Heating of the polar caps of old radio pulsars

The X-ray luminosity and temperature of the polar cap heated by the back flux of positrons from a radio pulsar with a period P ～ 1 s and a magnetic field B ~ 10¹² G have been estimated. An additional source of X-ray emission—a thin, hotter semiring on the polar-cap periphery—is shown to also exist. It is heated by the back flux of electrons from the light cylinder. Furthermore, the electric field near the hot semiring accelerates the ions of the surface layer that leave the neutron-star magnetosphere. The semiring area is smaller than the polar-cap area approximately by a factor of 100, i.e., at the same luminosity the temperature is higher by a factor of 3. The observed X-ray emission from old radio pulsars is the emission from thin hot polar-cap semirings. The emission from the polar caps themselves is strongly attenuated by interstellar absorption.     

The origin of Ganymede's polar caps

Since their discovery in Voyager images, the origin of the bright polar caps of Ganymede has intrigued investigators. Some models attributed the polar cap formation to thermal migration of water vapor to higher latitudes, while other models implicated plasma bombardment in brightening ice. Only with the arrival of Galileo at Jupiter was it apparent that Ganymede possesses a strong internal magnetic field, which blocks most of the plasma from bombarding the satellite's equatorial region while funneling plasma onto the polar regions. This discovery provides a plausible explanation for the polar caps as related to differences in plasma-induced brightening in the polar and the equatorial regions. In this context, we analyze global color and high resolution images of Ganymede obtained by Galileo, finding a very close correspondence between the observed polar cap boundary and the open/closed field lines boundary obtained from new modeling of the magnetic field environment. This establishes a clear link between plasma bombardment and polar cap brightening. High resolution images show that bright polar terrain is segregated into bright and dark patches, suggesting sputter-induced redistribution and subsequent cold trapping of water molecules. Minor differences between the location of the open/closed field lines boundary and the observed polar cap boundary may be due to interaction of Ganymede with Jupiter's magnetosphere, and our neglect of higher-order terms in modeling Ganymede's internal field. We postulate that leading-trailing brightness differences in Ganymede's low-latitude surface are due to enhanced plasma flux onto the leading hemisphere, rather than darkening of the trailing hemisphere. In contrast to Ganymede, the entire surface of Europa is bombarded by jovian plasma, suggesting that sputter-induced redistribution of water molecules is a viable means of brightening that satellite's surface.     

The nature of the residual Martian polar caps

A model of the behavior of the Martian polar caps is described which incorporates the heating effects of the atmosphere, as well as insolation and conduction. This model is used to try to match the observed regression curves of the polar caps, and it predicts that all the seasonally condensed CO₂ will be lost by around the summer solstice. The implication is that the residual caps are composed of water ice which, it is found by further modeling, should be stable during the Martian summers. However, it is also argued that this model may be too simplistic, and that the effects of wind in redistributing the seasonal condensate may lead to sufficient thickness of CO₂ in the central polar region to allow the year-long existence of CO₂ without significantly changing the retreat characteristics of the cap, and it is, therefore, concluded that at the present, the nature of the residual caps cannot be reliably determined.     

The entry of solar protons over the polar caps

Observations of solar protons at energies from 1 MeV to 360 MeV are examined in relation to the information that these particles give about the magnetosphere, magnetotail and magnetopause. Trajectory integrations in a realistic model of the geomagnetic field out to 25R_E and a tail field model fitted to observations from 15R_E to 80R_E are used to obtain a better understanding of the particle motion. The mean free path of protons in the tail is found to be 700R_E and 200R_E for 100 MeV and 1 MeV protons respectively, which indicates that trajectory calculations in a static field model are valid.     

The influence of the Martian polar caps on the diurnal tide

The large horizontal heating gradients that exist near the edge of the Martian polar caps during spring are shown to be capable of exciting large oscillations in the diurnal tide. To a lesser extent, the daily mass cycling between cap and atmosphere can also contribute. The calculations which demonstrate this are based on classical tidal theory as applied to the cylindrical coordinate system. This is done to facilitate the representation of the heating function. Results are presented for the horizontal surface winds only. They indicate a circulation at the cap edge somewhat analogous to the smaller scale terrestrial sea breeze. The amplitude of the zonal component is largest and is increased from 1 to 10 m sec^?1 by the modeled influence of the polar cap. When coupled with the basic flow these cap-edge tides can produce strong surface winds during spring. Such a mechanism may contribute to the ability of the south polar cap winds to generate the local dust storms observed near the cap edge at this season.     

Ridges and scarps in the equatorial belt of Mars

The morphology and distribution of ridges and scarps on Mars in the ± 30° latitude belt were investigated. Two distinct types of ridges were recognized. The first is long and linear, resembling mare ridges on the Moon; it occurs mostly in plains areas. The other is composed of short, anastomosing segments and occurs mostly in ancient cratered terrain and intervening plateaus. Where ridges are eroded, landscape configurations suggest that they are located along regional structures. The age of ridges is uncertain, but some are as young as the latest documented volcanic activity on Mars. The origins of ridges are probably diverse-they may result from wrinkling due to compression or from buckling due to settling over subsurface structures. The similar morphologic expressions of ridge types of various origins may be related to a similar deformation mechanism caused by two main factors: (1) most ridges are developed in thick layers of competent material and (2) ridges formed under stresses near a free surface.     

