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NASA NTRS2023

Prospects of Detecting Nonthermal Protons in Solar Flares via Lyman Line Spectroscopy: Revisiting the Orrall-Zirker Effect

Solar flares are efficient particle accelerators, with a substantial fraction of the energy released manifesting as nonthermal particles. While the role that nonthermal electrons play in transporting flare energy is well studied, the properties and importance of nonthermal protons are rather less well understood. This is in large part due to the paucity of diagnostics, particularly at the lower-energy (deka-keV) range of nonthermal proton distributions in flares. One means to identify the presence of deka-keV protons is by an effect originally described by Orrall & Zirker. In the Orrall–Zirker effect, nonthermal protons interact with ambient neutral hydrogen, and via charge exchange produce a population of energetic neutral atoms (ENAs) in the chromosphere. These ENAs subsequently produce an extremely redshifted photon in the red wings of hydrogen spectral lines. We revisit predictions of the strength of this effect using modern interaction cross sections, and numerical models capable of self-consistently simulating the flaring nonequilibrium ionization stratification, and the nonthermal proton distribution (and, crucially, their feedback on each other). We synthesize both the thermal and nonthermal emission from Ly α and Ly β, the most promising lines that may exhibit a detectable signal. These new predictions are weaker and more transient than prior estimates, but the effects should be detectable in fortuitous circumstances. We degrade the Ly β emission to the resolution of the Spectral Imaging of the Coronal Environment (SPICE) instrument on board Solar Orbiter, demonstrating that though likely difficult, it should be possible to detect the presence of nonthermal protons in flares observed by SPICE.

Solar flares↗

NASA NTRS2015

Characterization of the Inner Knot of the Crab: the Site of the Gamma-ray Flares?

One of the most intriguing recent discoveries has been the detection of powerful gamma-ray flares from the Crab Nebula. Such events, with a recurrence time of about once per year, can be so dramatic to make the system the brightest source in the gamma‐ray sky as occurred, e.g. in April 2011. These flares challenge our understanding of how pulsar wind nebulae work and defy current astrophysical models for particle acceleration. We present here our study of the inner knot located within a fraction of an arcsecond from the pulsar with the aim of characterizing the feature and asking if this might be the site of the origin of the gamma-ray flares. We took data using Keck, HST, and Chandra obtained as part of our multi‐wavelength campaign to identify the source of the enigmatic flares. We set an upper limit as to the gamma-ray flux from the knot. We also find that the dimensions, surface brightness, flux, etc. of the optical and infrared knot are all correlated with distance from the pulsar. This distance, in turn, varies with time. In addition to this most thorough characterization of the inner knot's properties, we examine the hypothesis that the knot may be the site of the flares by examining the knot separation versus the Fermi/LAT gamma-ray flux. Finally, as part of this research, we make use of a new approach employing singular value decomposition (SVD) for analyzing time series of images and compare the approach to more traditional methods. Our conclusions are only refined but not impacted by using the new approach.

Gamma-ray Flares↗

NASA NTRS2014

The Most Powerful Flaring Activity from the NLSy1 PMN J0948+0022

We report on multifrequency observations performed during 2012 December-2013 August of the first narrow-line Seyfert 1 galaxy detected in gamma rays, PMN J0948+0022 (z equal to 0.5846). A gamma-ray flare was observed by the Large Area Telescope on board Fermi during 2012 December-2013 January, reaching a daily peak flux in the 0.1-100 GeV energy range of (155 plus or minus 31)×10(exp −8) ph cm (exp −2) s (exp −1) on 2013 January 1, corresponding to an apparent isotropic luminosity of approximately 1.5×10)exp 48) erg s(exp −1). The gamma-ray flaring period triggered Swift and VERITAS observations in addition to radio and optical monitoring by OVRO, MOJAVE, and CRTS. A strong flare was observed in optical, UV, and X- rays on 2012 December 30, quasi-simultaneously to the gamma-ray flare, reaching a record flux for this source from optical to gamma rays. VERITAS observations at very high energy (E greater than 100 GeV) during 2013 January 6-17 resulted in an upper limit of F(sub greater than 0.2 TeV) less than 4.0 × 10(exp −12) ph cm(exp −2) s(exp −1). We compared the spectral energy distribution (SED) of the flaring state in 2013 January with that of an intermediate state observed in 2011. The two SEDs, modelled as synchrotron emission and an external Compton scattering of seed photons from a dust torus, can be modelled by changing both the electron distribution parameters and the magnetic field.

