Cap Boundary Research Articles

Flows in polar cap ionosphere could trigger auroral substorms

Auroral substorms, in which aurorae dramatically brighten and become highly dynamic, occur often and seem to be triggered by electric currents in the ionosphere at high latitude. However, scientists do not know the details of how auroral substorms begin and what drives their expansion and controls their duration. Plasma flows within the ionosphere's polar cap region had not generally been considered to be a driver of auroral substorms, but some recent studies have suggested that mesoscale plasma flows from the polar cap could cross the polar cap boundary and contribute to triggering of flows that may bring new plasma earthward and lead to the onset of substorms.

Possible connection of polar cap flows to pre- and post-substorm onset PBIs and streamers

[1] Recent analysis of a short period of observations has led to the hypothesis that enhanced meso-scale flows from well within the region of open polar cap field lines may cross the nightside polar cap boundary into the closed field line region and contribute to the triggering of equatorward (earthward) meso-scale flows across the ionospheric (equatorial) portion of plasma sheet fields lines and lead to PBIs and streamers. This includes the streamers that have been postulated to bring new plasma equatorward (earthward) and lead to substorm onset. Meso-scale structure of flow within the polar cap, often studied near the dayside polar cap boundary, has not previously been generally recognized as significant within the nightside polar cap. Here we have taken advantage of new capabilities to measure polar cap convection by the Resolute Bay incoherent scatter radar and the Rankin Inlet PolarDARN radar, coordinated with THEMIS all-sky imager observations, to study flow measurements from well within the polar cap to near the polar cap boundary. We present evidence that flow structures moving from the polar cap toward the nightside polar cap boundary may be important for triggering the flows that lead to substorm onset streamers. The new observations also have given evidence that the flow structures come from deep within the polar cap, and have given unexpected evidence that a continuation of flow structures moving from the polar cap toward the nightside polar cap boundary after substorm onset may be important in controlling the poleward expansion and duration of post-onset auroral activity.

Dayside and nightside contributions to cross-polar cap potential variations: the 20 March 2001 ICME case

Abstract. We investigate the association between temporal-spatial structure of polar cap convection and auroral electrojet intensifications during a 5-h-long interval of strong forcing of the magnetosphere by an ICME/Magnetic cloud on 20 March 2001. We use data from coordinated ground-satellite observations in the 15:00–20:00 MLT sector. We take advantage of the good latitudinal coverage in the polar cap and in the auroral zone of the IMAGE chain of ground magnetometers in Svalbard – Scandinavia – Russia and the stable magnetic field conditions in ICMEs. The electrojet events are characterized by a sequence of 10 min-long AL excursions to −1000/−1500 nT followed by poleward expansions and auroral streamers. These events are superimposed on a high disturbance level when the AL index remains around −500 nT for several hours. These signatures are different from those appearing in classical substorms, most notably the absence of a complete recovery phase when AL usually reaches above −100 nT. We concentrate on polar cap convection in both hemispheres (DMSP F13 data) in relation to the ICME By conditions, electrojet intensifications, and the global UV auroral configuration obtained from the IMAGE spacecraft. The temporal evolution of convection properties such as the cross-polar cap potential (CPCP) drop and flow channels at the dawn/dusk polar cap (PC) boundaries around the time of the electrojet events are investigated. This approach allows us to distinguish between dayside (magnetopause reconnection) and nightside (magnetotail reconnection) sources of the PC convection events within the context of the expanding-contracting model of high-latitude convection in the Dungey cycle. Inter-hemispheric symmetries/asymmetries in the presence of newly-discovered convection channels at the dawn or dusk side PC boundaries are determined.

