Signatures Of Reconnection Research Articles

Does the aurora provide evidence for the occurrence of antiparallel magnetopause reconnection?

Antiparallel merging at the magnetopause predicts a longitudinal bifurcation of the cusp during intervals of southeast or southwest interplanetary magnetic field (IMF) orientation. In the aurora this should manifest itself as a longitudinal gap in the midday sector, bracketed by bright auroral forms on either side. With the purpose of testing this prediction, we searched for an auroral signature in our database from the arctic islands of Svalbard, focusing on strong, isolated auroral brightenings belonging to the category of events which has been previously established as a signature of transient magnetopause reconnection (flux transfer events (FTEs)). We found some very interesting events, showing clear brightenings in the eastern and western parts of the field of view (FOV), with a gap in between. In this paper we present a 1 hour long interval of auroral observations in the 1100–1400 MLT sector, showing two strong, isolated auroral events, which were separated by 20 min. The IMF was steady and directed southeast (By = 3 nT; Bz = −3 nT; clock angle = 135°), in a solar wind of low speed (370 km s−1) and low dynamic pressure (0.2 nPa). Each auroral event is characterized by an initial, strong brightening of the cusp equatorward boundary, previously referred to as equatorward boundary intensification (EBI), which is followed by a westward and poleward moving auroral form (PMAF). The main point here is that the initial brightening is longitudinally bifurcated, with near‐simultaneous auroral brightenings in the eastern (1300–1400 MLT) and western (1100–1130 MLT) parts of the all‐sky camera FOV being separated by a ∼500 km wide “gap.” The brightening event is discussed as a possible signature of transient, antiparallel reconnection, taking place at high magnetopause latitudes, while the “gap” aurora is attributed to quasi steady component reconnection in the subsolar region.

Yohkoh-SXT Observations of reconnection

In this paper I review the different strong indications for the occurrence of reconnection in the solar corona observed of with Yohkoh-SXT. I make a distinction between the unique indicator of reconnection, i.e. the change in connectivity in the magnetic field, and non-unique signatures of reconnection, that also can originate from other processes. I review five different types of Yohkoh-SXT observations that indicate reconnection in the solar corona: erupting plasmoids, X-ray jets, formation of sigmoids through “linkage”, the existence of interconnecting loops, and the rigid rotation of coronal holes. I conclude that the presence of reconnection in the solar corona has been established with near certainty, but that higher resolution soft X-ray and EUV observations are needed to firmly establish the full physical picture for each of The phenomena described.

The location and rate of dayside reconnection during an interval of southward interplanetary magnetic field

Abstract. Using ionospheric data from the SuperDARN radar network and a DMSP satellite we obtain a comprehensive description of the spatial and temporal pattern of day-side reconnection. During a period of southward interplanetary magnetic field (IMF), the data are used to determine the location of the ionospheric projection of the dayside magnetopause reconnection X-line. From the flow of plasma across the projected X-line, we derive the reconnection rate across 7 h of longitude and estimate it for the total length of the X-line footprint, which was found to be 10 h of longitude. Using the Tsyganenko 96 magnetic field model, the ionospheric data are mapped to the magnetopause, in order to provide an estimate of the extent of the reconnection X-line. This is found to be ~ 38 RE in extent, spanning the whole dayside magnetopause from dawn to dusk flank. Our results are compared with previously reported encounters by the Equator-S and Geotail spacecraft with a reconnecting magnetopause, near the dawn flank, for the same period. The SuperDARN observations allow the satellite data to be set in the context of the whole magnetopause reconnection X-line. The total potential associated with dayside reconnection was ~ 150 kV. The reconnection signatures detected by the Equator-S satellite mapped to a region in the ionosphere showing continuous flow across the polar cap boundary, but the reconnection rate was variable and showed a clear spatial variation, with a distinct minimum at 14:00 magnetic local time which was present throughout the 30-min study period.Key words. Magnetospheric physics (magnetopause, cusp and boundary layers; magnetosphere-ionoshere interactions) – Space plasma physics (magnetic reconnection)

Solar wind coupling to and predictability of ground magnetic fields and their time derivatives

