The relationship between the strength of the IMF and the frequency of magnetic pulsations on the ground and in the solar wind

It has been established for some time that there is a correlation between the frequency of Pc 3–4 geomagnetic pulsations observed on the ground and the strength of the interplanetary magnetic field (IMF). The recent discovery of an apparently similar relationship between pulsations in the same frequency band in the solar wind and the strength of the IMF led to the suggestion that some magnetospheric Pc 3–4 pulsations have an exogenic source.In this paper we offer a statistical reappraisal of some of the earlier results, and an analysis of newly available ground and solar wind pulsation data sets, which suggest that on the basis of a frequency-field strength relationship alone, the case for an exogenic source is still unproven.We do, however, find support for the frequency-field strength relationship (for ground pulsations), which was the original basis for the Borok B index for prediction of the strength of the IMF. We also confirm that pulsation frequency is, at best, an imprecise predictor and show that any derived relationship is strongly dependent on the data sets used.     

Pi2 pulsations in the magnetosphere

Several substorms were observed at Explorer 45 in November and December 1971, and January and February 1972, while the satellite was in the evening quadrant near L = 5. These same substorms were identified in ground level magnetograms from auroral zone and low latitude stations. The satellite vector magnetic field records and rapid run ground magnetograms were examined for evidence of simultaneous occurrence of Pi2 magnetic pulsations. Pulsations which began abruptly were observed at the satellite during 7 of the 13 substorms studied and the pulsations occurred near the estimated time of substorm onset. These 7 pulsation events were also observed on the ground and 6 were identified in station comments as Pi2. All of the events observed were principally compressional waves, that is, pulsations in field magnitude. There were also transverse components elliptically polarized counter-clockwise looking along the field line. Periods observed ranged from 40 to 200 sec with 80 sec often the dominant period.     

Two simple methods for the determination of the resonance frequencies of magnetic field lines

It has previously been shown that application of the “gradient method” to simultaneous recordings of geomagnetic pulsation fields at two stations on a meridian can determine the resonant frequency of a magnetic field line, and that the distribution of resonant frequencies along the meridian can be calculated from three stations. It is shown here that if the D-component spectrum of the pulsations is taken to be a representation of the driving wave, the same information can be derived from one and two station measurements, respectively, albeit with some slight loss of accuracy. It is also suggested that the empirical method of inferring the intensity of the interplanetary magnetic field from the measurement of the period of ground magnetic pulsations would be more accurate if D-component observations only were used.     

Giant pulsations in the plasmasphere

It has been generally accepted up to now that giant pulsations (Pg) are auroral zone phenomena but here we present observations of a sequence of three Pg events on successive days at three stations well within the plasmasphere. Field line resonance behaviour is exhibited with one of the events clearly resonating at L ? 2.8. From the resonant frequency (10.4 mHz) equatorial mass densities are calculated and from these, and the measured azimuthai polarization at resonance, the inference is drawn that Pgs are oscillations in the fundamental guided poloidal mode. We suggest that the drift wave instability of the compressional Alfvén wave may be the source mechanism for Pgs and speculate how conditions for the instability may have arisen.     

High resolution method of direct measurement of the magnetic field lines'' eigen frequencies

The simultaneous observations of Pc4 geomagnetic pulsations at the two temporary stations, located along the geomagnetic meridian 50 km to the North and South from the observatory Borok (L = 2.8), have been used for the investigation of amplitude gradients of both H- and D-components of these pulsations. It has been discovered that the direction of a meridional component of the gradient H (grad_MH) depends on the frequency ƒ of a spectral component of pulsations. The grad_MD is directed more or less permanently northward independently from the frequency ƒ These results are the consequence of a local amplification of geomagnetic pulsations due to Alfvén waves resonance along the magnetic field lines. It has been demonstrated that the frequencies ƒ_R for which the northward direction of grad_MH is replaced by the southward one (with increasing ƒ) can be interpreted as the eigen frequencies of the field line which intersects the meridian in the middle between two temporary stations, i.e. in Borok.

