Letter to the editor The ionospheric response during an interval of Pc5 ULF wave activity

A preliminary analysis of Pc5, ULF wave activity observed with the IMAGE magnetometer array and the EISCAT UHF radar in the post midnight sector indicates that such waves can be caused by the modulation of the ionospheric conductivity as well as the wave electric field. An observed Pc5 pulsation is divided into three separate intervals based upon the EISCAT data. In the first and third, the Pc5 waves are observed only in the measured electron density between 90 and 112 km and maxima in the electron density at these altitudes are attributed to pulsed precipitation of electrons with energies up to 40 keV which result in the height integrated Hall conductivity being pulsed between 10 and 50 S. In the second interval, the Pc5 wave is observed in the F-region ion temperature, electron density and electron temperature but not in the D and E region electron densities. The analysis suggests that the wave during this interval is a coupled Alfven and compressional mode.     

Correlation between Pc1 electromagnetic activity and earthquakes

This work addresses the experimental study of an important aspect in the general problem of lithosphere-magnetosphere coupling, namely the correlation between the Pc1 electromagnetic waves (0.2–5 Hz) and the earthquakes. Using long time series of measurements from the catalogues of Pc1 and earthquakes, we revealed a new effect: the diurnal Pc1 activity in the middle latitudes is statistically higher the lower the diurnal global seismic activity. We assume that the influence of earthquake-related acoustic waves on the upper atmosphere suppresses the activity of the Pc1 waves.     

The ionosphere as an Alfven resonator in the Pc1 micropulsation range

Summary A local planar approximation of a stratified, inhomogeneous, anisotropic and dissipative ionosphere is presented as an Alfven quarter-wave plate resonator in the Pc1 micropulsation range. The frequency-amplitude structure of the resonance response of an isotropic Alfven wave on the Earth's surface and at a given altitude in the ionosphere is studied in comparison to standing waves in vacuum above an ideal conductor for a particular model of the ionosphere. An asymmetry in the response was found at both boundary of the resonator, at the Earth's surface and at the given altitude z₀. The results are related to the vertical changes of frequency dispersion in the dissipative medium of the ionosphere and to the vertical profile of the inhomogeneities of the resonator being considered.     

Note on the dependence of Pc3-4 activity on the solar wind velocity

Various models that describe the dependence of pulsation amplitude A on the solar wind velocity V are discussed. Phenomenological considerations and experimental data count in favour of a model of the form A/A₀=(V/V₀) exp (-V₀/V). The linear model A=a+bV which is common in the literature gives negative values of amplitude as V->0, which is strange from the physical point of view. The possibility of modifying the linear model by including critical phenomena is discussed.     

Relationships between magnetic storms and the Pc1 micropulsations

Using Pc1 data gathered at Ottawa (45.4°N, 75.6°W; L = 3.5) during the International Magnetospheric Study (IMS) period, relationships between ssc, Dst, and the occurrence of Pc1 pulsations are examined. It is found that the sudden compressions of the magnetoshere that took place in the postnoon period (13–22 hLT) frequently produced Pc1 pulsations at Ottawa. This pulsational activity took place about 25 to 125 hours after the occurrence of ssc’s of amplitude 5–25 nT and duration 2–6 min. Pc1’s also occur 20 to 40 hours after maximum Dst deviations in the range 50–110 nT, when the ring current has decayed to a considerable extent (5 nT < Dst < 25 nT). In agreement withHeacock andKivinen (1972), it appears that during the storm recovery phase energetic particles of the ring current with anisotropic pitch angle distribution interact with the surrounding cold plasma of the plasmasphere. When stable trapping limit is reached, proton cyclotron instability is triggered and pulsations in the Pc1 period range are generated.     

