Open Magnetic Field Research Articles

Element Abundances in Impulsive Solar Energetic-Particle Events

Impulsive solar energetic-particle (SEP) events were first distinguished as the streaming electrons that produce type III radio bursts as distinct from shock-induced type II bursts. They were then observed as the surprisingly enhanced 3He-rich SEP events, which were also found to have element enhancements rising smoothly with the mass-to-charge ratio A/Q through the elements, even up to Pb. These impulsive SEPs have been found to originate during magnetic reconnection in solar jets where open magnetic field lines allow energetic particles to escape. In contrast, impulsive solar flares are produced when similar reconnection involves closed field lines where energetic ions are trapped on closed loops and dissipate their energy as X-rays, γ-rays, and heat. Abundance enhancements that are power laws in A/Q can be used to determine Q values and hence the coronal source temperature in the events. Results show no evidence of heating, implying reconnection and ion acceleration occur early, rapidly, and at low density. Proton and He excesses that contribute their own power law may identify events with reacceleration of SEPs by shock waves driven by accompanying fast, narrow coronal mass ejections (CMEs) in many of the stronger jets.

Coronal voids and their magnetic nature

Context. Extreme ultraviolet (EUV) observations of the quiet solar atmosphere reveal extended regions of weak emission compared to the ambient quiescent corona. The magnetic nature of these coronal features is not well understood. Aims. We study the magnetic properties of the weakly emitting extended regions, which we name coronal voids. In particular, we aim to understand whether these voids result from a reduced heat input into the corona or if they are associated with mainly unipolar and possibly open magnetic fields, similar to coronal holes. Methods. We defined the coronal voids via an intensity threshold of 75% of the mean quiet-Sun (QS) EUV intensity observed by the high-resolution EUV channel (HRIEUV) of the Extreme Ultraviolet Imager on Solar Orbiter. The line-of-sight magnetograms of the same solar region recorded by the High Resolution Telescope of the Polarimetric and Helioseismic Imager allowed us to compare the photospheric magnetic field beneath the coronal voids with that in other parts of the QS. Results. The coronal voids studied here range in size from a few granules to a few supergranules and on average exhibit a reduced intensity of 67% of the mean value of the entire field of view. The magnetic flux density in the photosphere below the voids is 76% (or more) lower than in the surrounding QS. Specifically, the coronal voids show much weaker or no network structures. The detected flux imbalances fall in the range of imbalances found in QS areas of the same size. Conclusions. We conclude that coronal voids form because of locally reduced heating of the corona due to reduced magnetic flux density in the photosphere. This makes them a distinct class of (dark) structure, different from coronal holes.

Three-dimensional General-relativistic Simulations of Neutrino-driven Winds from Magnetized Proto–Neutron Stars

Formed in the aftermath of a core-collapse supernova or neutron star merger, a hot proto–neutron star (PNS) launches an outflow driven by neutrino heating lasting for up to tens of seconds. Though such winds are considered potential sites for the nucleosynthesis of heavy elements via the rapid neutron capture process (r-process), previous work has shown that unmagnetized PNS winds fail to achieve the necessary combination of high entropy and/or short dynamical timescale in the seed nucleus formation region. We present three-dimensional general-relativistic magnetohydrodynamical simulations of PNS winds which include the effects of a dynamically strong (B ≳ 1015 G) dipole magnetic field. After initializing the magnetic field, the wind quickly develops a helmet-streamer configuration, characterized by outflows along open polar magnetic field lines and a “closed” zone of trapped plasma at lower latitudes. Neutrino heating within the closed zone causes the thermal pressure of the trapped material to rise in time compared to the polar outflow regions, ultimately leading to the expulsion of this matter from the closed zone on a timescale of ∼60 ms, consistent with the predictions of Thompson. The high entropies of these transient ejecta are still growing at the end of our simulations and are sufficient to enable a successful second-peak r-process in at least a modest ≳1% of the equatorial wind ejecta.

