On the antenna beam shape reconstruction using planet transit

The calibration of the in-flight antenna beam shape and possible beamdegradation is one of the most crucial tasks for the upcoming Planck mission. We examine several effects which could significantly influence the in-flight main beam calibration using planet transit: the problems of the variability of the Jupiter’s flux, the antenna temperature and passing of the planets through the main beam. We estimate these effects on the antenna beam shape calibration and calculate the limits on the main beam and far sidelobe measurements, using observations of Jupiter and Saturn. We also discuss possible effects of degradation of the mirror surfaces and specify corresponding parameters which can help us to determine these effects.     

Local solar magnetic and velocity field development and related photospheric and chromospheric activity

We have compiled the results of our long-term studies of the local magnetic field and its activity development, derived from investigating sunspot group evolution, photoelectrically measured longitudinal magnetic and velocity fields, and measurements of sunspot proper motions. We estimate certain regularities according to which the magnetic and velocity fields, and photospheric, as well as chromospheric activities develop. We speculate about the physical background of such processes.Dedicated to Cornelis de Jager     

Magnetic fields of extragalactic radio sources: Testing cosmological models

Based on the synchrotron radiation mechanism with self-absorption, we estimate the magnetic fields of compact (～0 _. ^″ 001) structures of radio galaxies. Using a model radial dependence of the magnetic field, we estimate the field strength near the event horizon of a supermassive black hole. The latter turns out to be higher than that followed from the popular Blandford-Znajek mechanism. The magnetic fields are determined by taking into account evolution, which allows the redshift dependence of the magnetic field of the plasma surrounding a supermassive black hole to be derived. We show that various cosmological models can be tested in principle using magnetic field measurements of compact radio sources. We estimate the magnetic field of the farthest radio-loud quasar SDSS J0836+0054.     

Nonresonant generation of MHD disturbances during supernova explosions in an inhomogeneous interstellar medium

We analyze the nonresonant generation of large-scale magnetic inhomogeneities near a shock front by accelerated particles. The MHD disturbances are generated by the electric current excited by relativistic particles in the preshock medium in the presence of weak large-scale density inhomogeneities. The MHD modes considered can be amplified by other resonant and nonresonant mechanisms related to the presence of relativistic particles. We estimate the magnetic fields and the energies to which charged particles can be accelerated in different phases of the interstellar medium by taking into account the random magnetic fields generated by the mechanism considered.     

Cosmic microwave background power spectrum estimation with non-circular beam and incomplete sky coverage

Over the last decade, measurements of the cosmic microwave background (CMB) anisotropy have spearheaded the remarkable transition of cosmology into a precision science. However, addressing the systematic effects in the increasingly sensitive, high-resolution, 'full' sky measurements from different CMB experiments poses a stiff challenge. The analysis techniques must not only be computationally fast to contend with the huge size of the data, but the higher sensitivity also limits the simplifying assumptions which can then be invoked to achieve the desired speed without compromising the final precision goals. While maximum likelihood is desirable, the enormous computational cost makes the suboptimal method of power spectrum estimation using pseudo-C _l unavoidable for high-resolution data. The debiasing of the pseudo-C _l needs account for non-circular beams, together with non-uniform sky coverage. We provide a (semi)analytic framework to estimate bias in the power spectrum due to the effect of beam non-circularity and non-uniform sky coverage, including incomplete/masked sky maps and scan strategy. The approach is perturbative in the distortion of the beam from non-circularity, allowing for rapid computations when the beam is mildly non-circular. We advocate that it is computationally advantageous to employ 'soft' azimuthally apodized masks whose spherical harmonic transform die down fast with m . We numerically implement our method for non-rotating beams . We present preliminary estimates of the computational cost to evaluate the bias for the upcoming CMB anisotropy probes  ( l _max∼ 3000)  , with angular resolution comparable to the Planck surveyor mission. We further show that this implementation and estimate are applicable for rotating beams on equal declination scans, and can possibly be extended to simple approximations to other scan strategies.     

