ALBERT

Description: The solar wind is a highly turbulent and intermittent medium at frequencies between 10-4 and 10-1 Hz. Power spectra are used to look at fluctuations in the components of the magnetic field at high frequencies over a wide range of latitudes. Results show steady turbulence in the polar regions of the Sun and a more varied environment in the equatorial region. The magnetic field fluctuations exhibit anomalous scaling at high frequencies. Various models have been proposed in an attempt to better understand the scaling nature of such fluctuations in neutral fluid turbulence. We have used the Ulysses fast latitude scan data to perform a wide ranging comparison of three such models on the solar wind magnetic field data: the well-known P model, in both its Kolmogorov and Kraichnan forms, the lognormal cascade model and a model adapted from atmospheric physics, the G infinity model. They were tested by using fits to graphs of the structure function exponents g(q), by making a comparison with a non-linear measure of the deviation of g(q) from the non-intermittent straight line, and by using extended self similarity technique, over a large range of helio-latitudes. Tests of all three models indicated a high level of intermittency in the fast solar wind, and showed a varied structure in the slow wind, with regions of apparently little intermittency next to regions of high intermittency, implying that the slow wind has no uniform origin. All but one of the models performed well, with the lognormal and Kolmogorov P model performing the best over all the tests, indicating that inhomogeneous energy transfer in the cascade is a good description. The Kraichnan model performed relatively poorly, and the overall results show that the Kraichnan model of turbulence is not well supported over the frequency and distance ranges of our data set. The G infinity model fitted the results surprisingly well and showed that there may very well be important universal geometrical aspects of intermittency over many physical systems.

Description: The intertmittent nature of turbulence within solar wind plasma has been demonstrated by several studies of spacecraft data. Using magnetic field data taken in high speed flows at high heliographic latitudes by the Ulysses probe, the character of fluctuations within the inertia] range is discussed. Structure functions are used extensively. A simple consideration of errors associated with calculations of high moment structure functions is shown to be useful as a practical estimate of the reliability of such calculations. For data sets of around 300 000 points, structure functions of moments above 5 are rarely reliable on the basis of this test, highlighting the importance of considering uncertainties in such calculations. When unreliable results are excluded, it is shown that inertial range polar fluctuations are well described by a multifractal model of turbulent energy transfer. Detailed consideration of the scaling of high order structure functions suggests energy transfer consistent with a "Kolmogorov" cascade.

Description: We report on recent measurements of solitary waves made by the Wideband Plasma Wave Receiver located on each of the four Cluster spacecraft at 4.5-6.5RE (well above the auroral acceleration region) as they cross field lines that map to the auroral zones. These solitary waves are observed in the Wideband data as isolated bipolar and tripolar waveforms. Examples of the two types of pulses are provided. The time durations of the majority of both types of solitary waves observed in this region range from about 0.3 up to 5ms. Their peak-to-peak amplitudes range from about 0.05 up to 20mV/m, with a few reaching up to almost 70mV/m. There is essentially no potential change across the bipolar pulses. There appears to be a small, measurable potential change, up to 0.5V, across the tripolar pulses, which is consistent with weak or hybrid double layers. A limited cross-spacecraft correlation study was carried out in order to identify the same solitary wave on more than one spacecraft. We found no convincing correlations of the bipolar solitary waves. In the two cases of possible correlation of the tripolar pulses, we found that the solitary waves are propagating at several hundred to a few thousand km/s and that they are possibly evolving (growing, decaying) as they propagate from one spacecraft to the next. Further, they have a perpendicular (to the magnetic field) width of 50km or greater and a parallel width of about 2-5km. We conclude, in general, however, that the Cluster spacecraft at separations along and perpendicular to the local magnetic field direction of tens of km and greater are too large to obtain positive correlations in this region. Looking at the macroscale of the auroral zone at 4.5-6.5RE, we find that the onsets of the broadband electrostatic noise associated with the solitary waves observed in the spectrograms of the WBD data are generally consistent with propagation of the solitary waves up the field lines (away from Earth), or with particles or waves propagating up the field line, which leads to local generation of the solitary waves all along the field lines. A discussion of the importance of these solitary waves in magnetospheric processes and their possible generation mechanisms, through electron beam instabilities and turbulence, is provided.

