C. Guillermo Giménez de Castro

New telescopes for ground-based solar observations at submillimeter and mid-infrared

The solar submillimeter-wave telescope (SST) is the only one of its kind dedicated to solar continuous observations. Two radiometers at 0.740 mm (405 GHz), and four at 1.415 mm (212 GHz) are placed in the Cassegrain focal plane of the 1.5-m dish at El Leoncito high altitude site, San Juan, Argentina. The aperture efficiencies are close to design predictions: 20% and 35% for 2 and 4 arcminutes beam sizes at 405 and 212 GHz, respectively. The positioner absolute pointing accuracy is 10 arcseconds. Spectral coverage is complemented by ground-based mid-infrared telescopes developed for high cadence observations in the continuum 10 micron band (30 THz), using small apertures and room-temperature microbolometer cameras. Using the system, a new solar burst spectral component was discovered, exhibiting fluxes increasing for smaller wavelengths, separated from the well known microwave component. Rapid sub-second pulsations are common for all bursts. The pulsations onset times of appear to be connected to the launch times of CMEs. Active regions are brighter for shorter submillimeter-waves. Mid-IR bright regions are found closely associated with calcium plages and magnetic structures near the solar photosphere. Intense and rapid 10 micron brightening was detected on active centers in association with weak flares. These results raise challenging difficulties for interpretation.

Sub-terahertz, Microwaves and High Energy Emissions During the 6 December 2006 Flare, at 18:40 UT

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 18:30xa0UT, exhibited a particularly well-defined double spectral structure, with the sub-THz spectral component detected at 212 and 405xa0GHz by the Solar Submilimeter Telescope (SST) and microwaves (1u2009–u200918xa0GHz) 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 212xa0GHz 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.

Rapid Pulsations in Sub-THz Solar Bursts

A new solar burst emission spectral component has been found showing sub-THz fluxes increasing with frequency, spectrally separated from the well known microwave component. Rapid pulsations are found present in all events observed at the two frequencies of the solar submillimeter-wave telescope: 212 and 405 GHz. They were studied in greater detail for three solar bursts exhibiting the new THz spectral component. The pulse amplitudes are of about 5%-8% of the mean flux throughout the bursts durations, being comparable for both frequencies. Pulsations range from one pulse every few seconds to 8-10 per second. The pulse repetition rates (R) are linearly proportional to the mean burst fluxes (S), following the simple relationship S = kR, suggesting that the pulsations might be the response to discrete flare particle accelerator injections quantized in energy. Although this result is consistent with qualitative trends previously found in the GHz range, the pulse amplitude relative to the mean fluxes at the sub-THz frequencies appear to be nearly ten times smaller than expected from the extrapolation of the trends found in the GHz range. However there are difficulties to reconcile the nearly simultaneous GHz and THz burst emission spectrally separated components, exhibiting rapid pulsations with considerably larger relative intensities in the GHz range.

Launch of solar coronal mass ejections and submillimeter pulse bursts

[1]xa0The rapid solar spikes (100–500 ms) recently discovered at submillimeter waves bring new possibilities to investigate energetic processes near the solar surface that might have an important role in the launch and propelling of ionized mass away from the Sun. We present a study on the association between the launch time of coronal mass ejections (CMEs) observed by the LASCO instruments on the SOHO spacecraft and the onset of the new kind of rapid solar spikes (100–500 ms) observed at submillimetric waves (212 and 405 GHz) by the new Solar Submm-wave Telescope (SST). We investigated six submm-wave events, all found associated to CMEs. Seven related CME were identified. Five of them were associated with flares with large GOES class soft X-rays, presenting distinct time histories and associations at other energy ranges, and two of them were related to flares behind the solar limb, with simultaneous related activity observed in the visible solar disk. Ultraviolet images from EIT on SOHO show some kind of small or large-scale magnetic activity or brightening for all events. The extrapolation of apparent CME positions to the solar surface show that they occurred nearly coincident in time with the onset of submm-wave pulses for all six events. These results suggest that pulse bursts might be representative of an important early signature of CMEs, especially for events beginning near the center of the solar disk, sometimes identified as “halo” CMEs. They lead to several challenging questions relative to the physical nature of the pulses and its association to the launch and acceleration of coronal mass ejections. Although these evidences may favor multiple rapid energy releases at the origin near the solar surface, they require further research in order to better understand both diagnostics and model descriptions.

