Sunspot Magnetic Fields Research Articles

A magnetostatic sunspot model with ?twisted? field

It is shown that the internal structure of sunspots is affected only to a small degree by the presence of the azimuthal component of the sunspot magnetic field. The magnetostatic theory of sunspots developed by Schluter and Temesvary (1958) is extended to include such an effect. In the present study, the radial dependence of the azimuthal component of the field is adopted from the observation of Stepanov (1965). A typical sunspot of an effective temperature of 4000K is considered and its detailed structure is presented.

On Magnetic Fields in Sunspots and Active Regions

This review is restricted to a few topics of the extensive problem under discussion; it refers to the literature since 1967 only. The literature prior to 1967 is reviewed in Zwaan, 1968; Howard, 1967; Lüst, 1965; Kiepenheuer, 1968; Jäger, 1964; and Bray and Loughead, 1964.

Sunspot Magnetic Fields and Umbral Dots

The fine structure features of the umbral magnetic fields (i.e. large field gradients and changes of polarity in local regions) are considered as evidence of strong non-linear interactions between magnetic and thermal or mechanical forces in the umbra. It is suggested that the umbral dots observed in white light are the optical manifestation of these interactions. A three-dimensional radiative transfer analysis of possible models for these bright features is discussed and this enables one to place limits on the geometry of these features and on the non-radiative energy requirements of the models. Of the several models considered those which were compatible with convection as the source of this energy were found to be quite inconsistent with the available data. The most likely model was found to have a diameter of 200 km, a height of 50 km and an average emission of non-radiative energy of 4 × 103 erg s−1/cm−3 throughout this region. It is shown that this is two orders of magnitude greater than the energy available from Joule heating by locally twisted magnetic fields. However, if the energy flux transported through the umbra by Alfvén waves is partially dissipated in regions of locally twisted fields it is shown that the emission into a volume of the above dimensions is of the right order of magnitude.

Sunspot Magnetic Fields and High Energy Electrons in Flares

A balloon observation of an impulsive hard X-ray burst on September 27, 1969 showed the size of the source to be one arc minute or less. It was remarkably smaller than the associated Hα flare with a size of 3 arc min.The efficient acceleration of electrons and the trigger of the flares are suggested to be attributed to a large scale electric potential field caused by a gas motion near the photosphere. The primary cause of the onset of flares would be the acceleration of electrons. The electrons excite plasma waves which make the conductivity lower by several orders, so that the electromagnetic energy I2L stored before the onset of the flare would be suddenly converted into the heat due to the ohmic loss.

Observations of the Two-Level Structure of Sunspot Magnetic Fields

Inhomogeneity of magnetic field structure, ‘granulation’ in sunspot umbrae and fine structure of the Evershed motions lead to the conclusion that the sunspot umbra is composed of magnetic ropes (or plaits) with dimensions near the limit of resolution. Progress in the study of these ropes is closely connected with the possibility of obtaining extensive spectroscopic information about some selected regions on the solar disc. It is extremely interesting to obtain a picture of short-time-scale variations of the magnetic field strength and the radial velocity field in connection with the transfer of energy from the photosphere to the upper layers of the solar atmosphere (chromosphere, corona).

The Magnetic Fields and the Polarization of Radio Emission in the Active Center of October 1968

During the period of strong flare activity of the active center McMath No. 9740, from October 25 to November 2, 1968, 32 magnetograph records of longitudinal H∥ and 20 of transverse fields, H⊥, were obtained for this center with the Crimean magnetograph. Sometimes 5–7 daily records were made. Hα and K spectroheliograms, Hα cine film and routine determinations of sunspot magnetic fields were also obtained frequently. However the most important point is that all these optical observations were accompanied by simultaneous and continuous measurements of intensity and polarization of radio emission from this active center at a wavelength of 3 cm, recorded with the aid of the big 22-m radiotelescope of the Crimean Observatory (the resolution is 8′ for this wavelength), (Erushev and Zvetkov, 1970).

On the magnetic configuration of sunspot groups which produce solar proton flares

The configuration of sunspot magnetic fields is deduced by using the observational data on the development of type IV radio bursts and H α-brightening areas over the umbrae of sunspot groups. Since both polarity areas are existent together within the same umbrae, the magnetically neutral regions are usually produced along the areas separating the one polarity area from the other. In the eastern portion of the preceding sunspots, the gradient of sunspot magnetic fields is much steeper compared to all other portions and thus solar proton flares are triggered within or very near this portion. H α-brightening areas develop mainly above the preceding sunspots and their eastern portion of sunspot groups. This tendency seems to be closely related to the configuration of sunspot magnetic fields and the mechanism of solar proton flares.

