The formation of jets such as dynamic fibrils, mottles, and spicules in the solar chromosphere is one of the most important, but also most poorly understood, phenomena of the Sun's magnetized outer atmosphere. We use extremely high resolution observations from the Swedish 1 m Solar Telescope combined with advanced numerical modeling to show that in active regions these jets are a natural consequence of upwardly propagating slow-mode magnetoacoustic shocks. These shocks form when waves generated by convective flows and global p-mode oscillations in the lower lying photosphere leak upward into the magnetized chromosphere. We find excellent agreement between observed and simulated jet velocities, decelerations, lifetimes, and lengths. Our findings suggest that previous observations of quiet-Sun spicules and mottles may also be interpreted in light of a shock-driven mechanism.

Dynamic Fibrils Are Driven by Magnetoacoustic Shocks

The magnetic field strength within the polar caps of the sun is an important parameter for both the solar activity cycle and for our understanding of the interplanetary magnetic field. Measurements of the line-of-sight component of the magnetic field generally yield 0.1 to 0.2 mT near times of sunspot minimum. This paper reports measurements of the polar fields made at the Stanford Solar Observatory using the Fe I line at 525.02 nm. It is found that the average flux density poleward of 55 deg latitude is about 0.6 mT peaking to more than 1 mT at the pole and decreasing to 0.2 mT at the polar cap boundary. The total open flux through either polar cap thus becomes about 3 x 10 to the 14th Wb. It is also shown that observed magnetic field strengths vary as the line-of-sight component of nearly radial fields.

The strength of the sun's polar fields

We analyzed simultaneous EUV data from the Transition Region and Coronal Explorerand Ha 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 21 , 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 Ha surges that are correlated with these jets. A careful comparison, however, revealed that the Ha surges are not cospatial with the EUV jets. Instead, the EUV jets are identified with bright jetlike features in the Ha line center. Our results support a picture in which Ha 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 Ha surges.

EXTREME-ULTRAVIOLET JETS AND H a SURGES IN SOLAR MICROFLARES

Structure and development

Context. Helicity is a fundamental property of magnetic fields, conserved in ideal MHD. In flux rope topology, it consists of twist and writhe helicity. Despite the common occurrence of helical structures in the solar atmosphere, little is known about how their shape relates to the writhe, which fraction of helicity is contained in writhe, and how much helicity is exchanged between twist and writhe when they erupt. Aims. Here we perform a quantitative investigation of these questions relevant for coronal flux ropes. Methods. The decomposition of the writhe of a curve into local and nonlocal components greatly facilitates its computation. We use it to study the relation between writhe and projected S shape of helical curves and to measure writhe and twist in numerical simulations of flux rope instabilities. The results are discussed with regard to filament eruptions and coronal mass ejections (CMEs). Results. (1) We demonstrate that the relation between writhe and projected S shape is not unique in principle, but that the ambiguity does not affect low-lying structures, thus supporting the established empirical rule which associates stable forward (reverse) S shaped structures low in the corona with positive (negative) helicity. (2) Kink-unstable erupting flux ropes are found to transform a far smaller fraction of their twist helicity into writhe helicity than often assumed. (3) Confined flux rope eruptions tend to show stronger writhe at low heights than ejective eruptions (CMEs). This argues against suggestions that the writhing facilitates the rise of the rope through the overlying field. (4) Erupting filaments which are S shaped already before the eruption and keep the sign of their axis writhe (which is expected if field of one chirality dominates the source volume of the eruption), must reverse their S shape in the course of the rise. Implications for the occurrence of the helical kink instability in such events are discussed.

/pdf/the-writhe-of-helical-structures-in-the-solar-corona-4ooqx3kd1b.pdf

The writhe of helical structures in the solar corona

Recurrent and homologous jets were observed to the west edge of active region NOAA 11513 at the boundary of a coronal hole. We find two kinds of cancellations between opposite polarity magnetic fluxes, inducing the generation of recurrent jets. First, a satellite spot continuously collides with a pre-existing opposite polarity magnetic field and causes recurrent solar jets. Second, moving magnetic features, which emerge near the sunspot penumbra, pass through the ambient plasma and eventually collide with the opposite polarity magnetic field. Among these recurrent jets, a blowout jet that occurred around 21:10 UT is investigated. The rotation of the pre-existing magnetic field and the shear motion of the satellite spot accumulate magnetic energy, which creates the possibility for the jet to experience blowout right from the standard.

