D. H. Fairfield

The Cluster Magnetic Field Investigation

The Cluster mission provides a new opportunity to study plasma processes and structures in the near-Earth plasma environment. Four-point measurements of the magnetic field will enable the analysis of the three dimensional structure and dynamics of a range of phenomena which shape the macroscopic properties of the magnetosphere. Difference measurements of the magnetic field data will be combined to derive a range of parameters, such as the current density vector, wave vectors, and discontinuity normals and curvatures, using classical time series analysis techniques iteratively with physical models and simulation of the phenomena encountered along the Cluster orbit. The control and understanding of error sources which affect the four-point measurements are integral parts of the analysis techniques to be used. The flight instrumentation consists of two, tri-axial fluxgate magnetometers and an on-board data-processing unit on each spacecraft, built using a highly fault-tolerant architecture. High vector sample rates (up to 67 vectors s-1) at high resolution (up to 8 pT) are combined with on-board event detection software and a burst memory to capture the signature of a range of dynamic phenomena. Data-processing plans are designed to ensure rapid dissemination of magnetic-field data to underpin the collaborative analysis of magnetospheric phenomena encountered by Cluster.

Geotail observations of magnetic flux ropes in the plasma sheet

[1] Examination of Geotail measurements in the near-tail (X > � 30 RE) has revealed the presence of small flux ropes in the plasma sheet. A total of 73 flux rope events were identified in the Geotail magnetic field measurements between November 1998 and April 1999. This corresponds to an estimated occurrence frequency of � 1 flux rope per 5 hours of central plasma sheet observing time. All of the flux ropes were embedded within high-speed plasma sheet flows with 35 directed Earthward, hVxi = 431 km/s, and 38 moving tailward, hVxi = � 451 km/s. We refer to these two populations as ‘‘BBF-type’’ and ‘‘plasmoid-type’’ flux ropes. The flux ropes were usually several tens of seconds in duration, and the two types were readily distinguished by the sense of their quasisinusoidal Bz perturbations, i.e., � for the ‘‘BBF’’ events and ± for the ‘‘plasmoid’’ events. Most typically, a flux rope was observed to closely follow the onset of a high-speed flow within � 1–2 min. Application of the Lepping-Burlaga constant-a flux rope model (i.e., J = aB) to these events showed that approximately 60% of each class could be acceptably described as cylindrical, force-free flux ropes. The modeling results yielded mean flux rope diameters and core field intensities of 1.4 RE and 20 nT and 4.4 RE and 14 nT for the BBF and plasmoid-type events, respectively. The inclinations of the flux ropes were small relative to the GSM X–Y plane, but a wide range of azimuthal orientations were determined within that plane. The frequent presence of these flux ropes in the plasma sheet is interpreted as strong evidence for multiple reconnection X-lines (MRX) in the near-tail. Hence, our results suggest that reconnection in the near-tail may closely resemble that at the dayside magnetopause where MRX reconnection has been hypothesized to be responsible for the generation of flux transfer events. INDEX TERMS: 2740 Magnetospheric Physics: Magnetospheric configuration and dynamics; 2764 Magnetospheric Physics: Plasma sheet; 2744 Magnetospheric Physics: Magnetotail; 2788 Magnetospheric Physics: Storms and substorms

Geotail observations of the Kelvin‐Helmholtz instability at the equatorial magnetotail boundary for parallel northward fields

For several hours on March 24, 1995, the Geotail spacecraft remained near the duskside magnetotail boundary some 15 RE behind the Earth while the solar wind remained very quiet (V=330 km s−1, n=14–21 cm−3) with a very steady 11-nT northward magnetic field. Geotail experienced multiple crossings of a boundary between a dense (n= 19 cm−3), cool (Tp=40 eV), rapidly flowing (V=310 km s−1) magnetosheath plasma and an interior region characterized by slower tailward velocities (V=100 km s−1), lower but substantial densities (n=3 cm−3) and somewhat hotter ions (220 eV). The crossings recurred with a roughly 3-min periodicity, and all quantities were highly variable in the boundary region. The magnetic field, in fact, exhibited some of the largest fluctuations seen anywhere in space, despite the fact that the exterior magnetosheath field and the interior magnetosphere field were both very northward and nearly parallel. On the basis of an MHD simulation of this event, we argue that the multiple crossings are due to a Kelvin-Helmholtz instability at the boundary that generates vortices which move past the spacecraft. A determination of boundary normals supports Kelvin-Helmholtz theory in that the nonlinear steepening of the waves is seen on the leading edge of the waves rather than on the trailing edge, as has sometimes been seen in the past. It is concluded that the Kelvin-Helmholtz instability is an important process for transferring energy, momentum and particles to the magnetotail during times of very northward interplanetary magnetic field.

