Poleward Border Research Articles

Role and origin of the poleward Alfvénic arc

AbstractAlfvénic arcs are often found adjacent to the poleward border of the substorm bulge. They are created by precipitating narrow electron beams with a broad energy distribution, mainly below 1 keV, and dominated by Alfvénic structures of small scales and frequencies in the 1 Hz range. They are also associated with transverse ion heating. A balanced field‐aligned current system accompanying the poleward Alfvénic arc reveals its underlying structure being a broad channel of primary energy inflow of several tens of kilometers width. The associated magnetic shear stresses drive a flow along the poleward side of the substorm current wedge. Polar cap convection passes through this channel before entering the substorm bulge. The small‐scale electromagnetic structures are developing from the primary energy inflow by scale breaking and multiple reflections in the ionospheric Alfvén resonator. By their parallel electric field components these structures decouple effectively from the ionosphere. Their energy content is converted into kinetic energy of auroral particles in the topside ionosphere. A selected data set from FAST, DE 2, and Cluster is presented for characterizing the energetic and electromagnetic properties of the Alfvénic arc. Its function in the substorm can be summarized as a preconditioning of the polar cap plasma and magnetic field for entry into the magnetosphere through the poleward arc. This occurs under substantial density depletion.

Ps 6 disturbances: relation to substorms and the auroral oval

Abstract. Ps 6 disturbances and associated omega bands are often considered to be part of the phenomenology of the recovery phase of substorms. We note cases of the initiation of Ps 6 activity at or very near the time of onset, either of a substorm expansive phase, a pseudobreakup, or a poleward border intensification. Thus, we claim that Ps 6 disturbances need not be viewed primarily as phenomena of the recovery phase. This produces both the challenge of explaining Ps 6 within a broader context and the opportunity to use Ps 6 observations to better understand magnetospheric phenomenology, including expansive phase onsets. We further examine the position of the causative currents for Ps 6 and find that they may be located at either the equatorward or poleward border of the auroral oval, or within it. In the first case, the relationship of expansive phase onset and time delay to Ps 6 initiation appears to be very short. In the latter case, there is an association with poleward border intensification, but with a measurable time delay. We present HF radar data to discuss how the electric field at onset time favors the growth of Ps 6 current systems.Key words. Magnetospheric physics (Storms and substorms; Electric fields; MHD waves and instabilities)

Modulation of Jovian middle magnetosphere currents and auroral precipitation by solar wind-induced compressions and expansions of the magnetosphere: initial response and steady state

We present results from a theoretical model which has been used to investigate the modulation of the magnetosphere–ionosphere coupling currents in the Jovian middle magnetosphere by solar wind-induced compressions and expansions of the magnetosphere. We consider an initial system in which the current sheet field lines extend to 50 R J in the equatorial plane, and where the iogenic plasma in the current sheet undergoes steady outward radial diffusion under the influence of the ionospheric torque which tends to maintain corotation with the planet. We show using typical Jovian parameters that the upward-directed field-aligned currents flowing throughout the middle magnetosphere region in this system peak at values requiring the existence of significant field-aligned voltages to drive them, resulting in large precipitating energy fluxes of accelerated electrons and bright ‘main oval’ UV auroras. We then consider the changes in these parameters which take place due to sudden expansions or compressions of the magnetosphere, resulting from changes in the solar wind dynamic pressure. Two cases are considered and compared, these being first the initial response of the system to the change, determined approximately from conservation of angular momentum of the radially displaced plasma and frozen-in field lines, and second the subsequent steady state of steady outward radial diffusion applied to the compressed or expanded system. We show that moderate inward compressions of the outer boundary of the current sheet field lines, e.g. from 50 to 40 R J, are effective in significantly reducing the coupling currents and precipitation in the initial state, the latter then recovering, but only partly so, during the evolution to the steady state. Strong inward compressions, e.g. to 30 R J cause significant super-corotation of the plasma and a reversal in sense of the current system in the initial state, such that bright auroras may then be formed poleward of the usual ‘main auroral oval’ due to the ‘return’ currents. The sense of the currents subsequently reverts back to the usual direction as steady-state conditions are restored, but they are weak, and so is the consequent electron precipitation. For outward expansions of the current sheet, however, the field-aligned currents and electron precipitation are strongly enhanced, particularly at the poleward border mapping to the outer weak field region of the current sheet. In this case there is little evolution of the parameters between the initial expansion and the subsequent steady state. Overall, the results suggest that the Jovian middle magnetosphere coupling currents and resulting ‘main oval’ auroral acceleration and precipitation will be strongly modulated by the solar wind dynamic pressure in the sense of anti-correlation, through the resulting compressions and expansions in the size of the magnetosphere.

