High-latitude Dayside Research Articles

On the strength of E and F region irregularities for GNSS scintillation in the dayside polar ionosphere

We present results of the study conducted to quantify the relative contribution of different ionospheric regions to phase scintillation in Global Navigation Satellite Systems (GNSS) at the dayside high latitude ionosphere. By taking advantage of the scanning capability of the 32-m EISCAT radar in Svalbard (ESR) and its recurrent favourable location below the dayside auroral region, we developed a methodology to identify conjunctions between the radar and GNSS satellite signals in order to compare density irregularities identified by the radar with scintillation observed in GNSS signals. The analysis revealed that the dayside ionosphere contained irregularities predominantly in the F region with scintillation occurring 77% of the times. The likelihood of observing irregularities in the E region were comparatively less with a scintillation occurrence rate of 42%. The study therefore strongly suggests that the dayside F region is more structured than the E region and is the predominant source region for irregularities that cause scintillation at GNSS frequencies. The associated ionospheric conditions revealed enhanced F region electron and ion temperatures to be collocated with scintillation for majority of the times. This supports the fact that cusp/auroral dynamics play a crucial role in creating F region irregularities which can act as sources of scintillation in GNSS signals. The presented results provide a quantitative estimate of the effectiveness of irregularities and the associated ionospheric conditions in different regions of the dayside ionosphere during scintillation, which are relevant for high latitude modelling and instability studies as well as for space weather applications.

A unified model of cusp spot, High Latitude Dayside aurora (HiLDA)/(Space Hurricane), and 15MLT-PCA

Cusp spot, High Latitude Dayside aurora (HiLDA), ‘space hurricane’, and 15MLT-PCA (Polar Cap Arc observed around 15:00 Magnetic Local Time) are mesoscale auroral structures observed in the polar cap region. They share many common properties and, at the same time, have notable differences. A 15MLT-PCA is a polar cap arc connected to the auroral oval, but the others are auroral spots detached from the oval. A cusp spot differs from a HiLDA in local time location. A space hurricane differs from a HiLDA in having spiral-arm structures. Until now, relationships among these auroras have not been depicted clearly. Here we propose a unified model, based on lobe reconnection, that encompasses their similarities and differences. We then provide critical supporting evidence for the model. The model suggests that different reconnection sites under different IMF conditions result in these differently-appearing auroral forms. The model explains all the characteristic features of these auroras and has implications for understanding the dawn/dusk and inter-hemispheric asymmetries observed in their occurrence patterns. We anticipate confirmation of the model by observations to be made during the Solar wind-Magnetosphere-Ionosphere Link Explorer (SMILE) mission. Moreover, since the model indicates that these auroras appear on the footprint of the poleward cusp boundary, we expect data from the SMILE mission to make it possible to estimate the approximate shape of the cusp.

Transient Oscillations Near the Dayside Open‐Closed Boundary: Evidence of Magnetopause Surface Mode?

AbstractGeomagnetic pulsations in Pc5‐6 band (~3–20 min) are persistent feature of ULF activity at dayside high latitudes. Magnetopause surface eigenmodes may be suggested as potential mechanism of these pulsations. One might expect the ground response of these modes to be near ionospheric projection of the open‐closed field line boundary (OCB). Using data from instruments located at Svalbard we study transient geomagnetic response to impulsive “intrusion” of magnetosheath plasma into the dayside magnetosphere. These intrusions are triggered by modest changes of interplanetary magnetic field to southward, and observed as sudden shifts of equatorward red aurora boundary to lower latitudes and green line emission intensification. Each auroral disturbance is accompanied by burst of ~1.7–2.0‐mHz geomagnetic pulsations. Near‐cusp latitudinal structure of ULF pulsations is compared with instant location of equatorward boundary of the red aurora, assumed to be a proxy of the OCB. Optical OCB latitude has been identified using data from the meridian scanning photometer. The latitudinal maximum of the transient geomagnetic response tends to be located near disturbed OCB proxy, within the error ~1°–2° of the photometer and magnetometer methods. Recorded transient pulsations may be associated with the ground image of the magnetopause surface mode harmonic. Theoretical consideration indicates that after an initial excitation, surface large‐scale mode converts into localized Alfvén oscillations and thus can exist for limited time only. Therefore, MHD surface modes in realistic inhomogeneous plasma cannot be considered in isolation, but as a combined system of modes with discrete and continuous spectra with irreversible transformation between them.

