Main Impulsive Phase Research Articles

Episodic Energy Release during the Main and Post-impulsive Phases of a Solar Flare

When and where the magnetic field energy is released and converted in eruptive solar flares remains an outstanding topic in solar physics. To shed light on this question, here we report multiwavelength observations of a C9.4-class eruptive limb flare that occurred on 2017 August 20. The flare, accompanied by a magnetic flux rope eruption and a white light coronal mass ejection, features three post-impulsive X-ray and microwave bursts immediately following its main impulsive phase. For each burst, both microwave and X-ray imaging suggest that the nonthermal electrons are located in the above-the-loop-top region. Interestingly, contrary to many other flares, the peak flux of the three post-impulsive microwave and X-ray bursts shows an increase for later bursts. Spectral analysis reveals that the sources have a hardening spectral index, suggesting a more efficient electron acceleration into the later post-impulsive bursts. We observe a positive correlation between the acceleration of the magnetic flux rope and the nonthermal energy release during the post-impulsive bursts in the same event. Intriguingly, different from some other eruptive events, this correlation does not hold for the main impulse phase of this event, which we interpret as energy release due to the tether-cutting reconnection before the primary flux rope acceleration occurs. In addition, using footpoint brightenings at conjugate flare ribbons, a weakening reconnection guide field is inferred, which may also contribute to the hardening of the nonthermal electrons during the post-impulsive phase.

Implications for Additional Plasma Heating Driving the Extreme-ultraviolet Late Phase of a Solar Flare with Microwave Imaging Spectroscopy

Extreme-ultraviolet late phase (ELP) refers to the second extreme-ultraviolet (EUV) radiation enhancement observed in certain solar flares, which usually occurs tens of minutes to several hours after the peak of soft X-ray emission. The coronal loop system that hosts the ELP emission is often different from the main flaring arcade, and the enhanced EUV emission therein may imply an additional heating process. However, the origin of the ELP remains rather unclear. Here we present the analysis of a C1.4 flare that features such an ELP, which is also observed in microwave wavelengths by the Expanded Owens Valley Solar Array. Similar to the case of the ELP, we find a gradual microwave enhancement that occurs about 3 minutes after the main impulsive phase microwave peaks. Radio sources coincide with both foot points of the ELP loops and spectral fits on the time-varying microwave spectra demonstrate a clear deviation of the electron distribution from the Maxwellian case, which could result from injected nonthermal electrons or nonuniform heating to the footpoint plasma. We further point out that the delayed microwave enhancement suggests the presence of an additional heating process, which could be responsible for the evaporation of heated plasma that fills the ELP loops, producing the prolonged ELP emission.

Response of the magnetosphere–ionosphere system to sudden changes in solar wind dynamic pressure

The geomagnetic sudden commencement (SC) is a plasma and magnetic field disturbance in the magnetosphere–ionosphere region, which appears as a response of this system to a sudden change in the solar wind dynamic pressures associated with shock and tangential discontinuity of solar wind. As this mechanism seems to be simple, the SC has been widely investigated based on observations in the region from the surface of the Earth to the magnetosphere. A schematic model of the SC was presented by Araki (Solar wind sources of magnetospheric ultra-low-frequency waves. American Geophysical Union, Washington, DC, pp 183–200, 1994), who compiled many observational results and theories related to this phenomenon. Recent advances in supercomputing allow us to present new numerical results including three-dimensional global current systems of the SC which cannot be obtained only by direct observations and theoretical analysis. The simulation study not only confirms the results of a previous model on the initial response of the magnetosphere–ionosphere system to solar wind changes (the preliminary impulse) but also presents new findings on dynamical processes in the magnetosphere–ionosphere system in the period following the initial response (the main impulse). Furthermore, the simulation study reveals the SC sequence in the context of break and recovery of the steady magnetosphere–ionosphere convection system. The transient flow vortex in the flank magnetosphere in the main impulse phase plays an essential role in recovering the convection system. This report describes the SC process from the viewpoint of the state transition of the magnetosphere–ionosphere compound system Tanaka (Space Sci Rev 133:1, https://doi.org/10.1007/s11214-007-9168-4, 2007).

