Polar Cap Model Research Articles

Feasibility study on retrieving exoplanetary cloud cover distributions using polarimetry

As a new growing field, exocartography aims to map the surface features of exoplanets that are beyond the resolution of traditional observing techniques. While photometric approaches have been discussed extensively, polarimetry has received less attention despite its promising prospects. We demonstrate that the limb polarization of an exoplanetary atmosphere offers valuable insights into its cloud cover distribution. Specifically, we determine an upper limit for the polarimetric precision, which is required to extract information about the latitudinal cloud cover of temperate Jovian planets for scenarios of observations with and without host stars. To compute the scattered stellar radiation of an exoplanetary atmosphere and to study the polarization at various planetary phase angles, we used the three-dimensional Monte Carlo radiative transfer code POLARIS. When the planetary signal can be measured separately from the stellar radiation, information about the latitudinal cloud cover for polar cap models is accessible at polarimetric sensitivities of percent . In contrast, a precision of about e-3 ppm is required when the stellar flux is included to gain this information.

Solar Wind‐Magnetosphere Coupling During High‐Intensity Long‐Duration Continuous AE Activity (HILDCAA)

AbstractHigh‐Intensity Long‐Duration Continuous AE Activity (HILDCAA) intervals are driven by High Speed solar wind Streams (HSSs) during which the rapidly‐varying interplanetary magnetic field (IMF) produces high but intermittent dayside reconnection rates. This results in several days of large, quasi‐periodic enhancements in the auroral electrojet (AE) index. There has been debate over whether the enhancements in AE are produced by substorms or whether HILDCAAs represent a distinct class of magnetospheric dynamics. We investigate 16 HILDCAA events using the expanding/contracting polar cap model as a framework to understand the magnetospheric dynamics occurring during HSSs. Each HILDCAA onset shows variations in open magnetic flux, dayside and nightside reconnection rates, the cross‐polar cap potential, and AL that are characteristic of substorms. The enhancements in AE are produced by activity in the pre‐midnight sector, which is the typical substorm onset region. The periodicities present in the intermittent IMF determine the exact nature of the activity, producing a range of behaviors from a sequence of isolated substorms, through substorms which merge into one‐another, to almost continuous geomagnetic activity. The magnitude of magnetic fluctuations, dB/dt, in the pre‐midnight sector during HSSs is sufficient to produce a significant risk of Geomagnetically Induced Currents.

Universal Time Variations in the Magnetosphere and the Effect of CME Arrival Time: Analysis of the February 2022 Event that Led to the Loss of Starlink Satellites

AbstractWe study Universal Time (UT) variations in the magnetospheric response to Coronal Mass Ejection (CME) impacts, using the example of the two CMEs that led to the destruction of 38 out of 49 Starlink satellites in early February 2022. We employ the Expanding‐Contracting Polar Cap model to analyze the variation in the size of the ionospheric polar caps and an eccentric dipole model of the geomagnetic field and thereby quantify the UT variations caused by the inductive effect of the diurnal motions of the geomagnetic poles in a “geocentric‐solar” frame of reference (i.e., a frame with an X axis that points from the center of the Earth to the center of the Sun). The results show that use of a quasi‐steady convection model predicts a similar global power deposition into the thermosphere as that inferred here, but does not give the same division of that power between the northern and southern hemispheres. We demonstrate that, through the combined effects of the Russell‐McPherron dipole‐tilt mechanism on solar‐wind magnetosphere coupling and of the diurnal polar cap motions in a geocentric‐solar frame, the power deposited varies significantly with the arrival UT of the CMEs at Earth. We also show that in the events of early February 2022, both CMEs arrived at almost the optimum UT to cause maximum thermospheric heating.

