A magnetic core in the Sun? The solar rotator

The previously found solar distortion rotating rigidly and wave-like on the surface with a 12 day period is interpreted as the shape of the gravitational potential induced by the solar core distorted by an internal magnetic field and rotating rigidly with this period. The distortion does not have a symmetry axis and the necessary magnetic field is not compatible with the axial symmetry required of a quasi-static field locked in the rotating core. It is concluded that if the solar distortion is due to such a process the core is oscillating with a very long period, a toroidal oscillation with a period of the order of years.This research was supported in part by the National Science Foundation.     

The evolution of prominences and their relationship to active centers

The radiation field, emergent from an inhomogeneous atmosphere, may differ significantly from that calculated using a mean model for such an atmosphere. In the solar case, horizontal anisotropy of the granulation pattern leads to azimuthal dependence of the emergent intensity, and this appears as a latitude-dependent limb flux which may mimic oblateness. We examine this latitude-dependence for several two and three-dimensional models of the inhomogeneous solar atmosphere, with varying degrees of anisotropy in the granulation pattern. Elongation along an east-west axis of about 7% would yield a signal somewhat imperfectly mimicking an excess oblateness of 4 × 10^–5. Using the Babcock-Leighton model of the general solar magnetic field we show that some stretching of granules, of this order of magnitude, should be expected. However, it may vary with the solar activity cycle, and in any case the result is very sensitive to the parameters adopted. Even if study of granulation observations should exclude elongations as high as 7%, smaller essentially undetectable elongations may exist. We find that 1 % elongation can account for 25–50 % of a signal corresponding to excess oblateness 4 × 10^–5. We conclude that anisotropy of the granulation pattern may influence oblateness determinations; when this is considered together with other effects, much of the claimed oblateness may be eliminated.     

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.     

On the nature of photospheric magnetic fields beneath large coronal holes

We consider proposed mechanisms for the formation of coronal holes, and identify as crucial the issue whether the holes are permeated by rigidly rotating fields. It is suggested that the interaction between such a field and the differentially rotating, diffusive solar envelope will produce a fore aft asymmetry in the distribution of fields which emerge to the photosphere. An initial study is carried out in the context of an illustrative example, and the results indicate that the asymmetry may be observed for a certain range of parameters involving the properties of the solar envelope and the characteristic size of the emerging field pattern.     

Relativistic effects and solar oblateness from radar observations of planets and spacecraft

We used more than 250 000 high-precision American and Russian radar observations of the inner planets and spacecraft obtained in the period 1961–2003 to test the relativistic parameters and to estimate the solar oblateness. Our analysis of the observations was based on the EPM ephemerides of the Institute of Applied Astronomy, Russian Academy of Sciences, constructed by the simultaneous numerical integration of the equations of motion for the nine major planets, the Sun, and the Moon in the post-Newtonian approximation. The gravitational noise introduced by asteroids into the orbits of the inner planets was reduced significantly by including 301 large asteroids and the perturbations from the massive ring of small asteroids in the simultaneous integration of the equations of motion. Since the post-Newtonian parameters and the solar oblateness produce various secular and periodic effects in the orbital elements of all planets, these were estimated from the simultaneous solution: the post-Newtonian parameters are β = 1.0000 ± 0.0001 and γ = 0.9999 ± 0.0002, the gravitational quadrupole moment of the Sun is J₂ = (1.9 ± 0.3) × 10^?7, and the variation of the gravitational constant is ?/G = (?2 ± 5) × 10^?14 yr^?1. The results obtained show a remarkable correspondence of the planetary motions and the propagation of light to General Relativity and narrow significantly the range of possible values for alternative theories of gravitation.     

