Sun-aligned Arcs Research Articles

Polar cap arcs: Sun-aligned or cusp-aligned?

Polar cap arcs are often called sun-aligned arcs. Satellite observations reveal that polar cap arcs join together at the cusp and are actually cusp aligned. Strong ionospheric plasma velocity shears, thus field aligned currents, were associated with polar arcs and they were likely caused by Kelvin–Helmholtz waves around the low-latitude magnetopause under a northward IMF Bz. The magnetic field lines around the magnetopause join together in the cusp region so are the field aligned currents and particle precipitation. This explains why polar arcs are cusp aligned.

GPS phase scintillation at high latitudes during geomagnetic storms of 7–17 March 2012 – Part 2: Interhemispheric comparison

Abstract. During the ascending phase of solar cycle 24, a series of interplanetary coronal mass ejections (ICMEs) in the period 7–17 March 2012 caused geomagnetic storms that strongly affected high-latitude ionosphere in the Northern and Southern Hemisphere. GPS phase scintillation was observed at northern and southern high latitudes by arrays of GPS ionospheric scintillation and TEC monitors (GISTMs) and geodetic-quality GPS receivers sampling at 1 Hz. Mapped as a function of magnetic latitude and magnetic local time (MLT), the scintillation was observed in the ionospheric cusp, the tongue of ionization fragmented into patches, sun-aligned arcs in the polar cap, and nightside auroral oval and subauroral latitudes. Complementing a companion paper (Prikryl et al., 2015a) that focuses on the high-latitude ionospheric response to variable solar wind in the North American sector, interhemispheric comparison reveals commonalities as well as differences and asymmetries between the northern and southern high latitudes, as a consequence of the coupling between the solar wind and magnetosphere. The interhemispheric asymmetries are caused by the dawn–dusk component of the interplanetary magnetic field controlling the MLT of the cusp entry of the storm-enhanced density plasma into the polar cap and the orientation relative to the noon–midnight meridian of the tongue of ionization.

Climatology of GPS phase scintillation at northern high latitudes for the period from 2008 to 2013

Abstract. Global positioning system scintillation and total electron content (TEC) data have been collected by ten specialized GPS Ionospheric Scintillation and TEC Monitors (GISTMs) of the Canadian High Arctic Ionospheric Network (CHAIN). The phase scintillation index σΦ is obtained from the phase of the L1 signal sampled at 50 Hz. Maps of phase scintillation occurrence as a function of the altitude-adjusted corrected geomagnetic (AACGM) latitude and magnetic local time (MLT) are computed for the period from 2008 to 2013. Enhanced phase scintillation is collocated with regions that are known as ionospheric signatures of the coupling between the solar wind and magnetosphere. The phase scintillation mainly occurs on the dayside in the cusp where ionospheric irregularities convect at high speed, in the nightside auroral oval where energetic particle precipitation causes field-aligned irregularities with steep electron density gradients and in the polar cap where electron density patches that are formed from a tongue of ionization. Dependences of scintillation occurrence on season, solar and geomagnetic activity, and the interplanetary magnetic field (IMF) orientation are investigated. The auroral phase scintillation shows semiannual variation with equinoctial maxima known to be associated with auroras, while in the cusp and polar cap the scintillation occurrence is highest in the autumn and winter months and lowest in summer. With rising solar and geomagnetic activity from the solar minimum to solar maximum, yearly maps of mean phase scintillation occurrence show gradual increase and expansion of enhanced scintillation regions both poleward and equatorward from the statistical auroral oval. The dependence of scintillation occurrence on the IMF orientation is dominated by increased scintillation in the cusp, expanded auroral oval and at subauroral latitudes for strongly southward IMF. In the polar cap, the IMF BY polarity controls dawn–dusk asymmetries in scintillation occurrence collocated with a tongue of ionization for southward IMF and with sun-aligned arcs for northward IMF. In investigating the shape of scintillation-causing irregularities, the distributions of scintillation occurrence as a function of "off-meridian" and "off-shell" angles that are computed for the receiver–satellite ray at the ionospheric pierce point are found to suggest predominantly field-aligned irregularities in the auroral oval and L-shell-aligned irregularities in the cusp.

