Polar Cusp Region Research Articles

Simulation of the electron acoustic instability for a finite‐size electron beam system

Satellite observations at midaltitudes (≈ 20,000 km) near the earth's dayside polar cusp boundary layer indicate that the upward electron beams have a narrow latitudinal width up to 0.1°. In the cusp boundary layer where the electron population consists of a finite‐size electron beam in a background of uniform cold and hot electrons, the electron acoustic mode is unstable inside the electron beam but damped outside the electron beam. Simulations of the electron acoustic instability for a finite‐size beam system are carried out with a particle‐in‐cell code to investigate the heating phenomena associated with the instability and the width of the heating region. The simulations show that the finite‐size electron beam radiates electrostatic electron acoustic waves. The decay length of the electron acoustic waves outside the beam in the simulation agrees with the spatial decay length derived from the linear dispersion equation. The ambient cold electrons in a diffusion region surrounding the beam are heated to a higher temperature by absorbing the radiated electron acoustic waves, with the heating occurring mainly in the parallel direction. In the heat diffusion region, the temperature of the cold electrons decreases with the distance from the beam with a temperature gradient length smaller than the decay length of the wave energy. This feature of electron heating is modeled by using a heat diffusion equation derived from the second‐order theory. In solving the heat diffusion equation, the wave energy inside the beam is assumed to be saturated by trapping the beam and the cold electrons. The dominant wave number is determined by assuming that the wave frequency outside the beam is equal to the frequency corresponding to the maximum growth rate inside the beam. These results are discussed with respect to the DE 1 plasma and wave observations in the polar cusp region. Using the typical parameters in the cusp boundary layer, the spatial decay length of the electron acoustic waves is estimated to be in the range of 1–10 km and the heat diffusion region is about 5 km. The results suggest that the electron acoustic waves generated by cusp upward electron beams are probably confined in a small region surrounding the beams.

On a unified interpretation of the polar Slant E Condition (SEC) and other high E field related phenomena

Recent measurements by the incoherent scatter radar and by several other instruments in the polar cusp region at Sondre Stromfjord, Greenland, have shown features similar to previously published occurrences at lower, auroral‐zone latitudes of high electric field related electron heating events ascribed to an E region plasma instability effect. Our results further substantiate previous suggestions on a plasma instability and high electric field intensities in the E region being closely related to the ionogram Slant E Condition (SEC) and its two main signatures: the slant sporadic E (diffuse E region backscatter) and the lacuna (abnormal E region radiowave damping). Our observations suggest E region absorption as a souce of weak riometer deflections, and as a possible basis for the creation of the lacuna‐damping. The slant Es diffuse backscatter, most likely created by instability irregularities, has been found identical to diffuse radar aurora at HF and probably at higher frequencies too. It is suggested that the four phenomena, Slant E Condition, diffuse radar aurora, riometer E region absorption and E region electron heating events, are different, but often simultaneous effects related to a plasma instability and to high electric field and conductivity in the polar E region. It is emphasized that the routinely observed ionogram SEC phenomenon may be useful as a diagnostic tool for the study of the various associated phenomena mentioned above.

Characteristics of the Polar Cusp Region

Characteristics of the Polar Cusp Region

Simultaneous electric field changes in the equatorial electrojet in phase with polar cusp latitude changes during a magnetic storm

The Doppler frequency of the VHF radar signals backscattered from the drifting ionization irregularities in the equatorial electrojet is proportional to the east‐west electric field Ey in the equatorial electrojet. Using the VHF backscatter radar observations at Thumba during the geomagnetic storm of February 16, 1980, it is shown that the decreases and increases of the storm‐time electric fields in the equatorial electrojet are completely in phase with the decreases and increases in the mean latitude of the polar cusp region. This observation has important implications for understanding the magnetosphere‐ionosphere interactions, particularly the effect of the magnetospheric electric fields on the low latitude ionosphere.

Large‐ and small‐scale dynamics of the polar cusp

Photometric observations from two stations on Svalbard, Norway, have been used to map the location and dynamics of polar cusp auroras. Cases showing the behavior of cusp auroras and the local magnetic field related to changes in the interplanetary magnetic field (IMF) and irregularities in the solar wind plasma are presented. Dynamical phenomena with different time scales are studied. Southward and northward expansions of the midday sector of the auroral oval are discussed in relation to southward and northward turnings of the magnetosheath magnetic field and geomagnetic substorm activity. Examples showing no direct relationship to substorm activity are presented. Intensifications and rapid motions of discrete auroral structures in the cusp region are shown to be associated with local Pi‐type magnetic pulsations, each event lasting a few minutes. These small‐scale dynamical phenomena are discussed in relation to the concept of impulsive penetration of plasma irregularities across the dayside magnetopause, from the magnetosheath to the polar cusp region of the magnetosphere.

