Polar Cap Boundary Research Articles

The origin of Ganymede's polar caps

Since their discovery in Voyager images, the origin of the bright polar caps of Ganymede has intrigued investigators. Some models attributed the polar cap formation to thermal migration of water vapor to higher latitudes, while other models implicated plasma bombardment in brightening ice. Only with the arrival of Galileo at Jupiter was it apparent that Ganymede possesses a strong internal magnetic field, which blocks most of the plasma from bombarding the satellite's equatorial region while funneling plasma onto the polar regions. This discovery provides a plausible explanation for the polar caps as related to differences in plasma-induced brightening in the polar and the equatorial regions. In this context, we analyze global color and high resolution images of Ganymede obtained by Galileo, finding a very close correspondence between the observed polar cap boundary and the open/closed field lines boundary obtained from new modeling of the magnetic field environment. This establishes a clear link between plasma bombardment and polar cap brightening. High resolution images show that bright polar terrain is segregated into bright and dark patches, suggesting sputter-induced redistribution and subsequent cold trapping of water molecules. Minor differences between the location of the open/closed field lines boundary and the observed polar cap boundary may be due to interaction of Ganymede with Jupiter's magnetosphere, and our neglect of higher-order terms in modeling Ganymede's internal field. We postulate that leading-trailing brightness differences in Ganymede's low-latitude surface are due to enhanced plasma flux onto the leading hemisphere, rather than darkening of the trailing hemisphere. In contrast to Ganymede, the entire surface of Europa is bombarded by jovian plasma, suggesting that sputter-induced redistribution of water molecules is a viable means of brightening that satellite's surface.

Review of Ionospheric Effects of Solar Wind Magnetosphere Coupling in the Context of the Expanding Contracting Polar Cap Boundary Model

This paper reviews the coupling between the solar wind, magnetosphere and ionosphere. The coupling between the solar wind and Earth’s magnetosphere is controlled by the orientation of the Interplanetary Magnetic Field (IMF). When the IMF has a southward component, the coupling is strongest and the ionospheric convection pattern that is generated is a simple twin cell pattern with anti-sunward flow across the polar cap and return, sunward flow at lower latitudes. When the IMF is northward, the ionospheric convection pattern is more complex, involving flow driven by reconnection between the IMF and the tail lobe field, which is sunward in the polar cap near noon. Typically four cells are found when the IMF is northward, and the convection pattern is also more contracted under these conditions. The presence of a strong Y (dawn-dusk) component to the IMF leads to asymmetries in the flow pattern. Reconnection, however, is typically transient in nature both at the dayside magnetopause and in the geomagnetic tail. The transient events at the dayside are referred to as flux transfer events (FTEs), while the substorm process illustrates the transient nature of reconnection in the tail. The transient nature of reconnection lead to the proposal of an alternative model for flow stimulation which is termed the expanding/contracting polar cap boundary model. In this model, the addition to, or removal from, the polar cap of magnetic flux stimulates flow as the polar cap boundary seeks to return to an equilibrium position. The resulting average patterns of flow are therefore a summation of the addition of open flux to the polar cap at the dayside and the removal of flux from the polar cap in the nightside. This paper reviews progress over the last decade in our understanding of ionospheric convection that is driven by transient reconnection such as FTEs as well as by reconnection in the tail during substorms in the context of a simple model of the variation of open magnetic flux. In this model, the polar cap expands when the reconnection rate is higher at the dayside magnetopause than in the tail and contracts when the opposite is the case. By measuring the size of the polar cap, the dynamics of the open flux in the tail can be followed on a large scale.

