Main Sunspot Research Articles

A correlation between sunspot observations and solar Ca ii H&K activity proxies

ABSTRACT The magnetic phenomena on the solar surface have been the subject of several investigations over the last 400 yr. An early indicator of solar magnetic activity was sunspot counting. Currently, the main sunspot indicators are the international sunspot number, the sunspot group number, the total sunspot area, and the photometric sunspot index. Several improvements in observational techniques have allowed magnetic activity to be measured using solar/stellar spectra. Standard spectroscopic activity indicators are the $S_{\rm MW}$ index, based on the Ca ii H&K emission lines, and the chromospheric component $R^{\prime }_{\rm HK}$ index. In this context, we present a correlation between sunspot observations and solar Ca ii H&K activity proxies. We present our comparisons between the spectroscopic chromospheric activity proxies ($S_{\rm MW}$ and $R^{\prime }_{\rm HK}$) and the sunspot indicators over the last decades, using solar measurements (spectroscopic and spot proxy) performed on the same day. In general, our results indicate a linear fit between the chromospheric proxies and sunspot indicators. In addition, using the long-term sunspot group number records, we estimate an average spectroscopic proxy along the solar Maunder minimum (MM) phase, corresponding to $\mathinner {\langle {S_{\rm MW}}\rangle } = 0.167 \pm 0.013$ and $\mathinner {\log \langle {R^{\prime }_{\rm HK}}\rangle } = -4.913 \pm 0.363$. The estimated variability is $\sigma _{S_{\rm MW}} = 1.137 \times 10^{-7}$ and $\sigma _{\log R^{\prime }_{\rm HK}} = 2.704 \times 10^{-6}$. Our linear regression analysis, applied annually, suggests that the variability level of the chromospheric activity in the MM phase is significantly lower than in the normal period of activity and that this could be the result of linear regression on annually averaged data, combined with minimal sunspot activity during the solar MM phase. Further observations of MM analogues will be needed to test this hypothesis.

Expulsion of Counter Evershed Flows from Sunspot Penumbrae

In addition to the Evershed flow directed from the umbra toward the outer boundary of a sunspot, under special circumstances a counter Evershed flow (CEF) in the opposite direction also occurs. We aim to characterize the proper motions and evolution of three CEFs observed by the Solar Optical Telescope on board the Japanese Hinode spacecraft and the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We use state-of-the-art inversions of the radiative-transfer equation of polarized light applied to spectropolarimetric observations of the Fe i line pair around 630 nm. The three CEFs appeared within the penumbra. Two of the CEF structures, as part of their decay process, were found to move radially outwards through the penumbra parallel to the penumbral filaments with speeds, deduced from their proper motions, ranging between 65 and 117 m s−1. In these two cases, a new spot appeared in the moat of the main sunspot after the CEFs reached the outer part of the penumbra. Meanwhile, the CEFs moved away from the umbra, and their magnetic field strengths decreased. The expulsion of these two CEFs seems to be related to the normal Evershed flow. The third CEF appeared to be dragged by the rotation of a satellite spot. Chromospheric brightenings were found to be associated with the CEFs, and those CEFs that reached the umbra–penumbra boundary showed enhanced chromospheric activity. The two CEFs, for which line-of-sight velocity maps were available during their formation phase, appear as intrusions into the penumbra. They may be associated with magnetic flux emergence.

A forgotten sunspot record during the Maunder Minimum (Jean Charles Gallet, 1677)

ABSTRACT We have located a forgotten record of sunspot observations made by Jean Charles Gallet in the year 1677, in the middle of the Maunder Minimum (the period between 1645 and 1715 characterized by very low solar activity). Gallet observed one sunspot group from April 9 to 15 and another from October 1 to 6. While the group observed in April was also recorded by other astronomers of that time, no observer recorded the group observed by Gallet in 1677 October. Moreover, Gallet indicated the latter group was composed by four different spots and provided astrometric measurements of the main spot. We have calculated the latitudes corresponding to the main sunspot recorded in the October. The mean latitude is around 10º south. Therefore, this sunspot was observed in the southern hemisphere, as were most of the spots observed during the Maunder Minimum.

