Polar Cap Region Research Articles

Gamma-ray pulsars: polar cap or outer gap emission?

The power law spectra of the six knownγ-ray pulsars are seen to cut off at energies < 1 TeV so that no steady pulsed TeV component is observed from any knownγ-ray pulsar. In some cases we show that the cutoff is steeper than exponential, which is consistent with magnetic pair production above the polar cap region. The small upper limits on the ratio of TeV energy flux to optical - GeV energy flux is again consistent with the polar cap model, but appears to be inconsistent with inverse Compton controlled outer gaps.

Photon splitting in soft gamma repeaters

The exotic quantum process of photon splittingϒ →ϒϒ has great potential to explain the softness of emission in soft gamma repeaters (SGRs) if they originate in neutron stars with surface fields above the quantum critical fieldB cr = 4.413×1013 Gauss. Splitting becomes prolific at such field strengths: its principal effect is to degrade photon energies, initiating a cascade that softens gamma-ray spectra. Uniform field cascade calculations have demonstrated that emission could be softened to the observed SGR energies for fields exceeding about 1014 Gauss. Recently, we have determined splitting attenuation lengths and maximum energies for photon escape in neutron star environments including the effects of magnetospheric dipole field geometry. Such escape energies εesc suitably approximate the peak energy of the emergent spectrum, and in this paper we present results forε esc as a function of photon emission angles for polar cap and equatorial emission regions. The escape energy is extremely insensitive to viewing perspective for equatorial emission, arguing in favour of such a site for the origin of SGR activity.

Japan-EISCAT Collaboration on the Svalbard Radar: Scientific Significance.

In this paper, collaboration between the Japanese scientific community and the EISCAT Scientific Association on the new Svalbard radar project is briefly discussed. Some of the important research topics we intend to pursue in conjunction with the Svalbard radar are summarized. They are: identification of the polar cusp region, solar wind control of the polar cusp, electrodynamics in the polar cusp and polar cap regions, contracted-oval substorms, and thermospheric dynamics at high latitudes.

Resolved Shot Noise in the AM Her System V834 Cen

Cataclysmic variables of the AM Her type are close binary systems with accretion onto a highly magnetic (a few times 107 G) white dwarf. In these systems the magnetic field is strong enough to prevent the formation of an accretion disc. The accretion flow is instead funneled directly onto the polar caps where the plasma is shocked and radiates intense optical cyclotron radiation as well as bremsstrahlung at X-ray energies. The cyclotron radiation component, which dominates in the optical, is highly polarized (5-35%). The cyclotron emission exhibits intensity variations on a large range of time scales. This include quasi-periodic oscillations, flickering and long term changes between brightness states. The observed source intensity is also modulated by the white dwarf rotation which is synchronized with the orbital period (101 min for V834 Cen). The flickering is believed to be due to inhomogeneities in the accretion flow. These could either be density variations in a continous flow or it could be in the form of independent blobs accreting over the polar cap region. As blobs hit the white dwarf surface and radiate their accretion energy they should be seen as shot noise. For a theoretical discussion of the fate of such blobs see Frank et al. (1988). A modeling of the optical variability in AM Herculis, in terms of a shot noise process was made already by Panek (1980). It was found that a consistent model could be constructed by randomly occurring 708 – 908 rectangular shots, with on the average a few overlapping shots. The power spectrum had a v−2 shape above 0.02 Hz as expected, although it is not clear whether a break in the slope of the power spectrum was actually seen around 0.01 Hz. The shot overlap meant that individual shots could not be resolved directly in the light curve. This limits the precision and uniqueness by which shot parameters can be determined. However, in our observational data of AM Her objects we have found occasions when the character of the flickering has changed to become much more “flarelike”, suggesting that individual shots might be seen in these cases. This change is also associated with a change in the shape of the power spectrum, in particular a higher power level and a steepening at the high frequency end. Most importantly, there is a distinct break in the power spectra around 0.03 Hz, indicative of a characteristic time scale in the flickering noise. In addition to a power spectrum analysis we have also made a direct modeling of individual flares in the light curve. The distribution and correlations of the fitted pulse parameters were studied and compared with simulated data. The primary aim was to investigate whether the flares are really individual shots or produced by a random superposition of more frequent smaller amplitude shots.

