Traveling Compression Regions Research Articles

Reconnection Outflow Reversal Associated with Ion-scale Magnetic Flux Ropes in the Earth's Midtail: ARTEMIS Observations and Reconstruction Results

A rare reconnection outflow reversal in the Earth's midtail observed by the two Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) probes, is presented. During the event, two ion-scale magnetic flux ropes were separately observed by THC in the earthward and tailward reconnection outflows that were adjacent and accompanied, respectively, by the positive and negative normal magnetic field components to the current sheet. The two flux ropes were separated by ∼2.75 minutes and at the center of the flux ropes, the magnetic field strength was enhanced with a large core field. Comparison results of the convection and measured electric fields reveal that the ions and the magnetic fields were decoupled in the regions surrounding the two flux ropes. The two-dimensional magnetic field maps from the Grad–Shafranov reconstruction show that the diameters of the two flux ropes were similar, being ∼7.1 and ∼7.9 ion inertial lengths, but the aspect ratios of the width to the length were different, being ∼0.35 and ∼0.47. Moreover, one of the reconstructed field maps suggests that multiple x-lines may exist in the midtail reconnection and that the traveling compression region of the flux rope was seen at THB. The angle between the axial orientations of the two flux ropes was large, being ∼55°, and their axes were tilted away from the direction of the reconnection guide field, in agreement with the earlier studies of the magnetotail flux ropes.

Neptune’s Pole-on Magnetosphere: Dayside Reconnection Observations by Voyager 2

Abstract The “pole-on” configuration occurs when the polar magnetosphere of a planet is directed into the solar wind velocity vector. Such magnetospheric configurations are unique to the ice giant planets. This means that magnetic reconnection, a process that couples a magnetosphere to the solar wind, will be different at the ice giants when they are pole-on compared to other planets. The only available in situ measurements of a pole-on magnetosphere are from the Neptune flyby by Voyager 2, which we analyze in this paper. We show that dayside magnetopause conditions were conducive to magnetic reconnection. A plasma depletion layer in the magnetosheath adjacent to the magnetopause was observed. Plasma measurements inside the magnetospheric cusp show evidence of multiple reconnection taking place at the magnetopause before the spacecraft crossed the open–closed field line boundary. A possible traveling compression region from a nearby passing flux rope was also observed, providing further supporting evidence that multiple X-line reconnection occurred during the flyby. During a perfectly pole-on configuration, reconnection will not depend on the orientation of the interplanetary magnetic field, as is the case at other planetary magnetospheres. The rate of reconnection will not vary because the area of the dayside magnetopause where antiparallel shears occur will be approximately equal for all interplanetary magnetic field orientations. Therefore, we suggest that rotating into and out of the pole-on configuration will likely drive the “on–off”/“switch-like” dynamics observed in simulations. Consequently, the pole-on configuration is most likely an important rotational phase for driving ice giant magnetospheric dynamics.

Machine Learning Applications to Kronian Magnetospheric Reconnection Classification

The products of magnetic reconnection in Saturn’s magnetotail are identified in magnetometer observations primarily through characteristic deviations in the north–south component of the magnetic field. These magnetic deflections are caused by traveling plasma structures created during reconnection rapidly passing over the observing spacecraft. Identification of these signatures have long been performed by eye, and more recently through semi-automated methods, however these methods are often limited through a required human verification step. Here, we present a fully automated, supervised learning, feed forward neural network model to identify evidence of reconnection in the Kronian magnetosphere with the three magnetic field components observed by the Cassini spacecraft in Kronocentric radial–theta–phi coordinates as input. This model is constructed from a catalog of reconnection events which covers three years of observations with a total of 2093 classified events, categorized into plasmoids, traveling compression regions and dipolarizations. This neural network model is capable of rapidly identifying reconnection events in large time-span Cassini datasets, tested against the full year 2010 with a high level of accuracy (87%), true skill score (0.76), and Heidke skill score (0.73). From this model, a full cataloging and examination of magnetic reconnection events in the Kronian magnetosphere across Cassini's near Saturn lifetime is now possible.