Field-aligned currents of postnoon auroral arcs

[1] On 9 January 2008, postnoon (at about 15–16 magnetic local time [MLT]) poleward moving auroral arcs were observed over Svalbard during slightly negative interplanetary magnetic field (IMF) Bz and the IMF By gradually changing from near zero to about −3 nT. The arcs appeared from the western (dayside) horizon and developed toward the east coming to the postnoon sector. The steerable 32-m antenna of the European Incoherent Scatter (EISCAT) radar was directed at an elevation angle of 45° to the north-west at about 45° to geomagnetic west, allowing for measurement of azimuthal (along east-west) plasma flow in a range of latitudes. On the basis of this radar configuration, a new method has been developed and applied for calculating the field-aligned currents (FACs) in the vicinity of the arcs. The arcs were located in the region of the postnoon convection reversal, and the background upward FAC had a density of about 1 μA/m2. The optical arcs were of the order of 30–40 km wide and coincided with peaks of upward FAC. The return downward FAC and the southward electric field were observed poleward of the arcs. Equatorward of the arcs, secondary weaker arcs were detected so that the postnoon arcs appear to have a double structure. These features are surprisingly similar to the electric field and current patterns of morning Sun-aligned arcs. We suggest that the double arc structure can be explained by the nonlinear ionospheric feedback that results in splitting of the elongated magnetospheric plasma structures associated with the arcs.

Oblique ionogram features associated with off-great circle HF propagation at high and sub-auroral latitudes

Oblique ionograms received over northerly paths are often difficult to interpret, with many features not being of the form observed on the more familiar mid-latitude soundings. In particular, the ionograms contain unusual traces at frequencies close to and above the conventional or basic maximum usable frequency, i.e. above the junction frequency of the high and low angle modes. A series of experiments have been undertaken over a period of several years to investigate off-great-circle propagation mechanisms over high latitude and sub-auroral oval paths. In addition to measurements of the signal direction of arrival, oblique ionograms were also recorded over several of the paths. By these means, the characteristic ionogram signatures associated with off-great-circle propagation effects arising due to the presence of convecting patches of enhanced ionisation or sun-aligned arcs within the polar cap ionosphere, or to the presence of steep gradients to the side of the path within the mid-latitude trough region, have been identified.

The changing polar cap—Interball-2 and ground observations

A joint ground-spacecraft study of the polar cap from January to March, 1997 shows that the dawn and dusk boundaries identified on the meridian scanning photometer scans from Eureka, in the central polar cap, agree well with the boundaries seen by the Interball-2 UVAI images for northward interplanetary magnetic field (IMF) B z conditions. These boundaries are displaced duskward when B y is positive and dawnward when B y is negative. There are observable discrete emissions (above airglow) from the polar cap during much of the time. From four periods of extended observations, two contrasting types of polar caps emerge. For northward IMF B z the polar cap is constricted in the dawn–dusk direction. Sun-aligned arcs emerge from the auroral oval (from the dawn flank with B y positive and the dusk flank with B y negative) and drift across the polar cap. In some cases, but not all, they appear following transients in the IMF B y or B z . For southward IMF B z the central polar cap frequently has F-region plasma patches extending across much of the polar cap and drifting antisunward. Two auroral substorms occur, the first one (10 February 1997) showing a poleward auroral expansion along much of the oval and followed by a series of elongated, periodic F-layer patches. The second substorm (02 March 1997) has a poleward expansion confined to the midnight sector and the F-layer patches which subsequently appear are quite different (fainter and less extended), perhaps reflecting differing stability of ion flow patterns across the polar cap. While the extended views of the polar cap from the ground reveal much of the dynamics of polar arcs and patches in response to solar wind IMF changes, there are still unanswered questions as to the triggering mechanisms for both. And the onset and progress of auroral substorms in relation to F-layer patch formation and motion requires further investigation. One polar arc on January 22 recorded by the UVAI and the Eureka photometer permitted a determination of the UVAI sensitivity, indicating 1 CCD count corresponded to 1.5 R of 630 nm emission. This calibration value is specific to this type of aurora (low energy with a I630/I558 ratio of >1). The UVAI images showed that the arc was a theta aurora, indicating that such auroras can be excited by either energetic electrons or soft electrons.

