Pearl Pulsations Research Articles

Quiet Time Structured Pc1 Waves Generated During Transient Foreshock

AbstractWe study two events of structured Pc1 waves (pearls) observed by CARISMA magnetometers during a period of quiet magnetic (Kp ~1) and solar wind conditions on 1 August 2008. Bursts of proton precipitations and electromagnetic ion‐cyclotron waves were simultaneously observed by NOAA/POES and GOES satellites at dayside. Apparent correspondence was found between ground pearl pulsations and magnetospheric compressions observed by GOES and THEMIS satellites. We have found the Pc1 wave splitting into very close subfrequencies (within 0.1 Hz) that can be explained by weak magnetospheric compressions. The compressions are caused by pressure pulses originated from transient foreshock and interplanetary magnetic field discontinuities observed upstream of the bow shock by the THEMIS‐C probe. Thus, the observed pearls are compression‐related Pc1 waves generated under transient foreshock conditions.

In situ statistical observations of Pc1 pearl pulsations and unstructured EMIC waves by the Van Allen Probes

AbstractWe present here the first in situ statistical survey of structured Pc1 pearl pulsations compared with unstructured electromagnetic ion cyclotron (EMIC) waves observed by the Van Allen Probes spacecraft. This data set was compiled from observations spanning 8 September 2012 through 31 August 2015 and comprises over 1630 h of total EMIC wave activity, of which 291 h exhibited pearl structure. Additionally, we have identified 29 wave events demonstrating periodically oscillating wave packets, mostly about the magnetic equator, indicated by the reversal of Poynting flux along the background magnetic field. We have found several stark differences between Pc1 pearl pulsations and unstructured EMIC waves. While unstructured EMIC waves demonstrate the predicted behavior of a higher occurrence across the dayside with enhanced wave power at dusk, pearl pulsations occur uniformly across magnetic local time, with a small enhancement in the late morning sector. Pearl pulsations were more often observed during magnetospherically quiet periods, particularly in the late recovery period of geomagnetic storms. The mean excitation frequency of pearl pulsations was observed to be independent of the local ion cyclotron frequency, and individual wave investigations indicate that the modulation period also remained constant for the duration of the event over a finite range in L. We examine three possible generation mechanisms—the bouncing wave packet model, modulation by ultralow‐frequency Pc4 and Pc5 waves, and the formation of an ion cyclotron resonator—but are unable to definitively confirm the validity of any one model.

Cluster observations of reflected EMIC‐triggered emission

AbstractOn 19 March 2001, the Cluster fleet recorded an electromagnetic rising tone on the nightside of the plasmasphere. The emission was found to propagate toward the Earth and toward the magnetic equator at a group velocity of about 200 km/s. The Poynting vector is mainly oblique to the background magnetic field and directed toward the Earth. The propagation angle becomes more oblique with increasing magnetic latitude. Inside each rising tone is more field aligned for higher frequencies. Comparing our results to previous ray tracing analysis we conclude that this emission is a triggered electromagnetic ion cyclotron (EMIC) wave generated at the nightside plasmapause. We detect the wave just after its reflection in the plasmasphere. The reflection makes the tone slope shallower. This process can contribute to the formation of pearl pulsations.

In situ observations of Pc1 pearl pulsations by the Van Allen Probes

We present in situ observations of Pc1 pearl pulsations using the Van Allen Probes. These waves are often observed using ground-based magnetometers, but are rarely observed by orbiting satellites. With the Van Allen Probes, we have seen at least 14 different pearl pulsation events during the first year of operations. These new in situ measurements allow us to identify the wave classification based on local magnetic field conditions. Additionally, by using two spacecraft, we are able to observe temporal changes in the region of observation. The waves appear to be generated at an overall central frequency, as often observed on the ground, and change polarization from left- to right-handedness as they propagate into a region where they are resonant with the crossover frequency (where R- and L-mode waves have the same phase velocity). By combining both in situ and ground-based data, we have found that the region satisfying electromagnetic ion cyclotron wave generation conditions is azimuthally large while radially narrow. The observation of a similar modulation period on the ground as in the magnetosphere contradicts the bouncing wave packet mechanism of generation.

