Periodic modulation of Pc3 and Pc4 pulsations in the polar cap by interplanetary and atmospheric processes

The variations in the daily average energy of geomagnetic pulsations and noise in the Pc3 (20–60 mHz) and Pc4 (10–19 mHz) frequency bands in the polar cap have been studied based on the data from P5 Antarctic station (corrected geomagnetic latitude ?87°) from November 1998 to November 1999. The daily average pulsation energy has been calculated using the method for detecting the wave packets, the spectral amplitude of which is higher than the threshold level, from the dynamic spectrum. A spectral analysis of the energy of pulsations and noise in the Pc3 and Pc4 bands, performed using the maximal entropy method, has revealed periodicities of 18 days in the local winter and 26, 13, and 7–9 days during the local summer. The simultaneous and coherent variations with periods of 26, 13, and 7–9 days in the solar wind velocity and IMF orientation indicate that the variations in the Pc3–4 wave energy in the polar cap at a sunlit ionosphere are mainly controlled by the parameters of the interplanetary medium. The variations in the Pc3–4 wave energy with a period of 18 days are observed only during the local winter and are supposedly related to the variations in the ionospheric conductivity modulated by planetary waves.     

Pc3–4 ULF waves at polar latitudes

A search for Pc3–4 wave activity was performed using data from a trans-Antarctic profile of search-coil magnetometers extending from the auroral zone through cusp latitudes and deep into the polar cap. Pc3–4 pulsations were found to be a ubiquitous element of ULF wave activity in all these regions. The diurnal variations of Pc3 and Pc4 pulsations at different latitudes have been statistically examined using discrimination between wave packets (pulsations) and noise. Daily variations of the Pc3–4 wave power differ for the stations at the polar cap, cusp, and auroral latitudes, which suggests the occurrence of several channels of propagation of upstream wave energy to the ground: via the equatorial magnetosphere, cusp, and lobe/mantle. An additional maximum of Pc3 pulsations during early-morning hours in the polar cap has been detected. This maximum, possibly, is due to the proximity of the geomagnetic field lines at these hours to the exterior cusp. The statistical relation between the occurrence of Pc3–4 pulsations and interplanetary parameters has been examined by analyzing normalized distributions of wave occurrence probability. The dependences of the occurrence probability of Pc3–4 pulsations on the IMF and solar wind parameters are nearly the same at all latitudes, but remarkably different for the Pc3 and Pc4 bands. We conclude that the mechanisms of high-latitude Pc3 and Pc4 pulsations are different: Pc3 waves are generated in the foreshock upstream of the quasi-parallel bow shock, whereas the source of the Pc4 activity is related to magnetospheric activity. Hourly Pc3 power has been found to be strongly dependent on the season: the power ratio between the polar summer and winter seasons is 8. The effect of substantial suppression of the Pc3 amplitudes during the polar night is reasonably well explained by the features of Alfven wave transmission through the ionosphere. Spectral analysis of the daily energy of Pc3 and Pc4 pulsations in the polar cap revealed the occurrence of several periodicities. Periodic modulations with periods 26, 13 and 8–9 days are caused by similar periodicities in the solar wind and IMF parameters, whereas the 18-day periodicity, observed during the polar winter only, is caused, probably, by modulation of the ionospheric conductance by atmospheric planetary waves. The occurrence of the narrow-band Pc3 waves in the polar cap is a challenge to modelers, because so far no band-pass filtering mechanism on open field lines has been identified.     

