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The Effect of Background Subtraction on the Temperature of EIT Coronal Loops  

Piet Martens   Submitted: 2003-09-15 16:08

We have selected a sample of ten coronal loops that were clearly visible in the 171 Å passband of the SOHO/EIT, five on the limb and five on the disk. Our analysis was limited to 171/195/284 image triplets observations taken when the instrument cycles through the different passbands during routine operations. This cycle takes only a few minutes, so each of these non-flaring structures did not change significantly during the cycle. We chose five pixels along each loop, and five carefully selected background pixels. Temperature analysis was done four different ways: (1) standard EIT analysis on the five loop pixels with no background subtraction; (2) constant background subtraction for each EIT image; (3) pixel pair background subtraction; and (4) radial background array subtraction (this method works only for loops observed above the limb). Each method produced two estimates of temperature for each loop pixel, one from the 171-to-195 ratio and the second for the 195-to-284 ratio. Both ratios produced loops with a uniform temperature, but each ratio results in a statistically different temperature value, perhaps indicating that the plasma along the line of sight was not isothermal. Background subtraction did not affect the EIT temperature analysis, i.e., the results were the same with and without background subtraction. The results for loops on the limb were cleaner, i.e., had less scatter, than for loops on the disk. Finally, we did a similar temperature analysis with five randomly chosen pixels for each data set. The results were the same as for the loop pixels: two statistically different, uniform temperature structures. These findings indicate that EIT ratio analysis does not generate a physically meaningful value for the electron temperature.

Authors: J. T. Schmelz, J. E. Beene, K. Nasraoui, H. T. Blevins, P. C. H. Martens, and J. W. Cirtain
Projects: Soho-EIT

Publication Status: ApJ, in press
Last Modified: 2003-09-15 16:08
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The Origin of Prominences and Their Hemispheric Preferences for Chirality and the Skew of Overlying Loops  

Piet Martens   Submitted: 2003-09-15 12:42

I review a ``head-to-tail'' linkage model for the formation, evolution, and eruption of solar filaments. The magnetic field structure of the model is based upon the observation that filaments form exclusively in filament channels with no apparent magnetic connections low above the polarity inversion line. The formation of a filament in this configuration is driven by flux convergence and cancellation, which produces loop-like filaments segments with a half-turn. Filament segments of like chirality may connect and form long quiescent filaments. I analyze the topology of the field in and around filaments formed through ''head-to-tail'' linkage and find consistency with the observed hemispheric preference for the skew of the X-ray emitting loops that vault the prominence, contrary to what one would expect from differential rotation.

Authors: Piet Martens
Projects: Yohkoh-SXT

Publication Status: COSPAR Colloquia Series, Volume 13 (2002), P.C.H. Martens and D. Cauffman (eds.), pp. 135-138. (Elsevier: Dordrecht)
Last Modified: 2003-09-15 12:42
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SADE, The Starspot And Dynamo Explorer  

Piet Martens   Submitted: 2003-09-15 12:37

We propose a mission called SADE, the Starspot And Dynamo Explorer, to study dynamo activity in nearby late-type stars. The onboard instruments will be a Ca-K telescope for magnetically dominated chromospheric emission, and an X-ray grazing incidence telescope to study coronal emission. We design the mission for a life-time of 15 years or longer to capture a full activity cycle for most solar-type stars. We aim to firmly establish the spectrum of the relation between chromospheric and coronal emission in late-type stars, and capture one or more stars going into or coming out of a Maunder type minimum. Operation costs will be kept to a minimum by automating mission operations to a maximum, and have the science operations be carried out by students at Montana State University.

Authors: Martens, P.C.H.,Acton, L.W.A.,Klumpar, D., Kankelborg, C., Stern, R.A., Peres, G., and Culhane, J.L.
Projects: None

Publication Status: Advances in Space Research, in press
Last Modified: 2003-09-15 12:37
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Analysis of a Coronal Loop on the Limb  

Piet Martens   Submitted: 2003-09-15 12:34

The ability to accurately measure the temperature and density of the solar atmosphere is essential to understanding the physical nature of coronal loops. After data has been corrected to account for cosmic ray incidents on the CCD and instrumental effects, there are additional sources of error that must also be addressed. We applied a correction to the intensities for spectral lines to account for line of sight effects. Having performed these procedures, the corrected intensities from the Coronal Diagnostic Spectrometer are then used to produce differential emission measure curves with much improved error estimates.