Horizontal electrojet associated with the sun-aligned arcs

Analysis of two strong sun-aligned arcs over the Canadian Eureka Observatory (89° CGM) near the north magnetic pole and accompanying related ground magnetometer measurements has identified an electrojet current of ∼1×10 4 A flowing within the arcs in a sunward direction. The electrojet current was carried by low energy electrons created by impacting precipitation drifting at E× B/B 2 velocity within the arcs, where E is the dawn-to-dusk electric field. One of the arcs moved rapidly in a dawn to dusk direction. The measured arc velocity was 365 m/s. This agrees well with the velocity of 380 m/s inferred from the magnetic field signature of the electrojet current. This study suggests that such an electrojet is present whenever a polar arc is set up. However, a few conditions are required to observe clear ground magnetic signatures of the electrojet: (1) quiet local magnetic conditions; (2) a single sun aligned arc near or moving across zenith; and (3) arc excitation by ≥1 keV electrons.

The external magnetic sources over the polar caps - Feasible modelling unrealistic expectations

No average status can be defined for the sources of the external magnetic field over the polar caps, no index can provide with any such model, the observation of no single quantity or parameter can give the ultimate solution. Rather, every case history has to be considered independently. It is possible, however, to approach the problem from an interdisciplinary viewpoint, and to attempt to make an instant modelling of the electric currents that flow in the ionosphere and magnetosphere above the area of aeromagnetic prospecting. A few relevant previous such examples are discussed (such as Akasofu's inference on magnetospheric substorms derived by means of polar auroras, or the presently unfashionable Svalgaard vortex by means of the observed geomagnetic field and dealing with the pattern of the electric field within the magnetosphere, or the Sun-aligned auroral arcs inside the oval that are monitored by satellite, or, perhaps, the luminosity curve of polar auroras). It appears likely that some substantial achievement will be attained altogether with the progress in the understanding of the general pattern of a few typical recurrent configurations over the polar caps, in terms of a multidisciplinary input from different observations, either by ground-based observatories, or by space platforms.

Global aspects of plasma structures

This topical review provides an overview of the progress achieved under Project 3.1, entitled Global Aspects of Plasma Structures (GAPS) during the lifetime of the Solar Terrestrial Energy Program (STEP) from 1990–97. The mandate of the GAPS project covered middle and high latitude plasma structuring. However, given the requirement of limited length for this overview, only high latitude studies will be covered because of the particularly collaborative nature of the effort, made possible by an international program such as STEP. High latitude plasma structuring studies have progressed from joint experimental campaigns involving many locations and diagnostic techniques, and several focused international workshops that united experimenters and modelers. They have provided the groundwork for studying the macroscale (hundreds of km) and mesoscale (km and smaller) plasma structures at high latitudes under two distinct configurations of the interplanetary magnetic field (IMF). When the IMF is directed southward, we observe macroscale, enhanced density structures known as patches. We have documented much on their origin, modification by the electric field structure in the cusp, airglow signatures in the polar cap, interaction with the neutral medium, mesoscale structuring causing scintillations, convection through the polar cap, and eventual exit into the auroral oval. This has led to several modeling efforts, demonstrating patch formation via temporal changes in the large-scale flow configuration in the cusp. Additionally, we have successfully linked the climatology of the macroscale structure model to the mesoscale structure in the polar regions, an advance that may lead to truly predictive irregularity models for forecasting effects on communication and navigation systems during the upcoming solar maximum. For northward IMF conditions, we have advanced our ability to simulate Sun-aligned arcs using a magnetosphere–ionosphere (M–I) coupled model, driven by realistic magnitudes of electric fields, conductivities and currents. The simulation has been enabled by utilizing an extensive ground-based optical database supported by satellite measurements of their morphological characteristics, including their dawn-dusk motion, dependence on IMF By, and propensity for multiple structuring. We soon expect significant advances resulting from several newly established powerful instruments in the northern and southern polar regions.

