The evening diffuse radio aurora,field-aligned currents and particle precipitation

The relationship of the afternoon/evening diffuse radio aurora, proton and electron precipitation and field-aligned currents is studied with data from the auroral radar at Slope Point, New Zealand, and the ISIS 2 satellite. It is shown that there is a very close association between the radio aurora and (primarily downward) field-aligned currents, which confirms and extends previous work, but that there is no clear relation with either proton or electron precipitation.     

Time dependent studies of the aurora—II. Spectroscopic morphology

The temporal morphology of auroral spectral emission features is investigated. For a given energy distribution of bombarding electrons but a time varying flux magnitude, the emission rates of various auroral radiations exhibit a nonlinear time response due to the variety of reactions that contribute to excitation. Absolute intensities and intensity ratios of various spectroscopic features, therefore, can vary solely with atmospheric interaction effects, indicating the importance of considering the time history of a precipitation event when attempting to infer the characteristics of auroral electrons from optical measurements.     

Relationships between radio aurora,visual aurora and ionospheric currents during a sequence of magnetospheric substorms

In an earlier paper the latitudinal and longitudinal structure of ionospheric current flow during a sequence of magnetospheric substorms was presented (McDiarmid and Harris, 1976). In the present paper the relationships between the electrojets, the radio aurora observed at 48 MHz and the all-sky camera-recorded visual aurora are presented for the same substorm sequence. The previously described morphology of radio aurora during substorms is confirmed and the observed relationships can be explained.     

Electron precipitation and ionospheric radio absorption in the auroral zones

Auroral absorption, measured with riometers at various stations, has been compared with the fluxes of energetic particles detected simultaneously by a satellite passing overhead. The absorption and the particle flux are found to be correlated, the relation depending on the time of day. The observed relationship is discussed in the light of computations of the absorption, based on the rates of ion production previously given by Rees and on reasonable atmospheric models. A case is presented for more prolonged comparisons of this kind, the combination of a polar orbit with ground-based observations at the South Pole station being particularly fruitful in this type of experiment.     

Electron precipitation equatorward of the auroral oval and the mantle aurora in the midday sector

In the midday sector, the hard electron precipitation and the associated patchy aurora at geomagnetic latitude ～65° are the only auroral features (? 20 keV) located equatorward of the dayside auroral oval during intense and moderately disturbed geomagnetic conditions. We identify the patchy luminosity in the midday and late morning sectors as the active mantle aurora. The mantle aurora was found by Sanford (1964) using the IGY-IGC auroral patrol spectrographs and which was thought to be non-visual. The precipitating electrons reside mostly at energies greater than several keV with an energy flux of ? 0.1 erg cm^?2 s^?1 sr^?1 during geomagnetic active periods. This hard precipitation occurs in a region which is asymmetric in L.T. with respect to the noon meridian. The region extends from the morning sector to only early afternoon (13–14 M.L.T.) along the geomagnetic latitude circle of about 65–70°. The model calculation indicates that the mantle aurora is produced by the precipitation of the energetic electrons which drift azimuthally from the plasma sheet at the midnight sector to the dayside magnetopause during magnetospheric substorms.     

Effects of interplanetary magnetic field and magnetospheric substorm variations on the dayside aurora

Photometric observations of dayside auroras are compared with simultaneous measurements of geomagnetic disturbances from meridian chains of stations on the dayside and on the nightside to document the dynamics of dayside auroras in relation to local and global disturbances. These observations are related to measurements of the interplanetary magnetic field (IMF) from the satellites ISEE-1 and 3. It is shown that the dayside auroral zone shifts equatorward and poleward with the growth and decay of the circum-oval/polar cap geomagnetic disturbance and with negative and positive changes in the north-south component of the interplanetary magnetic field (B_z). The geomagnetic disturbance associated with the auroral shift is identified as the DP2 mode. In the post-noon sector the horizontal disturbance vector of the geomagnetic field changes from southward to northward with decreasing latitude, thereby changing sign near the center of the oval precipitation region. Discrete auroral forms are observed close to or equatorward of the ΔH = 0 line which separates positive and negative H-component deflections. This reversal moves in latitude with the aurora and it probably reflects a transition of the electric field direction at the polar cap boundary. Thus, the discrete auroral forms observed on the dayside are in the region of sunward-convecting field lines. A model is proposed to explain the equatorward and poleward movement of the dayside oval in terms of a dayside current system which is intensified by a southward movement of the IMF vector. According to this model, the Pedersen component of the ionospheric current is connected with the magnetopause boundary layer via field-aligned current (FAC) sheets. Enhanced current intensity, corresponding to southward auroral shift, is consistent with increased energy extraction from the solar wind. In this way the observed association of DP2 current system variations and auroral oval expansion/contraction is explained as an effect of a global, ‘direct’ response of the electromagnetic state of the magnetosphere due to the influence of the solar wind magnetic field. Estimates of electric field, current, and the rate of Joule heat dissipation in the polar cap ionosphere are obtained from the model.     

