S. Zou

Substorm triggering by new plasma intrusion: THEMIS all‐sky imager observations

[1] A critical, long‐standing problem in substorm research is identification of the sequence of events leading to substorm auroral onset. Based on event and statistical analysis of THEMIS all‐sky imager data, we show that there is a distinct and repeatable sequence of events leading to onset, the sequence having similarities to and important differences from previous ideas. The sequence is initiated by a poleward boundary intensification (PBI) and followed by a north‐south (N‐S) arc moving equatorward toward the onset latitude. Because of the linkage of fast magnetotail flows to PBIs and to N‐S auroras, the results indicate that onset is preceded by enhanced earthward plasma flows associated with enhanced reconnection near the pre‐existing open‐closed field line boundary. The flows carry new plasma from the open field line region to the plasma sheet. The auroral observations indicate that Earthward‐transport of the new plasma leads to a near‐Earth instability and auroral breakup ∼5.5 min after PBI formation. Our observations also indicate the importance of region 2 magnetosphere‐ionosphere electrodynamic coupling, which may play an important role in the motion of pre‐onset auroral forms and determining the local times of onsets. Furthermore, we find motion of the pre‐onset auroral forms around the Harang reversal and along the growth phase arc, reflecting a well‐developed region 2 current system within the duskside convection cell, and also a high probability of diffuse‐appearing aurora occurrence near the onset latitude, indicating high plasma pressure along these inner plasma sheet field lines, which would drive large region 2 currents.

Substorm triggering by new plasma intrusion: Incoherent‐scatter radar observations

Received 4 December 2009; revised 16 March 2010; accepted 30 March 2010; published 27 July 2010. [1] In the companion paper, we identified a repeatable sequence of events leading to substorm onset in THEMIS all‐sky imager observations: enhanced flows bring new plasma into the plasma sheet. The new plasma then moves earthward as a flow channel, bringing it to the near‐Earth plasma sheet and where it produces onset instability. New plasma entering the dusk (dawn) convection cell drifts equatorward and eastward and then around the Harang reversal, leading to pre‐midnight (near‐ and post‐midnight) onset. Here we present evidence supporting this sequence using incoherent scatter radar (ISR) ionospheric observations. Using the Sondrestrom ISR, we find that enhanced flows of new plasma commonly enter the plasma sheet from the polar cap ∼8 min prior to onset. These flows are related to poleward boundary intensification signatures, consistent with the inferences from the imagers. Using the Poker Flat ISR (PFISR), we find that shortly before onset, enhanced westward flows reach the subauroral polarization streams (SAPS) region equatorward of the Harang reversal (dusk‐cell onsets) or enhanced eastward flows enter the onset region from the poleward direction (dawn‐cell onset). PFISR proton precipitation signatures are consistent with the possibility that the enhanced flows consist of reduced‐entropy plasma sheet plasma, and that onset occurs poleward of much of the enhanced SAPS flow (dusk‐cell onsets) or equatorward of the enhanced eastward flows (dawn‐cell onsets). Consistency with reduced entropy plasma is seen only within the enhanced flows, leading us to suggest that intrusion of low‐entropy plasma may alter the radial gradient of entropy toward onset instability.

On the generation/decay of the storm‐enhanced density plumes: Role of the convection flow and field‐aligned ion flow

Storm-enhanced density (SED) plumes are prominent ionospheric electron density increases at the dayside middle and high latitudes. The generation and decay mechanisms of the plumes are still not clear. We present observations of SED plumes during six storms between 2010 and 2013 and comprehensively analyze the associated ionospheric parameters within the plumes, including vertical ion flow, field-aligned ion flow and flux, plasma temperature, and field-aligned currents, obtained from multiple instruments, including GPS total electron content (TEC), Poker Flat Incoherent Scatter Radar (PFISR), Super Dual Auroral Radar Network, and Active Magnetosphere and Planetary Electrodynamics Response Experiment. The TEC increase within the SED plumes at the PFISR site can be 1.4–5.5 times their quiet time value. The plumes are usually associated with northwestward E × B flows ranging from a couple of hundred m s−1 to > 1 km s−1. Upward vertical flows due to the projection of these E × B drifts are mainly responsible for lifting the plasma in sunlit regions to higher altitude and thus leading to plume density enhancement. The upward vertical flows near the poleward part of the plumes are more persistent, while those near the equatorward part are more patchy. In addition, the plumes can be collocated with either upward or downward field-aligned currents (FACs) but are usually observed equatorward of the peak of the Region 1 upward FAC, suggesting that the northwestward flows collocated with plumes can be either subauroral or auroral flows. Furthermore, during the decay phase of the plume, large downward ion flows, as large as ~200 m s−1, and downward fluxes, as large as 1014 m−2 s−1, are often observed within the plumes. In our study of six storms, enhanced ambipolar diffusion due to an elevated pressure gradient is able to explain two of the four large downward flow/flux cases, but this mechanism is not sufficient for the other two cases where the flows are of larger magnitude. For the latter two cases, enhanced poleward thermospheric wind is suggested to be another mechanism for pushing the plasma downward along the field line. These downward flows should be an important mechanism for the decay of the SED plumes.

