Initial Brightening Research Articles

Ejecta Evolution Following a Planned Impact into an Asteroid: The First Five Weeks

Abstract The impact of the Double Asteroid Redirection Test spacecraft into Dimorphos, moon of the asteroid Didymos, changed Dimorphos’s orbit substantially, largely from the ejection of material. We present results from 12 Earth-based facilities involved in a world-wide campaign to monitor the brightness and morphology of the ejecta in the first 35 days after impact. After an initial brightening of ∼1.4 mag, we find consistent dimming rates of 0.11–0.12 mag day−1 in the first week, and 0.08–0.09 mag day−1 over the entire study period. The system returned to its pre-impact brightness 24.3–25.3 days after impact though the primary ejecta tail remained. The dimming paused briefly eight days after impact, near in time to the appearance of the second tail. This was likely due to a secondary release of material after re-impact of a boulder released in the initial impact, though movement of the primary ejecta through the aperture likely played a role.

Evolution of coronal magnetic field parameters during X5.4 solar flare

The coronal magnetic field over NOAA Active Region 11,429 during a X5.4 solar flare on 7 March 2012 is modeled using optimization based Non-Linear Force-Free Field extrapolation. Specifically, 3D magnetic fields were modeled for 11 timesteps using the 12-min cadence Solar Dynamics Observatory (SDO) Helioseismic and Magnetic Imager photospheric vector magnetic field data, spanning a time period of 1 hour before through 1 hour after the start of the flare. Using the modeled coronal magnetic field data, seven different magnetic field parameters were calculated for 3 separate regions: areas with surface |Bz|≥ 300 G, areas of flare brightening seen in SDO Atmospheric Imaging Assembly imagery, and areas with surface |B| ≥ 1000 G and high twist. Time series of the magnetic field parameters were analyzed to investigate the evolution of the coronal field during the solar flare event and discern pre-eruptive signatures. The data shows that areas with |B| ≥ 1000 G and |Tw|≥ 1.5 align well with areas of initial flare brightening during the pre-flare phase and at the beginning of the eruptive phase of the flare, suggesting that measurements of the photospheric magnetic field strength and twist can be used to predict the flare location within an active region if triggered. Additionally, the evolution of seven investigated magnetic field parameters indicated a destabilizing magnetic field structure that could likely erupt.

Orphan optical flare as SOSS emission afterglow, localization in time

ABSTRACT We report on MASTER optical observations of an afterglow-like optical and X-ray transient AT2021lfa/ZTF21aayokph. We detected the initial steady brightening of the transient at 7σ confidence level. This allowed us to use smooth optical self-similar emission of GRBs model to constrain the explosion time to better than 14 min as well as to estimate its initial Lorentz factor Γ0 = 20 ± 10. Taking into consideration the low Γ0 and non-detection in gamma-rays, we classify this transient as the first failed GRB afterglow.

Unified Theory of the Arc Auroras: Formation Mechanism of the Arc Auroras Conforming General Principles of Convection and FAC Generation

AbstractThe arc aurora can be considered as the visualization of the field‐aligned current (FAC). In various cases, the numerically reproduced FAC morphologically matches well with that of the observed arc aurora. Such example can be seen in the sun‐aligned arc, the fan arc, and the theta aurora under the northward interplanetary magnetic field (IMF), and the substorm, the quiet arc, and the initial brightening under the southward IMF. The FAC reproduced by the global simulation is based on the general principle of the magnetohydrodynamic (MHD). In such general principle, the FAC is transmission of net charge, net charge is divE, and E is equivalent to motion. After all, the FAC is transmission of convection motion. In order to transmit the FAC to the dissipative ionosphere based on the MHD, formations of the dynamo and shear that leads from the magnetosphere to the ionosphere are essential. The dynamo and shear are reproduced so as to properly match with each convection structure. The comparison under various magnetospheric topologies between the global simulation and the arc aurora observation indicates that the arc aurora and the FAC occurs inevitably controlled by convection structure. Such a principle of the FAC and convection holds even for the onset FAC.