Reconnection-driven flares in 3D black hole magnetospheres

Context. Low-luminosity supermassive and stellar-mass black holes (BHs) may be embedded in a collisionless and highly magnetized plasma. They show nonthermal flares indicative of particles being accelerated up to relativistic speeds by dissipative processes in the vicinity of the BH. During near-infrared flares from the supermassive BH Sagittarius A* (Sgr A*), the GRAVITY Collaboration detected circular motion and polarization evolution, which suggest the presence of transient synchrotron-emitting hot spots moving around the BH. Aims. We study 3D reconnecting current layers in the magnetosphere of spinning BHs to determine whether plasma-loaded flux ropes which are formed near the event horizon could reproduce the hot spot observations and help constrain the BH spin. Methods. We performed global 3D particle-in-cell simulations in Kerr spacetime of a pair plasma embedded in a strong and large-scale magnetic field originating in a perfectly conducting disk in prograde Keplerian rotation. Results. A cone-shaped current layer develops which surrounds the twisted open magnetic field lines threading the event horizon. Spinning magnetic field lines coupling the disk to the BH inflate and reconnect a few gravitational radii above the disk. This quasi-periodic cycle accelerates particles, which accumulate in a few macroscopic flux ropes rotating with the outermost coupling magnetic field line. Once flux ropes detach, they propagate in the current layer following what appears as a rapidly opening spiral when seen face-on. A single flux rope carries enough relativistic electrons and positrons to emit synchrotron radiation at levels suitable to reproduce the peak-luminosity of the flares of Sgr A* but it quickly fades away as it flows away. Conclusions. Our kinematic analysis of the flux ropes motion favors a BH spin of 0.65 to 0.8 for Sgr A*. The duration of the flares of Sgr A* can only be explained provided the underlying magnetic loop seeded in the disk mid-plane has a finite lifetime and azimuthal extension. In this scenario, the hot spot corresponds to a spinning arc along which multiple reconnection sites power the net emission as flux ropes episodically detach.

Picoflare jets power the solar wind emerging from a coronal hole on the Sun.

Coronal holes are areas on the Sun with open magnetic field lines. They are a source region of the solar wind, but how the wind emerges from coronal holes is not known. We observed a coronal hole using the Extreme Ultraviolet Imager on the Solar Orbiter spacecraft. We identified jets on scales of a few hundred kilometers, which last 20 to 100 seconds and reach speeds of ~100 kilometers per second. The jets are powered by magnetic reconnection and have kinetic energy in the picoflare range. They are intermittent but widespread within the observed coronal hole. We suggest that such picoflare jets could produce enough high-temperature plasma to sustain the solar wind and that the wind emerges from coronal holes as a highly intermittent outflow at small scales.

The Solar Origin of an In Situ Type III Radio Burst Event

Solar type III radio bursts are generated by beams of energetic electrons that travel along open magnetic field lines through the corona and into interplanetary space. However, understanding the source of these electrons and how they escape into interplanetary space remains an outstanding topic. Here we report multi-instrument, multiperspective observations of an interplanetary type III radio burst event shortly after the second perihelion of the Parker Solar Probe (PSP). This event was associated with a solar jet that produced an impulsive microwave burst event recorded by the Expanded Owens Valley Solar Array. The type III burst event also coincided with the detection of enhanced in situ energetic electrons recorded by both PSP at 0.37 au and WIND at 1 au, which were located very closely on the Parker spiral longitudinally. The close timing association and magnetic connectivity suggest that the in situ energetic electrons originated from the jet’s magnetic reconnection region. Intriguingly, microwave imaging spectroscopy results suggest that the escaping energetic electrons were injected into a large opening angle of about 90°, which is at least nine times broader than the apparent width of the jet spire. Our findings provide an interpretation for the previously reported, longitudinally broad spatial distribution of flare locations associated with prompt energetic electron events and have important implications for understanding the origin and distribution of energetic electrons in interplanetary space.

