Gamma-ray continuum at
>
10
MeV photon energy yields information on
≳
0.2
– 0.3 GeV/nucleon ions at the Sun. We use the general-purpose Monte Carlo code FLUktuierenden KAskade (FLUKA) to model the ...transport of ions injected into thick and thin target sources, the nuclear processes that give rise to pions and other secondaries and the escape of the resulting photons from the atmosphere. We give examples of photon spectra calculated with a range of different assumptions about the primary ion velocity distribution and the source region. We show that FLUKA gives results for pion decay photon emissivity in agreement with previous treatments. Through the directionality of secondary products, as well as Compton scattering and pair production of photons prior to escaping the Sun, the predicted spectrum depends significantly on the viewing angle. Details of the photon spectrum in the
≈
100
MeV range may constrain the angular distribution of primary ions and the depths at which they interact. We display a set of thick-target spectra produced making various assumptions about the incident ion energy and angular distribution and the viewing angle. If ions are very strongly beamed downward, or ion energies do not extend much above 1 GeV/nucleon, the photon spectrum is highly insensitive to details of the ion distribution. Under the simplest assumptions, flares observed near disc centre should not display significant radiation above 1 GeV photon energy. We give an example application to Fermi Large Area Telescope data from the flare of 12 June 2010.
The Neupert effect is the empirical observation that the time evolution of non-thermal emission (e.g. hard X-rays) is frequently proportional to the time derivative of the thermal emission flux (soft ...X-rays), or, vice versa, that time integrated non-thermal flux is proportional to thermal flux. We analyzed the GOES M2.2 event SOL2011-02-14T17:25, and found that the 212 GHz emission plays quite well the role of the thermal component of the Neupert effect. We show that the maximum of the hard X-ray flux for energies above 50 keV is coincident in time with the time-derivative of the 212 GHz flux, within the uncertainties. The microwave flux density at 15.4 GHz, produced by optically thin gyrosynchrotron mechanism, and hard-X rays above 25 keV mark the typical impulsive phase, and have similar time evolution. On the other hand, the 12 GHz emission is delayed by about 25 seconds with respect of the microwave and hard X-ray peak. We argue that this delay cannot be explained by magnetic trapping of non-thermal electrons. With all the observational evidence, we suggest that the 212 GHz emission is produced by thermal bremsstrahlung, initially in the chromosphere, and shifting to optically thin emission from thehot coronal loops at the end of the gradual phase.
Solar maps at 212 and 405 GHz obtained by the Solar Submillimetric Telescope (SST) show regions of enhanced brightness temperature, which coincide with the location of active regions. A statistical ...study of the radio emission from these active regions was performed for the first time at such high frequencies during 23 days on June and July 2002, when the atmospheric opacity was low. The brightest regions on the maps were chosen for this study, where the brightness excess observed varies from 3 to 20% above quiet Sun levels (i.e., 200-1000 K) at both wavelengths. Sizes of the regions of enhanced emission calculated at half the maximum value were estimated to be between 2' and 7'. These sizes agree with observed sizes of active regions at other wavelengths such as Hα and ultraviolet. An important result is that the flux density spectra of all sources increase toward submillimeter frequencies, yielding flux density spectral index with an average value of 2.0. The flux density of the active region sources were complemented with that from maps at 17 and 34 GHz from the Nobeyama Radio Heliograph. The resulting spectra at all four frequencies were fit considering the flux density to be due to thermal bremsstrahlung from the active region. In the calculations, the source radius was assumed to be the mean of the measured values at 212 and 405 K. The effective temperatures of the radio emitting source, assumed homogeneous, obtained from this fit were 0.6-2.9 × 10^sup 4^ K, for source diameters of 2'-7'.PUBLICATION ABSTRACT
Observations of the Sun with the Atacama Large Millimeter Array have now started, and the thermal infrared will regularly be accessible from the NSF's Daniel K. Inouye Solar Telescope. Motivated by ...the prospect of these new data, and by recent flare observations in the mid infrared, we set out here to model and understand the source of the infrared continuum in flares, and to explore its diagnostic capability for the physical conditions in the flare atmosphere. We use the 1D radiation hydrodynamics code RADYN to calculate mid-infrared continuum emission from model atmospheres undergoing sudden deposition of energy by non-thermal electrons. We identify and characterise the main continuum thermal emission processes relevant to flare intensity enhancement in the mid- to far-infrared (2-200 \(\mu\)m) spectral range as free-free emission on neutrals and ions. We find that the infrared intensity evolution tracks the energy input to within a second, albeit with a lingering intensity enhancement, and provides a very direct indication of the evolution of the atmospheric ionization. The prediction of highly impulsive emission means that, on these timescales, the atmospheric hydrodynamics need not be considered in analysing the mid-IR signatures.
The presence of a solar burst spectral component with flux density increasing with frequency in the sub-terahertz range, spectrally separated from the well-known microwave spectral component, bring ...new possibilities to explore the flaring physical processes, both observational and theoretical. The solar event of 6 December 2006, starting at about 18:30 UT, exhibited a particularly well-defined double spectral structure, with the sub-THz spectral component detected at 212 and 405 GHz by the Solar Submilimeter Telescope (SST) and microwaves (1 – 18 GHz) observed by the Owens Valley Solar Array (OVSA). Emissions obtained by instruments onboard satellites are discussed with emphasis to ultra-violet (UV) obtained by the Transition Region And Coronal Explorer (TRACE), soft X-rays from the Geostationary Operational Environmental Satellites (GOES) and X- and
γ
-rays from the Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The sub-THz impulsive component had its closer temporal counterparts only in the higher energy X- and
γ
-rays ranges. The spatial positions of the centers of emission at 212 GHz for the first flux enhancement were clearly displaced by more than one arc-minute from positions at the following phases. The observed sub-THz fluxes and burst source plasma parameters were difficult to be reconciled with a purely thermal emission component. We discuss possible mechanisms to explain the double spectral components at microwaves and in the THz ranges.
