Local time asymmetry in the characteristics of Pc5 magnetic pulsations

The wave characteristics of Pc5 magnetic pulsations are analyzed with data of OGO-5, ISEE-1 and -2 satellites. The toroidal modes (δB_D >δB_H) of Pc5 pulsations are observed at a higher magnetic latitude in the dawnside outer magnetosphere. The compressional and poloidal modes (δB_z.dfnc; ～ δB_H >δB_D) of Pc5 pulsations are mostly observed near the magnetic equator in the duskside outer magnetosphere. This L.T. asymmetry in the occurrence of dominant modes of Pc5's in space can be explained by the velocity shear instability (Yumoto and Saito, 1980) in the magnetospheric boundary layer, where Alfvénic signals in the IMF medium are assumed to penetrate into the magnetospheric boundary layer along the Archimedean spiral. The asymmetrical behaviour of Pc5 pulsation activity on the ground across the noon meridian can be also explained by the ionospheric screening effect on the compressional Pc5 magnetic pulsations. The compressional modes with a large horizontal wave number in the duskside magnetosphere are expected to be suppressed on the ground throughout the ionosphere and atmosphere.     

Low-frequency upstream wave as a probable source of low-latitude Pc 3–4 magnetic pulsations

Daytime Pc 3–4 pulsation activities observed at globally coordinated low-latitude stations [SGC (L = 1.8,λ = 118.0°W), EWA(1.15,158.1°W), ONW(1.3,141.5°E)] are evidently controlled by the cone angle θ_XB of the IMF observed at ISEE 3. Moreover, the Pc 3–4 frequencies ($\text{?}$) at the low latitudes and high latitude (COL; L = 5.6 and λ = 147.9°W) on the ground and that of compressional waves at geosynchronous orbit (GOES 2; L = 6.67 and λ = 106.7°W) are also correlated with the IMFmagnitude(B_IMF).The correlation of $\text{?}$ of the compressional Pc 3–4 waves at GOES 2 against B_IMF is higher than those of the Pc 3–4 pulsations at the globally coordinated ground stations, i.e., γ = 0.70 at GOES 2, and (0.36,0.60,0.66,0.54) at (COL, SGC, EWA, ONW), respectively. The standard deviation ($\text{σ}{}_{\mathrm{n}}\text{= ± Δ?}\text{mHz}$) of the observed frequencies from the form $\text{?}$ (mHz) = 6.0 × B_IMF (nT) is larger at the ground stations than at GOES 2, i.e., $\text{Δ? = ± 6.6}\text{mHz at}$GOES 2, and ±(13.9, 9.1, 10.7, 12.1) mHz at (COL, SGC, EWA, ONW), respectively. The correlations between the IMF magnitude B_IMF and Pc 3–4 frequencies at the low latitudes are higher than that at the high latitude on the ground, which can be interpreted by a “filtering action” of the magnetosphere for daytime Pc 3–4 magnetic pulsations. The scatter plots of pulsation frequency $\text{?}$ against the IMF magnitude B_IMF for the compressional Pc 3–4 waves at GOES 2 are restricted within the forms $\text{? = 4.5 × B}{}_{\mathrm{<mtext>IMF</mtext>}}\text{and}\text{? = 7.5 × B}{}_{\mathrm{<mtext>IMF</mtext>}}$. The frequency distribution is in excellent agreement with the speculation ($\text{<ω}{}_{\mathrm{sc}}\text{Ω}{}_{\mathrm{i}}\text{= 0.3 ～ 0.5}$) of the spacecraft frame frequency of the magnetosonic right-hand waves excited by the anomalous ion cyclotron resonance with reflected ion beams with V₆ = 650 ～ 1150 km s^?1 in the solar wind frame observed by the ISEE satellite in the Earth's foreshock. These observational results suggest that the magnetosonic right-handed waves excited by the reflected ion beams in the Earth's foreshock are convected through the magnetosheath to the magnetopause, transmitted into the magnetosphere without significant changes in spectra, and then couple with various HM waves in the Pc 3–4 frequency range at various locations in the magnetosphere.     

