1,607 research outputs found
Charge-exchange limits on low-energy α-particle fluxes in solar flares
This paper reports on a search for flare emission via charge-exchange radiation in the wings of the Lyα line of He II at 304 Ă
, as originally suggested for hydrogen by Orrall and Zirker. Via this mechanism a primary α particle that penetrates into the neutral chromosphere can pick up an atomic electron and emit in the He II bound-bound spectrum before it stops. The Extreme-ultraviolet Variability Experiment on board the Solar Dynamics Observatory gives us our first chance to search for this effect systematically. The Orrall-Zirker mechanism has great importance for flare physics because of the essential roles that particle acceleration plays; this mechanism is one of the few proposed that would allow remote sensing of primary accelerated particles below a few MeV nucleon<sup>â1</sup>. We study 10 events in total, including the Îł-ray events SOL2010-06-12 (M2.0) and SOL2011-02-24 (M3.5) (the latter a limb flare), seven X-class flares, and one prominent M-class event that produced solar energetic particles. The absence of charge-exchange line wings may point to a need for more complete theoretical work. Some of the events do have broadband signatures, which could correspond to continua from other origins, but these do not have the spectral signatures expected from the Orrall-Zirker mechanism
The Absolute Abundance of Iron in the Solar Corona
We present a measurement of the abundance of Fe relative to H in the solar
corona using a technique which differs from previous spectroscopic and solar
wind measurements. Our method combines EUV line data from the CDS spectrometer
on SOHO with thermal bremsstrahlung radio data from the VLA. The coronal Fe
abundance is derived by equating the thermal bremsstrahlung radio emission
calculated from the EUV Fe line data to that observed with the VLA, treating
the Fe/H abundance as the sole unknown. We apply this technique to a compact
cool active region and find Fe/H
= 1.56 x 10^{-4}, or about 4 times its value in the solar photosphere.
Uncertainties in the CDS radiometric calibration, the VLA intensity
measurements, the atomic parameters, and the assumptions made in the spectral
analysis yield net uncertainties of order 20%. This result implies that low
first ionization potential elements such as Fe are enhanced in the solar corona
relative to photospheric values.Comment: Astrophysical Journal Letters, in pres
Investigating controls over methane production and bubbling from Interior Alaskan lakes using stable isotopes and radiocarbon ages
Thesis (M.S.) University of Alaska Fairbanks, 2010"Large uncertainties in first-order estimates of the magnitude of CHâ emissions from lakes (global lakes: 8-48 Tg CHâ yrâ»Âč Bastviken et al. 2004) result from variation in ebullition (bubbling) rates between and within lakes. Based on a comparison of two interior Alaska thermokarst lakes, I suggest that variation in CHâ ebullition observed within and between lakes can be explained by a few key differences in substrate quality and sediment density. Killarney Lake, which has a 130 cm-thick modern sediment package, emitted 120 mg CHâ Mâ»ÂČ dayâ»Âč produced from a mixture of modern C and permafrost C sources, while Goldstream Lake, a younger lake with only 2-5 cm of modern lake sediment, emitted more CHâ (183 mg CHâ Mâ»ÂČ dayâ»Âč) produced mostly from thawed permafrost. Incubated thawed permafrost supported production of substantially more CHâ (0.25 ± 0.04 mg CHâ g TCâ»Âč dâ»Âč) than did taberal lake sediments (0.08 ± 0.02 mg CHâ g TCâ»Âč dâ»Âč). Together, these lines of evidence support the importance of permafrost C availability as control on CHâ production and bubbling in thermokarst lakes. Stable isotope and radiocarbon values of contemporary interior Alaska thermokarst lake CHâ emissions reported in this study could help constrain contributions of thermokarst lakes to the global atmospheric CHâ budget. I show here that methanogens in close proximity to thermokarst utilized pore water derived from melted permafrost ice as a hydrogen source, and that [delta]DCHâ values reflected ancient [delta]D of precipitation. [Delta]DCHâ values from Alaskan thermokarst lakes were less-depleted than [delta]DCHâ values from Siberian lakes. Thus, thermokarst lake contributions to early Holocene atmospheric CHâ concentrations were likely higher than originally thought"--Leaf iiiAlaska ESPSoR, Center for Global Change Global Change Grants, Institute of Northern Engineering, Institute of Arctic Biology, IARC, DOE #DE- NT0005665, and NSF grants IPY #0732735 and OPP #06322641. Introduction and overview -- 1.1. Introduction -- 1.2. The interior environment -- Permafrost -- Organic matter inputs to Alaskan lake sediments -- Vegetation -- Study sites -- 1.3. Methanogenesis -- Physical and biological controls -- Pathway controls -- Methane oxidation -- 1.4. Stable