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G G Pavlov - One of the best experts on this subject based on the ideXlab platform.
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the compact central object in cassiopeia a a neutron star with hot Polar Caps or a black hole
The Astrophysical Journal, 2000Co-Authors: G G Pavlov, V E Zavlin, B Aschenbach, J Trumper, D SanwalAbstract:The central pointlike X-ray source of the Cassiopeia A supernova remnant was discovered in the Chandra first light observation and found later in the archival ROSAT and Einstein images. The analysis of these data does not show statistically significant variability of the source. Because of the small number of photons detected, different spectral models can fit the observed spectrum. The power-law fit yields the photon index γ = 2.6-4.1, and luminosity L(0.1-5.0 keV) = (2-60) × 1034 ergs s-1 for d = 3.4 kpc. The power-law index is higher, and the luminosity lower, than those observed from very young pulsars. One can fit the spectrum equally well with a blackbody model with T = 6-8 MK, R = 0.2-0.5 km, and Lbol = (1.4-1.9) × 1033 ergs s-1. The inferred radii are too small, and the temperatures too high, for the radiation to be interpreted as emitted from the whole surface of a uniformly heated neutron star. Fits with the neutron star atmosphere models increase the radius and reduce the temperature, but these parameters are still substantially different from those expected for a young neutron star. One cannot exclude, however, the possibility that the observed emission originates from hot spots on a cooler neutron star surface. An upper limit on the (gravitationally redshifted) surface temperature is T < 1.9-2.3 MK, depending on the chemical composition of the surface and the star's radius. Among several possible interpretations, we favor a model of a strongly magnetized neutron star with magnetically confined hydrogen or helium Polar Caps (T ≈ 2.8 MK, Rpc ≈ 1 km) on a cooler iron surface (T ≈ 1.7 MK). Such temperatures are consistent with the standard models of neutron star cooling. Alternatively, the observed radiation may be interpreted as emitted by a compact object (more likely, a black hole) accreting from a residual disk or from a late-type dwarf in a close binary.
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soft x rays from Polar Caps of the millisecond pulsar j0437 4715
Astronomy and Astrophysics, 1998Co-Authors: Vyacheslav E Zavlin, G G PavlovAbstract:We show that the soft X-ray spectra and light curves observedwiththeROSAT andEUV Efromtheclosestknown millisecond pulsar J0437-4715 can be interpreted as thermal ra- diation from two hot Polar Caps whose emitting layers (atmo- spheres) are comprised of hydrogen. The simplest model yields a uniform temperature of (0:8 0:9) 10 6 K within a cap radius of 0:7 0:9 km. The spectral ts indicate that the tem- peraturemaybenonuniformlydistributedalongtheCapsurface. The distribution can be approximated by a central core heated up to (1 2) 10 6 K within a radius of 0:2 0:4 km, sur- rounded by a colder rim with temperatures (3 5) 10 5 K extending out to 2 6 km. The Polar cap interpretation implies low column densities, (1 3) 10 19 cm 2 , and a high degree of ionization, > 20%, of the interstellar hydrogen towards the pulsar. The inferred bolometric luminosity of the Polar Caps, (1:0 1:6) 10 30 erg s 1 , is in excellent agreement with the predictions of the slot-gap model of radio pulsars developed by Arons and his coworkers. Similar Polar cap radiation should be emitted by other millisecond pulsars, although in some of them (e. g., PSR B1821-24) the soft X-ray flux is dominated by the nonthermal radiation from pulsar magnetospheres.
