A statistical study of the interplanetary magnetic field control of sporadic E-layer occurrence in the southern polar cap ionosphere

The influence of the interplanetary magnetic field (IMF) on the occurrence of sporadic E (Es)-layers in the southern polar cap ionosphere has been investigated. We statistically analysed ionogram and Doppler velocity observations made using a HF digital ionosonde located at Casey, Antarctica (66.3°S, 110.5°E; 81°S magnetic latitude) during the two summer campaign intervals 1 January to 18 February, and 1 November to 31 December 1997. The ionogram and Doppler velocity measurements were used to determine the Es-occurrence and electric field vectors (assuming E×B/B² drift), respectively. Concurrent IMF data were obtained from measurements made on board the Wind spacecraft. First, the gross properties of the IMF dependence of Es-formation were obtained: the occurrence rate was higher for negative B_y and/or positive B_z, and lower for positive B_y and/or negative B_z. To reconcile these gross properties with the electric field theory of Es-layer formation, the detailed diurnal variation of both Es-occurrence and the ionospheric electric field were obtained for different orientations of the IMF. The main statistical results are that: (1) the B_y component mainly controls the occurrence of the midnight Es-layers through its influence on the corresponding South West electric field; and (2) the B_z component mainly controls the occurrence of the evening Es-layers. However, the change in the occurrence rate for evening Es-layers was not related to the strength of the associated North West and North East electric fields. The total occurrence of Es-layers depended more on B_y than on B_z, owing to the dominance of B_y-controlled midnight Es-layers in the occurrence distribution. Nevertheless, the dependence of Es-occurrence on B_z was important. We suggest that the increase in Es-occurrence for positive B_z might be explained by the intermittent production of lower F-region ionisation by polar showers and squalls, which also increase in frequency and intensity for positive B_z. The importance of metallic ion transport within the ionosphere is also considered.     

Dependence of geomagnetic activity during magnetic storms on the solar wind parameters for different types of streams

The dependence of the maximal values of the |Dst| and AE geomagnetic indices observed during magnetic storms on the value of the interplanetary electric field (E _y ) was studied based on the catalog of the large-scale solar wind types created using the OMNI database for 1976–2000 [Yermolaev et al., 2009]. An analysis was performed for eight categories of magnetic storms caused by different types of solar wind streams: corotating interaction regions (CIR, 86 storms); magnetic clouds (MC, 43); Sheath before MCs (Sh_MC, 8); Ejecta (95); Sheath (Sh_E, 56); all ICME events (MC + Ejecta, 138); all compression regions Sheaths before MCs and Ejecta (Sh_MC + Sh_E, 64); and an indeterminate type of storm (IND, 75). It was shown that the |Dst| index value increases with increasing electric field E _y for all eight types of streams. When electric fields are strong (E _y > 11 mV m⁻¹), the |Dst| index value becomes saturated within magnetic clouds MCs and possibly within all ICMEs (MC + Ejecta). The AE index value during magnetic storms is independent of the electric field value E _y for almost all streams except magnetic clouds MCs and possibly the compressed (Sheath) region before them (Sh_MC). The AE index linearly increases within MC at small values of the electric field (E _y < 11 mV m⁻¹) and decrease when these fields are strong (E _y > 11 mV m⁻¹). Since the dynamic pressure (Pd) and IMF fluctuations (σB) correlate with the E _y value in all solar wind types, both geomagnetic indices (|Dst| and AE) do not show an additional dependence on Pd and IMF δB. The nonlinear relationship between the intensities of the |Dst| and AE indices and the electric field E _y component, observed within MCs and possibly all ICMEs during strong electric fields E _y , agrees with modeling the magnetospheric-ionospheric current system of zone 1 under the conditions of the polar cap potential saturation.     

Morning polar substorms and variations in the atmospheric electric field

The effects of morning magnetospheric substorms in the variations in near-Earth atmospheric electricity according to the observations of the electric field vertical component (E _z ), at Hornsund polar observatory (Spitsbergen). The E _z, data, obtained under the conditions of fair weather (i.e., in the absence of a strong wind, precipitation, and fog), are analyzed. An analysis of the observations indicated that the development of a magnetospheric substorm in the Earth’s morning sector is as a rule accompanied by positive deviations in E _z, independently of the Hornsund location: in the polar cap or at its boundary. In all considered events, Hornsund was located near the center of the morning convection vortex. In the evening sector, when Hornsund fell in the region of evening convection vortex, the development of a geomagnetic substorm was accompanied by negative deviations in E _z., It has been concluded that the variations in the atmospheric electric field E _z), at polar latitudes, observed during the development of magnetospheric substorms, result from the penetration of electric fields of polar ionospheric convection (which are intensified during a substorm) to the Earth’s surface.     