Nighttime sensitivity of ionospheric VLF measurements to X‐ray bursts from a remote cosmic source

On 22 January 2009, a series of X-ray bursts were emitted by the soft gamma ray repeater SGR J1550-5418. Some of these bursts produced enhanced ionization in the nighttime lower ionosphere. These ionospheric disturbances were studied using X-ray measurements from the Anti-Coincidence Shield of the Spectrometer for Integral onboard the International Gamma-Ray Astrophysics Laboratory and simultaneous phase and amplitude records from two VLF propagation paths between the transmitter Naval Radio Station, Pearl Harbor (Hawaii) and the receivers Radio Observatorio do Itapetinga (Brazil) and Estacao Antarctica Commandante Ferraz (Antarctic Peninsula). The VLF measurements have been obtained with an unprecedented high time resolution of 20u2009ms. We find that the illumination factor I (illuminated path length times the cosine of the zenith angle), which characterizes the propagation paths underlying the flaring object, is a key parameter which determines the sensitivity threshold of the VLF detection of X-ray bursts from nonsolar transients. For the present VLF measurements of bursts from SGR J1550-5418, it is found that for Iu2009≥u20091.8u2009Mm, all X-ray bursts with fluence in the 25u2009keV to 2u2009MeV range larger than F25_minu2009~u20091.0u2009×u200910−6 erg/cm2 produce a measurable ionospheric disturbance. Such a lower limit of the X-ray fluence value indicates that moderate X-ray bursts, as opposed to giant X-ray bursts, do produce ionospheric disturbances larger than the sensitivity limit of the VLF technique. Therefore, the frequency of detection of such events could be improved, for example by increasing the coverage of existing VLF receiving networks. The VLF detection of high-energy astrophysical bursts then appears as an important observational diagnostic to complement their detection in space. This would be especially important when space observations suffer from adverse conditions, like saturation, occultation from the Earth, or the passage of the spacecraft through the South Atlantic anomaly.

Sub-THz and Hα activity during the preflare and main phases of a Goes class M2 event

Radio and optical observations of the evolution of flare-associated phenomena have shown an initial and rapid burst at 0.4?THz only followed subsequently by a localized chromospheric heating producing an H? brightening with later heating of the whole active region. A major instability occurred several minutes later producing one impulsive burst at microwaves only, associated with an M2.0 GOES X-ray flare that exhibited the main H? brightening at the same site as the first flash.The possible association between long-enduring time profiles at soft X-rays, microwaves, H?, and sub-THz wavelengths is discussed. In the decay phase, the H? movie shows a disrupting magnetic arch structure ejecting dark, presumably chromospheric, material upward. The time sequence of events suggests genuine interdependent and possibly non-thermal instabilities triggering phenomena, with concurrent active region plasma heating and material ejection.

Search for continuum solar flare radiation in the terahertz range

A new solar burst emission spectral component has been found showing sub-THz fluxes increasing with frequency, spectrally separated from the well known microwave component bringing serious constraints for the interpretation. The THz spectral component might be the evidence of synchrotron emission from high energy electrons peaking somewhere in the far IR range. The knowledge of THz continuum spectral shape is therefore essential to investigate the nature of the emission mechanisms involved. Sub-THz solar flare flux data are available from observations carried out at El Leoncito high altitude observatory, at two atmosphere transmission windows (0.2 and 0.4 THz). We describe the results from developments addressed to the construction of solar flare radiometers at two frequencies to operate outside the terrestrial atmosphere (i.e. at 3 and 7 THz). The system has been planned as part of the SOLAR-T mission on a stratosphere long-duration balloon flight over Russia sometime in 2013–2016.

Formation of the thermal infrared continuum in solar flares

Observations of the Sun with the Atacama Large Millimeter Array have now started, and the thermal infrared will regularly be accessible from the NSFs 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

Analysis of Intermittency in Submillimeter Radio and Hard X-Ray Data During the Impulsive Phase of a Solar Flare

mu

Concept of partially overlapping multiple beams for spatial determination of solar bursts time transients at submm-IR wavelengths

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.