than "flares" per se.

Nanoflares, as defined by Aschwanden et al. (2000), have energies $<10^{26}$ erg and have been posited as the basic unit for coronal heating. Picoflares demonstrably extend this spectrum to $10^{20}$ erg, with occurrence rates up to 60 $\times$ higher than nanoflares observed from Earth orbit (Podladchikova et al., 11 Oct 2025).

6. Theoretical and Modeling Implications

The identification of two distinct picoflare mechanisms—looptop tearing-mode reconnection and footpoint anomalous resistivity—resolves the "energy partition paradox," previously hindering direct linkage between photospheric energy injection and coronal thermalization (Podladchikova, 26 Oct 2025). This dual-mechanism model unifies Parker’s nanoflare paradigm with observational evidence for both rapid coronal and slower transition-region energy dissipation. Electric fields mediate energy transfer, overcoming radiative losses and facilitating the cross-layer injection of magnetic energy.

A plausible implication is that future Solar Orbiter runs at even smaller heliocentric distances could reveal nano-picoflare events sufficient to balance the complete coronal energy budget, if the identified power-law slopes remain intact.

7. Outstanding Issues and Future Directions

Although picoflares contribute measurably to coronal heating and solar wind mass injection, their cumulative energy input remains insufficient ( $\sim1\%$ ) to account for the full quiet-Sun requirement absent an unbroken power-law distribution to still smaller energies (Podladchikova et al., 11 Oct 2025). The issue of detection thresholds, physical classification, and the nature of sub-picoflare plasma heating episodes persists. High-resolution, rapid-cadence EUV imaging remains essential for characterizing these smallest events; complementary X-ray and multiwavelength studies are required to bridge the empirical gap between thermal brightenings and canonical flare signatures (Chitta et al., 2023, Mirzoeva, 2015).

In summary, picoflares are now recognized as abundant, energetically consequential events, with distinct reconnection-driven and anomalous resistivity-driven mechanisms. Their discovery extends flare continuum theories, refines coronal heating models, and reveals a fundamentally intermittent picture of solar mass and energy outflow. Future advances will rely upon deeper observational reach and cross-instrument consistency to resolve their full role in solar atmospheric energetics.