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.     

Kp index correlations with solar-wind parameters during the first and second stages of a recurrent geomagnetic storm

The dependence of geomagnetic activity during a recurrent magnetic storm on the solar-wind magnetic field and plasma parameters has been studied. According to variations of solar-wind magnetic field strength B, a recurrent magnetic storm is divided into two stages: the first proceeding during the peak of B, and the second proceeding after the return of B to quiet level. The K_p index vs solar-wind parameters scattering diagrams for stages I and II differ significantly. In particular, the random scattering for stage I is much larger than for stage II. It was found that for stage I the K_p index correlates with B, with the sign and value of northsouth field component B_z and with the magnitude ΔB of field fluctuations, the situation being similar to that during sporadic magnetic storms, though the scale of the event is smaller. For stage II, the K_p index does not correlate with B, but strongly correlates with ΔB and weaker—with B_z. So geomagnetic activity at stage II is supported mainly by solar-wind magnetic field fluctuations. The dependence of the K_p index on plasma parameters (concentration of protons n, bulk velocity v and temperature T) is weak for both stages.     

Seasonal dependences of geomagnetic variations in the polar region in connection with large-amplitude annual Z-variation at the geomagnetic pole

A noticeable depression of the vertical component Z of the geomagnetic field is observed in the polar cap in summer. From the statistical analysis of the equivalent overhead current patterns for daily geomagnetic variations in the summer and winter polar regions for three different conditions of IMF (interplanetary magnetic field), it was concluded that the annual variation of geomagnetic Z in the vicinity of the geomagnetic pole is attributed to the relative spatial shift of the twin-vortex current patterns over the polar cap from summer to winter. In winterthe clockwise current vortex in the dawn sector extends almost over the entire polar cap (except for the periods when the B_z component of IMF has a large positive value), and this will result in the positive variation of the Z-value at the geomagnetic pole. In summer the counter-clockwise current vortex in the dusk sector always extends over the whole polar cap even when B_z of IMF is positiveso that the variation of Z becomes negative. The persistent existence of current vortex in the dawn sector is important for the further study of magnetospheric convection when B_z is positive.     

The warm current sheet model, and its implications on the temporal behaviour of the geomagnetic tail

A simple model current sheet is studied numerically. Consistent fields and particle trajectories, and their dependence on electron and proton temperature, convection velocity and normal field, B_z, linking through the current sheet, are presented and discussed. It is shown that the protons, which are the major current carriers, largely retain the decoupling of the motion in the x-y plane from the normal oscillations as in the ‘cold’ current sheet. The positive potential of the current sheet is shown to be sufficient to trap some energetic electrons, the motion of which enables the predominance of energetic electrons towards the dawn side of the tail to be understood. Semi-empirical relationships for the thickness and the potential of the current sheet are obtained.

The consequences of such a current sheet on the behaviour of the geomagnetic tail are investigated. Using Faraday's law and the consistent cross tail electric field it is shown that the effect of a southward turning of the interplanetary field is to lead to a decrease in B_z,an increase of the current sheet conductivity, and a growth of stored field energy, i.e. the current sheet blocks merging. The decrease of the resistance of the current sheet is limited by the finite width of the tail. Finally, it is pointed out that if the conditions which bring about the growth of field energy persist, then the collapse of the field lines characteristic of substorms may occur.     

On the role of the motion of the photospheric footpoints of the magnetic field lines

By means of a simple relation between the velocity v of the fluid particle and the velocity v_f of the photospheric footpoint of the magnetic field line v_z and B_z being respectively the components of v and the magnetic field B normal to the photospheric surface, it is shown formally that through the phtospheric surface the transport of all the quantities attributed to the magnetic field, such as the magnetic flux, the magnetic energy and the helicity, is independent of v_z, and v_f is the only kinematical quantity on which the transport depends. In addition, in the neighborhood of the neutral line the velocity v_l of the moving curve of constant B_z is found to be equal approximately to the component of v or v_f in the direction of v_l. Since v_l can be measured or extimated, so can the components of v and v_f near the neutral line.     

The effect of an electric field induced by a time-dependent ring current on the particle drift motion

The effect of an electric field induced by a rapidly decaying ring current on the motion of charged particles in the magnetosphere has been investigated using Euler potentials. For a model consisting of the earth dipole and the symmetric ring current, the electric field satisfies the condition E . B = 0.

Under this circumstance, the E × B drift of the particle can be identified as the motion of the magnetic field lines and vice versa. The time dependent electric field induced can be evaluated in a Spherical polar coordinate system by the formula

where and β are Euler potentials.

A model calculation on the particle drift velocity v_D = E × B/B² shows that the radial component of the drift velocity is in good agreement with those deduced from whistler duct studies.     

