Magnetic Meridian Plane Research Articles

Spaced direction finding of nighttime whistlers at low and equatorial latitudes and their propagation mechanism

The propagation mechanism of low‐latitude and equatorial latitude whistlers is investigated on the basis of spaced direction finding measurements in South China. Observations were made continuously in the local time (LT) interval from 0000h to 0400h during the period of January 5–11 1988 at the three stations (Zhanjiang (geomagnetic latitude 10.1°), Guilin (14.1°), and Wuchang (19.4°)) and two horizontal magnetic components and one vertical electric field component were simultaneously recorded over a wide frequency range to enable comprehensive direction finding. Two major occurrence peaks on January 5 and January 6 and two minor ones on January 9 and January 11 have been analyzed and the following experimental results have emerged. (1) The whistler occurrence at very low latitudes is generally very small compared with that at low latitude (geomagnetic latitude ≳20°), but once it occurs, the occurrence rate becomes comparable to that at low latitudes. (2) The whistler dispersion is single‐valued at any particular local time. (3) The ionospheric exit region of whistlers is very much restricted to the geomagnetic latitude range of 10°–14°, and there are no observed whistlers whose path latitude is between 14° and 20°. (4) The extent of their ionospheric exit region is very stable on different days, and the radius of the distribution is less than 40–50 km. (5) Surprisingly high occurrence of echo train whistlers is observed. (6) The propagation of whistlers in the Earth‐ionosphere waveguide after ionospheric transmission is more likely toward higher latitudes than toward the equator and the subionospheric propagation seems to exhibit a horizontal beaming around the magnetic meridian plane. We attempt to interpret these characteristics of the observations in terms of either nonducted or field‐aligned propagation, but discussion indicates that especially findings 4 and 5 are strongly indicative of field‐aligned propagation for very low latitude whistlers localized in geomagnetic latitudes of 10°–14°.

VHF transequatorial propagation via three dimensional waveguides

The three dimensional model of VHF propagation along ionospheric waveguides or ducts developed by Platt I. G. and Dyson P. L., 1989 [ J. atmos. terr. Phys. 51, 897–910] has been used to study transequatorial propagation (TEP) along equatorial bubble irregularities aligned along the earth's magnetic field. Bubbles with circular and elliptical cross-sections and different apex heights have been used to determine the region illuminated in the conjugate hemisphere by TEP and the power of the signals. The results show that the three dimensional waveguide model can explain many features of TEP, including those not well explained by the simpler waveguide models used previously. The calculations therefore confirm that TEP can be caused by guided propagation along equatorial bubble irregularities aligned along the magnetic field. The results show that a bubble located on the transmitter's longitude can illuminate a wide area of the conjugate hemisphere. A well defined maximum central power region occurs which in most cases is centred at a latitude slightly lower than the conjugate of the transmitter. Bubbles with circular cross-section can illuminate a region more than ten degrees in latitudinal extent and five degrees in longitude. Bubbles which are vertically elongated at their apex illuminate a much narrower region in longitude. TEP between points with relatively large separation in longitude can result from either leaky rays or ducts tilted out of the magnetic meridian plane. This explains some observations of TEP previously thought to be inconsistent with waveguide propagation. The variation of power loss with frequency depends on a number of factors such as bubble shape and height. Co-ordinated experiments involving a transmitter and several receivers in the conjugate hemisphere should enable properties of bubbles to be determined.