Observations of a quiet-time Pc5 wave in the outer magnetosphere

We report an observation of the radial profile of a Pc5 magnetic pulsation and the associated energetic electron flux oscillations from 10 to 18 Re, recorded by the IMP-5 satellite at 19.00 M.L.T. on 21 March 1970. The Pc5 pulsation was mainly compressional and occurred during extremely quiet geomagnetic conditions. Fluxes of energetic electrons detected above three energy thresholds (18, 45, and 80 keV) were found to oscillate out of phase with magnetic field intensity. One new result is that both the wave amplitude and the wave period increased with radial distance. Second, the electron flux oscillation amplitude was roughly proportional to magnetic field fluctuation amplitude and wave period. The wave event is found to be interpreted better as an ion drift wave because of lack of polarization reversal. The characteristics of energetic electron flux oscillations are shown to agree qualitatively with theoretical calculations of the kinetic perturbation of distribution functions by compressional waves.     

Do we need a surface wave approach to the magnetospheric resonances?

In this paper we study a possible existence of surface wave (SW) global modes of the outer magnetosphere. The SW modes are supported by two plasma discontinuities: the plasmapause and the boundary between the open and closed field lines of the magnetosphere. Conditions under which the SW global modes can propagate azimuthally and along the magnetic field lines are examined. The ionosphere at the ends of the field lines is considered as reflecting boundaries of these SW modes. As a result SW standing wave structures along the magnetic field fluxes can be formed. Two branches of SW modes are derived. The low frequency branch, f_s,1 falls in the Pc5 range, while the high frequency branch, f_s,2—in the Pc4 range, where f_s,1(2) is the fundamental SW global mode frequency. Their frequencies possess quantized properties in the following way: f≡(1,2,3, …)f_s,1(2). The high frequency SW branch, f_s,2 exists only for relatively great azimuthal wavenumbers k_⊥. It is pointed out that most of the SW global mode characteristics are similar to those of the FLR. These results are applied to 1.8 mHz global mode observations on 11 January 1997. Spectral, phase and polarization properties of this Pc5 pulsation event under northward IMF conditions are examined as we see them from ground-based (L’Aquila and TNB observatories) and satellite (POLAR and INTERBALL) observations.     

Pc3-4 geomagnetic pulsations at very low latitude in Brazil

In order to investigate Pc3-4 geomagnetic pulsations at very low and equatorial latitudes, L=1.0 to 1.2, we analyzed simultaneous geomagnetic data from Brazilian stations for 26 days during October-November 1994. The multitaper spectral method based on Fourier transform and singular value decomposition was used to obtain pulsation power spectra, polarization parameters and phase. Eighty-one (81) simultaneous highly polarized Pc3-4 events occurring mainly during daytime were selected for the study. The diurnal events showed enhancement in the polarized power density of about 3.2 times for pulsations observed at stations close to the magnetic equator in comparison to the more distant ones. The phase of pulsation observed at stations near the magnetic equator showed a delay of 48-62° in relation to the most distant one. The peculiarities shown by these Pc3-4 pulsations close to the dip equator are attributed to the increase of the ionospheric conductivity and the intensification of the equatorial electrojet during daytime that regulates the propagation of compressional waves generated in the foreshock region and transmitted to the magnetosphere and ionosphere at low latitudes. The source mechanism of these compressional Pc3-4 modes may be the compressional global mode or the trapped fast mode in the plasmasphere driving forced field line oscillations at very low and equatorial latitudes.     

Spectral studies of geomagnetic pulsations with periods between 20 and 120 sec,and their relationship to the plasmapause region

Digital spectrograms have been computed for 18 days of geomagnetic pulsation activity at three UK Earth current stations (L = 2.6?3.6).Three main conclusions are drawn: (1) There are days when the period of the dominant spectral amplitude is ordered according to the observatory latitude. The most frequently observed large amplitude spectral peaks are centred on 80, 60 and 45s for South Uist (L = 3.6), Eskdalemuir (L = 3.1) and East Anglia (L = 2.6). respectively. (2) There are other days when the period of the dominant spectral amplitude is the same at all the observatories. (3) When Pc 3 and 4 period waves have been detected together, the latitude dependence of the amplitudes supports the theory that the shorter period pulsation is enhanced in the plasmatrough while the longer period wave is enhanced within the plasmasphere.     

Variability of the period of the star DU Monocerotis,an RR Lyrae variable with the Blazhko effect

In 2012–2014 we obtained 3641 CCD frames of the fields of the RR Lyrae (AB subtype, P = 0.583 days) variable DU Mon with BV I _c filters using the 76-cm telescope of the South African Astronomical Observatory (SAAO) and the 1-m telescopes of the Las Cumbres Observatory Global Telescope Network (LCOGT). Our observations confirmed the presence of the Blazhko effect that we suspected previously and allowed its period to be determined, \({P_{Bl}} = 60_ \cdot ^d52 \pm 0_ \cdot ^d03\). Using all of the available observations, we constructed an O–C diagram spanning a time interval of 86 years that revealed at least one abrupt change in the pulsation period (a decrease by 15.26 s).     

