MULTISCALE DYNAMIC PROCESSES NEAR MAGNETOSPHERIC BOUNDARIES AND IN THE CUSP


The main aim of this study is to advance the understanding of plasma and magnetic energy transfer, which take place at magnetospheric boundaries and in the cusp, on scales from the ion gyro- radius to several R_E. By comparing models and theory with data obtained in the framework of the International Solar Terrestrial Program (ISTP), we study the hierarchy of processes from small scales dominated by turbulence and kinetic effects up to global scales controlled by MHD, with the stress on the global and intermediate scales, as the data from key missions Interball, Polar and Geotail advance. We focus on events when the solar wind and magnetospheric conditions, as well as the spacecraft positions, are favorable for addressing the scientific problems outlined above.

The team includes:

Jean Berchem University of California Los Angeles
Masaki Fujimoto Institute of Technology Tokyo
Stephen Fuselier LMATC Palo Alto
Rumi Nakamura MPE Garching
Zdenek Nemecek Charles University Prague
Arne Pedersen University Oslo (Chair)  
Jean-André Sauvaud CESR Toulouse
Sergey Savin IKI Moscow
Lev Zelenyi IKI Moscow


    Prior to the Cluster era we have been concentrated on the large-scale and middle-scale phenomena at the magnetospheric boundaries and into the cusp, as the available solar wind (WIND, IMP-8, ACE) data and that of Interball-1, 2, Magion-4, Polar, Geotail, Equator-S and DMSP provided opportunity to trace simultaneously different regions of outer magnetosphere. The global MHD simulation model, used in this study for inter-comparison and interpretation of the data, is based on a single fluid MHD description. The code solves the normalized resistive MHD equations as an initial value problem, using the solar wind data as the real-time input and projecting the output data onto traces of the other spacecraft.

    At the first stage of our work the representatives of four Space Agencies reported pre-selected cases using the key parameter data and after the discussions we have selected 10 new candidate events for further study.

    The first-level data for the selected events have been collected on the ISSI server prior to the second meeting, where we finally select a half of the events for detailed studies and publications and also rectified the particular problems for common publications with the usage of the known events and for statistical studies. A criterion for the event selection was applicability of the MHD code for a case in the real-time mode for the particular solar wind states.

    After the high-resolution data for the selected events and topics have been collected on the ISSI server and the MHD code has been run, at the final stage of our Project we discussed the prepared data in ISSI in details and have prepared the paper drafts and figures for publications.

    Taking into account multi-spacecraft (>10) and multi-nation (8) efforts in the data selection and processing and preparation of 8 papers for publication, we would like to suppose our Project under the financial, organizational, computer and scientific support from ISSI to be relatively fruitful, and we hope that at the current level of the knowledge and data availability the Project has promoted certain development in understanding of multi-scale processes near magnetopause and in the cusp. We would like especially appreciate the organizational support and participation in scientific discussion of the ISSI Directors Goetz Paschmann and Vittorio Manno, along with providing the exceptional environment for the fruitful work by the ISSI staff Diane Taylor, Gabriela Indermuehle and Dr. Xanier.

    In the report we briefly revise the papers, prepared for publications under umbrella of our Project and with substantial input from our team, starting from case studies and finishing by papers with reviews. We refer to the paper texts, attached to the Report, thus one can see the details in the papers per se.

    Paper 1 (click to see the paper text) deals with observations from the Polar spacecraft on 21 January 1998 in a region of closed magnetic field lines containing several distinct solar wind ion populations in the energy range from <10 eV to >200 keV/e. A global MHD simulation of this reproduces Geotail and Interball/Tail spacecraft observations in the outer magnetosphere and magnetosheath. These results demonstrate that the simulation faithfully reproduces the global magnetic field configuration of the magnetosheath and magnetosphere and provides confidence for the interpretation of the LLBL observations from Polar.

