136. Quasi-periodic spicule-like cool jets driven by Alfvén pulses

Author: Balveer Singh, A. K. Srivastava, K. Sharma, S. K. Mishra, B. N. Dwivedi
Affiliation(s): IIT (BHU) Varanasi; Indian Institute of Astrophysics; Rajiv Gandhi Institute of Petroleum Technology

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Introduction

Spicules are thin, cool jets triggering from the chromosphere, are among the most ubiquitous yet enigmatic features of the solar atmosphere. These spicules are a prime conduit for mass and energy exchange between the lower solar atmosphere. Their rapid rise–fall cycles, fine widths, and prevalence over network and plage suggest an impulsive origin tied to waves and shocks. Our study shows how impulsive Alfvén perturbations can non-linearly convert their transverse energy into field-aligned shocks that lift chromospheric plasma into the corona as quasi-periodically rise and fall. We develop a physics-based 2.5-dimensional model in which Alfvén pulses introduced as transient transverse velocity perturbations, non-linearly generate the field-aligned plasma motions via the ponderomotive force. These field-aligned plasma motions and induced magnetoacoustic shocks then rise through the transition region, lifting cool chromospheric material into the corona as narrow and cool spicule-like jets. In our 2.5D MHD simulations under single-fluid plasma regime, we demonstrate that multiple, strong-amplitude transverse velocity pulses that mimic Alfvén pulses of magnitudes between 50 and 90 km/s imposed at chromospheric height between 1.5 and 2.0 Mm transfer energy via the ponderomotive force, forming magnetoacoustic shocks that drive spicule-like jets with ~4 min recurrence.  These jets carry substantial instantaneous mass flux and kinetic energy density into the lower corona. The central question is whether such pulses, acting in a gravitationally, stratified solar atmosphere can non-linearly transfer energy to field-aligned motions, and generate quasi-periodic spicule-like jets with properties consistent with observations.

Model and Method

We solve the conservative ideal-MHD system with gravity using the PLUTO code (Godunov scheme, Roe solver, CFL=0.25, 3rd-order Runge–Kutta), in a box spanning x∈[−6,6] Mm and y∈[1,21] Mm at up to 7.8–15.6 km spatial resolution. The background atmosphere follows Avrett–Loeser temperature stratification, with a potential-like open magnetic field seeded by a 560 G subsurface pole plus a 5 G vertical component, giving |B|≈33 G at 1.5 Mm (i.e., Solar Chromosphere) and ≈22 G at the transition region (2.7 Mm). We impose Fifteen Gaussian Vz pulses (width 20 km; amplitudes 50–90 km s⁻¹) are injected randomly between 1.5–2.0 Mm and −5≤x≤5 Mm, and the system is evolved for 1000 s.

Results

  • Alfvén pulse to field-aligned motion pathway and jet formation. The applied Impulsive transverse pulses (50–90 km s⁻¹ at 1.5–2.0 Mm) are the main indirect driver to form the thin-cool spicule like jets in the model solar atmosphere. The transverse pulses mimic Alfvén nature (Vz, Bz polarization, and makes the angle of 180 degree between Vz, and Bz). The ponderomotive force drives field-aligned perturbations that steepen into shocks. These steepen into magnetoacoustic shocks that lift cool chromospheric plasma through the transition region, forming thin spicule‑like jets that track the expanding open field (Fig. 1, and 2). The driver–response sequence is evident in co‑temporal Vz-Vy diagnostics, with shock formation following the Alfvén excitation.
  • Kinematics, lifetimes, and recurrence. Distance–time maps along representative slits show multiple rise–fall episodes within the 0–1000 s run, where the first jets are typically the longest and densest jet (Fig. 2). Upflow and subsequent downflow signatures are clearly seen in vertical velocity (Vy) and density map, consistent with observed spicule behavior (Fig. 2, and 3). Typical jet lifetimes fall in the few‑minutes range, matching the driven periodicity.
  • Mass and energy transport to the low corona. The simulations recover substantial mass flux (order 10⁻⁶ g cm⁻² s⁻¹) above the transition region (Fig. 3, top panel), with significant kinetic energy density (KED) concentrated during maximum extension of the jets. Together, these show that the modeled spicules can locally supply mass and momentum into the inner corona, offering a physically grounded channel for intermittent coronal feeding.
  • Quasi‑periodicities around 4 min. Time–frequency analysis of the mass‑flux signal at ~3 Mm demonstrates dominant power in the 3–5 min band, with a globally significant peak close to 4 min (Fig. 4). This ties the recurrence to the driven shock dynamics and overlaps the canonical chromospheric p‑mode range, providing a natural explanation for the observed quasi‑periodic jetting in quiet‑Sun regions.

Conclusion

Our 2.5-D simulations establish a self‑consistent Alfvén‑pulse pathway for generating quasi‑periodic, spicule‑like thin, cool jets in a gravitationally, stratified quiet‑Sun atmosphere with open magnetic topology. In this scenario, strong Alfvén pulses act as an indirect driver, they do not push plasma upward directly, but non‑linearly transfer the momentum and generates the field‑aligned plasma motions and magntoacoustic shocks that repeatedly elevate chromospheric material above the transition region, yielding the observed rise-fall cycles and around 4‑minute periodicity. The modeled jets transport appreciable mass flux and kinetic energy density into the low corona, supporting a role for spicules in intermittent mass and energy exchange between chromosphere and corona. While the present study isolates the essential physics in ideal single‑fluid MHD, future work adding non‑ideal terms (e.g., partial ionization, ion–neutral coupling, conduction, radiative losses) will quantify how dissipative channels and multi‑species effects modulate shock efficiency, jet thermodynamics, and coronal impact, and will enable tighter confrontation with high‑resolution observations (SST, Solar Orbiter, DKIST).


Figure 1 The normalized density map of spicule-like jets at time t = 100 s. The transition region lies at a height of 2.7 Mm in the model solar atmosphere. The horizontal (x) and vertical (y) axes define the x–y plane. The z-axis is perpendicular to the x–y plane and is an ignorable direction in which the velocity perturbation (Vz) is applied. These jets are triggered by 15 different amplitudes of the Vz pulses. The transverse pulses mimic Alfvén pulses and are applied at chromospheric heights between 1.5 and 2.0 Mm. The amplitudes of these pulses are randomly chosen between 50 and 90 km s−1. These pulses generate field-aligned magnetoacoustic shocks, followed by mass motions exhibiting the features of spicule-like jets. The normalized velocity vectors are overplotted on the density map, showing the quasi-periodic rise and fall of the jets.

Figure 2 Distance–time diagram of logarithmic mass density (ρ) along the jet located at [specify height] Mm in Fig. 1, showing quasi-periodic rise and fall. Multiple episodes of upward and downward motion are evident, consistent with the observed dynamics of spicule-like jets.

Figure 3 Temporal evolution of vertical velocity (Vy, blue) and transverse velocity (Vz, red) at y = 3.0 Mm, above the transition region for the jet located at x = 0.0 Mm in Fig. 1. Co-temporal traces reveal shock steepening following Alfvén pulse driving, confirming the indirect pathway to jet formation.

Figure 4 Wavelet power spectrum of the mass-flux time series at y = 3.0 Mm (top panel) for the jet located at x = 0 Mm. Dominant power lies in the 3–5 min band (left-bottom panel), with a globally significant peak near 3.9 min (>95% confidence), linking jet recurrence to driven shock dynamics (right-bottom panel).

References:

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