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.     

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.     

Short-period interplanetary and polar magnetic field variations

It is shown that the dependence of the variations of vertical component of the polar cap magnetic field on the sector structure (actually, the azimuthal or Y component) of the interplanetary magnetic field as first discovered by Svalgaard (1968) and Mansurov (1969) extends to variations as brief as 1 hr or even less. The relation between sector structure dependent variations and substorm fields as indicated by the southward-directed component of the interplanetary magnetic field is investigated by comparing brief variations over selected intervals of time. The independence of the variations of the polar cap vertical and horizontal components suggests that there are at least two different current systems which produce brief variations in the polar cap. One of the current systems is related to the substonn field; the other is strongly seasonally dependent and is confined to the dayside sector of the Earth.     

On the relationship between polar faculae and large-scale magnetic field

We investigate a latitude–time distribution of polar faculae observed at Ussuriysk Observatory in years 1966–1986. The distribution is compared with the longitude-averaged (zonal) magnetic field of the Sun calculated from the data obtained at Mount Wilson Observatory in the years 1966–1976, and at Kitt Peak National Observatory during the period from 1976 to 1985. We found that slow, poleward-directed migration of the polar faculae zones occurring during the course of the solar cycle is not a continuous process, but it contains several episodes of appearance and fast poleward drift of new zones of polar faculae. At the rising phase of the solar cycle, new zones of polar faculae appear at latitudes as low as 40°, but the ones observed during the declining phase of the solar cycle originate at higher latitudes of 50–55°. Such episodes of appearance and fast migration of the polar faculae zones are associated with the poleward-directed streams of magnetic field originated at low latitudes. Moreover, we found some evidence for existence of an additional component of the polar faculae activity that reveals an equatorward migration during the course of the solar cycle. We also investigated a relationship between the number of polar faculae, n, and absolute magnetic flux _z of the zonal mode of the solar magnetic field. We found that within the polar zones of the Sun, substantial correlation between temporal variations of n and _z takes place both on the time scale of the solar cycle and on a shorter time scale of 2–4 years. The relationship between the number of polar faculae and magnetic flux may be approximated by a linear dependence n=0.12_z (where _z is expressed in 10²¹ Mx), except for time interval 1977 through 1980 for which the factor of proportionality is found to have a systematically larger value of 0.20.     

The structure of Galactic gas at high latitudes: The southern polar cap

We analyze the angular structure of the 21-cm interstellar neutral hydrogen emission at six and seven declinations in the northern (published previously) and southern polar caps of the Galaxy (Galactic latitudes from ?40° to ?90°), respectively, with an extent of 90° in right ascension. The RATAN-600 radio telescope has a beam width averaged over these regions of 2.′0×30′. One-dimensional power spectra for the angular distribution of interstellar neutral hydrogen emission were computed in each 6.3-km s^?1-wide spectral channel by using the standard Fast Fourier Transform (FFT) code and were smoothed over 1^h in right ascension. The Galactic latitude dependence of the mean parameters for the sky distribution of H I line emission at high latitudes was found to correspond to the distribution of gas in the form of a flat layer only in the northern region, while in the southern cap, the gas distribution is much less regular. In addition, the mean H I radial velocities are negative everywhere (?3.7±3.0 km s^?1 in the north and ?6.0±2.4 km s^?1 in the south). The power spectra of the angular fluctuations in the range of angular periods from 10′ to 6° appear as power laws. However, the spectral indices change greatly over the sky: from ?3 to ?1.2; on average, as the Galactic latitude increases and the H I column density decreases, the fluctuation spectrum of the interstellar gas emission becomes flatter. In the northern polar region, this behavior is much more pronounced, which probably stems from the fact that the gas column density in the south is generally a factor of 2 or 3 higher than that in the north. Therefore, the spectra are, on average, also steeper in the south, but the dependence on Galactic latitude is weaker. Using simulations, we show that the observed power-law spectrum of the H I emission distribution can be obtained in terms of not only a turbulent, but also a cloud model of interstellar gas if we use our previous spectra of the diameters and masses of H I clouds.     

The relationship between the interplanetary magnetic field inhomogeneous structure and the distribution of large-scale magnetic fields in the photosphere (1969–1975)

For the period 1969–1975, a study has been made of the dependence of the interplanetary magnetic field structure on the distribution and evolutionary properties of solar magnetic fields. By direct comparison of a sequence of synoptic charts of the photospheric magnetic field with the interplanetary magnetic field, and by applying the method of correlation analysis, it is shown that to areas with an unstable polarity of the interplanetary magnetic field there correspond regions with a complicated inverse polarity line that forms either narrow gulfs and islands against a background of the dominant polarity, or bipolar magnetic regions and their clusters. At the time of reconstruction of the photospheric magnetic field the correlation between the photospheric and interplanetary magnetic field element distributions worsens. An asymmetry of the correlation between the interplanetary and photospheric magnetic field structures of different hemispheres is found. During the period of study, the interplanetary field structure shows a better correlation with the distribution of the photospheric magnetic field at middle and lower latitudes (0°–40°) of the southern hemisphere.     

The role of fluctuations in the interplanetary magnetic field in determining the magnitude of substorm activity

When hourly averages of interplanetary magnetic-field parameters are compared to the AE index, it is found that, although the time-integral of the southward component is important in determining whether there will be a substorm, the magnitude of storm activity as measured by AE is related to the level of the variance of the field. For a given value of the time-integral of the southward component of the interplanetary magnetic field, the AE index increases as the variance increases. For periods immediately following storm sudden commencements, this simple relationship apparently breaks down indicating that a different triggering process or storm-generation mechanism may be at work. These observations imply that although it is impossible to differentiate between substorms at the Earth's surface, it may be possible to differentiate between the conditions in the solar wind that lead to a certain magnitude of magnetic activity.     

