Abstract

Flux transfer events appear to be manifestations of transient reconnection of the magnetospheric and magnetosheath magnetic fields at the Earth's magnetopause. It is clearly a significant magnetospheric phenomenon, yet the one aspect of dayside coupling that is, perhaps, most poorly understood. Many scientists have invoked FTEs to generate boundary motions on the magnetopause that can power ULF waves in the magnetosphere, to explain transient phenomena in the ionosphere, to be responsible for significant transfer of magnetic flux to the geomagnetic tail and the subsequent geomagnetic activity, and to power magnetospheric convection. This is not justified until we understand the FTE itself better. We propose to determine the magnetic and plasma structure of flux transfer events. The objective of this effort is to determine the source of plasmas within FTEs and how FTEs are formed. In this investigation we will determine the interior structure of flux transfer events and characterize both the field draping region and the reconnected flux tubes from combined magnetic field and plasma distribution function data from the ISEE-2 spacecraft.

The proposed work is an important one in magnetospheric research today and is hotly debated. The results of this study by themselves also provide the proof of concept that will enable us to prepare a very competitive proposal for follow-on funding.

This work will be directed jointly by Dr. Guan Le (UCLA) and Dr. John T. Gosling (LANL) and undertaken by graduate student Xiaowen Zhou as part of her dissertation research.

I. Proposed Research

The momentum and energy transfer to the Earth's magnetosphere from the solar wind is controlled primarily by reconnection that occurs in the vicinity of the subsolar point. Some of this reconnection appears to occur in a quasi-continuous fashion but sometimes reconnection appears to be quite discontinuous. One manifestation of time varying reconnection is believed to be the phenomenon called a Flux Transfer Event which is generally most discernible in the magnetic field near the magnetopause. The signature of a flux transfer event is a strong component of the magnetic field along the expected normal to the boundary with a bipolar signature, outward, then inward or vice versa. Other components of the field as well as the field magnitude vary in concert, but with a more varied signature. They can be observed both in the magnetosheath and in the magnetosphere near the magnetopause. The original suggestion for their appearance was that they represented the result of a temporary increase in the reconnection rate between magnetospheric and magnetosheath field lines. The resulting connected flux tube would go from the solar wind into the magnetosphere and down to the ionosphere as sketched in Figure 1 [Russell and Elphic, 1978]. As the flux tube moves tailward, the neighboring magnetic field drapes around it causing the characteristic FTE magnetic bi-polar signatures, as illustrated in Figure 2. Early statistical studies revealed that FTEs occur preferentially during nearly horizontal or southward interplanetary magnetic field (IMF) and the polarity of the bi-polar signatures is ordered by the geomagnetic equator, in a manner consistent with the global picture of a reconnected flux tube moving away from the equatorial region [Berchem and Russell, 1984; Rijnbeek et al., 1984; Southwood et al., 1986].

To date, the majority of studies of FTEs have focused primarily on their magnetic signatures. As a result, the plasma signatures of FTEs are relatively poorly understood. Within portions of many FTEs the plasma appears to be a mixture of magnetosheath and magnetospheric origins [Paschmann et al., 1982; Thomsen et al., 1987; Daly et al., 1981]. We know of only one way to obtain such a mixture: reconnection of magnetospheric and magnetosheath field lines at the magnetopause. At other times one apparently can not discern such a mixture of magnetosheath and magnetospheric plasmas on magnetic field lines within FTEs. Often the field lines where a mixture of magnetosheath and magnetospheric plasma is observed occur in the trailing portion of the FTEs [Elphic, 1995]. It is not presently understood why this should be the case. Our working hypothesis is that where a mixture of plasmas is observed within FTEs, the satellite directly samples reconnected flux tubes, but where this mixture is not observed the satellite samples field lines that are draped around the reconnected flux. Thus, most of the field lines in the leading portions of some FTEs may be draped about trailing reconnected field lines. This hypothesis, although attractive, needs to be tested against the observations.

Many puzzles also exist in the FTE magnetic signatures. For example, while the magnetic field component normal to the magnetopause surface consistently exhibit a bi-polar signature, the other two components and the field strength are much more variable and do not show consistent patterns. For many FTEs, the magnetic field strength shows a maximum near the center of the FTE signature. Such maximum can be readily explained in the field draping region. But some of FTEs shows a "crater" signature, or a local minimum [Luhr and Klocker, 1987; LaBelle et al., 1987] and some show no maximum or maximum. Evidence that the magnetic field inside FTEs sometimes twists about the tube axis has also been reported [Saunders et al., 1984]. Such variations in the magnetic field pattern also lead us to believe that there are two distinct regions within the identified FTEs. Lack of understanding of FTE structure is partially a result of the lack of combined field and plasma studies. No attempt has yet been made to examine the correlation between variations in plasma signatures and variations in magnetic field signatures.

