Observations Of Flux Transfer Events Research Articles

Sequential Observations of Flux Transfer Events, Poleward‐Moving Auroral Forms, and Polar Cap Patches

AbstractWe report the observation of solar wind‐magnetosphere‐ionosphere interactions using a series of flux transfer events (FTEs) observed by Magnetospheric MultiScale (MMS) mission located near the dayside magnetopause on 18 December 2017. The FTEs were observed to propagate duskward and either southward or slightly northward, as predicted under duskward and southward interplanetary magnetic field (IMF). The Cooling model also predicted a significant dawnward propagation of northward‐moving FTEs. Near the MMS footprint, a series of poleward‐moving auroral forms (PMAFs) occurred almost simultaneously with those FTEs. They propagated poleward and westward, consistent with the modeled FTE propagation. The intervals between FTEs, relatively consistent with those between PMAFs, strongly suggest a one‐to‐one correspondence between the dayside transients and ionospheric responses. The FTEs embedded in continuous reconnection observed by MMS and corresponding PMAFs individually occurred during persistent auroral activity recorded by an all‐sky imager strongly indicate that those FTEs/PMAFs resulted from the temporal modulation of the reconnection rate during continuous reconnection. With the decay of the PMAFs associated with the FTEs, patch‐like plasma density enhancements were detected to form and propagate poleward and then dawnward. Propagation to the dawn was also suggested by the Super Dual Auroral Radar Network (SuperDARN) convection and Global Positioning System (GPS) total electron content data. We relate the temporal variation of the driving solar‐wind and magnetospheric mechanism to that of the high‐latitude and polar ionospheric responses and estimate the response time.

Interball-1 observations of flux transfer events

Abstract. We present the results of a survey of 807 FTEs observed by Interball-1 on the mid- and high-latitude dayside and flank magnetopause. Dayside magnetosheath events show a strong tendency to occur for southward magnetosheath magnetic fields suggesting origin via component (or perhaps antiparallel) reconnection near the equatorial plane. Flank FTEs occur for both magnetosheath magnetic field orientations with only a slight preference for southward magnetosheath magnetic fields. These events are consistent with generation along an extended subsolar component reconnection line for all IMF orientations or a combination of reconnection along a subsolar component reconnection line for southward IMF and antiparallel reconnection at higher latitudes for northward IMF. The distribution of direct and reverse magnetosheath FTEs and the tilt angle of the reconnection line for dawnward and duskward IMF are in a good agreement with the theoretical predictions of the component merging model. The clear anticorrelation between the magnitude of the east/west (Bm) perturbation observed within magnetosphere FTEs versus Bm in the magnetosheath also demands an explanation of the FTEs in terms of reconnection along a tilted subsolar merging line, e.g. in terms of component merging.

Simultaneous observations of flux transfer events by THEMIS, Cluster, Double Star, and SuperDARN: Acceleration of FTEs

We present simultaneous observations of flux transfer events (FTEs) made by the THEMIS and Cluster spacecraft on 3 May 2007, along with supporting observations of fast flows in the dayside ionosphere observed by the SuperDARN radar network. The THEMIS spacecraft were in a string‐of‐pearls formation approximately 20,000 km long and crossed the postnoon magnetopause at low latitudes between 1200 UT (TH‐C) and 1430 UT (TH‐E). The Cluster spacecraft were situated in the magnetosheath at high latitudes in the Southern Hemisphere, approaching the magnetopause which was crossed at about 1600 UT. THEMIS observed “standard” polarity FTE signatures between 1100 and 1500 UT, while Cluster observed “reverse” polarity signatures at the same time. The two sets of signatures are consistent with being generated at the same small region of a subsolar reconnection line. Between 1100 and 1230 UT, the Double Star TC‐1 satellite was near the magnetopause closer to local noon but still ∼7 RE from the subsolar point. TC‐1 only observed a single FTE, suggesting that the variability of the reconnection rate differed between these two locations on the X line. Fast poleward ionospheric flows were observed in the noon and prenoon sector, at similar magnetic local times to the footprints of both Cluster and THEMIS, after 1200 UT. The long string formation of the THEMIS constellation allows the motion of the FTE structures to be tracked, and the acceleration is found to be small but consistent with a model prediction.