The relation of the cusp precipitating electron flux to the solar wind and interplanetary magnetic field

The temporal variation of the precipitating electron flux in the 47‐eV to 1‐keV energy range, detected in the low‐altitude cusp, is studied as a function of the solar‐wind parameters and the Bz component of the interplanetary magnetic field (IMF). It is found that the electron flux intensity in the polar cusp region depends nonlinearly on the density of the solar‐wind plasma and that it is higher in the negative IMF Bz period than in the positive IMF Bz period. Both these facts are consistent with the predictions of the existing models of magnetic field‐line merging at the magnetopause; however, the possibility that non‐linear diffusion processes predominate cannot be eliminated.

Magnetospheric flux erosion events are flux transfer events

Recent results have shown that magnetic reconnection at the Earth's dayside magnetopause manifests itself most frequently as an impulsive process. Haerendel et al.1, using data from the HEOS 2 spacecraft, were the first to notice short time scale magnetic field perturbations inside the magnetopause, which they interpreted as flux erosion events resulting from transient reconnection in the polar cusp regions. Using higher time resolution data from the ISEE 1 and 2 satellite pair, Russell and Elphic2 subsequently discovered certain characteristic magnetic field signatures in the magnetosheath, which they termed flux transfer events (FTEs). They also interpreted these events in terms of transient, localized reconnection. It has often been suggested3–7 that Haerendel et al.'s flux erosion events and Russell and Elphic's FTEs have a close physical connection, but proof of this has so far been lacking. Here we show that flux erosion events and flux transfer events are indeed the same physical phenomenon.

Nearly simultaneous observations of the conjugate polar cusp regions

The first simultaneous (within 6 min) observations of the low altitude polar cusp regions in the conjugate hemispheres are reported here based on two events detected by the DMSP-F2 and F4 satellites within the same geomagnetic local time sector. It is found that the electron spectra in the cusp are identical in the opposing hemispheres. In one case the observed latitudinal location and extent of the cusps are the same at the two hemispheres. However, in the other case the location of the equatorward boundary of the cusp regions differs by about 2° with drastically different spatial features. It is also found that in one of the events the plasma sheet electron precipitation regions overlap with the cusp regions at lower latitude in both hemispheres. The poleward boundary of these overlapping regions is located at the same latitude on either hemisphere, suggesting that this is the latitude of the last closed field line and that the cusp electrons are present on both closed and open magnetic field lines.

Case studies of the storm time variation of the polar cusp

The latitudinal variations of the polar cusp region were examined during three intense geomagnetic storms. The variations were compared with the intensity of storm time ring current inferred from the Dst index, with the magnitude of the north‐south component Bz of the interplanetary magnetic field and with substorm activity. The common feature is that the rapid equatorward shift occurred during the increase of the ring current growth and during the southward turning of the interplanetary magnetic field orientation. The equatorwardmost latitude of the cusp was reached before the peak of the ring current intensity, by a few to several hours, coinciding with the occurrence of the largest magnitude of the southward interplanetary magnetic field component. However, details of the polar cusp latitudinal movement differ from storm to storm. During the three storms studied, the poleward recovery commenced at the peak magnitude of the negative IMF Bz component, but the recovery proceeded without a clear relation to variations of the interplanetary Bz component, to the ring current intensity, or to the substorm activity. The lowest cusp latitude observed was at ∼61.7°, and the magnitude of this shift seems to be related to the magnitudes of −Bz. It is further observed that the approximate rates of the cusp macroscopic equatorward and poleward movements are about 3° and 1.5° per hour, respectively.

Intense, variable electric fields at ionospheric altitudes in the high latitude regions as observed by DE‐2

Intense, variable electric fields have been observed by DE‐2 using the double probe technique at ionospheric altitudes in high latitude regions. Measurements are made of both components of the electric field in the orbit plane (90° geographic inclination) at a rate of 16 samples/s (0.5 km spatial resolution). Small scale, spike‐like variations with amplitudes of over 100 mV/m are frequently seen in the polar cusp region and, to a lesser degree, also in the nightside auroral oval. Magnitudes up to 240 mV/m have been observed. These variations often occur over scale lengths of a few km or less. The fine structure in the cusp is consistent with the concept of limited regions of direct access within a larger overall region. It would appear that the intense electric fields which are observed at higher altitudes in the magnetosphere may penetrate to lower altitudes more extensively than indicated by previous measurements.

