Vasyl Yurchyshyn

ON THE MAGNETIC FLUX BUDGET IN LOW-CORONA MAGNETIC RECONNECTION AND INTERPLANETARY CORONAL MASS EJECTIONS

Wepresentthefirstquantitativecomparisonbetweenthetotalmagneticreconnectionfluxinthelowcoronainthe wake of coronal mass ejections (CMEs) and the magnetic flux in magnetic clouds (MCs) that reach 1 AU 2Y3 days after CME onset. The total reconnection flux is measured from flare ribbons, and the MC flux is computed using in situ observations at 1 AU, all ranging from 10 20 to 10 22 Mx. It is found that for the nine studied events in which the association between flares, CMEs,and MCs isidentified, the MCflux iscorrelatedwiththe total reconnection flux! r. Further, the poloidal (azimuthal) MC flux ! p is comparable with the reconnection flux ! r, and the toroidal (axial) MC flux ! t is a fraction of ! r. Events associated with filament eruption do not exhibit a different ! t, p-! r relation from events not accompanied by erupting filaments. The relations revealed between these independently measured physical quantities suggest that for the studied samples, the magnetic flux and twist of interplanetary magnetic flux ropes, reflected by MCs, are highly relevant to low-corona magnetic reconnection during the eruption. We discuss the implications of this result for the formation mechanism of twisted magnetic flux ropes, namely, whether the helical structure of themagnetic flux ropeis largelypre-existing or formed in situ by low-coronamagnetic reconnection.We alsomeasuremagneticfluxencompassedincoronaldimmingregions(! d)anddiscussitsrelationtothereconnection flux inferred from flare ribbons and MC flux.

Orientation of the magnetic fields in interplanetary flux ropes and solar filaments

Coronal mass ejections (CMEs) are often associated with erupting magnetic structures or disappearing filaments. The majority of CMEs headed directly toward the Earth are observed at 1 AU as magnetic clouds—the region in the solar wind where the magnetic field strength is higher than average and there is a smooth rotation of the magnetic field vectors. The three-dimensional structure of magnetic clouds can be represented by a force-free flux rope. When CMEs reach the Earth, they may or may not cause magnetic storms, alter Earths magnetic field, or produce the phenomena known as auroras. The geoeffectiveness of a solar CME depends on the orientation of the magnetic field in it. Two M-class solar flares erupted on 2000 February 17. The second flare occurred near a small active region, NOAA Active Region 8872. This eruption was accompanied by a halo CME. However, the February 17 CME did not trigger any magnetic activity when it arrived at the Earth. Another powerful flare, on 2000 July 14, was also associated with a halo CME, which caused the strongest geomagnetic activity of solar cycle 23. Using ACE measurements of the interplanetary magnetic fields, we study the orientation of the magnetic flux ropes in both sets of magnetic clouds and compare them with the orientation of the solar magnetic fields and disappearing filaments. We find that the direction of the axial field and helicity of the flux ropes are consistent with those of the erupted filaments. Thus, the geoeffectiveness of a CME is defined by the orientation and structure of the erupted filament and by its magnetic helicity as well. We also suggest that the geoeffectiveness of a CME can be forecasted using daily full-disk Hα and Yohkoh images and MDI magnetograms as well.

RAPID CHANGES OF MAGNETIC FIELDS ASSOCIATED WITH SIX X-CLASS FLARES

In this paper, we present the results of the study of six X-class flares. We found significant changes in the photospheric magnetic fields associated with all of the events. For the five events in 2001, when coronagraph data were available, all were associated with halo coronal mass ejections. Based on the analyses of the line-of-sight magnetograms, all six events had an increase in the magnetic flux of the leading polarity of order of a few times 1020 Mx while each event had some degree of decrease in the magnetic flux of the following polarity. The flux changes are considered impulsive because the changeover time, which we defined as the time to change from preflare to postflare state, ranged from 10 to 100 minutes. The observed changes are permanent. Therefore, the changes are not due to changes in the line profile caused by flare emissions. For the three most recent events, when vector magnetograms were available, two showed an impulsive increase of the transverse field strength and magnetic shear after the flares, as well as new sunspot area in the form of penumbral structure. One of the events in this study was from the previous solar cycle. This event showed a similar increase in all components of the magnetic field, magnetic shear, and sunspot area. We present three possible explanations to explain the observed changes: (1) the emergence of very inclined flux loops, (2) a change in the magnetic field direction, and (3) the expansion of the sunspot, which moved some flux out of Zeeman saturation. However, we have no explanation for the polarity preference; i.e., the flux of leading polarity tends to increase while the flux of following polarity tends to decrease slightly.