Solar Source of Interplanetary Magnetic Fields

Abstract : The mean magnetic field of the sun (seen as a star) during the interval March-June 1968 has an almost complete agreement with the polarity of the interplanetary magnetic field observed near the earth. This agreement could exist only if the solar source of the interplanetary field was part of a field pattern that was ordered over an appreciable portion of the solar disk. The result is consistent with a previously derived solar sector pattern in the weak, large-scale photospheric field in which the boundary between two solar sectors is at almost the same longitude independent of latitude over a range of at least 40 degrees N to 40 degrees S latitude. Comparison of the contribution of sunspot magnetic fields with the mean solar field indicates that the sunspot fields do not have a direct influence on the interplanetary field. (Author)

The fine structure of magnetic fields in sunspots and the dependence of the magnetic field on the spot area

The fine structure of magnetic fields in sunspots and the dependence of the magnetic field on the spot area

On the magnetic-field configuration in sunspots

During 1963–67 observations of the magnetic fields in sunspots have been obtained at the Oslo Solar Observatory. For the largest spots the detailed distribution of the magnetic-field strength is found. Based on calculations of line profiles made by the author (Kjeldseth Moe, 1967) also the direction of the magnetic field is derived. Observations of the magnetic field of the same spot at several positions on the solar disk give further information regarding the magnetic-field configuration. Our results are in fair agreement with those of Bumba (1962).

The Influence of the Magneto-active Plasma on Gyro-Synchrotron Emission Spectra in the Solar Atmosphere

The influence of a magneto-active plasma on the gyrosynchrotron emission from an accelerated electron is considered in detail. This influence for the right-handed waves is much, compared with that for the left-handed waves. The radiated intensity of both the right-and left-handed waves is generally smaller than that in vacuum, but the former occasionally become much higher within some special frequency range compared with the latter.Solar radio type IV bursts are emitted due to the interaction of sunspot magnetic fields with energetic electrons Moving in the solar atmosphere. Accordingly, the influence ofsolar atmospheric plasmas on the bursts is examined. Some investigations are made of this influence on the emission of the bursts, based on the estimation of both the intensity of sunspot magnetic fields and the plasma density in the flare regions. The spectral pattern of type IV radio bursts is briefly considered.

Motions of Magnetic Fields and Umbrae within a Spot Group

Over the past years considerable information has been acquired concerning the structure and magnitude of sunspot magnetic fields; however, there is a paucity of data concerning the motions of the seats of these fields, i.e., individual umbrae within a spot group. The well-known effects of differential rotation cause sunspots to diverge with time in longitude at a daily rate of, at most, a few degrees per solar rotation. On the other hand, a limited number of investigations have shown that individual umbrae can have proper motions of up to 100 to 200 m/s, or about ten times greater than effects of differential rotation. In addition, equally limited observations have shown that umbrae sometimes rotate about an axis through their centre; in an extreme case the rotation reached 30°/day. The last large sunspot group of the last cycle, having Central Meridian Passage 20.5 September, 1963, has been studied by means of photoheliograms and magnetograms obtained four days before and four days after CMP at the Crimean Astrophysical Observatory. The magnetograms show the distribution of both the transverse and longitudinal components of the magnetic field in the activity region. In this study we have studied umbral motions and their relation to the magnetic field.

The motion of an isolated charged particle in a simple force-free magnetic field

The motion of an isolated energetic charged particle in a particularly simple forcefree magnetic field with rectilinear lines of force is examined in detail. The equations of particle motion are solved exactly without assuming that the guiding centre approximation is valid. One component of the equation of particle motion is shown to be formally identical to the general equation of motion of a pendulum, which implies that three distinct types of particle motion can arise. Criteria are derived for these different types of motion and it is found that under certain conditions a particle travels predominantly across the magnetic lines of force rather than along them. The motion transverse to the magnetic field does not depend upon the existence of a neutral line or plane. It is suggested that some energetic charged particles may be able to escape from force-free magnetic fields by this process. The results are discussed briefly in relation to the trapping of solar cosmic rays in sunspot magnetic fields and the passage of galactic cosmic rays through interstellar space.

Micrometer and Photometric Observations of a Sunspot Velocity Field

High dispersion spectra (0.2 A/mm) were taken during the passage of one sunspot across the solar disk, for simultaneous micrometer and photometric investigations of a sunspot velocity field. The Fe I line at λ 5576 was selected for the investigation because it is unaffected by the sunspot magnetic field (Landé splitting factor g = 0). From detailed micrometer measures, solutions have been attempted for umbral and penumbral velocity components. The photometric analysis is based on a determination of the line contour λ 5576 across the sunspot for three disk positions. An attempt is made to relate the penumbral line asymmetry (line-flare) with visual measures of the line shift. The micrometer measurements show that the umbral motion is characterized by a small vertical velocity of descent (0.1 km/sec −1 ), while the predominantly radial, penumbral velocity field pattern appears to be both flattened and broadened during the passage of the spot from centre to limb. From the photometric measurements it is found that the line asymmetry is greatest at approximately the position of the maximum sight-line velocity within the spot, and appears more pronounced for spot positions nearer the solar limb. It is clear that in visual sunspot measures the eye does not set on the line-core but at a point towards the penumbral line-flare. The equivalent width of λ 5576 shows a marked increase from the photosphere to sunspot. The central intensity of the line within the spot increases with distance from the spot centre.