The Magnetic Storm of August 25–26, 2018: Dayside High Latitude Geomagnetic Variations and Pulsations

The features of daytime high latitude geomagnetic disturbances and geomagnetic pulsations during the recent strong magnetic storm on August 25–26, 2018, which occurred at the end of the decline phase of the 24th solar activity cycle with a very low level of solar flare activity, are considered. As a rule, during this phase of the solar activity cycle, magnetic storms are caused by high-speed solar wind flows from coronal holes; however, the magnetic storms during the decay of the 24th cycle were caused by coronal mass ejections (CMEs). It was shown that, despite very weak disturbances on the Sun and a low solar wind speed, a rather strong magnetic storm occurred in the Earth’s magnetosphere in August 2018 (Dst = –171 nT). The storm SC with a slight (~25 nT) jump in the Dst index caused quite intense daytime geomagnetic pulsations ipcl at the latitudes of the possible position of the daytime polar cusp. A feature of the recovery phase of this storm was the development of a magnetosphere substorm on a global scale, i.e., the appearance of a negative magnetic bay recorded simultaneously in the auroral night sector and in the polar latitudes of the day sector. A possible interpretation is considered.

The correspondence between dayside long-period geomagnetic pulsations and the open-closed field line boundary

Long-period pulsations in the nominal Pc5-6 band (periods about 3–15 min) have been known to be a persistent feature of dayside high latitudes. A mixture of broadband Irregular Pulsations at Cusp Latitudes (IPCL) and narrowband P≿5 waves is often observed. The mechanism and origin of IPCL have not been firmly established yet. Magnetopause surface eigenmodes were suggested as a potential source of high-latitude ULF waves with frequencies less than 2 mHz. A ground response to these modes is expected to be beneath the ionospheric projection of the open-closed field line boundary (OCB). To unambiguously resolve a possible association of IPCL with the magnetopause surface modes, multi-instrument observation data from Svalbard have been analyzed. We examine the latitudinal structure of high-latitude pulsations in the Pc5-6 band recorded by magnetometers covering near-cusp latitudes. This structure is compared with an instant location of the equatorward boundary of the cusp aurora, assumed to be a proxy of the OCB. The optical OCB latitude has been identified by an automatic algorithm, using data from the meridian scanning photometer at Longyearbyen, Svalbard. The comparison has shown that the latitudinal maximum of the broadband IPCL maximizes about 2°–3° deeper in the magnetosphere than the OCB optical proxy. Therefore, these pulsations cannot be associated with the ground image of the magnetopause surface modes. It is likely that an essentially non-dipole geometry of field lines and a high variability of the magnetopause region may suppress the excitation efficiency. The obtained result imposes important limitations on possible mechanisms of high-latitude dayside ULF variations.

Atmospheric loss from the dayside open polar region and its dependence on geomagnetic activity: implications for atmospheric escape on evolutionary timescales

Abstract. We have investigated the total O+ escape rate from the dayside open polar region and its dependence on geomagnetic activity, specifically Kp. Two different escape routes of magnetospheric plasma into the solar wind, the plasma mantle, and the high-latitude dayside magnetosheath have been investigated separately. The flux of O+ in the plasma mantle is sufficiently fast to subsequently escape further down the magnetotail passing the neutral point, and it is nearly 3 times larger than that in the dayside magnetosheath. The contribution from the plasma mantle route is estimated as ∼ 3. 9 × 1024exp(0. 45 Kp) [s−1] with a 1 to 2 order of magnitude range for a given geomagnetic activity condition. The extrapolation of this result, including escape via the dayside magnetosheath, indicates an average O+ escape of 3 × 1026 s−1 for the most extreme geomagnetic storms. Assuming that the range is mainly caused by the solar EUV level, which was also larger in the past, the average O+ escape could have reached 1027–28 s−1 a few billion years ago. Integration over time suggests a total oxygen escape from ancient times until the present roughly equal to the atmospheric oxygen content today.