powerful↗

NASA NTRS2015

A Unified Computational Model for Solar and Stellar Flares

We present a unified computational framework that can be used to describe impulsive flares on the Sun and on dMe stars. The models assume that the flare impulsive phase is caused by a beam of charged particles that is accelerated in the corona and propagates downward depositing energy and momentum along the way. This rapidly heats the lower stellar atmosphere causing it to explosively expand and dramatically brighten. Our models consist of flux tubes that extend from the sub-photosphere into the corona. We simulate how flare-accelerated charged particles propagate down one-dimensional flux tubes and heat the stellar atmosphere using the Fokker-Planck kinetic theory. Detailed radiative transfer is included so that model predictions can be directly compared with observations. The flux of flare-accelerated particles drives return currents which additionally heat the stellar atmosphere. These effects are also included in our models. We examine the impact of the flare-accelerated particle beams on model solar and dMe stellar atmospheres and perform parameter studies varying the injected particle energy spectra. We find the atmospheric response is strongly dependent on the accelerated particle cutoff energy and spectral index.

radiative transfer↗

NASA NTRS2023

Formation of the Lyman Continuum during Solar Flares

The Lyman continuum (LyC; <911.12 Å) forms at the top of the chromosphere in the quiet Sun, making LyC a powerful tool for probing the chromospheric plasma during solar flares. To understand the effects of nonthermal energy deposition in the chromosphere during flares, we analyzed LyC profiles from a grid of field-aligned radiative-hydrodynamic models generated using the RADYN code as part of the F-CHROMA project. The spectral response of LyC, the temporal evolution of the departure coefficient of hydrogen, b 1 , and the color temperature, T c , in response to a range of nonthermal electron distribution functions, were investigated. The LyC intensity was seen to increase by 4–5.5 orders of magnitude during solar flares, responding most strongly to the nonthermal electron flux of the beam. Generally, b 1 decreased from 10 2 –10 3 to closer to unity during solar flares, indicating a stronger coupling to local conditions, while T c increased from 8–9 to 10–16 kK. T c was found to be approximately equal to the electron temperature of the plasma when b1 was at a minimum. Both optically thick and optically thin components of LyC were found to be in agreement with the interpretation of recent observations. The optically thick layer forms deeper in the chromosphere during a flare compared to quiescent periods, whereas the optically thin layers form at higher altitudes due to chromospheric evaporation, in low-temperature, high-density regions propagating upward. We put these results in the context of current and future missions.

Solar flares↗

DOE OSTI2021

High-energy emission from a magnetar giant flare in the Sculptor galaxy

Magnetars are the most highly magnetized neutron stars in the cosmos (with magnetic field 10 13 –10 15 G). Giant flares from magnetars are rare, short-duration (about 0.1 s) bursts of hard X-rays and soft γ rays. Owing to the limited sensitivity and energy coverage of previous telescopes, no magnetar giant flare has been detected at gigaelectronvolt (GeV) energies. Here, we report the discovery of GeV emission from a magnetar giant flare on 15 April 2020 (refs. 3,4 and A. J. Castro-Tirado et al., manuscript in preparation). The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope detected GeV γ rays from 19 s until 284 s after the initial detection of a signal in the megaelectronvolt (MeV) band. Our analysis shows that these γ rays are spatially associated with the nearby (3.5 megaparsecs) Sculptor galaxy and are unlikely to originate from a cosmological γ-ray burst. Thus, we infer that the γ rays originated with the magnetar giant flare in Sculptor. Furthermore, we suggest that the GeV signal is generated by an ultra-relativistic outflow that first radiates the prompt MeV-band photons, and then deposits its energy far from the stellar magnetosphere. After a propagation delay, the outflow interacts with environmental gas and produces shock waves that accelerate electrons to very high energies; these electrons then emit GeV γ rays as optically thin synchrotron radiation. This observation implies that a relativistic outflow is associated with the magnetar giant flare, and suggests the possibility that magnetars can power some short γ-ray bursts.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2023

Radiative Reconnection-powered TeV Flares from the Black Hole Magnetosphere in M87

