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.     

Observations and interpretation of moving type IV solar radio bursts

Properties of 23 moving type IV bursts observed with the Culgoora Radioheliograph are summarized. Both shock and plasmoid models are examined. It is found that the theories invoking shocks have limited application and that plasmoid models have several problems with regard to plasmoid formation as well as with explanations for multiple sources and large values of circular polarization. While the synchrotron radiation mechanism is the most widely accepted for both shock and plasmoid models, it is possible that Langmuir wave emission processes may be important, at least in some events. To overcome some of the difficulties of the plasmoid theory, a new source model is proposed. This model involves synchrotron radiation from electror ; confined by rapid scattering through hydromagnetic wave particle interactions.Operated by the Association of Universities for Research in Astronomy, Inc. under contract AST-74-04129 with the National Science Foundation.     

Solar flare activity and the structure of the coronal neutral line

In this study we analyse the positions of major flares from 1978 and 1979, with respect to the magnetic structure of the solar corona, as described by a potential field model. We find that major flares exhibit no strong association with the neutral line at the chromospheric level. However, when we calculate the neutral line's position at higher and higher altitudes in the corona, we find that major flares show an increasing tendency to be found close to these high-altitude coronal neutral lines. The correlation between flares and higher-altitude coronal neutral lines reaches a maximum at an altitude of 0.35R , and thereafter decreases as the neutral line is moved out to the source surface at an altitude of 1.50R . This indicates that major flares are strongly associated with coronal structure at the 0.35R level ( 250 000 km) - an altitude surprisingly high in the corona. This reinforces the idea that flares are associated with large-scale coronal magnetic fields and also indicates that the region of coronal magnetic topology important to solar flare processes may be larger than previously thought.     

The coronal and interplanetary magnetic fields at the time of the solar eclipse of 7 March, 1970

The magnetic field in the outer corona and in interplanetary space has been calculated from the photospheric magnetic fields measured around the time of the 7 March, 1970 eclipse. The field-line maps are compared with eclipse photographs showing coronal structures out to about 12 r . The projected field lines as well as the observed streamers appear straight. This is caused by the rapid expansion of the outer corona and is not an indication of corotation. The calculations show that the angular velocity of the coronal plasma decreases rapidly with distance.The relation between magnetic fields and density enhancements is discussed. The field strength in the photosphere seems to determine the amount of mechanical heating of the lower corona. The density structure higher up in the corona will, however, depend decisively on the topology of the field, particularly on whether we are on open or closed field lines, and not simply on field strength.The calculations show a sector structure of the interplanetary field, which agrees well with spacecraft observations. Also the magnitudes of the observed and calculated interplanetary field agree after the Mt. Wilson magnetograph data have been corrected to account for the temperature and saturation effects in the spectral line Fei 5250 Å.On leave from the Astronomical Observatory, Lund, Sweden.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The gyro-synchrotron radiation from moving type IV sources in the solar corona

Calculations of the gyro-synchrotron emission are made for conditions which might be expected in moving type IV sources in the solar corona. Two simple models for an evolving source are treated: a uniform cube and an inhomogeneous sphere. The results suggest that most moving sources have the following features: (1) A rather strong magnetic field, 10 G, is carried out within the source. This is required to achieve the high degree of circular polarization often observed. (2) Synchrotron self-absorption causes the source to be optically thick at frequencies less than about 100 MHz, thus restricting the bandwidth of the radiation. The self-absorption decreases as the source moves outward and expands. The turnover frequency, which separates the optically thick and thin spectral regimes, moves rapidly to lower frequencies, accompanied by a change from low to high circular polarization. In the case of an inhomogeneous source, the source appears to be larger at the lower frequencies. (3) Razin-Tsytovich suppression cannot be an important factor in determining the characteristics of most sources.Exchange visitor from the Department of Astro-Geophysics, University of Colorado, Boulder, Colorado, U.S.A.     