Geomagnetic and solar activity dependence of ionospheric upflowing O+: FAST observations

This paper investigates the dependence of the occurrence frequency of ionospheric upflowing oxygen (O+) ions on the sunspot cycle and geomagnetic activity. We examine the upflows response to the geomagnetic disturbances as well as the influence of the ion energy factor in controlling the magnitude of the occurrence frequency and the net energy flux. We discuss the spatial distribution of the upflow occurrence frequency and construct a regression model as a function of the magnetic latitude. The results show an overall enhancement of the upflow occurrence frequency during magnetically disturbed periods and indicate that the high-occurrence area spreads out from the source regions during magnetically quiet periods. The high-occurrence areas are located at 70° magnetic latitude (mLat) in the dayside auroral oval zone and between 76–80° mLat in the dayside polar cusp region. In the nightside auroral oval zone, these areas are near 60° mLat, penetrating further equatorward to 55° mLat during magnetically disturbed periods. High energy (≥1 keV) upflowing ions are common in the nightside auroral oval zone while low energy (<1 keV) upflowing ions are found escaping from the high latitude dayside cusp region. A Gaussian function is shown to be a good fit to the occurrence frequency over the magnetic latitude. For high energy upflowing O+ ions, the occurrence frequency exhibits a single peak located at about 60° mLat in the nightside auroral oval zone while for low energy upflowing O+ ions, it exhibits two peaks, one near 60° mLat in the auroral oval zone and the other near 78° mLat in the cusp region. We study the solar activity dependence by analyzing the relationship between the upflow occurrence frequency and the sunspot number (\(R_{Z}\)). The statistical result shows that the frequency decreases with declining solar activity level, from ∼30 % at solar maximum to ∼5 % at solar minimum. In addition, the correlation coefficient between the occurrence frequency and \(R_{Z}\) is 0.9.

Modeling the ionosphere-thermosphere response to a geomagnetic storm using physics-based magnetospheric energy input: OpenGGCM-CTIM results

The magnetosphere is a major source of energy for the Earth's ionosphere and thermosphere (IT) system. Current IT models drive the upper atmosphere using empirically calculated magnetospheric energy input. Thus, they do not sufficiently capture the storm-time dynamics, particularly at high latitudes. To improve the prediction capability of IT models, a physics-based magnetospheric input is necessary. Here, we use the Open Global General Circulation Model (OpenGGCM) coupled with the Coupled Thermosphere Ionosphere Model (CTIM). OpenGGCM calculates a three-dimensional global magnetosphere and a two-dimensional high-latitude ionosphere by solving resistive magnetohydrodynamic (MHD) equations with solar wind input. CTIM calculates a global thermosphere and a high-latitude ionosphere in three dimensions using realistic magnetospheric inputs from the OpenGGCM. We investigate whether the coupled model improves the storm-time IT responses by simulating a geomagnetic storm that is preceded by a strong solar wind pressure front on August 24, 2005. We compare the OpenGGCM-CTIM results with low-earth-orbit satellite observations and with the model results of Coupled Thermosphere-Ionosphere-Plasmasphere electrodynamics (CTIPe). CTIPe is an up-to-date version of CTIM that incorporates more IT dynamics such as a low-latitude ionosphere and a plasmasphere, but uses empirical magnetospheric input. OpenGGCMCTIM reproduces localized neutral density peaks at approx. 400 km altitude in the high-latitude dayside regions in agreement with in situ observations during the pressure shock and the early phase of the storm. Although CTIPe is in some sense a much superior model than CTIM, it misses these localized enhancements. Unlike the CTIPe empirical input models, OpenGGCM-CTIM more faithfully produces localized increases of both auroral precipitation and ionospheric electric fields near the high-latitude dayside region after the pressure shock and after the storm onset, which in turn effectively heats the thermosphere and causes the neutral density increase at 400 km altitude.