isotopes -- Carbon isotopes -- Hydrogen isotopes -- 1.5. CHâ bubbling in northern lakes -- 1.6. Conclusion -- References -- Tables -- 2. A comparison of CHâ production and bubbling from two interior Alaskan thermokarst lakes -- Abstract -- 2.1. Introduction -- 2.2. Methods -- Physiography of study area -- Study lakes -- Sample collection and analysis -- Geophysics -- Anaerobic laboratory incubation -- Calculations -- 2.3. Results -- Whole-lake CHâ production -- Bubble fluxes and composition -- Production pathway -- Anaerobic incubation results -- Permafrost and sediment characteristics -- Geophysics -- Limnology -- 2.4. Discussion -- Temperature and production pathway -- Bubble gas composition variation -- Whole-lake CHâ production -- 2.5. Conclusion -- Acknowledgements -- References -- Figures -- Tables -- 3. Implications of [delta]DCHâ from Alaskan thermokarst lakes for past and present atmospheric CHâ budgets -- Abstract -- 3.1. Introduction -- 3.2. Methods -- Study site -- Sample collection and analysis -- Calculations -- 3.3. Results -- Bubble isotopic and elemental composition -- Water isotopes and H mixing model -- 3.4. Discussion -- 3.5. Conclusion -- Acknowledgements -- References -- Figures -- Tables -- Appendix
Flows in the solar atmosphere due to the eruptions on the 15th July, 2002
<p>Which kind of flows are present during flares? Are they compatible with the present understanding of energy release and which model best describes the observations? We analyze successive flare events in order to answer these questions. The flares were observed in the magnetically complex NOAA active region (AR) 10030 on 15 July 2002. One of them is of GOES X-class. The description of these flares and how they relate to the break-out model is presented in Gary & Moore (2004). The Coronal Diagnostic Spectrometer on board SOHO observed this active region for around 14 h. The observed emission lines provided data from the transition region to the corona with a field of view covering more than half of the active region. In this paper we analyse the spatially resolved flows seen in the atmosphere from the preflare to the flare stages. We find evidence for evaporation occurring before the impulsive phase. During the main phase, the ongoing magnetic reconnection is demonstrated by upflows located at the edges of the flare loops (while downflows are found in the flare loops themselves). We also report the impact of a filament eruption on the atmosphere, with flows up to 300 km s<sup>-1</sup> observed at transition-region temperatures in regions well away from the location of the pre-eruptive filament. Our results are consistent with the predictions of the break out model before the impulsive phase of the flare; while, as the flare progresses, the directions of the flows are consistent with flare models invoking evaporation followed by cooling and downward plasma motions in the flare loops.</p>
Hydrogen H line polarization in solar flares. Theoretical investigation of atomic polarization by proton beams considering self-consistent NLTE polarized radiative transfer
Context. We present a theoretical review of the effect of impact polarization
of a hydrogen H line due to an expected proton beam bombardment in
solar flares. Aims. Several observations indicate the presence of the linear
polarization of the hydrogen H line observed near the solar limb above
5% and preferentially in the radial direction. We theoretically review the
problem of deceleration of the beam originating in the coronal reconnection
site due to its interaction with the chromospheric plasma, and describe the
formalism of the density matrix used in our description of the atomic processes
and the treatment of collisional rates. Methods. We solve the self-consistent
NLTE radiation transfer problem for the particular semiempirical chromosphere
models for both intensity and linear polarization components of the radiation
field. Results. In contrast to recent calculations, our results show that the
energy distribution of the proton beam at H formation levels and
depolarizing collisions by background electrons and protons cause a significant
reduction of the effect below 0.1%. The radiation transfer solution shows that
tangential resonance-scattering polarization dominates over the impact
polarization effect in all considered models. Conclusions. In the models
studied, proton beams are unlikely to be a satisfying explanation for the
observed linear polarization of the H line.Comment: 11 pages, 11 figures, accepted for publication in A&
Fe XI emission lines in a high resolution extreme ultraviolet spectrum obtained by SERTS
New calculations of radiative rates and electron impact excitation cross
sections for Fe XI are used to derive emission line intensity ratios involving
3s^23p^4 - 3s^23p^33d transitions in the 180-223 A wavelength range. These
ratios are subsequently compared with observations of a solar active region,
