The behavior of the spotless active regions during the solar minimum 23-24
aa r X i v : . [ a s t r o - ph . S R ] M a r Living Around Active StarsProceedings IAU Symposium No. 328, 2016D. Nandi, A. Valio & P. Petit, eds. c (cid:13) The behavior of the spotless active regionsduring the solar minimum 23-24
Alexandre Jos´e de Oliveira e Silva and Caius Lucius Selhorst , IP&D - Universidade do Vale do Para´ıba (UNIVAP) - S˜ao Jos´e dos Campos, SP, Brazilemail: [email protected] NAT - Universidade Cruzeiro do Sul - S˜ao Paulo, SP, Brazilemail: [email protected]
Abstract.
In this work, we analysed the physical parameters of the spotless actives regionsobserved during solar minimum 23 – 24 (2007 – 2010). The study was based on radio maps at17 GHz obtained by the Nobeyama Radioheliograph (NoRH) and magnetograms provided bythe Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO).The results shows that the spotless active regions presents the same radio characteristics of aordinary one, they can live in the solar surface for long periods ( >
10 days), and also can presentsmall flares.
Keywords.
Sun: sunspots, actives regions - Sun: radio radiation - Sun: magnetograms
1. Introduction
The study of the solar magnetic field dynamics is very importante to understand thephenomena which may affect the Earth environment. These dynamics have been dailymonitored since the years ∼ F . F .
2. Data Analyses and Results
In this study, the active regions at 17 GHz were identified, just as Selhorst et al.(2014),that is: i) size greater than 150 pixels ( ∼ M SS (millionth solar surface)); ii) latitudesbetween ± ◦ ; iii) maximum brightness temperature ( T b max ) at least 40% greater thanthe quiet Sun value and iv) longitudes < ◦ . The active region size was calculatedconsidering the pixels with T b equal to 12000 K or more.During the last minimum (2007 – 2010), our analyses find 92 days without sunspots,that presented active regions at 17 GHz. Since, an active region can live for long periods,those spotless active regions were identified as 48 distinct ones. During their lives, some1 A. J. Oliveira e Silva & C. L. Selhorstof these active regions presented days with sunspot associated, these days were alsoanalysed (128 days). These weak active regions may also present flares, and a small one(class C1.1) was identified. N u m be r o f e v en t s Spotles active regionsActive regions with spots
Figure 1: The lifetime of spotless active regions (in days). In red were represented theactive regions that presented at least one day with sunspot associated.The lifetimes of the spotless active regions analysed are shown in Figure 1. Almost50% of them (23) were ephemeral ones and still visible for a maximum of three days. Theresults show that the spotless active regions present a short lifetime but can also live onthe solar surface for long periods ( >
10 days). All of these active regions living 5 daysor more presented at least one day with a sunspot associated to them.
Spotles active regionsActive regions with spots T b max − K)02468 A c t i v e r eg i on f l u x ( S F U ) Figure 2: The relation between the active region flux and their maximum brightnesstemperature ( T b max ). The size of circles are proportional to the percentage of activeregions in each group. Moreover, the active regions were separated in with or withoutspot, respectively, red and blue. The dashed lines are linear adjusts.The active regions presented a minimum brightness temperature of ∼ ∼ K for thats ones sunspots. Moreover, the the spotless ones showed an average area of325 pixels , whereas, those with sunspot were 35% greater (440 pixels ).In Figure 2, we compare active region flux (in SFU) with their maximum brightnesstemperature. The active regions were separated in groups by their T b max , every 2000 K,and the result shows the increase of the flux with T b max for both groups the spotlessregions (blue circles) and for those with associated sunspot (blue circles). The mean fluxdifference is 0.36 SFU, however, this difference reaches 0.85 SFU when the T b max is lower,where the largest number of spotless regions are concentrated, and decreases when thespotless active regions reached their maximum values ( ∼ potless active regions |B max | − G)1.01.52.02.53.03.54.04.55.0 M a x i m u m b r i gh t ne ss T e m pe r a t u r e ( T b m a x − K ) |B max | − G)0246810 A c t i v e r eg i on f l u x ( S F U ) a) b)Spotles active regionsActive regions with spots Active regions with spotsSpotles active regions Figure 3: Comparison of the (a) active region flux (SFU) and (b) mean brightness tem-perature in relation to | B | max .To analise the active regions magnetic fields intensities, the magnetograms obtainedby the MDI (Michelson Doppler Imager) were analysed. Due to sight line, the valuesof the maximum intensities of magnetic fields ( | B | max ) were corrected by dividing thevalue obtained in the magnetogram by (cos( Lat. ) × cos( Long. )) (Schad & Penn 2010).Here, we used the absolute maximum magnetic field intensity ( | B | max ) to characterisethe active regions.In the Figure 3, the active regions were grouped by their | B | max each 100 G. Thespotless regions are plotted in blue and the active regions with spots in red. For eachgroup, the maximum brightness temperature ( T b max ) and the active region flux wereaveraged for each group. The panel 3(a) shows a negative trend in the linear adjust forboth, active regions with spots and without them, that is, as magnetic fields increase theflux tends to decrease. The averaged flux of the active regions with spots is 0.81 SFUgreater than the spotless ones. In panel 3(b), T b max still constant with the increase ofthe magnetic field. Moreover, the active regions with spots are 3400 K hotter than thespotless ones.
3. Final Remarks
A total of 48 distinct active regions were analysed in the period 2007–2010. About 50%of them were ephemeral living a maximum of three days. On the other hand, those onesliving 5 days more presented at least one day with a sunspot. The active regions withsunspots are hotter and presented more flux than the spotless ones. However, the valueswere significantly smaller than the proposed by Livingston et al. (2012) for minimumnecessary for the spot formation (1500 G), that could be due to instrumental differences.
Acknowledgements
We would like to thank the Nobeyama Radioheliograph, which is operated by theNAOJ/Nobeyama Solar Radio Observatory. A.J.O.S. acknowledge the scholarship formCAPES. C.L.S. acknowledge financial support from the S˜ao Paulo Research Foundation(FAPESP), grant
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