Space Weather: Magnetic Substorms 7 November 2011

1. C/NOFS DMSP Space Weather: Magnetic Substorms 7 November 2011 William J. Burke Air Force Research Laboratory/Space Vehicles Directorate Boston College Institute for Scientific Research CRESS

2. Space Weather Course Overview • Lecture 1: Overview and Beginnings • Lecture 2: The Aurorae • Lecture 3: Basic Physics (painlessly administered) • Lecture 4: The Main Players • Lecture 5: Solar Wind Interactions with the Earth’s Magnetic Field • Lecture 6: Magnetic Storms • Lecture 7: Magnetic Substorms • Lecture 8: Magnetosphere – Ionosphere Coupling • Lecture 9 The Satellite Drag Problem • Lecture 10: Verbindung (to help make up for your rash decision not to take Wollen Sie Deutch Sprechen?)

3. Space WeatherMagnetic Substorms Overview • The subject matter of Lectures 6 through 8 are closely linked • In the previous two lectures we introduced the concepts of a super sonic solar wind carrying a weak interplanetary magnetic field (IMF) that interact with the Earth’s magnetic field. The interaction produces a magnetic cavity in the solar wind called the magnetosphere. • The consequent, distorted magnetic field of Earthno longer looks like a dipole. Two clefts or cusps develop on the dayside that separate magnetic flux that close on day and night side. Solar wind plasma has direct access to the ionosphere through the cusps. • In Dungey’s model the cusp separates closed and open flux • Today we first consider ionospheric and magnetospheric convection patterns during periods of southward IMF and how they are disruptedduring geomagnetic substorms. Dayside cusps

4. Space WeatherMagnetic Substorms Dungey model of magnetic merging-reconnection cycle • Simplified representation of high-latitude convection pattern. • Ionospheric plasma drifts on equipotentials away from the Sun across polar cap and back toward the Sun through auroral oval. • Associated two-cell current system: DP2. • Stations rotating under DP2 current see +/- BH before/after local midnight. Ionospheric footprint of convecting plasma & field

5. Space WeatherMagnetic Substorms • Kristian Birkeland was first to describe a pattern of nightside magnetic disturbance that he called “polar elementary storms.” - Signatures appear in high- but not low-latitude magnetometer data (unlike storms). • - Note that all stations saw negative magnetic perturbations => DP1. • - Initial slope was gently downward: => “growth phase” (30 min) • - Traces steepened sharply: => “expansion phase” • - Minimum excursion in BH : => “maximum epoch” • - Positive slope: => “recovery phase” - Typically all over in < 2 hours • Sydney Chapman introduced the now standard term “substorm” (~1960).

6. Space WeatherMagnetic Substorms • Auroral Substorm: In 1964 Syun-Ichi Akasofu pieced together all-sky images taken during the 1958 IGY that showed the development of aurorae during substorms. • T < 0 min: quiet-time, dim auroral arcs • T = 0-5 min: equatorward arc brightens • T = 5-10 min: poleward auroral expansion • T = 10-30 min: auroral breakup • T = 30-60 min: early recovery phase • T = 60-120 min: late recovery phase • • After more than 20 years of satellite imaging of aurora, Akasofu’s summary still accepted. • Figure from Akasofu Polar and Magnetospheric Substorms, (D. Reidel, 1968), summarizing IGY ground observations of discrete aurora.

7. Space WeatherMagnetic Substorms All the different sets of visible and extreme ultraviolet images of the high-latitude ionosphere acquired during magnetic substorms confirm the picture that Akasofu assembled in the mid 1960s. Integration times of 6 to 12 minutes Do not allow us to see the dynamics seen by the human eye on the ground. - Point made earlier by Størmer.

8. Space WeatherMagnetic Substorms • The first indication that something very different from predictions of the simple Dungey model came from auroral/plasma sheet particle measurements acquired by VELA satellites at about 14 RE in the magnetotail. • Ed Hones of LANL noticed that if VELA satellites were in the plasma sheet and ground magnetometers indicated that a substorm growth phase onset occurred, the satellites suddenly shifted to the open-field line lobes of the magnetotail and stayed there until early recovery. When VELA returned to the plasma sheet it was much hotter than was measured before the substorm process began. • He suggested an new model for loading the lobes with new fluxin the growth phase with a thinning of the plasma sheet and that a new reconnection line forms close to Earth. VELA

9. Space WeatherMagnetic Substorms • About the same time Bob McPherron ofUCLA argued that the single polarity of BH perturbations could be explained if a substorm system of field-aligned currents • flow into the ionosphere on the morning sideof local midnight and out on the evening side. • The development of McPherron’s postulated FACs have been verified in both satellite and ground based measurements. • Model correctly predicts that magnetic fields near local midnight at geostationary altitude should change from stretched and tail-like in the growth phase to nearly dipolar at the onset of the expansion phase. This too has been verified in CRRES measurements. • After 40 years, controversy still exists as to whether the near-Earth reconnection line or the new current system forms first.

10. Space Weather Magnetic Substorms This controversy is not within the observational community but among theoretical modelers. Everyone agrees the picture of substorm morphology in the magnetosphere is correct. Still they squabble endlessly (and I suspect enjoying every minute of it) concerning the nature of the underlying physics connected with triggering magnetotail current diversion to the ionosphere and the formation of a near-Earth reconnection. No one wants to admit we just don’t know!

11. Space Weather Magnetic Substorms

12. Space WeatherMagnetic Substorms Summary and Conclusions • During the GROWTH PHASE of substorms, the magnetotail configuration becomes stretched, and density of the cross-tail current becomes very large. – Energy is stored in the tail lobes during the growth phase. • Energy release begins suddenly at the onset of the EXPANSION PHASE. - Field lines that were very stretched and tail-like collapse to dipolar shapes. – Aurora suddenly brighten, and the ionospheric conduction currents in the westward electrojet intensifies greatly near local midnight. • – As expansion phases proceed, regions of depolarization, bright aurorae and intense electrojet currents expand to fill nightside auroral oval. - High-energy electrons cause severe charging of geosynchronous satellites. • Ionospheric currents/auroral activity gradually die in RECOVERY PHASE. • – Post-substorm plasma sheet is hotter than it was before. • - One or more substorms can occur in main phase of magnetic storms.