Afternoon Pc5 geomagnetic pulsations on the Earth’s surface and in the ionosphere (STARE radars)

Two cases when Pc5 geomagnetic pulsations were registered at the IMAGE Scandinavian network of stations and with STARE radars in the afternoon sector (1700–1800 MLT) during the recovery phase of the moderate magnetic storm are analyzed in detail. Using the ground-based observations, it has been indicated that classical quasimonochromatic resonance Pc5 pulsations were observed in the first case (on October 12, 1999; Kp = 5); in this case the maximal amplitude of the spectral maximum at a frequency of 2.5 mHz was registered at Φ ～ 65°. Two maximums were observed in the spectrum in the second case (on October 13, 1999; Kp = 4): ～2.5 mHz (the same maximum) and 2.9 mHz; in this case the maximal oscillation amplitude (2.5 mHz) shifted to Φ > 67°. These results were compared with the echo signal intensity simultaneously registered with the STARE Finland radar on a beam oriented along the 105° geomagnetic meridian. The spatial-temporal maps of the Pc5 pulsation amplitude latitudinal distribution (“keograms”), constructed based on the radar measurements in the wide range of geomagnetic latitudes (63°–70°) where the resolution was substantially higher than that of the ground-based observations, made it possible to detect two regions spaced in latitude (Φ ～ 65° and Φ ～ 67°–68°) with the simultaneous excitation of oscillations (double resonance?), between which the plasmapause projection was supposedly located.     

Geomagnetic pulsations and magnetic disturbances during the initial phase of a strong magnetic storm of May 15, 2005

A very strong magnetic storm of May 15, 2005, was caused by an interplanetary magnetic cloud that approached the Earths’ orbit. The sheath region of this cloud was characterized by a high solar wind density (～25–30 cm^?3) and velocity (～850 km/s) and strong variations (to ～20 nT) in the interplanetary magnetic field (IMF). It has been indicated that an atypical bay-like geomagnetic disturbance was observed during the initial phase of this storm in a large longitudinal region at high latitudes: from the morning to evening sectors of the geomagnetic local time. Increasing in amplitude, the magnetic bay rapidly propagated to the polar cap latitudes up to the geomagnetic pole. An analysis of the global space-temporal dynamics of geomagnetic pulsations in the frequency band 1–6 mHz indicated that most intense oscillations were observed in the morning sector in the region of the equivalent ionospheric current at latitudes of about 72°–76°. The wavelet structure of magnetic pulsations in the polar cap and fluctuations in IMF was generally similar to the maximum at frequencies lower than 4 mHz. This can indicate that waves directly penetrated into the polar cap from the solar wind.     

The recovery phase of the superstrong magnetic storm of July 15–17, 2000: Substorms and ULF pulsations

The geomagnetic observations, performed at the global network of ground-based observatories during the recovery phase of the superstrong magnetic storm of July 15–17, 2000 (Bastille Day Event, Dst = ?301 nT), have been analyzed. It has been indicated that magnetic activity did not cease at the beginning of the storm recovery phase but abruptly shifted to polar latitudes. Polar cap substorms were accompanied by the development of intense geomagnetic pulsations in the morning sector of auroral latitudes. In this case oscillations at frequencies of 1–2 and 3–4 mHz were observed at geomagnetic latitudes higher and lower than ～62°, respectively. It has been detected that the spectra of variations in the solar wind dynamic pressure and the amplitude spectra of geomagnetic pulsations on the Earth’s surface were similar. Wave activity unexpectedly appeared in the evening sector of auroral latitudes after the development of near-midnight polar substorms. It has been established that the generation of Pc5 pulsations (in this case at frequencies of 3–4 mHz) was spatially asymmetric about noon during the late stage of the recovery phase of the discussed storm as took place during the recovery phase of the superstrong storms of October and November 2003. Intense oscillations were generated in the morning sector at the auroral latitudes and in the postnoon sector at the subauroral and middle latitudes. The cause of such an asymmetry, typical of the recovery phase of superstrong magnetic storms, remains unknown.     