Tilt Angle of the Magnetic-Field Axis of Sunspots from Microwave Observations: Method and Measurement Results

The results of the measurements of the tilt of the magnetic-field axis of sunspots for active regions (ARs) with different morphological structures are presented. The observations are taken from the data archives of the RATAN-600 radio telescope and the Nobeyama Radioheliograph (NoRH) and processed with a new measurement method proposed by us earlier (Topchilo and Peterova, 2018). The method is based on the analysis of features of the microwave cyclotron source image above the sunspot and its dynamics with respect to the angle of view when the heliographic longitude of the sunspot changes due to the Sun’s rotation. We tested ten cases of observations and showed that the axis in most of them (~90%) is tilted toward the eastern limb and that the angle does not depend on the AR morphology. The degree of the tilt of the magnetic field axis of sunspots at the height of the lower part of the chromosphere–corona transition region (the boundary between them is at the height of ~2000 km) is ~1°–10° and increases with an increase in the height of the radiation source above the photosphere. Deviations from this rule are interpreted as the influence of neighboring ARs, as a result of which part of the magnetic flux of the main sunspot is closed, not on the tail of the AR as is the usual case, but on the neighboring, leading AR.

Automatic Detection of Sunspots and Extractionof Their Feature Parameters

Sunspots are solar features located in active regions of the Sun, whose number is an indicator of the Sun's magnetic activity. With a substantial increase in the quantity of solar image data, the automated detection and verification of various solar features have become increasingly important for the accurate and timely forecasts of solar activity and space weather. In order to use the high time-cadence SDO/HMI data to extract the main sunspot features for forecasting solar activities, we have established an automatic detection method of sunspots based on mathematical morphology, and calculated the sunspot group area and sunspot number. By comparing our results with those obtained from the Solar Region Summary compiled by NOAA/SWPC, it is found that the sunspot group areas and sunspot numbers computed with our algorithm are in good agreement with the active region values released by SWPC, and the corresponding correlation coefficients for the sunspot group area and sunspot number are 0.77 and 0.79, respectively. By using the method of this paper, the high time-cadence feature parameters can be obtained from the HMI data to provide the timely and accurate inputs for the solar activity forecast.

Distributed Electric Currents in Solar Active Regions

Using magnetographic data provided by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, we analyzed the structure of magnetic fields and vertical electric currents in six active regions (ARs) with different level of flare activity. We found that electric currents are well balanced over the entire AR: for all of them the current imbalance is below 0.1%, which means that any current system is closed within an AR. Decomposition of the transverse magnetic field vector into two components allowed us to reveal the existence of large-scale vortex structures of the azimuthal magnetic field component around main sunspots of ARs. In each AR, we found a large-scale electric current system occupying a vast area of an AR, which we call distributed electric current. For ARs obeying the Hale polarity law and the hemispheric helicity sign rule, the distributed current is directed upward in the leading part of an AR and it appears to be closing back to the photosphere in the following part of an AR through the corona and chromosphere. Our analysis of the time variations of the magnitude of the distributed electric currents showed that low-flaring ARs exhibit small variations of the distributed currents in the range of $\pm 20 \times 10^{12}$ A, whereas the highly flaring ARs exhibited significant slow variations of the distributed currents in the range of $30-95 \times 10^{12}$ A. Intervals of the enhanced flaring appear to be co-temporal with smooth enhancements of the distributed electric current.

Solar flare forecasting using morphological properties of sunspot groups

We describe a new tool developed for solar flare forecasting on the base of some sunspot group properties. Assuming that the flare frequency follows the Poisson statistics, this tool uses a database containing the morphological characteristics of the sunspot groups daily observed by the Equatorial Spar of INAF – Catania Astrophysical Observatory since January 2002 up today. By means of a linear combination of the flare rates computed on the base of some properties of the sunspot groups, like area, number of pores and sunspots, Zurich class, relative importance between leading spot and density of the sunspot population, and type of penumbra of the main sunspot, we determine the probability percentages that a flare of a particular energy range may occur. Comparing our forecasts with the flares registered by GOES satellites in the 1–8 Å X-ray band during the subsequent 24 h we measured the performance of our method. We found that this method, which combines some morphological parameters and a statistical technique, has the best performances for the strongest events, which are more interesting for their implications in the Earth environment.

