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.     

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.     

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 causes of recurrent geomagnetic storms

We studied the causes of recurrent geomagnetic activity by analyzing interplanetary magnetic field and plasma data from Earth-orbiting spacecraft in the interval from November 1973 to February 1974. This interval includes the start of two long sequences of geomagnetic activity and two corresponding corotating interplanetary streams. In general, the geomagnetic activity was related to an electric field which was primarily due to two factors: (1) the ordered, mesoscale pattern of the stream itself and (2) random, smaller-scale fluctuations in the southward component of the interplanetary magnetic field B_z. The geomagnetic activity in each recurrent sequence consisted of two successive stages. The first stage was usually the most intense and it occurred during the passage of the interaction region at the front of a stream. It was related to a V × B electric field which was large primarily because the amplitude of the fluctuations in B_z was large in the interaction region. It is suggested that these large amplitudes of B_z were primarily produced in the interplanetary medium by compression of ambient fluctuations as the stream steepened in transit to 1 A.U. The second stage of geomagnetic activity immediately following the first was associated with the highest speeds in the stream. It was, among other things, related to a V × B electric field which was large mainly because of the high speeds.     

The velocity field and X-ray spectrum of the flare of 1981 May 13

The H velocity field at 0516 UT during the eruption of the X1.5/3B flare in the active region E58 N11 (Boulder 3106) on 1981 May 13, obtained with the horizontal solar spectrograph of Yunnan Observatory is given in this paper. A comparative analysis of the velocity field with the magnetic field shows that the velocity field is related to the gradient and neutral line of the magnetic field and the brightness of the flare maximum changes in the velocity field of ±15 km/s occurs at the location of greatest magnetic field gradient.

The neutral line of the magnetic field (h = 0) basically matches the zero velocity line (v = 0) between the two bright ribbons. But they do not match between the two bright knots where the filament is twisted and ascends. The spectral lines show the sloping morphology, from which we deduced the dynamical parameters of the twist of the rising filament.     

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.     

Instantaneous triple-probe method for the direct display of plasma parameters in a spacecraft condition

It is proposed that the instantaneous triple-probe method can be applied to the direct display of parameters; particularly in the case of probe application during measurements carried out from spacecraft. The theory of the triple-probe method has been extended for the very high speed flowing collisionless plasma condition, i.e. in which the spacecraft speed U ≈ the most probable electron thermal velocity v_e > the most probable ion thermal velocity v_i. This condition is particularly important both in the case of re-entry of spacecraft and in the solar wind plasma. Experiments have been conducted in a free molecular beam plasma chamber for S_i = U/v_i > 10. The experimental results and their relationship to the appropriate theory are presented.     

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.     

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.     

A further study of the method of identifying absorption line systems in QSO spectra

The method of identifying absorption line systems in QSO spectra (Cui et al. 1983; Chen et al. 1983) is further developed here. Certain limitations of the method and their improvements are discussed. Certain other problems requiring further study are pointed out. The improved method is applied to PKS 0528-250, and gives two new absorption line systems Z_a = 0.065 and 0.0345 in addition to the four systems Z_a = 2.8110, 2.8130, 2.5275, 2.1410, consistent with the systems A₁A₂, B, C of Norton et al. (1980). However, the systems D₁, D₂, E, F and G of Chen and Norton (1984) are not recovered. The reason for this discrepancy is discussed.     

Changes in thermospheric molecular oxygen abundance inferred from twilight 6300 A airglow

When the local solar zenith angle, χ_L, is < 105° the 6300 A line is much stronger than expected on the basis of F region ionic recombination alone. Between 95 and 105° the additional intensity is quantitatively explained by production of O(¹D) from photolysis of O₂ in the Schumann-Runge continuum, (λλ 1300–1750 A) using current values for solar flux, atmospheric composition and quenching of O(¹D) by N₂. The Schumann-Runge (SR) component exhibits a large seasonal variation with a maximum in summer. We interpret this variation as implying a seasonal change in thermospheric O₂ abundance; the change seems largely to reflect a variation in O₂ density at the base of the diffusive regime although some contribution may come from changes in thermospheric temperature structure. Large changes in the SR component exist from day to day and with a 27 day period following a major magnetic storm. The photodissociation source becomes inadequate when x_l < 95°; at 90° more than half of the intensity comes from still another source which we identify as local photoelectron excitation of O atoms.     

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.     

Mangeto-optical effects in sunspots and their vector magnetic field

The system of transfer equations of the four Stokes parameters I, Q, U, V under the action of the magneto-optical effect (i.e. the Unno-Beckers equations) are numerically solved in this paper for the magneto-sensitive lines FeI λλ 6302.499 and 5324.191 using an appropriate sunspot model. The errors in the expressions for the coefficients _r and _W in Beckers' paper [2] have been corrected for. From the results of calculations, features of the profiles of the Stokes parameters dependent on the magnetic vector have been isolated. Our computations also show that the magneto-optical effect should be taken into consideration in the measurement of the vector magnetic fields.

In the fourth section of this paper we have established a simple and convenient method for obtaining-information on the magnetic vector (including the field strength B, its inclination to the line of sight γ and its azimuth χ) from the profiles of the Stokes parameters. It consists of three steps: (1) The value of B is determined from the distance of the highest point in the V-profile from the central line. (2) γ is then found from V_max, i.e maximum value of V. (3) Lastly, the angle χ is found from Q₀, i.e. the value of Q at line centre.     

