Nightside Reconnection Research Articles

The 17 March 2013 storm: Synergy of observations related to electric field modes and their ionospheric and magnetospheric Effects

AbstractThe main phase of the 17 March 2013 storm had excellent coverage from ground‐based instruments and from low‐ and high‐altitude spacecraft, allowing for evaluation of the relations between major storm time phenomena that are often considered separately. The shock impact with its concurrent southward interplanetary magnetic field (IMF) immediately drove dramatic poleward expansion of the poleward boundary of the auroral oval (implying strong nightside reconnection), strong auroral activity, and strong penetrating midlatitude convection and ionospheric currents. This was followed by periods of southward IMF driving of electric fields that were at first relatively smooth as often employed in storm modeling but then became extremely bursty and structured associated with equatorward extending auroral streamers. The auroral oval did not expand much further poleward during these two latter periods, suggesting a lower overall nightside reconnection rate than that during the first period and approximate balance with dayside reconnection. Characteristics of these three modes of driving were reflected in horizontal and field‐aligned currents. Equatorward expansion of the auroral oval occurred predominantly during the structured convection mode, when electric fields became extremely bursty. The period of this third mode also approximately corresponded to the time of largest equatorward motion of the ionospheric trough, of apparent transport of high total electron content (TEC) features into the auroral oval from the polar cap, and of largest earthward injection of ions and electrons into the ring current. The enhanced responses of the aurora, currents, TEC, and the ring current indicate a common driving of all these storm time features during the bursty convection mode period.

Effects of auroral potential drops on plasma sheet dynamics

AbstractThe reaction of the magnetosphere‐ionosphere system to dynamic auroral potential drops is investigated using the Lyon‐Fedder‐Mobarry global model including, for the first time in a global simulation, the dissipative load of field‐aligned potential drops in the low‐altitude boundary condition. This extra load reduces the field‐aligned current (j||) supplied by nightside reconnection dynamos. The system adapts by forcing the nightside X line closer to Earth, with a corresponding reduction in current lensing (j||/B = constant) at the ionosphere and additional contraction of the plasma sheet during substorm recovery and steady magnetospheric convection. For steady and moderate solar wind driving and with constant ionospheric conductance, the cross polar cap potential and hemispheric field‐aligned current are lower by approximately the ratio of the peak field‐aligned potential drop to the cross polar cap potential (10–15%) when potential drops are included. Hemispheric ionospheric Joule dissipation is less by 8%, while the area‐integrated, average work done on the fluid by the reconnecting magnetotail field increases by 50% within |y| < 8 RE. Effects on the nightside plasma sheet include (1) an average X line 4 RE closer to Earth; (2) a 12% higher mean reconnection rate; and (3) dawn‐dusk asymmetry in reconnection with a 17% higher rate in the premidnight sector.

Influence of ion outflow in coupled geospace simulations: 2. Sawtooth oscillations driven by physics‐based ion outflow

AbstractWe present the first simulations of magnetospheric sawtooth oscillations under steady solar wind conditions that are driven internally by heavy ion outflow from a physics‐based model. The simulations presented use the multifluid Lyon‐Fedder‐Mobarry magnetohydrodynamics model two‐way coupled to the ionosphere/polar wind model (IPWM). Depending on the type of wave‐particle interactions utilized within IPWM, the coupled simulations exhibit either sawtooth oscillations or steady magnetospheric convection. Contrasting the simulations that do and do not develop sawtooth oscillations yields insights into the relationship between outflow and sawtooth oscillations. The total outflow rate is not an adequate predictor of the convection mode that will emerge. The simulations that develop sawtooth oscillations are characterized by intense outflow concentrated in the midnight auroral region. This outflow distribution mass loads the tail reconnection region without excessively mass loading the dayside reconnection region and leads to an imbalance between the dayside and nightside reconnection rates.

