Precipitation of auroral particles in the central polar cap during a sudden commencement magnetic storm

All-sky camera observations from two stations in the inner (northern) polar cap and an auroral zone station are combined with photometer records from the polar cap station Nord in a study of the brilliant auroral display following the ssc of the storm of 7 November 1970. This display is the large, poleward expanding bulge of a substorm triggered by the ssc. It is composed of brilliant discrete forms embedded in low-intensity diffuse electron and proton aurora. The poleward edge of the diffuse electron aurora is 5° north of the discrete auroras and 3° north of the proton aurora. The intensity of the discrete aurora varies as the strength of the auroral electrojet as shown by magnetograms from auroral zone stations. Succeeding the retreating display a subvisible low-energy electron precipitation, which may be identified as the polar squall (Winningham and Heikkila, 1974) is observed over the polar cap during the main phase of the storm.In the early morning sector already existing diffuse auroras broaden towards the equator from the time of the ssc and at least during the following half hour.Ssc-triggered displays have been found (Feldstein, 1959) to withdraw from the inner polar cap as the initial (positive H) phase of the storm ends. A comparison of the records from seven low-latitude stations shows that during this particular storm the positive phase appears to be composed by two overlapping disturbances, i.e. the proper initial phase, which is generally thought to be due to compression of the inner magnetosphere and a series of positive bays accompanying the negative bays in auroral latitudes. These positive bays are observable over a great range of longitudes with a maximum of amplitude near midnight. As judged from the dayside magnetograms the initial (compression) phase ends at an early stage of the substorm. The observed coincidence between the withdrawal of the display and the cessation of the positive H phase of the storm is a consequence of the fact that the second component—the positive bays—and the auroral display over the polar cap are both signatures of the substorm activity.     

The excitation of the 557.7 nm line and Herzberg bands in the nightglow

Measurements of the emission intensities of the 557.7 nm line and Herzberg bands and of O(³P) concentrations carried out on two coordinated rocket flights at South Uist during the night of 8/9 September 1975 are presented. An examination of the 557.7 nm emission and $\text{O}\text{(}{}^3\text{P}\text{)}$ data on the basis of recent data on relevant rate coefficients has shown that the results can be interpreted on the basis of the Barth mechanism for the production of $\text{O}\text{(}{}^1\text{S}\text{)}$ atoms but not the Chapman mechanism. Evidence is provided that the A³Σ⁺_u state of O₂ could be responsible for the $\text{O}\text{(}{}^1\text{S}\text{)}$ production in the Barth mechanism. Values of the rate coefficients involved are deduced from a comparison of the 557.7 nm and Herzberg emission rates.     

Description of the Martian polar cap breeze

The surface temperature of the Martian polar caps is about 148 K (frost point temperature of CO₂ at a surface pressure of about 6 hPa), with the “desert” (frost-free) areas adjacent to the polar caps having much greater surface temperatures. The existence of this steep meridional gradient of temperature between the polar caps and the adjacent “desert” areas may produce in the atmosphere a baroclinic instability which generates an atmospheric circulation system similar in some aspects to the terrestrial sea breeze. We have called this circulation system the Martian polar cap breeze. In this paper, the phenomenology of the Martian polar cap breeze is developed on the basis of the indirect observational evidence. Along with friction and the Coriolis force, other factors influence the polar cap breeze: the prevailing wind, topography, irregularity of the polar cap-edge, and stability of the atmosphere. These factors are studied in a qualitative form, as well as the seasonal variations. In addition, the large-scale polar cap wind is presented as a different Martian atmospheric circulation system.     

Effects of the passage of an IMF discontinuity on the polar cap geometry and the formation of a polar cap arc

Changes of the geometry of the open field line region (namely, the polar cap) caused by the passage of a tangential IMF discontinuity are simulated using the model constructed by Akasofu and Roederer (1983). A singly-bounded open field line region tends to split into two, forming a narrow closed field line region and thus allowing the formations of a plasma sheet and of an auroral arc across the highest latitude region of the Earth. The three-dimensional geometry of some of the closed field lines in the narrow closed region is examined. In this connection, an interesting observation of the formation of an auroral arc over Thule, Greenland, is reported.     