Response of the Geospace System to the Solar Wind Dynamic Pressure Decrease on 11 June 2017: Numerical Models and Observations

AbstractOn 11 June 2017, a sudden solar wind dynamic pressure decrease occurred at 1437 UT according to the OMNI solar wind data. The solar wind velocity did not change significantly, while the density dropped from 42 to 10 cm−3 in a minute. The interplanetary magnetic field BZ was weakly northward during the event, while the BY changed from positive to negative. Using the University of Michigan Block Adaptive Tree Solarwind Roe Upwind Scheme global magnetohydrodynamic code, the global responses to the decrease in the solar wind dynamic pressure were studied. The simulation revealed that the magnetospheric expansion consisted of two phases similar to the responses during magnetospheric compression, namely, a negative preliminary impulse and a negative main impulse phase. The simulated plasma flow and magnetic fields reasonably reproduced the Time History of Events and Macroscale Interactions during Substorms and Magnetospheric Multiscale spacecraft in situ observations. Two separate pairs of dawn‐dusk vortices formed during the expansion of the magnetosphere, leading to two separate pairs of field‐aligned current cells. The effects of the flow and auroral precipitation on the ionosphere‐thermosphere (I‐T) system were investigated using the Global Ionosphere Thermosphere Model driven by simulated ionospheric electrodynamics. The perturbations in the convection electric fields caused enhancements in the ion and electron temperatures. This study shows that, like the well‐studied sudden solar wind pressure increases, sudden pressure decreases can have large impacts in the coupled I‐T system. In addition, the responses of the I‐T system depend on the initial convection flows and field‐aligned current profiles before the solar wind pressure perturbations.

The chain response of the magnetospheric and ground magnetic field to interplanetary shocks

AbstractIn response to interplanetary (IP) shocks, magnetic field may decrease/increase (negative/positive response) in nightside magnetosphere, while at high latitudes on the ground it has two‐phase bipolar variations: preliminary impulse and main impulse (MI). Using global MHD simulations, we investigate the linkage between the MI phase variation on the ground and the magnetospheric negative response to an IP shock. It is revealed that although the two phenomena occur at largely separated locations, they are physically related and form a response chain. The velocity disturbances near the flanks of the magnetopause cause the magnetic field to decrease, resulting in a dynamo which thus powers the transient field‐aligned currents (FACs). These FACs further generate a pair of ionospheric current vortex, leading to MI variations on the ground. Therefore, we report here the intrinsic physically related chain response of the magnetospheric and ground magnetic field to IP shocks, and thus link the magnetospheric sudden impulse (SI) and ground SI together.

Analysis of the coronal source of the partially limb-occulted flare on 2002 July 20

We carry out a detailed analysis of the X3.5 solar flare that occurred on 2002 July 20, which is the strongest partially limb-occulted flare ever observed by the RHESSI satellite. The main results are: (1) during the main impulsive phase that lasts ∼ 10 minutes, the motion of the thermal sources follows a U-shaped trajectory. Non-thermal sources move in a similar way, but in a series of larger zigzags. We further show that the non-thermal sources are actually leading the contraction motion. (2) During the main impulsive phase, X-ray sources at different energies continuously form a loop-like configuration, with the highest energy source (up to ∼ 100 keV) and the lowest energy source (down to ∼ 10 keV) being located at two ends. The entire loop-like configuration moves in a U-shaped trajectory, while keeping the order of descending energy from highest to lowest during motion. Two non-thermal hard X-ray sources with different energies are spatially well separated in the distribution. The unusual complexities of the X-ray emissions in the tenuous solar corona challenge interpretations using bremsstrahlung in a simple magnetic configuration.

Non-thermal “Burst-on-Tail” of Long-Duration Solar Event on 26 October 2003

Observations of a rare long-duration solar event of GOES class X1.2 from 26 October 2003 are presented. This event showed a pronounced burst of hard X-ray and microwave emission, which was extremely delayed (>60 min) with respect to the main impulsive phase and did not have any significant response visible in soft X-ray emission. We refer to this phenomenon as a "burst-on-tail". Based on TRACE observations of the growing flare arcade and some simplified estimation, we explain why a reaction of active region plasma to accelerated electrons may change drastically over time. We suggest that, during the "burst-on-tail", non-thermal electrons were injected into magnetic loops of larger spatial scale than during the impulsive phase bursts, thus resulting in much smaller values of plasma temperature and emission measure in their coronal volume, and hence little soft X-ray flux. The nature of the long gap between the main impulsive phase and the "burst-on-tail" is, however, still an open question.