Substorm related electron and proton bursts over the polar caps

We examined the energetic electron and proton data from different instruments on the dawn-dusk polar orbiting satellite AZUR during periods of high electrojet activity (AE > 500 γ) and find that there is a high probability of seeing during these periods relativistic electron bursts (?0.7 MeV) and in some cases also high-energy proton bursts (?250??500 keV). Fluxes, composition, energy spectra and spike forms are shown and are compared with similar burst events in the geomagnetic tail observed by other authors. It is suggested that the burst events discussed in this paper are the low-altitude signature of electron and proton bursts generated in the geomagnetic tail.     

The impact of meridional circulation on stellar butterfly diagrams and polar caps

Observations of rapidly rotating solar-like stars show a significant mixture of opposite-polarity magnetic fields within their polar regions. To explain these observations, models describing the surface transport of magnetic flux demand the presence of fast meridional flows. Here, we link subsurface and surface magnetic flux transport simulations to investigate (i) the impact of meridional circulations with peak velocities of  ≤125 m s⁻¹  on the latitudinal eruption pattern of magnetic flux tubes and (ii) the influence of the resulting butterfly diagrams on polar magnetic field properties. Prior to their eruption, magnetic flux tubes with low field strengths and initial cross-sections below  ∼300 km  experience an enhanced poleward deflection through meridional flows (assumed to be polewards at the top of the convection zone and equatorwards at the bottom). In particular, flux tubes which originate between low and intermediate latitudes within the convective overshoot region are strongly affected. This latitude-dependent poleward deflection of erupting magnetic flux renders the wings of stellar butterfly diagrams distinctively convex. The subsequent evolution of the surface magnetic field shows that the increased number of newly emerging bipoles at higher latitudes promotes the intermingling of opposite polarities of polar magnetic fields. The associated magnetic flux densities are about 20 per cent higher than in the case disregarding the pre-eruptive deflection, which eases the necessity for fast meridional flows predicted by previous investigations. In order to reproduce the observed polar field properties, the rate of the meridional circulation has to be of the order of 100 m s⁻¹, and the latitudinal range from which magnetic flux tubes originate at the base of the convective zone (≲50°) must be larger than in the solar case (≲35°).     

Relationships between the equatorial electrojet and polar magnetic variations

On moderately disturbed days when substorms occur frequently, the quiet day daily variation in the polar region (S_q^p) is enhanced. On such days, however, the quiet day variation along the dip equator appears to be suppressed, as well as being superposed with ‘fluctuations’.It is suggested that the enhancement of S_q^p is related to a partial suppression of the equatorial electrojet. The asymmetric ring current also causes an apparent suppression of the electrojet.On the other hand, the substorm-associated electric field which drives the eastward current in the auroral and subauroral zone (causing positive bays) in the afternoon sector appears to enhance the equatorial electrojet.Thus, magnetic variations along the dip equator are influenced by a number of processes in the magnetosphere.     

The interplanetary electric field, cleft currents and plasma convection in the polar caps

This report investigates the suggestion that the pattern of plasma convection in the polar cleft region is directly determined by the interplanetary electric field (IEF). Owing to the geometrical properties of the magnetosphere, the East-West component of the IEF will drive field-aligned currents which connect to the ionosphere at points lying on either side of noon, while currents associated with the North-South component of the IEF will connect the two polar caps as sheet currents centered at noon. The effects of the hypothesized IEF driven cleft current systems on polar cap ionospheric plasma convection are investigated through a series of numerical simulations. The simulations demonstrate that this simple electrodynamic model can account for the narrow “throats” of strong dayside antisunward convection observed during periods of southward interplanetary magnetic field (IMF) as well as the sunward convection observed during periods of strongly northward IMF. Thedawn-dusk shift of polar cap convection which is related to the B_y component of the IMF is also accounted for by the model.     

On the differential rotation of the polar caps of neutron stars

The model of a magnetized rotating neutron star with an electric current in the region of its fluid polar magnetic caps is considered. The presence of an electric current leads to differential rotation of the magnetic caps. The rotation structure is determined by the electric current density distribution over the surface. In the simplest axisymmetric configuration, the current flows in one direction near the polar cap center and in the opposite direction in the outer ring (the total current is zero for the neutron star charge conservation). In this case, two rings with opposite directions of rotation appear on the neutron star surface, with the inner ring always lagging behind the star’s main rotation. The differential rotation velocity is directly proportional to the electric current density gradient along the polar cap radius. At a width of the region of change in the electric current from 1 to 10² cm and a period ～1 s and a magnetic field B ～ 10¹² G typical of radio pulsars, the linear differential rotation velocity is ～10^?2–10^?4 cm s^?1 (corresponding to a revolution time of ～0.1–10 yr).     

