Since their discovery over 100 years ago, there have been many suggestions for the origin and development of solar spicules. Because the velocities of spicules are comparable to the sound and Alfven speeds of the low chromosphere, linear theory cannot fully describe them. Consequently, detailed tests of theoretical ideas had to await the development of computing power that only became available during the 1970s. This work reviews theories for spicules and spicule-like features over approximately the past 25 years, with an emphasis on the models based on nonlinear numerical simulations. These models have given us physical insight into wave propagation in the solar atmosphere, and have helped elucidate how such waves, and associated shock waves, may be capable of creating motions and structures on magnetic flux tubes in the lower solar atmosphere. So far, however, it has been difficult to reproduce the most-commonly-quoted parameters for spicules with these models, using what appears to be the most suitable input parameters. A key impediment to developing satisfactory models has been the lack of reliable observational information, which is a consequence of the small angular size and transient lifetime of spicules. I close with a list of key observational questions to be addressed with space-based satellites, such as the currently operating TRACE satellite, and especially the upcoming Solar-B mission. Answers to these questions will help determine which, if any, of the current models correctly explains spicules.

Solar Spicules: A Review of Recent Models and Targets for Future Observations – (Invited Review)

We investigate the statistical properties of Ellerman bombs in the dynamic emerging flux region NOAA Active Region 8844, underneath an expanding arch filament system. High-resolution chromospheric Hα filtergrams (spatial resolution 08), as well as photospheric vector magnetograms (spatial resolution 05) and Dopplergrams, have been acquired by the balloon-borne Flare Genesis Experiment. Hα observations reveal the first "seeing-free" data set on Ellerman bombs and one of the largest samples of these events. We find that Ellerman bombs occur and recur in preferential locations in the low chromosphere, either above or in the absence of photospheric neutral magnetic lines. Ellerman bombs are associated with photospheric downflows, and their loci follow the transverse mass flows on the photosphere. They are small-scale events, with typical size 18 × 11 , but this size depends on the instrumental resolution. A large number of Ellerman bombs are probably undetected, owing to limited spatial resolution. Ellerman bombs occur in clusters that exhibit fractal properties. The fractal dimension, with an average value ~1.4, does not change significantly in the course of time. Typical parameters of Ellerman bombs are interrelated and obey power-law distribution functions, as in the case of flaring and subflaring activity. We find that Ellerman bombs may occur on separatrix, or quasi-separatrix, layers, in the low chromosphere. A plausible triggering mechanism of Ellerman bombs is stochastic magnetic reconnection caused by the turbulent evolution of the low-lying magnetic fields and the continuous reshaping of separatrix layers. The total energies of Ellerman bombs are estimated in the range (1027, 1028) ergs, the temperature enhancement in the radiating volume is ~2 × 103 K, and the timescale of radiative cooling is short, of the order of a few seconds. The distribution function of the energies of Ellerman bombs exhibits a power-law shape with an index ~-2.1. This suggests that Ellerman bombs may contribute significantly to the heating of the low chromosphere in emerging flux regions.

/pdf/statistics-morphology-and-energetics-of-ellerman-bombs-1y8r2lcxl5.pdf

Statistics, Morphology, and Energetics of Ellerman Bombs

Magnetic flux emergence from the solar convection zone into the overlying atmosphere is the driver of a diverse range of phenomena associated with solar activity. In this article, we introduce theoretical concepts central to the study of flux emergence and discuss how the inclusion of different physical effects (e.g., magnetic buoyancy, magnetoconvection, reconnection, magnetic twist, interaction with ambient field) in models impact the evolution of the emerging field and plasma.

/pdf/flux-emergence-theory-45pkofupwr.pdf

Flux Emergence (Theory)

We analyzed simultaneous EUV data from the Transition Region and Coronal Explorer and Hα data from Big Bear Solar Observatory. In the active region studied, we found several EUV jets that repeatedly occurred where pre-existing magnetic flux was "canceled" by newly emerging flux of opposite polarity. The jets look like Yohkoh soft X-ray jets, but are smaller and shorter lived than X-ray jets. They have a typical size of 4000-10,000 km, a transverse velocity of 50-100 km s-1, and a lifetime of 2-4 minutes. Each of the jets was ejected from a looplike bright EUV emission patch at the moment that the patch reached its peak emission. We also found dark Hα surges that are correlated with these jets. A careful comparison, however, revealed that the Hα surges are not cospatial with the EUV jets. Instead, the EUV jets are identified with bright jetlike features in the Hα line center. Our results support a picture in which Hα surges and EUV jets represent different kinds of plasma ejection—cool and hot plasma ejections along different field lines—which must be dynamically connected to each other. We emphasize the importance of observed flux cancellation and a small erupting filament in understanding the acceleration mechanisms of EUV jets and Hα surges.