Association of Pi2 pulsations and pulsed reconnection: ground and Cluster observations in the tail lobe at 16 <i>R<sub>E</sub></i>

Abstract. Simultaneous measurements from the Cluster spacecraft and several ground stations (SAMNET, IMAGE, Kakioka, Hermanus) provide evidence for an association of Pi2 pulsations and pulsed reconnection in the magnetotail. On 8 September 2002, substorm-related Pi2 pulsations were recorded with the same waveform (same frequency) in the tail lobe at 16 RE and time-delayed on the ground (both nightside and dayside) spanning L values from 1.23 to 6.11. The tail lobe Pi2 pulsations were a series of nightside flux transfer event (NFTE) pulses propagating at a speed of 600–800 km/s towards Earth, which for the first time relates these two magnetospheric phenomena. NFTEs have previously been considered as the remote signature of tail reconnection. The first ground onset of the Pi2 pulsations occurred at high- and midlatitude ground stations with a time delay of ~30 s with respect to the tail lobe Pi2, followed by lower latitude ground stations. The largest pulsations were observed at high latitude (ten times larger than at low latitude) near the polar cap boundary. The polarization pattern of the ground Pi2s in the H-D plane was consistent with a periodically driven field-aligned current (FAC) system. In addition, fast mode waves must have also played a role in the inner magnetosphere because of propagation effects among ground stations and because of the simultaneous occurrence of dayside low-latitude Pi2. Auroral brightening occurred in the region of upflowing FAC, and the auroral electrojet expanded poleward together with the auroral bulge both of which are typical substorm signatures. Hence, we conclude that the substorm-related Pi2 pulsations in space at 16 RE and on the ground were remotely driven by pulsed reconnection in the magnetotail, that is, reconnection not only provided the energy but its temporal variations also determined the characteristic Pi2 frequency. Scenarios are discussed that address the connection of pulsed reconnection and the driven current system in the ionosphere. These results show that reconnection can be coupled to the ionosphere through what is phenomenologically known as Pi2 pulsations. As a corollary, it is shown that the time history of events fits within the modified NENL model of substorms.

Polar cap voltage saturation

Cross polar cap voltage saturation has been observed by various measurement techniques. An example of the saturation is shown in this paper and an explanation is proposed in terms of magnetospheric current behavior. The cross cap voltage saturation might be associated with either a voltage source or a current source effect since the cross cap potential and cross cap current are related by Ohm's law. The cross cap current is the difference between the Region 1 current influx to the polar cap boundary and the Region 2 current. Because, during high interplanetary magnetic field (IMF) −Bz conditions, the Region 2 current is significantly enhanced, the cross cap current, and hence cross cap voltage, is not anomalously higher than usual. Our measurements show agreement with Region 2 (partial ring current) enhancement during high IMF −Bz conditions.

Ionospheric signatures of the low-latitude boundary layer under conditions of northward IMF and small clock angle

Abstract. A case study is presented that concerns the footprints of the low-latitude boundary layer in the high-latitude ionosphere. The measurements were made near local magnetic noon in summertime under conditions of Bz>0 and small clock angle. Of particular interest are particle fluxes measured in the region by the NOAA-12 satellite that revealed energetic (>30 keV) electrons, characteristic of trapped particles, together with a population of softer precipitating magnetosheath particles. The particle energy-distribution was distinct from those identifying the central plasma sheet at lower latitudes. On its poleward side the layer extended to at least the latitude of the polar cap boundary as identified in ion flows and electron densities measured by the EISCAT Svalbard radar. It is proposed that the particles of the low-latitude boundary layer occurred on newly-closed magnetic field lines, which were formed by the closure of open polar cap field by lobe reconnection in both Northern and Southern Hemispheres.

Meso-scale aurora within the expansion phase bulge

Abstract. We present ground-based optical, riometer and magnetometer recordings together with Polar UVI and GOES magnetic field observations of a substorm that occurred over Canada on 24 November 1997. This event involved a clear optical onset followed by poleward motion of the aurora as a signature of an expanding auroral bulge. During the expansion phase, there were three distinct types of meso-scale (10–1000 km) auroral structures embedded in the bulge: at first a series of equatorward moving auroral arcs, followed by a well-defined spiral pair, and finally north-south directed aurora (a streamer). The spirals occurred several minutes after the onset, and indicate a shear in the field-aligned current. The north-south aligned aurora that formed about 10 min after the onset suggest bursty bulk flow type flows taking place in the central plasma sheet. Polar UVI observations of the polar cap location indicate that the southward drifting arcs were associated with magnetospheric activity within closed field lines, while the auroral streamer was launched by the bulge reaching the polar cap boundary, i.e. the mid-tail reconnection starting on the open field lines. The riometer data imply high energy electron precipitation in the vicinity of the the poleward moving edge of the auroral bulge, starting at the onset and continuing until the formation of the north-south structure. In this paper, we examine this evolving auroral morphology within the context of substorm theories.

