Figure 1.1: Schematic noon-midnight meridian cross-section of the terrestrial magneto- sphere (Russell, 1972).
The magnetosphere is a protective bubble surrounding the Earth formed by the in- teraction between the geomagnetic field and the IMF. It is key to life on Earth by firstly acting as a shield to prevent the atmosphere from being stripped away by the pressure of the solar wind. Secondly, it acts to guide harmful charged particles around the Earth so they generally do not reach the surface and endanger the biosphere. Without the magne-
tosphere, the human race would be subject to the same radiation threats as interplantary astraunauts. The most serious threat is thought to come from, the sporadic, solar pro- ton events (SPEs). If present in space at the time of an SPE, an astronaut would be bombarded with a large dose of harmful proton radiation, which is likely to cause acute radiation sickness and can be rapidly fatal (Seed, 2011). There are further health con- cerns from the bombardment of galactic cosmic rays which cause carcinogenesis of cells (Cucinotta and Durante, 2006) and an acceleration of neurological degeneration (Vazquez, 1998). For astonauts on month long missions (a typical mission length) experiencing an SPE is unlikely and exposure to GCRs is relatively short, however over a lifetime on Earth, without the protection of a magnetosphere, the likelihood of experiencing an SPE is sig- nificantly increased and the exposure to GCRs will be persistent. Global life expectancy would no doubt decrease significantly.
A schematic drawing of the magnetosphere is shown in figure 1.1 displaying a cross- section of the terrestrial magnetosphere along the noon-midnight meridian. The magne- tospheric magnetic field has large departures from the typical dipole shape magnetic field that is expected if the Earth was completely isolated. The dynamic pressure of the solar wind compresses the geomagnetic field on the dayside and on the nightside the field lines are stretched many Earth radii downstream forming the magnetotail. On the dayside a ‘bow shock’ is formed where the solar wind initially interacts with the geomagnetic field and is abruptly diverted around the magnetospheric cavity. Slightly Earthward of this feature is the magnetopause which marks the official boundary between the plasma of the solar wind and the magnetosphere. This typically sits roughly 10 Earth radii (Re) away
from the earth, at its subsolar location, although it can vary by 1-2 Re depending on the
rate of dayside magnetic reconnection and the solar wind dynamic pressure.
There are two types of magnetospheric configurations: closed and open. These are displayed in figure 1.2 a) and b) as diagrams of noon-midnight meridian cross-sections of the magnetosphere within the solar wind field which flows from left to right. The set-up of the configuration is controlled by whether the IMF is orientated northward (Bz >0) or southward (Bz <0). The solar wind’s interaction with the geomagnetic field
is strongest when there is a southward orientation (see subplot b)). This permits an ‘open’ magnetosphere where the geomagnetic field directly merges with the IMF (field lines labelled ‘C’) allowing direct access for solar wind particles into the magnetosphere, thus the term ‘open’ field lines. A northward orientation of the IMF (Bz >0), however,
1.1. Sun-Earth Interaction 33
Figure 1.2: Terrestrial and solar wind magnetic field configuration for, a) northward IMF and, b) a southward IMF and, c) the convection pattern over the polar cap (Dungey cycle) for a southward IMF. A: interplanetary magnetic field lines, B: IMF lines reconnecting to geomagnetic field line, C: open, D: closed and N: neutral points or reconnection sites, i.e., dayside reconnection point and near-Earth neutral line (Hargreaves, 1979).
creates a ‘closed’ magnetosphere (see subplot a)) where there are no shared field lines between the solar wind and geomagnetic field, thus all of the Earth’s field lines are closed (labelled as ‘D’).
In the special case of the open magnetosphere a magnetic flux tube convection about the Earth is established termed the Dungey cycle after its original discoverer. This is shown in figure 1.2 c) where the different stages of the cycle are labelled from 0 to 7. The Dungey cycle is due to antiparallel orientations (or at least orientations with relatively high magnetic field shear) of the southward IMF and the northward geomagnetic field (see stage 0) creating conditions which promote a process called magnetic reconnection. This process involves anti-parallel field lines breaking at a location where the field changes abruptly across relatively short spatial scales and rejoining to each other creating two new field lines. These can quickly convect away from the site of reconnection by a sudden release of energy stored in the tension of the magnetic field lines. This process is shown in figure 1.3.
There are two areas in the open magnetosphere where the conditions are right for reconnection. These are both labelled as ‘N’ for ‘neutal point’ (where locally B=0) on
Figure 1.3: A magnetic reconnection diagram showing antiparallel magnetic field lines from two different locations (lines at the top and bottom of figure) reconnecting to one another (at the ‘Neutral Line’ point in the centre) in order to form new field lines (at left and right of figure) which convect away from the reconnection site (Anderson, 2011). subplot b). The first one is at the front of the magnetopause, i.e., the dayside reconnection point, and the second one is at the tail end of the magnetosphere, i.e., the near Earth neutral line. The combination of these events drive a convection of magnetised plasma around the Earth (the Dungey cycle). The dayside reconnection event (stages 0-1) creates two ‘open’ field lines with their footprints originating in the Earth’s core and the other ends joined to the solar wind (this is part of the polar cusp region in figure 1.1). As the solar wind continues downstream it effectively drags the open field lines anti-sunward over the poles (stages 2-4) back to the tail of the magnetosphere (stage 5) where they meet again and, in a simple steady-state picture, the second reconnection event occurs (stages 6-7). The magnetic field lines are once again closed and convect back around opposite equatorward sides of the polar cap, i.e., the dawn and dusk flanks of the magnetosphere, until they meet once again at the dayside reconnection point and the convection cycle repeats itself.