Peering over the Sun's pole: behavior of its rotational vortex

A new method for measuring spectroscopically the rotation at the Sun's poles is described. Using solar CO lines at 4.666 µm, infrared spectra are recorded at a fixed limb distance of 4.8 arc sec while progressing along an arc ±5.7 deg from the Sun's rotational pole. Since the poles dip twice a year to about 7 arc sec from the limb, our observations can range either side of and through the vortex axis. Advantages to this technique are: (1) a low disturbing signal from supergranules owing to their superposition at the limb, (2) no ‘limb shift’ error since limb distance is constant and the CO lines have no known limb shift, (3) emphasis is on the quiet Sun since the CO molecule is confined there, (4) negligible scattered light in the IR (<1%), and (5) the improved seeing afforded by the IR. Although any definitive determination of solar rotation requires observations over an extended time span, our preliminary results suggest two features peculiar to the extreme pole: (1) the occasional apparent cessation of rotation, (2) some sort of singularity, again occasional, producing a sharp velocity signal (a vortex?) within 1 deg of the pole.     

An electric-current description of solar flares

The presently prevailing theories of solar flares rely on the hypothetical presence of magnetic flux tubes beneath the photosphere and the two subsequent hypotheses, their emergence above the photosphere and explosive magnetic reconnection, converting magnetic energy carried by the flux tubes to solar flare energy. In this paper, we discuss solar flares from an entirely different point of view, namely in terms of power supply by a dynamo process in the photosphere. By this process, electric currents flowing along the magnetic field lines are generated and the familiar ‘force-free’ fields or the ‘sheared’ magnetic fields are produced. Upward field-aligned currents thus generated are carried by downward streaming electrons; these electrons can excite hydrogen atoms in the chromosphere, causing the optical Hα flares or ‘low temperature flares’. It is thus argued that as the ‘force-free’ fields are being built up for the magnetic energy storage, a flare must already be in progress.     

A possible correlation between solar activity and Earth's oblateness

Recent observations demonstrate that Earth's dynamic oblateness (J₂), which has exhibited a decrease since 1979, suddenly increased around 1997 and the increase is still continuing. The decrease is attributed to post-glacial rebound from the mantle, and several causes, all of terrestrial nature, have been suggested to explain the sudden change in the trend. But the observations remain puzzling. On the other hand, close relationships are known to exist between many phenomena on the Earth with solar activity, and unusual behaviours of other planets have also been demonstrated to be correlated with solar activity. We show here that solar activity is significantly correlated with J₂, and is possibly responsible for the sudden increase in Earth's dynamic oblateness around 1997, the latter being due to an enormous increase in the correlation.     

Application of binary pulsars to axisymmetric bodies in the Elliptic R3BP

Binary systems hosting astrophysical compact objects such as white dwarfs and/or neutron stars provide excellent test beds for studying the impact of the oblateness of the main bodies in the restricted three-body problem (R3BP). The case is investigated when the primary bodies are non-luminous, non-spherical (oblate) bodies and the third body of infinitesimal mass is also an oblate spheroid. The existence of extra solar planets orbiting these systems constitutes a three-body problem which makes them excellent models for this axisymmetric ER3BP. The positions of the equilibrium points are affected by the oblateness parameters of the three-bodies; this is shown for double neutron star binaries. The triangular points are stable for 0<μ<μ _c ; where μ is the mass ratio (μ≤1/2) and μ _c is the critical mass value influenced by the eccentricity, semi major axis and oblateness factors. The size of the region of stability increases with decreasing values of the oblateness. The oblateness of the system’s bodies does not affect the nature of the stability of the collinear points since they remain unstable. Due to the almost equal masses of the primaries, our study shows that even the triangular points of these systems are unstable.     

Which One is the ‘GNEVYSHEV’ GAP?

