scholarly journals Spiral-shaped wavefronts in a sunspot umbra

2019 ◽  
Vol 621 ◽  
pp. A43 ◽  
Author(s):  
T. Felipe ◽  
C. Kuckein ◽  
E. Khomenko ◽  
I. Thaler
Keyword(s):  
Magnetic Field ◽  
Active Regions ◽  
Field Vector ◽  
Doppler Velocity ◽  
Data Set ◽  
Magnetic Field Topology ◽  
Magnetoacoustic Waves ◽  
Fabry Perot ◽  
Field Lines ◽  
The Magnetic Field

Context. Solar active regions show a wide variety of oscillatory phenomena. The presence of the magnetic field leads to the appearance of several wave modes whose behavior is determined by the sunspot thermal and magnetic structure. Aims. We aim to study the relation between the umbral and penumbral waves observed at the high photosphere and the magnetic field topology of the sunspot. Methods. Observations of the sunspot in active region NOAA 12662 obtained with the GREGOR telescope (Observatorio del Teide, Tenerife, Spain) were acquired on 2017 June 17. The data set includes a temporal series in the Fe I 5435 Å line obtained with the imaging spectrograph GREGOR Fabry-Pérot Interferometer (GFPI) and a spectropolarimetric raster map acquired with the GREGOR Infrared Spectrograph (GRIS) in the 10 830 Å spectral region. The Doppler velocity deduced from the restored Fe I 5435 Å line has been determined, and the magnetic field vector of the sunspot has been inferred from spectropolarimetric inversions of the Ca I 10 839 Å and the Si I 10 827 Å lines. Results. A two-armed spiral wavefront has been identified in the evolution of the two-dimensional velocity maps from the Fe I 5435 Å line. The wavefronts initially move counterclockwise in the interior of the umbra, and develop into radially outward propagating running penumbral waves when they reach the umbra-penumbra boundary. The horizontal propagation of the wavefronts approximately follows the direction of the magnetic field, which shows changes in the magnetic twist with height and horizontal position. Conclusions. The spiral wavefronts are interpreted as the visual pattern of slow magnetoacoustic waves which propagate upward along magnetic field lines. Their apparent horizontal propagation is due to their sequential arrival to different horizontal positions at the formation height of the Fe I 5435 Å line, as given by the inclination and orientation of the magnetic field.

scholarly journals Cusp-like plasma in high altitudes: a statistical study of the width and location of the cusp from Magion-4

2002 ◽  
Vol 20 (3) ◽  
pp. 311-320 ◽  
Author(s):  
J. Mĕrka ◽  
J. Šafránková ◽  
Z. Nĕmeček
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
High Altitudes ◽  
Data Set ◽  
Latitudinal Dependence ◽  
Merging Process ◽  
Low Altitude ◽  
Field Lines ◽  
The Magnetic Field ◽  
Dipole Tilt

Abstract. The width of the cusp region is an indicator of the strength of the merging process and the degree of opening of the magnetosphere. During three years, the Magion-4 satellite, as part of the Interball project, has collected a unique data set of cusp-like plasma observations in middle and high altitudes. For a comparison of high- and low-altitude cusp determination, we map our observations of cusp-like plasma along the magnetic field lines down to the Earth’s surface. We use the Tsyganenko and Stern 1996 model of the magnetospheric magnetic field for the mapping, taking actual solar wind and IMF parameters from the Wind observations. The footprint positions show substantial latitudinal dependence on the dipole tilt angle. We fit this dependence with a linear function and subtract this function from observed cusp position. This process allows us to study both statistical width and location of the inspected region as a function of the solar wind and IMF parameters. Our processing of the Magion-4 measurements shows that high-altitude regions occupied by the cusp-like plasma (cusp and cleft) are projected onto a much broader area (in magnetic local time as well as in a latitude) than that determined in low altitudes. The trends of the shift of the cusp position with changes in the IMF direction established by low-altitude observations have been confirmed.Key words. Magnetospheric physics (magnetopause, cusp and boundary layer; solar wind – magnetosphere interactions)

scholarly journals EUV Line Intensities and the Magnetic Field in Solar Active Regions

