The role of ionospheric O⁺ outflow in the generation of earthward propagating plasmoids

Abstract

Earthward propagating plasmoids in the Earth's magnetotail have been observed by satellites. Using a multifluid global magnetosphere simulation, earthward propagating plasmoids are reproduced when ionospheric O⁺ outflow is included in the global simulation. Controlled simulations show that without ionospheric outflow, the plasmoids generated in the magnetotail during a substorm-steady magnetospheric convection cycle only propagate tailward. With ionospheric outflow, earthward plasmoids can be induced through the modification of magnetotail reconnection at multiple X lines. When multiple X lines form in the magnetotail, plasmoids may be trapped between multiple reconnection sites. When magnetic reconnection rate is reduced at the near-Earth X line by the presence of ionospheric O⁺, the earthward exhaust flow of reconnection occurring at the midtail X line forces the plasmoid to propagate earthward. The propagation speed and spatial size of the simulated earthward plasmoid are consistent with observations from the Cluster satellites.

Key Points

• Earthward plasmoids can be induced by ionospheric O⁺ outflow
• Ionospheric O⁺ outflow changes the distribution of magnetotail reconnection
• Simulated earthward plasmoids are consistent with observations

1 Introduction

The formation and propagation of plasmoids play a significant role in geomagnetic substorm dynamics in the Earth's magnetosphere [e.g., Baker et al., 1996]. In the near-Earth neutral line scenario of geomagnetic substorms [e.g., Hones, 1977], a plasmoid forms when magnetic reconnection begins in the near-Earth magnetotail and proceeds to lobe field lines, which generates large magnetic islands disconnected from Earth's geomagnetic field. This plasmoid is then ejected tailward. Satellite observations have shown that these plasmoids are heated plasma sheet expansions that extend into the lobe, in which the thermal pressure is balanced by both the lobe magnetic pressure and the curvature force of the curved field lines surrounding the plasma [Slavin et al., 1984; Moldwin and Hughes, 1992a; Ieda et al., 1998]. Tailward propagating plasmoid structures have been observed in the deep magnetotail (−80R_E) using ISEE-3 [e.g., Hones et al., 1984; Slavin et al., 1984, 1993; Moldwin and Hughes, 1992a, 1992b], in the midmagnetotail (−35R_E) using IMP8 [e.g., Hones, 1977; Slavin et al., 1990], and in the near tail using Cluster [e.g., Zong et al., 2004, 2007; Tang et al., 2009] and Time History of Events and Macroscale Interactions during Substorms [e.g., Runov et al., 2009, 2011]. Plasmoids are usually considered as flux ropes since observations show that the B_y component inside a plasmoid is often enhanced [Hughes and Sibeck, 1987]. Therefore, in this paper, we do not distinguish plasmoids from flux ropes, rather we use the term plasmoid generically.

As demonstrated by Moldwin and Hughes [1991], most plasmoids observed in the magnetotail propagate tailward. However, examples of quasi-stagnant and earthward moving plasmoids have also been observed [e.g., Siscoe et al., 1984; Nishida et al., 1986; Moldwin and Hughes, 1992a; Eastwood et al., 2005; Zong et al., 2004, 2007]. A statistical study by Moldwin and Hughes [1994] showed that earthward propagating plasmoids are very rare, have a significantly smaller spatial size compared to tailward propagating plasmoids, and are correlated with small, isolated substorms during periods of relatively quiet geomagnetic activity. Moldwin and Hughes [1994] interpret the earthward moving plasmoids as being proto-plasmoids, i.e., plasmoids that are trapped inside closed magnetic field lines because magnetic reconnection at the near-Earth X line (NEXL) stopped before the lobe flux could be fully reconnected. As a consequence, the proto-plasmoid moves earthward due to the influence of earthward plasma flows from the distant neutral line. This hypothesis suggests that the traveling direction of plasmoids is determined by the interaction between multiple, competing reconnection X lines in the magnetotail. When reconnection at an X line (e.g., midtail X line) tailward of the NEXL dominates, both the NEXL and the trapped plasmoid move earthward [e.g., Eastwood et al., 2005]. However, the mechanism(s) that quenches magnetic reconnection at the NEXL (or alternatively that enhances reconnection at a distant X line) remains an open question.

