Search for high velocities in the disk counterpart of type II spicules

TL;DR

This study investigates the high-velocity dynamics of type II spicules on the solar disk using the IBIS instrument, revealing rapid events with Doppler shifts and linking them to the spicules' apparent limb motions, while explaining velocity discrepancies through modeling.

Contribution

It provides the first detection of high-velocity flows in the disk counterpart of type II spicules and offers a model explaining observed velocity differences.

Findings

Detected rapid, blue-shifted events with lifetimes under a minute.

Linked on-disk events to type II spicules based on spatial and temporal characteristics.

Explained velocity measurement discrepancies with a forward model considering line-of-sight effects.

Abstract

Recently, De Pontieu et al. (2007b) discovered a class of spicules that evolves more rapidly than previously known spicules, with rapid apparent motions of 50--150 km s${}^{-1}$, thickness of a few 100 km, and lifetimes of order 10--60 seconds. These so-called type II spicules have been difficult to study because of limited spatio-temporal and thermal resolution. Here we use the IBIS instrument to search for the high velocities in the disk counterpart of type II spicules. We have detected rapidly evolving events, with lifetimes that are less than a minute and often equal to the cadence of the instrument (19 secs). These events are characterized by a Doppler shift that only appears in the blue wing of the Ca II IR line. Furthermore the spatial extent, lifetime, and location near network, all suggest a link to type II spicules. However, the magnitude of the measured Doppler velocity is significantly lower than the apparent motions seen at the limb. We use Monte Carlo simulations to show that this discrepancy can be explained by a forward model in which the visibility on the disk of the high-velocity flows in these events is limited by a combination of line-of-sight projection and reduced opacity in upward propelled plasma, especially in reconnection driven jets that are powered by a roughly constant energy supply.