Auroral precipitation and atmospheric vorticity at the 500 mb level

On the assumption that, in the long term, auroral and associated particle precipitation is uniform in magnetic time it can be shown that, due to the differing geometries in the northern and southern hemisphere, there exist two regions of maximum particle precipitation in different time zones in U.T. These are the midnight location of the auroral oval between about 1500–1800 h U.T. in the northern hemisphere and between 0000–0600 h U.T. in the southern hemisphere. These times correspond with the maxima of the indices am and am¹ for the respective winter periods.The northern hemisphere auroral zone precipitation maximum lies above the Siberian winter low pressure region at 500 mb heights which is displaced from the geographic pole. It is suggested that this relative location is basic to the Wilcox boundary crossing-absolute vorticity correlation.The southern hemisphere auroral zone precipitation maximum lies above the Antarctic low pressure region and is near the geographic pole. This results in the lack of a similar correlation as found by Burns.     

Effect of parallel electric field on charged particle precipitation

Considering the presence of electric field parallel to geomagnetic field in the magnetosphere, the problem of wave-particle interaction has been considered. Dispersion equation of whistler mode wave in presence of parallel electric field has been derived. Using the effective dispersion equation, the wave-particle interaction has been reformulated to account for the effect of parallel electric field. Using charged particle energy spectrum and magnetospheric field and plasma models, the flux of electron precipitation has been computed. It is shown that the parallel electric field plays an important role and may work in simultaneity with other processes known for enhancement of electron precipitation.     

Influence of microscopic particle interaction models on the flux of atmospheric antiprotons

We study the effect of particle interaction models on the theoretical estimates of atmospheric antiproton flux by comparing the BESS observations of antiproton spectra with the spectra obtained by means of a full three dimensional Monte Carlo simulation program. For such a purpose, we use two popular microscopic interaction models, namely FLUKA and UrQMD, to simulate antiproton spectra at multiple observation levels. In this article, we further compare the atmospheric antiproton fluxes predicted by a few popular microscopic high energy particle interaction models with each other to get an idea about the influence of such models at energies beyond the BESS upper cutoff up to about 100 GeV. We find that the simulated antiproton flux has strong dependence on the choice of interaction models. The present analysis seems to further indicate that the theoretical prediction of galactic antiproton spectrum may be uncertain by an appreciable amount due to our limited knowledge of particle interaction characteristics.     

Hydromagnetic field line resonances and modulation of particle precipitation

Hydromagnetic wave and modulated particle precipitation data are reported from conjugate areas near the particledrift shell L ～ 4. A modulation of electrons precipitating from the magnetosphere is observed in the conjugate regions when the accompanying hydromagnetic wave period is ~ 90 s and the wave polarization is linear. When the wave period changes abruptly to ~ 30 s and the polarizations at the observing stations are no longer linear, the modulation of the precipitating electrons is no longer observed. The change in hydromagnetic wave characteristics does not appear to be related to interplanetary plasma and magnetic field conditions. Rather, it is proposed to arise from a change in the wave generation mechanism from an internal magnetospheric source near the inner edge of the plasmapause (lower frequency) to an externally driven source outside the magnetosphere (higher frequency). This observation of a change in the wave characteristics (frequency and polarization) associated with modulated electron precipitation appears to be related to two previous examples wherein modulated electron precipitation was reported to be closely associated with the existence of a wave resonance region near the observing site.     

Further studies of ionospheric and geomagnetic effects of sudden impulses

From riometer records for the sudden impulse event of 4 February 1969, it is shown that ionospheric absorption accompanying a sudden impulse has the same type of latitude and longitude variations found for sudden commencement events. In addition, an examination of magnetograms at College, Alaska shows that some positive sudden impulses may trigger negative bays around local midnight, similar to the recent results for sudden commencements.     

Rocket studies of sporadic-E ionization and ionospheric winds

Simultaneous observations of positive ion density (by means of positive ion probes) and high-altitude winds (by scattering of sunlight in trails of alkali metal vapour) were made in three rocket firings at Hammaguir, Algeria in order to study the correlation of sporadic-E layers and high-altitude wind shears. On one occasion, a sporadic-E layer was found to occur at 95 km where the E-W component of the wind was zero and the wind shear was in the sense predicted by the theory of Whitehead. However, on two occasions, layers were also observed near 115 km where the E-W wind component was zero, but the wind shear was in the opposite sense. This appears to suggest that some modification of the theory is required.     

