Temporal evolution of the chromospheric electron density in the flare of January 5, 1992

The analysis of the dynamic evolution of the chromospheric electron density during solar flares is fundamental for the testing of solar flare models. For this purpose we developed a digital imaging spectrograph for the observation of higher Balmer lines below 400 nm with a time resolution of 1 s and an algorithm for the determination of the electron density from the observed line profiles. On January 5, 1992 a M1/1N flare was observed in H, H and Caii H and the temporal evolution of the electron density was determined. The chromospheric electron density rises several times from less than 3 × 10¹⁹ to 1 × 10²⁰ m^–3 during the hard X-ray peaks.     

Multiwavelength Flare Observations: Temporal Evolution of the 20 August 1992 Flare

The 20 August 1992 flare around 14:28 UT was observed in H, H and Ca ii H with the imaging spectrographs at Locarno-Monti, Switzerland, with the radiotelescopes in Bern, and in soft and hard X-rays by the Yohkoh satellite. In this paper we discuss the analysis of the temporal and spatial evolution of this flare, well observed at chromospheric and coronal layers. We find that the chromospheric electron density shows well-correlated rises with the hard X-rays emphasizing the direct response of the chromosphere to the energy deposition. Although both footpoints of the loops show simultaneous rises of the electron density, non-thermal electron injection is only observed in one of the footpoints, while an additional heating mechanism, like thermal conduction, must be assumed for the other footpoint. However, it is puzzling that all the chromospheric observations in both footpoints are delayed by 3 s compared to the hard X-ray light curve. Although this would be compatible with the thermal heating of one footpoint, it is in contradiction to the non-thermal heating of the other one. Finally, we observed evidence that during the first part of the flare a thermal conduction front propagates at a speed of 2000 km s^-1 into a second loop, in which the energy release occurs in the second part of the flare.     

Multiwavelength observations of the impulsive flare of November 19, 1990

We present the observation and interpretation of a solar radio burst whose evolution of the source position at 48 GHz has been correlated with microwave spectral observations from 3.1 to 19.6 GHz and H imaging spectrograms. The event of November 19, 1990 showed 4 impulsive peaks in microwaves and 2 H kernels. There exists strong evidence that the impulsive emission has originated from nonthermal electrons including an electron beam during the rising phase of the third microwave peak. The complex evolution of the source position at 48 GHz is attributed to two inhomogeneous and spatially separated sources with changing relative brightness.     

Multiwavelength Observations of the 5 January 1992 Flare

The 5 January 1992 flare around 13:16 UT was observed in H, H, and Ca ii H with the imaging spectrographs at Locarno-Monti, Switzerland and in soft and hard X-rays by the Yohkoh satellite. In this paper we discuss the analysis of the temporal and spatial evolution of this flare well observed at chromospheric and coronal layers. We find that the strongest footpoint emission in the optical lines does not coincide with the sites of non-thermal electron injection and show that these footpoints are mainly heated by thermal conduction. The chromospheric electron density, determined from the H line profiles, shows several temporally well correlated rises with the hard X-ray intensity at the electron injection sites. Two of the flare loops clearly are associated with strong chromospheric evaporation, while very weak evaporation is observed in the loop with the strongest footpoint emission in the optical lines.