Abstract Active galactic nuclei in general, and the supermassive black hole in M87 in particular, show bright and rapid gamma-ray flares up to energies of 100 GeV and above. For M87, the flares show multiwavelength components, and the variability timescale is comparable to the dynamical time of the event horizon, suggesting that the emission may come from a compact region near the nucleus. However, the emission mechanism for these flares is not well understood. Recent high-resolution general-relativistic magnetohydrodynamic simulations show the occurrence of episodic magnetic reconnection events that can power flares near the black hole event horizon. In this work, we analyze the radiative properties of the reconnecting current layer under the extreme plasma conditions applicable to the black hole in M87 from first principles. We show that abundant pair production is expected in the vicinity of the reconnection layer, to the extent that the produced secondary pair plasma dominates the reconnection dynamics. Using analytic estimates backed by two-dimensional particle-in-cell simulations, we demonstrate that in the presence of strong synchrotron cooling, reconnection can produce a hard power-law distribution of pair plasma imprinted in the outgoing synchrotron (up to a few tens of MeV) and the inverse-Compton signal (up to TeV). We produce synthetic radiation spectra from our simulations, which can be directly compared with the results of future multiwavelength observations of M87* flares.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1972

Solar flares and solar wind helium enrichments - July 1965-July 1967.

It has previously been suggested that the very high relative abundances of helium occasionally observed in the solar wind mark the plasma accelerated by major solar flares. To confirm this hypothesis, we have studied the 43 spectra with He/H greater than 15% that were observed among 10,300 spectra collected by Vela 3 between July 1965-July 1967. Six new flare-enhancement events are discussed in this paper. It is concluded that the association of helium enhancements with major flares is real, nonrandom, and very strong. With this study, there are 12 cases of reliable associations between helium enhancements and flares reported in the literature. The general characteristics of these events are discussed. It is found that the flares are typically large and bright (2B or 3B), often they produce cosmic ray protons, and they are widely distributed in solar longitude. A qualitative discussion of some of the possibilities for the source of helium enhanced plasma is presented. It is suggested that the helium enriched plasma may be the piston producing the shock causing the Type II radio emission.

Hirshberg, J.↗

NASA NTRS1973

On the interplanetary shock waves associated with solar flares in the active region McMath no. 9740

The propagation pattern of shock waves emitted by solar flares which occured in the active region McMath No. 9740 during 23 October to 4 November 1968 is discussed. The solar flares were associated with type 2 and 4 radio bursts and with SSC geomagnetic storms. The flares and associated phenomena are summarized and the transmit times between the sun and the earth of the shock waves associated with the flares are shown. It is concluded that the interplanetary magnetic field controls the propagation of shock waves emitted by solar flares. It was also determined that the large scale configuration of the interplanetary magnetic field was not disturbed by the successive propagation of the shock waves.

Sakurai, K.↗

NASA NTRS1973

The extreme ultraviolet spectrum of solar flares

Extreme ultraviolet flare observations are discussed, with particular attention to relative variations of spectral features during flares. An approximately equal enhancement of most chromospheric, transition-zone, and coronal lines indicates that the thermal conduction may be important at temperatures between 100,000 and one million K in the flare plasma. However, enhancements sometimes appear to be somewhat larger for transition-zone lines and for density-sensitive lines arising from metastable levels. EUV lines characteristics of temperatures between 10,000 and 1.5 million K all show a nearly simultaneous rise in impulsive flares, coincident with the hard X-ray burst. Density increases by factors of 5 to 10 in the transition zone are inferred from emission-measure increases and from line ratios for small flares.

Noyes, R. W.↗

NASA NTRS1974

Chromospheric flares and sudden commencements of geomagnetic storms

A catalogue of flare activity was compiled during 1957-1967 (the solar activity cycle). By comparing all reliable SC during this period with chromospheric flares, the following conclusions are drawn; (1) There is no statistically significant correlation between SC and chromospheric flares. (2) The assumption that a shock wave propagates throughout the entire hemisphere is unjustified and contradicts the fact of recurrence of SC. (3) A statistically significant correlation was established between SC and chromospheric flares, that is, a relationship between a SC and the moment that a flare active region transits the Central Meridian. (4) SC are caused by shock waves or tangential discontinuities formed at the western boundary of the quasisteady directed corpuscular flux or at the boundary between sectors.

Nesmyanovich, A. T.↗

NASA NTRS1975

The landing flare: An analysis and flight-test investigation

Results are given of an extensive investigation of conventional landing flares in general aviation type airplanes. A wide range of parameters influencing flare behavior are simulated in experimental landings in a variable-stability Navion. The most important feature of the flare is found to be the airplane's deceleration in the flare. Various effects on this are correlated in terms of the average flare load factor. Piloting technique is extensively discussed. Design criteria are presented.