A blowout surge from the eruption of a miniature filament confined by large coronal loops

By means of Hα, EUV, soft X-ray, hard X-ray, and photospheric magnetic field observations, we report the surge-like eruption of a small-scale filament, called “blowout surge” according to recent observations, occurring on a plage region around AR 10876 on 1 May 2006. Along magnetic polarity reversal boundaries with obvious magnetic cancelations, the filament was located underneath a compact coronal arcade and close to one end of large coronal loops around the AR’s periphery. The filament started to erupt about 8min before the main impulsive phase of a small two-ribbon flare, which had two Hα blue-wing kernels connected by hard X-ray loop-top sources on the both sides of the filament. After the flare end, the filament further underwent a distant eruption following a path nearly along the preexisting large loops, and thus looked like an Hα surge and an EUV jet. During the eruption, a small coronal dimming was formed near the flare, while weak brightenings appeared around the remote end of the large loops. We interpret these joint observations as the filament eruption being confined and guided by the large loops. The filament eruption, initially embedded in one footpoint region of the large loops, can break away from the magnetic restraint of the overlying compact arcade, but might be still limited inside the large loops. As a result, the eruption took a surge form that can only expand laterally along the large loops rather than erupt radially.

Magnetic latitude and local time dependence of the amplitude of geomagnetic sudden commencements

Statistical analysis of the main impulse (MI) amplitude of geomagnetic sudden commencements (SCs) in a region from the middle latitudes to equator has been made using the long‐term geomagnetic field data obtained from the Yap (geomagnetic latitude, θ = 0.38°), Guam (θ = 5.22°), Okinawa (θ = 16.54°), Kakioka (θ = 27.18°), Memanbetsu (θ = 35.16°), and St. Paratunka (θ = 45.58°) stations. The magnetic local time (MLT) dependence of SC amplitude in the middle latitudes showed magnetic field variations produced by two‐cell ionospheric currents (DP 2‐type currents) which are driven by the dawn‐to‐dusk electric field accompanying a pair of field‐aligned currents (FACs). The effect of the DP 2‐type currents at least expands to the low latitude (θ = 16.54°). In this region, the DL part of SC produced by the enhanced Chapman‐Ferraro currents can be dominant, but the DP part of SC contaminated 7% of the DL one. On the other hand, at the daytime equator between 8:00 and 16:00 (MLT), the SC amplitude is considerably enhanced with its peak amplitude of 3.24 (normalized SYM‐H value) around 11:00 (MLT) due to the Cowling effect. Another interesting point is that the SC amplitude in the nighttime sector was enhanced at all the stations again, and its peak value increases with increasing magnetic latitude. This result suggests that the effect of the FACs associated with the MI phase of SC expands to the equator.

Swelling of a lava plug associated with a Vulcanian eruption at Sakurajima Volcano, Japan, as revealed by infrasound record: case study of the eruption on January 2, 2007

In order to clarify the time relation of the expansion of a gas pocket and failure of its overlying plug of lava during Vulcanian eruptions, infrasound records and video images of the Vulcanian eruption that occurred at Sakurajima volcano on January 2, 2007 were analyzed with respect to their origin times. Weak (≤3 Pa) and slowly increasing air pressure preceded the impulsive compression phase by 0.25–0.32 s, and a longer-period rarefaction phase of infrasound waves was recognized at all microphone stations. The velocity of the compression phase was assumed to be supersonic (ca. 400 m/s) up to 850 m above the crater bottom from other recent explosions. On the other hand, the propagation velocity of the preceding weak signal was regarded to be similar to the air sound velocity because the lack of impulsiveness is unlikely to be related to the main compression phase. Therefore, the estimated origin time of the main compression phase was delayed by 0.5–0.7 s from the preceding phase. The origin time of the preceding phase coincided with the onset of the isotropic expansion process of the pressurized gas pocket, which was obtained by the waveform inversion of the explosion earthquake. In contrast, the origin time of the main impulsive phase coincided with the time when the expansion rate reached its peak. This observation suggests that the volumetric increase of the gas pocket caused swelling of the surface of the crater bottom and its subsequent failure. When the expansion velocity exceeded a threshold level, the main impulsive compression phase radiated with a high velocity by the sudden releases of the pressurized gases. The volumetric change at the source was estimated to be 280–560 m3 from the preceding phase of the infrasound. This volume change indicates that the vertical displacement of the swelling ground was on the order of 1.0 m, assuming the radius of the lava plug was ca. 10 m.

