Solar Physics E-print Archive

Sergey Zharkov: Statistical properties of H-alpha flares in relation to sunspots and active regions in the cycle 23

Thu, 12 Nov 2009 12:49:00 MST

The statistical properties of H flare occurrences compared with those of sunspot and active region areas in the cycle 23. The flare numbers in the cycle 23 of all significances and locations are obtained from the Solar Geophysical Data (SGD) and the other data was taken from the automated Solar Feature Catalogs (SFC, http://solar.inf.brad.ac.uk). The average monthly flare occurrences during the whole cycle correlate closely with the total and cumulative areas of active regions and sunspots. The cumulative distribution of solar flare occurrences at different latitudes in the northern and southern hemispheres versus the time reveal a strong asymmetry with a domination of one or other hemispheres similar to the cumulative distributions of sunspots and active regions. Although the sunspot area asymmetry lags the flare occurrence asymmetry by about time of a few months at the ascending phase of the cycle and up to 12 months in the descending one. The latitudinal distribution of flare occurrences in the whole period reveal a well defined maximum at 18◦ in the northern and two maxima at 14◦ and 20◦ in the southern hemisphere. The longitudinal distributions of flare occurrence residuals of the running values and those averaged with a one year filter reveal a set of persistent longitudes in the opposite hemispheres lasting for about 1.0-1.5 years and changing quickly a few times over the cycle phases.

Valentina Zharkova: Diagnostics of energetic electrons with anisotropic distributions in solar flares I. Hard X-rays bremsstrahlung emission

Thu, 12 Nov 2009 12:39:00 MST

Aims. The paper aims are to simulate steady-state distributions of electrons beams precipitating in collisional and Ohmic losses with pitch angle anisotropy into a flaring atmosphere with converging magnetic field and to apply these to the interpretation of HXR photon spectra, directivity and polarization observed for different photon energies and positions of flares. Methods. Summary approximation method is applied to time-dependent Fokker-Planck equation splitting a temporal derivative equally between the derivatives in depth, energy and pitch angles and finding the solutions in forward and backward directions for each variable. Results. For softer beams there is a noticeable flattening of the photon spectra at lower energies caused by the self- induced electric field which increases for larger viewing angles. For the models with electric field the HXR emission with lower energies (30 keV) becomes directed mainly upwards at upper atmospheric levels owing to the increased number of particles moving upwards, while at the deeper layers it becomes again directed downwards. The polarization maximum shifts to higher energies with every precipitation depth approaching 25 keV for the models with pure collisions and 100 keV for the models with return currents. At deeper layers the polarization decreases because of the izotropization of electrons by collisions. The maximum polarization is observed at the viewing angle of 90◦ becoming shifted to lower angles for softer beams. The integrated polarization and directivity shows dependence on a magnetic field convergence for harder beams while for softer beam the directivity is strongly affected by the self-induced electric field changing from downward to upward one at upper atmospheric depths. Conclusions. The proposed precipitation model for an electron beam with wider pitch angle dispersion of 0.2 taking into account collisional and Ohmic losses allowed us to fit the double power law HXR photon spectra with a spectrum flattening at lower energies observed in the flares of 20 and 23 July 2002. The observed directivity of HXR photons of 20 keV derived for a large number of flares located from the disk center to limb is also well reproduced by the theoretical directivity calculated for an electron beam with a very narrow pitch angle dispersion of 0.02. The simulated polarization of such the narrowly-directed electron beam fits up to 90% of all the available polarimetric observations carried out at various locations on the solar disk.

Ilya Chertok: On the Correlation between Spectra of Solar Microwave Bursts and Proton Fluxes near the Earth

Wed, 11 Nov 2009 06:26:00 MST

Studies of the extreme solar proton event of January 20, 2005 intensified the contest over of a long-standing problem: are solar cosmic rays arriving at the Earth accelerated by solar flares or by shocks preceding rapidly moving coronal mass ejections? Among the most important questions is the relationship between the energy spectra of the solar cosmic rays and the frequency spectra of flare microwave bursts. Some studies of previous solar-activity cycles have shown that such a relationship does exist, in particular, for protons with energies of tens of MeV. The present work analyzes this relation using data for 1987-2008. For flare events observed in the western half of the disk, there is a significant correlation between the index d=log(J10/J100), (J10 and J100 are the peak proton fluxes at E > 10 and 100 MeV, respectively; d is equivalent to the power-law index of the integrated energy spectrum of 10-100 MeV protons detected near the Earth?s orbit) and radio burst parameters such as a ratio of peak fluxes S at two frequencies (for example, at 9 and 15 GHz) and a microwave peak frequency fm. Proton fluxes with hard (flat) energy spectra (d~<1.5) correspond to hard microwave frequency spectra (S9/S15~<1 and fm>15 GHz), while flares with soft radio spectra (S9/S15>~1.5 and fm~<5 GHz) result in proton fluxes with soft (steep) energy spectra (d>~1.5-2). It is also shown that powerful high-frequency bursts with the hardest radio spectra (fm~30 GHz) can point at acceleration of significant proton fluxes in flares occurring in strong magnetic fields. These results argue that solar cosmic rays (or at least their initial impulses) are mainly accelerated in flares associated with impulsive and post-eruptive energy release, rather than in shocks driven by coronal mass ejections.

