J. Kangas

Morphology and physics of short-period magnetic pulsations

This review is devoted to the main problems of experimental and theoretical investigations of geoelectromagnetic waves in the frequency range from 0.1 to 5 Hz. These waves constitute the short-period subclass of so-called geomagnetic pulsations. The short-period pulsations are represented by Pc1, Pc2, Pi1, Ipdp types and some subclassifications. The understanding of the pulsation mechanisms provides an insight into the structure and dynamics of the Earths magnetosphere. We focus our attention on Pc1 ‘pearl’ pulsations and on the classical (evening) Ipdp, for which basic physical concepts have been established. Other types and varieties are outlined also, but in less detail. In these cases, the physical mechanism is not always clear (as, for example, in the case of morning Ipdp), and/or the morphology is still to be determined carefully (Pc2 and discrete signals in polar cusps as typical examples).Short-period pulsations are a spontaneous, sporadic phenomenon which undergo a certain evolution in the course of a magnetic storm. We consider the storm-time variation as a natural ‘background’, and we use this background to collect the information about the pulsations in an orderly manner. At the same time, together with the transient storm-time variation of pulsation activity, quasi-periodic variations take place, which are connected with the Earths and Suns rotation, Earths orbital motion and solar cycle activity. The study of these regular variations allows us to have a new approach to the mechanisms of excitation and propagation of short-period geomagnetic pulsations.

On the morphology of auroral-zone X-ray events—I Dynamics of midnight events

Abstract Simultaneous multiple balloon measurements have been performed of X-ray bremsstrahlung from electrons ⩾30keV precipitated into the auroral zone during polar magnetic substorms. Observations in the midnight sector have shown that the precipitation region is normally narrow in the north-south direction, but has probably a large extension in the eastwest direction. It has been found that impulsive electron precipitation events frequently occur in the midnight sector near the onset of a negative bay. Indications of rapid poleward motion have been found for such events. During the growing phase of the negative bay, the precipitation region may move equatorward as often as poleward between L ⋍ 5 and L ⋍ 7 . Towards the end of the substorm the electron precipitation usually moves poleward to L ⋍ 7 or beyond. When magnetograms indicate the existence of an apparently well-defined electrojet, the precipitation region also seems to be well delimited, and its motions well correlated with simultaneous motions of the auroral electrojet. Following the initial impulsive precipitation, the energy spectrum of the precipitated electrons shows a gradual softening at least throughout the expanding phase of the substorm, irrespective of the direction of motion of the precipitation region. There seems to be a close agreement between the development of an X-ray substorm in the midnight sector and Akasofus picture of the dynamics of the auroral substorm.

On the morphology of auroral-zone X-ray events—II. Events during the early morning hours

Abstract Auroral-zone electron precipitation during early morning hours (0200–0600 hr magnetic local time) has been analysed with the aid of X-ray measurements from northern Scandinavia together with recordings of geomagnetic variations and cosmic noise absorption (CNA). The electron precipitation can be divided in two parts: one occurring close to the location of the electrojet, the other, when the electrojet is far away or absent. The main features of these two types of precipitation distinctly resemble those found earlier in the midnight hours and in the late-morning (SVA-events), respectively. Both types of precipitation may occur simultaneously in the early morning hours. The SVA-type precipitation may extend to very early local times, and the midnight-type precipitation towards dawn. Fast pulsations of the X-ray intensity were found in both types. The midnight-type precipitation apparently stems directly from the acceleration process. The SVA-precipitation was observed to be delayed with respect to the break-up phase in the midnight sector and showed characteristic variations of the energy spectrum in a sense as to support the assumption that drifting electrons were the cause of this phenomenon. It is proposed to call the part characteristic for local times around midnight ‘direct precipitation’ and the SVA-like part ‘drift precipitation’.

Quasiperiodic modulation of the Pc1 geomagnetic pulsations: An unsettled problem

Recently, it has been suggested that the standard model for interpretation of the Pc1 (pearl) pulsations, which is based on the concept of wave packet oscillating along the geomagnetic field lines by reflection from the ionosphere in the opposite hemispheres, is to be revised. The necessity to modify the standard model is clearly demonstrated by observations. However, it seems premature to reject this model completely, since the excitation of Pc1 pulsations is impossible without reflection of the waves from the ionosphere or from some turning points in the magnetosphere. We are inclined to believe that such turning points exist in the magnetosphere. Our hypothesis is that the Pc1 quasiperiodic wave packets observed over the Earths surface are the result of radiation from the magnetosphere-borne open resonator in which the oscillating ion cyclotron wave packets are generated owing to instability of the multicomponent magnetospheric plasma. We emphasize that much remains to be learned about the modulation of the Pc1 waves, and it should be realized that a number of unresolved questions refers equally to alternative approaches to this problem.