    Paper 2 describes the latter half of the magnetic cloud event on 10–11 January 1997. When the interplanetary magnetic field (IMF) was strongly northward and the solar wind density returned to a nominal value from an anomalously large one, two spacecraft, Geotail and Interball-Tail, were in the dusk-flank region and detected a change in the plasma sheet status from hot and tenuous to cold and dense. The change seen by these spacecraft making in situ observations is confirmed to be a global feature by DMSP observations at low altitude. Interball-Aurora and Polar were crossing the dusk-auroral oval. Injection of magnetosheath-like ions was detected by these spacecraft. The observations are related with the transport of magnetosheath plasma onto the magnetospheric field lines. Three candidate processes are discussed, but none of them turn out to be convincing, indicating the need for further study.

    Paper 3 The reconnection of the anti-parallel field lines at the smooth laminar MP was assumed to be the major mechanism of plasma penetration inside the MP. However, the weak dependence of the cusp and the over-cusp turbulent boundary layer (TBL) on the IMF direction and the observation of strong perturbations up to the electron inertial length indicate that there may also be different mechanisms. We assume that the flow into the MSH is broken by an obstacle in the shape of a step at the MP to form the TBL, where the ion kinetic energy transforms into heat. Near the plasma flow boundary (PF), separated from MP by stagnation region), the flow is locally accelerated to the energies Ekin higher than in the MSH. This can really be explained by the acceleration due to the reconnected magnetic-field tension. The reconnection is possible both near the geomagnetic equator and in the cusp locality. The small- scale fluctuating fields are reconnected efficiently in the TBL as well, as is evident from the breakdown of field-line freezing-in (by virtue of fluctuations on the electron inertial length scale). This allows plasma to penetrate inside the MP and provides efficient magnetic-flux transfer from the dayside of the magnetosphere to its nightside. Nevertheless, we assume that, in the essentially nonlinear situation occurring in the TBL, plasma percolation through the structured boundary makes the main contribution to the local mass transfer inside the MP. Taking the appropriate estimate of diffusion coefficient, one obtains Dp ~ 0.66 (dB/B0)ri 2Wi ~(5–10) ´ 10 9 m 2 /s for the typical MP parameters, where dB/B0 is the ratio of the perturbed magnetic field to its average value, and ri and Wi are the ion gyroradius and gyrofrequency, respectively. The resulting value; of (1–2) ´ 10 27 particles/s obtained for the flow through the northern and southern TBL is sufficient for filling the magnetosphere with solar plasma. Let us now turn to the nature of oscillations in TBL. Phase velocity is one of the properties that allows the low-frequency perturbations to be identified with the kinetic Alfvén waves (KAWs). We used the Interball-1 and Polar satellite data on the electric (E) and magnetic (B) fields in the TBL at August 26, 1995, May 5, 1996, June 19, 1998, and June 23, 1998 to verify that (a) like on April 2, 1996, the magnetic spectrum has two characteristic slopes and (b) the low-frequency phase velocity Vph = E/B is close to the Alfvén velocity VA and shows, on the average, a tendency toward the frequency dependence characteristic of the satellite flight through the KAW spatial structures, up to a frequency of several hertz (which is several times lower than the hybrid frequency). This dependence is expressed by the formula (E/B)2 ~ VA2 (1+( ri w/V)2) (1) where w is the frequency, V is the velocity of KAW structures relative to the satellite, and ( ri w/V)2 is the kinetic addition allowing for the finiteness of the ion gyroradius (KAW takes its name precisely from this fact). In most cases, the asymptotic behavior of (E/B) had the form ~ w, i.e., corresponded to Eq. (1). This, however, cannot be distinguished from the detection of waves with a constant wave vector k, because the Fourier transform of plane waves obeys the Maxwell equation kE ~ wB. Therefore, Eq. (1) does not allow the identification of KAW in the asymptotic region. The TBL is also characterized by the three-wave decay processes satisfying the condition f = fL + fK . In the frequency range of interest, the products of the appropriate three amplitudes show maxima up to 40% at frequencies fL ~ few mHz and over a continuous range of 1.5–80 mHz for fK. This signifies that the phase–frequency relations are fulfilled for the three-wave process (if the higher-order nonlinear processes are ignored) and the structures with the indicated frequencies fL decay in a broad range of frequencies fK and f. That is, the processes at these frequencies synchronize cascades in a broad frequency range. We thus assume that the inhomogenities in the incoming flow interact with the current layer of MP to generate KAWs, a part of which are reflected back, focused by the concave MP, and interact with the incoming flow. As a result, a number of cascades synchronized at the above-mentioned frequencies fL arise self-consistently. If the estimate of the upper limit of the characteristic scale at 1.5 mHz is carried out using VA, then L ~ VA/fL ~ (3–7)RE (Earth radii) is comparable with the TBL length, and L is also on the order of the radius of curvature of the unperturbed MP or the MSH thickness at the dayside. On the other hand, the presence of a maximum at ~1.5 mHz both in the MSH and in the cusp inside the MP also suggests that the observed process is global. To understand the nature of this resonance in more detail, it is necessary to carry out additional measurements at several points and at distances of both several thousand.