Eruption of a multi-flux-rope system in solar active region 12673 leading to the two largest flares in Solar Cycle 24

Context. Solar active region (AR) 12673 in 2017 September produced the two largest flares in Solar Cycle 24: the X9.3 flare on September 6 and the X8.2 flare on September 10. Aims. We attempt to investigate the evolutions of the two large flares and their associated complex magnetic system in detail. Methods. Combining observations from the Solar Dynamics Observatory and results of nonlinear force-free field (NLFFF) modeling, we identify various magnetic structures in the AR core region and examine the evolution of these structures during the flares. Results. Aided by the NLFFF modeling, we identify a double-decker flux rope configuration above the polarity inversion line (PIL) in the AR core region. The north ends of these two flux ropes were rooted in a negative- polarity magnetic patch, which began to move along the PIL and rotate anticlockwise before the X9.3 flare on September 6. The strong shearing motion and rotation contributed to the destabilization of the two magnetic flux ropes, of which the upper one subsequently erupted upward due to the kink-instability. Then another two sets of twisted loop bundles beside these ropes were disturbed and successively erupted within five minutes like a chain reaction. Similarly, multiple ejecta components were detected as consecutively erupting during the X8.2 flare occurring in the same AR on September 10. We examine the evolution of the AR magnetic fields from September 3 to 6 and find that five dipoles emerged successively at the east of the main sunspot. The interactions between these dipoles took place continuously, accompanied by magnetic flux cancellations and strong shearing motions. Conclusions. In AR 12673, significant flux emergence and successive interactions between the different emerging dipoles resulted in a complex magnetic system, accompanied by the formations of multiple flux ropes and twisted loop bundles. We propose that the eruptions of a multi-flux-rope system resulted in the two largest flares in Solar Cycle 24.

Precursors of Magnetic Flux Emergence in the Moat Flows of Active Region AR12673

AbstractWe report on observations of magnetic disturbances in active region AR12673 between 1 and 3 September 2017 seen as a disruption of the moat flow several hours before the onset of strong flux emergence near the main sunspot. The moat flow is commonly known as a radially oriented strong outflow of photospheric plasma surrounding sunspots, which ends abruptly and thus shapes an annular pattern around the penumbra. Using highly accurate methods of tracking this photospheric flow applied to Solar Dynamics Observatory/Helioseismic and Magnetic Imager data, we are able to describe the evolution of the moat surrounding the main sunspot of AR 12673. We find that several hours before the emergence of strong magnetic flux near the main sunspot, the moat boundaries are broken at these very same locations. This behavior is observed both on 1 and 3 September. There is no such behavior observed in the absence of flux emergence. These observational results pose the question of how often they occur in other active regions and whether the disruption of the moat flow might be, like in this case, an indication of impending enhanced magnetic activity or simply a coincidental event.

Successive X-class Flares and Coronal Mass Ejections Driven by Shearing Motion and Sunspot Rotation in Active Region NOAA 12673

Abstract We present a clear case study on the occurrence of two successive X-class flares, including a decade-class flare (X9.3) and two coronal mass ejections (CMEs) triggered by shearing motion and sunspot rotation in active region NOAA 12673 on 2017 September 6. A shearing motion between the main sunspots with opposite polarities began on September 5 and lasted even after the second X-class flare on September 6. Moreover, the main sunspot with negative polarity rotated around its umbral center, and another main sunspot with positive polarity also exhibited a slow rotation. The sunspot with negative polarity at the northwest of the active region also began to rotate counterclockwise before the onset of the first X-class flare, which is related to the formation of the second S-shaped structure. The successive formation and eruption of two S-shaped structures were closely related to the counterclockwise rotation of the three sunspots. The existence of a flux rope is found prior to the onset of two flares by using nonlinear force-free field extrapolation based on the vector magnetograms observed by Solar Dynamics Observatory/Helioseismic and Magnetic Image. The first flux rope corresponds to the first S-shaped structures mentioned above. The second S-shaped structure was formed after the eruption of the first flux rope. These results suggest that a shearing motion and sunspot rotation play an important role in the buildup of the free energy and the formation of flux ropes in the corona that produces solar flares and CMEs.

The relationship between the 5-min oscillation and 3-min oscillations at the umbral/penumbral sunspot boundary

Observations of the main sunspot of AR 11692 were carried out with the 1 m New Vacuum Solar Telescope (NVST) located on the Fuxian Solar Observatory (FSO) in \(H\alpha\) on March 13, 2013. The high cadence (up to 12 s) \(H\alpha\) intensity images help us to investigate the relationship between the 5-min oscillation and 3-min oscillation. It is found that running waves, periodically formed at the wave sources within umbra, propagate outward with the shape of partial arcs. The running waves run across the umbra–penumbra boundary and eventually disappear at the edge of penumbra. But there are obvious differences when we measure the period of running waves in different regions of a sunspot. The period is about 150 s when the running waves are located in umbra, which is a typical 3-min oscillation, and the period is about 300 s when the running waves are located in the penumbra, which is a typical 5-min oscillation. On the basis of time-slice images, we conclude that the waves form in the umbral region with the 5-min oscillation period, which can cause the brightness periodicity change in the umbra region with the 3-min period (in fact, is half of 5-min oscillation) and 5-min in the penumbra.