THEMIS observations of the dayside traveling compression region and flows surrounding flux transfer events

We present multipoint observations of Flux Transfer Events (FTEs) by the THEMIS spacecraft in a string‐of‐pearls configuration, at the near‐equatorial magnetopause. Common characteristics from a number of cases, examined at various local times, are exemplified here by a case study of FTE trains observed in the pre‐noon sector, on Aug. 18, 2007. We show that the magneto‐pause‐normal flow velocity is consistently towards and away from the magnetopause on the inbound and outbound transit of the FTE, on all spacecraft sufficiently away from the magnetopause. The velocity just outside the FTE at the time of closest proximity to the FTE core is opposite to the FTE direction of motion even when the FTE moves in the same direction as the adjacent sheath. The total pressure inside and just outside the FTE is consistently larger than expected from pressure balance at a nominal (unperturbed) magnetopause and therefore the result of local compression by the passing FTE. The multi‐spacecraft observation enables the reconstruction of the Dayside Traveling Compression Region (DTCR), whose result is consistent with previous theoretical results. Semi‐periodic compressional velocity oscillations were observed and are likely driven by nearby occurrences of FTE trains, implying recurrent reconnection with a similar periodicity.

Simultaneous Cluster and CUTLASS Observations of FTEs on 11 February 2004

AbstractThis paper analyzes the simultaneous observations of the Flux Gate Magnetometer (FGM) and Plasma Electron and Current Experiment (PEACE) on Cluster Spacecraft, and the CUTLASS Finland radar between 11:00 and 11:40 UT on 11 February 2004. In this interval, the Cluster spacecraft array encountered the northern hemisphere high altitude cusp, and crossed the magnetopause into the magnetosheath at about 11:18 UT. A large number of flux transfer events (FTEs) were observed by all 4 Cluster spacecraft. It is found that these FTEs appeared quasi‐periodically with a period of about 130 s. Since all 4 Cluster spacecraft observed these FTEs, we applied the four‐spacecraft techniques “Minimum Directional Derivative (or Difference)” (MDD) and “Spatio‐temporal Difference” (STD) to calculate the dimension, motion and scale of these FTEs. The inferred northwardly reconnected flux tubes for these FTEs are shown to move northward or north‐east and tailward with a quasi‐2‐D structure and a scale of (0.87 ~ 1.81)RE. The ionospheric footprint of the Cluster spacecraft during the boundary passage is shown to map centrally within the field‐of‐view of the CUTLASS Finland radar. It is shown that the backscatter power of CUTLASS data is enhanced in synchronously with the magnetospheric FTEs at the latitude of the Cluster footprint. These radar auroral features in the radar subsequently propagated poleward, forming classic “poleward‐moving radar auroral form” structures at higher latitudes. The FTE motions meanwhile are shown to be consistent with the expected motion of reconnected magnetic flux tubes over the surface of the magnetopause, arising from a predominantly sub‐solar reconnection site during the prevailing upstream conditions.

Flux transfer events on the high-latitude magnetopause: Interball-1 observations

Abstract. We present case and statistical studies of flux transfer events (FTEs) observed by Interball-1 on the high-latitude magnetopause. The case studies provide observations of FTEs in the cusp during periods of southward interplanetary magnetic field (IMF) orientation and on the magnetopause poleward of the cusp during periods of strongly northward IMF orientation. We interpret the former in terms of reconnection on the equatorial magnetopause and subsequent antisunward motion of FTEs into the cusps. We interpret the latter in terms of bursty antiparallel merging on the high-latitude magnetopause. A statistical survey demonstrates that events observed equatorward of the cusp show a marked tendency to occur for antiparallel (northward) magnetospheric and (southward) magnetosheath magnetic field orientations, whereas events observed poleward of the cusps tend to occur for either strongly parallel or antiparallel configurations. We suggest that this discrepancy implies that events observed poleward of the cusps originate both locally and on the equatorial magnetopause. Finally, we use the sense of the bipolar signature and the prevailing magnetic field orientation to demonstrate that almost all events move antisunward, i.e. that at these latitudes pressure gradients determine the motion of FTEs and not magnetic curvature forces.