Pc1 pearl waves with magnetosonic dispersion

Results of the analysis of 15 unusual Pc1 pearl wave events with inverse dispersion in comparison with the dispersion of well-known electromagnetic ion-cyclotron (EMIC) waves in the form of classic pearl pulsations are presented. Pulsations with the dynamical spectrum consisting of both falling tones only (first type) and events with structures, which start with the falling tones and then develop into rising tones (second type), have been discovered. The first type corresponds to the frequency dispersion of magnetosonic waves (R-waves), and the second type corresponds to the mixed frequency dispersion of R-waves and EMIC waves (L-waves). All events were observed during quiet geomagnetic periods. The duration of the events is about 20–30 min. For the interpretation of these phenomena, the cyclotron instability driven by energetic proton beams with relative mean velocity v₀ directed along the background magnetic field and corresponding to an energy ∼10–100 keV is considered. The interaction of such proton beams with waves having frequencies ω<ω_i (ω_i is the ion gyrofrequency) leads to the instability, which allows the fastest growth of electromagnetic oscillations with the dispersion of R-wave type. When the velocity of the proton beam decreases (v₀≈0), R-waves attenuate and L-waves (for the proton temperature T_⊥>T_∥) will be amplified. This instability is the reason for the generation of classic Pc1 pearl pulsations with the usual dispersion and allows explaining the transition of the dispersion from R- to L-waves.     

Satellite observations of Pc 1 pearl waves: The changing paradigm

Pc 1 pearls have been observed on the ground for about 70 years. During this time numerous publications have been written on the various properties of Pc 1 pearl waves, the related theory, and possible applications. Pc 1 waves with a clear pearl structure are only a fraction of all Pc 1 waves observed on ground, and this fraction depends on the latitude of observations, increasing from high to low latitudes. In fact, the spatial and temporal occurrence of Pc 1 pearls is closely connected with the location and development of the plasmapause. While it has been known roughly 40 years that Pc 1 waves are electromagnetic ion cyclotron waves generated by anisotropic, energetic ions in the near-equatorial magnetosphere, the formation of pearl structure is still largely in question. In situ observations of Pc 1 waves in the Earth's magnetosphere have been made since the 1970s by various satellites in different orbits. However, satellite observations of clear Pc 1 pearls are still rather few. Here we review a few crucial satellite-based observations of Pc 1 pearls, and evaluate their contribution to the understanding of pearl formation. We show that the long-held paradigm of the bouncing wave packet model is in serious contradiction with satellite observations and therefore outdated. Instead, observations support the idea that Pc 1 wave growth rate is successively modulated at the equator by long-period ULF waves.     

On the earthquake effects in the regime of Pc1

While searching for electromagnetic effects of the earthquakes, impulse-type signals in the frequency range of 0–5 Hz preceding the earthquake or following it have been detected. The advance or delay time is from 0 to 5 min. The signals are observed as single or pair impulses. It is supposed that the signals make a significant impact on the state of the magnetosphere and ionosphere. As a result, a sharp change in the regime of Pc1 geomagnetic pulsations is possible. These effects are analyzed on the basis of observations of the geomagnetic pulsations at the Borok Geophysical Observatory.     

The recording of Pc1 geomagnetic pulsations using a microcomputer preprocessing system

In the past slow-speed frequency-modulated analogue tape recording techniques have provided the most convenient method for recording ground-based Pc1 pulsation data on a continuous basis. With events occurring often only a few hours in a week, and a Nyquist frequency of 4–5 Hz, direct digital recording is not practical because of the bulk of data accumulated. However, with the recent availability of reasonably priced microcomputers and advanced signal processing techniques it is now possible to preprocess digital data in the field and store only events of interest. A two-component induction coil magnetometer incorporating a Z80 based 64K RAM microcomputer-floppy disk preprocessing system is described. It is capable of recording Pc1 signals in the 0.2–4 Hz band at middle-to-low latitudes with a minimum detectable signal level of 3 pT. The reliability and limitations of the preprocessing techniques utilizing FFT autospectral analysis to recognize Pc1 signals are discussed.     