Rocket electron density profiles over Volgograd, Heiss Island and Esrange during the DYANA campaign

The results of the determination of electron density ( N e ) profiles at Volgograd, Heiss Island and Esrange ranges are discussed. Some of these profiles were obtained at solar zenith angles <90°, but at times near the sunset, and others at night-time. The profiles obtained during the DYANA campaign were compared with others obtained in the past, with solar and geomagnetic indices as well as with the temperature and atmospheric circulation in the stratosphere and mesosphere. The daytime electron densities below 85 km over Volgograd do not show any appreciable connection to the temperature. An anomalous period of low electron densities observed from February to the beginning of March rather seems to be connected with the meridional and vertical components of the atmospheric wind. We can speculate that the high night-time N e -values at middle latitude may be considered as a manifestation of a post-storm effect. The relationship of the N e -values at higher altitudes (>80 km) and radio wave absorption seems to be connected with the atmospheric circulation and geomagnetic activity. The N e -values in the polar cap region (Heiss Island) are below those at the same altitude in the auroral zone (Esrange), but higher than at middle latitudes (Volgograd). The N e -values over Heiss Island seem to be connected with the geomagnetic activity and the meridional component of atmospheric circulation; the thermal control of the D-region cannot be excluded.

Historical Photometric Evidence for Volatile Migration on Triton

Analysis of CCD images of Triton obtained with the 1.5-m telescope on Palomar Mountain shows that in the time period surrounding the Voyager 2 encounter with the satellite (1985-1990), no changes in the satellite's visual albedo or color occurred. The published observations of Triton in the 0.35- to 0.60-μm spectral region obtained between 1950 and 1990 were reanalyzed to detect historical variability in both its albedo and visual color. Analysis of the photometry indicates that there is little, if any, change in Triton's visual geometric albedo. This result is consistent with the albedo pattern observed by Voyager and the change in sub-Earth latitude. Two distinct types of color changes are evident: a significant secular increase in the blue region of the visual spectrum since at least the 1950s, and the reported dramatic reddening of Triton's spectrum in the late 1970s. The latter change can be explained only by a short-lived geological phenomenon. Triton's changing pole orientation with respect to a terrestrial observer cannot explain the secular color changes. These changes imply volatile transport on a global scale on Triton's surface during the past 4 decades. We present two models which show that either removal of a red volatile from Triton's polar cap or deposition of a blue volatile in the equatorial regions can explain the secular color changes. A third possibility is that the changes are the result of the α-β phase transition of nitrogen and subsequent fracturing of the polar cap region (N. S. Duxbury and R. H. Brown, 1993, Science 261,748).

Middle atmosphere and lower thermosphere processes at high latitudes studied with the EISCAT radars

The middle and upper atmosphere and the ionosphere at high latitudes are studied with the EISCAT incoherent scatter radars in northern Scandinavia. We describe here the investigations of the lower thermosphere and the E-region, and the mesosphere and the D-region. In the auroral zone both these altitude regions are influenced by magnetospheric processes, such as charged particle precipitation and electric fields, which are measured with the incoherent scatter technique. Electron density, neutral density, temperature and composition are determined from the EISCAT data. By measuring the ion drifts, electric fields, mean winds, tides and gravity waves are deduced. Sporadic E-layers and their relation to gravity waves, electric fields and sudden sodium layers are also investigated with EISCAT. In the mesosphere coherent scatter occurs from unique ionization irregularities. This scatter causes the polar mesosphere summer echoes (PMSE), which are examined in detail with the EISCAT radars. We describe the dynamics of the PMSE, as well as the combination with aeronomical processes, which could give rise to the irregularities. We finally outline the future direction which is to construct the EISCAT Svalbard Radar for studying the ionosphere and the upper, middle and lower atmosphere in the polar cap region.