A survey of flux transfer events observed by Cluster during strongly northward IMF

During Cluster's annual dayside seasons (November–June) the four spacecraft cross the magnetopause at high latitudes near local noon, and at lower latitudes further along the flanks. During these crossings, observations of flux transfer events (FTEs), a signature of transient or variable‐rate magnetopause merging, are often made. We have compiled a survey of FTEs observed by Cluster in the 2002/3 dayside season. A significant number of FTEs, presented here, were observed under strongly northward IMF. Multi‐spacecraft techniques enable more accurate velocities to be calculated than previously possible. The observed velocities are consistent with a long, component merging X‐line emanating from the antiparallel merging site in the lobe, but require a relaxation of the antiparallel merging hypothesis to allow the X‐line to extend to regions of lower shear on the flank. The velocities observed at lower latitudes are not consistent with a subsolar X‐line.

First simultaneous observations of flux transfer events at the high-latitude magnetopause by the Cluster spacecraft and pulsed radar signatures in the conjugate ionosphere by the CUTLASS and EISCAT radars

Abstract. 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)

Time—dependent three‐dimensional Petschek—type reconnection: A case study for magnetopause conditions

We illustrate the implications of a generalized Petschek‐type reconnection model for conditions prevailing at the magnetopause. Petschek's model is generalized through the introduction of a space‐ and time‐varying reconnection rate. Furthermore, the model can incorporate skewed magnetic fields with different magnitudes, a velocity shear, and different densities on either side of the current sheet. Here we study a situation typical for the dayside magnetopause, in which all these features are present. On a qualitative level, it is shown that the physical manifestations of reconnection include phenomena which have been observed at the magnetopause, such as accelerated plasma flows, flux transfer events, and surface waves. To aid a quantitative and more detailed comparison with experimental data, we present the results in a format which is similar to that used to view the plasma bulk parameters and magnetic field data obtained from spacecraft measurements. In particular, at several fixed positions in space we analyze the time behavior of the magnetic field and plasma parameters resulting from a localized pulse of reconnection. This exercise reveals that although there is broad agreement between the observations and expectations based on the model, several discrepancies and unexplained features still remain, such as the increase in the total pressure associated with some observations of flux transfer events. These are presumably the result of various simplifying assumptions made in the model. We also reach the new conclusion that application of the stress balance relation does not necessarily guarantee the correct identification of a rotational discontinuity traversal, and we suggest an additional identifying criterion.

Time-dependent magnetic reconnection: Two- and three-dimensional MHD simulations

Observations of flux transfer events (FTEs) at the earth magnetospause and plasmoids in the magnetotail indicate that magnetic reconnections in the earth magnetosphere are time-dependent. Motivated by the observed flux transfer events, Lee and Fu proposed a multiple X-line reconnection (MXR) model for the dayside magnetopause. The multiple X-line reconnection is intrinsically a time-dependent process and is different from the classical single X-line reconnection (SXR) process. As a consequence of MXR, plasmoids or magnetic flux ropes are intermittently formed and convected out of the reconnection region. Two- and three-dimensional MHD simulations have been carried out to study the time-dependent magnetic reconnection process. The 2-D simulation results show that the time-dependent multiple X-line process tends to occur when the system length is long and the resistivity is small. Otherwise, the quasi-steady single X-line process takes place. It is found that the occurrence of multiple reconnection sites in the current sheet may be due to either (1) the development of tearing mode instability in the local slab model, or (2) the nonuniformity of the current sheet in the global model. In the three-dimensional case, the multiple X-line reconnection process leads to the formation of magnetic flux tubes or magnetic ropes. In the three-dimensional case, the field line topology at the ends of the magnetic rope is very complicated. The simulation shows that magnetic ropes formed through the MXR process usually have “frayed” ends. In addition, a strong tube-aligned plasma flow is observed during the formation of magnetic flux tubes in the 3-D simulation.

