Magnetopause Boundary Layer Research Articles

Quasi-periodic emissions (15–[formula omitted]) from the poles of Jupiter as a principal source of the large-scale high-latitude magnetopause boundary layer of energetic particle

In this study, we concentrate on the examination of the quasi-periodic behavior of 0.5–1.6MeV(∼40–400keV) protons (electrons) and of relativistic (>16MeV) electrons, observed at Jupiter by two different experiments onboard Ulysses, the HI-SCALE and COSPIN, respectively, within the large-scale south duskside magnetopause boundary layer (MPBL) of energetic particles, from ∼20:00UT, day 41, to 12:00UT, day 43. During those times, the observations confirm the transition of Ulysses to an important magnetospheric region with particle intensities comparable to the magnetodisk levels. A careful analysis of high time-resolution intensity, anisotropy and spectral measurements suggests the following: (1) The quasi-periodic ∼40min (QP-40) energetic electron (>∼40keV) and proton (>∼0.5MeV) emissions from the south pole were a characteristic phenomenon in the south duskside MPBL; an almost continuous QP 40-min variation has been confirmed in energetic electron observations. (2) When the QP-40 modulation was not evident in energetic particle intensities, it was still detectable in spectral and anisotropy observations. (3) The QP-40 emission is a principal, but not the only periodic, energetic electron and proton emission; other periodicities (∼15,∼20,∼30,∼50,∼60,∼80min) were also significant in energetic particle spectral and anisotropy observations. We infer that QP-40 emissions along with other periodic emissions (∼15,∼20,∼30,∼50,∼60,∼80min) were the principal source of the large-scale high-latitude MPBL of energetic particles during Ulysses outbound pass of Jupiter.

On a systematic spectral variation of energetic ions in the Jovian outer magnetosphere: HI-SCALE/Ulysses observations

An important finding of Ulysses flyby of Jupiter is the discovery of a large-scale energetic particle layer adjacent to the magnetopause. Previous studies discussed the particle flux and the anisotropy characteristics of this layer. This study examines the spectral characteristics of the large-scale magnetopause boundary layer of energetic particles. Examination of Ulysses’ observations in the Jovian outer magnetosphere reveals the following picture of energetic ions: (1) The ion flux increases and the spectrum hardens in the direction from the magnetosheath/magnetopause toward the outer magnetosphere. (2) At middle latitudes, the energy spectrum is continuous and is well described by a power law (dj/dE=kE−γ), with a spectral index γ ranging between ∼2.0 and ∼2.5 over the whole energy range (∼60 to ∼4000keV). (3) Close to the magnetopause the spectrum is described by a power law, but with a different slope at low (<∼400keV) and higher (>∼400keV) energies. (4) The ion spectrum near the magnetopause is in general softer than far from the magnetopause (γ≅∼2.6 to ∼3.4 at energies >∼400keV). (5) The spectral shape varies with a period of ∼5 or 10h. The observations are explained in terms of a periodic motion of the s/c within the large-scale boundary layer of energetic ions in the outer magnetosphere. This boundary was more evident in the duskside south magnetosphere and was found to extend in radial distances from ∼49Rj to 83Rj in this area.

Motion of the dusk flank boundary layer caused by solar wind pressure changes and the Kelvin‐Helmholtz instability: 10–11 January 1997

During an interval of steady northward IMF on 10–11 January 1997, Geotail and Interball spacecraft data at X = −5 to −15 RE on the dusk flank reveal magnetopause boundary layer motions caused by both solar wind pressure discontinuities and the Kelvin‐Helmholtz instability. Several large, sudden changes in solar wind density caused the magnetopause to move across both spacecraft. Relative timing of these crossings along with corresponding geosynchronous and ground magnetic field changes allow study of the propagation of pressure fronts in the magnetosheath and the associated wave propagation in the magnetosphere. The wave in the boundary layer propagates down the tail faster than the pressure front in the magnetosheath, producing boundary motion ahead of the magnetosheath front. This boundary layer wave can bulge out the magnetopause, causing a spacecraft in the magnetosheath to become immersed in the magnetosphere for as long as 2 or 3 min before the higher density magnetosheath plasma arrives and compresses the boundary. The tailward convecting magnetosheath pressure front moves more slowly near the magnetopause than it does further out in the magnetosheath, distorting the front and enhancing the likelihood that the boundary layer wave will outrun the magnetosheath front. Integrating the velocity perpendicular to the boundary shows that the boundary waves can have amplitudes of at least 1 or 2 Re. During periods of steady solar wind pressure, waves with periods of several minutes are observed in a boundary layer that are consistent with exitation of the Kelvin‐Helmholtz instability. Magnetograms from five ground magnetometer chains show both pulsations initiated by the pressure discontinuities and ongoing wave trains probably related to the KH instability.

