The Lifetime of Sunspot Tlatov, A. G.
Geomagnetism and Aeronomy,
12/2023, Letnik:
63, Številka:
8
Journal Article
Recenzirano
The lifetime of individual solar pores and sunspots is analyzed according to the data of the HMI/SDO space observatory observations in the 24th and 25th activity cycles. It is found that the lifetime ...of individual sunspots and pores
T
differs from the Gnevyshev–Waldmeier rule formulated for groups of sunspots. For regular sunspots, that is, spots with nuclei, there is a linear dependence on the maximum area
S
mx
:
T
sp
= –0.019(±0.2) + 0.027(±0.002)
S
mx
. For solar pores, the dependence of the lifetime on the area has a logarithmic form
T
por
= –0.24(±0.1) + 0.055(±0.014) log(
S
mx
). Possible mechanisms of disintegration of spots and pores are studied. The lifetime for regular spots is probably related to convective currents. It has been established that the average velocity of the matter flow, determined from observations of Doppler velocities, increases with a decrease in the area of sunspots. This can accelerate the decay rate of sunspots with a decrease in the area of sunspots. For solar pores, the lifetime can be determined by the heating mechanism.
A Reverse Shock in GRB 181201A Laskar, Tanmoy; Eerten, Hendrik van; Schady, Patricia ...
The Astrophysical journal,
10/2019, Letnik:
884, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We present comprehensive multiwavelength radio to X-ray observations of GRB 181201A spanning from 150 s to 163 days after the burst, comprising the first joint ALMA-VLA-GMRT observations of a ...gamma-ray burst (GRB) afterglow. The radio and millimeter-band data reveal a distinct signature at 3.9 days, which we interpret as reverse-shock (RS) emission. Our observations present the first time that a single radio-frequency spectral energy distribution can be decomposed directly into RS and forward shock (FS) components. We perform detailed modeling of the full multiwavelength data set, using Markov Chain Monte Carlo sampling to construct the joint posterior density function of the underlying physical parameters describing the RS and FS synchrotron emission. We uncover and account for all discovered degeneracies in the model parameters. The joint RS-FS modeling reveals a weakly magnetized ( 3 × 10−3), mildly relativistic RS, from which we derive an initial bulk Lorentz factor of Γ0 103 for the GRB jet. Our results support the hypothesis that low-density environments are conducive to the observability of RS emission. We compare our observations to other events with strong RS detections and find a likely observational bias selecting for longer lasting, nonrelativistic RSs. We present and begin to address new challenges in modeling posed by the present generation of comprehensive, multifrequency data sets.
ABSTRACT
A theory of the mean tilt of sunspot bipolar regions (the angle between a line connecting the leading and following sunspots and the solar equator) is developed. A mechanism of formation of ...the mean tilt is related to the effect of the Coriolis force on meso-scale motions of super-granular convection and large-scale meridional circulation. The balance between the Coriolis force and the Lorentz force (the magnetic tension) determines an additional contribution caused by the large-scale magnetic field to the mean tilt of the sunspot bipolar regions at low latitudes. The latitudinal dependence of the solar differential rotation affects the mean tilt, which can explain deviations from Joy’s law for the sunspot bipolar regions at high latitudes. The theoretical results obtained and the results from numerical simulations based on the non-linear mean-field dynamo theory, which takes into account conservation of the total magnetic helicity and the budget equation for the evolution of the Wolf number density, are in agreement with observational data of the mean tilt of sunspot bipolar regions over individual solar cycles 15–24.
Our analysis of groups of sunspots since the year 1610 till indicates that the Gnevyshev–Ohl rule (GO) displays cycles of inversion with the period of 200years. The latest inversion occurred in the ...Hale double cycle 22–23. Due to that, in several subsequent double cycles the odd cycles should be weaker than their preceding even cycles. Gleissberg cycles with the period of about 100years and variations with the period of 200years are manifested in variations of physical parameters of sunspots and are interconnected. We suggested that the secular minima of the solar activity occur in the vicinity of the extreme points of the 200-year cycles of inversion of the GO rule. The peak of the next secular minimum is expected between the years 2025÷2035. We studied the variations of the physical parameters of sunspots in a Gleissberg cycle. At the maximum phase of the Gleissberg cycle, the average area of groups and the average number of spots in a group reach their maximum. According to our forecast, the amplitude of the 25th solar activity cycle will be somewhat lower than that of the 24th.
We obtain the latitude-time distribution of the averaged tilt angle of solar bipoles. For large bipoles, which are mainly bipolar sunspot groups, the spatially averaged tilt angle is positive in the ...Northern solar hemisphere and negative in the Southern, with modest variations during the course of the solar cycle. We consider the averaged tilt angle to be a tracer for a crucial element of the solar dynamo, i.e. the regeneration rate of poloidal large-scale magnetic field from toroidal. The value of the tilt obtained crudely corresponds to a regeneration factor corresponding to about 10 per cent of rms velocity of solar convection. These results develop findings of Stenflo & Kosovichev concerning Joy's law, and agree with the usual expectations of solar dynamo theory. Quite surprisingly, we find a pronounced deviation from these properties for smaller bipoles, which are mainly solar ephemeral regions. They possess tilt angles of approximately the same absolute value, but of opposite sign compared to that of the large bipoles. Of course, the tilt data for small bipoles are less well determined than those for large bipoles; however, they remain robust under various modifications of the data processing.