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soft x rays from Polar Caps of the millisecond pulsar j0437 4715
arXiv: Astrophysics, 1997Co-Authors: Vyacheslav E Zavlin, G G PavlovAbstract:We show that the soft X-ray spectra and light curves observed with the ROSAT and EUVE from the closest known millisecond pulsar J0437--4715 can be interpreted as thermal radiation from two hot Polar Caps whose emitting layers (atmospheres) are comprised of hydrogen. The simplest model yields a uniform temperature of $(0.8-0.9)\times 10^6 K$ within a cap radius of $0.7-0.9 km$. The spectral fits indicate that the temperature may be nonuniformly distributed along the cap surface. The distribution can be approximated by a central core heated up to $(1-2)\times 10^6 K$ within a radius of $0.2-0.4 km$, surrounded by a colder rim with temperatures $(3-5)\times 10^5 K$ extending out to $2-6 km$. The Polar cap interpretation implies low column densities, $(1-3)\times 10^{19} cm^{-2}$, and a high degree of ionization, $> 20$%, of the interstellar hydrogen towards the pulsar. The inferred bolometric luminosity of the Polar Caps, $(1.0-1.6)\times 10^{30} erg s^{-1}$, is in excellent agreement with the predictions of the slot-gap model of radio pulsars developed by Arons and his coworkers. Similar Polar cap radiation should be emitted by other millisecond pulsars, although in some of them (e.~g., PSR B1821--24) the soft X-ray flux is dominated by the nonthermal radiation from pulsar magnetospheres.
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constrains on the mass and radius of pulsars from x ray observations of their Polar Caps
arXiv: Astrophysics, 1997Co-Authors: G G Pavlov, V E ZavlinAbstract:The properties of X-ray radiation from the Polar Caps predicted by the radio pulsar models depend on the surface chemical composition, magnetic field and star's mass and radius as well as on the cap temperature, size and position. Fitting the radiation spectra and light curves with the neutron star atmosphere models enables one to infer these parameters. We present here results obtained from the analysis of the soft X-ray radiation of PSR J0437-4715. In particular, with the aid of radio Polarization data, we put constraints on the pulsar mass-to-radius ratio.
A I Tsygan - One of the best experts on this subject based on the ideXlab platform.
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Emission of particles and photons in the pulsar Polar cap
International Astronomical Union Colloquium, 2020Co-Authors: A I TsyganAbstract:AbstractWe study emission of particles and photons from a pulsar Polar cap. The Goldreich-Julian model for the regime of free emission of charged particles from the neutron star surface is used. In this case the electric field is generated due to the general relativistic effect of dragging of inertial frames. The spectra and shapes of gamma-ray pulses, the parameters of the electron-positron plasma and the intensity of X-ray emission from hot spots in the Polar region of radio pulsars are discussed. The effect of non-dipole magnetic field on X-ray emission of Polar Caps is considered. It is shown that the increase of magnetic line curvature leads to much smaller temperatures and X-ray luminosities of the Polar Caps as compared with the purely dipole field.
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heating of the Polar Caps of old radio pulsars
Astronomy Letters, 2017Co-Authors: A I TsyganAbstract: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.
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on the differential rotation of the Polar Caps of neutron stars
Astronomy Letters, 2014Co-Authors: A I Tsygan, D P Barsukov, O A Goglichidze, D A ShalbykovAbstract: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).
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general relativistic electric potential drops above pulsar Polar Caps
Monthly Notices of the Royal Astronomical Society, 1992Co-Authors: A G Muslimov, A I TsyganAbstract:We study the general relativistic electrodynamics of an isolated, rotating, magnetic neutron star. We consider the region of a neutron star magnetosphere with steady, space charge limited flow along open magnetic field lines. The explicit solutions to the Maxwell equations are obtained. Being the simplest, this model enables one to carry out analytically a general relativistic treatment, and to demonstrate the influence of the effects of General Relativity on the creation of an electric field in the afore-mentioned region.
Bing Zhang - One of the best experts on this subject based on the ideXlab platform.
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Thermal radiation from hot Polar cap in pulsars
2020Co-Authors: George I Melikidze, Bing ZhangAbstract:Thermal radiation from hot Polar Caps is examined in radio pulsars with drifting subpulses. It is argued that if these subpulses correspond to sparking discharges of the inner acceleration region right above the Polar cap surface then a simple relationship between the observed subpulse drift rate in radio and thermal X-ray luminosity from the Polar cap heated by sparks should exist. This relationship is derived and tested in pulsars for which an appropriate good quality data is available.