A Comparative Analysis of Ground, Magnetospheric and Interplanetary Observations of Long Period Magnetic Oscillations

Continuous measurements of the geomagnetic field variations at ground stations are important to investigate several aspects of magnetospheric dynamics related to variations in the solar wind conditions which, ultimately, originate from the Sun. We present a comparative analysis of geomagnetic field measurements at several ground stations with simultaneous magnetospheric and interplanetary observations in order to understand the origin and characteristics of the observed fluctuations. The results suggest that long period geomagnetic field fluctuations can be directly driven by solar wind density fluctuations at the same frequencies via the modulation of the magnetopause current. We also discuss the possible occurrence of additional contributions related with cavity/waveguide resonances of the entire magnetosphere as well as those of resonance processes of the geomagnetic field lines.     

Geoeffectiveness of the interplanetary manifestations of coronal mass ejections and solar-wind stream–stream interactions

Coronal mass ejections and high-speed streams from the Sun, and related structures formed and evolved in interplanetary space, i.e. interplanetary manifestations of CMEs (ICMEs) and stream interaction regions (SIRs)/corotating interaction regions (CIRs), are mainly responsible for geomagnetic disturbances in the Earth’s magnetic environment. However, the presence or absence of associated/finer structures of ICMEs (e.g., shock/sheath, magnetic cloud) and SIRs/CIRs (forward and reverse shocks, stream interface) might influence their geoeffectiveness as these features within large-scale structures of ICMEs and SIRs display different and varying plasma and field characteristics. In this work, we analyze the solar-wind plasma and field parameters (plasma velocity, density and pressure, magnetic field, its north-south component and electric field) together with geomagnetic activity parameters (kp and Dst), applying the method of superposed epoch analysis. By systematically changing the time of passage of different features as epochs, e.g. discontinuities/shocks, CMEs/magnetic clouds in ICMEs and discontinuities/forward shocks in SIRs/CIRs, we study the relative geoeffectiveness of not only the large-scale structures (ICMEs/SIRs/CIRs), but of their finer features also. We critically analyze the differences in geoeffectiveness due to different structures and features, with distinct plasma/field characteristics, and we utilize these results to understand the mechanism during their interaction with geospace.     

Helioseismological constraint on solar axion emission

Helioseismological sound-speed profiles severely constrain possible deviations from standard solar models, allowing us to derive new limits on anomalous solar energy losses by the Primakoff emission of axions. For an axion-photon coupling g_ay 5 × 10⁻¹⁰ GeV⁻¹, the solar model is almost indistinguishable from the standard case, while g_ay 10 × 10⁻¹⁰ GeV⁻¹ is probably excluded, corresponding to an axion luminosity of about 0.20 L. This constraint on g_ay is much weaker than the well-known globular-cluster limit, but about a factor of 3 more restrictive than previous solar limits. Our result is primarily of interest to the large number of current or proposed search experiments for solar axions because our limit defines the maximum g_ay for which it is self-consistent to use a standard solar model to calculate the axion luminosity.     

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.     

Low amplitude anisotropic wave trains in cosmic rays

In the present study the occurrence of an unusual class of low amplitude anisotropic wave trains in the cosmic ray neutron intensity, which is distinctly different from the average diurnal variation as well as from other recognized types of low amplitude anisotropic wave trains are noted and the directional distribution in the interplanetary space determined. The major objective of this paper is to study the first three harmonics of low amplitude anisotropic wave trains of cosmic ray intensity over the period 1981–1994 for Deep River neutron monitoring station. The significant characteristic of these events is that the low amplitude wave trains shows a maximum intensity of diurnal component in a direction earlier than 18:00 h/co-rotational direction. It is noticed that these events are not caused either by the high-speed solar wind streams or by the sources on the Sun responsible for producing these streams such as polar coronal holes. However the possibility of occurrence of these events during high-speed solar wind streams cannot be denied. The occurrence of low amplitude events is dominant for positive polarity of B_z. The disturbance storm time index i.e. D_st, remains consistently negative only for majority of the low amplitude wave train events, which is never been reported earlier. The amplitude as well as direction of first two harmonics seems to remain unaffected with the variation in the D_st and A_p-index. However, the amplitude as well as direction of third harmonic found to deviates with the increase of D_st and A_p-index. The corotating streams produce significant deviations in cosmic ray intensity as well as in solar wind speed during low amplitude anisotropic wave train events.     