The geoelectric field at Vostok, Antarctica: its relation to the interplanetary magnetic field and the cross polar cap potential difference

The vertical geoelectric field measured at Vostok, Antarctica (78.5°S, 107°E, L=75.0) over the 13 month interval May 1979–May 1980 is correlated with the interplanetary magnetic field (IMF) components B_y and B_z at times when Vostok is connected to the dayside magnetosphere. No significant association with IMF B_x is found. The interaction of the solar wind and the Earth’s magnetic field generally results in anti-sunward plasma flow in the high-latitude, polar ionosphere driven by a dawn-to-dusk, cross polar cap potential difference pattern. Using the IZMEM model to infer the contribution of the cross polar cap potential difference to the potential difference between the ionosphere and the ground at Vostok for the measured IMF conditions, we show that this provides a viable mechanism for the IMF associations found. We demonstrate that the direct association of the geoelectric field with the cross polar cap potential difference is independent of a result (Park, 1976. Solar magnetic sector effects on the vertical atmospheric electric field at Vostok, Antartica. Geophysical Research Letters 3(8), 475–478) showing an 15% decrease in the vertical geoelectric field measured at Vostok, 1–3 days after the passage of IMF sector boundaries. Evidence is also presented supporting the Park result, for which a mechanism is yet to be confirmed.     

Does the polar cap disappear under an extended strong northward IMF?

The suggestion that the polar cap can completely disappear under certain northward IMF conditions is still controversial. We know that the size of the polar cap is strongly controlled by the interplanetary magnetic field (IMF). Under a southward IMF, the polar cap is usually large and filled with weak diffuse polar rain electrons. The polar cap shrinks under a northward IMF. Here we use the global auroral images and coincident particle measurements on May 15, 2005 to show that the discrete arcs (due to precipitation of both electrons and ions) expanded from the dayside oval to the nightside oval and filled the whole polar ionosphere after a long (8 h) and strong (～5–30 nT) northward IMF B_z, The observations suggested that the polar cap disappeared under a closed magnetosphere.     

Response of ionospheric electric fields to variations in the interplanetary magnetic field

The STARE system (Scandinavian Twin Auroral Radar Experiment) provides estimates of electron drift velocities, and hence also of the electric field in the high-latitude E-region ionosphere between 65 and 70 degrees latitude. The occurrence of drift velocities larger than about 400 m/s (equivalent to an electric field of 20 mV/m) have been correlated with the magnitude of the Interplanetary Magnetic Field (IMF) components B_z and B_y at all local times. Observation days have been considered during which both southward (B_z<0) and northward (B_z>0) IMF occurred. The occurrence of electric fields larger than 20 mV/m increases with increases in B_z magnitudes when B_z<0. It is found that the effects of southward IMF continue for some time following the northward turnings of the IMF. In order to eliminate such residual effects for B_z<0, we have, in the second part of the study, considered those days which were characterized by a pure northward IMF. The occurrence is considerably lower during times when B_z>0, than during those when B_z is negative. These results are related to the expansion and contraction of the auroral oval. The different percentage occurrences of large electric field for B_y>0 and B_y<0 components of the IMF during times when B_z>0, clearly display a dawn-dusk asymmetry of plasma flow in the ionosphere. The effects of the time-varying solar-wind speed, density, IMF fluctuations, and magnetospheric substorms on the occurrence of auroral-backscatter observations are also discussed.     

Variations in the polar cap area during intervals of substorm activity on 20–21 March 1990 deduced from AMIE convection patterns

The dynamic behaviour of the northern polar cap area is studied employing Northern Hemisphere electric potential patterns derived by the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure. The rate of change in area of the polar cap, which can be defined as the region of magnetospheric field lines open to the interplanetary magnetic field (IMF), has been calculated during two intervals when the IMF had an approximately constant southward component (1100- 2200 UT, 20 March 1990 and 1300–2100 UT, 21 March 1990). The estimates of the polar cap area are based on the approximation of the polar cap boundary by the flow reversal boundary. The change in the polar cap area is then compared to the predicted expansion rate based on a simple application of Faraday’s Law. Furthermore, timings of magnetospheric substorms are also related to changes in the polar cap area. Once the convection electric field reconfigures following a southward turning of the IMF, the growth rate of the observed polar cap boundary is consistent with that predicted by Faraday’s Law. A delay of typically 20 min to 50 min is observed between a substorm expansion phase onset and a reduction in the polar cap area. Such a delay is consistent with a synthesis between the near Earth neutral line and current disruption models of magnetospheric substorms in which the dipolarisation in the magnetotail may act as a trigger for reconnection. These delays may represent a propagation time between near geosynchronous orbit dipolarisation and subsequent reconnection further down tail. We estimate, from these delays, that the neutral X line occurs between \sim35R_E and \sim75R_E downstream in the tail.     