The texture of condensed CO2 on the martian polar caps

The widespread deposition of CO₂ ice on the martian polar caps in winter is readily visible from Earth and has been extensively studied from orbit. As the surface cools during polar night, CO₂ condenses directly out of the atmosphere at a rate that establishes equilibrium between radiative loss and latent heat of condensation. Since radiative loss is strongly geometry-dependent, the CO₂ frost will grow most rapidly on exposed surfaces and more slowly in depressions. Positive feedback will cause a dramatic enhancement of the relief of the underlying topography and a corresponding reduction in the average bulk density. The resulting surface will be highly textured and riddled with perforations.     

Thermal emission areas of heated neutron star polar caps

Observed hot spots on neutron stars are often associated with polar caps heated by the backflow of energetic electrons or positrons from accelerators on bundles of open magnetic field lines. Three effects are discussed that may be relevant to formation of hot spots and their areas. (1) The area of a polar cap is proportional to the ratio of the star’s surface dipole field to the local field at the polar cap. Because the field is coupled to the evolving spin of the superfluid core of the star, this ratio can depend on the stellar spin and its history. (2) The hot emission area may appear smaller to a distant observer when emitted X-rays propagate through electron-positron plasma created in the magnetosphere. The X-rays then change their energy spectrum because of cyclotron resonant scattering by pairs. (3) Hot spots may form on the star’s surface as a result of crust motions that are driven by the pull of core flux tubes pinned to the crust. Such motions twist the footprints of closed magnetic loops of the magnetosphere and induce an electric current in the loop, which will heat those footprints.     

The spatial distribution of pulsars and the spiral structure of the Galaxy

We have looked for and found a possible spatial correlation between the present pulsar distribution and the estimated locations of the spiral arms at earlier epochs. Through a detailed statistical analysis we find a significant correlation between the present distribution of pulsars and the mass distribution (in the spiral arms) expected about 60 Myr ago for a corotation resonance radius of 14kpc. We discuss the implications of this correlation for the minimum mass of the progenitors of pulsars. Interpreting the spread in the locations of pulsars with respect to the past locations of the spiral arms as predominantly due to their space velocities, we derive an average velocity for the pulsar population.     

An anticorrelation between polar and equatorial rotation of the solar photosphere

Published spectroscopic measurements of solar rotation are analyzed to show that when the rotation velocity increases at high latitudes it tends to decrease at low latitudes, and conversely. The high latitude velocities typically vary over only 20% of the range of those near the equator and the smallest variations of all occurred near latitude 60° during the rising portion of the previous solar cycle. The anticorrelation is consistent with a recent suggestion that differential rotation on the sun arises from photospheric wind systems whose strength is determined, ultimately, by oscillations within the Sun.     

On the supposed anticorrelation of solar polar and equatorial rotation rates

Howard and Harvey (1970) analyzed Mt. Wilson Doppler shifts to obtain a daily measure of the Sun's differential rotation. The data were fitted to give an angular velocity of the form = a + b sin² B + c sin⁴ B (B = heliographic latitude). Changes in a, b, c were found to be correlated (Howard and Harvey, 1970). Yoshimura (1972) used the anticorrelation of the b and c parameters to infer the existence of large-scale convection. Wolff (1975) used the b-c anticorrelation and a weak correlation between a and b to infer that variations of the Sun's polar and equatorial rotation rates are anticorrelated. In this paper, the anticorrelation of b and c is shown to be due to numerical coupling.     

Electric fields and currents in the earth's polar caps

A model for solar wind flow around the magnetopause incorporating a stagnation line at the frontside magnetopause is used to derive a formula for the electric field intensity and polar cap potential drop. These relationships are compared to experimental data from polar orbiting satellites. The relation between solar wind parameters and auroral arc velocity is also studied.     

Nuclear physics of reverse electron flow at pulsar polar caps

Protons produced in electromagnetic showers formed by the reverse electron flux are usually the largest component of the time-averaged polar cap open magnetic flux line current in neutron stars with positive corotational charge density. Although the electric field boundary conditions in the corotating frame are time independent, instabilities on both medium and short time-scales cause the current to alternate between states in which either protons or positrons and ions form the major component. These properties are briefly discussed in relation to nulling and microstructure in radio pulsars, pair production in an outer gap and neutron stars with high surface temperatures.     

The large-scale structure of the asteroid belt

We examine the distributions of 2888 numbered minor planets over orbital inclination, eccentricity, and semimajor axis, and define 19 zones which we believe adequately to isolate the selection biases in survey programs of the physical properties of minor planets. Six numbered asteroids have exceptional orbits and fall into no zone. We also call attention to rather sharp upper limits, which become increasingly stringent at larger heliocentric distances, on orbital inclinations and eccentricity.