Latitudinal variation of the polar cusp during a geomagnetic storm

Large amplitude latitudinal variation of the polar cusp position was observed during the intense geomagnetic storm of 15‐16 February 1980. The observation of the polar cusp, identified as the region of intense but extremely soft electron precipitation, was made by two nearly noon‐midnight orbit DMSP satellites over both northern and southern hemispheres. The latitudinal shift of the polar cusp is observed to be related to the intensity variation of the ring current indicated by the hourly Dst values. The polar cusp region moved from its normal location at ∼ 76° gm lat down to ∼ 62° gm lat at the peak of this storm. This movement took about 5 hours and was detected over both hemispheres. A drastic variation in the width of the cusp region was also observed; it is very narrow (∼ 1°) during the equatorial shift and expands to ≳ 5° during the poleward recovery. Variation of the polar cusp latitude with that of the Dst index was also seen during the period before the intense storm.

On the location of the source for the energetic electron layer at the polar magnetopause and its relation to magnetic reconnection

The existence of a layer of energetic (≳20 keV) electrons along the magnetopause, extending from the polar cusp region to the distant magnetotail, has been established by measurements on various spacecraft. We have performed a statistical study of electron flux and spectral measurements above 0.5 MeV observed in different regions of the magnetopause accessible to HEOS 2, with the aim of identifying the source for this magnetopause electron layer. The measurements along the polar magnetotail (i.e., adjacent to the plasma mantle), along the mid‐latitude dayside magnetopause (i.e., adjacent to the dayside plasma boundary layer), and on both sides of the magnetosheath/polar cusp interface (i.e., in the exterior polar cusp and in the entry layer) have been separately analyzed and are statistically compared. We conclude that escape of electrons from the magnetosphere can be excluded as a significant source for the tail magnetopause layer on the basis of the observed intensities, of the spectra, and of the frequency of appearance of electrons outside those magnetopause regions which lie adjacent to the outer trapping zone. The electron intensities in the exterior polar cusp region are found to be correlated with the north/south component of the interplanetary magnetic field (IMF). High intensity levels are observed when the IMF is directed southward (BZ <0), and low levels when the IMF points northward (BZ >0). The electron flux in the exterior cusp region at times of southward IMF and at the near‐earth polar magnetotail at times of northward IMF is well correlated with the number of field lines carried by the solar wind towards the magnetosphere (expressed by the product of solar wind speed and interplanetary field magnitude). We interpret this as strong evidence that acceleration processes associated with magnetic reconnection occur at the magnetopause at those times. Local acceleration near the high‐latitude magnetopause appears therefore to be a prime source for the magnetopause electron layer.

Electron precipitation in the midday auroral oval

Simultaneous observations of auroral displays and electron precipitations by the DMSP 33 satellite provide an excellent and unique opportunity to study precipitation characteristics of the midday auroral oval. Attention is given to two topics: (1) the nature of the ‘gap’ of the midday discrete auroras which is a permanent feature of the dayside auroral oval observed by both Isis 2 and DMSP satellites and (2) the relationship of this gap with the polar cusp region. Based on 2‐month (June, July 1975) observations of the midday auroras over the southern hemisphere, it is found that inside the ‘gap’ of the discrete auroras along the dayside auroral oval, soft electron precipitations with a magnetosheathlike spectrum were invariably detected. The spatial extent of this region was about few degrees in latitude and about 2–3 hours in local time near 1130 magnetic local time meridian. No significant electron precipitation was detected poleward of the instantaneous midday auroral oval. Typical plasma sheet and discrete auroral types of electron precipitations were detected in the other parts of the midday auroral oval. Therefore it is proposed that the ionospheric projection of the polar cusp is a small region of the instantaneous dayside auroral oval near the noon meridian, coinciding with the ‘gap’ of the midday discrete auroras.