Large-scale energy budget of impulsive magnetic reconnection: Theory and simulation.

We evaluate the large‐scale energy budget of magnetic reconnection utilizing an analytical time‐dependent impulsive reconnection model and a numerical 2‐D MHD simulation. With the generalization to compressible plasma, we can investigate changes in the thermal, kinetic, and magnetic energies. We study these changes in three different regions: (a) the region defined by the outflowing plasma (outflow region, OR), (b) the region of compressed magnetic fields above/below the OR (traveling compression region, TCR), and (c) the region trailing the OR and TCR (wake). For incompressible plasma, we find that the decrease inside the OR is compensated by the increase in kinetic energy. However, for the general compressible case, the decrease in magnetic energy inside the OR is not sufficient to explain the increase in thermal and kinetic energy. Hence, energy from other regions needs to be considered. We find that the decrease in thermal and magnetic energy in the wake, together with the decrease in magnetic energy inside the OR, is sufficient to feed the increase in kinetic and thermal energies in the OR and the increase in magnetic and thermal energies inside the TCR. That way, the energy budget is balanced, but consequently, not all magnetic energy is converted into kinetic and thermal energies of the OR. Instead, a certain fraction gets transfered into the TCR. As an upper limit of the efficiency of reconnection (magnetic energy → kinetic energy) we find η eff=1/2. A numerical simulation is used to include a finite thickness of the current sheet, which shows the importance of the pressure gradient inside the OR for the conversion of kinetic energy into thermal energy.

Flux transfer event observation at Saturn's dayside magnetopause by the Cassini spacecraft

AbstractWe present the first observation of a flux rope at Saturn's dayside magnetopause. This is an important result because it shows that the Saturnian magnetopause is conducive to multiple X‐line reconnection and flux rope generation. Minimum variance analysis shows that the magnetic signature is consistent with a flux rope. The magnetic observations were well fitted to a constant‐α force‐free flux rope model. The radius and magnetic flux content of the rope are estimated to be 4600–8300 km and 0.2–0.8 MWb, respectively. Cassini also observed five traveling compression regions (remote signatures of flux ropes), in the adjacent magnetosphere. The magnetic flux content is compared to other estimates of flux opening via reconnection at Saturn.

Reconnection events in Saturn's magnetotail: Dependence of plasmoid occurrence on planetary period oscillation phase

AbstractDuring its exploration of Saturn's magnetotail the Cassini magnetometer has detected many in situ examples of magnetic reconnection, in the form of plasmoids, traveling compression regions (TCRs), and dipolarizations. Meanwhile, many magnetospheric phenomena have been shown to be organized with particular regularity by planetary period oscillation systems driven separately from the Northern and Southern Hemispheres of the planet. Here we examine the relationship between the occurrence of plasmoids and TCRs and the magnetic phases of the northern and southern systems. We find a striking degree of organization of the events by both northern and southern phases, with events linked preferentially to intervals in which the magnetospheric plasma and field lines are displaced outward from the planet and the current sheet thinned, both effects being likely to favor the occurrence of reconnection and plasmoid‐related mass loss. Little evidence is found for significant visibility effects associated with north‐south motions of the plasma sheet.

Reconstruction of plasmoid and traveling compression region in the near-Earth magnetotail

Cluster spacecraft observed an earthward flowing plasmoid along with a travelling compression region (TCR) structure in southern plasma sheet boundary layer (PSBL) at 21:09 UT of September 19, 2001. We have reconstructed the two-dimensional topology of the magnetic field structure observed by C1 using Grad-Shafranov reconstruction method. Results show that C1 passed through part of a plasmoid, which compressed the lobe magnetic field and formed a TCR. The size of the whole plasmoid structure in X direction is estimated to be about 3 R E. Furthermore, using multi-spacecraft observations, we have found some detailed information about this structure. First, C1 observed bi-streaming electron components, which supports our suggestion that the spacecraft passed through closed field lines. Second, a small magnetic field perturbation within this plasmoid accompanied by slight decrease in electron flux suggests that a flux rope core might exist at the center of the plasmoid.

