IM-Waves and Ionospheric Heaters

One method by which we can influence the ionosphere-magnetosphere system, is by using high- frequency (HF) radars to heat electrons in the E-region. This heating lowers the ion-electron recombination rate,α, producing local enhancements in number density. In the context of IM- waves, these radars, called ionospheric heaters, open two exciting possibilities: (i) that we can use ionospheric heaters to experimentally verify IM-wave theory; and (ii) that IM-wave theory can help efforts to efficiently send Alfv´en waves into the magnetosphere by modification of the E-region.

5.5.4.1 Experimental Verification of IM-Wave Theory

First let us consider how a density perturbation, created by an ionospheric heater, would be ex- pected to behave as an IM-wave. The ideal conditions for such an experiment are low background number density so that IM-waves have a long lifetime (see discussion of the decay time in Section 5.5.1), and a strong background perpendicular electric field so that IM-waves evolve significantly during their lifetime. These conditions naturally occur at night in the downward return current re- gion adjacent to visible auroral arcs — the same conditions favoured by IFI experiments (Streltsov et al., 2010). If ionospheric heating creates an elliptical perturbation that is extended in theE_×B0 direction, then it will evolve as a 1D IM-wave, of the sort that we have explored extensively in this chapter.

Density perturbations, produced by heating, will evolve as ideal IM-waves provided they are much wider than the electron inertial length in the low-altitude magnetosphere. They should,

therefore, advect in the direction of the electric field at a speed given byvIM, and may steepen as

troughs catch up with crests, depending on the amplitude of the perturbations. Thus, an experi- mental programme might try the following types of driving, for which predictions are made:

1. Beam constantly on at a fixed location.

We expect to see E-region density increase at the heated location. Advection of ideal IM- waves will then create a tail, reminiscent of a wind-sock, and this will decay with an e- folding length (parallel to the electric field)λd= (vIMτd)−1.

2. Single pulse at a fixed location.

A single pulse will create a wavepacket, which advects as an ideal IM-wave. Observations of the wavepacket after the heating pulse should reveal advection, damping, and (depending on the amplitude of the wavepacket) wavebreaking. Here, the aim is to experimentally reproduce the results of Sections 5.2.3 and 5.4.1.

3. Pulsed driver at a fixed location.

By pulsing the driver, or modulating it with a periodic function, a wavetrain can be pro- duced, which again should exhibit the features of advection, decay by recombination, and possibly wavebreaking.

We can also imagine focusing the radar beam to an ellipse that is narrower than the electron inertial length in the magnetosphere. A wavepacket produced in this fashion should remain fairly stationary (in the direction parallel to the electric field), and oscillate atωIM, reproducing the

simulation presented in Section 5.3.3. Unfortunately, with present technology, it seems doubtful that an ionospheric heater could achieve the necessary tight focus (a few kilometres or less), or that observing instruments would reliably resolve these features. It nonetheless remains a possibility for the future.

5.5.4.2 Using IM-Waves to Send Waves into the Magnetosphere

Whilst we have mostly viewed IM-waves in terms of E-region number density, they are equally associated with field-aligned currents. They can therefore be used to produce FACs which prop- agate upwards in the form of shear Alfv´en waves. Depending on conditions, these waves may largely escape from the ionospheric Alfv´en resonator, which opens possibilities to efficiently send ULF Alfv´en waves into the magnetosphere.

