Pulsation Activity Research Articles

Ground-satellite correlations and geostationary orbit energy flow at substorm onset

The GEOS-2 spacecraft electric and magnetic field instruments have been used to measure the energy flow at geostationary orbit for three substorm onsets. The Poynting vector has been calculated in field aligned coordinates and shows very little field aligned energy flow at ULF frequencies. Instead a radial, damped, compressional wave-like energy flow is observed. For one onset both GEOS-2 and the BGS ground magnetometer array were available. This onset shows similar frequency pulsation activity in the auroral zone and in space, but activity of a different frequency and character at mid-latitudes. Again the space signature is almost totally compressional. It is concluded that whilst the ground auroral signature may be characterized by an Alfvén wave, most regions of space see a primarily compressional signal. This signal can propagate across field lines to stimulate the lower latitude activity

Observations of intense ULF pulsation activity near the geomagnetic equator during quiet times

The Active Magnetospheric Particle Tracer Explorers (AMPTE) CCE satellite frequently observed ULF pulsations in the Pc 3‐4 period range as it traveled from L = 3 to beyond L = 8 near local noon. In addition to the azimuthally polarized, harmonically structured pulsations that occurred frequently at all dayside magnetic latitudes sampled by the satellite (±16°), a distinctive class of pulsations appeared occasionally during quiet times when the satellite was within 1° or 2° of the geomagnetic equator. These pulsations, with increased amplitude (up to 12 nT peak to peak) and without multiple harmonic structure, were also transverse but with dominant power in the radial component. They were observed during magnetically quiet periods in regions characterized by intense fluxes of warm, strongly trapped light ions, predominantly H+, and often with streaming low‐energy plasma. We interpret the strong latitudinal localization of these pulsations to be due to equatorial mass loading or to partial reflection of Alfvén wave energy by latitudinal gradients in plasma density. In most cases we have identified a recently injected energetic particle population at the satellite's location as the probable source of wave energy. The similarity in occurrence between these events and the “giant” pulsations occasionally observed on the ground suggests that latitudinally localized plasma structures may be a necessary but not sufficient condition for the onset of these unusually long‐lived, highly localized pulsations.

Dynamic spectra of pulsation events at L ∼ 1.9 and L ∼ 3.3

High resolution dynamic spectra of 12 pulsation events are compared at NCK ( L ∼ 1.9) and KVB ( L ∼ 3.3), lying on the same meridian. Common features of the pulsation activity at the two stations include periods, period shifts and switches on and off. These parameters are investigated for the sample of pulsation activity and demonstrate that the two stations record activity from both a common and from different sources. Shell resonances are discussed in relation to the common source of activity.

Solar cycle effects in intervals of pulsations of diminishing periods pulsation activity

We report that intervals of pulsations of diminishing periods (IPDP) type plasma wave events which are associated with magnetospheric substorms are less frequent in the years of high solar activity than in years of low activity. This new result is based on two independent continuous data sets from 1974 to 1985. We show also that IPDP events occur throughout the recurrent magnetic storms in 1976, whereas they appear more sporadically in 1980, i.e., during high solar activity. It is concluded that several factors such as changes in plasmapause position, attenuation of horizontal ionospheric wave propagation, and heavy ion densities in the magnetosphere can contribute to the observed long‐term variability.

Greater than 0.1‐Hz ULF magnetic pulsations measured at South Pole, Antarctica

South Pole pulsation activity at frequencies greater than 0.1‐Hz is analyzed in this study and found to consist predominantly of large‐amplitude, irregular pulsations (Pi 1) closely correlated with overhead auroral activity. These pulsations therefore have two diurnal maxima related to the substorm polar expansion nightside aurora and cusp or dayside aurora. The dependence of this magnetic nightside Pi activity at South Pole on Kp and interplanetary magnetic field parameters is consistent with a strong substorm correlation. As found at lower latitudes, when pulsating aurora is evident in the photometric data there can be a good detailed correlation between the auroral luminosity fluctuations and the magnetic pulsations, suggesting that the pulsations are locally produced by overhead currents driven by the particle precipitation. Nightside Pi activity is often of larger‐amplitude than the dayside Pi and appears in bursts of pulsations having 5 to 20 minsduration. The burst‐like character arises because of the rapid poleward expansion of discrete auroral forms from the nightside auroral oval, as is evident in the South Pole photometric data. South Pole nightside auroral magnetic pulsations generally commence well after the onset of similar auroral magnetic pulsations recorded at Siple, Antarctica, a subauroral location. One third of the hours during which structured, band‐limited pulsations (Pc 1) are observed at the low‐latitude, Siple, Antarctica, location also contain Pc 1 observed at South Pole. When it is possible to identify similar structures in Pc 1 between South Pole and Siple they are in phase, in contrast to out of phase structures for opposite hemisphere stations. This supports horizontal ducting of Pc 1 from lower‐latitudes to South Pole. However, there often appears much more structure at South Pole than recorded at Siple, suggesting many paths of horizontal propagation from multiple lower‐latitude sources if the signals are not generated on South Pole field lines. The pronounced inverse correlation of South Pole Pc 1 with magnetic activity means that the magnetic field topology and/or ionospheric conditions associated with substorm activity preclude the possible propagation of Pc 1 to South Pole from lower‐latitudes when the magnetic activity is high.