Dynamo-generated magnetic fields at the surface of a massive star

Spruit has shown that an astrophysical dynamo can operate in the non-convective material of a differentially rotating star as a result of a particular instability in the magnetic field (the Tayler instability). By assuming that the dynamo operates in a state of marginal instability, Spruit has obtained formulae which predict the equilibrium strengths of azimuthal and radial field components in terms of local physical quantities. Here, we apply Spruit's formulae to our previously published models of rotating massive stars in order to estimate Tayler dynamo field strengths. There are no free parameters in Spruit's formulae. In our models of 10- and  50-M_⊙  stars on the zero-age main sequence, we find internal azimuthal fields of up to 1 MG, and internal radial components of a few kG. Evolved models contain weaker fields. In order to obtain estimates of the field strength at the stellar surface, we examine the conditions under which the Tayler dynamo fields are subject to magnetic buoyancy. We find that conditions for Tayler instability overlap with those for buoyancy at intermediate to high magnetic latitudes. This suggests that fields emerge at the surface of a massive star between magnetic latitudes of about 45° and the poles. We attempt to estimate the strength of the field which emerges at the surface of a massive star. Although these estimates are very rough, we find that the surface field strengths overlap with values which have been reported recently for line-of-sight fields in several O and B stars.     

Magnetic remote sensing of Europa’s ocean tides

Recent analyses of Galileo magnetometer and gravity data justifies approximations that allow estimates of the magnetic fields generated by Europa’s ocean tides to be made even though some of the ocean parameters that would generally be required are unavailable. We show solutions for the magnetic fields generated by published estimates of ocean tides on Europa and provide simple scaling formulas that can be used to estimate the magnetic-field amplitudes for other choices for the ocean tidal state. Because of the distinguished spatial/temporal form of these fields, it is expected that Europa’s ocean tides can be inferred from remote magnetic sensing by an orbiter of sufficient duration.     

Revised equipartition and minimum energy formula for magnetic field strength estimates from radio synchrotron observations

The commonly used classical equipartition or minimum‐energy estimate of total magnetic fields strengths from radio synchrotron intensities is of limited practical use because it is based on the hardly known ratio K of the total energies of cosmic ray protons and electrons and also has inherent problems. We present a revised formula, using the number density ratio K for which we give estimates. For particle acceleration in strong shocks K is about 40 and increases with decreasing shock strength. Our revised estimate for the field strength gives larger values than the classical estimate for flat radio spectra with spectral indices of about 0.5–0.6, but smaller values for steep spectra and total fields stronger than about 10 µG. In very young supernova remnants, for example, the classical estimate may be too large by up to 10×. On the other hand, if energy losses of cosmic ray electrons are important, K increases with particle energy and the equipartition field may be underestimated significantly. Our revised larger equipartition estimates in galaxy clusters and radio lobes are consistent with independent estimates from Faraday rotation measures, while estimates from the ratio between radio synchrotron and X‐ray inverse Compton intensities generally give much weaker fields. This may be explained e.g. by a concentration of the field in filaments. Our revised field strengths may also lead to major revisions of electron lifetimes in jets and radio lobes estimated from the synchrotron break frequency in the radio spectrum. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of IMF draping direction and crustal magnetic field location on Martian ion beams

Data from the Ion Mass Analyzer (IMA) sensor of the ASPERA-3 instrument suite onboard Mars Express and data from the Magnetometer/Electron Reflectometer (MAG/ER) on Mars Global Surveyor have been analyzed to determine whether ion beam events (IBEs) are correlated with the direction of the draped interplanetary magnetic field (IMF) or the proximity of strong crustal magnetic fields to the subsolar point. We examined 150 IBEs and found that they are organized by IMF draping direction. However, no clear dependence on the subsolar longitude of the strongest magnetic anomaly is evident, making it uncertain whether crustal magnetic fields have an effect on the formation of the beams. We also examined data from the IMA sensor of the ASPERA-4 instrument suite on Venus Express and found that IBEs are observed at Venus as well, which indicates the morphology of the Martian and Venusian magnetotails are similar.     