Optical and near-infrared spectroscopy of Nova V2891 Cygni: evidence for shock-induced dust formation

ABSTRACT We present multi-epoch optical and near-infrared observations of the highly reddened, Fe ii class slow Nova V2891 Cygni. The observations span 15 months since its discovery. The initial rapid brightening from quiescence, and the presence of an ∼35-d long pre-maximum halt, is well documented. The evidence that the current outburst of V2891 Cyg has undergone several distinct episodes of mass ejection is seen through time-varying P Cygni profiles of the O i 7773 Å line. A highlight is the occurrence of a dust formation event centred around approximately +273 d, which coincides with a phase of coronal line emission. The dust mass is found to be $\sim\!{0.83{-}1.25} \times 10^{-10}\ \mathrm{M}_{\odot }$. There is strong evidence to suggest that the coronal lines are created by shock heating rather than by photoionization. The simultaneous occurrence of the dust and coronal lines (with varying velocity shifts) supports the possibility that dust formation is shock induced. Such a route for dust formation has not previously been seen in a nova, although the mechanism has been proposed for dust formation in some core-collapse supernovae. Analysis of the coronal lines indicates a gas mass and temperature of 8.35–8.42 × 10−7 M⊙ and ∼(4.8–9.1) × 105 K, respectively, and an overabundance of aluminium and silicon. A Case B analysis of the hydrogen lines yields a mass of the ionized gas of (8.60 ± 1.73) × 10−5 M⊙. The reddening and distance to the nova are estimated to be E(B − V) = 2.21 ± 0.15 and d = 5.50 kpc, respectively.

Roles of the M‐I Coupling and Plasma Sheet Dissipation on the Growth‐Phase Thinning and Subsequent Transition to the Onset

AbstractThe plasma‐sheet dynamics leading to the substorm onset is analyzed from a global simulation. We calculated three cases, the normal (case 1), reduced magnetosphere‐ionosphere (M‐I) coupling (case 2) and reduced tail dissipation (case 3). The normal solution reproduces the thinning, reconnection, tail flow, and the dipolarization in the magnetosphere, and the AU/AL, the initial brightening, the beads, the pre‐onset arc, and the poleward expansion in the ionosphere. Case 2 shows that the M‐I coupling promotes the thinning by evacuating magnetic field from the plasma sheet. Case 3 shows that it is implausible to reproduce the AU/AL of the expansion phase only with numerical dissipation. The projection area of the growth phase plasma sheet is so narrow that the quiet arcs on the high‐latitude side are projected to the lobe. The key point to understand such narrow projection is to consider the origin of the field‐aligned current (FAC). The FAC occurs to transmit motion, and particle precipitation is the consequence. The upward FACs for the quiet arc and initial brightening, which produce the substorm sequence, are generated to transmit enhanced convection from the magnetosphere to the ionosphere. The substorm onset is the transmission of the convection transient. The onset FAC is generated by the near‐earth dynamo. Before the onset, a two‐stage reconnection occurs at the medium‐tail (MTNL) and the near‐earth (NENL) neutral lines through retreating nulls. The accompanying flow explains the model that assumes two causal areas for the onset in the near‐earth and middle tails.

An Eruptive Circular-ribbon Flare with Extended Remote Brightenings

Abstract We study an eruptive X1.1 circular-ribbon flare on 2013 November 10, combining multiwavelength observations with a coronal field reconstruction using a non-force-free field method. In the first stage, a filament forms via magnetic reconnection between two mildly twisted sheared arcades, which are embedded under the fan dome associated with a null point (NP). This reconnection seems to be driven by photospheric shearing and converging flows around the inner two arcade footpoints, consistent with the flare-related changes of the transverse field. The southern portion of the filament rises upward due to torus instability and pushes against the NP. The induced NP reconnection then generates the circular ribbon and the initial remote brightening in the west, as accelerated electrons precipitate along the fan and propagate outward along quasi-separatix surfaces with high values of the squashing factor (Q) in the envelope fields, which have a curtain-like shape here. In the second stage, the southern end of the flux rope breaks away from the surface, sequentially disrupts the dome and overlying fields, and erupts in a whipping-like fashion to become a partial halo coronal mass ejection. This leads to an enhanced flare emission and fast-moving remote brightenings at the footpoints of the magnetic curtain, which span a remarkably broad region and are also associated with coronal dimmings. This is a rare example of eruptive circular-ribbon flares, in which the evolution of a flux rope from its formation to successful eruption out of the dome and the resulting unusually extended remote brightenings are completely observed.

