Plasma Observations in the Distant Magnetotail During Intervals of Northward IMF

We examine a 6‐day traversal of the magnetotail by the ARTEMIS satellites during an interval of prolonged northward IMF. The electrostatic analyzer (ESA) onboard the ARTEMIS spacecraft measures high ion and electron fluxes at approximately 60 RE downtail in regions of the magnetotail which would normally be the magnetotail lobe, containing open flux evacuated of plasma. We interpret these observations as trapped plasma on closed magnetic flux indicating that the magnetotail is closed or partially closed but extends at least as far as ∼60 RE downtail. We find that the occurrence of plasma in the magnetotail and the closure of the magnetosphere results in distinct changes to the magnetotail structure including a reduction in the magnetic field strength and pressure as well as a narrowing of the tail by approximately 20 RE.


Introduction
Recent observations in the near-Earth magnetotail have suggested that the magnetosphere can become almost entirely closed during periods of low clock angle (Milan et al., 2023).In this study we examine the magnetotail structure at lunar orbit during such intervals, and suggest that a closed magnetotail can extend to at least 60 R E in length.
Dungey's open magnetosphere model (Dungey, 1961) has been highly successful in explaining many aspects of the magnetospheric structure and dynamics.In Dungey's model, the coupling between the upstream solar wind and the magnetosphere is well understood during periods of predominantly southward-directed interplanetary magnetic field (IMF).Reconnection between the IMF and the Earth's magnetic field occurring at the dayside subsolar point opens the magnetic field on the dayside resulting in an increase in the open flux content of the magnetosphere.The newly opened field lines convect anti-sunward into the magnetotail creating the open lobe regions north and south of the closed flux in the central plane of the magnetotail.The solar wind plasma is frozenin to the IMF; however as the IMF reconnects with the Earth's magnetic field resulting in open field lines in the lobe regions, the plasma escapes along the open field lines and back out to the solar wind leaving the magnetotail lobes evacuated of plasma.The polar caps which map to the magnetotail lobes are then also generally void of auroral emission.The open flux in the magnetotail is closed via reconnection in the nightside current sheet.The newly closed flux then convects back around to the dayside magnetosphere.The relative rate of ongoing dayside and nightside reconnection determines the open flux content of the magnetosphere and the polar caps and auroral ovals expand and contract as the open flux content varies (Cowley & Lockwood, 1992;Lockwood & Cowley, 1992;Milan et al., 2007Milan et al., , 2021)).
Under typical southward IMF conditions the magnetotail lobes contain approximately 0.5 GWb of open flux (Milan et al., 2004) and the open lobe field lines may extend to around 1,000 R E downtail (Dungey, 1965).The magnetotail is approximately 40 R E in diameter, although the diameter varies due to a number of factors including changes in the open flux content of the polar cap/lobes (Milan et al., 2004), the solar wind ram pressure on the near-Earth tail and the solar wind gas pressure on the distant tail.The stress balance between the solar wind pressure and the magnetic field pressure internal to the magnetotail requires a field strength of ∼10 nT in the distant tail, as predicted by the Tsyganenko magnetic field model (T96, Tsyganenko & Stern, 1996).
After periods of ongoing reconnection at the dayside subsolar point increasing the magnetospheric open flux content, a substorm can eventually initiate in the magnetotail.Substorms close large amounts of open flux and reduce the stress in the magnetotail (Akasofu, 1964;McPherron, 1970).Approximately 10 15 J of energy stored in the magnetosphere can be released during a substorm (Tanskanen et al., 2002).Substorm reconnection initiates in the tail at the near-Earth neutral line (NENL) (e.g., Baker et al., 1996;Hones, 1976) at about 20-30 R E downtail forming a plasmoid of closed magnetic field which is ejected downtail.As reconnection continues, the NENL propagates downtail to become a distant neutral line (DNL).