Magnetosphere-Ionosphere Coupling: Implications of Non-Equilibrium Conditions

The response times of the coupled magnetosphere-ionosphere-thermosphere system are, on average, greater than the autocorrelation timescales of solar wind forcing. This means that the system is rarely, if ever, in equilibrium. Departures from equilibrium are a key component of the Expanding-Contracting Polar Cap (ECPC) model of convection excitation in both the magnetosphere and ionosphere, driven by the Dungey reconnection cycle of opening and re-closing magnetospheric field lines. Averaging over sufficiently long timescales reduces data to the equivalent of steady-state conditions, which hides the physical mechanisms involved and allows us to map electric fields from interplanetary space to the ionosphere–but this is not valid, either physically or generally, because of magnetic induction effects. Only for transient phenomena on sufficiently short timescales do the mechanisms associated with non-equilibrium fully manifest themselves. Nevertheless, because of both ever-changing solar wind conditions and Earth’s dipole tilt, eccentricity and rotation, the magnetosphere is always tending towards a perpetually-evolving equilibrium configuration and there are important implications of transient events for understanding the general behavior of the coupled magnetosphere-ionosphere-thermosphere system and its response to solar wind forcing. We here discuss one example: as a consequence of the importance of departures from equilibrium inherent in the ECPC model, the solar wind dynamic pressure PSW influences the magnetosphere-ionosphere convection response to the generation of open field lines by reconnection in the dayside subsolar magnetopause. We here demonstrate this effect in a statistical survey of observations and show that it is as predicted by the ECPC model and that, through it, PSW has an influence on flux transport in the magnetosphere-ionosphere system.

The influence of small-scale magnetic field on the heating of J0250+5854 polar cap

The influence of surface small-scale magnetic field on the heating of PSR J0250+5854 polar cap is considered. It is assumed that the polar cap is heated only by reverse positrons accelerated in pulsar diode. It is supposed that pulsar diode is located near the star surface (polar cap model) and operates in the steady state space charge-limited flow regime. The reverse positron current is calculated in the framework of two models: rapid and gradually screening. To calculate the production rate of electron-positron pairs we take into account only the curvature radiation of primary electrons and its absorption in magnetic field. It is assumed that some fraction of electron-positron pairs may be created in bound state that can later be photoionized by thermal photons from star surface.

Young radio-loud gamma-ray pulsar light curve fitting

Context. Since the launch of the Fermi gamma-ray telescope, several hundred radio-loud gamma-ray pulsars have been detected, many belonging to millisecond pulsars but some belonging to the young pulsar population with spin periods longer than 30 ms. Aims. Observing simultaneously pulsed radio and gamma-ray emission from these stars helps to constrain the geometry and radiation mechanisms within their magnetosphere and to localize the multiple photon production sites. In this paper we fit the time-aligned gamma-ray light curves of young radio-loud gamma-ray pulsars. We assume a dipole force-free magnetosphere where radio photons emanate from high altitudes above the polar caps and gamma rays originate from outside the light cylinder, within the striped wind current sheet. Methods. We computed a full atlas of radio and gamma-ray pulse profiles depending on the magnetic axis obliquity and line-of-sight inclination with respect to the neutron star rotation axis. By applying a χ2 fitting technique, we were able to pin down accurately the magnetosphere geometry. Further constraints were obtained from radio polarization measurement following the rotating vector model, including aberration and retardation effects. Results. We find a good agreement between our model and the time-aligned single- or double-peaked gamma-ray pulsar observations. We deduce the magnetic inclination angle and the observer line of sight with respect to the rotation axis within a small error bar. The distinction between radio-loud or radio-quiet gamma-ray pulsars or only radio pulsars can entirely be related to the geometry of the associated emitting regions. Conclusions. The high-altitude polar cap model combined with the striped wind represents a minimalistic approach able to reproduce a wealth of gamma-ray pulse profiles for young radio pulsars. Based on self-consistent force-free simulations, it gives a full geometrical picture of the emission properties without resorting to detailed knowledge of the individual particle dynamics and energetics.

