The solar wind is magnetised plasma
The Sun continuously releases plasma containing protons, electrons and an embedded magnetic field. Upstream spacecraft near the L1 region monitor this flow before it reaches Earth. NOAA's real-time display may use data from the currently operational spacecraft and labels data quality.
A monitor is a sample at one location, not a complete map of the structure. Propagation to Earth takes time and an abrupt boundary can evolve while travelling.
Speed supplies context, not a threshold
Faster solar wind can deliver energy to the magnetosphere more rapidly and can accompany high-speed streams or CME structures. Speed alone does not tell whether magnetic coupling will be efficient. A fast stream with unfavourable magnetic orientation can produce less auroral response than a simplistic threshold suggests.
Density and magnetic-field strength also affect pressure and energy input. Read the trend over time and the quality flags rather than treating one momentary speed as a forecast.
Bz describes north–south orientation
Bz is the north–south component of the interplanetary magnetic field in a geocentric coordinate system. When Bz is southward for a sustained period, it can connect more effectively with Earth's oppositely directed dayside field and support energy transfer into the magnetosphere.
A negative reading is not a guarantee. Magnitude, duration, solar-wind speed, field strength and the magnetosphere's prior state all influence the response. Brief fluctuations can reverse before a strong auroral expansion develops.
Why long-range Bz is difficult
A CME can be observed leaving the Sun and modelled through interplanetary space, but its internal magnetic orientation near Earth is hard to know far ahead. Upstream measurement improves that knowledge only shortly before the structure arrives.
This is a fundamental reason to separate a multi-day CME arrival window from a near-real-time geomagnetic assessment. GreenHalos does not invent day-four-to-ten Bz values or convert event context into exact local visibility.
A useful dashboard reading order
Check the observation time and data quality first. Then compare the speed, Bz trend and total magnetic-field strength with current geomagnetic indices and the auroral oval. Finally add darkness and cloud for the location.
NOAA SWPC remains the operational authority. GreenHalos summarises first-party cached source data and provides explanations, but no single-variable rule such as ‘Bz below this number means aurora’ is presented as a prediction.
Evidence register
Primary sources
Sources were checked on . Each record identifies which sections it supports.
- Real Time Solar Wind NOAA Space Weather Prediction Center Supports: The solar wind is magnetised plasma; Speed supplies context, not a threshold; A useful dashboard reading order
- Space Weather Phenomena NOAA Space Weather Prediction Center Supports: Bz describes north–south orientation; Why long-range Bz is difficult
- Earth's Magnetosphere NASA Science Supports: Bz describes north–south orientation