A spatial and dynamic study of solar wind driving of high‐latitude ground magnetic fields and their time derivatives is given. Nonlinear, data‐derived basis functions are used to determine the optimal solar wind driven dynamic coupling function at high‐latitude locations. The method does not assume a solar wind driving function a priori so that an unbiased determination of the relative influence of different driving processes can be made. Using this method, the locations where high‐latitude models capture signatures of different driving processes, such as reconnection and the Kelvin‐Helmholtz instability, are revealed for both the amplitude of the field and its time derivative. The time rate of change of the field is a disturbance measure related to geomagnetically induced currents. For the amplitude of the ground magnetic field the primary driver is reconnection; the time rate of change of the field has signatures of both reconnection driving in the nightside sector and Kelvin‐Helmholtz driving in the prenoon sector. The independent influence of the solar wind ion density and azimuthal component of the magnetic field in driving magnetic field fluctuations is found to be small at auroral‐zone latitudes. The data‐derived coupling functions are also used to estimate the expected prediction error of a general class of models that specify the ground magnetic field or its time derivative given solar wind plasma measurements. Prediction efficiencies as small as zero and as large as 0.7 for both the amplitude of the field and its time derivative are possible. The prediction efficiency is highly dependent on spatial location and the direction of field being predicted.

RAS helps to reconnect scientists

agnetic reconnection may be defined as a change in connectivity of lines of force of a magnetic field in a plasma, an electrically neutral mixture of ions and electrons. Through this change, magnetic energy can be released, into heat and plasma flow. Reconnection is universally important: most of the universe is plasma and most plasmas generate magnetic fields. Magnetic reconnection occurs at the Sun and other stars, in accretion disks and in near-Earth space ‐ geospace. Geospace is the only natural environment in which reconnection can be observed, both locally by in situ spacecraft measurements and globally by remote sensing from the ground. The latter uses the dipole magnetic field as a lens that focuses signatures of reconnection from a vast region on the outer boundary of geospace (the magnetopause) to relatively small areas of the ionosphere near the poles. Signatures include the aurora, observed by optical instruments, and plasma convection and associated electrical currents measured by radars and magnetometers (figure 1). This RAS meeting aimed to improve understanding of reconnection in general by addressing two key questions: “What controls the location of magnetopause reconnection?” and “What controls the temporal variation of magnetopause reconnection?” Competing answers to these questions have been advanced through a range of satellite and ground-based observations and theoretical modelling. The meeting brought together about 50 researchers to discuss these ideas and identify ways towards resolving these questions. Talks and posters are summarized by the presenters as follows. Stan Cowley (Leicester University) gave a review of “Magnetic reconnection at the Earth’s magnetopause: theory and modelling”. The focus of this presentation was on how magnetic geometry and the motion of open flux tubes result in theoretical restrictions on where steady reconnection can occur on the magnetopause. He presented models that showed that steady reconnection can occur, in principle, over wide regions of the magnetopause (though not everywhere) when the interplanetary magnetic field (IMF) points southward; the question then remains as to where it occurs most frequently in that case. Two main schools of thought are that reconnection is mainly subsolar, or mainly where the fields are anti-parallel (generally at high latitudes in the vicinity of the cusps). Cowley outlined observations at the magnetopause and in the ionosphere that could distinguish between these scenarios.

Variations in the polar cap area during two substorm cycles

Abstract. This study employs observations from several sources to determine the location of the polar cap boundary, or open/closed field line boundary, at all local times, allowing the amount of open flux in the magnetosphere to be quantified. These data sources include global auroral images from the Ultraviolet Imager (UVI) instrument on board the Polar spacecraft, SuperDARN HF radar measurements of the convection flow, and low altitude particle measurements from Defense Meteorological Satellite Program (DMSP) and National Oceanographic and Atmospheric Administration (NOAA) satellites, and the Fast Auroral SnapshoT (FAST) spacecraft. Changes in the open flux content of the magnetosphere are related to the rate of magnetic reconnection occurring at the magnetopause and in the magnetotail, allowing us to estimate the day- and nightside reconnection voltages during two substorm cycles. Specifically, increases in the polar cap area are found to be consistent with open flux being created when the IMF is oriented southwards and low-latitude magnetopause reconnection is ongoing, and decreases in area correspond to open flux being destroyed at substorm breakup. The polar cap area can continue to decrease for 100 min following the onset of substorm breakup, continuing even after substorm-associated auroral features have died away. An estimate of the dayside reconnection voltage, determined from plasma drift measurements in the ionosphere, indicates that reconnection can take place at all local times along the dayside portion of the polar cap boundary, and hence presumably across the majority of the dayside magnetopause. The observation of ionospheric signatures of bursty reconnection over a wide extent of local times supports this finding.Key words. Ionosphere (plasma convection; polar ionosphere) – Magnetospheric physics (magnetospheric configuration and dynamics)