This paper is motivated by observational studies suggesting that ionospheric-sourced O⁺ ions may play a role in the development of earthward propagating plasmoids in the magnetotail. Using measurements from the Cluster satellites, Zong et al. [2007] observed O⁺ ions in the vicinity of earthward propagating plasmoids. They found that the O⁺ ions observed in the leading edge of the plasmoid behaved differently compared to the H⁺ ions, that is, the O⁺ ions flow both earthward and tailward while the H⁺ ions only flow earthward. This difference suggests that the plasmoid is largely formed of H⁺ ions that were preexisting in the plasma sheet while the O⁺ ions have been recently supplied by the ionosphere [Zong et al., 1997, 1998]. In the Zong et al. [2007] event, before 18:00 UT while the geomagnetic activity is low, little O⁺ exists in the plasma sheet owing to the low outflowing flux rates from the ionosphere [e.g., Cully et al., 2003; Lennartsson et al., 2004]. After 18:00 UT, the interplanetary magnetic field (IMF) mostly turns southward that opens magnetic flux tubes on the dayside at an increased rate, resulting in the accumulation of open magnetic flux in the lobes. The change in IMF to southward is likely to generate electromagnetic power in the dayside magnetosphere that can flow into the ionosphere which drives O⁺ ion upwelling and outflow [Strangeway et al., 2005]. Lennartsson et al. [2004] showed that statistically the O+ outflow fluxes are most prominent near the dayside cusp region. Given the 30–60 min transit time of O⁺ ions from the ionosphere to the magnetotail (depending on the outflow parallel velocity and the length of magnetotail flux tubes), it is probable that a plasmoid structure may have already begun to form by the time O⁺ reaches the plasma sheet, resulting in a limited population of O⁺ ions in the plasmoid with a larger population in the plasma around it. The Cluster observations raised two interesting questions regarding what are the impacts from these ionospheric O⁺ ions on the dynamics of plasmoids in the magnetotail and whether a direct connection exists between ionospheric O⁺ outflow and earthward moving plasmoids.

Global magnetohydrodynamics (MHD) simulations of the coupled solar wind-magnetosphere-ionosphere system have been used to investigate the formation and evolution of large, tailward propagating plasmoids [e.g., Farr et al., 2008; Honkonen et al., 2011; Zhu and Raeder, 2014]. These global models have also demonstrated a wide range of impacts of ionospheric-sourced ions on magnetosphere dynamics. For example, ionospheric O⁺ outflow may reduce the dayside magnetopause reconnection through the process of mass loading [Winglee et al., 2002; Glocer et al., 2009a, 2009b; Brambles et al., 2010], enhance the ring current in the inner magnetosphere during geomagnetic storms [e.g., Glocer et al., 2009a], and change the modes of magnetospheric convection from steady magnetospheric convection (SMC) to sawtooth oscillations during coronal mass ejection storms [Brambles et al., 2011, 2013]. Ouellette et al. [2013] showed that O⁺ ions landing in the vicinity of nightside reconnection act to reduce the local Alfvén speed and nightside reconnection rate. This result suggests a possible link between earthward plasmoids and multiple X line reconnection. Could the arrival and subsequent mass loading of O⁺ ions in the magnetotail be the reason why reconnection at the NEXL slows or ceases before all the lobe flux is reconnected?

Recently, Yu and Ridley [2013] produced an earthward moving plasmoid in a multifluid global MHD simulation when O⁺ outflow ions from the cusp were included in the simulation. Their simulations also suggest that ionospheric O⁺ ions may be important in the formation of earthward propagating plasmoids. While the global simulation does suggest a causal connection between ionospheric O⁺ outflow and earthward plasmoids, the physical processes associated with the generation and propagation of the earthward plasmoid were not analyzed in detail.

In this paper, we use controlled, multifluid Lyon-Fedder-Mobarry (MFLFM) global magnetospheric simulations to investigate roles of ionospheric O⁺ ions on the dynamics of earthward moving plasmoids in the magnetotail. Section 2 introduces the MFLFM global simulation and describes the ionospheric O⁺ outflow specifications used in the global model. In section 3, we compare and contrast the magnetotail plasmoid dynamics when a source of ionospheric O⁺ outflow is and is not included in the controlled global simulations. Section 4 summarizes the results.

