Polarization of solar active regions at 9.5 mm wavelength

A study of the circular polarization structure of solar active regions has been made from data obtained at 9.5 mm wavelength, using the 85 ft reflector and polarimeter at the Naval Research Laboratory Maryland Point Observatory. The angular resolution of the telescope at this wavelength is 1.6. All important active regions observed at 9.5 mm are bipolar in nature, the degree of polarization is about the same for both right and left circular components and it ranges up to about 4%. These oppositely polarized components correspond with the Mt. Wilson magnetic regions of opposite polarity; the line of zero polarization delineates clearly the neutral line between the regions of opposite polarity on magnetograms. Unipolar regions in magnetograms also show up as unipolar regions at 9.5 mm. Magnetic fields as low as 5–10 G on magnetograms manifest as distinctly polarized regions on 9.5 mm maps. A line of zero polarization seems to delineate the extent of absorption features observed at 9.5 mm in coincidence with H dark filaments.     

Polarization structure of a solar flare region at 9.5 mm wavelength

Polarization structure of an active region that produced a minor flare around 1900 UT on September 28, 1971 was measured at 9.5 mm wavelength using the 85-ft telescope of the Naval Research Laboratory Maryland Point Observatory. The angular resolution of the telescope at this wavelength is 1.6. The flare region underwent changes both in the degree of polarization as well as in its polarization structure before and after the start of the flare. These changes in the degree of polarization correspond to a decrease of longitudinal magnetic field of about 200 G at the chromospheric levels where the 9.5 mm radiation originates. Observations on the polarization structure of active regions for several days before and after September, 1971 are also presented.     

Relation between circular polarization of moving type IV bursts and polarity of photospheric magnetic fields

Two-dimensional, high-resolution observations of about 30 moving type IV bursts allow us to compare the polarization structure of the radio sources high in the corona with the distribution of magnetic fields measured at the photospheric level. Left- and right-handed circularly polarized moving type IV bursts are associated with active regions dominated by magnetic fields of plus and minus polarity respectively. The result suggests that the polarity of magnetic fields within the type IV source which moves high in the corona ( 1R above the photosphere) is closely related to the polarity of local magnetic fields at the photosphere. The above relation between the sense of polarization and the polarity of magnetic field is contrary to what would be expected from the generally accepted synchroton hypothesis. One way of resolving this conflict is to postulate that the magnetic field within the radio source has the opposite polarity to that of the ambient magnetic fields.     

Solar active regions at millimeter wavelengths

Some properties of solar active regions at 9, 3.5 and 1.2 mm wavelengths are discussed. The regions have excess brightness temperatures of up to 1000, 700 and 150 K at 9, 3.5 and 1.2 mm wavelengths. The background radiation at 3.5 mm is often seen to be absorbed in regions closely coincident with H dark filaments on the disk. Interpretation of this absorption as due to the large optical thickness of the overlying filamentary material leads to an estimate of electron density in the filaments. The 9 and 3.5 mm- regions show almost one-to-one correspondence with the Ca-plage regions as well as with the regions on magnetograms. The latter relationship suggests the possibility of measuring chromospheric magnetic fields from the measurement of polarization at millimeter wavelengths.     

Evolution of magnetic fields and mass flow in a decaying active region

Five days of coordinated observation were carried out from 24–29 September, 1987 at Big Bear and Huairou Solar Observatories. Longitudinal magnetic fields of an p sunspot active region were observed almost continuously by the two observatories. In addition, vector magnetic fields, photospheric and chromospheric Doppler velocity fields of the active region were also observed at Huairou Solar Observatory. We studied the evolution of magnetic fields and mass motions of the active region and obtained the following results: (1) There are two kinds of Moving Magnetic Features (MMFs). (a) MMFs with the same magnetic polarity as the center sunspot. These MMFs carry net flux from the spot, move through the moat, and accumulate at the moat's outer boundary. (b) MMFs in pairs of mixed polarity. These MMFs are not responsible for the decay of the spot since they do not carry away the net flux. MMFs in category (b) move faster than those of (a). (2) The speed of the mixed polarity MMFs is larger than the outflow measured by photospheric Dopplergrams. The uni-polar MMFs are moving at about the same speed as the Doppler outflow. (3) The chromospheric velocity is in approximately the opposite direction from the photospheric velocity. The photospheric Doppler flow is outward; chromospheric flow is inward. We also found evidence that downward flow appears in the photospheric umbra; in the chromosphere there is an upflow.     

