Magnetic field variations associated with umbral flashes and penumbral waves

Umbral flashes (UF) and running penumbral waves (RPWs) in sunspot chromospheres leave a dramatic imprint in the intensity profile of the Ca II 854.2 nm line. Recent studies have focussed on also explaining the observed polarization profiles, that show even more dramatic variations during the passage of these shock fronts. While most of these variations can be explained with an almost constant magnetic field as a function of time, several studies have reported changes in the inferred magnetic field strength during UF phases. In this study we investigate the origin of these periodic variations of the magnetic field strength by analyzing a time-series of high temporal cadence observations acquired in the Ca II line with the CRISP instrument at the Swedish 1-m Solar Telescope. In particular, we analyze how the inferred geometrical height scale changes between quiescent and UF phases, and whether those changes are enough to explain the observed changes in $B$. We have performed non-LTE data inversions with the NICOLE code of a time-series of very high spatio-temporal resolution observations in the Ca II and Fe I 630.15\630.25 nm lines. Our results indicate that the Ca II line in sunspots is greatly sensitive to magnetic fields at $\log\tau_{500}=-5$ during UFs and quiescence. However, this optical depth value does not correspond to the same geometrical height during the two phases. Our results indicate that during UFs and RPWs the $\log\tau=-5$ is located at a higher geometrical height than during quiescence. Additionally, the inferred magnetic field values are higher in UFs (~270 G) and in RPWs (~100 G). Our results suggest that opacity changes caused by UFs and RPWs cannot explain the observed temporal variations in the magnetic field, as the line seems to form at higher geometrical heights where the field is expected to be lower.Comment: Accepted in A&

Three-dimensional magnetic structure of a sunspot: comparison of the photosphere and upper chromosphere

We investigate the magnetic field of a sunspot in the upper chromosphere and compare it to the field's photospheric properties. We observed the main leading sunspot of the active region NOAA 11124 on two days with the Tenrife Infrared Polarimeter-2 (TIP-2) mounted at the German Vacuum Tower Telescope (VTT). Through inversion of Stokes spectra of the He I triplet at 1083.0 nm, we obtained the magnetic field vector of the upper chromosphere. For comparison with the photosphere we applied height-depended inversions of the Si I 1082.71 nm and Ca I 1083.34 nm lines. We found that the umbral magnetic field strength in the upper chromosphere is lower by a factor of 1.30-1.65 compared to the photosphere. The magnetic field strength of the umbra decreases from the photosphere towards the upper chromosphere by an average rate of 0.5-0.9 G km$^{-1}$. The difference in the magnetic field strength between both atmospheric layers steadily decreases from the sunspot center to the outer boundary of the sunspot, with the field (in particular its horizontal component) being stronger in the chromopshere outside the spot, suggestive of a magnetic canopy. The sunspot displays a twist that on average is similar in the two layers. However, the differential twist between photosphere and chromosphere increases rapidly towards the outer penumbral boundary. The magnetic field vector is more horizontal with respect to the solar surface by roughly 5-20$^\circ$ in the photosphere compared to the upper chromosphere. Above a lightbridge, the chromospheric magnetic field is equally strong as that in the umbra, whereas the lightbridge's field is weaker than its surroundings in the photosphere by roughly 1 kG. This suggests a cusp-like magnetic field structure above the lightbridge.Comment: 12 pages, 15 figures, accepted for publication in A&

The cause of spatial structure in solar He I 1083 nm multiplet images

Context. The He i 1083 nm is a powerful diagnostic for inferring properties of the upper solar chromosphere, in particular for the magnetic field. The basic formation of the line in one-dimensional models is well understood, but the influence of the complex 3D structure of the chromosphere and corona has however never been investigated. This structure must play an essential role because images taken in He i 1083 nm show structures with widths down to 100 km. Aims. To understand the effect of the three-dimensional temperature and density structure in the solar atmosphere on the formation of the He i 1083 nm line. Methods. We solve the non-LTE radiative transfer problem assuming statistical equilibrium for a simple 9-level helium atom that nevertheless captures all essential physics. As a model atmosphere we use a snapshot from a 3D radiation-MHD simulation computed with the Bifrost code. Ionising radiation from the corona is self-consistently taken into account. Results. The emergent intensity in the He i 1083 nm is set by the source function and the opacity in the upper chromosphere. The former is dominated by scattering of photospheric radiation and does not vary much with spatial location. The latter is determined by the photonionisation rate in the He i ground state continuum, as well as the electron density in the chromosphere. The spatial variation of the flux of ionising radiation is caused by the spatially-structured emissivity of the ionising photons from material at T = 100 kK in the transition region. The hotter coronal material produces more ionising photons, but the resulting radiation field is smooth and does not lead to small-scale variation of the UV flux. The corrugation of the transition region further increases the spatial variation of the amount of UV radiation in the chromosphere.Comment: Accepted for publication by A&

