Polar Cap Arcs Research Articles

Northward IMF and patterns of high‐latitude precipitation and field‐aligned currents: The February 1986 Storm

On February 7, 1986, during a major geomagnetic storm the Bz component of the interplanetary magnetic field (IMF) turned strongly northward for several hours. Data from the Defense Meteorological Satellite Program F6 and F7 satellites and the HILAT satellite were used to study the evolution of the pattern of high‐latitude precipitation and field‐aligned currents in response to this change. Prior to the northward IMF period, the auroral zone was observed down to mid‐latitudes and was very wide in latitude, and strong, large‐scale “region 1/region2” currents were clearly present (ΔB ∼ 1700 nanoteslas). Following the northward turning, the equatorward boundary of the auroral zone on the nightside contracted sharply poleward and polar cap arcs were observed. The strength of the region 1/region 2 currents decreased markedly and became immeasurably small at the time of the maximum contraction of the auroral oval. An NBZ current system was observed to grow and expand in the southern (summer) high latitude region over a period of more than 2 hours. Simultaneously, an irregular pattern of field‐aligned currents was observed in the northern (winter) hemisphere. During the contraction, the latitudinal width of the auroral region mapping to the central plasma sheet (CPS) decreased dramatically while the width of the area mapping to the boundary plasma regions (BPR) in the magnetosphere increased greatly. At the time of the maximum contraction the BPR extended up to a latitude of at least 87.1°. The NBZ currents expanded with and were entirely located within the BPR precipitation. Polar cap arcs were observed in both regions of BPR precipitation and polar cap precipitation and were not correlated with the location of the large‐scale field‐aligned currents. There was no indication of the CPS intruding to high latitudes, and thus no evidence for bifurcation of the magnetotail. The boundary between the CPS and the BPR showed little change. If this implies that the boundary between open and closed field lines contracted slowly or not at all, then a significant portion of the observed BPR precipitation was observed along open field lines. We interpret the pattern of particle precipitation and field‐aligned current and the implied plasma convection in terms of a distorted, two‐cell convection pattern driven by merging of closed field lines with the IMF as proposed by Crooker [1988]. When the IMF turned southward again, the pattern quickly reversed. The BPR contracted; the CPS precipitation regions expanded; the equatorward boundary of the auroral oval moved to lower latitudes; the NBZ currents disappeared in less than 30 min; and the region 1/region2 currents reappeared. Again, the BPR/CPS boundary did not move as rapidly and thus may indicate that the changes are due more to a reconfiguration within the magnetosphere than a change in the portion of the magnetosphere that is open or closed.

Intense keV energy polar rain

Polar rain is usually a weak, diffuse precipitation of electrons with intensities below 0.1 erg/cm² s and temperatures around 80 eV. Occasionally, it can have much higher fluxes and temperatures. We carried out an investigation using an automated search of the DMSP F7 polar cap precipitation over a 1100‐day interval to find all the days with reasonably intense (>106 eV/cm² s sr) fluxes in one or more ≥1‐keV electron channels above 80 MLAT. After we eliminated precipitation associated with polar cap arcs, 17 such days were found. For 11 days, the precipitation was at least 0.4 erg/cm² s, and on 4 days it reached a few ergs/cm² s (for example, n = 0.85/cm³, and kT = 2000 eV). A fairly uniform polar rain over the polar caps at this intensity approximately equals the hemispherical energy flux over the entire auroral oval. Like ordinary polar rain, the keV precipitation occurs in only one hemisphere and is generally accompanied by an often high‐density polar rain at more typical energies. No particular association with storms or poststorm quieting was evident in our sample; instead, the results appear to be consistent with the model of Fairfield and Scudder, in that there is an apparent association between intense keV polar rain and low solar‐wind densities. The keV electrons were not observed within the cusp proper, further supporting the concept of polar rain as a special component of the solar‐wind electron population. A persistent gap between the poleward edge of auroral electron precipitation and the occurrence of polar rain is observed; the gap is largest at dusk, smaller at dawn, still smaller near midnight, and nonexistent on the dayside. We interpret the field lines poleward of the auroraloval precipitation and equatorward of polar rain as recently closed field lines.

Evidence that polar cap arcs occur on open field lines.