Simultaneous observations of polar cap patches and Sun‐aligned arcs during transitions of the IMF

This paper presents the first observations of simultaneous polar cap patches and polar cap arcs in a single common 1000‐km field of view, and identifies a model that explains the interplanetary magnetic field (IMF) dependencies of the observed phenomenology. To study the characteristics of the polar cap optical emissions in the 630.0 nm line during transitions of the IMF Bz, we have scanned images taken at Qaanaaq, Greeland, between 1989 and 1994. We found that on a few occasions, when Bz changed from a south to a north orientation, a particular pattern of polar cap patches and Sun‐aligned arcs coexisted. No similar pattern of coexisting arcs and patches was found during north‐to‐south IMF transitions. The detailed analyses of three of these events are presented here in which patches and polar cap arcs are clearly identified to reside simultaneously within the Qaanaaq imager field of view. The digisonde located also at Qaanaaq is used to confirm that the optical patches correspond to enhancements in the number density and a simultaneous decrease of the hmF2 value. These two factors increase the capability of the imager to differentiate between patches and the background airglow. Data collected by the DMSP F8 satellite during one of the events reaffirm the appearance of polar cap precipitation during the Bz positive period. The J4 sensor on board DMSP F8 detected typical electron fluxes commonly associated with polar cap arcs. The coexistence of patches and arcs is due to a slower response of the patches in exiting the polar cap, and then the relatively sudden appearance of polar cap arcs presumably driven by dayside reconnection between the IMF and open flux drawn initially equatorward toward the cusp. This model, of dayside reconnection switching from equatorward of the cusp for Bz south to poleward of the cusp for Bz north, likewise explains why arcs and patches are seen by the imager to coexist for rapid Bz reversals only from south to north and not from north to south.

Simultaneous satellite and all-sky imager observations of polar cap phenomena

In order to understand the processes of formation and motion of sun-aligned polar cap arcs as well as patches, we have analyzed simultaneous observational data from Akebono (EXOS D) satellite and the all-sky imager at Qaanaaq, Greenland. The results demonstrate the correspondence between the localized electron precipitation signatures observed from Akebono and the sun-aligned arcs seen from the all-sky imager at Qaanaaq, and confirm that sun-aligned arc correspond to divE<0. The average energy of precipitating electrons is in the range from 100eV to 200eV. The magnetosheath/mantle is likely the source region of the precipitating electrons. For patches, most drifted anti-sunward, but some ceased to drift within the field of view of the all-sky imager. Comparison between patch movement and the plasma convection velocity demonstrates a general consistency between them.

Simultaneous Satellite and Ground Based Observations of Polar Cap Phenomena

In order to understand the processes of formation and motion of sun-aligned polar cap arcs as well as patches, we have analyzed simultaneous observations from the Akebono (EXOS-D) satellite and ground based instruments at Qaanaaq. Results demonstrated the correspondence between the localized electron precipitation signatures observed from Akebono and multiple sun-aligned arcs observed using an all sky camera (ASC) at Qaanaaq and have confirmed that sun-aligned arcs correspond to divE < 0. We also confirmed the time delay ofappearance and disappearance of sun-aligned arcs in response to IMF change. As for patches, most of the patches drifted anti-sunward, but some ceased to drift within field of view of the all sky camera. Results of comparison between patch movement and the convection demonstrated a general consistency between them.

Extended period of polar cap auroral display: auroral dynamics and relation to the IMF and the ionospheric convection

Abstract. An unusually extended period (5 h) of polar cap auroral display on 3 August 1986 is examined. Auroras have been investigated using ground-based data as well as measurements from the IMP-8 spacecraft in interplanetary space and simultaneous observations from the polar-orbiting satellites Viking and DE-1 in the northern and southern hemispheres, respectively. It is found that visible Sun-aligned arcs are located inside the transpolar band of the θ-aurora observed from the satellite in ultraviolet wavelengths. The transpolar band can contain several Sun-aligned arcs that move inside the band toward the morning or evening side of the auroral oval independent of the direction of the band movement. Intensifications of polar cap auroras with durations of up to about 30 min are observed. No change has been found in either IMF parameters or substorm activity that can be related to these intensifications. The θ-aurora occurred during a 2-h period when the B z-component of the IMF was negative. A tendency is noted for dawnward (duskward) displacement of the transpolar band when By&gt;0 (By&lt;0) in the southern hemisphere. Simultaneous observations of auroral ovals during interplanetary Bz&lt;0, By&lt;0 and Bx&gt;0 in both hemispheres and convection patterns for Bz&lt;0 and By&lt;0 have been displayed using satellite and ground-based measurements. It was found that the transpolar band of the &lt;theta&gt;-aurora in the sunlit hemisphere was situated in the region of large-scale downward Birkeland currents.