    Paper 4 is devoted to the experimental study of the singular regions at high-latitude magnetopause, where the magnetosheath (MSH) flow interaction with the exterior cusp result in the creation of the turbulent boundary layer (TBL). In the TBL magnetic fluctuations are of the order of the DC magnetic field and their power reaches 10-30% that of the thermal ion density. Early single spacecraft observations with Heos-2 and later Prognoz-7, 8, 10 have shown that the magnetopause (MP) position and MSH plasma flow structures are quite variable near the cusp, a magnetospheric region that is crucial for magnetosheath plasma entry. We here identify the MP as the innermost current sheet where the magnetic field turns from Earth-controlled to magnetosheath-controlled At the cusp the magnetopause is indented. The indentation was first detected by HEOS-2. The plasma in the vicinity of this indentation is highly disturbed and/or stagnant MSH plasma. The turbulent boundary layer (TBL) is a sub-region of the MSH/ cusp interface with nonlinear magnetic perturbations. It is located just outside and/or at the near cusp magnetopause and has recently been found to be a permanent feature. Here the energy density of the ultra low frequency (ULF) magnetic fluctuations is comparable to the ion kinetic, thermal, and DC magnetic field densities. The ULF power is usually several times larger than that in the MSH, and one or two orders of magnitude larger than that inside the magnetopause. Haerendel was the first one to introduce the turbulent boundary layer in cusp physics in a discussion on the interaction of the magnetosheath flow with the magnetopause at the flank of the tail lobe. This paper deals with the TBL during negative dominant By IMF conditions. In such configuration boundary layer is developed at high latitudes. We discuss spectral and statistical properties of the turbulence at the cusp/MSH interface on the basis of the data from Interball-1 on June 19, 1998, when Interball-1 and Polar crossed the southern and northern TBLs (i.e. winter and summer TBL, respectively). To distill inherent features of the disturbances in those typical TBL crossings, we compare the local data with results of Gasdynamic Convected Field Model (GDCFM) code. For the model we use Geotail data in solar wind (SW) as the input. We also discuss TBL fluctuation characteristic features in wavelet spectra and bi-spectra and in the magnetic field vector hodograms. Statistics of the magnetic field rotations in TBL is explored. Finally, we compare the in situ data with that from a virtual satellite, which crosses a model current sheet (from a kinetic code) at the state of developed nonlinear turbulence. Interball-1 data in the summer TBL are compared with that of Geotail in solar wind (SW) and Polar in the northern TBL. In the TBL two characteristic slopes are seen: ~ -1 at (0.004 -0.08) Hz and ~ -2.2 at (0.08-2) Hz. We present evidences that random current sheets with features of coherent solitons can result in: (i) Slopes of ~ -1 in the magnetic power spectra; (ii) Demagnetization of the SW plasma in ‘diamagnetic bubbles’; (iii) Nonlinear, presumably, 3-wave phase coupling with cascade features; (iiii) Departure from the Gaussian statistics. We discuss the above TBL properties in terms of intermittency and self-organization of nonlinear systems and compare them with kinetic simulations of reconnected current sheet at the nonlinear state. Virtual satellite data in the model current sheet reproduce valuable cascade-like, spectral and bi-spectral properties of the TBL turbulence.