The characteristics of polar cap arc occurrence are reviewed to show that the assumption of a closed magnetospheric magnetic field topology at very high latitudes when the IMF B sub z is strongly northward is difficult to reconcile with a wide variety of observational and theoretical considerations. In particular, we consider the implications of observations of particle entry for high and low energy electrons, magnetic flux conservation between the near and far tail, the time sequencing in polar cap arcs events, and the hemispherical differences in polar cap arc observations. These points can be explained either by excluding the need for a major topological magnetic field change from explanations of polar cap arc dynamics, or by assuming a long-tailed magnetosphere for all IMF orientations in which magnetic field lines eventually merge with solar wind field lines in either a smooth or a patchy fashion.

Plasma structuring in the polar cap.

Propagation experiments providing scintillation, total electron content and drift data in the field of view of an all-sky imager near the magnetic polar in Greenland are utilized to investigate the manner in which ionospheric plasma becomes structured within the polar cap. It is found that under IMF Bz southward conditions, large scale ionization patches which are convected through the dayside cusp into the polar cap get continually structured. The structuring occurs through the ExB gradient drift instability process which operates through an interaction between the antisunward plasma convection in the neutral rest frame and large scale plasma density gradients that exist at the edges of the ionization patches. It is shown that with the increase of solar activity the strength of the irregularities integrated through the ionosphere is greatly increased. Under the IMF Bz northward conditions, the plasma structuring occurs around the polar cap arcs in the presence of inhomogeneous electric field or disordered plasma convection. In that case, the irregularity generation is caused by the competing processes of non-linear Kelvin-Helmholtz instability driven by sheared plasma flows and the gradient drift instability process which operates in the presence of dawn-dusk motion of arc structures. The integrated strength of this classmore » of irregularities also exhibits marked increase with increasing solar activity presumably because the ambient plasma density over the polar cap is enhanced.« less

Relation of sun-aligned arcs to polar cap convection and magnetic disturbances

The relationship between the sun-aligned arcs and the polar cap convection is examined on the basis of the auroral data from an all-sky camera at Vostok station and the magnetic variation data from the meridional chain of stations in Antarctica. It is shown that the polar cap arcs associated with the transpolar band at θ-auroras are in a region of the sunward convection whereas the arcs occurring in the evening or morning limbs of θ-auroras lie in a region of the antisunward convection. There is a good agreement between the locations of the polar cap arcs observed simultaneously by a spacecraft imager and by a ground all-sky camera. A rare case is also analysed in the paper when the hook-shaped sun-aligned arcs appear near the border of the night-time oval at the beginning of a magnetic substorm.

Simultaneous relativistic electron and auroral particle access to the polar caps during interplanetary magnetic field Bz northward: A scenario for an open field line source of auroral particles

The large magnetic storm of February 1986 provides a unique opportunity to assess whether existing open‐closed theories of the magnetosphere can consistently account for precipitating particle signatures measured at 840 km under a wide variety of solar wind and solar particle conditions. Here we report the simultaneous observation of relativistic electrons associated with a solar proton event and of auroral ions and electrons associated with a polar cap arc event, made across the central polar cap using detectors on board the Defense Meteorological Satellite Program (DMSP) satellites. On February 7 while a solar proton event was still in progress, polar cap arc activity (interplanetary magnetic field (IMF) Bz large and positive, Kp = 3+) occurred prior to a prolonged period of oval arc activity (IMF Bz large and negative, Kp = 6−). Throughout these periods the following are identified: the equatorward edge of the relativistic electron precipitation; the outer zone electron poleward and equatorward boundaries; the equatorward auroral electron and ion boundaries; and the auroral electron transition boundary. The history of the boundaries is presented as the transition from intense polar cap activity to intense auroral oval activity takes place. The data clearly show that the polar cap region of relativistic electron precipitation does not change significantly as the sign of Bz changes. On the other hand, the boundary layer population, which contains auroral arcs and 1‐ to 10‐keV ion precipitation, expands to fill most of the polar cap for Bz northward and greatly contracts as Bz turns southward. The expansion is not symmetric between poles. The fact that in a broad region of the central polar cap, auroral arcs and relativistic electrons are found on the same field lines presents some difficulty for jointly maintaining theories that propose the formation of polar cap arcs strictly on closed field lines and theories that explain relativistic electron entry by means of direct access along open field lines. We argue that the data support polar cap arc formation on open field lines and present a scenario in which the tail lobe boundary plasma is the source of auroral particles at high latitudes.