Some theoretical models for solitary structures of boundary layer waves

Abstract. Solitary electrostatic structures have been observed in and around auroral zone field lines at various altitudes ranging from close to the ionosphere, to the magnetopause low-latitude boundary layer (LLBL) and cusp on the dayside, and to the plasma sheet boundary layer on the night-side. In this review, various models based on solitons/double layers and BGK modes or phase space holes are discussed in order to explain the characteristics of these solitary structures.

Slow magnetosonic solitons detected by the cluster spacecraft.

Experimental evidence is provided for the existence of slow-mode magnetosonic solitons in the col-lisionless plasma at the magnetopause boundary layer. The solitons were detected by the fleet of Cluster spacecraft at the dusk flank of the magnetosphere as magnetic field depressions (up to 85%) accom-panied with enhancement of the plasma density and temperature by a factor of 2. The solitons propagate 250 km/s with respect to the satellites and have perpendicular size of 1000-2000 km, which is a few ion inertial scale lengths. The comparison with numerical solutions of a theoretical model shows quantitative agreement between the model and observations.

Energetic ions, large diamagnetic cavities, and Chapman‐Ferraro cusp

Extremely large diamagnetic cavities with a size of as large as 6 RE have been observed in the dayside high‐altitude cusp regions. These diamagnetic cavities were associated with strong magnetic field turbulence. Associated with these cavities are &gt;40 keV ions that are more typical of the trapped ring current and radiation belt populations than the solar wind. The charge state distribution of these cusp cavity ions was indicative of their seed populations being a mixture of ionospheric and solar wind particles. In April 1999, the cusp diamagnetic cavities were observed by the Polar spacecraft almost in every orbit, indicating that such cavities are always there day by day. Some of the diamagnetic cavities were independent of the interplanetary magnetic field directions, suggesting that the cusp diamagnetic cavities are different from the magnetospheric sash predicted by MHD simulations. During a high solar wind pressure period on 21 April 1999, the Polar spacecraft observed lower energetic (&gt;20 keV/e) ion fluxes in the dayside high‐latitude magnetosheath than that in the neighboring cusp cavities. By their geometry cusp magnetic field lines are connected to all of the magnetopause boundary layers. These energetic particles in the cusp diamagnetic cavity together with the cusp's connectivity probably have significant global impacts on the geospace environment.

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.

Lower hybrid drift wave turbulence and associated electron transport coefficients and coherent structures at the magnetopause boundary layer

It is well known that at the Earth's magnetopause there is much scale size turbulence whose origin is not yet fully understood. Our objective here is to present a detailed investigation of short‐wavelength (in comparison with the ion gyroradius) lower hybrid drift (LHD) wave turbulence that is generated by sheared electron flows and ion beams. We employ here a hybrid approach, in which the ions are treated kinetically while the cold electrons are described by means of a fluid model, and derive the linear dispersion relation. The latter is analyzed analytically and numerically for different realistic situations. Introducing the mixing length hypothesis and mode coupling processes, we deduce the saturated wave spectra for nonthermal LHD wave fluctuations. The latter produce the anomalous resistivity and cross‐field particle diffusion in a nonuniform magnetoplasma. It is shown that the nonlinear equations governing the dynamics of weakly interacting LHD waves admit stationary solutions in the form of vortices which can also trap particles and transport them to large distances. The present study is relevant for understanding the properties of nonthermal fluctuations and associated transport processes and coherent vortex structures at the magnetopause current layer.

