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1.
C. E. Parnell 《Solar physics》2007,242(1-2):21-41
The magnetic fields within the solar atmosphere have a complex topology owing to the fragmentary nature with which they thread the solar surface. The topologies of the potential magnetic fields containing only a few (up to four) point photospheric sources have been classified. For small numbers of sources determining the connectivity of source pairs is equivalent to counting the number of flux domains. As the numbers of sources increase this, however, is no longer the case. Instead, a pair of connected sources can have more than one distinct flux domain linking them. We call these multiply connected source pairs. Pairs of nulls connected by more than one separator are called multiply connected null pairs. Multiply connected source and null pairs go hand-in-hand such that two separators connecting the same pair of nulls immediately implies multiple flux domains linking the same source pair and vice versa. It is found that multiply connected source pairs are common not only in fairly complex potential magnetic fields but more interestingly in the resistive-MHD evolution of both simple and complex magnetic fields. Magnetic energy release is often significant around separators. Thus fields with multiply connected source pairs, which naturally have more separators, (i) have more sites for intense energy release and (ii) are likely to release energy more quickly than other magnetic fields. Moreover, the combination of multiply connected source and null pairs can give rise to a situation where flux is reconnected repeatedly between two flux domains. 相似文献
2.
In two dimensions magnetic energy release takes place at locations where the magnetic field strength becomes zero and has an x-point topology. The x-point topology can collapse into two y-points connected by a current sheet when the advection of magnetic flux into the x-point is larger than the dissipation of magnetic flux at the x-point. In three dimensions magnetic fields may also contain singularities in the form of three-dimensional null points. Three-dimensional nulls are created in pairs and are therefore, at least in the initial stages, always connected by at least one field line – the separator. The separator line is defined by the intersection of the fan planes of the two nulls. In the plane perpendicular to a single separator the field line topology locally has a two dimensional x-point structure. Using a numerical approach we find that the collapse of the separator can be initiated at the two nulls by a velocity shear across the fan plane. It is found that for a current concentration to connect the two nulls along the separator, the current sheet can only obtain two different orientations relative to the field line structure of the nulls. The sheet has to have an orientation midway between the fan plane and the spine axis of each null. As part of this process the spine axes are found to lose their identity by transforming into an integrated part of the separator surfaces that divide space into four magnetically independent regions around the current sheet. 相似文献
3.
Using optically identical telescopes at different sites, we have measured the solar diameter with a drift-scan technique. In order to investigate the cause of the observed fluctuations, we not only compare observations made simultaneously by different observers at the same telescope, but also observations made simultaneously at two different sites. Our main results are: (a) The mean error of a single drift time measurement is ±0.08s(or ± 1.1) at Izaña and ±0.11 s (or ± 1.7) at Locarno; this closely corresponds to the angular resolution at those two sites under normal seeing conditions, (b) We find no correlation between observations at different sites; a significant correlation exists, however, between observations made simultaneously by different observers at the same site: This indicates that most of the observed fluctuations are due to atmospheric effects (image motion) rather than personality effects, (c) The mean solar semi-diameter derived from a total of 1122 observations made in 1990 (472 at Izaña, 650 at Locarno) is R = (960.56 ± 0.03) (Izaña: 960.51, Locarno: 960.59); this may be compared with R = (960.32 ± 0.02) which is obtained from a re-analysis of 1773 observations made in 1981 (Izaña: 960.16, Locarno: 960.38). Although a small residual increase of the solar diameter during the last ten years seems to be indicated, we conclude that most - if not all - of the observed variations are due to variable seeing conditions, and that there is still no conclusive evidence for a genuine solar variation with amplitudes in excess of about ±0.3. 相似文献
4.
We present results of a study of the green corona (530.3 nm, Fexiv) and photospheric magnetic fields over the period 1976–1999 when both quantities were observed by ground-based observatories. By comparing both the limb green-line intensities and photospheric magnetograms we have found a relation between the strength of magnetic field and coronal intensities. This relation enables us to extend solar surface magnetic fields since 1976 back to 1939. From 1947 to 1992 the magnetic field strength grew at the cycle maxima by a factor of 1.5–2. On the other hand, both the green corona intensity and magnetic field strength in the present cycle are smaller compared to cycle 22, by a factor of 2. No relationship was found between the green corona intensities and magnetic field polarity as was previously supposed. Behavior for the green corona intensities is different between high-latitude and mid-latitude regions, and this break occurs at the heliographic latitude of ± 45°. Homogeneous coronal data set cannot be directly used to derive `the tilt angle', even though some similarities between the green coronal holes, poleward branches in the green corona and prominences and the tilt angle can be found. 相似文献
5.