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.     

Observed connections between apparent polar cap features and the instantaneous diffuse auroral oval

Images of the instantaneous nightside auroral distribution reveal that at times the orientation of auroral oval arcs changes to become characteristic of polar cap arcs. These connecting arcs all terminate in the diffuse aurora in the midnight sector, and their separation from the equatorward boundary of the diffuse aurora generally increases away from the midnight termination. The occurrence of these features requires a northward interplanetary magnetic field (positive B_z) as well as low magnetic activity. The existence of connecting arcs and the observation that they are at times the poleward boundary of weak diffuse emission indicate that the poleward boundary of auroral emissions can be significantly modified during non-substorm periods. Such a distortion implies that there can be a modification of the standard convection pattern in the magnetosphere during periods of positive B_z to produce expanded regions of sunward convection in the high latitude ionosphere.     

Effect of field-aligned currents in the polar cap at the recovery phase of a magnetic storm

Unusually great fluctuations in the ΔB module of the geomagnetic field have been observed in the polar cap from the satellite Cosmos-321. They are explained by small-scale two-sheet field-aligned current systems which exist during the periods when magnetic fields having a considerable northward B_z(B_z 10 nT) component are observed in interplanetary space.     

The dawn polar cap boundary at high altitude

Comparison of hot plasma data from ATS-6 and GEOS-1 when the satellites were near dawn L.T. conjunction reveals the presence of strong gradients separating plasmas differing by more than two orders of magnitude in keV particle fluxes. These gradients are observed at off-equatorial geomagnetic latitudes of 25–30° on field lines outside the synchronous orbit. They are associated with magnetic storms and are distinct from magnetopause crossings. Interpretation of these events in terms of a boundary between magnetospheric and polar-cap plasma leads to the following conclusions: (1) the polar cap/lobe region is essentially devoid of keV plasma at these times; (2) the field lines defining this boundary are significantly distorted from a dipolar to a more stretched form consistent with the presence of a storm-ring current, (3) smaller substorm-scale motions are superposed on the gross motion of the boundary with some evidence present for structure in the plasma spatial profile, and (4) magnetosheath-like plasma finds access to the inner magnetosphere at dawn L.T., much as it does near noon, along polar-cap boundary-layer field lines which close through the low latitude magnetospheric boundary layer.     

Relationships between the equatorial electrojet and polar magnetic variations

On moderately disturbed days when substorms occur frequently, the quiet day daily variation in the polar region (S_q^p) is enhanced. On such days, however, the quiet day variation along the dip equator appears to be suppressed, as well as being superposed with ‘fluctuations’.It is suggested that the enhancement of S_q^p is related to a partial suppression of the equatorial electrojet. The asymmetric ring current also causes an apparent suppression of the electrojet.On the other hand, the substorm-associated electric field which drives the eastward current in the auroral and subauroral zone (causing positive bays) in the afternoon sector appears to enhance the equatorial electrojet.Thus, magnetic variations along the dip equator are influenced by a number of processes in the magnetosphere.     

The interplanetary electric field, cleft currents and plasma convection in the polar caps

This report investigates the suggestion that the pattern of plasma convection in the polar cleft region is directly determined by the interplanetary electric field (IEF). Owing to the geometrical properties of the magnetosphere, the East-West component of the IEF will drive field-aligned currents which connect to the ionosphere at points lying on either side of noon, while currents associated with the North-South component of the IEF will connect the two polar caps as sheet currents centered at noon. The effects of the hypothesized IEF driven cleft current systems on polar cap ionospheric plasma convection are investigated through a series of numerical simulations. The simulations demonstrate that this simple electrodynamic model can account for the narrow “throats” of strong dayside antisunward convection observed during periods of southward interplanetary magnetic field (IMF) as well as the sunward convection observed during periods of strongly northward IMF. Thedawn-dusk shift of polar cap convection which is related to the B_y component of the IMF is also accounted for by the model.     

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.     

Directional discontinuities in the interplanetary magnetic field

It is shown that the interplanetary magnetic field has different characteristics on different scales, and it is noted that a given physical theory may not be applicable or relevant on all scales. Four scales are defined in terms of time intervals on which the data may be viewed. Many discontinuities in the magnetic-field direction are seen on the mesoscale ( 4 days, 1 AU). The characteristics of such directional discontinuities which were observed by Pioneer 6 during the period December 16, 1965-January 4, 1966 are presented, with special emphasis on their distribution in time. Previously, it was suggested that such discontinuities are simply boundaries of spaghetti-like filaments extending from the sun to the earth. Here it is shown that on the mesoscale unique filaments with sharp boundaries containing well-ordered magnetic fields are not always seen although discontinuities are always present at 1 AU. Thus, the interplanetary medium appears to be discontinuous rather than filamentary. The filamentary model implies that discontinuities originate at the sun and are convected with the solar wind. The discontinuous model allows the additional possibility that the discontinuities form in the interplanetary medium far from the sun.     

The sources of the polar cap and low latitude bay-like disturbances during substorms

Examination of the polar cap and low-latitude bays during substorms shows that there are two types of disturbances, DP₁₁ and DP₁₂, which are different not only in their morphological features, but in their origin as well. The DP₁₂ disturbances are associated with pure ionospheric currents, whereas the DP₁₁ are though to be generated by the Birkeland type current system. This conclusion is based on examination of the following characteristics: (1) the seasonal changes of the DP₁₁ and DP₁₂ disturbances in the polar cap, (2) the seasonal variations of the low-latitude bay intensity at the conjugate points in the cases of the DP₁₁ and DP₁₂ disturbances, (3) the distribution of the intensity of the DP₁₁ and DP₁₂ disturbances in both northern and southern hemispheres along the midnight meridian.     

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.