Temporal variations of the magnetic field in sunspots

We obtained simultaneous spectra with a spatial resolution of 1/2 and a temporal resolution of 15 s in H, Ca ii-K, Caii 8542 Å, and three Fei lines of the sunspot group responsible for the large flares of August, 1972 (McMath No. 11976). A time series taken 1972, August 3 in the Fei 6173 Å Zeeman sensitive line was analyzed for oscillations of field strength and the angle between the field and the line of sight, and for changes of the field associated with the Ca ii-K umbral flashes discovered by Beckers and Tallant (1969). The power spectra show no significant peaks, conflicting with the results of Mogilevskii et al. (1972) who reported oscillations in the longitudinal component of the field strength with periods of 56, 90, and 150 s. Changes in the field were not observed to be correlated with the occurrence of umbral flashes. These results place restrictions on magnetic modes of energy transport between the photospheric layers and the chromospheric layers where the umbral flashes are observed.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Auroral boundary variations and the interplanetary magnetic field

This paper describes a DMSP data set of 150 auroral images during magnetically quiet times which have been analyzed in corrected geomagnetic local time and latitudinal coordinates and fit to offset circles. The fit parameters R (circle radius) and (X, Y) (center location) have been compared to the hourly interplanetary magnetic field (IMF) prior to the time of the satellite scan of the aurora. The results for variation of R with B_z, agree with previous works and generally show about a 1° increase of R with increase of southward B_z by 1 nT. The location of the circle center also has a clear statistical shift in the Southern Hemisphere with IMF B_y such that the southern polar cap moves towards dusk (dawn) with B_y > (B_y < 0).     

Flapping motions of the tail plasma sheet induced by the interplanetary magnetic field variations

Flapping motions of the magnetotail with an amplitude of several earth radii are studied by analysing the observations made in the near (x = ?25 ~ ?30 R_E and the distant (x? ?60 R_E) tail regions. It is found that the flapping motions result from fluctuations in the interplanetary magnetic field, especially Alfvénic fluctuations, when the magnitude of the interplanetary magnetic field is larger than ~10 γ and they propagate behind the Earth with the solar wind flow. Flappings tend to be observed in early phases of the magnetospheric substorm, and they have two fundamental modes with periods of ~200 and ~500 sec. In some limited cases a good correspondence with the long period micropulsations (Pc5) in the polar cap region is observed. These observational results are explained by the model in which the Alfvénic fluctuations in the solar wind penetrate into the magnetosphere along the connected interplanetary-magnetospheric field lines. The characteristics of the flapping reveal that the geomagnetic tail is a good resonator for the hydromagnetic disturbances in the solar wind.     

The electric field in axisymmetric pulsar magnetospheres

A knowledge of the non-corotational electric potential required to support particles in corotation with the star is used to deduce the global qualitative forms that the potential can take in axisymmetric pulsar magnetospheres.     

The effect of a magnetic field on an adiabatic explosion

As a paradigm for various explosive processes in the interstellar medium we consider the problem of an adiabatic explosion into a uniform magnetic field which is frozen to the gas. A typical numerical run is described and reveals the following features. The outer shock becomes oblate with respect to the field lines whereas the inner hot gas prolate density and temperature contours. During the later stages the shock weakens and the explosion comes into equilibrium with the interstellar medium. The dominant feature at this stage is a concentration of accelerated material at each pole. We then try to interpret this analytically, considering the magnetic field as a small perturbation to a spherical explosion. This enables us to derive a formula for the eccentricity, which is proportional tot ^6/5. However, the linear perturbation is singular at the centre and needs to be matched to a self-similar flow there, for which we give an approximation. This similarity solution is eventually important outside the region occupied by the material initially responsible for the explosion. We give some discussion of the various asymptotic regimes involved.     