Long-period oscillations of active region patterns: least-squares mapping on second-order curves

Active regions (ARs) are the main sources of variety in solar dynamic events. Automated detection and identification tools need to be developed for solar features for a deeper understanding of the solar cycle. Of particular interest here are the dynamical properties of the ARs, regardless of their internal structure and sunspot distribution. We studied the oscillatory dynamics of two ARs: NOAA 11327 and NOAA 11726 using two different methods of pattern recognition. We developed a novel method of automated AR border detection and compared it to an existing method for the proof-of-concept. The first method uses least-squares fitting on the smallest ellipse enclosing the AR, while the second method applies regression on the convex hull.} After processing the data, we found that the axes and the inclination angle of the ellipse and the convex hull oscillate in time. These oscillations are interpreted as the second harmonic of the standing long-period kink oscillations (with the node at the apex) of the magnetic flux tube connecting the two main sunspots of the ARs. In both ARs we have estimated the distribution of the phase speed magnitude along the magnetic tubes (along the two main spots) by interpreting the obtained oscillation of the inclination angle as the standing second harmonic kink mode. After comparing the obtained results for fast and slow kink modes, we conclude that both of these modes are good candidates to explain the observed oscillations of the AR inclination angles, as in the high plasma $\beta$ regime the phase speeds of these modes are comparable and on the order of the Alfv\'{e}n speed. Based on the properties of the observed oscillations, we detected the appropriate depth of the sunspot patterns, which coincides with estimations made by helioseismic methods. The latter analysis can be used as a basis for developing a magneto-seismological tool for ARs.

Statistical research of the umbral and penumbral oscillations

Observation of the main sunspot of AR11692 were carried out with the 1m New Vacuum Solar Telescope (NVST) located on the Fuxian Solar Observatory (FSO) with the Hα on 2013 march 13. The high cadence Hα intensity images show that running waves formed periodically within umbra, and propagate outwards with the shape of partial arcs. The running waves run across the umbra-penumbra boundary and disappear at the outer edge of the penumbra eventually. Confusingly, the period of the running waves within umbra is half of that within penumbra, i.e., the former is a typical 3-min oscillation with a period of 156s and the latter is typical 5-min oscillation with a period of 312s. To investigate the nature of the umbral and penumbral oscillations, we selected 300 sample points from the umbra and 150 sample points from the penumbra, respectively, of the sunspot to analyze the distributions of the extreme values related to the intensity profiles of the sample points. It is found that there are significant difference between the distributions of the extreme values related to umbral and penumbral oscillation intensity profiles. When a possible steady solar radiation, which was replaced by the average of troughs of the umbral oscillation intensity profile, was filtered from the umbral oscillation intensity, the distribution of the trough values is a normal distribution symmetrical about zero, the distribution of the crest values is a skewed distribution. When another possible steady solar radiation, which was replaced by the average of the penumbral oscillation intensity profile, was filtered from the penumbral oscillation intensity, while the distribution of the crest values and trough values were a pair of skewed distribution symmetrical about zero. According to the statistical characteristic of umbral and penumbral oscillations, an interpretation seemingly no having physical meaning was used to explain the statistical results. The possible interpretation for the phenomenon is that the pair of oscillations with different period in the intensity are produced by a same wave with a basis oscillation period of 312s. The basis wave can be expressed as Z(x,y,t)=A(x,y)cos(ωt+θ), where ω=(2π)/312 is the angular frequency of the wave. In the umbra, the intensity due to the wave is proportional to the square of the displacement of the wave, IU=a2Z2=a22A2(x,y)[1+cos(2ωt+2θ)], while the intensity in the penumbra due to the wave is proportional to the basis wave, IP=a1Z=A(x,y)cos(ωt+θ). Correspondingly, the period of the intensity within umbra is TU=156s, while the period of the intensity within penumbra is TP=312s.

