Substorm occurrence rates, substorm recurrence times, and solar wind structure

• Published in: Journal of geophysical research. Space physics 2017-03, Vol.122 (3), p.2973-2998
• Main Authors: Borovsky, Joseph E., Yakymenko, Kateryna
• Format: Article
• Language: English
• Subjects: Field strength; Geomagnetism; heliospheric magnetic field; Intervals; Mach number; Magnetic fields; Magnetosphere; Magnetospheres; Mathematical models; Mesoscale phenomena; periodic substorms; Populations; Properties (attributes); Ram pressure; random substorms; Saturation; Solar cycle; Solar cycles; Solar wind; Solar wind properties; Solar wind structure; Statistics; substorms; Wind structure
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1002/2016JA023625

Two collections of substorms are created: 28,464 substorms identified with jumps in the SuperMAG AL index in the years 1979–2015 and 16,025 substorms identified with electron injections into geosynchronous orbit in the years 1989–2007. Substorm occurrence rates and substorm recurrence‐time distributions are examined as functions of the phase of the solar cycle, the season of the year, the Russell‐McPherron favorability, the type of solar wind plasma at Earth, the geomagnetic‐activity level, and as functions of various solar and solar wind properties. Three populations of substorm occurrences are seen: (1) quasiperiodically occurring substorms with recurrence times (waiting times) of 2–4 h, (2) randomly occurring substorms with recurrence times of about 6–15 h, and (3) long intervals wherein no substorms occur. A working model is suggested wherein (1) the period of periodic substorms is set by the magnetosphere with variations in the actual recurrence times caused by the need for a solar wind driving interval to occur, (2) the mesoscale structure of the solar wind magnetic field triggers the occurrence of the random substorms, and (3) the large‐scale structure of the solar wind plasma is responsible for the long intervals wherein no substorms occur. Statistically, the recurrence period of periodically occurring substorms is slightly shorter when the ram pressure of the solar wind is high, when the magnetic field strength of the solar wind is strong, when the Mach number of the solar wind is low, and when the polar‐cap potential saturation parameter is high. Key Points Substorm occurrence rates and substorm recurrence times are studied and compared with the properties of the time‐varying solar wind at Earth The mesoscale structure of the solar wind magnetic field plays a role in the occurrence statistics of substorms The period of periodic substorms may be a combination of an intrinsic magnetospheric period plus variations of the solar wind magnetic field