Hemispheric Rule Research Articles

Magnetic Helicity of Solar Active Regions as Revealed by Vector Magnetograms and Coronal X-Ray Images

Abstract We have used photospheric vector magnetograms of 15 different solar active regions to calculate the current helicity parameter, $ \alpha _{\rm av}$ , and the linear force-free field (LFFF) parameter, $ \alpha _{\rm best}$ , that fits best the observed transverse field. The data were obtained with the Solar Magnetic Field Telescope at the Huairou Solar Observing Station, the National Astronomical Observatories of China, the Solar Flare Telescope of the National Astronomical Observatory of Japan, and the Haleakala Stokes Polarimeter at the Mees Solar Observatory, University of Hawaii, from 1997 to 2000. The agreement in sign of $ \alpha _{\rm av}$ between three vector magnetographs is better than 90%. For $ \alpha _{\rm best}$ , the agreement is 80%–90%. The line-of-sight magnetograms observed with the Michelson–Doppler Imager (MDI) on SOHO and coronal X-ray images observed with the Soft X-ray Telescope (SXT) on Yohkoh have been used to determine the constant $ \alpha _{\rm c}$ of the LFFF in the corona. The value of $ \alpha _{\rm c}$ corresponds to the extrapolated coronal field whose field lines best match, by visual inspection, the structure of coronal loops in X-ray images. It is found that the sign agreement between photospheric $ \alpha _{\rm av}$ or $ \alpha _{\rm best}$ and coronal $ \alpha _{\rm c}$ is lower (60%–85%). We consider the differences in measurements, observing conditions, data reduction methods, and limitation in LFFF extrapolation, and discuss their contributions to the dispersions in the hemispheric sign rule of helicity.

HEMISPHERIC HELICITY TREND FOR SOLAR CYCLE 24

Using vector magnetograms obtained with the Spectro-polarimeter (SP) on aboard Hinode satellite, we studied two helicity parameters (local twist and current helicity) of 64 active regions occurred in the descending phase of solar cycle 23 and the ascending phase of solar cycle 24. Our analysis gives the following results. (1) The 34 active regions of the solar cycle 24 follow the so-called hemispheric helicity rule, whereas the 30 active regions of the solar cycle 23 do not. (2) When combining all 64 active regions as one sample, they follow the hemispheric helicity sign rule as in most other observations. (3) Despite with the so-far most accurate measurement of vector magnetic field given by SP/Hinode, the rule is still weak with large scatters. (4) The data show evidence of different helicity signs between strong and weak fields, confirming previous result from a large sample of ground-based observations. (5) With two example sunspots we show that the helicity parameters change sign from the inner umbra to the outer penumbra, where the sign of penumbra agrees with the sign of the active region as a whole. From these results, we speculate that both the Sigma-effect (turbulent convection) and the dynamo have contributed in the generation of helicity, whereas in both cases turbulence in the convection zone has played a significant role.

A HEMISPHERIC HELICITY SIGN RULE INDICATED BY LARGE-SCALE PHOTOSPHERIC MAGNETIC FIELDS AT THREE PHASES OF SOLAR CYCLE 23

The hemispheric sign rule of current helicity is examined for large-scale magnetic fields, extending to 60° high in solar latitudes, at three different phases of solar cycle 23 and using three different instruments. The current helicity is calculated from the vector magnetic fields derived respectively from the longitudinal full-disk magnetograms obtained by the Michelson Doppler Imager on board the Solar and Heliospheric Observatory as well as the Kitt Peak Vacuum Telescope and the Synoptic Long-term Investigations of the Sun at the National Solar Observatory at Kitt Peak. We have used the same technique introduced by Pevtsov & Latushko to retrieve the vector magnetic field but have applied a different method to calculate the current helicity. Our analysis strengthens evidence that the large-scale magnetic fields show a clear and consistent current helicity pattern that follows the established hemispheric rule, that is, a positive helicity sign in the southern hemisphere and a negative helicity sign in the northern hemisphere. This hemispheric sign pattern is present everywhere in the global magnetic field, extending to 60° high in solar latitudes, independent of the instruments and parameters used, and is evident at both solar maximum and minimum phases.

Current helicity pattern of large-scale magnetic field on the photosphere

Following Pevtsov and Latushko, we study the current helicity pattern of the large-scale magnetic field on the photosphere. We use the same technique as theirs to derive the vector magnetic field (Br, Bθ, Bϕ) from full-disk longitudinal magnetograms based on the assumption that large-scale magnetic fields evolve rather slowly and the variations of the longitudinal magnetic fields within certain time duration are caused by the changing position angles only. Different from their study, we have calculated the current helicity maps directly from the derived vector magnetograms, rather than from obtaining the latitudinal variation of hc by ignoring the role of Bθ component and averaging Br and Bϕ over all solar longitudes. This approach significantly strengthens the evidence of the hemispheric rule presented in the reconstructed vector magnetic field. Our study shows that the established hemispheric sign rule, that is, positive helicity sign in the southern hemisphere and negative helicity sign in the northern hemisphere, is applicable everywhere in the global magnetic field, namely, also evident in weak fields outside active regions, and that the obtained sign pattern is independent of the longitudinal magnetograms and the parameters that we have used.