A Survey of 25 Years' Transpolar Voltage Data From the SuperDARN Radar Network and the Expanding‐Contracting Polar Cap Model

AbstractWe use 214,410 hourly observations of transpolar voltage, ΦPC, from 25 years' observations by the Super Dual Auroral Radar Network radars to confirm the central tenet of the expanding‐contracting polar cap model of ionospheric convection that ΦPC responds to both dayside and nightside reconnection voltages (ΦD and ΦN). We show that ΦPC not only increases at a fixed level of the nightside auroral electrojet AL index with increasingly southward interplanetary magnetic field (IMF) (identifying the well‐known effect of ΦD on ΦPC) but also with increasingly negative AL at a fixed southward IMF (identifying a distinct effect of ΦN on ΦPC). We also study the variation of ΦPC with time elapsed since the IMF last pointed southward, Δt, and show that low/large values occur when (−AL) is small/large. Lower numbers of radar echoes, ne, mean that the “map‐potential” reanalysis technique used to derive ΦPC is influenced by the model used: we present a sensitivity study of the effect of the threshold of ne required to avoid this. We show that for any threshold ne, ΦPC falls to about 15 kV for Δt greater than about 15 h, indicating any viscous‐like voltage ΦV is considerably smaller than this. It is shown that both ΦPC and (−AL) increase with increased solar wind dynamic pressure pSW, but not as much as the midlatitude geomagnetic index am. We conclude pSW increases both ΦD and ΦN through increasing the magnetic shear across the relevant current sheet but has a larger effect on midlatitude geomagnetic indices because of the effect of additional energy stored in the tail lobes.

Pair production in the millisecond pulsar J0030+0451

ABSTRACT The electric field accelerating electrons in the Timokhin–Arons polar-cap model as applied to millisecond pulsars is so high that the electron Lorentz factors are limited either by radiation reaction or by the Breit–Wheeler process. In the former case, it is possible to obtain an upper limit for curvature-radiation momentum components perpendicular to the local magnetic field that is independent of the flux-line radius of curvature. The threshold value for single-photon conversion to pairs is high but is possibly reached in J0030+0451. However, owing to the high polar-cap temperature reported, direct pair production by the Breit–Wheeler process is probably more important. If the existence of coherent radio emission in millisecond pulsars is assumed to need a high-multiplicity pair plasma, then it follows that the primary electrons also produce gamma-rays in the Fermi-LAT energy band for which the magnetosphere is completely transparent. The absence of these, in phase with the radio emission, would be an immediate indication that ultra-high-energy electrons and an active Timokhin–Arons polar cap are not present in J0030+0451.

Magnetospheric Flux Throughput in the Dungey Cycle: Identification of Convection State During 2010

AbstractWe quantify the contributions of different convection states to the magnetic flux throughput of the magnetosphere during 2010. To do this we provide a continuous classification of convection state for the duration of 2010 based upon observations of the solar wind and interplanetary magnetic field, geomagnetic indices, and field‐aligned currents measured by the Active Magnetosphere and Planetary Electrodynamics Response Experiment. Convection states are defined as (1) quiet, (2) weak activity, substorm (3) growth, (4) expansion and (5) recovery phases, (6) substorm driven phase (when relatively steady magnetospheric convection occurs), (7) recovery bays (when recovery phase is accompanied by a negative excursion of the AL electrojet index), and (8) periods of multiple intensifications (storm‐time periods when continuous short‐period AL activity occur). The magnetosphere is quiet for 46% of the time, when very little convection takes place. The majority of convection occurs during growth and driven phases (21% and 38%, respectively, of open magnetic flux accumulation by dayside reconnection). We discuss these results in the context of the expanding/contracting polar cap model of convection, and describe a framework within which isolated substorms and disturbances during periods of more continuous solar wind‐magnetosphere driving can be understood.

The influence of positronium photoionization rate on the heating of J0250+5854 polar cap

The influence of positronium photoionization rate on the heating of PSR J0250+5854 polar cap is considered. It is assumed that the polar cap is heated only by reverse positrons accelerated in pulsar diode. It is supposed that pulsar diode is located near the star surface (polar cap model) and operates in the steady state space charge-limited flow regime. The influence of a small-scale magnetic field on the electric field inside the pulsar diode is taken into account. The reverse positron current is calculated in the framework of two models: rapid and gradually screening. To calculate the production rate of electron-positron pairs we take into account only the curvature radiation of primary electrons and its absorption in magnetic field. It is assumed that some fraction of electron-positron pairs may be created in bound state that can later be photoionized by thermal photons from star surface.

