Interferometric phase velocity measurements in the auroral electrojet

A double-probe electric field detector and two spatially separated fixed-bias Langmuir probes were flown on a Taurus-Tomahawk sounding rocket launched from Poker Flat Research Range in March 1982. Interesting wave data have been obtained from about 10s of the downleg portion of the flight during which the rocket passed through the auroral electrojet. Here the electric field receiver and both density fluctuation (δn/n) receivers responded to a broad band of turbulence centered at 105 km altitude and at frequencies generally below 4 kHz. Closer examination of the two (δn/n) turbulent waveforms reveals that they are correlated, and from the phase difference between the two signals, the phase velocity of the waves in the rocket reference frame is inferred. The magnitude and direction of the observed phase velocity are consistent either with waves which travel at the ion sound speed (C_s) or with waves which travel at the electron drift velocity. The observed phase velocity varies by about 50% over a 5 km altitude range—an effect which probably results from shear in the zonal neutral wind, although unfortunately no simultaneous neutral wind measurements exist to confirm this.     

Radio scintillations produced by irregularities at sporadic-E level

Different observers assign differing degrees of importance to sporadic-E irregularities in producing amplitude variations (scintillations) in radio star and satellite transmissions. The most reliable index of the effectiveness of sporadic-E in this respect has been found to be the difference between critical and blanketing frequencies. It is shown how an apparent contradiction between the results of different observers may be resolved.     

The seasonal structure of ionosonde Es parameters and meteoroid deposition rates

A comparison covering more than three decades is made between the seasonal variation of radar meteor influx and seasonal variations in the occurrence probabilities of ionosonde sporadic-E parametersƒ₀E_s and ƒ_bE_s for different diurnal intervals at two Southern Hemisphere stations. The analyses show that for medium intensity 3E_m type sporadic-E no clear correlation with major Southern Hemisphere meteor shower activity exists. This finding which does not support some earlier shorter period surveys suggests the need for further work into the aeronomy of E_s source ions.     

Lower midlatitude ionospheric disturbances and the perkins instability

This paper presents a preliminary study of ionospheric disturbances at dip latitudes less than 40° as seen by the ion drift meter and the retarding potential analyzer on board Atmosphere Explorer E during a period of low solar activity. The altitude of observation was relatively low, mostly below 300 km. The emphasis is on the midlatitude region, where some features resemble equatorial bubbles; no clear demarcation in latitude could be recognized between the bubbles and other midlatitude disturbances. Excellent evidence was found that the Perkins instability is responsible for very structured disturbances which were frequently observed. In most cases where regions of inward and outward E × B drift were encountered, diffusive motion up or down the field line partially cancelled the vertical component of the cross-field drift. In some cases cancellation was almost perfect, but in others it appeared not to occur at all (probably cases involving rapid changes). Within the larger structures caused by the Perkins instability there were places where the secondary gradient drift instability was also active.     

Directional currents in nocturnal E-region layers

In the mid-latitude E-region, the wind-shear mechanism produces thin ionized layers at levels where the vertical ion velocity is zero. We show that such layers conduct electric current only towards the magnetic equator, and not in the zonal direction. We surmise that this property may influence the electric field distribution in the nocturnal ionosphere, and possibly also the coupling between ion drifts and neutral air winds in the F-region. Detailed case studies of nocturnal layers located near the peak of ion Pedersen conductivity (around 130km) are needed to test this idea.     

110 km neutral zonal wind patterns

In the height range between 105 and 115 km sporadic E formation is due exclusively to the zonal (E-W) neutral winds and both theory and experiment indicate sporadic E will occur very close to a reversal point of this zonal wind. By studying the observed heights of sporadic E-layers from a global distribution of stations we can deduce some of the regular properties of the zonal winds at 110 km. The semidiurnal zonal wind pattern is shown to be well defined, is principally the 2,2 mode, and agrees well with theoretical predictions. The diurnal zonal wind pattern is less clearly defined and does not closely resemble any theoretical mode. Steady components agree with those found by other methods.     