The possible applications of a gradient method of measurement of the magnetic field lines' eigen frequencies are discussed.     

The statistical magnetic signature of magnetospheric substorms

The characteristic magnetic signatures of magnetospheric substorms both on the ground and in space have been determined from the analysis of ～1800 substorm events. The timing and properties of these events were objectively determined according to explicit mathematical criteria by a computer pattern-recognition program. This program processed daily magnetograms from a mid-latitude network of geomagnetic observatories.Ground data analyzed, using onsets determined in this manner, included the AE indices and individual magnetograms at different local times in the auroral zone and at midlatitudes. Superposed epoch averages of these data confirm the local time magnetic substorm signatures, determined in earlier studies of fewer events, and demonstrate the validity of the computerized onset determination procedure.Superposed epoch averages of the interplanetary magnetic field (IMF) associated with the onsets demonstrates both a distinct southward component prior to the onsets and a dependence of the substorm amplitude on the integrated preceding southward IMF flux. Superposed epoch averages of the tail lobe magnetic field magnitude and vector components demonstrates field magnitude changes and rotations in association with the substorm onsets. These lobe field changes are consistent with the growth-phase model of substorm activity and with variations in the magnetopause flaring angle.     

Effects of solar wind parameters on the development of magnetospheric substorms

Effects of solar wind parameters on the development of substorms during the events of southward interplanetary magnetic field (IMF) lasting more than one hour were studied. Analysis on 175 events with average magnitude of the southward component of IMF larger than l·5γ as observed in July–December 1965 lead to the following results: (1) The total auroral electrojet (AEJ) current associated with the southward IMF event is approximately proportional to the time integral of the magnitude of the southward component. (2) The azimuthal component of IMF also affects the AEJ development. AEJ about twice as intense were observed when IMF was directed duskward than when IMF was directed dawnward. (3) AEJ intensity is strongly affected by the solar wind velocity during the southward IMF events, the intensity being approximately proportional to the square of the velocity. (4) No indication was found that the angle between the Sun-Earth line and the Earth's dipole axis plays any role on the development of substorms if effects of the solar wind parameters as described above are eliminated.     

Correlations between ground observations of Pi 2 geomagnetic pulsations and satellite plasma density observations

Ground observations of Pi 2 geomagnetic pulsations are correlated with satellite measurements of plasma density for three time intervals. The pulsations were recorded using the IGS network of magnetometer stations and the plasma density measurements were made on board GEOS-1 and ISEE-1. Using the technique of complex demodulation, the amplitude, phase and polarisation characteristics of the Pi 2 pulsations are observed along two meridional profiles; one from Eidar, Iceland (L = 6.7) to Cambridge, U.K. (L = 2.5) and the other from Tromso, Norway (tL = 6.2) to Nurmijarvi, Finland (L = 3.3). The observed characteristics of the Pi 2 pulsations are then compared with the plasma density measurements. Close relationships between the plasmapause position and the position of an ellipticity reversal and a variation in H component phase are observed. A small, secondary amplitude maximum is observed on the U.K./Iceland meridian well inside the position of the projection of the equatorial plasmapause. The primary maxima on the two meridians, in general occur close to the estimated position of the equatorward edge of a westward electrojet. Using the plasma density measurements, the periods of surface waves at the plasmapause for two intervals are estimated and found to be in good agreement with the dominant spectral peaks observed at the ground stations near the plasmapause latitude and within the plasmasphere. The polarisation reversal, together with phase characteristics, spectral evidence and the agreement between the theoretical and observed periods leads to the suggestion that on occasions a surface wave is excited on the plasmapause as an intermediate stage in the propagation of Pi 2 pulsations from the auroral zone to lower latitudes.     

