40 research outputs found
Nanoflare Properties throughout Active Regions: Comparing SDO/AIA Observations with Modeled Active Region Light Curves
Coronal plasma in active regions is typically measured to be at temperatures near ~1-3 MK. Is the majority of the coronal plasma in hydrostatic equilibrium, maintained at these temperatures through a form of quasi-steady heating, or is this simply a measure of the average temperature of widely varying, impulsively heated coronal plasma? Addressing this question is complicated by the fact that the corona is optically thin: many thousands of flux tubes which are heated completely independently are contributing to the total emission along a given line of sight. There is a large body of work focused on the heating of isolated features - coronal loops - which are impulsively heated, however it is the diffuse emission between loops which often comprises the majority of active region emission. Therefore in this study we move beyond isolated features and analyze all of the emission in an entire active region from all contributing flux tubes. We investigate light curves systematically using SDO/AIA observations. We also model the active region corona as a line-of-sight integration of many thousands of completely independently heated flux tubes. The emission from these flux tubes may be time dependent, quasi-steady, or a mix of both, depending on the cadence of heat release. We demonstrate that despite the superposition of randomly heated flux tubes, different distributions of nanoflare cadences produce distinct signatures in light curves observed with multi-wavelength and high time cadence data, such as those from SDO/AIA. We conclude that the majority of the active region plasma is not maintained in hydrostatic equilibrium, rather it is undergoing dynamic heating and cooling cycles. The observed emission is consistent with heating through impulsive nanoflares, whose energy is a function of location within the active region
Signatures of Steady Heating in Time Lag Analysis of Coronal Emission
Among the many ways of investigating coronal heating, the time lag method of
Viall & Klimchuk (2012) is becoming increasingly prevalent as an analysis
technique complementary to those traditionally used. The time lag method cross
correlates light curves at a given spatial location obtained in spectral bands
that sample different temperature plasmas. It has been used most extensively
with data from the Atmospheric Imaging Assembly on the Solar Dynamics
Observatory. We have previously applied the time lag method to entire active
regions and surrounding quiet Sun and create maps of the results (Viall &
Klimchuk 2012; Viall & Klimchuk 2015). We find that the majority of time lags
are consistent with the cooling of coronal plasma that has been impulsively
heated. Additionally, a significant fraction of the map area has a time lag of
zero. This does not indicate a lack of variability. Rather, strong variability
must be present, and it must occur in phase in the different channels. We have
shown previously that these zero time lags are consistent with the transition
region response to coronal nanoflares (Viall & Klimchuk 2015; Bradshaw & Viall
2016), but other explanations are possible. A common misconception is that the
zero time lag indicates steady emission resulting from steady heating. Using
simulated and observed light curves, we demonstrate here that highly correlated
light curves at zero time lag are not compatible with equilibrium solutions.
Such light curves can only be created by evolution.Comment: 10 pages, 3 figures. Accepted to ApJ July 5 201
A Survey of Nanoflare Properties in Active Regions Observed with the Solar Dynamics Observatory
In this paper, we examine 15 different active regions (ARs) observed with the Solar Dynamics Observatory and analyze their nanoflare properties. We have recently developed a technique that systematically identifies and measures plasma temperature dynamics by computing time lags between light curves. The time lag method tests whether the plasma is maintained at a steady temperature, or if it is dynamic, undergoing heating and cooling cycles. An important aspect of our technique is that it analyzes both observationally distinct coronal loops as well as the much more prevalent diffuse emission between them. We find that the widespread cooling reported previously for NOAA AR 11082 is a generic property of all ARs. The results are consistent with impulsive nanoflare heating followed by slower cooling. Only occasionally, however, is there full cooling from above 7 megakelvins to well below 1 megakelvin. More often, the plasma cools to approximately 1-2 megakelvins before being reheated by another nanoflare. These same 15 ARs were first studied by Warren et al. We find that the degree of cooling is not well correlated with the reported slopes of the mission measure distribution. We also conclude that the Fe (sup XVIII)-emitting plasma that they measured is mostly in a state of cooling. These results support the idea that nanoflares have a distribution of energies and frequencies, with the average delay between successive events on an individual flux tube being comparable to the plasma cooling timescale
Evidence for Widespread Cooling in an Active Region Observed with the SDO Atmospheric Imaging Assembly