Statistical Distributions of Speeds of Coronal Mass Ejections

We studied the distribution of plane-of-sky speeds determined for 4315 coronal mass ejections (CMEs) detected by the Large Angle and Spectrometric Coronagraph Experiment on board the Solar and Heliospheric Observatory (SOHO LASCO). We found that the speed distributions for accelerating and decelerating events are nearly identical and to a good approximation they can be fitted with a single lognormal distribution. This finding implies that, statistically, there is no physical distinction between the accelerating and the decelerating events. The lognormal distribution of the CME speeds suggests that the same driving mechanism of a nonlinear nature is acting in both slow and fast dynamical types of CMEs.

Earthshine observations of the Earth's reflectance

Regular photometric observations of the moons “ashen light” (earthshine) from the Big Bear Solar Observatory (BBSO) since December 1998 have quantified the earths optical reflectance. We find large (∼5%) daily variations in the reflectance due to large-scale weather changes on the other side of the globe. Separately, we find comparable hourly variations during the course of many nights as the earths rotation changes that portion of the earth in view. Our data imply an average terrestrial albedo of 0.297±0.005, which agrees with that from simulations based upon both changing snow and ice cover and satellite-derived cloud cover (0.296±0.002). However, we find seasonal variations roughly twice those of the simulation, with the earth being brightest in the spring. Our results suggest that long-term earthshine observations are a useful monitor of the earths albedo. Comparison with more limited earthshine observations during 1994–1995 show a marginally higher albedo then.

On the Relation between Filament Eruptions, Flares, and Coronal Mass Ejections

We present a statistical study of 106 filament eruptions, which were automatically detected by a pattern recognition program implemented at Big Bear Solar Observatory using Hα full-disk data from 1999 to 2003. We compare these events with Geostationary Operational Environmental Satellite soft X-ray time profiles, solar-geophysical data (SGD) solar event reports, Michelson Doppler Imager magnetograms, and Large Angle and Spectrometric Coronagraph (LASCO) data to determine the relationship between filament eruptions and other phenomena of solar activity. (1) Excluding eight events with no corresponding LASCO data, 55% or 56% of 98 events were associated with coronal mass ejections (CMEs). (2) Active region filament eruptions have a considerably higher flare association rate of 95% compared to quiescent filament eruptions with 27%, but a comparable CME association rate, namely, 43% for active region filament eruptions and 54% for quiescent filament eruptions. (3) 54% or 68% of 80 disk events were associated with new flux emergence. In addition, we derived the sign of magnetic helicity and the orientation of the magnetic field associated with seven halo CMEs and demonstrated that the geoeffectiveness of a halo CME can be predicted by these two parameters.

Flare Activity and Magnetic Helicity Injection by Photospheric Horizontal Motions

We present observational evidence that the occurrence of homologous flares in an active region is physically related to the injection of magnetic helicity by horizontal photospheric motions. We have analyzed a set of 1 minute cadence magnetograms of NOAA AR 8100 taken over a period of 6.5 hours by Michelson Doppler Imager (MDI) on board Solar and Heliospheric Observatory (SOHO). During this observing time span, seven homologous flares took place in the active region. We have computed the magnetic helicity injection rate into the solar atmosphere by photospheric shearing motions, and found that a signicant amount of magnetic helicity was injected during the observing period. In a strong M4.1 flare, the magnetic helicity injection rate impulsively increased and peaked at the same time as the X-ray flux did. The flare X-ray flux integrated over the Xray emission time strongly correlates with the magnetic helicity injected during the flaring interval. The integrated X-ray flux is found to be a logarithmically increasing function of the injected magnetic helicity. Our results suggest that injection of helicity and abrupt increase of helicity magnitude play a signicant role in flare triggering.