Three-dimensional inversion of corona structure and simulation of solar wind parameters based on the photospheric magnetic field deduced from the Global Oscillation Network Group

In this research, the Potential Field Source Surface–Wang–Sheeley–Arge (PFSS–WSA) solar wind model is used. This model consists of the Potential Field Source Surface (PFSS) coronal magnetic field extrapolation module and the Wang–Sheeley–Arge (WSA) solar wind velocity module. PFSS is implemented by the POT3D package deployed on Tianhe 1A supercomputer system. In order to obtain the three–dimensional (3D) distribution of the coronal magnetic field at different source surface radii (Rss), the model utilizes the Global Oscillation Network Group (GONG) photospheric magnetic field profiles for two Carrington rotations (CRs), CR2069 (in 2008) and CR2217 (in 2019), as the input data, with the source surface at Rss = 2Rs, Rss = 2.5Rs and Rss = 3Rs, respectively. Then the solar wind velocity, the coronal magnetic field expansion factor, and the minimum angular distance of the open magnetic field lines from the coronal hole boundary are estimated within the WSA module. The simulated solar wind speed is compared with the value for the corona extrapolated from the data observed near 1 AU, through the calculations of the mean square error (MSE), root mean square error (RMSE) and correlation coefficient (CC). Here we extrapolate the solar wind velocity at 1 AU back to the source surface via the Parker spiral. By comparing the evaluation metrics of the three source surface heights, we concluded that the solar source surface should be properly decreased with respect to Rss = 2.5Rs during the low solar activity phase of solar cycle 23.

Particle Acceleration and Their Escape into the Heliosphere in Solar Flares with Open Magnetic Field

Energetic particle populations in the solar corona and in the heliosphere appear to have different characteristics even when produced in the same solar flare. It is not clear what causes this difference: properties of the acceleration region, the large-scale magnetic field configuration in the flare, or particle transport effects, such as scattering. In this study, we use a combination of magnetohydrodynamic and test-particle approaches to investigate magnetic reconnection, particle acceleration, and transport in two solar flares: an M-class flare on 2013 June 19, and an X-class flare on 2011 September 6. We show that in both events, the same regions are responsible for the acceleration of particles remaining in the coronal and being ejected toward the heliosphere. However, the magnetic field structure around the acceleration region acts as a filter, resulting in different characteristics (such as energy spectra) acquired by these two populations. We argue that this effect is an intrinsic property of particle acceleration in the current layers created by the interchange reconnection, and therefore, may be ubiquitous, particularly, in noneruptive solar flares with substantial particle emission into the heliosphere.

CME propagation through the heliosphere: Status and future of observations and model development

The ISWAT (International Space Weather Action Teams) heliosphere clusters H1 and H2 have a focus on interplanetary space and its characteristics, especially on the large-scale co-rotating and transient structures impacting Earth. Solar wind stream interaction regions, generated by the interaction between high-speed solar wind originating in large-scale open coronal magnetic fields and slower solar wind from closed magnetic fields, are regions of compressed plasma and magnetic field followed by high-speed streams that recur at the ∼27 day solar rotation period. Short-term reconfigurations of the lower coronal magnetic field generate flare emissions and provide the energy to accelerate enormous amounts of magnetised plasma and particles in the form of coronal mass ejections into interplanetary space. The dynamic interplay between these phenomena changes the configuration of interplanetary space on various temporal and spatial scales which in turn influences the propagation of individual structures. While considerable efforts have been made to model the solar wind, we outline the limitations arising from the rather large uncertainties in parameters inferred from observations that make reliable predictions of the structures impacting Earth difficult. Moreover, the increased complexity of interplanetary space as solar activity rises in cycle 25 is likely to pose a challenge to these models. Combining observational and modeling expertise will extend our knowledge of the relationship between these different phenomena and the underlying physical processes, leading to improved models and scientific understanding and more-reliable space-weather forecasting. The current paper summarizes the efforts and progress achieved in recent years, identifies open questions, and gives an outlook for the next 5–10 years. It acts as basis for updating the existing COSPAR roadmap by Schrijver et al. (2015), as well as providing a useful and practical guide for peer-users and the next generation of space weather scientists.