2 Simulation Information

2.1 The MFLFM Global Model

The Lyon-Fedder-Mobarry (LFM) global simulation model has been used extensively to study the solar wind-magnetosphere-ionosphere interaction. The numerical methods of the LFM model are described by Lyon et al. [2004] and Merkin and Lyon [2010] with details of its precipitation model and stand alone conductance model described by Wiltberger et al. [2009] and Zhang et al. [2015]. The multifluid adaption of the LFM code can solve for multiple ion fluids allowing distinct ionospheric-sourced ion fluids to be tracked. The finite volume techniques allow the high-resolution MFLFM model to complete the calculation on a nonorthogonal grid that is adapted to magnetotail studies such as plasma sheet reconnection. Reconnection in the code occurs via numerical resistivity [Lyon et al., 2004], wherein the frozen-in condition is violated when magnetic gradients reach the grid size, which is approximately 0.25 R_E near the typical X line in the magnetotail. The reconnection electric field is typically of the order of 0.1v_AB_in, where v_A and B_in are the inflow Alfvén speed and magnetic field strength, respectively, indicating that simulated reconnection in LFM is fast Petschek like rather than slow Sweet-Parker. This practically universal scaling indicates that the changes in the rate of reconnection are not caused by changes in numerical dissipation and that the numerical resistivity is simulating large-scale aspects of reconnection appropriately [Ouellette et al., 2010, 2013]. Fedder and Lyon [1987] and Fedder et al. [1995] have also shown that the rate of reconnection in the magnetosphere is controlled by the solar wind conditions and magnetosphere-ionosphere coupling rather than numerically controlled by the simulation cell sizes.

2.2 O⁺ Outflow Simulation in MFLFM

The simulations in this paper use idealized, solar wind (SW) and interplanetary magnetic field (IMF) conditions that facilitate easier analysis and interpretation of complex magnetospheric dynamics. These solar wind conditions are designed to be similar to the SW/IMF conditions that are found during quiet isolated substorms typical of earthward propagating plasmoids [Moldwin and Hughes, 1992a] and during the 28 October 2002 event when earthward plasmoids were observed by the Cluster satellites [Zong et al., 2004, 2007]. In 28 October 2002 event, the IMF B_z varies with time and was mostly northward before 18:00 UT. After 18:00 UT, IMF B_z is predominantly southward with a variable magnitudes of approximately −4 nT. The average SW velocities V_x, V_y, and V_z were −466 km/s, 36 km/s, and 18 km/s, respectively, and the SW number density was 3.2 cm⁻³ and the IMF B_y was −1 nT. In this event, the earthward moving plasmoid was observed around 19:40 UT, approximately 1 h and 40 min after the IMF turns predominantly southward. The IMF B_z condition during the 28 October 2002 event is shown in Figure 1, in which the observation time of the earthward plasmoid is shaded in green. Given the uncertainties in the propagation of upstream solar wind and the lack of a first-principle, physics-based O⁺ outflow models (especially for transversely accelerated O⁺ ions), we chose idealized SW/IMF driving conditions rather than those from the 28 October 2002 event to investigate the role of ionospheric O⁺ on magnetotail dynamics. The IMF B_z condition used in the test simulations is shown in Figure 1. In the test simulations, after the magnetosphere was preconditioned [e.g., Lotko et al., 2014], IMF B_z was set to be +5 nT for 4 h (20:00–00:00 simulation time, ST) and then −5 nT for 2 h (00:00–02:00 ST). The IMF B_x and B_y components were set to zero, and the SW V_x = −400 km/s, V_y = V_z = 0. The SW number density and sound speed were set to be 5 cm⁻³ and 40 km/s, respectively. To simplify the analysis, the dipole tilt was set to zero in order to remove hemispheric asymmetries, and the ionospheric Pedersen conductance was set to be spatially uniform of 5 mhos. The use of uniform ionospheric conductance removes the asymmetries in the magnetotail reconnection [Lotko et al., 2014].