Authors: Cirtain, J.W., Martens, P.C.H., and Winter, H.D.
Projects: None

Publication Status: Advances in Space Research, in press
Last Modified: 2003-09-15 12:34
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Yohkoh-SXT Observations of Reconnection (Invited)  

Piet Martens   Submitted: 2003-09-15 12:23

In this paper I review the different strong indications for the occurrence of reconnection in the solar corona observed of with Yohkoh-SXT. I make a distinction between the unique indicator of reconnection, i.e. the change in connectivity in the magnetic field, and non-unique signatures of reconnection, that also can originate from other processes. I review five different types of Yohkoh-SXT observations that indicate reconnection in the solar corona: erupting plasmoids, X-ray jets, formation of sigmoids through ''linkage'', the existence of interconnecting loops, and the rigid rotation of coronal holes. I conclude that the presence of reconnection in the solar corona has been established with near certainty, but that higher resolution soft X-ray and EUV observations are needed to firmly establish the full physical picture for each of the phenomena described.

Authors: P.C.H. Martens
Projects: Yohkoh-SXT

Publication Status: Advances in Space Research, in press
Last Modified: 2003-09-15 12:23
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Blind Deconvolution of the SXT PSF Core Part  

Piet Martens   Submitted: 2003-09-15 12:16

We explore a blind iterative deconvolution algorithm for the determination of the core part of the point spread function of the soft X-ray telescope aboard the Yohkoh spacecraft. The algorithm has been adapted and modified to deal with the in-flight recorded X-ray images of solar flares. Particular care has been taken to achieve good data selection and initial conditions in order to improve the algorithm performance and convergence. We show an example of a deconvolved point spread function core profile and compare it with ground calibration data.

Authors: S. Gburek, J. Sylwester, and P.C.H. Martens
Projects: Yohkoh-SXT

Publication Status: COSPAR Colloquia Series, Volume 13 (2002), P.C.H. Martens and D. Cauffman (eds.), pp. 417-418. (Elsevier: Dordrecht)
Last Modified: 2003-09-15 12:16
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Multi-Wavelength Observations of Coronal Structure and Dynamics - Yohkoh 10th Anniversary Meeting  

Piet Martens   Submitted: 2003-09-15 12:11

These are the Proceedings of the Yohkoh 10th Anniversary Meeting, a COSPAR Colloquium held in Kona, Hawaii, USA, on January 20-24, 2002. There were 133 participants from 14 countries. The Colloquium was sponsored by (in no specific order) COSPAR, NASA, the MSU Solar Physics Group, LMSAL, ISAS, the Solar Physics Research Corporation, Elsevier Science, and the NSF. Thanks to the largesse of these sponsors the conference organizers were able to provide financial assistance in part or in whole to 30 participants. The meeting had originally been planned for September 2001 at the same location, but had to be postponed due to the difficulty in travel following the terrible events of September 11, 2001. We appreciate the understanding and flexibility of the participants and of the conference hotel in rescheduling this meeting. The title of the meeting was ''Multi-Wavelength Observations of Coronal Structure and Dynamics''. We reviewed the many and varied advances in our understanding of the dynamic solar atmosphere in the past ten years of observations by Yohkoh, often in collaboration with SOHO, TRACE, Ulysses, and many ground-based observatories. The Scientific Organizing Committee made a concerted effort to invite younger scientists to present the Invited Reviews. This was a success and we have a large number of excellent review papers in this volume. This policy had the additional positive effect of freeing up several of the senior scientists to present exciting new results from their own research in contributed talks. The large number of different sessions in these Proceedings reflects the large variety in science topics that are being addressed with Yohkoh data. In all, these Proceedings reflect the quality, breadth, and depth of solar physics research inspired by the decade of Yohkoh observations.