Method to locate the polar cap boundary in the nightside ionosphere and application to a substorm event

Abstract. In this paper we describe a new method to be used for the polar cap boundary (PCB) determination in the nightside ionosphere by using the EISCAT Svalbard radar (ESR) field-aligned measurements by the 42-m antenna and southward directed low-elevation measurements by the ESR 32 m antenna or northward directed low-elevation measurements by the EISCAT VHF radar at Tromsø. The method is based on increased electron temperature (Te) caused by precipitating particles on closed field lines. Since the Svalbard field-aligned measurement provides the reference polar cap Te height profile, the method can be utilised only when the PCB is located between Svalbard and the mainland. Comparison with the Polar UVI images shows that the radar-based method is generally in agreement with the PAE (poleward auroral emission) boundary from Polar UVI. The new technique to map the polar cap boundary was applied to a substorm event on 6 November 2002. Simultaneous measurements by the MIRACLE magnetometers enabled us to put the PCB location in the framework of ionospheric electrojets. During the substorm growth phase, the polar cap expands and the region of the westward electrojet shifts gradually more apart from the PCB. The substorm onset takes place deep within the region of closed magnetic field region, separated by about 6–7° in latitude from the PCB in the ionosphere. We interpret the observations in the framework of the near-Earth neutral line (NENL) model of substorms. After the substorm onset, the reconnection at the NENL reaches within 3 min the open-closed field line boundary and then the PCB moves poleward together with the poleward boundary of the substorm current wedge. The poleward expansion occurs in the form of individual bursts, which are separated by 2–10 min, indicating that the reconnection in the magnetotail neutral line is impulsive. The poleward expansions of the PCB are followed by latitude dispersed intensifications in the westward electrojet with high latitudes affected first and lower latitudes later. We suggest that reconnection bursts energize plasma and produce enhanced flows toward the Earth. While drifting earthward, part of the plasma population precipitates to the ionosphere producing latitude-dispersed enhancements in the WEJ.

On the profile of intense high-altitude auroral electric fields at magnetospheric boundaries

Abstract. The profile of intense high-altitude electric fields on auroral field lines has been studied using Cluster data. A total of 41 events with mapped electric field magnitudes in the range between 0.5–1 V/m were examined, 27 of which were co-located with a plasma boundary, defined by gradients in particle flux, plasma density and plasma temperature. Monopolar electric field profiles were observed in 11 and bipolar electric field profiles in 16 of these boundary-associated electric field events. Of the monopolar fields, all but one occurred at the polar cap boundary in the late evening and midnight sectors, and the electric fields were typically directed equatorward, whereas the bipolar fields all occurred at plasma boundaries clearly within the plasma sheet. These results support the prediction by Marklund et al. (2004), that the electric field profile depends on whether plasma populations, able to support intense field-aligned currents and closure by Pedersen currents, exist on both sides, or one side only, of the boundary.

Monitoring the auroral electrojet from polar cap stations

A method for monitoring the auroral electrojet from measurements at polar cap stations was developed. Since in the polar cap the auroral electrojet produces the magnetic field in both horizontal and vertical components, the method is accounting for all three components of the magnetic field. We studied statistically the correlation between hourly mean values of auroral electrojet AL index and magnetic disturbances at polar cap boundary for each magnetic field component and found that the best correlation takes place for total magnetic field disturbance T. Then we calculated mean values of the total magnetic field disturbances for each UT hour at four stations distributed along the polar cap boundary. The obtained values we called the T index of geomagnetic activity. We found high correlation between hourly mean values of the T index and the auroral electrojet AL/AE indices (correlation coefficients are ∼0.9) for any time when at least one of the stations was located at the nightside. This index also shows good correlation with solar wind parameters. We suggest that the T index might be an important independent measure for deriving the auroral electrojet, which might be useful in the cases when ground‐based measurements in the auroral zone are impossible as it takes place in the Southern Hemisphere. Since the T index is derived from an independent magnetometer network, it might also give important additional information on geomagnetic activity in both hemispheres.

