G: Microflares and Nanoflares,
Paolo Grigis (10)

Last Updated Fri Dec 5 16:24:42 2008

1: Jeffrey W. Brosius (Jeffrey.W.Brosius@nasa.gov), Catholic University at NASA/GSFC [E]
[soi]
[poster] We observed a solar microflare over a wide temperature range with three instruments aboard the SOHO spacecraft(CDS, EIT, MDI), TRACE (1600 A), GOES, and RHESSI. Extreme-ultraviolet spectra were obtained at a rapid cadence (9.8s) with CDS in stare mode that included emission lines originating from the chromosphere (~0.01 MK) and transition region (TR), to coronal and flare (~8MK) temperatures. Light curves derived from the CDS spectra and TRACE images (obtained with a variable cadence around 34 s) reveal two precursor brightenings before the microflare. After the precursors, chromospheric and TR emission are the first to increase, consistent with energy deposition by nonthermal electrons. The initial slow rise is followed by a brief (20 s) impulsive EUV burst in the chromospheric and TR lines, during which the coronal and hot flare emission gradually begin to increase. Relative Doppler velocities measured with CDS are directed upward with maximum values around 20 km/s during the second precursor and impulsive peak, indicating gentle chromospheric evaporation. The log of the electron density derived from an O IV line intensity ratio (~0.16 MK) increased from 10.4 during quiescent times to 11.7 at the impulsive peak. The X-ray emission observed by RHESSI peaked after the impulsive peak at chromospheric and TR temperatures and revealed no evidence of emission from nonthermal electrons. Spectral fits to the RHESSI data indicate a maximum temperature near 13 MK, consistent with a slightly lower temperature deduced from the GOES data. Magnetograms from MDI show that the microflare occurred in and around a growing island of negative magnetic polarity embedded in a large area of positive magnetic field. The microflare was compact, covering an area of 40 million square km in the EIT image at 195A, and about 26 million square km in the RHESSI image. TRACE images suggest that the microflare filled small loops.
2: Qingrong Chen (qrchen@stanford.edu), Stanford University [E]
[soi] We focus on the generation of nonthermal electrons, which may possibly exist in the quiet solar corona.
[poster] Recent observations (e.g., Ralchenko et al. 2007) imply that electrons in the quiet solar corona may have a suprathermal component superposed on the Maxwellian distribution of the typical corona temperature. In this poster, we investigate the possibility of generating a nonthermal tail from an initially Maxwellian spectrum in the quiet corona. By introducing stochastic turbulence of some generic form, we determine the evolution of electron spectrum towards a steady state condition by solving the time-dependent Fokker-Planck equation. At each time step, we evaluate the energy loss rate and adjust the level of turbulence to achieve the steady state condition. Our preliminary results indicate that a nonthermal tails can develop, however, in most cases these tails are too weak to account for the observations.
3: Steven Christe (schriste@ssl.berkeley.edu), Space Sciences Lab, U.C. Berkeley [E]
[soi] We present an in-depth statistical survey of all X-ray flares positions observed by RHESSI between March 2002 and March 2007, a total of >25,000 events, an order of magnitude larger then previous studies. The microflares were found using a new flare-finding algorithm designed to search the 6-12 keV count-rate when RHESSI's full sensitivity was available in order to find the smallest events. Larger flares included in the official RHESSI flare list are also included. The flare centroid position are found at the peak time and as a function of energy. Flares are found to occur only in active regions, not in the "quiet" Sun. We find that the distribution of flares centroid positions in the 6-12 keV band is consistent with a minimum height of 4.8 Mm (6.6 arcsec) above the optical limb and an exponential height distribution with a scale height of 7.7 Mm (10.6 arcsec). Flare positions within active regions are also considered.
[none]
4: Paolo Grigis (pgrigis@cfa.harvard.edu), Smithsonian Astrophysical Observatory [E]
[soi] My contribution to working group G would be the presentation of high cadence Hinode observations of nanoflares in soft X-rays and EUV and a study of their statistical properties in space and time, as well as their relation with the underlying magnetic field structure.
[talk]
5: Iain Hannah (iain@astro.gla.ac.uk), University of Glasgow [E]
[soi] Interested in microflares (observed primarily with RHESSI but also Hinode and TRACE), microflare statistics and nanoflaring activity outwith active regions.
[none]
6: Aase-Marit Janse (janse@ucar.edu), The National Center for Atmospheric Research [E]
