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Hard X-Ray Timing Experiments with HESSI  

Markus J Aschwanden   Submitted: 1999-11-04 18:01

We design three high-precision timing experiments that can be pursued with HESSI: (1) relative timing of magnetically conjugate footpoint hard X-ray sources (similar to previous studies by Sakao), (2) relative timing between footpoint and looptop or above-the-loop-top sour-ces (in the case of Masuda-ty- Masuda-type flares), and (3) relative timing between electrons (in hard X-rays) and protons (in gamma-rays). Re-analyzing Sakao's simultaneity measurements between conjugate footpoints in 14 flares we confirm his results and find an average uncertainty of {sigma} auapprox pm 250 ms for HXT (with a signal-to-noise ratio of 10: 1), which can be improved down to {sigma} au- {sigma} auapprox pm 22 ms for HESSI (for a signal-to-noise ratio of 100: 1). These timing experiments are expected to convey new information on the particle kinematics in solar flares.

Authors: Markus J. Aschwanden
Projects:

Publication Status: ASP Conf.Ser., High Energy Solar Physics: Anticipating HESSI, (R.Ramaty and N.Mandzhavidze eds.), subm. 1999 Nov 4.
Last Modified: 1999-11-04 18:01
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Particle Acceleration and Kinematics in Solar Flares and the Solar Corona'' (Invited Review)  

Markus J Aschwanden   Submitted: 1999-10-06 21:04

          and the Solar Corona'' (Invited Review)We review theoretical models of particle acceleration applied to solar flares (DC electric fields, wave-turbulence stochastic acceleration, shock acceleration acceleration) and confront these models with new observational findings from the {sl Compton Gamma Ray Observatory (CGRO), Solar Maximum Mission (SMM), Yohkoh}, and radio observations. Remote sensing of energetic particles via hard X-ray bremsstrahlung, gyrosynchrotron emission, and beam-driven plasma emission requi- requires a self-consistent modeling of the particle kinematics in the solar flare plasma, including acceleration, injection, propagation, trapping, and energy loss of the particles. New insights in these kinematic processes have been obtained, besides modeling of energetic particle spectra, increasingly from sub-second timing studies, e.g. in the context of Masuda's discovery of above-the-loop-top hard X-ray sources, from electron time-of-flight delay measurements, from the relative timing of propagation to magnetically conjugate footpoints, from the relative timing of particle signatures in interacting flare loops with quadrupolar geometry, and from the relative timing of gyrosynchrotron emission and hard X-ray signatures in magnetic traps. We anticipate significant progress to be made with the {sl High Energy Solar Spectroscopic Imager (HESSI)}, to be launched in 2000.           ``Magnetic Fields and Solar Processes'', Florence, Italy,          12-18 Sept 1999, ESA SP-448, Dec 1999, in press.

Authors: Aschwanden,M.J.
Projects:

Publication Status: in Proc. of the Ninth European Meeting on Solar Physics,
Last Modified: 1999-10-06 21:04
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Radio and Hard X-Ray Observations of Flares and their Physical Interpretation'', (Invited Review)  

Markus J Aschwanden   Submitted: 1999-10-06 21:04

        Interpretation'', (Invited Review)We review a selection of observations in radio, hard X-rays (HXR) and soft X-rays (SXR) that constrain geometrical and physical requirements for solar flare models. Guided by observations of interacting flare loops we discuss a flare model based on shear-driven quadrupolar reconnection, which explains single-loop and double-loop flares in a unified picture. We interpret various observational findings in the light of this unified flare model: {- topology and geometry of interacting flare loops, } {- localization of particle acceleration region, } {- scale invariance of electron time-of-flight path and flare loop geometry, } {- density and magnetic field diagnostic in accelerati- acceleration region, } {- bi-directionality of injected electron beams, } {- electron beam trajectories and correlated HXR pulses, } {- bifurcation of directly-precipitating and trap-precipitating electrons, } {- density and magnetic field diagnostic of trap region, } {- elementary time scales and dynamics in acceleration region. }         on

Authors: Aschwanden,M.J.
Projects:

Publication Status: in Proc. Nobeyama Symposium
Last Modified: 1999-10-06 21:04
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Do EUV Nanoflares Account for Coronal Heating?  

Markus J Aschwanden   Submitted: 1999-10-06 21:04

Recent observations with EUV imaging instruments such as SoHO/EIT and TRACE have shown evidence for flare-like processes at the bottom end of the energy scale, in the range of Ethapprox 1024-1027 erg. Here we compare these EUV nanoflares with soft X-ray microflares and hard X-ray flares across the entire energy range. From the observations we establish empirical scaling laws for the flare loop length, L(T) propto T, the electron density, n_e(T) propto T^2, from which we derive scaling laws for the loop pressure, p(T) propto T^3, and the thermal energy, Eth propto T^6. Extrapolating these scaling laws into the {sl picoflare regime} we find that the pressure conditions in the chromosphere constrain a height level for flare loop footpoints, which scales with heq(T) propto T-0.5. Based on this chromospheric pressure limit we predict a lower cutoff of flare loop sizes at Lminlapprox 5 Mm and flare energies Eminlapprox 1024 erg. We show evidence for such a rollover in the flare energy size distribution from recent TRACE EUV data. Based on this energy cutoff imposed by the chromospheric boundary condition we find that the energy content of the heated plasma observed in EUV, SXR, and HXR flares is insufficient (by 2-3 orders of magnitude) to account for coronal heating.         and Transition Region'', Monterey, California, 24-27 August 1999),        Solar Physics, Dec issue, in press.