Figure 1

The IMF B_z condition during the 28 October 2002 earthward plasmoid event, the observation time of the earthward plasmoid (from Zong et al. [2007]) is shaded in green. The idealized IMF B_z condition used in the test simulations is shown in red, the corresponding onset time of the simulated earthward plasmoid is shaded in brown.

The impacts of ionospheric O⁺ ions on magnetotail dynamics are investigated by comparing a “baseline” simulation with no ionospheric O⁺ outflow and an “outflow” simulation where the O⁺ ion fluids are introduced at the low-altitude (inner) boundary of the magnetospheric simulation domain. The ionospheric O⁺ outflow in the MFLFM simulation is specified in the first active computational shell (approximately 2.1 R_E geocentric) at the inner boundary of the magnetosphere domain. This approach is similar to the one used by Brambles et al. [2010]. In these simulations, O⁺ ions outflow from a fixed patch, representing the cusp region. The outflowing number flux is taken to be constant in space and time rather than specifying a causally regulated outflow as Brambles et al. [2011] and Glocer et al. [2009a]. The cusp outflow is turned on 20 min after the southward IMF arrives at the Earth to account for the time-delayed response of both the magnetosphere and the ionosphere. In LFM simulations, the dayside magnetospheric configuration including the cusp position takes approximately 20 min to adapt to a change in upstream solar wind conditions [e.g., Zhang et al., 2011, 2013].

For these solar wind driving conditions, the contribution from the nightside auroral region will be negligible until the magnetotail activity is enhanced after substorm onset (typically occurs in the LFM simulation 1–1.5 h after the southward turning of the IMF, depending on the magnitude of SW velocity and IMF B_z); and thus, no ions from nightside sources will populate the plasma sheet within the studied time frame. At the approximately 2.1 R_E geocentric distance, the outflowing O⁺ population is set to have a total number density of 160 cm⁻³, with an average parallel velocity of 20 km/s and sound speed of 15 km/s, resulting in an outflow flux of 3.2 × 10⁸ cm⁻² s⁻¹ or 2.6 × 10⁹ cm⁻² s⁻¹ dipole mapped down to the ionosphere (100 km altitude) assuming the flux is conserved along magnetic flux tubes (Flux/B is constant). This outflow flux is consistent with the statistical value of 10⁸−10⁹ cm⁻² s⁻¹ reported by Lennartsson et al. [2004]. The resulting integrated hemispheric O⁺ outflow flux was 3.8 × 10²⁵ s⁻¹ and is consistent with the statistical integrated hemispheric outflow rate of approximately 10²⁵ observed by both Lennartsson et al. [2004] and Cully et al. [2003] for the moderate southward IMF driving conditions used in the controlled simulations.

The outflow patch is centered at noon, spanning 2.2 h in magnetic local time (MLT) and between magnetic latitudes of 73° to 76° at ionospheric altitudes (100 km). The measured cusp location in the LFM simulation for these solar wind driving conditions varies between 74° and 75° depending upon the method used to determine the center of the cusp [Zhang et al., 2013]. The fixed location and extent of the cusp outflow patch are consistent with observations. Cusp widths of 3° and 4° in latitude are consistent with observations by Zhou et al. [2000] and Palmroth et al. [2001], respectively, and an azimuthal width of 1.8–2.7 MLT is consistent with observations by Merka et al. [2002].

3 Results

The following analysis compares the response of the magnetotail for simulations with and without inclusion of ionospheric O⁺ outflow. Figure 2 displays magnetotail dynamics in the baseline simulation (Figure 2a) and the outflow simulation (Figure 2b) along a nightside radial cut plane at 0130 MLT. This cut plane was chosen to highlight the formation of tailward/earthward moving plasmoids in the simulations without and with O⁺ outflow. The closed, open, and solar wind magnetic field lines are shown in red, blue, and green, respectively. The merging electric field (where B_x is the magnetic field strength in the x direction of the Solar Magnetospheric (SM) coordinate, μ₀ is the vacuum permeability, and ρ is the total plasma density) shown in each panel using a black dashed line is averaged from the reconnection inflow regions over z_SM = ±0.5R_E in the 0130 MLT cut plane. This average value is an indication of the local reconnection rate. In Figure 2b, the O⁺ mass density in the 0130 MLT cut plane is shown using a scaled red color.