Lightning as an embryonic source of VLF hiss

Data from the DE 1 satellite show that lightning‐generated whistlers often trigger hiss emissions that endure for up to 10‐ to 20‐s periods. The data consist of the measured electric and magnetic fields in the frequency range of 1.5 kHz to 6.0 kHz, during 22 DE 1 passes during the period December 28, 1986, to January 18, 1987. The 22 passes were nearly identical in terms of their projections on a magnetic meridional plane, and they covered L shells of 3.4 to 5.1 and geomagnetic latitude of 20°N to 40°S in the afternoon (∼1400–1500 MLT) sector. The geographic longitudes of the 22 passes were within ±50° of 80° W, and the geomagnetic activity during the period covered was relatively quiet (ΣKp<20 for most of the days). The whistler‐triggered hiss emissions were observed on 16 of the passes, and they generally exhibited the following characteristics: (1) emission spectra were wide band (1–2 kHz) and rather structureless, (2) well‐defined and sustained fading patterns were observed at twice the spin frequency over 10‐ to 20‐s periods, (3) the spin fading characteristics of the triggered hiss bursts were similar to those reported for background plasmaspheric hiss (Sonwalkar and Inan, 1988), indicating a large wave normal angle with respect to the ambient magnetic field. Our results indicate that lightning‐generated whistlers may be an important embryonic source for magnetospheric hiss and that whistlers and emissions triggered by them often constitute the dominant wave activity in the ∼1.5‐ to 6‐kHz range on L shells of 3.5 to 5 in the afternoon sector during geomagnetically quiet periods. Through cyclotron and Landau resonant scattering, it is likely that these lightning‐generated waves play a dominant role in the loss of ∼0.5‐ to 50‐keV electrons trapped on these field lines in the afternoon sector. Through anisotropic proton instability, these waves can also interact with ring current protons in the range of several tens of keV leading to a loss mechanism for ring current protons.

Auroral height-measuring system designed for real-time operation

The altitude and altitude distribution of auroral emissions, such as the [N+2] λ427.8-nm emission, are one result of processes in the ionosphere and magnetosphere during periods of auroral activity. A bistatic height-measuring system (HMS) has been developed with the primary objective of determining the altitude and latitude of auroral emissions in the magnetic meridian plane through Kiruna, Sweden. Since significant morphological redistributions occur in the aurora within a few seconds, HMS was designed to measure auroral altitudes with a time resolution of 1.5 s. The base line between the two stations was short (13 km), which generated similar-looking meridian profiles of the measured auroral emissions. The meridian profiles were obtained with intensified photodiode arrays, operated in a monochromatic imaging mode. The hardware and software configurations of HMS and essential assumptions for the triangulation procedure are described. HMS is especially adapted to measure during periods of disturbances in auroral arcs, e.g., eastward-traveling folds. The estimated optimal accuracies of the determinations of the latitudinal position of an auroral arc and the altitude of an identified auroral point are ±0.01° and ±1 km, respectively.

Wave normal direction and spectral properties of whistler mode hiss observed on the DE 1 satellite

A new study of magnetospheric hiss as a spatially and temporally enduring phenomenon is undertaken using a recently developed formalism that allows the representation of hiss by a field distribution function (FDF). This formalism explicitly takes into account the whistler mode relationships and the linear and spin motion of the satellite, so that on a spin‐stabilized satellite, it becomes possible to measure the wave propagation direction(s) from the observed fading patterns in the received electromagnetic field data. We have analyzed hiss signals received by electric and magnetic field antennae aboard the DE 1 satellite during a ∼3‐hour period on September 23, 1983. A band of hiss at frequencies <2 kHz was observed continuously from 0236 UT to 0539 UT over a range of geomagnetic latitudes from λm = 45°N to λm = 20°S and L shells of L = 4.3 to L = 5.3. Electron density deduced from in situ and remote measurements indicates that during this time the DE 1 satellite was near the boundary of the plasmasphere. Observations can be summarized as follows: (1) The general character of the electric and magnetic field spectrum remained the same throughout the 3‐hour‐long period, exhibiting an intensity peak at 1550 Hz. (2) The intensities of both the electric and the magnetic field decreased during this interval by ∼30 dB. (3) Well‐defined fading patterns at half the spin period (3.04 s) were observed throughout this period. These patterns were stable over a time scale of ∼1 min. (4) In addition, a slow fading pattern with a time scale of 30 s was observed approximately 30% of the time. (5) The hiss intensity also showed fading over a time scale of ∼10‐15 min. The spin fading patterns led to the measurement of wave propagation direction(s). The results of our analysis can be summarized as follows: (1) Near the geomagnetic equator (within ∼ ±3° latitude) we observe a wave normal angle of 60° ± 5° with respect to the local geomagnetic field. (2) Away from the equator, wave normal directions range from 30° to 80° with respect to the local geomagnetic field. (3) All wave normal directions observed over the 3‐hour‐long period were within ∼ ±45° of the plane normal to the local magnetic meridional plane. Results indicate that the hiss source radiates with initial wave normal angles in the range 30° < θ0 < θg (Gendrin angle) and 90° < ϕ0 < 270°, contrary to the common assumption that the source emits with wave normal angles closely aligned with the geomagnetic field. The FDF formulation has also permitted elucidation of the statistical nature of hiss. The slow time fading (∼30 s) is interpreted in terms of a coherence bandwidth Δω of about 0.2 rad/s.