2001 ◽  
Vol 203 ◽  
pp. 276-279
Author(s):  
J. Ireland ◽  
A. Fludra
Keyword(s):  
Magnetic Field ◽  
Extreme Ultraviolet ◽  
Active Regions ◽  
Central Meridian ◽  
Coronal Emission ◽  
Data Set ◽  
Line Intensities ◽  
Doppler Imager ◽  
Synoptic Observations ◽  
The Magnetic Field

The Coronal Diagnostic Spectrometer (CDS) on SOHO carries out daily synoptic observations of the Sun in four EUV (extreme ultraviolet) spectra: He I 584 Å, O V 630 Å, Mg IX 368 Å and Fe XVI 360 Å, over a 4 arcmin-wide strip along the solar central meridian. Using 53 active regions observed in this data set along with co-temporally observed SOHO-MDI (Michelson Doppler Imager) magnetograms we study the correlation of the chromospheric, transition region and coronal emission with the photospheric magnetic field for meridional active regions, probing the relation between the radiative output and magnetic observables. We also establish empirical, quantitative relations among intensities of different lines, and between intensities and the magnetic field flux.

scholarly journals Characterization of the umbra–penumbra boundary by the vertical component of the magnetic field

2020 ◽  
Vol 638 ◽  
pp. A25
Author(s):  
P. Lindner ◽  
R. Schlichenmaier ◽  
N. Bello González
Keyword(s):  
Magnetic Field ◽  
Near Infrared ◽  
Statistical Study ◽  
Vertical Component ◽  
Fundamental Property ◽  
Field Vector ◽  
Data Sets ◽  
Data Set ◽  
Geometric Difference ◽  
The Magnetic Field

Context. The vertical component of the magnetic field was found to reach a constant value at the boundary between penumbra and umbra of stable sunspots in a recent statistical study of Hinode/SP data. This finding has profound implications as it can serve as a criterion to distinguish between fundamentally different magneto-convective modes operating in the sun. Aims. The objective of this work is to verify the existence of a constant value for the vertical component of the magnetic field (B⊥) at the boundary between umbra and penumbra from ground-based data in the near-infrared wavelengths and to determine its value for the GREGOR Infrared Spectrograph (GRIS@GREGOR) data. This is the first statistical study on the Jurčák criterion with ground-based data, and we compare it with the results from space-based data (Hinode/SP and SDO/HMI). Methods. Eleven spectropolarimetric data sets from the GRIS@GREGOR slit-spectograph containing fully-fledged stable sunspots were selected from the GRIS archive. SIR inversions including a polarimetric straylight correction are used to produce maps of the magnetic field vector using the Fe I 15648 Å and 15662 Å lines. Averages of B⊥ along the contours between penumbra and umbra are analyzed for the 11 data sets. In addition, contours at the resulting B⊥const are drawn onto maps and compared to intensity contours. The geometric difference between these contours, ΔP, is calculated for each data set. Results. Averaged over the 11 sunspots, we find a value of B⊥const = (1787 ± 100) gauss. The difference from the values previously derived from Hinode/SP and SDO/HMI data is explained by instrumental differences and by the formation characteristics of the respective lines that were used. Contours at B⊥ = B⊥const and contours calculated in intensity maps match from a visual inspection and the geometric distance ΔP was found to be on the order of 2 pixels. Furthermore, the standard deviation between different data sets of averages along umbra–penumbra contours is smaller for B⊥ than for B∥ by a factor of 2.4. Conclusions. Our results provide further support to the Jurčák criterion with the existence of an invariable value B⊥const at the umbra–penumbra boundary. This fundamental property of sunspots can act as a constraining parameter in the calibration of analysis techniques that calculate magnetic fields. It also serves as a requirement for numerical simulations to be realistic. Furthermore, it is found that the geometric difference, ΔP, between intensity contours and contours at B⊥ = B⊥const acts as an index of stability for sunspots.