Low-frequency upstream wave as a probable source of low-latitude Pc 3–4 magnetic pulsations

Daytime Pc 3–4 pulsation activities observed at globally coordinated low-latitude stations [SGC (L = 1.8,λ = 118.0°W), EWA(1.15,158.1°W), ONW(1.3,141.5°E)] are evidently controlled by the cone angle θ_XB of the IMF observed at ISEE 3. Moreover, the Pc 3–4 frequencies ($\text{?}$) at the low latitudes and high latitude (COL; L = 5.6 and λ = 147.9°W) on the ground and that of compressional waves at geosynchronous orbit (GOES 2; L = 6.67 and λ = 106.7°W) are also correlated with the IMFmagnitude(B_IMF).The correlation of $\text{?}$ of the compressional Pc 3–4 waves at GOES 2 against B_IMF is higher than those of the Pc 3–4 pulsations at the globally coordinated ground stations, i.e., γ = 0.70 at GOES 2, and (0.36,0.60,0.66,0.54) at (COL, SGC, EWA, ONW), respectively. The standard deviation ($\text{σ}{}_{\mathrm{n}}\text{= ± Δ?}\text{mHz}$) of the observed frequencies from the form $\text{?}$ (mHz) = 6.0 × B_IMF (nT) is larger at the ground stations than at GOES 2, i.e., $\text{Δ? = ± 6.6}\text{mHz at}$GOES 2, and ±(13.9, 9.1, 10.7, 12.1) mHz at (COL, SGC, EWA, ONW), respectively. The correlations between the IMF magnitude B_IMF and Pc 3–4 frequencies at the low latitudes are higher than that at the high latitude on the ground, which can be interpreted by a “filtering action” of the magnetosphere for daytime Pc 3–4 magnetic pulsations. The scatter plots of pulsation frequency $\text{?}$ against the IMF magnitude B_IMF for the compressional Pc 3–4 waves at GOES 2 are restricted within the forms $\text{? = 4.5 × B}{}_{\mathrm{<mtext>IMF</mtext>}}\text{and}\text{? = 7.5 × B}{}_{\mathrm{<mtext>IMF</mtext>}}$. The frequency distribution is in excellent agreement with the speculation ($\text{<ω}{}_{\mathrm{sc}}\text{Ω}{}_{\mathrm{i}}\text{= 0.3 ～ 0.5}$) of the spacecraft frame frequency of the magnetosonic right-hand waves excited by the anomalous ion cyclotron resonance with reflected ion beams with V₆ = 650 ～ 1150 km s^?1 in the solar wind frame observed by the ISEE satellite in the Earth's foreshock. These observational results suggest that the magnetosonic right-handed waves excited by the reflected ion beams in the Earth's foreshock are convected through the magnetosheath to the magnetopause, transmitted into the magnetosphere without significant changes in spectra, and then couple with various HM waves in the Pc 3–4 frequency range at various locations in the magnetosphere.     

Longitudinal characteristics of Pc5 magnetic pulsations

Data from an East-West line of magnetometer stations stretching approximately along 67° geomagnetic latitude from western Alberta (290° geomagnetic longitude) to western Quebec (350° geomagnetic longitude) in Canada have been used to study the longitudinal characteristics of Pc5 geomagnetic pulsations. This paper concerns the analysis of 3 days' data of relatively intense pulsational activity which occurred around the middle of October in 1976. The intensity variations of Pc5 activity on longitude and time clearly show that the activity is localized in longitude in the morning sector and confused in the afternoon sector. Pulsational activity in the morning sector for two of the events studied appears to be markedly enhanced across the dawn terminator and midway through the pre-noon quadrant. A study of the longitudinal phase variation indicates that the eastern stations lead in phase before noon and lag in phase after noon. This implies that the signals propagate away from noon toward the dawn-dusk meridian. A systematic reversal in the sense of polarization in the horizontal plane was observed when the line of stations rotated across noon. The polarization characteristics in the vertical planes of the events recorded by stations in eastern Canada between 318° and 350° geomagnetic longitude appear to be stationary with respect to time suggesting that the polarization characteristics of pulsations are influenced by geoelectric structures. The implications of these morphological features will be discussed.     