Multipoint observations of plasma phenomena made in space by Cluster

Plasmas are ubiquitous in nature, surround our local geospace environment, and permeate the universe. Plasma phenomena in space give rise to energetic particles, theaurora, solar flares and coronal mass ejections, as well as many energetic phenomena in interstellar space. Although plasmas can be studied in laboratory settings, it is often difficult, if not impossible, to replicate the conditions (density, temperature, magnetic and electric fields, etc.) of space. Single-point space missions too numerous to list have described many properties of near-Earth and heliospheric plasmas as measured bothin situand remotely (seehttp://www.nasa.gov/missions/#.U1mcVmeweRYfor a list of NASA-related missions). However, a full description of our plasma environment requires three-dimensional spatial measurements. Cluster is the first, and until data begin flowing from the Magnetospheric Multiscale Mission (MMS), the only mission designed to describe the three-dimensional spatial structure of plasma phenomena in geospace. In this paper, we concentrate on some of the many plasma phenomena that have been studied using data from Cluster. To date, there have been more than 2000 refereed papers published using Cluster data but in this paper we will, of necessity, refer to only a small fraction of the published work. We have focused on a few basic plasma phenomena, but, for example, have not dealt with most of the vast body of work describing dynamical phenomena in Earth's magnetosphere, including the dynamics of current sheets in Earth's magnetotail and the morphology of the dayside high latitude cusp. Several review articles and special publications are available that describe aspects of that research in detail and interested readers are referred to them (see for example, Escoubet et al. 2005Multiscale Coupling of Sun-Earth Processes, p. 459, Keith et al. 2005Sur. Geophys.26, 307–339, Paschmann et al. 2005Outer Magnetospheric Boundaries: Cluster Results, Space Sciences Series of ISSI. Berlin: Springer, Goldstein et al. 2006Adv. Space Res.38, 21–36, Taylor et al. 2010The Cluster Mission: Space Plasma in Three Dimensions, Springer, pp. 309–330 and Escoubet et al. 2013Ann. Geophys.31, 1045–1059).

A statistical study on O&amp;lt;sup&amp;gt;+&amp;lt;/sup&amp;gt; flux in the dayside magnetosheath

Abstract. Studies on terrestrial oxygen ion (O+) escape into the interplanetary space have considered a number of different escape paths. Recent observations however suggest a yet insufficiently investigated additional escape route for hot O+: along open magnetic field lines in the high altitude cusp and mantle. Here we present a statistical study on O+ flux in the high-latitude dayside magnetosheath. The O+ is generally seen relatively close to the magnetopause, consistent with observations of O+ flowing primarily tangentially to the magnetopause. We estimate the total escape flux in this region to be ~ 7 × 1024 s−1, implying this escape route to significantly contribute to the overall total O+ loss into interplanetary space.

Determining the mechanism of cusp proton aurora

Earth's cusp proton aurora occurs near the prenoon and is primarily produced by the precipitation of solar energetic (2–10 keV) protons. Cusp auroral precipitation provides a direct source of energy for the high-latitude dayside upper atmosphere, contributing to chemical composition change and global climate variability. Previous studies have indicated that magnetic reconnection allows solar energetic protons to cross the magnetopause and enter the cusp region, producing cusp auroral precipitation. However, energetic protons are easily trapped in the cusp region due to a minimum magnetic field existing there. Hence, the mechanism of cusp proton aurora has remained a significant challenge for tens of years. Based on the satellite data and calculations of diffusion equation, we demonstrate that EMIC waves can yield the trapped proton scattering that causes cusp proton aurora. This moves forward a step toward identifying the generation mechanism of cusp proton aurora.