A numerical model of the ionospheric dynamo—III electric current at equatorial and low latitudes

Using the general dynamo model and its special cases derived in a previous paper, the distributions of three dimensional electric current density in a magnetic meridional plane in the equatorial and low latitude ionosphere are computed. The winds generating the ionospheric dynamo are tide-like and locally periodic, similar to those in an internal gravity wave. Very large (several μA m −2) field-aligned current density is obtained in the equatorial region at places of sharp vertical gradients of the wind velocity. The currents generated by locally periodic winds of latitudinal wavelength less than several hundred kilometers do not significantly affect the normal equatorial electrojet.

A theoretical model of artificial spread F echoes

Four invariants of the ray trajectory are found for a ray propagating in a horizontally stratified ionosphere under the density perturbation of HF wave‐induced field‐aligned irregularities. The reflection height of the ray can then be determined with the aid of those invariants. The results show that the reflection height of the ray varies drastically (namely, strong spread F echoes) in the presence of irregularities that polarize in the magnetic meridian plane. By contrast, the reflection height is not affected (namely, no spread F echoes) by those irregularities that polarize in the direction perpendicular to the meridian plane. Spread F is quite insensitive to the magnetic dip angle θ0 in the region from 20° to 70°. The dependence of spread F on the scale length of the irregularity has also been examined for the case θ0 = 50°. It is found that spread F is not caused by irregularity with scale length less than about 100 m.

Relative contribution of ionospheric conductivity and electric field to the auroral electrojets

Data from continuous scans of the Chatanika radar beam along the magnetic meridian plane are used to determine the latitudinal profile of height‐integrated ionospheric conductivities and horizontal electric fields, from which the latitudinal distribution of ionospheric currents is deduced. The observations cover invariant latitudes between 62° and 68°, where the IMS Alaska meridian chain of magnetometers was also in operation. Although the conductivities and the electric fields are interrelated, the relative importance of the two in driving the eastward and westward auroral electrojet currents can be assessed. It is found that for moderate and large current densities (i.e., ≳0.2 A/m), the northward electric field strength increases as the magnitude of the eastward electrojet in the evening sector increases. The height‐integrated Hall conductivity stays generally at the level of 10 mhos even when the current density becomes as large as 1 A/m. However, when the eastward electrojet is small, substantial electric fields of 10–20 mV/m may still exist as if the magnetosphere has a persistent voltage source. There appear to be two distinct components to the westward electrojet. In the midnight and early morning sestors (<0300 MLT) intensity is characterized by a weak southward electric field and a high Hall conductivity, whereas its late morning portion ( >0300 MLT) is dominated by a strong southward electric field.

Interpretation of ionograms in the vicinity of the dayside auroral oval by ray tracing

A ray‐tracing program based on the Haselgrove equations and the Appleton‐Hartree formula has been developed and used to synthesize sweep‐frequency ionograms. We have looked at ionograms taken in summer periods at high‐latitude ionosonde stations in the vicinity of the dayside auroral oval. Especially the traces on ionograms from oblique reflections from an electron density enhancement have been studied. The enhanced ionization is simulated either by a two‐dimensional Gaussian distribution or by a double exponential function similar to the bottom part of a Chapman function. The enhancements are aligned with the earth's magnetic field direction. Ray paths in the magnetic meridian plane are calculated and discussed in connection with computed ionograms and observed ionograms from Canada and Greenland. The ray‐tracing exercise was done in order to estimate characteristics of the electron density enhancement from observed ionograms. By varying the parameters in the models of the ionosphere to obtain agreement between observed and synthesized ionograms we have been able to analyze temporal and spatial progress of an electron density enhancement that could be a feature of the auroral oval like the field‐aligned current sheets in the cusp region.