In this open magnetosphere configuration, there is a careful balance between the two reconnection events. The dayside reconnection feeds open flux into the magnetosphere and the nightside process removes it, converting it back to closed flux. If the dayside
1.1. Sun-Earth Interaction 35 reconnection rate exceeds nightside, then open flux loading occurs which may eventually be dissipated explosively via a dramatic episode of nightside reconnection, i.e., geomag- netic substorms. When geomagnetic substorms, of typical duration of a few hours, occur persistently for a few days the disturbance can be classified as a full geomagnetic storm.
In the other magnetospheric configuration (shown in figure 1.2 a)), i.e., in the case of a closed magnetosphere (Bz >0), the Dungey cycle is not established. This is due to there
being no dayside reconnection. At the dayside subsolar location, the northward IMF field lines (labelled ‘A’) run parallel in the same direction as the closed geomagnetic field lines, hence, no reconnection occurs here. However, there is a small amount of reconnection promoted on the nightside of the magnetorsphere. As the solar wind flows downstream (beyond the dayside subsolar point), the field lines (A) drape around the top and bottom of the magnetospheric cavity and across its entire length (tailward). The draped field lines become antiparallel with the tailside closed geomagnetic field lines (D). Here, reconnection occurs on both the top and bottom, creating a new closed field line (which now loops round the dayside) and, also creating a new IMF field line (A) on the night-side (similar to the original IMF field lines (A) on the dayside). The rate of reconnection in this case, is far smaller than in the case of an open magnetosphere.
On time scales I am concerned with (>1sec), the convecting magnetic field lines are considered electrostatic equipotentials. This is due to the fact that the conductivities along them are so high that any slight charge imbalance would be immediately neutralised by a field-aligned current, hence maintaining an apparent zero electric potential gradient This condition may be violated in the so-called auroral acceleration region. Within this region, the Cluster spacecraft collected data from 4000 and 7000km altitude along the same magnetic field line to show a significant potential drop of 500V (Forsythe et al, 2012).
An implication of this phenomenon is that electrostatic equipotential surfaces are traced out by these convecting field lines which map down in altitude from the mag- netosphere to the upper ionosphere. Due to the near-collisionless plasma in the F region, the frozen-in assumption is appropriate for describing the plasma flow. This plasma mo- tion is termed ~E × ~B drift which defines the direction of horizontal flow in the upper thermosphere, i.e., perpendicular to both the electric and magnetic field directions, hence along the equipotential surfaces. For IMF Bz<0, the motion is antisunward over the poles
the polar cap creating two convection cells. For IMF Bz >0, the flow is less predictable
and there can be multiple smaller convection cells set-up. Figure 1.4 shows an average of SuperDARN radar (see section 1.7.3) data (Chisham et al, 2007) from 2002 (plot produced by myself) on a 200x200 magnetic latitude - magnetic local time grid (MLAT-MLT), i.e., a bird’s-eye view of the magnetic pole with the sun situated at infinity off the top of the diagram. The black vectors represent the plasma drift, the blue lines show the electrostatic equipotential lines and the red circle shows Svalbard’s (one of UCL’s observing sites) path around the magnetic pole at ∼75◦ latitude.
Figure 1.4: SuperDARN plasma drift vectors as black arrows (calculated from electro- static potential data by myself with Ruohoniemi and Baker (2007) method and plotted by myself) averaged over entire year of 2002 plotted on an MLAT-MLT grid. The elec- trostatic equipotentials are in blue and Svalbard’s track around the magnetic pole is in red. Data courtesy of University of Leicester (Chisham et al, 2007) and the SuperDARN collaboration (see section 1.7.3).
The cross-polar cap jet is on open field lines, following dayside reconnection, whereas the sunward return flow is carried on the closed field lines after nightside reconnection. The boundary between open and closed magnetic field lines separates these antisunward and sunward flows and manifests itself in the atmosphere as the visible light shows of the
1.1. Sun-Earth Interaction 37 auroral oval.
As the two magnetospheric reconnection processes are also mechanisms which acceler- ate charged particles into the atmosphere, these create aurora. The dayside reconnection is associated with flux transfer events (FTEs) and is responsible for the cusp aurora (see ‘Polar Cusp’ region in figure 1.1), whereas the tailside reconnection creates part of the nightside aurora. The cusp aurora occurs at a higher altitude than the night-side aurora due to the day-side magnetic field lines’ smaller magnetic tension force relative to the nightside’s. Therefore, the dayside reconnection supplies less kinetic energy to accelerate the particles (mostly electrons and protons) causing them to penetrate not so deeply into the atmosphere (∼240km altitude). These particles are called ‘soft’ precipitation with typical energies of ∼hundreds eV. However, the tailside magnetic tension is enormous, so upon a reconnection event, the field lines act as a ‘great elastic band’ catapulting the plasma Earthward with vast amounts of energy so that the charged particles can penetrate the atmosphere further down to lower altitudes. This is called ‘hard precipitation’ with typical energies >keV (typically at ∼110km altitude). Due to the chemistry prevailing at this altitude, this hard precipitation creates a green airglow which is the most common observational signature of aurora, whereas the soft precipitation creates a red glow which is fainter in the sky and so harder to observe with the naked eye.