John Harvey: Sun-as-a-Star, Chromospheric Lines, 1974-2009

Tue, 10 Nov 2009 15:01:00 MST

We update the McMath-Pierce spectral line scan archives for Ca II K, He I 10830 A, Ca II 8542 A, and H I 6562 A, both for full disk and center disk. The Ca K3 intensity feature displays a peak-to-peak activity cycle modulation of ~37% and He 10830 A about 100%. SOLIS observations of the Ca K index suggest a cycle 23-24 minimum in late 2008 followed by a 0.7% rise by late 2009. Other McMath-Pierce indices display no evidence yet of cycle 24. Center-disk Ca K index measurements (optically averaged over the central 2-arcmin), show no particular response to cycle activity, i.e. the quiet disk is constant. A similar null response is found for the center-disk Wilson-Bappu effect.

Guillaume Aulanier: FORMATION OF TORUS-UNSTABLE FLUX ROPES AND ELECTRIC CURRENTS IN ERUPTING SIGMOIDS

Fri, 06 Nov 2009 05:48:00 MST

We analyze the physical mechanisms that form a three-dimensional coronal flux rope and later cause its eruption. This is achieved by a zero-beta MHD simulation of an initially potential, asymmetric bipolar field, which evolves by means of simultaneous slow magnetic field diffusion and sub-Alfvenic, line-tied shearing motions in the photosphere. As in similar models, flux-cancellation driven photospheric reconnection in a bald-patch separatrix transforms the sheared arcades into a slowly rising and stable flux rope. A bifurcation from a bald-patch to a quasi-separatrix layer (QSL) topology occurs later on in the evolution, while the flux rope keeps growing and slowly rising, now due to shear-driven coronal slip-running reconnection, which is of tether-cutting type and takes place in the QSL. As the flux rope reaches the altitude at which the decay index −d lnB/d ln z of the potential field exceeds ∼ 3/2, it rapidly accelerates upward while the overlying arcade eventually develops an inverse tear-drop shape, as observed in coronal mass ejections (CMEs). This transition to eruption is in accordance with the onset criterion of the torus instability. Thus we find that photospheric flux-cancellation and tether-cutting coronal reconnection do not trigger CMEs in bipolar magnetic fields, but are key pre-eruptive mechanisms for flux ropes to build up and to rise to the critical height above the photosphere at which the torus instability causes the eruption. In order to interpret recent Hinode X- Ray Telescope observations of an erupting sigmoid, we produce simplified synthetic soft X-ray images from the distribution of the electric currents in the simulation. We find that a bright sigmoidal envelope is formed by pairs of J-shaped field lines in the pre-eruptive stage. These field lines form through the bald-patch reconnection, and merge later on into S-shaped loops through the tethercutting reconnection. During the eruption, the central part of the sigmoid brightens due to the formation of a vertical current layer in the wake of the erupting flux rope. Slip-running reconnection in this layer yields the formation of flare loops. A rapid decrease of currents due to field line expansion, together with the increase of narrow currents in the reconnecting QSL, yields the sigmoid hooks to thin in the early stages of the eruption. Finally, a slightly rotating erupting loop-like feature (ELLF) detaches from the center of the sigmoid. Most of this ELLF is not associated with the erupting flux rope, but with a current shell which develops within expanding field lines above the rope. Only the short, curved end of the ELLF corresponds to a part of the flux rope. We argue that the features found in the simulation are generic for the formation and eruption of soft X-ray sigmoids.

Ashley Crouch: Resolving the Azimuthal Ambiguity in Vector Magnetogram Data with the Divergence-Free Condition: Application to Discrete Data

Wed, 04 Nov 2009 20:57:00 MST

We investigate how the divergence-free property of magnetic fields can be exploited to resolve the azimuthal ambiguity present in solar vector magnetogram data, by using line-of-sight and horizontal heliographic derivative information as approximated from discrete measurements. Using synthetic data we test several methods that each make different assumptions about how the divergence-free property can be used to resolve the ambiguity. We find that the most robust algorithm involves the minimisation of the absolute value of the divergence summed over the entire field of view. Away from disk centre this method requires the sign and magnitude of the line-of-sight derivatives of all three components of the magnetic field vector.

