Evidence of Widespread Hot Plasma in a Nonflaring Coronal Active Region from Hinode/X-Ray Telescope

The Astrophysical Journal (Impact Factor: 6.28). 05/2009; 698(1):756. DOI: 10.1088/0004-637X/698/1/756
Source: arXiv

ABSTRACT Nanoflares, short and intense heat pulses within spatially unresolved magnetic strands, are now considered a leading candidate to solve the coronal heating problem. However, the frequent occurrence of nanoflares requires that flare-hot plasma be present in the corona at all times. Its detection has proved elusive until now, in part because the intensities are predicted to be very faint. Here, we report on the analysis of an active region observed with five filters by Hinode/X-Ray Telescope (XRT) in 2006 November. We have used the filter ratio method to derive maps of temperature and emission measure (EM) both in soft and hard ratios. These maps are approximate in that the plasma is assumed to be isothermal along each line of sight. Nonetheless, the hardest available ratio reveals the clear presence of plasma around 10 MK. To obtain more detailed information about the plasma properties, we have performed Monte Carlo simulations assuming a variety of nonisothermal EM distributions along the lines of sight. We find that the observed filter ratios imply bi-modal distributions consisting of a strong cool (log T ~ 6.3 – 6.5) component and a weaker (few percent) and hotter (6.6 < log T < 7.2) component. The data are consistent with bi-modal distributions along all lines of sight, i.e., throughout the active region. We also find that the isothermal temperature inferred from a filter ratio depends sensitively on the precise temperature of the cool component. A slight shift of this component can cause the hot component to be obscured in a hard ratio measurement. Consequently, temperature maps made in hard and soft ratios tend to be anti-correlated. We conclude that this observation supports the presence of widespread nanoflaring activity in the active region.

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The solar corona is orders of magnitude hotter than the underlying photosphere, but how the corona attains such high temperatures is still not understood. Soft X-ray (SXR) emission provides important diagnostics for thermal processes in the high-temperature corona, and is also an important driver of ionospheric dynamics at Earth. There is a crucial observational gap between ~0.2 and ~4 keV, outside the ranges of existing spectrometers. We present observations from a new SXR spectrometer, the Amptek X123-SDD, which measured the spatially-integrated solar spectral irradiance from ~0.5 to ~5 keV, with ~0.15 keV FWHM resolution, during sounding rocket flights on 2012 June 23 and 2013 October 21. These measurements show that the highly variable SXR emission is orders of magnitude greater than that during the deep minimum of 2009, even with only weak activity. The observed spectra show significant high-temperature (5-10 MK) emission and are well fit by simple power-law temperature distributions with indices of ~6, close to the predictions of nanoflare models of coronal heating. Observations during the more active 2013 flight indicate an enrichment of low first-ionization potential (FIP) elements of only ~1.6, below the usually-observed value of ~4, suggesting that abundance variations may be related to coronal heating processes. The XUV Photometer System Level 4 data product, a spectral irradiance model derived from integrated broadband measurements, significantly overestimates the spectra from both flights, suggesting a need for revision of its non-flare reference spectra, with important implications for studies of Earth ionospheric dynamics driven by solar SXRs.
    The Astrophysical Journal Letters 03/2015; 802(1):L2. DOI:10.1088/2041-8205/802/1/L2 · 5.60 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present spatially resolved EUV spectroscopic measurements of pervasive, faint Fe XIX 592.2 Å line emission in an active region observed during the 2013 April 23 flight of the Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS-13) sounding rocket instrument. With cooled detectors, high sensitivity, and high spectral resolution, EUNIS-13 resolves the lines of Fe XIX at 592.2 Å (formed at temperature T 8.9 MK) and Fe XII at 592.6 Å (T 1.6 MK). The Fe XIX line emission, observed over an area in excess of 4920 arcsec2 (2.58 × 109 km2, more than 60% of the active region), provides strong evidence for the nanoflare heating model of the solar corona. No GOES events occurred in the region less than 2 hr before the rocket flight, but a microflare was observed north and east of the region with RHESSI and EUNIS during the flight. The absence of significant upward velocities anywhere in the region, particularly the microflare, indicates that the pervasive Fe XIX emission is not propelled outward from the microflare site, but is most likely attributed to localized heating (not necessarily due to reconnection) consistent with the nanoflare heating model of the solar corona. Assuming ionization equilibrium we estimate Fe XIX/Fe XII emission measure ratios of ~0.076 just outside the AR core and ~0.59 in the core.
    The Astrophysical Journal 07/2014; 790(2):112. DOI:10.1088/0004-637X/790/2/112 · 6.28 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Small scale flare-like brightenings around active regions are among the smallest and most fundamental of energetic transient events in the corona, providing a testbed for models of heating and active region dynamics. In a previous study, we modeled a large collection of these microflares observed with Hinode/XRT using EBTEL and found that they required multiple heating events, but could not distinguish between multiple heating events on a single strand, or multiple strands each experiencing a single heating event. We present here a similar study, but with EUV data of Active Region 11520 from the High Resolution Coronal Imager (Hi-C) sounding rocket. Hi- C provides an order of magnitude improvement to the spatial resolution of XRT, and a cooler temperature sensitivity, which combine to provide significant improvements to our ability to detect and model microflare activity around active regions. We have found that at the spatial resolution of Hi-C (approximately 0.3 arcseconds), the events occur much more frequently than expected (57 events detected, only 1 or 2 expected), and are most likely made from strands of order 100 km wide, each of which is impulsively heated with multiple heating events. These findings tend to support bursty reconnection as the cause of the energy release responsible for the brightenings.
    The Astrophysical Journal 08/2014; 794(2). DOI:10.1088/0004-637X/794/2/119 · 6.28 Impact Factor

Full-text (2 Sources)

Available from
Jun 2, 2014