Description: Through case studies involving Cluster waveform observations, solitary waves in the form of bipolar and tripolar pulses have recently been found to be quite abundant in the near-Earth dayside magnetosheath. We expand on the results of those previous studies by examining the distribution of solitary waves from the bow shock to the magnetopause using Cluster waveform data. Cluster's orbit allows for the measurement of solitary waves in the magnetosheath from about 10 RE to 19.5 RE. Our results clearly show that within the magnetosheath, solitary waves are likely to be observed at any distance from the bow shock and that this distance has no dependence on the time durations and amplitudes of the solitary waves. In addition we have found that these same two quantities show no dependence on either the ion velocity or the angle between the ion velocity and the local magnetic field direction. These results point to the conclusion that the solitary waves are probably created locally in the magnetosheath at multiple locations, and that the generation mechanism is most likely not solely related to ion dynamics, if at all. To gain insight into a possible local generation mechanism, we have examined the electron differential energy flux characteristics parallel and perpendicular to the magnetic field, as well as the local electron plasma and cyclotron frequencies and the type of bow shock that Cluster is behind, for several time intervals where solitary waves were observed in the magnetosheath. We have found that solitary waves are most likely to be observed when there are counterstreaming (~parallel and anti-parallel to the magnetic field) electrons at or below about 100eV. However, there are times when these counterstreaming electrons are present when solitary waves are not. During these times the background magnetic field strength is usually very low (

Description: Bipolar pulses of ~ 25-100 µs in duration have been observed in the wave electric field data obtained by the Wideband plasma wave instrument on the Cluster spacecraft in the dayside magnetosheath. These pulses are similar in almost all respects to those observed on several spacecraft over the last few years. They represent solitary potential structures, and in this case, electron phase space holes. When the time series data containing the bipolar pulses on Cluster are transformed to the frequency domain by a windowed FFT, the pulses appear as typical broad-band features, extending from the low-frequency cutoff of the bandpass filter, ~ 1 kHz, up to as great as 20-40 kHz in some cases, with decreasing intensity as the frequency increases. The upper frequency cutoff of the broad band is an indication of the individual pulse durations (1/f). The solitary potential structures are detected when the local magnetic field is contained primarily in the spin plane, indicating that they propagate along the magnetic field. Their frequency extent and intensity seem to increase as the angle between the directions of the magnetic field and the plasma flow decreases from 90°. Of major significance is the finding that the overall profile of the broad-band features observed simultaneously by two Cluster spacecraft, separated by a distance of over 750 km, are strikingly similar in terms of onset times, frequency extent, intensity, and termination. This implies that the generation region of the solitary potential structures observed in the magnetosheath near the bow shock is very large and may be located at or near the bow shock, or be connected with the bow shock in some way.

Description: In order to estimate the dissipation time-scale in magnetic turbulence in the plasma sheet a novel method is introduced for classification of velocity dependent patterns of two-point probability density functions' shapes near their maxima. For the first time, we provide evidence for Reynolds number (velocity) dependent widening of the inertial range in the plasma sheet. Since spectral widening of the inertial range is a generic feature of many turbulent flows, its examination can facilitate the recognition of intermittent turbulence in the plasma sheet.

Description: Between December 1997 and March 1998 Equator-S made a number of excursions into the dawn-side magnetosheath, over a range of local times between 6:00 and 10:40 LT. Clear mirror-like structures, characterised by compressive fluctuations in |B| on occasion lasting for up to 5 h, were observed during a significant fraction of these orbits. During most of these passes the satellite appeared to remain close to the magnetopause (within 1–2 Re), during sustained compressions of the magnetosphere, and so the characteristics of the mirror structures are used as a diagnostic of magnetosheath structure close to the magnetopause during these orbits. It is found that in the majority of cases mirror-like activity persists, undamped, to within a few minutes of the magnetopause, with no observable ramp in |B|, irrespective of the magnetic shear across the boundary. This suggests that any plasma depletion layer is typically of narrow extent or absent at the location of the satellite, at least during the subset of orbits containing strong magnetosheath mirror-mode signatures. Power spectra for the mirror signatures show predominately field aligned power, a well defined shoulder at around 3–10 x 10 –2 Hz and decreasing power at higher frequencies. On occasions the fluctuations are more sinusoidal, leading to peaked spectra instead of a shoulder. In all cases mirror structures are found to lie approximately parallel to the observed magnetopause boundary. There is some indication that the amplitude of the compressional fluctuations tends to be greater closer to the magnetopause. This has not been previously reported in the Earth's magnetosphere, but has been suggested in the case of other planets.Key words. Magnetospheric physics (magnetosheath; plasma waves and instabilities; magnetopause, cusp and boundary layers)

Description: 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)

Description: For the first time during the mission, the Anisotropy Telescopes instrument on board the Ulysses spacecraft measured constant zero anisotropy of protons in the 1.3-2.2 MeV energy range, for a period lasting more than three days. This measurement was made during the energetic particle event taking place at Ulysses between 25 November and 15 December 1998, an event characterised by constant high proton fluxes within a region delimited by two interplanetary forward shocks, at a distance of 5.2 AU from the Sun and heliographic latitude of 17°S. We present the ATs results for this event and discuss their possible interpretation and their relevance to the issue of intercalibration of the two telescopes.Key words: Interplanetary physics (energetic particles) - Solar physics, astrophysics and astronomy (energetic particles) - Space plasma physics (instruments and techniques)

Description: 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)