Micropulsation Activity Research Articles

Simultaneous observations of Pc 1 micropulsation activity and stratospheric electrodynamic perturbations on 27 January 2003

The 2nd Polar Patrol Balloon campaign (2nd-PPB) was carried out at Syowa Station in Antarctica during 2002–2003. Identical stratospheric balloon payloads were launched as close together in time as allowed by weather conditions to constitute a cluster of balloons during their flights. A very pronounced negative ion conductivity enhancement was observed at 32 km in the stratosphere below the auroral zone on 27 January 2003 from 1500 to 2200 UT. During this event, the conductivity doubled for an interval of about 7 h. This perturbation was associated with an extensive Pc 1 or Pi 1 wave event that was observed by several Antarctic ground stations, balloon PPB 10, and the Polar spacecraft. No appreciable X-ray precipitation was observed in association with this event, which would point to >60 Mev proton precipitation as a possible magnetosphere–stratosphere coupling mechanism responsible for the conductivity enhancement. Such precipitation is consistent with the wave data. During the latter half of the event, E z was briefly positive. There was a tropospheric Southern Ocean storm system underneath the balloon during this interval. If the event was associated with this storm system and not energetic proton precipitation, the observations imply an electrified Southern Ocean storm and major perturbations in stratospheric conductivity driven by a tropospheric disturbance. This event represents a poorly understood source for global circuit current. Precipitating energetic proton data from Akebono and NOAA POES spacecraft show significant >16 MeV precipitation was occurring at the location of PPB 8 but not PPB 10, suggesting that proton precipitation was, in fact, the responsible coupling mechanism.

Pc5 micropulsation power at conjugate high‐latitude locations

The micropulsation power, integrated over the Pc5 frequency range, has been calculated for the horizontal component of the geomagnetic field at two high‐latitude conjugate locations: Dumont D'Urville (corrected geomagnetic coordinates: 80.61°S, 235.76°E) and Mould Bay (corrected geomagnetic coordinates: 80.85°N, 272.65°E). Because of the different distances between the geographic and the geomagnetic poles in each hemisphere, the comparison between the Pc5 power observed at Dumont D'Urville and at Mould Bay shows the relative importance of geomagnetic and solar illumination effects in driving low‐frequency micropulsation activity. In particular, similarities observed at the two sites can be explained in terms of their common geomagnetic characteristics, while differences can be attributed to the different sunlight or solar zenith angle configurations. Results show that the local summer Pc5 power is statistically higher in the northern hemisphere than in the southern one. This hemispherical difference is smaller for the local equinoxes, and it is only very slight or absent for local winters. These findings are interpreted in terms of the proportionality between the Pc5 power and the ionospheric conductance, which is higher at Mould Bay owing to more permanent and direct sunlight conditions during local summers and equinoxes. Thus the different geographic coordinates affect the Pc5 power at the two considered sites so much so that their effect is visible regardless of the geomagnetic similarities. However, the influence of the geomagnetic activity on Pc5 power is found to be more significant than these geographical effects or than the seasonal effects. In fact, for Kp < 2 the difference in simultaneous observations at Mould Bay and at Dumont D'Urville is <∣10∣ nT2 with an occurrence >70%. The magnetic local time modulation of Pc5 power is similar in both hemispheres, since it is determined by the equivalent geomagnetic characteristics and regardless of possible geographical differences. In particular, the occurrence of higher Pc5 power observed at Mould Bay than at Dumont D'Urville is not localized in one specific magnetic local time sector. The present observations are compared with previous results regarding solar illumination effects on geomagnetic activity and on auroral brightness. Finally, no solar cycle effect is observed on either the Pc5 power level, on its hemispherical dependence, or on its daily modulation.