Ionospheric signatures of internal reconnection for northward interplanetary magnetic field: Observation of “reciprocal cells” and magnetosheath ion precipitation

Magnetic reconnection that involves overdraped lobe field lines is called internal reconnection since it occurs inside the magnetopause. When the interplanetary magnetic field (IMF) is due northward and the Earth's dipole is tilted significantly, internal reconnection occurs in the winter hemisphere, not only between a summer lobe field line and a winter lobe field line but also between a summer lobe field line and a closed field line. The latter internal reconnection drives “reciprocal cells” in the winter ionosphere that circulate exclusively in the closed field line region. The reciprocal cells are intimately related to the lobe cells in the summer ionosphere in that in the steady state, the reconnection voltage associated with merging of IMF and open field lines is equal to the sum of the lobe cell potential and the reciprocal cell potential. In this paper we present observations of convection patterns consistent with those expected for reciprocal cells, using ionospheric radar and low‐altitude satellite data. We also show the concurrence of lobe cells and reciprocal cells. The observations of reciprocal cells provide support for the internal reconnection between a summer lobe field line and a closed field line. In addition, we show that equatorward of the polar cap boundary, magnetosheath‐like ions are drifting from noon toward the flankside in both hemispheres. We suggest that these ions are of magnetosheath origin and that they entered the closed region of the magnetosphere through the rotational discontinuity associated with internal reconnection. These magnetosheath‐like ion observations strongly support the occurrence of internal reconnection.

IMF effect on sporadic-E layers at two northern polar cap sites: Part I – Statistical study

Abstract. In this paper we investigate the relationship between polar cap sporadic-E layers and the direction of the interplanetary magnetic field (IMF) using a 2-year database from Longyearbyen (75.2 CGM Lat, Svalbard) and Thule (85.4 CGM Lat, Greenland). It is found that the MLT distributions of sporadic-E occurrence are different at the two stations, but both are related to the IMF orientation. This relationship, however, changes from the centre of the polar cap to its border. Layers are more frequent during positive By at both stations. This effect is particularly strong in the central polar cap at Thule, where a weak effect associated with Bz is also observed, with positive Bz correlating with a higher occurrence of Es. Close to the polar cap boundary, at Longyearbyen, the By effect is weaker than at Thule. On the other hand, Bz plays there an equally important role as By, with negative Bz correlating with the Es occurrence. Since Es layers can be created by electric fields at high latitudes, a possible explanation for the observations is that the layers are produced by the polar cap electric field controlled by the IMF. Using electric field estimates calculated by means of the statistical APL convection model from IMF observations, we find that the diurnal distributions of sporadic-E occurrence can generally be explained in terms of the electric field mechanism. However, other factors must be considered to explain why more layers occur during positive than during negative By and why the Bz dependence of layer occurrence in the central polar cap is different from that at the polar cap boundary.