Slowly varying component spectrum of the solar radio emission at millimetre wavelengths

This paper deals with the observed data on the solar S-component sources at millimetre wavelengths. The observations were made in 1968 and 1969 using the 22-m radio telescope of the Crimean Astrophysical Observatory at six wavelengths: 2, 4, 6, 8, 13 and 17 mm. The enhanced intensity of the solar active region in comparison with the quiet Sun level varies proportionally to ^–2 if the wavelength is within the range of 2 ÷ 6 mm. In the wavelength band of 6 ÷ 17 mm almost flat spectra of the solar S-component sources is observed. Assuming the bremsstrahlung mechanism of the radio emission for the quiet Sun and the solar active regions an attempt has been made to treat the above presented data. It appears that the most probable explanation of the 2 ÷ 6 mm spectrum is that the S-component sources are opaque. In the 6 ÷ 17 mm wavelength band there are two possibilities: the active region may be either transparent or opaque. But in the last case the source brightness temperature must be proportional to ². Some differences in the spectra of the sources, identified with flocculi and with bipolar sunspot groups, were mentioned. The cold regions (as compared with the quiet Sun) were observed up to = 2 mm and identified with the filaments. However, its visibility falls when the wavelength decreases.     

Source structure of gradual rise and fall bursts at 17 GHz

From 200 GRF (gradual rise and fall) bursts which have been recorded with the 17 GHz interferometer at Nobeyama, we deduce the following characteristics of GRF bursts: (1) Sources of GRF bursts are broader, less circularly polarized than those of impulsive bursts. (2) The sources are potentially of bipolar structure and have the peak brightness near the position at which the sense of circular polarization changes. (3) The association of GRF bursts with type III bursts, which are indicative of nonthermal electron acceleration, is significantly poorer than that of impulsive bursts.It is suggested that the sources of GRF bursts or generally of thermal bursts lie relatively high in the solar atmosphere possibly near the top of magnetic loops or arches which divide two regions of opposite magnetic polarity.     

The imaging vector magnetograph at Haleakala

We describe an instrument we have built and installed at Mees Solar Observatory on Haleakala, Maui, to measure polarization in narrow-band solar images. Observations in Zeemansensitive photospheric lines have been made for nearly all solar active regions since the instrument began operations in 1992. The magnetograph includes a 28-cm aperture telescope, a polarization modulator, a tunable Fabry-Pérot filter, CCD cameras and control electronics. Stokes spectra of a photospheric line are obtained with 7 pm spectral resolution, 1 arc sec spatial resolution over a field 4.7 arc min square, and polarimetric precision of 0.1%. A complete vector magnetogram observation can be made every eight minutes. The flexibility of the instrument encourages diverse observations: besides active region magnetograms we have made, for example, composite vector magnetograms of the full solar disk, and H polarization movies of flaring regions.     

Radio mapping of the heliospheric current sheet

Synoptic plots of solar radio noise storms in the interval 1973 to 1984 are described. The dividing line between opposite noise storm polarities appears to be a good representation of the heliospheric current sheet out to displacements in latitude of ± 50° from the solar equator. This result is surprising, because noise storms are closely associated with closed magnetic field regions near sunspots. The possibility that noise storm polarity is determined by mode coupling high in the corona, where field lines are open, can be ruled out by the available evidence. This leads us to conclude that it is the clustering in longitude of active region complexes which determines the sector structure of the interplanetary magnetic field.     