Saturn's dynamic magnetotail: A comprehensive magnetic field and plasma survey of plasmoids and traveling compression regions and their role in global magnetospheric dynamics

AbstractWe present a comprehensive study of the magnetic field and plasma signatures of reconnection events observed with the Cassini spacecraft during the tail orbits of 2006. We examine their “local” properties in terms of magnetic field reconfiguration and changing plasma flows. We also describe the “global” impact of reconnection in terms of the contribution to mass loss, flux closure, and large‐scale tail structure. The signatures of 69 plasmoids, 17 traveling compression regions (TCRs), and 13 planetward moving structures have been found. The direction of motion is inferred from the sign of the change in the Bθ component of the magnetic field in the first instance and confirmed through plasma flow data where available. The plasmoids are interpreted as detached structures, observed by the spacecraft tailward of the reconnection site, and the TCRs are interpreted as the effects of the draping and compression of lobe magnetic field lines around passing plasmoids. We focus on the analysis and interpretation of the tailward moving (south‐to‐north field change) plasmoids and TCRs in this work, considering the planetward moving signatures only from the point of view of understanding the reconnection x‐line position and recurrence rates. We discuss the location spread of the observations, showing that where spacecraft coverage is symmetric about midnight, reconnection signatures are observed more frequently on the dawn flank than on the dusk flank. We show an example of a chain of two plasmoids and two TCRs over 3 hours and suggest that such a scenario is associated with a single‐reconnection event, ejecting multiple successive plasmoids. Plasma data reveal that one of these plasmoids contains H+ at lower energy and W+ at higher energy, consistent with an inner magnetospheric source, and the total flow speed inside the plasmoid is estimated with an upper limit of 170 km/s. We probe the interior structure of plasmoids and find that the vast majority of examples at Saturn show a localized decrease in field magnitude as the spacecraft passes through the structure. We take the trajectory of Cassini into account, as, during 2006, the spacecraft's largely equatorial position beneath the hinged current sheet meant that it rarely traversed the center of plasmoids. We present an innovative method of optimizing the window size for minimum variance analysis (MVA) and apply this MVA across several plasmoids to explore their interior morphology in more detail, finding that Saturn's tail contains both loop‐like and flux rope‐like plasmoids. We estimate the mass lost downtail through reconnection and suggest that the apparent imbalance between mass input and observed plasmoid ejection may mean that alternative mass loss methods contribute to balancing Saturn's mass budget. We also estimate the rate of magnetic flux closure in the tail and find that when open field line closure is active, it plays a very significant role in flux cycling at Saturn.

On the azimuthal size of flux ropes near lunar orbit

We present Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) dual‐probe observations of flux ropes in the Earth's magnetotail near lunar orbit. On 15 July 2011 between 0400 and 0500 UT, the ARTEMIS probes (P1 and P2) are separated by ∼ (9/10/3) RE(XGSW/YGSW/ZGSW). GSW denotes the Geocentric Solar Wind coordinate system and differs from the GSM coordinate system in that its X axis is antiparallel to the solar wind. P1 is near midnight and P2 in the postmidnight sector at ∼ −13 RE YGSW. During a ∼ 50 min interval on 15 July 2011, P1 crossed the current sheet and encountered a flux rope thereafter. During the same interval, P2 observed only one flux rope near the time P1 crossed the current sheet but no flux rope or traveling compression region (TCR) for P1's subsequent flux rope observation. A Tsyganenko‐Fairfield model and minimum variance analysis during the current sheet crossing are used to infer the current sheet location with respect to the probes. We find the distance between P2 and the plasma sheet boundary to be less than 3 RE. Under these circumstances, P2 would be expected to observe a TCR if the flux rope observed by P1 extended to the postmidnight location of P2. The lack of such observations indicates that, contrary to previous models and simulation results, flux ropes may be spatially confined in the dusk‐dawn direction and do not extend across the entire cross section of the tail near lunar orbit.