Magnetospheric dynamics during periods of predominantly northward-directed IMF are less well understood.Under northward IMF reconnection occurs at higher latitudes tailwards of the cusp rather than near the subsolar point (Dungey, 1963).High latitude reconnection, often referred to as lobe reconnection, can occur simultaneously in both hemispheres.If there is a significant B Y component in the IMF, reconnection occurs on different field lines in both hemispheres resulting in no change to the open flux content of the magnetosphere.This is referred to as single lobe reconnection as the reconnection process is essentially occurring independently in each hemisphere.If the B Y IMF component is approximately zero then high latitude reconnection in both hemispheres may occur on the same magnetic field lines resulting in a closure of flux and an overall decrease in the magnetospheric open flux content.The B Y ≈ 0 case is known as dual-lobe reconnection (DLR).Simulations for a purely northward IMF have shown that the magnetotail becomes dominated by closed magnetic flux and can be extremely truncated (e.g., Fryer et al., 2023;Li et al., 2021).Estimations of the range of clock angles over which dual-lobe reconnection can occur vary with observational studies suggesting that DLR occurrence is limited to clock angles between θ c = ±10° (Imber et al., 2006) while others suggest a much larger range of θ c = ±60° ( Lavraud et al., 2006) or even θ c = ±90° (Twitty et al., 2004).Simulations have suggested that northward IMF with a non-zero B Y component results in a twist in the magnetotail with open magnetospheric field line regions forming in the dawn and dusk sectors with field lines rooted in the southern and northern hemisphere cusps, respectively (Fryer et al., 2023;Li et al., 2021Li et al., , 2022)), with both studies noting evidence for reconnection occurring in the magnetotail between the two lobe regions.The simulation by Fryer et al. (2023) suggested that reconnection between the dusk and dawn lobes resulted in a significant increase of the closed field line region between the two lobes extending ∼70 R E downtail and spanning Z ∼ ±20 R E .
During periods of northward IMF the magnetosphere has been observed to have a very different structure.In the inner magnetosphere, a cold (<1 keV), dense (∼1-2 cm 3 ) plasma sheet forms as a result of solar wind and magnetosheath plasma entry into the magnetosphere during high latitude reconnection (Li et al., 2008;Øieroset et al., 2005).Observations have shown the depletion and disappearance of open lobe field lines in the magnetotail after 1 hr of northward IMF and a mixed particle population of both plasma sheet and solar wind strahl electrons far downtail (X = 125 R E ) (Øieroset et al., 2008).Øieroset et al. (2008) suggested there was evidence of reconnection between the IMF and the nightside plasma sheet after the depletion of open lobe field lines but argued that the magnetotail consisted of regions of both open and closed flux.
Given the different magnetospheric structure under northward IMF, it can be misleading to refer to lobe regions of the magnetotail during northward IMF as "lobes" tend to refer to regions of open flux.Similarly, the polar caps are regions encircled by the auroral ovals which map to the open flux content of the magnetosphere lobes.While there may be regions of open flux in the magnetotail under northward IMF, overall we expect the magnetosphere to be dominated by closed flux.In this study, when we refer to the magnetotail lobes and the polar caps we specifically mean regions of open flux.When referring to similar regions during northward IMF, we will refer to the polar regions instead of the polar caps and clarify when we observed closed flux in regions where we would expect to see open magnetospheric lobes, evacuated of plasma.
