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On the Role of the Background Overlying Magnetic Field in Solar Eruptions  

Alexander Nindos   Submitted: 2012-02-16 03:35

The primary constraining force that inhibits global solar eruptions isprovidedby the overlying background magnetic field. Using magnetic field datafrom both the Helioseismic and Magnetic Imager aboard Solar DynamicsObservatoryand the spectropolarimeter of the Solar Optical Telescope aboard Hinode,we study the long-term evolution of the background field in activeregionAR11158 that produced three major coronal mass ejections (CMEs). The CMEformation heights were determined using EUV data. We calculated thedecayindex -(Z/B)(dB/dz_ of the magnetic field B (i.e. how fast the fielddecreases with height, z) related to each event from the time of theactive region emergenceuntil well after the CMEs. At the heights of CME formation, the decayindices were 1.1-2.1. Prior to two of the events, there were extendedperiods (ofmore than 23 hours) where the related decay indices at heights abovethe CMEformation heights either decreased (up to -15%) or exhibited smallchanges. Thedecay index related to the third event increased (up to 118%) atheights above20 Mm within an interval that started 64 hours prior to the CME. Themagneticfree energy and the accumulated helicity into the corona contributedthe mostto the eruptions by their increase throughout the flux emergence phase(by factorsof more than 5 and more than two orders of magnitude, respectively). Ourresults indicate that the initiation of eruptions does not dependcritically on thetemporal evolution of the variation of the background field with height.

Authors: A. Nindos, S. Patsourakos, T. Wiegelmann
Projects: None

Publication Status: ApJ (Letters), in press
Last Modified: 2012-02-17 08:42
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Pulsating Solar Radio Emission  

Alexander Nindos   Submitted: 2006-02-01 04:29

A status report of current research on pulsating radio emission is given, based on working group discussions at the CESRA 2004 workshop. Quasi-periodic pulsations have been observed at all wavelength ranges of the radio band. Usually, they are associated with flare events; however since the late 90s, pulsations of the slowly-varying component of the Sun's radio emission have also been observed. Radio pulsations show a large variety in their periods, bandwidths, amplitudes, temporal and spatial signatures. Most of them have been attributed to MHD oscillations in coronal loops, while alternative interpretations consider intrinsic oscillations of a nonlinear regime of kinetic plasma instabilities or modulation of the electron acceleration. Combined radio spectroscopic observations with radio imaging and X-ray/EUV data have revived interest in the subject. We summarize recent progress in using radio pulsations as a powerful tool for coronal plasma and magnetic field diagnostics. Also the latest developments on the study of the physical processes leading to radio emission modulation are summarized.

Authors: A. Nindos, H. Aurass
Projects: None

Publication Status: Proceedings of CESRA Workshop 2004 The high-energy solar corona: waves, eruptions, particles, to be published in Lecture Notes in Physics, 2006 (paper accepted for publication on July 12, 2005)
Last Modified: 2006-02-01 04:29
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Magnetic Helicity and Coronal Mass Ejections  

Alexander Nindos   Submitted: 2005-11-02 03:52

Magnetic helicity is a quantity that descibes the chiral properties ofmagnetic structures. It has the unique feature that it is probably theonly physical quantity which is approximately conserved even inresistive MHD. This makes magnetic helicity an ideal tool for theexploration of the physics of coronal mass ejections (CMEs). CMEscarry away from the Sun twisted magnetic fields and the concept ofhelicity can be used to monitor the whole history of a CME event fromthe emergence of twisted magnetic flux from the convective zone to theeruption and propagation of the CME into interplanetary space. Idiscuss the sources of the helicity shed by CMEs and the role ofmagnetic helicity in the initiation of CMEs.

Authors: A. Nindos
Projects:

Publication Status: Proc. Chapman Conf. on "Solar Energetic Plasmas & Particles" , (Washington, DC: AGU), in press [submitted 2/28/2005, accepted 7/31/2005]
Last Modified: 2005-11-02 03:52
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The Association of Big Flares and Coronal Mass Ejections: What Is the Role of Magnetic Helicity?  

