Relationship between flare occurrence and the Hale Sector Boundary Konstantina Loumou (k.loumou.1@research.gla.ac.uk)1, Iain G. Hannah1, Hugh S. Hudson1,2 (1) University of Glasgow, (2) University of California, Berkeley Session 4 Solar flares: Abrupt releases of magnetic energy that occur in places of highly concentrated magnetic field. What is the Hale Sector Boundary? The Interplanetary Magnetic Field (IMF) is divided in four sectors with alternating polarities. We are interested in their boundaries. Sunspots also change their polarity with the course of the solar cycle. The Hale Sector Boundary (HSB) will be the part of the boundaries that indicates same change in polarity as that of the sunspots. Importance: Dittmer(1975) and Svaalgard et al. (2011) indicated a correlation between the presence of the HSB and flare occurrence. Structure Above: Synoptic map of the IMF, taken from a Potential Field Source Surface (PFSS) extrapolation model. The sector boundary is shown with dark blue curve while the light blue vertical line shows the position of the Central Meridian. Movie created with data from http://gong2.nso.edu/products/ Above, right: Synoptic map of the IMF, taken from a PFSS model extrapolation model. The sector boundary is shown with the red curve while the light green point out the HSBs. The blue stars show the location of flares taking place the day stated on top of the image. Cartoon of the Sun and the projected IMF. The blur and yellow regions indicate the polarity of the IMF while the clouds show the polarity of the sunspots. The sector boundary is indicated with a “baseball-like” curve and the Hale part pointed out with a thick, red curve. Histogram: Fraction of flares occurring over the distance from a HSB, in a sum of 73712 flares over the last 14 years. 30% of flares were found to occur in about 30 degrees distance from the HSB. We use a large database, covering data from 14 years to extent their work. We confirm their conclusions also for the current solar cycle. We investigate the structure of the HSB in space and its behaviour with time. Konstantina Loumou k.loumou.1@research.gla.ac.uk University of Glasgow Slide 1 of 2

Relationship between flare occurrence and the Hale Sector Boundary Konstantina Loumou (k.loumou.1@research.gla.ac.uk)1, Iain G. Hannah1, Hugh S. Hudson1,2 (1) University of Glasgow, (2) University of California, Berkeley Session 4 Our work: Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) data from 2/2002 to 2/2016. RHESSI observes in hard X-rays and can detect micro-flares, resulting to a large database. Flares carefully checked for needed recalibration. Any GOES fluxes missing for initial data were added separately. Created Potential Field Source Surface (PFSS) model extrapolations of the Interplanetary Magnetic Field. We over-plotted the sector boundary as well as the location of flares occurring +/- 1 day of that date . Calculated the distance of the heliographic longitude of the flares from the one of the Hale Sector Boundary (HSB). Additionally, we wanted to see how stable the boundary is with time. We produced maps of the Carrington longitude of the boundary at Central Meridian over the Carrington Rotation for the time period of our study. The sectors seemed to suggest an alteration from a 2-sector structure to a 4-sector one. Created a PFSS image of the sector boundary as viewed from Earth. It was done for every year on an arbitrary day, since 2002. It is obvious that the same complex structure shown in the Carrington map cannot be easily detected from Earth. Behaviour with time Conclusions: Previous studies showed that flares prefer to occur at the location of the HSB. We expanded the research for cycle 24 and for the whole solar disk. Found a well organised sector boundary for the decay phase of cycle 23 but more complicated for cycle 24. An interesting alternation from a structure of two to four sectors, back to two again, appears. Studying the structure of the sector boundary as obtained from magnetic field extrapolations we obtained similar results. Questioning how many flares prefer to occur at the location of the Sector boundary we found that 30% of the flares that past 14 years did. Concentration of flares on the HBS is strong may provide us with a good statistical tool for anticipating flare and CME location. Left: Map of Carrington longitude with Carrington rotation of the boundary when it was at heliographic longitude 0, for cycles 23 and 24. The dashed line indicates the beginning of cycle 24. The boundary appears to be disorganised at the beginning of the cycles while it reaches a more stable configuration at the declining phase. Moreover, the structure of the sector seems to change from a a 2-sector structure (2002-2005 to a 4-sector one (2005-2009). -/+ and +/-: Change in polarity of the sectors of the Interplanetary magnetic field Right: Annual PFSS map of the sector boundary from ecliptic view. The alternation from 2 to 4 sectors seen above can be easily seen here, indicating a potential bias in statistics of measurements done from Earth. Konstantina Loumou k.loumou.1@research.gla.ac.uk University of Glasgow Slide 2 of 2