A numerical simulation of a negative sudden impulse

A numerical experiment of magnetospheric response to a negative pressure impulse in the solar wind is carried out by using a MHD model of the solar wind-magnetosphere-ionosphere coupled system. The numerical simulation confirms mirror-image relationship of the ionospheric and magnetospheric signatures between the negative and positive impulses, which has been suggested by previous observations. The plasma processes associated with the negative impulse are again divided into the three phases—the preliminary impulse phase, and the first and second main impulse phases in terms of the ionosphere-magnetosphere coupling. The SC transient cell convection in the second main impulse phase is related to the Region 2 current in the case of the negative impulse. In the last, we discuss a possible model for the auroral brightening at the onset of the negative impulse.

Double Loop Configuration of a Flaring Region from Microwave, Extreme‐Ultraviolet, and X‐Ray Imaging Data

We use extensive multiwavelength data to study a flare belonging to the interacting-loop class of events identified by Hanaoka. The class of flares is identified morphologically from the presence of two well-separated radio sources in 17 GHz images (in this event, 160'' apart), with only one source showing soft X-ray emission. This event shows many of the other properties apparently shared by this class of flares: a gradual rise showing many subsidiary peaks in both radio and hard X-ray light curves with a quasi-oscillatory nature, the presence of a bright compact X-ray-emitting loop in the main flare source, a delay of the radio emission from the remote source relative to the main X-ray-emitting source, higher circular polarization in the radio emission of the remote source than in the main source, and stronger photospheric magnetic field in the remote source. The new results of our analysis are that we are able to show, using a sequence of magnetograms, that the magnetic field in the main flare site changes sharply at the time of the flare, and further we argue that the remote site is magnetically connected to the main flare site only up to the time of the main impulsive phase, at which point we believe the magnetic connection to the remote site was broken and further flare manifestations are largely confined to the main flare site. This severing of the magnetic connection between two well-separated active regions may be an intrinsic part of the energy release in this flare. The region around the main flare site also exhibits rotation in the magnetogram in the period leading up to the flare. Radio and hard X-ray oscillations with periods of order 5-10 s are observed in the rise phase of this event. If they are due to transverse oscillations of the flare loop at the Alfven speed, then the density in the loop is inferred to be of order 1011 cm-3 and to increase with time as expected.

High-resolution observation and detailed photometry of a great H? two-ribbon flare

A great Hα two-ribbon flare of 12 October, 1981 was observed with the Domeless Solar Telescope at the Hida Observatory and its detailed photometry was made with a two dimensional microdensitometer. The principal results are as follows: (1) The impulsive phase of the flare started with the progressive brightenings of flare points forming the front lines of the Hα two ribbons at both sides of the magnetic neutral line. These are followed by the explosive expansion of Hα two ribbons at the main impulsive phase. (2) Three typical shapes of Hα light curves were found. The type 1 light curve is characterized by the primary impulsive rise and rapid fall of intensity. The light curve of type 3 has no impulsive component but has a very gradual maximum. The type 2 profile attains the main gradual maximum with a few small impulsive peaks. These different types of light curves are made by different heating mechanisms, those are electron precipitation, heat conduction and soft X-ray radiation respectively. (3) The light curve of total intensity, which was made by integrating Hα - 1.0 A intensities of the whole main Hα flare region, shows a primary impulsive peak and a later gradual maximum. The former peak coincides in time with that of the hard X-ray emission. The latter maximum is well correlated with the soft X-ray maximum. (4) The brightest flare points with time profiles of type 1 are closely related to the impulsive hard X-ray emissions of highest energy.