obtained during the 1995 flight Solar EUV Research Telescope and Spectrograph
(SERTS). The version of SERTS flown in 1995 incorporated a multilayer grating
that enhanced the instrumental sensitivity for features in the 170 - 225 A
wavelength range, observed in second-order between 340 and 450 A. This
enhancement led to the detection of many emission lines not seen on previous
SERTS flights, which were measured with the highest spectral resolution (0.03
A) ever achieved for spatially resolved active region spectra in this
wavelength range. However, even at this high spectral resolution, several of
the Fe XI lines are found to be blended, although the sources of the blends are
identified in the majority of cases. The most useful Fe XI electron density
diagnostic line intensity ratio is I(184.80 A)/I(188.21 A). This ratio involves
lines close in wavelength and free from blends, and which varies by a factor of
11.7 between N_e = 10^9 and 10^11 cm^-3, yet shows little temperature
sensitivity. An unknown line in the SERTS spectrum at 189.00 A is found to be
due to Fe XI, the first time (to our knowledge) this feature has been
identified in the solar spectrum. Similarly, there are new identifications of
the Fe XI 192.88, 198.56 and 202.42 A features, although the latter two are
blended with S VIII/Fe XII and Fe XIII, respectively.Comment: 21 pages, 9 gigures, accepted for publication in the Astrophysical
Journa
Extreme Ultra-Violet Spectroscopy of the Lower Solar Atmosphere During Solar Flares
The extreme ultraviolet portion of the solar spectrum contains a wealth of
diagnostic tools for probing the lower solar atmosphere in response to an
injection of energy, particularly during the impulsive phase of solar flares.
These include temperature and density sensitive line ratios, Doppler shifted
emission lines and nonthermal broadening, abundance measurements, differential
emission measure profiles, and continuum temperatures and energetics, among
others. In this paper I shall review some of the advances made in recent years
using these techniques, focusing primarily on studies that have utilized data
from Hinode/EIS and SDO/EVE, while also providing some historical background
and a summary of future spectroscopic instrumentation.Comment: 34 pages, 8 figures. Submitted to Solar Physics as part of the
Topical Issue on Solar and Stellar Flare
Observation and Modeling of the Solar Transition Region: II. Solutions of the Quasi-Static Loop Model
In the present work we undertake a study of the quasi-static loop model and
the observational consequences of the various solutions found. We obtain the
most general solutions consistent with certain initial conditions. Great care
is exercised in choosing these conditions to be physically plausible (motivated
by observations). We show that the assumptions of previous quasi-static loop
models, such as the models of Rosner, Tucker and Vaiana (1978) and Veseckey,
Antiochos and Underwood (1979), are not necessarily valid for small loops at
transition region temperatures. We find three general classes of solutions for
the quasi-static loop model, which we denote, radiation dominated loops,
conduction dominated loops and classical loops. These solutions are then
compared with observations. Departures from the classical scaling law of RTV
are found for the solutions obtained. It is shown that loops of the type that
we model here can make a significant contribution to lower transition region
emission via thermal conduction from the upper transition region.Comment: 30 pages, 3 figures, Submitted to ApJ, Microsoft Word File 6.0/9
Prospects of Detecting Non-thermal 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 non-thermal particles. While the role
that non-thermal electrons play in transporting flare energy is well studied,
the properties and importance of non-thermal protons is rather less well
understood. This is in large part due to the paucity of diagnostics,
particularly at the lower-energy (deka-keV) range of non-thermal proton
distributions in flares. One means to identify the presence of deka-keV protons
is by an effect originally described by \cite{1976ApJ...208..618O}. In the
Orrall-Zirker effect, non-thermal 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 non-equilibrium ionization stratification, and the non-thermal
proton distribution (and, crucially, their feedback on each other). We
synthesize both the thermal and non-thermal emission from \lya\ and \lyb, the
most promising lines that may exhibit a detectable signal. These new
predictions are are weaker and more transient than prior estimates, but the
effects should be detectable in fortuitous circumstances. We degrade the \lyb\
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 non-thermal protons
in flares observed by SPICE.Comment: Accepted for publication in The Astrophysical Journa
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