Polar cap aurorae tell the time

Abstract North of ∼75 degrees magnetic latitude, deep within the polar cap, auroral forms oriented in the Sun-Earth direction have long been observed. However, it was not until the 1980s that the improved sensitivity of our optical instruments showed that such auroral forms are present about half the time, when the interplanetary magnetic field is close to zero or pointed northward. These auroral forms are always aligned towards the Sun. Within the central polar cap, when the Sun is well below the horizon, one can virtually tell the time by noting their direction as the Earth rotates under these special auroral forms; in effect, we have a gigantic polar sundial. Sun-aligned arcs throw new light on the nature of the Earth's upper polar atmosphere and its coupling to the solar wind.

Parameterized study of the ionospheric modification associated with sun‐aligned polar cap arcs

Sun‐aligned (SA) arcs are a prevalent feature of the polar cap ionosphere during northward interplanetary magnetic field conditions. These arcs are, like the auroral arc, a complex electrodynamic system coupling the ionosphere and magnetosphere. The electron precipitation and convection electric field associated with this system modify the polar cap ionospheric plasma distribution. In the past decade, a wealth of observational information has indicated the complexity of these ionospheric modifications, but only a limited number of model studies have been carried out to elucidate the changes associated with SA arcs. In this investigation, an extensive parametric study has been conducted to identify and order these ionospheric modifications. Specifically, we are interested in modeling the local evolution and decay of ionospheric modification due to a source consisting of an idealized, though representative, SA arc structure. The Utah State University time‐dependent ionospheric model (TDIM) has been used to define the ionospheric physics and its dependencies on both solar cycle and season. An SA arc model has been developed. This model enables one to systematically vary the key arc parameters; electric field strength, width, electron precipitation energy flux, and electron precipitation characteristic energy. Our findings indicate that the changes associated with the E and F regions are quite different. F region structures have relatively long lifetimes and are significantly affected by transport. This leads to a significant role for cross‐arc convection. The E region is much less affected by transport due to short chemical recombination times and thus, E region modifications are confined to the arc structure itself where precipitation and enhanced electric fields are present. Furthermore, the response of the F region to changes in the arc parameters is not linear, and the largest ionospheric modifications occur for intermediate arc widths and electric field strengths. An important indicator of F region modification is the total electron content (TEC) for flux tubes convecting through SA arc structures. The magnitude and variation of the TEC enhancement with various arc parameters can be an important indicator of real processes which are occurring in the arc structure. In addition to TEC we examine the variation of Te, Ti, Ne, ΣP, and ΣH for various arc parameters.

Influence of horizontal inhomogeneity in the ionosphere on the reflection of Alfvén waves

A study of the reflection of Alfvén waves at a horizontally inhomogeneous ionosphere has been carried out. In this study, the Alfvén speed above the ionosphere is assumed to be uniform and the ionosphere is treated as a height‐integrated conducting slab. Analytical and numerical results indicate that the horizontal nonuniformity of the ionospheric conductivity can lead to a rotation of the reflected wave fields and cause field‐aligned currents that originate in the ionosphere. A strong conductivity nonuniformity in the direction perpendicular to the incident wave field, large Hall to Pedersen conductivity ratios, and low conductivity values lead to a large rotation of the reflected wave field in the range from a few to 40°. The implications of the theoretical results for some ionospheric phenomena are presented, including comments on the Harang discontinuity and Sun‐aligned arcs.