Dependence of the polar cap geometry on the IMF

The geometry of the open field line region in the polar region is computed for a variety of the interplanetary magnetic field (IMF) orientation. The open field line region can be identified as the area bounded by the auroral oval, namely the polar cap. The polar cap geometry varies considerably with the orientation of the IMF and magnitude, particularly when the IMF B_z component is positive and large. The corresponding exit points of the open field lines on the magnetopause are also examined. The results will be a useful guide in interpreting various upper atmospheric phenomena in the highest latitude region of the Earth and also in observing chemical releases outside the magnetopause.     

Interannual variability of Mars' south polar cap

Telescopic data on the twentieth-century regressions of Mars' south polar cap have been reexamined for evidence of interannual variability. Several regressions, particularly that of 1956, are found to differ significantly from the mean. The possibility of correlations with major dust storms is explored.     

A classification of polar cap auroral arcs

Global auroral imagery obtained by DMSP satellites during the years 1972–1979 over both the northern and southern high latitude polar regions were examined to study the morphology of the discrete arcs known as polar cap arcs. Based upon their morphology, the polar cap arcs can be generally classified into three types viz. (1) the distinctly sun-aligned polar cap arcs—Type 1 arcs, (2) the morning/evening polar cap arcs expanded from the auroral oval—Type 2 arcs and (3) the hook shaped arcs connecting the polar cap arc with the oval arc (including the hitherto unreported oppositely oriented hook shaped arcs)—Type 3 arcs. Concurrent auroral electrojet indices (AE) and interplanetary magnetic field (IMF) data were used to study the occurrence of the polar cap arcs. It was found that Type 1 arcs were observed mostly during low geomagnetic activity conditions, bright Type 2 arcs during the recovery phase of the substorms and Type 3 arcs do not occur during the recovery phase of the substorm. Over both hemispheres, the polar cap arcs were observed mostly during northward IMF. Furthermore, Type 1 arcs were obeserved over the northern polar cap during mostly negative B_x periods and over the southern polar cap during mostly positive B_x periods. The latter observation suggests that these types of arcs may be non-conjugate.     

A numerical simulation of the Martian polar cap breeze

A mathematical model for the Martian polar cap breeze was constructed in part from work previously done by others on the terrestrial sea breeze. With this model a numerical simulation corresponding to the Southern Hemisphere winter season was made. The results obtained with the proposed model show that the Martian polar cap breeze is a well defined system with some similarities to the terrestrial sea breeze. At the time of maximum intensity, the largest values of vertical velocities are about 10 cm/s and occur at heights between 850–1250 m. The largest values of horizontal velocities are about 15 m/s. A polar cap breeze front is clearly discernible in the results. The rate of advance of this front is at an average of about 10 km/h.     

A numerical model of the Martian polar cap winds

An investigation of the Martian polar cap winds and their response to a variety of factors is carried out by a series of numerical experiments based on a zonally symmetric primitive equation model. These factors are the seasonal thermal forcing, mass exchange between polar caps and atmosphere, large-scale topography, and polar cap size. The thermal forcing sets up a circulation whose surface winds adjust to achieve angular momentum balance, with low-latitude easterlies and high-latitude westerlies. The maximum westerlies occur roughly where the horizontal temperature gradients are largest. This pattern changes when cap and atmosphere exchange mass. Corriolis forces acting on the net outflow or inflow produce easterlies at the surface during spring (outflow) and westerlies during winter (inflow). Topography appears to have a small effect, but cap size does play a role, the circulation intensity increasing with cap size. Peak surface winds occur when outflow or inflow is a maximum and are 20 m sec^?1 during spring and 30 m sec^?1 during winter for the northern hemisphere. The model results show that surface winds near the edge of a retreating polar cap are substantially enhanced, a result which is consistent with the Viking observations of local dust storm activity near the edge of the south polar cap during spring. The results also indicate that the surficial wind indicators near the south pole are formed during spring and those near the north pole during winter. The implication is that the high-latitude dune fields in the northern hemisphere are formed at a time when the terrain is being covered with frost. It is therefore suggested that the saltating particles are “snowflakes” which have formed by the mechanism proposed by Pollack etal. The model results for the winter simulation, which have formed by the mechanism transport by large-scale eddies, compare favorably with general circulation model (GCM) calculations. This suggests that the eddy transports may be less important than those associated with the net mass flow, and that 2-D climate modeling may be more succesful for Mars than Earth.     

