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Satellites like DSCOVR are used to predict solar storms.

How does that work? If the satellite is orbiting L1 and "watches" the sun, how can the signal be back on earth before a storm arrives?

How are these storms predicted exactly? Are the models like for weather on earth?

Can these storms be predicted several days before, like with weather on earth?

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You may be asking about a couple of different things here.

1 Events at the surface of the sun

Events at the Sun itself may or may not head in the direction of the Earth, but this is step one of a prediction system. I believe there are two approaches:

a) look at what happened about a month ago and predict that it will be still there when that part of the Sun rotates back into view. I think this has been tried for a while as it is conceptually straightforward.

b) try and forecast an event (e.g. flare or CME) from the appearance of other measurable features on the Sun. I believe this is pretty involved in modelling terms and has not yet come of age (anyone know better?)

2 Events close to the Earth

A geomagnetic storm is the term given to phenomena measurable at the Earth but caused by previous events on the Sun. Whilst a flare (e.g. X rays) might travel quickly to Earth the accompanying release of particulate matter can take of the order of three days to transit.

a) Having a clear view of the sun in several wavebands does therefore allow quite an advanced warning but does not prove that the event seen at the sun is heading towards the Earth (cue more modelling work) or will connect magnitically with the Earth's field when it does (local measurements needed).

b) Having instruments at L1 for sampling of the local environment there may well say much more about the disturbances in the local region of space but unfortunately at not much advanced a warning (a few hours).

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Solar flares travel at about 2000 km/s. Radio travels at 300,000 km/s, so a spacecraft at L1 (1.5 million km out) like DISCOVR gives about 12 minutes of warning when the solar flare reaches it.

Spacecraft like SOHO observe the sun, and these observations are used by e.g. NOAA to try and predict flares:

Current methods of flare prediction are problematic, and there is no certain indication that an active region on the Sun will produce a flare. However, many properties of sunspots and active regions correlate with flaring. For example, magnetically complex regions (based on line-of-sight magnetic field) called delta spots produce the largest flares. A simple scheme of sunspot classification due to McIntosh, or related to fractal complexity.[57] is commonly used as a starting point for flare prediction.[58] Predictions are usually stated in terms of probabilities for occurrence of flares above M or X GOES class within 24 or 48 hours.

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