A mathematical magnetospheric field model with independent physical parameters

A quantitative magnetospheric magnetic field model has been calculated in three dimensions. The model is based on an analytical solution of the Chapman-Ferraro problem. For this solution, the magnetopause was assumed to be an infinitesimally thin discontinuity with given geometry. The shape of the dayside magnetopause is in agreement with measurements derived from spacecraft boundary crossings. The magnetic field of the magnetopause currents can be derived from scalar potentials. The scalar potentials result from solutions of Laplace's equation with Neumann's boundary conditions. The boundary values and the magnetic flux through the magnetopause are determined by all magnetic sources which are located inside and outside the magnetospheric cavity. They include the Earth's dipole field, the fields of the equatorial ring current and tail current systems, and the homogeneous interplanetary magnetic field. In addition, the flux through the magnetopause depends on two constants of interconnection which provide the possibility of calculating static interconnection between magnetospheric and interplanetary field lines. Realistic numerical values for both constants have been derived empirically from observed displacements of the polar cusps which are due to changes in the orientation of the interplanetary field. The transition from a closed to an open magnetosphere and vice versa can be computed in terms of a change of the magnetic boundary conditions on the magnetopause. The magnetic field configuration of the closed magnetosphere is independent of the amount and orientation of the interplanetary field. In contrast, the configuration of the open magnetosphere confirms the observational finding that field line interconnection occurs primarily in the polar cusp and high latitude tail regions. The tail current system reflects explicitly the effect of dayside magnetospheric compression which is caused by the solar wind. In addition, the position of the plasma sheet relative to the ecliptic plane depends explicitly on the tilt angle of the Earth's dipole. Near the tail axis, the tail field is approximately in a self-consistent equilibrium with the tail currents and the isotropic thermal plasma. The models for the equatorial ring current depend on the D st -parameter. They are self-consistent with respect to measured energy distributions of ring current protons and the axially symmetric part of the magnetospheric field.

The distribution of ion beams and conics below 8000 km

Measurements from electrostatic analyzers aboard the polar‐orbiting S3‐3 satellite have been tabulated to form a synoptic picture of the occurrence of upgoing 90eV to 3.9keV auroral ions. In this survey, a distinction is made between ion distributions having peak fluxes along B (beams) and those exhibiting flux maxima that are not field‐aligned (conics). It is shown that both beams and conics are common auroral phenomena, whose frequencies of occurrence in latitude, local time, and altitude have a marked dependence on magnetic activity. During quiet conditions (Kp ≲3) conical ion distributions are observed with constant frequency in altitude above 1000 km and appear to be associated with the daytime polar cusp region. In contrast, quiet time ion beams have a maximum occurrence frequency in the premidnight sector. Ion beams are observed primarily above 5000 km, with a frequency increasing with altitude up to the satellite apogee at 8000 km. During disturbed times, ion beams are a persistent phenomenon, mainly confined to the dusk sector, while conical distributions are observed uniformly in local time with a frequency that increases steadily in altitude. The results of this study, which are shown to be consistent with previous surveys of upward flowing ions if no distinction is made between conical and field‐aligned distributions, provide information relating to auroral ion acceleration processes.

Large scientific releases

Mass-injection experiments in space plasmas have been conducted for the last twenty years. These injections trace or stain chemical or physical processes, facilitating diagnosis of the natural state of the space plasma; artificially perturb the space plasma away from equilibrium, isolating and controlling selected parameters; simulate natural or artificial states of space plasmas; and utilize the advantages of space as a laboratory to study fundamental plasma physics. We use the Lagopedo ionospheric-depletion experiments to illustrate the special operational aspects of active experiments, including weather, logistics, communications, and real-time diagnostics. The various objectives and techniques of mass-injection experiments are described by example. The CAMEO experiment, a thermite barium release from a satellite over the nightside polar cap, is an excellent example of the use of barium injections to trace upward ion acceleration. The Periquito Dos experiment provided a “snapshot” view of convection electric fields in the dayside polar cusp region. Project Waterhole, an artificial depletion of the topside auroral ionosphere, attempted to modify the equilibrium character of the field-aligned currents and apparently shut off the aurora in a small space-time volume. The Trigger experiment is another example of an active perturbation experiment, wherein the auroral ionospheric transverse conductivities were modified via a cesium injection. The Buaro experiment, a shaped-charged barium injection perpendicular to the local geomagnetic field, resulted in an ion-beam/background-plasma system being displaced from equilibrium, permitting diagnostics of collisionless coupling of the ion beam to the background plasma.

Longitudinal position of polar cap plasma flow entry region

The polar slant E condition (SEC) whose most prominent feature is the Ess ionogram trace is taken as evidence of an ion acoustic instability driven by strong electric fields and thus as evidence of strong plasma flow. The occurrence of SEC in the northern polar cusp region strongly suggests that the throat region of rotational plasma flow into the polar cap does not move in local time when the sign of the IMF By changes.

