One of the most intriguing high-energy phenomena at the Sun is that of Long Duration Gamma-Ray Flares (LDGRFs), characterized by gamma-ray emission above ~ 50 MeV that is both delayed and long-lasting.  The highest energy emission has generally been attributed to pion decay from interactions of high-energy protons with ambient solar material.  The source of these accelerated protons, however, has been a challenge to explain, in part due to the extreme energies and long-durations of these events.  Observations of LDGRFs suggest that particle acceleration occurs within large volumes extending to high altitudes, either by stochastic acceleration within large coronal loops or by back-precipitation from CME-driven shocks.  Despite numerous multi-point and multi-wavelength observations of these elusive high-energy flares, the question of how and where particles are being energized remains open as well as how those particles are transported to the Sun to subsequently radiate.  This talk focuses on the current state of research regarding LDGRFs, challenges facing a more complete picture, and possible paths forward. 

The sustained gamma-ray emission (SGRE) events from the Sun are associated with an ultrafast halo coronal mass ejection and a type II radio burst in the decameter-hectometric (DH) wavelengths.  The SGRE duration is linearly related to the type II burst duration indicating that >300 MeV protons required for SGREs are accelerated by the same shock that accelerates tens of keV electrons required for type II bursts. When well connected, the associated SEP event has a hard spectrum, indicating a copious population of high-energy protons. In one of the SGRE events observed on 2014 January 7 by Fermi/LAT, the SEP event detected by GOES has a very soft has spectrum with not many particles beyond ~100 MeV, which contradicts the presence of the SGRE (implying the presence of significant number of >300 MeV protons). Furthermore, the durations of the type II burst and the SGRE agree with the known linear relationship between them (Gopalswamy et al. 2018, ApJ 868, L19). We show that the soft spectrum is due to poor magnetic connectivity of the shock nose to an Earth observer.  Even though the location of the eruption (S15W11) is close to the disk center, the CME propagated non-radially making Earth encounter the CME flank rather than the nose. High-energy particles are accelerated near the nose, so they do not reach GOES but they do precipitate to the vicinity of the eruption region to produce SGRE. This study provides further evidence that SGRE is caused by protons accelerated in shocks and propagating sunward to interact with the atmospheric ions.

Nonthermal sources located above bright flare arcades, referred to as the "above-the-loop-top" sources, have been often suggested as the primary electron acceleration site in major solar flares. The X8.2 limb flare on 2017 September 10 features such an above-the-loop-top source, which was observed in both microwaves and hard X-rays (HXRs) by the Expanded Owens Valley Solar Array and the Reuven Ramaty High Energy Solar Spectroscopic Imager, respectively. By combining the microwave and HXR imaging spectroscopy observations with multifilter extreme ultraviolet and soft X-ray imaging data, we derive the coronal magnetic field and energetic electron distribution of this source over a broad energy range from <10 keV up to ~MeV during the early impulsive phase of the flare. The source has a strong magnetic field of over 800 G. The best-fit electron distribution of the source consists of a thermal "core" from ~25 MK plasma. A nonthermal power-law "tail" joins the thermal core at ~16 keV with a spectral index of ~3.6, which breaks down at above ~160 keV to >6.0. Temporally resolved analysis suggests that the electron distribution above the break energy rapidly hardens with the spectral index decreasing from >20 to ~6.0 within 20 s, or less than ~10 Alfvén crossing times in the source. These results provide strong support for the above-the-loop-top source as the primary site where an ongoing bulk acceleration of energetic electrons is taking place very early in the flare energy release.

We investigate 16 solar energetic electron (SEE) events measured by WIND/3DP with a double-power-law spectrum and the associated western hard X-ray (HXR) flares measured by RHESSI with good count statistics, from 2002 February to 2016 December. In all the 16 cases, the presence of an SEE power-law spectrum extending down to ~5 keV at 1 AU implies that the SEE source would be high in the corona, at heliocentric distance of >1.3 solar radii, while the footpoint or footpoint-like emissions shown in HXR images suggest that the observed HXRs are likely produced mainly by thick-target bremsstrahlung processes very low in the corona. We find that the power-law spectral index of HXR-producing electrons, estimated under the relativistic thick-target bremsstrahlung model, is significantly larger than (similar to) and positively correlated with the observed high-energy spectral index of SEEs, in 8 cases (8 cases). In addition, the estimated number of SEEs is only ∼10^-4-10^-2 of the estimated number of HXR-producing electrons at energies above 30 keV, but with a positive correlation between the two numbers. These results suggest that in these cases, SEEs are likely formed by upward-traveling electrons from an acceleration source high in the corona, while their downward-traveling counterparts may undergo a secondary acceleration before producing HXRs via thick-target bremsstrahlung processes. In addition, the associated ³He/⁴He ratio is positively correlated with the observed high-energy spectral index of SEEs, indicating a possible relation of the ³He ion acceleration with high-energy SEEs.

