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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 NTRS1986

Hard X-ray imaging and the relative contribution of thermal and nonthermal emission in flares

The question of whether the impulsive 25 to 100 keV X-ray emission from solar flares is thermal or nonthermal has been a long-standing controversy. Both thermal and nonthermal (beam) models have been developed and applied to the hard X-ray data. It now seems likely that both thermal and nonthermal emission have been observed at hard X-ray energies. The Hinotori classification scheme, for example, is an attempt to associate the thermal-nonthermal characteristics of flare hard X-ray emission with other flare properties. From a theoretical point of view, it is difficult to generate energetic, nonthermal electrons without dumping an equal or greater amount of energy into plasma heating. On the other hand, any impulsive heating process will invariably generate at least some nonthermal particles. Hence, strictly speaking, although thermal or nonthermal emission may dominate the hard X-ray emission in a given energy range for a given flare, there is no such thing as a purely thermal or nonthermal flare mechanism.

Holman, G. D.↗
NASA NTRS2007

Motion of 3-6 keV Nonthermal Sources Along the Legs of a Flare Loop

Observations of nonthermal X-ray sources me critical to studying electron acceleration and transport in solar flares. Strong thermal emission radiated from the preheated plasma before the flare impulsive phase often makes it difficult to detect low-energy X-ray sources that are produced by relatively low-energy nonthermal electrons. Knowledge of the distribution of these low-energy nonthermal electrons is particularly important in determining the total nonthermal electron energy in solar flares. We report on an 'early impulsive flare' in which impulsive hard X-ray emission was seen early in the flare before the soft X-ray emission had risen significantly, indicating limited plasma pre-heating. Early in the flare, RHESSI < 25 keV images show coronal sources that moved first downward and then upwards along the legs of a flare loop. In particular, the 3-6 keV source appeared as a single coronal source at the start of the flare, and then it involved into two coronal sources moving down along the two legs of the loop. After nearly reaching the two footpoints at the hard X-ray peak, the two sources moved back up to the looptop again. RHESSI images and light curves all indicate that nonthermal emission dominated at energies as low as 3-6 keV. We suggest that the evolution of both the spectral index and the low-energy cutoff of the injected electron distribution could result in the accelerated electrons reaching a lower altitude along the legs of the dense flare loop and hence result in the observed downward and upward motions of the nonthermal sources.

Sui, Linhui↗
NASA NTRS2016

Global Energetics of Solar Flares: Nonthermal Energies - Part III

This study entails the third part of a global flare energetics project, in which Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) data of 191 M and X-class flare events from the first 3.5 years of the Solar Dynamics Observatory mission are analyzed. We fit a thermal and a nonthermal component to RHESSI spectra, yielding the temperature of the differential emission measure (DEM) tail, the nonthermal power-law slope and flux, and the thermal nonthermal cross-over energy eco. From these parameters, we calculate the total nonthermal energy E(sub nt) in electrons with two different methods: (1) using the observed cross-over energy e(sub co) as low-energy cutoff, and (2) using the low-energy cut off e(sub wt) predicted by the warm thick-target bremsstrahlung model of Kontar et al. Based on a mean temperature of T(sub e) = 8.6 MK in active regions, we find low-energy cutoff energies of e(sub wt) = 6.2 +/-1.6 keV for the warm-target model, which is significantly lower than the cross-over energies e(sub co) = 21 +/- 6 keV. Comparing with the statistics of magnetically dissipated energies E(sub mag) and thermal energies E(sub th) from the two previous studies, we find the following mean (logarithmic) energy ratios with the warm-target model: E(sub nt) = 0.41E(sub mag), E(sub th) = 0.08 E(sub mag), and E(sub th) = 0.15 E(sub nt). The total dissipated magnetic energy exceeds the thermal energy in 95% and the nonthermal energy in 71% of the flare events, which confirms that magnetic reconnection processes are sufficient to explain flare energies. The nonthermal energy exceeds the thermal energy in 85% of the events, which largely confirms the warm thick-target model.