The Gelfreikh–Lubyshev effect according to microwave observations of sunspots

We report the results of RATAN-600 radio telescope observations of the fine structure of the source of cyclotron microwave emission (SCMR) located in the solar corona above the main sunspot of NOAA11899 active region. Compared to earlier and mostly episodic observations of the SCMR, our regular observations with RATAN-600 radio telescope showed rather conclusively that the variation of the structure of the SCMR as a function of the angle of view in the present case is of geometric nature. The behavior of image variations generally agree with the computations performed by Gelfreikh and Lubyshev in terms of the simplest model of the solar atmosphere above the sunspot. The results of their computations are widely used for interpreting observations.

MULTI-WAVELENGTH STUDY OF A DELTA-SPOT. I. A REGION OF VERY STRONG, HORIZONTAL MAGNETIC FIELD

ABSTRACT Active region NOAA 11035 appeared in 2009 December, early in the new solar activity cycle. This region achieved a delta sunspot (δ spot) configuration when parasitic flux emerged near the rotationally leading magnetic polarity and traveled through the penumbra of the largest sunspot in the group. Both visible and infrared imaging spectropolarimetry of the magnetically sensitive Fe i line pairs at 6302 and 15650 Å show large Zeeman splitting in the penumbra between the parasitic umbra and the main sunspot umbra. The polarized Stokes spectra in the strongest field region display anomalous profiles, and strong blueshifts are seen in an adjacent region. Analysis of the profiles is carried out using a Milne–Eddington inversion code capable of fitting either a single magnetic component with stray light or two independent magnetic components to verify the field strength. The inversion results show that the anomalous profiles cannot be produced by the combination of two profiles with moderate magnetic fields. The largest field strengths are 3500–3800 G in close proximity to blueshifts as strong as 3.8 km s−1. The strong, nearly horizontal magnetic field seen near the polarity inversion line in this region is difficult to understand in the context of a standard model of sunspot magnetohydrostatic equilibrium.

The evolution of arch filament systems and moving magnetic features around a sunspot

Context. Arch filament systems (AFSs) are usually considered as the chromospheric manifestations of the emerging flux regions (EFRs) seen in Hα observations. Moving magnetic features (MMFs) look similar to EFRs in magnetograms, but often appear in the decaying phase of an active region (AR) and behave differently from EFRs. A possible relation between AFS and MMF would be important for revealing a common mechanism for building up basic structures on the Sun.Aims. Based on Hα and magnetic field observations with high spatial resolution, we study the evolution of MMFs around a sunspot, as well as their related AFSs from birth to death.Methods. The multiwavelength observations from the New Vacuum Solar Telescope (NVST) and the Solar Dynamic Observatories (SDO) are co-aligned in the spatial and the temporal sense. MMFs appeared near the northern end of a light bridge (LB). Their related AFSs were carefully identified and traced from their appearance to disappearance based on Hα , EUV data, and magnetograms.Results. In the main sunspot of AR NOAA 11711 during April 1−4, 2013, many slow-speed MMFs with a polarity opposite to that of the sunspot appeared from the close vicinity of the northern end of a LB. Different from other smaller MMFs around the sunspot, these MMFs were always related to arch filaments and eventually formed AFSs with three twisting branches. The total flux involved in the AFSs was estimated to be about 2.7 × 1021 Mx. The largest MMF “M1” evolved into a small pore that led to an intensity reduction in the continuum intensity images. The appearance and evolution of the AFSs near the sunspot seems to be controlled by MMFs emanating from the penumbra. Owing to continual magnetic cancellation between the MMFs and their surrounding opposite flux, the AFSs gradually disintegrated and finally disappeared.Conclusions. The appearance and evolution of the AFSs near the sunspot seem to be controlled by these MMFs emanating from the penumbra.