Luminosity of a radio pulsar and its new emission death line

We investigated the pulsar radio luminosity (L), emission efficiency (ratio of radio luminosity to its spin-down power ), and death line in the magnetic field (B) versus spin period (P) diagram. We found that the dependence of pulsar radio luminosity on its spin-down power () is very weak, shown as , which deduces an equivalent inverse correlation between emission efficiency and spin-down power as . Furthermore, we examined the distributions of radio luminosity of millisecond and normal pulsars and found that for the similar spin-down powers, the radio luminosity of millisecond pulsars is about one order of magnitude lower than that of the normal pulsars. The analysis of pulsar radio flux suggests that these correlations are not due to a selective effect but are intrinsic to the pulsar radio emission physics. Their radio radiations may be dominated by the different radiation mechanisms. The cut-off phenomenon of currently observed radio pulsars in B – P diagram is usually referred as the “pulsar death line”, which corresponds to erg s−1 and is obtained by the cut-off voltage of electron acceleration gap in the polar cap model of pulsar proposed by Ruderman and Sutherland. Observationally, this death line can be inferred by the actual observed pulsar flux S ≥ 1mJy and 1 kpc distance, together with the maximum radio emission efficiency of 1%. However, the observation data show that the 37 pulsars pass over the death line, including the recently observed two pulsars with long periods of 23.5 s and 12.1 s, which violate the prediction of the polar cap model. At present, the actual observed pulsar flux can reach 0.01mJy by FAST telescope. This will arise the observational limit of spin-down power of pulsars as low as erg s−1. This means that the new death line is downward shifted two orders of magnitude, which might be favorably referred as the “observational limit–line”. Accordingly, the pulsar theoretical model for the cut-off voltage of gap should be heavily modified.

X-ray light curves from realistic polar cap models: inclined pulsar magnetospheres and multipole fields

ABSTRACT Thermal X-ray emission from rotation-powered pulsars is believed to originate from localized ‘hotspots’ on the stellar surface occurring where large-scale currents from the magnetosphere return to heat the atmosphere. Light-curve modelling has primarily been limited to simple models, such as circular antipodal emitting regions with constant temperature. We calculate more realistic temperature distributions within the polar caps, taking advantage of recent advances in magnetospheric theory, and we consider their effect on the predicted light curves. The emitting regions are non-circular even for a pure dipole magnetic field, and the inclusion of an aligned magnetic quadrupole moment introduces a north–south asymmetry. As the quadrupole moment is increased, one hotspot grows in size before becoming a thin ring surrounding the star. For the pure dipole case, moving to the more realistic model changes the light curves by $5\!-\!10{{\, \rm per\, cent}}$ for millisecond pulsars, helping to quantify the systematic uncertainty present in current dipolar models. Including the quadrupole gives considerable freedom in generating more complex light curves. We explore whether these simple dipole+quadrupole models can account for the qualitative features of the light curve of PSR J0437−4715.

Systematical study of pulsar light curves with special relativistic effects

We systematically study pulsar light curves, taking into account the special relativistic effect, i.e., the Doppler factor due to the fast spin of the neutron stars, together with the time delay, which comes from the difference of the travel times depending on the position of the spots. For this purpose, first we derive the basic equations with the general expression of the metric for the static, spherically symmetric spacetime, where for simplicity we adopt the pointlike spot approximation for the antipodal spots associated with the magnetic polar cap model. Then, we calculate the light curves from the neutron star models in general relativity, with various angle between rotational and magnetic axes and the inclination angle. As the results, unlike the case for a slowly rotating stellar model, we find that the light curve from a fast rotating stellar model depends not only the stellar compactness but also the stellar radius. We also find that the amplitude of the light curve becomes larger as the stellar radius increases and as the stellar compactness decreases. Thus, via careful observations of the light curves from the rotating neutron star, one would determine the stellar compactness together with the stellar radius, if it rotates fast enough.