Interball-1 observations of flux transfer events

Abstract. We present the results of a survey of 807 FTEs observed by Interball-1 on the mid- and high-latitude dayside and flank magnetopause. Dayside magnetosheath events show a strong tendency to occur for southward magnetosheath magnetic fields suggesting origin via component (or perhaps antiparallel) reconnection near the equatorial plane. Flank FTEs occur for both magnetosheath magnetic field orientations with only a slight preference for southward magnetosheath magnetic fields. These events are consistent with generation along an extended subsolar component reconnection line for all IMF orientations or a combination of reconnection along a subsolar component reconnection line for southward IMF and antiparallel reconnection at higher latitudes for northward IMF. The distribution of direct and reverse magnetosheath FTEs and the tilt angle of the reconnection line for dawnward and duskward IMF are in a good agreement with the theoretical predictions of the component merging model. The clear anticorrelation between the magnitude of the east/west (Bm) perturbation observed within magnetosphere FTEs versus Bm in the magnetosheath also demands an explanation of the FTEs in terms of reconnection along a tilted subsolar merging line, e.g. in terms of component merging.

High-latitude plasma convection during Northward IMF as derived from in-situ magnetospheric Cluster EDI measurements

Abstract. In this study, we investigate statistical, systematic variations of the high-latitude convection cell structure during northward IMF. Using 1-min-averages of Cluster/EDI electron drift observations above the Northern and Southern polar cap areas for six and a half years (February 2001 till July 2007), and mapping the spatially distributed measurements to a common reference plane at ionospheric level in a magnetic latitude/MLT grid, we obtained regular drift patterns according to the various IMF conditions. We focus on the particular conditions during northward IMF, where lobe cells at magnetic latitudes &gt;80° with opposite (sunward) convection over the central polar cap are a permanent feature in addition to the main convection cells at lower latitudes. They are due to reconnection processes at the magnetopause boundary poleward of the cusp regions. Mapped EDI data have a particular good coverage within the central part of the polar cap, so that these patterns and their dependence on various solar wind conditions are well verified in a statistical sense. On average, 4-cell convection pattern are shown as regular structures during periods of nearly northward IMF with the tendency of a small shift toward negative clock angles. The positions of these high-latitude convection foci are within 79° to 85° magnetic latitude and 09:00–15:00 MLT. The MLT positions are approximately symmetric ±2 h about 11:30 MLT, i.e. slightly offset from midday toward prenoon hours, while the maximum (minimum) potential of the high-latitude cells is at higher magnetic latitudes near their maximum potential difference at ≈−10° to −15° clock angle for the North (South) Hemisphere. With increasing clock angle distances from ≈IMFBz+, a gradual transition occurs from the 4-cell pattern via a 3-cell to the common 2-cell convection pattern, in the course of which one of the medium-scale high-latitude dayside cells diminishes and disappears while the other intensifies and merges with the opposite main cell of the same polarity to form the large "round-shaped" convection cell when approaching a well-known IMFBy-dominated configuration. Opposite scenarios with interchanged roles of the respective cells occur for the opposite turning of the clock angle and at the Southern Hemisphere. The high-latitude dayside cells become more pronounced with increasing magnitude of the IMF vector.

A Study of Orientation and Motion of Flux Transfer Events Observed at High-Latitude Dayside Magnetopause

This paper investigates 13 FTEs, which were detected by Cluster II in magnetosheath near the cusp region during February to March 2001. The orientation of the FTE tube axis is derived by virtue of minimum variance analysis (MVA) using magnetic field data from the four Cluster spacecraft. Of the 13 events, 6 axis directions are determined reliably, most of them are closer to L axis than to M axis in the boundary normal coordinate system LMN. This implies that there are some differences between them and those events observed at low-latitude, whose axes tend to lie in east-west directions. The DeHoffmann-Teller (HT) frame for 13 events are also obtained, and there are good HT frames for all FTEs, the correlation coefficients between the convection electric field and the HT electric field are larger than 0.96 except two of them are about 0.9. This means these events pass spacecraft as a moving quasi-static structure at V HT (velocity of the HT frame). The motion of FTE flux tubes in the direction perpendicular to its axis is not convected with the ambient magnetosheath plasma, the velocity of FTE tube may be slower or faster than the ambient plasma. The motion direction of FTE tube is about the same with the ambient plasma, but there may be a small angle between them. The plasma velocity in HT frame approximates to zero for ten FTEs, and is only about 14% of the local Alfven speed for the others, they are all not satisfied with theWalen relation. The slope of Walen plot is positive in northern hemisphere and negative in southern hemisphere, this implies that the plasma flows along the magnetic field into the magnetosphere (parallel magnetic field in the northern hemisphere and anti-parallel magnetic field in the southern hemisphere).