Global hybrid simulation of the dayside reconnection layer and associated field‐aligned currents

A two‐dimensional global hybrid simulation is carried out to study the structure of the magnetopause boundary layer in the noon‐midnight meridian plane associated with the dayside reconnection. In the simulation the bow shock, magnetosheath, and magnetopause are formed self‐consistently by supersonic solar wind passing the geomagnetic field. The dayside reconnection is triggered by imposing a current‐dependent resistivity at the subsolar magnetopause. A large‐amplitude rotational discontinuity is formed in the quasi‐steady reconnection layer in the northern and southern hemispheres. The rotational discontinuity possesses an electron sense, or right‐hand polarization in the magnetic field as the discontinuity is formed from the X line, and the magnetic structures in the magnetopause northward and southward of the X line are asymmetric in the cases with a nonzero By component of the interplanetary magnetic field (IMF). However, later the rotational discontinuity tends to evolve to a structure with a smallest field rotational angle and thus may reverse its sense of the field rotation. The Walen relation is tested for electron and ion flows in the magnetopause rotational discontinuities with left‐hand and right‐hand polarizations. Field‐aligned currents are generated in the magnetopause rotational discontinuity during the reconneetion, and nearly 5% of the magnetopause currents propagate with Alfvén waves along the field lines into the polar ionosphere. In addition, a weak secondary rotational discontinuity or Alfvén wave pulse propagates from the reconnection site into the magnetosphere. The sense of the field‐aligned currents is investigated for various values of the IMF By. The Hall effects on the structure of the magnetopause boundary layer and the field‐aligned currents are discussed.

The shearing instability of ion flow at the high-latitude tail of magnetopause boundary layer

Shearing instability of ion flow in an inhomogeneous plasma background in the magnetopause boundary layer at the high-latitude magnetotail is studied in this paper. By considering tail-aligned currents, we find that the instability excitation strongly depends on the disturbed wavelength. A quasi-critical wave number for instability is obtained. For relatively long perturbations, the instability tends to be excited at the inner edge of the boundary layer. The stable surface waves at the magnetopause and the K-H instability at the inner edge of the boundary layer can exist at the same time. This may contribute to the continuous transfer of momentum toward the magnetosphere.

Coordinated Cluster and ground-based instrument observations of transient changes in the magnetopause boundary layer during an interval of predominantly northward IMF: relation to reconnection pulses and FTE signatures

Abstract. We study a series of transient entries into the low-latitude boundary layer (LLBL) of all four Cluster spacecraft during an outbound pass through the mid-afternoon magnetopause ( [ XGSM, YGSM, ZGSM ] ≈ [ 2, 7, 9 ] RE). The events take place during an interval of northward IMF, as seen in the data from the ACE satellite and lagged by a propagation delay of 75 min that is welldefined by two separate studies: (1) the magnetospheric variations prior to the northward turning (Lockwood et al., 2001, this issue) and (2) the field clock angle seen by Cluster after it had emerged into the magnetosheath (Opgenoorth et al., 2001, this issue). With an additional lag of 16.5 min, the transient LLBL events correlate well with swings of the IMF clock angle (in GSM) to near 90°. Most of this additional lag is explained by ground-based observations, which reveal signatures of transient reconnection in the pre-noon sector that then take 10–15 min to propagate eastward to 15 MLT, where they are observed by Cluster. The eastward phase speed of these signatures agrees very well with the motion deduced by the cross-correlation of the signatures seen on the four Cluster spacecraft. The evidence that these events are reconnection pulses includes: transient erosion of the noon 630 nm (cusp/cleft) aurora to lower latitudes; transient and travelling enhancements of the flow into the polar cap, imaged by the AMIE technique; and poleward-moving events moving into the polar cap, seen by the EISCAT Svalbard Radar (ESR). A pass of the DMSP-F15 satellite reveals that the open field lines near noon have been opened for some time: the more recently opened field lines were found closer to dusk where the flow transient and the poleward-moving event intersected the satellite pass. The events at Cluster have ion and electron characteristics predicted and observed by Lockwood and Hapgood (1998) for a Flux Transfer Event (FTE), with allowance for magnetospheric ion reflection at Alfvénic disturbances in the magnetopause reconnection layer. Like FTEs, the events are about 1 RE in their direction of motion and show a rise in the magnetic field strength, but unlike FTEs, in general, they show no pressure excess in their core and hence, no characteristic bipolar signature in the boundary-normal component. However, most of the events were observed when the magnetic field was southward, i.e. on the edge of the interior magnetic cusp, or when the field was parallel to the magnetic equatorial plane. Only when the satellite begins to emerge from the exterior boundary (when the field was northward), do the events start to show a pressure excess in their core and the consequent bipolar signature. We identify the events as the first observations of FTEs at middle altitudes.Key words. Magnetospheric physics (magnetopause, cusp and boundary layers; magnetosphere-ionosphere interactions; solar wind-magnetosphere interactions)