Magnetic Topology and the Structure of the Slow Solar Wind

We present a 3D reduced magnetohydrodynamic (RMHD) model of reflection driven Alfvén wave turbulence in an open magnetic field positioned near the solar equator. The non-linear interactions between outward and inward propagating waves generate turbulence. The RMHD equations describing the turbulence include the effects of solar wind outflow velocity on the dissipation of waves. For Alfvén wave turbulence to be a viable mechanism for heating the corona and accelerating the slow solar wind, there must be sufficient counter-propagating waves to generate the turbulence. In our previous study of the fast wind, we showed that the Alfvén wave turbulence model creates the energy needed for accelerating the fast solar wind, when observed density fluctuations are included in the model. Here, we will explore whether the conditions of the equatorial corona, a source of slow solar wind, are capable of generating sufficient Alfvén wave turbulence to account for the acceleration of the slow wind. We show that for a specific sets of model parameters, the energy from the Alfvén wave turbulence model is smaller by an order of magnitude than the energy needed to heat and accelerate the slow solar wind.

Различия в отклике на геомагнитные возмущения, вызванные КВМ и ОКВ

Utilizing 1-minute resolution data on the geomagnetic indices SYM-H, AE, solar wind parameters (velocity Vsw and density Np), and z-component Bz of the interplanetary magnetic field (IMF) during solar cycles 23 and 24, we have statistically analyzed the correlations between geomagnetic activity (storms and substorms), Vsw, Np, Bz, and energy coupling functions of solar wind and Earth’s magnetosphere. For the selected 131 CME-driven storms, SYM-H stronger depends on Vsw and B than other parameters, whereas the selected 161 CIR-driven storms have nearly the same dependence on the solar wind electric field, the rate of open magnetic flux dφ/dt, and the reconnection electric field Ekl. Thus, the solar wind electric field and the dayside magnetic reconnection are likely to have different contributions for storms of the two types. During storms of different types, the substorm intensity AE relies mainly on the IMF Bz, rate of open magnetic flux and reconnection electric field.

Differences in the response to CME and CIR drivers of geomagnetic disturbances

Utilizing 1-minute resolution data on the geomagnetic indices SYM-H, AE, solar wind parameters (velocity Vsw and density Np), and z-component Bz of the interplanetary magnetic field (IMF) during solar cycles 23 and 24, we have statistically analyzed the correlations between geomagnetic activity (storms and substorms), Vsw, Np, Bz, and energy coupling functions of solar wind and Earth’s magnetosphere. For the selected 131 CME-driven storms, SYM-H stronger depends on Vsw and B than other parameters, whereas the selected 161 CIR-driven storms have nearly the same dependence on the solar wind electric field, the rate of open magnetic flux dφ/dt, and the reconnection electric field Ekl. Thus, the solar wind electric field and the dayside magnetic reconnection are likely to have different contributions for storms of the two types. During storms of different types, the substorm intensity AE relies mainly on the IMF Bz, rate of open magnetic flux and reconnection electric field.

Suppression of bremsstrahlung losses from relativistic plasma with energy cutoff.

We study the effects of redistributing superthermal electrons on bremsstrahlung radiation from hot relativistic plasma. We consider thermal and nonthermal distribution of electrons with an energy cutoff in the phase space and explore the impact of the energy cutoff on bremsstrahlung losses. We discover that the redistribution of the superthermal electrons into lower energies reduces radiative losses, which is in contrast to nonrelativistic plasma. Finally, we discuss the possible relevance of our results for open magnetic field line configurations and prospects of the aneutronic fusion based on proton-boron-11 (p-B11) fuel.

Interchange reconnection as the source of the fast solar wind within coronal holes

The fast solar wind that fills the heliosphere originates from deep within regions of open magnetic field on the Sun called ‘coronal holes’. The energy source responsible for accelerating the plasma is widely debated; however, there is evidence that it is ultimately magnetic in nature, with candidate mechanisms including wave heating1,2 and interchange reconnection3–5. The coronal magnetic field near the solar surface is structured on scales associated with ‘supergranulation’ convection cells, whereby descending flows create intense fields. The energy density in these ‘network’ magnetic field bundles is a candidate energy source for the wind. Here we report measurements of fast solar wind streams from the Parker Solar Probe (PSP) spacecraft6 that provide strong evidence for the interchange reconnection mechanism. We show that the supergranulation structure at the coronal base remains imprinted in the near-Sun solar wind, resulting in asymmetric patches of magnetic ‘switchbacks’7,8 and bursty wind streams with power-law-like energetic ion spectra to beyond 100 keV. Computer simulations of interchange reconnection support key features of the observations, including the ion spectra. Important characteristics of interchange reconnection in the low corona are inferred from the data, including that the reconnection is collisionless and that the energy release rate is sufficient to power the fast wind. In this scenario, magnetic reconnection is continuous and the wind is driven by both the resulting plasma pressure and the radial Alfvénic flow bursts.