Aschwanden, Markus J.↗
NASA NTRS2025

Nonthermal Velocities in a Solar Active Region Observed by SERTS

We present measurements of coronal nonthermal Doppler velocities in NOAA Active Region 7870 observed by the Solar EUV Research Telescope and Spectrograph during its 1995 sounding rocket flight (SERTS-95). The instrument included a multilayer-coated toroidal diffraction grating that enhanced its sensitivity in second order at wavelengths between about 171 and 225 Å. Analyses of spectra from this flight were previously published but did not address the issue of nonthermal velocities; we do so here for spectra averaged over the 123װ .8 segment of the slit for which the brightest portion of the region observed by SERTS was recorded. All emission lines are fit with Gaussian profiles. With post-flight laboratory spectra of He ii and Ne II-IIINe we derive a first-order instrumental width (full width at half-maximum intensity) of Finst,1 = 50.6 ± 3.1 mÅ and a second-order instrumental width of Finst,2 = 25.3 ±1.5 mÅ. For emission lines of Fe x−Fe xv, formed at 6.05 ≤ log T(K) ≤ 6.35, we find nonthermal velocities between 26.1 and 35.0 km s −1 , independent of temperature; this range corresponds to Vnonth = 30.6 ± 4.5 km s −1 . For the two unblended Fe xii lines at 192.394 and 193.509 Å closest to the maximum sensitivity of SERTS-95, we have Vnonth = 33.7 ± 1.3 km s −1 . When solar observations are used to derive or confirm instrumental widths for spectrometers in orbit, those observations should compensate for nonthermal velocities such as the ones reported here in order to extract instrumental widths that are free of nonthermal broadening.

Jeffrey W Brosius↗
NASA NTRS1973

Thermal and nonthermal X-ray bursts observed from OSO-7

The UCSD solar X-ray experiment on the OSO-7 observes X-rays in the 2 to 15 keV range with a proportional counter and in the 10.6 to 300 keV range with a scintillation counter. The dynamic range and instrument parameters are chosen to separate X-ray emission due to a hot thermal plasma from that due to nonthermal electron populations for a large number of X-ray events associated with small flares. Many different morphological classes have been observed in the 200 or so principal events observed between October 1971 and June 1972. For events which have both a thermal and nonthermal component, we can perform an analysis which shows that in not all events is it possible for the nonthermal electrons to energize the thermal plasma. We observe many events which have no nonthermal component, at least one event which was purely power law on initiation, and several events which had a long, slow decay of the hard X-rays.

Peterson, L. E.↗
NASA NTRS1993

Acceleration efficiency in nonthermal sources and the soft gamma rays from NGC 4151 observed by OSSE and SIGMA

We show that the recent observations of the Seyfert galaxy NGC 4151 in hard X-rays and soft gamma rays by the OSSE and SIGMA detectors on board CGRO and GRANAT, respectively, are well explained by a nonthermal model with acceleration of relativistic electrons at an efficiency of less than 50 percent and with the remaining power dissipated thermally in the source (the standard nonthermal e(+/-) pair model assumed 100 percent efficiency). Such an acceleration efficiency is generally expected on physical grounds. The resulting model unifies previously proposed purely thermal and purely nonthermal models. The pure nonthermal model for NGC 4151 appears to be ruled out. The pure thermal model gives a worse fit to the data than our hybrid nonthermal/thermal model.

Zdziarski, Andrzej A.↗
NASA NTRS1994

Nonthermal processes in flaring X-ray-bright points

X-ray-bright point (XBPs) are known to show variability on a number of timescales, including impulsive X-ray brightenings. The relationship between these XBP 'flares' and normal solar flares is poorly known. A fundamental question is whether nonthermal acceleration of particles takes place in XBP flares. We address this issue by searching for nonthermal radio emission at metric wavelengths from flaring XBPs identified in Yohkoh soft x-ray telescope (SXT) data. Unequivocal evidence for type III-like radio bursts, usually attributed to beams of nonthermal electrons on open field lines, is found. This suggests that XBP flares are similar to normal flares and can indeed accelerate nonthermal populations of energetic particles.

Kundu, M. R.↗
NASA NTRS1999

A Study of Nonthermal X-Ray and Radio Emission from the O Star 9 Sgr

The observed X-ray and highly variable nonthermal radio emission from OB stars has eluded explanation for more than 18 years. The most favorable model of X-ray production in these stars (shocks) predicts both nonthermal radio and X-ray emission. The nonthermal X-ray emission should occur above 2 keV and the variability of this X-ray component should also be comparable to the observed radio variability. To test this scenario, we proposed an ASC/VLA monitoring program to observe the OB star, 9 Sgr, a well known nonthermal, variable radio source and a strong X-ray source. We requested 625 ks ASCA observations with a temporal spacing of approximately 4 days which corresponds to the time required for a density disturbance to propagate to the 6 cm radio free-free photosphere. The X-ray observations were coordinated with 5 multi-wavelength VLA observations. These observations represent the first systematic attempt to investigate the relationship between the X-ray and radio emission in OB stars.