The flow field in a force-free magnetic field

We have obtained a complete set of the zeroth-order equations for a force-free field at large magnetic Reynolds numbers. One of the equations has often been overlooked in previous works. We discuss the question of uniqueness of solution of the Cauchy problem and outline a general method of solution in the plane and axisymmetric cases.     

Search for long term variations of the interplanetary magnetic field

A statistical study is made of the long term variations of the interplanetary magnetic field parameters collected in the years 1964 to 1973 by 12 spacecraft (IMP's, Pioneers and HEOS). Although temporal fluctuations are observed on field components and magnitudes no clear solar cycle variation is found. The same conclusion holds for the statistical distributions and variances of these parameters. A search for possible heliographic latitude effects on the field also leads to a negative conclusion.     

Concerning five-minute variations of the global magnetic field of the Sun

For the purpose of identifying five-min oscillations we analyze long-term continuous observations of the solar magnetic field (with a duration from 3 to 11 hours) with 0.5 D spatial resolution obtained with the STOP telescope (Solar Telescope for Operative Predictions) at the Sayan observatory in 1987 and 1989. It is shown that global magnetic field fluctuations with such periods seem to be real, but the character of corresponding power spectra is strongly dependent on the mean field strength in the magnetograph aperture.     

The stability of a velocity shear in the presence of an inhomogeneous magnetic field

The stability of a velocity shear in the presence of a parallel but non-uniform magnetic field is considered in general terms. Two special cases are then investigated; (i) the well known case of a plane interface at which a discontinuity in the magnetic field coincides with the velocity shear; (ii) an axially symmetric flow in which discontinuities in the magnetic and velocity fields occur at a cylindrical surface whose axis is parallel to the flow. In the first case the flow is stabilized if the rms Alfvén velocity of the magnetic field exceeds the shear velocity; a result consistent with that obtained by other writers. In the second case it is shown that the discontinuity in the magnetic field increases the stability of the system. The significance of this result for the stability of the flux ropes associated with sunspots in the solar convection zone is considered.     

A modeling of magnetic field variations during magnetospheric substorms

Magnetic field variations in the noon-midnight plane during the magnetospheric substorm are studied in terms of changes of three current systems: the dynamo-driven current on the magnetopause, the cross-tail current and the field-aligned current-auroral electrojet system. The field-aligned current is assumed to be generated as a result of interruption and subsequent diversion of the cross-tail current to the ionosphere. It is concluded that the available observations are consistent with a large increase of the three currents.     

Burial of the polar magnetic field of an accreting neutron star – II. Hydromagnetic stability of axisymmetric equilibria

The theory of polar magnetic burial in accreting neutron stars predicts that a mountain of accreted material accumulates at the magnetic poles of the star, and that, as the mountain spreads equatorward, it is confined by, and compresses, the equatorial magnetic field. Here, we extend previous, axisymmetric, Grad–Shafranov calculations of the hydromagnetic structure of a magnetic mountain up to accreted masses as high as   M _a= 6 × 10⁻⁴ M_⊙  , by importing the output from previous calculations (which were limited by numerical problems and the formation of closed bubbles to   M _a < 10⁻⁴ M_⊙  ) into the time-dependent, ideal-magnetohydrodynamic code zeus-3d and loading additional mass on to the star dynamically. The rise of buoyant magnetic bubbles through the accreted layer is observed in these experiments. We also investigate the stability of the resulting hydromagnetic equilibria by perturbing them in zeus-3d . Surprisingly, it is observed that the equilibria are marginally stable for all   M _a≤ 6 × 10⁻⁴ M_⊙  ; the mountain oscillates persistently when perturbed, in a combination of Alfvén and acoustic modes, without appreciable damping or growth, and is therefore not disrupted (apart from a transient Parker instability initially, which expels <1 per cent of the mass and magnetic flux).     

