Coupling between the ionosphere and the magnetosphere

Transcription

1 Chapter 6 Coupling between the ionosphere and the magnetosphere It s fair to say that the ionosphere of the Earth at all latitudes is affected by the magnetosphere and the space weather (whose origin is in the solar wind and in the energetic particles and radiation from the Sun). Especially strong effect the space weather has on the ionosphere of the high latitudes (auroral regions) and that of the polar cap. This chapter is still under development in 2010 and will be extended later to include e.g. a short summary of the solar wind (which should be familiar to most of the students from course Avaruusfysiikan perusteet), magnetospheric electric fields and some dynamical phenomena of coupling (auroral substorms etc.). 6.1 Magnetospheric topology As seen above, the result of the high field-aligned conductivity is that electric fields are mapped from the magnetosphere to the ionosphere. The magnetosphere is largely variable; it responds actively to the properties of the solar wind and the interplanetary magnetic field. These variations are also seen in the current systems and electric fields of the ionosphere. To first order the Earth s magnetic field is that of a dipole whose axis is tilted with respect to the spin axis of Earth by about 11. However, since Earth is immersed in the expanding atmosphere of Sun, called the solar wind, the Earth s magnetic field is confined in a cavity called the magnetosphere (Fig. 6.1). As the solar wind is supersonic and super-alfvénic at 1 AU, a bow shock forms upstream of Earth (not shown in the figure). The solar wind is slowed and compressed before flowing around the magnetopause. The dayside magnetic field is compressed and an extended magnetic tail forms on the nightside. The shape of the magnetosphere implies the presence of magnetospheric currents. The magnetopause is formed by a current sheet that separates the solar wind magnetic field from the magnetospheric magnetic field. In order to produce a long tail, current must flow from dawn to dusk side at the centre of the tail (Fig. 6.1). This is known as the cross-tail current. At the center of the cross-tail current, the magnetic 119

2 120 CHAPTER 6. COUPLING field revers direction, that is why it is called the neutral sheet. The cross-tail current continues to a current flowing around the tail surface. It is important to note, that the magnetic field lines connect different magnetospheric regions to different latitudes in the ionosphere. At low and middle latitudes the field lines resemble dipole field lines and they are connected to a plasma region called the plasmasphere, which contains cool dense plasma (Fig. 6.2). This region has a high plasma content since the magnetic flux tubes fill from below with ionospheric plasma produced in the sunlit ionosphere (see section 3.11). The plasma sheet contains less dense plasma than the plasmasphere, but the energies of electrons (typically 0.6 kev) and ions ( typically 4 kev) are much higher. Plasma sheet is located on closed extended field lines and the plasma sheet particles carry the cross-tail current. A relatively thin layer known as the plasma sheet boundary layer (PSBL) is observed at the interface between the tail lobes and the plasma sheet. The ionospheric mappings of both the plasma sheet and the PSBL form the main part of the nightside auroral oval. In the PSBL, both the density and energy of particles is less than in the plasma sheet. 13The outermost pat of the magnetosphere contain magnetic field lines that extend to hundreds of R e (Earth radius, 1 R e = 6370 km) tailward. These field lines are called open field lines. In the ionosphere, the tail lobes correspond to regions known as polar caps. In the northern tail lobe, the 1.3magnetic MAGNETOSPHERE field lines go toward Earth whereas in the southern tail lobe the field lines point away from Earth. The shape of the magnetosphere is caused by the interaction between the geomagnetic Figure 1.2 field shows and a the sketch interplanetary of the structure magnetic of the field magnetosphere. (IMF) carried The by surface the solar of wind the (Fig. magnetosphere, 6.3). Because the the magnetopause solar wind is ishighly governed conductive, by the magnetopause the interplanetary currents, magnetic also field called B sw Chapman-Ferraro is carried withcurrents the velocity on theofdayside. solar Onwind the nightside, v sw so that thean magnetopause electric field Tail lobe Cross-tail current Plasma sheet Ring current Magnetopause current Figure 1.2. The most important currents systems in the Earth s magnetosphere are the magnetopause current, the cross-tail current in the middle of the magnetotail and the ring Figure 6.1: Magnetosphere of the Earth. current in the inner magnetosphere. Illustration by Teemu Mäkinen. currents encircle the lobes of the magnetotail, where the magnetic field lines are open.