A slowly moving plasmoid associated with a filament eruption

We report the imaging observations of a slowly moving type IV burst associated with a filament eruption. This event was preceded by weak type III burst activity and was accompanied by a quasi-stationary continuum that persisted for several hours. The starting times and speeds of moving type IV burst and the erupting filament are nearly the same, implying a close physical relation between the two. The moving type IV burst is interpreted as gyrosynchrotron emission from a plasmoid containing a magnetic field of 1–2 G and nonthermal electrons of density 10⁵–10⁶ cm^–3 with a relatively low average energy of 50 keV.     

On neutral sheets in the solar wind

The structure and dynamics of neutral sheets in the solar wind is examined. The internal magnetic topology of the sheet is argued to be that of thin magnetic tongues greatly distended outward by the expansion inside the sheet. Due to finite conductivity effects, outward flow takes place across field lines but is retarded relative to the ambient solar wind by the reverse J×B force. The sheet thickness as well as the internal transverse magnetic field are found to be proportional to the electrical conductivity to the inverse one third power. Estimating a conductivity appropriate for a current carried largely by the ions perpendicular to the magnetic field, we find sheet dimensions of the order of 500km representative for the inner solar corona. For a radial field of strength 1/2G at 2R , the transverse field there is about 2 × 10^–3G and decreases outward rapidly.The energy release in the form of Joulean dissipation inside the sheet is estimated. It is concluded that ohmic heating in current sheets is not a significant source of energy for the overall solar wind expansion, mainly because these structures occupy only a small percentage of the total coronal volume. However, the local energy release through this mechanism is found to be large - in fact, over 7 times that expected to be supplied by thermal conduction. Therefore, ohmic heating is probably a dominant energy source for the dynamical conditions within the sheet itself.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

80 MHz observations of a moving type IV solar burst,March 1, 1969

The 80 MHz emission from a moving type IV source has been observed as the source moved from 2 to 51/4 R from the centre of the Sun. The emission came from a plasma cloud ejected in association with an extensive solar prominence. The cloud appeared to move with a speed ( 270 km/sec) approximating the local Alfvén velocity in the corona. At 2 R the emission was from a single unpolarized source, while at 5 R it was from two sources strongly circularly polarized in opposite senses. The physical conditions inside and outside the source and the emission mechanisms are discussed.     

On the variability timescales and magnetic field in OJ 287

The continuum spectrum of OJ 287, like most other BL Lac objects, is featureless- no emission or absorption lines are observed. However, OJ 287 shows variations at different timescales in flux and polarization at various wavelength bands. Using the available variability data one can estimate the sizes of the emission regions in the source from light travel time arguments. We assume the emission mechanism to be synchrotron radiation by high energy electrons with single power law energy distribution. Theoretical synchrotron spectrum in the frequency range 10¹¹–10¹⁷ H _z is compared with the observed spectral shape, obtained from new multifrequency quasi-simultaneous observations, to estimate the lower and upper cut off frequencies. These frequencies are used to obtain theoretical values of the variability timescales and magnetic field in the emission region. We obtain a value of 0.93 G for the magnetic field and 5.184×10⁴ sec for the cooling time from the quiescent continuum spectrum. The shock-in-jet model explains the spectrum where shocks accelerate the particles and amplify the magnetic field in the jet. This timescale is compared with the one obtained from observed short timescale variability (20 minutes) with proper beaming correction. The short timescale variations (200 minutes in the source frame), possibly caused by an additional, flaring, component of the source, are also used to calculate compressed magnetic field. The observed and theoretically estimated variability timescales and the shape of the spectrum suggest that there are more than one emission components in OJ 287.     