In order to understand the processes that lead to the formation of FTEs, we need to characterize further the properties of FTEs, especially their interior structures. The joint data bases at UCLA (magnetic field) and LANL (plasma) from the ISEE mission together with the data analysis expertise at both institutions should enable us to make important inroads into understanding the structure as well as the generation of FTEs. As outlined below we intend to study the structure of combined magnetic and plasma signatures within FTEs using high resolution magnetic field and plasma data. We will determine the field and plasma characteristics in both the field draping region and inside the reconnected flux tube. Especially we will examine the field and plasma data inside the reconnected flux tube to determine the origins of the plasmas. We also will conduct a more general study of the relative mix of plasma components within FTE flux tubes to determine what controls the percentage of plasma from each source. We have successfully used the combined plasma and magnetic field data from ISEE 1 and 2 to study the magnetopause and boundary layer [Le et al., 1994; 1996].

II. Objectives and Significance

The objective of this effort is to determine the source of plasmas within FTEs and to learn how FTEs are formed. In this investigation we will determine the interior structure of flux transfer events and will characterize both the field draping region and the reconnected flux tubes using the combined magnetic field and plasma distribution function data from the ISEE-2 spacecraft. The plasma data should also allow identification of any ionospheric electrons that are present within FTEs to determine if the reconnected flux tube is in fact connected to the ionosphere as shown in Figure 1.

The proposed work is an most important topic in magnetospheric research today. Flux transfer events are a significant magnetospheric phenomenon, yet they are the one aspect of dayside coupling that is, perhaps, most poorly understood. FTEs appear to result from transient reconnection at the dayside magnetopause. Many scientists invoke FTEs to generate boundary motions on the magnetopause that power ULF waves in the magnetosphere, to explain transient phenomena in the ionosphere, to be responsible for significant transfer of magnetic flux from the dayside to the geomagnetic tail, and to power magnetospheric convection. FTEs have been the subject of a large number of theoretical papers [e.g., Lee and Fu, 1985; Scholer, 1988; Southwood et al., 1988; Smith et al., 1992] and are of great current interest in the space plasma community. Given this interest, it is essential to understand their observed physical nature better.

