Earth’s magnetosphere and its interaction with the solar wind (as seen in auroras, convection, and currents)
Steve Milan Radio and Space Plasma Physics Group Department of Physics and Astronomy University of Leicester Birkeland Centre for Space Sciences University of Bergen IMAGE FUV IMAGE data courtesy of Stephen Mende, Harald Frey and the IMAGE FUV team
Solar wind-magnetosphere-ionosphere-atmosphere coupling
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Magnetic reconnection University of plasma density and the Dungey cycle Alberta Ionospheric convection It takes 4 hours for the ionosphere to convect across the polar cap and 8 hours to return
We can calculate that the magneto- tail is 1000 RE long
Corotation in the inner magnetosphere SuperDARN Plasma populations in the magnetosphere
magnetosheath Plasma populations in the magnetosphere Plasma populations in the magnetosphere
Magnetosheath Plasmasphere Plasma sheet Ring current
The auroral oval is highly variable in “polar brightness cap” and size The “substorm” Substorm Substorm
5 June growth/ loading FPC 1998 0.9 GWb
0.6 GWb Open flux expansion, recovery/ 0.3 GWb Polar UVI unloading
0.0 GWb
Wind
IMF BZ
Milan et al. (2003)
[email protected] Substorm Substorm
5 June growth/ loading FPC 1998 0.9 GWb
0.6 GWb Open flux expansion, recovery/ 0.3 GWb Polar UVI unloading
0.0 GWb
Wind
IMF BZ
Milan et al. (2003)
[email protected] Substorm Substorm
5 June growth/ loading FPC 1998 0.9 GWb
0.6 GWb Open flux expansion, recovery/ 0.3 GWb Polar UVI unloading
0.0 GWb
Wind
IMF BZ
Milan et al. (2003)
[email protected] Substorm Substorm
5 June growth/ loading FPC 1998 0.9 GWb
0.6 GWb Open flux expansion, recovery/ 0.3 GWb Polar UVI unloading
0.0 GWb
Wind
IMF BZ
Milan et al. (2003)
[email protected] The expanding/contracting polar cap
dF PC = Φ − Φ dt D N
Faraday (1831) [email protected] Siscoe and Huang (1985) [email protected] Cowley and Lockwood (1992) Milan (2013) Open flux auroral control of intensity
intensity open flux Superposed epoch analyses of auroral intensity, open flux, AU, AL AU and AL, Sym-H, and SW-coupling
during 40 substorms Sym-H
Substorms binned
by open flux at onset Vday
[email protected] Open flux auroral control of intensity
intensity open flux Superposed epoch analyses of auroral intensity, open flux, AU, AL AU and AL, Sym-H, and SW-coupling
during 40 substorms Sym-H
Substorms binned
by open flux at onset Vday
[email protected] Open flux variations on substorm and storm time-scales
Proxy for open flux content *
Proxy for ring current intensity
data gaps due to * orbit of IMAGE [email protected] Milan (2009) Milan et al. (2009b) Superposed epoch -20 min -20 min analysis of convection 69 kV 41 kV - High latitude substorms have prompt convection response -10 min -10 min - Low latitude substorms have 69 kV 43 kV convection decrease at onset Substorm electrodynamics influenced by auroral bulge onset onset conductivity 71 kV 40 kV
Convection velocity in onset region +10 min +10 min 51 kV 42 kV
+20 min +20 min 62 kV 46 kV onset latitude onset
+50 min +50 min 55 kV 48 kV Grocott et al. (2009) Solar wind-magnetosphere-ionosphere-atmosphere coupling • The solar wind-magnetosphere interaction distorts the dipolar field of the Earth resulting in currents at the magnetopause and in the magnetotail
• This interaction also leads to dynamics which are communicated to the ionosphere by further current systems
Milan et al. (2016) Birkeland currents from AMPERE (Active Magnetosphere and Planetary Electrodynamics Response Experiment)
Region 2
Region 0
Region 1
Iijima and Potemra (1978) After Anderson et al. (2000), Waters et al. (2001) AMPERE and SuperDARN
[email protected] Courtesy Lasse Clausen AMPERE and SuperDARN
[email protected] Courtesy Lasse Clausen Oksavik et al. (2006) Sub-Auroral Polarization Streams (SAPS) • Fast westward convection flows in the mid-latitude ionosphere, associated with storm processes – inner magnetosphere dynamics • AMPERE observes bifurcations of the region 2 currents, esp. at dusk, associated with substorms • In the pre-midnight sector this forms an up/down current pair, consistent with a flow channel
2nd June 2011
00:00 04:48 09:36 14:24 19:12 24:00 [email protected] Sangha et al. (in prep.) Sub-Auroral Polarization Streams (SAPS) • Association of fast westwards flow stream in the pre-midnight sector with bifurcated (downward) region 2 current
• Auroras collocated with upwards current regions • Ionospheric conductance (or lack thereof) and inner magnetosphere dynamics (partial ring current) both play a role in the generation of fast flow Solar wind-magnetosphere coupling Reconnection location
IMF Bz < 0, By = 0 IMF Bz > 0, By > 0
Reconnection with closed field lines Reconnection with open field lines Northwards IMF “cusp spot”
Milan et al. (2000) Northwards IMF “cusp spot”
IMF BY
spot MLT
Milan et al. (2000) Transpolar arcs - theta aurora Transpolar arcs - theta aurora The arc grows across the polar It fades into the cap from a It moves across nightside aurora brightening of the polar cap as the IMF turns the nightside as IMF BY southwards once auroral oval switches again
Milan et al. (2005) Transpolar arcs - theta aurora Dayside vortical Azimuthal flows flows indicate The TPA is pushed indicate magnetic that the TPA towards the night- reconnection in a moves under side as standard twisted the influence of Dungey cycle flows magnetotail “lobe stirring” add new open flux
Milan et al. (2005) Conclusions • The open magnetosphere model provides a holistic picture which explains not only the magnetic and plasma structure of the magnetosphere, but also the dynamics • Many, originally unsuspected, magnetospheric phenomena fit naturally into the open magnetosphere picture • The inner magnetosphere and ionospheric conductance are also important for understanding the dynamics of the magnetosphere [email protected]
Milan et al. (2000b) Milan et al. (2000b) Milan et al. (2000b)
The auroral substorm The geomagnetic storm Solar wind-magnetosphere-ionosphere-atmosphere coupling
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[email protected] Milan et al. (2009)