Waldron, Wayne L.↗
NASA NTRS2002

Observation of Nonthermal Emission from the Supernova Remnant IC443 with RXTE

In this paper we present analysis of X-ray spectra from the supernova remnant IC443 obtained using the PCA on RXTE. The spectra in the 3 - 20 keV band are well fit by a two-component model consisting of thermal and nonthermal components. We compare these results with recent results of other X-ray missions and discuss the need for a cut-off in the nonthermal spectrum. Recent Chandra and XMM-Newton observations suggest that much of the nonthermal emission from IC443 can be attributed to a pulsar wind nebula. We present the results of our search for periodic emission in the RXTE PCA data. We then discuss the origin o f the nonthermal component and its possible association with the unidentified EGRET source.

Sturner, S. J.↗
NASA NTRS2007

Nonthermal X-ray Spectral Flattening toward Low Energies in Early Impulsive Flares

The determination of the low-energy cutoff to nonthermal electron distributions is critical to the calculation of the nonthermal energy in solar flares. The most direct evidence for low-energy cutoffs is flattening of the power-law, nontherma1 X-ray spectra at low energies. However, because of the plasma preheating often seen in flares, the thermal emissions at low energies may hide such spectral flattening of the nonthermal component. We select a category of flares, which we call "early impulsive flares", in which the > 25 keV hard X-ray (HXR) flux increase is delayed by less than 30 s after the flux increase at lower energies. Thus, the plasma preheating in these flares is minimal, so the nonthermal spectrum can be determined to lower energies than in flares with significant preheating. Out of a sample of 33 early impulsive flares observed by the Ramaty High Energy Solar Spectroscopy Imager (RHESSI), 9 showed spectral flattening toward low energies. In these events, the break energy of the double power-law fit to the HXR spectra lies in the range of 10-50 keV, significantly lower than the value we have seen for other flares that do not show such early impulsive emissions. In particular, it correlates with the HXR flux. After correcting the spatially-integrated spectra for albedo from isotropically emitted X-rays and using RHESSI imaging spectroscopy to exclude the extended albedo halo, we find that albedo associated with isotropic or nearly isotropic electrons can only account for the spectral flattening in 3 flares near Sun center. The spectral flattening in the remaining 6 flares is found to be consistent with the existence of a low-energy cutoff in the electron spectrum, falling in the range of 15-50 keV, which also correlates with the HXR flux.

Holman, Gordon D.↗
NASA NTRS1973

Characteristics of nonthermal electrons accelerated during the flash phase of small solar flares

Observations of impulsive hard X-rays and other flash phase emissions from small solar flares are analyzed in order to determine the characteristics of energetic electrons accelerated during the flash phase. The electron spectrum and its time variation are deduced from a model of the X-ray source in which the electron injection is continuous, and the electron energy loss is primarily due to collisions with the ambient plasma and escape into the corona. The instantaneous electron spectrum in the X-ray source, as well as the acceleration spectrum, are found to be nonthermal. The observations are consistent with a flare model having the following properties: (1) the acceleration region is located in the lower corona where the ion density is less than 10 to the 9th power per cubic centimeter, (2) the total kinetic energy of the nonthermal electron is approximately 10 percent of the total flare energy, so that the efficiency of the acceleration process is very high, (3) the nonthermal electrons (and protons) provide energy for all flash phase emissions, (4) the acceleration is a continuous process with a time constant less than 1 second, or a continuous series of impulses each lasting less than 1 second and a total duration of approximately 100 seconds, (5) the electron spectrum continuously hardens during the increasing phase of the X-ray burst and softens during the decreasing phase and, (6) the more energetic flares to not necessarily produce a harder electron spectrum.