Non-stationary Alfvén resonator: new results on Pc1 pearls and IPDP events

Abstract We analyse a Pc1 pearl event observed by the Finnish search-coil magnetometer network on 15 December 1984, which subsequently developed into a structured IPDP after a substorm onset. The EISCAT radar was simultaneously monitoring the mid- to high-latitude ionosphere. We have calculated the ionospheric resonator properties during the different phases of the event using EISCAT observations. Contrary to the earlier results, we find that the Pc1/IPDP (Interval of Pulsations of Diminishing Period) frequency observed on the ground corresponds to the maximum of the transmission coefficient rather than that of the reflection coefficient. This casts strong doubts on the bouncing wave packet model of Pc1 pearls. Instead, we present evidence for an alternative model of pearl formation in which long-period ULF waves modulate the Pc1 growth rate. Moreover, we propose a new model for IPDP formation, whereby the ionosphere acts as an active agent in forming the IPDP signal on the ground. The model calculations show that the ionospheric resonator properties can be modified during the event so that the resonator eigenfrequency increases according to the observed frequency increase during the IPDP phase. We suggest that the IPDP signal on the ground is a combined effect of the frequency increase in the magnetospheric wave source and the simultaneous increase of the resonator eigenfrequency. The need for such a complicated matching of the two factors explains the rarity of IPDPs on the ground despite the ubiquitous occurrence of EMIC waves in the magnetosphere and the continuous substorm cycle.

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.

Electron precipitation associated with a sudden commencement of a geomagnetic storm

Abstract Balloon observations of X-rays produced by precipitated electrons were made in the morning sector of the auroral zone at the time of the geomagnetic storm sudden commencement (SSC) of July 27, 1966. The impulsive precipitation event lasted 4 min, both the rise and fall times being nearly 1 min. On a shorter time scale a pronounced variation with a period of 1.8–1.9 sec existed, which occurred together with hm-emissions of the same period range. Besides the rapid fluctuations in the precipitation a 50-sec period was also present. The energy spectrum of the observed X-ray flux was rather steep, characterized by an e-folding energy of E0 = 18–22 keV. The SSC apparently triggered a polar magnetic substorm in the midnight sector of the auroral zone.

On the dependence of the Farley-Buneman turbulence level on ionospheric electric field

The mean electron density fluctuation amplitude of the Farley-Buneman E-region wave turbulence has been determined using coherent radar backscatter intensity and drift velocity measurements made with the STARE system, together with ionospheric electric field and electron density measurements obtained with the EISCAT incoherent scatter radar facility. The observations made with the two radar systems were obtained in nearly a common volume in the E-region. It is shown that the turbulence level increases from 1.5 to 2.7% as the electric field intensity increases from 20 to 35 mV m−1. It stays approximately constant as E > 35 mVm−1. This increase and saturation of the wave turbulence level is discussed in the frame of existing theories.

Oxygen cyclotron harmonic waves in the deep plasmasphere during magnetic storms

A new approach to the generation of storm-associated ULF waves with discrete spectra (ion cyclotron harmonic waves) observed in the equatorial plasmasphere is presented. It is proposed that the appearance of waves with phase velocities smaller than the Alfven velocity is connected with a strong dispersion of magnetosonic waves near the bi-ion frequency occurring in the presence of oxygen ions of ionospheric origin. The waves are generated by an instability involving hot oxygen ions with loss cone or ring like distributions. Such ions are found in the magnetosphere during magnetic storms. A simple analytical model of this instability is elaborated. It is shown that ULF wave observations on board Akebono satellite are in a reasonable agreement with the present theoretical approach.

IPDP source regions and resonant proton energies

Abstract We report the first satellite observations of protons involved in the generation of IPDP (intervals of pulsations of diminishing periods). Fifteen IPDP events observed on the ground during the period December 1971-February 1972 were correlated with particle data obtained on the Explorer 45 satellite. The analyses suggest that the IPDP events were generated by the proton cyclotron instability with westward drifting protons with energies of 1–100 keV; the approximate locations of the instability regions at the onset (low-frequency) of IPDP generation were generally near the plasmapause and were at L values between 4.7 and 5.5 with the majority at values between 5.0 and 5.3 in the afternoon/evening sector between 1700 and 2300 LT.