The Contribution of Microbunching Instability to Solar Flare Emission in the GHz to THz Range of Frequencies

Recent solar flare observations in the sub-terahertz range have provided evidence of a new spectral component with fluxes increasing for larger frequencies, separated from the well-known microwave emission that maximizes in the gigahertz range. Suggested interpretations explain the terahertz spectral component but do not account for the simultaneous microwave component. We present a mechanism for producing the observed double spectra. Based on coherent enhancement of synchrotron emission at long wavelengths in laboratory accelerators, we consider how similar processes may occur within a solar flare. The instability known as microbunching arises from perturbations that produce electron beam density modulations, giving rise to broadband coherent synchrotron emission at wavelengths comparable to the characteristic size of the microbunch structure. The spectral intensity of this coherent synchrotron radiation (CSR) can far exceed that of the incoherent synchrotron radiation (ISR), which peaks at a higher frequency, thus producing a double-peaked spectrum. Successful CSR simulations are shown to fit actual burst spectral observations, using typical flaring physical parameters and power-law energy distributions for the accelerated electrons. The simulations consider an energy threshold below which microbunching is not possible because of Coulomb repulsion. Only a small fraction of the radiating charges accelerated to energies above the threshold is required to produce the microwave component observed for several events. The ISR/CSR mechanism can occur together with other emission processes producing the microwave component. It may bring an important contribution to microwaves, at least for certain events where physical conditions for the occurrence of the ISR/CSR microbunching mechanism are possible

Characteristics of nighttime West-to-East VLF waves propagation using the South America VLF Network (SAVNET)

Abstract We present the monitoring of ~ 4 years of daily VLF amplitude curves using two parallel propagation paths from the South America VLF Network (SAVNET). During this period, the time of occurrence (Terminator Times) of deep amplitude minima were measured. We discuss typical properties of West-to-East VLF subionospheric propagation like the seasonal effect on the temporal evolution of Terminator Times, the fact that they are simultaneously detected on parallel propagation paths, and the derivation of the undisturbed nighttime ionospheric height. The method known as the terminator time method is also a promising way of looking for possible seismic-electromagnetic effects

Spread F echoes variability along solar flux and seasonality conditions over the 50-MHz radar on Christmas Island/ Espalhe a variabilidade dos ecos de F ao longo do fluxo solar e das condições de sazonalidade no radar de 50 MHz na Ilha Christmas

Perturbations in the amplitude (or phase) of radio signals are caused by irregularities in the ionospheric electron density. Ionospheric structures can disrupt the propagation of radio waves, and are commonly observed, for instance, by ionospheric vertical radio sounders (ionosondes). For historical reasons, the signatures of ionospheric irregularities in different types observations (optical, radio, in-stu) are referred to as equatorial spread F (ESF). Previous studies show that the occurrence rate can be greatly affected by solar and magnetic conditions. Observations during the period of 2003 to 2012 of nighttime echoes made from the 50-MHz radar on Christmas Island (2.0o N, 157.4o W) have revealed time and altitude pattern distribution along the descending phase of solar cycle 23 and the recent extended solar minimum phase. We present the study of spread F echoes as a function of solar flux conditions and seasonality in order to quantify their variability. Under higher solar flux conditions, echoes reach higher altitudes but decay earlier. Only during solar minimum conditions the echoes exist throughout the whole night, since the post?reversal anti?zonal background electric field is weaker. Thus, irregularities during solar maximum will be dominated by dynamics near the time of the PRE. Since the population of irregularities during solar minimum exists throughout the whole night, post?reversal ionospheric conditions may play a role in the morphology of plasma irregularities, especially with the coincidentally weaker PRE during solar minimum. Peak time occurrence of echoes along the current period show a well defined pattern, meanwhile the peak altitude occurrence of echoes show a slight regular pattern