Observations of nightside magnetic reconnection during substorm growth and expansion phases

The temporal and spatial variability of nightside magnetic reconnection is described using two‐dimensional ionospheric measurements during the growth phase and early expansion phase of a single substorm. Two techniques (A and B) are used to address both the localized (across ∼15° longitude at ∼2300 magnetic local time (MLT)) and the large‐scale reconnection rate, using ionospheric data that provides the component of the electric field tangential to the polar cap boundary (PCB) in the stationary boundary frame. Technique A uses localized high‐resolution measurements derived from 630 nm all‐sky imager data at Rankin Inlet and ionospheric convection vectors obtained from line‐of‐sight velocity data from the Kapuskasing and Saskatoon Super Dual Auroral Radar Network (SuperDARN) HF radars. Technique B uses lower resolution global measurements, obtained by combining Polar VIS imager data and a velocity field derived using SuperDARN global convection mapping. A third technique (C) estimates the nightside reconnection potential from the dayside reconnection potential and the variation in the polar cap area. Technique A reveals standing wave‐like variation of period ∼16 min in the reconnection rate in the late growth phase (spatial noncontinuity in the x‐line). The localized measurements of the reconnection rate vary between 0 and 50 mV/m during both the growth and expansion phases. Technique B shows the expansion of the x‐line toward the duskside during the first 15 min of the expansion phase, from a width of ∼4 hours of MLT to ∼7 hours MLT.

Experimental investigation of auroral generator regions with conjugate Cluster and FAST data

Abstract. Here and in the companion paper, Hamrin et al. (2006), we present experimental evidence for the crossing of auroral generator regions, based on conjugate Cluster and FAST data. To our knowledge, this is the first investigation that concentrates on the evaluation of the power density, E·J, in auroral generator regions, by using in-situ measurements. The Cluster data we discuss were collected within the Plasma Sheet Boundary Layer (PSBL), during a quiet magnetospheric interval, as judged from the geophysical indices, and several minutes before the onset of a small substorm, as indicated by the FAST data. Even at quiet times, the PSBL is an active location: electric fields are associated with plasma motion, caused by the dynamics of the plasma-sheet/lobe interface, while electrical currents are induced by pressure gradients. In the example we show, these ingredients do indeed sustain the conversion of mechanical energy into electromagnetic energy, as proved by the negative power density, E·J<0. The plasma characteristics in the vicinity of the generator regions indicate a complicated 3-D wavy structure of the plasma sheet boundary. Consistent with this structure, we suggest that at least part of the generated electromagnetic energy is carried away by Alfvén waves, to be dissipated in the ionosphere, near the polar cap boundary. Such a scenario is supported by the FAST data, which show energetic electron precipitation conjugated with the generator regions crossed by Cluster. A careful examination of the conjunction timing contributes to the validation of the generator signatures.

Motion of the dayside polar cap boundary during substorm cycles: II. Generation of poleward-moving events and polar cap patches by pulses in the magnetopause reconnection rate

Abstract. Using data from the EISCAT (European Incoherent Scatter) VHF and CUTLASS (Co-operative UK Twin-Located Auroral Sounding System) HF radars, we study the formation of ionospheric polar cap patches and their relationship to the magnetopause reconnection pulses identified in the companion paper by Lockwood et al. (2005). It is shown that the poleward-moving, high-concentration plasma patches observed in the ionosphere by EISCAT on 23 November 1999, as reported by Davies et al. (2002), were often associated with corresponding reconnection rate pulses. However, not all such pulses generated a patch and only within a limited MLT range (11:00-12:00 MLT) did a patch result from a reconnection pulse. Three proposed mechanisms for the production of patches, and of the concentration minima that separate them, are analysed and evaluated: (1) concentration enhancement within the patches by cusp/cleft precipitation; (2) plasma depletion in the minima between the patches by fast plasma flows; and (3) intermittent injection of photoionisation-enhanced plasma into the polar cap. We devise a test to distinguish between the effects of these mechanisms. Some of the events repeat too frequently to apply the test. Others have sufficiently long repeat periods and mechanism (3) is shown to be the only explanation of three of the longer-lived patches seen on this day. However, effect (2) also appears to contribute to some events. We conclude that plasma concentration gradients on the edges of the larger patches arise mainly from local time variations in the subauroral plasma, via the mechanism proposed by Lockwood et al. (2000).

