Penumbral micro-jets at high spatial and temporal resolution

Sunspot observations in chromospheric spectral lines have revealed the existence of short-lived linear bright transients, commonly referred to as penumbral micro-jets (PMJs). Details on the origin and physical nature of PMJs are to large extend still unkown. We aim to characterize the dynamical nature of PMJs to provide guidance for future modelling efforts. We analyze high spatial (0.1 arcsec) and temporal resolution (1 s) Ca II H filtergram (0.1 nm bandwidth) observations of a sunspot obtained on two consecutive days with the Swedish 1-m Solar Telescope. We find that PMJs appear to be the rapid brightening of an already existing (faint) fibril. The rapid brightening is the fast increase (typically less than 10 s) in intensity over significant length (several 100s of km) of the existing fibril. For most PMJs, we cannot identify a clear root or source from where the brightening appears to originate. After the fast onset, about half of the PMJs have a top that is moving with an apparent velocity between 5 and 14 km/s, most of them upwards. For the other PMJs, there is no significant motion of the top. For about a third of the PMJs we observe a splitting into two parallel and co-evolving linear features during the later phases of the lifetime of the PMJ. We conclude that mass flows can play only limited role in the onset phase of PMJs and that it is more likely that we see the effect of a fast heating front.Comment: Accepted for publication in Astronomy & Astrophysics. Movies are available at http://folk.uio.no/rouppe/pmj_highcadence

Microjets in the penumbra of a sunspot

Penumbral Microjets (PMJs) are short-lived jets found in the penumbra of sunspots, first observed in wide-band Ca H-line observations as localized brightenings, and are thought to be caused by magnetic reconnection. Earlier work on PMJs has been focused on smaller samples of by-eye selected events and case studies. It is our goal to present an automated study of a large sample of PMJs to place the basic statistics of PMJs on a sure footing and to study the PMJ Ca II 8542 Angstrom spectral profile in detail. High spatial resolution and spectrally well-sampled observations in the Ca II 8542 Angstrom line obtained from the Swedish 1-m Solar Telescope (SST) are reduced by a Principle Component Analysis and subsequently used in the automated detection of PMJs using the simple learning algorithm k-Nearest Neighbour. PMJ detections were verified with co-temporal Ca H-line observations. A total of 453 tracked PMJ events were found, or 4253 PMJs detections tallied over all timeframes and a detection rate of 21 events per timestep. From these, an average length, width and lifetime of 640 km, 210 km and 90 s were obtained. The average PMJ Ca II 8542 Angstrom line profile is characterized by enhanced inner wings, often in the form of one or two distinct peaks, and a brighter line core as compared to the quiet Sun average. Average blue and red peak positions were determined at -10.4 km/s and +10.2 km/s offsets from the Ca II 8542 Angstrom line core. We found several clusters of PMJ hotspots within the sunspot penumbra, where PMJ events occur in the same general area repeatedly over time. Our results indicate smaller average PMJs sizes and longer lifetimes compared to previously published values, but with statistics still in the same orders of magnitude. The investigation and analysis of the PMJ line profiles strengthen the proposed heating of PMJs to transition region temperatures.Comment: Figures 1, 2, 3, 4 and 11 exhibited artifacts in some pdf-readers, and have been replotted with new graphical settings to remedy this. Apart from slight changes in sizing and fonts, the figures are the same. The arXiv abstract has had tex-syntax removed for better readabilit