Flows,flares, and formation of umbrae and light bridges in BBSO region No. 1167

We present high-resolution observations of the large active region BBSO No. 1167 (Boulder No. 5060) which cast new light on the structure of sunspot regions. We obtained excellent data, highlighted by videomagnetograms (VMG) obtained with our 65-cm telescope, which give unprecedented spatial resolution, about 0.5' for much of two 11-hr periods. This permitted us to see details of the field evolution and flows in the AR. The H filtergrams and D3 filtergrams permit study of these magnetic changes compared to spots and chromospheric structure.The region was a huge but simple active region (CMP July 2, 1988) in which we observed rapid flux emergence for several days. Because the new flux generally matched the old, there were few large flares. However, there were 14 flares on June 28 and 29, mostly in two sites. The first site was a spot which already existed when the active region appeared on the east limb. This site showed little change of magnetic structure during our observing period. The second site is an area disturbed by new flux emergence, which included a spot which formed and disappeared in two days, and a rapidly moving p spot. Flares ocurring at one site almost always produced footpoints at the other. The delay between flash phases of the same flare at the two sites ranges from 40 to 160 s.The magnetograms show complex fine structure, with some closely interwined regions of opposite polarity. In a region of new flux emergence, positive (leading polarity) flux flows along elongated channels immersed in the negative flux. Moving magnetic features occur around all of the spots.We point out other interesting aspects of this large region: (1) While there is extensive penumbra around the main umbrae, there is also significant penumbra apparently unrelated to any spot. These unusual penumbrae are either due to flux returning to the surface, flux left behind by the moving umbra, or associated with pores that appear and disappear. (2) We observed umbrae to move faster than the accompanying penumbrae, and concluded that penumbrae are not a simple extension of the umbra. (3) We found that combining spots of the same polarity do not completely merge, but are always separated by a thin light bridge. This means that the emerging flux loops are discrete entities.     

On the large-scale diffuse magnetic field of the Sun – II. The Contribution of Active Regions

Dikpati and Choudhuri (1994, 1995) developed a model for the poleward migration of the weak diffuse magnetic field on the Sun's surface. This field was identified with the poloidal component produced by the solar dynamo operating at the base of the convection zone, and its evolution was studied by considering the effects of meridional circulation and turbulent diffusion. The earlier model is extended in this paper by incorporating the flux from, the decay of tilted active regions near the solar surface as an additional source of the poloidal field. This extended model can now explain various low-latitude features in the time-latitude diagram of the weak diffuse fields. These low-latitude features could not be accounted for in the earlier model, which was very successful in modeling the behavior at high latitudes. The time-latitude diagrams show that regions of a particular polarity often have tongues of opposite polarity. Such tongues can be produced in the theoretical model by incorporating fluctuations in the source term arising out of the decaying active regions.     

Structures of chromospheric magnetic fields in the solar flare producing active region 5747

In this paper, the formation and the measurement of the H line in chromospheric magnetic fields are discussed. The evolution of the chromospheric magnetic structures and the relation with the photospheric vector magnetic fields and chromospheric velocity fields in the flare producing active region AR 5747 are also demonstrated.The chromospheric magnetic gulfs and islands of opposite polarity relative to the photospheric field are found in the flare-producing region. This probably reflects the complication of the magnetic force lines above the photosphere in the active region. The evolution of the chromospheric magnetic structures in the active region is caused by the emergence of magnetic flux from the sub-atmosphere or the shear motion of photospheric magnetic fields. The filaments separate the opposite polarities of the chromospheric magnetic field, but only roughly those of the photospheric field. The filaments also mark the inversion lines of the chromospheric Doppler velocity field which are caused by the relative motion of the main magnetic poles of opposite polarities in the active region under discussion.     