Tracking the edge of the south seasonal polar cap of Mars

The advance and retreat of the polar caps were one of the first observations that indicated Mars had seasons. Because a large portion of the atmosphere is cycled in and out of the seasonal caps during the year, the frost deposits play a significant role in regional and global atmospheric circulation. Understanding the nature of the seasonal polar caps is imperative if we are to understand the current Martian climate. In this study, we track the seasonal cap edges as a function of season and longitude for the fall and winter seasons (MY27), using data from the Planetary Fourier Spectrometer (PFS) onboard the Mars Express (MEX) ESA mission. Making use of the rapid rise (decrease) in surface temperature that occurs when CO₂ ice is removed (deposited), in a first approach, we defined the advancing cap edge to be where the surface temperature drops below 150 K, and the retreating cap edge where the surface temperature rises above 160 K. In this case, starting from Ls∼50°, the edge progression speed start to be longitude dependent. In the hemisphere that extends form the eastern limit of the Hellas basin to the western limit of the Argyrae basin (and containing the two) the edges progression speed is about a half than that of the other hemisphere; the cap is thus asymmetric and, unexpectedly, no CO₂ ice seems to be present inside the basins. This is because the above mentioned surface temperatures used in this approach to detect the cap edges are not adequate (too low) for the high-pressure regions inside the basins where, following the Clausius–Clapeyron's law, the CO₂ condensation temperature can be several degrees higher than that of the adjacent lower-pressure regions. In the second, final approach, special attention has been given to this aspect and the advancing and retreating cap edges are defined where, respectively, the surface temperatures drop below and rise above the CO₂ condensation temperature for the actual surface pressure values. Now, the results show an opposite situation than the previous one, with the progression speed being higher and the cap more extended (up to −30° latitude) in the hemisphere containing the two major Martian basins. During the fall season, up to Ls∼50° the South Martian polar cap consists of CO₂ frost deposits that advance towards lower latitudes at a constant speed of 10° of latitude per 15 degrees of Ls. The maximum extension (−40° latitude) of the South polar cap occurs somewhere in the 80°–90° Ls range. At the winter solstice, when the edges of the polar night start moving poleward, the cap recession has already started, in response to seasonal changes in insolation. The CO₂ ice South polar cap will recede with a constant speed of ∼5° of latitude every 25° degrees of Ls during the whole winter. The longitudinal asymmetries reduce during the cap retreat and completely disappear around Ls=145°.     

PFS-MEX observation of ices in the residual south polar cap of Mars

The Mars Express spacecraft has a highly inclined orbit around Mars and so has been able to observe the south pole of Mars in illuminated conditions at the end of the southern summer (L_s=330). Spectra from the planetary Fourier spectrometer (PFS) short wavelength (SW) channel were recorded over the permanent ice cap to study its composition in terms of CO₂ ice and H₂O ice. Models are fitted to the observed data, which include a spatial mixture of soil (not covered by ice) and CO₂ frost (with a specific grain size and a small amount of included dust and H₂O ice). Two different kinds of spectra were observed: those over the permanent polar cap with almost pure CO₂ ice, negligible water ice, no soil fraction required, and bright; and those over mixed terrain (at the edge of the cap or near troughs) containing a significant soil spatial fraction, more water ice and smaller CO₂ grain size. The amount of water ice given by fits to scaled albedo models is less than 10 ppm by weight. When using multi-stream reflectance models with the appropriate lighting geometry, the water amount must be 2-5 times greater than the albedo fit (less than 50 ppm). At the periphery of the residual polar cap, we found a region almost completely covered by water frost, modeled as a mixture of micron-sized and sub-mm sized grains. Our result using a granular mixture of micron-sized grains of water ice and dust with the CO₂ grains is different from the modeling of OMEGA polar cap observations using molecular mixtures.     