Formation of polar cap arcs

It is shown that polar cap arcs of typical electron energy ≤ 1 keV can be produced by the downward acceleration of polar rain electrons by parallel potential drops on open field lines. The formation of these arcs is the result of current response to meso‐scale velocity shear structures in the ionospheric convection that is not matched at the magnetosphere. The theory predicts the formation of a pair of meso‐scale field‐aligned currents (one upward and one downward) for each ionospheric shear structure. The theory does not require a bifurcated magnetic configuration for each polar cap arc; hence, it can account for the frequently encountered multiple polar cap arcs without cutting up the magnetosphere into a large number of open and closed magnetic field regions.

Distribution of auroral arcs during quiet geomagnetic conditions

Auroral arcs observed from the Greenland all‐sky camera network during quiet intervals (AE<50 nT) have been ordered in corrected geomagnetic latitude/time mass plots for different values of AE. The arcs are distributed in a pattern which is shown to coincide with the precipitation belt of auroral electrons determined by the DMSP satellites. This belt is known to be composed of two parts: an equatorial part (average energy of >500 eV) and a poleward, low‐energy part. Previous studies have shown that the arc pattern is composed of two subpatterns, too, the “polar cap arc pattern” and the “oval arc pattern.” It is demonstrated that the “polar cap arc pattern” is situated in the poleward, low‐energy part of the precipitation belt, connected to the low‐latitude boundary layer, whereas the “oval arc pattern” is in the equatorial higher energy belt, connected to the plasma sheet. The dividing line between the two arc patterns is associated with the boundary of trapped ≥40‐keV electrons. The designation “polar cap arc pattern” is shown to be ambiguous, wherefore it is proposed to replace it by the term “high‐latitude arc pattern.”

Convection and field-aligned currents, related to polar cap arcs, during strongly northward IMF (11 January 1983)

Electric and magnetic fields and auroral emissions have been measured by the Intercosmos-Bulgaria-1300 satellite on 10–11 January 1983. The measured distributions of the plasma drift velocity show that viscous convection is diminished in the evening sector under IMF B y < 0 and in the morning sector if IMF B y > 0. A number of sun-aligned polar cap arcs were observed at the beginning of the period of strongly northward IMF and after a few hours a θ-aurora appeared. The intensity of ionized oxygen emission [O +( 2P), 7320 Å] increased significantly reaching up to several kilo-Rayleighs in the polar cap arc. A complicated pattern of convection and field-aligned currents existed in the nightside polar cap which differed from the four-cell model of convection and NBZ field-aligned current system. This pattern was observed during 12 h and could be interpreted as six large scale field-aligned current sheets and three convective vortices inside the polar cap. Sun-aligned polar cap arcs may be located in regions both of sunward and anti-sunward convection. Structures of smaller spatial scale correspond to the boundaries of hot plasma regions related to polar cap arcs. Obviously these structures are due to S-shaped distributions of electric potential. Parallel electric fields in these S-structures provide electron acceleration up to 1 keV at the boundaries of polar cap arcs. The pairs of field-aligned currents correspond to those S-structures: a downward current at the external side of the boundary and an upward current at the internal side of it.

Coherent mesoscale convection patterns during northward interplanetary magnetic field

All‐sky imaging photometer (ASIP) and coincident DE 2 satellite plasma drift and particle data have been combined to study polar ionospheric convection in the presence of subvisual intensity, soft‐particle excited (F region), 6300‐Å, Sun‐aligned polar cap arcs. Coincident DE‐B drift meter data identify these arcs electrodynamically as lines of negative electric field divergence. Based on conductivities, derived from the DE‐B (low‐altitude plasma instrument) measured particle fluxes, and the measured electric field gradients, the divergence of horizontal current across these particle impact excited arcs is in good quantitative agreement with upward Birkeland currents carried by the measured particle fluxes. Although velocity structure can be found without arcs, given the ASIP identified condition of stable weak (hundreds of rayleighs) 6300‐Å Sun‐aligned arcs in the polar cap, electric field negative divergence is consistently found. These arcs (of the order of 100 km in width) are found by ASIPs in the polar cap about half the time under Bz ≥ 0 interplanetary magnetic field conditions. They are regularly seen by ASIP data to extend 1000 to over 2000 km in the sunward direction, and to persist in time often for over an hour. We are thus led to conclude that velocity gradients of this noon‐midnight elongated scale are typical of Bz ≥ 0 conditions. We further conclude that combined polar ASIP images and electrostatic potentials calculated along transpolar satellite tracks offer a valuable diagnostic for polar ionospheric convection studies under Bz ≥ 0 conditions. The ASIP time continuous two dimensionality and the satellite equipotential scaling allow individual “snapshots” of these polar convection boundaries. Application here demonstrates highly anisotropic temporally stable convection with greater order than has previously been suspected.