Figure 2

Magnetic field lines in (a) the baseline simulation and (b) the outflow simulation along a nightside radial cut plane at 0130 MLT. The closed, open, and solar wind magnetic field lines are shown in red, blue, and green, respectively. The merging electric field shown in each panel using a black dashed line is averaged from z_SM = ±0.5R_E in the 0130 MLT cut plane. The O⁺ mass density in the 0130 MLT cut plane is shown using a scaled red color in Figure 2b.

In the baseline simulation without O⁺ outflow (Figure 2a), a cold-dense plasma sheet (CDPS) formed during the northward IMF driving before 0:00 ST and is still present at 1:00 Simulation Time (ST) (Figure 2a-i). Multiple magnetotail reconnection sites start to develop near x_SM=−15R_E and −45R_E. By 1:15 ST (Figure 2a-ii), multiple X lines have formed in the magnetotail near x_SM=−85R_E, −50R_E, and −20R_E, and plasmoids are formed between these X lines. The decrease of the open magnetic indicates that a substorm onset occurs near this simulation time (see Figure 4). Nightside reconnection occurs at all three X lines; however, the merging rate decreases as the distance from the X line to the Earth increases resulting in all plasmoids propagating tailward. Starting from 1:20 ST (Figure 2a-iii), the magnetotail reconnection is completely dominated by the NEXL around x_SM=−20R_E and the packet of multiple plasmoids are ejected tailward. After 1:30 ST (Figures 2a-v–2a-vii), the reconfiguration of the magnetotail due to the substorm allows a balance between dayside and nightside reconnection to be established, and the magnetosphere settles into a steady magnetospheric convection (SMC) mode, resulting in no further plasmoids being formed. This substorm-SMC cycle is consistent with the statistical observations reported by Kissinger et al. [2012].

In the controlled simulation with O⁺ outflow (Figure 2b), the magnetotail dynamics are similar to the baseline simulation at 1:00 ST (Figure 2b-i) and 1:15 ST (Figure 2b-ii). After 1:15 ST, the ionospheric O⁺ ion outflows start to land in the equatorial magnetosphere, as shown in Figure 2b-iii, and modify the distribution of magnetotail reconnection. As a consequence, the magnetotail dynamics diverges from the baseline simulation significantly. In contrast to the baseline simulation, between 1:15 and 1:30 ST, the plasmoid that formed between the NEXL and mid tail X line (MTXL) lines becomes quasi-stagnant instead of moving tailward as those in the baseline simulation. Starting from 1:30 ST, magnetotail reconnection is shifted from the NEXL to the MTXL as evidenced by the enhancement of the merging rate at the MTXL and its rapid reduction at the NEXL. At 1:32 ST (Figure 2b-iv), the magnetotail reconnection in the 0130 MLT cut plane is dominated by the MTXL and the trapped plasmoid starts to move earthward (the green shaded band in Figure 2b). Note that the earthward moving plasmoid is a part of a flux rope structure formed near 1:30 ST. The three-dimensional structure of the earthward moving plasmoid at 01:32:00 ST is shown in Figure 3. The closed, open, and IMF field lines are shown in red, blue, and green, respectively. The color in the equatorial plane indicates the plasma bulk velocity v_x, and the black contour is the B_z = 0 contour which, owing to hemispheric symmetry in this controlled simulation, indicates the location of magnetic reconnection [Ouellette et al., 2010, 2013].

Figure 3

The 3-D topology of the earthward moving plasmoid at 1:32 ST in the outflow simulation. The closed, open, and solar wind magnetic field lines are shown in red, blue, and green, respectively. The black contour shows that B_z = 0 line, and the color in the equatorial plane indicates the bulk velocity of magnetospheric plasma.

