We performed statistical and event studies of linear polarization in the H-alpha line during solar flares. The statistical study revealed that, among 71 H-alpha flares analyzed, including 64 GOES flares, only one event shows significant linear polarization signals. Such an infrequent occurrence of significant linear polarization in solar flares is consistent with the result by Bianda et al. (2005), who studied 30 flares and found no polarization signals. In the event showing the significant polarization, the maximum degree of linear polarization was 1.16±0.06%, and the average direction of the polarization deviated by -142.5±6.0 degrees from the solar north. The observed polarization degrees and the directions are consistent with the preceding reports. These strong linear polarization signals did not appear at major flare ribbons, nor did they correlate with either hard or soft X-ray emissions temporally or spatially. Instead they appeared at a minor flare kernel, which corresponds to one of the footpoints of a coronal loop. The active region caused coronal dimming after the soft X-ray peak. The observed flare show no direct evidence that the linear polarization is produced by high energy particles, which are often considered to generate the polarization. On the other hand, our study suggests the possibility that coronal mass ejections, which have been often observed in flares showing linear polarization signals, play an important role for exciting linear polarization at H-alpha flare kernels. 

White-light flare (WLF) is a phenomenon in which has enhancement of visible continuum in a solar flare. Although the emission mechanism of WLFs has not been well understood, previous studies have suggested that their origin is non-thermal electrons and that a strong magnetic field exists in the acceleration site (Watanabe et al., 2017). We tried to estimate the coronal magnetic field strength by using the turn-over frequency of gyro-synchrotron emission in microwaves (Dulk 1985). We used the Nobeyama Radio Polarimeters (NoRP) during the period from 2011 to 2017. We found 29 events which were simultaneously observed with white-light (SDO/HMI continuum) and microwave (NoRP) which had loop-top microwave source in the image observed with Nobeyama Radioheliograph. However, we couldn't find any difference in turn-over frequencies with or without white-light emission. In order to investigate the relationship between white-light emission and coronal magnetic field, this method might be too simple because the turn-over frequency is determined by not only the magnetic field strength but also the electron density (Dulk 1985).To overcome this problem, next, we tried to compare field strength and temperature of white-light emission region. The former/latter can be derived from SDO/HMI magnetogram/three continuum bands of Hinode/SOT. We performed a statistical analysis for 35 Hinode white-light events and then found the field strength of white-light emission region were correlated with the maximum brightness of the white-light emission, and the amount of the white-light enhancement was approximately proportional to the fourth power of the radiation temperature. Assuming that the magnetic field strength at the acceleration site in the corona is somehow proportional to the photospheric magnetic field strength at the white-light emitting region, this result suggested the strong acceleration process which produces WLFs takes place at the strong magnetic field region in the corona.

Solar flares are powerful particle accelerators. During a flare, magnetic reconnection occurs and it can accelerate electrons and ions as well as trigering other particle acceleration processes such as turbulence acceleration in closed loops or diffusive shock acceleration at shocks associated with outpropagating jets. To discern different acceleration mechanisms, one can examine the release times of particles of different energies, both downward and upward. Such a study is made possible by the recently developed Fractonal Velocity Dispersion Analysis (FVDA). I show a few events of which particle release times at the flare are carefully obtained. Constraints on the underlying particle acceleration mechanisms will be discussed.

HXR and radio observations provide primary diagnostics of the acceleration and transport of energetic electrons in solar flares. Nonthermal looptop sources suggest that particle acceleration takes place above the top of flare loops and the fast-mode flare termination shock (TS) is one of the promising candidates as the acceleration mechanism. Here we investigate the acceleration of energetic electrons in solar flares by combining a large-scale magnetohydrodynamic simulation of a solar flare with a particle acceleration and transport model. We find that the accelerated electrons are concentrated in the looptop region due to the acceleration at the TS and confinement by the magnetic trap structure, in agreement with HXR and microwave observations. Numerous plasmoids can be produced in the reconnection current sheet and interact dynamically with the TS. We find that the energetic electron population varies rapidly in both time and space due to plasmoid-shock interactions. Our simulations have strong implications to the interpretation of nonthermal looptop sources, as well as the commonly observed fast temporal variations in flare emissions.

