SPA-Solar and Heliospheric Physics General Contributions I
Presiding: G M Mason, Department of Physics, University of Maryland at College Park; J Zhang, George Mason University
SH31A-01 08:30h
Solar Hard X-ray Emissions in the Decay Phase of Gradual X-class Flares
Solar hard X-ray emissions are most often observed during the impulsive phase of solar flares and are produced by non-thermal electrons that lose their energy in the chromosphere by collisions. During large gradual flares, however, HXR emissions can last well into the decay phase showing progressively hardening X-ray spectra (e.g. Cliver et al. 1986). Here we present X-ray observations of such events obtained by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). RHESSI provides for the first time high resolution (1 keV) spectral observations with simultaneous imaging down to 2". In about half of all X-class flares seen by RHESSI, enhanced HXR emission lasting well into decay phase (up to two hours after the flare onset) is observed. These emissions are generally ~10 times fainter and show much less time variations than HXR bursts occurring during the impulsive phase. The X-ray spectra are found to be hard and can generally be fitted with broken power law functions. First results from one of the most prominent events (2005 January 19, 8UT) show clear spectral hardening. Over 15 minutes, the spectral power law index hardens below the break (from 2.6 to 2.1) as well as above it (from 3.3 to 2.6). Furthermore, the break energy increases from 40 to 60 keV. Imaging reveals that the emission come from footpoints of post flare loops suggesting that it is produced in a thick target. No HXR emission is seen to originate from the post flare loops. We are presently investigating the importance of trapping of energetic electrons in these events.
SH31A-02 08:45h
RHESSI Observations of Hard X-ray Footpoint Motions in Solar Flares
Solar HXR bremsstrahlung from energetic electrons accelerated in the impulsive phase of a flare is observed to be primarily from the footpoints of magnetic loops. Standard magnetic reconnection models predict increasing separation of the footpoints during the flare as longer and larger loops are produced. If the reconnection process results in accelerated electrons, the HXR footpoints should show this motion.The motion is only apparent; it is due to the HXR emission shifting to footpoints of neighboring newly reconnected field lines. The speed of footpoint separation reflects the rate of magnetic reconnection and should be roughly proportional to the total energy deposition in the footpoints. We studied footpoint motions in several large flares with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and compared the motion with the energy deposition rate in the footpoints. A correlation between the motion and the energy deposition rate is found during some periods in these flares, but not always.
SH31A-03 09:00h
RHESSI soft X-ray imaging spectroscopy of a flare
We apply RHESSI imaging spectroscopy to a well-observed solar flare on 26~April~2003. This GOES M2-class flare (N20W69) exercised all three of the RHESSI shutter states, and was simultaneously observed by the RESIK high-resolution X-ray spectrometer on board the CORONAS-F spacecraft (Dennis et al., 2005). It thus provides an excellent opportunity to study the behavior of the high-throughput RHESSI observations of the Fe~emission-line complex at ~6.7~keV. The equivalent width of this feature has a unique dependence on the assumed isothermal temperature of the source (Phillips, 2004). Comparing this feature-derived temperature between the onset and decay phases of the event, we note a discrepancy between it and the temperature derived directly from the continuum. We analyze the causes of this discrepancy in terms of image morphology, non-isothermality, non-equilibrium excitation, and instrument properties.
SH31A-04 09:15h
UVCS Observations of Slow Plasma Flow in the Corona Above Active Regions
The elusive source of slow solar wind has been the subject of ongoing discussion and debate. Observations of solar wind speed near the Earth orbit, first with IPS (interplanetary scintillation) and later with Ulysses in situ measurements, have suggested that some slow solar wind may be associated with active regions (Kojima & Kakinuma 1987; Woo, Habbal & Feldman 2004). The ability of SOHO UVCS Doppler dimming measurements to provide estimates of solar wind speed in the corona (Kohl et al. 1995) has made it possible to investigate the distribution of flow near the Sun. In this paper, we will present results confirming that active regions are one of the sources of slow wind. Insight into the relationship between coronal streamers, active regions and plasma flow will also be discussed.
SH31A-05 09:30h
Non-linear force-free field modeling: model techniques, boundary conditions, hares, and hounds
Understanding the conditions under which solar magnetic fields can destabilize to erupt in flares and coronal mass ejections requires a quantitative understanding of the coronal magnetic field and of the currents that it carries. The increased availability of vector magnetograms, together with EUV and X-ray coronal images, should provide adequate constraints to model the coronal field, and thus to visualize its 3D geometry and to measure the available free energy and helicity. Non-linear force-free fields (NLFFF) are likely a useful model to use when extrapolating the solar surface field upward into the coronal volume. It may even be possible to use the observed trajectories of coronal loops, evident in EUV images of the corona, as a further constraint. We present initial results of a team effort to understand the intricacies of NLFFF modeling: we discuss and evaluate comparisons of NLFFF models computed with different models and applications of boundary conditions, and look ahead to full coronal field modeling for the upcoming Solar-B and SDO missions.
SH31A-06 09:45h
N-S asymmetry, area and longitudinal distributions of sunspots and their magnetic fields in different phases on the solar cycle 23.
The statistical analysis of sunspot area and their magnetic field distributions are presented for the 1996-2004 data in the Solar Feature Catalogues (SFC) automatically extracted from the SOHO/NDI white light solar images and magnetograms. The number of sunspots is found to increase exponentially with the area decrease with a slightly increasing index from the solar minimum to its maximum. The N-S asymmetry in sunspot area distributions and its periodicity for different phases of the solar cycle and hemispheres is investigated. There is also a strong North-South asymmetry in the longitudes of sunspots appearance that varies with the phase of the solar cycle. The magnetic field distributions for the total and excess fluxes on the sunspot heliospheric longitude and latitude are presented for different phases of the solar cycle. These statistical properties of sunspots and their magnetic field are discusses in compariosn with those predicted by the turbulent dynamo theory.
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