Four‐fluid MHD simulations of the plasma and neutral gas environment of comet 67P/Churyumov‐Gerasimenko near perihelion

AbstractThe neutral and plasma environment is critical in understanding the interaction of the solar wind and comet 67P/Churyumov‐Gerasimenko (CG), the target of the European Space Agency's Rosetta mission. To serve this need and support the Rosetta mission, we have developed a 3‐D four‐fluid model, which is based on BATS‐R‐US (Block‐Adaptive Tree Solarwind Roe‐type Upwind Scheme) within SWMF (Space Weather Modeling Framework) that solves the governing multifluid MHD equations and the Euler equations for the neutral gas fluid. These equations describe the behavior and interactions of the cometary heavy ions, the solar wind protons, the electrons, and the neutrals. This model incorporates different mass loading processes, including photoionization and electron impact ionization, charge exchange, dissociative ion‐electron recombination, and collisional interactions between different fluids. We simulated the plasma and neutral gas environment near perihelion in three different cases: an idealized comet with a spherical body and uniform neutral gas outflow, an idealized comet with a spherical body and illumination‐driven neutral gas outflow, and comet CG with a realistic shape model and illumination‐driven neutral gas outflow. We compared the results of the three cases and showed that the simulations with illumination‐driven neutral gas outflow have magnetic reconnection, a magnetic pileup region and nucleus directed plasma flow inside the nightside reconnection region, which have not been reported in the literature.

GPS scintillations in the high latitudes during periods of dayside and nightside reconnection

AbstractWe use an automated procedure to identify periods of enhanced dayside reconnection followed by enhanced nightside reconnection in measurements of the polar cap size by the Active Magnetosphere and Planetary Electrodynamics Response Experiment between January 2010 and December 2012; we find 490 such events. We investigate the dynamics of the spatial distributions of the total electron content (TEC) and phase scintillations of Global Positioning System (GPS) signals across the northern polar region and here report three important findings: (1) While a TEC enhancement (due to polar cap patches) propagates across the polar cap during these events, this enhancement is not associated with significant GPS phase scintillations. (2) Instead, a significant impact on GPS signal quality is first found when the TEC enhancements cross the nightside auroral boundary. (3) In combination with upward field‐aligned currents, these TEC enhancements cause the strongest GPS phase scintillations. We conclude that polar cap patches are not, as previously thought, a space weather threat inside the polar cap but instead reveal their biggest impact once they reach the nightside auroral oval, in particular when combined with upward field‐aligned currents.

Direct observations of the full Dungey convection cycle in the polar ionosphere for southward interplanetary magnetic field conditions

AbstractTracking the formation and full evolution of polar cap ionization patches in the polar ionosphere, we directly observe the full Dungey convection cycle for southward interplanetary magnetic field (IMF) conditions. This enables us to study how the Dungey cycle influences the patches' evolution. The patches were initially segmented from the dayside storm enhanced density plume at the equatorward edge of the cusp, by the expansion and contraction of the polar cap boundary due to pulsed dayside magnetopause reconnection, as indicated by in situ Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations. Convection led to the patches entering the polar cap and being transported antisunward, while being continuously monitored by the globally distributed arrays of GPS receivers and Super Dual Auroral Radar Network radars. Changes in convection over time resulted in the patches following a range of trajectories, each of which differed somewhat from the classical twin‐cell convection streamlines. Pulsed nightside reconnection, occurring as part of the magnetospheric substorm cycle, modulated the exit of the patches from the polar cap, as confirmed by coordinated observations of the magnetometer at Tromsø and European Incoherent Scatter Tromsø UHF radar. After exiting the polar cap, the patches broke up into a number of plasma blobs and returned sunward in the auroral return flow of the dawn and/or dusk convection cell. The full circulation time was about 3 h.