Solar Orbiter, a joint ESA/NASA mission, is to study the Sun and inner heliosphere in greater detail than ever before. Launched in February 2000, Solar Orbiter already completed its first orbit in reaching perihelion of 0.5 au from the Sun in June 2000.Understanding the physical processes operating in Solar Energetic Particle (SEP) events is a major goal of the Solar Orbiter mission because of the importance of acceleration processes in solar system and astrophysical sites, and because of the potential impact of these events on space hardware. The Energetic Particle Detector (EPD) investigation on Solar Orbiter is a suite of four different sensors plus the instrument control unit to measure the energetic particles from slightly above solar wind energies to hundreds of MeV/nucleon. We report here data from Suprathermal Ion Spectrograph (SIS) sensor of the EPD that covers the energy range of 0.1 – 10 MeV/nucleon for H-Fe with high mass resolution during the first orbit. SIS observed several ³He-rich SEP events inside of 1 au.  Even though these events were small, their spectral forms, ³He content, and association with type III bursts convincingly identifies them as ³He-rich SEP events with properties similar to those previously observed at 1 au.

The Integrated Science Investigation of the Sun (IS☉IS) suite on board NASA’s Parker Solar Probe (PSP) observed six distinct enhancements in the intensities of suprathermal-through-energetic (~0.03-3 MeV nucleon^-1) He ions associated with corotating or stream interaction regions during its first two orbits. Our results from a survey of the time-histories of the He intensities, spectral slopes, and anisotropies, and the event-averaged energy spectra during these events show: 1) In the two strongest enhancements, seen at 0.35 au and 0.85 au, the higher energy ions arrive and maximize later than those at lower energies. In the event seen at 0.35 au, the He ions arrive when PSP was away from the SIR trailing edge and entered the rarefaction region in the high-speed stream; 2) The He intensities are either isotropic or show sunward flows in the spacecraft frame; and 3) In all events, the energy spectra between ~0.2–1 MeV nucleon^-1 are power-laws of the form ∝E^-2. In the two strongest events, the energy spectra are well represented by flat power-laws between ~0.03–0.4 MeV nucleon^-1 modulated by exponential roll-overs between ~0.4–3 MeV nucleon^-1. We conclude that the SIR-associated He ions originate from sources or shocks beyond PSP’s location rather than from acceleration processes occurring at nearby portions of local compression regions. Our results also suggest that rarefaction regions that typically follow the SIRs facilitate easier particle transport throughout the inner heliosphere such that low energy ions do not undergo significant energy loss due to adiabatic deceleration, contrary to predictions of existing models.

We propose a pan-spectrum fitting formula of suprathermal particles, J=AE^-β1[1+(E/E₀)^α]^{((β1-β2)/α)},  where J is the particle flux (or intensity), E is the particle energy, A is the amplitude coefficient, E₀ represents the spectral transition energy, α(>0) describes the sharpness and width of spectral transition around E₀, and the power-law index β₁ (β₂) gives the spectral shape before (after) the transition. This formula incorporates many commonly used spectrum functions as special cases. When α goes to infinity (zero), this spectral formulabecomes the classical double-power-law (logarithmic-parabola) function. When both β₂ and E₀ approach infinity and α is equal to 1, this formula can be simplified to the Ellison-Ramaty function. Under some other specific parameter conditions, this formula can be transformed to the Kappa or Maxwellian distribution. Considering the uncertainties in both particle intensity and energy, we improve the fitting method and fit this pan-spectrum formula well to the representative energy spectra of various suprathermal particle phenomena including SEPs (electrons, protons, ³He, and heavier ions), ESPs, bow-shocked electrons, solar wind suprathermal electrons, anomalous cosmic rays, and hard X-rays. Therefore, this pan-spectrum fitting formula would help us comparatively examine the properties of energy spectrum of different suprathermal particle phenomena typically with a single energy break.