MESSENGER observations of dipolarization events in Mercury's magnetotail

Several series of large dipolarization events are documented from magnetic field observations in Mercury's magnetotail made by the MESSENGER spacecraft. The dipolarizations are identified by a rapid (∼1 s) increase in the northward component of the magnetic field, followed by a slower return (∼10 s) to pre‐onset values. The changes in field strength during an event frequently reach 40 nT or higher, equivalent to an increase in the total magnetic field magnitude by a factor of ∼4 or more. The presence of spatially constrained dipolarizations at Mercury provides a key to understanding the magnetic substorm process in a new parameter regime: the dipolarization timescale, which is shorter than at Earth, is suspected to lead to efficient non‐adiabatic heating of the plasma sheet proton population, and the high recurrence rate of the structures is similar to that frequently observed for flux ropes and traveling compression regions in Mercury's magnetotail. The relatively short lifetime of the events is attributed to the lack of steady field‐aligned current systems at Mercury.

MESSENGER and Mariner 10 flyby observations of magnetotail structure and dynamics at Mercury

The first (M1), second (M2), and third (M3) MESSENGER flybys of Mercury traversed the planet's magnetotail from 1.25 to 3.25 RM downstream of the planet, where RM is Mercury's radius (2440 km). The encounters took place under northward, southward, and variable‐polarity interplanetary magnetic field (IMF), respectively. The magnetic field strength B in Mercury's magnetotail follows a power law decrease with increasing antisunward distance ∣X∣, B ∼ ∣X∣G, with G varying from −5.4 for northward to −1.6 for southward IMF. Low‐latitude boundary layers (LLBLs) containing strong northward magnetic field were detected at the tail flanks during two of the flybys. The observed thickness of the LLBL was ∼33% and 16% of the radius of the tail during M1 and M3, respectively, but the boundary layer was completely absent during M2. Clear signatures of tail reconnection are evident in the M2 and M3 magnetic field measurements. Plasmoids and traveling compression regions were observed during M2 and M3 with typical durations of ∼1–3 s, suggesting diameters of ∼500–1500 km. Overall, the response of Mercury's magnetotail to the steady southward IMF during M2 appeared very similar to steady magnetospheric convection events at Earth, which are believed to be driven by quasi‐continuous reconnection. In contrast, the M3 measurements are dominated by tail loading and unloading events that resemble the large‐scale magnetic field reconfigurations observed during magnetospheric substorms at Earth.

Cluster observations of a transient signature in the magnetotail: implications for the mode of reconnection

Abstract. Travelling compression regions (TCRs) are perturbations in the magnetotail lobe magnetic field caused by structures moving Earthward or tailward within the plasma sheet. Previous works have suggested that these structures are created by either time-dependant reconnection occurring at a single X-line, forming a flux-bulge-type structure, or space-variant reconnection at multiple X-lines, forming flux-rope-type structures. In this study we examine an event in which Cluster 2 observed a TCR while the 3 remaining Cluster spacecraft observed the underlying magnetic structure at a range of distances from the neutral sheet. The magnetic structure has a velocity of (99, 154, −31) km s−1 in GSM (|V| = 186 km s−1), an estimated size of 1.19 RE along the direction of travel and a size between 1.94 and 2.86 RE in the direction perpendicular to the current sheet. As the structure passes the spacecraft, Cluster 1 and Cluster 4 observed a bipolar signature in BZ, plasma-sheet-like plasma and field-aligned electron flows. Cluster 3 passed closest to the centre of the structure and observed two separate reductions in the plasma density (with field-aligned electron flows); these drop-outs in the plasma sheet were possibly created by the actions of X-lines. The second drop-out in the plasma sheet also includes a reversal of the ion flow, a signature consistent with the passage of a reconnecting X-line past the spacecraft. Between the X-lines, the plasma outflow from the X-lines caused an increase in pressure which led to a localised expansion of the plasma and also the observations at Cluster 1 and Cluster 4 and the TCR. Our observations do not uniquely match either of the flux rope or the flux bulge predictions although the observation of two plasma sheet drop-outs (interpreted as X-lines, one active, one dormant) with plasma-sheet-like between them and only one TCR is a situation expected in multiple X-line reconnection.

