The magnetic power spectrum analysis provides an effective way to understand the observed distribution of the photospheric magnetic fields and their interaction with plasma motions. We aim to investigate the power spectra using spherical harmonic decomposition of SOHO/MDI and SDO/HMI synoptic magnetograms for cycles 23 and 24. We find that the calibration factor between MDI and HMI power spectral density is spatial scale-dependent. The magnetic power spectra show two peaks at the AR scale (l≈30) and supergranular scale (l≈120). The power law indices between these show a good anti-correlation with the amplitude of magnetic activity.

It is believed that the tilt in the bipolar magnetic regions (BMRs) is produced due to a torque induced by the Coriolis force, acting on the diverging flow from the apex of the rising flux tube of the toroidal field in the solar convection zone (SCZ).The BMRs with a strong magnetic field are expected to have reduced tilt as they rise very quickly in the SCZ. This effect can provide the required nonlinear quenching mechanism to suppress the growth of magnetic filed in the dynamo models. Here, we use the magnetograms of the Michelson Doppler Imager (1996–2011) and Helioseismic and Magnetic Imager (2010–2018) to automatically detect the BMRs and look for the signature of tilt quenching. Based on the Bayesian inference method, our results show that the posterior distribution of quenching parameters is Gaussian, and the mean of this distribution agrees with the earlier findings.

Properties of bipolar magnetic regions (BMRs), particularly, the tilt angle play critical roles in generating the observed polar magnetic field and its reversal. Hence, a long-term study of BMR over its lifetime is crucial not only to understand the solar dynamo but also to identify the origin of the properties of BMR. In our work, we have developed an automatic algorithm to detect and track the BMRs from the line-of-sight (LOS) magnetograms of Michelson Doppler Imager (MDI) for the period of Solar Cycle 23 over its lifetime/disk passage. Here, we present the details of our algorithm and the features of BMR, particularly the tilt angle, magnetic field strength and lifetime.

The Aditya-L1 is the first space-based solar observatory of the Indian Space Research Organization (ISRO). The spacecraft will carry seven payloads providing uninterrupted observations of the Sun from the first Lagrangian point. Aditya-L1 comprises four remote sensing instruments, viz. a coronagraph observing in visible and infrared, a full disk imager in Near Ultra-Violet (NUV), and two full-sun integrated spectrometers in soft X-ray and hard X-ray. In addition, there are three instruments for in-situ measurements, including a magnetometer, to study the magnetic field variations during energetic events. Aditya-L1 is truly a mission for multi-messenger solar astronomy from space that will provide comprehensive observations of the Sun across the electromagnetic spectrum and in-situ measurements in a broad range of energy, including magnetic field measurements at L1.

Solar flares are an explosive manifestation of the complex magnetic structuring of active regions in the solar atmosphere. The photospheric magnetic field is found to change rapidly, abruptly, and significantly during flaring events. Previous studies are mainly based on line-of-sight or low-cadence data. In this work, we focus on the temporal and spatial evolution of the permanent changes in the magnetic field of solar flares from high-cadence vector data (135 seconds) of the imaging system (dopplergrams and magnetograms) of the SDO/HMI instrument. The highly energetic events under analysis occurred during the solar cycle 24, covering low and high energy ranges, according to GOES classification. This investigation also stands as a crucial input for the characterization and understanding of sunquakes.

Regular observations of the solar magnetic field are available only for about the last five cycles. Thus, to understand the origin of the variation of the solar magnetic field, it is essential to reconstruct the magnetic field for the past cycles, utilizing the proxies of the magnetic field from other data sets. Long-term uniform observations for the past 100 yrs, as recorded at the Kodaikanal Solar Observatory (KoSO), in multi-wavelengths provide such an opportunity. Various automatic techniques have been developed to extract these features from KoSO data. We analyzed the properties of these extracted features to understand global solar magnetism in the past.