3 6.2. CURRENT SYSTEMS AND GLOBAL CONVECTION 121 Figure 6.2: Plasma regions of the magnetosphere in the noon-midnight meridian plane. E sw = v sw B sw is created. When the interplanetary magnetic field has a southward component, the electric field has a direction from dawn to dusk. In this case also the geomagnetic field lines are connected to the solar wind in the way shown in Fig Then the electric field produced by the solar wind dynamo is mapped along the magnetic field lines to polar regions. The electric field driving the neutral sheet current is oriented from dawn to dusk and the field inside the tail lobes has the same direction. This electric field is mapped to the polar cap where its direction is from dawn to dusk. In the F region the electric field causes plasma flow at a velocity E B/B 2, i.e. from the dayside to the nightside over the polar cap. This plasma flow makes a return flow sunward at latitudes corresponding roughly the auroral oval. The result is that the plasma flow makes a double-cell convection pattern. This convection electric field (E a Fig. 6.4) points in the auroral ovalnorthward in the evening sector and southward in the morning sector. 6.2 Current systems and global convection The electric fields in the polar F region are mapped downwards to the E region altitudes in the manner described in Section 5.7. There they drive currents and these currents create magnetic fields which can be observed at the ground level. Magnetic observations can be used to calculate the equivalent current systems at high latitudes. Since the equivalent currents are mainly of the Hall type, the equivalent current flow is expected to be in the direction opposite to the F region plasma flow. The quiet-time equivalent currents in the polar regions are called the Sq p (solar quiet polar) system. From the plasma flow in Fig. 6.4 one would also expect a double-cell structure. The cells in the true Sq p system are not aligned along the noonmidnight meridian. This is caused by geometry of the interplanetary magnetic field and the the solar wind. The polar cap equivalent current system mainly depends on

4 122 CHAPTER 6. COUPLING v sw B sw bow shock E B sw = -v sw x B sw sw magnetopause E sw solar wind v sw B sw Figure 6.3: Magnetospheric convection during southward IMF. Figure 6.4: Plasma convection in the northern high latitude ionosphere and associated convection electric fields.

5 6.2. CURRENT SYSTEMS AND GLOBAL CONVECTION 123 Figure 6.5: Contours showing the equivalent currents for different orientations of the IMF Bz and By components (Friis-Christensen et al., 1985).

6 124 CHAPTER 6. COUPLING Figure 6.6: The IMAGE north-south chain of magnetograms measured in northern Scandinavia. the B z component of the interplanetary magnetic field (the z axis points northwards of the ecliptic plane). This is shown in Fig The orientation of the cells is twisted by about 3 hours from the noon-midnight meridian. The currents are weak when B z > 0 and are greatly intensified when B z gets negative values. This is associated with the fact that merging of the interplanetary and magnetospheric electric fields takes place when B z < 0, i.e. when the IMF has a southward component. The effect of the B y component is smaller (the y axis points towards of the dusk side of the Earth). Fig. 6.6 shows observations from a chain of magnetometers, extending from Bjørnøya in the Arctic ocean down to Pello in northern Finland. The X (northward) component of the geomagnetic field shows a similar behaviour at all sites. In the afternoon and evening the magnetic disturbance in X shows a positive bay, but after 20 UT (about 22:30 local time) a negative bay. The interpretation is that, in the afternoon-evening sector, there is an eastward current causing the magnetic variation and, when the magnetic variation changes sign close to midnight, the current direction turns westwards. If we assume that the magenetic disturbance is mainly

7 6.2. CURRENT SYSTEMS AND GLOBAL CONVECTION 125 Figure 6.7: Schematic figure of high-latitude currents (left) and plasma flow (right) on the night side. due to the Hall current, this would mean that the electric field is northward in the afternoon and evening and southward around midnight and morning. This is in agrement with the sketch in Fig The evening sector current is known as the eastward electrojet (EEJ) and the morning sector current as the westward electrojet (WEJ). The region between the two electrojets is called the Harang discontinuity (HD). Perpendicular to the eastward electrojet, a current flows northwards in the evening sector. It is connected to a downward field-aligned current in the south and to an upward field-aliged current in the north. In the morning sector the currents flow in opposite directions.q A downward field-aligned current flows at the polar cap boundary. It is divided into a current flowing over the polar cap and an equatorward current flowing in the region of the westward electrojet. The equatorward current is finally connected to an upward field-aligend. The field-aligned at the poleward side of the electrojet are called Region 1 currents and those at the equator side are called Region 2 currents. Upward currents are caused by electrons precipitating to the ionosphere from the magnetosphere. Energetic electrons produce ionisation which increases the conductivity and thus also enhances the currents. Then conductivities are not horizontally homogeneous, and therefore Pedersen currents also contribute to the electrojets and Hall currents are partly connected to the field-aligend currents. Fig. 6.8 shows the average patterns of the field-aligned currents measured by satellites when the interplanetary magnetic field points southwards. In reality the magnetosphere and the currents in the polar ionosphere are not stationary. The magnetosphere is not a passive circuit which transfers energy from solar wind to high-latitude ionosphere. Actually the magnetosphere is a system which is greatly activated when the interplanetary magnetic field turns southwards. Violent