Some basic questions on helicity inferred from vector magnetograms

This paper summarizes current helicity measurements in the solar active regions (ARs). There is a basic agreement with the “hemispheric sign rule (HSR)” of the current helicity among different vector magnetographs through two solar cycles, but there is a large dispersion of the fraction of ARs following the HSR. In our sample, there are 50%–78% ARs in solar cycle 22 and 44%–79% ARs in cycle 23 following the HSR. A variation is also found in the fraction of the ARs following the HSR between different instruments even when the same ARs are selected. The difference also exists for the same instrument when the selected ARs are different. There are some differences in the variation of HSR with solar cycle for the individual helicity parameter inferred from different instruments. Factors which influence the correlation of different data sets are analyzed.

3D Evolution of a Filament Disappearance Event Observed by STEREO

A filament disappearance event was observed on 22 May 2008 during our recent campaign JOP 178. The filament, situated in the Southern Hemisphere, showed sinistral chirality consistent with the hemispheric rule. The event was well observed by several observatories, in particular by THEMIS. One day, before the disappearance, Hα observations showed up- and down-flows in adjacent locations along the filament, which suggest plasma motions along twisted flux rope. THEMIS and GONG observations show shearing photospheric motions leading to magnetic flux canceling around barbs. STEREO A, B spacecraft with separation angle 52.4°, showed quite different views of this untwisting flux rope in He ii 304 Å images. Here, we reconstruct the three-dimensional geometry of the filament during its eruption phase using STEREO EUV He ii 304 Å images and find that the filament was highly inclined to the solar normal. The He ii 304 Å movies show individual threads, which oscillate and rise to an altitude of about 120 Mm with apparent velocities of about 100 km s−1 during the rapid evolution phase. Finally, as the flux rope expands into the corona, the filament disappears by becoming optically thin to undetectable levels. No CME was detected by STEREO, only a faint CME was recorded by LASCO at the beginning of the disappearance phase at 02:00 UT, which could be due to partial filament eruption. Further, STEREO Fe xii 195 Å images showed bright loops beneath the filament prior to the disappearance phase, suggesting magnetic reconnection below the flux rope.

TIME VARIATION AND STATISTICAL STUDIES OF MAGNETIC HELICITY INJECTION IN SOLAR MAGNETIC REGIONS

In this study, we analyzed the helicity injection in solar active regions statistically with the method proposed by Kusano et al. We investigated the variation in helicity injection as an active region evolves and the statistical distribution of helicity injection against the magnetic flux (Φ) and the latitude. The temporal evolution of active regions is divided into three phases (emergence, active, and decay phases). We studied one emergence-phase region, 13 active-phase regions, and five decay-phase regions. The total number of helicity injection rates we obtained is 1476. We found the following results. The dispersions (standard deviations) in the helicity injection rates are proportional to Φ–0.47 in the active-phase regions and to Φ–0.34 in the decay-phase regions. The dispersion for the emergence-phase region, 0.046 day–1, is larger than those for the active-phase regions, 0.004-0.03 day–1, and those for the decay-phase regions, 0.004-0.01 day–1. The average helicity injection rates of the active-phase regions show the latitude dependence, which is consistent with the rule of helical structures in the solar atmosphere, while those of the decay-phase regions are inconsistent with the hemispheric rule. The average helicity injection rates for the active-phase regions, 0.003-0.03 day–1, are larger than that for the emergence-phase region, 0.0045 day–1, and those for the decay-phase regions, <0.002 day–1. These dependences of the helicity injection rates give us clues on the generation mechanism of helicity and the evolution of the magnetic field in the solar convection zone.

On the Solar Cycle Variation of the Hemispheric Helicity Rule

We study the statistical significance of observed temporal variations of the solar active-region hemispheric helicity rule, as measured by the latitudinal gradient of the best-fit linear force-free-field parameter, -->dα/d . Using data from four different vector magnetographs, we compute and compare average annual -->dα/d values for these instruments for 19 years from solar cycles 21, 22, and 23. We find that although every instrument shows the wrong sign for the hemispheric rule in some years, there is no agreement among the instruments on which years are abnormal. None of the four data sets shows annual values of -->dα/d departing from the hemispheric helicity rule by more than 3 σ. We conclude that because the hemispheric helicity rule is a weak tendency with significant scatter, an annual subset of active regions is likely to produce statistically unreliable results.