Convection in the polar cap after the interplanetary magnetic field becomes northward

As shown by Iwasaki (1971); Maezawa (1976); Kuznetsov and Troshichev (1977) and other investigators, the electric field and the plasma convection in the polar cap change their direction after an appearance of a significant northward component of the interplanetary magnetic field. Two possible mechanisms of this phenomenon may be suggested: (i) the direct penetration of the dusk-to-dawn electric field from the solar wind into the magnetosphere, and (ii) the generation of the observed electric field and convection in a process of the decay of the three-dimensional current system which existed before the appearance of the northward interplanetary field. The latter mechanism implies that the value of the electric field generated in the polar cap will decrease with time after the appearance of the northward interplanetary magnetic field. The results of the experimental investigation show such a decrease which favours the second mechanism.     

Monitoring the perennial martian northern polar cap with MGS MOC

We have used the Mars Global Surveyor Mars Orbiter Camera Wide Angle (MGS MOC WA) dataset to study albedo trends on the martian northern residual cap. Six study regions were selected, the Chasma Boreale source region, three regions near the center of the cap (“fish hook” region, latitude = 87°; “bottle opener” region, latitude = 87°, “steep-shallow” region, latitude = 85°), and two lower latitude regions (crater, latitude = 77°, and polar outlier, latitude = 82°), and the albedos of these six regions were examined. These regions were chosen due to their good temporal coverage in the MOC dataset, as well as having been studied by other researchers (Bass et al., 2000, Icarus 144, 382-396; Calvin and Titus, 2004, Lunar Planet. Sci. XXXV, Abstract 1455). The picture which emerges is complex. Most areas experience a combination of darkening and brightening through the northern summer; only one area consistently brightens (the polar outlier region). A good deal of interannual repeatability in each region's albedo behavior is seen, however. Possible causes for the observed complex behaviors include dust deposition from late summer storms, sintering of frost grains over the course of the summer, and cold trapping of volatiles on bright, cold surfaces.     

On the nature of polar cap magnetic activity during undisturbed periods

The correlation between the polar cap geomagnetic variations (H-traces) and the changes of the azimuthal (Y_SE) and vertical (Z_SE) components of the interplanetary magnetic field (IMF) during undisturbed periods is examined. It is shown that peak-to-peak correlation between Y_SE and geomagnetic horizontal component variations may be generally observed in the daytime cusp region, independently of the magnitude and polarity of the Z_SE. The existence of the DP₃ disturbances associated with the northward component Z_SE > 0 is confirmed. It is shown that the disturbances due to the vertical component of the IMF dominate in the region near the pole. In so far as the southward component of the IMF generates both polar cap disturbances and geomagnetic substorms, the disturbances in the region near the pole, associated with Z_SE < 0, may be regarded as a precursor of a substorm. On this basis a new index of the polar cap magnetic activity PC_L, characterizing the changeability of the magnetic field is proposed. It is shown that the increase of the PC_L index is followed in 1–2 hr by a substorm in 70% of events considered.     

Hook-shaped arcs in dayside polar cap and their relation to the IMF

A special type of auroral forms has been revealed in the southern polar cap on the basis of Vostok station data. There are hook-shaped arcs consisting of sun-aligned polar cap arcs which convert into latitude-oriented oval arcs. These hook-shaped arcs are seen in the southern polar cap only when B_z, component of the IMF is northward and B_x > 0. The sun-aligned arcs are grouped in the prenoon sector when B_y is positive and they are displaced in the afternoon sector when B_y is negative. When B_y is near zero the sun-aligned arcs are set almost symmetrically relative to the noon meridian. If the hook-shaped arcs display a convection pattern, the occurrence of twin hooks would seem to be in favour of a throat form of the sunward plasma flows exiting the polar cap.     