Revisiting substorm events with preonset aurora

Abstract. Nishimura et al. (2010) proposed a new plasma intrusion or preonset aurora scenario of substorm triggering. In this scenario, a substorm is triggered by a fast earthward flow generated at the distant neutral line which corresponds to a preonset auroral streamer or arc in the ionosphere propagating from the auroral poleward boundary to the initial auroral brightening site, i.e., “preonset aurora”. In the present paper, we revisited three substorm events reported as being triggered by such a mechanism related to preonset auroras, based on THEMIS ground-based all-sky imager data. Unlike previous studies, we examined the arrival timing of the preonset aurora relative to the three steps of auroral onset arc development (initial brightening, enhancement of the wave-like structure, and poleward expansion) to make the role of the preonset aurora in the auroral steps clearer. Our detailed timing analysis found that preonset auroral streamers reached the auroral onset arc but away from the initial brightening site after initial brightening for two events, while no preonset aurora reaching the initial brightening site could be identified for the other event. This result suggests that the processes associated with auroral streamers are unlikely to affect at least initial brightening, even if we consider not only the presence and arrival timing and location of the auroral streamers but also the scale of the corresponding flow and flow vortices. We list a series of open questions for testing the preonset aurora scenario further in future studies. Keywords. Magnetospheric physics (storms and substorms; auroral phenomena; magnetotail)

A Case Study of Near‐Earth Magnetotail Conditions at Substorm and Pseudosubstorm Onsets

AbstractWhile a substorm involves auroral poleward expansion after initial brightening, a pseudosubstorm (pseudobreakup) subsides without progressing to poleward expansion. To understand what makes this difference, we studied near‐Earth magnetotail conditions at a pseudosubstorm onset and the subsequent substorm onset, using multipoint Time History of Events and Macroscale Interactions during Substorms spacecraft data. In the present event, near‐Earth magnetic reconnection possibly occurred before initial brightening for both pseudosubstorm and substorm. In the near‐Earth magnetotail at X ∼− 10 RE, the ion β, ion pressure, and ion and total (ion plus magnetic) pressure gradient projection along two closely located spacecraft were smaller and magnetic field lines were less stretched around the pseudosubstorm initial brightening than around the substorm initial brightening. Dipolarization did not occur for the pseudosubstorm, whereas it began just before poleward expansion for the substorm. These observations suggest that conditions of the near‐Earth magnetotail possibly affect whether the initial action develops into a full‐fledged substorm.

Simultaneous observation of auroral substorm onset in Polar satellite global images and ground-based all-sky images

Substorm onset has originally been defined as a longitudinally extended sudden auroral brightening (Akasofu initial brightening: AIB) followed a few minutes later by an auroral poleward expansion in ground-based all-sky images (ASIs). In contrast, such clearly marked two-stage development has not been evident in satellite-based global images (GIs). Instead, substorm onsets have been identified as localized sudden brightenings that expand immediately poleward. To resolve these differences, optical substorm onset signatures in GIs and ASIs are compared in this study for a substorm that occurred on December 7, 1999. For this substorm, the Polar satellite ultraviolet global imager was operated with a fixed-filter (170 nm) mode, enabling a higher time resolution (37 s) than usual to resolve the possible two-stage development. These data were compared with 20-s resolution green-line (557.7 nm) ASIs at Muonio in Finland. The ASIs revealed the AIB at 2124:50 UT and the subsequent poleward expansion at 2127:50 UT, whereas the GIs revealed only an onset brightening that started at 2127:49 UT. Thus, the onset in the GIs was delayed relative to the AIB and in fact agreed with the poleward expansion in the ASIs. The fact that the AIB was not evident in the GIs may be attributed to the limited spatial resolution of GIs for thin auroral arc brightenings. The implications of these results for the definition of substorm onset are discussed herein.