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x ray pulsar radiation from Polar Caps heated by back flow bombardment
Monthly Notices of the Royal Astronomical Society: Letters, 2007Co-Authors: George I Melikidze, Bing ZhangAbstract:We consider the problem of the thermal X-ray radiation from the hot Polar Caps of radio pulsars that show evidence of E×B subpulse drift in radio band. In our recent Paper I, using the partially screened gap (PSG) model of inner acceleration region we derived a simple relationship between the drift rate of subpulses observed in a radio-band and the thermal X-ray luminosity from Polar Caps heated by the back-flow particle bombardment. This relationship can be tested for pulsars in which the so-called carousel rotation time P4, reflecting the E×B plasma drift, and the thermal X-ray luminosity Lx from the hot Polar cap are known. To test the model we used only two available pulsars: PSRs B0943+10 and B1133+16. They both satisfied the model prediction, although due to low photon statistics the thermal component could not be firmly identified from the X-ray data. Nevertheless, these pulsars were at least consistent with PSG pulsar model. In this Letter we consider two more pulsars: PSRs B0656+14 and B0628−28, the data for which have recently become available. In PSR B0656+14 the thermal radiation from the hot Polar cap was clearly detected, and PSR B0628−28 also seems to have such a component. In all cases for which both P4 and Lx are presently known, the PSG pulsar model seems to be fully confirmed. Other available models of inner acceleration region fail to explain the observed relationship between radio and X-ray data. The pure vacuum gap model predicts too high Lx and too low P4, while the space charge limited model predicts too low Lx and the origin of the subpulse drift has no natural explanation.
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high energy accelerators above pulsar Polar Caps
arXiv: Astrophysics, 1999Co-Authors: Renxin Xu, G J Qiao, Bing ZhangAbstract:Similar to the terrestrial collision accelerators of electron-antielectron, another kind of accelerator is above a positively or negatively charged pulsar Polar cap. In the case of pulsars with magnetic axis parallel (anti-parallel) to rotational axis, relativistic antielectron (electron) with Lorentz factor about 10^6 hit the electrons in the Polar Caps. These scenarios are investigated both for pulsars being bare strange stars and for pulsars being neutron stars. Such a study may be valuable to differentiate neutron stars and bare strange stars observationally.
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two photon annihilation in the pair formation cascades in pulsar Polar Caps
Astronomy and Astrophysics, 1998Co-Authors: Bing Zhang, G J QiaoAbstract:The importance of the photon-photon pair produc- tion process ( + 0 ! e + + e ) to form pair production cas- cades in pulsar Polar Caps is investigated within the framework of the Ruderman-Sutherland vacuum gap model. It is found that this process is unimportant if the Polar Caps are not hot enough, but will play a non-negligible role in the pair formation cascades when the Polar cap temperatures are in excess of the critical tem- peratures, Tcri, which are around410 6 K when P =0 :1s and will slowly increase with increasing periods. Compared with the B process, it is found that the two-photon annihilation pro- cess may ignite a central spark near the magnetic pole, where B sparks can not be formed due to the local weak curva- tures. This central spark is large if the gap is dominated by the "resonant ICS mode". The possible connection of these central sparks with the observed pulsar "core" emission components is discussed.
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two photon annihilation in the pair formation cascades in pulsar Polar Caps
arXiv: Astrophysics, 1998Co-Authors: Bing Zhang, G J QiaoAbstract:The importance of the photon-photon pair production process ($\gamma+ \gamma^{\prime}\to e^{+}+e^{-}$) to form pair production cascades in pulsar Polar Caps is investigated within the framework of the Ruderman-Sutherland vacuum gap model. It is found that this process is unimportant if the Polar Caps are not hot enough, but will play a non-negligible role in the pair formation cascades when the Polar cap temperatures are in excess of the critical temperatures, $T_{cri}$, which are around $4\times 10^6K$ when $P=0.1$s and will slowly increase with increasing periods. Compared with the $\gamma-B$ process, it is found that the two-photon annihilation process may ignite a central spark near the magnetic pole, where $\gamma-B$ sparks can not be formed due to the local weak curvatures. This central spark is large if the gap is dominated by the ``resonant ICS mode''. The possible connection of these central sparks with the observed pulsar ``core'' emission components is discussed.