Cusp Modeling and Observations at Low Altitude

Cusp properties have been investigated with an open-field line particle precipitation model and Defense Meteorological Satellite Program (DMSP) satellite observations. Particular emphasis is placed on the effects of IMF B_y, since previous studies focus mostly on IMF B_z. The model-data comparisons for various IMF configurations show that the model captures the large-scale features of the particle precipitation very well, not only in the cusp region, but also in other open-field line regions such as the mantle, polar rain, and open-field line low-altitude boundary layer (LLBL). When the IMF is strongly duskward/dawnward and weakly southward, the model predicts the occurrence of double cusp near noon: one cusp at lower latitude and one at higher latitude. The lower latitude cusp ions originate from the low-latitude magnetosheath whereas the higher latitude ions originate from the high-latitude magnetosheath. The lower latitude cusp is located in the region of weak azimuthal E × B drift, resulting in a dispersionless cusp. The higher latitude cusp is located in the region of strong azimuthal and poleward E × B drift. Because of a significant poleward drift, the higher latitude cusp dispersion has some resemblance to that of the typical southward IMF cusp. Occasionally, the two parts of the double cusp have such narrow latitudinal separation that they give the appearance of just one cusp with extended latitudinal width. From the 40 DMSP passes selected during periods of large (positive or negative) IMF B_y and small negative IMF B_z, 30 (75%) of the passes exhibit double cusps or cusps with extended latitudinal width. The double cusp result is consistent with the following statistical results: (1) the cusp’s latitudinal width increases with |IMF B_y| and (2) the cusp’s equatorward boundary moves to lower latitude with increasing |IMF B_y|.     

Effect of the IMF B y component on the ionospheric flow overhead at EISCAT: observations and theory

We have analysed a database of 300 h of tristatic ionospheric velocity measurements obtained overhead at Tromsø (66.3° magnetic latitude) by the EISCAT UHF radar system, for the presence of flow effects associated with the y-component of the IMF. Since it is already known that the flow depends upon IMF B_z, a least-squares multivariate analysis has been used to determine the flow dependence on both IMF B_y and B_z simultaneously. It is found that significant flow variations with IMF B_y occur, predominantly in the midnight sector (2100/0300 MLT), but also pre-dusk (1600/1700 MLT), which are directed eastward for IMF B_y positive and westward for IMF B_y negative. The flows are of magnitude 20/30 m s^–1 nT^–1 in the midnight sector, and smaller, 10/20 m s^–1 nT^–1, pre-dusk, and are thus associated with significant changes of flow of order a few hundred m s^–1 over the usual range of IMF B_y of about ±5 nT. At other local times the IMF B_y-related perturbation flows are much smaller, less than 5 m s^–1 nT^–1, and consistent with zero within the uncertainty estimates. We have investigated whether these IMF B_y-dependent flows can be accounted for quantitatively by a theoretical model in which the equatorial flow in the inner magnetosphere is independent of IMF B_y, but where distortions of the magnetospheric magnetic field associated with a penetrating component of the IMF B_y field changes the mapping of the field to the ionosphere, and hence the ionospheric flow. We find that the principal flow perturbation produced by this effect is an east-west flow whose sense is determined by the north-south component of the unperturbed flow. Perturbations in the north-south flow are typically smaller by more than an order of magnitude, and generally negligible in terms of observations. Using equatorial flows which are determined from EISCAT data for zero IMF B_y, to which the corotation flow has been added, the theory predicts the presence of zonal perturbation flows which are generally directed eastward in the Northern Hemisphere for IMF B_y positive and westward for IMF B_y negative at all local times. However, although the day and night effects are therefore similar in principle, the model perturbation flows are much larger on the nightside than on the dayside, as observed, due to the day-night asymmetry in the unperturbed magnetospheric magnetic field. Overall, the model results are found to account well for the observed IMF B_y-related flow perturbations in the midnight sector, in terms of the sense and direction of the flow, the local time of their occurrence, as well as the magnitude of the flows (provided the magnetic model employed is not too distorted from dipolar form). At other local times the model predicts much smaller IMF B_y-related flow perturbations, and thus does not account for the effects observed in the pre-dusk sector.     