Figure 3 shows that the plasmoid has an earthward velocity of approximately 350 km/s, which is consistent with observations made by the Cluster spacecraft [Zong et al., 2007]. The large earthward flow in the flank regions of the nightside magnetosphere is a consequence of magnetotail reconnection, which is possibly due to the use of uniform ionospheric conductances [Lotko et al., 2014]. Note that the earthward moving plasmoid has a smaller spatial size (approximately 5 × 3R_E in the cut plane) compared to the tailward plasmoid (greater than 20 × 10R_E in the cut plane) shown in the baseline simulation. The size of the earthward plasmoid is consistent with an estimation of approximately 5.04 R_E derived from the Cluster observations [Zong et al., 2004]. The tailward plasmoid in the simulation moves at speeds close to the solar wind, the details about the tailward plasmoid dynamics are not the focus of this study and can be found in McGregor et al. [2014]. As the plasmoid moves earthward (Figures 2b-v and 2b-vi), the spatial size decreases quickly due to the reconnection between the plasmoid and the geomagnetic fields. The lifetime of the earthward plasmoid is also much shorter compared to the tailward plasmoid in the baseline simulation. After 1:35 ST (Figure 2b-vii), the earthward plasmoid disappears and the magnetosphere settles into an SMC state, with a new NEXL that appears closer to the Earth compared to that in the baseline simulation. The NEXL is closer to Earth in the outflow simulation, which is consistent with previous outflow simulation results reported by Brambles et al. [2010] and Wiltberger et al. [2010]. These simulations suggest that ionospheric O⁺ outflow can induce earthward moving plasmoids in the magnetotail by changing the distribution of magnetic reconnection in the tail. The earthward propagating plasmoid is formed through processes which reduces the reconnection rate at the NEXL (earthward of the plasmoid) and moves reconnection to the MTXL (tailward of the plasmoid), which forces the plasmoid earthward.

The tailward motion of the plasmoid may be stalled by the reduction of the nightside reconnection rate at the NEXL. This reduction is hinted in the equatorial merging rate shown in Figure 2, but the dynamic nature of the magnetotail makes it impossible to make conclusions simply based upon snapshots of cut planes; therefore, it is important to examine the integrated nightside reconnection rate in order to show the impacts of O⁺ outflow on the magnetotail dynamics more quantitatively. Figure 4a shows the comparison of open polar cap magnetic flux between the baseline and outflow simulations. The open magnetic flux is calculated by tracing field lines from the inner boundary of the magnetosphere domain (2.1 R_E geocentric) to determine the open/closed boundary of the geomagnetic field. The magnetic flux enclosed by this boundary is then integrated using the magnetic fields from the global simulation. In these controlled simulations, the dayside reconnection potential is largely controlled by the solar wind [Lopez et al., 2010]. Modification to the dayside reconnection potential through magnetopause mass loading of the O⁺ ions does not occur in the time frame of this simulation as the O⁺ ions have not yet to be transported from the magnetotail to the magnetopause. Therefore, the dayside reconnection remains approximately the same throughout both simulations. As a consequence, the difference in the simulated open flux is solely controlled by changes in the nightside reconnection rate. As shown in Figure 4a, after 01:15 ST, onset of nightside reconnection occurs in both simulations as accumulated open flux in the lobes begins to be reconnected. Between 01:20 and 01:25 the rate of closure of open flux is reduced by approximately 20% in the outflow simulation compared to the baseline simulation. This timing is consistent with the transit time of the O⁺ ions traveling from the inner boundary to the equatorial plane, which suggests that the O⁺ ions are acting to reduce the nightside reconnection. The O⁺ ions mass load the reconnection inflow decreasing the Alfvén velocity, the merging rate, and the reconnection electric field [Ouellette et al., 2013]. This reduction of nightside reconnection due to O⁺ ions is also evident in the temporal variation of the magnetic pressure in the lobes. Figure 4b shows the variations in the lobe magnetic pressure recorded at x_SM = −35R_E, z_SM = 25R_E, and y_SM = 0R_E from both simulation. In both simulations, the lobe magnetic pressure reaches a peak of 0.092 nPa at 01:15 ST and then decreases as the lobe magnetic field flux tubes are reconnected. In the baseline simulation, the rate of decrease in magnetic pressure is relatively constant between 01:15 and 01:35 ST; however, in the outflow simulation, the reduction in the lobe magnetic field stalls at 01:23 ST and does not start to decrease again until 01:30 ST. By 01:38 ST in the outflow simulation, both the polar cap open flux and lobe magnetic pressure have recovered to that in the baseline simulation suggesting that after 01:30 ST the reconnection rate is enhanced. The enhancement of reconnection rate after 1:30 ST in the outflow simulation is possibly a consequence of balancing the dayside reconnection rate since the nightside reconnection rate is reduced at 1:20 ST. The physical processes associated with the enhanced magnetotail reconnection are unknown, which requires further investigation. The timing is slightly different between Figures 4a and 4b due to the fact that the open magnetic flux is an integrated quantity while the magnetic lobe pressure is taken at one specific location in the magnetosphere. The temporal variations in lobe pressure at different radial distances give similar profiles with small variations (±2 min) in timing. These results provide compelling evidence that in the outflow simulation, the presence of the O⁺ ions landing in the equatorial plane acts to reduce the nightside reconnection rate, causing the plasmoid to become quasi-stagnant.