The Eruption from a Sigmoidal Solar Active Region on 2005 May 13

This paper presents a multiwavelength study of the M8.0 flare and its associated fast halo CME that originated from a bipolar NOAA AR 10759 on 2005 May 13. The source active region has a conspicuous sigmoid structure at the TRACE 171 A channel as well as in the SXI soft X-ray images, and we mainly concern ourselves with the detailed process of the sigmoid eruption, as evidenced by the multiwavelength data ranging from Hα, WL, EUV/UV, radio, and hard X-rays (HXRs). The most important finding is that the flare brightening starts in the core of the active region earlier than that of the rising motion of the flux rope. This timing clearly addresses one of the main issues in the magnetic eruption onset of sigmoid, namely, whether the eruption is initiated by an internal tether cutting to allow the flux rope to rise upward, or a flux rope rises due to a loss of equilibrium to later induce tether cutting below it. Our high time cadence SXI and Hα data show that the first scenario is relevant to this eruption. As in other major findings, we have the RHESSI HXR images showing a change of the HXR source from a confined footpoint structure to an elongated ribbon-like structure after the flare maximum, which we relate to the sigmoid-to-arcade evolution. The radio dynamic spectrum shows a type II precursor that occurred at the time of expansion of the sigmoid and a drifting pulsating structure in the flare rising phase in HXRs. Finally, type II and III bursts are seen at the time of maximum HXR emission, simultaneous with the maximum reconnection rate derived from the flare ribbon motion in UV. We interpret these various observed properties with the runaway tether-cutting model proposed by Moore et al. in 2001.

LOW-LATITUDE CORONAL HOLES AT THE MINIMUM OF THE 23rd SOLAR CYCLE

Low- and mid-latitude coronal holes (CHs) observed on the Sun during the current solar activity minimum (from 2006 September 21, Carrington rotation (CR) 2048, to 2009 June 26, CR 2084) were analyzed using Solar and Heliospheric Observatory/Extreme ultraviolet Imaging Telescope and STEREO-A SECCHI EUVI data. From both the observations and Potential Field Source Surface modeling, we find that the area occupied by CHs inside a belt of ±40 ◦ around the solar equator is larger in the current 2007 solar minimum relative to the similar phase of the previous 1996 solar minimum. The enhanced CH area is related to a recurrent appearance of five persistent CHs, which survived during 7–27 solar rotations. Three of the CHs are of positive magnetic polarity and two are negative. The most long-lived CH was being formed during 2 days and existed for 27 rotations. This CH was associated with fast solar wind at 1 AU of approximately 620 ± 40 km s −1 .T he three-dimensional magnetohydrodynamic modeling for this time period shows an open field structure above this CH. We conclude that the global magnetic field of the Sun possessed a multi-pole structure during this time period. Calculation of the harmonic power spectrum of the solar magnetic field demonstrates a greater prevalence of multi-pole components over the dipole component in the 2007 solar minimum compared to the 1996 solar minimum. The unusual large separation between the dipole and multi-pole components is due to the very low magnitude of the dipole component, which is three times lower than that in the previous 1996 solar minimum.

Rapid Changes in the Longitudinal Magnetic Field Related to the 2001 April 2 X20 Flare

Big Bear Solar Observatory observed the X20 flare that occurred at approximately 21:50 UT on 2001 April 2 with its standard complement of instruments. In this paper, we discuss the evolution of high-resolution and high-cadence longitudinal magnetograph observations in the region of the flare. The data reveal that there was a significant increase in the magnetic field on the limbward side of the neutral line of the active region at the location of the flare, while the magnetic field on the side of the neutral line closer to the disk center remained constant. We discuss possible rearrangements in the active regions magnetic field that could lead to the observed changes.