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Extended period of polar cap auroral display: auroral dynamics and relation to the IMF and the ionospheric convection

An unusually extended period (5 h) of polar cap auroral display on 3 August 1986 is examined. Auroras have been investigated using ground-based data as well as measurements from the IMP-8 spacecraft in interplanetary space and simultaneous observations from the polar-orbiting satellites Viking and DE-1 in the northern and southern hemispheres, respectively. It is found that visible Sun-aligned arcs are located inside the transpolar band of the θ-aurora observed from the satellite in ultraviolet wavelengths. The transpolar band can contain several Sun-aligned arcs that move inside the band toward the morning or evening side of the auroral oval independent of the direction of the band movement. Intensifications of polar cap auroras with durations of up to about 30 min are observed. No change has been found in either IMF parameters or substorm activity that can be related to these intensifications. The θ-aurora occurred during a 2-h period when the B z-component of the IMF was negative. A tendency is noted for dawnward (duskward) displacement of the transpolar band when By>0 (By<0) in the southern hemisphere. Simultaneous observations of auroral ovals during interplanetary Bz<0, By<0 and Bx>0 in both hemispheres and convection patterns for Bz<0 and By<0 have been displayed using satellite and ground-based measurements. It was found that the transpolar band of the <theta>-aurora in the sunlit hemisphere was situated in the region of large-scale downward Birkeland currents.
Delay coordinates: a sensitive indicator of nonlinear dynamics in single charged particle motion in magnetic reversals

The delay coordinate technique is examined as an indicator of the regime of particle dynamics for the system of single charged particle motion in magnetic reversals. Examples of numerically integrated trajectories in both static (zero electric field) and time dependent (corresponding nonzero induction electric field) simple models for magnetic reversals are considered. In the static case, the dynamics can in principle be directly classified by constructing Poincaré surfaces of section; here we demonstrate that whilst the Poincaré surface contains the relevant information to classify the dynamics, the corresponding delay coordinate plot can provide a far more sensitive indication of the onset of nonregular behaviour. In the case of nonperiodic time dependence considered here Poincaré plots cannot in general be constructed directly. Nevertheless, delay coordinate plots can still reveal details of the phase space portrait of the system, and here are shown to indicate whether segments of stochastic motion exist in a given trajectory. It is anticipated that the delay coordinate plot technique as realized here will be a valuable tool in characterizing the behaviour in large numbers of trajectories that are evolved in time-dependent systems, thereby giving us insight into the evolution of the distribution function as a whole, either in prescribed fields or in self-consistent numerical simulations.
Magnetopause magnetic barrier parameters in dependence on the solar wind magnetic field orientation

Some theories predict the magnetosheath magnetic field strength will decrease and the density increase just outside the dayside magnetopause as the interplanetary magnetic field turns southward. Two studies have recently reported results which confirm these expectations. In contrast, we briefly review our own theoretical predictions which indicate that precisely the opposite effect is expected. We survey new and previously reported magnetosheath observations and demonstrate that they are consistent with the predictions of our model. The conflicting results indicate a need for further theoretical and observational work.
The self-similar, non-linear evolution of rotating magnetic flux ropes