Sunspot Magnetic Fields and Loop Prominences

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The Penumbral Filaments as Convection Rolls.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Penumbral Filaments as Convection Rolls. Danielson, Robert F. Abstract Linearized equations describing the onset of convection in a horizontal layer of conducting fluid in the presence of a magnetic field have been derived by Chandrasekhar (Phil. Mag. 43, 501, 1952). From these equations growth rates of convection cells (of various sizes and shapes) have been calculated as a function of the magnetic field and the Rayleigh number for two cases: (1) a vertical magnetic field and (2) a horizontal magnetic field. The penumbral filaments are long narrow structures having a width of 300 km or less and a length of 5000 km or more. Their lifetime is roughly five times that of the granulation. It is proposed that most of these filaments can be explained as convection rolls (i.e., long narrow convection cells) with the axis of the roll parallel to the horizontal component of the sunspot magnetic field. The growth rate of convection cells in a magnetic field of 10 gauss or less (corresponding to the granulation) is compared with the growth rate of convection rolls in a magnetic field of 1000 gauss (corresponding to the penumbral filaments). The growth rate of the rolls is less than, but comparable to, the growth rates of the cells if the effective Rayleigh number in the penumbra is equal to the effective Rayleigh number in the granulation. The growth rate of the rolls will be five times less than the growth rate of the cells if the effective Rayleigh number in the penumbra is only a factor of 2 or 3 smaller than in the granulation. Hence it appears that both the lifetime and shape of the penumbral filaments can be accounted for by convection rolls. Publication: The Astronomical Journal Pub Date: March 1961 DOI: 10.1086/108362 Bibcode: 1961AJ.....66...42D full text sources ADS |

On the Relativistic Electrons in the Solar Atmosphere

We consider the various processes, in solar atmosphere, of the relativistic electrons ejected in association with flares and examine how these electrons will lose their energies.In view of the above examination, we can show that these electrons will be trapped in the sunspot magnetic fields (∼100 gauss) and then lose the majority of their energies through the synchrotron radiation. So, the radio waves emitted by these synchrotron radiation processes will be observed as the type IV outbursts.

The Spatial Analysis of Magnetic Fields in Sunspots

A review of existing methods of determining the strength and direction of magnetic fields in sunspots, as a function of position and depth within the spot shows that these have given fragmentary and somewhat discordant results. In particular, little is known of the direction of the magnetic vector. A new method of determining this quantity is described. By means of a Babinet compensator, polarization fringe systems are produced in the components of a normal Zeeman triplet. From the phase difference of the σ -component fringes, the constants of the elliptical polarization at each point in the spot are determined, and the orientation of the lines of force deduced. The method minimizes the effect of instrumental polarization; the reduction is simple and fairly short. An experimental test on four sunspots has been carried out, using the Oxford 35 m solar tower and 12 m spectrograph with a Babcock grating. Measurements of field direction were made, using the triplet λ 6173 (Fe I ); the associated field strengths were determined from the same line and from λ 6149, a weak line of Fe II . The resulting field-strength variation is in agreement with Broxon's parabolic law, but in contrast with Joy and Nicholson's measures of direction, 40 per cent of the flux is found to be confined within a cone of unit solid angle. Comparisons of field-strength at λ 6173 (Fe I ) and λ 6149 (Fe II ) suggest that the gradient in depth does not exceed 1 oersted/km.

28. The magnetic fields of sunspots and the evershed effect

The observational data on the magnetic fields and the physical parameters in sunspots indicate in a qualitative way how the magnetic lines of force run relative to the isobaric surfaces. If matter is confined to move only along the lines of force it is shown that for sufficient tilt between these lines and the isobars, matter will be accelerated outward along the lines of force. The flow corresponding to this forced convection works as a cooling cap for the core of the spot. It is indicated how a stationary state may be reached with the outward velocity adapted to the temperature difference between the spot and the photosphere.

The magnetic fields of sunspots

The occurrence of sunspots in pairs with opposite magnetic polarities can be explained if it is assumed that lines of magnetic force encircle the axis of rotation in opposite directions a little below the surface of the Sun. Material moving upwards will carry a loop of field through the surface and give a south magnetic pole on the same circle of latitude. Such a field is to be expected from the interaction of the Sun's non-uniform rotation with a meridional field. The progression of the spots in latitude requires a slow meridional circulation. The dynamics of the non-uniform rotation and the meridional circulation are not understood; it is possible that the equatorial parts of the Sun are coupled magnetically to a more rapidly rotating interior.