Modeling subauroral polarization streams during the 17 March 2013 storm

The subauroral polarization streams (SAPS) are one of the most important features in representing magnetosphere-ionosphere coupling processes. In this study, we use a state-of-the-art modeling framework that couples an inner magnetospheric ring current model RAM-SCB with a global MHD model Block-Adaptive Tree Solar-wind Roe Upwind Scheme (BATS-R-US) and an ionospheric potential solver to study the SAPS that occurred during the 17 March 2013 storm event as well as to assess the modeling capability. Both ionospheric and magnetospheric signatures associated with SAPS are analyzed to understand the spatial and temporal evolution of the electrodynamics in the midlatitude regions. Results show that the model captures the SAPS at subauroral latitudes, where Region 2 field-aligned currents (FACs) flow down to the ionosphere and the conductance is lower than in the higher-latitude auroral zone. Comparisons to observations such as FACs observed by Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE), cross-track ion drift from Defense Meteorological Satellite Program (DMSP), and in situ electric field observations from the Van Allen Probes indicate that the model generally reproduces the global dynamics of the Region 2 FACs, the position of SAPS along the DMSP, and the location of the SAPS electric field around L of 3.0 in the inner magnetosphere near the equator. The model also demonstrates double westward flow channels in the dusk sector (the higher-latitude auroral convection and the subauroral SAPS) and captures the mechanism of the SAPS. However, the comparison with ion drifts along DMSP trajectories shows an underestimate of the magnitude of the SAPS and the sensitivity to the specific location and time. The comparison of the SAPS electric field with that measured from the Van Allen Probes shows that the simulated SAPS electric field penetrates deeper than in reality, implying that the shielding from the Region 2 FACs in the model is not well represented. Possible solutions in future studies to improve the modeling capability include implementing a self-consistent ionospheric conductivity module from inner magnetosphere particle precipitation, coupling with the thermosphere-ionosphere chemical processes, and connecting the ionosphere with the inner magnetosphere by the stronger Region 2 FACs calculated in the inner magnetosphere model.

Reply to comment by Harald U. Frey on “Substorm triggering by new plasma intrusion: THEMIS all‐sky imager observations”

[1] The sequence of events leading to substorm onset has been a long outstanding issue in magnetosphere‐ionosphere coupling physics. Nishimura et al. [2010a, hereafter N2010] proposed a potential resolution to this problem based on observations from the all‐sky imager (ASI) array [Mende et al., 2008] of the THEMIS project [Angelopoulos, 2008], which provides high spatial and temporal resolution auroral images with broad latitudinal and longitudinal coverage. Using the capability of the imager array, we found a repetitive precursor auroral sequence prior to substorm auroral onset. The preonset auroral sequence reported by N2010 is initiated by a poleward boundary intensification (PBI), which is followed by an approximately north‐south (N‐S) oriented arc (also referred to as an auroral streamer) moving equatorward toward the onset latitude and sometimes turning into an east‐ west (E‐W) oriented luminosity enhancement propagating azimuthally. It finally leads to onset instability in the near‐ Earth plasma sheet and is observed as auroral onset. Because of the linkage of fast magnetotail flows to PBIs and to N‐S auroras, this sequence gives strong support to the idea that onset instability develops following enhanced plasma flows from the open‐closed boundary toward the near‐Earth plasma sheet [Lyons et al., 2010a, 2010b]. [2] Frey [2010, hereafter F2010] has commented on our study of the auroral sequence leading to substorm onset. The main issues raised by F2010 are as follows. [3] 1. Time differences of auroral intensifications less than 30 min are too short to be called two separated onsets. Thus some of the events considered by N2010 are not onsets but are just intensifications of earlier substorms, and inclusion of such intensifications might affect our statistical results. [4] 2. A large number of substorms were missed in the N2010 analysis, based on a comparison to the event list by F2010. [5] In this reply, we first show that the events separated by short time intervals are indeed auroral onsets and that the preonset sequence found by N2010 is commonly seen in both first and subsequent onsets occurring within ∼30 min. Then we show that over half of the events in the F2010 list not included in the N2010 study are not substorms but other types of auroral phenomena, and that the majority of the remainder had onsets that were not within the field of views (FOVs) of available imagers. We further demonstrate that the N2010 event list covers most of the F2010 substorm onsets. Finally, we show statistical results using only isolated events and provide evidence that the preonset sequence found by N2010 is common for isolated substorms, and has essentially the same high occurrence probability as for all events.