Cluster observes the Earth’s magnetopause: coordinated four-point magnetic field measurements

Abstract. The four-spacecraft Cluster mission has provided high-time resolution measurements of the magnetic field from closely maintained separation distances (200–600 km). Four-point coverage of the Earth’s magnetopause began on the 9 and 10 November 2000 when all spacecraft first exited the dusk-side magnetosphere at about 19:00 LT, providing extensive coverage of the near flank magnetosheath and magnetopause boundary layer on re-entry to the magnetosphere. The traversals on this occasion were caused by the arrival of an intense CME at the Earth, which produced a large compression of the magnetopause and high magnetic activity. The magnetopause traversals represent an unprecedented data set, allowing detailed analysis of the local magnetic structure (gradients) and dynamics of the magnetopause boundary. By performing minimum variance analysis (MVA) on the magnetic field data from all four spacecraft, we demonstrate that the magnetopause was planar on the scale of the spacecraft separation scales and that the transverse scale size of the magnetopause boundary layer was 1000–1100 km. We also show that the motion of the boundary (defined by the magnetic shear at the current layer), is changing over the sequence of spacecraft crossings so that acceleration of the magnetopause can be very high in this region of the magnetosphere. Indeed, the magnetopause speed reaches the order of 300 km/s in response to the arrival of the interplanetary shock. Using MVA coordinates, we have identified a number of magnetospheric and magnetosheath FTE signatures, which are sampled simultaneously by all spacecraft at different distances from and on either side of the magnetopause. The signatures show a variation of scale with distance from the boundary.Key words. Magnetospheric physics (magnetopause, cusp and boundary layers) Space plasma physics (discontinuities; magnetic reconnection)

Four point measurements of electrons using PEACE in the high-altitude cusp

Abstract. We present examples of electron measurements from the PEACE instruments on the Cluster spacecraft in the high-latitude, high-altitude region of the Earth’s magnetosphere. Using electron density and energy spectra measurements, we examine two cases where the orbit of the Cluster tetrahedron is outbound over the northern hemisphere, in the afternoon sector approaching the magnetopause. Data from the magnetometer is also used to pinpoint the position of the spacecraft with respect to magnetospheric boundaries. This preliminary work specifically highlights the benefit of the multipoint measurement capability of the Cluster mission. In the first case, we observe a small-scale spatial structure within the magnetopause boundary layer. The Cluster spacecraft initially straddle a boundary, characterised by a discontinuous change in the plasma population, with a pair of spacecraft on either side. This is followed by a complete crossing of the boundary by all four spacecraft. In the second case, Cluster encounters an isolated region of higher energy electrons within the cusp. The characteristics of this region are consistent with a trapped boundary layer plasma sheet population on closed magnetospheric field lines. However, a boundary motion study indicates that this region convects past Cluster, a characteristic more consistent with open field lines. An interpretation of this event in terms of the motion of the cusp boundary region is presented.Key words. Magnetospheric physics (magnetopause, cusp and boundary layers; solar wind-magnetosphere interactions)

Cluster PEACE observations of electrons during magnetospheric flux transfer events