Recent progress in understanding Pc1 pearl formation

We discuss recent progress in understanding mechanisms of formation of Pc1 pearl emissions. This problem remains unsettled in spite of many years of experimental and theoretical studies. Modern satellite observations by e.g. Polar and Cluster still do not reveal the whole picture experimentally since they do not stay long enough in the generation region to give a full account of all the spatio-temporal structure belonging to a single Pc1 event. Ground-based observations have also been extremely helpful, especially in combination with spacecraft data, but still existing experimental information allows one to advocate different scenario of Pc1 nature. On the other hand, a complete self-consistent theory taking into account all factors significant for Pc1 generation remains to be developed. Several mechanisms are discussed with respect to formation of Pc1 pearl spectrum, among them are nonlinear modification of the ionospheric reflection by precipitating energetic protons, modulation of ion-cyclotron instability by long-period (Pc3/4) pulsations, reflection of waves from layers of heavy-ion gyroresonances, and nonlinearities of wave generation process. We show that each of these mechanisms have their attractive features and explains certain part experimental data but any of them, if taken alone, meets some difficulties when compared to observations. We conclude that development of a refined nonlinear theory and further correlated analysis of modern satellite and ground-based data is needed to solve this very intriguing problem.     

Ground-based Pc5 ULF wave power: Solar wind speed and MLT dependence

Using over 20 years of ground-based magnetometer data from the CANOPUS/CARISMA magnetometer array, we present a statistical characterisation of Pc5 ultra-low frequency (ULF) power in the 2–10 mHz band as a function of magnetic local time (MLT), L-shell, and solar wind speed. We examine the power across L-shells between 4.2 and 7.9, using data from the PINA, ISLL, GILL and FCHU stations, and demonstrate that there is a significant MLT dependence in both the H- and D-component median 2–10 mHz power during both fast (>500 km/s) and slow (<500 km/s) solar wind speeds. The H-component power consistently dominates over D-component power at all MLTs and during both fast and slow solar wind. At the higher-L stations (L>5.4), there are strong MLT power peaks in the morning and midnight local time sectors; the morning sector dominating midnight during fast solar wind events. At lower L-shells, there is no evidence of the midnight peak and the 2–10 mHz power is more symmetric with respect to MLT except during the fastest solar wind speeds. There is little evidence in the ground-based power of a localised MLT peak in ULF power at dusk, except at the lowest L-shell station, predominantly in the H-component. The median 2–10 mHz power increases with an approximate power law dependence on solar wind speed, at all local times across the L-shell domain studied in both components. The H-component power peaks at the latitude of the GILL station, with significantly lower power at both higher and lower L-shells. Conversely, the D-component power increases monotonically. We believe that this is evidence for 2–10 mHz power accumulating at auroral latitudes in field line resonances. Finally, we discuss how such ULF wave power characterisation might be used to derive empirical radiation belt radial diffusion coefficients based on, and driven by, the solar wind speed dependence of ULF wave power.     

Period characteristics of Pc1 pulsations observed at Ottawa

IMS data from Ottawa, Canada are analyzed to study the propagation characteristics of Pc1 pulsations. The majority of pulsations observed possessed periods of 1 second and lasted less than an hour. Shorter-period Pc1s are observed during the summer than during the winter. Periods of pulsations are also shorter during the noon hours than in the morning, and shorter during intervals of high magnetic activity. The diurnal variation of period at Ottawa is different from that at high-latitude stations. For Pc1s the calculated ratio of the spacing period to the pulse period at Ottawa is 86, in good agreement with values found for both higher- and lower-latitude stations. An IPDP (intervals of pulsations of diminishing periods) event occurred on April 19, 1977. The analysis supports the view that the energy dispersion of storm time protons, as well as the earthward movement of the instability region due to increasing magnetic activity, are involved in the production of such events. Earth Physics Branch Contribution No. 1087.     