Authors: Piet Martens and Dave Cauffman (eds.)
Projects: RHESSI,Soho-EIT,Soho-MDI,Yohkoh-BCS,Yohkoh-HXT,Yohkoh-SXT,Yohkoh-WBS

Publication Status: COSPAR Colloquia Series, Volume 13 (2002), ISBN: 0-08-044060-6 . (Elsevier: Dordrecht).
Last Modified: 2003-09-15 12:11
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The Inadequacy of Temperature Measurements in the Solar Corona through Narrowband Filter and Line Ratios  

Piet Martens   Submitted: 2003-09-15 11:40

We analyze the determination of coronal line-of-sight temperatures with the technique of narrowband filter ratios that is currently employed for data obtained with the Transition Region and Coronal Explorer and the EUV Imaging Telescope on board the Solar and Heliospheric Observatory. We demonstrate that the simple fact that the observed differential emission measure curves in coronal loops have a broad plateau everywhere along the length of the loop leads to the finding of isothermal loops with different temperatures for each pair of filters. We show that none of the temperatures thus obtained correctly describe the state of the loop plasma, which instead must be characterized by the full differential emission measure per pixel. We conclude that the recent discovery of a new class of isothermal loops is probably a mere artifact of the narrowband filter ratio method and show that the shift in the location of the plateau in the differential emission measure along the loop indicates significant heating near the loop tops.

Authors: Martens, P. C. H.; Cirtain, J. W.; Schmelz, J. T.
Projects: None,Soho-EIT

Publication Status: ApJL 557, L115 (2002)
Last Modified: 2003-09-15 11:40
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Measurements of Flux Cancellation During Filament Formation  

Piet Martens   Submitted: 2003-09-15 11:31

We study the process of flux cancellation and filament formation in a nest of three decaying Active Regions, using data from SoHO MDI and EIT, and Hα images from Meudon and Big Bear. We find that there are no apparent EUV loops connecting the twp poles of a cancelling feature prior to and during cancellation, suggesting an absence of coronal magnetic connectivity between these opposite polarity flux patches. We further find that the cancellation occurs at the ends of the Hα sections of the filament and is accompanied by a noticeable increase in Hα intensity and linkage of the Hα sections, but that the locations of the links remain the weakest in Hα absorption. We present our measurements of the amount of flux cancelled at each site and and show it is in agreement with an estimate of the axial flux contained in the filament. We also observe two events of flux emergence, and find that they do not influence the filament formation in this case. We compare our results with similar measurements in recent papers and find agreement for the amounts of cancelled flux per patch, except for one case in a young emerging Active Region, for which we provide an alternative interpretation. We conclude that our measurements of flux cancellation are consistent with both the scenarios in which the filament is formed through ''head-to-tail'' linkage, as well as the scenario in which filament flux tubes emerge as a whole from below the photosphere, but that only the former scenario is consistent with the apparent absence of coronal magnetic links between the cancelling magnetic patches.

Authors: Paul Wood and Piet Martens
Projects: Soho-EIT,Soho-MDI

Publication Status: Solar Physics, in press
Last Modified: 2003-09-15 11:31
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Coronal Heating by Resonant Absorption: the Effects of Chromospheric Coupling  

Piet Martens   Submitted: 2001-10-24 13:17

We present the first 2.5 dimensional numerical model calculations of the nonlinear wave dynamics and heating by resonant absorption in coronal loops with thermal structuring of the transition region and higher chromosphere. The numerical calculations were done with the Versatile Advection Code. The transition region can move freely and is transparent for mass motions from chromosphere to corona. The loops are excited at the chromospheric level by linearly polarized monochromatic Alfvén waves. We find that the efficiency of resonant absorption can be much lower than in equivalent line-tied coronal loop models. The inefficiency is due to the fast rate at which slow magnetosonic waves are nonlinearly generated in the chromosphere and transition region. This leads to considerable transfer of energy from the Alfvén wave to the magnetosonic waves. Consequently, only a relatively small fraction of the Poynting flux that is injected into the loop system at the chromospheric level is available at the coronal level. Cavity leakage and detuning also have a negative impact on the efficiency, but less so than the nonlinear energy transfer. Inclusion of radiative and conductive losses improves the efficiency of resonant absorption. While the efficiency of resonant absorption heating is low, our results indicate that heating by compression and dissipation of the slow magnetosonic waves and shocks can easily lead to a temperature rise of a few percent, and for larger driver amplitudes even to a rise over 10%. Hence, our results support the idea of indirect coronal heating through the nonlinear generation of magnetosonic waves that was put forward more than 20 yr ago. Furthermore, the large transition region and coronal density oscillations that are associated with the slow magnetosonic waves provide an explanation for some observed coronal and transition region loop extreme-ultraviolet intensity variations.