We study the evolution of the decaying active region NOAA 12708, from the photosphere up to the corona using high resolution, multi-wavelength GREGOR observations taken on May 9, 2018. We utilize spectropolarimetric scans of the 10830 Å spectral range by the GREGOR Infrared Spectrograph (GRIS), spectral imaging time-series in the Na ID2 spectral line by the GREGOR Fabry-Pérot Interferometer (GFPI) and context imaging in the Ca IIH and blue continuum by the High-resolution Fast Imager (HiFI). Context imaging in the UV/EUV from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) complements our dataset. The region under study contains one pore with a light-bridge, a few micro-pores and extended clusters of magnetic bright points. We study the magnetic structure from the photosphere up to the upper chromosphere through the spectropolarimetric observations in He II and Si I and through the magnetograms provided by the Helioseismic and Magnetic Imager (HMI). The high-resolution photospheric images reveal the complex interaction between granular-scale convective motions and a range of scales of magnetic field concentrations in unprecedented detail. The pore itself shows a strong interaction with the convective motions, which eventually leads to its decay, while, under the influence of the photospheric flow field, micro-pores appear and disappear. Compressible waves are generated, which are guided towards the upper atmosphere along the magnetic field lines of the various magnetic structures within the field-of-view. Modelling of the He i absorption profiles reveals high velocity components, mostly associated with magnetic bright points at the periphery of the active region, many of which correspond to asymmetric Si I Stokes-V profiles revealing a coupling between upper photospheric and upper chromospheric dynamics. Time-series of Na ID2 spectral images reveal episodic high velocity components at the same locations. State-of-the-art multi-wavelength GREGOR observations allow us to track and understand the mechanisms at work during the decay phase of the active region.

The Sun is our dynamic host star due to its magnetic fields causing plentiful of activity in its atmosphere. From high energetic flares and coronal mass ejections (CMEs) to lower energetic phenomena such as jets and fibrils. Thus, it is of crucial importance to learn about formation and evolution of solar magnetic fields. These fields cover a wide range of spatial and temporal scales, starting on the larger end with active regions harbouring complex sunspots, via isolated pores, down to the smallest yet resolved elements – so-called magnetic bright points (MBPs). Here, we revisit the various manifestations of solar magnetic fields by the largest European solar telescope in operation, the 1.5-meter GREGOR telescope. We show images from the High-resolution Fast Imager (HiFI) and spectropolarimetric data from the GREGOR Infrared Spectrograph (GRIS). Besides, we outline resolved convective features inside the larger structures – so-called light-bridges occurring on large to mid-sized scales.

Polar magnetic field, as a component produced by the global dynamo, is thought to be the remant of toroidal magnetic field transported poleward from Sun’s active belt. With the improvement of instruments, more and more observations are challenging the viewpoint. Recently, we identify the bipolar magnetic emergences (BMEs) in the polar region, and find that the distribution of the magnetic axes for these BMEs shows random state, which does not follow the Joy’s law of active region. The result implies the possible existence of local dynamo in the solar polar region.

Planetary transits are used to measure the solar radius since the beginning of the 18th century and are the most accurate direct method to measure potentially long-term variation in the solar size. Historical measures present a range of values dominated by systematic errors from different instruments and observers. Atmospheric seeing and black drop effect contribute as error sources for the precise timing of the planetary transit ground observations. Both Solar and Heliospheric Observatory (SOHO) and Solar Dynamics Observatory (SDO) made observations of planetary transits from space to derive the solar radius. The International Astronomical Union approved the resolution B3 in 2015, defining a nominal solar radius of precisely 695,700 km. In this work, we show that this value is off by more than 300 km, which is one order of magnitude higher than the error of the most recent solar radius observations.

We report on multi-wavelength ultraviolet (UV) high-resolution observations taken with the IRIS satellite during the emergence phase of an emerging flux region embedded in the unipolar plage of active region NOAA 12529. These data are complemented by measurements taken with the spectropolarimeter aboard the Hinode satellite and by observations from SDO.