Seckel, E.↗

NASA NTRS1974

Millimeter radio evidence for containment mechanisms in solar flares

Recent theories of solar flares are reviewed with emphasis on the aspects of pre-flare heating. The heating evident at 3.3-mm wavelength is analyzed in the form of daily maps of the solar disk and synoptic maps compiled from the daily maps. It is found that isotherms defining antenna temperature enhancements of 340 K correspond in shape and location to facular areas reported by Waldmeier. Maximum enhancements occur over sunspots or near neutral lines of the longitudinal magnetic fields which indicates heating associated with chromospheric currents. These enhancements are correlated with flare importance number and are observed to increase during several days preceding flaring. This evidence for a containment mechanism in the chromosphere is collated with current theories of solar flares.

Mayfield, E. B.↗

NASA NTRS1976

Gamma-ray and microwave evidence for two phases of acceleration in solar flares

Relativistic electrons in large solar flares produce gamma ray continuum by bremsstrahlung and microwave emission by gyrosynchrotron radiation. Using observations of the 1972, August 4 flare, the electron spectrum and the physical properties of the common emitting region of these radiations were evaluated. Information was also obtained on energetic protons in this flare by using gamma ray lines. From the electron spectrum, the proton-to-electron ratio, and the time dependences of the microwave emission, the 2.2 MeV line and the gamma ray continuum, it was concluded that in large solar flares relativistic electrons and energetic nuclei are accelerated by a mechanism which is different from the mechanism which accelerates approximately less than 100 keV electrons in flares.

Bai, T.↗

NASA NTRS1975

Ultraviolet emission line profiles of flares and active regions

A preliminary description of ultraviolet spectra of active regions and flares, photographed from Skylab by the Naval Research Laboratory's UV spectrograph, is given. The findings can be summarized as follows: (1) Line profiles of medium-ionized lines (transition-zone lines) show the most pronounced broadenings and shifts in flares and flare-like events; (2) typical full widths at half maximum of these lines correspond to Doppler velocities of 70 km/s; (3) shifts of the same magnitude can be observed; (4) intersystem lines are not broadened or shifted; (5) forbidden coronal lines and intersystem lines become enhanced in the flare spectrum at the moment when the turbulence seen in the allowed transitions disappears; and (6) a very broad line at 1354.2 A, which appears only in flare spectra, seems to be the forbidden transition 3P(1)-3P(0) of Fe XXI.

Brueckner, G. E.↗

NASA NTRS1975

Evidence for X-ray emission from flare stars observed by ANS

Observations that detected the first X-ray emission from flare stars are described. An X-ray flare was detected from YZ CMi at 0.28 keV and approximately 1-7 keV, although no optical or radio coverage was available. During a very large optical flare from UV Ceti, X-ray emission at (only) 0.28 keV was detected. Upper limits for X-ray emission from several small optical flares of UV Ceti are presented. Implications for X-ray flare models, the diffuse X-ray background, and low-energy cosmic-ray flux are mentioned.

Heise, J.↗

NASA NTRS1975

The solar-flare infrared continuum - Observational techniques and upper limits

Exploratory observations at 20 microns and 350 microns have determined detection thresholds for solar flares in these wavelengths. In the 20-micron range, solar atmospheric fluctuations (the 'temperature field') set the basic limits on flare detectability at about 5 K; at 350 microns, extinction in the earth's atmosphere provides the basic limitation of about 30 K. These thresholds are low enough for the successful detection of several infrared-emitting components of large flares. The upper limits obtained for subflares indicate that the thickness of the H-alpha flare region does not exceed approximately 10 km. This result confirms the conclusion of Suemoto and Hiei (1959) regarding the small effective thickness of the H-alpha-emitting regions in solar flares.

Hudson, H. S.↗

NASA NTRS1975

Observations of X-rays from flare stars with ANS

Observations during which X-ray emission from flare stars was detected for the first time, are described. An X-ray flare was detected from YZ CMi at 0.28 keV and 1-7 keV, although no optical or radio coverage was available. During a very large optical flare from UV Ceti, X-ray emission at 0.28 keV only was detected. Upper limits for X-ray emission from several small optical flares of UV Ceti are obtained. Implications for X-ray flare models, the diffuse X-ray background, and low-energy cosmic-ray flux are examined.

Grindlay, J.↗