{"id":32,"date":"2019-08-29T12:21:45","date_gmt":"2019-08-29T10:21:45","guid":{"rendered":"http:\/\/www.issibern.ch\/teams\/solactregars\/?page_id=32"},"modified":"2019-08-29T12:21:45","modified_gmt":"2019-08-29T10:21:45","slug":"rationale","status":"publish","type":"page","link":"https:\/\/www.issibern.ch\/teams\/solactregars\/main-page\/rationale\/","title":{"rendered":"Project description"},"content":{"rendered":"\n

\nProblem\nstatement<\/strong><\/em><\/p>\n\n\n\n

\nSpace weather<\/em>\nis the incessant forcing exerted by solar magnetic activity on the\nspace environment of Earth and other bodies in the solar system. This\nforcing perturbs the Earth\u2019s magnetic sphere of influence\n(geospace) on timescales of hours and days. These perturbations can\nbe detrimental, and potentially catastrophic, to terrestrial\ninfrastructure and human assets in orbit, beyond the atmosphere [1].\nSpace climate<\/em>,\nin turn, is due to the Sun\u2019s variability over timescales of\ndecades, centuries and millennia, influencing the long-term\noccurrence frequencies of such space weather events [2,3]. An ability\nto predict space climate variations is clearly important for the\nplanning of future space missions, as well as for the incorporation\nof realistic solar forcing variations in models of future terrestrial\nclimate change.<\/p>\n\n\n\n

\nThe immediate cause of space climate\nvariations is the semi-regular nature of the 11-year cyclic\nvariations in solar activity: the amplitude and length of successive\nsolar cycles show quite large variations. The second half of the 20th\ncentury was characterized by a series of strong solar cycles known as\nthe Modern Maximum. This era of strong solar activity has abruptly\nended in the first decade of the 21st century. The ongoing Solar\nCycle 24 which started in 2008, peaked in 2014 with a significantly\nlower peak amplitude, somewhat below the long-term average and\nsimilar to the solar cycles seen in the early 20th century.<\/p>\n\n\n\n

\nThis marked change has prompted increased\ninterest in the origin of cycle-to-cycle variations in solar activity\nand in possibilities of predicting the amplitude of upcoming solar\ncycles, most notably Cycle 25. Experience has shown that the best\ncandidate for a physical precursor of the amplitude of an upcoming\ncycle is the peak strength of the solar polar magnetic fields (or\nalternatively, the solar dipole moment), reached typically around the\ntime of solar minimum [4].<\/p>\n\n\n\n

\nThe critical issue still open is how, in\nturn, the amplitude of the dipole field is determined by solar\nactivity in the previous cycles. Observations clearly indicate that\nthe polar fields are built up by the poleward transport of trailing\npolarity magnetic flux from bipolar active regions. (\u201cTrailing\u201d\nand \u201cleading\u201d refer to the direction of solar rotation. Trailing\npolarity is normally positioned at slightly higher latitudes than\nleading polarity, and it is the same polarity for ARs on the same\nhemisphere in the same solar cycle.) This transport is mainly due to\na meridional flow, so variations in the meridional flow have been\ninvoked as a factor in intercycle activity variations [5-7]. \n<\/p>\n\n\n\n

\nAn alternative possibility has been\nhighlighted by Cameron et al. [8] who stressed the importance of the\ntilt angle of bipolar active regions relative to the east-west\ndirection. Clearly, for zero tilt, leading and trailing polarity flux\nwould be transported towards the poles in equal rates, resulting in\nno net change in the polar flux. An increasing tilt angle will then\nlead to an increasing polar field strength. This opens two intriguing\npossibilities. On the one hand, some studies of the observational\nrecord indicate that tilt angles are anticorrelated to cycle\namplitude [9]. The origin of this anticorrelation may be related to\nthe dynamics of the emerging flux loop or to the meridional inflows\ntowards the active latitude zone associated with the torsional\noscillation pattern, the amplitude of which is determined by the\nlevel of solar activity. This tilt\nquenching<\/em> is a potentially\nimportant nonlinear feedback effect of solar activity level on the\ntilt angles, and thereby on the buildup of polar fields that will\nserve as seed fields for the next cycle.<\/p>\n\n\n\n

\nOn the other hand, a random scatter of\ntilt angles around the mean value determined by the above process\nwill introduce a significant degree of stochasticity in the process.\nAs the total magnetic flux in the polar cap is comparable to the\nmagnetic flux in a single large active region, in some cases even one\nAR can have a major distorting effect of polar flux buildup. Large\nexceptional or rogue<\/em>\nactive regions disobeying Joy’s law or Hale’s polarity rules [10,11]\ncan potentially play havoc with the buildup of polar fields,\nespecially if they emerge near the equator as in this case a higher\nfraction of one polarity can diffuse across the equator, avoiding\ncancellation with its opposite polarity counterpart. It has been\nshown in a dynamo model that in extreme cases such freak events may\neven trigger longer episodes of unusually low or unusually high\nactivity, i.e. grand minima or grand maxima [12].<\/p>\n\n\n\n

\nIn view of the above developments, for\nimproving solar activity forecasts on a decadal scale (i.e. for space\nclimate forecasting) we need to<\/p>\n\n\n\n