Simultaneous ground-based observations of polar cap arcs and spacecraft measurements of particle precipitation

In order to investigate the particles which produce the polar cap aurora at the Vostok station in Antarctica, charged particle data obtained by the DMSP satellites for some days in a period from April to August 1985 were surveyed. Due to the satellite orbit the local time range in which the data were available was the morning sector. For all the events when sun-aligned arcs were observed on the ground the simultaneous DMSP measurements on almost the same field line showed an increased integral number flux J. > 10 8 (cm 8/s/sr) −1 of the precipitating electrons with energy E e > 200 eV. The electron spectra with double peaks are typical of intense electron precipitation in the polar cap arcs. The most noticeable feature of ion spectra in the polar cap arcs is the prominent minimum in ion flux in the energy range 0.1 < E i < 1 keV in contrast with the oval precipitation ; this feature gives the possibility to separate the polar arcs from the aurora in the oval. In some events the satellite crossed the system of two widely separated arcs ; one of them was a sun-aligned arc whereas the other was circular at constant latitude according to the Vostok data. The analysis of the DMSP electron and ion precipitation data has shown that in these events the latitude-oriented arcs are located in the polar cap and not in the auroral oval.

Intensification and fading of auroral arcs in the dusk‐midnight sector of the polar cap

Observations of the aurora from South Pole station (magnetic latitude = −74.2°) have been used to study the intensification and fading of polar arcs observed near the dusk meridian. Most of the cases examined have the following features in common: (1) a preexisting auroral form intensifies for about 10 min; (2) this activation is followed by a pronounced decrease of luminosity; (3) the auroral fade terminates after 30–60 min with the onset of intense aurora which sweeps rapidly overhead. The brief intensifications involve O (630.0 nm) enhancements of 0.1–0.3 kR and N2+ (427.8 nm) enhancements of 0.3–1.5 kR giving an emission ratio which in each case is consistent with the precipitation of electrons with an average energy of 3–12 keV, assuming a Maxwellian distribution. The auroral brightening is not accompanied by riometer absorption. Ground magnetic perturbations of the order 20–40 nT occur, and in about half the instances, weak broadband impulsive hiss is present. The availability of all‐sky camera, auroral electrojet (AE) index and interplanetary magnetic field (IMF) data for some of the cases enables the following additional characterizations of these events. The preexisting form is a Sun‐ or oval‐aligned arc (or part of a multiple arc system) which disappears following the activation; equatorward drift of the arc (or system) accompanies the luminosity change. There is some evidence to suggest that the arc is poleward of the auroral oval. The brief intensification and/or the onset of fading occurs during the growth phase or near the start of the expansive phase of a substorm; termination of the fade is near the maximum in AE and is probably indicative of the beginning of the recovery phase of the substorm. For all three cases for which IMF data were available the onset of fading occurred 20–30 min after Bz turned southward. Two of these cases were preceded by auroral arc enhancement when Bx was sunward; in the third case there was no prior enhancement when Bx was zero or slightly antisunward. By was negative (dawnward) in each case. Sun‐aligned arcs are a common feature of the polar cap during northward Bz but disappear during the increasingly disturbed conditions that accompany southward Bz. The present results suggest that brief intensifications of southern hemisphere polar cap arcs near dusk may be linked in part to the sunward orientation of the IMF which favors enhanced electron fluxes in the southern lobe of the magnetotail.

Dynamics of the quiet polar cap.

Work in the past has established that a few percent of the time, under northward interplanetary magnetic field and thus magnetically quiet conditions, sun aligned arcs are found in the polar cap with intensities greater than the order of a kilo Rayleigh in the visible. Here we extend this view. We first note that imaging systems with sensitivity down to tens of Rayleighs in the visible find sun aligned arcs in the polar cap far more often, closer to half the time than a few percent. Furthermore, these sun aligned arcs have simple electrodynamics. They mark boundaries between rapid antisunward flow of ionospheric plasma on their dawn side and significantly slower flow, or even sunward flow, on their dusk side. Since the sun aligned arcs are typically the order of 1000km to transpolar in the sun-earth direction, and the order of 100km or less in the dawn-dusk direction, they demarcate lines of strongly anisotropic ionospheric flow shears or convection cells. The very quiet polar cap (strongly northward IMF) is in fact characterized by the presence of sun aligned arcs and multiple highly anisotropic ionospheric flow shears. Sensitive optical images are a valuable diagnostic with which to study polar ionospheric convection under these poorly understood conditions.

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.