8 126 CHAPTER 6. COUPLING Figure 6.8: The average pattern of Region1/Region2 currents when the IMF is southward. The cusp currents at high latitudes at noon depend highly on the IMF B y component (Iijima and Potemra, 1976). changes, known as magnetospheric substorms, take place in the magnetosphere. In the ionosphere, these changes are observed as magnetic and auroral substorms and changes in the size of the polar cap. The energy released in the ionosphere during substorms originates from the solar wind, but the magnetosphere has an active role in storing the energy and releasing it in explosive bursts. The temporal changes in substorms follow follow a certain pattern which we shall not discuss in detail here. An essential feature in the substorm is that the cross-tail current disrupts para) b) Figure 6.9: Disruption of the cross-tail current and formation of the substorm current wedge as 3D (a) and 2D (b) presentation.

9 6.2. CURRENT SYSTEMS AND GLOBAL CONVECTION 127 Figure 6.10: Auroral images by the IMAGE satellite FUV instrument during the Bastille day storm. tially and connects via field-aligned currents to the ionosphere. In the dawn side of the tail, the current flows to the ionosphere, where it is connected to a strong westward current known as substorm electrojet. In the premidnight sector this current is connected to an upward field-aligned current. The whole current system is known as the substorm current wedge (Fig. 6.9). The same happens both in the northern and southern hemispheres, i.e. field-aligned currents emerge from the cross-tail current towards both hemispheres. The substorm current system evolves with time. It starts within a narrow region close to the midnight and typically expands toward west, east and poleward with time, forming a substorm bulge. The whole area of the substorm bulge is filled with particle precipitation from the magnetosphere to the ionosphere which increases electron densities there and produces auroral luminosity. The western edge of the auroral bulge is known as the westward travelling surge (WTS) and there the most intense particle precipitation and upward current take place. Temporal evolution of a substorm sequence measured by a polar orbiting IMAGE satellite is shown in Fig The second panel shows an onset of a substorm, in this unusually disturbed case not only in the night sector (upper part of the figure), but also in the day sector. The third panel shows clearly the WTS and the auroral bulge. During the course of the event, the bulge expands westward, toward dayside.

10 128 CHAPTER 6. COUPLING 6.3 Cowling channel model of auroral electrojets The auroral oval (one in both hemispheres) is a region of higher conductance than its surroundings. The auroral oval exists at all times, even during the northward IMF, but is then much weaker and smaller in size. As Fig. 6.7 shows, the oval is typically during southward IMF connected to field-aligned currents close to its edges. Inside the oval, more narrow structures of enhanced precipitation exist, like auroral arcs, which typically have their own current systems with both horisontal and field-aligned currents. When auroral precipitation takes place in the ionosphere, there are two basic ways the ionosphere-magnetosphere system may behave. One is to generate enhanced currents, including field-aligned currents, and the other is to produce a polarization electric field inside the high-conducting area. Let s study the case of high-conductance slab, which is restricted in the north-south direction (y), but extended in the east-west direction (x) in Fig The magnetic field is vertical. For simplicity, let us assume that conductances are zero outside slab. If the electric field in the slab has both the x and y components, the height-integrated horizontal current in the slab is ( ) ( ) ( ) Jx ΣP Σ = H Ex. (6.1) J y Σ H If we assume that no field-aligned currents can flow at the edges of the slab, a polarization electric field in the north-south direction must form so that the northward current is zero J y = Σ H E x + Σ P E y = 0. (6.2) Note that E y is now the total electric field, which can be interpreted to be a sum of a convection and polarization electric field. From the eq. above, we can get a necessary relationship between the components of the electric field Σ P E y E y = Σ H Σ P E x. (6.3) Then the horizontal current can flow only in the direction of the high-conductance y J B x Figure 6.11: Schematic figure of enhanced conductance slab, which is restricted in the north-south direction (y), but extended in the east-west direction (x).