III. Approach Flux transfer events are identified mainly from a bi-polar signature in the magnetic field component normal to the magnetopause boundary. Plasma mixing signatures are present only in a subset of FTEs. Even within that subset, mixing signatures are present only in a part of the magnetic signature. Our hypothesis is that the plasma mixing signatures arise when the spacecraft passes through the actual reconnected flux tube. We associate the absence of a mixing signature with field line draping around the reconnected flux tube. Such a hypothesis can be verified by examining the combined magnetic field and plasma data within FTEs and characterizing the FTE's interior magnetic and plasma structures. Flux Transfer Events (FTEs) have been identified in the ISEE 1 and 2 magnetometer data. A complete ISEE FTE data base from the full ISEE mission has been established from our previous studies. First we will select events with plasma mixing signatures based on the ISEE-2 FPE plasma moment data. We will examine the high resolution magnetic field and plasma distribution functions for these events. Specifically, we will determine the source of the plasmas within the region with plasma mixing signatures for FTEs found in both the magnetosheath and the magnetosphere. In our preliminary study, we have found a strong depletion of hot magnetospheric plasma and inflow of magnetosheath plasma within magnetospheric FTEs, and outflow of magnetospheric plasma in the magnetosheath FTEs. These observations are consistent with the reconnected open flux tube interpretation. Figure 3 shows an example of a magnetospheric FTE, in which the plasma mixing signature is present in a portion of the magnetic signature. The magnetic field strength increases initially in the identified field draping region. Figure 4 shows the plasma distribution functions taken in the field draping region and within the reconnected flux tube for them same event. The plasma in the field draping region is essentially the same as the magnetospheric plasma. But we see a strong depletion of the hot magnetospheric plasma, especially electrons, and entry of magnetosheath plasma within the reconnected flux tube. We will examine the relative mix of plasma components within the reconnected flux tubes to determine what controls the percentage of plasma from each source. The magnetic field in the plasma mixing region does not always show a maximum in the field strength, indicating the field topology is different from the draping region. More detailed study, especially in comparison with theoretical predictions, is needed to understand the field topology. We will examine the field signature outside the region of plasma mixing signatures to look for draping signatures, i.e., to determine if the magnetic field strength increase is associated with the signature in the normal component as the spacecraft approaches the flux tube, and if there is a consistent pattern in the other two components as predicted from the field draping. The draping signature has been seen in our preliminary study. Meanwhile we will select events without plasma mixing signatures. We will determine if these FTE signatures are caused by the spacecraft encountering only the field line draping region. We will compare the magnetic signatures of FTEs without plasma mixing signatures to those of field draping region in the FTEs with plasma mixing signatures. Then, to further study the plasma data, we will search our magnetometer records for FTEs with field lines that were tilted closer than 55o to the ecliptic plane so that any ionospheric electrons, which would be strongly field aligned, would be detected by the ñ55o field-of-view of the plasma experiment. Since FTEs are often accompanied by large deflections of the magnetic field, we expect to be able to find several suitable FTEs both inside and outside the magnetosphere. We will especially wish to examine FTEs during periods of large spacecraft separation in 1978 when the plasma instruments on both ISEE 1 and ISEE 2 were operating to look for differences in the plasma properties in different regions of the FTE. We note that any such study using plasma data is likely to produce new results because very little has been done to characterize the FTEs outside of the two limited studies by Paschmann et al. [1982] and Thomsen et al. [1987]. IV. Proposing Team The UCLA Principal Investigator, Guan Le, will supervise the day-to-day research of the student Xiaowen Zhou who will undertake this project as part of her thesis research. She will assist the student to obtain the data from the ISEE magnetometer and provide guidance in the use of analysis techniques. The Laboratory Principal Investigator, John T. Gosling, will assist with the overall conduct of the research. In particular, he will assist in obtaining needed ISEE Fast Plasma Experiments (FPE) data and provide advice in the analysis and interpretation of these data. Graduate student Xiaowen Zhou will conduct the bulk of the research project as part of her graduate dissertation research in the Earth and Space Sciences Department at UCLA. Others who we expect will be involved include: C. T. Russell (UCLA) who is Xiaowen Zhou's academic advisor and R. C. Elphic (LANL) who has strong interest and much experience in studying flux transfer events. V. Facilities to be Used a) Facilities at Los Alamos The plasma data to be used in this project reside uniquely at Los Alamos. They also require computer programs for access that are LANL unique. Thus, the plasma oriented portion of the effort must be done at Los Alamos. Moreover, the expertise in interpreting these data lie entirely with the LANL PI and his colleagues who will be consulted in the execution of this project. b) Facilities at UCLA The high resolution magnetometer data to be used in this project reside uniquely at UCLA. The analysis routines for the magnetometer data also reside at UCLA, although these may be accessed remotely. VI. Future of Project We have a small grant from NASA that maintains our magnetopause data base but does not allow student support. We plan to propose next year to the NSF GEM program to expand our study of the magnetopause. This grant will help us to prepare a good proposal. Figure Captions Figure 1 Schematic of the reconnection model of an FTE. Figure 2 Schematic illustrating how the ambient field drapes around the an FTE cylindrical flux tube traveling in the X direction. Figure 3. Magnetic field in boundary normal coordinates and plasma moment data for an example of magnetospheric FTE. Figure 4. Ion and electron distribution functions observed within the magnetospheric FTE in Figure 3.  References Berchem, J., and C. T. Russell, Flux transfer events at the magnetopause: Spatial distribution and controlling factors, J. Geophys. Res., 89, 6689-6703, 1984. Daly, P. W., D. J. Williams, C. T. Russell, and E. Keppler, Particle signature of magnetic flux transfer events at the magnetopause, J. Geophys. Res., 86, 1628-1632, 1981. Elphic, R. C., Observations of flux transfer events: A review, in Physics of the Magnetopause, Geophysical Monograph 90, edited by P. Song, B. U. O. Sonnerup and M. F. Thomsen, American Geophysical Union, Washington DC, 1995. LaBelle, J., R. A. Treumann, G. Haerendel, O. H. 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Saunders, What are flux transfer events?, Planet. Space Sci., 36, 503-508, 1988. Thomsen, M. F., J. A. Stansberry, S. J. Bame, S. A. Fuselier, and J. T. Gosling, Ion and electron velocity distributions within flux transfer events, J. Geophys. Res., 92, 12,127-12,136, 1987.