Interplanetary Magnetic Field Discontinuities Research Articles

Statistical Study of Hot Flow Anomaly Induced Ground Magnetic Ultra‐Low Frequency Oscillations

AbstractPc5 ULF waves play an important role in transporting energy and particles in the coupled magnetospheric and ionospheric system. They are known to be initiated by dynamic pressure fluctuations upstream of the magnetopause, including those induced by hot flow anomalies (HFAs). However, the role of HFAs in generating magnetospheric and ground magnetic Pc5 ULF oscillations has not been investigated statistically yet. Thus, in this paper, we investigate the contribution of HFAs to ground magnetic Pc5 ULF oscillations and analyze how the characteristics of HFAs influence these oscillations, based on the coordinated observations between the THEMIS probes and the ground magnetometers at high latitudes during the years 2008, 2009 and 2019. We find that HFAs can serve as a notable source of ground magnetic Pc5 ULF oscillations, with about 18.9% of Interplanetary Magnetic Field (IMF) discontinuity‐induced HFAs associated with discernible enhancements in Pc5 ULF wave power, whereas spontaneous HFAs play a comparatively minor role in generating these oscillations. Furthermore, we observe that the cores of HFAs are likely to contribute more significantly to modulating the induced ground magnetic Pc5 ULF oscillations than their compressed boundaries. More dynamic pressure reductions within HFA cores correspond to stronger ground magnetic Pc5 ULF oscillations. Additionally, HFAs can propagate with the IMF discontinuity along the bow shock, continuously generating ground magnetic Pc5 ULF oscillations during their propagation. This research sheds light on the mechanisms underlying Pc5 ULF wave generation and underscores the role of HFAs in driving magnetospheric‐ionospheric interactions.

Orientation of IMF Discontinuity Normals Across the Solar Cycles

AbstractTransient events like hot flow anomalies and foreshock bubbles are common in the Earth's foreshock. These foreshock transients may play an important role in the solar wind‐magnetosphere interaction. They typically occur when backstreaming ions accumulate at the intersection of interplanetary magnetic field (IMF) discontinuities with the bow shock. Discontinuity orientations play a key role in determining when and where transients form in the foreshock and strike the magnetopause. They also play a role in determining the amplitude, and significance, of individual transients. We investigate properties of the IMF discontinuity normals across two solar cycles. We compare Advanced Composition Explorer (ACE) IMF discontinuity observations for 2001, 2002, 2012, and 2014 (solar maximum) and 2008, 2009, 2019, and 2020 (solar minimum). We employ both the Minimum variance analysis (MVA) and Cross‐product methods to determine phase front normals. Most discontinuity normals point earthward and dawnward at longitudes ranging from 180° to 250°. A greater range of normal can be seen during solar minimum than during solar maximum. Normals lie further from the Sun‐Earth line when solar wind densities are low and velocities are high. This provides a natural explanation for the observed tendency of transients to occur for low solar wind densities and high velocities in terms of longer discontinuity interaction times with the bow shock.

Ion Acceleration by Foreshock Bubbles: Magnetospheric Multiscale Observations

AbstractForeshock bubbles (FBs) occur when interplanetary magnetic field discontinuities encounter the Earth's foreshock. These transient (∼1 to 5 min) features exhibit depressed densities and magnetic field strengths, enhanced temperatures, and deflected plasma flows trailed by a region of enhanced plasma density and magnetic field strength. Ions can be accelerated inside the FBs through the Fermi acceleration process. Hybrid simulations and test particle calculations predict that the maximum energies of ions accelerated by FBs reach 5.6 times the solar wind ram energy (Esw). We identify 23 FBs from September 2015 to January 2020 Magnetospheric Multiscale spacecraft observations. Most FBs (17 of 23) occurred upstream of the dusk‐side bow shock and above the ecliptic. The FBs occurred for Alfvn Mach numbers ranging from 5 to 15, with 11 FBs having an Alfvn Mach number near 10. To investigate ion energization inside the cores of the FBs we compare the proton spectra observed by the Hot Plasma Composition Analyzer and Energetic Ion Spectrometer before (upstream), during (core), and after (downstream) the FBs. The proton intensities at energies from Esw (the solar wind ram energy, ) up to about 5.6Esw are greater inside than outside 19 of 23 FBs, confirming that FBs can accelerate particles to these energies. The proton flux intensities at energies between Esw and 5.6Esw in the core of the FBs are consistent with results from global hybrid simulations for ion energization from FBs through second‐order Fermi acceleration.

