NuSTAR Observations of 11 Microflares: Difference between revisions

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{{Infobox Nugget
|name = Nugget
|title = NuSTAR Observations of 11 Microflares
|number = 401
|first_author = Jessie DUNCAN
|second_author =
|publish_date = 22 February 2021
|next_nugget = Solar FLUKA
|previous_nugget = {{#ask: [[Category:Nugget]] [[RHESSI Nugget Index::400]]}}
}}


== Introduction ==
Solar flares have been observed across over seven orders of magnitude
in estimated
[https://www.solarmonitor.org 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.
This motivates investigation of the faintest events currently observable.
== Hard X-Ray Solar Observation with NuSTAR ==
In Ref. [1], this work, we examined 11 microflares with the
[https://www.nustar.caltech.edu NuSTAR]
hard X-ray telescope, seeking to characterize the hottest plasma
and to detect accelerated electrons via their
[https://en.wikipedia.org/wiki/Bremsstrahlung bremsstrahlung] emission.
See also RHESSI Nugget
[https://sprg.ssl.berkeley.edu/~tohban/wiki/index.php/NuSTAR_detects_X-ray_flares_in_the_quiet_Sun 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.
[[File:401f1.png|600px|thumb|center|<b>Figure 1:</b>
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, a two-temperature spectral fit
was justified, but in all cases the spectra
are basically consistent with thermal emission.
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.
[[File:401f2.png|600px|thumb|center|<b>Figure 2:</b>.
NuSTAR microflares from this study (pink circles) are shown in
context with other NuSTAR microflares (black triangles), as well as
[http://foxsi.umn.edu 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 is a
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] [https://ui.adsabs.harvard.edu/abs/2021ApJ...908...29D "NuSTAR Observation of Energy Release in 11 Solar Microflares"]
[2] [https://ui.adsabs.harvard.edu/abs/2020ApJ...891L..34G "Accelerated Electrons Observed Down to <7 keV in a NuSTAR Solar Microflare"]

Latest revision as of 20:19, 24 February 2021

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