Spectral characteristics of low latitude Pc3 geomagnetic pulsations in south-east Australia

Geomagnetic pulsations in the Pc3 period range (10–45 sec) were recorded simultaneously in south-east Australia using an array of four low latitude induction coil magnetometers over a longitudinal range of 17° atL=1·8 to 2·7. Geomagnetic data from March 25 to September 21, 1982 were digitized and spectral studies were carried out from Grey scale digital (GSD) dynamic spectra and contour sonagrams. Similar wave spectra were generally seen at the three stationsWM, BH andNC in the same latitude with Pc3 waves mostly being observed in the 40–45 mHz band. In addition the higher latitude station (LN) also showed lower frequency Pc4 bands (≈ 10mHz) on some days but higher frequency bands (60–80mHz) on others. Sometimes completely different wave frequencies were observed on theX andY components with the stations in the same latitude generally showing similar spectra on the corresponding components. Dynamic spectra also indicated that wave energy often turned off and on abruptly over short intervals of time implying solar wind control and Pc3 pulsation activity. A band harmonic structure was occasionally seen in wave spectra at the azimuthal stations mainly on theX component during the local afternoon with a harmonic spacing (Δf) of ≈ 10 mHz.

Relations between morning sector Pi 1 pulsation activity and particle and field characteristics observed by the De 2 satellite

Impulsive short‐period magnetic field fluctuations (Pi 1) observed by ground instrumentation in the morning sector auroral zone have in recent years been convincingly tied to the presence overhead of pulsating aurora. We here present simultaneous records from ground‐based magnetometer, photometer, and riometer instrumentation at Siple, Antarctica, and its conjugate point, Roberval, Quebec, and low‐altitude particle and magnetic and electric field data from the DE‐2 spacecraft. Satellite magnetometer observations support earlier findings of a link between morning sector Pi 1 and the presence of a region 2 Birkeland current sheet. The addition of energetic electron and electric field observations provides a more complete characterization of the environment of Pi 1 pulsation events. Intense fluxes of energetic electrons (from 10 keV to over 35 keV) are observed simultaneously with Pi 1 pulsations and region 2 Birkeland currents. When intense (>10−5 ergs/cm² s sr eV) fluxes of lower energy electrons (E < 1 keV) are also present, we observe strong region 2 Birkeland currents and strong Pi 1 pulsations; when lower energy electrons are less abundant, region 2 Birkeland currents and Pi 1 pulsations are considerably weaker. No Pi 1 pulsations have been found in regions without intense fluxes of energetic electrons. Simultaneous observations of magnetic and electric fields in space indicate complex localized current and field structures, which are likely to be the source of the magnetic pulses observed on the ground.

Transient magnetic disturbances from solar flare explosions and associated pulsations of type IV radio bursts

We have proposed a mechanism of arise of transient magnetic disturbances from solar flare explosion which can lead to understanding of observed pulsations of type IV radio emission with period of 0.3–3.0 s. According to the proposed mechanism the pulsation activity of the radio emission results from MHD waves accompanying the expanding diamagnetic plasma produced by the explosive flare material.

On the immediate effect of cone-angle changes on geomagnetic pulsations

In spite of many studies on the effect of cone-angle on geomagnetic pulsations, this question continues to be discussed. In the present paper data are presented on the immediate effect of cone-angle changes on geomagnetic pulsations, i.e. effects are studied when the cone-angle (the angle between the Sun-Earth direction and the direction of the interplanetary magnetic field, IMF) changed from an unfavourable/favourable value to a favourable/unfavourable one. It is concluded that the cone-angle influences ground pulsation activity quite clearly (switch-offs and switch-ons) and in most respects earlier results are confirmed or extended.

