Abstract. We advance 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 comparison of those data within the broader context of three-year magnetopause (MP) crossings by Interball-1. Multi-spacecraft boundary layer studies reveal that in ∼80% of the cases the interaction of the magnetosheath (MSH) flow with the high latitude MP produces a layer containing strong nonlinear turbulence, called the turbulent boundary layer (TBL). The TBL contains wave trains with flows at approximately the Alfvén speed along field lines and "diamagnetic bubbles" with small magnetic fields inside. A comparison of the multi-point measurements obtained on 29 May 1996 with a global MHD model indicates that three types of populating processes should be operative:-large-scale (∼few R E ) anti-parallel merging at sites remote from the cusp; -medium-scale (few thousand km) local TBL-merging of fields that are anti-parallel on average;Correspondence to: S. Savin (ssavin@iki.rssi.ru) -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 Alfvénic phase velocities with characteristic dispersion indicate the kinetic Alfvénic nature of the TBL waves. The three-wave 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 Alfvén waves are proposed for forming the larger scale "organizing" structures, which in turn synchronize all nonlinear cascades within the TBL in a self-consistent manner. This infers a qualitative differ-184 S. Savin et al.: Magnetosheath-cusp interface ence 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.

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Abstract. The plasma-wave experiment ASPI (analysis of spectra of plasma waves and instabilities) on board the INTERBALL spacecraft is a combined wave diagnostics experiment. It performs measurements of the DC and AC magnetic ®eld vector by¯ux-gate and searchcoil sensors, the DC and AC electric ®eld vector by Langmuir double probes and the plasma current by Langmuir split probe. Preliminary data analysis shows the low noise levels of the sensors and the compatibility of new data with the results of previous missions. During several months of in-orbit operation a rich collection of data was acquired, examples of which at the magnetopause and plasma sheet are presented in second part of the paper. Scienti®c objectivesThe INTERBALL project is orientated towards the investigation of the interaction between di erent parts of the magnetosphere in relation to changes in the solar wind and ionosphere. First INTERBALL-1 orbits passed through the solar wind, bow shock, magnetosheath and magnetopause regions. Several months later the orbit apogee shifted to the near-Earth magnetotail, so that INTERBALL-1 observed the tail lobes and the plasma sheet. These orbit parameters allow the neutral sheet region to be studied for several hours.Plasma waves and¯uctuations play a crucial role in the highly collisionless space plasma. Waves and¯uctu-ations are of particular importance at the plasma boundaries such as bow shock, magnetopause, neutral sheet, and plasma sheet boundary layer. Wave-particle, interactions in the plasma result in processes like: (1) anomalous transport (pitch-angle and spatial di usion, conductivity, viscosity); (2) energy redistribution and plasma heating; (3) generation of anisotropic distribution functions and their relaxation; (4) triggering of large-scale instabilities.The speci®c objectives of the ASPI wave and ®eld experiment on board INTERBALL-1 are:1. The study of the¯uctuation characteristics in di erent regions of the magnetosphere and the use of these data as high-time-resolution indicators of plasma phenomena. 2. The determination of the micro-scale plasma characteristics (e.g. di usion and anomalous transport coe cients, wave-wave and wave-particle coupling).

The most widespread instrument used today for the measurement of quasistationary and slowly fluctuating vector magnetic fields is a flux-gate magnetometer (FGM). The most important parameter characterizing the magnetometer quality is its magnetic noise-its threshold sensitivity or its own noise level (NL). Based on the results of experimental research, we may state that the FGM NL mainly depends on the quality of the magnetic material used for FGM sensor core. The ''solid liquid'' model explaining the nature of magnetic noise is proposed and substantiated. It is demonstrated that special attention has to be paid to the annealing of the core. A new effect-termed gammamagnetic normalization-is discovered and discussed. It is shown that the magnetometer NL depends not only on the core length and volume but also on the excitation mode of the core. Besides, the ways to improve other factors, such as power consumption and thermal drift which must be taken into account in order to create a FGM with the highest possible performance, are discussed. Some examples are given of the parameters of present advanced FGMs for geophysical uses.

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