    Paper 5 Multi-spacecraft tracing of the high latitude magnetopause (MP) and boundary layers and Interball-1 statistics indicate that: (a) The turbulent boundary layer (TBL) is a persistent feature in the region of the cusp and ‘sash’, a noticeable part of the disturbances weakly depends on the interplanetary magnetic field By component; TBL is a major site for the magnetosheath (MSH) plasma penetration inside the magnetosphere through percolation and local reconnection. (b) The TBL disturbances are mainly inherent with the characteristic kinked double-slope spectra and, most probably, 3-wave cascading. The bi-spectral phase coupling indicates self-organization of the TBL as the entire region with features of the non-equilibrium multi-scale and multi-phase system in the near-critical state. (c) We’ve found the different outer cusp topologies in summer/ winter periods: the summer cusp throat is open for the decelerated MSH flows, the winter one is closed by the distant MP with large-scale (~several Re) diamagnetic ‘plasma ball’ inside the MP; the ‘ball’ is filled from MSH through the patchy merging rather than large-scale one. (d) A mechanism for the energy release and mass inflow is the local TBL reconnection, which operates at the larger scales for the average anti-parallel fields and at the smaller scales for the nonlinear fluctuating fields; the latter is operative throughout the TBL. The remote from TBL anti-parallel reconnection seems to happen independently.

    Paper 6 (Text, Figs) The cusp represents a place where the magnetosheath plasma can directly penetrate into the magnetosphere. Since the main transport processes are connected with merging of the interplanetary and magnetospheric field lines, the interplanetary magnetic field (IMF) orientation plays a decisive role in the formation of the high­altitude cusp. Situations when IMF BZ is the major IMF component are rather rare. The present case study reveals the importance of horizontal IMF components on the global magnetospheric configuration as well as on small­scale processes at the cusp­magnetosheath interface. We have used simultaneous measurements of several spacecraft operating in different regions of interplanetary space and two closely spaced satellites (INTERBALL­1/MAGION­4) crossing the cusp­magnetosheath boundary to show the connection between the short and large­scale phenomena. In the northern hemisphere, observations suggest a presence of two spots of cusp­like precipitation supplied by reconnection occurring simultaneously in both hemispheres. A source of this bifurcation is the positive IMF B Y component further enhanced by the field draping in the magnetosheath. This magnetic field component shifts the entry point far away from the local noon but in opposite sense in either hemisphere.

    Paper 7 advances the achievements of Interball-1 and other contemporary missions in exploration of the magnetosheath-cusp interface. Extensive discussion of published results is accompanied by presentation of new data from a case study and a consideration of those data within the broader context of one-year Interball-1 statistics. Multi-spacecraft boundary layer studies reveal that in ~83 % of the cases the interaction of the magnetosheath (MSH) flow with the high latitude magnetopause produces a layer containing strong nonlinear turbulence, called the turbulent boundary layer (TBL). The TBL contains wave trains with flows at approximately the Alfven speed along field lines and ‘diamagnetic bubbles' with small magnetic fields inside. A comparison of the multi-point measurements obtained on May 29, 1996 with a global MHD model indicates that three types of populating processes should be operative: - large-scale (~ few Re) antiparallel merging at sites remote from the cusp; - medium-scale (few thousand km) local TBL-merging of fields that are antiparallel on average; - small-scale (few hundred km) bursty reconnection of fluctuating magnetic fields, representing a continuous mechanism for MSH plasma inflow into the magnetosphere, which could dominate in quasi-steady cases. The lowest frequency (~ 1-2 mHz) TBL fluctuations are traced throughout the magnetosheath from the post-bow shock region up to the inner magnetopause border. The resonance of these fluctuations with dayside flux tubes might provide an effective correlative link for the entire dayside region of the solar wind interaction with the magnetopause and cusp ionosphere. The TBL disturbances are characterized by kinked, double-sloped wave power spectra and, most probably, three-wave cascading. Both elliptical polarization and nearly Alfvenic phase velocities with characteristic dispersion indicate the kinetic Alfvenic nature of the TBL waves. Bi-spectral phase coupling could effectively support the self-organization of the TBL plasma by means of coherent resonant-like structures. The estimated characteristic scale of the ‘resonator’ is of the order of the TBL dimension over the cusps. Inverse cascades of kinetic Alfven waves are proposed for forming the larger scale ‘organizing’ structures, which in turn synchronize all non-linear cascades within the TBL in a self-consistent manner. This infers a qualitative difference from the traditional approach, wherein the MSH/cusp interaction is regarded as a linear superposition of magnetospheric responses on the solar wind or MSH disturbances. We propose that the TBL represents an open system in a non-equilibrium steady state. In this regard we discuss the applicability of the self-organized criticality concept to the multi-scale processes in this region.