Assessing the role of oxygen on ring current formation and evolution through numerical experiments

We address the effect of ionospheric outflow and magnetospheric ion composition on the physical processes that control the development of the 5 August 2011 magnetic storm. Simulations with the Space Weather Modeling Framework are used to investigate the global dynamics and energization of ions throughout the magnetosphere during storm time, with a focus on the formation and evolution of the ring current. Simulations involving multifluid (with variable H+/O+ ratio in the inner magnetosphere) and single‐fluid (with constant H+/O+ ratio in the inner magnetosphere) MHD for the global magnetosphere with inner boundary conditions set either by specifying a constant ion density or by physics‐based calculations of the ion fluxes reveal that dynamical changes of the ion composition in the inner magnetosphere alter the total energy density of the magnetosphere, leading to variations in the magnetic field as well as particle drifts throughout the simulated domain. A low oxygen to hydrogen ratio and outflow resulting from a constant ion density boundary produced the most disturbed magnetosphere, leading to a stronger ring current but misses the timing of the storm development. Conversely, including a physics‐based solution for the ionospheric outflow to the magnetosphere system leads to a reduction in the cross‐polar cap potential (CPCP). The increased presence of oxygen in the inner magnetosphere affects the global magnetospheric structure and dynamics and brings the nightside reconnection point closer to the Earth. The combination of reduced CPCP together with the formation of the reconnection line closer to the Earth yields less adiabatic heating in the magnetotail and reduces the amount of energetic plasma that has access to the inner magnetosphere.

Plasmapause formation at Saturn

AbstractCassini observations during a rapid, high‐latitude, dawnside pass from Saturn's lobe to inner magnetosphere on 25 June 2009 provide strong evidence for the formation of a “plasmapause” at Saturn by Vasyliunas‐type nightside reconnection of previously mass‐loaded flux tubes. A population of hot, tenuous plasma that lies between the lobe and the dense inner magnetospheric plasma is consistent with a region formed by very recent injection from a reconnection region in the tail, including low density, high temperature, supercorotational flow, a significant O+ content, and the near‐simultaneous observation of enhanced Saturn kilometric radiation emissions. The sharp boundary between that region and the cool dense inner magnetospheric plasma thus separates flux tubes that were involved in the reconnection from those that successfully traversed the nightside without mass loss. This event demonstrates that tail reconnection can strip off inner magnetospheric plasma in to at least dipole L = 8.6. Clear evidence of flux tube interchange driven by the sharp boundary is found, both inward moving flux tubes of hotter plasma and, for the first time, the outward moving cool population. The outward moving cool regions have azimuthal sizes less than 1 RS, were probably created within the past 1.2 h, and have outflow speeds greater than about 5 km/s. At the outer edge of the reconnected region, there is also a possible signature of Dungey‐type lobe reconnection following the initial Vasyliunas‐type reconnection. Observations from this event are entirely consistent with previously described global MHD simulations of tail reconnection, plasmoid departure, and Saturnward injection of reconnected flux.

A superposed epoch analysis of the regions 1 and 2 Birkeland currents observed by AMPERE during substorms

AbstractWe perform a superposed epoch analysis of the evolution of the Birkeland currents (field‐aligned currents) observed by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) during substorms. The study is composed of 2900 substorms provided by the SuperMAG experiment. We find that the current ovals expand and contract over the course of a substorm cycle and that currents increase in magnitude approaching substorm onset and are further enhanced in the expansion phase. Subsequently, we categorize the substorms by their onset latitude, a proxy for the amount of open magnetic flux in the magnetosphere, and find that Birkeland currents are significantly higher throughout the epoch for low‐latitude substorms. Our results agree with previous studies which indicate that substorms are more intense and close more open magnetic flux when the amount of open flux is larger at onset. We place these findings in the context of previous work linking dayside and nightside reconnection rate to Birkeland current strengths and locations.