Kane, S. R.↗
NASA NTRS1976

Lyman-alpha emission from nonthermal proton beams

Nonthermal fast protons penetrating an atmosphere containing neutral hydrogen will produce some nonthermal fast neutrals which will radiate Doppler-shifted photons. The hydrogen line profiles observed from such an atmosphere will thus have nonthermal, partially polarized wings that contain information on the flux, energy spectrum, and direction of the incident proton beam. This paper develops the theory of this effect and applies it to proton beams from impulsive solar flares impacting on the sun's atmosphere. Calculations of the L-alpha profile from the region of impact have been made for the Vernazza-Avrett-Loeser solar atmosphere assuming proton energy fluxes and power-law spectra similar to those inferred for the electron beams believed responsible for hard X-ray bursts. The resulting profiles show that the effect should be detectable and that it could serve as a diagnostic for flare protons near their place of origin on the sun.

Orrall, F. Q.↗
NASA NTRS1979

On the nonthermal excitation and polarization of X-ray lines during solar flares

The energy distributions of nonthermal electrons are derived from hard X-ray spectra taken during the impulsive phase of two 2B flares in February 1969. They are used to calculate the fluxes of nonthermally excited X-ray lines of hydrogen-like and helium-like ions. These fluxes are compared to the total line fluxes observed at the same time with crystal spectrometers. The nonthermal excitation is found to give only small contributions to the total line intensities. This implies that the impact polarization which is to be expected for anisotropic velocity distributions of the energetic electrons, will be low. Nevertheless it should be feasible to detect line polarization during the impulsive phase of strong X-ray flares.

Haug, E.↗
NASA NTRS1985

Observations of Nonthermal Radio Emission from Early-type Stars

As a part of a wider survey of radio emission from O, B, and Wolf-Rayet (WR) stars, five new stars whose radio emission is dominated by a nonthermal mechanism of unknown origin were discovered. From statistics of distance-limited samples of stars, it is estimated that the minimum fraction of stars which are nonthermal emitters is 25% for the OB stars and 10% for the WR stars. The characteristics of this new class of nonthermal radio emitter are investigated.

Abbott, D. C.↗
NASA NTRS1987

G0.18-0.04 - Interaction of thermal and nonthermal radio structures in the arc near the galactic center

Using a high-resolution radio-continuum image made at a frequency of 4.8 GHz, the radio source G0.18-0.04 located at the intersection of the galactic center Arc with the galactic plane is discussed. In this direction, the radio Arc is composed of a set of linear, nonthermal filamentary structures superimposed on a narrow, sickle-shaped, thermal feature which crosses them. The radio image suggests a physical interaction between the thermal and nonthermal structures. It is argued that the magnetic field is dynamically important and that its strength is estimated to be very large (0.001 G). Furthermore, it is suggested that the ionization of G0.18-0.04 is caused by collisional ionization rather than ultraviolet photons from massive stars. The discussion presents possible ways in which some of the energy of the relativistc particles or the magnetic field in the nonthermal filaments might be extracted to ionize ambient gas in the galactic plane.

Yusef-Zadeh, Farhad↗
NASA NTRS1989

Hydrogen and deuterium loss from the terrestrial atmosphere - A quantitative assessment of nonthermal escape fluxes

A comprehensive one-dimensional photochemical model extending from the middle atmosphere (50 km) to the exobase (432 km) has been used to study the escape of hydrogen and deuterium from the earth's atmosphere. The model incorporates recent advances in chemical kinetics as well as atmospheric observations by satellites, especially the Atmosphere Explorer C satellite. The results suggest that the escape fluxes of both H and D are limited by the upward transport of total hydrogen and total deuterium at the homopause. About one fourth of total hydrogen escape is thermal, the rest being nonthermal. It is shown that escape of D is nonthermal and that charge exchange and polar wind are important mechanisms for the nonthermal escape of H and D.

Yung, Yuk L.↗
NASA NTRS1990

Comparison of thermal and nonthermal hard X-ray emission in electron-heated solar flares

Direct nonthermal bremsstrahlung and electronbeam-energized hot plasma thermal bremsstrahlung calculations are presently conducted on the basis of numerical simulation results for solar atmospheric response to heating during solar flares by nonthermal electron beams. It is established that while emission is dominated by the thermal component below a certain 'cross-over' energy, the direct brehmsstrahlung component becomes more important at higher energies. The dependence of the cross-over energy on the parameters of the electron energy input is also investigated, and the minimum photon energy which should be observable with future instruments in order that the nonthermal component not be swamped by the self-consistent thermal counterpart created by beam heating.

Li, Peng↗