Estimate of the Upper Limit on Hot Plasma Differential Emission Measure (DEM) in Non-Flaring Active Regions and Nanoflare Frequency Based on the Mg xii Spectroheliograph Data from CORONAS-F/SPIRIT

Abstract

The nanoflare-heating theory predicts steady hot plasma emission in the non-flaring active regions. It is hard to find this emission with conventional non-monochromatic imagers (such as Atmospheric Imaging Assembly or X-Ray Telescope), because their images contain a cool temperature background. In this work, we search for hot plasma in non-flaring active regions using the Mg xii spectroheliograph onboard Complex Orbital Observations Near-Earth of Activity on the Sun (CORONAS)-F/SPectroheliographIc X-ray Imaging Telescope (SPIRIT). This instrument acquired monochromatic images of the solar corona in the Mg xii 8.42Å line, which emits only at temperatures higher than 4 MK. The Mg xii images contain the signal only from hot plasma without any lowtemperature background. We studied the hot plasma in active regions using the SPIRIT data from 18 -28 February 2002. During this period, the Mg xii spectroheliograph worked with a 105-second cadence almost without data gaps. The hot plasma was observed only in the flaring active regions. We do not observe any hot plasma in non-flaring active regions. The hot plasma column emission measure in the non-flaring active region should not exceed 3 × 10 24 cm −5 . The hot Differential Emission Measure (DEM) is less than 0.01 % of the DEM of the main temperature component. Absence of Mg xii emission in the non-flaring active regions can be explained by weak and frequent nanoflares (delay less than 500 seconds) or by very short and intense nanoflares that lead to non-equilibrium ionization.

High Resolution Soft X-ray Spectroscopy and the Quest for the Hot (5–10 MK) Plasma in Solar Active Regions

“…The upper limit of the emission measure at 10 MK is about three orders of magnitude lower than the peak at 3 MK. Reva et al (2018) used CORONAS-F/SPIRIT Mg XII images to estimate an upper limit of the emission measure around 10 MK about four orders of magnitude lower than the peak value around 3 MK, which was constrained by SoHO EIT imaging. However, the information from Mg XII is somewhat limited as this ion in equilibrium is formed over a very broad temperature range, from 4 to well over 15 MK.…”

Monochromatic X-Ray Imagers of the Sun Based on the Bragg Crystal Optics

“…During these years, the Sun was in the maximum of the activity, and plenty of information about hot plasma dynamics was recorded. These data were used to study large flares (Grechnev et al, 2006;Urnov et al, 2007;Shestov et al, 2010;Reva et al, 2015), micro-activity (Reva et al, 2012), and coronal heating (Reva et al, 2018).…”

“…For the quiet Sun, we used the data obtained by Brooks et al (2009). For the active region, we used the modified DEM of the non-flaring active region obtained by Reva et al (2018). The DEM obtained by Reva et al (2018) has a high-temperature tail.…”

“…For the active region, we used the modified DEM of the non-flaring active region obtained by Reva et al (2018). The DEM obtained by Reva et al (2018) has a high-temperature tail. It is caused by the limited sensitivity of the instruments, and it is an upper limit on the DEM of the hot plasma.…”

KORTES Mission for Solar Activity Monitoring Onboard International Space Station

“…Hence the MXI images are free from unwanted low-temperature background usually present on images of analogous telescopes (e.g., SDO/AIA, Hidone/XRT, RHESSI). This feature makes MXI an ideal choice to observe plasma at temperatures above 4 MK (Reva et al, 2012(Reva et al, , 2015(Reva et al, , 2018Bogachev, 2013, 2017a,b).…”