Auroral kilometric radiation: Time‐averaged source location

The location of the average generation region of auroral kilometric radiation is found by studying average electric field strengths as a function of spacecraft position in narrow frequency bands centered at 178, 100, and 56.2 kHz. A combined 5 years of data from the University of Iowa plasma wave experiments on satellites Hawkeye 1 and Imp 6 provide the basis for determining the average electric field strengths. Hawkeye 1 was in a highly elliptical, polar orbit with an apogee near 21 RE over the northern polar region, and Imp 6 was in a highly elliptical, near‐equatorial orbit with an apogee of 33 RE. Together these satellites provide extensive coverage from 3 to 21 RE in the northern hemisphere and inside of 3 RE in the southern hemisphere. Intense sources of auroral kilometric radiation are found in the northern and southern hemispheres. Their locations are near 65° invariant latitude in their respective hemispheres, between 22 and 24 hours magnetic local time, and near 2.5 RE. The total time‐averaged power generation is found to be about 107 W, assuming a spectral bandwidth of 200 kHz. Propagation effects limit the emission cone of auroral kilometric radiation in a given hemisphere to roughly 4.1 sr at 178 kHz, 2.2 sr at 100 kHz, and 1.5 sr at 56.2 kHz. Evidence that the polar cusp region is illuminated at distances as close as 4 RE suggests the possibility that previously observed polar cusp sources are the result of scattering from field‐aligned density irregularities.

Expected charge states of energetic ions in the magnetosphere

This paper reviews major developments in our understanding of the physics of energetic heavy ions in the Earth's plasma environment during the past four years (1974–1977). Emphasis is placed on processes that influence or are influenced by the ion charge states. This has been a period of growing awareness of the important role heavy ions play in space plasmas. Large fluxes of helium ions and even heavier ions have been observed at the geostationary altitude and in the heart of the radiation belts. Such ions have also been observed on low latitude rockets and satellites, and oxygen ion precipitation exceeding that of protons has been reported. In the outer parts of the Earth's plasma envelope there is mounting evidence for significant fluxes of heavy ions: in the magnetotail, the magnetosheath and in the polar cusp regions. In the inner magnetosphere there is a limited theoretical understanding of equatorially mirroring ions, but generally only radial diffusion at one pitch angle and pitch angle diffusion at one L- shell have been studied; for ions the coupled equations are yet unsolved even for the simplest case of only one charge state (protons). Theoretical modeling of the charge state structures of geophysical heavy ion populations is in part frustrated by the lack of adequate laboratory measurements of the pertinent charge exchange cross sections. A first attempt has, however, been made to treat the charge state transformation processes in the radiation belts for equatorially mirroring atomic oxygen ions. Wave-particle interactions in the magnetosphere become much more complex in multi component and multi charge state plasmas where hybrid resonances and wave-particle interaction induced non-linear species-species coupling could be important. Heavy ion plasma physics in the Earth's magnetosphere and in the magnetospheres of other planets should be a field of fruitful study for both experimentalists and theoreticians in the years ahead.

Plasma waves in the polar cusp: Observations from Hawkeye 1

In this paper we investigate the characteristics of plasma waves observed by the Hawkeye 1 spacecraft in the vicinity of the polar cusp. The primary types of plasma waves associated with the polar cusp are (1) a band of ULF‐ELF magnetic noise extending from a few hertz to several hundred hertz, (2) a broad‐band electrostatic emission extending from a few hertz to about 30–100 kHz, with maximum intensities at about 10–50 Hz, (3) electrostatic electron cyclotron waves near the electron gyrofrequency, and (4) whistler mode auroral hiss emissions. Of these various types of waves, only the ULF‐ELF magnetic noise is uniquely associated with the cusp in the sense that the noise can be used as a reliable indicator of the polar cusp region. All of the other types of plasma waves occur in regions adjacent to the polar cusp as well as in the cusp itself. Spectrum measurements often show that the ULF‐ELF magnetic noise extends up to, but does not exceed, the local electron gyrofrequency fg−. This upper cutoff strongly suggests that the noise consists of whistler mode electromagnetic waves. The mechanism for generating these waves remains highly uncertain, however, since the electron angular distribution in the cusp is usually not sufficiently anisotropic to account for these waves by the well‐known whistler mode cyclotron resonance instability. Other mechanisms, such as turbulence generated by the Kelvin‐Helmholtz instability or by a drift wave instability, have also been suggested to generate this noise. The broad‐band electrostatic noise is believed to be caused by a current‐driven electrostatic instability (ion cyclotron or ion acoustic) of the type widely believed to occur in auroral field‐aligned current systems. The mechanisms for generating electron cyclotron waves and auroral hiss emissions are believed to be reasonably well understood, on the basis of previous studies of these emissions in other regions of the magnetosphere.