It is important to understand the complex topology of the magnetic field in the solar corona in order to be able to comprehend
the mechanisms which give rise to phenomena such as coronal loop structures and x-ray bright points. A key feature of the
magnetic topology is a separator. A magnetic separator is a field line which connects two magnetic null points, places where
the magnetic field becomes zero. A stable magnetic separator is important as it is the intersection of two separatrix surfaces.
These surfaces divide the magnetic field lines into regions of different connectivity, so a separator usually borders four
regions of field-line connectivity. This work examines the topological behaviour of separators that appear in a magnetic field
produced by a system of magnetic sources lying in a plane (the photosphere). The questions of how separators arise and are
destroyed, the topological conditions for which they exist, how they interact and their relevance to the coronal magnetic
field are addressed. 相似文献
6.
Cotemporal Nii 676.8 nm full-disk magnetograms from the Michelson Doppler Interferometer (MDI) instrument on SOHO and the Advanced Stokes Polarimeter (ASP) are quantitatively compared using observations of active region AR 8218, a large negative polarity sunspot group observed at S20 W22 on 13 May 1998. MDI produces flux density estimates based on a polarized line center-of-gravity algorithm using moderate spectral resolution filtergrams with approximately 4 arc sec angular resolution. The magnetograms are formed by an on-board image processor and sent to the ground where they are calibrated using an empirical model to produce flux density maps. The ASP uses high spectral resolution Stokes polarimetric observations to produce very high precision vector magnetic field maps at angular resolution values on the order of 1 arc sec in good seeing. We use ASP inversion results to create a reference ASP `longitudinal magnetic flux density map' with which to calibrate the MDI full-disk magnetograms. The magnetograms from each instrument are scaled to a common reference frame and co-aligned with an accuracy of about 1.6 arc sec. Regions of invalid data, poor field-of-view overlap, and sunspots are masked out in order to calibrate MDI predominately on the relatively vertical `weak-field' plage magnetic elements. Pixel-to-pixel statistical comparisons are used to determine an MDI magnetogram linear calibration relative to reference ASP flux density values. We find that the current Level-1.5 MDI full-disk calibration gives flux density values lower on average by a factor of 0.64±0.013 compared to the ASP reference in active region plage. In sunspot regions (penumbra and umbra) the factor is 0.69±0.007. 相似文献
7.
The evolution of coronal and chromospheric structures is examined together with magnetograms for the 1B flare of January 19, 1972. Soft X-ray and EUV studies are based on the OSO-7 data. The H filtergrams and magnetograms came from the Sacramento Peak Observatory. Theoretical force-free magnetic field configurations are compared with structures seen in the soft X-ray, EUV and H images. Until the flare, two prominent spots were connected by a continuous dark filament and their overlying coronal structure underwent an expansion at the sunspot separation rate of 0.1 km s–1. On January 19, the flare occurred as new magnetic fields emerged at 1019 Mx h–1 beneath the filament, which untwisted and erupted as the flare began. The pre-flare coronal emissions remained unchanged during the flare except for the temporary addition of a localized enhancement that started 5 min after flare onset. EUV lines normally emitted in the upper transition region displayed a sudden enhancement coinciding in time and location with a bright H point, which is believed to be near the flare trigger or onset point. The EUV flash and the initial H brightening, both of which occurred near the center of the activated filament, were followed by a second EUV enhancement at the end of the filament. The complete disruption of the filament was accompanied by a third EUV enhancement and a rapid rise in the soft X-ray emission spatially coincident with the disappearing filament. From the change of magnetic field inferred from H filtergrams and from force-free field calculations, the energy available for the flare is estimated at approximately 1031 erg. Apparently, changes in the overlying coronal magnetic field were not required to provide the flare energy. Rather, it is suggested that the flare actually started in the twisted filament where it was compressed by emerging fields. Clearly, the flare started below the corona, and it appears that it derived its energy from the magnetic fields in or near the filament.NCAR is sponsored by NSF. 相似文献
8.