Correlation of Alfvén wave Poynting flux in the plasma sheet at 4–7 RE with ionospheric electron energy flux

A comparison of Poynting flux in the plasma sheet at geocentric distances of 4–7 RE to the energy flux of magnetically conjugate precipitating electrons at 100‐km altitude is presented. We have investigated 40 plasma sheet crossings by the Polar satellite, including both cases with large in situ values of Poynting flux (∼1 ergs cm−2 s−1) and cases with low values (≤0.1 ergs cm−2 s−1). The values correspond to ∼125 and ∼12 ergs cm−2 s−1, respectively, when mapped along converging magnetic field lines to 100 km. The north‐south component of the electric field and the east‐west component of the magnetic field were the primary source of the Poynting flux. On the basis of the phase relationship and ratio of E and B, the majority of Poynting flux events were identified as Alfvén waves. The Poynting flux measured at high altitudes by Polar was correlated with the intensity of the conjugate auroral emission in the ultraviolet frequency range, which can be used to estimate energy deposition due to precipitating electron beams. The electron energy flux during times of intense Poynting flux in the plasma sheet exceeded 20 ergs cm−2 s−1. In the absence of strong Poynting flux in the plasma sheet, electron precipitation was small (≤5 ergs cm−2 s−1). The mapped Poynting flux was in almost all events larger by a factor of 1–10 than the ionospheric electron energy flux. These results show that Alfvénic Poynting flux in the midtail region is associated with and capable of powering localized regions of magnetically conjugate auroral emissions. Furthermore, the large Poynting flux events observed at the outer edge of the plasma sheet were conjugate to the poleward border of the active auroral regions, giving further evidence that at least some of the discrete aurora connects to the plasma sheet boundary layer.

Identification of substorm expansive phase onsets

Over the years, many different particle and electromagnetic signatures have been invoked to identify the onset of what is called a “substorm.” In this paper I shall first clarify what is generally meant by the term “substorm onset,” and I shall show that some disturbances identified as substorm onsets should be treated with care. I shall review the substorm in terms of its two recognized characteristic components: directly driven and storage/release. I shall then discuss the importance of pseudobreakups in deciding what constitutes a legitimate substorm onset and, in doing so, will illustrate the subjective nature of present substorm onset identification techniques. I shall stress that substorm‐like disturbances can take place both near the equatorward edge of the midnight sector oval (mapping to the near‐Earth magnetotail inside ∼−12 RE) and near the poleward edge of the evening sector oval (mapping to regions much farther downtail). I shall present case studies that illustrate that some poleward edge disturbances have magnetic and auroral signatures that may be incorrectly interpreted as legitimate expansive phase onsets. On the basis of this study, I suggest that what may appear to be a lengthy series of closely spaced expansive phases may, in fact, be a single expansive phase followed by an arbitrarily long period of poleward border activity.