Global simulation study for the time sequence of events leading to the substorm onset

AbstractWe have developed a global simulation code which gives numerical solutions having an extremely high resolution. The substorm solution obtained from this simulation code reproduces the precise features of the substorm onset in the ionosphere. It can reproduce the onset that starts from the equatorward side of the quiet arc, two step development of the onset, and the westward traveling surge (WTS) that starts 2 min after the initial brightening. Then, we investigated the counter structures in the magnetosphere that correspond to each event in the ionosphere. The structure in the magnetosphere promoting the onset is the near‐Earth dynamo in the inner magnetospheric region away from the equatorial plane. The near‐Earth dynamo is driven by the field‐aligned pressure increase due to the parallel flow associated with the squeezing, combined with equatorward field‐perpendicular flow induced by the near‐Earth neutral line (NENL). The dipolarization front is launched from the NENL associated with the convection transient from the growth phase to the expansion phase, but neither the launch nor the arrival of the dipolarization front coincides with the onset timing. The arrival of flow to the equatorial plane of the inner magnetosphere occurs 2 min after the onset, when the WTS starts to develop toward the west. The expansion phase is further developed by this flow. Looking at the present result that the onset sequence induced by the near‐Earth dynamo reproduces the details of observation quite well, we cannot avoid to conclude that the current wedge is a misleading concept.

Rotating Magnetic Structures Associated with a Quasi-circular Ribbon Flare

Abstract We present the detection of a small eruption and the associated quasi-circular ribbon flare during the emergence of a bipole occurring on 2015 February 3. Under a fan dome, a sigmoid was rooted in a single magnetic bipole, which was encircled by negative polarity. The nonlinear force-free field extrapolation shows the presence of twisted field lines, which can represent a sigmoid structure. The rotation of the magnetic bipole may cause the twisting of magnetic field lines. An initial brightening appeared at one of the footpoints of the sigmoid, where the positive polarity slides toward a nearby negative polarity field region. The sigmoid displayed an ascending motion and then interacted intensively with the spine-like field. This type of null point reconnection in corona led to a violent blowout jet, and a quasi-circular flare ribbon was also produced. The magnetic emergence and rotational motion are the main contributors to the energy buildup for the flare, while the cancellation and collision might act as a trigger.

Statistical properties of substorm auroral onset beads/rays

AbstractAuroral substorms are often associated with optical ray or bead structures during initial brightening (substorm auroral onset waves). Occurrence probabilities and properties of substorm onset waves have been characterized using 112 substorm events identified in Time History of Events and Macroscale Interactions during Substorms (THEMIS) all‐sky imager data and compared to Rice Convection Model–Equilibrium (RCM‐E) and kinetic instability properties. All substorm onsets were found to be associated with optical waves, and thus, optical waves are a common feature of substorm onset. Eastward propagating wave events are more frequent than westward propagating wave events and tend to occur during lower‐latitude substorms (stronger solar wind driving). The wave propagation directions are organized by orientation of initial brightening arcs. We also identified notable differences in wave propagation speed, wavelength (wave number), period, and duration between westward and eastward propagating waves. In contrast, the wave growth rate does not depend on the propagation direction or substorm strength but is inversely proportional to the wave duration. This suggests that the waves evolve to poleward expansion at a certain intensity threshold and that the wave properties do not directly relate to substorm strengths. However, waves are still important for mediating the transition between the substorm growth phase and poleward expansion. The relation to arc orientation can be explained by magnetotail structures in the RCM‐E, indicating that substorm onset location relative to the pressure peak determines the wave propagation direction. The measured wave properties agree well with kinetic ballooning interchange instability, while cross‐field current instability and electromagnetic ion cyclotron instability give much larger propagation speed and smaller wave period.

Substorm simulation: Quiet and N‐S arcs preceding auroral breakup

AbstractAuroral breakup at the onset of substorm expansion is sometimes preceded by auroral forms known as quiet arcs and N‐S arcs. Observations have shown that both the auroral forms tend to move equatorward, and the initial brightening takes place in or near one of the quiet arcs. The auroral forms attract great attention, but generation of auroral forms and their association with the initial brightening are poorly understood. Recent global magnetohydrodynamic simulations are capable of producing upward field‐aligned currents (FACs) that resemble the auroral forms in both shape and temporal evolution. Based on the simulation results, we propose the following scenarios: (1) When the convection electric field is weak (northward interplanetary magnetic field (IMF)), the high‐pressure region is elongated from the plasma sheet toward higher latitudes and is structured by a coupling between the magnetosphere and the ionosphere (interchange‐like instabilities). (2) When the convection electric field is strong (southward IMF), the structured high‐pressure region moves equatorward (toward the plasma sheet). Upward currents are generated around it, which can be observed as arcs in the ionosphere. The upward current can be tentatively intensified in the course of the equatorward movement before the formation of a near‐Earth neutral line (NENL). (3) The NENL releases magnetic tension and results in the enhancement of plasma pressure at off‐equator in the near‐Earth region. Sudden formation of the off‐equatorial high‐pressure region generates the onset current system that manifests initial brightening. Our scenario can explain the observational fact that poleward arcs remained undisturbed at the onset.