Observations and modeling of multi‐frequency VHF and GHz scintillations in the equatorial region

Amplitude scintillation spectra and statistics obtained simultaneously at VHF (257.55 MHz), L band (1541.5 MHz), and C band (3945.5 MHz) from the MARISAT communications satellite (15°W) are presented. The measurements were made at Ascension Island (7°58′S, 14°25′W, 30°S DIP) over a high elevation propagation path within a few degrees of the magnetic meridian plane. The multi‐frequency data and the unique propagation geometry enable us to study the structures of the equatorial ionospheric irregularities closely. Numerical simulation is used to model a specific event. Comparisons between the modeling results and the observations indicate that the data are consistent with the recently measured (in situ) two‐component power law irregularity spectrum with a shallow large‐scale regime and a steeper small‐scale regime, and a spectral break at about 1‐km scale size.

Latitudinal and magnetic flux tube extension of the equatorial spread F irregularities

A comparative study has been carried out of the spread F occurrence characteristics at the magnetic equatorial station, Fortaleza (4°S, 38°W, dip latitude 1.8°S) and the low‐latitude station, Cachoeira Paulista (22°S, 45°W, dip latitude 14°S) located with a relatively small longitudinal difference, in Brazil. The results show strong correlation between the occurrences of spread F events at the two stations for most of the time. While all the spread F events over Cachoeira Paulista are accompained by such events over Fortaleza, the converse is not true. Also, the spread F onset times over the equator are systematically earlier than those over the low latitude. Another interesting result is that while the spread F durations at the two stations are approximately linearly related, an event over Cachoeira Paulista is observed only for those that have durations more than 4–5 hours over Fortaleza. Possible implications of these results on the plasma bubble rise velocities in the equatorial ionosphere are discussed. It seems possible also to predict the approximate duration of a spread F event over the equator by observations carried out at a low‐latitude station situated close to a common magnetic meridional plane.

A thermal oscillating two-stream instability

A theory for the oscillating two-stream instability, in which the Ohmic heating of the electrons constitutes the nonlinearity, is developed for an inhomogeneous and magnetized plasma. Its possible role in explaining short-scale, field-aligned irregularities observed in ionospheric heating experiments is emphasized. The theory predicts that the initial growth of such irregularities is centered around the level of upper hybrid resonance. Furthermore, plane disturbances nearly parallel to the magnetic meridian plane have the largest growth rates. Expressions for threshold, growth rate, and transverse scale of maximum growth are obtained. Special attention is paid to the transport theory, since the physical picture depends heavily on the kind of electron collisions which dominate. This is due to the velocity dependence of collison frequencies, which gives rise to the thermal forces.

Simultaneous rocket and radar measurements of currents in an auroral arc

A detailed study of electric field, current and conductivities associated with an auroral arc was made in a coordinated rocket and radar experiment in Alaska on March 9, 1978. The payload, designated 29.007 UE, was launched at 1013 p.m. local time. It penetrated the diffuse aurora on the upleg and at apogee traversed field lines connected to a stable auroral arc of 40 kR intensity. Among the instruments carried by the payload were a vector magnetometer, a set of electrostatic double probes and a set of electron and proton spectrometers. Simultaneous electron density and line‐of‐sight velocity measurements were made by the Chatanika radar operating in an elevation scan mode in the magnetic meridian plane. Both the radar and rocket measurements indicated that the zonal electric field was westward and approximately constant across the arc with a magnitude of about 7 mV/m. Small differences between the rocket and radar zonal electric field measurements indicated the presence of upward drifting ions in the region of the arc. The meridional field was large and northward equatorward of the arc, but negligible within the arc. Conductivities computed from measured fluxes of energetic electrons agreed well with the conductivities derived from the radar measurements of electron density. The electric field and conductivity measurements indicated that the zonal currents were eastward equatorward of the arc and westward within the arc. These electrojet currents agreed well with those inferred from the rocket magnetometer data. Better agreement was obtained when a westward neutral wind was added. The westward wind was also consistent with differences between the rocket and radar meridional electric fields. The meridional currents computed from the electric field measurements were northward over the entire region. Since the northward current was enhanced within the arc, a pair of oppositely directed field‐aligned current sheets associated with the arc edges was inferred. However, these current sheets were not detected by the rocket‐borne vector magnetometer. The magnetometer measured a downward current in the diffuse aurora equatorward of a wide region of upward current within which the arc was embedded. The differences in the field‐aligned current patterns inferred from the two methods are attributed either to a time dependent field‐aligned current profile or east‐west current divergence in the region of the Harang discontinuity.