We study, in the ideal MHD approximation, the non-linear evolution of cylindrical magnetic flux tubes differentially rotating about their symmetry axis. Our force balance consists of inertial terms, which include the centrifugal force, the gradient of the axial magnetic pressure, the magnetic pinch force and the gradient of the gas pressure. We employ the "separable" class of self-similar magnetic fields, defined recently. Taking the gas to be a polytrope, we reduce the problem to a single, ordinary differential equation for the evolution function. In general, two regimes of evolution are possible; expansion and oscillation. We investigate the specific effect rotation has on these two modes of evolution. We focus on critical values of the flux rope parameters and show that rotation can suppress the oscillatory mode. We estimate the critical value of the angular velocity Ωcrit, above which the magnetic flux rope always expands, regardless of the value of the initial energy. Studying small-amplitude oscillations of the rope, we find that torsional oscillations are superimposed on the rotation and that they have a frequency equal to that of the radial oscillations. By setting the axial component of the magnetic field to zero, we study small-amplitude oscillations of a rigidly rotating pinch. We find that the frequency of oscillation ω is inversely proportional to the angular velocity of rotation Ω; the product ωΩbeing proportional to the inverse square of the Alfvén time. The period of large-amplitude oscillations of a rotating flux rope of low beta increases exponentially with the energy of the equivalent 1D oscillator. With respect to large-amplitude oscillations of a non-rotating flux rope, the only change brought about by rotation is to introduce a multiplicative factor greater than unity, which further increases the period. This multiplicative factor depends on the ratio of the azimuthal speed to the Alfvén speed. Finally, considering interplanetary magnetic clouds as cylindrical flux ropes, we inquire whether they rotate. We find that at 1 AU only a minority do. We discuss data on two magnetic clouds where we interpret the presence in each of vortical plasma motion about the symmetry axis as a sign of rotation. Our estimates for the angular velocities suggest that the parameters of the two magnetic clouds are below critical values. The two clouds differ in many respects (such as age, bulk flow speed, size, handedness of the magnetic field, etc.), and we find that their rotational parameters reflect some of these differences, particularly the difference in age. In both clouds, a rough estimate of the radial electric field in the rigidly rotating core, calculated in a non-rotating frame, yields values of the order mV m–1.
Solar wind velocity distribution on the heliospheric current sheet during Carrington rotations 1787-1795

The solar wind velocity distribution in the heliosphere is best represented using a v-map, where velocity contours are plotted in heliographic latitude-longitude coordinates. It has already been established that low-speed regions of the solar wind on the source surface correspond to the maximum bright regions of the K-corona and the neutral line of the coronal magnetic field. In this analysis, v-maps on the source surface for Carrington rotations (CRs) 1787–1795, during 1987, have been prepared using the interplanetary scintillation measurements at Research Institute of Atmospherics (RIA), Nagoya Univ., Japan. These v-maps were then used to study the time evolution of the low-speed (leq450 km s–1) belt of the solar wind and to deduce the distribution of solar wind velocity on the heliospheric current sheet. The low-speed belt of the solar wind on the source surface was found to change from one CR to the next, implying a time evolution. Instead of a slow and systematic evolution, the pattern of distribution of solar wind changed dramatically at one particular solar rotation (CR 1792) and the distributions for the succeeding rotations were similar to this pattern. The low-speed region, in most cases, was found to be close to the solar equator and almost parallel to it. However, during some solar rotations, they were found to be organised in certain longitudes, leaving regions with longitudinal width greater than 30° free of low-speed solar wind, i.e. these regions were occupied by solar wind with velocities greater than 450 km s–1. It is also noted from this study that the low-speed belt, in general, followed the neutral line of the coronal magnetic field, except in certain cases. The solar wind velocity on the heliospheric current sheet (HCS) varied in the range 300–585 km s–1 during the period of study, and the pattern of velocity distribution varied from rotation to rotation.
Longitudinal effects of ionospheric responses to substorms at middle and lower latitudes: a case study

An ionospheric model is used to simulate total electron content (TEC) disturbance events observed at middle and lower latitude sites near 75°W and 7°E longitudes. Within this longitudinal range, daytime TEC disturbances show patterns that are correlated with substrom activity seen in both auroral electrojet and ring current behavior. In modeling studies of the observed ionospheric effects, both electric field and neutral wind perturbations are examined as possible mechanisms. The morphological features of the required electric field perturbations near drawn and dusk are compared with those at other times to examine the local time characteristics of magnetospheric influence. Large-scale traveling atmospheric disturbances (TADs), an alternative candidate for the disturbance source, are also characterized and compared with known thermospheric behavior.
External gravity oscillations in Lake Onega