A general association between discrete auroras and ion precipitation from the tail

Using two‐dimensional observations from the spinning, polar‐orbiting S3‐3 satellite, we have compared the locations of regions that we identify as discrete auroral arcs with regions of isotropic ion precipitation. Regions of discrete auroral arcs are defined to be regions containing particle distributions consistent with field‐aligned potential drops ≳0.5 kV. Polar cap arcs and local times near noon (0900‐1500 MLT) are excluded from this study. Our most important result is that the inferred locations of discrete auroral arcs are almost exclusively confined to the region of isotropic ion precipitation at all local times studied. In the rare exceptions, the ion pitch angle distributions approach isotropy. This association is what is expected if magnetic field lines containing discrete auroral arcs thread the tail current sheet at distances where ion motion violates the guiding center approximations. We have also found that discrete arc regions are generally associated with spatial structure and boundaries in the precipitating ions. Additionally, we occasionally observe upward going, return‐current electrons, and these are also associated with the structured, isotropic ion precipitation.

Simultaneous observations of Sun‐aligned polar cap arcs in both hemispheres by Exos‐C and Viking

On September 25, 1986, the EXOS‐C satellite traversed an intense electron precipitation in the southern polar cap, while the Viking satellite simultaneously obtained image data of the polar cap arc in the northern hemisphere. The energy spectrum of the precipitation, measured by instrumentation aboard EXOS‐C, was very similar to that of adjacent (typical) auroral arcs, and the precipitation in the southern polar cap was observed in the same local time sector in which the arc was found in the northern polar cap. Observations seem to support the view that the polar cap arc occurs on closed field lines and is conjugate in both hemispheres.

Features of the polar cap aurorae in the southern polar region

The data of the all-sky camera and photometer at Vostok station in the Antarctic nearpole region have been used for analysis of the auroral features in the southern polar cap. It is shown that the sun-aligned arcs appear in the polar cap with a delay-time of about an hour relative to the northward turning of the IMF and that they can remain for many hours provided the IMF is invariable. The distribution of these aurorae in the dayside polar cap displays a θ-shaped pattern which is clearly separated from the latitude-oriented arcs in the auroral oval. Disappearance of the sun-aligned arcs after the southward turning of the IMF occurs much faster: in 10–15 min. The observation of the polar cap arcs immersed in a wide band of diffuse luminescence implies that the sun-aligned arcs should be located within the θ-aurora transpolar band. It is concluded that the distribution of the aurorae in the polar cap, auroral oval and in the day-side cusp requires a closed structure for the magnetosphere. The convection pattern corresponding to a closed magnetosphere suggests the formation of closed cavities with a stagnation plasma regime in the distant tail of the magnetosphere.

Ionospheric currents associated with a Sun‐aligned arc connected to the auroral oval

On the evening of 14 November, 1985, the Sondrestrom incoherent scatter radar made measurements of ionization and drifts associated with a sun‐aligned polar cap arc near local midnight. All‐sky photographs at 630.0 nm showed that the arc, which appeared to move westward across the Sondrestrom meridian, was connected to the auroral oval. The radar measured height‐integrated Hall and Pedersen conductances in the arc of 2 to 3 mhos. The plasma drift within the arc was anti‐sunward with velocities greater than 1 km/sec, indicating a westward electric field across the arc of about 60 mV/m. The radar data imply a model current system that includes a northward Hall current within the sun‐aligned arc and a westward electrojet in the auroral oval to the south. The model currents produce ground magnetic perturbations that are consistent with those measured by stations in the Greenland magnetometer chain; in particular the sun‐aligned arc is detectable by a negative deflection in the D component of the magnetometers. Divergence of the currents at the edges of the auroral forms yield field‐aligned currents that are upward on the west edge of the arc and downward on the east edge. An upward current is required at the point where the polar cap arc joins the auroral oval because of the gradient in the conductivity and electric field at that location. This upward current implies the presence of enhanced electron precipitation which can account for the observed brightening of polar cap arcs at the point where they intersect the auroral oval.