Analysis of a Failed Eclipse Plasma Ejection Using EUV Observations

The photometry of eclipse white-light (W-L) images showing a moving blob is interpreted for the first time together with observations from space with the PRoject for On Board Autonomy (PROBA-2) mission (ESA). An off-limb event seen with great details in W-L was analyzed with the SWAP imager (Sun Watcher using Active pixel system detector and image Processing) working in the EUV near 174 Å. It is an elongated plasma blob structure of 25 Mm diameter moving above the east limb with coronal loops under. Summed and co-aligned SWAP images are evaluated using a 20-h sequence, in addition to the 11 July, 2010 eclipse W-L images taken from several sites. The Atmospheric Imaging Assembly (AIA) instrument on board the Solar Dynamics Observatory (SDO) recorded the event suggesting a magnetic reconnection near a high neutral point; accordingly, we also call it a magnetic plasmoid. The measured proper motion of the blob shows a velocity up to \(12~\mbox{km}\,\mbox{s}^{-1}\). Electron densities of the isolated condensation (cloud or blob or plasmoid) are photometrically evaluated. The typical value is \(10^{8}~\mbox{cm}^{-3}\) at \(r=1.7~\mathrm{R}_{\odot}\), superposed on a background corona of \(10^{7}~\mbox{cm}^{-3}\) density. The mass of the cloud near its maximum brightness is found to be \(1.6\times10^{13}\) g, which is typically \(0.6\times10^{-4}\) of the overall mass of the corona. From the extrapolated magnetic field the cloud evolves inside a rather broad open region but decelerates, after reaching its maximum brightness. The influence of such small events for supplying material to the ubiquitous slow wind is noticed. A precise evaluation of the EUV photometric data, after accurately removing the stray light, suggests an interpretation of the weak 174 Å radiation of the cloud as due to resonance scattering in the Fe IX/X lines.     

Coronal density and temperature gradients

Mean density models of the solar corona show evidence for two distinctive density regimes characterized by different density gradients. High density gradients are identified with regions of predominantly open magnetic lines of force and low density gradients are identified with regions of predominantly closed magnetic lines of force. Spectroscopic data yielding equivalent widths of forbidden lines of Fe x and Fe xiv strongly suggest that the coronal temperature for r > 2.5 R decreases considerably less rapidly in equatorial regions than r ^–2/7, which is the decrease predicted by conduction models with open field lines.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

A statistical study of moving type IV bursts based on Culgoora radioheliograph observations

Thirty-one moving type IV (IV(M)) bursts recorded with the Culgoora radioheliograph are examined to deduce their characteristic features, such as spatial distribution, projected velocity, etc., and their relation to other phenomena. The distribution of the projected velocity suggests that less than 15% of the total IV(M) bursts have fast velocities (>1000 km s^–1), almost equal to MHD shock velocity, and that the remaining IV(M) bursts have slower velocities (400 km s^–1) and are probably not associated with MHD shock waves. Most of the slow IV(M) bursts (and 70% of the total IV(M) bursts) are of an isolated plasmoid type. Even if they are associated with minor H flares, IV(M) bursts of the isolated-plasmoid type have 10³¹ ergs in the form of magnetic energy. They are in many cases closely associated with extended flare-continuum sources; this seems plausible if the flare continuum is interpreted as an interaction of a plasmoid with a large-scale magnetic arch.The association of IV(M) bursts with energetic proton events seems to be poor - contrary to expectation.     

The High Altitude Observatory Coronagraph/Polarimeter on the Solar Maximum Mission

The High Altitude Observatory Coronagraph/Polarimeter, to be flown on the National Aeronautics and Space Administration's Solar Maximum Mission satellite, is designed to produce images of the solar corona in seven wavelength bands in the visible spectral range. The spectral bands have been chosen to specifically exclude or include chromospheric spectral lines, so as to allow discrimination between ejecta at high (coronal) and low (chromospheric) temperatures, respectively. In addition, the instrument features spectral filters designed to permit an accurate color separation of the F and K coronal components, and a narrow band (5.5 Å) filter to observe the radiance and polarization of the Fe xiv 5303 Å line. The effective system resolution is better than 10 arc sec and the instrument images a selected quadrant (or smaller field) on an SEC vidicon detector. The total height range that may be recorded encompasses 1.6 to more than 6.0R (from Sun center). The instrument is pointed independently of the SMM spacecraft, and its functions are controlled through the use of a program resident within the onboard spacecraft computer. Major experimental goals include: (a) Observation of the role of the corona in the flare process and of the ejecta from the flare site and the overlying corona; (b) the study of the direction of magnetic fields in stable coronal forms, and, perhaps, ejecta; and (c) examination of the evolution of the solar corona near the period of solar maximum activity.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The structure of the white-light corona and the large-scale solar magnetic field