Simultaneous Cluster and IMAGE observations of cusp reconnection and auroral proton spot for northward IMF

On March 18, 2002, under northward interplanetary magnetic field (IMF) and high (∼15 nPa) solar wind dynamic pressure conditions, Cluster observed reconnection signatures and the passage of an X‐line at the large (∼175°) magnetic‐shear high‐latitude magnetopause (MP). The observations are consistent with the occurrence of a reconnection site tailward of the cusp and in the vicinity of the spacecraft. At the same time IMAGE observed a bright spot poleward of the dayside auroral oval resulting from precipitating protons into the atmosphere. The intensity of the proton spot is consistent with the energy flux contained in the plasma jets observed by Cluster. Using the Tsyganenko‐01 magnetic field model with enhanced solar wind pressure, the Cluster MP location is mapped to the vicinity of the IMAGE proton spot. Mapping the auroral spot out to the MP implies an X‐line of at least 3.6 RE in yGSM. In addition to confirming the reconnection source of the dayside auroral proton spot, the Cluster observations also reveal sub‐Alfvénic flows and a plasma depletion layer in the magnetosheath next to the MP, in a region where gas dynamic models predict super‐Alfvénic flows.

Comment on “An ionospheric convection signature of antiparallel reconnection” by Coleman et al.

Comment on “An ionospheric convection signature of antiparallel reconnection” by Coleman et al.

Multi-instrument study of the dynamic cusp during dominant IMF <i>B<sub>y</sub></i> conditions

Abstract. We present multi-instrument observations using the meridian scanning photometer (MSP) at NyAlesund, the EISCAT Svalbard radar (ESR) and the CUTLASS Finland HF radar, to investigate the dynamics of the cusp region during pulsed reconnection events. The optical data obtained from the MSP indicate the presence of several poleward-moving auroral forms (PMAFs) which have been previously identified as the auroral signature of pulsed reconnection. Furthermore, the optical green line (557.7 nm) luminosity indicates a loss of emission equatorward of the location of the onset of the PMAFs, characteristic of magnetospheric plasma escaping to the magnetosheath along newly opened field lines. This reduction in green line luminosity creates a "dark region", the equatorward edge of which is found to lie close to the boundary between high and low spectral widths observed by the CUTLASS Finland radar. High spectral widths on the dayside have previously been identified as a good indicator of cusp backscatter. Both of these boundaries have been suggested to provide an accurate representation of the location of the open/closed field line boundary. The ESR observations show enhancements in electron density and electron temperature occurring in conjunction with the optical PMAFs. The observations demonstrate some correspondence with the theoretical predictions of Davis and Lockwood (1996), who used an auroral precipitation model to predict ESR observations in the vicinity of the cusp. However, the limitations of this model are apparent under conditions of large plasma flows in the ionosphere. Finally, convection velocities obtained from the HF radar data illustrate a flow regime similar to that predicted to be driven by strong IMF By, as described by Cowley and Lockwood (1992), demonstrating an initial azimuthal flow followed by a rotation to more poleward directions.Key words. Ionosphere (ionosphere-magnetosphere interactions; particle precipitation) – Magnetospheric physics (magnetopause, cusp and boundary layers)

Signatures of collisionless magnetic reconnection

Simulations of magnetic reconnection in simple systems without a guide field predict the formation of a spatially antisymmetric out‐of‐plane magnetic field signature and a symmetric perturbation in the plasma pressure. Here we show, using two fluid simulations and analytic arguments, that these signatures are not in fact a generic feature of fast reconnection. Rather, in the strong guide field limit the pressure perturbation becomes antisymmetric, while the perturbation in the out‐of‐plane field can be either symmetric for βx ∼ 1 or antisymmetric for βx ≫ 1, where βx is the plasma β defined using the reconnection magnetic field just upstream of the reconnection region.

