Phase variation of ULF pulsations along the geomagnetic field-line

Some points are discussed concerning possible sources of confusion in earlier theory on ULF pulsations with finite ionosphere conductances. Behaviour of the time-integrated Poynting vector in this theory is examined in detail. It is shown that, whenever ionosphere conductances are asymmetric, the time-integrated Poynting vector is zero at a point (termed the “null-point”) displaced away from the equatorial plane. The wave phase behaviour along the field-line is consistent with a picture of travelling-wave components originating at the null-point and carrying energy to the ionospheres.Higher harmonics of resonances with asymmetric conductances are discussed from the point of view of comparing electric field phase using measurements from geostationary satellites and STARE-type auroral radars. The null-point behaviour suggests that some surprising phase differences may be obtained in certain cases between equatorial plane and ionosphere.     

Quarter-wave ULF pulsations

Recent theoretical work has predicted the possible existence of “quarter-wave” ULF pulsation resonances, in which the wave electric field has a near-node in one ionosphere and an antinode in the conjugate ionosphere. Eigenvalues are derived for quarter-wave toroidal and guided poloidal resonances for a range of L-values and plasma density distributions. From these eigenvalues, resonant periods can be obtained.Three pulsation events with anomalously long periods (when interpreted as half-waves) are examined in the light of these results. It is decided that only one event is a good candidate for quarter-wave status; this event seems likely to be a driven resonance effectively in the quarter-wave guided poloidal mode.     

Long period Pc5 pulsations

The characteristics of long period Pc5 pulsations (frequency 3·33-1·67 mHz; period 300–600 sec) for stations in the subauroral, auroral and polar zones are studied for 1967. These pulsations occur mainly in the auroral and polar zones with one morning and one evening peak; in the cusp region they occur most frequently near local noon. The evening peak gets stronger and appears farther away from noon with increasing geomagnetic activity. Periods are shorter and amplitudes larger in the morning compared to the evening hours. Only in a small latitudinal belt (60–70°) do the periods tend to increase with latitude. Amplitudes are almost always maximum near the central line of the auroral zone and drop much more sharply towards lower latitudes than towards higher latitudes. Considerable diurnal variations and also variations with magnetic activity are found to exist in the occurrence-latitude and amplitudelatitude profiles. In all the three regions the occurrence and the amplitude of these pulsations increase with magnetic activity to a certain level after which results become uncertain. Periods either do not change very much or at some stations decrease as activity increases.     

Nonlinear plasma density modification by the ponderomotive force of ULF pulsations at the dayside magnetosphere

We investigate analytically and numerically a nonlinear modification of the magnetospheric plasma density under the action of the ponderomotive force induced by ULF traveling waves, using the nonlinear stationary force balance equation. This equation is applied to both the dipole and dayside magnetosphere having one and two minima of the geomagnetic field near the magnetospheric boundary. The separate and joint actions of the ponderomotive, centrifugal, and gravitational forces on the density distribution are shown.     

The relationship between ULF geomagnetic pulsations and ionospheric Doppler oscillations: Model predictions

This paper is a continuation of the work of Sutcliffe and Poole (1989, J. geophys. Res. 94, 13,505) which described the mechanisms which connect ground-based geomagnetic pulsation measurements with simultaneously observed Doppler velocity oscillations (V^*) in the ionosphere. We concentrate on a presentation of model predictions that show the extreme variability of V^* with parameters such as radio sounding frequency, electron concentration profile, magnetoionic mode, geomagnetic field inclination, scale length and pulsation frequency. The potential to use these results in a diagnostic fashion is also discussed.     

Ionospheric Joule dissipation as a damping mechanism for high latitude ULF pulsations: Observational evidence

On 9 January 1979 an SI-excited pulsation event was observed by the Scandinavian Magnetometer Array. The pulsation period shows a clear variation with latitude which suggests decoupled oscillations of individual magnetic field shells. The pulsation amplitudes exhibit an e-fold decay with the damping rate γ varying both in longitudinal and latitudinal directions. Assuming Joule heating in the ionosphere as the dominant damping mechanism (and thus γ ～ Σ^?1_p) approximate height-integrated Pedersen conductivity profiles were calculated which fit well with previously observed Σ_p distributions. This is interpreted as observational evidence for ionosopheric Joule dissipation as the major damping mechanism for high-latitude ULF-pulsations.     