Vertical magnetic field gradient in the photospheric layers of sunspots

We investigate the vertical gradient of the magnetic field of sunspots in the photospheric layer. Independent observations were obtained with the SOT/SP onboard the Hinode spacecraft and with the TIP-2 mounted at the VTT. We apply state-of-the-art inversion techniques to both data sets to retrieve the magnetic field and the corresponding vertical gradient. In the sunspot penumbrae we detected patches of negative vertical gradients of the magnetic field strength, i.e.,the magnetic field strength decreases with optical depth in the photosphere. The negative gradient patches are located in the inner and partly in the middle penumbrae in both data sets. From the SOT/SP observations, we found that the negative gradient patches are restricted mainly to the deep photospheric layers and are concentrated near the edges of the penumbral filaments. MHD simulations also show negative gradients in the inner penumbrae, also at the locations of filaments. Both in the observations and simulation negative gradients of the magnetic field vs. optical depth dominate at some radial distances in the penumbra. The negative gradient with respect to optical depth in the inner penumbrae persists even after averaging in the azimuthal direction, both in the observations and, to a lesser extent, also in MHD simulations. We interpret the observed localized presence of the negative vertical gradient of the magnetic field strength in the observations as a consequence of stronger field from spines expanding with height and closing above the weaker field inter-spines. The presence of the negative gradients with respect to optical depth after azimuthal averaging can be explained by two different mechanisms: the high corrugation of equal optical depth surfaces and the cancellation of polarized signal due to the presence of unresolved opposite polarity patches in the deeper layers of the penumbra.Comment: 17 pages, 25 figures, accepted for publication in A&

Characterization and formation of on-disk spicules in the Ca II K and Mg II k spectral lines

We characterize, for the first time, type-II spicules in Ca II K 3934\AA\ using the CHROMIS instrument at the Swedish 1-m Solar Telescope. We find that their line formation is dominated by opacity shifts with the K$_{3}$ minimum best representing the velocity of the spicules. The K$_{2}$ features are either suppressed by the Doppler-shifted K$_{3}$ or enhanced via an increased contribution from the lower layers, leading to strongly enhanced but un-shifted K$_{2}$ peaks, with widening towards the line-core as consistent with upper-layer opacity removal via Doppler-shift. We identify spicule spectra in concurrent IRIS Mg II k 2796\AA\ observations with very similar properties. Using our interpretation of spicule chromospheric line-formation, we produce synthetic profiles that match observations.Comment: 10 pages, 8 figures, accepted for publication in Astronomy and Astrophysics Letter

Observations of Ellerman bomb emission features in He I D3 and He I 10830 {\AA}

Context. Ellerman bombs (EBs) are short-lived emission features, characterized by extended wing emission in hydrogen Balmer lines. Until now, no distinct signature of EBs has been found in the He I 10830 {\AA} line, and conclusive observations of EBs in He I D 3 have never been reported. Aims. We aim to study the signature of EBs in neutral helium triplet lines. Methods. The observations consist of 10 consecutive SST/TRIPPEL raster scans close to the limb, featuring the H$\beta$, He I D3 and He I 10830 {\AA} spectral regions. We also obtained raster scans with IRIS and make use of the SDO/AIA 1700 {\AA} channel. We use Hazel to invert the neutral helium triplet lines. Results. Three EBs in our data show distinct emission signatures in neutral helium triplet lines, most prominently visible in the He I D3 line. The helium lines have two components: a broad and blue-shifted emission component associated with the EB, and a narrower absorption component formed in the overlying chromosphere. One of the EBs in our data shows evidence of strong velocity gradients in its emission component. The emission component of the other two EBs could be fitted using a constant slab. Our analysis hints towards thermal Doppler motions having a large contribution to the broadening for helium and IRIS lines. We conclude that the EBs must have high temperatures to exhibit emission signals in neutral helium triplet lines. An order of magnitude estimate places our observed EBs in the range of $T\sim 2\cdot 10^4-10^5$ K.Comment: 15 pages, 14 figure