Timing and localization of reconnection signatures - Is there a substorm model problem?

[1] Observations place ionospheric substorm onset signatures on field lines that map to the near-Earth plasma sheet, and field-aligned currents (FACs) are observed before dipolarization, equatorward of open field lines. The near-Earth neutral line (NENL) model, however, appears to suggest perturbations to originate from the x-line on newly reconnected field lines, which map to much higher latitudes. Its order of dipolarization, FACs, and subsequent ionospheric signatures contradicts the observations. Using large-scale kinetic simulations, we demonstrate that these “flaws” of the NENL model are misconceptions and that the timing and location of signatures in the model are indeed consistent with observations. In a thinned current sheet, energetic ions are virtually unmagnetized, get further energized when passing through the neutral sheet, and travel earthward quickly and close to the neutral sheet, thus providing a pathway for early substorm signatures at relatively low latitudes.

A multi-instrument approach to mapping the global dayside merging rate

Abstract. For the first time three different methods have been used to calculate the global merging rate during the same substorm growth phase. The ionospheric plasma drift was monitored by six of the Northern Hemisphere SuperDARN radars, allowing the convection pattern to be studied over 12 h of magnetic local time. The radars observed reconnection signatures on the dayside simultaneously with substorm signatures on the nightside. The three methods to calculate the global merging rate are: (i) the equatorward expansion of radar backscatter on the nightside, which provides an estimate of the rate of polar cap expansion, while upstream WIND measurements gave an estimate of the reconnection electric fields; (ii) the derivation of the dayside boundary normal plasma flow velocity and an estimate of the extent of the ionospheric merging gap, from radar observation of dayside reconnection; (iii) utilizing the map-potential technique to map the high-latitude plasma flow and cross polar cap potential (Ruohoniemi and Baker, 1998), allowing the global dayside merging rate to be calculated. The three methods support an extensive magnetopause X-line length of between 30 ± 12RE and 35 ± 15 RE (assuming a single X-line and constant merging rate). Such close agreement between the different methods of calculation are unexpected, especially as the length of the magnetopause X-line is not well known.Key words. Magnetospheric physics (magnetopause, cusp and boundary layers; magnetosphere – ionosphere interactions; solar-wind magnetosphere interactions)

Radio signature of magnetic reconnection and bi-directional shock waves in a flare-CME event on April 15, 1998

A flare-CME event on April 15, 1998 is studied with data of Nobeyama Radio Polarimeters (NoRP) and Heliograph (NoRH), the radio spectrometers of Chinese National Astronomical Observatories (1.0–2.0 GHz and 2.6–2.8 GHz), and the Astrophysical Institute of Postdam (200–800 MHz), as well as the data of YOHKOH, SOHO, BATSE, and GOES. There were strong fluctuations superposed on the initial phase of the BATSE hard X-ray burst, and the radio burst at 1.0–2.0 GHz with a group of type III-like positive and negative frequency drift pairs, which may be interpreted as the process of magnetic reconnection or particle acceleration in corona. A type II-like burst with a series of pulsations at 200–800 MHz followed the maximum phase of the radio and hard X-ray burst, and slowly drifted to lower frequencies with typical zebra feature. After 10 min of that, a similar dynamic spectrum was recorded at 2.6–3.8 GHz, where the type II-like signal drifted to higher frequencies with a series of pulsations and zebra structures. The polarization sense was strongly RCP at 2.6–3.8 GHz, and weakly LCP at 1.0–2.0 GHz, which was confirmed by the observations of NoRP. The radiation mechanism of these pulsations may be caused by the electron cyclotron maser instability. The local magnetic field strength and source height are estimated based on the gyro-synchrotron second harmonic emission. The ambient plasma density is calculated from the YOHKOH/SXT data. The ratio between the electron plasma frequency and gyro-frequency is around 1.3, which corresponds to the reversal value from extraordinary mode (LCP) to ordinary mode (RCP). Moreover, both the time scale and the modularity of an individual pulse increase statistically with the increase in the burst flux, which may be explained by the acceleration process of non-thermal electrons in the shock wave-fronts propagated upward and downward. Therefore, the radio observations may provide an important signature that flare and CME are triggered simultaneously by magnetic reconnection and are associated with the formation of bi-directional shock waves.

