Solar Fine-Scale Structures. I. Spicules and Other Small-Scale, Jet-Like Events at the Chromospheric Level: Observations and Physical Parameters

Abstract

Over the last two decades the uninterrupted, high resolution observations of the Sun, from the excellent range of telescopes aboard many spacecraft complemented with observations from sophisticated ground-based telescopes have opened up a new world producing significantly more complete information on the physical conditions of the solar atmosphere than before. The interface between the lower solar atmosphere where energy is generated by subsurface convection and the corona comprises the chromosphere, which is dominated by jet-like, dynamic structures, called mottles when found in quiet regions, fibrils when found in active regions and spicules when observed at the solar limb. Recently, space observations with Hinode have led to the suggestion that there should exist two different types of spicules called Type I and Type II which have different properties. Groundbased observations in the Ca ii H and K filtergrams reveal the existence of long, thin emission features called straws in observations close to the limb, and a class of short-lived events called rapid blue-shifted excursions characterized by large Doppler shifts that appear only in the blue wing of the Ca ii infrared line. It has been suggested that the key to understanding how the solar plasma is accelerated and heated may well be found in the studies of these jet-like, dynamic events. However, while these structures are observed and studied for more than 130 years in the visible, but also in the UV and EUV emission lines and continua, there are still many questions to be answered. Thus, despite their importance and a multitude of observations performed and theoretical models proposed, questions regarding their origin, how they are formed, their physical parameters, their association with the underlying photospheric magnetic field, how they appear in the different spectral lines, and the interrelationship between structures observed in 2 Please give a shorter version with: \authorrunning and \titlerunning prior to \maketitle quiet and active regions on the disk and at the limb, as well as their role in global processes has not yet received definitive answers. In addition, how they affect the coronal heating and solar wind need to be further explored. In this review we present observations and physical properties of small-scale jet-like chromospheric events observed in active and quiet regions, on the disk and at the limb and discuss their interrelationship.

The magnetic field in the solar atmosphere

“…This magnetic reconnection between newly emerging magnetic flux and the ambient field plays a major role for jets has been confirmed in numerical experiments recently by Moreno-Insertis and Galsgaard (2013). For a recent review on small-scale, jet-like events at chromospheric levels, we refer to Tsiropoula et al (2012), as well as to Sect. 6.2.4 for polar jets.…”

Active region upflows

“…(for a review see Tsiropoula et al 2012). The IBIS images were overlayed with the EIS contour of the blueshifted coronal emission to determine the region associated with the AR upflow.…”

Transmission and conversion of magnetoacoustic waves on the magnetic canopy in a quiet Sun region

“…The plasma-β parameter (with β defined as the ratio of the gas pressure to the magnetic pressure) is used to distinguish between these two regimes, and the condition β = 1 defines the location of the magnetic canopy. In and above the magnetic canopy layer, flux tubes become highly inclined, and the chromosphere in Hα appears as a mesh pattern of elongated dark structures, most of them related to mottles (see Tsiropoula et al 2012, for a comprehensive review on their properties).…”