Martin Hosner

The interaction between localized fast plasma jets, called bursty bulk flows (BBF) or flow bursts and ambient magnetic field plays an important role in the complex chain of solar wind-magnetosphere-ionosphere coupling processes. In particular the transition region, where the magnetic field configuration changes from dipolar-like configuration to tail-like configuration and where near-Earth flow braking/bouncing processes take place, is a key region of fundamental processes such as the particle energization and wave-particle interaction. These processes, associated with magnetic and pressure disturbances, drive enhanced energy and momentum transfer from the nightside outer magnetosphere along Earth’s magnetic field lines down to the ionosphere. Across the field lines, particle injections further affect the inner magnetosphere dynamics, constituting a source population for the radiation belt and the ring current.In this presentation we stress the importance of observations of BBFs and dipolarization fronts by multi-point measurements in an extensive region covering both equatorial and off-equatorial locations, and simultaneously at ion and fluid scales for understanding the energy transport processes. These allows us to monitor both the field-aligned and perpendicular evolution of the flux tube and enable to study the coupling with the ionosphere. By showing several examples of observations from previous studies of different scales of disturbances and fortuitous multi-spacecraft configuration at different scales, the 3D nature of the interaction between the BBF and ambient plasma, and its relationship to ionosphere including field-aligned current and aurora will be discussed.

In the Earth's magnetotail bursty bulk flows are often associated with dipolarizing magnetic flux bundles. The leading edges of such earthward‐moving flux bundles are called dipolarization fronts (DF). In the present study we investigate the characteristics of ion‐scale oscillations embedded inside DFs, using observations from NASA's Magnetospheric Multiscale mission (MMS) between 2017 and 2022. By applying wavelet and spectral analysis methods to multi‐point measurements, we obtain a statistical picture of the magnetic and electric field fluctuations from parameters such as the polarization, propagation direction and velocity in the central and outer plasma sheet. More than 96% of the observed events show enhanced power around the proton cyclotron frequency. The power maximum occurred either at the beginning or end of the DF. For a large portion of DF events, in particular those in the central plasma sheet, the power enhancement is due to the DF itself as expected from the ion scale‐thin DFs. However, ion‐scale fluctuations embedded in the DF are observed for the majority of the cases in the outer plasma sheet. While some limited cases are associated with electromagnetic ion cyclotron waves, most of these fluctuations can be interpreted as static co‐moving structures, generated by local Hall or field‐aligned currents. There are quite a number of fluctuations suggesting a rippled dipolarization front predominantly for the events in the outer plasma sheet.

In the Earth’s magnetotail fast earthward plasma flows (so-called Bursty Bulk Flows) are often associated with dipolar magnetic flux bundles. The leading edges of such earthward-moving flux bundles are called dipolarization fronts (DF). Previous studies have reported wave signatures around the local proton cyclotron frequency during selected events, using data from the Cluster, THEMIS and MMS missions. In the present study, we examine characteristics of these ion-scale waves during several hundred DF events, observed by NASA’s Magnetospheric Multiscale mission (MMS) between 2017 and 2022. By applying a wavelet analysis to the database, we not only obtain a statistical picture of the waves from spectral parameters such as the polarization, propagation direction, and electromagnetic character. It also allows us to map their temporal distribution around the DFs, and to obtain a global picture about the occurrence of such waves across different regions of the magnetotail, including the central plasma sheet, the outer plasma sheet, and the plasma sheet boundary layer. Based on the analysis we discuss possible generation mechanisms of the waves.

Recent THEMIS and MMS spacecraft observations confirmed the kinetic simulations showing that the kinetic Ballooning/Interchange Instability (BICI) may lead to off-equator electromagnetic ion-cyclotron waves. The ion-cyclotron waves appeared to ripple the growing ballooning-interchange heads and to erode and thin the magnetotail current sheet at ion scales. As this wave activity may be important for initiating magnetic reconnection in the magnetotail, we aim at collecting a number of high-resolution MMS ion observations with clear signatures of the electromagnetic ion-cyclotron waves, and at investigating the particle behavior caused by the waves. Particularly, we analyze the properties of the electromagnetic ion cyclotron waves and search for signatures of wave interaction with resonant particles using partial ion distribution functions and their moments.

In the Earth’s Magnetotail fast earthward plasma flows (so-called Bursty Bulk Flows) with a duration of several minutes and velocities of several hundreds of km/s are regularly observed in in-situ measurements. These flows are frequently accompanied by embedded co-moving, and more dipolar-shaped, magnetic flux bundles. The leading edge of such flux bundles is called a Dipolarization Front (DF). Such Earthward moving flux bundles and DFs can be formed by magnetic reconnection in the Magnetotail, as well as by the Ballooning/Interchange Instability (BICI). For the latter mechanism, simulations of the BICI suggest that, away from the central plasma sheet, the BICI heads are accompanied by electromagnetic ion cyclotron waves, and such wave signatures were indeed observed by the MMS and THEMIS spacecraft between the neutral sheet and the magnetospheric lobes.In the present study we use years of data from NASA’s Magnetospheric Multiscale Mission (MMS) to compile a database of several hundred events. We analyze this database with the goal of a statistical description of such flux bundles with respect to ion cyclotron wave occurrence, and compare them to several parameters, such as magnetic field and plasma conditions, to further enhance the understanding of the large-scale context and origin of magnetotail dipolarizations.