The diurnal variation of atomic hydrogen

This note critically examines the relative importance of several effects which influence the diurnal variation of atomic hydrogen abundance near the critical level.It is pointed out that the neglect of exospheric hydrogen in a recent theoretical treatment causes an overestimation of the diurnal variation at high exospheric temperatures, and an underestimation at low exospheric temperatures. The fluxes due to lateral flow are large compared to other fluxes only to the extent that the actual diurnal variation is very different from the diurnal variation corresponding to zero net lateral flow, which does not seem to be the case in the real atmosphere. Two effects which are probably important are charge exchange reactions with thermal oxygen ions, resulting in a diurnal exchange with the plasmasphere; and charge exchange reactions with high velocity protons, resulting in enhanced escape and diurnal variation.     

Period variation of Pc5 pulsations at high latitudes

Geomagnetic data for the year 1967 from seven Canadian observatories, spanning the subauroral, auroral and polar zones, have been analysed to investigate the characteristic variation of Pc5 period with several geophysical variables. Pulsations in the whole spectrum of Pc5 (period range 150–600 s) were found to occur at all of the observatories. Those with smaller periods occurred more frequently at lower latitudes while those with longer periods occurred more frequently at higher latitudes. Daily variation of the periods of Pc5 showed little change with seasons or with magnetic activity. Periods, in general, had two daily maxima which appeared at different local times in different zones. A predominant morning peak was noted at all stations except Baker Lake, where a mid-day maximum of the period was found. The Pc5 periods tended to increase with geomagnetic activity at lower latitude stations, and to decrease with activity at stations in the polar cap for low to moderateKp levels. At high activity levels these trends appeared to reverse, though results are less certain. In different seasons and for the whole year the periods increased almost linearly with latitude. However when similar analysis was done for individual hours of the day and for different magnetic activity groups, this linear relationship between period and geomagnetic latitude was not evident. Efforts to detect a 27-day recurrence tendency of Pc5 periods did not succeed.Contributions from the Earth Physics Branch No. 495.     

On the adiabatic pulsations of uniformly rotating bodies

The problem of the oscillations and stability of compressible Maclaurin spheroids is reconsidered, on the basis of the third-order virial equations, in an arbitrarily rotating frame of reference. In contrast with the work of Kochhar and Trehan (1974), it is found that the frequencies evaluated in a rotating frame and those evaluated in an inertial frame are related to one another in a very simple way. Numerical calculations made for a wide range of the adiabatic exponent further clarify the effect of compressibility on the natural frequencies.     

On fast magnetosonic coronal pulsations

The linear properties of the fast magnetosonic modes of a coronal loop modelled as a smooth density inhomogeneity in a uniform magnetic field are compared with the case of a step function slab. It is shown that the group velocityC _gof the modes, important in determining the structure of impulsively excited wave packets, possesses a minimum for a wide class of profile including the slab, with the exception of the Epstein profile for which the minimum inC _gmoves out to infinity. Results for the simple step profile are thus of wider validity, and likely to be applicable to coronal loops.     

Characteristics of enhanced and low-amplitude cosmic-ray diurnal variation

The occurrence of a large number of high- and low-amplitude cosmic-ray diurnal wave trains during the two solar cycles (20 and 21) over the years 1965–1990 has been examined as a function of solar activity. The high-amplitude days with the time of maximum in the 18:00 hr corotation direction do not indicate any significant correlation with solar activity. But, the low-amplitude days are inversely correlated with solar activity and the time of maximum shifts to earlier hours ( 15:00 hr direction). The slope of the power-specrum density roughly characterized by power spectral index n in the high-frequency range 3.5 x 10^–5 Hz to 8.3 x 10^–4 Hz (time scales of 20 min to 8 hr) is different for the two classes of events. A suggestion is made that the enhanced and low-amplitude cosmic-ray diurnal variations are produced by different types of interplanetary magnetic field distributions.     