11 6.4. EQUATORIAL ELECTROJET 129 slab and it is J x = Σ P E x Σ H E y = Σ P E x + Σ H Σ H Σ P E x = Hence we have arrived at the Cowling conductance ( Σ P + Σ2 H Σ P ) E x. (6.4) Σ C = Σ P + Σ2 H Σ P, (6.5) which now gives the enhanced east-west current density in the slab. Even though it is known that Region 1 and 2 currents exist, the Cowling channel model has been suggested to the auroral electrojets (the eastward and the westward electrojet) and also for the substorm electrojet. Partial polarisation may take place, but it s not entirely clear how important it is in various situations. 6.4 Equatorial electrojet Close to magnetic equator, the geomagnetic field is horizontal and the ionosphere is a horizontal conducting slab. According to eq. (5.52), a northward electric field E x creates a northward current given by j x = σ E x and an eastward electric field E y creates an eastward current j y = σ C E y. In principle, a strong meridional (northward) current should be generated by E x, since σ has a high value (see Fig. 5.9). However, at a few degrees from the equator values of σ decrease and this current will produce a polarisation electric field that prevents the flow of the meridional current. No steep gradients limit the zonal current. In addition σ P σ H at about 100-km height, so that the Cowling conductivity σ C = σ P + σ 2 H/σ P gets very high values there (see the height-integrated values in Fig. 5.9). The global tidal winds create a zonal component of the tidal electric field, which has been measured by the incoherent scatter radar facility at Jicamarca, Peru. This electric field is very small in amplitude, 1 mv/m, and it is eastward on the dayside and westward in the nightside. The resulting current is the equatorial electrojet which flows eastwards in the daytime. At night a weaker westward equatorial electrojet is observed. The equatorial electrojet flows at low altitudes; rocket measurements have showed that it lies at altitudes between 90 and 130 km, which is determined by the height profile of the Cowling conductivity. Fig shows an example of magnetic observations from a set of sites on both sides of the magnetic equator (this is from west Africa, where the magnetic equator lies on the northern hemisphere). The figure shows the latitudinal variation of the two magnetic components H (meridional) and Z (vertical) and the declination D. A different curve has been plotted for each hour. The results indicate that the H component is greatly increased in daytime around the magnetic equator, which is a manifestation of an eastward current. The equatorial electrojet was originally found from this kind of magnetic observations. Note that this figure is in accordance with The S q current system shown in Fig

12 130 CHAPTER 6. COUPLING Figure 6.12: Magnetograms showing the equatorial electrojet. Figure 6.13: Equatorial electrojet current densities inferred from 2600 passes of the CHAMP satellite over the magnetic equator between 11:00 and 13:00 local time (http : //geomag.org/info/equatorial e lectrojet.html)

13 6.5. JOULE HEATING BY IONOSPHERIC CURRENTS Joule heating by ionospheric currents The magnetosphere and ionosphere exchange energy in the form of electromagnetic energy flux accompanied by electric fields and field-aligned currents, by particle fluxes, and by plasma waves. In the following, we will concentrate on the first form of energy, the electromagnetic energy. The exchange of energy between plasma and electromagnetic field is described by Poynting s teorem W t + S + j E = 0, (6.6) where W = (ɛ 0 E 2 + B 2 /µ 0 )/2 is the energy density in the electromagnetic field, S = E B/µ 0 is the Poynting vector and j E is the energy exchange per unit volume per unit time between the field and plasma. In a steady state, the equation above becomes as S + j E = 0, (6.7) indicating that any plasma volume gaining energy (j E > 0) is a sink of electromagnetic energy flux ( S < 0). Thus currents flowing in the direction of the electric field dissipate energy. The ionospheric Joule heating is calculated in the frame of reference of the neutral atmosphere. When the neutral atmosphere is moving at a velocity u, the electric field in the frame of reference of the neutral atmosphere E is given by Lorentz transformation E = E + u B (6.8) and then the electric field in the Earth-fixed frame of reference is E = E u B. The current density in the ionosphere doesn t depend on the frame of reference and is according to (5.2) j = σ (E + u B), (6.9) where σ is the conductivity tensor. Now, the electromagnetic energy exchange rate in the ionosphere can be expressed as follows by using eq. (6.8) j E = j E j (u B) = j E + u (j B). (6.10) The first term on the right hand side, j E is the Joule heating rate and the second term u (j B) is the mechanical energy transfer rate to the neutral gas. By using eq. (6.8) and (6.9), Joule heating rate becomes as j E = σ P {E + (u B)} 2. (6.11) The Joule heating defined this way corresponds to heating due to collisions between ions and neutrals and the thermal energy is distributed roughly evenly between these species. The effect of Joule heating can be seen in increased ion temperature T i.

14 132 CHAPTER 6. COUPLING The Joule heating rate in the equation above depends on height as follows q j (z) = σ P (z){e + (u(z) B)} 2 (6.12) since large-scale E maps with a constant value to the E region and B can be considered as constant within the rather small altitude range (about km) that contributes to Joule heating. The neutral wind height profile u(z) in the E region is often unknown. It can be ignored if u E/B, which is often the case in the auroral ionosphere. In that case the equation above is simplified to q j (z) = σ P (z)e 2 (6.13) and then the height dependence of Joule heating rate is the same as for Pedersen conductivity. In this case, the height-integrated Joule heating rate Q j can be expressed by using the height-integrated Pedersen conductivity Σ P Q j = z2 z 1 q j (z)dz = z2 z 1 σ P (z)dz E 2 = Σ P E 2 (6.14) where z 1 is at the base of the E region and z 2 is at or below the F region maximum. The unit of Q j is W/m 2.