Long-period geomagnetic pulsations in the quasi-conjugate arctic and antarctic regions during the magnetic storm of April 16–17, 1999

Based on the observations in six pairs of almost conjugate high-latitude stations in the Arctic and Antarctic regions, the spectral and spatial-temporal structures of long-period geomagnetic pulsations (f = 2–5 mHz) during the magnetic storm of April 16–17, 1999, which is characterized by a high (up to 20 nPa) solar wind dynamic pressure, have been studied. It has been indicated that the magnetic storm sudden commencement is accompanied by a symmetrical excitation of np pulsations near the dayside polar cusps with close amplitudes. Under the conditions when IMF B _z > 0 and B _y < 0, strong magnetic field variations with the periods longer than 15–20 min were observed only in the northern polar cap. When IMF B _z and B _y became close to zero, geomagnetic pulsation bursts in both hemispheres were registered simultaneously but differed in the spectral composition and spatial distribution. In the Northern Hemisphere, pulsations were as a rule observed in a more extensive latitude region than in the Southern Hemisphere. In the Northern Hemisphere, the oscillation amplitude maximum was observed at higher latitudes than in the Southern Hemisphere. The pulsation amplitude at geomagnetic latitude lower than 74° was larger in the Arctic Regions than in the Antarctic Regions. This can be explained by sharply different geographic longitudes in the polar cap and latitudes in the auroral zone, which results in a different ionospheric conductivity affecting the amplitude of geomagnetic pulsations.     

Penetration of Pc5 geomagnetic pulsations to unusually low latitudes during the recovery phase of a superstrong magnetic storm of October 31, 2003

Based on the data of the ground observations, the global distributions of the Pc5 geomagnetic pulsation amplitudes during the recovery phase of the superstorm of October 31, 2003, have been mapped, and an unusually deep penetration of these pulsations into the inner magnetosphere has been found out. Thus, two more zones with identical dynamic spectra and oscillation amplitudes from the polar to equatorial latitudes have been detected in the postnoon sector simultaneously with morning classical Pc5 pulsations in the narrow (～63°–68° CGM) latitudinal band extended along longitude. The higher-latitude zone as if continues the morning band, and the lower-latitude zone is characterized by the maximal intensity at latitudes of ～50°–57° CGM. The oscillation amplitudes are of the same order of magnitude in both zones. The zones are spatially separated by a very narrow latitudinal amplitude minimum and by a change in the phase and sense of rotation of the wave polarization vector. The pulsation spectra in the morning and daytime sectors are different, which indicates that the nature of the morning and postnoon oscillations is different.     

Global Pi3 geomagnetic pulsations as a response of large variations in the solar wind and IMF during the magnetic storm of August 5, 2011

Spatial-temporal and spectral features of ground geomagnetic pulsations in the frequency range of 1–5 mHz at the initial phase of a strong magnetic storm of the 24th cycle of solar activity (August 5–6, 2011, with a Dst-variation in the storm maximum of ?110 nT) are analyzed. Large opposite in sign amplitudes of variations in IMF parameters (from ?20 to +20 nT) at a high velocity of the solar wind (～650 km/s) accompanied by intense bursts in solar-wind density (up to ～50 cm^?3) were distinctive feature of interplanetary medium conditions causing the storm. Geomagnetic Pi3 pulsations global in longitude and latitude and in-phase in the middle and equatorial latitudes were found. The onset of pulsation generation was caused by a pulse of dynamic pressure of the solar wind (～20 nPa), i.e., by a considerable compression of the magnetosphere. The maximum (2–3 mHz) in the amplitude spectrum of near-equatorial pulsations coincided with the maximum of pulsations in the daytime polar cap. After the next jump of the dynamic pressure of the solar wind (～35 nPa), an additional maximum appeared in the pulsation spectrum in the frequency band of ～3.5–4.5 mHz. Global pulsations suddenly stopped after a sharp decrease in the solar-wind dynamic pressure and corresponding extension of the magnetosphere. The obtained results are compared with the time dynamics of the position and shape of the plasmapause.     

