Planned talk schedule. Substorm models. Reading: Chapter 9 - SW-Magnetospheric Coupling from Russell book (posted)

Transcription

1 Reading: Chapter 9 - SW-Magnetospheric Coupling from Russell book (posted) Today: Example of dynamics/time variation Review of intro to auroral substorms Substorm models How do we know a substorm is occurring? Substorms in magnetotail: examples MHD models Planned talk schedule Wed Nov 16 - Tom on geomagnetic storms Mon. Nov 21 - Xiangwei on radiation belts Mon. Nov 28 - Sahel on helioseismology Mon Dec 5 - Caitlin on stellar seismology Wed Dec 7 - Tony on pulsar magnetospheres One more planned guest lecturer: Prof Tom Jones on space plasma and plasma astrophysics simulations (shock & shock acceleration, Kelvin- Helmholtz, etc) on Wed 11/23 Akasofu picture of the aurora during substorms: (a) (b) (c) (d) (e) (f) Quiet auroral arc before substorm Equatorward edge of aurora intensifies Westward traveling surge forms Poleward expansion of surge Aurora begins to fade; patchy pulsating aurora forms on dawn Auroral oval retreats to pre-substorm locations Substorm models

2 Near-Earth Neutral Line Scenario for Substorms NENL model assumes that reconnection begins within current sheet close to Earth (2). Reconnection region expands (3-6) and ejects a plasmoid that moves down the tail (7-8). Current sheet refills to initial configuration (9-10) Hones, 1974 Current Disruption Scenario for Substorms Flux Balance in Convection Lui, 1994 Current disruption scenario proposes that an instability leads to the disruption of the cross-tail current and its subsequent diversion into field-aligned current. Subsequently, a rarefaction wave propagates tailward eventually leading to reconnection. Thus this is in inside-out model as opposed to the outside-in NENL model. Possible modes include ballooning instability or lower hybrid waves. Where is magnetic flux stored? The dayside magnetosphere (closed field lines), the tail lobes (open field lines), and the nightside plasma sheet (closed field lines) Ulikely that the rate of change of flux in all regions are balanced These rates depend on the merging rate M, the transport rate T, the reconnection rate R, and the return rate R A steady state tail occurs when all rates are equal McPherron, 2008

3 Example of magnetic flux storage and the rates of reconnection at the frontside magnetopause (M), in the tail (R), and transfer (convection from dayside to nightside) During growth phase, M is increasing, flux on the dayside decreases, magnetic flux builds up in the tail lobes and plasma sheet thins. During expansion phase, R increases, magnetic flux decreases in the tail lobes and plasma sheet expands. Russell/Strangeway Russell/Strangeway Ionospheric and field-aligned currents Baker et al.,

4 Ionospheric Currents Substorm DP-2 consists of two electrojets (east and west) flowing towards midnight DP-1 is a current system centered at local midnight that flows within the region of bright aurora Cause of the DP-1 Magnetic Perturbations At expansion onset a portion of the tail current is diverted along field lines to the ionosphere The current is down post midnight, westward across the auroral bulge, and upward out of the westward traveling surge The ionospheric segment causes the sudden increase in negative X perturbations The field-aligned segments produce effects at midlatitudes as indicated by diagram at bottom This 3-D system is called the substorm current wedge McPherron, 2008 McPherron, 2008 Complete current system Iijima and Potemra, 1978 Russell/Strangeway

5 AL, AU and AE AU and AL indices give upper and lower envelope of horizontal component of magnetic field from an array of magnetometers. AE=AU-AL AU is a measure of the eastward auroral electrojet flowing in the dayside auroral oval and coupled to magnetopause currents. AU is generally a measure of the directly driven response of the magnetosphere to the changes of the interplanetary medium (solar-wind) conditions. AE as indicator of substorm phase and activity AL is a measure of the westward auroral electroject that intensifies during substorms. Russell et al., Chap. 9, 2011 Russell et al., Chap. 9, 2011 Relationship of substorms to IMF Substorm and reconnection: ISEE example Superposed epoch study of 54 isolated substorms showing the average percentage of the maximum value of the AE index and north-south component of the interplanetary magnetic field. The time scale is measured in hours from the onset of the expansion phase (Foster et al., 1971). From Russell, 1974

6 Keograms - alternate way to see auroral motions Scanning photometer keogram of Growth and Expansion Phase Mapping of onset arc to tail At the beginning of the keogram two bands of weak aurora are evident Both move equatorward with time. Occasional bright spots and several N-S streamers emanate from the poleward arc The onset is signaled by a sudden brightening in the equatorward arc and a rapid poleward expansion At the end of the expansion there is repeated brightening of the poleward edge of the expansion Baker et al., 1996

7 Comparing timing of current disruption and reconnection THEMIS: Mission to Study Substorms Onset in UV image at 0408:19(+/- 1min) Reconnection onset at ~18 Re ~7 minutes earlier than auroral intensification and geosynchronous changes THEMIS Mission THEMIS is a 5-satellite mission that will string the satellites out along the tail with 1, 2, and 4 day periods so that they will line up every 4 days and be able to determine the time history of events in substorms. All sky camera and satellite locations with time delays

8 Plasmoids in the deep-tail: THE TRAVELING COMPRESSION REGION CHARACTERISTICS Identification Occurrence Frequency Duration Amplitude Time Of Flight Speed* Approximate Size OBSERVED VALUES Bt pulse, Bipolar Bz 3-4 per day 5-20 minutes 10-30% km/sec Re * Assuming release at onset at 20 Re From Slavin et al. [1984] in McPherron, 2008 Statistical comparison of onset and plasmoids

9 Movie, compiled from sequential IBEX images, is what appears to be the first dynamic plasma sheet event ever imaged. While a plasmoid is one explanation for the event, other possibilities include acceleration of ions from compression of the plasma sheet or an injection of new plasma caused by a reconnection event further back in the tail, with particles streaming back toward Earth. IBEX team, SWRI Why substorm? The Expandable Tippy Thundermug model of substorm behavior by Davis (1970). This model continues the earlier concepts of the Van Allen belts as a leaky bucket and a splash catcher, while illustrating many of the phenomena known to occur in the magnetosphere. In Russell,1974