Structure and polarization of active region microwave emission

We present observations of active region radio emission at 6.16 cm wavelength, obtained with an angular resolution of 3 by 10 arc using the Westerbork Synthesis Radio Telescope (WSRT) during the action interval May 20–27, 1980 of the Solar Maximum Year (SMY). We present maps in both total intensity (I) and circular polarization (V) of three regions (Hale numbers 16850, 16863, and 16864) and provide a detailed comparison of these maps with on- and off-band H pictures and with magnetograms. The strongest sources were associated with neutral lines and soft X-ray arcades. We present evidence that these neutral lines were characterized by having their two opposite polarities close to each other, implying a high magnetic field gradient, and by their association with arch filament systems. The sunspot associated radio sources had a relatively simple structure in region 16850; however for the large spots of regions 16864 and 16863 the emission had a patchy appearance with a tendency of the peaks to lie over the penumbra. In the V maps we observed for the first time two islands, polarized in the sense of the ordinary mode, which were located inside the sunspot associated sources and were associated with intrusions of opposite polarity field into the penumbra. These structures can be accounted for if the electron temperature along the line of sight is not a monotonically increasing function of height, but has a maximum near the second harmonic level. Finally we give a detailed analysis of observations of the inversion of the sense of circular polarization in region 16863. We find that the large scale structure of the magnetic field can be approximated by a dipole with its axis inclined by 11° with respect to the photosphere and with a dipole moment of about 2 × 10³¹ cgs units; the depolarization line is located at a height of 0.16–0.19 R above the dipole, where the estimated intensity of the magnetic field is 10–20 G.     

Installation of solar chromospheric telescope at the Indian Astronomical Observatory,Merak

We report the observations of the solar chromosphere from a newly commissioned solar telescope at the incursion site near Pangong Tso lake in Merak (Leh/Ladakh). This new \(\hbox {H}_{\alpha }\) telescope at the Merak site is identical to the Kodaikanal \(\hbox {H}_{\alpha }\) telescope. The telescope was installed in the month of August 2017 at the Merak site. The telescope consists of a 20-cm doublet lens with additional re-imaging optics. A Lyot filter with 0.5 Å passband isolates the Balmer line of the hydrogen spectra to make the observations of the solar chromosphere. The observations made in \(\hbox {H}_{\alpha }\) wavelength delineates the magnetic field directions at the sunspot and the quiet regions. A CCD detector records the images of the chromosphere with a pixel resolution of 0.27\(^{\prime \prime }\) and covers 9.2\(^{\prime }\) field-of-view. This telescope has a good guiding system that keeps the FoV in the intended position. We report the development of control software for tuning the filter unit, control detector system, observations and calibration of the data to make it useful for the scientific community. Some preliminary results obtained from the Merak \(\hbox {H}_{\alpha }\) telescope are also presented. This high altitude facility is a timely addition to regularly obtain \(\hbox {H}_{\alpha }\) images around the globe.     

Weak-Field Magnetogram Calibration using Advanced Stokes Polarimeter Flux-Density Maps – I. Solar Optical Universal Polarimeter Calibration

Cotemporal Fei 630.2 nm magnetograms from the Solar Optical Universal Polarimeter (SOUP) filter 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. The SOUP instrument produces Stokes V/I `filter magnetograms' with wide field of view and spatial resolution below 0.5 arc sec in good seeing, but low spectral resolution. In contrast, the ASP uses high spectral resolution to produce very high-precision vector magnetic field maps at spatial resolution values on the order of 1 arc sec in good seeing. We use ASP inversion results to create an ASP `longitudinal magnetic flux-density map' with which to calibrate the less precise SOUP magnetograms. The magnetograms from each instrument are co-aligned with an accuracy of about 1 arc sec. Regions of invalid data, poor field-of-view overlap, and sunspots are masked out in order to calibrate SOUP predominately on the relatively vertical `weak-field' plage magnetic elements. Pixel-to-pixel statistical comparisons are used to determine the SOUP magnetogram linear calibration constant relative to ASP flux-density values. We compare three distinct methods of scaling the ASP and SOUP data to a common reference frame in order to explore filling factor effects. The recommended SOUP calibration constant is 17000 ± 550 Mx cm^–2 per polarization percent in plage regions. We find a distinct polarity asymmetry in SOUP response relative to the ASP, apparently due to a spatial resolution effect in the ASP data: the smaller, less numerous, minority polarity structures in the plage region are preferentially blended with the majority polarity structures. The blending occurs to a lesser degree in the high-resolution SOUP magnetogram thus leading to an apparent increase in SOUP sensitivity to the minority polarity structures relative to the ASP. One implication of this effect is that in mixed polarity regions on the Sun, lower spatial resolution magnetograms may significantly underestimate minority polarity flux levels, thus leading to apparent flux imbalances in the data. ^*Visiting Astronomer, National Solar Observatory, operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under cooperative agreement with the National Science Foundation. The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Loss of magnetic tension in pre-flare magnetic configurations