Irregularity structures in the cusp/cleft and polar cap regions

A case study of the temporal behavior of ionospheric scintillations and their frequency spectra in the cusp/cleft and polar cap regions is presented. These measurements were made at Søndrestrom and Thule, Greenland, using the 243‐MHz transmissions from quasi‐stationary satellites during a coupling energetic and dynamics of atmospheric regions (CEDAR) high‐latitude plasma structure (HLPS)/solar terrestrial energy program (STEP) global aspects of plasma structures (GAPS) campaign. During this campaign, the incoherent scatter radar (ISR) observations were also performed at Søndrestrom, which defined the dynamic ionospheric environment in the cusp/cleft region. The availability of the radar results has enhanced this case study. It is found that scintillations at Søndrestrom are abruptly enhanced about an hour before magnetic noon when the propagation path to the satellite entered the cusp/cleft region. Subsequently, a series of enhanced and reduced scintillation activity was detected. The enhanced scintillation structures were found to be asymmetric, with sharp leading edges and diffuse trailing edges. Spaced‐antenna scintillation measurements at Søndrestrom detected considerable velocity shear, and the frequency spectra showed flat low‐frequency portions, implying the presence of turbulent plasma flows. A comparison with the ISR observations indicates that the temporal variation of scintillation was caused by the poleward convection of alternate regions with high‐ and low‐ionization density, the density depletions being caused by channels of high zonal flows associated with velocity shear. The level of scintillation observed in the low‐density regions imply the presence of small‐scale irregularities with considerable irregularity amplitude. In contrast to the above behavior, the polar cap scintillations exhibit deep minima between the transit of successive “patches” of ionization, and their frequency spectra imply the absence of turbulent plasma flows. It is postulated that in the cusp/cleft and polar cap regions, the gradient‐drift instability mechanism generates the observed small‐scale irregularities associated with discrete density enhancements, whereas a shear‐driven instability, such as the nonlinear collisional Kelvin‐Helmholtz (K‐H) instability mechanism, may generate the irregularities in the intervening low‐density regions.

Relation of electric field and particle precipitation in the polar cap region inferred from Akebono observations

Relation of electric field and particle precipitation in the polar cap region inferred from Akebono observations

Riometer observations at Sanae (L = 4.0) related to solar proton events

Atmospheric ionization by protons of integral intensity above a cutoff of 0.86 GV at Sanae is too small to record a riometer absorption above noise even during the large ground‐level enhancements (GLEs) of September 29 and October 19 and 24, 1989. However, linear increases may be depicted in the absorptions of 20‐, 30‐, and 51.4‐MHz cosmic radio noise that ranged from 0.25 dB per hour at 30 MHz for a GLE with a rising phase of 26% per hour to 0.81 dB per hour for a GLE of 360% per hour. Lowering of cutoff rigidity by magnetospheric ring currents to give direct access to solar flare protons of rigidity below 0.86 GV or precipitation of energetic magnetospheric electrons due to magnetic activity may be ruled out. Mutual consistency appears between the rates in ground‐level solar proton enhancements and the rates of increases in absorptions at all three of the frequencies for all three events. This suggests that nonrelativistic solar protons should have traversed the geomagnetic field lines to be precipitated at Sanae. This traversal may be due to equatorward gradient and curvature drifts from the polar cap region, a westward longitudinal drift along L = 4.0, and subsequent precipitation into the atmosphere at Sanae due to the South Atlantic Magnetic Anomaly. Preceding these linear increases, solar flare effects with absorption maxima coincident with the maxima of solar flare X rays and the 2.2‐MeV gamma ray line have been observed. The rates of recovery in absorption that are faster than that of the 0.5‐ to 4‐Å X rays suggest that these absorptions are due to ionization caused by still shorter wavelength electromagnetic radiation.

Measurement of direct current electric fields and plasma flow speeds in Jupiter's magnetosphere

During the encounter of Ulysses with Jupiter, we have measured two components of the dc electric field and deduced from them the flow speed in the Io torus, as well as the presence of a polar cap region and what we interpret as a cleft region. Within the torus the flow speed is approximately equal to the speed of a plasma corotating with Jupiter but has significant deviations. The dominant deviations have an apparent period of the order of Jupiter's rotation period, but this might be a latitudinal effect. Other important periods are about 40 min and less than 25 min.

Akebono (EXOS D) observations of small‐scale electromagnetic signatures relating to polar cap precipitation

We have analyzed small‐scale electric and magnetic field variations and electron fluxes which were obtained by the Akebono (EXOS D) satellite in the polar cap region. An electron precipitation most often coincides with a large gradient of flow velocity in which div E is negative (type 1). However, sometimes a steep negative div E is seen only at the edge of a wider region of electron precipitation (type 2). We also observed cases where no particle precipitation occurred in the region of negative div E (type 3). For both type 1 and type 2 cases, a common signature is a spikelike enhancement in the electric field and associated plasma flow in which electron precipitation can be seen on a side where div E is negative. The direction of the spikelike flow is not unique; both sunward flow and antisunward flow are observed. Hence the location of the upward current and the downward current with respect to the noon‐night direction depends on the flow direction in the spike.

