Ionospheric storms are storms which contain varying densities of energised electrons in the ionosphere as produced from the Sun. Ionospheric storms are caused by geomagnetic storms. They are categorised into positive and negative storms, where positive storms have a high density of electrons and negative storms contain a lower density. The total electron content (TEC) is used to measure these densities, and is a key variable used in data to record and compare the intensities of ionospheric storms. Ionospheric storm occurrences are strongly linked with sudden increases of solar wind speed, where solar wind brings energised electrons into the upper atmosphere of Earth and contributes to increased TEC. Larger storms form global visibility of auroras. Auroras are most commonly seen in the Arctic Circle; however, large ionospheric storms allow for them to be visible at somewhat lower latitudes. The most intense ionospheric storm occurred in 1859, commonly named the "solar storm of 1859" or the "Carrington Event." The Carrington Event was named after Richard Carrington, an English astronomer who observed the irregular sun activity that occurred during the Carrington Event. The intensity of the storm brought the visibility of the aurora to lower latitudes, and it was reportedly seen in places such as Florida and the Caribbean. Ionospheric storms can happen at any time and location. F-region and D-region ionospheric storms are also considered main categories of ionospheric storms. The F-region storms occur due to sudden increases of energised electrons instilled into Earth's ionosphere. The F-region is the highest region of the ionosphere. Consisting of the F1 and F2 layers, its distance above the Earth's surface is approximately 200–500 km. The duration of these storms are around a day and reoccur every approximately 27.3 days. Most ionospheric abnormalities occur in the F2 and E layers of the ionosphere. D-region storms occur immediately after F-region storms, and are referred to as the "Post-Storm Effect," the duration of it spanning for a week after the F-region storm's occurrence.
Historical occurrences The largest ionospheric storm occurred during the Carrington event on August 28, 1859 and caused extensive damages to various parts including the sparking of fires in railway signals and telegraph wires. The substantial density of energised electrons produced by the storm caused these electrical overloads and shortages. Occurrences of storms in the last 35 years have been consolidated and measured in maximum Ap which records the average daily geomagnetic activity during ionospheric storms. There are higher levels of geomagnetic activity with high maximum Ap counts. Ap counts in terms of geomagnetic activity from 0-7 are considered "quiet," 8-15 "unsettled," 16-29 "active," 30-49 "minor storm," 50-99 "major storm," and above 100 classified as a "severe storm." Minor storms in the last 35 years ranging from 30-49 Ap occurred on 13 September 1999 (46), 11 October 2008 (34), 11 March 2011 (37), 9 October 2012 (46) and on 19 February 2014 (43). Major storms ranging from 50-99 Ap occurred on 6 April 2000 (82), 7 April 2000 (74), 11 April 2001 (85), 18 April 2002 (63), 20 April 2002 (70), 22 January 2004 (64), 18 January 2005 (84), 5 April 2010 (55), 9 March 2012 (87), 15 July 2012 (78) and on 1 June 2013 (58). Severe storms equalling or exceeding 100 Ap occurred on 8 February 1986 (202), 9 February 1986 (100), 13 March 1989 (246), 14 March 1989 (158), 17 November 1989 (109), 10 April 1990 (124), 7 April 1995 (100), 31 March 2001 (192), 6 November 2001 (142), 18 August 2003 (108), 29 October 2003 (204), 30 October 2003 (191), 20 November 2003 (150), 27 July 2004 (186), 8 November 2004 (140) and on 10 November 2004 (161). In recent accounts, the St Patrick's Day storms in March 2013 and 2015 caused a strong negative phase in the F2 ionospheric region. The March 2013 and 2015 storms were also long-lasting, spanning for over 6 hours. The June 2015 Southern Hemisphere winter storm had a shorter duration, lasting between 4 and 6 hours, and producing a positive effect in the ionosphere. It is difficult to determine the exact location and time for the occurrences of ionospheric storms, their effects being dependent on season, their varying starting points, compositional changes in the ionosphere and the travelling ionospheric disturbances (TIDs) in relation to gravity waves having varying impacts on different locations.
Phases of ionospheric storms In the commencement of an ionospheric storm, due to geomagnetic disturbances in the ionosphere, the storm will become positive for a brief duration. Then, it will become a negative phase storm, and revert to a recovery phase where electron density neutralises.
Positive phase The positive phase of an ionospheric storm will last for around the first 24 hours. In this phase, electron density in the ionosphere, particularly in higher altitude layers such as F1 and F2 will increase. Ionisation in the positive phase will be less apparent due to the increase of electron density. Positive phase ionospheric storms have a longer duration and are more prevalent in winter. During the positive phase of large ionospheric storms, the altitude of ionospheric F-region increases, resulting in the massive tongue-shaped plasma anomaly spreading anti-sunward over the geomagnetic pole, which can be observed by ground radars, as well as by satellites and the GPS system. Even for the largest geomagnetic storms, such as the 20 November 2003 superstorm, modern general circulation models are able to simulate positive ionospheric anomalies.
Negative phase The negative phase of an ionospheric storm will occur directly after the storm's positive phase and last one to two days after the positive phase decreases in electron density to "below its quiet time reference level." Negative phases decrease the electron density of the storm. They also span for longer durations and appear more often during summer.
Recovery phase The recovery phase of the ionospheric storm occurs after the negative phase ends and neutralises the electron density. A time scale of 12 hours to 1 day can be used in accordance with the Thermosphere Ionosphere General Circulation Model (TIGCM) as a means of calculating the precise time of electron density restabilising post-storm.
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