Substorm probabilities are best predicted from solar wind speed

Abstract Most measures of magnetospheric activity – including auroral power (AP), magnetotail stretching, and ring current intensity – are best predicted by solar wind-magnetosphere coupling functions which approximate the frontside magnetopause merging rate. However radiation belt fluxes are best predicted by a simpler function, namely the solar wind speed, v. Since most theories of how these high energy electrons arise are associated with repeated rapid dipolarizations such as associated with substorms, this apparent discrepancy could be reconciled under the hypothesis that the frequency of substorms tracks v rather than the merging rate – despite the necessity of magnetotail flux loading prior to substorms. Here we investigate this conjecture about v and substorm probability. Specifically, a continuous list of substorm onsets compiled from SuperMAG covering January 1, 1997 through December 31, 2007 are studied. The continuity of SuperMAG data and near continuity of solar wind measurements minimize selection bias. In fact v is a much better predictor of onset probability than is the overall merging rate, with substorm odds rising sharply with v. Some loading by merging is necessary, and frontside merging does increase substorm probability, but nearly as strongly as does v taken alone. Likewise, the effects of dynamic pressure, p, are smaller than simply v taken by itself. Changes in the solar wind matter, albeit modestly. For a given level of v (or Bz), a change in v (or Bz) will increase the odds of a substorm for at least 2 h following the change. A decrease in driving elevates substorm probabilities to a greater extent than does an increase, partially supporting external triggering. Yet current v is the best single predictor of subsequently observing a substorm. These results explain why geomagnetically quiet years and active years are better characterized by low or high v (respectively) than by the distribution of merging estimators. It appears that the flow of energy through the magnetosphere is determined by frontside merging, but the burstiness of energy dissipation depends primarily on v.

[1]  T. Pulkkinen,et al.  Magnetospheric convection during intermediate driving: Sawtooth events and steady convection intervals as seen in Lyon-Fedder-Mobarry global MHD simulations , 2007 .

[2]  Mats Nilsson,et al.  Energy exchange and water budget partitioning in a boreal minerogenic mire , 2013 .

[3]  G. Reeves,et al.  Long‐term variations in solar wind velocity and radiation belt electrons , 2013 .

[4]  P. Newell,et al.  Solar wind driving and substorm triggering , 2011 .

[5]  C. Clauer,et al.  Magnetospheric sawtooth events during the solar cycle 23 , 2013 .

[6]  C. Russell,et al.  Proxy studies of energy transfer to the magnetosphere , 1991 .

[7]  B. Sonnerup,et al.  Magnetopause reconnection rate , 1974 .

[8]  Patricia H. Reiff,et al.  Empirical polar cap potentials , 1997 .

[9]  H. Singer,et al.  Storm‐substorm relationship: Variations of the hydrogen and oxygen energetic neutral atom intensities during storm‐time substorms , 2005 .

[10]  J. Drake,et al.  Theory and simulation of Kelvin-Helmholtz instability in the geomagnetic tail , 1996 .

[11]  Jesper Gjerloev,et al.  The SuperMAG data processing technique , 2012 .

[12]  M. Friel,et al.  Substorm behavior of the auroral electrojet indices , 2004 .

[13]  D. Weimer,et al.  Improved calculations of interplanetary magnetic field phase front angles and propagation time delays , 2008 .

[14]  Frederick J. Rich,et al.  A nearly universal solar wind-magnetosphere coupling function inferred from 10 magnetospheric state variables , 2007 .

[15]  C. Meng,et al.  Auroral precipitation power during substorms: A Polar UV Imager‐based superposed epoch analysis , 2001 .

[16]  E. Tanskanen.  A comprehensive high‐throughput analysis of substorms observed by IMAGE magnetometer network: Years 1993–2003 examined , 2009 .

[17]  R. McPherron,et al.  Steady magnetospheric convection: Statistical signatures in the solar wind and AE , 2002 .

[18]  N. G. Popova,et al.  Observational evidence of the loading‐unloading substorm scheme , 2005 .

[19]  T. Pulkkinen,et al.  Solar wind electric field driving of magnetospheric activity: Is it velocity or magnetic field? , 2007 .

[20]  R. Horne,et al.  Favored regions for chorus‐driven electron acceleration to relativistic energies in the Earth's outer radiation belt , 2003 .

[21]  C. Russell,et al.  An empirical relationship between interplanetary conditions and Dst , 1975 .

[22]  G. Cunningham,et al.  On the relationship between relativistic electron flux and solar wind velocity: Paulikas and Blake revisited , 2010 .