Theory of electric currents in ionospheric E-layers

The magnetic disturbance associated with East-West current in the ionosphere is calculated in terms of the production and loss of ionisation. This is physically equivalent to a conventional equation of the type j = [σ]E, but may be preferred in many experimental circumstances. The relationship between the deformation of an ionospheric layer and the electric current, or magnetic disturbance in it, is explored in detail. Applications to mid-latitude sporadic-E, the equatorial electrojet, night-E, deformation of mid-latitude E-layer by quiet and disturbed currents and deformation of the E-layer by auroral electrojets are considered. Under a wide range of conditions, vertical backscatter devices can be used to find the altitude profile of the East-West component of ionospheric electric current by measuring the deformations of the vertical profiles of electron density.     

Ridge belt tectonics of Ganiki Planitia on Venus

The main major ridge belts of Ganiki Planitia on Venus (Lama, Ahsonnutli and Pandrosos Dorsa) are part of the fan-shaped ridge belt complex along the 200 parallel of longitude. These ridge belts with evidence of crustal shortening support the idea of a large-scale E-W compression. The ridge belt patterns indicate a N-S shear component. These forces are explained by a triangular planitia area which compressed by surrounding terrains. The crustal shortening and ridge belt formation indicates compressional plate movement stresses in the uppermost lithosphere.Three sizes of ridge belt structure are to be found within Ganiki Planitia. (1) The ridge belt spacing of 200–400 km can be used to estimate the depth of the major uppermost homogeneous layer of Venus. There are numerous volcanic coronae, paterae and montes located along the main ridge belts or at their junctions. (2) Mid-size ridge groups or subbelts are to be found within the major ridge belts. These are formed by more local responses to tectonic stresses in the stratified uppermost crust. A wavelength of 40–70 km can be seen as a result of bending of the crustal strata and may relate to its thickness. (3) Small individual ridges are connected with most local stresses, defining places where the surface layers broke along the crests of large ridge belts or mid-scale subbelts. Radial and concentric mare ridge-like structures around coronae indicate that corona formation was effective at a sufficiently close vicinity to fault the surface.     

Highly supersonic molecular flows in wind-clump boundary layers

The gradual acceleration, through viscous coupling to a wind, of molecular hydrogen in turbulent boundary layers around obstacle clumps is proposed to be responsible for the widths of the emission features with full widths at zero intensity of greater than 100 km s^–1.     

The role of neutral winds and ionospheric electric field in forming stable sporadic E-layers

Using the combined measurements from a rocket flight through a stable intense sporadic E-layer, we examine the shortcomings of conventional wind shear theory of ion layer formation, principally in underestimating the role of the ambient ionospheric electric field. Our results imply that the ionospheric electric field may control the stability and precise location of such ionisation layers within a region of convergent ion flow.     

The Stratification of Jupiter's Troposphere at the Galileo Probe Entry Site

Galileo Probe Atmospheric Structure Investigation (ASI) pressure and temperature sensor data acquired during the parachute descent phase have been used to derive the static stability structure of Jupiter's troposphere at pressure levels of 0.5-22 bars using three techniques. The first approach utilizes both the p-sensor and T-sensor data, but since the p-sensor's zero offset was significantly affected by the thermal anomaly in the probe, two other approaches using only T-sensor data have also been developed. By making the physically reasonable assumptions of equilibrium descent for the probe and hydrostatic balance of the atmosphere, an algorithm for deriving the background static stability from T-sensor measurements alone is developed. Regions with static stability 0.1-0.2 K km⁻¹ are found at 0.5-1.7 bars, 3-8.5 bars, and 14-20 bars. Between these layers, regions of weaker static stability are present. Mean molecular weight gradients due to the vertical variation of water vapor abundance near the 11-bar pressure level appear to stabilize the atmosphere at this level. Oscillatory structures with vertical wavelength ∼15-30 km and amplitude ∼0.1-0.2 K are observed in the T-sensor data. For pressure <2 bars, these eddies are well above the noise level of the measurements and are consistent with the predictions of linear gravity wave theory for a wave with horizontal phase speed c_x=160 m s⁻¹ with respect to System III propagating through the static stability derived from the T-sensor data alone. They provide quantitative confirmation of the static stability derived from T-sensor data in the troposphere where p<2 bars. The observed static stability structure shows an inverse correlation with the regions of wind shear observed by the Doppler Wind Experiment: regions of highest shear in the horizontal wind appear to be associated with regions of lowest static stability. The particulate population detected by other experiments on the probe shows some correlation with the uppermost layer of static stability, suggesting enhanced solar energy deposition at these levels may play a role in producing the positive static stability.     