Magnetic local time variation and scaling of poleward auroral boundary dynamics

AbstractThe balance of dayside and nightside reconnection processes within the Earth's magnetosphere and its effect on the amount of open magnetic flux threading the ionosphere is well understood in terms of the expanding‐contracting polar cap model. However, the nature and character of the consequential fluctuations in the polar cap boundary are poorly understood. By using the poleward auroral luminosity boundary (PALB), as measured by the FUV instrument of the IMAGE spacecraft, as a proxy for the polar cap boundary, we have studied the motion of this boundary for more than 2 years across the complete range of magnetic local time. Our results show that the dayside PALB dynamics are broadly self‐similar on timescales of 12 min to 6 h and appear to be monofractal. Similarity with the characteristics of solar wind and interplanetary magnetic field variability suggests that this dayside monofractal behavior is predominantly inherited from the solar wind via the reconnection process. The nightside PALB dynamics exhibit scale‐free behavior at intermediate time scales (12–90 min) and appear to be multifractal. We propose that this character is a result of the intermittent multifractal structure of magnetotail reconnection.

Day‐night coupling by a localized flow channel visualized by polar cap patch propagation

We present unique coordinated observations of the dayside auroral oval, polar cap, and nightside auroral oval by three all-sky imagers, two Super Dual Auroral Radar Network (SuperDARN) radars, and Defense Meteorological Satellite Program (DMSP)-17. This data set revealed that a dayside poleward moving auroral form (PMAF) evolved into a polar cap airglow patch that propagated across the polar cap and then nightside poleward boundary intensifications (PBIs). SuperDARN observations detected fast antisunward flows associated with the PMAF, and the DMSP satellite, whose conjunction occurred within a few minutes after the PMAF initiation, measured enhanced low-latitude boundary layer precipitation and enhanced plasma density with a strong antisunward flow burst. The polar cap patch was spatially and temporally coincident with a localized antisunward flow channel. The propagation across the polar cap and the subsequent PBIs suggests that the flow channel originated from dayside reconnection and then reached the nightside open-closed boundary, triggering localized nightside reconnection and flow bursts within the plasma sheet.

The relation between transpolar potential and reconnection rates during sudden enhancement of solar wind dynamic pressure: OpenGGCM‐CTIM results

AbstractThis study investigates how solar wind energy is deposited into the magnetosphere‐ionosphere system during sudden enhancements of solar wind dynamic pressure (Psw), using the coupled Open Geospace General Circulation Model–Coupled Ionosphere Thermosphere Model (OpenGGCM‐CTIM) 3‐D global magnetosphere‐ionosphere‐thermosphere model. We simulate three unique events of solar wind pressure enhancements that occurred during negative, near‐zero, and positive interplanetary magnetic field (IMF) Bz. Then, we examine the behavior of the dayside and nightside reconnection rates and quantify their respective contributions to cross polar cap potential (CPCP), a proxy of ionospheric plasma convection strength. The modeled CPCP increases after a Psw enhancement in all three cases, which agrees well with observations from the Defense Meteorological Satellite Program spacecraft and predictions from the assimilative mapping of ionospheric electrodynamics technique. In the OpenGGCM‐CTIM model, dayside reconnection increases within 9–13 min of the pressure impact, while nightside reconnection intensifies about 13–25 min after the pressure increase. As the strong Psw compresses the dayside magnetosheath and, subsequently, the magnetotail, their magnetic fields intensify and activate stronger antiparallel reconnection on the dayside magnetopause first and near the central plasma sheet second. For southward IMF, dayside reconnection contributes to the CPCP enhancement 2–4 times more than nightside reconnection. For northward IMF, the dayside contribution weakens, and nightside reconnection contributes more to the CPCP enhancement. We find that high‐latitude magnetopause reconnection during northward IMF produces sunward ionospheric plasma convection, which decreases the typical dawn‐to‐dusk ionosphere electric field. This results in a weaker dayside reconnection contribution to the CPCP during northward IMF.

Modeling Birkeland currents in the expanding/contracting polar cap paradigm

AbstractWe present a simple mathematical model of the region 1 and 2 Birkeland current system intensities for differing dayside and nightside magnetic reconnection rates, consistent with the expanding/contracting polar cap paradigm of solar wind‐magnetosphere‐ionosphere coupling. The current intensities are shown to be dependent on the cross‐polar cap potential, which is the average of the dayside and nightside reconnection rates. Current intensities are expected to maximize on the dayside or the nightside when magnetopause or magnetotail reconnection dominates. Current intensities are also dependent on the ionospheric conductance, the width of the merging gaps, the width of the ionospheric convection return flow region, and on the size of the polar cap.