We demonstrate that magnetic tension vanishes at regions of large magnetic shear on the polarity inversion line. The characteristics of these tension-free fields depend on the density of the medium and, therefore, change as a consequence of instabilities which modify the density. These instabilities may possibly evolve into solar flares. We suggest this as a possible explanation for the observed occurrence of flares at locations of large magnetic shear along the polarity inversion line.     

Magnetic fields in flares and active prominences

The locations of flares and chromospheric absorption features on May 21 and 23, 1967, are compared with a series of H magnetograms. Each of the four major flares included in the study developed as double emission ribbons lying at positions of steep field gradient on opposite sides of the boundary between regions of opposite magnetic polarity. At certain stages, the flare outlines followed closely the isogauss contours of the longitudinal field. A fluctuating field of 75 gauss was measured directly in the importance two flare of May 21. Modifications in the magnetic structure of the active region followed the flares of May 23.     

Sources of the Slowly-Varying Component of Solar Microwave Emission and their Relationship with their Host Active Regions

Daily surveys of the solar disc made at 2.8 cm wavelength over the period 1–13 November 1981, complemented by magnetograms and H filtergrams, are used to examine the relationship between sources of the slowly varying component of solar radio emission and properties of their host regions. Two classes of source are noted: diffuse and compact. Sources are designated compact when smaller than 40. The diffuse sources may be explained in terms of free–free thermal emission from trapped plasma in loops overlying the active region. The great majority of compact sources can be accounted for in terms of gyroresonance from thermal electrons in the strong magnetic fields overlying sunspots. A small minority are less amenable to this explanation. They are associated with magnetic complexity and dynamism, lie close to magnetic polarity reversals, and could be non-thermal. Microwave sources are an evolutionary feature common to all but the smallest active regions.     

Studies of the solar chromosphere from millimetre and sub-millimetre observations

Spectroheliograms were obtained in bands centred at 1.2 mm, 0.8 mm and 0.4 mm wavelength during 1969 and 1971. In order to obtain photometrically valid data, a specialized set of reduction techniques was employed, obviating the effects of severe differential attenuation across the disc by atmospheric water vapour and of emission noise from the atmosphere, before taking out the instrumental spread functions of the telescope and detector.Comparison of our maps with those of other observers at 3 mm and 8.6 mm wavelength suggests that the chromospheric brightness temperature increments above active regions show a monotonic increase with increasing height above the photosphere over the normal increase in brightness temperature in the quiet chromosphere. Within the limit of angular resolution of 3 available, no evidence was recorded of normal limb brightening in our three passbands but the presence of isophotes at 50% of the central disc temperature consistently circumscribing the optical limb implies a narrow spike of sub-millimetre brightening close to the limb.     

Polarization of hard X-rays from solar flares

The polarization of hard solar X-radiation (> 10 keV) is calculated on the assumption that electrons get a non-isotropic velocity distribution in the initial phase of a flare. The brems-strahlung generated by nonthermal electrons spiralling around magnetic field lines with discrete pitch angles is considerably polarized if observed at approximately right angles to the magnetic field. In the energy range from 10 to 50 keV the degree of polarization is not strongly dependent on the photon energy. For pitch-angle distributions of the form sin² and cos², the polarization has opposite signs; it decreases appreciably at high photon energies. The observation of X-ray polarization will be useful in deducing the physical conditions in flares.