Multi-scale features of solar terrestrial coupling in the cusp ionosphere

The large scale dynamics of the cusp ionosphere is directly controlled by the solar wind and the orientation of the interplanetary magnetic field. The IMF BY controlled east–west movement of the poleward moving forms (PMAFs), separating from the cusp, is consistent with the magnetic tension pull of newly opened flux, which is a key feature tying the auroral phenomenon of PMAFs to flux transfer events (FTEs) at the magnetopause. The central region of an FTE is a mesoscale flow channel, bounded by a pair of Birkeland current sheets on each flank. Systematic observations by EISCAT Svalbard Radar, meridian scanning photometers and all-sky imagers have provided us further insight into key processes of the cusp ionosphere such as how FTEs/PMAFs control ion upflow events, and how transient magnetopause reconnection processes segment a stream of high density solar EUV plasma into polar cap patches at the polar cap boundary. We have found a close causal relationship between formation of plasma patches and onset of scintillation by a previously unrecognized process. We have discovered a new category of flow channels, reversed flow channel events (RFEs) that may appear related to a signature of Birkeland currents arcs, and may be signature of the MI-coupling/inverted V rather than mapping directly to the magnetopause. This review paper concentrates on a niche of cusp studies made by high-resolution radar and optical measurements.

IMF <i>B</i><sub><i>y</i></sub> effects in the plasma flow at the polar cap boundary

Abstract. We used the dataset obtained from the EISCAT Svalbard Radar during 2000–2008 to study statistically the ionospheric convection in a vicinity of the polar cap boundary as related to IMF By conditions separately for northward and southward IMF. The effect of IMF By is manifested in the intensity and direction of the azimuthal component of ionospheric flow. The most significant effect is observed on the day and night sides whereas on dawn and dusk the effect is essentially less prominent. However, there is an asymmetry with respect to the noon-midnight meridian. On the day side the intensity of By-related azimuthal flow is maximal exactly at noon, whereas on the night side the maximum is shifted toward the post-midnight hours (~03:00 MLT). On the dusk side the relative reduction of the azimuthal flow is much larger than that on the dawn side. Overall, the magnetospheric response to IMF By seems to be stronger in the 00:00–12:00 MLT sector compared to the 12:00–24:00 MLTs. Quantitative characteristics of the IMF By effect are presented and partly explained by the magnetospheric electric fields generated due to the solar wind and also by the position of open-closed boundary for different IMF orientation.

Dependence of spectral width of ionosphericFregion HF echoes on electric field

[1] The EISCAT Svalbard radar (ESR) monitors plasma parameters in the ionospheric region that is frequently located near the polar cap boundary. The SuperDARN radar at Hankasalmi, Finland, detects coherent echoes from this region, and these echoes typically show increased spectral width. We consider data of joint ESR and SuperDARN observations to show that the spectral width of HF echoes tends to increase with the ionospheric electric field. This relationship is explained in terms of nonlinear evolution of the E × B gradient drift instability with energy cascade from hundreds of meter wavelengths to meter wavelengths. We assume that decameter waves (seen by the Hankasalmi radar) with relatively large amplitude decay through a three-wave interaction with shorter wavelengths and estimate that the decay time of the decameter waves/irregularities is determined by the parameters of the shorter-wavelength structures. We associate the decameter wave decay time with the correlation time, and thus the spectral width, of HF echoes.

EISCAT-Cluster observations of quiet-time near-Earth magnetotail fast flows and their signatures in the ionosphere

Abstract. We report observations of a sequence of quiet-time Earthward bursty bulk flows (BBFs) measured by the Cluster spacecraft in the near-tail plasma sheet (XGSM ~ −12 to −14 RE) in the evening sector, and by simultaneous high-resolution measurements in the northern conjugate ionosphere by the EISCAT radars, a MIRACLE all-sky camera and magnetometers, as well as a meridian-scanning photometer (MSP) in the Scandinavian sector on 17 October 2005. The BBFs at Cluster show signatures that are consistent with the plasma "bubble" model (Chen and Wolf, 1993, 1999), e.g. deflection and compression of the ambient plasma in front of the Earthward moving bubble, magnetic signatures of a flow shear region, and the proper flows inside the bubble. In addition, clear signatures of tailward return flows around the edges of the bubble can be identified. The duskside return flows are associated with significant decrease in plasma density, giving support to the recent suggestion by Walsh et al. (2009) of formation of a depleted wake. However, the same feature is not seen for the dawnside return flows, but rather an increase in density. In the ionosphere, EISCAT and optical measurements show that each of the studied BBFs is associated with an auroral streamer that starts from the vicinity of the polar cap boundary, intrudes equatorward, brakes at 68–70° aacgm MLAT and drifts westward along the proton oval. Within the streamer itself and poleward of it, the ionospheric plasma flow has an equatorward component, which is the ionospheric manifestation of the Earthward BBF channel. A sharp velocity shear appears at the equatorward edge of a streamer. We suggest that each BBF creates a local velocity shear in the ionosphere, in which the plasma flow poleward of and inside the streamer is in the direction of the streamer and southeastward. A northwestward return flow is located on the equatorward side. The return flow is associated with decreased plasma densities both in the ionosphere and in the magnetosphere as measured by EISCAT and Cluster, respectively. In summary, we present the first simultaneous high-resolution observations of BBF return flows both in the plasma sheet and in the ionosphere, and those are in accordance with the bubble model. The results apply for the duskside return flows, but the manifestation of dawnside return flows in the ionosphere requires further studies. Finally, EISCAT measurements indicate increased nightside reconnection rate during the ~35-min period of BBFs. We suggest that the observed temporal event of IMF rotation to a more southward direction produces enhanced open flux transport to the nightside magnetotail, and consequently, the nightside reconnection rate is increased.

