Solar microflares: a case study on temperatures and the Fe XVIII emission. (arXiv:1905.08579v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Mitra_Kraev_U/0/1/0/all/0/1">Urmila Mitra-Kraev</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zanna_G/0/1/0/all/0/1">Giulio Del Zanna</a>

In this paper, we discuss the temperature distribution and evolution of a
microflare, simultaneously observed by Hinode XRT, EIS, and SDO AIA. We find
using EIS lines that during peak emission the distribution is nearly isothermal
and peaked around 4.5 MK. This temperature is in good agreement with that
obtained from the XRT filter ratio, validating the use of XRT to study these
small events, invisible by full-Sun X-ray monitors such as GOES. The increase
in the estimated Fe XVIII emission in the AIA 94 {AA} band can mostly be
explained with the small temperature increase from the background temperatures.
The presence of Fe XVIII emission does not guarantee that temperatures of 7 MK
are reached, as is often assumed. We also revisit with new atomic data the
temperatures measured by a SoHO SUMER observation of an active region which
produced microflares, also finding low temperatures (3 – 4 MK) from an Fe XVIII
/ Ca XIV ratio.

In this paper, we discuss the temperature distribution and evolution of a
microflare, simultaneously observed by Hinode XRT, EIS, and SDO AIA. We find
using EIS lines that during peak emission the distribution is nearly isothermal
and peaked around 4.5 MK. This temperature is in good agreement with that
obtained from the XRT filter ratio, validating the use of XRT to study these
small events, invisible by full-Sun X-ray monitors such as GOES. The increase
in the estimated Fe XVIII emission in the AIA 94 {AA} band can mostly be
explained with the small temperature increase from the background temperatures.
The presence of Fe XVIII emission does not guarantee that temperatures of 7 MK
are reached, as is often assumed. We also revisit with new atomic data the
temperatures measured by a SoHO SUMER observation of an active region which
produced microflares, also finding low temperatures (3 – 4 MK) from an Fe XVIII
/ Ca XIV ratio.

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