Pc1 geomagnetic pulsations as a potential hazard of the myocardial infarction

The analysis of 85,800 events (1979–1981) of Moscow ambulance calls, related to the myocardial infarction (MI), demonstrates a seasonal variation with the profound summer minima and winter maxima. Similar results were obtained by analyzing the 25-year (1970–1995) statistical monthly data on the death from infarction in Bulgaria. The estimated high correlation coefficient (0.84) between Moscow and Bulgarian data suggests a common reason. There is a great number of clinical and statistical studies confirming that the MI number rises during geomagnetic disturbances, which have a maximum of occurrence near equinox, not in winter. In order to explain this contradiction we suggest that one of the critical additional factors, which affect a human cardiovascular system, could be geomagnetic Pc1 pulsations at frequencies comparable with the human heart beat rate. The MI variations as well as the Pc1 pulsations exhibit a summer minimum. The comparative analysis of the Moscow ambulance MI data and Pc1 pulsations recorded at the geophysical observatory in Borok is presented. It is shown that in about 70% of the days when an anomalously great number of ambulance calls (AMI) has been registered Pc1 pulsations have been recorded. In the winter season the probability of the simultaneous AMI and Pc1 occurrence was 1.5 times larger than their accidental coincidence. Moreover, it was found that the effects of magnetic storms and Pc1 in AMI were much higher in winter than in summer. We suggest that the seasonal variation of the production of the pineal hormone melatonin leads to a winter instability in the human organisms and increases the sensitivity of the patient to the “negative” influence of Pc1 geomagnetic pulsations in winter.     

Dependence of ground-based Pc5 ULF wave power on F10.7 solar radio flux and solar cycle phase

Utilising fifteen (1990–2005) years of ground-based magnetometer data from four magnetometer stations, we characterise the statistical dependence of the Pc5 ULF wave power spectra on variations in F10.7 solar radio flux and on solar cycle phase. We show that the median Pc5 ULF wave power spectra can be characterised as a power-law with a localised Gaussian centred at a specific frequency superimposed on the power-law spectrum. Further, we demonstrate that the location of the Gaussian in frequency systematically varies with both solar cycle phase and F10.7 and is more pronounced during high-speed solar wind intervals. We postulate that the localised power spectrum enhancement (or Gaussian) is a manifestation of the local eigenfrequency of field line resonances in the Earth's magnetosphere and that the variation in the location of the Gaussian occurs as a result of increased ionospheric outflow during periods of enhanced F10.7 and active solar activity.     

Temporal characteristics and medical aspects of Pc1 geomagnetic pulsations

This paper is devoted to the morphology of Pc1 geomagnetic pulsations (frequency range 0.2–5.0 Hz). This study is based on the series of continuous observations of Pc1 pulsations during more than three solar cycles (July 1957–December 1995). The main attention is given to the temporal characteristics of Pc1 activity, i.e. daily, seasonal and cyclic variations, and also the relationship of Pc1 activity with magnetic storms, sector structure of the interplanetary magnetic field and parameters of the solar wind. The results may be used in the studies of medicobiologic aspects of the problem of solar–terrestrial relations.     

Climate change impact on Caspian Sea wave conditions in the Noshahr Port

Excessive usage of fossil fuels and high emission of greenhouse gases have increased the earth’s temperature and consequently have led to changes in wind and wave regimes. The main effects of climate change on oceans are warming of the ocean water, melting of ice, acidification of ocean water, and change in the ocean currents. The main effects of climate change on coastal regions are change in the coast hydrodynamics, sea level rise, change in wave height, coastal erosion, coastal structure damage, food shortage, and storms. Due to the importance of waves in the coastal zone and its effect on erosion and sedimentation, it is necessary to study wave changes. In this study, the effect of climate change on wave specifications was evaluated in the southern coast of the Caspian Sea in Noshahr Port. To simulate wave parameters, the third generation spectral Simulating WAves Nearshore (SWAN) model was used. Wave modeling was carried out using the SWAN numerical model for two 30-yearly periods, including the control period (1984 to 2014) and the future period (2051 to 2080). For wave modeling in the control period, the European Center for Average Weather Forecast wind field was used, and for the future period, a downscaled wind field from Coordinated Regional Downscaling Experiment projection, which was sponsored by World Climate Research Programme, based on the most recent emission scenarios RCP2.6, RCP4.5, and RCP8.5, was used. The model results were calibrated and verified with buoy-recorded data. The effect of the climate change on the wave parameters was evaluated by studying the differences between the patterns in three scenarios and the control period. Results showed that the 30-year maximum significant wave height will increase because of climate change, and the wave direction will not change. In addition, the intensity of storms will increase in the future.