Field-aligned beams and reconnection in the jovian magnetotail

The release of plasma in the jovian magnetotail is observed in the form of plasmoids, travelling compression regions, field-aligned particle beams and flux-rope like events. We demonstrate that electrons propagate along the magnetic field lines in the plasma sheet boundary layer (PSBL), while close to the current sheet center the electron distribution is isotropic. The evidences of the counterstreaming electron beams in the PSBLs are also presented. Most of the field-aligned energetic ion beams are associated with the field-aligned electron beams and about half of them have the bipolar fluctuation of the meridional magnetic field component. Moreover they often show a normal velocity dispersion for the different species which fits well in the scenario of particle propagation from a single source. All features above are observed during jovian reconfiguration events which are typically bonded with plasma flow reversals. From all these characteristics, which are based on energetic particle and magnetic field measurements, we believe that the reconfiguration processes in the jovian magnetotail are associated with reconnection.

A THEMIS survey of flux ropes and traveling compression regions: Location of the near-Earth reconnection site during solar minimum

A statistical study of flux ropes and traveling compression regions (TCRs) during the Time History of Events and Macroscale Interactions during Substorms (THEMIS) second tail season has been performed. A combined total of 135 flux ropes and TCRs in the range GSM X approx -14 to -31 R(sub E) were identified, many of these occurring in series of two or more events separated by a few tens of seconds. Those occurring within 10 min of each other were combined into aggregated reconnection events. For the purposes of this survey, these are most likely the products of reconnect ion occurring simultaneously at multiple, closely spaced x-lines as opposed to statistically independent episodes of reconnection. The 135 flux ropes and TCRs were grouped into 87 reconnection events; of these, 28 were moving tailward and 59 were moving Earthward. The average location of the near-Earth x-line determined from statistical analysis of these reconnection events is (X(sub GSM), Y*(sub GSM)) = (-30R(sub E), 5R(sub E)), where Y* includes a correction for the solar aberration angle. A strong east-west asymmetry is present in the tailward events, with >80% being observed at GSM Y* > O. Our results indicate that the Earthward flows are similarly asymmetric in the midtail region, becoming more symmetric inside - 18 R(sub E). Superposed epoch analyses indicate that the occurrence of reconnection closer to the Earth, i.e., X > -20 R(sub E), is associated with elevated solar wind velocity and enhanced negative interplanetary magnetic field B(sub z). Reconnection events taking place closer to the Earth are also far more effective in producing geomagnetic activity, judged by the AL index, than reconnection initiated beyond X approx -25 R(sub E).

Magnetic field structure of large-scale plasmoid generated by the fast reconnection mechanism in a sheared current sheet

Abstract. On the basis of the spontaneous fast reconnection model, three-dimensional magnetic field profiles associated with a large-scale plasmoid propagating along the antiparallel magnetic fields are studied in the general sheared current sheet system. The plasmoid is generated ahead of the fast reconnection jet as a result of distinct compression of the magnetized plasma. Inside the plasmoid, the sheared (east-west) field component has the peak value at the plasmoid center located at x=XC, where the north-south field component changes its sign. The plasmoid center corresponds to the so-called contact discontinuity that bounds the reconnected field lines in x<XC and the field lines without reconnection in x>XC. Hence, contray to the conventional prediction, the reconnected sheared field lines in x<XC are not spiral or helical, since they cannot be topologically connected to the field lines in x>XC. It is demonstrated that the resulting profiles of magnetic field components inside the plasmoid are, in principle, consistent with satellite observations. In the ambient magnetic field region outside the plasmoid too, the magnetic field profiles are in good agreement with the well-known observations of traveling compression regions (TCRs).

Impulsive magnetic pulsations and electrojets in the loop footpoint driven by the fast reconnection jet

It is well known that magnetic pulsations of long periods impulsively occur in accordance with the sudden onset of geomagnetic substorms and drastic enhancement of electrojets in the ionosphere. On the basis of the spontaneous fast reconnection model, the present paper examines the physical mechanism by which both magnetic pulsations and strong electrojets are impulsively driven by the fast (Alfvenic) reconnection jet. When a large-scale plasmoid [or traveling compression region (TCR)], directly caused by the fast reconnection jet, collides with the magnetic loop footpoint, strong electrojets are impulsively driven in a finite extent in the loop footpoint in accordance with the evolution of the current wedge and the generator current circuit. Simultaneously, magnetohydrodynamic (Alfven) waves, accompanied by the TCR, are reflected from the electrojet layer, leading to impulsive magnetic pulsations ahead of the loop footpoint because of the interaction (or resonance) between the reflected waves and the waves traveling toward the footpoint. The pulsations propagate outward in all directions from the source region of the wave reflection, and the pulsation periods are typically estimated to be of several tens of seconds.