Observing the auroral emission and dynamics near the poles is crucial to understand the larger scale structure of the magnetosphere under any IMF orientation.During a period of northward IMF, Y. Zhang et al. (2009) observed discrete auroral arcs in the polar region which expanded to fill the polar ionosphere.The authors suggested that it could indicate a complete disappearance of the polar cap due to the closure of the magnetosphere.Milan et al. (2020) and Milan et al. (2022Milan et al. ( , 2023) ) have suggested that high latitude reconnection occurring simultaneously in both hemispheres (i.e., DLR) can close significant amounts of open flux and can even result in an almost entirely closed magnetosphere, consistent with simulations which predict a closed or partially closed magnetosphere during northward IMF (e.g., Fryer et al., 2023;Song et al., 1999;Usadi et al., 1993).Periods of DLR are associated with distinct auroral signatures including horse-collar auroras (HCAs) which form as a result of a poleward contraction of the dawn and dusk auroral oval creating a tear-drop shaped polar cap (Bower et al., 2022;Hones et al., 1989;Milan et al., 2020) and cusp-aligned arcs (CAAs) which are observed as weak auroral arcs that cover the polar regions and are aligned toward the cusp or extend to intersect the cusp (Y.Zhang et al., 2016;Q.-H. Zhang et al., 2020).During a period of prolonged northward IMF, Milan et al. (2022) observed auroral signatures, ionospheric flows and field-aligned current patterns that indicated that high latitude reconnection was occurring in both hemispheres.A horse-collar auroral configuration was observed after which the polar region filled with multiple CAAs.In a nearly closed magnetosphere, regions of the magnetotail which are usually open and evacuated of plasma (i.e., the lobes) instead contain a higher closed flux content and trapped plasma which can be accelerated to produce auroral emission such as CAAs in the polar region.Milan et al. (2022) suggested that bursty or patchy high latitude reconnection can result in flow shears that accelerate trapped plasma via field-aligned currents forming the auroral arcs.Alternatively, Q.-H.Zhang et al. (2020) suggested that Kelvin-Helmholtz waves on the magnetotail flanks could introduce flow shears, resulting in CAAs.Wang et al. (2023) observed conjugate HCA signatures and the near disappearance of the polar cap during a period of prolonged northward IMF.Wang et al. (2023) interpreted the formation of the HCA signatures as being a result of simultaneous high latitude reconnection in both hemispheres supporting the Milan et al. (2022) model.Simulations of this period by Wang et al. (2023) estimated that the magnetosphere was closed and that the closed field lines only extended 28 R E downtail, however this was not verified by spacecraft observations.Observations and simulations of the length of the closed magnetosphere under northward IMF have varying estimates between ∼28 R E (Wang et al., 2023) to 200 R E (Fairfield et al., 1996).Milan et al. (2023) have studied a prolonged period of predominantly northward IMF during 15 days in October 2011 during which time several intervals of CAAs were observed.Milan et al. (2023) examined high density plasma in both the high latitude (Z ± 13 R E ) northern and southern magnetotail observed simultaneously by both the Cluster and Geotail satellites coincident with the CAA observations.The observed plasma was interpreted as trapped plasma on closed magnetotail field lines in regions which would typically be expected to be open lobe field lines evacuated of plasma.In this study we use ARTEMIS data to examine plasma populations in the distant magnetotail in the northern hemisphere during the same period as Milan et al. (2023).The ARTEMIS observations show occurrences of high density ion and electron populations coinciding with high density plasma observed by Cluster which orbits closer to Earth but extends far out of the ecliptic plane (Z ∼ 13 R E , X ∼ 14 R E ), indicating trapped plasma on closed flux in the magnetotail as far as ∼60 R E downtail.Although the magnetosphere is almost entirely closed, the ARTEMIS spacecraft remain within the magnetosphere as they traverse the magnetotail and as such the magnetotail is not truncated to within 60 R E .

Observations
The Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission launched in 2007 as an array of five identical satellites, named A through E (Angelopoulos, 2008).THEMIS-A, D and E orbited close to Earth with an apogee of ∼10 R E , THEMIS-C had an apogee of ∼20 R E and THEMIS-B had a more distant apogee of ∼30 R E (Frey et al., 2008) allowing the THEMIS mission to cover a range of locations within the magnetosphere to study complex magnetospheric dynamics.In 2010 THEMIS-B and C were re-positioned into lunar orbit and became known as Acceleration, Reconnection, Turbulence and Electrodynamics of the moon's Interaction with the Sun (ARTEMIS) P1 and P2, providing observations of the distant magnetotail (∼60 R E downtail), the solar wind, and the lunar space and planetary environment (Angelopoulos, 2011;Sibeck et al., 2011;Sweetser et al., 2011).