Correction to “Plasma energization on auroral field lines as observed by the Viking spacecraft,” and to “polar cap arcs observed by the Viking satellite”

Correction to “Plasma energization on auroral field lines as observed by the Viking spacecraft,” and to “polar cap arcs observed by the Viking satellite”

Polar cap arcs observed by the Viking satellite

The Viking orbit has been well suited for studies of polar cap phenomena in the altitude range 5000 to 13500 km. A variety of particle features have been observed. Signatures of acceleration below the spacecraft altitude by field‐aligned potentials such as narrow beams of upflowing ions and an increased electron loss cone angle are typically observed. Polar arc events are caused by accelerated particle distributions in regions with characteristics similar to those in the plasma sheet boundary layer. The shape of the electron angular distributions indicates that the arcs occur on closed magnetic field lines. Many "polar cap" arcs are better described as the poleward edge of an expanded oval during magnetic quiet conditions.

Oval intensifications associated with polar arcs

The existence of large scale auroral forms in the high latitude region poleward of the auroral oval has generated much interest in recent years because of the implications these forms have for magnetospheric topology. Often these so called polar cap arcs are observed to be connected to auroral regions which are identified with plasma sheet particle populations. These connection points are sometimes associated with intensifications in the auroral distribution. Observations with the Viking UV Imager show that these intensifications are highly dynamic on time scales of minutes, being sometimes suggestive of substorms. The polar arcs may move in response to the dynamics of the intensification or else remain spatially fixed.

Simultaneous measurements of polar cap electron distributions in opposite hemispheres

On January 16, 1983, the NOAA 6 and NOAA 7 satellites traversed polar cap auroral arcs in the northern hemisphere while the DMSP‐F6 satellite traversed similar polar cap arcs in the southern hemisphere. All of these encounters were in the morning local time sector and were made within 5 min in universal time of one another. The interplanetary magnetic field at this time had a strong northward Bz. The satellite observations of auroral electrons indicated that the precipitation exhibited a high degree of similarity, indicating a linkage between the two hemispheres. In particular, the source electron distributions, inferred from the satellite observations of precipitating electrons, were identical to within experimental error. For the comparison made when the satellites were closest to the same magnetic local time‐invariant magnetic latitude locations and nearly simultaneous in universal time, the electron spectrums in the northern and southern hemispheres displayed a peak near 1 keV, suggesting a common source and a similar acceleration process for these electrons.

Hook-shaped arcs in dayside polar cap and their relation to the IMF

A special type of auroral forms has been revealed in the southern polar cap on the basis of Vostok station data. There are hook-shaped arcs consisting of sun-aligned polar cap arcs which convert into latitude-oriented oval arcs. These hook-shaped arcs are seen in the southern polar cap only when B z , component of the IMF is northward and B x > 0. The sun-aligned arcs are grouped in the prenoon sector when B y is positive and they are displaced in the afternoon sector when B y is negative. When B y is near zero the sun-aligned arcs are set almost symmetrically relative to the noon meridian. If the hook-shaped arcs display a convection pattern, the occurrence of twin hooks would seem to be in favour of a throat form of the sunward plasma flows exiting the polar cap.

Model of oval and polar cap arc configurations

A model of oval and polar cap arc configurations has been formulated by combining the theory of antiparallel magnetic merging and theories of the evening discrete arc and of the morningside arc systems. Aside from incorporating the verified and yet‐to‐be‐verified predictions of the individual component theories this unified model leads to definite predictions of not only arc configurations but also characteristics of the plasmas that produce the various types of arcs. The model predicts two types of polar cap arcs characterized by the plasma sources: the magnetosheath arc and the plasma sheet arc. Their occurrence should be governed by the By component of the interplanetary magnetic field, and the same type should not occur simultaneously in both hemispheres.