An analysis of Pc3 and Pc4 pulsations at Terra Nova Bay (Antarctica)

Abstract. We conducted a statistical analysis of the physical characteristics of the micropulsation activity (Pc3 and Pc4 range) detected, during the austral summer 1994–95, at Terra Nova Bay (Antarctica, corrected latitude 80.0°S), a station which is few degrees poleward of those where most of the Antarctic measurements in these frequency ranges have been performed. The emerging overview suggests that the correspondence between the pulsation power and the external parameters (solar wind speed, interplanetary magnetic field magnitude and orientation) is significantly stronger than at somewhat lower latitudes. The day-to-day power variability was found to be strictly related to the general level of the geomagnetic activity, and the power level sharply maximizes at local magnetic noon. In the Pc4 range peaks of correlation with the SW speed are found in the dawn and dusk sides of the Earth's magnetosphere and the daily variation of the polarization pattern is closely consistent with that found at auroral latitudes and at lower frequencies. In the Pc3 range the correlation coefficient between the pulsation power and the SW speed has maximum values in the local morning, and the frequency of selected events reveals a strong IMF control during closed magnetospheric conditions. The local time dependence of the correlation coefficient between the pulsation power and the cone angle reveals an additional control by the IMF orientation, which becomes more explicit around local noon.Key words: Magnetospheric physics (magnetopause, cusp, and boundary layers)

An analysis of Pc3 and Pc4 pulsations at Terra Nova Bay (Antarctica)

We conducted a statistical analysis of the physical characteristics of the micropulsation activity (Pc3 and Pc4 range) detected, during the austral summer 1994–95, at Terra Nova Bay (Antarctica, corrected latitude 80.0°S), a station which is few degrees poleward of those where most of the Antarctic measurements in these frequency ranges have been performed. The emerging overview suggests that the correspondence between the pulsation power and the external parameters (solar wind speed, interplanetary magnetic field magnitude and orientation) is significantly stronger than at somewhat lower latitudes. The day-to-day power variability was found to be strictly related to the general level of the geomagnetic activity, and the power level sharply maximizes at local magnetic noon. In the Pc4 range peaks of correlation with the SW speed are found in the dawn and dusk sides of the Earth's magnetosphere and the daily variation of the polarization pattern is closely consistent with that found at auroral latitudes and at lower frequencies. In the Pc3 range the correlation coefficient between the pulsation power and the SW speed has maximum values in the local morning, and the frequency of selected events reveals a strong IMF control during closed magnetospheric conditions. The local time dependence of the correlation coefficient between the pulsation power and the cone angle reveals an additional control by the IMF orientation, which becomes more explicit around local noon.Key words: Magnetospheric physics (magnetopause, cusp, and boundary layers)

Pc3 activity at low geomagnetic latitudes: A comparison with solar wind observations

On an hourly time-scale the different roles of the solar wind and interplanetary magnetic field (IMF) parameters on ground micropulsation activity can be better investigated than at longer timescales. A long-term comparison between ground measurements made at L'Aquila ( L ≌ 1.6) and IMP 8 observations confirms the solar wind speed as the key parameter for the onset of pulsations even at low latitudes, although additional control of the energy transfer from the interplanetary medium to the Earth's magnetosphere is clearly exerted by the cone angle. Above ≌-20 mHz the frequency of pulsations is confirmed to be closely related to the IMF magnitude while, in agreement with model predictions, the IMF magnitude is related to the amplitude of the local fundamental resonant mode. We provide an interesting example in which high resolution measurements simultaneously obtained in the foreshock region and on the ground show that external transversal fluctuations do not penetrate deep into the low latitude magnetosphere.

HF Doppler Observations of Small Scale Plasma Motions in the Equatorial Electrojet

Recordings of the variations in phase path, P of reflections, at vertical incidence, from blanketing sporadic-E layers over Kodaikanal (dip 3.5°N) revealed small amplitude, quasi-sinusoidal oscillations of 45-180 s periods in the time rate of change of phase path, Ṗ (Doppler frequency). Geomagnetic micropulsation activity at ground level in the Pc4 range (45-150 s) was not evident at Trivandrum (dip 0.6°S, located ∼ 200 km south of Kodaikanal) on all the 26 occasions (out of the total 29 events studied) when geomagnetic data simultaneous with the phase path observations were available. Besides, the short period quasi-periodic fluctuations in Ṗ were also consistently seen in the data of equatorial sporadic E reflections. We interpret the observed pulsations in Ṗ as representative of those in the vertical electron drift associated with the longitudinal electrostatic drift waves generated in the equatorial electrojet by the gradient drift plasma instability mechanism.