We compare large-scale filtergrams of a hitherto neglected class 1B flare with previously published vector magnetograms and maps of photospheric longitudinal electric current density (Hagyard et al., 1985). The vector magnetic fields were mapped simultaneously with the eruption of this flare. We find a coincidence, to within the ±2 registration accuracy of the data, between the flare kernels and the locations of maximum shear and of peak values in the longitudinal electric current density. The kernels brighten in a way which implies that the preflare heating and the main release of flare energy are spatially coincident within the limits of resolution (2). A pronounced magnetic shear exists in the vertical direction at the location of the strongest flare kernels. We provide evidence that the electric currents could be maintained by the energy stored in the sheared transverse magnetic field and that the amount of energy released is proportional to the amount stored. These circumstances are consistent with theories in which flares are triggered by plasma instabilities due to surplus electric currents. 相似文献
9.
Multiple moving magnetic structures in the solar corona 总被引：1，自引：0，他引：1
We report the study of moving magnetic structures inferred from the observations of a moving type IV event with multiple sources. The ejection contains at least two moving radio emitting loops with different relative inclinations. The radio loops are located above multiple H flare loops in an active region near the limb. We investigate the relationship between the two systems of loops. The spatial, temporal and geometrical associations between the radio emission and near surface activities suggest a scenario similar to coronal mass ejection (CME) events, although no CME observations exist for the present event. From the observed characteristics, we find that the radio emission can be interpreted as Razin suppressed optically thin gyrosynchrotron emission from nonthermal particles of energy 100, keV and density 102–105 cm–3 in a magnetic field 2 G. 相似文献
10.
Simultaneous observations of chromospheric (H) and photospheric (Fei 5324.19 Å) magnetograms in quiet solar regions enable us to study the spatial configuration of the magnetic field in the solar atmosphere. With the typical spatial resolution of the Huairou magnetograph, the photospheric and chromospheric magnetic structures of the quiet Sun maintain a very similar pattern. Moreover, the vertical magnetic flux is almost the same from the photosphere to the chromosphere. As an intermediate step, we analyze the formation of the working lines used by the Huairou video magnetograph of the Beijing Astronomical Observatory. The Stokes V contribution function of H and Fei 5324.19 Å are calculated. It is found that our H magnetograms provide the distribution of the chromospheric magnetic field at a height some 1000–1500 km above the photosphere. 相似文献
11.
Observations of a coronal disturbance on 1973 January 11 commencing at 18h01m UT are described. The event is homologous with an earlier disturbance from the same region of the corona. The observations suggest that a cloud of coronal gas containing 4 × 1039 electrons propagated outwards to 5 R
behind a piston-driven shock wave travelling at a velocity of 800 to 1200 km s–1.On leave from Division of Radiophysics, CSIRO, Sydney, Australia. 相似文献
12.
We have used a 5.5 min time-sequence of spectra in the Fe i lines 5576 (magnetically insensitive), 6301.5 and 6302.5 (magnetically sensitive) to study the association of concentrated magnetic regions and velocity in the quiet Sun. After the elimination of photospheric oscillations we found downflows of 100–300 m s –1, displaced by about 2 from the peaks of the magnetic field; this velocity is comparable to downflow velocity associated with the granulation and of the same order or smaller than the oscillation amplitude. Quasi-periodic time variations of the vertical component of the magnetic field up to ± 40% were also found with a period near 250 s, close to the values found for the velocity field. Finally we report a possible association of intensity maxima at the line center with peaks of the oscillation amplitude. 相似文献
13.