Interpretation of observations on July 24, 1996 detected by Interball-1 as pulsed reconnection at low latitude magnetopause

A method for investigating reconnection events is presented. The approach is based on advantages of non-linear spectral analysis named by the Method of Global Minimum and magnetic field measurements. The technique allows to reveal a presence and plasma properties of different particle species, waves, and time intervals of development of non-linear processes in the reconnection layers. We put into practice the approach to study the event on July 24, 1996 detected by Interball-1 and interpreted as gross deformation of the dayside magnetopause of about 5 R E caused by a process at the bow shock by (Sibeck et al., 1998). Our method of spectral analysis of the data reveals that plasma detected during the event consists of ions both solar wind and ionosphere origin. The spectrum of the magnetic field data is determined by periods caused by gyro-motions of different species of the solar wind plasma (Fe +6, O +6, He ++) and the ionosphere ions (N ++, He +), and power non-stationary (transient) oscillations at period T∼120 sec. (frequency∼8 mHz. We reveal the exchange of plasma from the solar wind to the magnetosphere during the event too. Based on results of our analysis we suggest that the simplest explanation of the discussed event is that these are signatures of transient reconnection of interplanetary and terrestrial magnetic fields.

Evidence of Posteruption Reconnection Associated with Coronal Mass Ejections in the Solar Wind

Using a coupled 2.5-dimensional, time-dependent MHD model of the solar corona and inner heliosphere, we have simulated the eruption and evolution of a coronal mass ejection containing a flux rope all the way from the Sun to 1 AU. Although idealized, we find that the simulation reproduces many generic features of magnetic clouds. In this paper we report on a new, intriguing aspect of these comparisons. Specifically, the results suggest that jetted outflow, driven by posteruptive reconnection underneath the flux rope, occurs and may remain intact out to 1 AU and beyond. We present an example of a magnetic cloud with precisely these signatures and show that the velocity perturbations are consistent with reconnection outflow. We suggest that other velocity and/or density enhancements observed trailing magnetic clouds may be signatures of such reconnection and, in some cases, may not be associated with prominence material, as has previously been suggested.

Ionospheric signatures of split reconnection X‐lines during conditions of IMF Bz < 0 and |By| ∼ |Bz|: Evidence for the antiparallel merging hypothesis

We present measurements of the convection electric field in the dayside ionosphere during extended intervals when the interplanetary magnetic field (IMF) has a negative Bz component and a significant By component (|By| ∼ |Bz|). Under these conditions, distinct differences in the ionospheric projection of the magnetopause reconnection X‐line are predicted by the antiparallel and subsolar merging hypotheses. Close to solstice, in the winter hemisphere the antiparallel merging hypothesis predicts two ionospheric merging lines which are significantly separated either side of noon. This prediction is not expected from any subsolar merging hypothesis (e.g., component merging). Using the method of global convection mapping with line‐of‐sight velocity data from the SuperDARN HF radar network, we present evidence of two distinct merging regions on the dayside magnetopause under these IMF and seasonal conditions. The two merging regions manifest themselves in the ionosphere as two regions of convection flow with a strong poleward component separated by a region of convection flow with either a weak poleward or equatorward component corresponding to an adiaroic portion of the polar cap boundary. The spatiotemporal nature of the actual convection scenario can be complicated, but the predicted pattern represents a recurring feature of the convection electric field under these IMF and seasonal conditions. These observations lend support to the hypothesis that antiparallel merging is the dominant form of magnetopause reconnection during these steady state IMF intervals. This suggests that the magnetic shear between magnetosheath and magnetospheric magnetic field lines plays the major role in determining the location of reconnection on the dayside magnetopause.