Alexander Nindos   Submitted: 2004-12-06 03:18

Recently, M. D. Andrews found that approximately 40% of M-class flares between 1996 and 1999, classified according to GOES X-ray flux, are not associated with coronal mass ejections (CMEs). Using 133 events from his data set for which suitable photospheric magnetograms and coronal images were available, we studied the preflare coronal helicity of the active regions that produced big flares. The coronal magnetic field of 78 active regions was modeled under the ``constant α ' linear force-free field assumption. We find that in a statistical sense the preflare value of α and coronal helicity of the active regions producing big flares that do not have associated CMEs is smaller than the coronal helicity of those producing CME-associated big flares. A further argument supporting this conclusion is that for the active regions whose coronal magnetic field deviates from the force-free model, the change of the coronal sign of α within an active region is twice as likely to occur when the active region is about to produce a confined flare than a CME-associated flare. Our study indicates that the amount of the stored preflare coronal helicity may determine whether a big flare will be eruptive or confined.

Authors: A. Nindos, M.D. Andrews
Projects: None

Publication Status: ApJ (Letters), 616, L175 (December 1 2004)
Last Modified: 2004-12-06 07:04
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The Configuration of Simple Short-Duration Solar Microwave Bursts  

Alexander Nindos   Submitted: 2004-03-09 05:01

Using data from the Nobeyama Radioheliograph (NoRH) we study the source configuration of four simple short-duration 17 and 34 GHz bursts which have also been observed partially by the Yohkoh Soft X-ray Telescope (SXT). Two events are consistent with a single flaring loop configuration. In one of them the flaring loop is resolved in the SXT images. We derive a self-consistent model for this event by comparing the radio observations with gyrosynchrotron model loop calculations. Our best-fit model is able to reproduce both the observed flaring loop shape as well as the fluxes and structures of the radio emission at the peak of the event. The flaring loop is relatively small having a footpoint separation of 16arcsec and maximum height of 7.7arcsec. The variation of the magnetic field along the loop is small (800 G at the footpoints and 665 G at loop top) and the loop is filled with electrons with energies up to 10 MeV. The other two bursts show two radio sources; one source being cospatial with a compact bright soft X-ray loop associated with a patch of parasitic magnetic polarity whose photospheric magnetic flux increases before the flares while the other source is not prominent at any other wavelength range. The two sources are connected with diffuse loop-like soft X-ray emission. We infer that these bursts originate from the interaction of two sets of loops with different sizes. Therefore the simple short duration microwave bursts we studied do not always appear in the same configuration. Contrary to previous results not all of them appear as single-loop events. It is possible that some events are caused by two interacting loops.

Authors: M.R. Kundu, A. Nindos, V.V. Grechnev
Projects: None

Publication Status: A&A, in press
Last Modified: 2004-03-09 05:01
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The Magnetic Helicity Budget of Solar Active Regions and Coronal Mass Ejections  

Alexander Nindos   Submitted: 2003-05-26 07:59

We compute the magnetic helicity injected by transient photospheric horizontal flows in 6 solar active regions associated with halo coronal mass ejections (CMEs) that produced major geomagnetic storms and magnetic clouds (MCs) at 1 AU. The velocities are computed using the local correlation tracking (LCT) method. Our computations cover time intervals of 110-150 hours and in 4 active regions the accumulated helicities due to transient flows are factors of 8-12 larger than the accumulated helicities due to differential rotation. As has been first pointed out by Demoulin & Berger (2003), we suggest that the helicity computed with the LCT method yields not only the helicity injected from shearing motions but also the helicity coming from flux emergence. We compare the computed helicities injected into the corona with the helicities carried away by the CMEs using the MC helicity computations as proxies to the CME helicities. If we assume that the length of the MC flux tubes is ell=2 AU then the total helicities injected into the corona are a factor of 2.9-4 lower than the total CME helicities. If we use the values of ell determined by the condition for the initiation of the kink instability in the coronal flux rope or ell=0.5 AU then the total CME helicities and the total helicities injected into the corona are broadly consistent. Our study, at least partially, clears up some of the discrepancies in the helicity budget of active regions because the discrepancies appearing in our paper are much smaller than the ones reported in previous studies. However they point out the uncertainties in the MC/CME helicity calculations and also the limitations of the LCT method which underestimates the computed helicities.