Spatial variations of background magnetic field polarity distribution on the Sun

Based on an analysis of the latitude and longitude regularities in the distribution of the sign of the background magnetic field (BMF) on the solar surface for 14 years (1969–1982), a classification of BMF distributions in the form of synoptic maps, is proposed. That the low- and high-latitude BMF distributions are differing in character appears to be due to the difference in the character of the BMF rotation at low and high latitudes. It is shown that as low-latitude BMF details make contact with the high-latitude field of the same polarity, the former grow in area. The low- and high-latitude fields come into contact through high-latitude field details protruding into lower latitudes as far as 10 to 15 ° below 40 ° latitude, and they are referred to as bulges. Bulges and low-latitude fields of like polarity are moving with respect to each other along the E-W line at a mean rate of 10 to 15 ° per rotation. Bulges of the same polarity in the same hemisphere are moving with respect to each other 3–5 ° per rotation, on the average. The above-mentioned properties of the structure, interaction and rotation of the low- and high-latitude magnetic field details, taken together, provide a qualitative scheme for the formation and variation of the longitude-regular (sectoral) BMF distribution in the latitude range 40 S-40 N.     

The instability of a horizontal magnetic field in an atmosphere stable against convection

The theoretical problem posed by the buoyant escape of a magnetic field from the interior of a stably stratified body bears directly on the question of the present existence of primordial magnetic fields in stars. This paper treats the onset of the Rayleigh-Taylor instability of the upper boundary of a uniform horizontal magnetic field in a stably stratified atmosphere. The calculations are carried out in the Boussinesq approximation and show the rapid growth of the initial infinitesimal perturbation of the boundary. This result is in contrast to the extremely slow buoyant rise of a separate flux tube in the same atmosphere. Thus for instance, at a depth of 1/3R beneath the surface of the Sun, a field of 10² G develops ripples over a scale of 10³ km in a characteristic time of 50 years, whereas the characteristic rise time of the same field in separate flux tubes with the same dimensions is 10¹⁰ years. Thus, the development of irregularities proceeds quickly, soon slowing, however, to a very slow pace when the amplitude of the irregularities becomes significant. Altogether the calculations show the complexity of the question of the existence of remnant primordial magnetic fields in stellar interiors.This work was supported in part by the National Aeronautics and Space Administration under Grant NGL 14-001-001     

The accretion of matter by a collapsing star in the presence of a magnetic field

The exact nonstationary solution for the variation of the magnetic field in the Schwarzschild metric with a given spherically symmetric flow is obtained. Initially a homogeneous magnetic field increases with time, changing into a quasi-radial field. On the assumption of equipartition between the magnetic and kinetic energies of a falling gas, in the relativistic case, estimates of the stationary field and the intensity of synchrotron radiation are presented. A considerable part of the radiation is formed in the relativistic regionr?(2.5 to 7.7)r _g (r _g is the gravitational radius of a black hole). Estimates are made for radiation from the relativistic region in the case of disc type accretion.     

The magnetic field in a protostellar cloud

General forms of theB-p relation are investigated in both the isothermal and the non-isothermal regions. The magnetic flux dissipation either by ambipolar diffusion or by Ohmic dissipation has been studied. The rates of heating due to the magnetic dissipation processes have been calculated in comparison with the rate of compressional heating.The magnetic field strength is derived as a function of flux/mass ratio, mass, density, and geometry of the isothermal cloud. In the non-isothermal region, the temperature is added to the above-mentioned variables.It has been found that the magnetic flux starts to dissipate via ambipolar diffusion at neutral density ofn>3×10⁹ cm^–3. Ambipolar diffusion continues effective until reaching densities ofn>10¹¹ cm^–3, where Ohmic dissipation dominates. Under some conditions, the electrons evaporate from the grain surface atn>10¹³ cm^–3, while the ions are still adsorbed on the grain surfce. In this case, the magnetic flux loss returns to be influenced by ambipolar diffusion.The rates of heating by both Ohmic dissipation _OD and ambipolar diffusion _AD are found to be smaller than the rate of compressional heating _C in case of magnetic dissipation. Assuming that the magnetic field is frozen in the medium, then _C is smaller than both _OD and _AD . The above results of heating were found in the non-isothermal region.     