On large plasmoid formation in a global magnetohydrodynamic simulation

Abstract. We investigate plasmoid formation in the magnetotail using the global magnetohydrodynamic (MHD) simulation GUMICS-4. Here a plasmoid implies a major reconfiguration of the magnetotail where a part of the tail plasma sheet is ejected downstream, in contrast to small Earthward-propagating plasmoids. We define a plasmoid based solely on the structure of the closed (connected to the Earth at both ends) magnetic field line region. In this definition a plasmoid is partly separated from the ordinary closed field line region by lobe field lines or interplanetary field lines resulting from lobe reconnection. We simulate an event that occurred on 18 February 2004 during which four intensifications of the auroral electroject (AE) index occurred in 8 h. Plasmoids form in the simulation for two of the four AE intensifications. Each plasmoid forms as a result of two consecutive large and fast rotations of the interplanetary magnetic field (IMF). In both cases the IMF rotates 180 degrees at 10 degrees per minute, first from southward to northward and some 15 min later from northward to southward. The other two AE intencifications however are not associated with a plasmoid formation. A plasmoid does not form if either the IMF rotation speed or the angular change of the rotation are small. We also present an operational definition for these fully connected plasmoids that enables their automatic detection in simulations. Finally, we show mappings of the plasmoid footpoints in the ionosphere, where they perturb the polar cap boundary in both hemispheres.

Development of parallel electric fields at the plasma sheet boundary layer

[1] Many measurements of auroral particles, in particular recent measurements from the FAST satellite, indicate that the auroral electron distribution is often broad in energy and field-aligned in pitch angle. Such electrons are seen in conjunction with strong kinetic Alfven waves with small perpendicular wavelength, and so the aurora produced by these electrons has been termed the “Alfvenic aurora.” The Alfvenic aurora is predominant at the polar cap boundary of the aurora as well as in the auroral arc that brightens during substorm onset. The process of forming parallel electric fields in Alfven waves has been investigated by means of three-dimensional two-fluid simulations. Waves with small perpendicular wavelengths can be produced by phase mixing when perpendicular gradients are present in the plasma. At lower altitudes, Alfven waves can interact with the ionospheric Alfven resonator and phase mix to scales of a few kilometers. At higher altitudes, the electron thermal speed becomes comparable or larger than the Alfven speed, and parallel electric fields due to the Landau resonance can develop. This has been modeled in the fluid code by an approximation to the plasma dispersion function used in kinetic theory. Phase mixing is most effective when there are strong gradients, such as at the plasma sheet boundary layer. Simulations indicate that these processes can produce parallel electric fields on scales of a few kilometers (in the cold plasma case) to a few tens of kilometers (in the warm plasma case), comparable to the scale sizes of auroral arcs.

