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Coronal Mass Ejections (CMEs) are large expulsions of plasma and magnetic field from the Sun’s corona. They can eject billions of tons of coronal material and carry an embedded magnetic field (frozen in flux) that is stronger than the background solar wind interplanetary magnetic field (IMF) strength. CMEs travel outward from the Sun at speeds ranging from slower than 250 kilometers per second (km/s) to as fast as near 3000 km/s. The fastest Earth-directed CMEs can reach our planet in as little as 15-18 hours. Slower CMEs can take several days to arrive. They expand in size as they propagate away from the Sun and larger CMEs can reach a size comprising nearly a quarter of the space between Earth and the Sun by the time it reaches our planet.The more explosive CMEs generally begin when highly twisted magnetic field structures (flux ropes) contained in the Sun’s lower corona become too stressed and realign into a less tense configuration – a process called magnetic reconnection. This can result in the sudden release of electromagnetic energy in the form of a solar flare; which typically accompanies the explosive acceleration of plasma away from the Sun – the CME. These types of CMEs usually take place from areas of the Sun with localized fields of strong and stressed magnetic flux; such as active regions associated with sunspot groups. CMEs can also occur from locations where relatively cool and denser plasma is trapped and suspended by magnetic flux extending up to the inner corona - filaments and prominences. When these flux ropes reconfigure, the denser filament or prominence can collapse back to the solar surface and be quietly reabsorbed, or a CME may result. CMEs travelling faster than the background solar wind speed can generate a shock wave. These shock waves can accelerate charged particles ahead of them – causing increased radiation storm potential or intensity.Important CME parameters used in analysis are size, speed, and direction. These properties are inferred from orbital satellites’ coronagraph imagery by SWPC forecasters to determine any Earth-impact likelihood. The NASA Solar and Heliospheric Observatory (SOHO) carries a coronagraph – known as the Large Angle and Spectrometric Coronagraph (LASCO). This instrument has two ranges for optical imaging of the Sun’s corona: C2 (covers distance range of 1.5 to 6 solar radii) and C3 (range of 3 to 32 solar radii). The LASCO instrument is currently the primary means used by forecasters to analyze and categorize CMEs; however another coronagraph is on the NASA STEREO-A spacecraft as an additional source.Imminent CME arrival is first observed by the Deep Space Climate Observatory (DSCOVR) satellite, located at the L1 orbital area. Sudden increases in density, total interplanetary magnetic field (IMF) strength, and solar wind speed at the DSCOVR spacecraft indicate arrival of the CME-associated interplanetary shock ahead of the magnetic cloud. This can often provide 15 to 60 minutes advanced warning of shock arrival at Earth – and any possible sudden impulse or sudden storm commencement; as registered by Earth-based magnetometers.Important aspects of an arriving CME and its likelihood for causing more intense geomagnetic storming include the strength and direction of the IMF beginning with shockarrival, followed by arrival and passage of the plasma cloud and frozen-in-flux magnetic field. More intense levels of geomagnetic storming are favored when the CME enhanced IMF becomes more pronounced and prolonged in a south-directed orientation. Some CMEs show predominantly one direction of the magnetic field during its passage, while most exhibit changing field directions as the CME passes over Earth. Generally, CMEs that impact Earth’s magnetosphere will at some point have an IMF orientation that favors generation of geomagnetic storming. Geomagnetic storms are classified using a five-level NOAA Space Weather Scale. SWPC forecasters discuss analysis and geomagnetic storm potential of CMEs in the forecast discussion and predict levels of geomagnetic storming in the 3-day forecast.*Images courtesy of NASA and the SOHO and STEREO missions National Oceanic and Atmospheric AdministrationNational Weather ServiceNational Centers for Environmental PredictionSpace Weather Prediction Center325 Broadway, Boulder CO 80305DisclaimerPrivacy PolicyAbout NOAA's National Weather ServiceCareers in WeatherHOMEABOUT SPACE WEATHERImpactsEarth's ClimateElectric Power TransmissionGPS SystemsHF Radio CommunicationsSatellite CommunicationsSatellite DragPartners and StakeholdersCommercial Service ProvidersFederal AgenciesInternational OrganizationsInternational Service ProvidersSpace Weather ResearchPhenomenaAuroraCoronal HolesCoronal Mass EjectionsEarth's MagnetosphereF10.7 cm Radio EmissionsGalactic Cosmic RaysGeomagnetic StormsIonosphereIonospheric ScintillationRadiation BeltsSolar EUV IrradianceSolar Flares (Radio Blackouts)Solar Radiation StormSolar WindSunspots/Solar CycleTotal Electron ContentAdditional InfoNOAA Space Weather ScalesCustomer Needs & Requirements StudyPRODUCTS AND DATAForecasts27-Day Outlook of 10.7 cm Radio Flux and Geomagnetic Indices3-Day Forecast3-Day Geomagnetic ForecastForecast DiscussionPredicted Sunspot Numbers and Radio FluxReport and Forecast of Solar and Geophysical ActivitySolar Cycle ProgressionSpace Weather Advisory OutlookUSAF 45-Day Ap and F10.7cm Flux ForecastWeekly Highlights and 27-Day ForecastReportsForecast VerificationGeoalert - Alerts, Analysis and Forecast CodesGeophysical AlertSolar and Geophysical Event ReportsUSAF Magnetometer Analysis ReportModelsAurora - 30 Minute ForecastCTIPe Total Electron Content ForecastD Region Absorption Predictions (D-RAP)Geospace Geomagnetic Activity PlotGeospace Ground Magnetic Perturbation MapsGeospace Magnetosphere MoviesNorth American (US Region) Total Electron ContentNorth American Total Electron ContentRegional Geoelectric (1D)Geoelectric Field 1-Minute (Empirical EMTF - 3D Model)Geoelectric 3D-1D ComparisonRelativistic Electron Forecast ModelSEAESRTSTORM Time Empirical Ionospheric CorrectionWSA-Enlil Solar Wind PredictionWAM-IPEObservationsBoulder MagnetometerGOES Electron FluxGOES MagnetometerGOES Proton FluxGOES Solar Ultraviolet Imager (SUVI)GOES X-ray FluxLASCO CoronagraphPlanetary K-indexReal Time Solar WindSatellite EnvironmentSolar Synoptic MapSpace Weather OverviewStation K and A IndicesSummariesSolar & Geophysical Activity SummarySolar Region SummarySummary of Space Weather ObservationsAlerts, Watches and WarningsAlerts, Watches and WarningsNotifications TimelineExperimentalACE Real-Time Solar WindSolar TErrestrial RElations Observatory (STEREO)Data AccessDASHBOARDSAviationElectric PowerEmergency ManagementGlobal Positioning SystemRadioSatellitesSpace Weather EnthusiastsMEDIA AND RESOURCESEducation and OutreachGlossaryNews ArchiveNewsroomSUBSCRIBEANNUAL MEETINGFEEDBACK