Accelerator measurement of NaI response to medium energy neutrons and application to a Satellite-Borne Spectrometer

We report on the response of a prototype detector to medium energy neutrons. The neutrons were produced by n-p scattering of a neutron beam on a hydrogen target. The measurements provide unique data on the efficiency and response of large NaI scintillators to neutrons in the energy range 36–709 MeV. We apply the results to the high-energy mode of the Gamma-Ray Spectrometer (GRS) on the Solar Maximum Mission satellite by estimating its efficiency for neutron detection. This estimate is compared to earlier Monte Carlo calculations of the GRS efficiency.     

The stability of small flux tubes

We have investigated the influence of stationary velocity fields, twists of the field lines and changes of gas pressure within flux tubes on the interchange instability of magnetic flux tubes. A small flux tube is found to be stable. All three factors mentioned above can stabilize tubes with all fluxes. We estimate that, for the solar case, a change of gas pressure in flux tubes plays an important role in stabilizing magnetic flux tube.     

Dichroic circular polarization analyzer for the Main Stellar Spectrograph of the 6-m telescope

We describe a dichroic circular polarization analyzer with a double image slicer providing seven slices for each polarization developed for the Main Stellar Spectrograph of the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. The analyzer is designed for measuring stellar magnetic fields using the Zeeman effect and represents an upgraded version compared to earlier used polarization analyzers. We report the parameters of the analyzer and estimate the accuracy of measurements performed with it.     

The Apparent Reversal of Fields at the Solar Limb

We show how the apparent reversal of longitudinal magnetic fields near the limb is a projection effect and may be used to estimate the divergence angle of the magnetic canopy. The limb distance at which the apparent reversal is not seen is the limiting angle of divergence of lines of force emerging from the surface. We have surveyed a number of polar limb magnetograms, where the unipolar field makes this easier, and found the divergence angle to be about 20° from the vertical.     

The propagation of a dense quasi-neutral ion beam across a magnetized plasma

Propagation of a quasi-neutral narrow ion beam across a magnetised cold plasma is investigated in slab geometry. This problem is of interest in connection with artificial beam injection experiments and with naturally appearing plasma injections into magnetic fields as astrophysical jets. Several different cases are discussed briefly where the beam is assumed either slow or fast. For fast beams it is shown that they propagate due to generation of a polarisation electric field even in the case of presence of a background plasma. Slow beams can depolarise by currents flowing into the beam along the field lines and providing the required electrons for charge neutralisation. Some implications of the model are discussed in the context of recent active beam injection experiments into space plasma.     

Beam-return current systems in nonthermal solar flare models

Previous investigations of return currents driven by suprathermal electron beams in solar flares have been based both conceptually and mathematically on analyses of electron beams in the laboratory environment. However, the physics of laboratory electron beams is fundamentally different from the physics of solar flare electron beams. Consider first the laboratory beam, which is injected into the plasma from an external source and is, therefore, modeled as a semi-infinite charged rigid rod. The longitudinal electrostatic field of such a charged rod has no preferred direction and therefore cannot drive a return current. Consequently, in the laboratory the return current is established inductively through the appearance of the changing magnetic field associated with the rising beam current, there being no offsetting displacement current term in such a geometry. It subsequently decays on the resistive time-scale; because of this decay, the net current of the system increases, and the lifetime of the electron beam becomes limited by self-pinching effects. Therefore, in the laboratory, the beam/return current system cannot reach a steady state.By contrast, the electron beam in the solar flare forms in situ and the longitudinal electrostatic field is produced by charge separation. Such an electrostatic field does have a preferred direction and so can drive a cospatial return current. Further, the magnetic field generated by the beam current is always close to being offset by either the magnetic field associated with the displacement current (E/t) or the electrostatically-driven return current; hence, inductive fields are never important. Thus, in the solar flare the return current is principally established by electrostatic fields; the return current is continuously driven and does not decay resistively. Thus, if the acceleration mechanism drives a steady beam current, then the beam/return current system rapidly achieves a steady state. We present in this paper analytic expressions for the approach to this state.Presidential Young Investigator.     