Observations of polar patches generated by solar wind Alfvén wave coupling to the dayside magnetosphere

A long series of polar patches was observed by ionosondes and an all-sky imager during a disturbed period (K_p = 7- and IMF B_z <0). The ionosondes measured electron densities of up to 9 × 10¹¹ m⁻³ in the patch center, an increase above the density minimum between patches by a factor of ≈4.5. Bands of F-region irregularities generated at the equatorward edge of the patches were tracked by HF radars. The backscatter bands were swept northward and eastward across the polar cap in a fan-like formation as the afternoon convection cell expanded due to the IMF B_y > 0. Near the north magnetic pole, an all-sky imager observed the 630-nm emission patches of a distinctly band-like shape drifting northeastward to eastward. The 630-nm emission patches were associated with the density patches and backscatter bands. The patches originated in, or near, the cusp footprint where they were formed by convection bursts (flow channel events, FCEs) structuring the solar EUV-produced photoionization and the particle-produced auroral/cusp ionization by segmenting it into elongated patches. Just equatorward of the cusp footprint Pc5 field line resonances (FLRs) were observed by magnetometers, riometers and VHF/HF radars. The AC electric field associated with the FLRs resulted in a poleward-progressing zonal flow pattern and backscatter bands. The VHF radar Doppler spectra indicated the presence of steep electron density gradients which, through the gradient drift instability, can lead to the generation of the ionospheric irregularities found in patches. The FLRs and FCEs were associated with poleward-progressing DPY currents (Hall currents modulated by the IMF B_y) and riometer absorption enhancements. The temporal and spatial characteristics of the VHF backscatter and associated riometer absorptions closely resembled those of poleward moving auroral forms (PMAFs). In the solar wind, IMP 8 observed large amplitude Alfvén waves that were correlated with Pc5 pulsations observed by the ground magnetometers, riometers and radars. It is concluded that the FLRs and FCEs that produced patches were driven by solar wind Alfvén waves coupling to the dayside magnetosphere. During a period of southward IMF the dawn-dusk electric field associated with the Alfvén waves modulated the subsolar magnetic reconnection into pulses that resulted in convection flow bursts mapping to the ionospheric footprint of the cusp.     

ELF polar chorus and magnetic storms

The data of continuous observations of ELF emissions (polar chorus) at South Pole Antarctic observatory (Φ = ?74.02°) for 1997–1999 and during the superstrong magnetic storms of October and November 2003 are analyzed. It has been established that an increase in polar chorus is as a rule observed during the initial and recovery phases of a magnetic storm at positive values of the IMF vertical component (IMF B _z > 0). Under such conditions, South Pole is located in the region of closed field lines. It has been found that the generation of polar chorus at South Pole abruptly ceases during the storm main phase after the IMF B _z southward turning and beginning of an intense substorm in the nightside auroral zone, probably, because this observatory appears in the region of projection of the open magnetosphere due to the expansion of the polar cap.     

The effects of IMF and convection on thermal ion outflow in magnetosphere-ionosphere coupling

We study the influence of the interplanetary magnetic field (IMF) and convection electric field on the rate and destination of polar wind and other thermal (low-energy) ion outflows, and its resulting effects on magnetosphere–ionosphere coupling, using single-particle trajectory simulations in conjunction with ion velocity distribution measurements on Akebono and IMF and ionospheric convection data. We find that the ions preferentially feed the dusk sector of the plasma sheet when the IMF is duskward (B_y>0), and are more evenly distributed in the plasma sheet when the IMF is dawnward. The flow of oxygen ions originating from the noon or dusk sectors of the polar cap has a higher probability of reaching the magnetosphere and beyond compared with that from the dawn or midnight sectors, due to the increased centrifugal acceleration associated with the larger magnetic field curvature near noon and the increased convection electric field in the dusk sector. The flow is enhanced and confined to lower L-shells at times of strongly southward IMF, compared with that at times of northward IMF. The outflow rate to both the plasma sheet and the magnetotail correlates strongly with the ion temperature. As a result, the IMF and the convection electric fields affect both the overall magnitude and the detailed distribution of mass transfer from the ionosphere to the magnetosphere in magnetosphere–ionosphere coupling.     