A reciprocity theorem relating currents and fields in the presence of a plane-stratified ionosphere

We consider the plane-wave spectrum generated by two arbitrary current distributions, J 1( r) and J 2( r), in free space below, or above, a plane stratified ionosphere. We apply the 2×2 ionospheric reflection or transmission matrices R( k) or T ±( k) defined in terms of the TE or TM modal amplitudes, and perform an inverse Fourier transform on the reflected or transmitted spectral components, to obtain the resultant fields, E 1( r) and E 2( r), reflected from, or transmitted through the ionosphere. A scattering theorem is then applied which connects the scattering matrices in a given problem with those in a ‘conjugate’ problem, namely, R ±(k) = R ̃ ±(k c) , T ±(k) = T ̃ ∓(k c) , where the propagation vector k c is the mirror image of k with respect to the vertical, magnetic east-west plane. It is shown that ∝ E 1( r) · J 2( r) d 3 r = ∝ E 2 c ( r) · J 1 c ( r) d 3 r where E i c ( r), i = 1, 2, represents the respective fields generated by the image or mirrored, current systems J i c ( r), i.e. mirrored with respect to the vertical magnetic meridian plane. It is noted that whereas the Lorentz reciprocity theorem relates currents and fields in the presence of (or inside) a given medium with those in the presence of a transposed medium (i.e. in the presence of a hypothetical ionosphere in which the geomagnetic field has been reversed), the result here derived relates instead a given and a mirrored pair of current systems, and their associated wave fields, in the same physical system.

Superthermal electrons and Bernstein waves in Jupiter's inner magnetosphere

A theoretical model for generation of banded electrostatic emissions by low density, superthermal electrons is developed for application to Jupiter's magnetosphere. The model employs a power law form for the energy dependence and a loss cone pitch angle distribution of the superthermals to drive convective instability of Bernstein modes. We concentrate on instability in the upper hybrid band and on lower harmonic bands below the upper hybrid frequency. A direct correspondence between spectral features of the 3/2's band and resonant superthermal electrons is found. The concept of a critical flux j⊥* of resonant electrons able to provide 10 e‐foldings of electric field amplification yields an explicit relation j⊥* ∼p in terms of the background thermal electron pressure. This result is used to construct a theoretical/empirical model of thermal electron density and temperature from 6–20 RJ in the Jovian magnetosphere which suggests that the ion and electron temperatures satisfy Te <Ti ∼10Te in this region. Finally, wave ray paths are computed for propagation in the magnetic equator and in the magnetic meridional plane of a dipole magnetic field. These ray paths suggest that intense wave activity is tightly confined to a small latitudinal extent, Λ≲±4°, about the magnetic equator.

Saturn's magnetosphere and its interaction with the solar wind

Pioneer 11 vector helium magnetometer observations of Saturn's planetary magnetic field, magnetosphere, magnetopause, and bow shock are presented. Models based on spherical harmonic analyses of measurements inside 8 Rs reveal that the planetary field has a high degree of symmetry about the rotation axis. The vector dipole moment of 0.2 G Rs³ has a tilt angle less than 1° and is offset along the polar axis 0.04±0.02 Rs. Equatorial offsets derived from the models show substantial variability and could be consistent with a very small offset. Beyond 10 Rs, near the noon meridian, the field topology is characteristic of a dipole field being compressed by high‐speed solar wind. There is no evidence of plasma outflow, i.e., a planetary wind. Near the dawn meridian the field lines in the outer magnetosphere are stretched‐out into a nearly equatorial orientation. Crossings of a thin current sheet are observed, apparently caused by motions driven from outside the magnetosphere. The field above and below the current sheet spirals out of the magnetic meridian plane at large distances to point tailward and parallel to the magnetopause. The location of the magnetopause is consistent with a shape that is similar to that of the earth but perhaps more blunt, as suggested by the attitude of the magnetopause near dawn. Near both the noon and dawn magnetopause the field in the magnetosheath equals or exceeds the field in the magnetosphere. The noon observations suggest a piling‐up of magnetosheath field lines adjacent to the magnetopause. Large impulsive field compressions are observed in the magnetosheath near noon. Multiple crossings of the bow shock are observed, and the absence of significant changes in field direction shows that it is quasi‐perpendicular. The speeds of motion of the shock toward and away from Saturn are estimated to be 150 and 50 km/s, respectively. A shock thickness of ∼2000 km is inferred.