Numerous spacecraft missions, theories and numerical modelling have studied the Kelvin-Helmholtz instability (KHI) excited at the Earth’s magnetopause at different scales. Important insights into particle transport and mixing as well as energy conversion related to the KHI were obtained from these studies and linked to processes such as magnetic reconnection and plasma turbulence.Recently, Blasl et al. (2022, 2023) and Nakamura et al. (2022 a, b) reported the first observations of the KHI during southward Interplanetary Magnetic Field (IMF) conditions from the Magnetospheric Multiscale (MMS) mission together with fully-kinetic Particle-In-Cell simulations designed for this event. The unprecedented resolution of the MMS mission together with large-scale kinetic simulation runs enabled a multi-scale study of the KHI. Their results showed the onset and evolution of secondary instabilities such as the Rayleigh-Taylor Instability responsible for globally deforming the vortex structures as well as the Lower-Hybrid Drift Instability (LHDI) leading to plasma mixing along the spine region of the vortices. Additionally, at 1 out of the 11 KH wave crossings they reported the observation of an electron-scale reconnecting current sheet at the interface between this mixing region and the magnetospheric plasma, as suggested by simulation results.In this study, we revisit this previously reported MMS KH event during southward IMF and study the remaining 10 vortex structures in detail. Especially, we will characterize the signatures of LHDI-induced plasma mixing from both simulations and observations at different evolutionary stages and discuss the prevalence and signatures of small-scale current sheets at these vortex structures. First results suggest the coexistence of vortex structures at different evolutionary stages in this KH event, which will be discussed in detail. A short discussion on the influence of the IMF on this mechanism will be given at the end of the presentation.

We examine characteristics of the boundaries of 11 Kelvin‐Helmholtz vortex crossings observed by MMS on 23 September 2017 under southward IMF conditions. At both the leading and trailing edges, boundary regions of mixed plasma are observed together with lower‐hybrid wave activity. We found that thicker boundary regions feature a higher number of sub‐ion scale current sheets, of which only one shows clear reconnection signatures. Moreover, the lower‐hybrid waves along the vortex spine region are identified as an effective mechanism for plasma transport with an estimated diffusion coefficient of s. Comparisons with 3D simulations performed under the same conditions as the MMS event suggest that the extension of the boundary regions as well as the number of current sheets are related to different evolutionary stages of the vortices. Such observations can be explained by changes in the upstream magnetic field conditions.

When the velocity shear between the two plasmas separated by Earth's magnetopause is locally super‐Alfvénic, the Kelvin‐Helmholtz (KH) instability can develop. A crucial role is played by the interplanetary magnetic field (IMF) orientation, which can stabilize the velocity shear. Although, in a linear regime, the instability threshold is equally satisfied during both northward and southward IMF orientations, in situ measurements show that KH instability is preferentially excited during the northward IMF orientation. We investigate this different behavior by means of a mixing parameter which we apply to two KH events to identify both boundaries and the center of waves/vortices. During the northward orientation, the waves/vortex boundaries have stronger electrons than ions mixing, while the opposite is observed at their center. During the southward orientation, instead, particle mixing is observed predominantly at the boundaries. In addition, stronger local ion and electron non‐thermal features are observed during the northward than the southward IMF orientation. Specifically, ion distribution functions are more distorted, due to field‐aligned beams, and electrons have a larger temperature anisotropy during the northward than the southward IMF orientation. The observed kinetic features provide an insight into both local and remote processes that affect the evolution of KH structures.

We examine characteristics of the boundaries of 11 Kelvin-Helmholtz vortex crossings observed by MMS on September 23, 2017 under southward IMF conditions. At both the leading and trailing edges, boundary regions of mixed plasma are observed together with lower-hybrid wave activity. We found that thicker boundary regions feature a higher number of sub-ion scale current sheets, of which only one shows clear reconnection signatures. Moreover, the lower-hybrid waves along the vortex spine region are identified as an effective mechanism for plasma transport with an estimated diffusion coefficient of D ≈ 109 m2/s. Comparisons with 3D simulations performed under the same conditions as the MMS event suggest that the extension of the boundary regions as well as the number of current sheets are related to different evolutionary stages of the vortices. Such observations can be explained by changes in the upstream magnetic field conditions.

When the velocity shear between the two plasmas separated by Earth’s magnetopause is locally super-Alfvénic, the Kelvin-Helmholtz (KH) instability can develop. A crucial role is played by the interplanetary magnetic field (IMF) orientation, which can stabilize the velocity shear. Although, in a linear regime, the instability threshold is equally satisfied during both northward and southward IMF orientations, in-situ measurements show that KH instability is preferentially excited during the northward IMF orientation. We investigate this different behavior by means of a mixing parameter which we apply to two KH events to identify both boundaries and the center of waves/vortices. During the northward orientation, the waves/vortex boundaries have stronger electrons than ions mixing, while the opposite is observed at their center. During the southward orientation, instead, particle mixing is observed predominantly at the boundaries. In addition, stronger local ion and electron non-thermal features are observed during the northward than the southward IMF orientation. Specifically, ion distribution functions are more distorted, due to field-aligned beams, and electrons have a larger temperature anisotropy during the northward than the southward IMF orientation. The observed kinetic features provide an insight into both local and remote processes that affect the evolution of KH structures.