Editing Geomagnetic storm (section)

== Instruments ==
[[Magnetometer]]s monitor the auroral zone as well as the equatorial region. Two types of [[radar]], coherent scatter and incoherent scatter, are used to probe the auroral ionosphere. By bouncing signals off ionospheric irregularities, which move with the field lines, one can trace their motion and infer magnetospheric convection.

Spacecraft instruments include:

* Magnetometers, usually of the flux gate type. Usually these are at the end of booms, to keep them away from magnetic interference by the spacecraft and its electric circuits.<ref name="Magnetometry">{{cite web | last = Snare | first = Robert C. | title = A History of Vector Magnetometry in Space | publisher = University of California | url = http://www-ssc.igpp.ucla.edu/personnel/russell/ESS265/History.html | access-date = 2008-03-18 | archive-url = https://web.archive.org/web/20120520150421/http://www-ssc.igpp.ucla.edu/personnel/russell/ESS265/History.html | archive-date = 2012-05-20 | url-status = dead }}</ref>
* Electric sensors at the ends of opposing booms are used to measure potential differences between separated points, to derive electric fields associated with convection. The method works best at high plasma densities in low Earth orbit; far from Earth long booms are needed, to avoid shielding-out of electric forces.
* Radio sounders from the ground can [[Ionospheric sounding|bounce radio waves]] of varying frequency off the ionosphere, and by timing their return determine the electron density profile—up to its peak, past which radio waves no longer return. Radio sounders in low Earth orbit aboard the Canadian [[Alouette 1]] (1962) and [[Alouette 2]] (1965), beamed radio waves earthward and observed the electron density profile of the "topside ionosphere". Other radio sounding methods were also tried in the ionosphere (e.g. on [[IMAGE (spacecraft)|IMAGE]]).
* Particle detectors include a [[Geiger counter]], as was used for the original observations of the [[Van Allen radiation belt]]. [[Scintillator detectors]] came later, and still later "channeltron" [[electron multiplier]]s found particularly wide use. To derive charge and mass composition, as well as energies, a variety of [[Mass spectrometry|mass spectrograph]] designs were used. For energies up to about 50&nbsp;keV (which constitute most of the magnetospheric plasma) [[time-of-flight spectrometer]]s (e.g. "top-hat" design) are widely used.{{Citation needed|date=December 2008}}

Computers have made it possible to bring together decades of isolated magnetic observations and extract average patterns of electrical currents and average responses to interplanetary variations. They also run simulations of the global magnetosphere and its responses, by solving the equations of [[magnetohydrodynamics]] (MHD) on a numerical grid. Appropriate extensions must be added to cover the inner magnetosphere, where magnetic drifts and ionospheric conduction need to be taken into account. At polar regions, directly linked to the [[solar wind]], large-scale ionospheric anomalies can be successfully modeled, even during geomagnetic super-storms. <ref>{{cite journal | author=Pokhotelov D. |display-authors=et al. | title = Polar tongue of ionisation during geomagnetic superstorm |journal = Ann. Geophys. |volume=39 |pages=833–847 |year=2021 |issue=5 |doi=10.5194/angeo-39-833-2021 |bibcode=2021AnGeo..39..833P |doi-access=free |url=https://elib.dlr.de/144151/1/angeo-39-833-2021.pdf }}</ref> At smaller scales (comparable to a degree of latitude/longitude) the results are difficult to interpret, and certain assumptions about the high-latitude forcing uncertainty are needed. <ref>{{cite journal | author=Pedatella N. |display-authors=et al. | title = Effects of High-Latitude Forcing Uncertainty on the Low-Latitude and Midlatitude Ionosphere |journal = J. Geophys. Res. |volume=123 |pages=862–882 |year=2018 |issue=1 |doi=10.1002/2017JA024683 |bibcode=2018JGRA..123..862P |doi-access= |s2cid=133846779 }}</ref>