In the photosphere, we observe the appearance of opposite emerging polarities, separating from each other, and cancellation with a pre-existing flux concentration of the plage.

In the upper atmospheric layers, recurrent brightenings resembling UV bursts, with counterparts in all UV/EUV filtergrams, are identified in the EFR site. In addition, plasma ejections are observed at chromospheric level. Most important, we unravel a signature of plasma heated up to 1 MK detecting Fe XII emission in the core of the brightening sites.

Comparing these findings with previous observations and numerical models, we suggest evidence of several long-lasting, small-scale magnetic reconnection episodes between the new bipolar EFR and the ambient field.

Sunspots are the most prominent feature of the solar magnetism in the photosphere. Although they have been widely investigated in the past, their structure remains poorly understood. Indeed, due to limitations in observations and the complexity of the magnetic field estimation at chromospheric heights, the magnetic field structure of sunspot above the photosphere is still uncertain. Improving the present knowledge of sunspot is important in solar and stellar physics, since spot generation is seen not only on the Sun, but also on other solar-type stars. In this regard, we studied a large, isolated sunspot with spectro-polarimeteric measurements that were acquired at the Fe I 6173 nm and Ca II 8542 nm lines by the spectropolarimeter IBIS/DST under excellent seeing conditions lasting more than three hours. Using the Non-LTE inversion code NICOLE, we inverted both line measurements simultaneously, to retrieve the three-dimensional magnetic and thermal structure of the penumbral region from the bottom of the photosphere to the middle chromosphere. Our analysis of data acquired at spectral ranges unexplored in previous studies shows clear spine and intra-spine structure of the penumbral magnetic field at chromopheric heights. Our investigation of the magnetic field gradient in the penumbra along the vertical and azimuthal directions confirms results reported in the literature from analysis of data taken at the spectral region of the He I 1083 nm triplet.

Due to its unique 400-year duration, the sunspot number is a central reference for understanding the long-term evolution of solar activity and its influence on the Earth environment and climate. Here, we outline current data recovery work. For the sunspot number, we find historical evidence of a disruption in the source observers occurring in 1947–48. For the sunpot group number, recent data confirm the clear southern predominance of sunspots during the Maunder Minimum, while the umbra-penumbra ratio is similar to other epochs. For the Dalton minimum, newly recovered historical observations confirm a higher activity level than in a true Grand Minimum.

The observed convective flows on the photosphere (e.g., supergranulation, granulation) play a key role in the Babcock-Leighton (BL) process to generate large scale polar fields from sunspots fields. In most surface flux transport (SFT) and BL dynamo models, the dispersal and migration of surface fields is modeled as an effective turbulent diffusion. We present the first kinematic 3D FT/BL model to explicitly incorporate realistic convective flows based on solar observations. The results obtained are generally in good agreement with the observed surface flux evolution and with non-convective models that have a turbulent diffusivity on the order of 3 × 1012 cm2 s−1 (300 km2 s−1). However, we find that the use of a turbulent diffusivity underestimates the dynamo efficiency, producing weaker mean fields and shorter cycle.

We utilized line-of-sight magnetograms acquired by HMI/SDO to derive the value of turbulent magnetic diffusivity in undisturbed photosphere. Two areas, a coronal hole area (CH) and an area a super-granulation pattern, SG, were analyzed. The behavior of the turbulent diffusion coefficient on time scales of 1000-40000 s and spatial scales of 500-6000 km was explored. Small magnetic elements in both CH and SG areas disperse in the same way and they are more mobile than the large elements. The regime of super-diffusivity is found for small elements (the turbulent diffusion coefficient K growths from 100 to 300 km2 s−1). Large magnetic elements disperse differently in the CH and SG areas. Comparison of these results with the previously published shows that there is a tendency of saturation of the diffusion coefficient on large scales, i.e., the turbulent regime of super-diffusivity gradually ceases so that normal diffusion with a constant value of K ≈ 500 km2 s−1 might be observed on time scales longer than a day. The results show that the turbulent diffusivity should not be considered in modeling as a scalar, the flux- and scale-dependence is obvious.