Response of the Jovian Magnetosphere‐Ionosphere System to the Interplanetary Magnetic Field Discontinuity: A Simulation Study

AbstractThe response of the magnetosphere‐ionosphere (MI) system at Jupiter to the interplanetary magnetic field (IMF) tangential discontinuity is studied via global Magnetohydrodynamics (MHD) simulations. Our results show that the IMF discontinuity, which has a transition front perpendicular to the Sun‐Jupiter line, has strong effects on the Jovian MI system depending on local times and radial distances to Jupiter. The magnetospheric significant response to the discontinuity is delayed by ∼2–6 hr after the encounter of the discontinuity front with the bow shock, while the delay time for the ionospheric field‐aligned currents (FACs) is ∼3–8 hr. The outer magnetosphere, particularly at dayside, tends to respond slightly quicker than the inner regions. At the ionosphere, the response of downward FACs is found to occur earlier than that of the upward ones by a time ranging from less than 1 hr to several hours, which is also more prominent at dayside sectors. Using a new MI mapping method with Jupiter's JRM09 magnetic field model, we reproduce for the first time the ionospheric FACs consistent with the morphology of the observed ultraviolet (UV) main auroral emissions. This indicates a potential application of the simulated upward FACs on the morphology of the UV main auroral ovals.

On Differentiating Multiple Types of ULF Magnetospheric Waves in Response to Solar Wind Periodic Density Structures.

Identifying the nature and source of ultra‐low frequencies (ULF) waves (f ⪅ 4 mHz) at discrete frequencies in the Earth's magnetosphere is a complex task. The challenge comes from the simultaneous occurrence of externally and internally generated waves, and the ability to robustly identify such perturbations. Using a recently developed robust spectral analysis procedure, we study an interval that exhibited in magnetic field measurements at geosynchronous orbit and in‐ground magnetic observatories both internally supported and externally generated ULF waves. The event occurred on 9 November 2002 during the interaction of the magnetosphere with two interplanetary shocks that were followed by a train of 90 min solar wind periodic density structures. Using the Wang‐Sheeley‐Arge model, we mapped the source of this solar wind stream to an active region and a mid‐latitude coronal hole just prior to crossing the Heliospheric current sheet. In both the solar wind density and magnetospheric field fluctuations, we separated broad power increases from enhancements at specific frequencies. For the waves at discrete frequencies, we used the combination of satellite and ground magnetometer observations to identify differences in frequency, polarization, and observed magnetospheric locations. The magnetospheric response was characterized by: (a) forced breathing by periodic solar wind dynamic pressure variations below ≈1 mHz, (b) a combination of directly driven oscillations and wave modes triggered by additional mechanisms (e.g., shock and interplanetary magnetic field discontinuity impact, and substorm activity) between ≈1 and 4 mHz, and (c) largely triggered modes above ≈4 mHz.

Quiet Time Structured Pc1 Waves Generated During Transient Foreshock

AbstractWe study two events of structured Pc1 waves (pearls) observed by CARISMA magnetometers during a period of quiet magnetic (Kp ~1) and solar wind conditions on 1 August 2008. Bursts of proton precipitations and electromagnetic ion‐cyclotron waves were simultaneously observed by NOAA/POES and GOES satellites at dayside. Apparent correspondence was found between ground pearl pulsations and magnetospheric compressions observed by GOES and THEMIS satellites. We have found the Pc1 wave splitting into very close subfrequencies (within 0.1 Hz) that can be explained by weak magnetospheric compressions. The compressions are caused by pressure pulses originated from transient foreshock and interplanetary magnetic field discontinuities observed upstream of the bow shock by the THEMIS‐C probe. Thus, the observed pearls are compression‐related Pc1 waves generated under transient foreshock conditions.

Interaction of the Interplanetary Shock and IMF Directional Discontinuity in the Solar Wind

AbstractThe solar wind is not stationary and variations of the magnetic field magnitude and/or direction and plasma parameters are frequent. Interactions of fast forward shocks and interplanetary magnetic field discontinuities with the bow shock were analyzed in details theoretically as well as experimentally. However, an oblique interaction between a fast‐forward interplanetary shock and a directional interplanetary magnetic field discontinuity was studied only within the framework of the ideal magnetohydrodynamics. A fortunate position of six spacecraft on 22 June 2015 facilitates detailed observations of an interplanetary shock modification due to the interaction with the directional discontinuity in the solar wind. We present a multispacecraft analysis of the event supported with magnetohydrodynamic (MHD) modeling showing that this interaction can lead to a bifurcation of the velocity jump across the shock.