Flux pile-up and plasma depletion at the high latitude dayside magnetopause during southward interplanetary magnetic field: a cluster event study

Abstract. An event of strong flux pile-up and plasma depletion at the high latitude magnetopause tailward of the cusp has been analyzed based on observations by the suite of Cluster spacecraft. The multi-satellite analysis facilitates the separation of temporal and spatial features and provides a direct estimate for the strength of the plasma depletion layer for this event. A doubling of the magnetic field strength and a forty percent reduction of the density are found. Our analysis shows that roughly half of the total magnetic field increase occurs within 0.6 RE of the magnetopause and another quarter within a distance of 1.2 RE. In addition, the plasma depletion signatures exhibit temporal variations which we relate to magnetopause dynamics. Keywords. Magnetospheric physics (Magnetopause, Cusp and boundary layers; Magnetosheath; Solar windmagnetosphere interactions)

High‐latitude Joule heating response to IMF inputs

We evaluate the response of the high‐latitude Joule heating to orientation and magnitude of the interplanetary magnetic field (IMF). Approximately 9000 individual Joule heating patterns derived from data assimilation for the northern hemisphere were used to develop averaged and hemispherically integrated Joule power maps for the northern hemisphere north of 40° magnetic latitude. Hemispherically integrated Joule heating increases with IMF magnitude when the IMF is southward, but is relatively unchanged with increased IMF magnitude when the IMF is north. Only a small IMF By effect is evident in the Joule heating patterns. We quantify the IMF effects with linear fits of integrated Joule heating as a function of IMF magnitude for eight different IMF clock angle bins in the GSM Y‐Z plane. During extreme northward IMF conditions, Joule heating is restricted to the high latitude dayside. During extreme southward IMF conditions, Joule heating is located predominantly in the auroral region, with increased heating in the morning sector compared to the evening sector. This additional heating may lead to the increased incidence of gravity waves reported in the morning sector. Our estimate of the increase of hemispherically integrated Joule heating with IMF magnitude during southward IMF periods is 13GW/nT.

Seasonal dependence of localized, high‐latitude dayside aurora (HiLDA)

The FUV instrument on the IMAGE spacecraft frequently observes intense ultraviolet (UV) emissions from a localized High Latitude Dayside Aurora (HiLDA) poleward of the general auroral oval location [Frey et al., 2003a]. It has been shown that this aurora is entirely created by high‐energy precipitating electrons, which have probably been accelerated in a quasi‐static field‐aligned electric potential. Here we extend the previous case study to an investigation of the HiLDA occurrence in the Northern Hemisphere over more than 2 years and compare it with the averaged solar wind plasma and interplanetary magnetic field (IMF) properties. HiLDA occurrence is strongly biased toward low solar wind density and IMF with positive Bz, strong positive By (clock angles around 70°), and negative Bx components. Such IMF conditions are favorable for lobe reconnection in the Northern Hemisphere, and the creation of a dominating dusk convection cell with an upward field‐aligned current in its center. Additionally, we investigate the seasonal occurrence of this phenomenon, which shows a maximum in the Northern Hemisphere during sunlit summer months and an almost complete absence in the dark winter. In contrast to the daylight‐suppressed aurora in the auroral oval, the HiLDA cannot be the result of ionospheric feedback due to the stronger ionospheric conductance in sunlight. Instead, in agreement with ionospheric convection models, it is caused by the asymmetry of field‐aligned currents in different seasons, which result from the different dipole tilt angles in summer and winter. We further discuss two scenarios for how the low solar wind density can enhance the field‐aligned parallel potential over the dusk convection cell.