IMF effect on the polar cap contraction and expansion during a period of substorms

Abstract. The polar cap boundary (PCB) location and motion in the nightside ionosphere has been studied by using measurements from the EISCAT radars and the MIRACLE magnetometers during a period of four substorms on 18 February 2004. The OMNI database has been used for observations of the solar wind and the Geotail satellite for magnetospheric measurements. In addition, the event was modelled by the GUMICS-4 MHD simulation. The simulation of the PCB location was in a rather good agreement with the experimental estimates at the EISCAT longitude. During the first three substorm expansion phases, neither the local observations nor the global simulation showed any poleward motions of the PCB, even though the electrojets intensified. Rapid poleward motions of the PCB took place only in the early recovery phases of the substorms. Hence, in these cases the nightside reconnection rate was locally higher in the recovery phase than in the expansion phase. In addition, we suggest that the IMF Bz component correlated with the nightside tail inclination angle and the PCB location with about a 17-min delay from the bow shock. By taking the delay into account, the IMF northward turnings were associated with dipolarizations of the magnetotail and poleward motions of the PCB in the recovery phase. The mechanism behind this effect should be studied further.

Temporal and spatial dynamics of the regions 1 and 2 Birkeland currents during substorms

We use current density data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) to identify the location of maximum region 1 current at all magnetic local times (MLTs). We term this location the R1 oval. Comparing the R1 oval location with particle precipitation boundaries identified in DMSP data, we find that the R1 oval is located on average within 1° of particle signatures associated with the open/closed field line boundary (OCB) across dayside and nightside MLTs. We hence conclude that the R1 oval can be used as a proxy for the location of the OCB. Studying the amount of magnetic flux enclosed by the R1 oval during the substorm cycle, we find that the R1 oval flux is well organized by it: during the growth phase the R1 oval location moves equatorward as the amount of magnetic flux increases whereas after substorm expansion phase onset significant flux closure occurs as the R1 current location retreats to higher latitudes. For about 15 min after expansion phase onset, the amount of open magnetic flux continues to increase indicating that dayside reconnection dominates over nightside reconnection. In the current density data, we find evidence of the substorm current wedge and also show that the dayside R1 currents are stronger than their nightside counterpart during the substorm growth phase, whereas after expansion phase onset, the nightside R1 currents dominate. Our observations of the current distribution and OCB movement during the substorm cycle are in excellent agreement with the expanding/contracting polar cap paradigm.

Properties of outflow‐driven sawtooth substorms

The mechanisms by which ionospheric O+ stretches the plasma sheet causing variations in polar cap flux during magnetospheric sawtooth substorms are investigated using the multi‐fluid Lyon‐Fedder‐Mobarry simulation code. O+ outflow is induced in the simulation by Alfvénic Poynting flux flowing to low altitude during the sawtooth expansion and growth phases. The O+ fluid populates the plasma sheet and causes it to stretch tailward in response to the increased mass density. The ionospheric outflow thus functions as a feedback loop in the magnetosphere‐ionosphere system wherein O+ ions released during the expansion phase alter the magnetospheric configuration and enable the development of the next substorm. We also find that the nightside reconnection rate is strongly dependent on the position of the tail merging line. As the x‐line moves tailward, the inflow speed and magnetic field strength are reduced, creating an imbalance between dayside and nightside merging. This imbalance is responsible for the buildup and release of open magnetic flux in the magnetosphere during the sawtooth cycle.