This paper presents a procedure for reconstructing the longitudinal magnetic field from observational data of the complete profile of spectral magnetic lines. The method is based on a fast search ...algorithm for curve fitting for complete profiles of magnetically sensitive lines 6301.5 Å and 6302.5 Å. The magnetic field is calculated as the distance between the centers of the
I
-profiles of different polarization components. The centers are defined as the midpoints of the Gaussians approximating the
I
+
V
,
I
–
V
profiles. This approach makes it possible to carry out calculations rather quickly. The presented method makes it possible to reduce the noise in maps of a large-scale magnetic field and improve the accuracy of measurements, primarily of weak fields. The method has been tested to reconstruct the large-scale magnetic field from observations with the STOP full-disk magnetograph telescope (Kislovodsk) in 2014–2022.
The Wang–Shelley–Arge (WSA) solar wind (SW) model is based on the idea that weakly expanding coronal magnetic field tubes are associated with fast SW sources and vice versa. The Flux-Tube Expansion ...factor (FTE) is used to determine the degree of expansion of magnetic tubes. The FTE is calculated based on a model of the coronal magnetic field, usually in the potential approximation. The second input parameter of the WSA model is the distance from the base of the magnetic tube on the photosphere to the boundary of the corresponding coronal hole (Distance to the Coronal Hole Boundary, DCHB). These two parameters of the coronal magnetic field are related empirically to the speed of the solar wind near the Sun. The WSA model has shortcomings and does not fully explain the mechanisms of SW formation. This paper presents an analysis of the degree of correlation of various parameters of the magnetic field (length of field lines, latitude of the base of field lines, etc.) with the observed velocity SW. The parameters are calculated in potential (PFSS) and nonpotential approximations based on three synoptic series of magnetographic observations: the Kislovodsk Solar Telescope for Operative Predictions (STOP), SDO/HMI, and WSO. We found that the FTE correlates relatively weakly with solar wind speed, in contrast to field line lengths and DCHB. We propose an alternative relation to the WSA model that relates the length of field lines, the DCHB, and the amplitude of the magnetic field at the source surface with the velocity SW. The presented relationship is not based on the FTE and shows a better correlation with observations compared to the WSA model. We also optimized the formula in the WSA model for the STOP magnetograph.
Based on the method of averaging the shape of sunspots identified in the HMI/SDO images in the 2010–2022 period, an analysis was made of the relative position of sunspot nuclei during their passage ...across the solar disk. To do this, in different ranges of sunspot areas, we constructed averaged boundaries of the photosphere-penumbra and the core-penumbra. We then tracked changes in sunspot shape at various distances from the center of the disk in the eastern and western hemispheres. A shift of the boundary of sunspot nuclei to the western edge of the sunspot has been observed. The relative value of the shift increases with the area of sunspots. We estimated the value of the Wilson depression for sunspots of various sizes. The magnitude of the depression varies from 100 km for small sunspots to 1000 km for large sunspots. There is some imbalance of the Wilson depression near the eastern and western boundaries of the core-penumbra, depending on the area of the sunspots and their latitude.
—
The possibilities for organizing a solar activity observation service for space weather forecasting are considered. At this stage, the most promising approach is the establishment of a ground-based ...observation network. This network should include solar magnetographs for observing large-scale magnetic fields of the Sun and patrol optical telescopes for detecting coronal mass ejections and solar flares. Magnetographic observations provide data for assessing recurrent solar wind streams. Patrol telescopes operating in continuous mode make it possible to detect eruptive moments and determine parameters of coronal mass ejections at the initial stage. The network can be complemented by other types of observations in radio and optical bands. The article discusses the composition of observation tools as well as methods and models of forecasting.
Abstract
Long-term evolution of areas with open configuration of magnetic field (coronal holes) on the Sun reconstructed on the basis of H-alpha synoptic charts for the period 1887-2016 was studied ...and compared with annual occurrence frequencies of magnetic storms with gradual (GC) commencements. It was found that correlation between yearly values of coronal hole (CH) areas and sunspot numbers with no time shift is negative and not strong, but increases up to ∼0.6-0.7 when CH areas are delayed by 4-5 years relative to sunspot numbers. Temporal variations of CH areas in the Northern and Southern hemispheres are characterized by dominant ∼11-year periodicities; however, they differ significantly on the multidecadal time scale. The wavelet spectra of CH areas in the Southern hemisphere, unlike those in the Northern one, reveal persistent periodicities of ∼30-35 years on the studied time interval. Similar periodicities of ∼30-35 years are observed in annual occurrences of GC magnetic storms which are caused by high-speed streams of solar wind from coronal holes. The results of cross wavelet analysis of annual occurrence frequencies of GC magnetic storms and areas of coronal holes revealed common periodicities ∼11, ∼35 and ∼60 years which confirmed a close link of these storms with the evolution of large-scale magnetic fields on the Sun.
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