Helicity Observations of Weak and Strong Fields

We report in this Letter our analysis of a large sample of photospheric vector magnetic field measurements. Our sample consists of 17,200 vector magnetograms obtained from 1997 January to 2004 August by the Huairou Solar Observing Station of the Chinese National Astronomical Observatory. Two physical quantities, α and current helicity, are calculated, and their signs and amplitudes are studied in a search for solar cycle variations. Differently from other studies of the same type, we calculate these quantities for weak (100 G 1000 G) fields separately. For weak fields, we find that the signs of both α and current helicity are consistent with the established hemispheric rule during most years of the solar cycle and that their magnitudes show a rough tendency of decreasing with the development of the solar cycle. The analysis of strong fields gives an interesting result: both α and current helicity present a sign opposite to that of weak fields. The implications of these observations for dynamo theory and helicity production are also briefly discussed.

Solar-Cycle Variation of Magnetic Helicity in Active Regions

Abstract The hemispheric sign rule of helicity and its long-term variation were studied. The data were obtained from the Solar Flare Telescope at Mitaka and the 65-cm solar telescope at Okayama. The data covered the period of 1983–2001, from the declining phase of solar cycle 21 to the rising phase of cycle 23. Although the hemispheric sign rule of helicity generally holds, we also found significant time variations in the yearly values of helicity during the observation period. The hemispheric sign rule of helicity is satisfied in the solar maximum phase, but may not be so in the solar minimum phase.

Advanced Automated Solar Filament Detection And Characterization Code: Description, Performance, And Results

We present a code for automated detection, classification, and tracking of solar filaments in full-disk Hα images that can contribute to Living With a Star science investigations and space weather forecasting. The program can reliably identify filaments; determine their chirality and other relevant parameters like filament area, length, and average orientation with respect to the equator. It is also capable of tracking the day-by-day evolution of filaments while they travel across the visible disk. The code was tested by analyzing daily Hα images taken at the Big Bear Solar Observatory from mid-2000 until beginning of 2005. It identified and established the chirality of thousands of filaments without human intervention. We compared the results with a list of filament proprieties manually compiled by Pevtsov, Balasubramaniam and Rogers (2003) over the same period of time. The computer list matches Pevtsov's list with a 72% accuracy. The code results confirm the hemispheric chirality rule stating that dextral filaments predominate in the north and sinistral ones predominate in the south. The main difference between the two lists is that the code finds significantly more filaments without an identifiable chirality. This may be due to a tendency of human operators to be biased, thereby assigning a chirality in less clear cases, while the code is totally unbiased. We also have found evidence that filaments obeying the chirality rule tend to be larger and last longer than the ones that do not follow the hemispherical rule. Filaments adhering to the hemispheric rule also tend to be more tilted toward the equator between latitudes 10∘ and 30∘, than the ones that do not.

The Current Helicity Parameter Hc is More Sensitive than αbest to Faraday Rotation

Observations have indicated that the net helicity sign of active regions is predominantly negative in the northern hemisphere and positive in the southern (see Table 1). From Table 1, we find that the hemispheric sign rule of helicity parameter αbest does not change with another solar cycle; but the helicity parameter Hc seems to show a weak opposite hemispheric preference for Huairou data in the solar cycle 23 and no preference for Mees data. How to explain such a phenomenon? We think one reason may be from the action of Faraday rotation. Faraday rotation will cause a counterclockwise rotation of the azimuth for a positive polarity field and vice versa. During the cycle 22, the polarity of leading sunspots is dominantly negative in the northern hemisphere and positive in the southern. The effect of Faraday rotation, which is determined mostly by a leading polarity sunspot, has a positive contribution to the percentage of current helicity signs in active regions and leads to the increase of the strength of the hemispheric sign rule. In the cycle 23, the polarity of most leading sunspots is positive in the northern hemisphere and Faraday rotation will decrease the percentage of current helicity signs. Consequently, the strength of the hemispheric rule should be weakened. On the other hand, Hc is more susceptible to Faraday rotation than αbest because it is mainly related to the areas where the line-of-sight field is strong. Therefore, if the effect of Faraday rotation is not completely removed in our observations, the hemispheric sign rule showed is weak in the cycle 23 than in the cycle 22 (for αbest), even opposite (for Hc).

Chirality of Chromospheric Filaments

We use the chromospheric full-disk Hα observations to study the chirality of 2310 filaments from 2000-2001. For each filament, we identify the spine and its barbs and determine the filament chirality as fraction of dextral/sinistral barbs of the total number of barbs. We find that 80.2% (558 out of 696) of quiescent filaments in the northern hemisphere are dextral and 85.5% (633 out of 740) of filaments in southern hemisphere are sinistral, in agreement with the well-known hemispheric helicity rule. Our data also show that the active-region filaments follow the same rule, though the hemispheric dependence is weaker: 74.9% (338 out of 451) of active-region filaments in the northern hemisphere are dextral, and 76.7% (297 out of 387) of filaments in the southern hemisphere are sinistral. We show that quiescent filaments formed on leading and returning arms of the same switchback exhibit the same chirality. We also investigate a possible change in the hemispheric rule with polarity reversal of the polar field, and we find no such change.