Gnevyshev [Solar Phys. 1, 107, 1967] showed that in solar cycle 19 (1954 –1965), the coronal line half-yearly average intensity at 5303 Å (green line) had actually two maxima, the first one in 1957 and the second in 1959–1960. In the present communication, the structures at solar maxima were reexamined in detail. It was noted that the two-peak structure of solar indices near sunspot (Rz) maxima was only a crude approximation. On a finer time scale (monthly values), there were generally more than three peaks, with irregular peak separations in a wide range of ～12± 6 months. The sequences were seen simultaneously (within a month or two) at many solar indices (notably the 2800 MHz radio flux) at and above the photosphere, and these can be legitimately termed ‘Gnevyshev peaks’ and ‘Gnevyshev gaps’. The open magnetic flux emanating from the Sun showed this sequence partially, some peaks matching, others not. In interplanetary space, the interplanetary parameters N (number density), V (solar wind speed), B (magnetic field) showed short-time peak structures but mostly not matching with the Rz peaks. Geomagnetic indices (aa, Dst) had peaked structures, which did not match with Rz peaks but were very well related to V and B, particularly to the product VB. The cosmic ray (CR) modulation also showed peaks and troughs near sunspot maximum, but the matching with Rz peaks was poor. Hence, none of these can be termed Gnevyshev peaks and gaps, particularly the gap between aa peaks, one near sunspot maximum and another in the declining phase, as this gap is qualitatively different from the Gnevyshev gap in solar indices.     

On the Possible Connection between Photospheric 5-Min Oscillation and Solar Flare Microwave Emission

Dynamic spectra of low-frequency modulation of microwave emission from solar flares are obtained. Data of 15 bursts observed in 1989–2000 with Metsähovi radio telescope at 37 GHz have been used. During 13 bursts a 5-min modulation of the microwave emission intensity was detected with the frequency of ν _I = 3.2± 0.24 (1σ) mHz. Five bursts revealed a 5-min wave superimposed on a ～1 Hz, linear frequency modulated signal generated, presumably, by coronal magnetic loop, this wave frequency is ν_fm = 3.38± 0.37 (1σ) mHz. Both intensity and frequency modulations detected are in good agreement with the data on 5-min global oscillations of photosphere and with the data on the umbral velocity oscillations observed in the vicinity of sunspots. Possible role of p-mode photospheric oscillations in modulation of microwave burst emission is discussed.     

Temporal changes of physical conditions in the photospheric layers of a solar flare

An M4.1/1B solar flare on November 5, 2004, is investigated. The Stokes I ± V profiles of nine photospheric Fe I, Fe II, Sc II, and Cr II lines are studied for three instants of this flare (11 ^h 35 ^m , 11 ^h 39 ^m , and 11 ^h 45 ^m UT). The magnetic fields in the flare were measured in two ways: using the center-of-gravity method and by comparing the observed profiles with the theoretical ones computed with Baranovsky’s code. Analysis of the profiles reveals that the magnetic field strength peaked in the upper photosphere (logτ₅₀₀ = ?2.7) at the flare maximum (11 ^h 35 ^m ); this peak was smeared and shifted into the deeper photospheric layers as the flare evolved. The semiempirical model of the flare has two layers with an enhanced temperature: in the upper and middle photosphere. These layers also shifted deep into the photosphere as the flare evolved. The turbulent velocities at the distribution maximum increased by almost a factor of 5 compared to those in the undisturbed photosphere, while the plasma density both increased and decreased by a factor of 3–6.     

Why the Sun may appear oblate

If the Sun loses angular momentum from its core, due to core contraction, into the solar wind at the observed rate, then an 0.7 day rotational period for the core of the Sun is required for temporal equilibrium. The rotational power released in the core contraction process can equal the observed magnetic energy released in the solar activity cycle if the Sun's core rotates with a period near 1.4 to 4 days. The rotational power released from a rotating object is , where is the torque on the object and is its angular velocity. Fitting this to the solar wind torque and core rotation rate provides an 0.5 to 5 day rotation period for the Sun's core. A gravitational Pannekoek-Rosseland electric field in the Sun makes the Ferraro theorem inapplicable in such a way that rather than a constant angular velocity with radius, an inverse square radial dependence occurs. This results in a two day rotational period for the region in the Sun where most of the angular momentum resides. The consistency of the above four methods suggests that the Sun's observed oblateness is due to a rapidly rotating solar core. The oblateness of the photosphere is estimated to be near 3.4×10^–5.     

