Polar Cap Structure Research Articles

Unsolved problems: Mesoscale polar cap flow channels’ structure, propagation, and effects on space weather disturbances

Dynamic mesoscale flow structures move across the open field line regions of the polar caps and then enter the nightside plasma sheet where they can cause important space weather disturbances, such as streamers, substorms, and omega bands. The polar cap structures have long durations (apparently at least ∼1½ to 2 h), but their connections to disturbances have received little attention. Hence, it will be important to uncover what causes these flow enhancement channels, how they map to the magnetospheric and magnetosheath structures, and what controls their propagation across the polar cap and their dynamic effects after reaching the nightside auroral oval. The examples presented here use 630-nm auroral and radar observations and indicate that the motion of flow channels could be critical for determining when and where a particular disturbance within the nightside auroral oval will be triggered, and this could be included for full understanding of flow channel connections to disturbances. Also, it is important to determine how polar cap flow channels lead to flow channels within the auroral oval, i.e., the plasma sheet, and determine the conditions along nightside oval/plasma sheet field lines that interact with an incoming polar cap flow channel to cause a particular disturbance. It will also be interesting to consider the generality of geomagnetic disturbances being related to connections with incoming polar cap flow channels, including the location, time, and type of disturbances, and whether the duration and expansion of disturbances are related to flow channel duration and to multiple flow channels.

An Estimation of the Geometrical Structure of Polar Cap and Emission Property of Radio Pulsar: A Treatment from an Analytical Approach

Pulsars are believed to be one of the most interesting objects in the universe. The emission mechanism of pulsars is still a conundrum to physicists, as there is no completely acceptable theory that can establish a consensus between theory and observation. Pulsars possess a gigantic magnetic field, to the order of 1012 Gauss, and generate a very powerful radio beam from the magnetic pole. However, the powerful radio beam is generated by some complicated coherent plasma processes and acceleration in the pulsar magnetosphere. The location of the origin of the radio waves has been predicted to come out exclusively from the polar cap zone, whose boundary is defined by the footprint of the last open field line. However, in this paper, we mainly try to generate the shape of the polar cap structure from an analytical solution and discuss how it gets distorted for different geometrical parameters due to the presence of perturbation such as polar cap current flow. Also, apart from that, we try to emphasize understanding the variation of radio emission height and polarization angle with respect to different geometry-related parameters as well as with frequency.

Polar cap precursor of nightside auroral oval intensifications using polar cap arcs

AbstractRecent radar and optical observations suggested that localized fast flows in the polar cap precede disturbances within the nightside auroral oval. However, how commonly this connection occurs has been difficult to examine due to limited coverage of radar flow measurements and diffuse and dim nature of airglow patches. Polar cap arcs are also associated with fast flows in the polar cap and appear much brighter than patches, allowing evaluation of the interaction between polar cap structures and nightside aurora more definitively. We have surveyed data during six winter seasons and selected quasi‐steady polar cap arcs lasting >1 h. Thirty‐four arcs are found, and for the majority (~85%) of them, as they extend equatorward from high latitude, their contact with the nightside auroral poleward boundary is associated with new and substantial intensifications within the oval. These intensifications are localized (< ~1 h magnetic local time (MLT)) and statistically occur within 10 min and ±1 h MLT from the contact. They appear as poleward boundary intensifications in a thick auroral oval or an intensification of the only resolvable arc within a thin oval, and the latter can also exhibit substantial poleward expansion. When radar echoes are available, they corroborate the association of polar cap arcs with localized enhanced antisunward flows. That the observed oval intensifications are major disturbances that only occur after the impingement of polar cap arcs and near the contact longitude suggest that they are triggered by localized fast flows coming from deep in the polar cap.