A well known behavior of EUV light curves of discrete coronal loops is that the peak intensities of cooler channels or spectral lines are reached at progressively later times. This time lag is understood to be the result of hot coronal loop plasma cooling through these lower respective temperatures. However, loops typically comprise only a minority of the total emission in active regions. Is this cooling pattern a common property of active region coronal plasma, or does it only occur in unique circumstances, locations, and times? The new SDO/AIA data provide a wonderful opportunity to answer this question systematically for an entire active region. We measure the time lag between pairs of SDO/AIA EUV channels using 24 hours of images of AR 11082 observed on 19 June 2010. We find that there is a time-lag signal consistent with cooling plasma, just as is usually found for loops, throughout the active region including the diffuse emission between loops for the entire 24 hour duration. The pattern persists consistently for all channel pairs and choice of window length within the 24 hour time period, giving us confidence that the plasma is cooling from temperatures of greater than 3 MK, and sometimes exceeding 7 MK, down to temperatures lower than approx. 0.8 MK. This suggests that the bulk of the emitting coronal plasma in this active region is not steady; rather, it is dynamic and constantly evolving. These measurements provide crucial constraints on any model which seeks to describe coronal heating
Measuring temperature - dependent propagating disturbances in coronal fan loops using multiple SDO/AIA channels and surfing transform technique
A set of co-aligned high resolution images from the Atmospheric Imaging
Assembly (AIA) on board the Solar Dynamics Observatory (SDO) is used to
investigate propagating disturbances (PDs) in warm fan loops at the periphery
of a non-flaring active region NOAA AR 11082. To measure PD speeds at multiple
coronal temperatures, a new data analysis methodology is proposed enabling
quantitative description of subvisual coronal motions with low signal-to-noise
ratios of the order of 0.1 %. The technique operates with a set of
one-dimensional "surfing" signals extracted from position-time plots of several
AIA channels through a modified version of Radon transform. The signals are
used to evaluate a two-dimensional power spectral density distribution in the
frequency - velocity space which exhibits a resonance in the presence of
quasi-periodic PDs. By applying this analysis to the same fan loop structures
observed in several AIA channels, we found that the traveling velocity of PDs
increases with the temperature of the coronal plasma following the square root
dependence predicted for the slow mode magneto-acoustic wave which seems to be
the dominating wave mode in the studied loop structures. This result extends
recent observations by Kiddie et al. (Solar Phys., 2012) to a more general
class of fan loop systems not associated with sunspots and demonstrating
consistent slow mode activity in up to four AIA channels.Comment: 23 pages, 8 figures, 2 table
SDO/AIA Light Curves and Implications for Coronal Heating: Model Predictions
It seems largely agreed that many coronal loops---those observed at a temperature of about 1 MK---are bundles of unresolved strands that are heated by storms of impulsive nanoflares. The nature of coronal heating in hotter loops and in the very important but largely ignored diffuse component of active regions is much less clear. Is it also impulsive or is it quasi steady? The spectacular new data from the Atmospheric Imaging Assembly (AIA) telescopes on the Solar Dynamics Observatory (SDO) offer an excellent opportunity to address this question. We analyze the light curves of coronal loops and the diffuse corona in 6 different AIA channels and compare them with the predicted light curves from theoretical models. Light curves in the different AIA channels reach their peak intensities with predictable orderings as a function the nanoflare storm properties. We show that while some sets of light curves exhibit clear evidence of cooling after nanoflare storms, other cases are less straightforward to interpret. Complications arise because of line-of-sight integration through many different structures, the broadband nature of the AIA channels, and because physical properties can change substantially depending on the magnitude of the energy release. Nevertheless, the light curves exhibit predictable and understandable patterns. This presentation emphasizes the modeling aspects of our study. A companion presentation emphasizes the observations
Evidence for Widespread Cooling in an Active Region Observed with the SDO Atmospheric Imaging Assembly
A well known behavior of EUV light curves of discrete coronal loops is that
the peak intensities of cooler channels or spectral lines are reached at
progressively later times than hotter channels. This time lag is understood to
be the result of hot coronal loop plasma cooling through these lower respective
temperatures. However, loops typically comprise only a minority of the total
emission in active regions. Is this cooling pattern a common property of active
region coronal plasma, or does it only occur in unique circumstances,
locations, and times? The new SDO/AIA data provide a wonderful opportunity to
answer this question systematically for an entire active region. We measure the
time lag between pairs of SDO/AIA EUV channels using 24 hours of images of AR
11082 observed on 19 June 2010. We find that there is a time-lag signal
consistent with cooling plasma, just as is usually found for loops, throughout
the active region including the diffuse emission between loops for the entire
24 hour duration. The pattern persists consistently for all channel pairs and
choice of window length within the 24 hour time period, giving us confidence
that the plasma is cooling from temperatures of greater than 3 MK, and
sometimes exceeding 7 MK, down to temperatures lower than ~ 0.8 MK. This
suggests that the bulk of the emitting coronal plasma in this active region is
not steady; rather, it is dynamic and constantly evolving. These measurements
provide crucial constraints on any model which seeks to describe coronal
heating.Comment: 17 pages text, 7 figures in main body, 5 Appendix figure