Changes in the ultra-low frequency wave field during the precursor phase to the Sichuan earthquake: DEMETER observations

Abstract. Electromagnetic phenomena observed in association with increases in seismic activity have been studied for several decades. These phenomena are generated during the precursory phases of an earthquake as well as during the main event. Their occurrence during the precursory phases may be used in short-term prediction of a large earthquake. In this paper, we examine ultra-low frequency (ULF) electric field data from the DEMETER satellite during the period leading up to the Sichuan earthquake. It is shown that there is an increase in ULF wave activity observed as DEMETER passes in the vicinity of the earthquake epicentre. This increase is most obvious at lower frequencies. Examination of the ULF spectra shows the possible occurrence of geomagnetic pearl pulsations, resulting from the passage of atmospheric gravity waves generated in the vicinity of the earthquake epicentre.

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 v 0 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 ( v 0≈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.

Discovery of the pearl waves by Eyvind Sucksdorff

Eyvind Sucksdorff (1899–1955) was an enthusiastic scientist who was the director of Sodankylä Geophysical Observatory (SGO) from 1927 to 1945. He continued magnetic measurements, which were started in 1913 when SGO was established. Sucksdorff observed events with periodically modulated amplitude in the registration of the new La Cour quick-run magnetometers in 1932–35. He interpreted these events to be due to short-period oscillations and called them “rapid micropulsations” or “pearl necklace” due to the shape of the signal in the registration. From the “pearl necklaces” he estimated the upper bound of the oscillation to be 2–3 s. Sucksdorff did not know the accurate values of the eigenperiods of the systems (H, D and Z components of the magnetometer). Later measurements have shown that they were 2–3 s. Nowadays, the pearl pulsations discovered by Sucksdorff 70 years ago are known as a subgroup of Pc1 magnetic pulsations. Sucksdorff published his observations in 1936. He studied both the annual and diurnal distributions of the new pulsations. Comparisons of the records made in Stockholm, Copenhagen and Sodankylä revealed for the first time the global features of Pc1 pulsations. Sucksdorff did not present any explanation for the pearl pulsations he had observed. Leiv Harang from the Auroral Observatory at Tromsø, Norway, published his analysis of rapid registrations made in Tromsø in 1932–36 in the same issue of the Terrestrial Magnetism and Atmospheric Electricity. He used the name “vibrations” for his short-period oscillations.

Nonstationary pearl pulsations as a signature of magnetospheric disturbances

Abstract. We analyse long-lasting (several hours) Pc1 pearl pulsations with decreasing, increasing or constant central frequencies. We show that nonstationary pearl events (those with either decreasing or increasing central frequency) are observed simultaneously with increasing auroral magnetic activity at the nightside magnetosphere while the stationary events (constant central frequency) correspond to quiet magnetic conditions. Events with decreasing central frequency are observed mostly in the late morning and daytime whereas events with increasing central frequency appear either early in the morning or in the afternoon. We explain the diurnal distribution of the nonstationary pearl pulsations in terms of proton drifts depending on magnetic activity, and evaluate the magnetospheric electric field based on the variation of the central frequency of pearl pulsations.Key words: Magnetospheric physics (magnetospheric configuration and dynamics; plasma waves and instabilities)

Nonstationary pearl pulsations as a signature of magnetospheric disturbances

We analyse long-lasting (several hours) Pc1 pearl pulsations with decreasing, increasing or constant central frequencies. We show that nonstationary pearl events (those with either decreasing or increasing central frequency) are observed simultaneously with increasing auroral magnetic activity at the nightside magnetosphere while the stationary events (constant central frequency) correspond to quiet magnetic conditions. Events with decreasing central frequency are observed mostly in the late morning and daytime whereas events with increasing central frequency appear either early in the morning or in the afternoon. We explain the diurnal distribution of the nonstationary pearl pulsations in terms of proton drifts depending on magnetic activity, and evaluate the magnetospheric electric field based on the variation of the central frequency of pearl pulsations.Key words: Magnetospheric physics (magnetospheric configuration and dynamics; plasma waves and instabilities)