Response of the low ionosphere to X-ray and Lyman-a solar flare emissions

International audience[1] Using soft X-ray measurements from detectors onboard the Geostationary Operational Environmental Satellite (GOES) and simultaneous high-cadence Lyman-a observations from the Large Yield Radiometer (LYRA) onboard the Project for On-Board Autonomy 2 (PROBA2) ESA spacecraft, we study the response of the lower part of the ionosphere, the D region, to seven moderate to medium-size solar flares that occurred in February and March of 2010. The ionospheric disturbances are analyzed by monitoring the resulting sub-ionospheric wave propagation anomalies detected by the South America Very Low Frequency (VLF) Network (SAVNET). We find that the ionospheric disturbances, which are characterized by changes of the VLF wave phase, do not depend on the presence of Lyman-a radiation excesses during the flares. Indeed, Lyman-a excesses associated with flares do not produce measurable phase changes. Our results are in agreement with what is expected in terms of forcing of the lower ionosphere by quiescent Lyman-a emission along the solar activity cycle. Therefore, while phase changes using the VLF technique may be a good indicator of quiescent Lyman-a variations along the solar cycle, they cannot be used to scale explosive Lyman-a emission during flares

Launch of solar coronal mass ejections and submillimeter pulse bursts

The 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.Fil: Kaufmann, Pierre. Universidade Estadual de Campinas; Brasil. Universidade Presbiteriana Mackenzie; BrasilFil: De Castro, C. Guillermo Giménez. Universidade Presbiteriana Mackenzie; BrasilFil: Makhmutov, Vladimir S.. Universidade Presbiteriana Mackenzie; Brasil. The Russian Academy Of Sciences; RusiaFil: Raulin, Jean Pierre. Universidade Presbiteriana Mackenzie; BrasilFil: Schwenn, Rainer. Max Planck Institute For Solar System Research; AlemaniaFil: Levato, Orlando Hugo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Complejo Astronómico "El Leoncito". Universidad Nacional de Córdoba. Complejo Astronómico "El Leoncito". Universidad Nacional de la Plata. Complejo Astronómico "El Leoncito". Universidad Nacional de San Juan. Complejo Astronómico "El Leoncito"; ArgentinaFil: Rovira, M.. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentin

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 (SST): 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 = k R, 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.Comment: 9 pages and 11 figures, submitted to Astrophys. J; Revised version accepted by ApJ on 4 March 200

Properties of fast submillimeter time structures during a large solar flare

We report properties of fast varying submillimeter emission during one of the strongest solar radio flares of solar cycle 23. Emission was obtained by the Solar Submillimeter-Wave Telescope at 212 and 405 GHz and compared with hard X-ray and γ-ray counts up to few tens of MeV photon energy ranges. We employ different methods to detect and characterize flux density variations and find that during the impulsive phase of the event, the closer in time to the peak flare, the higher the occurrence of the fastest and brightest time structures. The good comparison with hard X-ray and γ-ray count rates indicates that fast submillimeter pulses are the signatures of primary energetic injections. The characteristics of the fast spikes at 212 and 405 GHz, such as their flux density and localization, compared to those of the underlying slower impulsive component, also suggest that their nature is different.Fil: Raulin, Jean Pierre. Universidade Presbiteriana Mackenzie; BrasilFil: Kaufmann, Pierre. Universidade Estadual de Campinas; Brasil. Universidade Presbiteriana Mackenzie; BrasilFil: de Castro, Carlos G. Giménez. Universidade Presbiteriana Mackenzie; BrasilFil: Pacini, Alessandra A.. Universidade Presbiteriana Mackenzie; BrasilFil: Makhmutov, Vladimir S.. Russian Academy Of Sciences; RusiaFil: Levato, Orlando Hugo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Complejo Astronómico "El Leoncito". Universidad Nacional de Córdoba. Complejo Astronómico "El Leoncito". Universidad Nacional de la Plata. Complejo Astronómico "El Leoncito". Universidad Nacional de San Juan. Complejo Astronómico "El Leoncito"; ArgentinaFil: Rovira, Marta Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentin