Valentyna Abramenko

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.

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.

The Statistical Relationship between the Photospheric Magnetic Parameters and the Flare Productivity of Active Regions

Using line-of-sight Michelson Doppler Imager (MDI) magnetograms of 89 active regions and Solar Geophysical Data (SGD) flare reports, we explored, for the first time, the magnitude scaling correlations between three parameters of magnetic fields and the flare productivity of solar active regions. These parameters are (1) the mean value of spatial magnetic gradients at strong-gradient magnetic neutral lines, ()NL; (2) the length of strong-gradient magnetic neutral lines, LGNL; and (3) the total magnetic energy, (Bz) dA, dissipated in a layer of 1 m during 1 s over the active regions area. The MDI magnetograms of active regions used for our analysis are close to the solar central meridian (within ±10°). The flare productivity of active regions was quantified by the soft X-ray flare index for different time windows from the time interval of the entire disk passage down to +1 day from the time of the analyzed magnetogram. Our results explicitly indicate positive correlations between the parameters and the overall flare productivity of active regions, and imminent flare production as well. The correlations confirm the dependence of flare productivity on the degree of nonpotentiality of active regions.

Scaling Behavior of Structure Functions of the Longitudinal Magnetic Field in Active Regions on the Sun

In the framework of a refined Kolmogorov hypothesis, the scaling behavior of the Bz-component of the photospheric magnetic field is analyzed and compared with flaring activity in solar active regions. We use Solar and Heliospheric Observatory Michelson Doppler Imager, Huairou (China), and Big Bear measurements of the Bz-component in the photosphere for nine active regions. We show that there is no universal behavior in the scaling of the Bz-structure functions for different active regions. Our previous study has shown that scaling for a given active region is caused by intermittency in the field, (B)(), describing the magnetic energy dissipation. When intermittency is weak, the Bz field behaves as a passive scalar in the turbulent flow, and the energy dissipation is largely determined by the dissipation of kinetic energy in the active regions with low flare productivity. However, when the field (B)() is highly intermittent, the structure functions behave as transverse structure functions of a fully developed turbulent vector field, and the scaling of the energy dissipation is mostly determined by the dissipation of the magnetic energy (active regions with strong flaring productivity). Based on this recent result, we find that the dissipation spectrum of the Bz-component is strongly related to the level of flare productivity in a solar active region. When the flare productivity is high, the corresponding spectrum is less steep. We also find that during the evolution of NOAA Active Region 9393, the Bz dissipation spectrum becomes less steep as the active regions flare activity increases. Our results suggest that the reorganization of the magnetic field at small scales is also relevant to flaring: the relative fraction of small-scale fluctuations of magnetic energy dissipation increases as an active region becomes prone to producing strong flares. Since these small-scale changes seem to begin long before the start of a solar flare, we suggest that the relation between scaling exponents, calculated by using only measurements of the Bz-component, and flare productivity of an active region can be used to monitor and forecast flare activity.

Time Distributions of Large and Small Sunspot Groups Over Four Solar Cycles

Here we analyze solar activity by focusing on time variations of the number of sunspot groups (SGs) as a function of their modified Zurich class. We analyzed data for solar cycles 20-23 by using Rome (cycles 20 and 21) and Learmonth Solar Observatory (cycles 22 and 23) SG numbers. All SGs recorded during these time intervals were separated into two groups. The first group includes small SGs (A, B, C, H, and J classes by Zurich classification), and the second group consists of large SGs (D, E, F, and G classes). We then calculated small and large SG numbers from their daily mean numbers as observed on the solar disk during a given month. We report that the time variations of small and large SG numbers are asymmetric except for solar cycle 22. In general, large SG numbers appear to reach their maximum in the middle of the solar cycle (phases 0.45-0.5), while the international sunspot numbers and the small SG numbers generally peak much earlier (solar cycle phases 0.29-0.35). Moreover, the 10.7 cm solar radio flux, the facular area, and the maximum coronal mass ejection speed show better agreement with the large SG numbers than they do with the small SG numbers. Our results suggest that the large SG numbers are more likely to shed light on solar activity and its geophysical implications. Our findings may also influence our understanding of long-term variations of the total solar irradiance, which is thought to be an important factor in the Sun-Earth climate relationship.

Magnetic Field, Hα, and RHESSI Observations of the 2002 July 23 Gamma-Ray Flare

In this paper we examine two aspects of the 2002 July 23 gamma-ray flare by using multiwavelength observations. First, the data suggest that the interaction of the erupted field with an overlying large-scale coronal field can explain the offset between the gamma-ray and the hard X-ray sources observed in this event. Second, we pay attention to rapid and permanent changes in the photospheric magnetic field associated with the flare. MDI and BBSO magnetograms show that the following magnetic flux had rapidly decreased by 1 × 1020 Mx immediately after the flare, while the leading polarity was gradually increasing for several hours after the flare. Our study also suggests that the changes were most probably associated with the emergence of new flux and the reorientation of the magnetic field lines. We interpret the magnetograph and spectral data for this event in terms of the tether-cutting model.