Variations in the ULF index of daytime geomagnetic pulsations during recurrent magnetic storms

A new index of wave activity (ULF index) is applied to analyze daytime magnetic pulsations in the Pc5 range (f = 2–7 mHz) during ten successive recurrent magnetic storms (CIR (corotating interaction region) storms) of 2006. The most intense daytime geomagnetic Pc5 pulsations on the Earth’s surface in all phases of CIR storms are predominantly observed in the pre-noon sector at latitudes higher than 70°, while those in CME storms (storms initiated by coronal mass ejection (CME)) are observed at latitudes lower than 70°. A comparison of wave activity during CIR and CME storms has shown that the amplitude of Pc5 pulsations in CIR storms is much smaller than that in CME storms and the spectrum maximum is observed at lower frequencies and higher latitudes. At the same time, the mechanism of ULF wave generation during both types of magnetic storms seems to be similar, namely, resonance of magnetic field lines due to the development of the Kelvin-Helmholtz instability caused by an approach of a high-velocity solar wind stream to the Earth’s magnetosphere. Since resonance oscillations are excited only in the closed magnetosphere, the higher-latitude position of the Pc5 pulsation intensity maximum in CIR storms points to larger dimensions of the daytime magnetosphere during CIR storms as compared to CME storms.     

Properties of daytime long-period pulsations during magnetospheric storm commencement

Long-period geomagnetic pulsations during the SSC of July 14, 2012, are studied. The prenoon longitudinal sector (09:20–11:30) MLT, from the boundaries of which pulsations propagate azimuthally onto the dawn and dusk sides with an opposite polarization direction and increased amplitude, has been distinguished. The position of this sector relative to noon (a shift to the dawn side) depends on the front azimuthal inclination. It has been found that the polarization direction reverses in going from low (<30°) to middle/subauroral (≥50°) latitudes on the entire dayside. The geomagnetic pulsations mainly fluctuate near the f₁ = 2.9 and f₂ = 4.4 mHz frequencies. Fluctuations with frequency f₁, which coincide with the fluctuation frequency of the IMF х component, predominate at the polar cap latitudes (the open field line region) in the form of rapidly attenuating impulses and at low latitudes with a much smaller amplitude. Fluctuations with frequency f₂ are globally registered at all latitudes in the dayside sector below the magnetopause projection as a train of several fluctuations. It is assumed that fluctuations with frequency f₁ penetrate from the solar wind, and fluctuations with frequency f₂ are radial magnetopause oscillations.     

Planetary distribution of geomagnetic pulsations during a geomagnetic storm at solar minimum

We investigate the features of the planetary distribution of wave phenomena (geomagnetic pulsations) in the Earth’s magnetic shell (the magnetosphere) during a strong geomagnetic storm on December 14–15, 2006, which is untypical of the minimum phase of solar activity. The storm was caused by the approach of the interplanetary magnetic cloud towards the Earth’s magnetosphere. The study is based on the analysis of 1-min data of global digital geomagnetic observations at a few latitudinal profiles of the global network of ground-based magnetic stations. The analysis is focused on the Pc5 geomagnetic pulsations, whose frequencies fall in the band of 1.5–7 mHz (T ～ 2–10 min), on the fluctuations in the interplanetary magnetic field (IMF) and in the solar wind density in this frequency band. It is shown that during the initial phase of the storm with positive IMF Bz, most intense geomagnetic pulsations were recorded in the dayside polar regions. It was supposed that these pulsations could probably be caused by the injection of the fluctuating streams of solar wind into the Earth’s ionosphere in the dayside polar cusp region. The fluctuations arising in the ionospheric electric currents due to this process are recorded as the geomagnetic pulsations by the ground-based magnetometers. Under negative IMF Bz, substorms develop in the nightside magnetosphere, and the enhancement of geomagnetic pulsations was observed in this latitudinal region on the Earth’s surface. The generation of these pulsations is probably caused by the fluctuations in the field-aligned magnetospheric electric currents flowing along the geomagnetic field lines from the substorm source region. These geomagnetic pulsations are not related to the fluctuations in the interplanetary medium. During the main phase of the magnetic storm, when fluctuations in the interplanetary medium are almost absent, the most intense geomagnetic pulsations were observed in the dawn sector in the region corresponding to the closed magnetosphere. The generation of these pulsations is likely to be associated with the resonance of the geomagnetic field lines. Thus, it is shown that the Pc5 pulsations observed on the ground during the magnetic storm have a different origin and a different planetary distribution.     