M Jardine - One of the best experts on this subject based on the ideXlab platform.
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the impact of meridional circulation on stellar butterfly diagrams and Polar Caps
Monthly Notices of the Royal Astronomical Society, 2006Co-Authors: V Holzwarth, D H Mackay, M JardineAbstract: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 ≤ 25 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°).
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the impact of meridional circulation on stellar butterfly diagrams and Polar Caps
arXiv: Astrophysics, 2006Co-Authors: V Holzwarth, D H Mackay, M JardineAbstract: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 sub-surface and surface magnetic flux transport simulations to investigate (i) the impact of meridional circulations with peak velocities of <125m/s 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 about 300km experience an enhanced poleward deflection through meridional flows. 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% 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 on the order of 100m/s, and the latitudinal range from which magnetic flux tubes originate at the base of the convective zone (<50degrees) must be larger than in the solar case (<35degrees).
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Polar Caps on active stars magnetic flux emergence and transport
Monthly Notices of the Royal Astronomical Society, 2004Co-Authors: D H Mackay, Collier A Cameron, M Jardine, J F Donati, G A J HussainAbstract:In recent years, Zeeman Doppler imaging of rapidly rotating solar-like stars has shown high-latitude magnetic flux patterns of intermingled magnetic Polarities. Such high-latitude intermingling of positive and negative flux is inconsistent with our present understanding of how magnetic flux emerges on the Sun and is transported poleward. To determine how these patterns may arise, magnetic flux transport simulations are carried out. These simulations follow the evolution of the radial magnetic field at the surface of the star as new magnetic bipoles emerge and are advected poleward by the surface effects of differential rotation, meridional flow and supergranular diffusion. To produce intermingling of flux at high latitudes, key parameters such as the emergence profiles and transport coefficients are varied from presently used solar values. In doing so, it is found that, in order to explain the high-latitude intermingling, at least two of these parameters must be changed. First, the emergence profile must be extended to higher latitudes (λ = 50°-70° ), and secondly the value of the meridional flow must be increased by around a factor of 10 (∼100 m s -1 ). The results show that the observed intermingling of high-latitude flux can only occur through a flux emergence and transport process that is significantly different from that which occurs on the Sun. Observable features produced by both of these changes are considered, and the significance of the simulations to future observing programmes discussed. Finally the emergence profile and transport coefficients that best fit the observations of the young active star AB Dor (period = 0.514 d) are put forward.
Bernhard Lee Lindner - One of the best experts on this subject based on the ideXlab platform.
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the hemispherical asymmetry in the martian Polar Caps
Journal of Geophysical Research, 1993Co-Authors: Bernhard Lee LindnerAbstract:An energy balance model is used to study the behavior of CO2 ice on Mars. The effect of the solar zenith angle dependence of albedo is to lengthen CO2 ice lifetimes at the poles. Hemispherical asymmetries in cloud and dust abundance could result in the survival of seasonal CO2 ice through summer in the south and not in the north, in agreement with observations. CO2 ice observed in the summertime Polar cap in the south could be of recent origin, although a permanent CO2 Polar cap cannot be ruled out.
