Fine Structure Of Magnetic Fields Research Articles

Evolution of the Fine Structure of Magnetic Fields in the Quiet Sun: Observations from Sunrise/IMaX and Extrapolations

Observations with the balloon-borne Sunrise/Imaging Magnetograph eXperiment (IMaX) provide high spatial resolution (roughly 100 km at disk center) measurements of the magnetic field in the photosphere of the quiet Sun. To investigate the magnetic structure of the chromosphere and corona, we extrapolate these photospheric measurements into the upper solar atmosphere and analyze a 22-minute long time series with a cadence of 33 seconds. Using the extrapolated magnetic-field lines as tracer, we investigate temporal evolution of the magnetic connectivity in the quiet Sun’s atmosphere. The majority of magnetic loops are asymmetric in the sense that the photospheric field strength at the loop foot points is very different. We find that the magnetic connectivity of the loops changes rapidly with a typical connection recycling time of about 3±1 minutes in the upper solar atmosphere and 12±4 minutes in the photosphere. This is considerably shorter than previously found. Nonetheless, our estimate of the energy released by the associated magnetic-reconnection processes is not likely to be the sole source for heating the chromosphere and corona in the quiet Sun.

MAGNETIC LOOPS IN THE QUIET SUN

We investigate the fine structure of magnetic fields in the atmosphere of the quiet Sun. We use photospheric magnetic field measurements from {\sc Sunrise}/IMaX with unprecedented spatial resolution to extrapolate the photospheric magnetic field into higher layers of the solar atmosphere with the help of potential and force-free extrapolation techniques. We find that most magnetic loops which reach into the chromosphere or higher have one foot point in relatively strong magnetic field regions in the photosphere. $91%$ of the magnetic energy in the mid chromosphere (at a height of 1 Mm) is in field lines, whose stronger foot point has a strength of more than 300 G, i.e. above the equipartition field strength with convection. The loops reaching into the chromosphere and corona are also found to be asymmetric in the sense that the weaker foot point has a strength $B < 300$ G and is located in the internetwork. Such loops are expected to be strongly dynamic and have short lifetimes, as dictated by the properties of the internetwork fields.

Hinode Observations of a Vector Magnetic Field Change Associated with a Flare on 2006 December 13

Abstract Continuous observations of the flare productive active region 10930 were successfully carried out with the Solar Optical Telescope aboard the Hinode spacecraft during 2006 December 6 to 19. We focused on the evolution of photospheric magnetic fields in this active region, and the magnetic field properties at the site of the X3.4 class flare, using a time series of vector field maps with high spatial resolution. The X3.4 class flare occurred on 2006 December 13 at the apparent collision site between the large, opposite polarity umbrae. Elongated magnetic structures with alternatingly positive and negative polarities resulting from flux emergence appeared one day before the flare in the collision site penumbra. Subsequently, the polarity inversion line at the collision site became very complicated. The number of bright loops in Ca II H increased during the formation of these elongated magnetic structures. Flare ribbons and bright loops evolved along the polarity inversion line and one footpoint of the bright loop was located in a region having a large departure of the field azimuth angle with respect to its surroundings. SOT observations with high spatial resolution and high polarization precision revealed temporal change in the fine structure of magnetic fields at the flare site: some parts of the complicated polarity inversion line then disappeared, and in those regions the azimuth angle of the photospheric magnetic field changed by about 90°, becoming more spatially uniform within the collision site.

On the fine structure of magnetic fields in sunspot penumbrae

Recent observations have revealed the interlocking-comb structure of the magnetic field in the outer penumbra of a sunspot. We have argued that this structure owes its origin in part to downward pumping of magnetic flux by vigorous granular convection in the region surrounding the spot. Here we stress the difference between the inner and outer penumbra, and correct some misleading assertions in a recent paper by Spruit & Scharmer.

Magnetic flux separation in photospheric convection

Three-dimensional non-linear magnetoconvection in a strongly stratified compressible layer exhibits different patterns as the strength of the imposed magnetic field is reduced. There is a transition from a magnetically dominated regime, with small-scale convection in slender hexagonal cells, to a convectively dominated regime, with clusters of broad rising plumes that confine the magnetic flux to narrow lanes where fields are locally intense. Both patterns can coexist for intermediate field strengths, giving rise to flux separation: clumps of vigorously convecting plumes, from which magnetic flux has been excluded, are segregated from regions with strong fields and small-scale convection. A systematic numerical investigation of these different states shows that flux separation can occur over a significant parameter range and that there is also hysteresis. The results are related to the fine structure of magnetic fields in sunspots and in the quiet Sun.

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

The connection of fine-structure photospheric features in active regions with magnetic fields

1.The fine structure of the proton sunspot group of July 4–8, 1966 was studied on the basis of high-resolution heliograms. The comparison of the orientation between penumbral filaments and the transverse magnetic fields (observed by A.B. Severny and T.T. Tsap) shows that the direction of the filaments coincides in general with that of the magnetic field.2.Measurements of the magnetic fields of smallest pores (1·5″-2″) showed that the pores are always connected with strong magnetic field (in average 1400 gauss), which is localized at the same small area as the pore.3.Magnetic fields of faculae are concentrated in small elements with the dimension not exceeding 1·5″-3″. Magnetic-field strength H|| of about 45% of facular granules is within the limits of photographic measuring errors (approximately 25 gauss). For a quarter of all facular granules the strength H|| is from 25–50 gauss; about 30% of facular granules have H|| &gt; 50 gauss, and sometimes there appear faculae with field strength of about 200 gauss. The magnetic-field strength of facular granules, which are found directly above spots, is 10–20 times less than the field strength of spots. This field is 80–210 gauss only.4.All observational data mentioned above show that the appearance of the fine-structure features in active regions is directly connected with the fine structure of magnetic field of different strength and different orientation. The study of high-resolution heliograms gives additional information about the fine structure of the magnetic field.

On the structure of the magnetic field of sunspots

The model of the magnetic field of sunspots, taking account of fine structure of magnetic field in solar plasma, is considered. Small-scale subgranules with their own field form magnetic filaments in the external current-free field. The filaments are vertical in the umbra, while in the penumbra they run along the surface with sharp bends. In a number of spot umbra the relation between Doppler velocity and the field is established on polarized spectrograms. The π-component splitting in umbra is interpreted as a result of a weak background magnetic-field existence together with a large field of magnetic filaments. Spectrographic definition of the magnetic field in spot umbra is accomplished on the effect of magnetic-lines intensification and directly on spectrograms of low-excitation (Fe I, Ti I) and high-excitation (Fe II) lines. Magnetic field measured in low-excitation lines exceeds twice the field value obtained in high-excitation lines. This result has been considered in the light of the proposed model of sunspot field.