V. A. Troitskaya: the Founder of the Russian School of Geomagnetic Pulsations

A thorough investigation of short-period oscillations in the Earth’s magnetic field as a fundamental natural process of the magnetospheric plasma began in Russia after V.A. Troitskaya established two oscillatory regimes in the geomagnetic field, namely, the regimes of continuous (Pc) and irregular pulsations (Pi). For studying these pulsations, 19 stations recording the telluric currents were installed during the International Geophysical Year (IGY, 1957–1959) on Troitskaya’s initiative. One of these stations was the Borok station. Subsequently, Borok has become the basic site for investigating geomagnetic pulsations and the main center for studying the short-period pulsations (SPPs) in the Earth’s magnetic field. This is the Borok scientific station where the key fundamental regularities of different types of geomagnetic pulsations were established. Troitskaya led and actively participated these works. Troitskaya organized and conducted the first complex geomagnetic observations in the world at the conjugate points Sogra (Arkhangelsk region, Russia) and Kerguelen (Indian Ocean). These studies were initially tested at the Borok observatory, where it was established that the wave packets of Pc1 geomagnetic pulsations are alternately observed in the northern and southern hemispheres in contrast to the other pulsation types which simultaneously occur in both hemispheres. The studies carried out at Borok promoted the establishment of a new direction in geophysics—diagnostics of the state of the magnetosphere based on the ground observations of geomagnetic pulsations. The analysis of simultaneous observations of the geomagnetic pulsations at polar latitudes of the Arctic and Antarctic was also for the first time conducted at the Borok observatory. This analysis revealed the main characteristics of wave phenomena at the geomagnetic poles and in the vicinity of the projection of the dayside polar cusp. Thus, for the first time in the world, Troitskaya and her Borok colleagues established the key patterns of the oscillatory regimes in the geomagnetic field of the Earth. This laid the basis for the further experimental and theoretical investigations which have shown that SPPs play a leading role in the dynamics of the magnetospheric plasma. In this paper we also list of 60 of Troitskaya’s main publications.     

Statistical relations between the probability of occurrence of Pc3-4 pulsations at high latitudes in the antarctic regions and the solar wind and IMF parameters

Geomagnetic pulsation in the Pc3-4 bands have been studied at high Antarctic latitudes during the local summer. The statistical relation between the occurrence probability of Pc3 and Pc4 pulsations and the solar wind (SW) and IMF parameters has been revealed by verifying the hypothesis that an indication is identical in two distributions. Different dependences of the occurrence probability of high-latitude Pc3 and Pc4 pulsations on the IMF value and orientation and SW density and velocity have been found out. It has been indicated that these dependences remain unchanged in the range of geomagnetic latitudes from 66° to 87°. It has been established that the Pc3 observation probability at small (20°–50°) IMF cone angles (θ = cos^?1(B _x/|B|)) is a factor of 1.5 higher than the average statistical probability and depends on the IMF value, which confirms the hypothesis that the Pc3 source is the turbulent region upstream of the magnetospheric quasiparallel low shock. On the contrary, the probability of occurrence of Pc4 weakly depends on the IMF cone angle and is maximal at θ ～ 0° and ～90°. With increasing negative B _z values, the generation probability increases in the Pc4 band and tends to decrease in the Pc3 band. It has been found out for the first time that the dependence of the Pc4 occurrence probability on the IMF clock angle (? = tan^?2 (B/B _z) is identical in the regions of projections of closed and open field lines, whereas this dependence is different for Pc3. In the region of projections of closed field lines, the Pc3 occurrence probability increases at B _z < 0 and B _y > 0 (the condition under which the cusp shifts on the dawn side) and at B _y < 0 and B _z > 0 (which is typical of the formation of the low-latitude boundary plasma sheet). In the region of projections of open field lines such a probability increases at B _y < 0 and B _z < 0 (which results in the formation of the high-latitude boundary plasma sheet). Based on the discovered regularities, the conclusion has been made that the sources of generation of high-latitude Pc3 and Pc4 pulsations are different.     