Spatiotemporal features of the auroral acceleration region as observed by Cluster

The two major agents for producing aurora are generally believed to be the quasi-static parallel electric fields, accelerating electrons in the auroral acceleration region (AAR), and Alfven waves. The Cluster spacecraft quartet has made multi-spacecraft measurements in the AAR possible for the first time. Four event studies are included and discussed in this thesis, using Cluster data inside and at the top of the AAR, to address various open issues regarding the nature of the quasistatic electric potential structures, such as their altitude distribution, temporal and spatial variability, as well as their interactions with regions of Alfven wave activity.In Paper 1, Cluster data from the upper and lower parts of the AAR were used to determine the altitude and latitude distribution of the acceleration potential above the aurora and to address its stability in space and time. The acceleration potential pattern derived consisted of two equally broad and intense U-shaped electric potential structures and a narrower S-shaped potential structure located below. The acceleration potential distribution was found to be stable, both in intensity and width, over five minutes. Furthermore, the perpendicular (to the magnetic field) spatial scale of the electric field was found to be much smaller than that of the current in the lower AAR, but roughly equal in the upper AAR. Revealing of these features was possible only by combining data from the two Cluster spacecraft.In Paper 2, the spatial and temporal characteristics and development of two AAR structures were studied, benefiting from a magnetic conjunction between two of the Cluster spacecraft and a short time difference (~1 min) between the spacecraft crossings. The configuration allowed for estimating the characteristic times of development for the two structures and the parallel electric field and potential drop for the more stable one. Potential structure 1, having a perpendicular width of ~80 km was short-lived, developing in less than 40 seconds and decaying in one minute. The parallel potential drop between 1.13 and 1.3 RE altitudes increased, whereas that above 1.3 RE remained almost unchanged during this time. For potential structure 2, having a width of ~50 km, the parallel potential drop increased by a factor of 3 below 1.3 RE during ~40 seconds, after which it remained stable for at least a minute. Also here, the parallel potential drop above 1.3 RE remained roughly unchanged. An average parallel electric field was estimated to be 0.56 mV/m.In the auroral zone, the quasi-static properties are dominant in the plasma sheet (PS) and especially in the central plasma sheet (CPS), while the Alfvenic properties are generally strongest at the plasma sheet boundary layer (PSBL). Therefore it is of special interest to study the PSBL/CPS boundary, regarding how the two processes interact. In Papers 3 and 4 we addressed this matter. In each event, data from the Cluster fleet making an equatorward crossing of the highaltitude AAR were used together with a DMSP UV image of the oval. The particle and field data were used to infer the acceleration potentials of the observed arcs and their distribution in altitude and latitude. In Paper 3, observations demonstrate a quasi-static potential structure extending into the Alfvenic region of the polar cap boundary (PCB). The associated density cavity did not extend into the Alfvenic region, suggesting that the Alfvenic activity observed within the PCB region prevents the cavity formation. The results show that Alfvenic and quasi-static acceleration operate jointly in the PCB region. In Paper 4, Cluster passed over a system of East-West aligned auroral arcs. The upper extent of the AAR was found to be at least 0.5 RE higher in altitude than generally expected. Overlapping Alfvenic and quasi-static regions were found within the PSBL and inside Region 2 of downward currents. Growth of small-scale potential structures was observed in the PSBL and in the middle of Region 2, during the ~1.5 minutes between the Cluster 4 and Cluster 3 passages. During this period, the Alfvenic regions retreated both at the poleward and equatorward oval boundaries, while the quasi-static potentials intensified.

Dynamic temporal evolution of polar cap tongue of ionization during magnetic storm

During a magnetic storm on 14–16 December 2006, a polar cap tongue of ionization (TOI) was detected by an all‐sky imager (ASI) at Resolute Bay, Canada (74.73°N, 265.07°E). We investigate the temporal evolution and spatial structure of the TOI in detail by combining the optical data with other observations (e.g., solar wind, GPS total electron content, SuperDARN, and DMSP and NOAA POES satellites). The TOI was observed as a bright and elongated 630 nm airglow plume for 4 h during the main phase of the storm. This interval corresponded to a period of prolonged stable large‐amplitude southward IMF during a coronal mass ejection (CME). One to one and a half hours before the appearance of TOI, the polar cap boundary expanded rapidly far equatorward, and a positive ionospheric storm occurred. This implies that both the “expansion of the high‐latitude plasma convection” and “build up of the source plasma in the midlatitudes” are necessary conditions for the formation of a TOI. Because both of them were triggered by a major southward turning of the IMF, the prolonged large‐amplitude southward IMF orientation in the trailing part of the CME was primarily responsible for the generation of TOI. After its appearance, the TOI exhibited dynamic motion in the dawn to dusk direction. Simultaneous SuperDARN data suggest that a longitudinal progression of subauroral polarization stream controlled this dynamic motion. The optical TOI was found to be a continuous stream elongated in the noon‐midnight direction although it contained some mesoscale patterns. Absence of large‐scale temporal changes in the cusp plasma flow during the stable IMF period allowed the TOI to remain continuous without being broken into polar cap patches. The mesoscale structures within the TOI were probably produced by small‐scale velocity fluctuations in the cusp plasma flow. The TOI as visualized with the all‐sky airglow imager was found to be much more dynamic and much more complicated than we ever thought. The current study indicates that such a behavior of the TOI was presumably caused by a combination of temporal variations in the global‐scale plasma circulation system, expansion and contraction of the polar cap area, and plasma density changes in the dayside low to midlatitudes.

Ionospheric convection signatures of the interchange cycle at small interplanetary magnetic field clock angles

The purpose of this paper is to show a “proof of the existence” of the ionospheric situation that is expected for the interchange cycle, during periods of favorable interplanetary magnetic field (IMF) and dipole tilt conditions. To do so, we present three case studies of dayside high‐latitude ionospheric convection that is observed around the equinoxes (near‐zero dipole tilt) and at small IMF clock angles (one θc ∼ −30° event and two θc ∼ 30° events, where θc ≡ Arg(BZ + iBY)), using Super Dual Auroral Radar Network (SuperDARN)/Defense Meteorological Satellite Program (DMSP)/National Oceanic and Atmospheric Administration (NOAA) data in the Northern Hemisphere and, when available, DMSP data in the Southern Hemisphere. The convection pattern exhibits twin reverse cells in both hemispheres, but the constituents of each cell are different. In the Northern Hemisphere, for θc ∼ 30° (θc ∼ −30°), the center of the dawnside (duskside) cell is located poleward of the polar cap boundary, while the center of the duskside (dawnside) cell is located equatorward of the polar cap boundary. For θc ∼ 30°, we confirmed that the above‐mentioned dawn‐dusk relation reverses in the Southern Hemisphere. The north‐south asymmetric behavior of the conjugate reverse cells, on the dawnside and duskside each, is consistent with two independent interchange cycles that result from the coupling of IMF‐lobe reconnection in one hemisphere with lobe‐closed reconnection in the opposite hemisphere.

Monopolar and bipolar auroral electric fields and their effects

Abstract. Most of the high-altitude auroral electric fields observed by CLUSTER can be classified into monopolar and bipolar structures. The observations associate monopolar electric fields with polar cap boundary arcs, while bipolar fields tend to be linked to discrete arcs within the auroral oval and to polar cap arcs. The present paper proposes an explanation for this association based on a simple model of the magnetotail configuration and kinetic model computations. The paper introduces a quasi-electrostatic model to describe the auroral current system associated with monopolar and bipolar high-altitude fields. Analytic solutions are presented. The model gives indications about the location of the up- and downward field-aligned current regions, the ionospheric and magnetospheric convection along the arc, the acceleration or deceleration of precipitating particles, and the behaviour of escaping ionospheric ions.

Polar cap flow channel events: spontaneous and driven responses

Abstract. We present two case studies of specific flow channel events appearing at the dusk and/or dawn polar cap boundary during passage at Earth of interplanetary (IP) coronal mass ejections (ICMEs) on 10 January and 25 July 2004. The channels of enhanced (>1 km/s) antisunward convection are documented by SuperDARN radars and dawn-dusk crossings of the polar cap by the DMSP F13 satellite. The relationship with Birkeland currents (C1–C2) located poleward of the traditional R1–R2 currents is demonstrated. The convection events are manifest in ground magnetic deflections obtained from the IMAGE (International Monitor for Auroral Geomagnetic Effects) Svalbard chain of ground magnetometer stations located within 71–76° MLAT. By combining the ionospheric convection data and the ground magnetograms we are able to study the temporal behaviour of the convection events. In the two ICME case studies the convection events belong to two different categories, i.e., directly driven and spontaneous events. In the 10 January case two sharp southward turnings of the ICME magnetic field excited corresponding convection events as detected by IMAGE and SuperDARN. We use this case to determine the ground magnetic signature of enhanced flow channel events (the NH-dusk/By<0 variant). In the 25 July case a several-hour-long interval of steady southwest ICME field (Bz<0; By<0) gave rise to a long series of spontaneous convection events as detected by IMAGE when the ground stations swept through the 12:00–18:00 MLT sector. From the ground-satellite conjunction on 25 July we infer the pulsed nature of the polar cap ionospheric flow channel events in this case. The typical duration of these convection enhancements in the polar cap is 10 min.