Monte-Carlo simulations of globular cluster dynamics

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 2000.Includes bibliographical references (leaves 156-163).We present the results of theoretical calculations for the dynamical evolution of dense globular star clusters. Our new study was motivated in part by the wealth of new data made available from the latest optical, radio, and X-ray observations of globular clusters by various satellites and ground-based observatories, and in part by recent advances in computer hardware. New parallel supercomputers, combined with improved computational methods, now allow us to perform dynamical simulations of globular cluster evolution using a realistic number of stars (N - 10 - 106) and taking into account the full range of relevant stellar dynamical and stellar evolutionary processes. These processes include two-body gravitational scattering, strong interactions and physical collisions involving both single and binary stars, stellar evolution of single stars, and stellar evolution and interactions in close binary stars. We have developed a new numerical code for computing the dynamical evolution of a dense star cluster. Our code is based on a Monte Carlo technique for integrating numerically the Fokker-Planck equation. We have used this new code to study a number of important problems. In particular, we have studied the evolution of globular clusters in our Galaxy, including the effects of a mass spectrum, mass loss due to the tidal field of the Galaxy, and stellar evolution. Our results show that the direct mass loss from stellar evolution can significantly accelerate the total mass loss from a globular cluster, causing most clusters with low initial central concentrations to disrupt completely. Only clusters born with high central concentrations, or with relatively few massive stars, are likely to survive until the present and remain observable. Our study of mass segregation in clusters shows that it is possible to retain significant numbers of very-low-mass (m < 0.1M.) objects, such as brown dwarfs or planets, in the outer halos of globular clusters, even though they are quickly lost from the central, denser regions. This is contrary to the common belief that globular clusters are devoid of such low-mass objects. We have also performed, for the first time, dynamical simulations of clusters containing a realistic number of stars and a significant fraction of binaries. We find that the energy generated through binarybinary and binary-single-star interactions in the cluster core can support the system against gravothermal collapse on timescales exceeding the age of the Universe, explaining naturally the properties of the majority of observed globular clusters with resolved cores.by Kriten J. Joshi.Ph.D

Intensity and velocity oscillations in a flaring active region

Chromospheric oscillations can give us insight into the physical environment in the solar atmosphere, both in quiet Sun and flaring conditions. Many authors have reported increases in the prevalence of 3-minute oscillations which are thought to be excited by events which impact the chromosphere such as flares. In this study, we utilized the Ca II 8542 Å line to study the oscillatory behaviour of the chromosphere in an active region which underwent two B-class flares. We analysed oscillations in both intensity and velocity, and found different behaviours in both. Intensity oscillations were most prevalent over the umbrae of sunspots and magnetic pores in the active region, and the extent of the area which contained significant oscillations was found to decrease when comparing times after the flares to before. By measuring the evolution of the magnetic field, we found that this could be because the areas surrounding the umbrae were becoming more ‘penumbral’ with an increase to the magnetic field inclination. Velocity oscillations were found across the active region both before and after the flares but were observed clearly in areas which were brightened by the second flare. By comparing to EUV imaging, it was seen that strong chromospheric velocity oscillations with 3–4-minute periods occurred at the same time and location as a flare loop cooling 30 min after the second flare peak. This could be evidence of disturbances in the loop exciting a response from the chromosphere at its acoustic cut-off frequency

Effect of Polarimetric Noise on the Estimation of Twist and Magnetic Energy of Force-Free Fields

The force-free parameter $\alpha$, also known as helicity parameter or twist parameter, bears the same sign as the magnetic helicity under some restrictive conditions. The single global value of $\alpha$ for a whole active region gives the degree of twist per unit axial length. We investigate the effect of polarimetric noise on the calculation of global $\alpha$ value and magnetic energy of an analytical bipole. The analytical bipole has been generated using the force-free field approximation with a known value of constant $\alpha$ and magnetic energy. The magnetic parameters obtained from the analytical bipole are used to generate Stokes profiles from the Unno-Rachkovsky solutions for polarized radiative transfer equations. Then we add random noise of the order of 10$^{-3}$ of the continuum intensity (I$_{c}$) in these profiles to simulate the real profiles obtained by modern spectropolarimeters like Hinode (SOT/SP), SVM (USO), ASP, DLSP, POLIS, SOLIS etc. These noisy profiles are then inverted using a Milne-Eddington inversion code to retrieve the magnetic parameters. Hundred realizations of this process of adding random noise and polarimetric inversion is repeated to study the distribution of error in global $\alpha$ and magnetic energy values. The results show that : (1). the sign of $\alpha$ is not influenced by polarimetric noise and very accurate values of global twist can be calculated, and (2). accurate estimation of magnetic energy with uncertainty as low as 0.5% is possible under the force-free condition.Comment: 33 pages, 6 figures, 1 table; Accepted for publication in the Ap