Ring current development during the great geomagnetic storm of February 1986

TLDR

The 1986 February geomagnetic storm, the largest in recent decades, began on Feb 6, peaked at Dst −312 nT on Feb 9, and although it recovered quickly at first, full recovery took over a month, with both solar‑wind and ionospheric ions contributing to the storm‑time ring current. The study tracks ring‑current evolution during the storm using CHEM particle data from the AMPTE CCE spacecraft. The authors compare the ring‑current ion energy content with Dst‑derived expectations, leveraging nearly complete 1–310 keV composition coverage from CHEM, and monitor the composition over five days from pre‑storm quiet to early recovery. During the storm, H+ carried most of the energy except near maximum when O+ dominated (47 % vs 36 % for H+), contrasting with moderate storms where H+ dominated; the rapid initial Dst recovery (τ≈9.3 h) was largely due to loss of 75–100 keV O+ via charge exchange at L=2.5–3.0, suggesting that a >50 % O+ + N+ component generally exists near maximum of great storms.

Abstract

The largest geomagnetic storm in recent decades began with a sudden commencement on February 6, 1986, developed slowly over the next two days, and, with a rapid intensification late on February 8, reached a minimum Dst of −312 nT during the first hour of February 9. Initial recovery was rapid, but full recovery took more than a month. In this paper we follow the ring current development during the storm using particle measurements from the charge‐energy‐mass (CHEM) instrument on the Active Magnetospheric Particle Tracer Explorers (AMPTE) CCE spacecraft. We compare the energy content of the ring current ions with that expected from observed Dst values utilizing for the first time composition coverage over nearly the complete ring current energy range (1–310 keV/ e ). The ring current composition is followed for five days from prestorm quiet time to early recovery phase. Ions of both solar wind and ionospheric origin are important constituents of the storm time ring current. Although H + carries the majority of the energy during most of the storm, O + dominates near the storm's maximum phase, with 47% of the energy density compared with 36% in H + . This is in contrast with all of the more moderate storms which occurred during 1984–1985 in which H + ions contained most of the energy density near storm maximum. The very rapid initial Dst recovery (τ ∼ 9.3 hours) in this storm results largely from the rapid loss of 75‐ to 100‐keV O + via charge exchange in the inner portion of the ring current ( L = 2.5–3.0). Since it has been long observed that initial Dst recovery is much more rapid in great storms than in moderate storms, we suggest that a major (>50%) O + + N + ring current component generally exists near the maximum phase of great storms.