Evidence for thermospheric gravity waves in the southern polar cap from ground-based vertical velocity and photometric observations

Abstract. Zenith-directed Fabry-Perot Spectrometer (FPS) and 3-Field Photometer (3FP) observations of the λ630 nm emission (~240 km altitude) were obtained at Davis station, Antarctica, during the austral winter of 1999. Eleven nights of suitable data were searched for significant periodicities common to vertical winds from the FPS and photo-metric variations from the 3FP. Three wave-like events were found, each of around one or more hours in duration, with periods around 15 minutes, vertical velocity amplitudes near 60 ms–1 , horizontal phase velocities around 300 ms–1 , and horizontal wavelengths from 240 to 400 km. These characteristics appear consistent with polar cap gravity waves seen by other workers, and we conclude this is a likely interpretation of our data. Assuming a source height near 125 km altitude, we determine the approximate source location by calculating back along the wave trajectory using the gravity wave property relating angle of ascent and frequency. The wave sources appear to be in the vicinity of the poleward border of the auroral oval, at magnetic local times up to 5 hours before local magnetic midnight.Key words. Meteorology and atmospheric dynamics (thermospheric dynamics; waves and tides)

A case study of HF radar spectra and 630.0 nm auroral emission in the pre-midnight sector

Abstract. A comparison of HF radar backscatter observed by the CUTLASS Finland radar, meridian scanning photometer data from Longyearbyen, magnetic field variations from IMAGE stations, and particle precipitation measured by the DMSP F12 spacecraft is presented. The interval under discussion occurred in the pre-midnight local time sector, during a period of weakly northward interplanetary magnetic field. A region of HF backscatter, typically 8 degrees wide, occurred in the field of view of the CUTLASS Finland radar. A well defined gradient in the spectral width parameter was present, with mainly low (< 200 m s - 1 ) spectral widths in the lower latitude part of the scatter and predominantly large (> 200 ms - 1 ) spectral widths in the higher latitude part. The relationship between the spectral width and the red line (630.0 nm) emission measured by the meridian scanning photometer is considered. The poleward border of the red line emission, which has, in the past, been proposed as being representative of the polar cap boundary, was co-located to within 1° of magnetic latitude with the gradient in spectral width for part of the interval. Statistically, large spectral widths occurred poleward of the red line emission, while small spectral widths occurred within or equatorward of the red line emission. Near simultaneous DMSP particle observations in the 20 eV to 20 keV range indicate that the poleward border of the red line emission and the gradient in spectral width occurred at the same latitude as the transition from auroral oval to polar rain particle energies. We conclude that the large spectral widths were not caused by particle precipitation associated with the auroral oval. There were two periods of special interest when the relationship between the red line and the spectral width broke down. The first of these happened during enhanced red line and green line (557.7 nm) emission, with a drop out of the radar scatter and an enhanced, narrow westward electrojet. We conclude that this event was a magnetospheric substorm occurring at much higher than usual latitudes. The second period of special interest happened when equatorward moving bands of large spectral width occurred within the region of scatter. Up to 4 of these bands were present during an interval of 100 minutes. Associated with these narrow bands of large spectral width were narrow channels of enhanced westward ion velocities. We conclude that these equatorward moving bands of large spectral width may be related to reconnection processes in the tail. The observations demonstrate that the tail continues to be active even under low solar wind energy input conditions. Furthermore, we conclude that the gradient in the spectral width may be used as a proxy for the polar cap boundary, but only with extreme caution.Key words. Ionosphere (ionosphere-magnetosphere inter-actions; polar ionosphere) – Magnetospheric physics (storms and substorms)

Magnetospheric source region of discrete auroras inferred from their relationship with isotropy boundaries of energetic particles

Abstract. According to observations, the discrete auroral arcs can sometimes be found, either deep inside the auroral oval or at the poleward border of the wide (so-called double) auroral oval, which map to very different regions of the magnetotail. To find common physical conditions for the auroral-arc generation in these magnetotail regions, we study the spatial relationship between the diffuse and discrete auroras and the isotropic boundaries (IBs) of the precipitating energetic particles which can be used to characterise locally the equatorial magnetic field in the tail. From comparison of ground observation of auroral forms with meridional profiles of particle flux measured simultaneously by the low-altitude NOAA satellites above the ground observation region, we found that (1) discrete auroral arcs are always situated polewards from (or very close to) the IB of >30-keV electrons, whereas (2) the IB of the >30-keV protons is often seen inside the diffuse aurora. These relationships hold true for both quiet and active (substorm) conditions in the premidnight-nightside (18-01-h) MLT sector considered. In some events the auroral arcs occupy a wide latitudinal range. The most equatorial of these arcs was found at the poleward edge of the diffuse auroras (but anyway in the vicinity of the electron IB), the most poleward arcs were simultaneously observed on the closed field lines near the polar-cap boundary. These observations disagree with the notion that the discrete aurora originate exclusively in the near-Earth portion of plasma sheet or exclusively on the PSBL field lines. Result (1) may imply a fundamental feature of auroral-arc formation: they originate in the current-sheet regions having very curved and tailward-stretched magnetic field lines.