DYNAMICS IN SUNSPOT UMBRA AS SEEN IN NEW SOLAR TELESCOPE AND INTERFACE REGION IMAGING SPECTROGRAPH DATA

We analyse sunspot oscillations using Interface Region Imaging Spectrograph (IRIS) slit-jaw and spectral data and narrow-band chromospheric images from the New Solar Telescope (NST) for the main sunspot in NOAA AR 11836. We report that the difference between the shock arrival times as measured the Mg II k 2796.35\AA\ and Si IV 1393.76\AA\ line formation levels changes during the observed period and peak-to-peak delays may range from 40~s to zero. The intensity of chromospheric shocks also displays a long term (about 20~min) variations. NST's high spatial resolution \ha\ data allowed us to conclude that in this sunspot umbral flashes (UFs) appeared in the form of narrow bright lanes stretched along the light bridges and around clusters of umbral bright points. Time series also suggested that UFs preferred to appear on the sunspot-center side of light bridges, which may indicate the existence of a compact sub-photospheric driver of sunspot oscillations. The sunspot's umbra as seen in the IRIS chromospheric and transition region data appears bright above the locations of light bridges and the areas where the dark umbra is dotted with clusters of umbral dots. Co-spatial and co-temporal data from the Atmospheric Imaging Assembly on board Solar Dynamics Observatory showed that the same locations were associated with bright footpoints of coronal loops suggesting that the light bridges may play an important role in heating the coronal sunspot loops. Finally, the power spectra analysis showed that the intensity of chromospheric and transition region oscillations significantly vary across the umbra and with height, suggesting that umbral non-uniformities and the structure of sunspot magnetic fields may play a role in wave propagation and heating of umbral loops.

SUDDEN PHOTOSPHERIC MOTION AND SUNSPOT ROTATION ASSOCIATED WITH THE X2.2 FLARE ON 2011 FEBRUARY 15

The Helioseismic and Magnetic Imager provides 45 s cadence intensity images and 720 s cadence vector magnetograms. These unprecedented high-cadence and high-resolution data give us a unique opportunity to study the change of photospheric flows and sunspot rotations associated with flares. By using the differential affine velocity estimator method and the Fourier local correlation tracking method separately, we calculate velocity and vorticity of photospheric flows in the flaring NOAA AR 11158, and investigate their temporal evolution around the X2.2 flare on 2011 February 15. It is found that the shear flow around the flaring magnetic polarity inversion line exhibits a sudden decrease, and both of the two main sunspots undergo a sudden change in rotational motion during the impulsive phase of the flare. These results are discussed in the context of the Lorentz-force change that was proposed by Hudson et al. (2008) and Fisher et al. (2012). This mechanism can explain the connections between the rapid and irreversible photospheric vector magnetic field change and the observed short-term motions associated with the flare. In particular, the torque provided by the horizontal Lorentz force change agrees with what is required for the measured angular acceleration.

Anomalous polarization features of the cyclotron radiation source in the NOAA 7123 active region: The structure, dynamics, and reasons of occurrence

The research results of the unique NOAA 7123 active region (April 1992) are presented; its anomalous polarization features (long-term predominance of O-mode radiation above the central part of the main sunspot) were previously revealed by VLA observations with a resolution of (2–4)” at wavelengths of 3.6 and 6.4 cm (Vourlidas et al., 1997). Additional observations in a wider band of electromagnetic radiation, including, in particular, spectrum-polarization observations performed with the RATAN-600 and SSRT radio telescopes, are considered. The analysis of these data allowed for the conclusion that the anomaly was caused by the presence of a cold substance in the upper coronal layers, starting from the height where (6–7) cm radiation was generated. A possible transportation agent of this substance—Hα filament—one of the bases of which fell on the sunspot’s umbra-penumbra boundary, was found.

Inconsistency of the Wolf sunspot number series around 1848

Aims. Sunspot number is a benchmark series in many studies, but may still contain inhomogeneities and inconsistencies. In particular, an essential discrepancy exists between the two main sunspot number series, Wolf (WSN) and group (GSN) sunspot numbers, before 1848. The source of this discrepancy has so far remained unresolved. However, the recently digitized series of solar observations in 1825-1867 by Samuel Heinrich Schwabe, who was the primary observer of the WSN before 1848, makes such an assessment possible. Methods. We construct sunspot series, similar to WSN and GSN, but using only Schwabe's data. These series, called WSN-S and GSN-S, respectively, were compared with the original WSN and GSN series for the period 1835-1867 to look for possible inhomogeneities. Results. We show that: (1) The GSN series is homogeneous and consistent with the Schwabe data throughout the entire studied period; (2) The WSN series decreases by roughly ~20% around 1848 caused by the change of the primary observer from Schwabe to Wolf and an inappropriate individual correction factor used for Schwabe in the WSN; (3) This implies a major inhomogeneity in the WSN, which needs to be corrected by reducing its values by 20% before 1848; (4) The corrected WSN series is in good agreement with the GSN series. This study supports the earlier conclusions that the GSN series is more consistent and homogeneous in the earlier part than the WSN series.