[Paper Review] Observation of an Instability in a 'Quiescent' Prominence

G. Stellmacher, E. Wiehr|arXiv (Cornell University)|Oct 25, 2012

Solar and Space Plasma Dynamics1 references3 citations

TL;DR

This study presents the first observation of a bubble-like cavity propagating through a quiescent solar prominence, detected via Hα slit-yaw imaging and Ca II 8542 spectroscopy. The cavity exhibits suppressed Hα emission (reduced by >16×) and propagates at 7–17 km/s, consistent with MHD compressive waves; Doppler shifts up to 12 km/s indicate lateral outflow, suggesting a violation of lateral magnetic stability in the Kippenhahn-Schlüter model.

ABSTRACT

We present the detection of a bubble-like cavity traveling through a quiescent prominence. The H-alpha emission in the cavity is >16 times smaller than in its surroundings. The cavity propagates almost with the phase-velocity of MHD compressive waves. We suggest a disruption of the lateral magnetic stability. The Ca II 8542 spectra indicate a material outflow along the lines of force up to 12 km/s.

Motivation & Objective

• To investigate the physical origin of unexpected dynamic structures in quiescent prominences, traditionally considered stable.
• To determine whether observed bubble-like cavities represent true emission voids or Doppler shifts off-band.
• To assess the role of magnetic field instability in triggering prominence dynamics.
• To measure radial velocities and line broadening to infer non-thermal motions and outflows.
• To test the validity of the Kippenhahn-Schlüter magnetic stability criterion in quiescent prominences.

Proposed method

• High-resolution Hα slit-yaw imaging with a 0.5 Å passband to track the cavity's motion and morphology.
• Simultaneous Ca II 8542 spectroscopy using a two-step proximity-focused image intensifier to reduce exposure time to ~1 second.
• Kodalith film recording with contrast enhancement to analyze faint infrared emission lines.
• Spatial and spectral resolution of ~1 arcsec and 15 mÅ, enabling detection of Doppler shifts up to ±120 km/s.
• Estimation of Hα emission deficit in the cavity using continuum intensity as a lower detection limit (3×10⁻³ of disk center).
• Calculation of phase velocity of the disturbance using observed propagation speed and comparison with MHD wave theory (v_MHD = √(v_sound² + v_Alfven²)).

Experimental results

Research questions

• RQ1 What causes the formation and propagation of a bubble-like cavity in a quiescent prominence?
• RQ2 Is the observed cavity a true emission deficit or an artifact of Doppler shift?
• RQ3 What is the physical mechanism driving the observed lateral outflow of prominence material?
• RQ4 Does the disturbance violate the lateral magnetic stability criterion in the Kippenhahn-Schlüter model?
• RQ5 How do the observed velocities compare with theoretical MHD wave speeds and prominence sinking velocities?

Key findings

• The cavity exhibits Hα emission reduced by a factor of at least 16 compared to the surrounding prominence, confirming it as a true emission void.
• The cavity propagated at 7–17 km/s, consistent with the phase velocity of MHD compressive waves, requiring an Alfvén velocity between 3 and 13 km/s.
• Doppler shifts of up to 12 km/s were observed at the cavity boundary, indicating lateral outflow of prominence material along magnetic field lines.
• Non-thermal line broadening up to 5 km/s was measured, suggesting turbulent or multi-component flows within the prominence.
• The observed velocities align well with the sinking velocity of prominence matter through the corona (Unsöld, 1970), supporting the MHD wave interpretation.
• The instability is interpreted as a disruption of the lateral magnetic stability criterion in the Kippenhahn-Schlüter model, triggered by small disturbances.