Motion of the dayside polar cap boundary during substorm cycles: I. Observations of pulses in the magnetopause reconnection rate

Abstract. Using data from the EISCAT (European Incoherent Scatter) VHF radar and DMSP (Defense Meteorological Satellite Program) spacecraft passes, we study the motion of the dayside open-closed field line boundary during two substorm cycles. The satellite data show that the motions of ion and electron temperature boundaries in EISCAT data, as reported by Moen et al. (2004), are not localised around the radar; rather, they reflect motions of the open-closed field line boundary at all MLT throughout the dayside auroral ionosphere. The boundary is shown to erode equatorward when the IMF points southward, consistent with the effect of magnetopause reconnection. During the substorm expansion and recovery phases, the dayside boundary returns poleward, whether the IMF points northward or southward. However, the poleward retreat was much faster during the substorm for which the IMF had returned to northward than for the substorm for which the IMF remained southward - even though the former substorm is much the weaker of the two. These poleward retreats are consistent with the destruction of open flux at the tail current sheet. Application of a new analysis of the peak ion energies at the equatorward edge of the cleft/cusp/mantle dispersion seen by the DMSP satellites identifies the dayside reconnection merging gap to extend in MLT from about 9.5 to 15.5 h for most of the interval. Analysis of the boundary motion, and of the convection velocities seen near the boundary by EISCAT, allows calculation of the reconnection rate (mapped down to the ionosphere) from the flow component normal to the boundary in its own rest frame. This reconnection rate is not, in general, significantly different from zero before 06:45 UT (MLT<9.5 h) - indicating that the X line footprint expands over the EISCAT field-of-view to earlier MLT only occasionally and briefly. Between 06:45 UT and 12:45 UT (9.5<MLT<15.5 h) reconnection is continuously observed by EISCAT, confirming the (large) MLT extent of the reconnection footprint deduced from the DMSP passes. As well as direct control by the IMF on longer timescales, the derived reconnection rate variation shows considerable pulsing on timescales of 2-20 min during periods of steady southward IMF.

Ring current and the magnetosphere‐ionosphere coupling during the superstorm of 20 November 2003

We investigated the impact on the terrestrial ring current of a coronal mass ejection (CME) and the associated magnetic cloud that severely disturbed the Earth's magnetosphere on 20 November 2003. This CME decreased the Dst index to −472 nT, which makes it the second largest storm, based on the minimum Dst index values, observed between 1957 and 2004. Data from the DMSP, NOAA, and LANL satellites showed the unique characteristics of this storm; a polar cap potential that increased to at least 200 kV, a polar cap boundary that moved as low as about 60° MLAT, a plasma sheet density that increased to 5 cm−3 at L = 6.6 when the Dst index was near its minimum, and the inner edge of the plasma sheet ion population that penetrated into a region for which L ≤ 1.5. In order to study the dynamics of the ring current and the associated magnetosphere‐ionosphere coupling, we performed a ring current simulation that computed the evolution of the phase space density of the ring current ions and the closure of the electric current between the magnetosphere and the ionosphere. Major results were as follows: (1) The ring current, in terms of the Dst index and the inner edge of the plasma sheet, can result from the enhancement of the convection electric field, given the polar cap potentials used in the model; (2) The solar wind particles probably penetrated quickly into the geosynchronous altitude on the nightside with a lag of about 80 min, resulting in further enhancement of the ring current; (3) Dense geocoronal neutral hydrogen or a large coefficient of pitch angle diffusion (>10−4 s−1) is probably needed to account for the rapid motion of the inner edge of the plasma sheet (or the ring current) population to a higher L value; (4) Both the simulated and observed field‐aligned current (FAC) distributions show multiple current sheets, rather than the normally expected two current sheets. Fluctuations in the polar cap potential and the plasma sheet density are believed to cause the multiple sheets of field‐aligned currents; (5) The equatorward edge of the Region 2 type field‐aligned currents was observed to expand as low as 40° MLAT, which is consistent with the simulation; and (6) The convection pattern can be much more complicated than an average one due to a strong Region 2 FAC. A noticeable feature was the reversal of the zonal ionospheric plasma flow that emerged on the dawnside. In particular, a westward flow was observed in the equatorial region of the eastward plasma flow at dawn. Its speed had a local maximum of about 5° equatorward of the flow reversal. The flow reversal is thought to have resulted from the relatively strong shielding electric field.