Spontaneous hot flow anomalies at Mars and Venus

AbstractWe report the first observations of Spontaneous Hot Flow Anomalies (SHFAs) at Venus and Mars, demonstrating their existence in the foreshocks of other planets beyond Earth. Using data from the ESA Venus Express and the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, we present magnetic and plasma observations from events at both planets, exhibiting properties similar to “classical” Hot Flow Anomalies, with bounding shock‐like compressive regions and a hot and diffuse core. However, these explosive foreshock transients were observed without any attendant interplanetary magnetic field discontinuity, consistent with SHFAs observed at Earth and our hybrid simulations.

A multispacecraft event study of Pc5 ultralow‐frequency waves in the magnetosphere and their external drivers

AbstractWe investigate a quiet time event of magnetospheric Pc5 ultralow‐frequency (ULF) waves and their likely external drivers using multiple spacecraft observations. Enhancements of electric and magnetic field perturbations in two narrow frequency bands, 1.5–2 mHz and 3.5–4 mHz, were observed over a large radial distance range from r ~ 5 to 11 RE. During the first half of this event, perturbations were mainly observed in the transverse components and only in the 3.5–4 mHz band. In comparison, enhancements were stronger during the second half in both transverse and compressional components and in both frequency bands. No indication of field line resonances was found for these magnetic field perturbations. Perturbations in these two bands were also observed in the magnetosheath, but not in the solar wind dynamic pressure perturbations. For the first interval, good correlations between the flow perturbations in the magnetosphere and magnetosheath and an indirect signature for Kelvin‐Helmholtz (K‐H) vortices suggest K‐H surface waves as the driver. For the second interval, good correlations are found between the magnetosheath dynamic pressure perturbations, magnetopause deformation, and magnetospheric waves, all in good correspondence to interplanetary magnetic field (IMF) discontinuities. The characteristics of these perturbations can be explained by being driven by foreshock perturbations resulting from these IMF discontinuities. This event shows that even during quiet periods, K‐H‐unstable magnetopause and ion foreshock perturbations can combine to create a highly dynamic magnetospheric ULF wave environment.

Localized reconnection in the magnetotail driven by lobe flow channels: Global MHD simulation

AbstractRecent ionospheric measurements suggest polar cap flow channels often trigger nightside auroral brightening. However, measurements were limited to the ionosphere, and it was not understood if such flow channels can exist in the lobe and can trigger magnetotail reconnection in a localized cross‐tail extent. We examined if localized flow channels can form self‐consistently in a global MHD regime, and if so, how such flow channels originate and relate to localized magnetotail reconnection. We show that lobe convection became nonuniform with azimuthally narrow flow channels (enhanced dawn‐dusk electric fields) of ~3 RE cross‐tail width. The flow channels propagated from the dayside toward the plasma sheet as an interplanetary magnetic field (IMF) discontinuity swept tailward. The plasma sheet around the lobe flow channels became thinner with a similar cross‐tail extent and then localized reconnection occurred. These results suggest that localized flow channels can propagate tailward across the lobe and drive localized magnetotail reconnection, that the cross‐tail width of reconnection and resulting plasma sheet flow channels and dipolarization fronts are related to the width of inflow from the lobe, and that IMF discontinuities drive lobe flow channels.

Spontaneous hot flow anomalies at quasi‐parallel shocks: 1. Observations

We present Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations of a “Spontaneous Hot Flow Anomaly” (SHFA) upstream from the prenoon bow shock at 0431 UT on 12 August 2007. Although the SHFA exhibited the greatly heated and deflected solar wind plasmas used to identify hot flow anomalies (HFAs), it did not result from the standard mechanism invoked for the formation of HFAs, namely the interaction of an interplanetary magnetic field (IMF) discontinuity with the bow shock. We employ THEMIS A, B, C, and D observations to describe the evolution of the event from a proto‐SHFA exhibiting regions of depressed magnetic field strength and density but little evidence for plasma heating or flow deflection, to a well‐developed SHFA further downstream. These observations show that SHFA can be generated without the presence of an IMF discontinuity and are therefore a new category of HFAs.