We focus on observations from the electrostatic analyzer (ESA) (McFadden et al., 2008) and the fluxgate magnetometer (FGM) (Auster et al., 2008) onboard the ARTEMIS spacecraft to study the conditions in the northern lobe at ∼60 R E downtail as the spacecraft traversed the magnetotail during a 6-day period in October 2011.In this study, we focus on observations from the ARTEMIS P2 spacecraft although observations from ARTEMIS P1 are also included for completeness.We use data from the OMNI database (King & Papitashvili, 2005) to monitor the upstream solar wind conditions throughout the period.We also include data from the Cluster Ion Spectrometry instrument (C1/CIS-CODIF, Rème et al., 1997), as presented by Milan et al. (2023) to highlight the particles fluxes observed simultaneously at both ARTEMIS and Cluster at different locations in the magnetosphere.
Figure 1 shows the trajectory of the Cluster C1 satellite and the pair of ARTEMIS satellites as they traverse the magnetotail in lunar orbit between approximately Y = ±30 R E .The ARTEMIS spacecraft enter the magnetosphere on the dusk flank on 9th October 2011, traverse across the northern magnetotail and exit the magnetosphere on the dawn flank on 14th October 2011.The dotted line indicates the location of the magnetopause estimated by the Shue model (Shue et al., 1998).The blue diagonal dashed line indicates the mean location of the neutral sheet estimated from the Tsyganenko T96 model (Tsyganenko & Stern, 1996).Due to the significant tilt in the neutral sheet, the ARTEMIS orbit crosses the magnetotail considerably above the neutral sheet in the northern hemisphere at Z ∼ 5 R E while the Cluster orbit takes the spacecraft to Z ∼ 13 R E well below the neutral sheet in the southern hemisphere.
Figure 2 shows the observations from the ARTEMIS P2 satellite between 9-14 October 2011 (days 282-287).The top two panels (a) and (b) show the IMF components and clock angle in GSE coordinates, respectively from OMNI data.Panel (c) shows the predicted magnetic field as the ARTEMIS P2 spacecraft passes through the magnetotail from the Tsyganenko T96 model for a northward IMF of B Z = 5 nT (Tsyganenko & Stern, 1996).Panel (d) shows the magnetic field components measured by the FGM.Panel (e) shows the ion velocity components.Panel (f) shows the ion and electron density.Panel (g) shows the calculated plasma, magnetic and combined total pressure calculated from ESA and FGM data.Panels (h) and (i) show the ion and electron energy flux spectrograms observed by the ARTEMIS/ESA instrument.Panel (j) shows the ion flux spectrogram from CIS on Cluster (C1).Panel (k) shows the AU and AL electrojet indices which help in identifying substorm activity within the magnetosphere.Figure 3 shows the same observations from the ARTEMIS P1 spacecraft for the same period.The observations are presented in the same format as Figure 2. Milan et al. (2023) identified a period of prolonged northward IMF during which multiple instances of cuspaligned arcs (CAAs) were identified in the polar region using the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) instrument (Paxton et al., 1992) onboard the Defense Meteorological Spacecraft Program (DMSP) satellites.Milan et al. (2023) observed high plasma fluxes in the near-Earth tail in Cluster data that coincided with the observations of cusp-aligned arcs.In this study, we use ARTEMIS data to observe the structure and plasma characteristics further down the magnetotail at ∼60 R E during the same period of prolonged northward   2023) while the ARTEMIS spacecraft traversed the magnetotail for reference and are numbered 1-9.The estimated start and end times of the CAA observations are approximate due to the limitations of the DMSP SSUSI observations and the cadence of the DMSP orbits.We note that more CAAs may have occurred in this period but may not be clearly visible in the SSUSI data due to partial coverage of the polar cap.