Direct Observations of the Evolution of Polar Cap Ionization Patches

Patches of ionization are common in the polar ionosphere, where their motion and associated density gradients give variable disturbances to high-frequency (HF) radio communications, over-the-horizon radar location errors, and disruption and errors to satellite navigation and communication. Their formation and evolution are poorly understood, particularly under disturbed space weather conditions. We report direct observations of the full evolution of patches during a geomagnetic storm, including formation, polar cap entry, transpolar evolution, polar cap exit, and sunward return flow. Our observations show that modulation of nightside reconnection in the substorm cycle of the magnetosphere helps form the gaps between patches where steady convection would give a "tongue" of ionization (TOI).

Magnetotail origins of auroral Alfvénic power

The generation of Alfvénic Poynting flux in the central plasma sheet and its polar distribution at low altitude are studied using three dimensional global simulations of the solar wind‐magnetosphere‐ionosphere interaction. A 24‐hour event simulation (4–5 Feb 2004) driven by solar wind and interplanetary magnetic field data reproduces the global morphology of Alfvénic Poynting flux measured by the Polar satellite, including its dawn‐dusk asymmetry. Controlled simulations show that the dawn‐dusk asymmetry is regulated by the spatial variation in ionospheric conductance. The asymmetry disappears when the conductance is taken to be spatially uniform. The simulated Alfvénic Poynting flux is generated in the magnetotail by time‐variable, fast flows emerging from nightside reconnection. The simulated fast flows are more intense in the premidnight sector as observed; this asymmetry also disappears when the ionospheric conductance is spatially uniform. Analysis of the wave propagation in the plasma sheet source region, near xGSM ≈ −15 RE, shows that as the fast flow brakes, a portion of its kinetic energy is transformed into the electromagnetic energy of intermediate and fast magnetohydrodynamic waves. The wave power is dominantly compressional in the source region and becomes increasingly Alfvénic as it propagates along magnetic field lines toward the ionosphere.

Relationship between interplanetary parameters and the magnetopause reconnection rate quantified from observations of the expanding polar cap

Many studies have attempted to quantify the coupling of energy from the solar wind into the magnetosphere. In this paper we parameterize the dependence of the magnetopause reconnection rate on interplanetary parameters from the OMNI data set. The reconnection rate is measured as the rate of expansion of the polar cap during periods when the nightside reconnection rate is thought to be low, determined from observations by the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) Far Ultraviolet (FUV) imager. Our fitting suggests that the reconnection rate is determined by the magnetic flux transport in the solar wind across a channel approximately 4 RE in width, with a small correction dependent on the solar wind speed, and a clock angle dependence. The reconnection rate is not found to be significantly dependent on the solar wind density. Comparison of the modeled reconnection rate with SuperDARN measurements of the cross‐polar cap potential provides broad support for the magnitude of the predictions. In the course of the paper we discuss the relationship between the dayside reconnection rate and the cross‐polar cap potential.

Statistical study of the effect of solar wind dynamic pressure fronts on the dayside and nightside ionospheric convection

[1] Over the past few years, the prominent role of solar wind dynamic pressure in enhancing dayside and nightside reconnection and driving-enhanced ionospheric convection has been documented by both ground and spaceborne instruments. For a previous case study of an abrupt increase in solar wind dynamic pressure, Super Dual Auroral Radar Network (SuperDARN) measurements of plasma convection within the dayside polar ionosphere revealed an immediate enhancement of plasma convection. The convection enhancement variation closely follows the variation in solar wind pressure. The dayside enhancement was followed by a nightside convection increase about 40 min later, which has similar variation characteristics as seen on the dayside. We now use SuperDARN flow measurements during a large number of solar wind pressure enhancements to conduct a superposed epoch analysis of the effects of solar wind pressure fronts on the dayside and nightside ionospheric convection. The results for the dayside show an increase of convection for nearly all interplanetary magnetic field (IMF) Bz values. The response is more pronounced and immediate (within minutes) for southward IMF, with a duration of 20–30 min. The response time scales increase to 5–10 min for northward IMF, and the enhanced flows last for 30–50 min. We also find a significant enhancement of nightside convection, particularly for small values of IMF By, that follows about 10–15 min after the dayside response and can last for 40–50 min.