A high‐latitude, low‐latitude boundary layer model of the convection current system

Observations suggest that both the high‐ and low‐latitude boundary layers contribute to magnetospheric convection, and that their contributions are linked. In the interpretation pursued here, the high‐latitude boundary layer (HBL) generates the voltage while the low‐latitude boundary layer (LBL) generates the current for the part of the convection electric circuit that closes through the ionosphere. However, the interpretation has a self‐consistency problem: in their magnetospheric settings, the two boundary layer generators are ostensibly independent, but in the ionosphere, Ohm's law ties the voltage to the current, and, thereby, couples them. This paper gives a model that joins the high‐ and low‐latitude boundary layers consistently with the ionospheric Ohm's law. It describes an electric circuit linking both boundary layers, the region 1 Birkeland currents, and the ionospheric Pedersen closure currents. The model works by using the convection electric field that the ionosphere receives from the HBL to determine two boundary conditions to the equations that govern viscous LBL'ionosphere coupling. The result provides the needed self‐consistent coupling between the two boundary layers and fully specifies the solution for the viscous LBL‐ionosphere coupling equations. The solution shows that in providing the current required by the ionospheric Ohm's law, the LBL needs only a tenth of the voltage that spans the HBL. (This has led to undervaluing the LBL's role in convection.) The solution also gives the latitude profiles of the ionospheric electric field, parallel currents, and parallel potential. The parallel currents span the convection reversal and shift both north and south relative to the polar cap boundary as the strength of the transpolar potential changes, offering an empirical test of the model. It predicts that the plasma in the inner part of the LBL moves sunward instead of antisunward and that, as the transpolar potential decreases below about 40 kV, reverse polarity (region 0) currents appear at the poleward border of the region 1 currents. A possible problem with the model is its prediction of a thin boundary layer (∼1000 km), whereas thicknesses inferred from satellite data tend to be greater.

North‐south structures in the midnight sector auroras as viewed by the Viking imager

During auroral substorms, active auroras involving field‐aligned electric fields appear at the poleward border of the auroral oval. Equatorward of these active discrete auroras, a diffuse region of luminosity develops which often features imbedded localized structures which can have distinctively north‐south orientations. In this paper we study the behaviour of such longitudinally confined northsouth oriented auroral arc structures which may turn on and off sporadically during the substorm lifetime. In particular, we show observations which suggest that these structures may be the projection on the ionosphere of the drift paths of energetic electrons as they drift from the magnetotail into the more dipolar inner magnetosphere. Our results suggest that injection of auroral particles into the central plasma sheet occurs in quite localized regions over lengthy periods of time. The injected particles drift earthward in the central plasma sheet where those electrons with pitch angles close to the loss cone are sporadically caused to precipitate into the ionosphere causing the fingers of luminosity. North‐south aligned structures near the equatorward edge of the oval are believed to represent precipitation of a spatially localized population of energetic electrons for which the westward convective drift velocity in the evening sector nearly balances the eastward gradient and curvature drift velocity thus leading to purely earthward convective drift of the energetic electrons.