We present an investigation of solar energetic electron (SEE) events in solar cycle 23 and solar cycle 24 observed by the 3-D Plasma and Energetic Particles experiment on the WIND. We perform statistical analysis of energy spectrum of SEEs. For the SEE energy spectrum, we use general spectrum fitting method to fit the SEE energy spectrum in order to obtain self-consistent fitting parameters. We also use optical data such as SDO/AIA EUV data, SGD radio data, GOES SXR data and RHESSI HXR data to investigate the association between SEEs and solar optical phenomenon. The comparison of these properties of SEEs in the two solar cycles and solar cycle periodicity of SEEs will be accentuated.   

In large gradual solar energetic particle (SEP) events, particles are believed to be accelerated by fast CME-driven shocks via diffusive shock acceleration mechanism. Recent observations have shown double power-laws in the event-integrated differential spectra in many SEP events, with the break energy ordered by the ion charge-to-mass ratio (Q/A) for different species. In this work we perform numerical modeling of particle acceleration at coronal shocks propagating through a streamer-like coronal magnetic field by solving the Parker transport equation. We show that for all species the ion energy spectra integrated over the simulation domain resemble a double power-law, due to a mixture of two distinct populations accelerated in open and closed magnetic field regions where the particle acceleration rates are significantly different. We also find that the break energy has a power-law dependence on the ion’s Q/A, with the spectral index varying from 0.15 to 1.44 by considering different turbulence spectra. Because in observations the event-integrated spectra at 1 au may sample SEPs from different sources due to cross-field diffusion, we suggest that the mixing effect may be an important factor in the formation of double power-laws.

We present a statistical study of shock acceleration of ~1-100 keV solar wind suprathermal electrons at Earth’s bow shock, by using Wind 3D plasma and energetic particle measurements in ambient solar wind and Magnetospheric Multi-scale mission measurements in shock downstream. Among 74 shock cases (1 quasi-parallel shock, 73 quasi-perpendicular shocks) during 2015 October - 2017 January, there are 11 cases with a clear double-power-law spectrum, J∝ε^-β1 (J∝ε^-β2) when ε<<ε_tr (ε>>ε_tr), bending down at ε_tr~20-90 keV in downstream. The spectral indexes at ~0.8-10 keV, β1, range from 2.8 to 3.4, while the spectral indexes above ~40 keV, β1, range from 4 to 9, and all the spectral indexes show no correlations with those in ambient solar wind. These 11 cases have large flux enhancement J_dn/J_ab>300 and large shock angle θ_Bn, fast match number M_f and magnetic compression ratio r_B. To explain the formation of double-power-law spectrum in downstream, we use shock drift acceleration (SDA) model and find that the drift time T_d of suprathermal electrons in shock ramp is close to the plasma transit time T_tr of bulk plasma traveling through shock ramp. T_d/T_tr increases (decrease) with energy when ε<ε_tr (ε>ε_tr), and the gyration scale of electrons at ε_tr, D_g(ε_tr), is close to the ramp thickness D_ramp. These results make a quantitive explanation of the formation of double-power-law spectrum in shock downstream

The Deep Dielectric Charging Effect Monitor (DDCEM) has been designed to study the internal charging effect by measuring the charging currents and potentials inside the spacecraft. It is equipped on three Chinese navigation satellites in a circular Medium Earth Orbit (MEO) with 22000km average height and 55° inclination. Numerical simulation based on the Geant4-RIC method was used to evaluate the data of DDCEM. The data during May to November 2019 on one of the three satellites shows that the charging currents of DDCEM were negatively enhanced when the satellite moved into the outer radiation belt. The currents reached the negative maximum during a significant electron enhancement in September 2019. Positive currents were also detected besides negative currents that were caused by the deposition of electrons in the sensor. The causation of positive currents in the space environment may be that the low-energy electrons cannot penetrate the satellite skin and make it charging to negative potential, the reference ground of DDCEM that is connected to the satellite skin drops below zero by the low-energy electrons so that the output currents turn to positive. Ground experiment was used to simulate the causation of positive currents and the result verified our theory.