Lake Onega is located in the southern part of Karelia, in the north-west of Russia. We report data taken by various limnigraphs and current meters and isolate, by spectral analysis, the most conspicuous barotropic periods of these signals. The analysis that follows is based on the linearized shallow-water equations, that are solved for the free oscillations and thus identify the eigenperiods and corresponding mode structures for this lake. Computational results are presented for a finite difference representation for these surface-seiche equations applied to the entire Lake Onega including its bays. The grid consists of quadratic elements of 1000 m side length corresponding to 9344 active cells. The emerging matrix eigenvalue problem exceeds the storage and compuational capacity of standard PCs or workstations and thus requires use of the approximate Lanczos procedure to isolate the first ten eigenperiods and corresponding mode structures of the barotropic seiches, lying between 12.1 and 2.2 h. Comparison of computational results and inferences from the water level and current-meter records, disclose satisfactory agreement between theory and observation.
The measurement and extraction of the Nu6 atmospheric emission band of CFC-12 from interfering emission features

Ground-based thermal emission measurements of the zenith sky have been made at Peterborough, Ontario since January 1993. In this paper, the measurement of the Nu6 band of atmospheric CFC-12, an important greenhouse gas, is presented for a cold, clear day in January 1994. A spectrum of the non-CFC-12 emission features has been simulated using the FASCD3P radiation code and measured radiosonde profiles of temperature, pressure and humidity. This has enabled a satisfactory subtraction of the interfering emission features from the CFC-12 emission spectrum. A comparison of the observed and simulated Nu6-bands of CFC-12 shows good agreement at all frequencies of emission. From these spectra the total downward greenhouse radiative flux from the CFC-12 Nu6 emission based for a very cold day has been estimated to be 0.27 W m–2±10%.
Inversion and space-time-averaging algorithms for ScaRaB (Scanner for the Earth Radiation Budget). Comparison with ERBE

Establishment of a uniform long-term record of "top-of-the atmosphere" (TOA) Earth radiation budget (ERB) components, on a scale appropriate to the study of cloud radiation interactions, requires that the data obtained from different observation missions satisfy two basic conditions: (1) the broadband shortwave (SW:0.2–4 µm) and longwave (LW: 4–50 µm) radiances must be demonstrably made on the same absolute scale; and (2) the methods used first to convert the instantaneous (filtered) radiances into (unfiltered) SW and LW radiant fluxes, and then to perform the space-time integrations to yield regional monthly means, must be consistent. Here we consider mainly the second point, with regard to the ScaRaB/Meteor mission in orbit since 25 January 1994 and observing the Earth since 24 February 1994. The objective of this mission is to determine the TOA ERB components and so to provide a continuation of the NASA ERBE scanner mission (November 1984–February 1990). We show how results compatible with ERBE can be obtained by taking into account the instrumental characteristics and the satellite orbit parameters: spectral response of the broadband channels, Earth local time of observation. Considering the spectral response of the ScaRaB broadband channels, we show that no spectral correction is required in the longwave domain, whereas a correction of +4.5% must be applied in the shortwave domain for clear and partly cloudy ocean, in order to compensate for underestimation at the shortest wavelengths. Despite possible differences between ERBE and ScaRaB procedures in values assumed for certain parameters of the scene/cloud identifications, application of these procedures to the same set of ERBE data (spectrally corrected, i.e. "unfiltered" radiances) shows that scene identification agreement is close to 90% and that, where there is disagreement, resulting differences in LW fluxes are negligible, those in SW fluxes small. We show that regional and global mean quantities are in excellent agreement, considering that differences between (ERBS+NOAA-9) and (NOAA-9 only) results may be taken as illustrating time-sampling effects. We find that biases may occur from the undersampling, specifically for the night-time clear-sky estimation over land and desert. Preliminary results using ScaaB data of March 1994 show that clear-sky regional estimates may be less numerous than in ERBE scanner products, due to either the larger pixel size or the auxiliary parameters used in the scene identification, and that expected uncertainties in the global monthly mean values depend mainly on the instrument radiometric calibration.
First observation of mesospheric wind shear as high as 330 m s−1 km−1

Mesospheric wind profiles with an altitude resolution of 25 m have been obtained by means of radar tracking of foil chaff clouds. Such experiments were performed during winter 1990 at Biscarrosse, France (44°N, 1°W). On one flight, a wind shear as high as 330 m s–1 km–1 at 87.4 km and a region of dynamical instability between 86 and 88 km was measured. This wind shear is believed to be the largest value ever measured in the mesosphere. The region of dynamical instability results from a superposition of two wave motions, and is found to link well with enhanced turbulence and small-scale wave activity.