Recent results on solar activity at submillimeter wavelengths

Since the installation of the Solar Submillimeter Telescope (SST) in 1999 in the Complejo Astronómico El Leoncito (CASLEO, Argentina), the almost unexplored solar emissions at frequencies >100 GHz started to reveal new insights about thermal and non-thermal processes in active regions. SST operates at the frequencies of 212 and 405 GHz providing the unique opportunity to distinguish and investigate emission mechanisms. We present a review of the most relevant findings obtained. An statistical study made with observations of a selected sample of active regions shows that their flux density spectra increase with frequency. Rapid brightenings (pulses) are always observed both at 212 and 405 GHz in association to solar flares lasting for some tens to hundreds of milliseconds. They are well correlated between the two frequencies and have flux spectra either flat or increasing with frequency. The flux of submillimeter wave pulses remain within the same order of magnitude for different bursts, ranging typically 100–300 s.f.u. at 212 GHz and 500–1000 s.f.u. at 405 GHz. The time evolution of the pulse occurrence rate usually reproduces the time profile of the X-rays/γ-rays emission, and the bulk emission at submillimeter waves, when the latter is observable. There are examples of good correlation between individual pulses at submillimeter waves and hard X-rays/γ-rays. Submillimeter pulses are not restricted to flare events, but appear to be a general phenomenon that occurs over active regions as well. The starting time of the rapid submillimeter wave pulses is coincident or precedes the projected launch time of the coronal mass ejections. SST observations of the November 4, 2003 large flare revealed a new and yet unknown spectral component with intensities increasing towards even higher frequencies, appearing along with, but separated from the well-known microwave component.     

Solar flare particle injection spectra and spectra of flux maxima at the point of observation

Numerical models of impulsive solar flare particle events usually assume the radial diffusion coefficient to be independent of energy per nucleon, T, although the observations indicate a T^0.5 dependence (constant mean free path). The assumption of a constant diffusion coefficient results in a preservation of a power law injection spectrum at all radial distances throughout the event. We investigate the effect of an energy dependent diffusion coefficient on the spectrum of flux maxima at a fixed point in interplanetary space. This spectrum is harder than that of initial differential number densities close to the sun. Furthermore, the spectrum hardens with increasing radial distance which seems to be at variance with observations.     

Temporal evolution analysis of storm-enhanced density during an intense magnetic storm on March 2015

An intense Storm Enhancement Density (SED) event with the magnetic storm occurred on 17–24 March 2015 has been investigated. The morphological character of the SED during different phase of the magnetic storm is examined and compared with the non-storm time. Three intensity indexes, i.e., “general” SED index, “heavy” SED index and “severe” SED index, are defined to represent the intensity of SED respectively represented by the numbers of the ionospheric total electron content (TEC) grids with TEC > 60 TECu, TEC > 80 TECu and TEC > 100 TECu. The temporal evolution of the SED intensity indexes during a time span covering the non-storm time and the magnetic storm time have also been investigated. The SED exhibits a shape with two parallel slender troughs in the middle and low latitudes during the non-storm time and then gradually develops into an ellipse structure as the development of magnetic storm. The intensity of SED and the fluctuation of the TEC evolution are generally corresponding to the fluctuation of Dst index. The analyzing results enrich our understanding of the temporal and spatial evolution of the ionospheric SED.     

A study of L band scintillations during the initial phase of rising solar activity at an Indian low latitude station

The ionospheric scintillation and TEC (Total Electron Content) variations are studied using GPS (Global Positioning System) measurements at an Indian low latitude station Surat (21.16°N, 72.78°E; Geomagnetic: 12.90°N, 147.35°E), situated near the northern crest of the equatorial anomaly region. The results are presented for data collected during the initial phase of current rising solar activity (low to moderate solar activity) period between January 2009 and December 2011. The results show that within a total number of 656 night-time scintillation events, 340 events are observed with TEC depletions, Rate of change of TEC (ROT) fluctuations and enhancement of Rate of change of TEC Index (ROTI). A comparison of night-time scintillation events from the considered period reveal strong correlation amongst the duration of scintillation activity in S₄ index_, TEC depletion, ROT fluctuations and ROTI enhancement in the year 2011, followed by the year 2010 and least in 2009. The statistical analyses of scintillation activity with enhancement of ROTI also show that about 70–96% scintillation activity took place in equinox and winter months. Moreover, from a nocturnal variation in occurrence of scintillation with (S₄ ? 0.2) and enhancement of ROTI with (ROTI ? 0.5), a general trend of higher occurrence in pre-midnight hours of equinox and winter seasons is observed in both indices during the year 2011 and 2010, while no significant trend is observed in the year 2009. The results suggest the presence of F-region ionospheric irregularities with scale sizes of few kilometers and few hundred meters over Surat and are found to be influenced by solar and magnetic activity.