Vertical thermal O+ flows at 850 km in dynamic auroral boundary coordinates

Contemporary magnetosphere models now include species‐dependent dynamics. Energetic O+ has significant consequences for the energy stored in the ring current, the rate of reconnection, and perhaps the timing of substorm injections. The mechanism by which thermal O+ escapes from the top of the ionosphere and into the magnetosphere is not fully understood. Previous studies have used dynamic auroral boundary coordinates to describe the outflowing energetic O+ ions above the ionosphere. In this study we focus on the vertical flow of O+ ions at lower altitudes before they are accelerated to escape velocity. An algorithm has been devised to identify auroral zone boundaries using precipitating electron observations from the Defense Meteorological Satellite Program (DMSP) spacecraft. Vertical ion flows measured by the DMSP special sensor for ions electrons and scintillation ion drift meter and the retarding potential analyzer instruments aboard the F12 (noon‐midnight) and F13 (dawn‐dusk) spacecraft from 1997 to 1998 were projected into dynamic auroral boundary coordinates and used to investigate the dependence of Southern Hemisphere bulk flows on interplanetary magnetic field (IMF) and geomagnetic conditions. Initial results show that (1) net upward flows occur primarily in the auroral zone and net downward flows occur primarily in the polar cap, (2) there exists a strong upward flow at 9 magnetic local time (MLT) near the polar cap boundary, 3) the downward ion flow orientation is strongly dependent on IMF By, and 4) the auroral boundary does not coincide exactly with the upward/downward boundary for bulk flows.

Seasonal and interplanetary magnetic field–dependent polar cap contraction during substorm expansion phase

Recent observations from simultaneous imaging in two hemispheres have shown that the polar caps can attain considerably different shapes as the auroral ovals contract during the substorm expansion phase. We use images from 2770 substorms to study the evolution of the polar cap boundary location statistically. We show that, during the first 26 min after substorm expansion phase onset, the polar cap boundary location depends on seasons, interplanetary magnetic field (IMF) By, and IMF Bx. For different signs of By, with ∣By∣ > 3 nT, the asymmetry in polar cap boundary observed at onset increases during the expansion phase, consistent with an increase in tail reconnection of field lines with asymmetrical footpoints. When Bx > 2 nT and ∣By∣ < 2 nT, the polar cap boundary dawnward of the onset propagates slightly farther poleward compared to negative Bx conditions. In the sunlit hemisphere, the polar cap boundary evolves from a pronounced equatorward displacement at onset to an almost reversed displacement during the expansion phase, compared to substorms observed in darkness. Substorms in the dark hemisphere also have a much more pronounced bulge than substorms in the sunlit hemisphere. If the interpretation of the poleward auroral boundary as being coincident with the open/closed field line boundary (OCB) is correct, the seasonal differences in OCB locations imply seasonal differences in the ionospheric convection during the substorm expansion phase.

Plasma flow during the brightening of proton aurora in the cusp

On the basis of simultaneous observations from the Super Dual Auroral Radar Network (SuperDARN), the far ultraviolet instrument on the IMAGE spacecraft, and a magnetometer installed on the east coast of Greenland, we present the characteristics of plasma flow during a westward moving proton aurora in the cusp, observed on 28 July 2000. Data with a time resolution of 12 and 25 s from SuperDARN at Þykkvibær, Iceland, show that the flow having a poleward component, which was accompanied by a quick equatorward expansion of the flow region, occurred in the early stages (approximately 1.5 min) of the brightening. Data from the magnetometer from a ground station on the Greenland east coast, which is located a few hundred kilometers west of the radar's field of view, show perturbations that are consistent with equatorward flow overhead, coincident with the expansion. The presence of flow vectors in opposite directions suggests that vortical flow was present during the proton aurora. Evidence supporting the suggested flow pattern has been found in an event recorded by the Greenland west coast magnetometer chain and in a cusp pass of the DE 2 spacecraft in the literature. We attribute the moving proton aurora to the traveling bulge at the polar cap boundary, which is the footprint of a flux transfer event, and imply that the preexisting vortical flow may be intensified when it becomes inflow to the bulge.