Pi 3 magnetic pulsations associated with substorms

Using magnetic data from the North American IMS network at high latitudes, Pi 3 pulsations are analysed for a period of $\text{4}\text{1}\text{2}$ continuously-disturbed days. The data were obtained from 13 stations in the Alaska and Fort Churchill meridional chains and in the east-west chain along the auroral zone. In the past, Pi 3 pulsations associated with substorms have been classified into two sub-categories, Pi p and Ps 6. However, we find that Pi 3's which have longer periods than Pi p and which are different from Ps 6 are more commonly observed than these two special types. Power spectra, coherence and phase differences are compared among the stations. Results show that noticeable differences for latitudinal dependence of period and amplitude exist among midnight, morning and late-evening Pi 3 pulsations. Results for Pi 3 occurring near midnight indicate that the periods at which the power spectral density is a maximum are longest, and the amplitude largest, near the center of the westward auroral electrojet. On the other hand, for Pi 3 pulsations occurring in the morning, the periods at which the power spectral density is a maximum are longest, and the amplitude largest, near the poleward edge of the westward electrojet. Furthermore, for Pi 3 pulsations occurring in the late evening, their periods are longer and their amplitudes larger near both the Harang discontinuity and the poleward edge of the westward electrojet than near its center. Correlations between pairs of adjoining stations are better in the polar cap than at auroral latitudes. It is also found from hodograms that the sense of polarization often varies from one station to another for the same event, and that the time duration in which the same rotational sense is maintained is shorter near midnight than in the morning and late evening. It is suggested that the source regions of the morning and late-evening Pi 3's lie on the electrojet boundaries; that is at the Harang discontinuity (in the evening) and at the poleward edge of the westward electrojet (in the morning and evening). The generation of midnight Pi 3 pulsations, centered at a location within the westward auroral electrojet appears to be associated directly with the generation of that electrojet.     

THREE-DIMENSIONAL MHD SIMULATION OF INTERPLANETARY MAGNETIC FIELD CHANGES AT 1 AU CAUSED BY A SIMULATED SOLAR DISTURBANCE AND A TILTED HELIOSPHERIC CURRENT/PLASMA SHEET

A fully three-dimensional (3D), time-dependent, MHD interplanetary model has been used, for the first time, to study the relationship between one form of solar activity and transient variations of the north–south component, Bz, of the interplanetary magnetic field (IMF) at 1 AU during the active period of a representative solar cycle. Four cases of initial steady-state solar wind conditions, with different tilt angles of the heliospheric current sheet/plasma sheet (HCS/HPS) which is known to be inclined at solar maximum, are used to study the relationship between the location of solar activity and transient variations of the north–south IMF Bz component at 1 AU. We simulated the initialization of the disturbance as a density pulse at different locations near the solar surface for each case of initial steady-state condition and observed the simulated IMF evolution of B (= –Bz) at 1 AU. The results show that, for a given density pulse, the orientation of the corresponding transient variation of Bz has a strong relationship to the location of the density pulse and the initial conditions of the IMF. A recipe for prediction of the initial Bz turning direction is also presented in this study.In previous studies that used this recipe with only a flat HCS/HPS that was coincident with the solar equatorial plane, we found a prediction accuracy of 83% from a data set of 73 events during solar maximum. The present study that incorporates more realistic HCS/HPS tilt angles confirms the earlier work.Our study leads us to suggest that significant Bz values, associated with substantial post-shock temporal periods of hours at 1 AU, could be achieved if large energies (say, 10 32–10 33 erg) were released at the Sun in a flare or helmet de-stabilization process.     

