CORONAS/SPIRIT Mg XII and Nanoflares

Nugget
Number:	335
1st Author:	Anton Reva
2nd Author:
Published:	22 October 2018
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Introduction

The high temperature of the solar corona has several possible explanations, and correspondingly intense theoretical and observational work trying to identify the right one(s). In nanoflare heating theory, the corona is heated by a large number of small-scale flare-like events (nanoflares). If the nanoflare frequency is low, then some hot plasma should always exist in non-flaring active regions, since the apparently steady emission would reflect the lower average temperature of the nanoflare events. The detection of hot plasma in non-flaring active regions would therefore be indirect evidence of low-frequency nanoflare heating (a "smoking gun"). The absence of the hot emission can help to constrain the nanoflare frequency. The breadth of the distribution of emission measure if it is not isothermal, i.e. the "differential emission measure" (DEM), thus reflects the presence or absence of nanoflares.

It is hard to find faint hot-plasma emission with conventional non-monochromatic imagers (like AIA or XRT), because their images contain broad temperature contributions. This Nugget describes an upper limit on the hot-plasma DEM using direct observations of the hot plasma by the Mg XII spectroheliograph SPIRIT (Ref. [1]) onboard the CORONAS-F satellite observatory. This instrument imaged coronal hot plasma without any low-temperature background contamination. We compare the obtained limit with the result of recent numerical simulations and aim to place constraints on the parameters of the nanoflare-heating model (Refs. [2,3]).

Experimental Data

The Mg XII spectroheliograph obtained monochromatic images of the solar corona in the Mg XII 8.42 Â line. This line emits mainly at temperatures higher than 4 MK. The Mg XII images do not contain a solar limb (hence no contribution from the general corona) or any other low-temperature background.

We studied the period from 18-28 February 2002. At this time, the Mg XII spectroheliograph worked with a 105-second cadence almost without data gaps. In these observations, the Mg XII spectroheliograph registered binned images with a spatial resolution of 8 arc seconds and a 37-second exposure time.

Results

The Mg XII data (Figure 1) shows that only two types of hot objects were present on the Sun: the first are small isolated flare-like phenomena (Ref. [4]). During the period of observations, they occurred at a rate of about 20 per day. The second are large hot structures inside active regions. These are produced during flares or sequences of microflares. Their X-ray emission is highly variable. After the flare ended, these large structures faded away.

Figure 1: Hot plasma observed by the Mg XII spectroheliograph. Left: EIT image. Right: Mg XII spectroheliograph image (blue and green correspond to low intensities, red and yellow to high intensities). 1) Small flare-like objects. 2) Flaring active region with hot plasma. 3) Non-flaring active region without hot plasma.

Except for rare microflares, there was no hot plasma in non-flaring active regions. Below we discuss this fact and try to estimate the upper limit of the hot-plasma emission measure and nanoflare frequency, which may be consistent with this lack of signal.

To estimate the relative amount of the hot and warm (main temperature component) plasma, we reconstructed the DEM of the non-flaring active region NOAA 09833 (see Figure 2). We extracted the fluxes of the selected active region from the Mg XII, EIT 171 Â, 195 Â, and 284 Â channels. Using these four fluxes, we reconstructed the DEM with a genetic algorithm.

Figure 2: Non-flaring active region NOAA 09833, which was used to determine the DEM. a) EIT 171 Â image; b) EIT 195 Â image; c) EIT 284 Â image. d) Spectroheliograph Mg XII image. The rectangle marks the active region that was used for DEM measurements

The amount of plasma with T =5 MK (logT = 6.7) is four orders of magnitude lower than the main temperature component, and the amount of plasma with T = 10 MK is four to five orders lower (see Figure 3).

Figure 3: The DEM of the non-flaring active region marked in Figure 2. Red shows the DEMs obtained during different runs of a genetic algorithm, and black represents their median. The blue-dashdotted DEM-loci show EIT 171 Â, green-dotted DEM-loci are EIT 195 Â, red-dashed DEM-loci show EIT 284 Â, and purple-solid DEM-loci represent the Mg XII spectroheliograph.

At log T > 7.0, only the Mg XII flux constrains the DEM. The DEM in Figure 3 at log T > 7.0 shows the values that could be added to the DEM without increasing the Mg XII flux to a level greater than the noise. These values are the DEM upper limit.

Comparison with Simulations

Numerical simulations (Ref. [5]) show how a coronal loop should react to the sequence of nanoflares depending on the nanoflare frequency. The author found that for a low nanoflare frequency, the DEM has a hot component. The hot component vanishes for a high nanoflare frequency. For each different regimes and for different delays between nanoflares, the simulations provide DEMs. For each of these plots, we manually measured the ratio of the DEM at 10 MK (DEM10) to the DEM of the main temperature component (DEM_max). Then we used the obtained values to build a plot of how this ratio depends on the delay between the nanoflares (see Figure 4).

Figure 4 shows that the relative amount of hot plasma rapidly diminishes with the decrease of the delay between nanoflares. The Mg XII data show that this ratio should be lower than 0.0001. Therefore, the delay between the nanoflares should be shorter than 500 seconds.

Figure 4: The ratio of DEM10 to the DEM_max as a function of the average time between nanoflares. The data were taken from the simulations of Ref. [5]. The black line denotes the simulations where nanoflares have power-law energy distribution with a slope m = -2.5 and the delay between nanoflares is proportional to their energy. The red line denotes the simulations where nanoflares have a power-law energy distribution with a slope m = -1.5 and the delay between nanoflares is proportional to their energy. The blue line denotes the simulations where nanoflares have a power-law energy distribution with a slope m = -2.5 and the delay between nanoflares is fixed. The purple line denotes the Mg XII upper limit on the ratio of the DEMs of the hot and warm components.

Conclusions

The CORONAS-F/SPIRIT data constrain the high-temperature DEM levels in the solar corona, and do not find any evidence for the "smoking gun" signature expected for nanoflare heating. Because the instrument is uniquely monochromatic, and because of the extent of the data, these constraints are the most direct ones yet published. The details of this analysis appear in Ref. [6].