Properties of energetic water-group ions in the extended pick-up region surrounding comet Giacobini-Zinner

The properties of energetic (65–95 keV) cometary water-group ions in the extended solar wind pick-up region surrounding comet Giacobini-Zinner are examined using data from the EPAS instrument on the ICE spacecraft. In the outer part of this region, extending from cometocentric distances of several hundred thousand to a few million kilometres (the limit of pick-up ion detectability), it is found that large modulations of the ion flux occur (with J_MAX/J_MIN 10²-10³) which are related to the direction of the magnetic field. It is also found that the ions stream in a direction which is intermediate between the directions of the solar wind flow and the E × B drift, and that ions are present at energies somewhat above the local pick-up energy. These properties indicate that the waves which are excited by the unstable “ring-beam” pick-up ion velocity distributions do result in significant scattering of the ions in this region, both in pitch angle and in energy, but that they have insufficient amplitude to scatter the ions into near isotropy in the solar wind frame. Closer to the comet (but still upstream from the bow shock), the ion flux modulations are considerably reduced in amplitude and the ions respond less to the E × B drift, indicating that the ions are scattered nearer to isotropy in this region. Inbound, this transition takes place relatively abruptly at a distance of 4 × 10⁵ km in association with an increase in the solar wind speed, after which the ion flux increases, and ceases to be modulated by the field direction, while the streaming direction is continuously antisolar and unmodulated by the direction of the E × B drift. Outbound, weak vestiges of the ring-beam ion anisotropy are present in the region immediately upstream from the bow shock (at −1 × 10⁵ km), but these become more marked at distances in excess of t4 × 10⁵ km, increasing gradually with increasing distance from the comet. It is shown that the evolution of the ion properties is qualitatively consistent with expectations based on quasi-linear diffusion of the ions by the magnetosonic waves observed during the encounter.     

Midlatitude sporadic-E layers and vertical metallic ion drift profiles

An investigation of the relationship between the occurrence of midlatitude sporadic-E layers and convergent points in the ion drift profiles has been made using the 430 MHz incoherent scatter radar at the Arecibo Observatory. Electron concentration profiles were obtained using a 13 baud Barker coded pulse yielding 600 m range resolution, while a 5-pulse sequence with 2.4 km range resolution was used to obtain line-of-sight ion drift velocities. With some exceptions, observed sporadic-E layers occur near convergent points in the vertical metallic ion drift profile, and vertical motions of these layers follow the vertical motions of the convergent point.Vertical motions and intensity variations of observed sporadic-E layers are due to interaction between the mean wind, tidal waves, and gravity waves of different periods producing a predominantly downward motion of the layer. However, a sudden increase in the altitude of a sporadic-E layer has been observed. This is attributed to the disappearance of the convergent point—releasing the layer—followed by an ascent of the layer to the closest overlying convergent point.     

Observation of a thin Es-layer by the eiscat radar

High resolution E-region measurements carried out on 16 November 1983 using the EISCAT incoherent scatter radar are presented. The experiment was monostatic with a vertical radar beam, and it was based on a Barker-coded four-pulse code on one frequency channel and Barker-coded single pulses on three channels. The basic integration time was 15 s and the spatial resolution 450 m. The results reveal a short-lived but intense thin sporadic E-layer at 18:00–18:06 U.T. at an altitude of about 106 km. Both before and during the event, downward ion velocities of the order of 100 m s⁻¹ are observed above this height. A convergent null in the vertical ion speed is occasionally seen at the layer altitude. The layer occurrence is associated with auroral arcs drifting across the radar beam. Simultaneous observations of the STARE radar show an ionospheric electric field of 25–30 mV m⁻¹. The field always has a westward component, which is in accordance with the observed downward plasma flow. Most of the time when the layer is intense, the field points into the NW-sector. Theoretically, this field direction should create convergent vertical plasma motion. Therefore it is suggested that the observed E_s-layer is created by the action of the auroral electric field rather than by the wind shear mechanism.     