Seasonal and UT variations of the position of the auroral precipitation and polar cap boundaries

The data of the DMSP F7 spacecraft are used for studying the influence of the geomagnetic dipole tilt angle on the latitudinal position of auroral precipitation boundaries in the nighttime (2100–2400 MLT) and daytime (0900–1200 MLT) sectors. It is shown that, in the nighttime sector, the high-latitude zone of soft diffuse precipitation (SDP) and the boundary of the polar cap (PC) at all levels of geomagnetic activity are located at higher and lower latitudes relative to the equinox period in winter and summer, respectively. The position of boundaries of the diffuse auroral precipitation zone (DAZ) located equatorward from the auroral oval does not depend on the season. In the daytime sector, the inverse picture is observed: the SDP precipitation zone takes the most low-latitude and high-latitude positions in the winter and summer periods, respectively. The total value of the displacements from winter to summer of both the nighttime and daytime boundaries of the PC is ∼2.5°. A diurnal wave in the latitudinal position of the nighttime precipitation boundaries is detected. The wave is most pronounced in the periods of the winter and fall seasons, is much weaker in the spring period, and is almost absent in summer. The diurnal variations of the position of the boundaries are quasi-sinusoidal oscillations with the latitude maximum and minimum at 0300–0500 and 1700–2100 UT, respectively. The total value of the diurnal displacement of the boundaries is ∼2.5° of latitude. The results obtained show that, undergoing seasonal and diurnal variations, the polar cap is shifted as a whole in the direction opposite to the changes in the tilt angle of the geomagnetic dipole. The seasonal displacements of the polar cap and its diurnal variations in the winter period occur without any substantial changes in its area.

Comparison of the open‐closed separatrix in a global magnetospheric simulation with observations: The role of the ring current

The development of global magnetospheric models, such as Space Weather Modeling Framework (SWMF), which can accurately reproduce and track space weather processes has high practical utility. We present an interval on 5 June 1998, where the location of the polar cap boundary, or open‐closed field line boundary (OCB), can be determined in the ionosphere using a combination of instruments during a period encompassing a sharp northward to southward interplanetary field turning. We present both point‐ and time‐varying comparisons of the observed and simulated boundaries in the ionosphere and find that when using solely the coupled ideal magnetohydrodynamic magnetosphere‐ionosphere model, the rate of change of the OCB to a southward turning of the interplanetary field is significantly faster than that computed from the observational data. However, when the inner magnetospheric module is incorporated, the modeling framework both qualitatively, and often quantitatively, reproduces many elements of the studied interval prior to an observed substorm onset. This result demonstrates that the physics of the inner magnetosphere is critical in shaping the boundary between open and closed field lines during periods of southward interplanetary magnetic field (IMF) and provides significant insight into the 3‐D time‐dependent behavior of the Earth's magnetosphere in response to a northward‐southward IMF turning. We assert that during periods that do not include the tens of minutes surrounding substorm expansion phase onset, the coupled SWMF model may provide a valuable and reliable tool for estimating both the OCB and magnetic field topology over a wide range of latitudes and local times.

Interhemispheric observations of emerging polar cap asymmetries

In this paper we use simultaneous global UV images of the aurora in the two hemispheres to study differences in the polar cap boundary location. We show that the northern and southern auroral ovals circumvent the same amount of magnetic flux, providing additional evidence that the poleward boundary of the aurora coincides with the open/closed field line boundary. During a period of significant flux closure, large asymmetries in the polar cap boundaries developed between the hemispheres. The asymmetry was strongest in the regions where the polar caps contracted the most, suggesting that emerging interhemispheric polar cap asymmetries is an intrinsic phenomenon during substorm expansions, when magnetic flux closes rapidly in the tail. Utilizing the prolonged surveillance of the open/closed boundary location, we show that the growing asymmetries can be accounted for by differences in the ionospheric convection in the two hemispheres. The observations suggest that the differences in convection were due to seasonal differences between the hemispheres, and that the summer hemisphere responded more promptly to changes in magnetospheric convection than the winter hemisphere.