On fast pulsations in flare continua

Pulsations with periods down to 0.05 s have been observed in continuum emission following flares. In pulsating events several frequencies may be excited, simultaneously or in time succession. Pulsations occur both in classical type IV events and in short-lasting continuum events. The power spectrum shows no large variation during one or two hours. Some implications of fast pulsations for the generation mechanism are discussed.Proceedings of the Workshop on Radio Continua during Solar Flares, held at Duino (Trieste), Italy, 27–31 May, 1985.     

On the diurnal heat budget of the thermosphere

The potential significance of atmospheric waves as contributors to the diurnal heat budget of the thermosphere is recalled and discussed in the context provided by the work of Chandra and Sinha.     

On the interpretation of some observed features of geomagnetic pulsations

The evidence for a latitude-period relation for geomagnetic micropulsations is re-examined and the physical meaning of various experimental results is discussed. Coupling between the various components of the magnetic field fluctuations is also investigated.     

Short period variation of the photospheric magnetic field

The photospheric magnetic data recorded from August 12, 1959 to September 29, 1967 and averaged over Bartels rotation periods are treated as zonal terms of the solar magnetic field which is expanded in a series of spherical harmonics. Numerical analysis of the reduced data gives seven periods. Three of these seem to be essential in the superposed variation of the solar magnetic field. The first of them (17.74 yr) is thought to be a contribution from the magnetic cycle for the determination of which the data covering only 8 yr interval are of insufficient accurity. For this reason, a 22.2 yr period is favoured by the computations. The numerical values of the two shorter periods are deduced as 2.557 yr and 4.194 yr. The amplitudes and phase angles of the periodic terms in question are determined.     

The diurnal variation of polarization characteristics of the Earth treated as an exoplanet

The diurnal polarization variation of the Earth, treated as an exoplanet with an unresolved disk but resolvable from its host star, is presented in three wavelength bands centered at 490 nm, 670 nm and865 nm respectively according to French satellite-borne PARASOL data. We aim to estimate disk-integrated polarization of the Earth with a phase angle of 55?. It is shown that:(1) the linear polarization signal and its variation are ascribed to the combination of surface feature distribution and atmospheric conditions acting as a variable polarimetric modulator; and(2) the polarimetric wavelength dependence is strong due to the atmospheric wavelength sensitivity. During the period when the PARASOL data were acquired, the cloud coverage ranged from 44.0% to 57.7%, and the polarimetric diurnal variation amplitude was within 1.8%in the 490 nm band, 1.3% in the 670 nm band and 1.5% in the 865 nm band.     

The period variation of DY Hercules

In this paper the researchers collected 28 times of maximum light including 4 times of those observed at the Xinglong station, the National Astronomical Observation of China between May 27, 2004 and June 1, 2004 and 1 time of maximum light from a 60 cm telescope on May 4, 1984. It found that the O-C point distribution was more completely compared than in any papers published before. The period is decreasing at the rate of about (1.4±0.1)×10⁻⁸ per year, which should not be caused by stellar evolution. It might be only part of a binary orbital light-time variation or other unknown reason. The time scale is longer than Pocs and Szeidl’s suggestion; the star needs more observations before we can be certain of the exact light-time variation.     

A study of the diurnal variation in the thermosphere as derived by satellite drag

An analysis was performed on 29,574 densities derived from the drag of 10 satellites to determine simultaneously the parameters of the solar-activity effect in the thermosphere on the one hand, and the amplitude and shape of the diurnal-variation curve on the other. This paper reports on the study of the diurnal variation only.Although a considerable amount of smoothing is inherent in the drag method, it seemed useful to see whether we could detect any change in the shape of the diurnal-variation curve with height, latitude and solar activity. None was detected: the curve remains remarkably stable, symmetric, with a maximum at 14 hr 20 min L.S.T. and a minimum at 2 hr 20 min L.S.T. A systematic variation of the temperature range with height is observed when static models are used to derive it.