Probably, the long-term monitoring of the solar atmosphere started in Italy with the first telescopic observations of the Sun made by Galileo Galilei in the early 17th century. His recorded observations and science results, as well as the work carried out by other following outstanding Italian astronomers inspired the start of institutional programs of regular solar observations at the Arcetri, Catania, and Rome Observatories.

These programs have accumulated daily images of the solar photosphere and chromosphere taken at various spectral bands over a time span larger than 80 years. In the last two decades, regular solar observations were continued with digital cameras only at the Catania and Rome Observatories, which are now part of the INAF National Institute for Astrophysics. At the two sites, daily solar images are taken at the photospheric G-band, Blue (λ = 409.4 nm), and Red (λ = 606.9 nm) continua spectral ranges and at the chromospheric Ca II K and Hα lines, with a 2″ spatial resolution.

Solar observation in Italy, which benefits from over 2500 hours of yearly sunshine, currently aims at the operational monitoring of solar activity and long-term variability and at the continuation of the historical series as well. Existing instruments will be soon enriched by the SAMM double channel telescope equipped with magneto-optical filters that will enable the tomography of the solar atmosphere with simultaneous observations at the K I 769.9 nm and Na I D 589.0 nm lines. In this contribution, we present the available observations and outline their scientific relevance.

The tilt angle of a sunspot group is a critical quantity in the surface transport of magnetic flux and the solar dynamo. To contribute long-term databases of the tilt angle, we developed an IDL routine, which allows the user to interactively select and measure sunspot positions and areas on the solar disc. We measured the tilt angles of sunspot groups for solar cycles 19-24 (1954.6-2017.8), using the sunspot drawing database of Kandilli Observatory. The method is similar to that used in the discontinued Mt. Wilson and Kodaikanal databases, with the exception that sunspot groups were identified manually, which has improved the accuracy of the resulting tilt angles. We obtained cycle averages of the tilt angle and compared them with the values from other datasets, keeping the same group selection criteria. We conclude that the previously reported anti-correlation with the cycle strength needs further investigation.

We re-examined solar polar magnetic fields, using ground based synoptic photospheric magnetograms, during solar cycle 24. IThe signed polar magnetic fields showed an unusual hemispheric asymmetry in the polar field reversal process with a single unambigous reversal in the Southern hemisphere around late 2013 while the polar reversal in the Northern hemisphere started earlier around June 2012, but was completed only by the end of 2014. The examination of the unsigned polar magnetic fields in cycle 24 showed a continuing decline of fields in the Northern hemisphere whereas in the Southern hemisphere, it had partially recovered. However, the overall declining trend in solar polar fields, which began in the mid-1990’s, is still in progress. The continued decline seen in solar photospheric fields raises thequestion of whether we are heading towards a Grand or Maunder like solar minimum.

We analyze the flux emergence rate of solar active regions (ARs). Numerical simulations by other authors suggest that the flux emergence rate depends on the AR’s twist. To prove this statement observationally, we make a comparison of the flux emergence rate and twist of 215 emerging ARs. Our results confirm that the correlation exists: the higher the twist the higher the flux emergence rate of an AR. We suppose that the difference in the twist can be caused by chaotic influence of the convective plasma motions on the lifting magnetic flux tube.

The solar oscillation frequencies have shown variation over the solar activity cycle, which is believed to be the indicator of the structural and magnetic changes taking place in the Sun. The ground-based network of six identical solar telescopes in the Global Oscillation Network Group (GONG) program has been nearly-continuously observing the Sun since the last quarter of the year 1995 for Doppler imaging of the solar-disk aimed to study the oscillations and velocity flows on the surface of the Sun. In this work, we study the variations in the solar disk-integrated mean velocity flows on the solar surface as observed with the GONG over the complete Solar Cycle 23 and ongoing Cycle 24. The correlation analysis of these solar photospheric mean velocity flows relative to the various solar activity indicators is also discussed.