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.     

Does IMF By induce the cusp field-aligned currents?

A well established correlation exists between the IMF B_y and the cusp field-aligned and horizontal currents (Wilhjelm et al., 1978). The northern and southern cusp currents may be parts of one large scale current system (D'Angelo, 1980) flowing mainly at the magnetopause and driven by the z-component of the solar wind electric field. Primdahl and Spangslev (1981) suggested that the large scale current system seems to shield out the IMF B_y from the interior of the magnetosphere. This paper proposes that the currents are induced by the change of sign of B_y at the IMF sector boundary crossings, and argues that the time constant for decay of the currents may well be one week or larger. The percentage errors in inferring the IMF sector polarity from the Godhavn H magnetogram increases with increasing time since the last sector boundary crossing. This is in accordance with a steady decay of the induced currents. Finally experimental tests are proposed to demonstrate the feasability of and possibly distinguish between the mechanisms.     

On the heliographic latitude dependence of the solar wind velocity

Plasma data from Pioneers 6–7 and from a variety of satellites operating near the Earth are used to investigate the heliographic latitude dependence of the solar wind bulk speed near the sunspot maximum. No evidence is found for a latitude effect: the latitudinal gradient, if any, turns out to be 2 km (sec degree)^–1, to be compared with the gradient of 10 km (sec degree)^–1 observed in periods of low or moderate solar activity.     

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.     

North-south asymmetry of the polar solar wind and some characteristics of solar magnetic field

We have found correlated variations of the yearly averaged north-south asymmetry in the polar solar wind speed (_sol) and the ratio of the zonal quadrupolar to the zonal dipolar contribution in the inferred coronal magnetic field during the declining phase of sunspot cycle 21. A physically meaningful association between _sol and some polar solar magnetic field proxies is also observed during the low sunspot activity periods of the above cycle.     

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.     

Impact of neutron star oscillations on the accelerating electric field in the polar cap of pulsar

Pulsar “standard model”, that considers a pulsar as a rotating magnetized conducting sphere surrounded by plasma, is generalized to the case of oscillating star. We developed an algorithm for calculation of the Goldreich-Julian charge density for this case. We consider distortion of the accelerating zone in the polar cap of pulsar by neutron star oscillations. It is shown that for oscillation modes with high harmonic numbers (l,m) changes in the Goldreich-Julian charge density caused by pulsations of neutron star could lead to significant altering of an accelerating electric field in the polar cap of pulsar. In the moderately optimistic scenario, that assumes excitation of the neutron star oscillations by glitches, it could be possible to detect altering of the pulsar radioemission due to modulation of the accelerating field. This work was partially supported by RFBR grant 04-02-16720, and by the grants N.Sh.-5218.2006.2 and RNP-2.1.1.5940.     

Non-radial solar wind flows and IMF Bz during 1973-2003

The characteristics of latitudinal angles of solar wind flow (θ_v) observed near earth have been studied during the period 1973-2003. The average magnitude of θ_v shows distinct enhancements during the declining and maximum phases of the sunspot cycles. A close association of B_z component of IMF in the GSE system and the orientation of meridional flows in the solar wind is found which depends on the IMF sector polarity. This effect has been studied in typical geomagnetic storm periods. The occurrence of non-radial flows is also found to exhibit heliolatitudinal dependence during the years 1975 and 1985 as a characteristic feature of non-radial solar wind expansion from polar coronal holes.     

Magnetic field of the solar wind

Relationship between the geoefficiency of the solar flares as well as of the active regions passing the central meridian of the Sun and the configuration of the large scale solar magnetic field is studied.It is shown that if the tangential component of the large scale magnetic field at the active region or at the flare region is directed southwards, that region and that flare produce geomagnetic storm. In case when the tangential magnetic field is directed northward, the active region and the flares occurring at that region do not cause any geomagnetic disturbance.An index of the geoefficiency of the solar flares and of the active regions is proposed.     

Equivalent ionospheric current systems representing IMF sector effects on the polar geomagnetic field

Equivalent ionospheric current systems representing IMF sector effects on the geomagnetic field in high latitudes are examined for each of the twelve calendar months by spherical harmonic analyses of geomagnetic hourly data at 13 northern polar stations for seven years. The main feature of obtained equivalent current systems includes circular currents at about 80° invariant latitude mostly in the daytime in summer and reversed circular currents at about 70° invariant latitude mainly at night in winter. Field-aligned current distributions responsible for equivalent currents, as well as vector distributions of electric fields and ionospheric currents, are approximated numerically from current functions of equivalent current systems by taking assumed distributions of the ionospheric conductivity. Two sets of upward and downward field-aligned current pairs in the auroral region, and also a field-aligned current region near the pole show seasonal variations. Also, ionospheric electric-field propagation along geomagnetic field lines from the summer hemisphere to the winter hemisphere with auroral Hall-conductivity effects may provide an explanation for the winter reversal of sector effects.     