Concentrating on Figure 2, the OMNI IMF data in panels (a) and (b) shows that the IMF fluctuated between northward and southward throughout these periods.The IMF clock angle also fluctuated.At the beginning of the interval on 9 October (day 282), the ARTEMIS satellite is out in the solar wind evident from high ion velocities anti-sunward (V X ∼ 300 km/s) in panel (e), consistently high ion fluxes indicative of the solar wind beam in panel (h) and the variation in the magnetic field data measured by ARTEMIS in panel (d).Between approximately 0000 to 1200 on day 282, ARTEMIS P2 observes a mixture of solar wind and magnetosheath plasma (V X ∼ 300 km/s, density ∼10 cm 3 ) indicating that the dusk flank is highly variable and the ARTEMIS spacecraft crossed the magnetosheath multiple times.ARTEMIS P1 shown in Figure 3 observed a mixture of solar wind and magnetosheath plasma until approximately 0600 on day 283.The P2 satellite crossed into the dusk magnetosphere on day 282 at approximately 1200 UT where the x-component of the ion and electron velocities became very small (Figure 2e).The B X component of the magnetic field measured by ARTEMIS was negative indicating that ARTEMIS initially crossed into the southern lobe (B X = 10 nT), in agreement with the T96 model prediction of the spacecraft location, in panel (c).From approximately 1800 on day 282 to 1800 on day 283, the B X component of the magnetic field measured by ARTEMIS P2 turns positive (B X = 10 nT) but oscillates between positive and negative before remaining mostly positive from about 1800 on day 283.The oscillation could indicate that the spacecraft is crossing between the northern and southern hemisphere lobes or that it is continuing to cross between the magnetosheath and the lobes.A similar oscillation between positive and negative B X is observed in the P1 data in Figure 3d.From approximately 1800 on day 283, P2 observes a positive turning of the B X component, in agreement with the T96 prediction, which remains positive until approximately 1800 on day 286 indicating that the ARTEMIS spacecraft is traversing the magnetotail above the ecliptic plane in the northern hemisphere (typically, the northern magnetotail lobe) during this 4-day period.The neutral sheet is substantially tilted at the time of year (October) during these observations such that the ARTEMIS spacecraft cross the magnetotail considerably above the neutral sheet, well into the northern hemisphere as estimated from the T96 model, shown in Figure 1.ARTEMIS P2 crosses back into the magnetosheath at approximately 1800 on day 286 approximately 12 hr before the T96 model estimates the spacecraft will exit the magnetotail.As the spacecraft exits the magnetotail, it again observes signatures of both magnetosheath and solar wind plasma and likely crosses the magnetosheath multiple times.From the location of the satellite as it enters (Y ∼ +32 R E ) and exits (Y ∼ 27 R E ) the magnetotail flanks, we can estimate that the width of the magnetotail is a maximum of approximately 60 R E during this crossing.
Under typical southward IMF conditions the magnetotail lobe would be expected to contain open magnetic flux which is evacuated of plasma, with magnetic pressure dominating the pressure balance.During the ARTEMIS magnetotail crossing, there is evidence of the open magnetotail lobe structure during intermittent periods when the IMF is southward, for example, between 0600 and 1600 UT on day 286.During this period, the magnetic pressure dominates (Figure 2g) and the ARTEMIS and Cluster particle flux data (Figures 2h-2j) show empty lobe regions evacuated of plasma.However, during and following periods of northward IMF, both ARTEMIS spacecraft observe dense plasma (∼0.1-1 cm 3 ) containing high energy ions (on the order of 10 2 -10 4 eV) and high energy electrons (on the order of 10 2 -10 3 eV), as shown in panels (h) and (i).High ion fluxes are also observed simultaneously by Cluster (panel j) consistent with Milan et al. (2023) observations during this period.During the intervals of high particle fluxes, the plasma pressure makes a more significant contribution to the pressure balance shown in panel (g) and often corresponds with a decrease in the magnetic field magnitude from  B X ∼ 10 nT to B X ∼ 7-8 nT, for example, between 0000 and 1400 UT on the day 284 and around 1100-1600 UT on day 285.
The intervals of high ion and electron fluxes align with quiet, non-substorm periods in the AU and AL geomagnetic indices, shown in panel (k).After periods when the IMF has turned southward, for example, between 1400 and 2000 UT on day 284, the high particle fluxes disappear and the plasma pressure decreases returning to an evacuated lobe dominated by open magnetic flux with substorm activity also returning.
