M.A. Saunders

What are flux transfer events

Abstract We argue that surges in the reconnection rate on the magnetopause give rise to bubble-like regions of plasma containing a twisted field with energetic streaming particles in the outer layers. We propose that flux transfer events observed on spacecraft are the result of passage through or nearby such bubbles. All important observational features of flux transfer events fit qualitatively well with the model. Predictions of phenomena in the ionosphere associated with FTEs based on our model are substantially different from those predicted on earlier models such as Russell and Elphics; events could have a much larger footprint than believed hitherto. Correspondingly, predictions of the electromotive force imposed around the terrestrial polar cap by FTE occurrence may need upward revision.

Dayside ionospheric convection changes in response to long-period interplanetary magnetic field oscillations - Determination of the ionospheric phase velocity

Ground magnetic field perturbations recorded by the CANOPUS magnetometer network in the 7 to 13 MLT sector are used to examine how reconfigurations of the dayside polar ionospheric flow take place in response to north-south changes of the IMF. During the 6-hour interval in question IMF Bz oscillates between ±7 nT with about a 1-hour period. Corresponding variations in the ground magnetic disturbance are observed which we infer are due to changes in ionospheric flow. Cross correlation of the data obtained from two ground stations at 73.5° magnetic latitude, but separated by ∼2 hours in MLT, shows that changes in the flow are initiated in the prenoon sector (∼10 MLT) and then spread outward toward dawn and dusk with a phase speed of ∼5 km s−1 over the longitude range ∼8 to 12 MLT, slowing to ∼2 km s−1 outside this range. Cross correlating the data from these ground stations with IMP 8 IMF Bz records produces a MLT variation in the ground response delay relative to the IMF which is compatible with these deduced phase speeds. We interpret these observations in terms of the ionospheric response to the onset, expansion and decay of magnetic reconnection at the dayside magnetopause.

A survey of flux transfer events recorded by the UKS spacecraft magnetometer

Abstract The UKS spacecraft operated from August 1984 through to January 1985. During that time, it made multiple crossings of the magnetopause in local time sectors extending from mid-afternoon to just behind the dawn meridian. We have surveyed the magnetometer records from these magnetopause encounters and have compiled a catalogue of flux transfer events (FTEs using criteria identical to those employed by Rijnbeek et al. (1984, J. Geophys. Res. 89, 786) in their survey of ISEE spacecraft magnetometer data. Using the catalogue, we find that FTE occurrence determined from the UKS data set is substantially less than that detected using data from the early ISEE 1 2 spacecraft orbits. The UKS data set shows a correlation between FTE occurrence and southward external magnetic field, but there are several instances of passes in which no FTEs are detected but for which the external field was unam- biguousluy southward. The passes with the largest number of events are those for which the field outside the magnetopause has a large BM component. We conclude that the lower latitude of the UKS encounters is responsible for the discrepancy with the ISEE occurrence. The most likely source region appears to be near the subsolar region.

Interpretation of magnetic field perturbations in the earth's magnetopause boundary layers

Studies of the boundary layers in the vicinity of the Earths dayside magnetopause are important in determining the nature of the processes which couple the magnetosphere to the flowing solar wind, thereby driving magnetospheric convection. In this paper we examine theoretically the magnetic field and plasma properties expected in the boundary regions for various models involving either diffusion or reconnection at the boundary. For diffusion models the transport of magnetosheath momentum across the magnetopause will result in field shears on either side of the boundary, the field rotations being in opposite senses on either side relative to the undisturbed fields. The directions of these rotations depend upon location at the magnetopause relative to the momentum transfer region and to the noon meridian. In reconnection models the effect of the tension of the open boundary layer field lines must be taken into account in addition to the magnetosheath flow, but on the super-Alfvenic flanks of the magnetosphere the latter still dominates, so that qualitatively similar effects will occur in the two models. More detailed, quantitative or statistical studies are then required to distinguish the two models in this regime. In the sub-Alfvenic dayside region, however, open field tension effects will dominate in reconnection models such that boundary layer field and plasma properties will then be determined mainly by the magnetosheath magnetic field configuration. In particular the East-West flow in the magnetospheric boundary layer will be controlled largely by the East-West field in the magnetosheath, leading to flow reversals across the magnetopause in some quadrants of the magnetopause. This behaviour is directly related to the Svalgaard-Mansurov effect and is a signature unique to reconnection models. The boundary layer fields are also expected to tilt towards the field on the opposite side of the boundary in these models on the dayside. “Toward” tilting can also occur in this regime in diffusion models, but “away” tilting, a signature unique to dayside diffusion, should also occur equally frequently. Finally, we briefly discuss previously published high-resolution ISEE 1 and 2 data from the boundary regions in the light of our results. We find that “toward” tilting generally occurs in boundary region crossings previously identified as being reconnection-associated and we present some examples in which the above unique reconnection signature has been observed. During impulsive FTE-like events, however, the field may tilt in either direction, possibly as a result of field line twists, thus complicating our simple picture in this case. We also show that the “reverse draping” observations presented by Hones et al. (1982) approximately satisfy the open magnetopause stress balance conditions.