Local time variation in the response of Pc 5 pulsations in the morning sector to substorm expansive phase onsets near midnight

Since substorm expansive phase activity in the midnight sector and Pc 4,5 pulsation activity in the morning sector are both colocated with the auroral oval, it is logical to assume that sudden changes in plasma and field parameters in the oval would cause perturbations in the activity level of both types of activity. In an earlier study, Samson and Rostoker [1981] demonstrated that the frequency of Pc 4,5 pulsations on the dayside increased at the onset of a substorm expansive phase in the night sector. In that study, only pulsations recorded near noon were investigated for their frequency content. In this paper we consider the response of pulsation activity across the entire morning sector to substorm onsets near midnight. Using superposed epoch analyses we demonstrate that the conclusions of Samson and Rostoker [1981] are valid across the morning sector. However close to dawn, Pc 5 activity appears to be initiated at substorm onset rather than decreasing by shifting frequency toward the Pc 5 band, as appears to be the case closer to noon. We present a model of Pc 4,5 activity to explain this phenomenon in which the origin of the pulsations is proposed to be a Kelvin‐Helmholtz instability at the interface between the low‐latitude boundary layer and the central plasma sheet.

On the relationship between morning sector irregular magnetic pulsations and field aligned currents

Data from search coil magnetometer, riometer, and photometer instrumentation at Siple, Antarctica, are compared with vector magnetometer data from south polar passes of the MAGSAT satellite for three magnetically disturbed days in early 1980. Two kinds of association were found between irregular pulsation activity in the 0.5‐ to 40‐s frequency band (Pi 1) observed on the ground and magnetospheric‐ionospheric currents measured from space. (1) During each MAGSAT polar pass in the morning sector the level of Pi 1 activity correlates well with the intensities of three‐dimensional current systems. Fine structure was often observed in the field‐aligned currents during periods of intense Pi activity. Birkeland currents include 2‐ to 10‐s (16–80 km) structured perturbations evident in the transverse components of the field, most likely indicating filamentary currents. (2) The pulsations observed at Siple in the morning sector include an asymmetric class of fluctuations in dB/dt whenever the region 2 field‐aligned currents extend to or equatorward of the invariant latitude of Siple and do not have such components when the currents do not extend to that latitude. These observations support models suggesting that Pi 1 pulsations represent fluctuations of a three‐dimensional current system in the magnetosphere and are consistent with recent conductivity and current enhancement models for the ionospheric source of Pi 1 activity.

Development of an auroral absorption substorm: Studies of substorm related absorption events in the afternoon-early evening sector

We discuss the effects in ionospheric absorption of particle precipitation observed in the afternoon-early evening sector during substorms with onset in the midnight sector. All events considered here occurred during magnetically disturbed periods, K p > 3. For many of the substorm events a smooth southward moving absorption bay is seen in the midnight and evening sectors about 1 h preceeding the onset. The magnetic pulsation activity is low during this preceding bay. After substorm onset near magnetic midnight the precipitation region may expand with a sharp onset at the front towards the West in spatially confined regions at high and low L-values separately with about equal velocities. The observations are consistent with a model of westward expansion of the energetic electron precipitation in two regions, aligned parallel to the auroral oval, at high and low L-values of about L ⩾ 6 and L ⩽ 4.8. The westward expanding absorption activity correlates well with local magnetic variations. In magnetic pulsations PiB events are seen at high latitudes simultaneously with the westward moving onsets while at low latitudes IPDP pulsations are observed during the active part of the absorption events. Later in the substorm event a slowly varying absorption event (SVA) is sometimes observed at the lower L-values, L ∼ 3–4.

Simultaneous geomagnetic and radio auroral observations

Abstract—A correlative study was carried out using ground magnetometer data, which includes ordinary magnetograms and micropulsation recordings, and STARE radar auroral data from an ionospheric region above the magnetometer station. The observations are from a reasonably disturbed period which covers the evening, midnight and morning sectors. A close relationship was found to exist between the long period geomagnetic and backscatter amplitude variations which are mainly controlled by conductivity modifications in the E-region. The irregular P 1 pulsation activity is positively related to enhancements in backscatter intensity; the electric field strength was found to decrease considerably during P 1 intensifications. The relatively fast variations observed in backscatter amplitude simultaneously with impulsive P i -bursts, for short time intervals in the evening sector, are possibly associated with precipitation pulsations taking place during substormonsets. It issuggested that the major source of irregular P i -noiseis the auroral Hall current perturbed by electron density fluctuations which could possibly be due to the effects of the two stream and gradient drift plasma instabilities.