Optical and magnetic pulsations

Based on data from the PULSAUR-rocket (1980) and ground observations, a correlation study between optical and magnetic pulsations has been carried out. By use of All-Sky TV along with the measured flux of electrons we have also simulated the ground magnetic field. The simulation is based on a model of pulsating currents caused by conductivity changes in the ionosphere. Our simulated field well represents the observed field. The time delay between the optical and magnetic signal is discussed in relation to our model, and so is the lack of correlation between the high frequency component of the two types of pulsations.     

Influence of interplanetary magnetic field sector boundary passages on Jovian decametric radio bursts

Jovian decametric radio emission (DAM) observations from five stations operated by the Goddard Space Flight Centre (GSFC) and from the University of Colorado, Boulder, are used to explore the connection between DAM activity and the interplanetary magnetic field (IMF). Assuming that the IMF sector structure corotates with the Sun, IMF sector boundary crossing times at the orbit of Jupiter have been determined. It is found that in both the frequency ranges covered (16.7 MHz and 22.2 MHz), Jovian DAM activity increases as these sector boundaries pass Jupiter.     

Hard x-ray Pulsations in the Initial Phase of Flares

When analyzing light curves of hard X-ray bursts recorded by the Hard X-Ray Spectrometer on board the MTI satellite, we have found three events (all associated with major solar flares, two of them in the same active region) which show pulsations in the very initial phase of the burst. Periods of the pulsations range from 25 to 48 s. We compare them with other observations of pulsations of radio waves and in X-rays and conclude that pulsations of this kind have not been observed before. We mention several possible causes and prefer interactions between current-carrying loops as the most likely interpretation of the observed variations.     

Pcl pulsation polarization and electron precipitation at college,Alaska

In this paper we report on a study of the association between the polarization states of Pcl and ULF pulsations (primarily IPDP) and electron precipitation recorded at College, Alaska in 1983. This study indicates that there is no direct association of precipitating electrons with Pcl pulsations exhibiting left-hand polarization at the ground. We examined 33 pulsation events which occurred in the afternoon magnetosphere during the first four months of 1983. Twenty-five of the pulsations were right-hand polarized and eight were left-hand polarized. Particle precipitation was observed during 16 of the right-hand polarized events and four of the left-hand polarized events. These results indicate that the handedness of a pulsation event is not a good predictor of associated electron precipitation. Recent investigations have shown that the propagation of Pcl waves in a multicomponent plasma can account for mixed polarization signatures on source field lines. Our results then indicate that the identification of a source field line based upon polarization information alone may be incorrect.     

Permanent flare activity in the magnetosphere during periods of low magnetic activity in the auroral zone

Auroral, magnetic variation and pulsation data from the dense network in the nearmidnight portion of the auroral zone are used together with the measurements of suprathermal particles and electromagnetic fields by the IMP-8 and ISEE-1 spacecraft within the plasma sheet to study the characteristics of activity during two magnetically quiet periods on 3 March 1976 and 23 March 1979. Contrary to existing beliefs, we found clear signatures of numerous (5–10 events per hour) transient events, characterized by plasma flows, energetic particle bursts and E−B field variations. A close association of these events in the plasma sheet with the local auroral flares (LAFs) in the conjugate sector of the auroral zone is established for many events. We conclude that LAF (local auroral arc activation with associated Pi pulsations but extremely weak magnetic bays) have the same plasma sheet manifestations (apparently, the same physics) as the individual substorm intensifications during strong substorm expansion events, which differ from the studied quiet periods mainly by the strength and number of these intensifications. These transient phenomena seem to play an important role in the energetics of the quiet time magnetotail.     