Acceleration of energetic particles of the outer regions of planetary magnetospheres: Inferences from laboratory and space experiments

High energy particles, with energies above those attainable by adiabatic or steady-state electric field acceleration, have been observed in and around the outer regions of planetary magnetospheres. Acceleration by large amplitude sporadic cross-tail electric fields over an order of magnitude greater than steady-state convection fields is proposed as a source of these particles. It is suggested that such explosive electric fields will occur intermittently in the vicinity of the tail neutral line in the expansive phases of substorms. We use laboratory Double Inverse Pinch Device (DIPD) and satellite evidence to estimate this electric potential for substorms at Earth; values of 500 kV to 2 MV are calculated, in agreement with particle observations. It is further suggested that these particles, which have been accelerated in the night side magnetosphere, drift to the dayside on closed field lines, and under certain interplanetary conditions can escape to regions upstream of the bow shock.     

Is the supersonic turbulence observed in HII regions sustained by the presence of magnetic fields?

We describe a set of Hα emission line profiles from populations of H II regions in nearby spiral galaxies. These are characterized by a strong Gaussian central peak, and lower intensity higher velocity wing features. From the peak we extract a non-thermal velocity component, due to the internal turbulence of the region. The plot of the widths of these non-thermal components against Hα luminosity shows a lower envelope in line width, which we assign to regions in, or close to virial equilibrium, although the region mass derived on this assumption is higher than the mass obtained by summing all known mass components. We speculate that this discrepancy, as well as the supersonicity of the line widths, can be explained by the presence of turbulent magnetic fields within the H II regions, and make a very rough estimate of the fields implied, of order a few tens of microgauss.     

Directivity of low frequency solar type III radio bursts

The occurrence rate of type III solar bursts in the frequency range 4.9 MHz to 30 kHz is analyzed as a function of burst intensity and burst arrival direction. We find that (a) the occurrence rate of bursts falls off with increasing flux, S, according to the power law S ^–1.5, and (b) the distribution of burst arrival directions at each frequency shows a significantly larger number of bursts observed west of the Earth-Sun line than east of it. This western excess in occurrence rate appears to be correlated with the direction of the average interplanetary magnetic field, and is interpreted as beaming of the observed burst radiation along the magnetic field direction.Presently at the University of Maryland, College Park, Maryland.     

Induced magnetic field effects at planet Mercury

At Mercury's surface external magnetic field contributions caused by magnetospheric current systems play a much more important role than at Earth. They are subjected to temporal variations and therefore will induce currents in the large conductive iron core. These currents give rise to an additional magnetic field superposing the planetary field. We present a model to estimate the size of the induced fields using a magnetospheric magnetic field model with time-varying magnetopause position. For the Hermean interior we assume a two-layer conductivity distribution. We found out that about half of the surface magnetic field is due to magnetospheric or induced currents. The induced fields achieve 7-12% of the mean surface magnetic intensity of the internal planetary field, depending on the core size. The magnetic field was also modeled for a satellite moving along a polar orbit in the Hermean magnetosphere, showing the importance of a careful separation of the magnetic field measurements.     

Almost analytic models of ultramagnetized neutron star envelopes

Recent ROSAT measurements show that the X-ray emission from isolated neutron stars is modulated at the stellar rotation period. To interpret these measurements, one needs precise calculations of the heat transfer through the thin insulating envelopes of neutron stars. We present nearly analytic models of the thermal structure of the envelopes of ultramagnetized neutron stars. Specifically, we examine the limit in which only the ground Landau level is filled. We use the models to estimate the amplitude of modulation expected from non-uniformities in the surface temperatures of strongly magnetized neutron stars. In addition, we estimate cooling rates for stars with fields B  ∼ 10¹⁵ − 10¹⁶ G, which are relevant to models that invoke 'magnetars' to account for soft γ-ray emission from some repeating sources.