Observational Evidence of S-web Source of the Slow Solar Wind

From 2022 March 18 to 21, NOAA Active Region (AR) 12967 was tracked simultaneously by Solar Orbiter at 0.35 au and Hinode/EIS at Earth. During this period, strong blueshifted plasma upflows were observed along a thin, dark corridor of open magnetic field originating at the AR’s leading polarity and continuing toward the southern extension of the northern polar coronal hole. A potential field source surface model shows large lateral expansion of the open magnetic field along the corridor. Squashing factor Q-maps of the large-scale topology further confirm super-radial expansion in support of the S-web theory for the slow wind. The thin corridor of upflows is identified as the source region of a slow solar wind stream characterized by ∼300 km s−1 velocities, low proton temperatures of ∼5 eV, extremely high density >100 cm−3, and a short interval of moderate Alfvénicity accompanied by switchback events. When the connectivity changes from the corridor to the eastern side of the AR, the in situ plasma parameters of the slow solar wind indicate a distinctly different source region. These observations provide strong evidence that the narrow open-field corridors, forming part of the S-web, produce some extreme properties in their associated solar wind streams.

The Effects of the Polar Rain on the Polar Wind Ion Outflow From the Nightside Ionosphere

AbstractThe polar wind, consisting of low‐energy ions and electrons, is an outflow along the open magnetic field lines from the polar cap ionosphere to the magnetosphere. Previous studies found that both solar radiation and solar wind electromagnetic energy are the two main energy sources for the polar wind. The polar rain, being field‐aligned precipitating electrons from the solar wind to the polar cap, may provide additional energies for the polar wind. This scenario is complicated as simulation studies show that polar rain changes the electric potential structures over the polar cap ionosphere. It is unclear how the polar rain affects the polar wind ion outflow. In this study, we show a positive correlation between the polar wind and the polar rain. Meanwhile, the polar wind is generally diminished in regions with strong Earth's magnetic field, suggesting the |B| modulates the penetration depth of the polar rain through the magnetic mirror force and thus the energy dissipation of the polar rain. Therefore, the polar rain can be an additional energy source for the polar wind although the polar rain has generally smaller energies and intensities than the particle precipitations in the auroral regions.

Accounting for differential rotation in calculations of the Sun’s angular momentum-loss rate

Context. Sun-like stars shed angular momentum due to the presence of magnetised stellar winds. Magnetohydrodynamic models have been successful in exploring the dependence of this ‘wind-braking torque’ on various stellar properties; however the influence of surface differential rotation is largely unexplored. As the wind-braking torque depends on the rotation rate of the escaping wind, the inclusion of differential rotation should effectively modulate the angular momentum-loss rate based on the latitudinal variation of wind source regions. Aims. Here we aim to quantify the influence of surface differential rotation on the angular momentum-loss rate of the Sun, in comparison to the typical assumption of solid-body rotation. Methods. To do this, we exploited the dependence of the wind-braking torque on the effective rotation rate of the coronal magnetic field, which is known to be vitally important in magnetohydrodynamic models. This quantity has been evaluated by tracing field lines through a potential field source surface (PFSS) model, driven by ADAPT-GONG magnetograms. The surface rotation rates of the open magnetic field lines were then used to construct an open-flux weighted rotation rate, from which the influence on the wind-braking torque could be estimated. Results. During solar minima, the rotation rate of the corona decreases with respect to the typical solid-body rate (the Carrington rotation period is 25.4 days), as the sources of the solar wind are confined towards the slowly rotating poles. With increasing activity, more solar wind emerges from the Sun’s active latitudes which enforces a Carrington-like rotation. Coronal rotation often displays a north-south asymmetry driven by differences in active region emergence rates (and consequently latitudinal connectivity) in each hemisphere. Conclusions. The effect of differential rotation on the Sun’s current wind-braking torque is limited. The solar wind-braking torque is ∼10 − 15% lower during solar minimum, (compared with the typical solid body rate), and a few percent larger during solar maximum (as some field lines connect to more rapidly rotating equatorial latitudes). For more rapidly rotating Sun-like stars, differential rotation may play a more significant role, depending on the configuration of the large-scale magnetic field.