MAGNETIC HELICITY OF SOLAR ACTIVE REGIONS AND ITS IMPLICATIONS

We have studied the magnetic helicity of active regions by using the data from (1) the photo-electric magnetograph of the Okayama Observatory (1983-1995) and (2) the video magnetograph of NAOJ/Mitaka (1992-2000). The latitude distribution of helicity showed a tendency that the regions in the north (south) hemisphere have negative (positive) helicities, respectively, which is already known as the hemispheric sign rule. If we look into the sign of helicity as a function of time, the sign rule was less definite or was reversed sometimes in the sunspot minimum phase. We also studied the relation between the magnetic helicity and the sunspot tilt angles, and found that these two quantities are positively correlated, which is opposite to the expectation of a theoretical model. The implications of this cycle-phase dependence of helicity signs and the correlation between magnetic he Ii city and sunspot tilt angles are discussed.

Magnetic helicity and flare activity

We studied two active regions appeared in 2001 (NOAA 9415 and 9661), both of which have produced X-class flares. The time evolutions of magnetic flux and magnetic helicity were derived based on vector magnetograph observations. These regions satisfied the so-called hemispheric rule of magnetic helicity, namely regions in the northern (southern) hemisphere tend to have negative (positive) helicity. The magnetic helicity integrated over the regions evolved slowly and did not show abrupt changes at the time of the flares, although the distributions of magnetic helicity changed significantly over a few days in the regions. The appearance of localized regions whose helicity was opposite to that expected from the hemispheric rule was seen in both cases.

Flare Occurrence in the Solar Active Regions with Reversed Helicity Sign

Based on the Huairou Solar Observing Station dataset, we computed the current helicity for several hundreds of active regions and found that: (1) Active regions that do not follow the hemispheric helicity sign rule show more flare activity than normal active regions. (2) The relative number of active regions with reversed helicity sign is higher near sunspot maximum. (3) It appears that during solar cycle 22 the southern hemisphere has more the reversed-sign active regions and stronger flare activity than the northern hemisphere.

The hemispheric sign rule of current helicity during the rising phase of cycle 23

We compute the signs of two different current helicity parameters (i.e., αbest andH c) for 87 active regions during the rise of cycle 23 The results indicate that 59% of the active regions in the northern hemisphere have negative αbest and 65% in the southern hemisphere have positive. This is consistent with that of the cycle 22. However, the helicity parameterH cshows a weaker opposite hemispheric preference in the new solar cycle. Possible reasons are discussed.

Vector magnetic fields, subsurface stresses and evolution of magnetic helicity

Observations of the strength and spatial distribution of vector magnetic fields in active regions have revealed several fundamental properties of the twist of their magnetic fields. First, the handedness of this twist obeys a hemispheric rule: left-handed in the northern hemisphere, right-handed in the southern. Second, the rule is weak; active regions often disobey it. It is statistically valid only in a large ensemble. Third, the rule itself, and the amplitude of the scatter about the rule, are quantitatively consistent with twisting of fields by turbulence as flux tubes buoy up through the convection zone. Fourth, there is considerable spatial variation of twist within active regions. However, relaxation to a linear force-free state, which has been documented amply in laboratory plasmas, is not observed.

The Hemispheric Sign Rule of Current Helicity during the Rising Phase of Cycle 23

AbstractWe compute the signs of two different current helicity parameters (i.e., αbestandHc) for 87 active regions during the rise of cycle 23. The results indicate that 59% of the active regions in the northern hemisphere have negative αbestand 65% in the southern hemisphere have positive. This is consistent with that of the cycle 22. However, the helicity parameterHcshows a weaker opposite hemispheric preference in the new solar cycle. Possible reasons are discussed.

Studies of solar intranetwork fields and current helicity in active regions

In this paper we begin with a brief review of the most recent observational studies of intranetwork elements. Next by analyzing well observed quantities of other magnetic elements, we suggest that the magnetic flux of all elements is proportional to its lifetime, mainly depending on each size. Then we present an empirical way to extrapolate the size and magnetic field of intranetwork elements, and find that their average size is about 210 km and the corresponding field strength is 350 G. Finally, our studies of current helicity in active regions are summarized as follows: (1) most of the active regions in solar cycle 22 follow the hemispheric helicity sign rule, and the evolution of surface large-scale current helicity has a good correlation with solar activity; (2) the active regions with reversed helicity sign tend to emerge at certain longitudes; (3) the rapid and substantial changes of current helicity in an area or in its vicinity are most likely to trigger flares.