Characteristics of optical emissions and particle precipitation in polar cap arcs

Simultaneous optical and particle data from the ISIS-2 satellite are used to characterize polar cap arcs. Polar cap arcs are identified from two-dimensional geomagnetic transforms of the optical data along with precipitating electron data for the time at which the satellite is on the field line intersecting the arc. No precipitating protons were detected for any of the arc crossings. The pitch angle. distribution of the precipitating electrons is generally isotropic and the differential electron spectra show enhancements in the flux in the 300–750 eV energy range. The average energy of the precipitating electrons for the different arcs ranges from about 300 to 600 eV. A possible explanation of the observed precipitating particle characteristics is that parallel electric fields are accelerating polar rain type spectra at an altitude of several thousand km. For the arc crossings reported here the equivalent 4278 Å emission rate per unit energy deposition rate has a mean value of 162 R/(erg cm^?2 s^?1). Average 3914 Å intensities are about 0.8 kR while 6300 Å intensities range from 0.5 to 3 kR. Model calculations indicate that direct impact excitation is a minor source for the 5577 Å emission rate, but supplies approx. 40% of the 6300 Å emission.     

Subsurface radar sounding of the martian polar cap: radiative transfer approach

The problem of subsurface radar sounding of the martian polar caps [Ilyushin, 2004. Martian northern polar cap: layering and possible implications for radar sounding. Planet. Space Sci. 52, 1195–1207] is considered from the point of view of incoherent radiative transfer theory. Since it has been previously shown that the radar signal field within the polar cap has diffuse structure, there is a need for a statistical approach to the problem. Radiative transfer theory, which is now well developed, seems to be the most appropriate formalism for this approach.Several physical models of polar caps have been formulated. The asymptotic solutions for all proposed models are derived here. In the present paper only the case of orbital ground penetrating radar is considered, because it is of great interest in relationship to currently developed radar experiments. In principle, the approach is believed to be applicable to a wide class of short pulse and compressed chirp radar experiments, including both orbital and landed instruments and media more complicated than a simple plane parallel geometry. This work, however, is postponed to future papers.Techniques for retrieval of physical properties of polar caps from the radar measurements are proposed. From the observational data, the macroscopic parameters of the medium appearing in radiative transfer theory, i.e. the single scattering albedo and volume extinction coefficient can be estimated. These estimates put certain constraints on the physical parameters of the medium model introduced in the paper. With some additional information, known a priori or from other observations, these estimates can be used to retrieve physically meaningful information, for example, the average content of impurities in the ice.     

Observed characteristics of polar cleft Hα and OI emissions

Observations at Godhavn, Greenland show that the intersection of the polar cleft with the ionosphere can be recognized by simultaneous occurrence of hydrogen emissions (H_α, H_β>) and enhanced OI 6300 Å emission. The H_α-line reveals a characteristic narrow and symmetric Doppler profile which is interpreted as indicating that the solar wind protons retain their typical flux and energy spectrum all the way down to the ionosphere. The cleft intersection seems to cover the sector 04:00–22:00 geomagnetic time.     

The relationship between interplanetary quantities and magnetic activity in the southern polar cap

The polar cap magnetic activity MAGPC-index characterizing the intensity of disturbances affected by the IMF vertical component was derived from the antarctic station Vostok data in accordance to method of Troshichev et al. (1979a). The paper examines the statistical relationship between the 15-min values of this index and interplanetary quantities such as IMF components, solar wind velocity, interplanetary electric field and others. The results of the computation show a good correlation of MAGPC indices with interplanetary quantities including the IMF southward component. The best correlation is obtained for the merging electric field. The conclusion is : the MAGPC index derived from the background magnetic data may be used for monitoring of the convection electric field in the polar cap.