Authors: A. Nindos, J. Zhang, H. Zhang
Projects: None

Publication Status: ApJ, in press
Last Modified: 2003-05-26 07:59
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Photospheric Motions and Coronal Mass Ejection Productivity  

Alexander Nindos   Submitted: 2002-06-28 00:35

Shearing motions have been frequently used in MHD simulations of CME initiation but have hardly been reported from observations of CME-producing regions. In this paper we investigate whether the bulk of magnetic helicity carried away from the Sun by CMEs comes from helicity injected to the corona by such motions or by emerging magnetic flux. We use photospheric magnetic field observations of active region NOAA 9165 (AR 9165) which is an ideal candidate for such study because (1) it is the site of both new flux emergence and intense horizontal shearing flows; (2) it shows rapid development and rapid decay and for a few days it is the site of violent activity; (3) the horizontal motions occur when it is close to disk center, thus minimizing the errors involved in the relevant computations; (4) observations of a magnetic cloud associated with one of the CMEs linked to the active region are available. The computed helicity change due to horizontal shearing motions is probably the largest ever reported; it amounts to about the total helicity that the active region's differential rotation would have injected within 3 solar rotations. But the CMEs linked to the active region remove at least a factor of 4-64 more helicity than the helicity injected by horizontal shearing motions. Consequently the main source of the helicity carried away by the CMEs is the new magnetic flux that emerges twisted from the convective zone. Our study implies that shearing motions, even when they are strong, have little effect in the process of buildup of magnetic free energy that leads to the initiation of CMEs.

Authors: A. Nindos, H. Zhang
Projects:

Publication Status: ApJ, 573, L133
Last Modified: 2002-06-28 00:35
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Spatially resolved microwave oscillations above a sunspot  

Alexander Nindos   Submitted: 2002-06-28 00:29

Using high quality VLA observations, we detected for the first time spatially resolved oscillations in the microwave total intensity (I) and circular polarization (V) emission of a sunspot-associated gyroresonance (g-r) source. Oscillations were detected at 8.5 and 5 GHz during several time intervals of our 10-hour-long dataset. The oscillations are intermittent: they start suddenly and are damped somehow more gradually. Despite their transient nature when they are observed they show significant positional, amplitude and phase stability. The spatial distribution of intensity variations is patchy and the location of the patches of strong oscillatory power is not the same at both frequencies. The strongest oscillations are associated with a small region where the 8.5 GHz emission comes from the second harmonic of the gyrofrequency while distinct peaks of weaker oscillatory power appear close to the outer boundaries of the 8.5 and 5 GHz g-r sources, where the emissions come from the third harmonic of the gyrofrequency. Overall, the 5 GHz oscillations are weaker than the 8.5 GHz oscillations (the rms amplitudes of the I oscillations are 1.3-2.5 imes 104 K and 0.2-1.5 imes 105 K, respectively). At both frequencies the oscillations have periods in the three-minute range: the power spectra show two prominent peaks at 6.25-6.45 mHz and 4.49-5.47 mHz. Our models show that the microwave oscillations are caused by variations of the location of the third and/or second harmonic surfaces with respect to the base of the chromosphere-corona transition region (TR), i.e. either the magnetic field strength or/and the height of the base of the TR oscillates. The best-fit model to the observed microwave oscillations can be derived from photospheric magnetic field strength oscillations with an rms amplitude of 40 G or oscillations of the height of the base of the TR with an rms amplitude of 25 km. Furthermore small variations of the orientation of the magnetic field vector yield radio oscillations consistent with the observed oscillations.

Authors: A. Nindos, C.E. Alissandrakis, G.B. Gelfreikh, V.M. Bogod, C. Gontikakis
Projects:

Publication Status: A&A, 386, 658
Last Modified: 2002-06-28 00:29
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Abstracts by Author
On the Role of the Background Overlying Magnetic Field in Solar Eruptions
Pulsating Solar Radio Emission
Magnetic Helicity and Coronal Mass Ejections
The Association of Big Flares and Coronal Mass Ejections: What Is the Role of Magnetic Helicity?
The Configuration of Simple Short--Duration Solar Microwave Bursts
The Magnetic Helicity Budget of Solar Active Regions and Coronal Mass Ejections
Photospheric Motions and Coronal Mass Ejection Productivity
Spatially resolved microwave oscillations above a sunspot

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