Magnetospheric Multiscale mission (MMS) observations of the reconnection process have confirmed the close relationship between the formation of X-lines in a thinned cross-tail current sheet and the creation of ion-inertial scale flux ropes. By comparison, the generation of ion-scale flux ropes through turbulent reconnection remains far less well understood. Recently, we have reported the presence of ion-scale flux ropes with in the plasma sheet for which the polarity of the bi-polar Bz variations show the opposite correlation with their sunward or anti-sunward flow speed from what has been previously observed for larger diameter flux ropes. We term the flux ropes with a north-then-south (south-then-north) change in the L-component of the magnetic field in the minimum variance coordinate system as they travel tailward (sunward) as being “forward” (“reverse”) flux ropes. Our initial examination of the 2017 MMS tail measurements identified 117 ion-scale flux ropes. Of these 7 (~6%) were reverse-type and 110 (~94%) were forward-type. Analysis of the detailed structure of the forward and reverse flux ropes showed them to be quite similar except for the sense of the bi-polar Bz variations for the earthward and tailward moving flux ropes in the two classes. Here we present the results of a multi-year study of forward and reverse flux ropes in the MMS plamsa sheet data set. The structure of the forward and reverse flux rope populations will be determined using Grad-Shafranov reconstruction. Further correlations between flux rope-type and their structural properties and upstream solar wind conditions and internal magnetospheric dynamics will be sought. Possible mechanisms for the formation of the reverse flux ropes including on large-amplitude (delta-B/B) turbulent reconnection and post-formation interactions with ion-scale flow vortices will be investigated.

Magnetic flux ropes (FR) are a space plasma phenomenon characterized by a strong axial core field at the center surrounded by strong azimuthal fields. In the dayside Martian plasma environment, FRs act as both channels for atmospheric escape, and evidence for magnetic reconnection (MR). Using data from the magnetometer onboard the Mars Atmosphere and Volatile (MAVEN) spacecraft, we have identified 156 FRs in the dayside ionosphere of Mars. Martian FR formation is of interest due to the localized, crustal magnetic anomalies which interact directly with the solar wind. 90% of FRs within our database are consistent with having been formed via either multiple X-line external reconnection (ER) between the interplanetary magnetic field (IMF) and a crustal anomaly or single-X line internal reconnection (IR) of an overlapping crustal anomaly. Above ~300 km, the atmosphere of Mars is collisionless, but becomes increasingly collisional as altitude decreases. Ion-neutral collisions decrease the rate of MR in plasma environments, approaching that of Sweet-Parker reconnection, thus eliminating the possibility of the fast MR required for FR formation. In order to understand the role ion-neutral collisions play in FR formation, we identify both the particular anomaly that was the source for each individual FR in our database, and the altitude at which they formed. We examine orbits in which MAVEN both encountered a FR and sampled the solar wind. We use a spherical harmonic model of the crustal anomalies at various altitudes and project properties of the solar wind to identify the likely anomaly and altitude at which the FR was formed. We present evidence for FRs formed in both collisionless and collisional plasma regimes with ~80% forming above 300 km, and ~20% forming below 300 km, suggesting ion-neutral collisions may serve to decrease the rate of FR formation in the ionosphere of Mars.

Small-scale magnetic flux rope (SFR), one type of the structures in the solar wind, possesses helical magnetic field lines and has been studied via both simulation and observation for decades. Based on the Grad-Shafranov (GS)-based computer program, we have identified over 87,000 SFRs in about 39 years via five in-situ spacecraft missions in the solar wind. These missions are the Parker Solar Probe (PSP), Helios 1 & 2 , Advanced Composition Explorer (ACE), Ulysses, and Voyager 1 & 2. We establish the corresponding SFR databases and present the comprehensive analyses of SFR properties from 0.16 to 9.74 au in heliocentric distances and from -80.2 to 80.2 deg in heliographic latitudes. The SFR events in these databases comprise the most abundant small-scale, the intermediate size, and finally the large-scale flux ropes with a continuous size distribution. The SFR properties generally follow the solar wind characteristics. They exhibit power-law distributions between 0.29 and 8 au, but with different power-law indices. The SFR magnetic field resembles the trend dictated by the nominal Parker field. The occurrence of events with relatively high Alfvenicity is pronounced at times. They are observed not only in high-latitude regions of fast-speed wind, but also in the slow wind closer to the Sun. Additionally, we found that the number of SFRs detected near the Sun is much less than at larger radial distances, where magnetohydrodynamic (MHD) turbulence may act as the local source to produce these structures.