Daytime quasiperiodic geomagnetic pulsations during the recovery phase of the strong magnetic storm of May 15, 2005

Intense quasimonchromatic geomagnetic pulsations with a period of ～15 min, observed on the Earth’s surface in the near-noon sector at the beginning of the recovery phase of a very strong (Dst _min = ?260 nT) magnetic storm of May 15, 2005, are analyzed. The variations were registered at auroral latitudes only in the X field component, and wave activity shifted into the postnoon sector of the polar cap an hour later; in this case pulsations were observed in the X and Y field components. Within the magnetosphere the source of magnetic pulsations could be the surface waves on the magnetopause caused by the pulse of the solar wind magnetic pressure. Geomagnetic pulsations in the polar cap, observed in phase at different latitudes, could apparently reflect quasiperiodic variations in the NBZ system of field-aligned currents. Such variations can originate due to the series of pulsed reconnections in the postnoon outer cusp at large (～20 nT) positive B _z values and large (about ?40 nT) negative values of IMF B _x .     

Burst regimes of long-period irregular pulsations at frequencies of 2.0–6.0 mHz and substorm activity in the nightside magnetosphere

The results of studying the simultaneous observations of burst regimes of long-period irregular pulsations at frequencies of 2.0–6.0 mHz (the series of ipcl bursts) in the region of the dayside polar cusp and magnetic field disturbances in the nightside auroral oval are presented. The data on the magnetic field at Mirny (MIR, Φ = 76.93°; Λ = 122.92°) and Yellowknife (YKC, Φ = 69.94°; Λ = 294.38°) antipodal observatories as well as the AE index values (http://www.cetp.ipsl.fr/~isgi/homepag1.htm) have been used in an analysis. It has been found out that 87% (group I) and 13% (group II) of events were registered against a back-ground of substorm activity and a quiet nightside magnetosphere, respectively. It has been revealed that several morphological characteristics of the group-I and -II ipcl bursts differ depending on the conditions in the nightside magnetosphere. It has been indicated that the intervals between peaks and the amplitudes of ipcl bursts of both types are distributed according to the exponential and power laws. The results indicate that magnetospheric plasma turbulence develops in the region where burst regimes are formed. It is assumed that the substorm processes in the magnetotail manifest themselves in plasma turbulence in the dayside cusp.     

Unusual spatial-temporal dynamics of geomagnetic disturbances during the main phase of the extremely strong magnetic storm of November 7–8, 2004

The spatial dynamics of geomagnetic variations and pulsations, auroras, and riometer absorption during the development of the main phase of the extremely strong magnetic storm of November 7–8, 2004, has been studied. It has been indicated that intense disturbances were observed in the early morning sector of auroral latitudes rather than in the nighttime sector, as usually takes place during magnetic storms. The unusual spatial dynamics was revealed at the beginning of the storm main phase. A rapid poleward expansion of disturbances from geomagnetic latitudes of 65°–66° to 74°–75° and the development of the so-called polar cap substorm with a negative bay amplitude of up to 2500 nT, accompanied by precipitation of energetic electrons (riometer absorption) and generation of Pi2–Pi3 pulsations, were observed when IMF B _z was about ?45 nT. The geomagnetic activity maximum subsequently sharply shifted equatorward to 60°–61°. The spatial dynamics of the westward electrojet, Pi2–Pi3 geomagnetic pulsations, and riometer absorption was similar, which can indicate that the source of these phenomena is common.     

Analysis of latitudinal distribution of Pi2 geomagnetic pulsations using the generalized variance method

The spatial dynamics of bursts of geomagnetic Pi2-type pulsations during a typical event of a magnetospheric substorm (April 13, 2010) drifting to the pole was investigated using the method of generalized variance characterizing the integral time increment of the total horizontal amplitude of the wave at a given point in the selected time interval. The digital data of Scandinavian profile observations from IMAGE magnetometers with 10-second sampling and data of the INTERMAGNET project observations at the equatorial, middle-latitude and subauroral latitudes with a 1-second sampling were used in the analysis. It was shown that Pi2 pulsation bursts in a frequency band of 8–20 mHz appear simultaneously on a global scale: from the polar to equatorial latitudes with maximum amplitudes at latitudes of the maximum intensity of the auroral electrojet and with a maximum amplitude of geomagnetic pulsations Pi3 within a band of 1.5–6 mHz. The first (left-polarized) intensive Pi2 burst appeared at auroral latitudes several minutes after breakup, while the second (right-polarized) burst occurred 15 min after breakup but at higher (polar) latitudes where the substorm had displaced by that time. The direction of wave-polarization vector rotation was opposite for auroral and subauroral latitudes, but it was identical at the equator and in the subauroral zone. The pulsation amplitude at the equator was maximal in the night sector.     