A user's guide to locating flux transfer events in low‐altitude satellite measurements: An S3‐2 case study

Although flux transfer events (FTEs) have been observed frequently at high altitudes near the magnetopause their ionospheric signatures remain elusive. Vestiges of FTEs and other magnetopause phenomena have possibly been detected in transient magnetic field perturbations measured on the ground at cusp latitudes. Direct observations of FTEs by instrumentation on polar orbiting satellites have not yet been forthcoming. We have developed a methodology for use by investigators based on expected, simultaneous measurements of plasma drifts (electric fields), field‐aligned currents (magnetic fields) and precipitating particle fluxes. To test and illustrate this methodology we present the results of a search through the available S3‐2 data for electric and magnetic field signatures from the mid‐day sector displaying irregular structures of scales 100 to 200 km, consistent with ionospheric mapping of FTEs. Out of 36 orbits between December 1975 and April 1976 we found five possible events. An event selected for detailed analysis was observed approximately at 2219 UT on January 5, 1976 during an equatorward crossing of the northern dayside auroral region at about 1030 MLT. The qualitative electric and magnetic field fluctuations expected during a satellite encounter with an FTE at ionospheric altitudes are compared with single component measurements for S3‐2. Although an encounter with a fast stream in the plasma flow poleward of the cusp cannot be excluded, the data are consistent with expectations for the S3‐2 spacecraft having made a near center crossing of an ionospheric footprint of an FTE with a diameter of about 150 km moving eastward along the polar cap boundary with a velocity of 1.3 km/s. Criteria for distinguishing between FTEs and fast streams from satellite measurements of two components of the electric and magnetic field are presented.

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.

Multiple X line reconnection: 3. A particle simulation of flux transfer events

The multiple X line reconnection process, which was proposed by Lee and Fu to explain the flux transfer events observed at the dayside magnetopause, is studied by computer simulations based on a two‐dimensional magnetostatic particle code. A driven boundary condition is applied at the two sides of the simulation domain with an incoming plasma flow while a free boundary condition is used at the other two sides with an outflow. Repeated formation and convection of magnetic islands are observed in the simulation. Magnetic islands with various sizes are observed. Superthermal ions of energy 5–50 keV, with an energy spectrum f(E)∼E−1.9, are generated during the multiple X line reconnection process. It is found that the power spectrum of the normal magnetic field is given by a power law PB ∼ k−0.6, where k is the wave number. The convection speed of magnetic islands may exceed the Alfvén speed. The recurrence time of magnetic islands is found to be τ ≃ 100/ΩRo, where Ω is the ion gyrofrequency, Ro = V1/VA, V1 is the imposed inflow speed, and VA is the Alfvén speed. Our results are consistent with satellite observations of flux transfer events.

Multiple X line reconnection: 2. The dynamics

The plasma dynamics associated with both the single and multiple X line reconnection processes is studied based on an MHD simulation. It is found that the plasma dynamics in the single X line reconnection is generally consistent with the reconnection models by Petschek and Sonnerup. However, the outflow plasma speed is considerably smaller than the upstream Alfvén speed predicted by the theoretical models. In the case of multiple X line reconnection, repeated formation and convection of magnetic islands are observed with a recurrence time τ ≃ 10tA/Ro, where tA is the Alfvén transit time, and Ro = V1/VA is the imposed driving rate. The magnetic reconnection process is found to be impulsive and intermittent. An exponential growth of the reconnection rate is observed during the impulsive phase. The simulation results are generally consistent with satellite observations of flux transfer events (FTE's). The BN signature associated with the FTE's is observed in our simulation. Magnetic flux tubes with a cross section of ∼1 RE² are found to occur repeatedly every 5–15 min in our simulation.

Observations of reverse polarity flux transfer events at the Earth's dayside magnetopause

Observations of flux transfer events (FTEs) at the Earth's dayside magnetopause are presented which have plasma and magnetic field signatures reversed in sign from those previously reported. These FTEs are interpreted in terms of localized and transitory reconnection with the spacecraft located south of the reconnection site. They are associated with a previously reported interval of quasi-steady reconnection which is similarly located, indicating that a close physical connection exists between the two processes.

ISEE observations of flux transfer events at the dayside magnetopause

Magnetic field measurements from the ISEE 1 and 2 spacecraft are examined in the vicinity of the magnetopause near local noon on a typical pass when the magnetosheath field is southward. The data clearly show evidence for patchy impulsive reconnection. The flux transfer rate for these events is at least of the order of 1 to 2 × 1012 Maxwells per second, and possibly greater. This rate is similar to rates deduced for magnetopause erosion events. Not only are these observations relevant to the substorm process, but the impulsive nature of the flux transfer events leads to boundary oscillations that could also be the source of long period magnetic pulsations in the outer magnetosphere.