L. van Driel-Gesztelyi A. Hofmann P. Démoulin B. Schmieder G. Csepura 《Solar physics》1994,149(2):309-330
Through coordinated observations made during the Max'91 campaign in June 1989 in Potsdam (magnetograms), Debrecen (white light and H), and Meudon (MSDP), we follow the evolution of the sunspot group in active region NOAA 5555 for 6 days. The topology of the coronal magnetic field is investigated by using a method based on the concept of separatrices - applied previously (Mandriniet al., 1991) to a magnetic region slightly distorted by field-aligned currents. The present active region differs by having significant magnetic shear. We find that the H flare kernels and the main photospheric electric current cells are located close to the intersection of the separatrices with the chromosphere, in a linear force-free field configuration adapted to the observed shear. Sunspot motions, strong currents, isolated polarities, or intersecting separatrices are not in themselves sufficient to produce a flare. A combination of them all is required. This supports the idea that flares are due to magnetic reconnection, when flux tubes with field-aligned currents move towards the separatrix locations. 相似文献
14.
Hongqi Zhang 《Solar physics》1993,146(1):75-92
A series of H chromospheric magnetograms was obtained at various wavelengths near the line center with the vector video magnetograph at Huairou Solar Observing Station as a diagnostic of chromospheric magnetic structures. The two-dimensional distribution of the circular polarization light of the H line with its blended lines at various wavelength in active regions was obtained, which consists of the analyses of Stokes' profileV of this line. Due to the disturbance of the photospheric blended line Fei 4860.98 for the measurement of the chromospheric magnetic field, a reversal in the chromospheric magnetograms relative to the photospheric ones occurs in the sunspot umbrae. But in the quiet, plage regions, even penumbrae, the influence of the photospheric blended Fei 4860.98 line is not obvious. As regards the observation of the H chromospheric magnetograms, we can select the working wavelength between -0.20 and -0.24 from the line core of H to avoid the wavelengths of the photospheric blended lines in the wing of H.After the spectral analysis of chromospheric magnetograms, we conclude that the distribution of the chromospheric magnetic field is similar to the photospheric field, especially in the umbrae of the sunspots. The chromospheric magnetic field is the result of the extension of the photospheric field. 相似文献
15.
In this paper, we present a time series of Fei 5250.2 Å photospheric filtergrams and corresponding magnetograms in a quiet region. The relationship between fine structures of granulation and magnetic fields is analyzed. It is found that although most bright filigree features in photospheric filtergrams are related to corresponding magnetic features, they are generally not cospatial. It is also found that some bright features and their corresponding photospheric magnetic fields show fast changes within several minutes. 相似文献
16.
The equatorial latitude of auroral activity has been derived from both electron and optical observations with the DMSP satellites. Virtually all of the observations that were obtained during the 5-year interval June 1972-September 1977 have been used to construct a nearly continuous plot of invariant geomagnetic latitude versus time.This plot has two main characteristics: (1) A diurnal variation of approximately ± 5° which is associated with the precession of the Earth's magnetic dipole axis about the Earth's rotation axis; (2) an irregular variation of roughly 5–10° for intervals of one to several days associated with the occurrence of solar flares and coronal holes.With the help of a condensed, Bartels-type display of these measurements, we conclude that: (a) Modest auroral expansions (to ~ 60°) occur during the main body of high-speed streams from coronal holes; (b) great expansions (to < 55°) occur only during intervals of intense interplanetary magnetic fields such as may occur at the leading edge of a high-speed stream or at a flare-produced interplanetary shock. 相似文献
17.
Yun-Tung Lau 《Solar physics》1993,148(2):301-324
We study the magnetic field-line topology in a class of solar flare models with four magnetic dipoles. By introducing a series of symmetry-breaking perturbations to a fully symmetric potential field model, we show that isolated magnetic nulls generally exist above the photosphere. These nulls are physically important because they determine the magnetic topology above the photosphere. In some special cases, there may be a single null above the photosphere with quasi two-dimensional properties. For such a model, aquasi null line connects the null to the photosphere. In the limit of small non-ideal effects, boundary layers and current sheetsmay develop along the quasi null line and the associated separatrix surfaces. Field lines can then reconect across the quasi null line, as in two-dimensional reconnection. In a more general force-free case, the field contains a pair of nulls above the photosphere, with a field line (theseparator) connecting the two nulls. In the limit of small non-ideal effects, boundary layers and current sheets develop along the separator and the associated separatrix surfaces. The system exhibits three-dimensional reconnection across the separator, over which field lines exchange identity. The separatrices are related to preferable sites of energy release during solar flares. 相似文献
18.