The use of iron charge state changes as a tracer for solar wind entry and energization within the magnetosphere

Abstract. The variation of the charge state of iron [Fe] ions is used to trace volume elements of plasma in the solar wind into the magnetosphere and to determine the time scales associated with the entry into and the action of the magnetospheric energization process working on these plasmas. On 2–3 May 1998 the Advanced Composition Explorer (ACE) spacecraft located at the L1 libration point observed a series of changes to the average charge state of the element Fe in the solar wind plasma reflecting variation in the coronal temperature of their original source. Over the period of these two days the average Fe charge state was observed to vary from + 15 to + 6 both at the Polar satellite in the high latitude dayside magnetosphere and at ACE. During a period of southward IMF the observations at Polar inside the magnetosphere of the same Fe charge state were simultaneous with those at ACE delayed by the measured convection speed of the solar wind to the subsolar magnetopause. Comparing the phase space density as a function of energy at both ACE and Polar has indicated that significant energization of the plasma occurred on very rapid time scales. Energization at constant phase space density by a factor of 5 to 10 was observed over a range of energy from a few keV to about 1 MeV. For a detector with a fixed energy threshold in the range from 10 keV to a few hundred keV this observed energization will appear as a factor of ~103 increase in its counting rate. Polar observations of very energetic O+ ions at the same time indicate that this energization process must be occurring in the high latitude cusp region inside the magnetosphere and that it is capable of energizing ionospheric ions at the same time.Key words. Magnetospheric physics (magnetopause, cusp, and boundary layers; magnetospheric configuration and dynamics; solar wind-magnetosphere interactions)

The distant cusp and the surrounding magnetopause: A view in snapshots from polar

In 1996 and 1997 during months when the orbit apogee was at XGSM⩾0 (May–October 1996 and April–August 1997), NASA's Polar spacecraft was skimming the high-latitude dayside magnetopause whenever the solar wind dynamic pressure (p dyn) was larger than ∼ 4 nPa. Magnetopause crossings occurred mostly under a northward and/or strongly dominating duskward-dawnward interplanetary magnetic field and took place at the poleward or dawn/dusk edge of the distant cusp. Polar data reveal that the indentation of cusp magnetopause may be deep, ∼2.5 R E below the axisymmetric surface of empirical magnetopause models. A significant IMF BY-dependence for probability of magnetopause encounter by Polar in a given local time sector suggests a formation of localized valleys extending away from the cusp proper along the merging lines on the magnetopause. Multipoint data in the distant cusp with the spacecraft separation on a ∼1–2 R E spatial scale could help to confirm this finding.

Coordinated ground-based, low altitude satellite and Cluster observations on global and local scales during a transient post-noon sector excursion of the magnetospheric cusp

Abstract. On 14 January 2001, the four Cluster spacecraft passed through the northern magnetospheric mantle in close conjunction to the EISCAT Svalbard Radar (ESR) and approached the post-noon dayside magnetopause over Green-land between 13:00 and 14:00 UT. During that interval, a sudden reorganisation of the high-latitude dayside convection pattern occurred after 13:20 UT, most likely caused by a direction change of the Solar wind magnetic field. The result was an eastward and poleward directed flow-channel, as monitored by the SuperDARN radar network and also by arrays of ground-based magnetometers in Canada, Greenland and Scandinavia. After an initial eastward and later poleward expansion of the flow-channel between 13:20 and 13:40 UT, the four Cluster spacecraft, and the field line footprints covered by the eastward looking scan cycle of the Söndre Strömfjord incoherent scatter radar were engulfed by cusp-like precipitation with transient magnetic and electric field signatures. In addition, the EISCAT Svalbard Radar detected strong transient effects of the convection reorganisation, a poleward moving precipitation, and a fast ion flow-channel in association with the auroral structures that suddenly formed to the west and north of the radar. From a detailed analysis of the coordinated Cluster and ground-based data, it was found that this extraordinary transient convection pattern, indeed, had moved the cusp precipitation from its former pre-noon position into the late post-noon sector, allowing for the first and quite unexpected encounter of the cusp by the Cluster spacecraft. Our findings illustrate the large amplitude of cusp dynamics even in response to moderate solar wind forcing. The global ground-based data proves to be an invaluable tool to monitor the dynamics and width of the affected magnetospheric regions.Key words. Magnetospheric cusp, ionosphere, reconnection, convection flow-channel, Cluster, ground-based observations