ULF/ELF waves and turbulence in the high‐latitude dayside magnetosphere

The magnetospheric polar cusp has been recognized as a region of strong plasma wave activity. The paper briefly summarizes previous observations of waves in the high‐altitude cusp and the dayside magnetosheath. The main part of the work is concerned with low‐frequency waves found in and near the cusp at low altitudes (500–2000 km) and on the ground. Examining the ac electric field measurements made by polar‐orbiting satellites (mainly the DE‐2), the following groups of waves appear to be characteristic for the dayside high‐latitude magnetosphere: (1) ULF/ELF broadband noise extends from the low hertz range to approximately 1 kHz. The region of peak intensity is generally found to be consistent with the latitudinal extension of the cusp. (2) VLF hiss is found to be present in the cusp region, but generally extends outside the established projection of the cusp. (3) Narrow‐band ELF emissions, covering frequencies 200 Hz ≲f ≲ 1 kHz, are observed on the equatorward side of the cusp, in the closed field line regime. Ground‐based low‐frequency wave measurements at Λ = 75.4° show in the daytime the presence of strong micropulsation activity in the frequency range ∼ 1–100 mHz and polar ELF emissions, a narrow band of relatively unstructured noise at frequencies 300 Hz ≲ f ≲ 1 kHz. The latter is believed to correspond to the satellite‐observed narrow‐band ELF noise. The frequency range 1–7 kHz is, in general, characterized in both the satellite and ground recordings by very low emission activity in the time period ∼3 hours on either side of magnetic noon. This classification of the waves is independent of the level of magnetic disturbance. It is discussed how the characteristic appearance of the ULF/ELF noise and the narrow‐band ELF emissions can be used to localize the position of the polar cusp.

Associated geomagnetic and ionospheric variations

An investigation has been conducted into the association between geomagnetic micropulsation activity and ionospheric disturbances. It has been found that both correlated oscillations and associated irregularities may be observed at various times.

First results of micropulsation activity observed by sabre

Between September 1981 and March 1982 many Pc5 micropulsation events have been observed by the Wick component of the new SABRE auroral radar. A feature of these observations is that in addition to the characteristic poleward drifting regions of radar backscatter, equatorward moving regions are frequently observed. A consistent diurnal pattern in the direction of drift emerges from a statistical analysis of the data.

Latitudinal and Longitudinal Variation of Pc 4, 5 Pulsations and Implications Regarding Source Mechanisms

Early theoretical studies of geomagnetic pulsations dealt with the oscillation of individual magnetic lines of force in the toroidal mode. In recent years it has become increasingly clear that the pulsations are organized in the azimuthal direction so that treatment of an oscillating surface of field lines may need to be considered rather than oscillations of individual field lines. In this paper evidence for the azimuthal dependence of Pc 4, 5 pulsation characteristics will be presented. In addition, the latitudinal characteristics of the ground perturbation pattern and the local time variation of these pulsations will be discussed in the context of some of the source mechanisms proposed for long period micropulsation activity.

Auroral mechanisms and morphology

This past quadrennium has witnessed a considerable expansion of our knowledge and understanding of processes responsible for the aurora. Because of the number and diversity of contributions, a complete description is difficult, and many topics are presented here in summary form only. Instead, this review will emphasize the major accomplishments and outline the major problems that should be addressed during the next quadrennium.The scope of this review includes observations, analysis of observations and theoretical work which are likely to be relevant to the understanding of the aurora. Ground‐based observations reviewed include (l) auroral imagery, both by television and all‐sky camera, (2) gross magnetic variations indicative of ionospheric currents, (3) VLF hiss observations, and (4) effects of high‐energy electron impact measured through cosmic radio noise absorption and by observations of bremsStrahlung x‐rays from balloons. Not included in this review are spectroscopic observations, observation of micropulsation activity and auroral radar observations, as these observations seem to provide less direct diagnostic information on the basic auroral mechanism. However, incoherent scatter radar observations are utilized to the extent they provide information on ionospheric currents and electric fields.