Implications of the hydrodynamic analogue for the solar‐terrestrial interaction and the mapping of high latitude convection pattern into the magnetotail

In the past, attempts have been made to relate magnetospheric geometry to that expected for the hydrodynamic analogue involving high speed flow past an obstacle. For this analogue, the region behind the obstacle can be divided into two distinct regions ‐ a separation bubble immediately behind the obstacle and a laminar wake further downstream. The purpose of this paper is to examine the implications of such an analogue in terms of convective flow patterns in the magnetotail and how these flow patterns might be expected to map down to ionospheric levels. We contend that magnetic field lines which thread the plasma mantle never reach the region of sunward convection normally associated with the auroral oval. We further claim that, during extended periods of southward interplanetary magnetic field, magnetic flux piles up near the poleward border of the morning sector auroral oval and that return flow occurs across the polar cap only after the interplanetary magnetic field turns northward. This effect should be observed in comparing the cross polar cap potential drop to the potential drop across the auroral oval for periods of northward and southward interplanetary magnetic field. Our model also explains the origin of field‐aligned currents and theta auroras in what would conventionally be called the polar cap.

On the morphology of energetic (≥30keV) electron precipitation at the onset of negative magnetic bays

Multiple balloon recordings of bremsstrahlung X-rays supported by recordings of cosmic noise absorption have been used to study in detail energetic (≥30 keV) electron precipitation events occurring near local midnight at the onset of the expansion phase of magnetospheric substorms. This type of precipitation occurs during the first 5–10 min after bay onset and can usually be distinguished from the subsequent bay-associated precipitation by its characteristic time structure, variation in energy spectrum, and higher intensities. During this same interval, the poleward border of the precipitation region moves rapidly towards higher latitudes with speeds of typically 1–2 km/s, whereas the equatorward border seems to move slowly towards lower latitudes. The northward expansion starts just poleward of the lowest latitude reached during the slow equatorward motion of the preceding growth-phase precipitation. The previous narrow precipitation region may thus expand to as much as 10° of invariant latitude within a few minutes. Within the expanding region there are additional intrinsic temporal variations. As the flux of precipitating electrons tends to be most intense and most energetic near the poleward border, recordings made northward of the latitude where the poleward motion started tend to give the appearance of an impulsive precipitation event. The bay-onset precipitation starts abruptly at the onset of Pi 2 magnetic pulsations. Associated with these pulsations there are modulations of the flux of precipitating electrons. An intensified westward electrojet appears to have its center in the equatorward part of the precipitation region. It is suggested that the poleward expansion is associated with the expansion of the plasma sheet earthward of a newly formed X-type neutral line, and is caused by a sudden enhancement of field-line re-connection across the neutral sheet. The intense, more energetic electron precipitation at the poleward border of the precipitation region then takes place along the outer border of the expanding plasma sheet.

The expansive phase of magnetospheric substorms 2. The response at synchronous altitude of particles of different energy ranges

Several previous studies have shown that there are variations in the energetic particle populations at synchronous orbit during periods of substorm activity; however, in these investigations the precise location of the satellite with respect to the longitudinal regime experiencing expansion phase activity has been unknown. In this paper, data from the Lockheed particle detectors on ATS 5 in synchronous orbit and from the meridian line of magnetometers operated by the University of Alberta are correlated for periods of substorm activity where the position of the satellite with respect to the expansion phase regime is known. It is found that changes in the nature of the energetic particle signatures at ATS 5 are correlated with changes in the auroral electrojet structure during the development of the substorm expansive phase. In particular, it is found that marked increases in the fluxes of the energetic particles showing no dispersion among the energy channels occur only when the satellite is on field lines which map to the poleward border of the substorm-intensified westward electrojet. It is further found that when the satellite is on field lines which penetrate the heart of the substorm westward electrojet, one only observes steady high fluxes of energetic particles, and there are no sharp well-defined changes in fluxes associated with continuing impulsive intensifications of the electrojet at its poleward border. It is concluded that the substorm disturbance typically begins at a given latitude and propagates poleward in impulsive steps and that energetic electron enhancements are observed at ATS 5 when the poleward border of the electrojet intensifies in the latitude range of the ATS field line foot. This fact permits the mapping of field lines in the geographic equatorial plane to the earth's surface at specific instants during the substorm expansion phase.