Fluctuations of the repetition period of Pc1 Pearl pulsations

In this paper we investigate fluctuations of the repetition period of geomagnetic Pc1 pearl pulsations. Starting from calculated repetition period we present a general formula for the deviation of the repetition period as a function of wave frequency and stochastic parameters of the medium along the ray trace. We then apply this formula for a dipole magnetic field with a simple plasma distribution, and show that a linear correlation between repetition period and its deviation is predicted. This correlation and the frequency dependence of fluctuations are then compared with experimental values measured from selected Pc1 pearl events observed in Finland.

Recent observations of electromagnetic ion cyclotron waves in space

In the past ten years observations of electromagnetic ion cyclotron (EMIC) waves in space have been reported which have greatly increased our understanding of the occurrence distribution and dynamics of Pc 1 waves in the magnetosphere. The most prominent site of EMIC wave activity is now known to be the dayside outer magnetosphere. Transient compressions of the magnetosphere have been shown to be effective in stimulating proton cyclotron wave activity by enhancing the proton temperature anisotropy. New Poynting flux measurements from Viking and IRM both indicate that wave energy is directed toward the ionosphere, in agreement with earlier ATS-6 results. Observations from lower altitude have revealed properties of lower L pulsations that reflect behavior consistent with Pc 1 pearl pulsations including quasi-periodic packets, dispersion, and occurrence in a narrow L range but wide longitudinal extent. Finally, the response of thermal ions to EMIC waves has also been studied showing that He + responds by developing X-type distributions indicative of non-local heating at locations along the field line where the wave frequency approaches the He + gyrofrequency. Further progress is likely in a number of areas: new spacecraft data sets, especially Freja; ground-satellite correlation studies; detailed instability analysis of proton distributions in the source region; and analysis of wave characteristics to assess the wave vector orientation.

The ion cyclotron group delay for source regions near the plasmapause

Electromagnetic ion cyclotron waves are generated near the magnetic equator through an instability driven by the energetic ion temperature anisotropy. Many of these waves are observed on the ground as Pc 1 magnetic pulsations, which often exhibit a distinct wave packet structure (pearl pulsations). We use the warm dispersion relation to calculate the group delay of ion cyclotron waves from the equatorial source region near the plasmapause to the foot of the field line and compare the results to both cold theory and observations. Our results differ significantly from the cold plasma result for frequencies less than 0.85 fnof;cHe. In general we find that the presence of the ring current produces longer delays and greater wave dispersion. We model three particular ion cyclotron wave events each of which was observed simultaneously by DE 1 and the Air Force Geophysics Laboratory Magnetometer Network. Plasma data from three other instruments on DE 1 provide input parameters to the model. In two out of three cases our predicted delays agree with ground/satellite correlation delays. The third event does not agree as well with theory possibly due to the presence of a significantly greater O+ concentration during this event. The enhanced O+ concentration introduces an additional stop band below fnof;cHe along the propagation path which the linear theory is unable to model.