The Global Geospace Science Program and its investigations

The detailed study of the solar-terrestrial energy chain will be greatly enhanced with the launch and simultaneous operation of several spacecraft during the current decade. These programs are being coordinates in the United States under the umbrella of the International Solar Terrestrial Physics Program (ISTP) and include fundamental contributions from Japan (GEOTAIL Program) and Europe (SOHO and CLUSTER Programs). The principal United States contribution to this effort is the Global Geospace Science Program (GGS) described in this overview paper. Two spacecraft, WIND and POLAR, carrying an advanced complement of field, particle and imaging instruments, will conduct investigations of several key regions of ‘geospace’. This paper provides a general overview of the science objectives of the missions, the spacecraft orbits and the ground elements that have been developed to process and analyze the instrument observations.

Earthward flow bursts in the inner magnetotail and their relation to auroral brightenings, AKR intensifications, geosynchronous particle injections and magnetic activity

High-velocity magnetotail flow bursts measured by the Geotail Low Energy Plasma experiment in the premidnight equatorial region between 10 and 15 RE have been compared with other magnetospheric phenomena. These bursts, typically characterized by earthward velocities approaching 1000 km/s and lasting for times of the order of l min, are associated with magnetotail dipolarizations and large magnetic field fluctuations. Using supporting measurements of the International Solar Terrestrial Physics program it is found that the flow bursts are closely associated with auroral brightenings, AKR onsets, geosynchronous particle injections, and ground magnetic activity. Flow bursts for which Polar UVI images are available showed auroral brightenings that developed near the footpoint Geotail field line. AKR intensifications usually accompanied the flow bursts in close time coincidence, whereas dispersionless geosynchronous particle injections tended to be delayed by 1–3 min. Since flow bursts often exhibit the earliest onsets of these various phenomena, it seems likely that this chain of events is initiated in the tail beyond 15 RE, presumably by magnetic reconnection. It is concluded that flow bursts are a fundamental magnetotail process of limited spacial extent that are important in energy and magnetic flux transport in the magnetosphere. Magnetotail flow bursts are intimately connected to auroral acceleration processes and AKR generation at several thousand kilometer altitude and a full explanation of substorms will have to explain this relationship.

A nonlinear dynamical analogue model of geomagnetic activity

The solar wind-magnetosphere interaction is discussed within the framework of deterministic nonlinear dynamics. Linear prediction filter studies have shown that the magnetospheric response to energy transfer from the solar wind contains both directly driven and unloading components. These studies have also shown that the response is significantly nonlinear and, thus, the filter technique and other correlative techniques cannot give a complete description of that response. Phase space reconstruction studies have shown that the evolution of the nonlinear solar wind-magnetosphere system is dominated by only a few degrees of freedom; the system approaches a low-dimensional attractor on which its behavior can be described using a relatively simple nonlinear dynamical model. An earlier dripping faucet analogue model of the low-dimensional solar wind-magnetosphere system is briefly reviewed, and then a plasma physical counterpart to that model is constructed. A Faraday loop in the magnetotail is considered, and the relationship of electric potentials on the loop to changes in the magnetic flux threading the loop is developed. This approach leads to a model of geomagnetic activity which is similar to the earlier mechanical model but described in terms of the geometry and plasma contents of the magnetotail. The model is best characterized as an elementary time-dependent global convection model. The convection evolves within a magnetotail shape that varies in a prescribed manner in response to the dynamical evolution of the convection. The result is a nonlinear model capable of exhibiting a transition from regular to chaotic loading and unloading. The behavior of the model under steady loading and also some elementary forms of time-dependent loading is discussed. The model appears to properly account for all macrophysical aspects of magnetotail geomagnetic activity, it incorporates both the directly driven and the unloading components of geomagnetic activity, and it includes, in a fundamental way, the inherent nonlinearity of the solar wind-magnetosphere interaction.

Modeling the Growth-Phase of a Substorm Using the Tsyganenko Model and Multi-Spacecraft Observations - Cdaw-9

The CDAW-9 Event C focused upon the early part of 3 May 1986 when a large substorm onset occurred at 0111 UT. By modifying the Tsyganenko 1989 magnetic field model, the authors construct a model in which the near-Earth current systems are enhanced with time to describe the observed development of the tail magnetic field during the growth phase. The cross-tail current intensity and the thickness of the current sheet are determined by comparison with three spacecraft in the near-Earth tail. The location of the auroral bulge as recorded by the Viking imager is mapped to the equatorial current sheet. The degree of chaotization of the thermal electrons is estimated, and the consequences to the tail stability towards ion tearing are discussed. The authors conclude that the mapping of the brightening region in the auroral oval corresponds to the regions in the tail where the current sheet may be unstable towards ion tearing.