The large-scale density structure of the white-light solar corona has been compared to the organization of the solar magnetic field as identified by the appearance of neutral lines in the photosphere in order to examine whether any consistent relationship exists between the two. Data from the High Altitude Observatory's Mk-III K-coronameter have been used to describe the coronal density structure, and observations from several sources, beginning with observations from the University of Hawaii Stokes Polarimeter have been used to establish the magnetic field distribution. Stanford magnetograms as well as the neutral line inferred from potential field models have also been examined. During the period covering Carrington rotations 1717 to 1736 brightness enhancements in the low corona tend to lie over the global neutral sheet identified in the photospheric magnetic field. The brightest of these enhancements, however, are associated with neutral lines through active regions. These associations are not 1-1, but do hold both in stable and evolving conditions of the corona. We find a significant number of long-lived neutral lines, including filaments seen in H, for which there are not coronal enhancements.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

VLA,SOHO and TRACE Observations of Nonthermal Burst Activity,Coronal Mass Ejections,and EUV Transient Events

Very Large Array (VLA) observations of the Sun at 91 and 400 cm wavelength have been used to investigate the radio signatures of EUV heating events and coronal mass ejections (CMEs) detected by SOHO and TRACE. Our 91 cm observations show the onset of Type I noise storm emission about an hour after an EUV ejection event was detected by EIT and TRACE. The EUV event also coincided with the estimated start time of a CME detected by the LASCO C2 coronagraph, suggesting an association between the production of nonthermal particles and evolving plasma-magnetic field structures at different heights in the corona. On another day, our VLA 400 cm observations reveal weak, impulsive microbursts that occurred sporadically throughout the middle corona. These low-brightness-temperature (T _b=0.7–22×10⁶ K) events may be weak Type III bursts produced by beams of nonthermal electrons which excite plasma emission at a height where the local plasma frequency or its first harmonic equals the observing frequency of 74 MHz. For one microburst, the emission was contained in two sources separated by 0.7 R ₀, indicating that the electron beams had access to widely-divergent magnetic field lines originating at a common site of particle acceleration. Another 400 cm microburst occurred in an arc-like source lying at the edge of EUV loops that appeared to open outward into the corona, possibly signaling the start of a CME. In most instances the 400 cm microbursts were not accompanied by detectable EUV activity, suggesting that particles that produce the microbursts were independently accelerated in the middle corona, perhaps as the result of some quasi-continuous, large-scale process of energy release.     

Coronal emission line polarization

A discussion of a program for the computation of coronal emission line polarization is presented. The starting point is a general formulation of the scattering function for magnetic dipole transitions between any two total angular momentum levels, J J, J ± 1. Illustration of the behavior of the scattering function for different transitions is given. The integration of the scattering function over the solar disk and along the line of sight accounting for arbitrary distribution of magnetic fields as well as an inhomogeneous temperature and density structure of the corona is considered next.Sample results are presented for the numerical computation of the angle of maximum polarization and the degree of maximum polarization to be expected from idealized magnetic field configurations such as radial and dipole. A computation is included for a realistic field configuration predicted to exist at the time of the 1966 eclipse. The magnetic field input to the scattering calculation is based upon the potential field extension of photospheric magnetic fields. It is the purpose of the sample calculations to demonstrate how the measurement of emission polarization measurements can be interpreted in terms of the direction of coronal magnetic fields. Factors which lend ambiguity to such interpreations are clearly illustrated from the examples. These include the Hanle-effect depolarization and the depolarization at the Van Vleck angle.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Small-scale inhomogeneities in the solar corona: Evidence from meter-λ radio bursts