The first in situ observation of Kelvin‐Helmholtz waves at high‐latitude magnetopause during strongly dawnward interplanetary magnetic field conditions

We report the first in situ observation of high‐latitude magnetopause (near the northern duskward cusp) Kelvin‐Helmholtz waves (KHW) by Cluster on January 12, 2003, under strongly dawnward interplanetary magnetic field (IMF) conditions. The fluctuations unstable to Kelvin‐Helmholtz instability (KHI) are found to propagate mostly tailward, i.e., along the direction almost 90° to both the magnetosheath and geomagnetic fields, which lowers the threshold of the KHI. The magnetic configuration across the boundary layer near the northern duskward cusp region during dawnward IMF is similar to that in the low‐latitude boundary layer under northward IMF, in that (1) both magnetosheath and magnetospheric fields across the local boundary layer constitute the lowest magnetic shear and (2) the tailward propagation of the KHW is perpendicular to both fields. Approximately 3‐hour‐long periods of the KHW during dawnward IMF are followed by the rapid expansion of the dayside magnetosphere associated with the passage of an IMF discontinuity that characterizes an abrupt change in IMF cone angle,ϕ = acos , from ∼90° to ∼10°. Cluster, which was on its outbound trajectory, continued observing the boundary waves at the northern evening‐side magnetopause during sunward IMF conditions following the passage of the IMF discontinuity. By comparing the signatures of boundary fluctuations before and after the IMF discontinuity, we report that the frequencies of the most unstable KH modes increased after the discontinuity passed. This result demonstrates that differences in IMF orientations (especially inϕ) are associated with the properties of KHW at the high‐latitude magnetopause due to variations in thickness of the boundary layer, and/or width of the KH‐unstable band on the surface of the dayside magnetopause.

Hot flow anomalies at Venus

We present a multi‐instrument study of a hot flow anomaly (HFA) observed by the Venus Express spacecraft in the Venusian foreshock, on 22 March 2008, incorporating both Venus Express Magnetometer and Analyzer of Space Plasmas and Energetic Atoms (ASPERA) plasma observations. Centered on an interplanetary magnetic field discontinuity with inward convective motional electric fields on both sides, with a decreased core field strength, ion observations consistent with a flow deflection, and bounded by compressive heated edges, the properties of this event are consistent with those of HFAs observed at other planets within the solar system.

Time History of Events and Macroscale Interactions during Substorms observations of a series of hot flow anomaly events

A series of seven hot flow anomaly (HFA) events has been observed by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) C spacecraft just upstream from the subsolar bow shock from 0100 to 1300 UT on 19 August 2008. Both young (no shocks at edges, two distinct ion populations) and mature (strong shocks at edges, a single hot ion population) HFAs have been observed. Further upstream, THEMIS B observed four proto‐HFAs (density and magnetic field strength depletions, plasma heating but no flow deflections) which later developed into HFAs observed by THEMIS C. We present evidence indicating that electromagnetic right‐hand resonant ion beam instabilities heat ions inside HFAs. Observations of small‐amplitude perturbations (ΔB/B < 50%) consistent with the resonant ion beam instability in a proto‐HFA, 30 s electromagnetic waves (ΔB/B ∼ 1) in a young HFA, and magnetic pulsations in a mature HFA (ΔB/B ∼ 4) indicate that they are at early, middle, and late (nonlinear) stages of the electromagnetic right‐hand resonant ion beam instabilities. Both young and mature HFAs are associated with strong electromagnetic waves near the lower hybrid frequency (0.1–1 Hz). The lower hybrid waves are the likely source of the electron heating inside HFAs. THEMIS B observations of four proto‐HFAs which later developed into HFAs observed by THEMIS C indicate that these four HFAs might extend beyond 14 RE upstream from the bow shock, while the other three HFAs may extend between 5 and 14 RE upstream from the bow shock. We present an example of an HFA that lies displaced toward the side of the tangential discontinuity with a quasi‐parallel bow shock configuration rather than lying centered on the driving interplanetary magnetic field discontinuity.