The configuration of the loop-like structure of the solar atmosphere

An integral, governing steady flows in an isolated thin magnetic flux tube in the hydrostatic plane-stratified atmosphere, has been obtained. The integral, that we named as the shape integral, is expressed as (1 − M_A²)B cos θ = const. Here M_A² is the Alfven Mach number, B is the magnetic field strength and θ is the flux tube inclination to the horizontal. The shape integral should hold for most loop models because it represents just the momentum balance laws and has no relation to any energy balance mode. Its application to the isothermal and static cases is discussed and illustrated.     

High resolution method of direct measurement of the magnetic field lines'' eigen frequencies

The simultaneous observations of Pc4 geomagnetic pulsations at the two temporary stations, located along the geomagnetic meridian 50 km to the North and South from the observatory Borok (L = 2.8), have been used for the investigation of amplitude gradients of both H- and D-components of these pulsations. It has been discovered that the direction of a meridional component of the gradient H (grad_MH) depends on the frequency ƒ of a spectral component of pulsations. The grad_MD is directed more or less permanently northward independently from the frequency ƒ These results are the consequence of a local amplification of geomagnetic pulsations due to Alfvén waves resonance along the magnetic field lines. It has been demonstrated that the frequencies ƒ_R for which the northward direction of grad_MH is replaced by the southward one (with increasing ƒ) can be interpreted as the eigen frequencies of the field line which intersects the meridian in the middle between two temporary stations, i.e. in Borok.

The possible applications of a gradient method of measurement of the magnetic field lines' eigen frequencies are discussed.     

On the Kelvin-Helmholtz instability of structured plasma layers in the magnetosphere

The hydromagnetic Kelvin-Helmholtz (K-H) instability problem is studied for a three-layered system analytically by arriving at the marginal instability condition. As the magnetic field directions are taken to vary in the three regions, both the angle and finite thickness effects are seen on the instability criterion. When the relative flow speed of the plasmas on the two sides of the interfaces separating the inner and the surrounding layers is U < U_c, where U_c is the critical speed, the system is stable both for symmetric and asymmetric perturbations. However, unlike the case of the interface bounded by two semiinfinite media, U_c is no longer the minimum critical speed above which the system will be unstable for all wavenumbers; another critical speed U^* > U_c is introduced due to the finiteness of the system. When U_c < U < U^*, the instability can set in either through the symmetric or asymmetric mode, depending on the ratio of the plasma parameters and angle between the magnetic field directions across the boundaries. The instability arises for a finite range of wavenumbers, thus giving rise to the upper and lower cut-off frequencies for the spectra of hydromagnetic surface waves generated by the K-H instability mechanism. When U > U^*, both the modes are unstable for short wavelengths. The results are finally used to explain some observational features of the dependence of hydromagnetic energy spectra in the magnetosphere on the interplanetary parameters.     

Model of polarization state of compressional surface MHD waves in the low-altitude cusp

Examination of thermal plasma data obtained by low-altitude satellite measurements indicates that the intersection of the cusp in the dayside magnetosphere with the topside ionosphere creates a distinct plasma geometry at low altitudes. This region consists of one or two plasma discontinuities with steep boundaries. As a result of the plasma structuring in the cusp which commonly takes place in the winter hemisphere, the propagation of compressional surface MHD waves is supported. This point is illustrated by an analysis of the polarization state of compressional surface MHD waves propagating along a plasma layer with thickness a and ambient magnetic field B₀ parallel to the interfaces. The results obtained are applicable to the case of a single interface, which is derived in the limit a → ∞. In the general case the polarization of the compressional surface MHD waves in the plane transverse to the magnetic field B₀ is elliptical. This feature of the polarization state of the compressional surface modes does not follow from the former analysis by Edwin and Roberts (1982, Solar Phys. 76, 239) for a magnetic slab, because the disturbance components parallel to the interfaces and perpendicular to the magnetic field B₀ have not been examined. Although the absence of these components does not prove to be essential for deriving the exact dispersion equation for arbitrary wave directions of the surface modes, they must be included when considering polarization states. The surface mode polarization in the plasma layer changes its sense three times: at interfaces X = 0 and X = a and in the middle plane X = a/2. For the symmetrical (sausage) mode the wave disturbance component b_n transverse (normal) to the interfaces becomes zero in the middle plane; for the asymmetrical (kink) mode, the component b_t parallel to the interfaces and transverse to the ambient magnetic field is zeroed in the same plane. For a moving observer such as a satellite the polarization patterns which might be recorded change, depending on the velocity of the observer and the angles at which the layered cusp is traversed. An essential feature in the polarization of the compressional surface MHD modes is the presence of jumps in the magnetic disturbance component b_t at the interfaces. These jumps disappear only for propagation along the ambient magnetic field. In this particular case the component b_t vanishes and then the surface modes are undistinguishable from the body modes.     

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.     

Mechanisms for the rigid rotation of coronal holes

We show that the rotation of coronal holes can be understood in terms of a current-free model of the coronal magnetic field, in which holes are the footpoint locations of open field lines. The coronal field is determined as a function of time by matching its radial component to the photospheric flux distribution, whose evolution is simulated including differential rotation, supergranular diffusion, and meridional flow. We find that ongoing field-line reconnection allows the holes to rotate quasi-rigidly with their outer-coronal extensions, until their boundaries become constrained by the neutral line of the photospheric field as it winds up to form stripes of alternating magnetic polarity. This wind-up may be significantly retarded by a strong axisymmetric field component which forces the neutral line to low latitudes; it is also gradually halted by the cross-latitudinal transport of flux via supergranular diffusion and a poleward bulk flow. We conclude that a strong axisymmetric field component is responsible for the prolonged rigid rotation of large meridional holes during the declining phase of the sunspot cycle, but that diffusion and flow determine the less rigid rotation observed near sunspot maximum, when the holes corotate with their confining polarity stripes.