Leonid Didkovsky: Minima of Solar Cycles 22/23 and 23/24 as Seen in SOHO/CELIAS/SEM Absolute Solar EUV Flux

Wed, 04 Nov 2009 09:54:00 MST

Verified and updated calibrated absolute solar flux in the He II 30.4 nm spectral band-pass as measured by the Solar EUV Monitor (SEM) allows us to study variations of the solar EUV irradiance near the minima of Solar Cycles 22/23 and 23/24. Based on eight (1996 to 2007) NASA sounding rocket flights, a comparison of SEM data with the measurements from three independent EUV instruments was performed to verify and confirm the accuracy of the published SEM data. SEM calibrated data were analyzed to determine and compare minima for solar cycles 22/23 and 23/24. The minima points were calculated using SEM first order daily averaged flux smoothed by a running mean (RM) filter with the window of averaging equal to 365 days. These minima occurred on June 2, 1996 (22/23) and November 28, 2008 (23/24). The 23/24 minimum showed about 15% lower EUV flux in the 30.4 nm band-pass than the 22/23 minimum. The 365-day RM curve around the 23/24 minimum has significant asymmetry (fast decrease of the EUV flux to the minimum and a long, near-horizontal profile after the minimum). This profile is quite different from the much faster and symmetrical change of the flux around the 22/23 minimum. SEM flux was compared with both high spectral resolution (0.1 nm) Mg II index calculated from the Solar Radiation and Climate Experiment (SORCE) using the Solar Stellar Irradiance Comparison Experiment (SOLSTICE) data and with the NOAA composite Mg II index spectrum.

Sanjay Gosain: HINODE OBSERVATIONS OF COHERENT LATERAL MOTION OF PENUMBRAL FILAMENTS DURING A X-CLASS FLARE

Wed, 04 Nov 2009 03:08:00 MST

The X-3.4 class flare of 13 December 2006 was observed with a high cadence of 2 minutes at 0.2 arc-sec resolution by HINODE/SOT FG instrument. The flare ribbons could be seen in G-band images also. A careful analysis of these observations after proper registration of images show flare related changes in penumbral filaments of the associated sunspot, for the first time. The observations of sunspot deformation, decay of penumbral area and changes in magnetic flux during large flares have been reported earlier in the literature. In this Letter, we report lateral motion of the penumbral filaments in a sheared region of the -sunspot during the X-class flare. Such shifts have not been seen earlier. The lateral motion occurs in two phases, (i) motion before the flare ribbons move across the penumbral filaments and (ii) motion afterwards. The former motion is directed away from expanding flare ribbons and lasts for about four minutes. The latter motion is directed in the opposite direction and lasts for more than forty minutes. Further, we locate a patch in adjacent opposite polarity spot moving in opposite direction to the penumbral filaments. Together these patches represent conjugate foot-points on either side of the polarity inversion line (PIL), moving towards each other. This converging motion could be interpreted as shrinkage of field lines.

Leonid Didkovsky: EUV SpectroPhotometer (ESP) in Extreme Ultraviolet Variability Experiment

Tue, 03 Nov 2009 20:33:00 MST

The Extreme ultraviolet SpectroPhotometer (ESP) is one of five channels of the Extreme ultraviolet Variability Experiment (EVE) onboard the NASA Solar Dynamics Observatory (SDO). The ESP channel design is based on a highly stable diffraction transmission grating and is an advanced version of the Solar Extreme ultraviolet Monitor (SEM), which has been successfully observing solar irradiance onboard the Solar and Heliospheric Observatory (SOHO) since December 1995. ESP is designed to measure solar Extreme UltraViolet (EUV) irradiance in four first order bands of the diffraction grating centered around 19 nm, 25 nm, 30 nm, and 36 nm, and in a soft X-ray band from 0.1 to 7.0 nm in the zeroth order of the grating. Each band's detector system converts the photo-current into a count rate (frequency). The count rates are integrated over 0.25 sec increments and transmitted to the EVE Science and Operations Center for data processing. An algorithm for converting the measured count rates into solar irradiance and the ESP calibration parameters are described. The ESP pre-flight calibration was performed at the Synchrotron Ultraviolet Radiation Facility of the National Institute of Standards and Technology. Calibration parameters were used to calculate absolute solar irradiance from the Sounding Rocket flight measurements on 14 April 2008. These irradiances for the ESP bands closely match the irradiance determined for two other EUV channels flown simultaneously, EVE's Multiple Euv Grating Spectrograph (MEGS) and SOHO's Charge, Element and Isotope Analysis System / Solar EUV Monitor (CELIAS/SEM).

Iain Hannah: The effect of wave-particle interactions on low energy cutoffs in solar flare electron spectra

Mon, 02 Nov 2009 19:41:00 MST

Solar flare hard X-ray spectra from RHESSI are normally interpreted in terms of purely collisional electron beam propagation, ignoring spatial evolution and collective effects. In this paper we present self-consistent numerical simulations of the spatial and temporal evolution of an electron beam subject to collisional transport and beam-driven Langmuir wave turbulence. These wave-particle interactions represent the background plasma's response to the electron beam propagating from the corona to chromosphere and occur on a far faster timescale than coulomb collisions. From these simulations we derive the mean electron flux spectrum, comparable to such spectra recovered from high resolution hard X-rays observations of solar flares with RHESSI. We find that a negative spectral index (i.e. a spectrum that increases with energy), or local minima when including the expected thermal spectral component at low energies, occurs in the standard thick-target model, when coulomb collisions are only considered. The inclusion of wave-particle interactions does not produce a local minimum, maintaining a positive spectral index. These simulations are a step towards a more complete treatment of electron transport in solar flares and suggest that a flat spectrum (spectral index of 0 to 1) down to thermal energies maybe a better approximation instead of a sharp cut-off in the injected electron spectrum.