Signatures Of Reconnection Research Articles

A theory of patchy and intermittent reconnections for magnetospheric flux transfer events

We propose that the elbow‐shaped flux transfer events (FTEs) of Russell and Elphic (1979) are initiated by a specific normal mode of three‐dimensional tearing of the magnetopause current sheet, resulting in a three‐dimensional component reconnection driven by the solar wind. The patchy nature of the proposed three‐dimensional tearing reconnection is determined by the structure of the three‐dimensional tearing on the dayside magnetopause, leading to isolated reconnection sites; the intermittent nature is governed externally by the fluctuation of the interplanetary magnetic field (IMF) and internally regulated by the ionospheric line‐tying effect. The proposed 3DT reconnection can occur simultaneously with the single x line intermittent or quasi‐steady reconnection if the IMF drapes over the dayside magnetopause, which takes place when the solar wind speed exceeds the MHD fast‐mode speed. Magnetic field signatures of the 3DT reconnection are consistent with the elbow‐shaped FTE signatures. Electric fields associated with the elbow‐shaped FTEs can be expected to be spiky, peaking near the center of the localized FTE reconnection sites. The spiky electric field of FTEs is expected to produce channels of enhanced magnetospheric convection. The observed multiple field‐aligned current sheets and auroral arcs fanning out from the cusp region along enhanced convection channels can be expected as consequences of the proposed 3DT reconnection on the dayside magnetopause. The polar cap potential due to the elbow‐shaped FTEs is predicted to have a dependence on the IMF polar angle somewhat different from that of the single x line reconnection.

Observations of flux transfer events

A decade of research on flux transfer events (FTE's) has supported their interpretation as signatures of reconnection between the solar and terrestrial magnetic fields. We review some of the observational evidence. Another observational signature of reconnection has been studied in the literature : high speed plasma flows satisfying approximately stress balance calculations. We revisit a well-documented crossing of the magnetopause to show how these signatures, which are prima facie so diverse and which have hitherto been studied in isolation, can be understood in terms of unsteady Petschek reconnection occurring at the magnetopause. A review of some works on FTE's using data from the AMPTE spacecraft highlights the advances made possible by that mission.

Observations at the magnetopause and in the auroral ionosphere of momentum transfer from the solar wind

Recent radar studies of field-perpendicular flows in the auroral ionosphere, in conjunction with observations of the interplanetary medium immediately upstream of the Earth's bow shock, have revealed direct control of dayside convection by the B z component of the interplanetary magnetic field (IMF). The ionospheric flows begin to respond to both northward and southward turnings of the IMF impinging upon the magnetopause after a delay of only a few minutes in the early afternoon sector, rising to about 15 minutes nearer dawn and dusk. In both the polar cap and the auroral oval, the subsequent rise and decay times are of order 5–10 minutes. We conclude there is very little convection “flywheel” effect in the dayside polar ionosphere and that only newly-opened flux tubes impart significant momentum to the ionosphere, in a relatively narrow region immediately poleward of the cusp. These findings concerning the effects of quasi-steady reconnection have important implications for any ionospheric signatures of transient reconnection which should be considerably shorter-lived than thought hitherto. In order to demonstrate the difficulty of uniquely identifying a Flux Transfer Event (FTE) in ground-based magnetometer data, we present observations of an impulsive signature, identical with that expected for an FTE if data from only one station is studied, following an observed magnetopause compression when the IMF was purely northward. We also report new radar observations of a viscous-like interaction, consistent with an origin on the flanks of the magnetotail and contributing an estimated 15–30kV to the total cross-cap potential during quiet periods.

Conjugate spacecraft and ground-based studies of hydromagnetic phenomenon near the magnetopause

A brief review is presented of hydromagnetic phenomena measured by spacecraft and ground instruments at geomagnetic latitudes which connect through the dayside magnetopause. Impulsive magnetic events, proposed to be ionosphere signatures of sporadic dayside reconnection (flux transfer events), are shown to have accompanying optical and electrical signatures in the ionosphere. A direct coincidence between spacecraft-observed flux transfer events and ionosphere-based magnetic impulses has not as yet been found. Similar frequency wave-like magnetic variations have been observed between high and low altitude spacecraft and ground stations situated along the same flux tubes. These frequencies can correspond to the fundamental mode of a standing wave along a magnetopause field line.

Detailed observations of the plasma sheet during a substorm on April 24, 1979

This paper reports a detailed study of plasma, magnetic field, and energetic particle data acquired by the ISEE 1 and 2 satellites, about 20 RE from earth in the magnetotail plasma sheet during a well‐defined substorm on April 24, 1979. The paper addresses, particularly, the plasma flows and energetic particle anisotropies that were encountered during the various stages of the substorm's evolution and compares these to predictions of the neutral line model of substorms. We find that the observations support that model in considerable detail. The interval from substorm onset at earth to plasma dropout was only 4½ min, and we stress the importance of high time resolution in all measurements if a full understanding of substorm initiation is to be developed. We found that the energetic ions (up to 125 keV) and plasma ions, although they sometimes showed somewhat different angular distributions in the plasma rest frame, typically shared a common bulk speed, i.e., the bulk flow speed of the plasma, parallel and perpendicular to the magnetic field. Thus the anisotropies of the energetic ion fluxes that have sometimes led to their characterization as earthward or tailward “beams” were found to be largely a consequence of their bulk motion with the plasma. We identify the region at the lobe‐plasma sheet interface, which has been called the plasma sheet boundary layer, with the separatrix layer in the neutral line model. This is the region of field lines including, and just equatorward of, the lobe‐plasma sheet separatrix, which should carry the most recent dynamic signatures of reconnection. Our results seem to support that expectation. Several features of our observations were inconsistent with the substorm picture, called the boundary layer dynamics model, in which a satellite's observation of fast plasma and particle flow is due primarily to its envelopment by a preexisting boundary layer of flowing plasma.