Abstract. During the first quarter of 2001 the apogees of the Cluster spacecraft quartet precessed through midday local times. This provides the first opportunity for 4 spacecraft studies of the bow shock, magnetosheath and the dayside magnetopause current layer and boundary layers. In this paper, we present observations of electrons in the energy range ~ 10 eV–26 keV made by the Plasma Electron And Current Experiment (PEACE) located just inside the magnetopause boundary, together with supporting observations by the Flux Gate Magnetometer (FGM). During these observations, the spacecraft have separations of ~ 600 km. This scale size is of the order or less than the typical size of flux transfer events (FTEs), which are expected to be observed following bursts of reconnection on the dayside magnetopause. We study, in detail, the 3-D configuration of electron populations observed around a series of enhancements of magnetosheath-like electrons which were observed within the magnetosphere on 2 February 2001. We find that individual spacecraft observe magnetic field and electron signatures that are consistent with previous observations of magnetospheric FTEs. However, the differences in the signatures between spacecraft indicate that these FTEs have substructure on the scale of the spacecraft separation. We use these differences and the timings of the 4 spacecraft observations to infer the motions of the electron populations and thus the configuration of these substructures. We find that these FTEs are moving from noon towards dusk. The inferred size and speed of motion across the magnetopause, in one example, is ~ 0.8 RE and ~ 70 km s-1 respectively. In addition, we observe a delay in and an extended duration of the signature at the spacecraft furthest from the magnetopause. We discuss the implications of these 4 spacecraft observations for the structure of these FTEs. We suggest that these may include a compression of the closed flux tubes ahead of the FTE, which causes density and field strength enhancements; a circulation of open flux tubes within the FTE itself, which accounts for the delay in the arrival of the magnetosheath electron populations at locations deepest within the magnetosphere; and a possible trapping of magnetospheric electrons on the most recently opened flux tubes within the FTE.Key words. Magnetospheric physics (magnetopause, cusp and boundary layers; solar wind - magnetosphere interactions)

First simultaneous observations of flux transfer events at the high-latitude magnetopause by the Cluster spacecraft and pulsed radar signatures in the conjugate ionosphere by the CUTLASS and EISCAT radars

Abstract. Cluster magnetic field data are studied during an outbound pass through the post-noon high-latitude magnetopause region on 14 February 2001. The onset of several minute perturbations in the magnetospheric field was observed in conjunction with a southward turn of the interplanetary magnetic field observed upstream by the ACE spacecraft and lagged to the subsolar magnetopause. These perturbations culminated in the observation of four clear magnetospheric flux transfer events (FTEs) adjacent to the magnetopause, together with a highly-structured magnetopause boundary layer containing related field features. Furthermore, clear FTEs were observed later in the magnetosheath. The magnetospheric FTEs were of essentially the same form as the original "flux erosion events" observed in HEOS-2 data at a similar location and under similar interplanetary conditions by Haerendel et al. (1978). We show that the nature of the magnetic perturbations in these events is consistent with the formation of open flux tubes connected to the northern polar ionosphere via pulsed reconnection in the dusk sector magnetopause. The magnetic footprint of the Cluster spacecraft during the boundary passage is shown to map centrally within the fields-of-view of the CUTLASS SuperDARN radars, and to pass across the field-aligned beam of the EISCAT Svalbard radar (ESR) system. It is shown that both the ionospheric flow and the backscatter power in the CUTLASS data pulse are in synchrony with the magnetospheric FTEs and boundary layer structures at the latitude of the Cluster footprint. These flow and power features are subsequently found to propagate poleward, forming classic "pulsed ionospheric flow" and "poleward-moving radar auroral form" structures at higher latitudes. The combined Cluster-CUTLASS observations thus represent a direct demonstration of the coupling of momentum and energy into the magnetosphere-ionosphere system via pulsed magnetopause reconnection. The ESR observations also reveal the nature of the structured and variable polar ionosphere produced by the structured and time-varying precipitation and flow.Key words. Ionosphere (auroral ionosphere) Magentospheric physics (magnetopause, cusp and boundary layers; magnetosphere-ionosphere interactions)

Transient Magnetic Reconnection and Unstable Shear Layers

We study three-dimensional magnetic reconnection caused by the Kelvin-Helmholtz (KH) instability and differential rotation in subsonic and sub-Alfvenic flows. The flows, which are modeled by the resistive magnetohydrodynamic equations with constant resistivity, are stable in the direction of the magnetic field but unstable perpendicular to the magnetic field. Localized transient reconnection is observed on the KH time scale, and kinetic energy increases with decreasing resistivity. As in flux-transfer events in the Earth's magnetopause boundary layer, bipolar structures in the normal flux and bidirectional jetting away from reconnection zones are observed.