A multi-diagnostic spectral analysis of penumbral microjets

Penumbral microjets (PMJs) are short-lived, jet-like objects found in the penumbra of sunspots. They were first discovered in chromospheric lines and have later also been shown to exhibit signals in transition region (TR) lines. Their origin and manner of evolution is not yet settled. We perform a comprehensive analysis of PMJs through the use of spectral diagnostics that span from photospheric to TR temperatures to constrain PMJ properties. We employed high-spatial-resolution Swedish 1-m Solar Telescope observations in the Ca II 8542 Angstrom and H-alpha lines, IRIS slit-jaw images, and IRIS spectral observations in the Mg II h & k lines, the Mg II 2798.75 Angstrom & 2798.82 Angstrom triplet blend, the C II 1334 Angstrom & 1335 Angstrom lines, and the Si IV 1394 Angstrom & 1403 Angstrom lines. We derived a wide range of spectral diagnostics from these and investigated other secondary phenomena associated with PMJs. We find that PMJs exhibit varying degrees of signal in all of our studied spectral lines. We find low or negligible Doppler velocities and velocity gradients throughout our diagnostics and all layers of the solar atmosphere associated with these. Dark features in the inner wings of H-alpha and Ca II 8542 Angstrom imply that PMJs form along pre-existing fibril structures. We find evidence for upper photospheric heating in a subset of PMJs through emission in the wings of the Mg II triplet lines. There is little evidence for ubiquitous twisting motion in PMJs. There is no marked difference in onset-times for PMJ brightenings in different spectral lines. PMJs most likely exhibit only very modest mass-motions, contrary to earlier suggestions. We posit that PMJs form at upper photospheric or chromospheric heights at pre-existing fibril structures.Comment: 26 pages, 18 figures. Accepted for publication in Astronomy & Astrophysics. This version rectifies bad formatting caused by an oversized figure. In the previous version this caused the figure itself and subsequent figures to be misplaced in the text. No other changes beside figure sizing and placement was made. This altered the given page coun

On microjets in sunspot penumbrae

Penumbral microjets (PMJs) are intriguing flashes of light found in the twilight region of sunspots, the penumbra, and they have typical lengths of hundreds of kilometres. Quite some finesse is required to tease out the origins, precise properties, and how PMJs actually move through the Sun’s plasma. PMJs are likely caused by the process of magnetic reconnection, in which “tangled” magnetic field lines suddenly “snap” and realign, releasing energy. This process is typically associated with much larger solar flares. Accordingly, the study of PMJs offers insight into this process at a much smaller scale. The thesis focuses on highly detailed observations from the Swedish 1-m Solar Telescope and NASA’s Interface Region Imaging Spectrograph. A wide range of advanced methods were used, including an automated detection scheme for PMJs involving the machine-learning algorithm k-Nearest Neighbour. Multiple sets of observations were analysed using detailed spectral diagnostics, ranging across many spectral lines. Ultimately, the work accomplished great strides forward in the characterization of PMJs, especially in how they move through the atmosphere and how they evolve in time

Chromospheric Microjets in the Penumbra of a Sunspot

In this thesis chromospheric features called Penumbral Microjets (PMJs) are investigated in a sunspot. Observations of high resolution in space, time and wavelength from the Swedish 1-meter Solar Telescope (SST) are used to probe the Ca II 8542 Å-line. The goal is to obtain detailed spectral signatures of PMJs in the Ca II 8542 Å -line, as well as basic statistical measures such as lifetime and sizes. This is achieved by the implementation of an automated detection scheme employing dimensionality reduction methods and a simple learning algorithm which together reduce and classify the observations from the SST. The advantages of employing an automated numerical detection scheme are exploited to assemble a large sample of objects and extract statistically significant results from it. The obtained statistical values and spectra are then discussed and put in a wider context

High-resolution observations of the solar photosphere, chromosphere, and transition region: A database of coordinated IRIS and SST observations

NASA’s Interface Region Imaging Spectrograph (IRIS) provides high-resolution observations of the solar atmosphere through ultraviolet spectroscopy and imaging. Since the launch of IRIS in June 2013, we have conducted systematic observation campaigns in coordination with the Swedish 1 m Solar Telescope (SST) on La Palma. The SST provides complementary high-resolution observations of the photosphere and chromosphere. The SST observations include spectropolarimetric imaging in photospheric Fe  I lines and spectrally resolved imaging in the chromospheric Ca  II 8542 Å, H α , and Ca  II K lines. We present a database of co-aligned IRIS and SST datasets that is open for analysis to the scientific community. The database covers a variety of targets including active regions, sunspots, plages, the quiet Sun, and coronal holes