Pi 2 pulsations and the auroral electrojet

Measurements of the properties of Pi 2 pulsations along a magnetic meridian at high latitudes during a number of substorms have been analyzed for their relationship to the auroral electrojet. It is found that the maximum Pi 2 pulsation amplitudes are closely associated with the instantaneous position of the electrojet. That is, the average pulsation amplitude in the Pi 2 band as well as the amplitudes of pulsations at specific frequencies in the band have maximum amplitudes at latitudes close to the instantaneous electrojet location. Stations equatorward of the electrojet tend to observe a classical Pi 2 waveform concurrent with the onset of the substorm electrojet. Stations near the electrojet observe a broad spectrum of pulsations indicating a multiplicity of sources. Stations poleward of the initial electrojet position see little pulsation activity until the electrojet moves overhead. The appearance of large amplitude Pi 2 pulsations at a station which was poleward of the electrojet at the onset of a substorm appears to be coincident with the arrival of the poleward border of the electrojet.

A comparison of satellite observations of Birkeland currents with ground observations of visible aurora and ionospheric currents

Observations of Birkeland (field aligned) currents inferred from magnetic field perturbations observed at ∼800-km altitude by the Triad satellite are correlated with simultaneous ground-based auroral and magnetic observations. A detailed study of one satellite pass is presented in which the ground-based data are used to locate the position of the substorm-disturbed region with respect to the region of the upper atmosphere through which Triad passed. Two other cases are presented to show the repeatable character of the correlation between the Triad data and the auroral arc configuration. It is found that the poleward discrete arc existing at the time of the pass marks the northernmost boundary of the Birkeland current region and all the visual auroral arcs lie within the latitudinal regime occupied by the Birkeland current flow. The dominant feature of the Birkeland current flow in the evening sector is intense upward flow just equatorward of the poleward arc. A broad region of diffuse inward flowing current flows in the region further equatorward. The eastward electrojet in the evening sector flows in the region bounded by the Birkeland current flow, and the poleward border of the electrojet coincides with the poleward edge of the upward field aligned current flow.

The expansive phase of magnetospheric substorms: 1. Development of the auroral electrojets and auroral arc configuration during a substorm

A detailed description of the development of the auroral and magnetic signatures of a magnetospheric substorm is presented in this paper. The isolated nature of the event makes it ideal to demonstrate the character and uniqueness of the phases of a substorm, and it is found that the various perturbation patterns associated with each substorm phase dominate in different regions of the high-latitude ionosphere. It is shown that the ground-based signatures of the growth of isolated substorms are confined to the polar cap and to the evening sector and that these signatures are the same as those noted for the more obvious expansive phase in the average auroral zone. The expansive phase involves the growth of the auroral electrojet through a series of steplike poleward jumps of the northern border of the current system. The substorm continues to develop through a series of distortions and intensifications of the poleward border of the electrojet, each involving the generation of a bright auroral loop or surge. The major portion of the ionospheric current flow is apparently unaffected by the northern border activity of the auroral arcs; thus it is suggested that the energy feeding the substorm is dissipated by different mechanisms active in different regions of the night side magnetosphere.

Response of the polar electrojets in the evening sector to polar magnetic substorms

Several studies have established the existence of an eastward electrojet in the evening sector during periods of magnetospheric activity. It is shown that this electrojet may exist in such a state that it does not fluctuate in conjunction with the substorm-associated westward electrojet. However, it is also seen that the eastward electrojet may intensify at the time of the onset of a polar magnetic substorm. Studies of the evening sector auroral zone reveal that it is generally dominated by the eastward electrojet and that during substorm activity the westward electrojet in the midnight sector intensifies and expands into the evening sector and occupies a narrow latitudinal regime north of the poleward border of the eastward jet. The exact configuration of the electrojets in the evening sector is confirmed by using model current system calculations. It is shown that the growth of the eastward electrojet in the evening sector may be the signature of the onset of the expansive phase of a substorm in the midnight sector. Thus it is not possible to consider the growth of the eastward electrojet as a unique signature of the growth phase of a substorm.