Double-Cap Differential Codebook Structure for MU MISO Systems in Correlated Channels

We propose a double-cap differential codebook (DCB) design for multiuser (MU) multiple-input-single-output systems. The MU transmission uses signal-to-leakage-and-noise ratio precoding. The double-cap DCB is designed for temporally/ spatially correlated channels and is based on a combined spherical and polar cap structure. We consider an inner spherical cap and an outer polar cap structure with different radii. The design allows us to systematically alter the radii of the caps over the feedback time depending on the previously selected codeword. The proposed double-cap DCB does not require knowledge of temporal or spatial channel correlation at the base-station, thus removing the overhead of additional feedback from the user. Simulation results show the effectiveness of the proposed double-cap DCB over a single polar cap DCB in terms of sum capacity gain and quantization errors.

Structure of the intracellular part of the motility apparatus of halobacteria

The electron microscopic study of the structure of the motility apparatus of the archaea Halobacterium salinarium 4W12 and Natronobacterium magadii confirmed our earlier observation that the motility apparatus of halobacteria contains an intracellular disk-shaped lamellar structure (DLS). Polar cap structures (PCSs) isolated from the halobacterium were preliminarily identified as the DLSs. The PCSs in complexes with flagella were also isolated from the haloalkaliphilic bacterium N. magadii. The specific structure of the archaeal motility apparatus is discussed.

Signatures of pulsar polar-cap emission at the high-energy spectral cut-off

We address four unique signatures in pulsar gamma-ray radiation as predicted by polar-cap models. These signatures are expected to be present nearby the spectral high-energy cut-off at around several GeV. Their magnitude and, therefore, their observability depends strongly on the orientational factors, the rotation, as well as on the details of the polar-cap structure. These strong predictions are likely to be verified by the NASA’s future gamma-ray mission GLAST.

Polar cap area and boundary motion during substorms

The area of the polar cap as a function of local time and substorm phase was measured using images from the Polar Ultraviolet Imager (UVI) for different interplanetary magnetic field (IMF) orientations during three substorms in January 1997. We measured changes in the polar cap area and motion of the poleward and equatorward boundary of the auroral oval. It was found that the polar cap boundary is strongly influenced by thinning of the oval, decrease in polar cap structures, the poleward expansion of the substorm at midnight, and the fading of luminosity below the instrument sensitivity threshold. Generally, these effects dominate over the latitudinal motion of the auroral oval at its equatorward edge. A new feature is that the polar cap region clears of precipitation during the substorm growth phase, which expands the size of the polar cap but is not necessarily related to an expansion of the open flux region. Another finding is that the increase in polar cap area prior to onset can be independent of the strength of the southward IMF component. For one case the polar cap area increased while the southward component of the IMF was 0 ± 0.5 nT. These observations have strong implications for models that use the polar cap area to estimate the magnitude of energy storage in the lobe magnetic field and loss during substorms.

Initial response and complex polar cap structures of the aurora in response to the January 10, 1997 magnetic cloud

On January 10th, 1997, a magnetic cloud originating at the Sun was incident on the Earth. The initial disturbance to the magnetosphere, as reflected in the activities of the aurora, was measured by the Ultraviolet Imager on the Polar Spacecraft. During this event we have observed the development of several unusual unique auroral forms that to our knowledge are unexplained in current models and theories. The observations were made on a global scale with unprecedented spatial and temporal resolution. The first activation of the aurora at local noon occurred within minutes of the arrival of the shock at 0107 UT. The substorm onset was observed at 0334 UT. During the intervening time significant polar cap precipitation occurred.

The flagellar bundle of Halobacterium salinarium is inserted into a distinct polar cap structure

Flagellated envelopes of Halobacterium salinarium cells were prepared by lysis with taurodeoxycholate. After solubilization of the envelopes with Triton X-100 at high ionic strength, flagella and round patches from which numerous flagella emerged were isolated by gel filtration chromatography. We conclude that the flagellar bundle of H. salinarium is inserted into a differentiated polar cap structure.