Response of the magnetosphere‐ionosphere system to a sudden southward turning of interplanetary magnetic field

A sudden southward turning of interplanetary magnetic field (IMF) is simulated by the University of Michigan's BATS‐R‐US model. The main goal of this study is to determine the time delay as well as physical processes between when an IMF discontinuity reaches the bow shock, when reconnection is initiated at the magnetopause, and when the ionosphere starts to react. While observations or empirical models might give an estimate of the time delay for the propagation of the discontinuity from the bow shock to the magnetopause, the global MHD simulation provides a more comprehensive insight of responses of the magnetosphere‐ionosphere system. An idealized north‐to‐south IMF transition is modeled, using a solar wind velocity of 400 km/s. After the southward IMF encounters the bow shock, it takes about 6 min for the north‐to‐south IMF transition front to arrive at the subsolar geomagnetic field. The ionospheric response to this sudden southward IMF turning is delayed by another ∼4 min, during which the magnetosphere undergoes a conversion from cusp reconnection to subsolar reconnection and the Alfvén wave propagation to the ionosphere takes place. Thereafter, changes in the ionosphere and ground magnetic perturbations associated with the southward IMF are observed. These responses appear to be globally onset as described in many other studies. The time it takes from the encounter of the IMF transition with the bow shock to when the ionospheric reaction takes place varies with the solar wind speed, ranging from nearly 15 min for a solar wind speed of 300 km/s to just over 6 min for solar wind speeds of 600 km/s.

A study of particle flows in hot flow anomalies

Hot flow anomalies (HFAs) are distinct disturbances of the solar wind flow observed often in the bow shock vicinity. Both experiment and theory suggest that these disturbances are created at the intersection of the bow shock and tangential discontinuity (TD) of the interplanetary magnetic field (IMF). The discontinuity is swept along the bow shock by the solar wind flow and thus the resulting HFA containing hot, tenuous, and deflected plasma can be found not only in the subsolar region but also on the flanks. We have identified three HFAs in measurements of the INTERBALL-1 and MAGION-4 spacecraft. The first one was observed in the solar wind and two others created an interface between the solar wind and magnetosheath. We have analyzed the flow directions inside these HFAs in order to find a relationship between particle flows and orientations of original IMF discontinuities. Our results show that, beside the dominant plasma motion connected with the sweeping of the discontinuity, a significant velocity component directed along the discontinuity can be found. This suggests a highly elongated shape of the HFA cavity. The correlation length of particular HFA features (amplitude of bounding density enhancements, their duration, etc.) may be rather short; it is of the order of 1 R E in our case. We also suggest that HFA, which extends into the magnetosheath, modifies locally downstream the Mach number and causes a local displacement of the bow shock.

Steady state slow shock inside the Earth's magnetosheath: To be or not to be? 1. The original observations revisited

The original data that led to the slow mode transition (SMT) scenario for the diversion of the solar wind flow, in front of the dayside magnetoseath, are reexamined. It is shown that a number of SMT cases on the original list should be rejected because the corresponding solar wind data observed upstream from the magnetosheath display numerous density data gaps or have low time resolution inconsistent with accurate correlation study, or the SMT occurs between two successive crossings of the magnetopause. Therefore the SMT scenario is not established at a statistical level because of the limited number of well‐identified cases. Temporal variations of the interplanetary magnetic field (IMF) are shown to play a crucial role in the origin, duration and magnetic field topology of SMTs through the introduction of multiple time shifts in any correlations of magnetic field data obtained on two spacecraft whose distance in a plane perpendicular to the Sun‐Earth line is more than a few of tens Earth radius. These temporal IMF variations, correlated with solar wind density enhancements, induce a compressed magnetopause during SMT observations. The exogeneous nature of SMTs is established. Case studies reveal the different processes at the origin of the large enhancement of the density in SMTs with respect to the density upstream of the SMTs. These processes include: (1) an increase of the solar wind density, (2) variations in the density in the magnetosheath linked to increases of the Alfvén Mach number and to a new orientation of the IMF, and (3) a density gradient effect induced in the magnetosheath depletion layer by motion of the magnetopause. Other cases display proeminent density peaks linked to interplanetary magnetic field discontinuities imbedded in the SMTs. We also show that a previous analysis about stationarity of a SMT's front as well as of identification of slow modes propagating against the flow are not confirmed. The exogeneous scenario based on IMF and solar wind density variations for the origin and nature of the SMT phenomenon answers all the questions so far asked about this process.