Ulysses observations of anti-sunward flow on Jovian polar cap field lines

We examine the energetic (MeV) ion data obtained by the Anisotropy Telescopes instrument of the Ulysses COSPIN package during two northern high-latitude excursions prior to closest approach to Jupiter, when the spacecraft left the region of trapped fluxes on closed magnetic field lines at lower latitudes and entered a region of open field lines which we term the polar cap. During these intervals the ion fluxes dropped by 4–5 orders of magnitude to low but very steady values, and the ion spectrum was consistent with the observation of an essentially unprocessed interplanetary population. Ion anisotropies observed at these distances (within 16R_J, of Jupiter) indicate that in the low-latitude, high-flux regions the flows are principally azimuthail and in the sense of corotation, with speeds which are within a factor of 2 (in either direction) of rigid corotation. In the higher latitude trapped flux regions the flows rotate to become northward as the polar cap is approached, while in the polar cap itself the flows rotate further to become anti-corotational (and anti-sunward in the morning sector) and northward. These results provide primary evidence of the existence of solar wind-driven flows in the outer Jovian magnetosphere mapping to the high-latitude ionosphere. Investigation of concurrent magnetic data for the signatures of related field-aligned currents reveals only weak signatures with an amplitude of order 1 nT. The implication is that the height-integrated Pedersen conductivity of the ionosphere to which the spacecraft was connected was low, of order 0.01 mho or less. We also examine the ion observations during the two northern high-latitude excursions previous to those discussed above. These data indicate that the spacecraft approached but did not penetrate the open flux region during these intervals.     

On the correlation of coronal green-line intensity and solar wind velocity

Cross-correlation functions have been computed between green-line intensity (Kislovodsk) and Vela solar wind velocity January–June 1967. They are calculated separately for east and west limb observations in 5° latitude increments, and the solar wind velocites are correlated at their estimated emission times by correcting for the plasma Earth-Sun transit time using the observed velocities. The cross-correlation patterns appear to be dominated by two competing effects: a tendency of quasi-stationary green-line emission and solar wind velocity to anti-correlate; and a tendency of transient green-line emission and solar wind velocity enhancements to correlate positively. We also find evidence for simultaneous (same-day) emission brightenings over 2 to 4 limb quadrants. It is therefore recommended that, following a well-known practice in solar terrestrial studies, recurrent and transient events in both solar wind and green-line emissions should be studied separately.     

Variations of the IMF Bz component over a solar activity cycle for different regimes of solar wind formation

The behavioural features of the IMF B_z component for different solar wind velocity regimes have been studied. The study revealed a significant difference in variations of the B_z component between high-speed and low-speed regimes. Formation mechanisms for the IMF meridional component as well as the relationship of B_z with dynamical properties of the large-scale magnetic fields on the Sun are discussed.     

Measurement of the solar wind velocity with cometary tail rays

Cometary tail rays are traces of the magnetic fields caught in the cometary magnetosphere. Time variations of these rays give us a way to measure the local solar wind velocity at the location of a comet. We introduce a simple method for determining the radial velocity of the solar wind by observing the ray folding motion, and show an example of its application to comet P/Brorsen-Metcalf 1989o, which resulted in 340 ± 35 km s^–1.     

Test for detection of fine structure of the solar wind velocity

A new method of search and analysis of the fine structure in the velocity of interplanetary plasma irregularities is developed.     

The solar wind velocity in the eleven-year cycle no. 20 and the solar radar cross-section

The annual average values of the solar wind velocity over the period 1962–1972 were investigated on the basis of data obtained from different space probes. The comparison of the pattern of the annual average solar wind velocities observed by the Vela and Pioneer 6 satellites indicates that the pattern presented by Gosling et al. (1971) is realistic. The long-range trend in the solar wind velocity during the 11-year cycle is governed by the number and intensity of irregularities occurring in the corona. These irregularities may represent motions of mass or some types of MHD shock waves and they are responsible for the increased heating of the corona which then in turn causes an increase in the values of the solar radar cross-section and of the solar wind velocity. A close relation is demonstrated between the monthly and annual average values of the solar wind velocity and of the cross-section.     

On the dependence of the linear velocity of solar rotation on latitude and optical depth

The law of solar axial rotation is investigated by the spectroscopic method. Fayes formula: () = 2.03 – 1.6 sin followed from a great number of observations (800 different values of velocity derived from an investigation of 800 spectral lines of different intensity and of different elements). The dependence of the solar rotation on the depth in the atmosphere was investigated. For this purpose the optical depths of the formation of the core of 800 spectral lines were determined. The dependence of the rotational velocity on the heliographic latitude and the depth in the atmosphere is well described by the empirical formula (Equation (5) in the text).The main conclusion of this investigation is the existence of an anomaly in the rotational velocity of the Sun at heliographic latitude 25°. Moreover the amplitude of the deflexion of the rotational velocity at the above-mentioned latitude varies with the optical depth.