AMPTE observations of steady-state reconnection at the dayside magnetopause

Since the introduction of magnetic reconnection into magnetospheric physics by Dungey /1/, the argument over its existence at the dayside magnetopause has raged back - and - forth. Much indirect evidence for dayside reconnection has been compiled. For example, the dependence of magnetospheric processes on the IMF Bz component /2/ and IMF control of energetic solar particle access to the polar cap /3/. However, analysis of IMP6 /4/ and HEOS2 data /5/ failed to find any evidence of high speed ion flows at the magnetopause, an expected signature of magnetic reconnection /6/. It was thus left to the ISEE - 1 and - 2 spacecraft to provide the most powerful evidence for the existence of dayside reconnection. In particular, the observations of high speed plasma flows /7/, which have been shown to be consistent with reconnection models /8/, provide strong evidence. Recently, observations from the AMPTE spacecraft provide further evidence for steady - state reconnection and promise to further our knowledge of the reconnection process.

Plasma flow in the near and distant geomagnetic tail

Measurements of the bulk flow of plasma in the outer magnetosphere were first made a little over a decade ago with Los Alamos instruments on the Vela satellites. During the intervening years, as flow measurements have been made with improved instruments and by other satellites they have come to play a crucial role in the development of our understanding of the structure and dynamics of the magnetosphere. For example, they were the means of discovery of the magnetosphere's boundary layer and of plasma vortices within the plasma sheet. They were the essential ingredient in the identification of signatures of magnetic reconnection at the magnetopause. And they were indispensible in clarifying the complex phenomena in the magnetotail accompanying substorms and in showing that these phenomena are consistent with a substorm model involving magnetic reconnection at a near-earth neutral line. Most recently, magnetotail plasma flow measurements by the ISEE-3 satellite at distances as great as 230 R E have been instrumental in fixing the average location of the “distant” neutral line at ∼ 60 to 120 R E and in identifying plasmoids (i.e., severed sections of the plasma sheet), released during substorms and escaping down-tail. This paper reviews the features of magnetotail plasma flow, describes the most recent observations, and discusses their implications for magnetospheric physics.

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

Studies of the boundary layers in the vicinity of the Earth's 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-Alfvénic 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-Alfvénic 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.

Reconnection in the Jovian magnetosphere

In the nightside Jovian magnetosphere, Voyager 1 and 2 detected signatures of reconnection. The anti‐sunward streaming events, recognized and designated as magnetospheric winds by Krimigis et al., were clearly associated with the northward inclination of the magnetic field. The reconnection is likely to be due to the need to adjust the length of the extended nightside field lines to the dimension of the dayside magnetosphere, but it is not a steady‐state feature and shows considerable variability. In addition to these events, there were numerous occasions where northward and southward field polarities were mixed at the neutral sheet. These latter events were observed by Voyager 2 beyond about 80 RJ where the observed field across the neutral sheet exceeded the dipole field strength, and they probably represent the product of the tearing mode.

Plasma behavior during energetic electron streaming events: Further evidence for substorm‐associated magnetic reconnection

A recent study showed that streaming energetic (〉200 keV) electrons in Earth's magnetotail are statistically associated with southward magnetic fields and with enhancements of the AE index. It is shown here that the streaming electrons characteristically are preceded by a ∼15 minute period of tailward plasma flow and followed by a dropout of the plasma sheet, thus demonstrating a clear statistical association between substorms and the classical signatures of magnetic reconnection and plasmoid formation. Additionally, a brief upward surge of mean electron energy preceded plasma dropout in several of the events studied, providing direct evidence of localized, reconnection‐associated heating processes.

Observed signatures of reconnection in the magnetotail

Plasma and magnetic field data from Imp 6 are examined statistically to study the occurrence of reconnection in the near‐earth region (within ∼20 RE) of the magnetotail. It is confirmed that the southward polarity of the magnetic field observed in the low‐latitude magnetotail (within a few RE of the estimated neutral sheet position) is strongly coupled to a tailward flow of plasma, as expected from the near‐earth reconnection model. It is noted, however, that reconnection is not the unique source of the tailward flow, and plasma is seen to be flowing back and forth regardless of the occurrence of near‐earth reconnection. It is fast tailward flow having speeds greater than about 400 km/s that is mostly due to near‐earth reconnection. There is an indication that detectability of an associated signature in the auroral zone magnetic field is related to the reconnection rate.

On the expected signatures of reconnection in the magnetotail

This paper enumerates basic expected signatures of the reconnection process in the magnetotail. Agreements between expected and observed signatures are pointed out.