Dependence of Pc5 micropulsation power on conductivity variations in the morning sector

For many years it has been known the that most intense and continuous Pc5 micropulsation activity occurs in the local time quadrant between dawn and noon. Recently, Lam and Rostoker (1978) have shown that Pc5 pulsations occur in the latitudinal regime occupied by the westward auroral electrojet and have suggested that part of the oscillating current system responsible for the pulsations involves upward field-aligned current at the boundary between the sunlit and dark ionosphere at local dawn. In this paper, we show that power in the Pc5 micropulsation range is markedly enhanced as one moves across the dawn terminator at 100 km from the nightside to the dayside. It is further shown that there is a significant increase in pulsation strength at ∼0730 L.T.. The increase in Pc5 pulsation strength across the dawn terminator favors the concept that Pc5 micropulsations can be viewed as oscillations of a three-dimensional current loop involving downward current in the pre-noon sector diverging to flow in the ionosphere as part of the westward auroral electrojet and returning to the magnetosphere along field lines penetrating the ionosphere across the region separating the dark and sunlit ionosphere. We further suggest that the region of enhanced high energy electron precipitation shown by Hartz and Brice (1967) to maximize in the pre-noon quadrant is associated with the marked enhancement of Pc5 activity near 0730 L.T.

The relationship of pc 5 micropulsation activity in the morning sector to the auroral westward electrojet

Data from a line of magnetometers stretching along a corrected geomagnetic meridian ∼ 302°E through western Canada are used to study the relationship between the convection westward electrojet and Pc 5 micropulsations in the morning sector. It was found that the dominant spectral bands in the Pc 5 range occur within the same latitudinal range occupied by the electrojet. The intensity contours and the character of the polarization parameters clearly show that the Pc 5 activity tracks the westward convection electrojet. The Pc 5 activity is found to be enhanced in conjunction with rapid reconfigurations of the electrojet. Evidence of spatial oscillations of the borders of the electrojet and variations in the intensity of the electrojet is presented. It is concluded from our study that the Pc 5 activity in the morning sector is closely related to the convection westward electrojet and its associated three-dimensional current system.

Micropulsations recorded by an array of magnetic variometers

Data recorded at 10-sec intervals by an array of 25 Gough-Reitzel magnetic variometers have been studied for micropulsation activity. The instruments operated simultaneously over an area approximately 500 × 750 km in SE Australia, each instrument recording three components of the geomagnetic variation field. Concurrent recording by the instruments of several cycles of activity of period 7 min has enabled us to map the large-scale spatial variation of the micropulsation components. The resulting patterns are quite smooth; the strongest effect is the spatial variation of the vertical component, which exhibits a ‘coast effect’ similar to that observed at longer periods. Micropulsation data recorded by arrays of such instruments may yield valuable information on crustal conductivity structure.

Occurrence of Pc 4, 5 micropulsation activity at the polar cusp

Data from a line of magnetometers stretching along a corrected geomagnetic meridian ∼302°E through western Canada are used to study the latitude-dependent characteristics of Pc 4 and Pc 5 micropulsation activity. Though most of this activity follows a well-defined frequency-latitude dependence, a broadband class of Pc 4, 5 activity is found to occur at very high latitudes (Λ > 75°) during the daytime hours. It is suggested that this high-latitude magnetic activity is generated in the daytime polar cusp and that the observation of this activity from ground-based stations can be used to determine the latitude of the dayside polar cusp. Three possible mechanisms for the generation of the Pc 4, 5 activity in the polar cusp are the Alfvén-wave instability, the drift-wave instability, and the Kelvin-Helmholtz instability.