Effects of E-region electric fields on F-region parameters at the magnetic equator

The effects of E-region electric fields on F-region parameters like height of constant electron density contours (h_N) and semi-thickness, at an equatorial station, Trivandrum, are investigated. The E-region east-west electric field (E_y) has been deduced using the horizontal magnetic field values from the ground magnetograms. It is found that the semi-thickness parameter and h_N follow closely the E-region electric field (E_y) variations in the forenoon and noon periods. In the afternoon, it is shown that there is close association between h_N and E_y, when the electric field variations are large. On a day-to-day basis also, h_N and the E-region electric field show very good correlation in the forenoon and noon hours. It is also shown that higher levels are affected more than the lower levels in the F-region by E_y.     

Convection and field-aligned currents, related to polar cap arcs, during strongly northward IMF (11 January 1983)

Electric and magnetic fields and auroral emissions have been measured by the Intercosmos-Bulgaria-1300 satellite on 10–11 January 1983. The measured distributions of the plasma drift velocity show that viscous convection is diminished in the evening sector under IMF B_y < 0 and in the morning sector if IMF B_y > 0. A number of sun-aligned polar cap arcs were observed at the beginning of the period of strongly northward IMF and after a few hours a θ-aurora appeared. The intensity of ionized oxygen emission [O⁺(²P), 7320 Å] increased significantly reaching up to several kilo-Rayleighs in the polar cap arc. A complicated pattern of convection and field-aligned currents existed in the nightside polar cap which differed from the four-cell model of convection and NBZ field-aligned current system. This pattern was observed during 12 h and could be interpreted as six large scale field-aligned current sheets and three convective vortices inside the polar cap. Sun-aligned polar cap arcs may be located in regions both of sunward and anti-sunward convection. Structures of smaller spatial scale correspond to the boundaries of hot plasma regions related to polar cap arcs. Obviously these structures are due to S-shaped distributions of electric potential. Parallel electric fields in these S-structures provide electron acceleration up to 1 keV at the boundaries of polar cap arcs. The pairs of field-aligned currents correspond to those S-structures: a downward current at the external side of the boundary and an upward current at the internal side of it.     

Response time of the high-latitude dayside ionosphere to sudden changes in the north-south component of the IMF

The time scale of the response of the high-latitude dayside ionospheric flow to changes in the North-South component of the interplanetary magnetic field (IMF) has been investigated by examining the time delays between corresponding sudden changes. Approximately 40 h of simultaneous IMF and ionospheric flow data have been examined, obtained by the AMPTE-UKS and -IRM spacecraft and the EISCAT “Polar” experiment, respectively, in which 20 corresponding sudden changes have been identified. Ten of these changes were associated with southward turnings of the IMF, and 10 with northward turnings. It has been found that the corresponding flow changes occurred simultaneously over the whole of the “Polar” field-of-view, extending more than 2° in invariant latitude, and that the ionospheric response delay following northward turnings is the same as that following southward turnings, though the form of the response is different in the two cases. The shortest response time, 5.5 ± 3.2 min, is found in the early- to mid-afternoon sector, increasing to 9.5 ± 3.0 min in the mid-morning sector, and to 9.5 ± 3.1 min near to dusk. These times represent the delays in the appearance of perturbed flows in the “Polar” field-of-view following the arrival of IMF changes at the subsolar magnetopause. Overall, the results agree very well with those derived by Etemadi et al. (1988, Planet. Space Sci. 36, 471) from a general cross-correlation analysis of the IMF B_z and “Polar” beam-swinging vector flow data.     

Deactivation of N2A Σumolecules in the aurora

Recent rocket observations of the N₂ V-K (Vegard-Kaplan) system in the aurora have been reinterpreted using an atmospheric model based on mass spectrometer measurements in an aurora of similar intensity at the same time of year. In contrast to the original interpretation, we find that population by cascade from the C³Π_u and B³Π_g states in the A³Σ_u⁺v=0,1 levels, as calculated using recently measured electron excitation cross sections, accurately accounts for the observed relative emission rates (IV-K/12PG_0.0). In addition there is no need to change the production rate of A ³ Σ _u⁺ molecules relative to that of C³Π_uv=0 as a function of altitude in order to fit the profile of the deactivation probability to the atmospheric model. Quenching of A ³ Σ _u⁺ molecules at high altitudes is dominated by atomic oxygen. The rate constants for the v=0 and v=1 levels are 8 × 10⁻¹¹ cm³ sec⁻¹ and 1.7 × 10⁻¹⁰ cm³ sec⁻¹ respectively, as determined using the model atmosphere mentioned above. Recent observations with a helium cooled mass spectrometer suggest that conventional mass spectrometer measurements tend to underestimate the atomic oxygen relative concentration. The rate coefficients may therefore be too large by as much as a factor of 3. Below 130 Km we find that it is possible to account for the deactivation in bright auroras by invoking large nitric oxide concentrations, similar to those recently observed mass spectrometrically and using a rate constant of 8 × 10⁻¹¹ cm³ sec⁻¹ for both the v=1 levels. This rate constant is very nearly the same as that measured in the laboratory (7 × 10⁻¹¹ cm³ sec⁻¹). Molecular oxygen appears not to play a significant role in deactivating the lower A ³ Σ _u⁺ levels.     

