TheoriesandSimulationsinSubstormResearch:AReview

Boththeoryandsimulationhaveplayedimportantrolesindefiningandilluminatingthekeymechanismsinvolvedinsubstorms.Basictheoriesofmagneticreconnectionandofinterchangeandballooninginstabilitiesweredevelopedmorethan50yearsago,andtheseplasmaphysicalconceptshavebeencentralindiscussionsofsubstormphysics.Avastamountofresearchonreconnection,includingboththeoreticalandcomputationalstudies,hashelpedprovideapictureofhowreconnectionoperatesinthecollisionlessenvironmentofthemagnetosphere.Still,however,wedonotfullyunderstandhowkeymicroscaleprocessesandlarge-scaledynamicsworktogethertodeterminethelocationandrateofreconnection.Whileinthelasttwentyyears,ithasbecomeclearthatinterchangeprocessesareimportantfortransportingplasmathroughtheplasmasheetintheformofburstybulkflowsandsubstormexpansions,westillhavenotreachedthepointwheresimulationsareabletorealisticallyanddefensiblyrepresentalloftheimportantaspectsofthephenomenon.Morethantwodecadesagoitwassuggestedthattheballooninginstability,thebasictheoryforwhichdatesfromthe1950s,mayplayanimportantroleinsubstorms.Nowthemajorityofexpertsagreethatregionsoftheplasmasheetareoftenlinearlyunstabletoideal-MHDballooning.However,itisalsoclearthatkineticeffectsintroduceimportantmodificationstotheMHDstabilitycriterion.Itisstilluncertainwhetherballooningplaysaleadingroleinsubstormsorhasjustaminorpart.Amongthedifferenttypesofsimulationsthathavebeenappliedtothesubstormproblem,globalMHDcodesareuniqueinthat,inasense,theyrepresenttheentireglobalsubstormphenomenon,includingcouplingtothesolarwindandionosphere,andtheimportantmechanismsofreconnection,interchange,andballooning.However,theyhavenotyetprogressedtothepointwheretheycanaccuratelyrepresentthewholephenomenon,becausegrid-resolutionproblemslimittheaccuracywithwhichtheycansolvetheequationsofidealMHDandthecouplingtotheionosphere,andtheycannotaccuratelyrepresentsmall-scaleprocessesthatviolateidealMHD.