Some peculiarities in excitation and propagation of the gravity waves in solar photosphere

Using the data of observations in the Fe I line, the spatial-time variations of pressure in solar photosphere are reproduced, and local internal gravity waves are identified through the filtration. Based on the power spectra of gravity waves, some integral features of excitation and propagation of g-modes in real solar photosphere are studied which are excited by the dynamical processes near the boundary region of penetrative convection.     

Rigidly rotating modes of the solar magnetic field

Discrete rigidly rotating components (modes) of the large-scale solar magnetic field have been investigated. We have used a specially calculated basic set of functions to resolve the observed magnetic field into discrete components. This adaptive set of functions, as well as the expansion coefficients, have been found by processing a series of digitized synoptic maps of the background magnetic field over a 20-year period. As a result, dependences have been obtained which describe the spatial structure and the temporal evolution of the 27-day and 28-day rigidly rotating modes of the Sun's magnetic field.The spatial structure of the modes has been compared with simulations based on the known flux-transport equation. In the simulations, the rigidly rotating modes were regarded as stationary states of the magnetic field whose rigid rotation and stability were maintained by a balance between the emergence of magnetic flux from stationary sources located at low latitudes and the horizontal transport of flux by turbulent diffusion and poleward directed meridional flow. Under these assumptions, the structure of the modes is determined solely by the horizontal velocity field of the plasma, except for the low-latitude zone where sources of magnetic flux concentrate. We have found a detailed agreement between the simulations and the results of the data analysis, provided that the amplitude of the meridional flow velocity and the diffusion constant are equal to 9.5 m s^–1 and 600 km² s^–1, respectively.The analysis of the expansion coefficients has shown that the rigidly rotating modes undergo rapid step-like variations which occur quasi-periodically with a period of about two years. These variations are caused by separate surges of magnetic flux in the photosphere, so that each new surge gives rise to a rapid replacement of old large-scale magnetic structures by newly arisen ones.     

On altitude structure of centimeter-wave radio emission of solar active regions

A method is presented for the direct measurement of the heights of the radio emission of solar active regions when they are located at the limb in order to reconstruct the vertical structure of the magnetic field in solar active regions. The method involves an analysis of radio source positions in the scans based on high frequency resolution one-dimensional centimeter-wave measurements performed on the RATAN-600 radio telescope. Radio sources are difficult to identify at many frequencies when observed at the limb at zero position angle because of abrupt signal variations at the solar limb. To eliminate edge effects on the scan, special observing periods are used (near vernal and autumnal equinoxes), when the source at the limb is located far from the scan edge because of the large position angle of the Sun. As a result of these observations, the spectra of relative heights are constructed for a number of sources for the period from 2007 through 2012. Source heights are shown to generally increase with wavelength. The height difference between the 5 and 2 cm emission is equal to 5.2 ± 2.0 Mm, and the corresponding height difference between the 8 and 2 cm emission is equal to 9.6 ± 3.0 Mm. It is shown that such characteristics can be obtained for a field generated by a dipole submerged under the photosphere at a depth of 17 Mm irrespective of the possible reduction of relative altitudes to absolute altitudes.     

Observations of Ganymede,Callisto, Ceres,Uranus, and Neptune at 3.33 mm wavelength

We have measured the 3.33 mm wavelength disk brightness temperatures of Ganymede (136 ± 21°K), Callisto (95 ± 17°K), Ceres (137 ± 25°K), Uranus (125 ± 9°K), and Neptune (126 ± 9°K). Our observations of Ganymede are consistent with the radiation from a blackbody in solar equilibrium, whereas Callisto's microwave spectrum indicates a surface similar to that of the Moon. The disk temperature for Ceres agrees with that expected from a rapidly rotating blackbody. The millimeter temperatures of Uranus and Neptune greatly exceed solar equilibrium values, implying atmospheres with large temperature gradients.     