Staged cooling of a fusion-grade plasma in a tokamak thermal quench

In tokamak disruptions where the magnetic connection length becomes comparable to or even shorter than the plasma mean-free-path, parallel transport can dominate the energy loss and the thermal quench of the core plasma goes through four phases (stages) that have distinct temperature ranges and durations. The main temperature drop occurs while the core plasma remains nearly collisionless, with the parallel electron temperature dropping in time t as and a cooling time that scales with the ion sound wave transit time over the length of the open magnetic field line. These surprising physics scalings are the result of effective suppression of parallel electron thermal conduction in an otherwise bounded, quasineutral, and collisionless plasma, which is different from what are known to date on electron thermal conduction along the magnetic field in a nearly collisionless and quasineural plasma.

Slow solar wind sources

Context. The origin of the slow solar wind is still an open issue. One possibility that has been suggested is that upflows at the edge of an active region can contribute to the slow solar wind. Aims. We aim to explain how the plasma upflows are generated, which mechanisms are responsible for them, and what the upflow region topology looks like. Methods. We investigated an upflow region using imaging data with the unprecedented temporal (3 s) and spatial (2 pixels = 236 km) resolution that were obtained on 30 March 2022 with the 174 Å channel of the Extreme-Ultraviolet Imager (EUI)/High Resolution Imager (HRI) on board Solar Orbiter. During this time, the EUI and Earth-orbiting satellites (Solar Dynamics Observatory, Hinode, and the Interface Region Imaging Spectrograph, IRIS) were located in quadrature (∼92°), which provides a stereoscopic view with high resolution. We used the Hinode/EIS (Fe XII) spectroscopic data to find coronal upflow regions in the active region. The IRIS slit-jaw imager provides a high-resolution view of the transition region and chromosphere. Results. For the first time, we have data that provide a quadrature view of a coronal upflow region with high spatial resolution. We found extended loops rooted in a coronal upflow region. Plasma upflows at the footpoints of extended loops determined spectroscopically through the Doppler shift are similar to the apparent upward motions seen through imaging in quadrature. The dynamics of small-scale structures in the upflow region can be used to identify two mechanisms of the plasma upflow: Mechanism I is reconnection of the hot coronal loops with open magnetic field lines in the solar corona, and mechanism II is reconnection of the small chromospheric loops with open magnetic field lines in the chromosphere or transition region. We identified the locations in which mechanisms I and II work.

Behavior of the solar coronal holes around the maximum activity of the cycle 24

The behavior of coronal holes (CHs) as open magnetic field lines and a substrate for the high-speed exit of solar winds is an essential aspect of solar activity. This paper studies the statistical properties of CHs around the maximum activity of cycle 24 (13 May 2010 to 31 October 2017) observed by the Atmospheric Imaging Assembly at 193 Å images. We showed that the size of CHs follows the power-law distributions indicating the scale-free behavior of CHs. The butterfly diagram of CHs demonstrates that the overall number of CHs shows a similar distribution in the two hemispheres. Large CHs with sizes ⩾2.57×105 arcsec2 (top 3% of observed CHs) more commonly appear at latitudes above 30∘ closer to the poles. We found that the largest CHs with sizes ⩾7.71×105 arcsec2 (top 0.03% of observed CHs) occur in the northern hemisphere. The appearance and disappearance of large CHs exhibit the asymmetrical behavior between the two hemispheres. Also, it is observed that CHs tend to have a latitudinal drift towards the poles in each hemisphere. A significant correlation between the number of sunspots and the eccentricity of equatorial CHs appeared near the solar disk center revealed that most of the CHs have elongated shapes at the maximum solar activity. In contrast, they have less abnormality during solar minimum. Applying detrended fluctuation and rescaled range analyses to the time series of CHs, we realized that the CHs have a long-term correlation and form a self-organized system.