Short-term prediction and a new method of classification of Pc 1 pulsation occurrences

Because of their known tendency to occur in the interval 2–7 days after the start of a geomagnetic storm, Pc 1 pulsations (0.2–5 Hz) are particularly well suited for a method of occurrence prediction based on the comparison of running means of a geomagnetic activity index. By comparing the running mean of a short interval (～ 2 days) of activity data with the mean of a longer interval (～ 5 days), it is possible to isolate the intervals of declining activity that contain a large proportion (if 66%) of Pc 1 pulsation occurrences. Assuming the real time availability of a daily activity index, predictions can be made for 3–10 days ahead of the probability of Pc 1 occurrences. The method of prediction generalizes the previous observations on the relation between Pc 1 pulsations and geomagnetic storms, and one of its important features is its ability to divide Pc 1 pulsation occurrences into a unified system of categories. It is probable that this system can be exploited to provide new information about the pulsations.     

High-latitude Pc1–2 geomagnetic pulsations and their connection with location of the dayside polar cusp

The result of investigating high-latitude Pc1–2 pulsations are presented in this paper. They show that these unstructured oscillations are typical in intervals of low magnetic activity for regions of projections of the dayside cusp on the Earth's surface. The morphological properties of these pulsations, namely the character of their diurnal variations and dependence of their amplitude and frequency of occurrence on magnetic activity on different latitudes, suggest methods of utilization for tracing the location of the equatorial boundary of the dayside cusp. It is suggested that Pc1–2 pulsations are generated mainly in the dayside magnetosheath on field lines, crossing the magnetopause and entering in the dayside cusp. The possible mechanism of generation is the ion-cyclotron instability of plasma of finite pressure (β_⊥ ? 1) and with anisotropic temperature (T_⊥ > T_∥).     

Time of flight calculations for high latitude geomagnetic pulsations

High latitude geomagnetic field lines differ significantly from a dipole geometry. Time of flight calculations using the Mead-Fairfield (1975) model of the geomagnetic field are presented for different tilt angles and K_p conditions. Typical standing wave periods of geomagnetic pulsations are estimated for three different magnetospheric cold plasma regions, corresponding to waves guided in (i) the plasmatrough, (ii) the extended plasmasphere and (iii) regions of enhanced proton density (detached plasma) within the plasmatrough.Pc4/5 pulsation studies at high latitudes are briefly reviewed and some new results from Tromso are given. Many of the observations reveal hydromagnetic waves whose location and period are consistent with ducting in a region of enhanced plasma density within the plasmatrough.     

Multipoint observations of low latitude ULF Pc3 waves in south-east Australia

Geomagnetic pulsations recorded on the ground are the signatures of the integrated signals from the magnetosphere. Pc3 geomagnetic pulsations are quasi-sinusoidal variations in the earth’s magnetic field in the period range 10–45 seconds. The magnitude of these pulsations ranges from fraction of a nT (nano Tesla) to several nT. These pulsations can be observed in a number of ways. However, the application of ground-based magnetometer arrays has proven to be one of the most successful methods of studying the spatial structure of hydromagnetic waves in the earth’s magnetosphere. The solar wind provides the energy for the earth’s magnetospheric processes. Pc3–5 geomagnetic pulsations can be generated either externally or internally with respect to the magnetosphere. The Pc3 studies undertaken in the past have been confined to middle and high latitudes. The spatial and temporal variations observed in Pc3 occurrence are of vital importance because they provide evidence which can be directly related to wave generation mechanisms both inside and external to the magnetosphere. At low latitudes (L < 3) wave energy predominates in the Pc3 band and the spatial characteristics of these pulsations have received little attention in the past. An array of four low latitude induction coil magnetometers were established in south-east Australia over a longitudinal range of 17 degrees at L = 1.8 to 2.7 for carrying out the study of the effect of the solar wind velocity on these pulsations. Digital dynamic spectra showing Pc3 pulsation activity over a period of about six months have been used to evaluate Pc3 pulsation occurrence. Pc3 occurrence probability at low latitudes has been found to be dominant for the solar wind velocity in the range 400–700 km/s. The results suggest that solar wind controls Pc3 occurrence through a mechanism in which Pc3 wave energy is convected through the magnetosheath and coupled to the standing oscillations of magnetospheric field lines.     