scholarly journals Flare induced penumbra formation in the sunspot of NOAA 10838

2010 ◽  
Vol 6 (S273) ◽  
pp. 303-307
Author(s):  
Sreejith Padinhatteeri ◽  
Sankarasubramanian K.
Keyword(s):  
Magnetic Field ◽  
Active Region ◽  
Magnetic Fields ◽  
Doppler Velocity ◽  
Solar Telescope ◽  
Vector Magnetic Field ◽  
Data Set ◽  
Intensity Images ◽  
Field Lines ◽  
The Way

AbstractWe have observed formation of penumbrae on a pore in the active region NOAA10838 using Dunn Solar Telescope at NSO, Sunpot, USA. Simultaneous observations using different instruments (DLSP, UBF, Gband and CaK) provide us with vector magnetic field at photosphere, intensity images and Doppler velocity at different heights from photosphere to chromosphere. Results from our analysis of this particular data-set suggests that penumbrae are formed as a result of relaxation of magnetic field due to a flare happening at the same time. Images in Hα show the flare (C 2.9 as per GOES) and vector magnetic fields show a re-orientation and reduction in the global α value (a measure of twist). We feel such relaxation of loop structures due to reconnections or flare could be one of the way by which field lines fall back to the photosphere to form penumbrae.

scholarly journals Helicity proxies from linear polarisation of solar active regions

2020 ◽  
Vol 641 ◽  
pp. A46 ◽  
Author(s):  
A. Prabhu ◽  
A. Brandenburg ◽  
M. J. Käpylä ◽  
A. Lagg
Keyword(s):  
Magnetic Field ◽  
Active Regions ◽  
Field Vector ◽  
Magnetic Helicity ◽  
Solar Dynamics Observatory ◽  
Solar Dynamics ◽  
Good Proxy ◽  
The Magnetic Field ◽  
Linear Polarisation ◽  
Polarisation Measurements

Context. The α effect is believed to play a key role in the generation of the solar magnetic field. A fundamental test for its significance in the solar dynamo is to look for magnetic helicity of opposite signs both between the two hemispheres as well as between small and large scales. However, measuring magnetic helicity is compromised by the inability to fully infer the magnetic field vector from observations of solar spectra, caused by what is known as the π ambiguity of spectropolarimetric observations. Aims. We decompose linear polarisation into parity-even and parity-odd E and B polarisations, which are not affected by the π ambiguity. Furthermore, we study whether the correlations of spatial Fourier spectra of B and parity-even quantities such as E or temperature T are a robust proxy for magnetic helicity of solar magnetic fields. Methods. We analysed polarisation measurements of active regions observed by the Helioseismic and Magnetic Imager on board the Solar Dynamics observatory. Theory predicts the magnetic helicity of active regions to have, statistically, opposite signs in the two hemispheres. We then computed the parity-odd EB and TB correlations and tested for a systematic preference of their sign based on the hemisphere of the active regions. Results. We find that: (i) EB and TB correlations are a reliable proxy for magnetic helicity, when computed from linear polarisation measurements away from spectral line cores; and (ii) E polarisation reverses its sign close to the line core. Our analysis reveals that Faraday rotation does not have a significant influence on the computed parity-odd correlations. Conclusions. The EB decomposition of linear polarisation appears to be a good proxy for magnetic helicity independent of the π ambiguity. This allows us to routinely infer magnetic helicity directly from polarisation measurements.

The Photospheric Flow near the Flare Locations of Active Regions

2000 ◽  
Vol 179 ◽  
pp. 249-250
Author(s):  
Debi Prasad Choudhary
Keyword(s):  
Magnetic Field ◽  
Active Regions ◽  
Field Of View ◽  
Image Motion ◽  
Line Center ◽  
Flare Activity ◽  
Data Set ◽  
Zero Crossing ◽  
Left And Right ◽  
The Magnetic Field