It is well established that O+ is a significant component of the storm-time ring current, indicating that the ionosphere is a strong source. However, many questions remain on how the source population at the inner edge of the plasma sheet changes as a function of time, leading to the ionospheric dominance of the ring current during storm times. Both the solar wind and the ionosphere contribute to the plasma sheet population, but their relative contributions vary as a function of time.  In addition, there are two regions in the ionosphere, the dayside cusp and the nightside aurora, that can contribute to the nightside plasma sheet, and their contributions can also vary with distance down the tail, as well as time.  Cusp outflow can be driven by enhanced solar wind pressure, and enhanced dynamic pressure can precede a storm main phase by many hours. Nightside outflow is driven by auroral activity which normally peaks during the main phase of the storm. An initial study of changes to the near-earth plasma sheet using AMPTE/CHEM data shows that the transition from solar wind to ionospheric source can occur quite sharply during storms, with the ionospheric contribution becoming dominant during the storm main phase.  However, the AMPTE measurements do not include the <1 keV ions that are critical for identifying the source.  In this paper, we use measurements from MMS/HPCA and Van Allen Probes/HOPE to investigate how the different sources to the plasma sheet vary with distance down the tail, and whether the same storm-time changes observed in the 6-8 Re plasma sheet with AMPTE/CHEM are also observed further down the tail.

 An impressive body of work has been devoted to the escape of H+, He+, and O+ ions from Earth’s ionosphere, and their circulation and redistribution throughout the terrestrial magnetosphere. However, the transport and energization of N+ in addition to that of O+, has not been considered by most studies, even though a number of direct and indirect measurements made to describe the ionic composition of the ionosphere-magnetosphere system, have established that N+ is a significant ion species in the ionosphere and its presence in the magnetosphere is significant. A wide range in the magnitude of the N+ to O+ density ratio has been reported, with significant variations not only with geomagnetic activity, but also with solar cycle, season, time of day, latitude, etc. These variations suggest that N+ and O+, while relatively close in mass, they obey different chemical and energization processes, and possibly follow different paths of energization, therefore the nitrogen ions flow out from the terrestrial ionosphere depends strongly on the external driver.  We discuss here the relative transport and energization of nitrogen and oxygen ions, as they are transported from the low altitude ionosphere throughout the global magnetosphere, and their impact on the magnetospheric dynamics.  Numerical simulations suggest that polar wind ionic composition is of the utmost importance, as the inclusion of N+ alters the dynamics of ionospheric outflow, and the ring current development and the decay. Furthermore, tracking the loss of nitrogen ions could provide clues regarding a planet's ability to sustain life, as the clement climate conditions on the Earth's surface are facilitated by its nitrogen-dominated atmosphere.

The response of the high-altitude cusp to an interplanetary (IP) shock is studied with the joint observations from ACE at L1 Lagrange point, THEMIS E in the subsolar magnetosheath and four Cluster spacecraft travelling inbound in the polar region during 22:30-24:00 UT on 7 September 2017. After the shock arrival with strong IMF B_z, triple cusps were observed by Cluster C4 when there were high solar wind dynamic pressures. It is attributed to the same cusp moving back and forth three times, because its each appearance clearly corresponded to the dayside magnetosphere compression indicated by THEMIS E going into solar wind from magnetosheath. Energetic ions (O⁺, He⁺ and H⁺) with energy up to MeV were observed in the moving cusps, which were mainly distributed at pitch angles of 90^o-180^o. There are two populations separated at 10 keV for O⁺ and only one for both He⁺ and H⁺. The kappa index of the low-energy population is similar with He⁺ and H⁺, while the high-energy exhibits a harder tail and its flux ratio of O⁺/H⁺ is increasing with energy. These observations are consistent with the impulsive acceleration by electric fields, which is mass-dependent. This study provides a new acceleration mechanism for cusp energetic ions, especially for O⁺.