Auroral X‐ray images from DMSP‐F6

X‐ray images of auroral arcs for consecutive north and south polar crossings of the DMSP‐F6 satellite are presented. On January 18, 1983, the north polar region was in the earth's shadow and the south polar region was in complete sunlight. The X‐ray images of both polar cap regions show features of the dawn and dusk auroral oval that are quite similar. However a strong polar cap structure appears only in the north pole image. Near simultaneous measurements of precipitating electrons obtained by instruments aboard the NOAA‐6 and NOAA‐7 satellites crossing the south pole, show no indication of polar auroral structures. These examples illustrate the utility of X‐ray imaging in both sunlight and in darkness for studying auroral boundaries and morphology.

Inhomogeneous pulsar polar CAP structure and radiation processes

Temporal development of the avalanche process over the pulsar polar cap due to the electron-positron pair production is investigated. The plasma produced from a single seed charge forms a series of thin but high-density ribbons which move toward the magnetic pole. Each ribbon is spatially and temporally modulated, with characteristic scales which correspond to those of radio waves. These modulations are intimately related with the observed radio emission. The small transverse scales of the ribbon may be responsible for the observed microstructure of pulsar signals. In addition, sheets of charges which can emit optical radiation are produced, but for various reasons they are not likely to be related to the observed optical radiation. Several mechanisms which control the plasma density within the ribbon are discussed. Neither the turbulent resistivity nor the charge redistribution within the ribbon is likely to control the density. It is suggested that the accumulation of charges over the neutron star surface and/or over the acceleration zone are responsible in terminating the avalanche process.

Uneven illumination of the polar caps by solar protons: Comparison of different particle entry models

Recent solar proton measurements over the polar caps have shown up some startling features during times of interplanetary particle anisotropies. Explanations of the observations have been based on diffusive entry into the magnetotail, direct entry by motion along trajectories without scattering, and models incorporating both entry mechanisms. The direct entry mode is based on the computation of particle orbits, of points of access, and asymptotic directions at the magnetopause and in the tail. Although different research groups have used trajectory calculations in order to explain observed polar cap structure, there is considerable disagreement on the computed trajectories and especially on the proposed access mechanisms of solar particles to these trajectories at the magnetopause. In this paper the different models are evaluated, and it is found that the nonadiabatic reconnection model is in good accord with the observations, whereas other models show serious inconsistencies.

Nonadiabatic particle motion in the magnetosphere

Using calculations in a model geomagnetic fleld, the influence of nonadiabatic particle motion on the polar cap structure of polar protons is investigated. Breakdown of the first invariant, due to excessive field curvature and gradients near the last closed field line, results in an effect that cannot be differentiated from strong pitch angle scattering using low-altitude detectors, although no time variations of the magnetic field are considered. The results obtalned thus form a basis to which wave-particle interactions and other scattenng effects must be added. In a companion paper, these results are applied to observations of solar protons over the polar caps, and it is shown that the quiettime behavior of these particles is indeed approximately given by the constraints due to the spatial variations of the geomagnetic field.

Study of the magnetosphere using energetic solar particles

A review is presented of the study of the magnetosphere using energetic solar particles. After a historical introduction, present knowledge about the geomagnetic field topology is shortly described in order to assess the contribution, which energetic particle measurements can make towards the study of the magnetosphere. The following historical review of satellite observations is discussed in terms of their contributions to a phenomenological classification of polar cap structures. Subsequent to about 1969 detailed trajectory calculations of particles in model magnetospheric fields were available which could be used in the interpretation of the measurements so that a strict historical treatment is not tractable. After a short description of the trajectory calculation method in various model magnetospheric fields the role of different interplanetary conditions in determining intensity structures in the geomagnetic tail and over the polar caps is discussed. Firstly, polar cap observations during different interplanetary conditions are described in terms of their contribution to the understanding of the magnetospheric topology, and secondly, observations in the distant magnetosphere and magnetotail are reported and compared with relevant theory.