Figure 4

Comparisons of (a) open polar cap magnetic flux and (b) lobe magnetic pressure recorded at x_SM = −35R_E, z_SM = 25R_E, and y_SM = 0R_E between the baseline and outflow simulations.

The timing of the enhancement in nightside reconnection shown in temporal profiles of the open flux and lobe magnetic pressure is consistent with when the plasmoid begins to move earthward and reconnection becomes intensified at the MTXL. The change in the reconnection location is explored in Figure 5 which shows the distributions of O⁺ mass density, total mass density, the average merging rate, and in the equatorial plane at 1:29:30 ST just before the plasmoid starts to move earthward and at 1:33:00 ST when the plasmoid is moving earthward in the outflow simulation. The average merging rate ( ) is calculated the same as that in Figure 2 and is illustrative of the reconnection electric field, which is averaged from two x-y cut planes with z = ± 0.5R_E. The earthward moving plasmoid is highlighted in Figure 5 by the red rectangle. For figure clarity, the locations of NEXL and MTXL are only indicated in the plot with the average merging rate (Figure 5-iii) . As shown in Figure 5-i, at 01:29 ST, a significant number of O⁺ ions have accumulated in the equatorial plane especially in the near-midnight region of the NEXL. As a consequence, the local reconnection rate is reduced through the mass loading process [Borovsky, 2013; Ouellette et al., 2013]. The ionospheric O⁺ ions do not have direct access to the core region of the plasmoid because the plasmoid was formed before the ionospheric O⁺ ions arrived at the equatorial plane and when it was magnetically connected to the IMF (see Figure 2). As a result, the earthward plasmoid contains little O⁺ plasma and mostly consists of H⁺ plasma from the cold-dense plasma sheet that formed during northward IMF while the ambient plasma has a higher concentration of O⁺ ions. This low O⁺/H⁺ density ratio in the earthward moving plasmoid is consistent with the Cluster observations [Zong et al., 2007]. At 01:29 ST in the outflow simulation, the merging rate dominates at the NEXL in regions where is large and the total mass density is low. This mass loading of the midnight NEXL forces reconnection to the dawn and dusk flanks where the density is lower and reconnection is more favorable. Reconnection at the MTXL is also present tailward of the plasmoid that acts to maintain the plasmoid in a quasi-stagnant state.

Figure 5

The distributions of O⁺ mass density, total mass density, the average merging rate, and in the equatorial plane at 1:29:30 ST before the plasmoid starts to move earthward and at 1:33:00 ST when the plasmoid is moving earthward in the outflow simulation. The average merging rate ( ) is averaged from two x-y cut planes with z = ± 0.5R_E. The line contours in each panel indicate the locations with B_z = 0. The earthward moving plasmoid is highlighted using a red rectangle.