Substorm simulation: Formation of westward traveling surge

AbstractAuroral substorm expansion is characterized by initial brightening of aurora, followed by a bulge expanding in all directions, and a westward traveling surge (WTS). On the basis of the result obtained by a global magnetohydrodynamic simulation, we propose a scenario for the onset and the subsequent formation of WTS. (1) Near‐Earth neutral line releases magnetic tension in the near‐Earth plasma sheet to compress plasma and accelerate it earthward. (2) Earthward, perpendicular flow is converted to parallel flow in the near‐Earth tail region. (3) Plasma moves earthward parallel to a field line. The plasma pressure is additionally enhanced at off‐equator with an expanding slow‐mode variation. (4) Flow vorticities coexist near the off‐equatorial high‐pressure region. Resultant field‐aligned current (FAC) is connected to the ionosphere, which may manifest initial brightening. (5) Due to continued earthward flow, the high‐plasma pressure region continues to expand to the east and west. (6) The ionospheric conductivity continues to increase in the upward FAC region, and the conductivity gradient becomes steeper. (7) The convergence of the Hall current gives rise to divergent electric field near the steep gradient of the conductivity. (8) Due to the divergent electric field, magnetospheric plasma moves counterclockwise at low altitude (in the Northern Hemisphere). (9) The additional flow vorticity generates a localized upward FAC at low altitudes, which may manifest WTS, and redistributes the ionospheric current and conductivity. Thus, WTS may be maintained in a self‐consistent manner, and be a natural consequence of the overflow of the Hall current.

Substorm simulation: Insight into the mechanisms of initial brightening

AbstractInitial brightening of the aurora is an optical manifestation of the beginning of a substorm expansion and is accompanied by large‐amplitude upward field‐aligned currents (FACs). Based on global magnetohydrodynamic simulation, we suggest the possible generation mechanism of the upward FAC that may manifest the initial brightening. (1) A formation of the near‐Earth neutral line (NENL) releases the tension force that accelerates plasma earthward. (2) The earthward (perpendicular) flow is converted to a field‐aligned flow when flow braking takes place. (3) A high‐pressure region propagates earthward along a field line. (4) The off‐equatorial high‐pressure region pulls in and discharges ambient plasma, which generates a flow vorticity around it. (5) Region 1‐sense FAC is generated in the upper part of the off‐equatorial high‐pressure region. (6) The upward FAC is connected with the ionosphere in the center of the Harang discontinuity, causing the initial brightening. Additional dynamo is generated in the near‐Earth region, which transmits electromagnetic energy. Upward FAC that manifests the initial brightening seems to be necessarily originated in the near‐Earth off‐equatorial region where the magnitude of the perpendicular (diamagnetic) current is relatively small in comparison with that of the FAC. Near the equatorial plane, the perpendicular current is comparable to or larger than FAC so that a current line is diverted from a magnetic field line and that the FAC generated near the equatorial plane is not necessarily connected with the ionosphere. The proposed mechanism occurs regardless of the location of the NENL and may explain some of auroral forms.

Vortices at the magnetic equator generated by hybrid Alfvén resonant waves

We performed three-dimensional magnetohydrodynamic simulations of shear Alfvén waves in a full field line system with magnetosphere-ionosphere coupling and plasma non-uniformities. Feedback instability of the Alfvén resonant modes showed various nonlinear features under the field line cavities: (i) a secondary flow shear instability occurs at the magnetic equator, (ii) trapping of the ionospheric Alfvén resonant modes facilitates deformation of field-aligned current structures, and (iii) hybrid Alfvén resonant modes grow to cause vortices and magnetic oscillations around the magnetic equator. Essential features in the initial brightening of auroral arc at substorm onsets could be explained by the dynamics of Alfvén resonant modes, which are the nature of the field line system responding to a background rapid change.