Pc5 pulsations associated with ring current proton drifts: Stare radar observations

Detailed properties of a Pc5 pulsation with large azimuthal wavenumber observed using the STARE radar have recently been reported. A further four examples of this type of pulsation are presented, and it is shown that their properties are generally similar to those of the first example. However, there are some differences, the most important being that the variation of azimuthal phase velocity with latitude is significantly different for different time intervals during individual events, so that a mean phase velocity for a given latitude cannot be defined.When mapped to the equatorial plane in a dipole geomagnetic field, the variation of azimuthal phase velocity with L resembles the gradient-curvature drift of energetic protons in only a few time intervals within the events. The results are interpreted in terms of current theories of drift and bounce resonance of energetic particles with hydromagnetic waves. It is found that no single theory explains all aspects of the observations.     

Global Pc5 wave activity observed using SuperDARN radars and ground magnetometers during an extended period of northward IMF

Observations are presented of long-lived global Pc5 ULF wave activity observed at a wide range of local times. The event was monitored in the high latitude ionosphere (∼60–80° magnetic latitude) by several SuperDARN HF radars and 5 magnetometer chains in Scandinavia, Greenland, Canada, Alaska and Russia. The event coincided with a protracted period (∼36 h) of northward interplanetary magnetic field (IMF). The study focuses on 4 h during which distinct dawn/dusk asymmetries in the wave characteristics were observed with multiple field line resonance (FLR) structures observed in the dawn flank at 1.7, 2.6, 3.3, 4.2 and 5.4 mHz and compressional oscillations in the dusk flank at 1.7 and 2.3 mHz. The data indicated an anti-sunward propagation in both the dawn and dusk flanks and a low azimuthal m number (∣m∣∼6) suggesting a generation mechanism external to the Earth's magnetosphere. A sudden increase in the solar wind dynamic pressure followed by a period of strongly northward, B_z dominated IMF, coincides with the observations and also a large increase in Pc5 wave power observed in the dawn flank. The observed enhancements in the wave activity and FLR structures are thought to be due to a Kelvin–Helmholtz driven waveguide mode. Additionally, there is no evidence that the frequencies of the FLRs are intrinsic to the solar wind. It thus seems that the frequencies were determined by the dimensions of the magnetospheric cavity.     

CCD BVI Photometry of the Southern Open Clusters Pismis 23 and BH 222

CCD BVI Johnson–Cousins photometry of the open cluster candidates Pismis 23 and BH 222 is presented. Both the analysis of the colour-magnitude diagrams and star counts performed in the regions of these two objects support their physical reality. For Pismis 23 we derive E(B?V) = 2.0 ± 0.1, E(V?I) = 2.6 ± 0.1, a distance from the Sun d= (2.6 ± 0.6) kpc and an age of (300 ± 100) Myr, while for BH 222 we obtain E(V?I) = 2. 4 ± 0.2, d= (6.0 ± 2.7) kpc and (60 ± 30) Myr. Both objects, located beyond the Sagittarius arm, are among the most reddened and distant open clusters known in the direction towards the Galactic centre.     

160 m Pulsations in the magnetosphere of the Earth possibly caused by oscillations of the Sun

The measurements of the amplitudes envelope of Pc 3–4 geomagnetic micropulsations obtained at the Borok Geophysical Observatory were analysed by the cosinor method to search for magnetospheric pulsations with a period of about 160 m. 216 days of observations in 1974–1978 were used. It was found that Pc 3–4 amplitudes are modulated by the period 160.010 m with a stable phase. The maximum of the Pc 3–4 amplitudes follows approximately 20 m after the maximum of the solar expansion velocity (for the center of the disk) in the optical observations of Severny et al. This modulation of the Pc 3–4 amplitudes could be caused by the presence of an oscillating component in solar UV radiation over the wavelength range 100–900 Å. The amplitude of the UV flux variation may be as large as 2–4%.     