A hydromagnetic vortex seen by ISEE-1 and 2

Abstract Magnetometer and plasma data from the dual ISEE spacecraft are combined in a study of the initial plasma vortex event reported by Hones et al . (1978) in the dawn plasma sheet. The event is a transient hydromagnetic wave of two cycles duration with a six minute period. Large amplitude compressional and transverse magnetic components were present. Particle and magnetic pressure oscillations were in strict antiphase, but did not balance. When combined with the plasma velocity data these properties show that substantial Earthward field-aligned flows of electromagnetic energy and heat flux occurred during the vortex. The nett energy flow perpendicular to B was in the antisolar direction. This event possesses hydromagnetic features unique to a hot plasma environment.

Hydromagnetic vortices. I: The 11 december 1977 event

Abstract Through a synthesis of magnetometer, plasma, energetic particle and electric field data from the ISEE satellite pair, we describe the characteristics of the initial (11 December 1977) magnetotail plasma vortex event reported by Hones et al. (1978). The event is associated with a hot (β ∼ 1) compressional hydromagnetic wave and apparent vortical motion is seen because at two points in the flow cycle the flow is field-aligned. The behaviour of the energetic ions receives special study : when combined with the thermal flow measurements energy dispersion is evident in the field-aligned flow, while the large pitch angle energetic ions reveal the presence of gradients. We argue that these gradients are wave-induced and use the data to determine the perpendicular wave wavelength together with the speed and direction of transverse wave propagation.

The morphology of dayside Birkeland currents

Abstract Intense (10 5 A) electric currents flow into and from the Earths two polar ionospheres near magnetic noon. These currents, called Birkeland or magnetic field-aligned currents, are the agent by which momentum couples from the flowing solar wind plasma to drive plasma motions in the high latitude ionosphere. Coupling is strongest when the interplanetary magnetic field (IMF) has a southward component and when this occurs there exist two principal regions of Birkeland current near magnetic noon called the region 1 and the cusp systems. We present a simple model bringing theoretical order to the many patterns proposed previously for the morphology of these dayside Birkeland currents as observed by orbiting satellites in the topside polar ionosphere. Specifically we show that the cusp Birkeland current system is not a latitudinally separate region but is instead the extension in longitude of the region 1 Birkeland current from either dawn or dusk; which particular one depends on the sign of the east-west ( Y ) component of the IMF. The presence of an IMF Y -component therefore leads to two region 1 current systems near magnetic noon, with the poleward one being that previously called the ‘cusp’ system.

Hydromagnetic vortices. II: Further dawnside events

We show that the 11 December 1977 plasma vortex event—the subject of a multi-instrument investigation in Paper I (Saunders et al., 1983)—was neither atypical nor uncommon, by describing the magnetic and plasma characteristics of three further vortices recorded within 3 weeks of, and at similar locations to, the 11 December study. One of the new events has added interest since magnetic pulsations were seen simultaneously on the ground in the vicinity of the satellite magnetic “footprint”.

Magnetosheath, magnetopause and low latitude boundary layer research, 1987–1989

Abstract Important developments over the past two years (1987–1989) in studies of the magnetosheath, the magnetopause and the magnetopause boundary layers are reviewed. The pace of research shows no sign of abating, but progress has become focused on certain topics. These include flux transfer event reconnection and the relevance of solar wind dynamic pressure variations for magnetopause and polar cusp dynamics. A promising development in both these areas is the use of ground observations under the polar cusp ionosphere for monitoring solar wind-magnetosphere coupling processes at the magnetopause. In contrast to these progress areas, the magnetosheath and the low latitude boundary layer remain topics which are being unjustifiably neglected. The controversies and the key remaining questions are highlighted and suggestions are made on how to proceed further.

Mass-loading and the formation of the Venus tail

Abstract Despite its lack of an intrinsic magnetic field Venus has a well defined magnetotail, containing about 3 megawebers of magnetic flux in a tail about 4 R V across with perhaps a slightly elliptical cross section. This tail arises through the mass-loading of magnetic flux tubes passing by the planet. Mass-loading can occur due to charge exchange and photo-ionization as well as from the diffusion of magnetic field into the ionosphere. Various evidence exists for the mass-loading process, including the direct observation of the picked up ions with both the Venera and Pioneer Venus plasma analyzers.