Estimating the magnetic energy inside traveling compression regions

Abstract. We investigate a series of six TCRs (traveling compression regions), appearing in the course of a small substorm on 19 September 2001. Except for two of these TCRs, all Cluster spacecraft were located in the lobe and detected the typical signatures of TCRs, i.e., compressions in |B| and bipolar Bz variations. We use these perturbations in Bz for calculations on the magnetic energy inside the TCR and compare the amount of magnetic field energy with the kinetic energy inside the underlying plasma bulge. According to results obtained from theory, the amount of magnetic energy inside TCRs is about two times higher than the kinetic plasma energy inside the accompanied plasma bulge. We verify this theoretical result by first investigations of the magnetic field energy inside TCRs. The calculations lead to a magnetic energy in the order of 1010 Joule per RE for each of the TCRs.

Modeling of substorms and flares by the fast reconnection mechanism

Abstract The 3D spontaneous fast reconnection model is applied to well-known signatures of geomagnetic substorms and solar flares. First, it is applied to the traveling compression regions (TCRs) associated with plasmoids propagating down the tail plasma sheet, known as a definite signature of geomagnetic substorms, and the in-situ satellite observations can be precisely explained, both qualitatively and quantitatively. Then, it is demonstrated that the magnetospheric current wedge drastically evolves through field-aligned currents to link the tail current to the auroral electrojet. It is also found that the well-known morphological features of two-ribbon flares can be explained by the fast reconnection model. In particular, the joule heating, associated with the flare current wedge, is shown to be important for the two-ribbon heating. Therefore, it is suggested that both solar flares and geomagnetic substorms result from the same physical mechanism, i.e., the fast reconnection mechanism.

Plasmoid formation for multiple onset substorms: observations of the Japanese Lunar Mission "Kaguya"

Abstract. The Japanese Lunar Mission "Kaguya" carried out its first magnetic field and plasma measurements in the Earth's magnetotail on 22 December 2007. Fortuitously, three well-defined multiple onset substoms took place. Kaguya was located in the premidnight magnetotail at radial distances of 56 RE and observed plasmoids and/or traveling compression regions (TCRs). Although the present study is based on limited data sets, important issues on multiple onset substorms can be examined. Each onset in a series of onsets releases a plasmoid, and magnetic reconnection likely proceeds to tail lobe field lines for each onset. Since the duration of each plasmoid is less than 5 min, these observations imply that magnetic reconnection for each onset can develop fully to the tail lobe field lines and be quenched within this timescale.

First application of a Petschek‐type reconnection model with time‐varying reconnection rate to THEMIS observations

We present a method to determine the location of the reconnection site and the amount of reconnected magnetic flux out of an analytical time‐dependent reconnection model and apply this method to disturbances observed on 2 February 2008 at about 0200 and 0815 UT by THEMIS B (P1). During these events, P1 detected two tailward propagating traveling compression regions, associated with typical variations in Bz and Bx. We find the reconnection site to be located at about −16 RE for the event at 0200 UT and −17.5 RE for the event at 0815 UT. These locations are consistent with simple timing considerations with respect to disturbances detected by the inner THEMIS spacecraft. The amount of reconnected flux in our 2‐D model can be found to be in the order of 108 nT m for both events. The calculations for the reconnection site's location are done by using two approaches, i.e., by using the Bz and the Bx signals, yielding consistent results. The reconnected flux can be determined using Bz and vz. Also, these results are in good agreement. A comparison between the disturbances detected by P1 and the modeled variations shows that our model describes disturbances in the magnetic field and the background plasma very well.