There are other intervals where high ion and electron fluxes are observed at ARTEMIS but not at Cluster for example, between 1800 UT on day 283 to 0000 UT on day 284 and 2200 UT on day 285-0600 UT on day 286.Both of these intervals seem to follow substorm activity indicated by the AU/AL indices in panel (k), particularly the second interval between 2200 UT on day 285-0600 UT on day 286.The plasma characteristics in these intervals differ from periods where plasma is observed coincidentally at ARTEMIS and Cluster and are more filamentary in structure with different ion and electron energies.The ion velocities are also higher during these periods with both Earthward and tailward flows of |V X | ∼ 300-400 km/s.At approximately 2200 UT on day 285, ARTEMIS P2 observes a high density tailward flow which could indicate a passing plasmoid flux rope ejected downtail following the substorm.The observed particle densities then decrease; however Earthward flows continue to be observed at lower velocities, which could indicate reconnection continuing to actively occur but the x-line is now located further downtail than the spacecraft at a distant neutral line.Both earthward and tailward directed flows have previously been observed and studied using ARTEMIS data with tailward flows primarily associated with the substorm expansion phase and NENL activity (Nishimura et al., 2013).

Case Study: Cusp-Aligned Arc Interval on 11th October 2011
In the next section, we focus in detail on two intervals of coincident high plasma fluxes observed at ARTEMIS and Cluster which occur between 2300 UT on 10 October (day 283) to 0100 UT on 12 October (day 285).
Figure 4 shows the ARTEMIS P2 data centered on 11 October 2011.During this period, the IMF B Z component in panel (a) is northward with a southward turning just after 1300 UT.Between approximately 2300 UT on day 283-1400 UT on day 284, there is a reasonably consistent flux of ions and electrons (density ∼0.1 cm 3 ) which at times contribute an equal amount of pressure to the magnetic pressure in the magnetotail, for example, between 0700-0800 UT and 1100-1200 UT.Between 1200 and 1400 UT there is a significant reduction in the B X component of the magnetic field measured by ARTEMIS.This may be due to the magnetotail being compressed by the solar wind.Between approximately 1400-1900 UT, the AU and AL indices indicate that the magnetosphere progresses into a substorm growth phase suggesting that dayside reconnection is now occurring near the subsolar point as a result of the southward turning in the upstream IMF leading to an increase in the open flux content of the magnetosphere in the lobe regions.After 1400 UT and the southward IMF turning, the conditions observed by ARTEMIS return to a more typical lobe-type structure.The lobes become largely evacuated of plasma, the plasma pressure decreases and the B X component of the magnetic field increases in magnitude as it is no longer being reduced by the presence of plasma.The substorm expansion phase onset occurs shortly before 1600 UT.At approximately 1700 and 1800 UT there are two brief observations of high energy ions observed at ARTEMIS which may be substorm-related plasma passing the spacecraft.
At approximately 1530 UT, the upstream IMF B Z component turns northward.The magnetotail lobe maintains an open field structure with no plasma observed until approximately 2100 UT where the magnetotail fills with high energy ions and electrons again.During this second interval of high plasma fluxes, similar characteristic properties are observed including an increase in the observed electron and ion density, a reduction in the B X component of the magnetic field measured at ARTEMIS due to the presence of plasma and an equal contribution of plasma and magnetic pressure to the magnetotail pressure balance.In this interval the high particle fluxes observed at  ARTEMIS disappear shortly after 0000 UT however the high particle fluxes observed at Cluster seem to persist for longer until 0200 UT.