Pc5 modulation of high energy electron precipitation: particle interaction regions and scattering efficiency

Abstract. Using the NORSTAR riometer and CANOPUS magnetometer arrays we have investigated the modulation of high energy electron precipitation by ULF waves in the Pc5 frequency band. We conducted two separate studies of Pc5 activity in the riometers. The first is an independent survey of three riometer stations in the Churchill line (one at each sub-auroral, auroral, and typical polar cap boundary latitudes) in which we identified all riometer Pc5-band pulsations over 11 years. All had a corresponding magnetometer pulsation implying that a magnetic pulsation, is a necessary condition for a riometer pulsation (in the Pc5 Band). We find seasonal and latitude dependencies in the occurrence of riometer pulsations. By a factor of two, there are more riometer pulsations occurring in the fall-winter than the spring-summer. At higher latitudes there is a tendency towards noon pulsations during the spring-summer, suggesting that the criteria for riometer pulsations is affected by the dipole tilt. Our second study was based on the previous magnetometer study of Baker et al. (2003). Using the database of Pc5 activity from that study we were able to select the riometer Pc5 pulsations which adhere to the strict Pc5 definition in the magnetometer. We find that roughly 95% of the riometer pulsations occurred in the morning sector compared to 70% in the magnetometer. Given a magnetometer pulsation at Gillam in the morning sector, there is a 70% chance of there being a corresponding riometer pulsation. The morning sector probabilities at Rankin (geomagnetic (PACE) latitude 74°) and Pinawa (61°) are 3% and 5%, respectively. These statistics suggest there is a localized region in the pre-noon magnetosphere where Pc5 band ULF activity can modulate high energy electron precipitation. We also find that riometer pulsations display a Kp selection towards mid (i.e. 3–4) activity levels which mimics the product of the Kp dependence of high-energy electron fluxes on the dawn side (from CRRES) and all magnetic Pc5 activity. A superposed epoch analysis revealed that the elevated electron flux needed to produce a riometer pulsation is most likely provided by substorm injections on the nightside. We also find that the amplitude of modulated precipitation correlates well with the product of the background absorption and the magnetic pulsation amplitude, again leading to the idea that a riometer pulsation needs both favorable magnetospheric electron flux conditions and large enough magnetic Pc5 wave activity. We further separate our pulsations into field line resonances (FLRs), and non-field line resonances (non-FLRs), as identified in the Baker et al. (2003) survey. We find that FLRs are more efficient at modulating particle precipitation, and non-FLRs display an amplitude cutoff below which they do not interact with the high energy electron population. We conclude that the high energy electron precipitation associated with Pc5 pulsations is caused by pitch angle scattering (diffusion) rather than parallel acceleration. We suggest two future studies that are natural extensions of this one. Keywords. Energetic Particles/Precipitating; Wave-Particle Interactions; Auroral Phenomena

Characteristics of quasi-static potential structures observed in the auroral return current region by Cluster

Abstract. Temporal and spatial characteristics of intense quasi-static electric fields and associated electric potential structures in the return current region are discussed using Cluster observations at geocentric distances of about 5 Earth radii. Results are presented from four Cluster encounters with such acceleration structures to illustrate common as well as different features of such structures. The electric field structures are characterized by (all values are projected to 100 km altitude) peak amplitudes of ≈1V/m, bipolar or unipolar profiles, perpendicular scale sizes of ≈10km, occurrence at auroral plasma boundaries associated with plasma density gradients, downward field-aligned currents of ≈10µA/m2, and upward electron beams with characteristic energies of a few hundred eV to a fewkeV. Two events illustrate the temporal evolution of bipolar, diverging electric field structures, indicative of positive U-shaped potentials increasing in magnitude from less than 1kV to a few kV on a few 100s time scale. This is also the typical formation time for ionospheric plasma cavities, which are connected to the potential structure and suggested to evolve hand-in-hand with these. In one of these events an energy decay of inverted-V ions was observed in the upward field-aligned current region prior to the acceleration potential increase in the adjacent downward current region, possibly suggesting that a potential redistribution took place between the two current branches. The other two events were characterized by intense unipolar electric fields, indicative of S-shaped potential contours and were encountered at the polar cap boundary. The total observation time for these events was typically 10-20s, too short for monitoring the evolution of the structure, but yet of interest for revealing their short term stability. The locations of the two bipolar events at the poleward boundary of the central plasma sheet and of the two unipolar events at the polar cap boundary, suggest that the special profile shape depends on whether plasma populations, dense enough to support upward field-aligned currents and closure of the return current, exist on both sides, or on one side only, of the boundary.