Correlated irregular magnetic pulsations and optical emissions observed at Siple Station, Antarctica

Pi 1 magnetic pulsations observed at Siple Station, Antarctica, contain an unusual class of fluctuations. They are large amplitude, 0.2–1 nT/s, one‐sided pulsations in dB/dt with variable direction. The pulsations are from 2 to 10 s wide and occur in an irregular train of pulses that lasts, sporadically, up to an hour. An earlier study (Arnoldy et al., 1982) reported that in 1979 data these pulses were often closely associated with riometer and auroral pulsations. The present report extends this work by using 1980 data and describes the variety in observation of these pulsations. The pulsations are observed from 2200 local time until noon with most frequent occurrence near dawn. They usually occur during or shortly after a burst of high amplitude pulsation activity within a long duration (several hours) interval of large or moderate amplitude Pi. Although the direction of the pulsations is typically west or southwest other directions are also observed, with occasionally a shift of direction (for example, from north to south) occurring in a few minutes. We interpret these Pi 1 pulsations to be the result of local enhancements in ionospheric and field‐aligned currents associated with precipitating electrons responsible for the associated pulsations in auroral luminosity. No evidence was found for a sinusoidal component of these magnetic pulsations, travelling as a wave in the magnetosphere.

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.

Pc 1 pulsation activity at Ottawa

Pulsations in the Pc 1 period range recorded at Ottawa for three years during the IMS are analyzed to study some of their occurrence characteristics. The diurnal occurrence pattern depends on the seasons and on the prevailing magnetic activity conditions. In both cases the morning peak shifts to earlier hours and the evening peak shifts to later hours with increasing conduction (electron density) in the ionosphere. The largest number of events were recorded in the winter months and about as many in the equinoctial months. A large majority of the events are inferred to occur, at Ottawa, during the recovery phase of magnetic storms. The enhancements of Pc 1's noted at different levels of magnetic activity seem to be controlled by the location of the plasmapause and also by the propagation conditions in the ionosphere. The existence of a relationship between the occurrence of Pc 1 pulsations and the sector polarity of the interplanetary magnetic field remains to be established.

Morphology of spatial patterns in Pi 1-2 magnetic field pulsation activity - a review.

The source regions of Pi 1-2 activity in the night-morning sector consists of two regions or “spirals”. The night region consists of impulsive bursts, i. e., called Pi B events, that are correlated with auroral breakups, the Harang discontinuity, and lower frequency magnetic agitation. The morning region consists of more continuous Pi activity called Pi C events; these are associated to negative bays in the H component of the magnetic field and to pulsating aurora. A cleft and a dayside polar cap region of Pi 1-2 activity are also recognized. These regions form a domain with characteristics different from those for the night and morning region domain.

Possible changes in natural Pc 1 pulsation activity caused by BART

In a previous preliminary study of the diurnal variation of Pc 1 pulsation activity at Stanford during four months in 1975, evidence was obtained for changes in the rate of occurrence of the pulsations around the times when service was started and terminated by the San Francisco Bay Area Rapid Transit (BART) system (Fraser‐Smith et al., 1979). The present study extends this earlier work by analyzing Pc 1 pulsation data recorded at Stanford during 1976. Spectrograms of N‐S geomagnetic activity were prepared for the complete year and 15‐minute intervals containing Pc 1 pulsation activity were tabulated. The diurnal variations of the Pc 1 activity (as measured by these 15‐minute intervals) for weekdays, during which BART was in operation, and weekends, during which BART was not normally in operation, were compared. Correlations of the observed differences with the BART schedule suggest once again that the ultralow‐frequency electromagnetic noise produced by BART may be influencing the occurrence of Pc 1 pulsations along the Stanford geomagnetic meridian.

Changes of pulsation activity during two solar cycles

Monthly and yearly averages of the pulsation activity in the mid-latitude station Nagycenk are compared to solar wind velocity and ionospheric-plasmaspheric electron concentration data. It is found that pulsation amplitudes are correlated with solar wind velocities with the exception of some month around December in solar maximum years, when they are significantly lower than computed from the corresponding solar wind velocities. This decrease can be caused either by a cutoff of the magnetospheric shell resonances or by local ionospheric damping. In addition to these effects, pulsations amplitudes slightly depend also on the geomagnetic activity and have a semi-annual activity change with maxima around equinoxes.

Kelvin‐Helmholtz instability and the variation of geomagnetic pulsation activity

It is shown that the observed local time variation of dayside geomagnetic micropulsations is consistent with the Kelvin‐Helmholtz generation mechanism operating at the magnetopause. The variation of the angle between the interplanetary magnetic field and the magnetopause around the magnetosphere causes variations in the magnetosheath magnetic field, which in turn lead to local time variations in micropulsation amplitudes. Morning sector pulsations are expected to be larger than afternoon sector pulsations. Furthermore, large‐amplitude pulsations are expected to be more frequently observed when the angle between the interplanetary magnetic field and the solar wind velocity in front of the bow shock is small.