Gareth Chisham

A statistical study of the open magnetic flux content of the magnetosphere at the time of substorm onset

In this paper we determine the probability of substorm onset as a function of open magnetic flux in the magnetosphere by comparing the occurrence distribution of open flux observed at all times with that observed at the time of substorm onset. The open magnetic flux is measured in 12735 auroral images of the ionospheric polar cap from the IMAGE WIC detector. The probability of substorm onset is found to be negligible for fluxes below ~0.3 GWb, increases almost linearly until ~0.9 GWb, and is undefined above this. We also demonstrate that those substorms which show a clear particle injection signature at geosynchronous orbit, as measured by the LANL spacecraft, occur, on average, with higher values of open flux than those showing no activity. We discuss these results in the context of various hypotheses for substorm onset.

An ionospheric convection signature of antiparallel reconnection

This paper sets out a critical test of the antiparallel merging hypothesis. For the conflicting theories of antiparallel and subsolar reconnection, we model the location of reconnection regions on the dayside magnetopause, their ionospheric footprints, and the resulting ionospheric convection patterns. We show that antiparallel reconnection, under particular seasonal and solar wind conditions, gives rise to a distinctive ionospheric convection signature. Specifically, around midwinter with a quasi-steady solar wind and IMF Bz < 0 and |By | ∼ |Bz |, we predict equatorward flow in the noon sector with poleward flow either side of noon if the antiparallel merging hypothesis is correct. In contrast, we predict poleward flow in the noon sector in midwinter under these solar wind conditions if the subsolar reconnection hypothesis is correct and in other seasons under both hypotheses. We go on to present radar and spacecraft data for an interval which satisfies the above seasonal and solar wind criteria, demonstrating that the convection signature of antiparallel merging is present. This is not consistent with the subsolar merging hypothesis.

Multisatellite and ground-based observations of a tailward propagating Pc5 magnetospheric waveguide mode

Using data from the Active Magnetospheric Particle Tracer Explorers (AMPTE) Charge Composition Explorer (CCE) satellite near local noon, and the AMPTE Ion Release Module (IRM) and United Kingdom Subsatellite (UKS) on the morning flank, we investigate the tailward propagation of a compressional Pc5 wave (380 s period) on October 28, 1984, and suggest that the observation represents a magnetospheric waveguide mode. The observed wave is a transient oscillation lasting about five wave cycles with an observed time of flight from CCE to IRM/UKS of 180 s. The first three or four cycles at the leading edge of the event display a remarkable wave packet coherence between CCE and IRM/UKS despite their large separation in the magnetosphere (∼ 6.2 RE). A time of flight analysis suggests a waveguide mode group speed ∼ 220 km s−1, being much less than the local Alfven speed and comparable to the expected sub-Alfvenic propagation speed of a waveguide mode. The ground-based signature of the event is also observed near local noon by the European Incoherent Scatter magnetometer cross, displaying the same period and lasting for the same number of cycles. The ground-based data show no evidence of a field line resonance; the event is monochromatic, and there is little variation of amplitude and polarization with latitude. An increase in solar wind ram pressure of ∼ 30% is observed by IMP 8 upstream in the solar wind just prior to the event, and this may have provided an impulsive energy source for the waveguide mode. This is the first time the signature of a particular waveguide mode harmonic has been observed by satellites which are widely spaced in the magnetosphere. Moreover, we present the first unambiguous observation of the downtail propagation of a waveguide mode. The observation also provides possible evidence of dispersion in the Earths outer magnetospheric waveguide.

The interplanetary magnetic field influences mid-latitude surface atmospheric pressure

The existence of a meteorological response in the polar regions to fluctuations in the interplanetary magnetic field (IMF) component By is well established. More controversially, there is evidence to suggest that this Sun–weather coupling occurs via the global atmospheric electric circuit. Consequently, it has been assumed that the effect is maximized at high latitudes and is negligible at low and mid-latitudes, because the perturbation by the IMF is concentrated in the polar regions. We demonstrate a previously unrecognized influence of the IMF By on mid-latitude surface pressure. The difference between the mean surface pressures during times of high positive and high negative IMF By possesses a statistically significant mid-latitude wave structure similar to atmospheric Rossby waves. Our results show that a mechanism that is known to produce atmospheric responses to the IMF in the polar regions is also able to modulate pre-existing weather patterns at mid-latitudes. We suggest the mechanism for this from conventional meteorology. The amplitude of the effect is comparable to typical initial analysis uncertainties in ensemble numerical weather prediction. Thus, a relatively localized small-amplitude solar influence on the upper atmosphere could have an important effect, via the nonlinear evolution of atmospheric dynamics, on critical atmospheric processes.