Extended AbstractThe observation of the photospheric velocity field along with the magnetic field is very important for understanding the origin and evolution of these locations of active regions. Earlier measurements have shown a general down flow with velocities of 0.2 to 0.3 km s−1in the active regions along with few locations of upflows. The localised upflows are observed in the light bridges and emerging flux regions with different speeds (Beckers & Schröter 1969). The flow patterns of flare locations in the active regions are observed by using the tower vector magnetograph (TVM) of Marshall Space Flight Centre. The line-center-magnetogram (LCM) technique has been employed to determine the active region velocities (Giovanelli & Ramsay 1971). The LCM is based on finding the wavelength in the line profile where two opposite circularly polarised Zeeman-split components change sign. If the material in the magnetic field of different locations have relative line of sight velocities, their cross-over wavelength will be seen to be Doppler shifted. In order to use the LCM with TVM, a series of Stoke-V images are made as a function of wavelength and their cross-over wavelength at each pixel is determined. We have observed 12 active regions between June 25th and August 25th, 1998. Three of these active regions (NOAA 8253, 8264 and 8307) show flare activity associated with the flux emergence and/or changes in magnetic shear during their disk passage. The images of a selected field of view in left and right circularly polarised Zeeman components in the wavelength range of 5250.12 to 5250.30 Å are obtained at 10 mÅ steps. The time taken for obtaining one set of observations is about 10–15 minutes. In this mode of operation, the start and end wavelengths are specified and the filter is tuned at desired wavelength steps. In one observing sequence, two sets of left and right circularly polarized images are produced as a function of wavelength. These sets of images are processed and merged following a certain procedure to produce a data cube. The most important requirement for the Doppler shift measurements is the repeatability of the wavelength steps. In the recent improvement, the filter tuning was achieved with accuracy better than 0.3 mÅ by using an optical encoder. However, it has been shown that insufficient spectral resolution would lead to spurious zero-crossing shift of asymmetric Stokes-V. This effect of spectral smearing in the case of observations with TVM and the present data analysis procedure has been estimated by simulation. The individual images are flat fielded and registered in order to remove the pixel sensitivity variation over the field of view and image motion during the observations. The two Zeeman components are subtracted to obtain a set of difference images as a function of wavelength. These processed images are merged to make Stokes-V data cubes, with two spatial and one-wavelength dimensions. The integrated Stokes-V profiles are obtained by averaging the profiles of the pixels with magnetic field values higher than a certain cut-off value depending on the noise level in each data set.

scholarly journals Chromospheric condensations and magnetic field in a C3.6-class flare studied via He I D3 spectro-polarimetry

2019 ◽  
Vol 621 ◽  
pp. A35 ◽  
Author(s):  
Tine Libbrecht ◽  
Jaime de la Cruz Rodríguez ◽  
Sanja Danilovic ◽  
Jorrit Leenaarts ◽  
Hiva Pazira
Keyword(s):  
Magnetic Field ◽  
Leading Edge ◽  
Field Vector ◽  
Line Of Sight ◽  
Substantial Part ◽  
Magnetic Field Topology ◽  
Chromospheric Line ◽  
The Impact ◽  
The Magnetic Field ◽  
Class Flare

Context. Magnetic reconnection during flares takes place in the corona, but a substantial part of flare energy is deposited in the chromosphere. However, high-resolution spectro-polarimetric chromospheric observations of flares are very rare. The most used observables are Ca II 8542 Å and He I 10830 Å. Aims. We aim to study the chromosphere during a C3.6 class flare via spectro-polarimetric observations of the He I D3 line. Methods. We present the first SST/CRISP spectro-polarimetric observations of He I D3. We analyzed the data using the inversion code HAZEL, and estimate the line-of-sight velocity and the magnetic field vector. Results. Strong He I D3 emission at the flare footpoints, as well as strong He I D3 absorption profiles tracing the flaring loops are observed during the flare. The He I D3 traveling emission kernels at the flare footpoints exhibit strong chromospheric condensations of up to ∼60 km s−1 at their leading edge. Our observations suggest that such condensations result in shocking the deep chromosphere, causing broad and modestly blueshifted He I D3 profiles indicating subsequent upflows. A strong and rather vertical magnetic field of up to ∼2500 G is measured in the flare footpoints, confirming that the He I D3 line is likely formed in the deep chromosphere at those locations. We provide chromospheric line-of-sight velocity and magnetic field maps obtained via He I D3 inversions. We propose a fan-spine configuration as the flare magnetic field topology. Conclusions. The He I D3 line is an excellent diagnostic to study the chromosphere during flares. The impact of strong condensations on the deep chromosphere has been observed. Detailed maps of the flare dynamics and the magnetic field are obtained.