Field-aligned currents in the winter and summer hemispheres caused by IMF <Emphasis Type="Italic">By</Emphasis>

Structures controlled by the IMF By sign and season of the year have been detected based on the decomposition of field-aligned current maps constructed using magnetic field measurements on polar low-orbiting satellites. It has been indicated that field-aligned currents have identical structures, composed of the main polar circular current and the return current at the polar cap dayside boundary, at any By sign in the summer hemisphere. Two different types of structures are implemented under winter conditions depending on the By sign. For the northern winter, it is the polar circular current and the return current at the polar cap nightside boundary at By < 0; current sheets are strongly stretched along latitudes below 80° MLat, and only small part of the current is in the noon sector of the polar cap. For the summer winter, the corresponding structures are implemented at opposite By signs. The intensities of the field-aligned currents, originating as a result of the interhemispheric asymmetry and flowing along closed geomagnetic field lines near the polar cap boundary, have been estimated. The maximum of the interhemispheric current density is 0.25 μA m⁻² in the summer and 0.1 μA m⁻² in the winter; the total current is 5 × 10⁵ and 5 × 10⁴ A, respectively.     

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

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 -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 -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 B_y>0 (B_y<0) in the southern hemisphere. Simultaneous observations of auroral ovals during interplanetary B_z<0, B_y<0 and B_x>0 in both hemispheres and convection patterns for B_z<0 and B_y<0 have been displayed using satellite and ground-based measurements. It was found that the transpolar band of the -aurora in the sunlit hemisphere was situated in the region of large-scale downward Birkeland currents.     

Dependence of geomagnetic disturbances on extreme values of the solar wind <Emphasis Type="Italic">E</Emphasis><Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> component

The dependence of the zonal geomagnetic indices (AE, Ap, Kp, Kn, and Dst) on the solar wind parameters (the electric field E _y component, dynamic pressure P _d and IMF irregularity σB) has been studied for two types of events: magnetic clouds and high-speed streams. Based on the empirical relationships, it has been established that the AE, Ap, Kp, and Kn indices are directly proportional to the E _y value at E _y < 12 mV m^?1 and are inversely proportional to this value at E _y > 12 mV m^?1 for the first-type events. On the contrary, the dependence of Dst on E _y is monotonous nonlinear. A linear dependence of all geomagnetic indices on E _y is typical of the second-type events. It has been indicated that the specific features of geoeffectiveness of magnetic clouds and high-speed solar wind streams are caused by the dependence of the electric field potential across the polar cap on the electric field, solar wind dynamic pressure, and IMF fluctuations.     

Investigation of the broadband ELF turbulence by observations of the FAST satellite

Scaling properties of variable electric fields in the topside ionosphere have been investigated on scales s from ∼30 m to 2 km by FAST electric field observations with sample rate of 512 s⁻¹, in sixteen events of the broadband ELF turbulence. It is shown that down to scales of a few hundred meters, the power of turbulent electric fluctuations is a power law, ∼s ^α. Scaling index α derived from the slope of logarithmic diagrams (LD) constructed by the discrete wavelet transform of data can be estimated as α = 2.2 ± 0.3, which is close to α estimate earlier reported for scales 1–30 km by electric field observations of the Dynamics Explorer 2 satellite. The behavior of α index is analyzed near the scale of the order of electron inertial length λ_e = c/ω₀ (ω₀ being the electron plasma frequency). At altitudes considered (700–2500 km), λ_e makes 100–900 m. We demonstrate that at scales ≤λ_e, a decrease of LD slope and deviation from the power law are typically observed. As pointed out in the discussion, this feature cannot be identified as a transition to the diffusion range, where dissipation of the turbulence occurs.     