Discussion
The structure and dynamics of the magnetosphere and the associated auroral signatures during periods of northward IMF are less well understood compared to the southward IMF case.In this study, we have examined a 6-day traversal of the magnetotail by the ARTEMIS spacecraft at approximately 60 R E downtail.During the traversal, the B Z component of the IMF fluctuated but was predominently northward.The ARTEMIS spacecraft traversed the tail above the ecliptic plane and considerably above the tilted neutral sheet in the northern hemisphere in a region that would be expected to be the magnetotail lobe under typical southward IMF conditions consisting of open magnetic flux evacuated of plasma.However, the ARTEMIS spacecraft observed high density plasma of energies between 10 2 -10 4 eV during or following intervals of northward IMF.The plasma in the magnetotail is coincident with simultaneous observations of similar plasma signatures observed in Cluster and Geotail data during the same period investigated by Milan et al. (2023).Using pitch angle data available from the CIS and PEACE instruments onboard Cluster, Milan et al. (2023) showed that the observed ion fluxes had pitch angles of 0°and 180°indicating two counter streaming ion populations whereas the electron population showed a double loss-cone distribution.Both of these pitch angle distributions indicate that the plasma is trapped on closed field lines.Simulations of the magnetotail under northward IMF also show that the magnetotail becomes dominated by closed magnetic flux with a closed field line region spanning Z ∼ ± 20 R E and extending approximately 70 R E downtail (Fryer et al., 2023).We interpret our observations in this study as the same trapped flux observed by Milan et al. (2023) extending at least 60 R E downtail.These observations of the more distant magnetotail support the interpretation of Milan et al. (2023) that the plasma is trapped on closed magnetic field lines during periods where the magnetosphere is almost-entirely closed as a result of dual-lobe reconnection occurring under northward IMF conditions.Milan et al. (2023) suggested that the trapped plasma population in the magnetotail observed by Cluster and Geotail could be the source plasma for cusp-aligned arc (CAA) signatures that were observed simultaneously in the polar region in their study.The CAA emission has been found to have inverted-V precipitation signatures suggesting that the plasma is accelerated down into the polar cap by field-aligned currents associated with flow shears in the convection pattern (Q.-H.Zhang et al., 2020;Milan et al., 2022).
In the ARTEMIS data presented in this study, distinct changes to the structure of the magnetotail were observed during the intervals of northward IMF.ARTEMIS observed a 20-30% decrease in the B X component of the tail magnetic field from approximately 10 nT to 7-8 nT due to the presence of plasma contributing to the outward pressure balance.As the ARTEMIS spacecraft entered and exited the tail, multiple magnetosheath crossings were observed coincident with intervals of IMF Bz fluctuating between northward and southward.These observations suggest that the magnetotail flanks move inwards during the northward IMF intervals to approximately Y ∼± 20 R E and the tail narrowed from ∼60 R E to approximately 40 R E when the magnetosphere was more closed.The estimated widths of the magnetotail are in agreement with a statistical study by Mieth et al. (2019) who found that the width of the magnetopause at lunar distances varied between 20 and 60 R E depending on the solar wind conditions.The results from this study support those of Milan et al. (2004) who suggested that the flux content and structure of the magnetosphere are also important factors in modulating the width of the magnetotail.
Although the magnetosphere is closed or almost closed during periods of northward IMF, the ARTEMIS spacecraft remained within the magnetotail indicating that the length of the magnetotail was not truncated to less than 60 R E in this case.Wang et al. (2023) estimated that the magnetotail could be truncated to less than 30 R E when the magnetosphere is closed, in contrast to our observations; however we note that the study by Wang et al. (2023) modeled an event in which the magnetosphere was closed following a large coronal mass ejection where the upstream IMF B Z component exceeded 15 nT which is larger than during our period of observation.We suggest that the plasma population observed in the magnetotail helps to maintain the pressure balance of the tail pushing outward against the solar wind pressure and maintaining the magnetotail to at least 60 R E .
The intervals during which high density plasma flux was observed in the magnetotail corresponded well with quiet, non-substorm time periods as also reported by Milan et al. (2023).The upstream B Z component of the IMF was consistently less than 10 nT but typically less than 5 nT indicating that the IMF does not need to be strongly northward-oriented for dual-lobe reconnection and the near-closure of the magnetosphere to occur.During the observations of high plasma flux density, the IMF clock angle varied but was typically less than 90°, in agreement with estimations that dual-lobe reconnection may occur with IMF clock angles of up to θ c = ±90° (Twitty et al., 2004).The end of intervals of high plasma fluxes tended to align with substorm growth phases or the accumulation of open flux in the magnetosphere due to a southward turning of the IMF suggesting that the magnetopause reconnection site returns to the subsolar point.Following this, the typical evacuated open lobe structure quickly reformed and the B X component of the magnetic field within the lobe increased to balance the external solar wind pressure on the magnetopause without the presence of the additional plasma pressure.