Plasmoid ejection and auroral brightenings

Geotail plasma and magnetic field observations of 24 plasmoids between 21 and 29 RE have been compared with Polar ultraviolet observations of auroral brightenings. Both single and multiple plasmoids almost always corresponded to brightenings, but the brightenings were sometimes weak and spatially limited and did not always grow to a global substorm. Even a case where a plasmoid event occurred with fast postplasmoid flow corresponded to a weak brightening but no substorm. Some brightenings did not correspond to plasmoids, but these brightenings were observed away from the longitude of Geotail, indicating that plasmoids have a small longitudinal extent in the near tail. The plasmoids were occasionally observed before the brightenings but more frequently were observed 0–2 min after the brightenings, with the delays probably due to the transit time to the observation point. It seems likely that formation of a near-Earth neutral line causes each brightening in the polar ionosphere, but these formations do not always lead to a full-fledged substorm. What additional circumstance causes development of a full, large-scale substorm remains an open question.

Geotail observations of substorm onset in the inner magnetotail

On April 26, 1995, while Geotail was in the near-equatorial magnetotail at 13 RE and 2300 LT, a substorm onset occurred that was documented by ground magnetograms, auroral kilometric radiation, and magnetic field and particle data from four spacecraft at and near geosynchronous orbit. Although Geotail was initially outside a greatly thinned current sheet, plasma sheet thickening associated with the substorm dipolarization quickly caused Geotail to move into the plasma sheet where it observed field-aligned earthward moving ions with velocities of 400 km/s. During the subsequent few minutes as the magnetic field became more northward, the velocities increased with particles moving increasingly into the energy range of the energetic particle experiment. These flows culminated with 1-min worth of earthward flow of 2000 km/s that was perpendicular to the northward B field. Such flow, probably the largest ever detected at 13 RE, was confirmed by the observation of an intense dc electric field of 50 mV/m (0.3 megavolts/RE). This large field is probably inductive, caused by reconnection that occurred tailward of the spacecraft, and related to the acceleration processes associated with particle injection at geosynchronous orbit. Energy and magnetic flux conservation arguments suggest that this rapid flow has a small cross-tail dimension of the order of 1 RE. The data appear to support a simulation of Birn and Hesse [1996] which showed rapid earthward flows from a reconnection line at 23 RE that caused a tailward expansion of a region of dipolarized flux. Subsequent to the onset, Geotail observed plasma vortices with typical velocities of 50–100 km/s that occurred in a high-beta plasma sheet with a 15-nT northward magnetic field. The vortices were punctuated by occasional flow bursts with velocities up to 400 km/s, one of which was accompanied by a violently varying magnetic field where north/south field components were as large as 30 nT and as small as −8 nT.

Unusual locations of Earth's bow shock on September 24–25, 1987: Mach number effects

ISEE 1 and IMP 8 data are used to identify 19 crossings of Earths bow shock during a 30-hour period following 0000 UT on September 24, 1987. Apparent standoff distances for the shock are calculated for each crossing using two methods and the spacecraft location; one method assumes the average shock shape, while the other assumes a ram pressure-dependent shock shape. The shocks apparent standoff distance, normally ∼ 14 RE, is shown to increase from near 10 RE initially to near 19 RE during an 8-hour period, followed by an excursion to near 35 RE (where two IMP 8 shock crossings occur) and an eventual return to values smaller than 19 RE. The Alfven MA and fast magnetosonic Mms Mach numbers remain above 2 and the number density above 4 cm−3 for almost the entire period. Ram pressure effects produce the initial near-Earth shock location, whereas expansions and contractions of the bow shock due to low Mach number effects account, qualitatively and semiquantitatively, for the timing and existence of almost all the remaining ISEE crossings and both IMP 8 crossings. Significant quantitative differences exist between the apparent standoff distances for the shock crossings and those predicted using the observed plasma parameters and the standard model based on Spreiter et al.s (1966) gasdynamic equation. These differences can be explained in terms of either a different dependence of the standoff distance on Mach number at low MA and Mms, or variations in shock shape with MA and Mms (becoming increasingly “puffed up” with decreasing MA and Mms, as expected theoretically), or by a combination of both effects. Global MHD simulations, to be presented elsewhere, confirm that both effects occur and are significant. Ram pressure-induced changes in the shocks shape are discussed but found to be quantitatively unimportant for the shock crossings analyzed. Approximate estimates of both the deviation of the shocks standoff distance from the standard model and of the shocks shape are determined independently (but not consistently) for Mms ∼ 2.4. The estimates imply substantial changes in standoff distance and/or shock shape at low MA and Mms. Mach number effects can therefore be quantitativwely important in determining and predicting the shape and location of the bow shock, even when MA and Mms remain above 2. This study confirms and generalizes previous studies of Mach number effects on Earths bow shock. Statistical studies and simulations of the bow shocks shape and location should be performed as a function of Mach number, magnetic field orientation, and ram pressure.