A number of inconsistencies between simple theory and observations of solar radio bursts indicate that mode-mode coupling in the solar corona is much stronger than predicted. The inconsistencies include the absence of predicted reversal of the sense of polarization in a type 1 storm at CMP, and the anomalously weak polarization of type II and type III emission. The strong mode coupling could be explained in terms of small scale inhomogeneities (L _N? 100 km) throughout the relevant regions of the corona. The relevant regions are those with open magnetic field lines overlying active regions. It is suggested that the coronal plasma is confined to magnetically self-pinched sheets, and it is pointed out that another inconsistency, namely the anomalously small amount of Faraday variation in type III bursts, could be explained if the value of n _e B in the inter-sheet region were two orders of magnitude less than in the sheets.     

The Shape of the Outer Corona during Cycle 21

The Naval Research Laboratory (NRL) Solwind coronagraph recorded the outer corona at elongations 2_5 R to 10 R during the 6 1/2-year interval from March 1979, before solar maximum, to the beginning of solar minimum in September 1985. During the minimum period, when the solar magnetic field was dipole-like, the observed corona consisted of the equatorial streamer belt that is characteristic of solar minimum, and that is interpreted as an edgewise view of a nearly flat current sheet or coronal disk lying near the plane of the heliographic equator. The observed disk was a radial projection from the magnetic neutral line that was computed for the 2.5 R source surface surrounding the Sun. At earlier times, shortly after solar maximum, the observed corona often consisted of a single coronal disk similar to that at solar minimum, but strongly tilted to the heliographic equator. Again this disk projected from a tilted magnetic neutral line that was computed for the 2.5 R source surface. Solar rotation allowed this coronal disk to be viewed in all aspects. In the edgewise view it appeared as a tilted streamer belt. In the broadside view the more flower-like pattern of solar maximum was observed. The latter view was interpreted as a non-uniform distribution of coronal material in the thin coronal disk. There were many intervals during the declining phase of the solar cycle when the computed magnetic neutral line at 2.5 R remained relatively simple but was not the source of an observable coronal disk. This latter result was probably because of the limitations of plane-of-sky observations, combined with short-term changes in the corona. Altogether, a single coronal disk, either flat or somewhat convoluted, was recognizable during only one third of the year lifetime of the coronagraph.     

Mechanisms of optical,X-ray andγ-radiation from Crab pulsar

The synchrotron mechanism of radiation from the Crab pulsar has been investigated on the assumption that the mechanism acts in a source moving with relativistic velocity round a neutron star. A detailed matching has been made of the theoretical spectra of synchrotron radiation from relativistic electrons with the results of measurements of the radiation flux from the Crab pulsar in the infrared, optical and X-ray ranges. The parameters of the radiating region (intensity of the magnetic field, source dimensions, density and lifetime of radiating electrons) have been found. They are expressed through the ratio of the energy density of the magnetic field in the source to that of radiating electrons. The level of Compton-radiation in this region is estimated. Possible values of at which the level will correspond to the available results of measurements of the-radiation flux from the Crab pulsar are given. An estimate is presented for the surface magnetic field of the neutron star which does not contradict those obtained from considerations of the magnetic flux conservation when compressing the object up to the neutron star dimensions.     

OJ 287: Polarization and photometric behaviour during 1971–76

The results are given of polarimetric and photometric observations of BL Lacertae-type object OJ 287 for 1972–76. These, and all other data available from the literature, are used in a comparative analysis of polarimetric and photometric properties of the object. The variations of time-scales from several years to several hours are noted. The variability is caused by the flaring up and fading of separate sources (hot spots) of polarized (synchrotron) radiation. The existence of a preferable direction of polarization (₀=80°) is an indication of a stable magnetic field. It may be used as an argument in favour of the single-body hypothesis of Lacertids. The behaviour of OJ 287 during a 6-day interval may be explained by intensity changes of a single source with constant parameters of polarization. It is found that, for this source,p=42.8%, ₀=101°. The night variations of brightnes and parameters of polarization limit the dimensions of the source responsible for this variability (R<-5×10¹⁴ cm) and enable us to estimate its degree of polarization (p50%).