Spatial structure of ionospheric convection velocities in regions of open and closed magnetic field topology

We present a spatial structure function analysis of ionospheric velocity, measured by the Halley Super Dual Auroral Radar Network (SuperDARN) radar over five years. We show evidence for scale-free velocity fluctuations for separations from 45 km to ∼1000 km. For larger separations a deviation from scale-free structure is seen that we interpret as the influence of the global convection pattern. We find evidence for scale-free structure in regions of both open and closed field line topology, though with different power-law exponents. The measured power-law exponents poleward and equatorward of the open-closed field line boundary are 0.31 and 0.39 respectively. We propose that the scale-free spatial structure of ionospheric velocity fluctuations on open magnetic field lines is a reflection of the spatial structure of the solar wind and the scale-free spatial structure of ionospheric velocity fluctuations on closed magnetic field lines is dictated by the internal dynamics of the magnetosphere-ionosphere system.

The response of the HF radar spectral width boundary to a switch in the IMF By direction: Ionospheric consequences of transient dayside reconnection?

In the high-latitude dayside ionosphere, the movement of the HF radar spectral width boundary (SWB) provides a good proxy for the movement of the open-closed field line boundary around magnetic local noon. By studying the dynamics of the spectral width boundary we can investigate features of the dayside ionospheric response to changes in the interplanetary magnetic field (IMF). The high temporal and spatial resolution of the Super Dual Auroral Radar Network (SuperDARN) HF radars make them good tools to study these features. In this paper, we use the Halley HF radar in Antarctica to study the equatorward motion of the SWB which appears to occur in response to a large change in the direction of IMF By. The spectral width boundary initially moves equatorward in the form of a U-shaped bulge close to magnetic local noon. This bulge then expands longitudinally to earlier and later magnetic local times. Merged velocity vectors from two Antarctic HF radars describe the flow velocity variation in the boundary region. The flow equatorward of the boundary follows the contours of the boundary as it expands. The flow poleward of the boundary is directed at more oblique angles to the boundary. This study represents the first clear two-dimensional observation of the formation of an equatorward bulge on the polar cap boundary which may be associated with changes in dayside reconnection and presents a unique observation of the variation of the ionospheric flow in the locality of the boundary. We discuss the possible interpretations of this event and its consequences to our present understanding of the ionospheric response to changes in the IMF.

Solar wind‐driven geopotential height anomalies originate in the Antarctic lower troposphere

We use National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data to estimate the altitude and time lag dependence of the correlation between the interplanetary magnetic field component, By, and the geopotential height anomaly above Antarctica. The correlation is most statistically significant within the troposphere. The peak in the correlation occurs at greater time lags at the tropopause (∼6–8 days) and in the midtroposphere (∼4 days) than in the lower troposphere (∼1 day). This supports a mechanism involving the action of the global atmospheric electric circuit, modified by variations in the solar wind, on lower tropospheric clouds. The increase in time lag with increasing altitude is consistent with the upward propagation by conventional atmospheric processes of the solar wind-induced variability in the lower troposphere. This is in contrast to the downward propagation of atmospheric effects to the lower troposphere from the stratosphere due to solar variability-driven mechanisms involving ultraviolet radiation or energetic particle precipitation.

Poleward‐moving HF radar flow bursts in the cusp: Transient changes in flow speed or direction?

Poleward-moving line-of-sight velocity “flow bursts” have been observed in the cusp by two southern hemisphere SuperDARN HF radars with overlapping fields-of-view. This has allowed the estimation of unambiguous two-dimensional velocity vectors in the vicinity of the “flow bursts”. Rather than showing enhancements in the flow magnitude, the velocity vectors suggest that the line-of-sight velocity enhancements are a result of a change in the direction of the flow associated with latitudinal motion of the convection reversal boundary. These observations may have important implications for understanding the ionospheric footprint of flux transfer events, and also illustrate that caution is needed when interpreting line-of-sight velocity data from single radars.

IMF clock angle control of multifractality in ionospheric velocity fluctuations

We present an analysis of 8 years of meridional line-of-sight ionospheric plasma velocity measurements from the Halley SuperDARN radar which investigates the effect of the interplanetary magnetic field (IMF) clock angle on the scaling exponents of the first three order velocity structure functions. We only use velocity measurements made poleward of the open/closed magnetic field line boundary in the nightside ionosphere. The measured scaling exponents are consistent with multifractal Kraichnan-Iroshnikov turbulence for all clock angles but with varying intermittency that decreases to zero during purely northward IMF conditions. We thus propose that intermittency is inherited from the solar wind but also discuss other possible reasons for this relationship. Citation: Abel, G. A., M. P. Freeman, and G. Chisham (2009), IMF clock angle control of multifractality in ionospheric velocity fluctuations, Geophys. Res. Lett., 36, L19102, doi:10.1029/2009GL040336.

On the probability distributions of SuperDARN Doppler spectral width measurements inside and outside the cusp

The Doppler spectral width of backscatter from the SuperDARN HF radars has been routinely used to identify the ionospheric cusp. The probability distribution of spectral width values has been described as Gaussian ( normal) in the cusp and exponential equatorward of it. Here we re-examine the empirical distributions and show that they are better described by a log-Levy distribution in both the cusp and non-cusp regions, with the same Levy index a approximate to 1.85 but varying Levy location parameter. The spectral width distribution may also be log-Levy at other magnetic local times, but with generally different parameters.