https://iopscience.iop.org/article/10.1086/311910/pdf

Extreme-ultraviolet jets and hα surges in solar microflares

Since their discovery over 100 years ago, there have been many suggestions for the origin and development of solar spicules. Because the velocities of spicules are comparable to the sound and Alfven speeds of the low chromosphere, linear theory cannot fully describe them. Consequently, detailed tests of theoretical ideas had to await the development of computing power that only became available during the 1970s. This work reviews theories for spicules and spicule-like features over approximately the past 25 years, with an emphasis an the models based on nonlinear numerical simulations. These models have given us physical insight into wave propagation in the solar atmosphere and have helped elucidate how such waves, and associated shock waves, may be capable of creating motions and structures on magnetic flux tubes in the lower solar atmosphere. So far, however, it has been difficult to reproduce the most commonly quoted parameters for spicules with these models, using what appears to be the most suitable input parameters. A key impediment to developing satisfactory models has been the lack of reliable observational information, which is a consequence of the small angular size and transient lifetime of spicules. I close with a list of key observational questions to be addressed with space-based satellites, such as the currently operating Transition Region and Coronal Explorer (TRACE) satellite, and especially the upcoming Solar-B mission. Answers to these questions will help determine which, if any, of the current models correctly explains spicules.

Solar Spicules: A Review of Recent Models and Targets for Future Observations

High resolution on- and off-band Hα filtergrams of disk solar surges obtained with the Vacuum Tower Telescope of the Sacramento Peak Observatory have been compared to magnetic data.

The magnetic properties of solar surges

Curves of velocity as a function of height have been determined from time sequences of on- and off-band Hα filtergrams of four surges. The surges behave as predicted by the melon-seed mechanism. However, in the outward-moving material the deceleration taking place after the maximum velocity has been reached, is stronger than if gravity alone is acting in three of four cases; during the return phase, the acceleration toward the solar surface is less than free-fall in two of three cases. The braking force, opposing the motion, is proportional to the velocity of the surge material. Non-force-free effects in the magnetic field supporting the surge provide a reasonable explanation for the braking.

The dynamics of solar surges

One hundred seventy-eight Ellerman bombs were identified and studied with on- and off-band Hα filtergrams of two active centers, one near disk center and the other near the east limb. The photographs, taken through a 1/4 A bandpass filter, occasionally attained resolution as fine as 0.3″. The mean duration of bombs at Hα-2 A was about 13 min near disk center and 11 min near the limb; these times increase slightly when we observe closer to the core of Hα. Eighty-six percent of the bombs in the near-limb region and 56% in the disk-center region were seen to be accompanied by ejections of dark material; the ejections were 3–30″ long. The ejection length appears to be proportional to the bomb size times the bomb lifetime.

Some statistical properties of Ellerman bombs

Computed current-free magnetic fields are compared to the loop prominence associated with the west limb proton flare of 18 November 1968. Successive sets of fitting fieldlines closely resemble the loop prominence system throughout its growth and lifetime. The successive position bases of the fieldlines reproduce the drift rate of spreading two-ribbon flares.

The magnetic configuration of the November 18, 1968 loop prominence system

We have made detailed comparisons of coronal structure, as photographed at A 5303 A, with magnetic lines of force, as computed from measurements of the longitudinal component of the underlying photospheric magnetic fields. Coronal fields were computed under the assumption that the space above the active regions is current-free. Out of 36 regions for which there exist both magnetic and coronal data at the Sacramento Peak Observatory, we found only four suitable for analysis. Using a light-pen attachment to a digital computer, we were able to choose lines of force whose footpoints in the photosphere best matched those of the lines of force which were suggested by local intensity variations in the corona. The lines of force thus computed give excellent agreement with the apparent height and shape of coronal fieldlines. In particular the magnetic field in the loop structures of 2 November 1969 and 18–19 November 1968 falls from several hundred gauss at 10000–20000 km above the photosphere to 2–20 G at heights of 100000–150000 km. These computed values are in general agreement with other measurements. Spectra of the loops reveal that the direction of the line-of-sight component of the motion agrees with that anticipated from a comparison of the computed fieldline orientation and the observed motion in the plane of observation. We conclude that coronal magnetic fields above slowly changing active regions are nearly the same as the vacuum potential fields derived from underlying photospheric field sources (principally the larger sunspots in a region) and that calculations of this type may be relied upon to give the magnetic fields for studies of coronal dynamics.

J. René Roy

Papers

The magnetic properties of solar surges

The dynamics of solar surges

Some statistical properties of Ellerman bombs

The magnetic configuration of the November 18, 1968 loop prominence system

Coronal Magnetic Fields above Active Regions