Statistical identification of solar wind origins of magnetic impulse events

We have compiled a complete list of magnetic impulse events (MIEs) during the 8‐year period of 1995–2002 covering solar minimum to solar maximum using fluxgate magnetometer data obtained at the South Pole. Wavelet analysis enables us to detect 825 distinct MIEs automatically with high confidence. Interplanetary magnetic field (IMF) discontinuities are also detected automatically for the same period from Wind and ACE satellite data. From an examination and comparison of the two lists, we find that monthly and seasonal variations in IMF discontinuity occurrences have a significant correlation (0.4–0.5) with those of MIEs. We also find that MIEs tend to occur during intervals of low density, high‐speed solar wind streams, and/or radial IMF. No preferences for MIE occurrences are found for solar wind pressure jumps or IMF Bz southward turnings. From detailed minimum variance analysis of 36 IMF discontinuities with one‐to‐one correspondence to MIEs, ∼70% of the IMF discontinuities are found to be tangential discontinuities. The hot flow anomaly (HFA) mechanism can explain at most ∼50% of the MIEs; bursty reconnection and pressure pulses can explain the production of at most ∼30% and ∼20% of the MIEs, respectively. All of the observations and associations are consistent with HFAs or foreshock cavities being the main cause of MIEs.

Probing the magnetic polarity structure of the heliospheric current sheet

We use solar wind heat‐flux electrons to determine the solar magnetic polarities of the interplanetary magnetic field (IMF) close to the heliospheric current sheet (HCS) around the time of the last solar minimum in 1995–1996. At that time the tilt angle of the HCS was very low and solar activity was minimal, allowing the Wind spacecraft to probe the polarities of the fields close to the HCS during a time relatively free of transient interplanetary coronal mass ejections (ICMEs). During the three periods we examined, all solar polarity boundaries predicted from Stanford source surface (SS) maps were observed. During 34 days in which the SS magnetic neutral line skimmed within 3° of the ecliptic, only six tangential excursions into opposite solar polarities were observed. The distribution of the durations of magnetic polarity sectors was very similar to that reported earlier for solar maximum in 1978–1982, showing no increase in sectors which might be expected for a spacecraft trajectory roughly tangential to a corrugated HCS separating regions of opposite polarity. The heat‐flux electron pitch angle distributions, intervals of magnetic false polarities, high‐latitude field excursions, and orientations of minimum variance vectors at polarity boundaries were all similar to those observed away from the boundaries. About one third of the polarity boundaries were displaced from the large‐angle changes of the IMF direction usually thought to define the HCS. These observations suggest a single, globally smooth HCS in the form of a corrugated ribbon much less complex than and often separated from a thicker and more structured surrounding current sheet system formed by IMF discontinuities.

Magnetopause poleward of the cusp: Comparison of plasma and magnetic signature of the boundary for southward and northward directed interplanetary magnetic field

A coherent data set of high-latitude dayside magnetopause encounters by old (Heos 2, Hawkeye, Prognoz 7, 8) and new (Polar, Interball Tail, Cluster) spacecraft is needed to build a realistic model of the magnetopause (MP) including an indentation in the cusp. In building such a coherent data set a caution is necessary as the dayside magnetopause at high-latitudes may be less clearly defined than in the case of observations at low latitudes. It is due to expected presence of bundles of newly-reconnected magnetic field lines forming an extended boundary layer on the magnetosheath (MS) side of the magnetopause in the cusp region. Moreover, numerical magnetohydrodynamic (MHD) models of the solar wind-magnetosphere interaction predict that under northward interplanetary magnetic field (IMF) an additional thin current sheet should form inside the magnetopause at high latitudes on the dayside (e.g., Wu, 1983; Palmroth et al., 2001). Such a thin currect sheet is absent in empirical magnetosphere models. This internal current sheet, if a real one, may be mistaken for the magnetopause if magnetic field data are only taken into account and/or plasma data are unavailable. The Interball-Tail orbit allows for a full transition of magnetopause boundary layers at high-latitudes. We compare plasma and magnetic field signatures of the magnetopause poleward of the cusp for southward and northward IMF. The distance between the magnetic signature of the magnetopause (the current layer) and a cold and laminarly antisunward flowing MS plasma (so called free-flow MS) was found to be 0.5 to 1 R E , at least. These observations were made under nominal solar wind of v∼350 km/s and p dyn =1 to 4 nPa. We also observed several transient magnetic field reversals in the cusp related to pulses of solar wind dynamic pressure and/or the IMF discontinuity arrival. These transient reversals occurred at the same distance to the model MP as well defined full MP crossing, so most probably they represent just short encounters with the magnetopause current layer. Our analysis suggests that an indentation of the magnetopause with a subtle structure dependent on the local magnetic shear would explain and allow to predict the magnetic configuration in the high-altitude cusp.