Ground-based observations of the auroral zone and polar cap ionospheric responses to dayside transient reconnection

Abstract. Observations from the EISCAT VHF incoherent scatter radar system in northern Norway, during a run of the common programme CP-4, reveal a series of poleward-propagating F-region electron density enhancements in the pre-noon sector on 23 November 1999. These plasma density features, which are observed under conditions of a strongly southward interplanetary magnetic field, exhibit a recurrence rate of under 10 min and appear to emanate from the vicinity of the open/closed field-line boundary from where they travel into the polar cap; this is suggestive of their being an ionospheric response to transient reconnection at the day-side magnetopause (flux transfer events). Simultaneous with the density structures detected by the VHF radar, poleward-moving radar auroral forms (PMRAFs) are observed by the Finland HF coherent scatter radar. It is thought that PM-RAFs, which are commonly observed near local noon by HF radars, are also related to flux transfer events, although the specific mechanism for the generation of the field-aligned irregularities within such features is not well understood. The HF observations suggest, that for much of their existence, the PMRAFs trace fossil signatures of transient reconnection rather than revealing the footprint of active reconnection itself; this is evidenced not least by the fact that the PMRAFs become narrower in spectral width as they evolve away from the region of more classical, broad cusp scatter in which they originate. Interpretation of the HF observations with reference to the plasma parameters diagnosed by the incoherent scatter radar suggests that as the PMRAFs migrate away from the reconnection site and across the polar cap, entrained in the ambient antisunward flow, the irregularities therein are generated by the presence of gradients in the electron density, with these gradients having been formed through structuring of the ionosphere in the cusp region in response to transient reconnection.Key words. Magnetospheric physics (magnetosphere-ionosphere interaction) – ionosphere (ionospheric irregularities; plasma density and temperature)

EISCAT Svalbard radar observations of ionospheric signatures of magnetopause reconnection during a changing IMF Bz polarity

Abstract. Observations by the EISCAT Svalbard radar are presented that show the response of the spatial structure of the ionosphere in the dayside cusp region to a rotational trend in the IMF clock angle. Over a period of one hour, the clock angle increased from about 45° to some 150°, moving the likely location of the magnetopause reconnection site from the high-latitude lobe to near the equatorial plane. Increased topside electron temperatures measured by the ESR identified footprints of the reconnection process. Temporal changes in the spatial distribution of the temperature reflected the change from lobe to equatorial reconnection. Discrete spatial enhancements in ion temperature were found resulting from ion-neutral frictional heating in the fast flows where it was likely that field lines were being convected from the reconnection locations. The corresponding electron density structuring is interpreted in terms of the particle precipitation, field-aligned currents and convection flows driven by the IMF.Key words. Ionosphere (ionosphere – magnetosphere interactions; plasma temperature and density; polar ionosphere)

Equator-S observation of reconnection coupled to surface waves

In the morning sector of its near equatorial orbit Equator-S observed large magnetic oscillations in the Pc 5 frequency range. The poloidal part of the observed oscillations is usually interpreted as being caused by surface waves of the magnetopause driven by a Kelvin-Helmholtz instability (KHI). We found, however, that these oscillations are sometimes additionally superposed with reconnection signatures. This occurs during periods of northward interplanetary magnetic field. In fact, the Walen relation as a test for reconnection is satisfied for many short periods of time. This indicates transient and thin layers of reconnection, embedded in the large scale motion. We compared our observations with appropriate magnetohydrodynamic simulations allowing reconnection in the case of shear flows. Comparing the simulation results with the magnetic field and plasma observations we found good agreement of simulations and observations. We conclude that Equator-S has observed reconnection embedded in Kelvin-Helmholtz instability generated vortices in its nonlinear phase.

Statistics of the mid‐altitude cusp observed by Polar

The Polar GGS space mission has allowed the northern hemispheric polar cusp region to be probed in a variety of ways to further our understanding and knowledge of the processes that occur within this region. The discovery of pulsed, energy‐dispersed ion injections on newly‐opened field lines (Pulsed Particle Signatures, or PPSs), the magnetospheric plasma signatures of transient reconnection, is one such advancement in our knowledge. Utilising a ∼22 months subset of Polar's mission, we perform a statistical study of the dayside traversals of the cusp region by the spacecraft to determine the extent and location of PPSs as well as the prevailing Interplanetary Magnetic Field (IMF) conditions under which they occur. In this paper we report upon the broad extent of these signatures in latitude and local time regardless of IMF orientation, and show that PPSs occur under both southward and northward IMF conditions in high percentages, although favouring southward directed IMF. These results are discussed in terms of previous statistical studies of reconnection signatures, and in terms of lobe and low latitude reconnection.