The idea of heating the E-region to send ULF (or VLF) Alfv´en waves into the magnetosphere is well established and has been confirmed by several experiments. Possible applications for such a technique are numerous, for example, ULF Alfv´en waves have the potential to: interact

with energetic particles in the magnetosphere (Inan et al., 1985); move plasma along field-lines by the (non-linear) ponderomotive force (Streltsov and Lotko, 2008); ‘tag’ magnetic field-lines with a signature recognisable by spacecraft (Robinson et al., 2000), aiding the mapping of Earth’s magnetic field; or be used for ‘active’ magnetoseismology, where measurements of the waves’ propagation (e.g. travel time to the conjugate ionosphere) are exploited to reveal properties of the magnetosphere. One can even imagine that it might be possible to trigger geomagnetic substorms by sending a wave into a marginally-stable region of the magnetosphere-ionosphere system: whilst this last idea is very speculative, the concept of actively influencing the timing of storms is also very appealing, whether storms are triggered early to reduce their impact (e.g. when less energy has accumulated) or to make the best use of scientific resources (e.g. to coincide with satellite orbits).

How do we get the biggest effect (strongest FAC) for a given heating power? The properties of IM-waves suggests an answer. Recall that an ionospheric heater creates a localised region where the recombination coefficient has been lowered. In the absence of a background electric field, the low recombination coefficient produces an E-region density enhancement that remains stationary and decays when heating is switched off; this does not create FACs because there is no incident magnetic field perturbation to reflect. In the presence of a background electric field, however, FACs are created, and the initial density enhancement advects as an IM-wave. Therefore, if the beam remains stationary, then the initial density perturbation will advect into unheated areas where the recombination coefficient is larger, damping the IM-wave. The damping problem can be avoided, however, if the heating region tracks at the IM-advection speedvIM. This way, the

density enhancement remains inside the region of suppressed recombination at all times, ensuring that it has the greatest possible amplitude, with correspondingly strong FACs.

We can also use ionospheric feedback instability to further increase the E-region density pertur- bation, and hence boost the strength of the upgoing FACs, by carefully choosing the width of our heating region. Figure 5.10 illustrates the desired situation for a Gaussian heating profile: IFI will cause growth if the wavepacket advects a distance FWHM/2in the time,T, taken for an Alfv´en wave to complete on full cycle inside the magnetospheric cavity. We therefore wish to choose the width of the heating region so that

FWHM = 2T

vIM

. (5.78)

This analysis is in excellent agreement with recent results presented by Streltsov and Pedersen (2010), who used computer simulations to search for effective methods to produce FACs using ionospheric heating. These authors observed that E-region density features naturally move in the direction of the electric field (at a speed of74.3ms−1in their simulation), and that stronger FACs are produced when the region of ionospheric heating is tracked at this speed. By linking their

results with an IM-wave interpretation, we are able to propose a formula for the advection speed (vIM), and to explain why efficiency improves if the heating region tracks at this speed, adding

significant value to their findings. Recent correspondence has confirmed that74.3ms−1 agrees with the value ofvIMcomputed for their simulations, withvAevaluated at the base of the F-region

Coupled Magnetosphere-Ionosphere Dynamics

Driven by Field-Aligned Currents

6.1

Introduction

This chapter seeks to answer, “How does the ionosphere react to a large-scale current system, given that the ionosphere also modifies magnetospheric current?” The ground has been carefully prepared: self-consistent models and codes have been developed (Chapter 3); steady-states have been closely examined (Chapter 4); and ionosphere-magnetosphere waves have been studied and developed as a means of understanding ionosphere-magnetosphere dynamics (Chapter 5). Now we apply these tool to ionosphere-magnetosphere dynamics when the system is driven by large- scale field-aligned currents originating from the outer magnetosphere.

The chapter is organised in three main sections, with the first division according to sustainability of the downward current density: the steady-states of Chapter 4 show a changeover when the current density drawn by the magnetosphere exceeds that which the ionosphere can supply by ionisation, so we treat the sustainable case,_|2ji|< jcfor all downward current, in its own Section

6.2. When|2ji| > jc in a downward current region, the dynamics become significantly more

complex, with the formation and broadening of an ionospheric plasma-density cavity. It is useful to first examine the dynamics of broadening for an ideal magnetosphere, which we treat in Section 6.3, establishing key ideas before bringing in inertial effects in Section 6.4. As usual, the different sections are tied together in a discussion to conclude the chapter.