Pi 2 pulsations and the eastward electrojet: A case study

We report the results of a case study of two Pi 2 pulsations observed near the eastward electrojet by the Scandinavian Magnetometer Array. The power of the two Pi 2 pulsations, calculated using a standard Fast Fourier Transform method, peaks near the centre of the eastward electrojet. For both events there is a strong latitudinal gradient in the power poleward of the equatorward border of the electrojet. The sense of polarisation is predominantly clockwise at the northern stations and anticlockwise at the southern stations although the reversal from clockwise to anticlockwise does not occur at a constant latitude. For the first event the polarisation reversal occurs at higher latitudes in the western half of the array; for the second the polarisation reversal occurs at higher latitudes at the edges of the array. The polarisation reversal does not appear to be related to the location of the eastward electrojet. Equivalent current vectors of the Pi 2 pulsations, obtained by rotating the band pass filtered data through 90°, exhibit clear vortex structures in both events. The vortices change sense of direction at half the period of the Pi 2 pulsation. A simple model for the ionospheric electric field in accord with the field line resonance theory reconstructs the basic features of the observed Pi 2 equivalent current system. We thus conclude that Pi 2 signatures in the region of the eastward electrojet and far away from the auroral break-up region are governed by the field line resonance mechanism.     

Multipoint observations of low latitude ULF Pc3 waves in south-east Australia

Geomagnetic pulsations recorded on the ground are the signatures of the integrated signals from the magnetosphere. Pc3 geomagnetic pulsations are quasi-sinusoidal variations in the earth’s magnetic field in the period range 10–45 seconds. The magnitude of these pulsations ranges from fraction of a nT (nano Tesla) to several nT. These pulsations can be observed in a number of ways. However, the application of ground-based magnetometer arrays has proven to be one of the most successful methods of studying the spatial structure of hydromagnetic waves in the earth’s magnetosphere. The solar wind provides the energy for the earth’s magnetospheric processes. Pc3–5 geomagnetic pulsations can be generated either externally or internally with respect to the magnetosphere. The Pc3 studies undertaken in the past have been confined to middle and high latitudes. The spatial and temporal variations observed in Pc3 occurrence are of vital importance because they provide evidence which can be directly related to wave generation mechanisms both inside and external to the magnetosphere. At low latitudes (L < 3) wave energy predominates in the Pc3 band and the spatial characteristics of these pulsations have received little attention in the past. An array of four low latitude induction coil magnetometers were established in south-east Australia over a longitudinal range of 17 degrees at L = 1.8 to 2.7 for carrying out the study of the effect of the solar wind velocity on these pulsations. Digital dynamic spectra showing Pc3 pulsation activity over a period of about six months have been used to evaluate Pc3 pulsation occurrence. Pc3 occurrence probability at low latitudes has been found to be dominant for the solar wind velocity in the range 400–700 km/s. The results suggest that solar wind controls Pc3 occurrence through a mechanism in which Pc3 wave energy is convected through the magnetosheath and coupled to the standing oscillations of magnetospheric field lines.     

Statistical studies of geomagnetic pulsations with periods between 10 and 70 sec and their relationship to the plasmapause region

Geomagnetic pulsations, in the period range 10–150 sec, have been analysed from five stations; Eskdalemuir (L = 3.1), Lerwick (L = 4.0), St. Anthony (L = 4.9), Sodankyla (L = 5.3) and Tromsø (L = 6.6). The results of 12 observatory years' worth of data are presented in the form of contour maps showing the frequency of occurrence of the pulsations as a function of K_p index and of local time. The maps show that a ground based observatory is more likely to record shorter period oscillations (pc 3) when the geomagnetic field line linking the station with the southern hemisphere passes through the plasmatrough than when the observatory field line links the plasmasphere. The peak occurrence of pc 3 for the observatories considered is at 08:45 hr ± 1 hr LT and is related to the observatory L value and the average night-time K_p index by the equation, L = 8.1 ? 1.2K_p. At Eskdalemuir, the spectrum is broader band than the other stations and tends to divide into two peaks; the pc 3 (20 sec) peak tends to occur when the plasmapause has moved in close to the observatory; while the pc 4 (60 sec) peak occurs when the K_p values have been lower and the plasmapause is further away at higher latitudes.