[23]  T. Moretto,et al.  Coordinated observations demonstrating external substorm triggering , 1997 .

[24]  M. Freeman,et al.  A minimal substorm model that explains the observed statistical distribution of times between substorms , 2004 .

[25]  R. Wolf,et al.  Is steady convection possible in the Earth's magnetotail? , 1980 .

[26]  S. Wing,et al.  External versus internal triggering of substorms: An information‐theoretical approach , 2014 .

[27]  Daniel N. Baker,et al.  Solar cycle changes, geomagnetic variations, and energetic particle properties in the inner magnetosphere , 2008 .

[28]  David G. Sibeck,et al.  Magnetopause shape as a bivariate function of interplanetary magnetic field Bz and solar wind dynamic pressure , 1993 .

[29]  V. Angelopoulos.  The role of impulsive particle acceleration in magnetotail circulation , 1996 .

[30]  C. Russell,et al.  Substorm and interplanetary magnetic field effects on the geomagnetic tail lobes , 1975 .

[31]  P. Newell,et al.  Substorm and magnetosphere characteristic scales inferred from the SuperMAG auroral electrojet indices , 2011 .

[32]  J B Blake,et al.  Electron Acceleration in the Heart of the Van Allen Radiation Belts , 2013, Science.

[33]  F. Mozer,et al.  Comparison of S3-3 polar cap potential drops with the interplanetary magnetic field and models of magnetopause reconnection , 1983 .

[34]  Steven K. Morley,et al.  No evidence for externally triggered substorms based on superposed epoch analysis of IMF Bz , 2009 .

[35]  R. Horne,et al.  Substorm dependence of chorus amplitudes: Implications for the acceleration of electrons to relativistic energies , 2001 .

[36]  L. Lyons,et al.  A new theory for magnetospheric substorms , 1995 .

[37]  S. Wing,et al.  Substorm cycle dependence of various types of aurora , 2010 .

[38]  A. Lui,et al.  A synthesis of magnetospheric substorm models , 1991 .

[39]  P. Newell,et al.  SuperMAG‐based partial ring current indices , 2012 .

[40]  C. Russell,et al.  Characteristics of ion flow in the quiet state of the inner plasma sheet , 1993 .

[41]  D. Baker,et al.  Quantifying the radiation belt seed population in the 17 March 2013 electron acceleration event , 2014 .

[42]  D. Baker,et al.  Gradual diffusion and punctuated phase space density enhancements of highly relativistic electrons: Van Allen Probes observations , 2014 .

[43]  Nancy U. Crooker,et al.  Dayside merging and cusp geometry , 1979 .

[44]  Statistical Characteristics of the Heliospheric Plasma and Magnetic Field at the Earth's Orbit during Four Solar Cycles 20-23 , 2013, 1301.2929.

[45]  C. Meng,et al.  Two‐component auroras , 2002 .

[46]  N. Crooker,et al.  On the low correlation between long‐term averages of solar wind speed and geomagnetic activity after 1976 , 1993 .

[47]  T. Pulkkinen,et al.  Statistical survey on sawtooth events, SMCs and isolated substorms , 2009 .

[48]  Steven K. Morley,et al.  On the association between northward turnings of the interplanetary magnetic field and substorm onsets , 2007 .

[49]  P. Newell,et al.  Statistical comparison of isolated and non‐isolated auroral substorms , 2012 .

[50]  R. Elphic,et al.  The driving of the plasma sheet by the solar wind , 1998 .

[51]  Jesper Gjerloev,et al.  Evaluation of SuperMAG auroral electrojet indices as indicators of substorms and auroral power , 2011 .

[52]  P. Newell,et al.  Space climate implications from substorm frequency , 2013 .

[53]  Kan Liou,et al.  Polar Ultraviolet Imager observation of auroral breakup , 2010 .

[54]  M. Freeman,et al.  A statistical study of the open magnetic flux content of the magnetosphere at the time of substorm onset , 2009 .

[55]  Masahisa Sugiura,et al.  Auroral electrojet activity index AE and its universal time variations. , 1966 .

[56]  Lou‐Chuang Lee,et al.  Energy coupling function and solar wind‐magnetosphere dynamo , 1979 .

[57]  J. Gosling,et al.  On the high correlation between long‐term averages of solar wind speed and geomagnetic activity , 1977 .

[58]  R. McPherron,et al.  Occurrence frequencies of IMF triggered and nontriggered substorms , 2003 .