Pulsations in the solar wind and on the ground

Measurements of the pulsation activity recorded by the HEOS-1 satellite in the solar wind upstream from the Earth's bow shock are compared with records of Pc 3–4 activity at the Soviet Borok Observatory. Selecting only events in the 0300–1000 U.T. range most suitable for observing at Borok, we obtained eight events with closely similar periods at the satellite and at Borok, while another event showed similar onset times but had rather different dominant periodicities at the two locations.The time delay between the start of the event at HEOS and at Borok depends on the distance between the satellite and the bow shock in a way which suggests that the pulsation activity at the satellite is produced by protons which are counter-streaming along the interplanetary field lines as a result of reflection or energisation at the shock. When the interplanetary field is directed away from the Sun, the period of the disturbance at Borok is most closely similar to the period at HEOS and both are inversely proportional to the magnitude of the interplanetary magnetic field. This coincident dominant periodicity at HEOS and Borok did not seem to exist during periods of Sunward directed interplanetary field.These results are discussed in terms of the possible generating mechanisms for Pc 3–4 activity.     

Parameters of superflares on G-Type stars observed with the Kepler space telescope

We continued the analysis of 279 G-type stars with superflares (energies in the range of 10³³–10³⁶ erg). We calculated the SFL parameter (part of the stellar surface which emits in the flare). The SFL estimates were derived from the relation connecting this value with the amplitude of the flare and its radiation on the assumption of the blackbody character of the emission at times close to its maximum. Most SFL values are in the range of 0–0.1, with values of 0.2–0.4 for some strong flares. Dependence of SFL on effective temperature for stars with superflares is similar to that found earlier for the spottedness parameter S. The SFL distribution reaches its maximum in the temperature range of about 5100–5250 K and decreases with the effective temperature increase. We suggested an assumption on the presence of bimodal distribution in the “SFL–rotation period” relation with a gap for objects with rotation periods P of about 10 days. For stars with P less than 10 days, the given data can indicate a decrease in flare areas with the P increase. Our analysis showed that significant changes both in flare energy and in flare areas can be achieved with small changes in spottedness S for one and the same star.     

Some properties of geomagnetic field pulsations in the 0.1–1.0-Hz frequency range: A quantitative description and comparison with the satellite and ground-based observations

By using an image-dipole magnetic field model for a variety of plasma density profiles we have studied the latitude effect of the 0.1–1.0-Hz hydromagnetic wave propagation in the Earth's magnetosphere. On comparing the results of signal group delay time calculations for dipole and model magnetic fields with ground and satellite observations we obtain some propagation characteristics of Pc1s and localize the regions of their generation. Our results show that most high-latitude Pc1 events are generated in the outer magnetosphere in accordance with ground and satellite observations and theoretical considerations. The non-dipole geometry of the geomagnetic field in the outer magnetosphere (at geomagnetic latitudes φ₀ > 66°, L > 6) has a significant effect on the hydromagnetic wave propagation.     

Source structure and dynamics of Pc 1 pulsations at low latitudes

Structured Pc 1 signals propagate in the ionospheric F2 region duct from their secondary sources at high latitudes to lower latitudes. Propagation directions to low latitude stations can be inferred from measurements of polarization parameters. The analysis of five events recorded at two low latitude stations (L = 1.9) are presented. Direction of arrival measurements are used to investigate the spatial and temporal structure of Pc 1 sources. Results show a close relationship between the structure of events identified in the frequency-time representation and direction of arrival measurement patterns. Multiple sources are sometimes indicated.     

Saturn's rings. I. Optical thickness of rings A,B, D and structure of ring B

A photometric study of high-resolution (～0″.3) plates of Saturn taken at the Lowell Observatory in 1943 and 1945 is presented. N-S scans were taken over both the planet and rings. The excess brightness due to the planet seen through the rings is found by taking the difference between the central meridian (CM) scans and scans displaced by 5″.7. Adopting a value for the albedo of the planet, it is possible to obtain the optical thickness, τ_CM(r). In particular, for the regions of maximum brightness in rings A and B, we find τ_CM(I_{A max}) = 0.38 ± 0.11 and τ_CM(I_{B max}) = 0.61 ± 0.11. Observations by Barnard made in 1890 show evidence of ring D, recently discovered by Guerin (1969). The value for the optical thickness of this ring is τ_D(I_{D max}) = 0.03 ± 0.01. Ring B exhibits a pronounced (7–10%) decrease in brightness from the extremity of the major axis to the CM. After considering several possible explanations, we conclude that the ring particles are nonspherical and are in synchronous rotation around the planet with their long axis toward it. The mean value for the ratio of major to minor axis for the particles at 15″ is $\text{(a/b)}\text{? 1.08}$. Because of the shape and orientation of the particles, the optical thickness at the extremity of the major axis and at the CM are different for any saturnicentric latitude B ≠ 90°. Under these circumstances, only a minimum value for τ at the extremity can be derived.