    Paper 8 surveys the processes near magnetopause, the majority of which has been discussed in the previous papers, presented in this Report. It proceeds the analysis of event on June 19, 1998 and present new materials for January 27, 1997. We discuss data from Interball-1,2, Magion-4 (Interball-1 sub-satellite), Geotail and Polar on January 27, 1997 over a winter cusp and compare them with results of Spreiter model of the solar wind (SW) interaction with the magnetosphere. The main question we address here is whether the turbulent boundary layer and the outer cusp throat play a substantial role in the magnetosheath plasma flow interaction with the high latitude MP. The multi-point data provide evidences that downstream the cusp the MSH flow is decelerated and heated as compared with the near-equatorial MSH flow. While being permanent, the ion heating in TBL is too weak to account for the ion energies in the lower altitude boundary cusp and ‘sash’, detected by Interball-2 and Polar respectively. Another example of the winter TBL encounter by Interball-1 on June 19, 1998 serves to demonstrate the asymmetry of boundary layers for positive (sunward) Earth magnetic dipole tilts in summer and that of the negative (anti-sunward) tilts in winter. The difference has been established by comparison with simultaneous Polar data in the summer stagnation region. We reproduce most interesting results from the previous studies and analyze detailed dynamics of the ion energy and of Poynting flux to clarify the pattern of nonlinear interactions in the upstream TBL. The wave packets, going upstream MSH flow from MP, occur to stimulate partial randomization of the flow far in front of MP, the SW driving plays the minor role. The interaction with the upstream waves launches downstream current sheets, which confine super-Alfvenic tailward jets. That signifies the cascade-like non-linear energy transformation in TBL. We exhibit de-magnetized large-scale ‘plasma balls’ inside winter MP and study their statistics versus that of heated MSH plasma outside MP in summer. Finally, we discuss the presented and published Interball-1 data in relation to the MSH plasma penetration and acceleration both due to plasma percolation and turbulent heating and due to laminar large-scale reconnection of anti-parallel magnetic fields. We present both statistical and two case studies of magnetosheath (MSH) interaction with the high latitude magnetopause (MP) on the basis of Interball-1 and other ISTP spacecraft data. The results of our data analysis strongly indicate that the TBL fluctuations, instead of being random, are phase-coupled and ‘organized’ by the cascades of nonlinear, presumably 3-wave, interactions. The selected coherent wave trains are capable of synchronizing interactions throughout the TBL, somewhat resembling a global TBL resonance. Multiplying the characteristic period of the ‘organizing’ wave mode by the MSH Alfven speed we get 3-4 Re as a proxy for the characteristic scale. This is close to the diameter of the TBL or outer cusp throat and can be attributed to a standing nonlinear wave, trapped in the outer cusp throat. The quasi-coherent structures control the spectral shape and result in non-Gaussian statistical characteristics of the disturbances, that conforms the fluctuation intermittency. We suggest that multi-scale TBL processes play at least a comparable role to those of reconnection remote from the cusp in the solar wind energy transformation and population of the magnetosphere by the MSH plasma. The TBL transforms the MSH flow energy including deceleration and heating of the flow downstream the high latitude cusp. The plasma-plasma interaction over cusp throat operates via reflected waves, which ignite the chaotization of ~ 40% upstream kinetic energy, the sub-Alfvenic flow decay launches the TBL nonlinear cascades along with the jets accelerated downstream up to 3 Alfvenic Mach numbers

May 2003