Authors: A.J.C. Belien, P.C.H. Martens, and R. Keppens
Projects:

Publication Status: ApJ, 525, 478 (1999)
Last Modified: 2001-10-24 13:17
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Highly Energetic Physical Processes and Mechanisms for Emissions from Astrophysical Plasmas  

Piet Martens   Submitted: 2001-10-24 10:09

The symposium started with reviews on past, current, and future satellite observations in high-energy and solar astrophysics, followed by theoretical papers which were more directly related to these observations, on high-energy phenomena in, and emission from, astrophysical plasmas. The meeting proceeded to cover more theory-oriented, basic physical problems, such as the effects of magnetic fields, particle acceleration, and physical processes in relativistic astrophysical plasmas. The afternoon of the last day was reserved for a panel discussion and conference summary. This symposium included invited review and specialty papers, a panel discussion, a conference summary, and contributed papers (mostly posters but some oral). The spirit of the meeting was to bring together researchers from different subfields of high-energy astrophysics to share their expertise in studying emission processes of common interest in astrophysical plasmas.

Authors: P.C.H. Martens, S. Tsuruta, and M.A. Weber (eds.)
Projects:

Publication Status: Proceedings of IAU Colloquium 195, Publ. Astron. Soc. Pacific (2000).
Last Modified: 2001-10-24 10:09
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Chromospheric Damping of Alfvén Waves  

Piet Martens   Submitted: 2001-10-24 09:48

We analytically study the damping of Alfvén mode oscillations in the chromosphere and in coronal loops. In the partially ionized chromosphere the dominant damping process of Alfvén waves is due to collisions between ions and neutrals. We calculate the damping time for Alfvén waves of a given frequency, propagating through model chromospheres of various solar structures such as active region plage, quiet sun, and the penumbra and umbra of sunspots. For a given wave frequency, the maximum damping always occurs at temperature minimum heights and in the coldest structure(s), i.e., the umbra of sunspots. Energy dissipation due to ion-neutral damping of Alfvén waves with an energy flux of 107 ergs cm-3 s- 1 can play a considerable role in the energy balance of umbrae, quiet sun, and plage for Alfvén wave periods of the order, respectively, 50, 5, and 0.5 s. We also consider Alfvén waves in coronal loops and the leakage of wave energy through the footpoints. We assume a three-layer model of coronal loops with constant Alfvén speed vA (and no damping) in the corona, vA varying exponentially with height in the dissipative chromosphere, and vA again constant in the photosphere at the end of the loop. We find an exact analytical solution in the chromospheric part. Using these solutions, we estimate the leakage of wave energy from the coronal volume through the footpoint regions of the loop and find that the presence of a moderate amount of chromospheric damping can enhance the footpoint leakage. We apply this result to determine the damping time of standing waves in coronal loops. The enhanced footpoint leakage also has implications for theories of coronal heating based on resonant absorption. Finally, we find exact expressions for the damping of Alfvén waves launched in the photosphere and upward propagating through the chromosphere and into the corona. The partially ionized chromosphere presents an effective barrier for upward propagating Alfvén waves with periods less than a few seconds.

Authors: B. De Pontieu, P.C.H. Martens, and H.S. Hudson
Projects:

Publication Status: ApJ., 558, 85 (2001)
Last Modified: 2001-10-24 09:48
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Origin and Evolution of Filament-Prominence Systems  

Piet Martens   Submitted: 2001-05-23 16:27

We present a 'head-to-tail' linkage model for the formation, evolution, and eruption of solar filaments. The magnetic field structure of our model is based upon the observation that filaments form exclusively in filament channels with no apparent magnetic connections above the polarity inversion line. The formation of a filament in this configuration is driven by flux convergence and cancellation, which produces loop-like filaments segments with a half-turn. Filament segments of like chirality may connect and form long quiescent filaments. Such filaments are stabilized through footpoint anchoring until further cancellation at the footpoints causes their eruption. The eruption restores the original filament channel so that filament formation may resume immediately. We then demonstrate that the combined workings of Hale's polarity law, Joy's law, and differential rotation introduce a strong hemispheric preference in the chirality of filaments formed poleward of the sunspot belt, in agreement with observations. We analyze the magnetic fine structure of filaments formed through our model and find consistency with the observed hemispheric preference for barb orientation and a simple explanation for barb formation. Finally we consider the flux tubes retracted below the surface in the process of filament formation. We show that every cancellation event that generates a filament obeying the hemispheric chirality preference, injects a flux tube below the surface with a poloidal field opposite to that of the ongoing cycle. We suggest that this pattern of submergence of flux represents the specific mechanism for the reversal of the poloidal flux in a Babcock-Leighton-Durney type model for the solar dynamo.

