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Abstract Solar flares, driven by prompt release of free magnetic energy in the solar corona 1,2 , are known to accelerate a substantial portion (ten per cent or more) 3,4 of available electrons to high energies. Hard X-rays, produced by high-energy electrons accelerated in the flare 5 , require a high ambient density for their detection. This restricts the observed volume to denser regions that do not necessarily sample the entire volume of accelerated electrons 6 . Here we report evolving spatially resolved distributions of thermal and non-thermal electrons in a solar flare derived from microwave observations that show the true extent of the acceleration region. These distributions show a volume filled with only (or almost only) non-thermal electrons while being depleted of the thermal plasma, implying that all electrons have experienced a prominent acceleration there. This volume is isolated from a surrounding, more typical flare plasma of mainly thermal particles with a smaller proportion of non-thermal electrons. This highly efficient acceleration happens in the same volume in which the free magnetic energy is being released 2 .

Aims. We analyse particle, radio, and X-ray observations during the first relativistic proton event of solar cycle 25 detected on Earth. The aim is to gain insight into the relationship between relativistic solar particles detected in space and the processes of acceleration and propagation in solar eruptive events. Methods. To this end, we used ground-based neutron monitor measurements of relativistic nucleons and space-borne measurements of electrons with similar speed to determine the arrival times of the first particles at 1 AU and to infer their solar release times. We compared the release times with the time histories of non-thermal electrons in the solar atmosphere and their escape to interplanetary space, as traced by radio spectra and X-ray light curves and images. Results. Non-thermal electrons in the corona are found to be accelerated in different regions. Some are confined in closed magnetic structures expanding during the course of the event. Three episodes of electron escape to the interplanetary space are revealed by groups of decametric-to-kilometric type III bursts. The first group appears on the low-frequency side of a type II burst produced by a coronal shock wave. The two latter groups are accompanied at higher frequencies by bursts with rapid driftsmore »to both lower and higher frequencies (forward- or reverse-drifting bursts). They are produced by electron beams that propagate both sunward and anti-sunward. The first relativistic electrons and nucleons observed near Earth are released with the third group of type III bursts, more than ten minutes after the first signatures of non-thermal electrons and of the formation of the shock wave in the corona. Although the eruptive active region is near the central meridian, several tens of degrees east of the footpoint of the nominal Parker spiral to the Earth, the kilometric spectrum of the type III bursts and the in situ detection of Langmuir waves demonstrate a direct magnetic connection between the L1 Lagrange point and the field lines onto which the electron beams are released at the Sun. Conclusions. We interpret the forward- and reverse-drifting radio bursts as evidence of reconnection between the closed expanding magnetic structures of an erupting flux rope and ambient open magnetic field lines. We discuss the origin of relativistic particles near the Earth across two scenarios: (1) acceleration at the CME-driven shock as it intercepts interplanetary magnetic field lines rooted in the western solar hemisphere and (2) an alternative where the relativistic particles are initially confined in the erupting magnetic fields and get access to the open field lines to the Earth through these reconnection events.« less

Abstract Waves and oscillations are important solar phenomena, not only because they can propagate and dissipate energy in the chromosphere, but also because they carry information about the structure of the atmosphere in which they propagate. The nature of the 3 minute oscillations observed in the umbral region of sunspots is considered to be an effect of propagation of magnetohydrodynamic waves upward from below the photosphere. We present a study of sunspot oscillations and wave propagation in NOAA Active Region 12470 using an approximately 1 hr long data set acquired on 2015 December 17 by the Atacama Large Millimeter/submillimeter Array (ALMA), the Goode Solar Telescope (GST) operating at the Big Bear Solar Observatory, the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, and the Interface Region Imaging Spectrograph. The ALMA data are unique in providing a time series of direct temperature measurements in the sunspot chromosphere. The 2 s cadence of ALMA images allows us to well resolve the 3 minute periods typical of sunspot oscillations in the chromosphere. Fourier analysis is applied to ALMA Band 3 (∼100 GHz, ∼3 mm) and GST H α data sets to obtain power spectra as well as oscillation phase information. We analyzedmore »properties of the wave propagation by combining multiple wavelengths that probe physical parameters of solar atmosphere at different heights. We find that the ALMA temperature fluctuations are consistent with that expected for a propagating acoustic wave, with a slight asymmetry indicating nonlinear steepening.« less

Abstract We present both the observation and the magnetohydrodynamics (MHD) simulation of the M2.4 flare (SOL2017-07-14T02:09) of NOAA active region (AR) 12665 with a goal to identify its initiation mechanism. The observation by the Atmospheric Image Assembly (AIA) on board the Solar Dynamics Observatory (SDO) shows that the major topology of the AR is a sigmoidal configuration associated with a filament/flux rope. A persistent emerging magnetic flux and the rotation of the sunspot in the core region were observed with Magnetic Imager (HMI) on board the SDO on the timescale of hours before and during the flare, which may provide free magnetic energy needed for the flare/coronal mass ejection (CME). A high-lying coronal loop is seen moving outward in AIA EUV passbands, which is immediately followed by the impulsive phase of the flare. We perform an MHD simulation using the potential magnetic field extrapolated from the measured pre-flare photospheric magnetic field as initial conditions and adopting the observed sunspot rotation and flux emergence as the driving boundary conditions. In our simulation, a sigmoidal magnetic structure and an overlying magnetic flux rope (MFR) form as a response to the imposed sunspot rotation, and the MFR rises to erupt like a CME.more »These simulation results in good agreement with the observation suggest that the formation of the MFR due to the sunspot rotation and the resulting torus and kink instabilities were essential to the initiation of this flare and the associated coronal mass ejection.« less

Magnetic flux ropes are the centerpiece of solar eruptions. Direct measurements for the magnetic field of flux ropes are crucial for understanding the triggering and energy release processes, yet they remain heretofore elusive. Here we report microwave imaging spectroscopy observations of an M1.4-class solar flare that occurred on 2017 September 6, using data obtained by the Expanded Owens Valley Solar Array. This flare event is associated with a partial eruption of a twisted filament observed in Hαby the Goode Solar Telescope at the Big Bear Solar Observatory. The extreme ultraviolet (EUV) and X-ray signatures of the event are generally consistent with the standard scenario of eruptive flares, with the presence of double flare ribbons connected by a bright flare arcade. Intriguingly, this partial eruption event features a microwave counterpart, whose spatial and temporal evolution closely follow the filament seen in Hαand EUV. The spectral properties of the microwave source are consistent with nonthermal gyrosynchrotron radiation. Using spatially resolved microwave spectral analysis, we derive the magnetic field strength along the filament spine, which ranges from 600 to 1400 Gauss from its apex to the legs. The results agree well with the nonlinear force-free magnetic model extrapolated from the preflare photosphericmore »magnetogram. We conclude that the microwave counterpart of the erupting filament is likely due to flare-accelerated electrons injected into the filament-hosting magnetic flux rope cavity following the newly reconnected magnetic field lines.

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