Studies on the latitudinal distribution of ground-based geomagnetic pulsations and fluctuations in the interplanetary medium using discrete mathematical analysis methods

Ground-based geomagnetic Pc5 (2–7 mHz) pulsations, caused by the passage of dense transients (density disturbances) in the solar wind, were analyzed. It was shown that intensive bursts can appear in the density of the solar wind and its fluctuations, up to Np ～ 30–50 cm³, even during the most magnetically calm year in the past decades (2009). The analysis, performed using one of the latest methods of discrete mathematical analysis (DMA), is presented. The energy functional of a time-series fragment (called “anomaly rectification” in DMA terms) of two such events was calculated. It was established that fluctuations in the dynamic pressure (density) of the solar wind (SW) cause the global excitation of Pc5 geomagnetic pulsations in the daytime sector of the Earth’s magnetosphere, i.e., from polar to equatorial latitudes. Such pulsations started and ended suddenly and simultaneously at all latitudes. Fluctuations in the interplanetary magnetic field (IMF) have turned up to be less geoeffective in exciting geomagnetic pulsations than fluctuations in the SW density. The pulsation generation mechanisms in various structural regions of the magnetosphere were probably different. It was therefore concluded that the most probable source of ground-based pulsations are fluctuations of the corresponding periods in the SW density.     

Ground images at high latitudes of ULF wave processes in the outer magnetosphere

Magnetohydrodynamic (MHD) wave processes in space and laboratory plasmas make possible the resonant conversion of global compressional modes into localized Alfven oscillations. In this way MHD signals excited by local non-steady processes in space can convey information to the ground. This review focuses on long-period ultra-low-frequency (ULF) phenomena specific to high latitudes, including those in the cusp region, nightside oval, and polar cap, and highlights several unresolved problems related to the physics of ULF waves at these latitudes. Several new results are highlighted: (1) In the high-altitude cusp an MHD waveguide/resonator without conductive boundaries could occur, where hydromagnetic turbulence penetrating from the magnetosheath can accumulate and transform into Alfven waves escaping along the field lines. (2) Broadband long-period wave activity near the cusp exhibits temporal structure that is as yet unexplained, as well as frequent long intervals of non-conjugate levels of wave power. (3) Occurrence of field-aligned potential drops in the auroral acceleration region may cause severe damping of Pc5/Pi2 waves as compared with ionospheric Joule dissipation or dispersive leakage. (4) Specific long-period irregular variations in the nightside polar cap of still unknown origin exist, which might be related to the flapping of the magnetotail or turbulent trans-polar convection. Finally, we point out what remains unknown, to what extent our current knowledge is incomplete, what observations have been rare and what future research could focus on.     

Estimation of storm-time level of day-side wave geomagnetic activity using a new <Emphasis Type="Italic">ULF</Emphasis> index

The level of wave geomagnetic activity in the morning and daytime sectors of auroral latitudes during strong magnetic storms with Dst _min varying from ?100 to ?150 nT in 1995–2002 have been studied using a new ULF index of wave activity proposed in [Kozyreva et al., 2007]. It has been detected that daytime Pc5 pulsations (2–6 mHz) are most intense during the main phase of a magnetic storm rather than during the recovery phase as was considered previously. It has been indicated that morning geomagnetic pulsations during the substorm recovery phase mainly contribute to daytime wave activity. The appearance of individual intervals with the southward IMF B _z component during the magnetic storm recovery phase results in increases in the ULF index values.