A Collective Study of 11 NuSTAR Microflares

From RHESSI Wiki

Jump to: navigation, search


Nugget
Number: 401
1st Author: Jessie DUNCAN
2nd Author: Lindsay GLESENER
Published: 22 February 2021
Next Nugget: Solar FLUKA
Previous Nugget: A Solar FRB
List all



Contents

Introduction

Solar flares have been observed across over seven orders of magnitude in estimated GOES soft X-ray (SXR) flux, with fainter events occurring far more frequently than brighter ones. While the energy content of larger flares is insufficient to heat the solar corona, the combined effect of a large ensemble of extremely faint flaring events is a strong candidate heating mechanism. These theorized events (nanoflares) would be below even NuSTAR's sensitivity limits, but these pioneer a new domain as described here.

The accepted standard model for large flares involves energy released from reconnection between twisted magnetic field lines. Nanoflares likely originate from a similar process at a much smaller energy scale. While we can't observe nanoflares directly, we can investigate flare behavior over a wide range of magnitudes (including their temporal and spatial properties, as well as the incidence or absence of emission from nonthermally-accelerated particles). Consistency in these properties between large and very faint events provides evidence that the same processes are at work across the entire observed range, which in turn can help us discern what we might expect at the nanoflare scale. These theorized events (nanoflares) would be below our current observational limits.

Hard X-Ray Solar Observation with NuSTAR

In Ref. [1], this work, we examined 11 microflares with the NuSTAR hard X-ray telescope, seeking to characterize the hottest plasma and to detect accelerated electrons via their bremsstrahlung emission. See also RHESSI Nugget No. 319.

NuSTAR is most ideal for observing faint microflares (GOES A-class or below). We considered four microflares observed shortly before the Great American Eclipse (21 August 2017), and seven observed on 29 May 2018. Ten are new to the literature, and one was previously examined in Ref. [2] due to being the first NuSTAR microflare with confirmed nonthermal emission.

Temporal and Spatial Properties

HXR emission in large flares commonly exhibits a fast rise followed by a gradual fall (an impulsive time profile), consistent with a quick initial release of energy to plasma heating and particle acceleration followed by a longer cooling interval. Additionally, higher-energy HXR emission in larger events generally peaks earlier in time than lower-energy emission, suggesting a transfer of energy from accelerated particle populations and smaller, hotter plasma volumes into heating of the surrounding chromospheric plasma. The majority of the eleven microflares displayed both of these large-flare properties.

Large flares often show spatially distinct thermal and nonthermal sources, resulting in differing centroids of flare emission in different HXR energy ranges. Plasma temperature gradients across the flare site could also cause an energy-dependent centroid difference. To look for this type of spatial complexity, quiet-time active region emission must first be removed. Suitable quiet times were found for only 2/11 microflares, neither of which displayed an energy-dependent difference in emission centroid. Images of one of these (and five other microflare images) are shown in Figure 1.

Figure 1: NuSTAR emission contours plotted over AIA 94A context images, showing variable morphologies among six of the 11 events. NuSTAR experiences an uncertainty of a few arcminutes in absolute astrometry when observing the sun, which has been corrected by co-aligning the data to the AIA context. Centroids of emission in each energy range are marked. Only the bottom rightmost event (start time: 16:18 UT on 29 May 2018) has had quiescent background emission removed; in this event no difference is seen in emission centroids in the two NuSTAR energy ranges.

Microflare Spectroscopy

In all but three of our events, the microflare spectra were best-fit by combining two isothermal plasma models at different temperatures, implying consistency with a fully thermal picture. Of the three remaining events, one was the confirmed nonthermal microflare (Ref. [2]), which had spectra completely inconsistent with a fully thermal interpretation. The other two were similarly well-fit by the model combining two isothermal sources, as well as one combining a single isothermal source with a nonthermal broken power law distribution. While this is not conclusive proof of the presence of a nonthermal source in these two microflares, we note that the nonthermal energy content calculated in these events was more than an order of magnitude larger than the thermal energy content (see Ref. [1]). This result is consistent with the idea of a non-thermal energy source that source that could power the observed thermal distribution. Figure 2 summarizes all of the events in the NuSTAR domain of faint microflares as a "flux-ratio" diagnostic diagram.

Figure 2:. NuSTAR microflares from this study (pink circles) are shown in context with other NuSTAR microflares (black triangles), as well as FOXSI (stars) and RHESSI (red) events. The flare from Ref. [2] is included as a pink triangle. The vertical axis is a measure of flux, while the horizontal axis gives spectral hardness, a proxy for temperature. This representation allows for comparison between spectral shapes of different events. The blue fit line shows a linear fit to all of the thermal NuSTAR flares with significant counts at 8 keV.

Conclusions

In general, we found good consistency between the temporal evolution of these flares and that commonly seen in much larger HXR events. Our spatial analysis was limited by properties of this particular dataset (few sufficient quiet times for background subtraction). With the exception of the previously published nonthermal microflare, the events were dominantly thermal, but the spectra of some of the larger microflares were consistent with a picture involving a nonthermal energy source. Looking at the brightness vs. hardness diagram (Figure 2), we see that the confirmed nonthermal microflare has a spectral shape that is not at all unusual when compared to the others.

It's possible that the smaller microflares may also have nonthermal aspects that are more challenging to disentangle from their thermal emission. The range of magnitudes in HXR flux explored here seems to include the transition between events where nonthermal emission is dominant and those where it is largely indistinguishable from the thermal. This is an especially crucial regime to explore in order to develop our understanding of particle acceleration in very small-scale events. This will begin with further NuSTAR studies, but ultimately will require a statistical approach using a solar-dedicated focusing HXR instrument.

For much more information about this study, see Ref. [1].

References

[1] "NuSTAR Observation of Energy Release in 11 Solar Microflares"

[2] "Accelerated Electrons Observed Down to <7 keV in a NuSTAR Solar Microflare"

Personal tools
Namespaces
Variants
Actions
Navigation
Toolbox