The diurnal variation of the polarisation of 25-mHz (pc 3) micropulsations in the horizontal plane

An experimental technique is described which enables the polarisation parameters of large amounts of micropulsation activity to be rapidly determined. Using this method, results are presented for the diurnal variations in the ellipticity, sense of rotation, degree of polarisation and ellipse orientation of 322 hr of activity at a frequency of 25 mHz.

Relations between geomagnetic micropulsations and magnetotail field changes

The magnetic field in the tail region observed by the Imp 1 satellite was compared with geomagnetic-micropulsation activities detected by ground-based observations at Ralston, Alberta, Canada. The tail field strength was found to decrease, typically by one to a few gammas, in simultaneity with the onset of a Pi 2 micropulsation activity detected on the ground. Several tens of such examples were obtained during continuous rapid-run micropulsation observations for about a week. During the same time interval, four Pc 3 micropulsation events took place with clearly identifiable onsets—an unusual feature for Pc 3. The tail field followed a characteristic pattern of evolution in each of the four events. In a period of about one-half hour to one and one-half hours before a Pc 3 onset the tail field strength gradually built up, and with the onset it abruptly decreased by several gammas. Whereas a sudden onset-time decrease in the tail field strength is common to Pi 2 and Pc 3, Pc 3 activity is found to diminish in sympathy with a gradual tail field decrease over a time of about twenty minutes or more, provided that the tail field decrease exceeds about ten gammas.

Broad Band Micropulsation Activity at a Geomagnetic Midlatitude Station

The relationships of broad band geomagnetic micropulsations (periods from 20-300sec.) at Strawberry Hill, Concord, Mass, to some magnetospheric, interplanetary, and solar phenomena are examined to ascertain possible locations of energy sources for their excitation. The micropulsation activity is characterized by a one-hour, quasi-logarithmic index, M, which is chiefly a measure of hydromagnetic (HM) wave amplitudes. The strong diurnal variation in the HM wave energy continues to elude adequate explanation. However, it is apparent that the excitation of these waves is favored in the daylight hemisphere. High micropulsation activity levels generally obtain during periods of sustained magnetic disturbance, but several exceptions preclude the existence of a direct cause-and-effect relationship. Indirect evidence suggests the importance of magnetospheric convection in controlling the micropulsation activity level. Daily micropulsation activity levels are more closely related to interplanetary magnetic field variability than to daily averages of the solar wind velocity. No significant relationship between the sign of θ (the latitude angle of the interplanetary magnetic field vector in solar ecliptic coordinates) and micropulsation activity is indicated by the present study. Superposed epoch analysis reveals an association of highly disturbed micropulsation days with the central meridian passage (CMP), 3 to 5 days earlier, of solar regions characterized by low levels of calcium plage activity. It is suggested that high micropulsation activity levels are more closely associated with the CMP of unipolar magnetic regions (UMR) than are high geomagnetic activity levels.

The Power Spectral Density technique applied to micropulsation analysis

An empirical investigation has been conducted on the practical methods of obtaining the spectra of micropulsation activity, and the interpretational limits which can be applied to the detailed use of spectra computed by the Power Spectral Density (PSD) method. Results indicate that sufficiently high resolution may be obtained reliably from the PSD method for it to be regarded as a useful improvement over handscaling methods. The limitation is shown to remain in the selection of events rather than in computational or instrumental properties.

The 2-0.1 Hz polar cap micropulsation activity

Magnetic field micropulsations, T = 0·2–500 sec, were recorded at Kanak, Greenland, for several years, simultaneously with micropulsation recordings taken at sites in Alaska and Finland. Some characteristics of short-period micropulsation activity at the pole station are described and compared to activity at auroral-zone sites. A strong seasonal variation in periodically-structured Pc 1 reception is found at Kanak and is attributed to a seasonal variation in propagation conditions in the polar cap F-layer horizontal waveguide. Unstructured Pc 1–2 activity was found to be especially prominent within the polar cap.