A method is presented for constructing the coronal magnetic field from photospheric magnetograms and observed coronal loops. A set of magnetic field lines generated from magnetogram data is parameterized and then deformed by varying the parameterized values. The coronal flux tubes associated with this field are adjusted until the correlation between the field lines and the observed coronal loops is maximized. A mathematical formulation is described which ensures that (i) the normal component of the photospheric field remains unchanged, (ii) the field is given in the entire corona over an active region, (iii) the field remains divergence-free, and (iv) electric currents are introduced into the field. It is demonstrated that a parameterization of a potential field, comprising a radial stretching of the field, can provide a match for a simple bipolar active region, AR 7999, which crossed the central meridian on 1996 November 26. The result is a non-force-free magnetic field with the Lorentz force being of the order of 10–5.5 g cm s–2 resulting from an electric current density of 0.079 A m–2. Calculations show that the plasma beta becomes larger than unity at a relatively low height of 0.25 r supporting the non-force-free conclusion. The presence of such strong non-radial currents requires large transverse pressure gradients to maintain a magnetostatic atmosphere, required by the relatively persistent nature of the coronal structures observed in AR 7999. This scheme is an important tool in generating a magnetic field solution consistent with the coronal flux tube observations and the observed photospheric magnetic field. 相似文献
19.
We analyzed 689 high-resolution magnetograms taken daily with the NSO Vacuum Telescope on Kitt Peak from 1975 to 1991. Motions in longitude on the solar surface are determined by a one-dimensional crosscorrelation analysis of consecutive day pairs. The main sidereal rotation rate of small magnetic features is best fit by = 2.913(±0.004) – 0.405(±0.027) sin2
– 0.422(±0.030) sin4
, in µrad s–1, where is the latitude. Small features and the large-scale field pattern show the same general cycle dependence; both show a torsional oscillation pattern. Alternating bands of faster and slower rotation travel from higher latitudes toward the equator during the solar cycle in such a way that the faster bands reach the equator at cycle minimum. For the magnetic field pattern, the slower bands coincide with larger widths of the crosscorrelations (corresponding to larger features) and also with zones of enhanced magnetic flux. Active regions thus rotate slower than small magnetic features. This magnetic torsional oscillation resembles the pattern derived from Doppler measurements, but its velocities are larger by a factor of more than 1.5, it lies closer to the equator, and it leads the Doppler pattern by about two years. These differences could be due to different depths at which the different torsional oscillation indicators are rooted.Operated by the Association of Universities for Research in Astronomy Inc. under cooperative agreement with the National Science Foundation. 相似文献
20.
The rotation of the corona can be determined either directly by using Doppler methods or indirectly by using tracers, i.e., structures within the corona. In this study the rotational characteristics of the corona are determined using coronal holes as tracers, for the period 1978–1991. The coronal data used here are from an atlas of coronal holes mapped in Hei 10830 data. A comparison is made between our results and previous determinations of the coronal rotation rate, e.g., by Sime (1986), using white-light K-coronameter observations, by Timothy, Krieger, and Vaiana (1975), using soft X-ray observations, and by Shelke and Pande (1985) and Navarro-Peralta and Sanchez-Ibarra (1994), using Hei 10830 data. For the atlas of coronal holes used in this study the nature of the coronal hole distributions in number and latitude, in yearly averages, has been determined. These distributions show that at solar minimum the polar coronal holes dominate and the few non-polar holes are confined to a narrow band near the equator. At solar maximum, however, mid-latitude coronal holes dominate, with a large spread in latitudes. Given these distributions we consider the differential rotation data only as an average over a solar cycle. This removes spurious effects caused by having only a small number of coronal holes contributing to the results, or by having a narrow latitude band for the observations, thus limiting the results to that narrow latitude band. By considering these coronal holes as tracers of the differential rotation we show that the mid-latitude corona rotates more rigidly than the photosphere, but still exhibits significant differential rotation, with an equatorial rate of 13.30 ± 0.04° day–1, and at 45° latitude a rate of 12.57 ± 0.13° day–1. These results are comparable, within errors, to the Sime (1986) results which have an equatorial rate of approximately 13.2 ± 0.2° day–1 and a rate of approximately 12.9 ± 0.3° day–1 at 45° latitude. 相似文献