Eigenmode scattering relations for plane-stratified gyrotropic media

We consider the 4×4 scattering matrixS for a plane wave incident on a plane-stratified gyrotropic multilayer slab, which is assumed to have a single symmetry axis (the magnetisation vector in a ferrite sheet, or the external magnetic field direction in a plane-stratified magnetoplasma). In the given (original) problem the plane of incidence is at an azimuthal angle ϕ with respect to the magnetic meridian plane (the plane containing the normal to the stratification, and the gyrotropic symmetry axis), and there is a corresponding “conjugate” set of wave fields, in which the plane of incidence is at an azimuthal angle (π−ϕ), with a corresponding conjugate scattering matrixS′. Adjoint wave fields, obtained by reversing the magnetic symmetry vector, are used to yield the eigenmode amplitudes required in the definition of the scattering matrices. At an interface between two adjacent layers the scattering matrices are shown to be uniquely determined by the characteristic wave polarisations, and this is used to prove that the given and conjugate scattering matrices,S andS′, for the overall multilayer system are mutually transposed, i.e.\(S = \tilde S'\).

The second Z-propagation window

ELLIS1,2investigated the condition in the ionosphere under which an upgoing ordinary radio wave is converted, through a coupling process, into an upgoing extraordinary wave in the Z mode. (Propagation in the Z mode requires the presence of both an ionised gas and a magnetic field; hence such waves cannot be received in free space.) This occurs for a wave incident on the ionosphere from below with its wave normal close to a critical direction in the magnetic meridian plane. This direction is such that when the wave reaches the level where X = 1, which is normally a complete reflector for the ordinary wave, its wave-normal direction has become close to the magnetic field line direction. The symbols X and Y are as usually defined in magnetoionic theory (see Ratcliffe3). The level X = 1 becomes semi-transparent for a cone of angles centred on the critical direction. This cone is called the ‘Ellis window’. The Z-mode waves are reflected at a higher level where X = 1+Y. Here I wish to draw attention to a window which is relevant to the escape of Z-mode radiation produced naturally in the topside ionosphere

Excitation of the earth-ionosphere waveguide by downgoing whistlers—III. Wave normal not in the magnetic meridian.

Calculations relating to the excitation of the Earth-ionosphere waveguide by downgoing whistlers carried out in previous papers are generalized to the case where the plane of incidence is not in the magnetic meridian. It is found that the results are very insensitive to the orientation of the plane of incidence. Thus previous results applying in the magnetic meridian plane can be applied to stations widely separated in longitude. In particular stations spaced in longitude by up to 2000 km should be able to receive the same whistlers.

Theory of heating in the lower ionosphere by obliquely incident waves from a powerful point transmitter

In the study of wave interaction it is necessary to know the power absorbed per unit volume of the anisotropic ionospheric plasma, from the disturbing radio wave radiated by a powerful point transmitter. This depends on height, range and direction. The problem is first considered when there are only two obliquely upgoing waves. The two magnetoionic components that reach the same real point in the ionosphere are incident at different complex angles. They are added, with allowance for their relative phase, to give an interference pattern in three dimensions. The time average of the power absorbed per unit volume is found from the total electric intensity and the resulting current density. It is equal to minus the divergence of the time averaged Poynting vector. To study these effects and to illustrate the methods of calculation, a semi-infinite homogeneous cold electron plasma with a sharp lower boundary is used as a model of the ionosphere. Calculation for points in the magnetic meridian plane is simplest and is explained first. It is then shown that for other directions a similar method can be used by rotating the coordinate axes about the vertical through a suitable complex angle. The effect of including two further waves, that have been reflected at a higher level, is also studied. Some typical results are presented.