Ionospheric convection during the magnetic storm of 20–21 March 1990

We report on the response of high-latitude ionospheric convection during the magnetic storm of March 20–21 1990. IMP-8 measurements of solar wind plasma and interplanetary magnetic field (IMF), ionospheric convection flow measurements from the Wick and Goose Bay coherent radars, EISCAT, Millstone Hill and Sondrestrom incoherent radars and three digisondes at Millstone Hill, Goose Bay and Qaanaaq are presented. Two intervals of particular interest have been identified. The first starts with a storm sudden commencement at 2243 UT on March 20 and includes the ionospheric activity in the following 7 h. The response time of the ionospheric convection to the southward turning of the IMF in the dusk to midnight local times is found to be approximately half that measured in a similar study at comparable local times during more normal solar wind conditions. Furthermore, this response time is the same as those previously measured on the dayside. An investigation of the expansion of the polar cap during a substorm growth phase based on Faraday’s law suggests that the expansion of the polar cap was nonuniform. A subsequent reconfiguration of the nightside convection pattern was also observed, although it was not possible to distinguish between effects due to possible changes in B_y and effects due to substorm activity. The second interval, 1200–2100 UT 21 March 1990, included a southward turning of the IMF which resulted in the B_z component becoming -10 nT. The response time on the dayside to this change in the IMF at the magnetopause was approximately 15 min to 30 min which is a factor of \sim2 greater than those previously measured at higher latitudes. A movement of the nightside flow reversal, possibly driven by current systems associated with the substorm expansion phases, was observed, implying that the nightside convection pattern can be dominated by substorm activity.     

Analysis of high-latitude current systems during the BEAR experiment based on the IZMEM model

Based on the model of large-scale high-latitude current systems developed at IZMIRAN (IZMEM model), it has been indicated that auroral electrojets and current systems concentrated in the polar cap were the generators of long-period geomagnetic variations during the BEAR experiment on the electromagnetic field registration at the Scandinavian test site on June 1–July 15, 1998. Precisely circumpolar current systems, prevailing in the high-latitude ionosphere during the periods of a quiet magnetospheric state, which is characterized by the presence of the northern vertical (B _z >0) component of the IMF vector in the solar wind, are responsible for the magnetotelluric fields.     

Polar cap particle precipitation and aurora: Review and commentary

Polar rain has a beautiful set of symmetry properties, individually established, but not previously discussed collectively, which can be organized by a single unifying principle. The key polar rain properties are favored hemisphere (controlled by the interplanetary magnetic field B_x), dawn/dusk gradient (IMF B_y), merging rate (IMF B_z or more generally dΦ_MP/dt), nightside/dayside gradient, and seasonal effect. We argue that all five properties involve variants on a single theme: the further downstream a field line exits the magnetosphere (or less directly points toward the solar wind electron heat flux), the weaker the polar rain. This effect is the result of the requirements of charge quasi-neutrality, and because the ion thermal velocity declines and the tailward ion bulk flow velocity rises moving down tail from the frontside magnetopause.Polar cap arcs (or more properly, high-latitude sun-aligned arcs) are largely complementary to the polar rain, occurring most frequently when the dayside merging rate is low, and thus when polar rain is weak. Sun-aligned arcs are often considered as originating either in the polar rain or the expansion of the plasma sheet into the polar cap. In fact three quite distinct types of sun-aligned high-latitude arcs exist, two common, and one rare. One type of arc occurs as intensifications of the polar rain, and is common, but weak, typically <0.1 ergs/cm² s, and lacks associated ion precipitation. A second category of Sun-aligned arcs with energy flux >0.1 ergs/cm² s usually occurs adjacent to the auroral oval, and includes ion precipitation. The plasma regime of these common, and at times intense, arcs is often distinct from the oval which they abut. Convection alone does not specify the open/closed nature of these arcs, because multiple narrow convection reversals are common around such arcs, and the arcs themselves can be embedded within flows that are either sunward or anti-sunward. These observational facts do not neatly fit into either a plasma sheet origin or a polar rain origin (e.g., the necessity to abut the auroral oval, and the presence of ions does not fit the properties of polar rain, which can in any event be nearly absent for northward interplanetary magnetic field). One theory is that such arcs are associated with merging tailward of the cusp. Both of these common types of sun-aligned arcs fade within about 30 min of a southward IMF turning.The third, and rarest, category of sun-aligned arcs are intense, well detached from the auroral oval, contain plasma sheet origin ion precipitation as well as electrons, and persist for hours after a southward turning. These intense detached sun-aligned arcs can rapidly cross the polar cap, sometimes multiple times. Most events discussed in the literature as “theta-aurora” do not fit into this category (for example, although they may appear detached in images, they abut the oval in particle data, and do not have the persistence of detached events under southward IMF turnings). It is possible that no single theory can account for all three types of sun-aligned arcs.Solar energetic particle (SEP) events are at times used to demarcate polar cap open/closed boundaries. Although this works at times, examples exist where this method fails (e.g., very quiet conditions for which SEP reaches below L=4), and the method should be used with caution. Finally, it is shown that, although it is rare, the polar cap can at times completely close.