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Simulations of the seasonal Polar Caps on Mars
1992Co-Authors: Bernhard Lee LindnerAbstract:One of the most puzzling mysteries about the planet Mars is the hemispherical asymmetry in the Polar Caps. Every spring the seasonal Polar cap of CO2 recedes until the end of summer, when only a small part, the residual Polar cap, remains. During the year that Viking observed Mars, the residual Polar cap was composed of water ice in the Northern Hemisphere (Kieffer et al., Science, 194, 1341, 1976), but was primarily carbon dioxide ice in the Southern Hemisphere (Kieffer, J. Geophys. Res., 84, 8263, 1979). Scientists have sought to explain this asymmetry by modeling observations of the latitudinal recession of the Polar cap and seasonal variations in atmospheric pressure (since the seasonal Polar Caps are primarily frozen atmosphere, they are directly related to changes in atmospheric mass). These models reproduce most aspects of the observed annual variation in atmospheric pressure fairly accurately. Furthermore, the predicted latitudinal recession of the northern Polar cap in the spring agrees well with observations, including the fact that CO2 ice is predicted to completely sublime away. However, these models all predict that the carbon dioxide ice will also sublime away during the summer in the Southern Hemisphere, unlike what is observed. It is shown here how the radiative effects of ozone, clouds, and airborne dust, light penetration into and through the Polar cap, and the dependence of albedo on solar zenith angle affect CO2 ice formation and sublimation, and how they help explain the hemispherical asymmetry in the residual Polar Caps. These effects have not been studied with prior Polar cap models. The combination of the effects of solar zenith angle on albedo and the radiative effects of clouds and dust act to extend the lifetime of CO2 ice on the south pole relatively more than on the north pole, possibly explaining the hemispherical asymmetry in the residual Polar Caps without the need of a hemispherical asymmetry in Polar cap albedo. This does not imply that a hemispherical asymmetry in Polar cap albedo does not exist, but that one is not necessary. Observations of the regression of the Polar Caps and the annual cycle in atmospheric pressure are reproduced fairly well by the model, as shown in the figures, although further improvement is needed. When CO2 ice is retained at the south pole, the model predictions of the annual cycle in atmospheric pressure have a phase shift relative to the data, no matter what model input parameters are used. We are investigating other processes not included in prior Polar cap models.
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Why is the north Polar cap on Mars different than the south Polar cap
1992Co-Authors: Bernhard Lee LindnerAbstract:One of the most puzzling mysteries about the planet Mars is the hemispherical asymmetry in the Polar Caps. Every spring the seasonal Polar cap of CO2 recedes until the end of summer, when only a small part, the residual Polar cap, remains. During the year that Viking observed Mars, the residual Polar cap was composed of water ice in the northern hemisphere but was primarily carbon dioxide ice in the southern hemisphere. Scientists have sought to explain this asymmetry by modeling observations of the latitudinal recession of the Polar cap and seasonal variations in atmospheric pressure (since the seasonal Polar Caps are primarily frozen atmosphere, they are directly related to changes in atmospheric mass). These models reproduce most aspects of the observed annual variation in atmospheric pressure fairly accurately. Furthermore, the predicted latitudinal recession of the northern Polar cap in the spring agrees well with observations, including the fact that the CO2 ice is predicted to completely sublime away. However, these models all predict that the carbon dioxide ice will also sublime away during the summer in the southern hemisphere, unlike what is observed. This paper will show how the radiative effects of ozone, clouds, airborne dust, light penetration into and through the Polar cap, and the dependence of albedo on solar zenith angle affect CO2 ice formation and sublimation, and how they help explain the hemispherical asymmetry in the residual Polar Caps. These effects have not been studied with prior Polar cap models.
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The Martian Polar cap: Radiative effects of ozone, clouds, and airborne dust
Journal of Geophysical Research, 1990Co-Authors: Bernhard Lee LindnerAbstract:The solar and thermal flux striking the Polar cap of Mars is computed for various ozone, dust, and cloud abundances and for three solar zenith angles. Ozone does not significantly affect the total energy budget of the Polar cap. Hence the observed hemispherical asymmetry in ozone abundance causes only an insignificant hemispherical asymmetry in the Polar Caps. Vertical optical depths of dust and cloud ranging from zero to 1 cause little change in the total flux absorbed by the Polar cap near its edge but increase the absorbed flux significantly as one travels poleward. Hemispherical asymmetries in dust abundance, cloud cover, and surface pressure combine to cause a significant hemispherical asymmetry in the total flux absorbed by the residual Polar Caps, which helps to explain the dichotomy in the residual Polar Caps on Mars. Other processes which affect the energy budget of the Polar cap are proposed and reviewed, particularly with respect to their interaction with the radiative effects of clouds and dust.