[soi] We have developed mathematical models to demonstrate the inevitability of current-sheet formation in magnetic fields governed by the ideal hydromagnetic induction equation, as described by the Parker theory. This process, central to the heating of the solar corona, is radically different in fully three-dimensional fields as compared with two-dimensional fields. Magnetic neutral points or separatrix flux surfaces are necessary for sheet formation in two-dimensional fields. In fully three-dimensional fields, current sheets form readily even in the complete absence of neutral points and separatrix surfaces, and, these sheets may form densely throughout the field in response to changes in the magnetic volume. This general result is established for cylindrical fields that are topologically untwisted, including the first direct demonstration of sheet formation in the absence of any magnetic neutral point. We suggest that current sheets can form densely to produce reconnection throughout the field in response to changes in the magnetic volume. This may clarify the role current sheets have in solar flares. Traditionally the dissipation of a single current sheet was thought to be the origin of a flare. However, several quantitative analyses of observed CME-related, soft X-ray flares have suggested that they are due to reconnection heatings that persisted over extended times and occurred spatially pervasively throughout the emitting plasmas. The dense production of current sheets suggested by our model is a possible origin of these observationally inferred reconnections
[poster] We have developed mathematical models to demonstrate the inevitability of current-sheet formation in magnetic fields governed by the ideal hydromagnetic induction equation, as described by the Parker theory. This process, central to the heating of the solar corona, is radically different in fully three-dimensional fields as compared with two-dimensional fields. Magnetic neutral points or separatrix flux surfaces are necessary for sheet formation in two-dimensional fields. In fully three-dimensional fields, current sheets form readily even in the complete absence of neutral points and separatrix surfaces, and, these sheets may form densely throughout the field in response to changes in the magnetic volume. This general result is established for cylindrical fields that are topologically untwisted, including the first direct demonstration of sheet formation in the absence of any magnetic neutral point. We suggest that current sheets can form densely to produce reconnection throughout the field in response to changes in the magnetic volume. This may clarify the role current sheets have in solar flares. Traditionally the dissipation of a single current sheet was thought to be the origin of a flare. However, several quantitative analyses of observed CME-related, soft X-ray flares have suggested that they are due to reconnection heatings that persisted over extended times and occurred spatially pervasively throughout the emitting plasmas. The dense production of current sheets suggested by our model is a possible origin of these observationally inferred reconnections
7: Susanna Parenti (s.parenti@oma.be), Royal Observatory of Belgium [J]
[soi] WG G: EUV signatures of small scale heating in loops: I will review a theoretical work developed by Parenti et.al aimed at testing the conservation of the statistical properties of a nanoflare heating in the EUV emission of a loop. I will show how such properties depend on the fine scale of the loop, on the width of the instrument's band used to measure the emission, and on the isoelectronic group of the spectral line adopted for the investigation.
[none]
8: Isroil Sattarov (isattar@astrin.uzsci.net), Tashkent State Pedagogical University [C]
[soi] I am interested in how coronal bright points are developed in different latitudinal belts for period nearby and in solar minimum. We used SOHO/EIT data for 1996-2008. We have received complete cycle of coronal bright points for the whole of Sun and different latitudinal belts.
[poster] Temporal variations of coronal bright points number in EIT/SoHO data from 195 pass band for 1996-2008 are studied. For identification of CBPs the method developed by authors is used. It is found that: cycle variation of CBPs number is complete by July 2008; cycle variations of CBPs number are different at different latitudes.
9: Toshifumi Shimizu (shimizu@solar.isas.jaxa.jp), ISAS/JAXA [C]
[soi] High resolution photospheric magnetic field vectors derived with Hinode SOT/SP have been studies for the exact footpoints of several X-ray multiple-loop-type microflares and chromospheric ejections well observed at a sunspot light bridge. This study would give a hint in discussions on the origin of microflaring activities and its implication to coronal heating.
[posternone] High resolution photospheric magnetic field vectors derived with Hinode SOT/SP have been studies for several multiple-loop-type microflares etc. This topics can be presented in the meeting (oral rather than poster).
10: Paola Testa (ptesta@cfa.harvard.edu), SAO [I]
[soi] A study of the thermal properties of coronal plasma in non-flaring active region using simultaneous Hinode XRT and EIS observations will be presented. The multi-filter XRT dataset together with EIS spectra including its entire wavelength range allow to accurately determine the thermal structure of the X-ray emitting active region plasma, and to investigate the presence of hot plasma in non-flaring regions. These results will be discussed in the context of coronal heating models.
[none]

Second Choice

D. Shaun Bloomfield, Gianna Cauzzi, Lindsay Glesener, Gordon Hurford, Philip Judge, Wei Liu, Anna Maria Massone, Ryan Milligan, Susanna Parenti, Michele Piana, Marco Prato, Richard Schwartz, ShiChao TANG, Ignacio Ugarte-Urra, Nicole Vilmer, Tetsuya Watanabe, Peter Young,