Atmospheric expansion from Joule heating

An expression for the vertical velocity of the neutral atmosphere in the F-region is derived for Joule heating by the electric field that drives the auroral electrojet. When only vertical expansion is allowed, it is found that the vertical wind must always increase monotonically with altitude. The heating rate is proportional to the F-region ion density, so that appreciable heating, even during high electric fields, requires some production mechanism of ionization such as auroral secondary ionization or solar photoionization, in the lower F-region. Once started at night, when an ionizing source is present in the lower F-region, the expansion of the atmosphere transports ionization upward, thereby increasing the heating rate, and hence the expansion rate, i.e. positive feedback. Electric field strengths and F-region ion densities of 50 mV/m and 2 × 10¹¹e/m³, respectively, will produce vertal neutral wind speeds of several tens of m/sec in the 300–500 km altitude range. During periods of high magnetic activity, i.e. high electric field, Joule heating can produce large increases in the relative N₂ concentration in the upper F-region; computations made with a simple model suggest that tenfold increases can occur at 400 km altitude $\text{1}\text{2?1}\text{hr}$ after the onset of magnetic activity, a result in agreement with satellite observations. When the Joule heating theory is applied to incoherent scatter data taken during one period of high heating, the horizontal electric field in the F-region is found to decrease markedly, possibly approaching zero as the field penetrates a weak, discrete auroral arc; the decrease began 10–20 km from the arc.     

Ion flow and momentum transfer in the Venus plasma environment

An analysis of ion data from 390 Venus Express, VEX, orbits demonstrates that the flow of solar wind- and ionospheric ions near Venus is characterized by a marked asymmetry. The flow asymmetry of solar wind H⁺ and ionospheric O⁺ points steadily in the opposite direction to the planet’s orbital motion, and is most pronounced near the Pole and in the tail/nightside region. The flow asymmetry is consistent with aberration forcing, here defined as lateral forcing induced by the planet’s orbital motion. In addition to solar wind forcing by the radial solar wind expansion, Venus is also subject a lateral/aberration forcing induced by the planet’s orbital motion transverse to the solar wind flow.The ionospheric response to lateral solar wind forcing is analyzed from altitude profiles of the ion density, ion velocity and ion mass-flux. The close connection between decreasing solar wind H⁺ mass-flux and increasing ionospheric O⁺ mass-flux, is suggestive of a direct/local solar wind energy and momentum transfer to ionospheric plasma. The bulk O⁺ ion flow is accelerated to velocities less than 10 km/s inside the dayside/flank Ionopause, and up to 6000 km in the tail. Consequently, the bulk O⁺ outflow does not escape, but remains near Venus as a fast (km/s) O⁺ zonal wind in the Venus polar and nightside upper ionosphere. Furthermore, the total O⁺ mass-flux in the Venus induced magnetosphere, increases steadily downward to a maximum of 2 × 10⁻¹⁴ kg/(m² s) at ≈400 km altitude, suggesting a downward transport of energy and momentum. The O⁺, and total mass-flux, decay rapidly below 400 km. With no other plasma mass-flux as replacement, we argue that the reduction of ion mass-flux is caused by ion-neutral drag, a transfer of ion energy and momentum to neutrals, implying that the O⁺ plasma wind is converted to a neutral (thermosphere) wind at Venus. Incidentally, such a neutral wind would go in the same direction as the Venus atmosphere superrotation.     