Dynamical polar wind and its response to kinetic ion heating

The dynamic polar wind and its response to kinetic ion heating are examined in this paper. We used a time‐dependent hydrodynamic polar wind code to study the dynamic flow characteristics of an H+ and O+ polar wind from 200 km to ∼6 ‐ 7 RE. We found that the polar wind ion relaxation times are comparable to the time required for the ions to propagate at the ion speed of sound over the altitudinal range considered. Although there may be large transient O+ fluxes lasting ∼15 min during the early stage of expansion of the unheated polar wind, these fluxes cannot be sustained because of the large O+ gravitational potential. We subsequently studied how polar wind ion heating affects the ion outflow. Our previous work showed that O+ and H+ polar wind ions can become heated due to ion beam instabilities in the polar cap region. We incorporate these kinetic effects into our hydrodynamic model by specifying ion temperature profiles in the ion momentum equations. Two limiting cases were considered, (1) preferential O+ and (2) preferential H+ ion heating. We found that ion heating does not have much effect on the escape of H+ ions because of the limiting nature of the H+ escape fluxes. However, O+ heating can substantially increase the upward O+ pressure gradient in the heating region. This leads to increased O+ ion escape from the polar ionosphere. Thus O+ heating is a viable explanation for the unexpectedly large O+ fluxes which are frequently observed in the polar magnetosphere.

Particles and fields measurements at Neptune with Voyager 2

The particles and fields complement of instruments on the Voyager 2 spacecraft performed a comprehensive set of measurements during the encounter with the Neptune system on August 24 through 28, 1989. These included measurements of the magnetic field, plasma (10 ev ≤ E ≤ 6 keV), energetic and high energy particles (2 keV ≤ E ≤ 5 MeV), plasma waves (10 Hz to 50 kHz) and radio emissions (∼ 20 to ∼ 1300 kHz); additional information relating to UV emissions was provided by the ultraviolet spectrometer. The first results of these measurements are reviewed in this paper and may be summarized as follows: (a) The planetary magnetic field outside ∼ 4 R N may be described by an offset (∼ 0.55 R N), tilted (47°), dipole of moment 0.133 Gauss-R 3 N; inside that distance the field is dominated by higher order terms. (b) Plasma densities were generally low (∼ 5 × 10 −3 cm −3), except at magnetic equatorial crossings when densities up to ∼ 1 cm −3 were seen; best fits (but not unique) to corotating Maxwellians are obtained for two components, H + and N +. (c) Energetic ions (≳ 28 keV) and electrons (≳ 22 keV) were seen throughout the magnetosphere, while higher energy (≳ 200 keV) particles were confined inside the orbital radius of Triton. Spectrally soft electron and ion enhancements during spacecraft passage through the polar cap region resembled those at Earth's auroral latitudes but particle angular distributions did not. (d) A variety of plasma wave emissions were seen, including chorus, hiss, electron cyclotron waves, and upper hybrid resonance in the inner magnetosphere. (e) Radio wave bursts in the range 100 to 1300 kHz, narrow band and strongly polarized, have enabled determination of the planetary rotation period as 16.11 ± 0.05 hours; smooth emissions in the range 20 to 865 kHz were also observed. (f) Weak auroral emissions in H Lyman β (1025 Å) have been tentatively identified on the dayside of Neptune with total radiated power of 5 × 10 7W. (g) The interaction between the magnetosphere of Neptune and Triton's ionosphere is sub-Alfvénic. The magnetic field orientation with respect to the rotation axis was such that the spacecraft entered a “pole-on” magnetosphere, fortuitously sampling the cusp region of an outer planet for the first time. The measured flux of soft electrons and ions over the polar region of ∼ 2 × 10 −3 erg/cm 2 sec results in an estimated power input of ∼ 3 × 10 7 W, i.e. substantially less than that at other planets. Possible reasons for these differences are discussed.