The influence of the positronium photoionization rate on the polar cap X-ray luminosity of radio pulsars

The influence of the positronium photoionization rate on the polar cap X-ray luminosity of old radio pulsars is considered. It is assumed that the polar cap is heated only by reverse positrons accelerated in the pulsar diode. It is supposed that the pulsar diode is in a stationary state with the lower plate located near the star surface (polar cap model) occupies all the pulsar tube cross section and operates in the regime of steady space charge by the limited electron flow. The influence of a small-scale magnetic field on the electric field inside the pulsar diode is taken into account. The reverse positron current is calculated in the framework of two models: rapid and gradual screening. To calculate the production rate of electron-positron pairs we take into account only the curvature radiation of primary electrons and its absorption in the magnetic field. It is assumed that some fraction of electron-positron pairs is created in a bound state (positronium). Later, such positroniums are photoionized by thermal photons from the polar cap.

Pulse profiles from a pulsar in scalar-tensor gravity

The pulse profile from a neutron star in scalar-tensor theory of gravity is examined for several stellar models, where we assume the existence of the antipodal hot spots on the neutron star based on the polar cap model. Then, we find that the pulse profile from the scalarized neutron star in scalar-tensor gravity is almost the same as that in general relativity, i.e., without a scalar field, if the stellar compactness of the both stars is very similar. That is, the existence of the scalar field does not directly change the pulse profile from the neutron star, while the stellar compactness is crucial for determining the pulse shape even in the scalar-tensor gravity. Additionally, we find that the pulse shape from the scalarized neutron star is more or less similar to that from the neutron star with the same mass in general relativity, while the ratio of the minimum amplitude to the maximum amplitude in the pulse profile depend strongly on the coupling constant in scalar-tensor gravity, depending on the angle between the rotational and magnetic axes and the angle between the rotational axis and the direction to the observer. So, the direct observation of the pulse profile together with the additional observation of the stellar mass, one may extract the imprint of the gravitational theory in strong field regime.

Testing nowcasts of the ionospheric convection from the expanding and contracting polar cap model

AbstractThe expanding/contracting polar cap (ECPC) model, or the time‐dependent Dungey cycle, provides a theoretical framework for understanding solar wind‐magnetosphere‐ionosphere coupling. The ECPC describes the relationship between magnetopause reconnection and substorm growth phase, magnetotail reconnection and substorm expansion phase, associated changes in auroral morphology, and ionospheric convective motions. Despite the many successes of the model, there has yet to be a rigorous test of the predictions or nowcasts made regarding ionospheric convection, which remains a final hurdle for the validation of the ECPC. In this study we undertake a comparison of ionospheric convection, as measured in situ by ion drift meters on board DMSP (Defense Meteorological Satellite Program) satellites and from the ground by SuperDARN (Super Dual Auroral Radar Network), with motions nowcasted by a theoretical model. The model is coupled to measurements of changes in the size of the polar cap made using global auroral imagery from the IMAGE FUV (Imager for Magnetopause to Aurora Global Exploration Far Ultraviolet) instrument, as well as the dayside reconnection rate, estimated using the OMNI data set. The results show that we can largely nowcast the magnitudes of ionospheric convection flows using the context of our understanding of magnetic reconnection at the magnetopause and in the magnetotail.

SEARCH FOR DIFFERENCES BETWEEN RADIO-LOUD AND RADIO-QUIET GAMMA-RAY PULSAR POPULATIONS WITH FERMI-LAT DATA

ABSTRACT Observations by the Fermi-Large Area Telescope (LAT) have enabled us to explore the population of non-recycled gamma-ray pulsars with a set of 112 objects. It was recently noted that there are apparent differences in the properties of radio-quiet and radio-loud subsets. In particular, the average observed radio-loud pulsar is younger than the average radio-quiet one and is located at lower Galactic latitude. Even so, the analysis based on the full list of pulsars may suffer from selection effects. Namely, most radio-loud pulsars are first discovered in the radio band, while radio-quiet ones are found using the gamma-ray data. In this work we perform a blind search for gamma-ray pulsars using the Fermi-LAT data alone, using all point sources from the 3FGL catalog as the candidates. Unlike our previous work, the present catalog is constructed with a semi-coherent method based on the time-differencing technique and covers the full range of characteristic ages down to 1 kyr. The search resulted in a catalog of 40 non-recycled pulsars, 25 of which are radio-quiet. All pulsars found in the search were previously known gamma-ray pulsars. We find no statistically significant differences in age or in distributions in Galactic latitude for the radio-loud and radio-quiet pulsars, while the distributions in rotation period are marginally different with a statistical probability of . The fraction of radio-quiet pulsars is estimated as . The results are in agreement with the predictions of the outer magnetosphere models, while the polar cap models are disfavored.