Categorization of the Time Sequence of Events Leading to Substorm Onset Based on THEMIS All-Sky Imager Observations

The sequence of events leading to substorm auroral onset has been a long-standing issue in substorm research. Based on statistical studies using THEMIS all-sky imager data, we have recently reported evidence that most substorm onset events are preceded by a pre-onset auroral form which is a distinct north-south arc originating from an poleward boundary intensification (PBI) and reaches the auroral onset region just before onset. This onset sequence was found to be a repetitive process; it is detected in 84% of 249 events between November 2007 and April 2008. A high occurrence of PBIs (84%) emphasizes an abrupt flux transport across the open-closed field line as initiation of the onset sequence. Here we present a variation of the onset sequence we have previously reported and two less frequently observed types of onset time sequence: poleward boundary contact and Harang aurora deformation. While poleward boundary contact events also start with PBIs, the auroral oval width becomes much narrower (∼2° MLAT) prior to onset, indicating that the plasma sheet is thin and the nightside magnetic separatrix is located closer to the near-Earth onset region. Harang auroral deformation events are not associated with an observed PBI, but the equatorward portion of a pre-existing Harang aurora bends equatorward, which indicates a rapid convection change leading to onset. All of those three categories of events suggest that new plasma intrusion toward onset location changes the pressure profile in the near-Earth region and leads to onset instability.

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

The main phase of the 2013 March 17 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 IMF immediately drove dramatic poleward expansion of the poleward boundary of the auroral oval (implying strong nightside reconnection), strong auroral activity, and strong penetrating mid-latitude 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 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 stormtime features during the bursty convection mode period.

PFISR observation of intense ion upflow fluxes associated with an SED during the 1 June 2013 geomagnetic storm

The Earths ionosphere plays an important role in supplying plasma into the magnetosphere through ion upflow/outflow, particularly during periods of strong solar wind driving. An intense ion upflow flux event during the 1 June 2013 storm has been studied using observations from multiple instruments. When the open-closed field line boundary (OCB) moved into the Poker Flat incoherent scatter radar (PFISR) field of view, divergent ion fluxes were observed by PFISR with intense upflow fluxes reaching ~1.9 × 1014 m−2 s−1 at ~600 km altitude. Both ion and electron temperatures increased significantly within the ion upflow, and thus, this event has been classified as a type 2 upflow. We discuss factors contributing to the high electron density and intense ion upflow fluxes, including plasma temperature effect and preconditioning by storm-enhanced density (SED). Our analysis shows that the significantly enhanced electron temperature due to soft electron precipitation in the cusp can reduce the dissociative recombination rate of molecular ions above ~400 km and contributed to the density increase. In addition, this intense ion upflow flux event is preconditioned by the lifted F region ionosphere due to northwestward convection flows in the SED plume. During this event, the OCB and cusp were detected by DMSP between 15 and 16 magnetic local times, unusually duskward. Results from a global magnetohydrodynamics simulation using the Space Weather Modeling Framework have been used to provide a global context for this event. This case study provides a more comprehensive mechanism for the generation of intense ion upflow fluxes observed in association with SEDs.

Midlatitude Plasma Bubbles Over China and Adjacent Areas During a Magnetic Storm on 8 September 2017

This paper presents observations of postsunset super plasma bubbles over China and adjacent areas during the second main phase of a storm on 8 September 2017. The signatures of the plasma bubbles can be seen or deduced from (1) deep field-aligned total electron content depletions embedded in regional ionospheric maps derived from dense Global Navigation Satellite System networks, (2) significant equatorial and midlatitudinal plasma bite-outs in electron density measurements on board Swarm satellites, and (3) enhancements of ionosonde virtual height and scintillation in local evening associated with strong southward interplanetary magnetic field. The bubbles/depletions covered a broad area mainly within 20∘–45∘N and 80∘–110∘E with bifurcated structures and persisted for nearly 5 hr (∼13–18 UT). One prominent feature is that the bubbles extended remarkably along the magnetic field lines in the form of depleted flux tubes, reaching up to midlatitude of around 50∘N (magnetic latitude: 45.5∘N) that maps to an altitude of 6,600 km over the magnetic equator. The maximum upward drift speed of the bubbles over the magnetic equator was about 700 m/s and gradually decreased with altitude and time. The possible triggering mechanism of the plasma bubbles was estimated to be storm time eastward prompt penetration electric field, while the traveling ionospheric disturbance could play a role in facilitating the latitudinal extension of the depletions.

Effects of sudden commencement on the ionosphere: PFISR observations and global MHD simulation

Sudden commencement (SC) induced by solar wind pressure enhancement can produce significant global impact on the coupled magnetosphere-ionosphere (MI) system, and its effects have been studied extensively using ground magnetometers and coherent scatter radars. However, very limited observations have been reported about the effects of SC on the ionospheric plasma. Here we report detailed Poker Flat Incoherent Scatter Radar (PFISR) observations of the ionospheric response to SC during the 17 March 2015 storm. PFISR observed lifting of the F region ionosphere, transient field-aligned ion upflow, prompt but short-lived ion temperature increase, subsequent F region density decrease, and persistent electron temperature increase. A global magnetohydrodynamic (MHD) simulation has been carried out to characterize the SC-induced current, convection, and magnetic perturbations. Simulated magnetic perturbations at Poker Flat show a satisfactory agreement with observations. The simulation provides a global context for linking localized PFISR observations to large-scale dynamic processes in the MI system.