Viking magnetic and electric field observations of periodic Pc 1 waves: Pearl pulsations

Pearl pulsations, with an average repetition period of 60 s, were recorded using the magnetic and electric field experiments on the polar‐orbiting Viking satellite. The wave event occurred on September 30, 1986, during Viking orbit 1212 at 1030 MLT, from L = 3.6 to L = 4.1, and at an altitude of 13,500 km. Electron density observations obtained from Viking show that the waves were generated at the plasmapause and at lower amplitudes in the plasmasphere. The wave Poynting flux, calculated using the magnetic and electric field, indicated that the waves generally were propagating downward toward the ionosphere, although upward Poynting fluxes were observed. Clear evidence of upward propagating waves, associated with downward propagating waves reflected at the ionosphere, was not observed. Linear convective growth rates suggest that the anisotropic ions which provide the free energy have a perpendicular temperature around 15 keV. The repetition period, calculated using the measured electron density and magnetic field strength at Viking, is consistent with the double‐hop transit time for ion cyclotron waves which propagate along field lines from one hemisphere to the other. However, the absence of upward propagating wave packets implies that the upper limit of the wave ionospheric reflection coefficient is on the order of 10 to 20%. Alternative mechanisms for producing the observed repetition are also investigated and include a periodic generation model of pearl pulsations at the ion bounce period.

Role of the plasmapause and ionosphere in the generation and propagation of pearl pulsations

The source of Pc 1 (pearl) pulsations observed in the course of the local morning hours on 7 December 1977 has been determined by the amplitude and group delay methods. The frequency of pulsations exhibit the typical diurnal variation with the maximum frequency during dawn hours. The source location of pearls during every 1-h interval is compared with the position of the plasmapause inferred from the GEOS I measurements and from previous statistical analysis. It is shown that the source of high-frequency pulsations ( f > 1 Hz) is well inside the plasmapause whereas low-frequency pulsations ( f < 1 Hz) occur near the plasmapause. The source of pulsations is displaced to higher L-values in the course of the local morning hours and this displacement is associated with the decrease of the frequency of pulsations. The source displacement is much more pronounced than the simultaneous movement of the plasmapause position. These observations imply that the model of the Pc1 generation which locates the source only at the plasmapause has serious shortcomings. A model is discussed which takes into account the generation of Pc1 pulsations also well inside the plasmapause and the properties of the waveguide propagation of waves in the ionspheric duct.

A study of pearl pulsations in the equatorial region of India

Pearl pulsations in the equatorial region, despite their relative paucity, are known to exhibit definite features in their diurnal, seasonal and annual occurrence patterns. In the present study, attempts are made to examine and bring out in detail some more aspects of these pulsations recorded at Choutuppal (Geomagnetic latitude: 7°28′N) over a period of nine years during 1967–1975. The mid-period ( t) of pearls is found to bear a linear relation to the pearl repetition period ( T) and is of the form T = 70 t + 11.2. The periods of pearls are seen to be influenced by the average magnetic activity level prevailing during the preceding few days rather than that existing on the day of pearl occurrence. There is an increase in the mid-frequency of pearls that occurred succeeding a magnetic disturbance particularly when the disturbance persisted over some days. The amplitude ratio ( H x / H y ) averaged for each two-hourly interval is seen to attain a value of unity around local midnight and decrease towards the dawn and morning hours. From these results, namely the dependence of mid-frequency of pearls on the average magnetic activity (represented by ( Σ Kp >) during the preceding few days and the occurrence of shorter period pearls even after a few days following a magnetic storm, it is suggested that the plasmapause takes much longer time to re-establish to the quiet time position. In the alternative, it may be visualized that possible presence of localized regions of high plasma density gradients inside the plasmasphere after the cessation of a storm might provide favourable conditions for the generation of shorter period pearls.