Detailed dayside auroral morphology as a function of local time for southeast IMF orientation: implications for solar wind-magnetosphere coupling

Abstract. In two case studies we elaborate on spatial and temporal structures of the dayside aurora within 08:00-16:00 magnetic local time (MLT) and discuss the relationship of this structure to solar wind-magnetosphere interconnection topology and the different stages of evolution of open field lines in the Dungey convection cycle. The detailed 2-D auroral morphology is obtained from continuous ground observations at Ny Ålesund (76° magnetic latitude (MLAT)), Svalbard during two days when the interplanetary magnetic field (IMF) is directed southeast (By>0; Bz<0). The auroral activity consists of the successive activations of the following forms: (i) latitudinally separated, sunward moving, arcs/bands of dayside boundary plasma sheet (BPS) origin, in the prenoon (08:00-11:00 MLT) and postnoon (12:00-16:00 MLT) sectors, within 70-75° MLAT, (ii) poleward moving auroral forms (PMAFs) emanating from the pre- and postnoon brightening events, and (iii) a specific activity appearing in the 07:00-10:00 MLT/75-80° MLAT during the prevailing IMF By>0 conditions. The pre- and postnoon activations are separated by a region of strongly attenuated auroral activity/intensity within the 11:00-12:00 MLT sector, often referred to as the midday gap aurora. The latter aurora is attributed to the presence of component reconnection at the subsolar magnetopause where the stagnant magnetosheath flow lead to field-aligned currents (FACs) which are of only moderate intensity. The much more active and intense aurorae in the prenoon (07:00-11:00 MLT) and postnoon (12:00-16:00 MLT) sectors originate in magnetopause reconnection events that are initiated well away from the subsolar point. The high-latitude auroral activity in the prenoon sector (feature iii) is found to be accompanied by a convection channel at the polar cap boundary. The associated ground magnetic deflection (DPY) is a Svalgaard-Mansurov effect. The convection channel is attributed to effective momentum transfer from the solar wind-magnetosphere dynamo in the high-latitude boundary layer (HBL), on the downstream side of the cusp.

The location of the open-closed magnetic field line boundary in the dawn sector auroral ionosphere

Abstract. As a measure of the degree of coupling between the solar wind-magnetosphere-ionosphere systems, the rate at which the size of the polar cap (the region corresponding to ionospheric termini of open magnetic flux tubes) varies is of prime importance. However, a reliable technique by which the extent of the polar cap might be routinely monitored has yet to be developed. Current techniques provide particularly ambiguous indications of the polar cap boundary in the dawn sector. We present a case study of space- and ground-based observations of the dawn-sector auroral zone and attempt to determine the location of the polar cap boundary using multi-wavelength observations of the ultraviolet aurora (made by the IMAGE FUV imager), precipitating particle measurements (recorded by the FAST, DMSP, and Cluster 1 and 3 satellites), and SuperDARN HF radar observations of the ionospheric Doppler spectral width boundary. We conclude that in the dawn sector, during the interval presented, neither the poleward edge of the wideband auroral UV emission (140-180nm) nor the Doppler spectral width boundary were trustworthy indicators of the polar cap boundary location, while narrow band UV emissions in the range 130-140nm appear to be much more reliable.