The Dynamic Spectrum of Interplanetary Scintillation: First Solar Wind Observations on LOFAR

The LOw Frequency ARray (LOFAR) is a next-generation radio telescope which uses thousands of stationary dipoles to observe celestial phenomena. These dipoles are grouped in various ‘stations’ which are centred on the Netherlands with additional ‘stations’ across Europe. The telescope is designed to operate at frequencies from 10 to 240 MHz with very large fractional bandwidths (25?–?100 %). Several ‘beam-formed’ observing modes are now operational and the system is designed to output data with high time and frequency resolution, which are highly configurable. This makes LOFAR eminently suited for dynamic spectrum measurements with applications in solar and planetary physics. In this paper we describe progress in developing automated data analysis routines to compute dynamic spectra from LOFAR time–frequency data, including correction for the antenna response across the radio frequency pass-band and mitigation of terrestrial radio-frequency interference (RFI). We apply these data routines to observations of interplanetary scintillation (IPS), commonly used to infer solar wind velocity and density information, and present initial science results.     

The motion of major planets from observations 1769–1988 and some astronomical constants

Modern planetary theories may be considered as a realisation of a four-dimensional dynamical reference frame. The existence of secular trends between the dynamical system and the adopted system of the Fundamental Catalogue (as well as between time scales involved) has been studied by discussing planetary observations of different types and by comparison with a numerical theory constructed for the time span 1769–1988. Parameters of the theory were fitted to radar ranging data for 1961–1988 for inner planets and to meridian observations of 18^th–20^th centuries for outer planets. Then a set of the inner planet optical observations, which includes USNO meridian observations, transits through the solar disk and occultations of fundamental stars are discussed. The main results are the following:

1. Radar data were used to estimate the time derivative? of the gravitational constantG (in another interpretation, the secular trend between the atomic and dynamic time scales): $$\dot G/G = (0.37 \pm 0.45) \times 10^{ - 11} /y.$$ This estimation, being statistically insignificant, gives some physically meaningful restriction to?.
2. From the same data a new estimation of relativistic effects in the motion of Mercury was obtained, which has confirmed the Einstein value of the perihelion advance with the error 0″.06/cy. So in the frame of Einstein's theory the value of solar dynamic oblateness cannot be larger than 2×10^?6.
3. The analysis of time behavior of residuals in the inner planet longitudes shows secular trends. It is demonstrated that these trends may be explained by combined action of a linear trenddT of Brouwer's time scale (which is adopted as a standard for reduction of observations before 1959) and the error in Newcomb's value of the constant of precession. From USNO meridian observations fordT the following estimate was obtained:dT=?14.5±2.1 sec/cy with the corresponding correction,dp, to Newcomb's precessiondp=0″.46±0″.13/cy. The estimate ofdT is in good agreement with the value ofdT determined from transits of Mercury and Venus through the solar diskdT=?12.9±1.3 sec/cy which does not depend on any precession error.
4. As a by-product, new accurate ephemerides of the outer planets are obtained over the time interval 1769–1988, the average residuals being presented.

     

Oblateness,radius, and mean stratospheric temperature of Neptune from the 1985 August 20 occultation

The occultation of a bright (K∼6) infrared star by Neptune revealed a central flash at two stations and provided accurate measurements of the limb position at these and several additional stations. We have fitted this data ensemble with a general model of an oblate atmosphere to deduce the oblateness e and equatorial radius a₀ of Neptune at the 1-μbar pressure level, and the position angle p_n of the projected spin axis. The results are e=0.0209±0.0014, a₀=25269±10 km, p_n=20.1°±1°. Parameters derived from fitting to the limb data alone are in excellent agreement with parameters derived from fitting to central flash data alone (E. Lellouch, W.B. Hubbard, B. Sicardy, F. Vilas, and P. Bouchet, 1986, Nature 324, 227–231), and the principal remaining source of uncertainty appears to be the Neptune-centered declination of the Earth at the time of occultation. As an alternative to the methane absorption model proposed by Lellouch et al., we explain an observed reduction in the central flash intensity by a decrease in temperature from 150 to 135°K as the pressure rises from 1 to 400 μbar. Implications of the oblateness results for Neptune interior models are briefly discussed.