To understand magnetosphere-ionosphere conditions that result in thermal emission velocity enhancement (STEVE) and subauroral ion drifts (SAID) during the substorm recovery phase, we present substorm aurora, particle injection and current systems during two STEVE events. Those events are compared to substorm events with similar strength but without STEVE. We found that the substorm surge and intense upward currents for the events with STEVE reach the dusk, while those for the non-STEVE substorms are localized around midnight. THEMIS satellite observations show that location of particle injection and fast plasma-sheet flows for the STEVE events also shifts duskward. Electron injection is stronger and ion injection is weaker for the STEVE events compared to the non-STEVE events. SAID are measured by SuperDARN during the STEVE events, but the non-STEVE events only showed latitudinally wide subauroral polarization streams (SAPS) without SAID. To interpret the observations, Rice Convection Model (RCM) simulations with injection at pre-midnight and midnight have been conducted. The simulations successfully explain the stronger electron injection, weaker ion injection, and formation of SAID for injection at pre-midnight, because injected electrons reach the pre-midnight inner magnetosphere and form a narrower separation between the ion and electron inner boundaries. We suggest that substorms and particle injections extending far duskward away from midnight offer a condition for creating STEVE and SAID due to stronger electron injection to pre-midnight. 

In this study, we using the Van Allen Probes data statistically investigate the features of magnetic dips by the means of superposed epoch analysis. Based on the values of electron and proton plasma betas, we categorize the dips into two types: electron-driven dips and proton-driven dips. The global distributions of dips are obtained. Superposed epoch analysis on two types of magnetic dips suggests the correlation between the magnetic fluctuations and plasma betas. Moreover, the occurrences of butterfly pitch angle distributions of relativistic electrons driven by the magnetic dips are confirmed by the statistical results. Our results reveal the statistical characteristics of magnetic dips and build up the relationship among the magnetic fluctuations and background plasma parameters, indicating the potentially important role of magnetic dips in the dynamics of the inner magnetosphere.

Flux transfer events (FTEs) are commonly observed on the magnetopause as a result of intermittent magnetic reconnection. FTEs are usually accompanied by ionospheric signatures of poleward moving auroral forms (PMAFs), which then decay and are followed by occurrence of polar cap patches, i.e., locally enhanced plasma density structures in the polar cap. The sequence of FTEs, PMAFs, and polar cap (PC) patches represents the dynamic coupling processes between the solar wind, magnetosphere, and ionosphere. In this study, using Multiscale Atmosphere-Geospace Environment (MAGE) model simulations, we investigate the development of FTEs, PMAFs, and PC patches during 18 December 2017 event reported by Hwang et al. [2020] (2019JA027674). The MAGE model consists of the Grid Agnostic MHD with Extended Research Applications (GAMERA) global MHD model of the magnetosphere, Rice Convection Model (RCM) of the ring current model, the Thermosphere-Ionosphere Electrodynamics General Circulation Model (TIEGCM), all coupled via an electrostatic ionospheric potential solver (REMIX). Using results from the GAMERA simulation, we characterize the distribution of reconnection rate on the magnetopause and its variations with solar wind conditions. We identify ionospheric signatures of FTEs using field line tracing. We compare propagation direction and speed of FTEs on the magnetopause and PMAFs in the polar cap. We further compare the longitudinal extent of PMAF and FTEs and track the formation of PC patches and their relationship with PMAFs.  The simulation results will be compared against satellite and ground-based observations such as MMS, all-sky imagers, and SuperDARN radar network. This work provides a multiscale dynamic view of the solar wind-magnetosphere-ionosphere coupling from the perspective of global geospace modeling with the capability to resolve key mesoscale features such as FTEs, PMAFs, and PCs.

During southward IMF Bz period, the convection electric field penetrates into the magnetosphere and the conjugate ionosphere. In the meanwhile, energetic particles in the equatorial magnetosphere gradually form the ring current, which slowly shields the convection electric field from penetrating deep into the inner magnetosphere. The interplay between the penetration and shielding processes forms a highly dynamic environment for the conjugate ionosphere and affects the ionospheric density structures there, such as the equatorial plasma bubble. In this talk, we will present our recent progress on understanding the impact of the penetration and shielding electric fields on the low and mid-latitude ionosphere density structures, including large-scale equatorial ionization anomaly and meso-scale equatorial plasma bubble. Observations from multiple instruments and results from numerical simulations will be presented.