Substorm onset process: Ignition of auroral acceleration and related substorm phases

AbstractThe substorm onset process was studied on the basis of the vertical evolution of auroral acceleration regions derived from auroral kilometric radiation (AKR) spectra and Pi pulsations on the ground. The field‐aligned auroral acceleration at substorm onset demonstrated two distinct phases. Low‐altitude acceleration (h~3000–5000 km), which accompanied auroral initial brightening, prebreakup Pi2, and direct current of ultralow frequency (DC‐ULF) pulsation were first activated and played an important role (precondition) in the subsequent substorm expansion phase onset. Prebreakup Pi2 is suggestive of the ballooning mode wave generation, and negative decrease in DC‐ULF suggests increasing field‐aligned current (FAC). We called this stage the substorm initial phase. A few minutes after this initial phase onset, high‐altitude acceleration, which accompanied auroral breakup and poleward expansion with breakup Pi1 and Pi2 pulsations, suddenly broke out in an altitude range from 8000 to 16,000 km. Thus, substorm expansion onset originated in the magnetosphere‐ionosphere (M‐I) coupling region, i.e., substorm ignition in the M‐I coupling region. Statistical investigations revealed that about 65% of earthward flow bursts observed in the plasma sheet were accompanied by enhanced low‐altitude AKR, suggesting that flow braking of bursts causes FAC and resulting low‐altitude field‐aligned acceleration in the M‐I coupling region. On the basis of these observations, we propose a substorm onset scenario in which FAC that originated from the braking of plasma flow bursts first enhances low‐altitude acceleration (substorm initial phase onset) and then the increasing FAC induces current‐driven instability in the M‐I coupling region, which leads to high‐altitude acceleration and resulting substorm expansion phase onset.

THE PECULIAR BALMER DECREMENT OF SN 2009ip: CONSTRAINTS ON CIRCUMSTELLAR GEOMETRY

We present optical and near-IR spectroscopic observations of the luminous blue variable SN 2009ip during its remarkable photometric evolution of 2012. The spectra sample three key points in the SN 2009ip lightcurve, corresponding to its initial brightening in August (2012-A) and its dramatic rebrightening in early October (2012-B). Based on line fluxes and velocities measured in our spectra, we find a surprisingly low I(H-alpha)/I(H-beta) ~ 1.3-1.4 in the 2012-B spectra. Such a ratio implies either a rare Case B recombination scenario where H-alpha, but not H-beta, is optically thick, or an extremely high density for the circumstellar material of n_e > 10^13 cm^(-3) The H-alpha line intensity yields a minimum radiating surface area of >~20,000 AU^2 in H-alpha at the peak of SN 2009ip's photometric evolution. Combined with the nature of this object's spectral evolution in 2012, a high circumstellar density and large radiating surface area imply the presence of a thin disk geometry around the central star (and, consequently, a possible binary companion), suggesting that the observed 2012-B rebrightening of SN 2009ip can be attributed to the illumination of the disk's inner rim by fast-moving ejecta produced by the underlying events of 2012-A.

Relationship between auroral substorm and ion upflow in the nightside polar ionosphere

We investigated ionospheric ion upflow during an auroral substorm using simultaneous European Incoherent Scatter radar and IMAGE satellite data. Approximately 6 min after an initial brightening identified with data from the IMAGE wideband imaging camera instrument, ion upflow was seen and the electron temperature became enhanced, too. The ion upflow, with a velocity of about 150 m/s, and the electron temperature enhancement lasted for about 25 min. During the poleward expansion phase, surges of large upward ion velocity and flux, and high ion and electron temperatures occurred over Longyearbyen. The upward ion flux reached 2 × 1014 m−2s−1. Naturally enhanced ion‐acoustic lines (NEIALs) were seen near the poleward edge of the expanded auroral oval both near the end of expansion phase 17 min after onset and also later in the recovery phase. The NEIALs seemed to be accompanied by another type of enhanced echoes, obliquely to the local geomagnetic field. Data from the Low Energy Neutral Atom instrument on the IMAGE satellite show that energetic neutral oxygen reaches the IMAGE satellite about 40 min after the initial brightening, and oxygen continues to get detected during the recovery phase. We propose that ion upflow at the poleward edge of the auroral oval during the expansion phase is related to ion/neutral outflow with energy below 18–27 eV, whereas during the recovery phase of a substorm upward ions are accelerated up to about 60 eV and flow out in the entire polar region.