Jovian plasma sheet morphology: particle and field observations by the Galileo spacecraft

We present results from an investigation of the plasma sheet encounter signatures observed in the Jovian magnetosphere by the Energetic Particles Detector (EPD) and Magnetometer (MAG) onboard the Galileo spacecraft. Maxima in ion flux were used to identify over 500 spacecraft encounters with the plasma sheet between radial distances from Jupiter from 20 to 140R_J during the first 25 orbits (4 years of data). Typical signatures of plasma sheet encounters show a characteristic periodicity of either 5 or 10 hours that is attributed to an oscillation in the relative distance between the spacecraft and the plasma sheet that arises from the combination of planetary rotation and offset magnetic and rotational axes. However, the energetic particle and field data also display much variability, including instances of intense fluxes having little to no periodicity that persist for several Jovian rotation periods. Abrupt changes in the mean distance between the plasma sheet and the spacecraft are suggested to account for some of the transitions between typical flux periodicities associated with plasma sheet encounters. Additional changes in the plasma sheet thickness and/or amplitude of the plasma sheet displacement from the location of the spacecraft are required to explain the cases where the periodicity breaks down but fluxes remain high. These changes in plasma sheet characteristics do not display an obvious periodicity; however, the observations suggest that dawn/dusk asymmetries in both the structure of the plasma sheet and the frequency of anomalous plasma sheet encounters are present. Evidence of a thin, well-ordered plasma sheet is found out to 110R_J in the dawn and midnight local time sectors, while the dusk magnetosphere is characterized by a thicker, more disordered plasma sheet and has a potentially more pronounced response to an impulsive trigger. Temporal variations associated with changing solar wind conditions are suggested to account for the anomalous plasma sheet encounters there.     

The consequences of high latitude particle precipitation on global thermospheric dynamics

A previous comparison of experimental measurements of thermospheric winds with simulations using a global self-consistent three-dimensional time-dependent model confirmed a necessity for a high latitude source of energy and momentum acting in addition to solar u.v. and e.u.v. heating. During quiet geomagnetic conditions, the convective electric field over the polar cap and auroral oval seemed able to provide adequate momentum input to explain the thermospheric wind distribution observed in these locations. However, it seems unable to provide adequate heating, by the Joule mechanism, to complete the energy budget of the thermosphere and, more importantly, unable to provide the high latitude input required to explain mean meridional winds at mid-latitudes. In this paper we examine the effects of low energy particle precipitation on thermospheric dynamics and energy budget. Modest fluxes over the polar cap and auroral oval, of the order of 0.4 erg cm ⁻²/s, are consistent with satellite observations of the particles themselves and with photometer observations of the OI and OII airglow emissions. Such particle fluxes, originating in the dayside magnetosheath cusp region and in the nightside central plasma sheet, heat the thermosphere and modify mean meridional winds at mid-latitudes without enhancing the OI 557.7 line, or the ionization of the lower thermosphere (and thus enhancing the auroral electrojets), neither of which would be consistent with observations during quiet geomagnetic conditions.     

Observations of the plasmapause and diffuse aurora

Simultaneous auroral and whistler data from SANAE, Antarctica, show that the separation between the equatorward boundary of the diffuse aurora and the plasmapause lies between zero and 0.25 L. There is also some evidence to suggest that auroral precipitation occurs, at least partly, on closed field lines.     

Jupiter's ring system: New results on structure and particle properties

A reexamination of the Voyager images has yielded a refined understanding of Jupiter's diffuse ring system. The system is composed of a relatively bright narrow ring and inner toroidal halo, in addition to the exterior “gossamer” ring discussed elsewhere (Showalter et al., 1985, Nature 316, 526–528). The previously suspected inner disk is absent. The main ring is ∼7000 km wide and has an abrupt outer boundary at a radius of 129,130 ± 100 km. Visible in the ring are several narrow bright features, which may bear some relationship to Adrastea and Metis; these features appear to be narrower and relatively brighter in backscatter. The smallest ring particles obey a power law size distribution, and have an optical depth of 1–6 × 10⁻⁶ for grains up to 100 μm in radius. The largest bodies are dark, rough, and red, and of comparable total optical depth. The halo arises at the bright ring's inner boundary and rapidly expands inward to a ∼20,000-km full thickness, but remains symmetric about the ring plane. It disappears from sight at a radius of 90,000 km, roughly halfway between the main ring and the planet's cloudtops. The halo particles are not predominantly Rayleigh scatterers; they appear to obey a size distribution similar to that of the micron-sized population in the main ring, and comprise a similar optical depth.     

Comparative studies of E-region ionospheric drifts and meteor winds

Ionospheric drifts using total reflections from the E-region have been compared with neutral winds measured by meteor radar. Close agreement was found when both measurements were made in a common volume of atmosphere. Even with a separation of 700 km between the measuring regions the results were very similar. It is concluded that the drift technique does measure the movement of the neutral atmosphere in the altitude range 95–120 km. The agreement between measurements from widely separated regions indicates the horizontal scale of the wind structure is at least 700 km.     

The effect of particle precipitation events on the neutral and ion chemistry of the middle atmosphere: II. Odd hydrogen

A one dimensional time-dependent model of the neutral and ion chemistry of the middle atmosphere has been used to examine the production of odd hydrogen (H, OH, and HO₂) during charged particle precipitation. At altitudes above about 65 km, odd hydrogen production depends on the ionization rate, and the atomic oxygen and water vapor densities. Odd hydrogen production is shown to exhibit diurnal and other time dependent variations during such an event at these altitudes, and the assumption that two odd hydrogen particles are always produced per ionization is reexamined.