Chen receives First Scarf Award

At the AGU 1992 Spring Meeting in Montreal, Margaret Wei‐Mei Chen will be presented with the Space Physics and Aeronomy section's F.L. Scarf Award for outstanding dissertation research that contributes directly to solar‐planetary science. Chen will receive a cash award of $1,000 and present an invited paper on the subject of her dissertation in a SPA‐Magnetospheric Physics session at the meeting.Chen received her Ph.D. in physics from the University of California, Los Angeles, in September 1990. Her dissertation on “The Polar Wind and Ionic Outflow” involved the oretical analysis, numerical simulations, and comparison with observational data. Advised by Maha Ashour‐Abdalla, Chen incorporated kinetic processes such as ion heating into a macroscopic hydrodynamical model of the polar wind in order to investigate “non‐classical” effects in the polar wind. She also worked closely on this subject with Thomas E. Holzer of the NCAR High Altitude Observatory. In collaboration with William K. Peterson of Lockheed and Thomas E. Moore of NASA Marshall Space Flight Center, Chen analyzed measurements from the Dynamics Explorer 1 spacecraft of upflowing ion streams in the polar cap region. These data were used to guide the theoretical modeling.

Light curves of rotating, oscillating neutron stars

A technique has been developed for computing the light curve produced by a rotating, oscillating neutron star that emits radiation from circular polar cap regions, as are thought to exist in pulsars, X-ray binaries, and perhaps X-ray bursters. Several examples of light curves produced by single, low-order (l = 1, 2) oscillation modes are given. A Gaussian beaming function is used to simulate typical radio pulsar beam widths in order to investigate a neutron star oscillation model for subpulse drift in pulsars. X-ray bursts and X-ray pulsars have also been simulated to assess the possibility of detecting such oscillations in these sources with XTE and AXAF.

Closing Comments: Theory

Though it had been circulating through the grapevine for more than ten years, it is now widely known that the ion work function on the neutronstar surface is so low that the surface may not even be a solid (Kössl et al. 1988). In this meeting the issue surfaced in the review by Arons, and came up in many discussions by the participants. A view was expressed that since a real “professional” calculation is not yet available, one should reserve judgement on this issue. However, three different methods lead to about the same value, a fraction of a keV, for the ion work function. Furthermore, the polar cap region in pulsars will be hot and will have strong electromagnetic fields. Thus easy extraction of ions from the stellar surface appears most likely.

An Empirical Theory of Pulsar Emission

AbstractA system of pulsar profile classification is used as a starting point to study the emission characteristics of pulsars. Two types or mechanisms of pulsar radiation are identified which combine geometrically to produce five major species of profile. The core emission, which forms a pencil beam of radiation, is apparently produced close to the stellar surface throughout the entire polar cap region by low γ particles. The conal emission, which consists of a hollow conical beam, then seems to be emitted at heights of 10 to 20 stellar radii by currents of high γ particles travelling along some of the most peripheral of the “open” field lines.

Increase in the PSC‐formation probability caused by high‐flying aircraft

The saturation temperature Tsat for the formation of polar stratospheric clouds (PSC) strongly depends on the local partial pressures of nitric acid and water vapour, and thus is sensitive to NOx and H2O‐injection due to exhaust from aircraft in the stratosphere. The present paper investigates this effect, using daily stratospheric temperature data from the northern hemisphere compiled over the last 25 years by the Free University of Berlin. For Type‐I PSC the data were examined for temperatures T &lt; Tsat ‐ 3 K, which would allow for a supercooling of about the 3 K measured in a previous arctic winter balloon mission, in effect corresponding to a large HNO3‐supersaturation. We show that this is required to overcome the heterogeneous nucleation energy barrier. We compare the analyses of both a background atmosphere and one perturbed by a fleet of 600 stratospheric aircraft flying at ∼22 km altitude. The result is that between December and March in the polar cap region there might be more than a doubling in the occurence of Type‐I PSCs and an even stronger increase of Type‐II PSCs, and accordingly a substantial enhancement in the potential ozone destruction by chlorine radicals.

Accretion powered pulsars - Continuum spectra and light curves of settling accretion mounds

The transfert technique, decribed in Burnard D. J. et al. (1988, Astrophys. J., 324), is applied to model the polarimetric spectra expected from a mound of accreting plasma settling subsonically onto the polar cap regions of an X-ray pulsar. An analytical model of the accretion flow near the polar caps of the neutron stars is employed and it is demonstrated how the emission from such an accretion mound can be computed from a series of one-dimensional slabs of nonuniform material. The results of the calculations for these slabs are compared with those obtained from a simple mode-coupled modified blackbody model.