scholarly journals Mode Coupling in the Solar Corona. III. Alfvén and Magnetoacoustic Waves

1977 ◽  
Vol 30 (4) ◽  
pp. 495 ◽  
Author(s):  
DB Melrose
Keyword(s):  
Magnetic Field ◽  
Solar Corona ◽  
Acoustic Waves ◽  
Mode Coupling ◽  
Stratified Medium ◽  
Magnetoacoustic Waves ◽  
Field Lines ◽  
Hydromagnetic Waves ◽  
Special Case ◽  
The Magnetic Field

Coupling between Alfven waves and fast mode waves obliquely incident on a stratified medium is treated using the method of Clemmow and Heading (1954) within the framework of the cold plasma approximation. A result due to Frisch (1964) is rederived in the special case of vertical incidence. The coupling is strongest for nearly parallel (to the magnetic field lines) propagation, and the coupling ratio may be approximated by Q = (00 /0)" where 0 is the angle between the wave vector and the magnetic field lines, while og = A/L, with A the wavelength and L the scalelength of the inhomogeneity. This result may be of significance in connection with the heating of the solar corona by the dissipation of waves generated initially as acoustic waves in the photosphere, and perhaps with the propagation of hydromagnetic waves in the interplanetary medium.

Emergence of Twisted Magnetic Flux Related Sigmoidal Brightening

2000 ◽  
Vol 179 ◽  
pp. 263-264
Author(s):  
K. Sundara Raman ◽  
K. B. Ramesh ◽  
R. Selvendran ◽  
P. S. M. Aleem ◽  
K. M. Hiremath
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Magnetic Flux ◽  
Ultra Violet ◽  
Active Regions ◽  
Morphological Properties ◽  
Energy Storage And Conversion ◽  
The Sun ◽  
X Rays ◽  
The Magnetic Field

Extended AbstractWe have examined the morphological properties of a sigmoid associated with an SXR (soft X-ray) flare. The sigmoid is cospatial with the EUV (extreme ultra violet) images and in the optical part lies along an S-shaped Hαfilament. The photoheliogram shows flux emergence within an existingδtype sunspot which has caused the rotation of the umbrae giving rise to the sigmoidal brightening.It is now widely accepted that flares derive their energy from the magnetic fields of the active regions and coronal levels are considered to be the flare sites. But still a satisfactory understanding of the flare processes has not been achieved because of the difficulties encountered to predict and estimate the probability of flare eruptions. The convection flows and vortices below the photosphere transport and concentrate magnetic field, which subsequently appear as active regions in the photosphere (Rust & Kumar 1994 and the references therein). Successive emergence of magnetic flux, twist the field, creating flare productive magnetic shear and has been studied by many authors (Sundara Ramanet al.1998 and the references therein). Hence, it is considered that the flare is powered by the energy stored in the twisted magnetic flux tubes (Kurokawa 1996 and the references therein). Rust & Kumar (1996) named the S-shaped bright coronal loops that appear in soft X-rays as ‘Sigmoids’ and concluded that this S-shaped distortion is due to the twist developed in the magnetic field lines. These transient sigmoidal features tell a great deal about unstable coronal magnetic fields, as these regions are more likely to be eruptive (Canfieldet al.1999). As the magnetic fields of the active regions are deep rooted in the Sun, the twist developed in the subphotospheric flux tube penetrates the photosphere and extends in to the corona. Thus, it is essentially favourable for the subphotospheric twist to unwind the twist and transmit it through the photosphere to the corona. Therefore, it becomes essential to make complete observational descriptions of a flare from the magnetic field changes that are taking place in different atmospheric levels of the Sun, to pin down the energy storage and conversion process that trigger the flare phenomena.

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