HIGHEST RESOLUTION OBSERVATIONS OF THE QUIETEST SUN

Highest resolution observations made with the new 1.6 m aperture solar telescope in Big Bear Solar Observatory during this time of historic inactivity on the Sun reveal new insights into the small-scale dynamics of the Suns photosphere. The telescopes unprecedented resolution enabled us to observe that the smallest scale photospheric magnetic field seems to come in isolated points in the dark intergranular lanes, rather than the predicted continuous sheets confined to the lanes, and the unexpected longevity of the bright points implies a deeper anchoring than predicted. Further, we demonstrated for the first time that the photospheric plasma motion and magnetic fields are in equipartition over a wide dynamic range, and both cascade energy to ever-smaller scales according to classical Kolmogorov turbulence theory. Finally, we discovered tiny jet-like features originating in the dark lanes that surround the ubiquitous granules that characterize the solar surface.

SIGNATURE OF AN AVALANCHE IN SOLAR FLARES AS MEASURED BY PHOTOSPHERIC MAGNETIC FIELDS

We analyzed time variations of turbulent parameters of the photospheric magnetic field of four active regions obtained during the course of major solar flares using longitudinal magnetograms from the Big Bear Solar Observatory and from SOHO/MDI full-disk measurements. Analysis of the data indicated that, before each flare, the degree of intermittency of the magnetic field had been increasing for 6-33 minutes and reached a maximum value approximately 3-14 minutes before the peak of the hard X-ray emission for each event. This result seems to suggest the existence in an active region of a turbulent phase prior to a solar flare. We also found that the maximum of the correlation length of the magnetic energy dissipation field tends to follow (or to occur nearly simultaneously) with the peak of the hard X-ray emission. The data suggest that the peak in the correlation length might be a trace of an avalanche of coronal reconnection events. We discuss the results in the framework of the concept of self-organized criticality.

MAXIMUM CORONAL MASS EJECTION SPEED AS AN INDICATOR OF SOLAR AND GEOMAGNETIC ACTIVITIES

We investigate the relationship between the monthly averaged maximal speeds of coronal mass ejections (CMEs), international sunspot number (ISSN), and the geomagnetic Dst and Ap indices covering the 1996-2008 time interval (solar cycle 23). Our new findings are as follows. (1) There is a noteworthy relationship between monthly averaged maximum CME speeds and sunspot numbers, Ap and Dst indices. Various peculiarities in the monthly Dst index are correlated better with the fine structures in the CME speed profile than that in the ISSN data. (2) Unlike the sunspot numbers, the CME speed index does not exhibit a double peak maximum. Instead, the CME speed profile peaks during the declining phase of solar cycle 23. Similar to the Ap index, both CME speed and the Dst indices lag behind the sunspot numbers by several months. (3) The CME number shows a double peak similar to that seen in the sunspot numbers. The CME occurrence rate remained very high even near the minimum of the solar cycle 23, when both the sunspot number and the CME average maximum speed were reaching their minimum values. (4) A well-defined peak of the Ap index between 2002 May and 2004 August was co-temporal with the excess of the mid-latitude coronal holes during solar cycle 23. The above findings suggest that the CME speed index may be a useful indicator of both solar and geomagnetic activities. It may have advantages over the sunspot numbers, because it better reflects the intensity of Earth-directed solar eruptions.

EVOLUTION OF OPTICAL PENUMBRAL AND SHEAR FLOWS ASSOCIATED WITH THE X3.4 FLARE OF 2006 DECEMBER 13

The rapid and irreversible decay of penumbrae related to X-class flares has been found in a number of studies. Since the optical penumbral flows are closely associated with the morphology of sunspot penumbra, we use state-of-the-art Hinode data to track penumbral flows in flaring active regions as well as shear flows close to the flaring neutral line. This paper concentrates on AR 10930 around the time of an X3.4 flare on 2006 December 13. We utilize the seeing-free solar optical telescope G-band data as a tracer to obtain the horizontal component of the penumbral and shear flows by local correlation tracking, and Stokes-V data to register positive and negative magnetic elements along the magnetic neutral line. We find that: (1) an obvious penumbral decay appears in this active region intimately associated with the X3.4 flare; (2) the mean magnitude of the horizontal speeds of the penumbral flows within the penumbral decay areas temporally and spatially varies from 0.6 to 1.1 km s?1; (3) the penumbral flow decreases before the flare eruption in two of the four penumbral decay areas; (4) the mean shear flows along the magnetic neutral line of this ?-sunspot started to decrease before the flare and continue to decrease for another hour after the flare. The magnitude of this flow apparently dropped from 0.6 to 0.3 km s?1. We propose that the decays of the penumbra and the penumbral flow are related to the magnetic rearrangement involved in the coronal mass ejection/flare events.