Features of energetic ions near the compressed magnetosphere

A possible cusp acceleration mechanism has been investigated during a major geomagnetic storm on May 4, 1998 when the magnetosphere was compressed and eroded. At 5:00–12:05 UT, the WIND spacecraft was about 213 R E upstream from the Earth, the GEOTAIL was in the post dusk magnetosheath, and the POLAR traveled in its outbound orbit from the equatorial radiation belt to the high-altitude dayside cusp and crossed the magnetopause into the magnetosheath. Many instances of in situ ion energization (to >MeV) were observed by POLAR during this period. Simultaneous observations indicated that no comparable flux was observed by WIND. Ion fluxes measured by POLAR were higher than that measured by GEOTAIL, indicating ion source regions near or within the magnetosphere. The measured ions show some interesting features: (1) In the radiation belt, in the magnetopause boundary layer, and in the dayside cusp, most 1– 200 keV/ e fluxes were around pitch angles of 90°; while in the magnetosheath, the fluxes came from a broad hemispherical direction between ∼sunward and earthward. (2) At around 6:00 UT, the MeV ion flux was lower in the magnetosheath than in the adjacent magnetosphere. (3) There were two CEP (cusp energetic particle) events with 2–3 orders of magnitudes enhancements of MeV ion fluxes. (4) The MeV ions in one CEP event had a peak flux higher than that of the intense outer radiation belt in the equatorial plane. (5) The 100 keV/ e O <+3 ions which originated from the ionosphere were observed in both the outer cusp and the magnetopause boundary layer. (6) There were no large magnetic field fluctuations upstream before the bow shock. These observations suggest a likely source of substantial particle energization exists in the cusp region.

Interplanetary shocks, magnetopause boundary layers and dayside auroras: The importance of a very small magnetospheric region

Dayside near-polar auroral brightenings occur when interplanetary shocks impinge upon the Earth's magnetosphere. The aurora first brightens near local noon and then propagates toward dawn and dusk along the auroral oval. The propagation speed of this wave of auroral light is ∼10 km s-1 in the ionosphere. This speed is comparable to the solar wind speed along the outer magnetosphere. The fundamental shock-magnetospheric interaction occurs at the magnetopause and its boundary layer. Several physical mechanisms transferring energy from the solar wind directly to the magnetosphere and from the magnetosphere to the ionosphere are reviewed. The same physical processes can occur at other solar system magnetospheres. We use the Haerendel (1994) formulation to estimate the acceleration of energetic electrons to 50 keV in the Jovian magnetosphere/ionosphere. Auroral brightenings by shocks could be used as technique to discover planets in other stellar systems.

“Broadband” plasma waves in the boundary layers

Boundary layers are commonly encountered in space and astrophysical plasmas. For example, interaction of solar wind plasma with the planets and comets produces magnetopause and cometopause boundary layers, respectively. Generally, the boundary layers are formed when plasmas with different characteristics interact with each other. The plasma sheet boundary layer in the Earth's magnetotail is formed owing to the interaction of hot dense plasma in the plasma sheet region with the rarefied plasma of the lobe region. Boundary layers are the site where energy and momentum are exchanged between two distinct plasmas. Boundary layers occurring in space plasmas can support a wide spectrum of plasma waves spanning a frequency range of a few mHz to 100 kHz and beyond. The purpose of this review is to describe the main characteristics and the possible generation mechanisms of the broadband plasma waves (with frequencies of &gt; 1 Hz) observed in the Earth's magnetopause boundary layer, the Jovian magnetopause boundary layer, the plasma sheet boundary layer, and the Earth's polar cap boundary layer. The rapid pitch angle scattering of energetic particles via cyclotron resonant interactions with the waves can provide sufficient precipitated energy flux to the ionosphere to create the dayside aurora at Earth and a weak high‐latitude auroral ring at Jupiter. In general, the broadband plasma waves may play an important part in the processes of local heating/acceleration of the boundary layer plasma. Recent exciting high time resolution results on the broadband plasma waves coming from Geotail, Polar, and FAST will be discussed.

Sun-Earth connection: Boundary layer waves and auroras

Boundary layers are the sites where energy and momentum are exchanged between two distinct plasmas. Boundary layers occurring in space plasmas can support a wide spectrum of plasma waves spanning a frequency range of a few mHz to 100 kHz and beyond. The main characteristics of the broadband plasma waves (with frequencies >1 Hz) observed in the magnetopause, polar cap, and plasma sheet boundary layers are described. The rapid pitch angle scattering of energetic particles via cyclotron resonant interactions with the waves can provide sufficient precipitated energy flux to the ionosphere to create the diffused auroral oval. The broadband plasma waves may also play an important role in the processes of local heating/acceleration of the boundary layer plasma.