Authors: Aschwanden,M.J.
Projects:

Publication Status: (Contribution to the TRACE workshop ``Physics of the Solar Corona
Last Modified: 1999-10-06 21:04
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Solar Flares: Energetic Particles  

Markus J Aschwanden   Submitted: 1999-10-06 21:04

Solar Flares: Nonthermal electrons         Institute of Physics and Macmillan Publishing,         in press.

Authors: Aschwanden,M.J.
Projects:

Publication Status: in Encyclopedia of Astronomy, (ed. E.Priest),
Last Modified: 2002-04-03 04:52
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Time Variability of the Quiet Sun Observed with TRACE. I. Instrumental Effects, Event Detection, and Discrimination of EUV Nanoflares  

Markus J Aschwanden   Submitted: 1999-10-06 21:02

The {sl Transition and Coronal Explorer (TRACE)} observed a ``Quiet Sun'' region on 1999 Feb 17 from 01: 30 UT to 10: 00 UT with full resolution (0.5'' pixel size), high cadence (125 s), and deep exposures (65 s and 46 s) in the 171 ang and 195 ang wavelengths. We start our investigation of the time variability of ``Quiet Sun'' images with a detailed analysis of instrumental and nonsolar effects, such as orbital temperature variations, filtering of particle radiation spikes, spacecraft pointing jitter, and solar rotation tracking. We quantify the magnitude of various noise components (photon Poisson statistics, data digitization, data compression, readout noise) and establish an upper limit for the data noise level, above which temporal variability can safely be attributed to solar origin. We develop a pattern recognition code which extracts spatio-temporal events with significant variability, yielding a total of 3131 events in 171 ang and 904 events in 195 ang . We classify all 904 events detected in 195 ang according to flare-like characteristics and establish a numerical flare criterion based on temporal, spatial, and dynamic cross-correla- cross-correlation coefficients between the two observed temperatures (0.9 MK and 1.4 MK). This numerical criterion matches the visual flare classification in 83% and can be used for automated flare search. Using this flare discrimination criterion we find that only 35% (and 25%) of the events detected in 171 (and 195) ang represent flare-like events. The discrimination of flare events leads to a frequency distribution of peak fluxes, N(Delta F) propto Delta F-1.83pm0.07 at 195 ang, that is significantly flatter than the distribut- distribution of all events. A sensitive discrimination criterion of flare events is therefore important for microflare statistics and for conclusions on their occurrence rate and efficiency for coronal heating.

Authors: Aschwanden,M.J., Nightingale,R., Tarbell,T. S., and Wolfson,C.J.
Projects:

Publication Status: ApJ 535, 1027-1046, (2000)
Last Modified: 2000-08-31 07:47
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Time Variability of the Quiet Sun Observed with TRACE. II. Physical Parameters, Temperature Evolution, and Energetics of EUV Nanoflares  

Markus J Aschwanden   Submitted: 1999-10-06 21:02

We present a detailed analysis of the geometric and physical parameters of 281 EUV nanoflares, simultaneously detected with the {sl TRACE} telescope in the 171 and 195 ang wavelengths. The detection and discrimination of these flare-like events is detailed in Paper I. We determine the loop length l, loop width w, emission measure EM, the evolution of the electron density n_e(t) and temperature Te(t), the flare decay time { au}decay, and calculate the radiative loss time { au}loss, the conductive loss time { au}cond { au}cond, and the thermal energy Eth. The findings are: (1) EUV nanoflares in the energy range of 1024-1026 erg represent miniature versions of larger flares observed in soft X-rays and hard X-rays, scaled to lower temperatures (Te lapprox 2 MK), lower densities (n_e lapprox 109 cm-3), and somewhat smaller spatial scales (lapprox 2-20 Mm); (2) The cooling time { au}decay is compatible with the radiative cooling time { au}rad, but the conductive cooling time scale { au}cond is about an order of magnitude shorter, suggesting repetitive heating cycles in time intervals of a few minutes; (3) The frequency distribution of thermal energies of EUV nanoflares, N(E)approx 10-46(E/1024)-1.8 [s-1 cm-2 erg-1] matches that of SXR microflares in the energy range of 1026-10^- 1026-1029, and exceeds that of nonthermal energies of larger flares observed in HXR by a factor of 3-10 (in the energy range of 1029-1032 erg. Discrepancies of the power-law slope with other studies, which report higher values in the range of aschwanden@lmsal.com: =2.3-2.6 (Krucker & Benz, or Parnell & Jupp), are attributed to methodical differences in the detection and discrimination of EUV microflares, as well as to different model assumptions in the calculation of the electron density. Besides the insufficient power of nanoflares to heat the corona, we find also other physical limits for nanoflares at energies lapprox 1024 erg, such as the area coverage limit, the heating temperature limit, the lower coronal density limit, and the chromospheric loop height limit. Based on these quantitative physical limitations, it appears that coronal heating requires other energy carriers that are not luminous in EUV, SXR, and HXR.

Authors: Aschwanden,M.J., Tarbell,T.D., Nightingale,R.W., Schrijver,C.J., Title,A.,
Projects:

Publication Status: 2000, ApJ 535, 1047-1065
Last Modified: 2000-08-31 07:59
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Hard X-Ray Timing Experiments with HESSI
Particle Acceleration and Kinematics in Solar Flares and the Solar Corona'' (Invited Review)
Radio and Hard X-Ray Observations of Flares and their Physical Interpretation'', (Invited Review)
Do EUV Nanoflares Account for Coronal Heating?
Solar Flares: Energetic Particles
Time Variability of the Quiet Sun Observed with TRACE. I. Instrumental Effects, Event Detection, and Discrimination of EUV Nanoflares
Time Variability of the Quiet Sun Observed with TRACE. II. Physical Parameters, Temperature Evolution, and Energetics of EUV Nanoflares

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