Long-period magnetic pulsations generated in the magnetospheric boundary layers

Long-period hydromagnetic waves can be excited by the velocity shear instability in the magnetospheric boundary layers, where the penetrated bulk flow of the solar wind comprises a fairly strong velocity shear. Model spaces of the boundary layers are considered to estimate amplification rates on the HM waves in the low-latitude flank-side and in the dayside high-latitude and mantle-side boundary layers, where the ambient magnetic field is assumed to be perpendicular and parallel to the bulk flow of the solar wind, respectively. Wave characteristics of the HM waves are also investigated for the k-vector almost normal to the magnetopause.The localized HM waves in the Pc 3–4, Pc 4–5 and Pc 6 frequency ranges, of which group velocities are mostly parallel to the plane in the ambient magnetic field and the bulk flow directions, i.e., parallel to the magnetopause, are sufficiently amplified in the dayside low- and high-latitude, in the low-latitude flank-side, and in the mantle-side boundary layers, respectively. A left-handed toroidal (transverse) and a right-handed poloidal (compressional) mode of long-period $\text{(T ? 120}\text{s}\text{)Ω}{}_{\mathrm{A}}$-wave are generated in the dawn- and the duskflank boundary layers, respectively, where the k-vector of Alfvénic signals was assumed to be almost in the Archemedean spiral direction. The localized compressional HM waves in the Pc 3–4 range indicate both lefthanded and right-handed polarizations in the dayside boundary layer, which are functions of the k-vector of the waves and the sense of the velocity shear. The variance directions of perturbation fields of the HM waves in the magnetospheric boundary layers tend to be nearly parallel to the magnetopause. These localized HM waves can propagate into the high-latitude ionosphere. We conclude that the localized HM waves driven by the velocity shear instability in the magnetospheric boundary layers are the most probable source of the daytime Pc 3–5 magnetic pulsations in the outer magnetosphere.     

Two-stream instability in a collisionless plasma

This paper discusses the development of two-stream instability in a collisionless plasma. The plasma is described by velocity moments of Vlasov equation where heat flow tensor has been neglected. A dispersion relation for arbitrary propagation is derived for a collisionless electron fluid. Special cases of propagation parallel and perpendicular to the field lines are discussed. Growth rate is computed for parameters representative of the shear layers of solar wind at one AU. It is found that the shear layers are likely to be overstable.     

Solar wind interactions with Comet 19P/Borrelly

The Plasma Experiment for Planetary Exploration (PEPE) made detailed observations of the plasma environment of Comet 19P/Borrelly during the Deep Space 1 (DS1) flyby on September 22, 2001. Several distinct regions and boundaries have been identified on both inbound and outbound trajectories, including an upstream region of decelerated solar wind plasma and cometary ion pickup, the cometary bow shock, a sheath of heated and mixed solar wind and cometary ions, and a collisional inner coma dominated by cometary ions. All of these features were significantly offset to the north of the nucleus-Sun line, suggesting that the coma itself produces this offset, possibly because of well-collimated large dayside jets directed 8°-10° northward from the nucleus as observed by the DS1 MICAS camera. The maximum observed ion density was 1640 ion/cm³ at a distance of 2650 km from the nucleus while the flow speed dropped from 360 km/s in the solar wind to 8 km/s at closest approach. Preliminary analysis of PEPE mass spectra suggest that the ratio of CO⁺/H₂O⁺ is lower than that observed with Giotto at 1P/Halley.     

General features of the magnetic field of the equatorial electrojet measured by the POGO satellites

The magnitude of the equatorial electrojet signature, S, is a measure of its magnetic field at the location of the satellite recording the signature. The general features of the large quantity of the magnetic field data of the electrojet observed by the series of POGO satellites from 1967 to 1970 have been studied here. We have compared the position of the axis of the electrojet as indicated by the position of the minimum of the electrojet signature with the position of the dip equator on the Earth's surface, and we find no significant latitudinal shift of the electrojet axis from the dip equator on the Earth. Apart from the expected decrease of the magnetic field of the electrojet with altitude above the electrojet, we have found unexpected cases in which the field increases with altitude. More surprisingly, we have discovered that the magnitude of S oscillates with altitude having maxima at about 460km and 635km and minima at about 580km and 725km, with a mean wavelength of 160 ± 29 km. It is suggested that this could be caused by additional weak current layers flowing above the main electrojet at about 110 km altitude. It is also pointed out that Onwumechili's model based on a single current system of the equatorial electrojet predicts field oscillation with altitude. The model therefore shows that a field oscillating with altitude can also result from a single complicated system of current unaided by additional current layers.