Persistent particle anisotropies and magnetospheric models

The influence of an interplanetary particle anisotropy on the asymmetry of solar particle entry into the magnetotail is analysed in the diffusion as well as in the reconnection model. By time dependent diffusion calculations with an asymmetric boundary condition in a cylindrical tail lobe it can be shown that a north-south interplanetary anisotropy leads in the open as well as in the closed magnetosphere to essentially the same polar cap structures when observed with a dawn-dusk polar orbiting satellite. However, depending on the satellite orbit, an east-west interplanetary anisotropy can serve to distinguish between rival magnetospheric models. Comparison of our diffusion calculations with polar cap measurements during an east-west interplanetary anisotropy, as presented in Morfill and Scholer (1972), show a large discrepancy, whereas an open tail model fits these observations best.

Polar-cap structures of solar protons observed during the passage of interplanetary discontinuities

Particle measurements from the low-altitude polar-orbiting satellite GRS-A/Azur and from the Imp 5 satellite inside the magnetosphere near the magnetopause and particle and magnetic-field measurements in interplanetary space from the Heos A1 satellite are presented for the solar-proton event of March 1970. Comparison of interplanetary magnetic-field and particle data indicate that a flux increase lasting 20 min is associated with an interplanetary magnetic filament. A following sharp flux increase is attributed to an interplanetary magnetic discontinuity. As the discontinuity with different proton fluxes on both sides is swept over the earth, the particles immediately enter the magnetosphere near the neutral sheet, and the higher flux is observed without time delay in the quasi-trapping region over the polar cap. The central polar cap is still dominated by particles entering via the distant magnetospheric tail. The discontinuity is convected along the tail and ultimately reaches the region where field lines originating in the central polar cap are connected to or intermingled with the interplanetary field. Filling up of the polar cap in less than 2 hours to the higher flux level results in a maximum ‘tail length’ of about 550 RE.

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 25 R E and a tail field model fitted to observations from 15 R E to 80 R 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 700 R E and 200 R E for 100 MeV and 1 MeV protons respectively, which indicates that trajectory calculations in a static field model are valid. Structure over the polar caps at low altitudes due to interplanetary anisotropies was observed for protons > 30 MeV during the solar flares of November 18th 1968 and February 25th 1969. Apart from an easily explained direct impact zone structure, fluxes corresponding to peak anisotropic interplanetary intensity were observed to come from deep in the magnetotail. A magnetopause model of the generalised rotational discontinuity type, with a diverging tail field, is proposed and it is shown that this allows particle access into the tail with conservation of the distribution function. Particles from the direction of maximum flux (usually the Parker spiral direction) reverse their direction of motion during multiple magnetopause crossings. They eventually stay in the tail with small pitch angles and are thus observable at low altitudes. Observations show that 1 MeV protons become isotropic at the magnetopause. It is suggested that this is due to irregularities in the magnetopause, possibly caused by Kelvin Helmholtz instabilities, which are of comparable dimensions to the Larmor radius of these particles. Intensity differences over the polar cap of ▪40 per cent are expected due to the overall radial intensity gradient, and rapid local gradients can produce even greater differences. It is proposed here that sharp local intensity changes bounded by tangentional discontinuities, which convect past the Earth with the solar wind account for most of the observed polar cap intensity variations for 1 MeV protons. Anisotropies in flux produce gradients across the tail which can also be observed over the polar caps. Particle trajectories crossing the neutral sheet are analysed, and it is shown that dispersion of trajectories produces broadening of sharp interplanetary intensity changes when observed in the tail, not by a diffusion process, but through different trajectory lengths. Results of trajectory integrations at three energies are presented. These can be used to analyse polar cap structure in future work. Results of nonadiabatic particle motion in the pseudo-trapping region are shown. This motion produces effects similar to strong pitch angle scattering although it takes place in a static field.