At 01:33 ST (Figure 5b) the plasmoid is no longer stagnant and is propagating earthward. The plasmoid moves earthward owing to an increase in the merging rate at the MTXL relative to the NEXL as shown in Figure 5b-iii. The enhanced merging rate at the MTXL is due to an increase in the B_x component at the MTXL (Figure 5b-iv), while the reduced merging rate at the NEXL is due to the increase in O⁺ density from cusp outflow (Figure 5b-i). The exact reason why the reconnection shifts to the MTXL at this time is unknown. Magnetotail reconnection in the LFM simulation becomes prominent at locations with the fastest reconnection rate. It seems plausible that when the lobe magnetic pressure achieves a secondary peak at 1:29 ST (see Figure 4b) and the plasma sheet becomes sufficiently thinned at the MTXL, reconnection becomes more favorable at that location. As a result, the exhaust flows of the enhanced reconnection at the MTXL force the plasmoid to move earthward, after which time the open flux and lobe magnetic pressure start to decrease to the level of the baseline simulation.

4 Summary and Discussions

The controlled simulation with idealized cusp O⁺ outflow suggests a possible connection between ionospheric heavy ions (O⁺) and earthward moving plasmoids. When multiple X lines are formed in the magnetotail, plasmoids may be trapped between multiple reconnection sites. If the reconnection rate is reduced at the NEXL by ionospheric O⁺, reconnection can become more favorable at the MTXL. This development is consistent with the observed formation of earthward propagating plasmoids as suggested by Moldwin and Hughes [1994]. In the scenario described here, the ionospheric O⁺ ions are responsible for quenching reconnection at the NEXL. This hypothesis is also consistent with that proposed by Lopez et al. [2010] to describe the response of the dayside reconnection potential to localized mass loading from plasmaspheric plumes. They suggested that when the dayside merging line is locally mass loaded at a certain location, the reconnection rate locally decreases but the accompanying pileup of magnetic flux causes the merging rate to increase in neighboring regions. In the magnetotail, an extra degree of freedom exists so that reconnection can take place at variable radial distances as well as in MLT owing to multiple X lines. In this regard, the tail presents an additional level of complexity that allows for the generation of earthward propagating plasmoids.

In these simulations, the timing of cusp O⁺ outflow reaching the equatorial plane plays an important role in inducing earthward plasmoids. Additional numerical experiments show that when the cusp O⁺ outflow is delayed by 15 min in the simulation (turned on at 00:35 ST) instead of 00:20UT, no earthward plasmoids are generated because by the time O⁺ ions arrive in the equatorial magnetosphere, the trapped plasmoids are already ejected tailward and the magnetosphere has already begun to settle into an SMC mode as shown in the baseline simulation. This numerical experiment on the timing of initiating cusp O⁺ outflow also suggests the importance of the CDPS in the formation of earthward moving plasmoids. When dayside reconnection brings magnetic flux into the lobe, the CDPS is compressed toward the x axis and multiple X lines may form during the transient stage of the magnetotail. This role of the CDPS in the formation of earthward moving plasmoids is also consistent with the fact that the observed earthward plasmoids occur during relative quiet periods when a CDPS had time to develop in the magnetotail. This result differs from the sequence of events described by Yu and Ridley [2013]. An earthward propagating plasmoid formed in their simulation when ionospheric outflow was initiated after the system had settled into a SMC mode. It is unclear in their simulations how the O⁺ causes the compression of the plasma sheet and subsequent generation of multiple X lines. Wiltberger et al. [2010] used similar upstream driving conditions and O⁺ outflow specifications to that used in this study; however, their outflow was turned on when the southward IMF hit the Earth rather than being delayed. In their report, they did not investigate the physical processes associated with whether earthward propagating plasmoids were generated by the ionospheric-sourced ions. Note that the dependence of earthward plasmoid on the transit time of ionospheric outflow and the existence of CDPS suggest that these earthward propagating plasmoids are not common features of the coupled magnetosphere-ionosphere system. Given the fact that the simulations use idealized driving conditions and controlled specifications of ionospheric outflow, in the real magnetosphere, these conditions may not induce earthward plasmoids as shown in the test simulations. Thus, the occurrence of earthward plasmoids may be even lower. The rareness of earthward plasmoids is clearly illustrated by satellite observations [e.g., Moldwin and Hughes, 1994].

Acknowledgments

The research was supported by the following projects: NASA grants NNX11AO59G, NNX11AJ10G, and NSFAGS-1404599. Computing resources were provided by the CISL at the National Center for Atmospheric Research (NCAR) under Project UDRT0006. NCAR is sponsored by the NSF. Simulation data are available from B. Zhang ([email protected]).