Properties of the He I 10830 Å Line in Solar Flares

Abstract

We study the properties of the He I 10830 A line in nine selected solar flares, using spectral data obtained with the Multi-channel Infrared Solar Spectrograph (MISS) at Purple Mountain Observatory (PMO) and photospheric images from the Michelson Doppler Imager (MDI) onboard the Solar and Heliospheric Observatory (SOHO). Our results indicate that, over an area of 3''– 8'', the He I 10830 A line shows emission exceeding the continuum in nearby quiet region when the Geostationary Operations Environmental Satellite (GOES) X-ray class of the flare reaches a threshold value (C4.5). The He I 10830 A line emission is detected only in the kernels of the Hα brightenings, but is not associated with the size of the flare. It is found that, whenever the He I 10830 A line shows excess emission over the nearby continuum both the Hα and the Ca II 8542 A lines display enhanced intensities exceeding their preflare intensities. The He I 10830 A line emission can occasionally extend into the umbra of the involved sunspot, which is inconsistent with previous studies. The weak component of He I 10830 A line changes from emission to absorption earlier than does the main component. Our results favor the photoionization-reconnection mechanism for the excitation of the He I 10830 A line.