We present an analysis of intermittent processes occurred during the impulsive phase of the flare SOL2012-03-13, using hard X-rays and submillimeter radio data. Intermittency is a key characteristic ...in turbulent plasmas and have been a analyzed recently for hard X-rays data only. Since in a typical flare the same accelerated electron population is believed to produce both hard X-rays and gyrosynchrotron, we compare both time profiles searching for intermittency signatures. For that we define a cross-wavelet power spectrum, that is used to obtain the Local Intermittency Measure or LIM. When greater than three, the square LIM coefficients indicate a local intermittent process. The LIM\(^2\) coefficient distribution in time and scale helps to identify avalanche or cascade energy release processes. We find two different and well separated intermittent behaviors in the submillimeter data: for scales greater than 20 s, a broad distribution during the rising and maximum phases of the emission seems to favor a cascade process; for scales below 1 s, short pulses centered on the peak time, are representative of avalanches. When applying the same analysis to hard X-rays, we find only the scales above 10 s producing a distribution related to a cascade energy fragmentation. Our results suggest that different acceleration mechanisms are responsible for tens of keV and MeV energy ranges of electrons.
We present the first digital CCD images and long-slit spectroscopy of the optical ring nebula around the Wolf–Rayet star θ Mus. The CCD images obtained through narrow-band filters centred at O iii ...and Hα show that the nebula has a filamentary structure, similar to supernova remnants, mainly seen in O iii. A spatial detachment between O iii and Hα images suggests excitation stratification, or multiple rings. An analysis of the physical conditions in the nebula was performed by means of long-slit CCD spectra. The spectral images show that the nebula is of low density and medium excitation. By means of quotients of recombination and collisional spectral line fluxes we determine that the principal excitation mechanism is photoionization. We have determined the electronic temperature and density, and chemical abundances for the oxygen at different sites within the nebula. Nebular chemical abundances are found to be similar to the Galactic ISM, indicating that the nebula is mainly composed of swept up material.
We present new spectroscopy in the optical range and 21-cm H i data covering the Ruprecht 55 (Ru 55) field in the Puppis window where several authors have proposed the existence of one (or two) ...clusters. We have determined new MK spectral types for about 50 stars in the region, finding 43 OB-type stars among them. LS 985 was found to be an O9 V + O9.5 III binary and it is the earliest type of star in our observed sample. We have identified a stellar OB association (Ru 55), which is most likely related to a depletion detected in our H i data, as: (i) they are located at the same distance (6 kpc), within observational errors; (ii) both have similar radial velocities (∼67 km s−1); (iii) current OB stars could have provided the energy needed to blow the cavity; (iv) the dynamical time-scale for the hole buildup matches the age estimated for the earliest OB stars; and (v) LS 985 might be responsible for ionizing the H i cavity inner walls close to it.
Optical depth measurements at 45 and 90 GHz in CASLEO Valle Silva, J.F.; Giménez de Castro, C. Guillermo; Passarelli, Celi ...
Journal of atmospheric and solar-terrestrial physics,
03/2020, Letnik:
199
Journal Article
Recenzirano
The optical depth at 45 and 90 GHz have been obtained from calibration data of two solar radio telescopes located in El Leoncito Astronomical Complex (CASLEO), during the years 2012–2013. The ...observed period and the temporal resolution of the measurements is sufficient to show seasonal patterns. Using the precipitable water vapor content (PWV) obtained with a Sun-photometer for aerosols measurements, it was found a good correlation of PWV with the optical depth in both frequencies. With the water vapor content and data from a meteorological station, the atmosphere over CASLEO was modeled using ATM 2009, an Earth’s atmospheric model. The results of the model allows to evaluate the range of optical depth measurements and to predict the contributions from H2O and O2, the main components of optical depth at these frequencies. It was found a reasonable agreement at 45 GHz when the PWV is lower than 5 mm, but notable discrepancies for the entire range of water vapor content at 90 GHz. An empirical relation is established between the water vapor contribution to the measured optical depth for the observed frequencies. Our results show that CASLEO, despite being a site with 2500 meters high, is appropriate for solar observations in the millimeter range and is still useful for observations in the submillimeter at the epoch of the year with low water vapor content.
•Atmospheric absorption at 45/90 GHz correlates with precipitable water vapor.•Atmospheric models used to predict the contributions of water vapor and oxygen.•Refinement of atmospheric models using instruments operating in several frequencies.
During the rising phase of the August 30, 2002 X1.5 flare a short pulse with a total duration of 8 seconds was observed. Its background-subtracted radio spectrum ranges only from 5 to 12 GHz with a ...maximum flux density of approximately 900 s.f.u. at 7 GHz and a steep optically thin spectral index α ≃ 8. Maximum degree of polarization at 7 GHz is around 5%. The hard X-ray pulse emission above the background in the range of 30–150 keV observed by RHESSI is coincident in time with the microwave observation. Hard X-ray images reveal very compact (∼ 10″) footpoint sources. Below 30 keV, a thermal source is observed.
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