A common feature of magnetic reconnection is the formation, even under relatively steady driving conditions of magnetic flux ropes, either via primary formation mechanisms such as multiple X-line reconnection, or via secondary instabilities at a single X-line. As well as causing time variability in the reconnection outflow, they may act as sites for particle acceleration, and provide a useful tracer of reconnection-type processes in planetary missions where the full properties of the plasma may not be measured. Of particular interest is the ultimate fate of flux ropes, and their lifetime as a function of the ambient plasma conditions and the larger scale system they are embedded in.  In the context of the Earth’s magnetosphere, observations far from the reconnection site are more difficult to obtain, and so solar wind reconnection provides the opportunity to diagnose the properties of flux rope formation and evolution in extended reconnecting current sheets. Here we review observations of flux ropes in solar wind current sheets, comparing them to observations in the Earth’s magnetosphere. In particular, we present new Solar Orbiter magnetic field observations of an ion-scale flux rope confined within a bifurcated solar wind current sheet, and consider its properties in the context of the enhance turbulence that is observed in the current sheet. Based on comparisons with Wind, this event is interpreted in terms of a reconnection scenario, and the implications of the observations are discussed by considering other heliospheric observations.

We will present the recently reported discovery and analysis of seven new flux rope observations at Saturn’s dayside magnetopause by the Cassini spacecraft and analyze the measurements of all eight known flux ropes (Jasinski et al., 2021). Since the high plasma-beta conditions at Saturn’s magnetopause are likely to suppress reconnection from occurring we investigate how flux ropes will differ in comparison to other planets. The measured ion-scale flux ropes have diameters close to or above the ion inertial length d_i~1–27 (median and mean values of 5 and 8), considerably lower than typical flux ropes found at Earth’s magnetopause. The magnetic flux contents are 4–461 kWb (median and mean values of 16 and 77 kWb), considerably smaller (<0.1%) than average flux opened during magnetopause compression events at Saturn. This is in contrast to Earth and Mercury where dayside flux ropes can contribute significantly to magnetospheric flux transfer. Therefore, at Saturn, flux ropes represent a negligible proportion of the amount of open magnetic flux transferred. Due to the likely suppression of the two main growth-mechanisms (continuous multiple x-line reconnection and coalescence), we conclude that adiabatic expansion is the likely (if any) candidate to grow the size of flux ropes at Saturn. Electron energization is observed inside, due to either Fermi acceleration or parallel electric fields. Due to diamagnetic suppression of reconnection at Saturn’s magnetopause, we suggest that the typical size of flux ropes at Saturn is most likely very small, and that there may be more d_i-scale flux ropes present in the Cassini magnetometer data that have not been found due to their brief and unremarkable magnetic signatures.

Solar magnetic flux ropes (MFRs) are the eruptive core structure of coronal mass ejections (CMEs) and thus are also the core topic for  CME research. In general,  eruptive MFRs are thought to be created either by direct flux emergence from below photosphere or successive magnetic reconnection before and during solar eruptions. Here, we provides an alternative creation scenario:  the rapid magnetic twist transport in coronal magnetic fields can also directly results in the formation of a newborn large-scale MFR. Specifically, we investigate the origin of an Earth-directed CME and its related magnetic coupling process with stereoscopic and multi-bands observations from EUV passbands of SDO/AIA and STEREO. Our results show that this whole event starts with a small-scale active-region filament whose eruption occurs at a coronal geyser site due to flux emergence and cancellation. By interacting with an overlying null-point configuration, this erupting filament first breaks one of its legs and triggers an unwinding blowout jet. The release of magnetic twist in its jet spire is estimated at around 1.5−2.0 turns. This prominent twist transport in jet spire rapidly creates a newborn large-scale flux rope from the jet base to a remote site. As a result, the newborn large-scale flux rope erupts into the outer coronae causing a stellar-sized CME bubble. Moreover, two sets of distinct flare post-flare loops form in its source region in sequence, indicating this eruptive event couples with twice flare reconnection. This observation not only provides a new hint for understanding for the formation of large-scale CME flux ropes, but support the view that that some large-scale coronal eruptive phenomena may originate from the magnetic coupling among different magnetic activities.

Small-scale flux ropes, with estimated diameters of ∼500 km and that pass over the MESSENGER spacecraft on timescales of seconds or less, are a common feature in Mercury’s magnetosphere. These magnetic structures, sometimes referred to as plasmoids, are believed to form as a result of rapid transient reconnection in the magnetopause or cross-tail current sheet at Mercury and the other planets. Here shows the magnetic flux ropes with a duration of several seconds or even longer, corresponding to a spatial scale of ∼0.5–1 planetary radius, can occur at Mercury. For unusually plasmoids structures in Mercury’s magnetotail, their spatial scales in the north–south direction exceed Mercury’s radius of 2440 km, and their time durations are comparable to or longer than the average Dungey cycle time of ∼200 s. They have a more loop-like magnetic structure than the more common helical-like flux rope topology. The formation of these macroscale flux ropes at a small dimensional magnetosphere will be discussed.