Neutrino luminosity by the ordinary URCA process in an intense magnetic field

The neutrino luminosity by the ordinary URCA process in a strongly magnetized electron gas is computed. General formulae are presented for the URCA energy loss rates for an arbitrary degree of degeneracy. Analytic expressions are derived for a completely degenerate, relativistic electron plasma in the special case of neutron-proton conversion. Numerical results are given for more general cases.The main results are as follows: the URCA energy loss rates are drastically reduced for the regime of great degeneracy by a factor up to 10^–3 for 1, andT ₉10, where =H/H _q ,H _q =m ² c ³/eh=4.414×10¹³ G. In the non-degenerate regime the neutrino luminosity is enhanced approximately linearly with for the temperature range 1T ₉10. Possible applications to white dwarfs and neutron stars are briefly discussed.We have been recently informed that in Gamow home-dialect (Odessa dialect) URCA means thief — (Private communication from Prof. G. Wataghin).     

The spontaneous magnetic field direction in an anisotropic MHD dynamo

The phenomenon of magnetic field generation in an astrophysical plasma in the frame of developed magnetohydrodynamic (MHD) turbulence is considered. The functional quantum field renormalization group approach is applied to helical anisotropic MHD developed turbulence which is stabilized by the self-generated homogeneous magnetic field. The purpose of the study is to calculate the value as well as direction of the magnetic field in the stochastic dynamo model. The generated magnetic field is determined by ignoring divergent rotor part of Green function of the magnetic field. It is shown that the magnetic field direction is connected with unique existing vector n describing the anisotropic turbulence forcing.     

The concentration of the large-scale solar magnetic field by a meridional surface flow

We calculate analytical and numerical solutions to the magnetic flux transport equation in the absence of new bipolar sources of flux, for several meridional flow profiles and a range of peak flow speeds. We find that a poleward flow with a broad profile and a nominal 10 m s^–1 maximum speed concentrates the large-scale field into very small caps of less than 15° half-angle, with average field strengths of several tens of gauss, contrary to observations. A flow which reaches its peak speed at a relatively low latitude and then decreases rapidly to zero at higher latitudes leads to a large-scale field pattern which is consistent with observations. For such a flow, only lower latitude sunspot groups can contribute to interhemispheric flux annihilation and the resulting decay and reversal of the polar magnetic fields.     

Analysis of sudden variations in photospheric magnetic fields during a large flare and their influences in the solar atmosphere

The solar active region NOAA 11719 produced a large two-ribbon flare on 2013 April 11. We have investigated sudden variations in the photospheric magnetic fields in this active region during the flare by employing magnetograms obtained in the spectral line Fe I 6173 A? acquired by the Helioseismic and Magnetic Imager(HMI) onboard the Solar Dynamics Observatory(SDO) spacecraft. The analysis of the line-of-sight magnetograms from HMI show sudden and persistent magnetic field changes at different locations of the active region before the onset of the flare and during the flare. The vector magnetic field observations available from HMI also show coincident variations in the total magnetic field strength and its inclination angle at these locations. Using the simultaneous Dopplergrams obtained from HMI, we observe perturbations in the photospheric Doppler signals following the sudden changes in the magnetic fields in the aforementioned locations. The power spectrum analysis of these velocity signals shows enhanced acoustic power in these affected locations during the flare as compared to the pre-flare condition. Accompanying these observations, we have also used nearly simultaneous chromospheric observations obtained in the spectral line Hα 6562.8 A? by the Global Oscillation Network Group(GONG) to study the evolution of flareribbons and intensity oscillations in this active region. The Hα intensity oscillations also show enhanced oscillatory power during the flare in the aforementioned locations. These results indicate that the transient Lorentz force associated with sudden changes in the magnetic fields could drive localized photospheric and chromospheric oscillations, like the flare-induced oscillations in the solar atmosphere.