Authors: Petrus C. Martens & Cornelis Zwaan
Projects:

Publication Status: ApJ, 558, 872 (2001)
Last Modified: 2001-10-24 09:37
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Can Streamer Blobs Prevent the Buildup of the Interplanetary Magnetic Field?  

Piet Martens   Submitted: 1999-11-16 21:04

Coronal mass ejections continuously drag closed magnetic field lines away from the Sun, adding new flux to the interplanetary magnetic field (IMF). We propose that the outward-moving blobs that have been observed in helmet streamers are evidence of ongoing, small-scale reconnection in streamer current sheets, which may play an important role in the prevention of an indefinite buildup of the IMF. Reconnection between two open field lines from both sides of a streamer current sheet creates a new closed field line, which becomes part of the helmet, and a disconnected field line, which moves outward. The blobs are formed by plasma from the streamer that is swept up in the trough of the outward-moving field line. We show that this mechanism is supported by observations from Solar and Heliospheric Observatory (SOHO)/Large Angle and Spectrometric Coronagraph. Additionally, we propose a thorough statistical study to quantify the contribut- contribution of blob formation to the reduction of the IMF and indicate how this mechanism may be verified by observations with SOHO/Ultraviolet Coronagraph Spectrometer and the proposed NASA Solar Terrestrial Relations Observatory and ESA Polar Orbiter missions.

Authors: M.K. van Aalst, P.C.H. Martens, and A.J.C. Belien
Projects:

Publication Status: ApJL 511, L125 (1999)
Last Modified: 1999-11-16 21:04
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The Preflare Phase  

Piet Martens   Submitted: 1999-11-16 21:03

Encyclopedia article; no abstract

Authors: P.C.H. Martens
Projects:

Publication Status: Encyclopedia of Astronomy and Astrophysics, 2000, D. Emerson and E.R. Priest (eds.), The Institute of Physics Publishing
Last Modified: 2001-10-24 09:40
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The Scaling of Solar Flare Hard X-ray Emission to Other Flaring Objects in the Universe  

Piet Martens   Submitted: 1999-11-16 21:03

Fletcher and Martens have successfully modeled solar hard X-ray sources observed at the top and footpoints of flaring magnetic loops with a Fokker-Planck type particle transport code. I show here that there are invariances in the Fokker-Planck equations that make these results applicable to environments with vastly different physical parameters, such as hard X-ray flares in accretion disks in active galactic nuclei, and in RS CVn and ALGOL type binaries.

Authors: Martens, P.C.H.
Projects:

Publication Status: IAU Colloquium 195, P.C.H. Martens, S. Tsuruta, and M.A. Weber (eds.) 2000, Publ. Astron. Soc. Pacific.
Last Modified: 2001-10-24 16:38
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Abstracts by Author
The Effect of Background Subtraction on the Temperature of EIT Coronal Loops
The Origin of Prominences and Their Hemispheric Preferences for Chirality and the Skew of Overlying Loops
SADE, The Starspot And Dynamo Explorer
Analysis of a Coronal Loop on the Limb
Yohkoh-SXT Observations of Reconnection (Invited)
Blind Deconvolution of the SXT PSF Core Part
Multi-Wavelength Observations of Coronal Structure and Dynamics -- Yohkoh 10th Anniversary Meeting
The Inadequacy of Temperature Measurements in the Solar Corona through Narrowband Filter and Line Ratios
Measurements of Flux Cancellation During Filament Formation
Coronal Heating by Resonant Absorption: the Effects of Chromospheric Coupling
Highly Energetic Physical Processes and Mechanisms for Emissions from Astrophysical Plasmas
Chromospheric Damping of Alfven Waves
Origin and Evolution of Filament-Prominence Systems
Can Streamer Blobs Prevent the Buildup of the Interplanetary Magnetic Field?
The Preflare Phase
The Scaling of Solar Flare Hard X-ray Emission to Other Flaring Objects in the Universe

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