Young and middle age pulsar light-curve morphology: Comparison ofFermiobservations withγ-ray and radio emission geometries

Thanks to the huge amount of gamma-ray pulsar photons collected by the Fermi Large Area Telescope since June 2008, it is now possible to constrain gamma-ray geometrical models by comparing simulated and observed light-curve morphological characteristics. We assumed vacuum-retarded dipole pulsar magnetic field and tested simulated and observed morphological light-curve characteristics in the framework of two pole emission geometries, Polar Cap (PC), radio, and Slot Gap (SG), and Outer Gap (OG)/One Pole Caustic (OPC) emission geometries. We compared simulated and observed/estimated light-curve morphological parameters as a function of observable and non-observable pulsar parameters. The PC model gives the poorest description of the LAT pulsar light-curve morphology. The OPC best explains both the observed gamma-ray peak multiplicity and shape classes. The OPC and SG models describe the observed gamma-ray peak-separation distribution for low- and high-peak separations, respectively. This suggests that the OPC geometry best explains the single-peak structure but does not manage to describe the widely separated peaks predicted in the framework of the SG model as the emission from the two magnetic hemispheres. The OPC radio-lag distribution shows higher agreement with observations suggesting that assuming polar radio emission, the gamma-ray emission regions are likely to be located in the outer magnetosphere. The larger agreement between simulated and LAT estimations in the framework of the OPC suggests that the OPC model best predicts the observed variety of profile shapes. The larger agreement between observations and the OPC model jointly with the need to explain the abundant 0.5 separated peaks with two-pole emission geometries, calls for thin OPC gaps to explain the single-peak geometry but highlights the need of two-pole caustic emission geometry to explain widely separated peaks.

Polar caps in the presence of an induction field

Following the early paper of Goldreich & Julian (1969), polar-cap models have usually assumed that the closed sector of a pulsar magnetosphere corotates with the neutron star. Recent work by Melrose & Yuen has been a reminder that in an oblique rotator, the induction field arising from the time-varying magnetic flux density cannot be completely screened. The principal consequence is that the plasma does not corotate with the star. Here it is shown that the physics of the polar cap is not changed at the altitudes of the radio emission source. But the presence of a plasma drift velocity in the corotating frame of reference does provide a mechanism whereby the net charge of the star can be maintained within a stable band of values. It also shows directly how electron injection and acceleration occur in the outer gap of the magnetosphere. It is consistent with radio-loud pulsars in the Fermi LAT catalogue of gamma-emitters all having positive polar-cap charge density.

Blind search for radio-quiet and radio-loud gamma-ray pulsars with Fermi-LAT data

The Fermi Large Area Telescope (LAT) has observed more than a hundred of gamma-ray pulsars, about one third of which are radio-quiet, i.e., not detected at radio frequencies. The most of radio-loud pulsars are detected by Fermi LAT by using the radio timing models, while the radio-quiet ones are discovered in a blind search. The difference in the techniques introduces an observational selection bias and, consequently, the direct comparison of populations is complicated. In order to produce an unbiased sample, we perform a blind search of gamma-ray pulsations using Fermi-LAT data alone. No radio data or observations at optical or X-ray frequencies are involved in the search process. We produce a gamma-ray selected catalog of 25 non-recycled gamma-ray pulsars found in a blind search, including 16 radio-quiet and 9 radio-loud pulsars. This results in the direct measurement of the fraction of radio-quiet pulsars ɛRQ = 64 ± 10%, which is in agreement with the existing estimates from the population modeling in the outer magnetosphere model. The Polar cap models are disfavored due to a lower expected fraction and the prediction of age dependence. The age, gamma-ray energy flux, spin-down luminosity and sky location distributions of the radio-loud and radio-quiet pulsars from the catalog do not demonstrate any statistically significant difference. The results indicate that the radio-quiet and radio-loud pulsars belong to one and the same population. The catalog shows no evidence for the radio beam evolution.