https://iopscience.iop.org/article/10.1088/0004-637X/815/1/71/pdf

Recurrent Solar Jets Induced by a Satellite Spot and Moving Magnetic Features

https://iopscience.iop.org/article/10.3847/0004-637X/817/2/117/pdf

Observations of oppositely directed umbral wavefronts rotating in sunspots obtained from the New Solar Telescope of BBSO

The mode-switching phenomenon of PSR B0329+54 is investigated based on the long-term monitoring from 2003 September to 2009 April made with the Urumqi 25 m radio telescope at 1540 MHz. At that frequency, the change of relative intensity between the leading and trailing components is the predominant feature of mode switching. The intensity ratios between the leading and trailing components are measured for the individual profiles averaged over a few minutes. It is found that the ratios follow normal distributions, where the abnormal mode has a greater typical width than the normal mode, indicating that the abnormal mode is less stable than the normal mode. Our data show that 84.9% of the time for PSR B0329+54 was in the normal mode and 15.1% was in the abnormal mode. From the two passages of eight-day quasi-continuous observations in 2004, supplemented by the daily data observed with the 15 m telescope at 610 MHz at Jodrell Bank Observatory, the intrinsic distributions of mode timescales are constrained with the Bayesian inference method. It is found that the gamma distribution with the shape parameter slightly smaller than 1 is favored over the normal, log-normal, and Pareto distributions. The optimal scale parameters of the gammamore » distribution are 31.5 minutes for the abnormal mode and 154 minutes for the normal mode. The shape parameters have very similar values, i.e., 0.75{sup +0.22}{sub -0.17} for the normal mode and 0.84{sup +0.28}{sub -0.22} for the abnormal mode, indicating that the physical mechanisms in both modes may be the same. No long-term modulation of the relative intensity ratios was found for either mode, suggesting that the mode switching was stable. The intrinsic timescale distributions, constrained for this pulsar for the first time, provide valuable information to understand the physics of mode switching.« less

https://iopscience.iop.org/article/10.1088/0004-637X/741/1/48/pdf

Long-term monitoring of mode switching for psr b0329+54

We analyze the magnetic configurations of three super active regions, NOAA 10484, 10486 and 10488, observed by the Huairou Multi-Channel Solar Tele- scope (MCST) from 2003 October 18 to November 4. Many energetic phenomena, such as flares (including a X-28 flare) and coronal mass ejections (CMEs), occurred during this period. We think that strong shear and fast emergence of magnetic flux are the main causes of these events. The question is also of great interest why these dramatic eruptions occurred so close together in the descending phase of the solar cycle.

http://iopscience.iop.org/article/10.1088/1009-9271/3/6/491/pdf

Three Super Active Regions in the Descending Phase of Solar Cycle 23

We identify 356 transequatorial loops (TLs) from the data set of Yohkoh Soft X-Ray Telescope (SXT) in the period of solar cycles 22 and 23. The classification of the TLs can be made on two bases. One is according to the magnetic polarities of the footpoints of TLs, and the other is according to the number of TLs in the same region. Based on the first criterion, TLs fall into two categories: PTLs in which the magnetic polarities of the footpoints are the same as the preceding polarities and FTLs in which the footpoint polarities are the same as the following polarities of active regions, respectively. It is found that PTLs have a preference; about 66% of the TLs are PTLs, and this preference of PTLs is independent of the solar cycle. The percentage of FTLs is about 34%. Based on the second criterion, TLs are also divided into two categories: the number of TLs in a region is either single (STLs) or multiple (MTLs). In addition, we find that the number of TLs, PTLs, and FTLs have good correlations with solar cycle indices. By comparing the number of TLs and the number of active regions in each year, we obtain the ratio between them. The separation of footpoints and their yearly variations are calculated, and we find that our result is consistent with sporer's law.

Jie Chen

Papers

Recurrent Solar Jets Induced by a Satellite Spot and Moving Magnetic Features

Observations of oppositely directed umbral wavefronts rotating in sunspots obtained from the New Solar Telescope of BBSO

Long-term monitoring of mode switching for psr b0329+54

Three Super Active Regions in the Descending Phase of Solar Cycle 23

A Statistical Study of Transequatorial Loops