On the asymmetric nature of micropulsations—I. the spectrum

This paper is based on the postulate that the natural electromagnetic radiation observed in the micropulsation band is accounted for by the eigenmodes of a resonant cavity in the Earth's outer atmosphere, just as the adjacent ELF part of the spectrum is explained by resonances in the Earth-ionosphere cavity. The inner edge of the plasma sheet (the Alfvén layer) forms an effective resonant cavity which we call the Alfvénsphere. Its complex medium is characterized by two parameters, effective conductivity, and effective Alfvén speed: its quasi-stationary states are specified by two state parameters, effective cavity size, and effective time scale for magnetospheric processes, and in principle, they can be evaluated from the power spectra of observed micropulsations. Because of the complex geometry of the cavity and the fact that the vector hydromagnetic wave equation for an asymmetric electric field is not simply separable in spherical and orthogonal dipole coordinates (and the spatial boundary value problem is virtually insoluble), a model is developed which contains the essential physics and admits of tractable equations. A coupling scheme is defined and discussed which permits one to study the eigenvalue equation under conditions of weak and strong coupling as well as the uncoupled case. Emphasis is placed on the most difficult weakly-coupled case because the results can be readily compared with the uncoupled case. The complex dispersion relation-ship is presented and complex eigenvalues are calculated. It is shown that for any mode ( v, i, m), the fundamental ( i = 1) appears at the highest latitude and the highest harmonic ( i = i max) appears at the lowest latitude. Further it is shown that the fundamental and harmonics are split into multiplet frequency states, clustered at different latitudes, and ordered at a particular latitude by the asymmetric label m. This property is used to explain beating and atitudinal and longitudinal variations in pearl pulsations. It is demonstrated that the east-west magnetic component of the perturbed magnetic field (for any mode) has two spatial resonances (logarithmic and asymmetric) and this feature can be used to derive and interpret the T cos 2 Θ = const law. This in turn suggests a method for ordering the east-west component power spectra for a station at any latitude below 70° N mag. in terms of v, and evaluating the corresponding phenomenological state parameters. The inescapable conclusion appears to be that there is no intrinsic difference between the ‘different’ classes of pulsations; they are simply the excited eigenmodes of the Alfvénsphere for different quasi-stationary states.

Correlation between occurrence of pearl pulsations and interplanetary magnetic field sector boundaries

A significant correlation between the occurrence of pearl pulsations at College, Alaska, and interplanetary magnetic field sector boundaries has been observed. Pearl pulsation data published by the Geophysical Institute, University of Alaska, and sector data published by Wilcox and Ness have been used to show the following: pearl pulsations are more likely to occur on the day a sector boundary passes the earth than in the middle of a sector. Pearl pulsations occur with maximum probability near 1500 local time on all sector days but are restricted almost entirely to local afternoon in the middle of a sector. On the average, fewer pearl pulsations are observed during magnetically disturbed times than quiet times, but in the afternoon pearl occurrence is enhanced by geomagnetic disturbance. Pearls begin and end later in the day as the level of magnetic activity increases.

Auroral-zone pearl pulsations

An analysis has been made of Pc 1 band micropulsations observed in the auroral zone during one solar rotation at Flin Flon, Manitoba, August-September 1964. The Pc 1 band micropulsations were subdivided into pearl pulsations (PP), impulsive activity (IMP), and short irregular pulsations (SIP) this paper is concerned primarily with PP. We have observed: (1) there is no correlation of PP with electron precipitation indicated by bremsstrahlung recorded at balloon altitude; (2) PP type activity occurs at the time of a sudden commencement and can have an hourly repetition of occurrence for 5 or 6 hours thereafter; (3) the Kp and AP indices are too coarse for adequate representation of geomagnetically disturbed times when studying the dependence of PP occurrence on geomagnetic activity; (4) there may be a slight increase of PP period with increase in K; (5) PP have higher amplitudes when K values are higher; (6) Pc 3, 4 band activity may be used as a good predictor for PP occurrence, since, for days rated as disturbed by Pc 3, 4 activity, there is an inverse correlation between PP and Pc 3, 4. For days rated as quiet by Pc 3, 4 activity, PP may occur at any time during the daylight hours; (7) PP are distributed throughout the daylight hours on quiet days, whereas they tend to appear only within an hour or two of 1800 LMT on disturbed days. We can interpret these observations in terms of a hydromagnetic wave packet origin for PP with the occurrence of PP dependent on (1) triggering mechanism; (2) amplification mechanism; (3) guiding along field lines; (4) transmission through the ionosphere.