Gnevyshev peaks in geomagnetic indices

Sunspots have a major 11-year cycle, but the years near the maximum show two or more peaks called Gnevyshev peaks. It was noticed that in cycle 23, the double peaks in sunspot numbers are reflected in the electromagnetic radiations and coronal mass ejections (CMEs) in the solar atmosphere. But, in the interplanetary space, the ICMEs (interplanetary CMEs) show peaks not all coinciding with the peaks of sunspot numbers. Also, there are stream interaction regions (SIR), including co-rotating interaction regions (CIR), which evolve quite differently from sunspot numbers. In the geomagnetic indices, the peaks are related mainly to the peaks in SIRs, indicating that geomagnetic indices have no direct relationship with most of the phenomena at the Sun but are responding only to the interplanetary blobs due to SIRs, which are more predominant in the declining phase of sunspot activity.     

The relation between low-latitude neutral density variations near 400 km and magnetic activity indices

Neutral density data were obtained near 400km (1600 LT) from a microphone density gauge on OGO-6 from 0°G to 40°N magnetic latitude for 25 September–3 October 1969. Several geomagnetic storms occurred during this period (a_p varied from 0 to 207). Least-squares fits were made to data points on density-a_p and density-D_st scatter diagrams, where the density values selected were delayed in time behind a_p and D_st. An equation representing the least-squares fit was computed for each delay time. The equation of best fit (and the corresponding time delay between the density and the magnetic index which resulted in this best fit) was found by choosing the equation that gave the minimum standard error. For example, the best fit at 10°N geomagnetic latitude occurred for a_p at t — 3 hr, where t is the time of the density values. The implications of the time differences associated with the best fits at various latitudes and longitudes are discussed with regard to the time delays involved in geomagnetic heating of the neutral upper atmosphere.

A low-latitude density bulge has been found between 0°N and 40°N whose magnitude varies with a_p. DeVries (1972b) has independently discovered this daytime phenomenon. If the bulge is a semi-permanent feature near the equinoxes because of the enhanced geomagnetic activity, this may help explain the semi-annual effect in density, which was uncovered first in the drag data from low inclination satellites.     

Nature of solar-cycle and heliomagnetic-polarity dependence of cosmic rays, inferred from their correlation with heliomagnetic spherical surface harmonics in the period 1976–1985

Correlation of cosmic-ray intensity (I) with the solar magnetic field expanded into the spherical surface harmonics, B_ns(n 9), by Hoeksema and Scherrer has been studied using the following regression equation:

, where are subgroups of B_ns classified in ascending order of n, and τ_i is the time lag of I behind correlation coefficient between the observed and simulated intensities (I_obs, I_sml) in the period 1976–1985 is 0.87 and considerably better than that derived from any single index of solar activity. The lag time τ₃ is greater than others, indicating that the higher order magnetic disturbances effective to the cosmic-ray modulation have a longer lifetime in space than the lower order disturbances. The rigidity spectrum of the cosmic-ray intensity variation responsible for A_I due to the dipole moment is harder than those for others (A₂,A₃), indicating that the lowest order (i.e. largest scale) magnetic disturbances can modulate cosmic rays more effectively than the higher order disturbances. As another result of the present analysis, it has been found that the intensity depends also on the polarity of the polar magnetic field of the Sun; the residual (I_obs−I_sml) of the simulation changes its sign from positive to negative with a time lag (0–5 Carrington rotation periods) behind the directional change of the solar magnetic dipole moment from northward to southward, and has a softer rigidity spectrum than A_iS. The dependence is consistent with the result having been obtained in the previous period, 1936–1976, by one (K.N.) of the present authors. The polarity dependence can be found also in the 22-year variation of the time lags obtained every solar cycle in the period 1936–1985. The theoretical interpretation of these polarity dependences is discussed on the basis of the diffusion-convection-drift model.     

The structure of the solar flare stream magnetic field

The structure of the interplanetary magnetic field within the flare streams as well as associated variations of the geomagnetic disturbancy are considered. It is shown that in the main body of the flare stream the magnetic field is determined by the configuration of the large scale magnetic field on the Sun at the flare region. Within the head part of the flare stream the magnetic field represents by itself the compressed field of the background solar wind and hence is determined by the distribution of the super large scale solar magnetic field outside the flare region.A certain asymmetry in the parameters of the magnetic field within the streams associated with geoeffective and non-effective flares is shown to exist.