Previous studies have analyzed observations of uncharacteristically hot plasma in regions typically expected to be the open magnetotail lobes during periods of northward IMF indicating trapped plasma on closed field lines (e.g., Coxon et al., 2021;Fear et al., 2014) which has been linked to polar cap auroral emission such as transpolar arcs (e.g., Fryer et al., 2021) and cusp-aligned arcs (Milan et al., 2023).

Conclusions
In this study we have examined a 6-day traversal of the northern magnetotail by the ARTEMIS spacecraft at approximately 60 R E downtail during a period of prolonged northward IMF.Coincident with the intervals of northward IMF, we observe a high flux of plasma in regions of the magnetotail which usually contain open lobe field lines evacuated of plasma.We interpret these observations as trapped plasma on closed magnetic field lines suggesting that the magnetosphere is almost entirely closed down to ∼60 R E downtail.However, the extent of the magnetotail is not truncated to within the lunar orbit.When the IMF was northward, we also observed a narrowing in the width of the magnetotail as well as a decrease in the magnetic field strength and magnetic pressure in the magnetotail due to the presence of the plasma making a significant contribution to the outward pressure balance with the solar wind.This may also explain why the magnetotail extends to at least 60 R E in this case, rather than being truncated.The results of this study support the case for trapped plasma on closed magnetic field lines during periods of northward IMF as a result of simultaneous high latitude reconnection occurring in both hemispheres (i.e., dual-lobe reconnection) and the near-closure of the magnetosphere proposed by Milan et al. (2020), Milan et al. (2022), and Milan et al. (2023).

Figure 1 .
Figure 1. Figure showing the location of the Cluster (C1), ARTEMIS P1 and P2 satellites between 9th-14th October 2011 in Geocentric Solar Ecliptic (GSE) coordinates.The Cluster spacecraft orbit Earth, shown in magenta while the ARTEMIS satellites are in lunar orbit as they traverse the magnetotail.The ARTEMIS P1 trajectory is shown in purple to yellow and the P2 trajectory is shown in blue to green.The satellite trajectories in the X-Y and Y-Z plane are shown in panel (a) and (b), respectively.The blue dashed diagonal line indicates the mean location of the neutral sheet.

Figure 2 .
Figure 2. ARTEMIS P2 observations during the 6-day period between 9-14 October 2011 (days 282-287).(a) shows the IMF components in GSE coordinates, (b) shows the IMF clock angle, (c) shows the predicted magnetic field as the ARTEMIS P2 spacecraft passes through the magnetotail from the Tsyganenko T96 model, (d) shows the measured magnetic field components from the FGM instrument onboard P2, (e) shows the ion velocity components measured by the spacecraft, (f) shows the ion and electron density, (g) shows the pressure, (h) and (i) show the ion and electron energy flux spectrograms from ARTEMIS P2.The blue line in panel (i) indicates the spacecraft potential.(j) shows the ion flux spectrogram from CIS on Cluster (C1) and (k) shows the AU and AL electrojet indices.The turquoise boxes indicate periods where a positive CAA observation was identified by Milan et al. (2023).

Figure 3 .
Figure 3. ARTEMIS P1 observations during the 6-day period between 9-14 October 2011.(a) shows the IMF components in GSE coordinates, (b) shows the IMF clock angle, (c) shows the predicted magnetic field as the ARTEMIS P1 spacecraft passes through the magnetotail from the Tsyganenko T96 model, (d) shows the measured magnetic field components from the FGM instrument onboard P1, (e) shows the ion velocity components measured by the spacecraft, (f) shows the ion and electron density, (g) shows the pressure, (h) and (i) show the ion and electron energy flux spectrograms from ARTEMIS P1.The blue line in panel (i) indicates the spacecraft potential.(j) shows the ion flux spectrogram from CIS on Cluster (C1) and (k) shows the AU and AL electrojet indices.The turquoise boxes indicate periods where a positive CAA observation was identified by Milan et al. (2023).

Figure 4 .
Figure 4. ARTEMIS P2 observations from 2200 UT on 10 October 2011 (day 283) to 0200 UT on 12 October 2011 (day 285).The data is presented in the same format as the previous figures.