Showing papers by "Ryan O. Milligan published in 2008"
TL;DR: In this paper, RHESSI and Hinode observations of a GOES B-class flare are combined to investigate the origin of 15 MK plasma, and the absence of any detectable hard X-ray emission coupled with weak blueshifted emission lines (indicating upward velocities averaging only 14 km/s) suggests that this was a result of direct heating in the corona, as opposed to nonthermal electron precipitation causing chromospheric evaporation.
Abstract: RHESSI and Hinode observations of a GOES B-class flare are combined to investigate the origin of 15 MK plasma. The absence of any detectable hard X-ray emission coupled with weak blueshifted emission lines (indicating upward velocities averaging only 14 km/s) suggests that this was a result of direct heating in the corona, as opposed to nonthermal electron precipitation causing chromospheric evaporation. These findings are in agreement with a recent hydrodynamical simulation of microflare plasmas that found that higher temperatures can be attained when less energy is used to accelerate electrons out of the thermal distribution. In addition, unusual redshifts in the 2 MK Fe xv line (indicating downward velocities of approx.14 km/s) were observed cospatial with one of the flare ribbons during the event. Downflows of such high-temperature plasma are not predicted by any common flare model.
34 citations
TL;DR: In this article, the temporal evolution of temperature, emission measure, energy loss and velocity in a C-class solar flare from both an observational and theoretical perspective were derived by following the systematic cooling of the plasma through the response functions of a number of instruments -- RHESSI (>5 MK), GOES-12 (5-30 MK), TRACE 171 A (1 MK) and SOHO/CDS (~0.03-8 MK).
Abstract: This paper investigates the temporal evolution of temperature, emission measure, energy loss and velocity in a C-class solar flare from both an observational and theoretical perspective. The properties of the flare were derived by following the systematic cooling of the plasma through the response functions of a number of instruments -- RHESSI (>5 MK), GOES-12 (5-30 MK), TRACE 171 A (1 MK) and SOHO/CDS (~0.03-8 MK). These measurements were studied in combination with simulations from the 0-D EBTEL model. At the flare on-set, upflows of ~90 km s-1 and low level emission were observed in Fe XIX, consistent with pre-flare heating and gentle chromospheric evaporation. During the impulsive phase, upflows of ~80 km s-1 in Fe XIX and simultaneous downflows of 20 km s-1 in He I and O V were observed, indicating explosive chromospheric evaporation. The plasma was subsequently found to reach a peak temperature of ~13 MK in approximately 10 minutes. Using EBTEL, conduction was found to be the dominant loss mechanism during the initial ~300s of the decay phase. It was also found to be responsible for driving gentle chromospheric evaporation during this period. As the temperature fell below ~8 MK, and for the next ~4,000s, radiative losses were determined to dominate over conductive losses. The radiative loss phase was accompanied by significant downflows of <40 km s-1 in O V. This is the first extensive study of the evolution of a canonical solar flare using both spectroscopic and broad-band instruments in conjunction with a hydrodynamic model. While our results are in broad agreement with the standard flare model, the simulations suggest that both conductive and non-thermal beam heating play important roles in heating the flare plasma during the impulsive phase of at least this event.
1 citations